Eutrophication management in surface waters using lanthanum modified bentonite: A review

Combined measures in lake restoration - A powerful approach as exemplified from Lake Groote Melanen (the Netherlands)

Controlling lake eutrophication is a challenge. A case-specific diagnostics driven approach is recommended that will guide to a suite of measures most promising in restoration of eutrophic lakes as exemplified by the case of the shallow lake Groote Melanen, The Netherlands. A lake system analysis identified external and internal nutrient load as main reasons for poor water quality and reoccurring cyanobacterial blooms in the lake. Based on this analysis, a package of restoration measures was implemented between January 2015 and May 2016. These measures included fish removal, dredging, capping of peat rich sediment with sand and an active barrier (lanthanum-modified bentonite), diversion of two inlet streams, reconstruction of banks, and planting macrophytes. Dredging and sand capping caused temporarily elevated turbidity and suspended solids concentrations, while addition of the lanthanum-modified clay caused a temporary exceedance of the Dutch La standard for freshwaters. Diversion of inflow streams caused 35 % less water inflow and larger water level fluctuations, but the lake remained water transporting with strongly improved water quality as was revealed by comparing five years pre-intervention water quality data with five years' post-intervention data. Total phosphorus concentration in the water column was reduced by 93 % from 0.47 mg P l-1 before the intervention to 0.03 mg P l-1 after the intervention, total nitrogen by 66 % from 1.27 to 0.21 mg N l-1, total chlorophyll-a by 75 % from 68 to 16 µg l-1, cyanobacteria chlorophyll-a by 88 % from 32 to 4 µg l-1. Turbidity had declined by 58 % from 23.5 FTU to on average 9.9 FTU. No cyanobacteria blooms were recorded over the entire post-intervention monitoring period (2016-2021). Submerged macrophytes increased from complete absence before intervention to around 10 %-15 % coverage after intervention. Repeated fish removal lowered the fish stock to below 100 kg ha-1 with 12 % of bream and carp remaining. Hence, the package of cohesive measures that was based on a thorough diagnosis resulted in rapidly, strongly and enduringly improved water quality. This case provides evidence for the power of combining measures in restoring eutrophic lakes.

Environmental impacts of selected metal cations for phosphorus capture in natural waters: A synthesis

Cultural eutrophication from excessive human-related nutrient (phosphorus, P, and nitrogen, N) inputs is a major concern for water quality. Because P historically was regarded as the critical nutrient in controlling noxious algal/plant growth, P became the focus of "capturing" techniques, with emphasis on removal performance rather than environmental impacts. Here, we synthesize a literature review of known environmental effects linked to use of metal-cation-based P-capturing materials under eutrophic conditions in freshwaters. P-capturing materials with functional cations based on aluminum (Al), calcium (Ca), iron (Fe), lanthanum (La), and magnesium (Mg) were reviewed in terms of their ecotoxicity, persistence, and bioaccumulation-standard criteria used to evaluate environmental risks of chemical substances. We found very few published studies on environmental impacts of metal-cation-based P-capturing materials under eutrophic conditions. Available reports indicated that environmental effects vary depending on the selected material, dose, target organism(s), and experimental conditions. The Al-based materials had the potential to negatively impact various biota; several Fe-based materials caused various levels of toxicity in a limited group of aquatic organisms; La-based materials can bioaccumulate and some were linked to various harmful effects on biota; and Mg-based materials also adversely affected various organisms. The limited number of published studies underscores the need for further research to characterize the environmental impacts of these materials. Results can be used to guide future work and can assist resource managers in sustainable management strategies. Among various research needs, future assessments should assess the impacts of chronic exposures on sensitive species under realistic field conditions in eutrophic waters.

Longevity and efficacy of lanthanum-based P remediation under changing dissolved oxygen availability in a small eutrophic lake

We set out to study the seasonal variations in porewater phosphorus and lanthanum concentrations in the dated sediment cores from a small eutrophic lake that has been treated with Phoslock, a lanthanum-modified bentonite (LMB) amendment. Three sites were sampled when the hypolimnion was either oxygenated or anoxic: (i) the lake's deepest point, (ii) a littoral site receiving inflows from the catchment, and (iii) a littoral site influenced by nearby septic tanks. Phosphate (PO43--P), lanthanum (La), iron (Fe), dissolved organic carbon (DOC) and sulfate (SO42-) were measured in porewater samples. An inverse diagenetic model was used to quantify fluxes of dissolved elements across the sediment-water interface as well as the net rate of their reactions along the porewater concentration gradients. Results show that porewater P and Fe underwent strong seasonal dynamics, while La did not. P fluxes, 20-fold higher at the deepest site than elsewhere in the basin, were influenced by anoxic conditions in the hypolimnion during summer and winter, suggesting that P mobility remained sensitive to redox fluctuations despite the addition of La. At the deepest site, fluxes of P across the sediment-water interface increased from 1 to 9 × 10-9 μmol cm-2 s-1 between spring and summer, while the rate of P production to the porewater also increased a hundredfold. These increases were concurrent with Fe mobilization. Finally, sediment dating shows that the fraction of P sequestered by La is buried under freshly deposited sediment at a rate of 2-3 mm per year. These results indicate that external P fluxes and erosion control remain crucial to maintain the longevity of the LMB treatment.

Lanthanum-modified pyroaurite as a geoengineering tool to simultaneously sink Microcystis cyanobacteria and immobilize phosphorus in eutrophic water

Excessive phosphorus (P) in eutrophic water induces cyanobacterial blooms that aggravate the burden of in-situ remediation measures. In order to ensure better ecological recovery, Flock & Lock technique has been developed to simultaneously sink cyanobacteria and immobilize P but requires a combination of flocculent and P inactivation agent. Here we synthesized a novel lanthanum-modified pyroaurite (LMP), as an alternative for Flock & Lock of cyanobacteria and phosphorus at the background of rich humic acid and suspended solids. LMP shows a P adsorption capacity of 36.0 mg/g and nearly 100 % removal of chlorophyll-a (Chl-a), turbidity, UV254 and P at a dosage (0.3 g/L) much lower than the commercial analogue (0.5 g/L). The resultant sediment (98.2 % as immobile P) exhibits sound stability without observable release of P or re-growth of cyanobacteria over a 50-day incubation period. The use of LMP also constrains the release of toxic microcystins to 1.4 μg/L from the sunk cyanobacterial cells, outperforming the commonly used polyaluminum chloride (PAC). Similar Flock & Lock efficiency could also be achieved in real eutrophic water. The outstanding Flock & Lock performance of LMP is attributable to the designed La modification. During LMP treatment, La acts as not only a P binder by formation of LaPO4, but also a coagulant to create a synergistic effect with pyroaurite. The controlled hydrolysis of surface La(III) over pyroaurite aided the possible formation of La(III)-pyroaurite networking structure, which significantly enhanced the Flock & Lock process through adsorption, charge neutralization, sweep flocculation and entrapment. In the end, the preliminary economic analysis is performed. The results demonstrate that LMP is a versatile and cost-effective agent for in-situ remediation of eutrophic waters.

Phosphorus immobilization/release behavior of lanthanum-modified bentonite amended sediment under the dual effects of pH and dissolved organic carbon

Lanthanum modified bentonite (LMB) is typical P-inactivating agent that has been applied in over 200 lakes. Dissolved organic carbon (DOC) and high pH restrict the phosphorus (P) immobilization performance of LMB. However, the P immobilization/release behaviors of LMB-amended sediment when suspended to overlying water with high pH and DOC have not yet been studied. In the present work, batch adsorption and long-term incubation experiments were performed to study the combined effects of pH and DOC on the P control by LMB. The results showed that the coexistence of low concentration of DOC or preloading with some DOC had a negligible effect on P binding by LMB. In the presence of DOC, the P adsorption was more pronounced at pH 7.5 and was measurably less at pH 9.5. Additionally, the pH value was the key factor that decided the P removal at low DOC concentration. The increase in pH and DOC could significantly promote the release of sediment P with a higher EPC0. Under such condition, a higher LMB dosage was needed to effectively control the P releasing from sediment. In sediment/water system with intermittent resuspension, the alkaline conditions greatly facilitated the release of sediment P and DOC, which increased from 0.087 to 0.581 mg/L, and from 11.05 to 26.56 mg/L, respectively. Under the dual effect of pH and DOC, the P-immobilization performance of LMB was weakened, and a tailor-made scheme became essential for determining the optimum dosage. The desorption experiments verified that the previously loaded phosphorus on LMB was hard to be released even under high pH and DOC conditions, with an accumulative desorption rate of less than 2%. Accordingly, to achieve the best P controlling efficiency, the application strategies depending on LMB should avoid the high DOC loading period such as the rainy season and algal blooms.

Ferric- and calcium-loaded red soil assist colonization of submerged macrophyte for the in-situ remediation of eutrophic shallow lake: From mesocosm experiment to field enclosure application

Eutrophication and its resulting harmful algal blooms greatly reduce the ecosystem services of natural waters. The use of modified clay materials to assist the phytoremediation of eutrophic water is a promising technique. In this study, ferric chloride and calcium hydroxide were respectively loaded on red soil for algal flocculation and phosphorus inactivation. A two-by-two factorial mesocosm experiment with and without the application of ferric- and calcium- loaded red soil (FA), and with and without planting the submerged macrophyte Vallisneria natans was conducted for the in-situ repair of eutrophic water and sediment. Furthermore, field enclosure application was carried out to verify the feasibility of the technology. At the end of the mesocosm experiment, the total phosphorus, total nitrogen, and ammonia nitrogen concentrations in water were reduced by 81.8 %, 63.3 %, and 62.0 %, respectively, and orthophosphate phosphorus concentration in the sediment-water interface decreased by 90.2 % in the FA + V. natans group compared with those in the control group. The concentration and proportion of chlorophyll-a in cyanobacteria decreased by 89.8 % and 71.2 %, respectively, in the FA + V. natans group. The content of active phosphorus in V. natans decreased and that of inert phosphorus increased in the FA + V. natans group, compared with those in the V. natans alone group, thus may reducing the risk of phosphorus release after decomposing of V. natans. The sediment bacterial diversity index did not change significantly among treatments. Field enclosure application have also been successful, with chlorophyll-a concentration in the water of treated enclosure decreased from above 200 μg/L to below 10 μg/L, and phosphorus concentration in the water decreased from >0.6 mg/L to <0.02 mg/L. These results demonstrated that the FA in combination with submerged macrophyte planting had great potential for the in-situ remediation of eutrophic water, especially those with severe algal blooms.

The combined effects of lanthanum-modified bentonite and Vallisneria spiralis on phosphorus, dissolved organic matter, and heavy metal(loid)s

The use of lanthanum-modified bentonite (LMB) combined with Vallisneria spiralis (V∙s) (LMB + V∙s) is a common method for controlling internal phosphorus (P) release from sediments. However, the behaviors of iron (Fe) and manganese (Mn) under LMB + V∙s treatments, as well as the associated coupling effect on P, dissolved organic matter (DOM), and heavy metal(loid)s (HMs), require further investigations. Therefore, we used in this study a microelectrode system and high-resolution dialysis technology (HR-Peeper) to study the combined effects of LMB and V∙s on P, DOM, and HMs through a 66-day incubation experiment. The LMB + V∙s treatment increased the sediment DO concentration, promoting in-situ formations of Fe (III)/Mn (IV) oxyhydroxides, which, in turn, adsorbed P, soluble tungsten (W), DOM, and HMs. The increase in the concentrations of HCl-P, amorphous and poorly crystalline (oxyhydr) oxides-bound W, and oxidizable HMs forms demonstrated the capacity of the LMB + V∙s treatment to transform mobile P, W, and other HMs forms into more stable forms. The significant positive correlations between SRP, soluble W, UV254, and soluble Fe (II)/Mn, and the increased concentrations of the oxidizable HMs forms suggested the crucial role of the Fe/Mn redox in controlling the release of SRP, DOM, and HMs from sediments. The LMB + V∙s treatment resulted in SRP, W, and DOM removal rates of 74.49, 78.58, and 54.78 %, which were higher than those observed in the control group (without LMB and V∙s applications). On the other hand, the single and combined uses of LMB and V·s influenced the relative abundances of the sediment microbial communities without exhibiting effects on microbial diversity. This study demonstrated the key role of combined LMB and V∙s applications in controlling the release of P, W, DOM, and HMs in eutrophic lakes.

Contrasting effects of a traditional material of polyaluminum chloride and an emerging material of lanthanum carbonate capping on sediment internal phosphorus immobilization

Polyaluminum chloride (PAC) is a traditional material used for immobilizing sediment internal phosphorus (P) in field-scale experiment. Lanthanum carbonate (LC) is an emerging material which have been used in immobilizing sediment internal P in laboratory. To promote LC in practice, the premise is that it does have advantages over traditional material when used. Herein, a 90-day incubation experiment was conducted comparing the effectiveness and mechanism of LC and PAC capping in controlling sediment internal P. The results of isotherm experiment and XPS analysis indicated that the adsorption mechanism of P onto LC and PAC involved ligand exchange and formation of inner-sphere La/Al-O-P complexes. The incubation experiment revealed that PAC capping was more effective in reducing pore water soluble reactive phosphorus (SRP), exhibiting a reduction of up to 81.32 % but showed a decrease trend. However, LC capping resulted in a reduction of pore water SRP up to 52.84 % and maintained stability. On average, LC and PAC capping reduced SRP flux by 0.27 and 0.32 μg·m-2d-1, respectively relative to the control sediment. Moreover, LC capping facilitated the formation of Fe(III)/Mn(IV) oxyhydroxides, leading to an increased P adsorption, whereas PAC capping facilitated the reduction of Fe(III)/Mn(IV) minerals with P release. Additionally, LC capping resulted in the reduction of a higher ratio of mobile P/TP to stable P forms than PAC capping, as compared to the control. In contrast to PAC capping which converted mobile P to stable NaOH-rP, LC capping transformed mobile P and NaOH-rP into more stable HCl-P and ResP. Both LC and PAC capping caused variations in sediment bacterial communities. Nevertheless, PAC capping heightened the risk of Co, Ni, Cu, and Pb releases in sediment compared to LC capping. In summary, this study suggested that LC capping surpassed PAC capping in immobilizing sediment internal P.

Combining dredging with modified zeolite thin-layer capping to control nitrogen release from eutrophic lake sediment

Dredging is widely used to control internal sediment nitrogen (N) pollution during eutrophic lake restoration. However, the effectiveness of dredging cannot be maintained for long periods during seasonal temperature variations. This study used modified zeolite (MZ) as a thin-layer capping material to enhance dredging efficiency during a year-long field sediment core incubation period. Our results showed that dredging alone more effectively reduced pore water N, N flux, and sediment N content than MZ capping but showed more dramatic changes during the warm seasons. The N flux in dredged sediment in summer was 1.8 and 2.5 times that in spring and autumn, respectively, indicating a drastic N regeneration process in the short term. In contrast, the combination method reduced the extra 10% pore water N, 22% N flux, and 8% sediment organic N content compared with dredging alone and maintained high stability during seasonal changes. The results indicated that the addition of MZ to the surface of dredged sediment not only enhanced the control effect of dredging by its adsorption capacity but may also smooth the N regeneration process via successive accumulation (in the channel of the material) and activation of bacteria for months, which was evidenced by the variation in microbial diversity in the MZ treatment. As a result, the combination of dredging with modified zeolite simultaneously enhanced the efficiency and stability of the single dredging method in controlling sediment N content and its release, exhibiting great prospects for long-term application in eutrophic lakes with severe pollution from internal N loading.

The effects of different doses of lanthanum-modified bentonite in combination with a submerged macrophyte (Vallisneria denseserrulata) on phosphorus inactivation and macrophyte growth: A mesocosm study

The combination of chemical phosphorus (P) inactivation and submerged macrophyte transplantation has been widely used in lake restoration as it yields stronger effects than when applying either method alone. However, the dose effect of chemical materials on P inactivation when used in combination with submerged macrophytes and the influences of the chemicals used on the submerged macrophytes growth remain largely unknown. In this study, we investigated P inactivation in both the water column and the sediment, and the responses of submerged macrophytes to Lanthanum modified bentonite (LMB) in an outdoor mesocosm experiment where Vallisneria denseserrulata were transplanted into all mesocosms and LMB was added at four dosage levels, respectively: control (LMB-free), low dosage (570 g m-2), middle dosage (1140 g m-2), and high dosage (2280 g m-2). The results showed that the combination of LMB dosage and V. denseserrulata reduced TP in the water column by 32%-38% compared to V. denseserrulata alone, while no significant difference was observed among the three LMB treatments. Porewater soluble reactive P, two-dimensional diffusive gradient in thin films (DGT)-labile P concentrations, and P transformation in the 0-1 cm sediment layer exhibited similar trends along the LMB dosage gradient. Besides, LMB inhibited plant growth and reduced the uptake of mineral elements (i.e., calcium, manganese, iron, and magnesium) in a dosage-dependent manner with LMB. LMB may reduce plant growth by creating a P deficiency risk for new ramets and by interfering with the uptake of mineral elements. Considering both the dose effect of LMB on P inactivation and negative effect on macrophyte growth, we suggest a "small dosage, frequent application" method for LMB application to be used in lake restoration aiming to recover submerged macrophytes and clear water conditions.

Nanoparticles for Mitigation of Harmful Cyanobacterial Blooms

With the rapid advancement of nanotechnology and its widespread applications, increasing amounts of manufactured and natural nanoparticles (NPs) have been tested for their potential utilization in treating harmful cyanobacterial blooms (HCBs). NPs can be used as a photocatalyst, algaecide, adsorbent, flocculant, or coagulant. The primary mechanisms explored for NPs to mitigate HCBs include photocatalysis, metal ion-induced cytotoxicity, physical disruption of the cell membrane, light-shielding, flocculation/coagulation/sedimentation of cyanobacterial cells, and the removal of phosphorus (P) and cyanotoxins from bloom water by adsorption. As an emerging and promising chemical/physical approach for HCB mitigation, versatile NP-based technologies offer great advantages, such as being environmentally benign, cost-effective, highly efficient, recyclable, and adaptable. The challenges we face include cost reduction, scalability, and impacts on non-target species co-inhabiting in the same environment. Further efforts are required to scale up to real-world operations through developing more efficient, recoverable, reusable, and deployable NP-based lattices or materials that are adaptable to bloom events in different water bodies of different sizes, such as reservoirs, lakes, rivers, and marine environments.

Enhanced nutrient control efficiency in sediments using modified clay inactivation coupled with aquatic vegetation in the confluence area of a eutrophic lake

Developing a long-term method for controlling sediment N and P release is important for enabling lake restoration. In this study, inactivation methods using lanthanum-modified clay, modified zeolite, or planting aquatic vegetation and their combinations were used in the control internal sediment loading (pore water N and P concentrations and their fluxes), and the efficacies of the methods were analyzed. The results indicated that compared to the control sediment, the addition of P sorbent, which was La and Al co-modified attapulgite (ACLA), and N sorbent, which was NaCl-modified zeolite (modified zeolite), planting of aquatic vegetation Vallisneria spiralis (V. spiralis), and a combination of sorbents and plants effectively reduced the porewater nutrient content and its fluxes across the sediment-water interface. However, the reduction in pore water nutrients and flux were superior when using a combination of clay inactivation and aquatic planting. The poorest sediment N and P control was achieved by planting V. spiralis alone. The addition of La and Al co-modified attapulgite (ACLA) and modified zeolite efficiently reduced N and P in the sediment, but the N and P sorbents did not achieve long-lasting nutrient release control. The high efficiency obtained by the combination of modified clay-based inactivation and V. spiralis was likely due to the strong chemical sorption capacity of clay and oxygenation by the rhizosphere of aquatic vegetation. These results show that a combination of chemical and ecological methods would be the most effective approach to remediate polluted sediments in the long term.

Complexation-based selectivity of organic phosphonates adsorption from high-salinity water by neodymium-doped nanocomposite

Organic phosphonates have been widely used in various industries and are ubiquitous in wastewaters, and efficient removal of phosphonates is still a challenge for the conventional processes because of the severe interferences from the complex water constitutions. Herein, an Nd-based nanocomposite (HNdO@PsAX) was fabricated by immobilizing hydrated neodymium oxide (HNdO) nanoparticles inside a polystyrene anion exchanger (PsAX) to remove phosphonates from high-salinity aqueous media. Batch experiments demonstrated that HNdO@PsAX had an excellent adsorption capacity (∼90.5 mg P/g-Nd) towards a typical phosphonate (1-hydrox-yethylidene-1,1-diphosphonic acid, HEDP) from the background of 8 g/L NaCl, whereas negligible HEDP adsorption was achieved by PsAX. Attractively, various coexisting substances (humic acid, phosphate, citrate, EDTA, metal ligands, and anions) exerted negligible effects on the HEDP adsorption by HNdO@PsAX under high salinity. FT-IR and XPS analyses revealed that the inner-sphere complexation between HEDP and the immobilized HNdO nanoparticles is responsible for HEDP adsorption. Fixed-bed experiments further verified that HNdO@PsAX was capable of successively treating more than 4500 bed volumes (BV) of a synthetic high-salinity wastewater (1.0 mg P/L of HEDP), whereas only ∼2 BV of effective treatment capacity was received by PsAX. The exhausted HNdO@PsAX was amenable to a complete regeneration by a binary NaOHNaCl solution without significant loss in capacity. The capability in removing other organic phosphonates and treating a real electroplating wastewater by HNdO@PsAX was further validated. Generally, HNdO@PsAX exhibited a great potential in efficiently removing phosphonates from high-salinity wastewater.

The role of bioturbation triggered by benthic macroinvertebrates in the effectiveness of the Floc & Lock technique in mitigating eutrophication

Anthropogenic activities have led to excessive loading of phosphorus and nitrogen into water bodies, leading to eutrophication and promoting the growth of cyanobacteria, posing a threat to the health of humans and aquatic animals. Techniques such as Floc & Lock have been developed to mitigate eutrophication by reducing phosphorus concentrations in water and preventing algal blooms. However, little attention has been given to the impact of phosphorus resuspension by sediment-associated organisms such as benthic macroinvertebrates, on the effectiveness of this technique. Here, we experimentally evaluated whether the presence of snails Melanoides tuberculata (Müller, 1774) and larvae of Chironomus sancticaroli (Strixino and Strixino, 1981) affects the efficiency of the Floc & Lock technique. Snails and chironomid larvae are benthic macroinvertebrates commonly found in high abundance in eutrophic reservoirs. Specifically, we tested the hypotheses that (i) the presence of benthic macroinvertebrates reduces the efficiency of coagulants and clays in removing phosphorus and algal biomass from the water column, and (ii) this effect is species-dependent, as some organisms such as the snails, revolve the substrate and resuspend sedimented particles, while other ones, such as chironomid larvae, aid in the removal of phosphorus from the water column by depositing them in the sediment. Our findings revealed that the impact of benthic macroinvertebrates on the effectiveness of the Floc & Lock technique is species-dependent. Chironomid larvae positively influenced the efficiency of the technique by aiding in the removal of total phosphorus, soluble reactive phosphorus, and algal biomass from the water column, depositing them in the sediment. In contrast, the presence of snails had the opposite effect, resulting in increased phosphorus concentration and algal biomass in the water. Surprisingly, the snails consumed the flocs formed by the coagulant and clay within a short time interval of 72 h, raising concerns due to the presence of toxic cyanobacterial biomass in these flocs. Our study emphasizes the importance of considering benthic macroinvertebrates and their impact on the effectiveness of eutrophication management techniques.

Comparison of dredging, lanthanum-modified bentonite, aluminium-modified zeolite, and FeCl2 in controlling internal nutrient loading

The eutrophic Bouvigne pond (Breda, The Netherlands) regularly suffers from cyanobacterial blooms. To improve the water quality, the external nutrient loading and the nutrient release from the pond sediment have to be reduced. An enclosure experiment was performed in the pond between March 9 and July 29, 2020 to compare the efficiency of dredging, addition of the lanthanum-modified bentonite clay Phoslock® (LMB), the aluminum-modified zeolite Aqual-P™ (AMZ) and FeCl2 to mitigate nutrient release from the sediment. The treatments improved water quality. Mean total phosphorus (TP) concentrations in water were 0.091, 0.058, 0.032, 0.031, and 0.030 mg P L-1 in controls, dredged, FeCl2, LMB and AMZ treated enclosures, respectively. Mean filterable P (FP) concentrations were 0.056, 0.010, 0.009, 0.005, and 0.005 mg P L-1 in controls, dredged, FeCl2, LMB and AMZ treatments, respectively. Total nitrogen (TN) and dissolved inorganic nitrogen (DIN) were similar among treatments; lanthanum was elevated in LMB treatments, Fe and Cl in FeCl2 treatments, and Al and Cl in AMZ treatments. After 112 days, sediment was collected from each enclosure, and subsequent sequential P extraction revealed that the mobile P pool in the sediments had reduced by 71.4%, 60.2%, 38%, and 5.2% in dredged, AMZ, LMB, and FeCl2 treatments compared to the controls. A sediment core incubation laboratory experiment done simultaneously with the enclosure experiment revealed that FP fluxes were positive in controls and cores from the dredged area, while negative in LMB, AMZ and FeCl2 treated cores. Dissolved inorganic nitrogen (DIN) release rate in LMB treated cores was 3.6 times higher than in controls. Overall, the applied in-lake treatments improved water quality in the enclosures. Based on this study, from effectiveness, application, stakeholders engagement, costs and environmental safety, LMB treatment would be the preferred option to reduce the internal nutrient loading of the Bouvigne pond, but additional arguments also have to be considered when preparing a restoration.

Who does better for in-situ eutrophic remediation in anoxic environment improvement and nutrient removal: MgO2 versus CaO2

The long-term insufficient dissolved oxygen (DO), excessive nitrogen (N) and phosphorus (P) have become the main causes of the troublesome eutrophication. Herein, a 20-day sediment core incubation experiment was conducted to comprehensively evaluate the effects of two metal-based peroxides (MgO₂ and CaO₂) on eutrophic remediation. Results indicated that CaO₂ addition could increase DO and ORP of the overlying water more effectively and improve the anoxic environment of the aquatic ecosystems. However, the addition of MgO₂ had a less impact on pH of the water body. Furthermore, the addition of MgO₂ and CaO₂ removed 90.31% and 93.87% of continuous external P in the overlying water respectively, while the removal of NH₄⁺ was 64.86% and 45.89%, and the removal of TN was 43.08% and 19.16%. The reason why the capacity on NH₄⁺ removal of MgO₂ was higher than that of CaO₂ is mainly that PO₄^3- and NH₄⁺ can be removed as struvite by MgO₂. Compared with MgO₂, mobile P of the sediment in CaO₂ addition group was reduced obviously and converted to more stable P. Notably, the microbial community structure of sediments was optimized by MgO₂ and CaO₂, which showed that the relative abundance of anaerobic bacteria decreased and that of aerobic bacteria increased significantly, especially some functional bacteria involved in the nutrient cycle. Taken together, MgO₂ and CaO₂ have a promising application prospect in the field of in-situ eutrophication management.

Comparative study of sediment phosphorus immobilization via the addition of lanthanum-modified and thermal-modified drinking water treatment sludge

Lanthanum-modified drinking water treatment sludge (DTSLa) and thermal-modified drinking water treatment sludge (TDTS) were prepared from drinking water treatment sludge(DTS). The adsorption properties of DTSLa and TDTS on phosphate in water and the effects on the controlled release and morphology of phosphorus in sediment at different dosages (0%, 2.5%, 5%) were discussed. Combining with SEM, BET, XRD, FTIR, and XPS characterization methods, the immobilization mechanism of DTSLa and TDTS on phosphorus in sediment was explored. The addition of TDTS can transform NH₄Cl-P (loosely sorbed P), BD-P (bicarbonate-dithionite extractable P), and Org-P (organic P) into stable NaOH-rP (metal oxide-bound P) in sediment, and the conversion amount will increase with the increase of TDTS supplemental amount. DTSLa converted NH₄Cl-P, BD-P, Org-P, and NaOH-rP to more stable HCl-P (calcium-bound P). At the same time, the content of WSP (water-soluble phosphorus) and olsen-P (NaHCO₃ extractable P) in sediment can be reduced by the addition of DTSLa and TDTS, reducing the risk of the release of phosphorus from the sediment to the overlying water. In addition, phosphorus can be directly removed from the interstitial water by DTSLa and TDTS, so as to reduce the phosphorus concentration gradient between the overlying water and the interstitial water, thus inhibiting the release of phosphorus from interstitial water to overlying water. The results showed that DTSLa is better than TDTS in terms of its adsorption capacity and adsorption effect on endogenous phosphorus in water, so DTSLa is more suitable to be used as a sediment conditioner to control the phosphorus content in water and sediment.

Simultaneous removal of phosphate, calcium, and ammonia nitrogen in a hydrogel immobilized reactor with bentonite/lanthanum/PVA based on microbial induced calcium precipitation

In recent years, it is urgent to solve nitrogen and phosphorus pollution in domestic wastewater. The target strain Pseudomonas sp. Y1 was immobilized using polyvinyl alcohol (PVA) matrix coupled with bentonite and lanthanum (La), respectively, to fabricate four hydrogel materials that used to construct bioreactors. The optimal operating parameters and dephosphorization mechanism were discussed, and the effects of hydrogel materials and different loads on the performance of the bioreactor were contrastively analyzed. The results manifested that when the hydraulic retention time (HRT) was 6.0 h, the C/N was 6.0, and the Ca²⁺ concentration was 100.0 mg L^-1, the bioreactors had the best heterotrophic nitrification-aerobic denitrification (HNAD) and biomineralization capacity, and the maximum removal efficiencies of Ca²⁺, PO₄^3--P, and NH₄⁺-N were 82.57, 99.17, and 89.08%, respectively. The operation data indicated that the addition of bentonite significantly promoted HNAD, and the bioreactor had stronger dephosphorization ability in the presence of La. The main phosphorous removal mechanisms were confirmed to be adsorption and co-precipitation. Finally, high-throughput sequencing results indicated that Pseudomonas accounted for the paramount proportion in the bioreactor, and the prediction of functional genes indicated that the C/N of 6.0 is more favorable for the expression of nitrogen removal-related functional genes in the bioreactor system. This study highlights the superiority of microbial induced calcium precipitation (MICP) combined with PVA hydrogel, and provides a theoretical basis for simultaneous nitrogen and phosphate removal of wastewater.

Effect of common ions aging treatment on adsorption of phosphate onto and control of phosphorus release from sediment by lanthanum-modified bentonite

The objective of this work was to explore the influence of combined aging treatment using Na⁺, Ca²⁺, Cl^-, HCO₃^- and SO₄^2- on the adsorption of phosphate (H_iPO₄^i-3) onto and the restraint of internal phosphorus (P) migration into overlying water (OW) by lanthanum modified bentonite (LMB). To achieve this aim, the adsorption characteristics and mechanisms of H_iPO₄^i-3 onto the raw and aged LMBs (named as R-LMB and A-LMB, respectively) were comparatively studied, and the effects of R-LMB and A-LMB treatments (addition and capping) on the migration of P from sediment to OW were comparatively investigated. The results showed that the combined aging treatment of R-LMB with Na⁺, Ca²⁺, Cl^-, HCO₃^- and SO₄^2- inhibited the adsorption of H_iPO₄^i-3. Similar to R-LMB, the precipitation of H_iPO₄^i-3 with La³⁺ to form LaPO₄ and the ligand exchange between CO₃^2- and H_iPO₄^i-3 to form the inner-sphere lanthanum-phosphate complexes are the important mechanisms for the H_iPO₄^i-3 uptake by A-LMB. The R-LMB addition and capping can be effective in the suppression of endogenous P release to OW under hypoxia conditions. The inactivation of diffusive gradient in thin film-unstable P (DGT-UP) and potentially mobile P (PM-P) in sediment acted as a key role in the restraint of internal P release to OW by the R-LMB addition, and the immobilization of DGT-UP and PM-P in the topmost sediment played a key role in the interception of endogenous P migration into OW by the R-LMB capping. Although the Na⁺/Ca²⁺/Cl^-/HCO₃^-/SO₄^2- combined aging treatment had a certain negative effect on the efficiencies of LMB addition and capping to hinder the liberation of P from sediment into OW, the A-LMB addition and capping still can be effective in the control of sediment internal phosphorus pollution to a certain degree. The results of this work indicate that LMB has a high potential to be used as a capping/amendment material to control internal phosphorus pollution.

Control of phosphorus release from sediment by hydrous zirconium oxide combined with calcite, bentonite and zeolite

This study investigated the effectiveness and mechanism for the control of internal phosphorus (P) liberation from sediment by hydrous zirconium oxide (HZrO₂) combined with calcite, bentonite and zeolite. The results suggested that coexisting calcite, calcium-modified bentonite (CaBT) and calcium-modified zeolite (CaZ) all had the ability to promote the adsorption of phosphate (PO₄^3-) onto HZrO₂. The mechanisms of PO₄^3- elimination by HZrO₂/calcite mixture involved the adsorption of PO₄^3- on calcite, the precipitation of PO₄^3- with Ca²⁺, and the inner-sphere complexation of PO₄^3- with HZrO₂. The amendment of sediment with HZrO₂/calcite, HZrO₂/CaBT or HZrO₂/CaZ mixture can effectively prevent the sedimentary P release, and the immobilization of mobile P in the sediment and the uptake of dissolved reactive P (DRP) from the interstitial water by the amendment material played a key role in the control of P release from sediment by the combined amendment. Capping sediment with HZrO₂/calcite, HZrO₂/CaBT or HZrO₂/CaZ mixture also can effectively intercept sediment P release, and the formation of P static layer attributed to the uptake of interstitial water DRP and DGT (diffusive gradient in thin-films)-unstable P in the upper sediment by the capping material was a key to the inhibition of sedimentary P migration into the overlying water by the combined capping. The great majority of P immobilized by the HZrO₂/calcite, HZrO₂/CaBT or HZrO₂/CaZ combined covering layer is stable P and it has a low re-releasing risk under dissolved oxygen-deficit and pH 5-9 condition. The stability of P bound by the combined covering layer was larger than that by the single HZrO₂ covering layer. The results of this research show that the combined use of HZrO₂ and calcite, HZrO₂ and CaBT, or HZrO₂ and CaZ as a capping material has great potential in the reduction of sediment P loading.

Effects on the migration and speciation of heavy metals by combined capping and biochemical oxidation during sediment remediation

Capping and oxidation by lanthanum-modified bentonite (LMB) and calcium nitrate (CN) has a dual effect of deep phosphorus (P)/arsenic (As) clearance and surface P/As blockade. However, little information is available on the effect of LMB and CN on heavy metals. In this study, we hypothesize that LMB and CN exerted the same synergistic effect on heavy metals as P and As. We verified this through Rhizon samplers, diffusive gradients in thin films technology (DGT) and planar optode (PO) methods. The results showed that individual and combined LMB and CN treatments temporarily decreased but eventually increased the dissolved oxygen of the sediment-water interface (SWI). DGT-labile sulfide in the surface 110 mm sediment, soluble Fe(II) and DGT-labile Fe(II) in the surface 80 mm sediment were eliminated within 30 days by CN and LMB + CN treatments. A temporary sharp increase in soluble Fe, Mn, Co, and DGT-labile Mn, Co, Cu, and Ni was observed in CN and LMB + CN groups probably due to sulfide oxidation and carbonate dissolution. LMB + CN group showed a less-intense increase in DGT-labile metals and less metal release than the CN group (inferred from the total metal content). This indicates that LMB and CN had a synergistic effect on heavy metals. When using the LMB + CN treatment, LMB partly adsorbed and blocked metal release in sulfide and carbonate bound forms and finally transformed them into Fe and Mn oxides and residual forms. We suggest that CN should be combined with capping agents (at an appropriate pH) to compact sediments and block metal exchange at the SWI.

Comparison of magnetite, hematite and goethite amendment and capping in control of phosphorus release from sediment

The characteristics and mechanism of phosphate adsorption onto magnetite, hematite and goethite were comparatively studied, and the effects of magnetite, hematite and goethite amendment and capping on endogenous phosphorus (P) liberation from sediment into overlying water (OW) were comparatively investigated. The adsorption of phosphate onto magnetite, hematite and goethite mainly obeyed the inner-sphere complexation mechanism, and the phosphate adsorption capacity decreased in the order of magnetite > goethite > hematite. The magnetite, hematite and goethite amendment all can decrease the risk of endogenous Prelease into OW under anoxic conditions, and the inactivation of diffusion gradients in thin films-labile P in sediment made a great contribution to the restraint of endogenous P release into OW by the magnetite, hematite and goethite amendment. The efficiency of endogenous P release restraint by the iron oxide addition decreased in the order of magnetite > goethite > hematite. The magnetite, hematite and goethite capping all can be effective for the suppression of endogenous P release from sediment into OW under anoxic conditions, and most of P immobilized by the magnetite, hematite and goethite capping layers is relatively or very stable. The results obtained from this work suggest that magnetite is more suitably used a capping/amendment material to prevent P release from sediment than hematite and goethite, and magnetite capping is a promising approach for hindering sedimentary P release into OW.

Controlling internal nitrogen and phosphorus loading using Ca-poor soil capping in shallow eutrophic lakes: Long-term effects and mechanisms

Clean soil is a potential capping material for controlling internal nutrient loading and helping the recovery of macrophytes in eutrophic lakes, but the long-term effects and underlying mechanisms of clean soil capping under in-situ conditions remain poorly understood. In this study, a three-year field capping enclosure experiment combining intact sediment core incubation, in-situ porewater sampling, isotherm adsorption experiments and analysis of sediment nitrogen (N) and phosphorus (P) fractions was conducted to assess the long-term performance of clean soil capping on internal loading in Lake Taihu. Our results indicate that clean soil has excellent P adsorption and retention capacity as an ecologically safe capping material and can effectively mitigate NH₄⁺-N and SRP (soluble reactive P) fluxes at the sediment-water interface (SWI) and porewater SRP concentration for one year after capping. The mean NH₄⁺-N and SRP fluxes of capping sediment were 34.86 mg m^-2 h^-1 and -1.58 mg m^-2 h^-1, compared 82.99 mg m^-2 h^-1 and 6.29 mg m^-2 h^-1 for control sediment. Clean soil controls internal NH₄⁺-N release through cation (mainly Al³⁺) exchange mechanisms, while for SRP, clean soil can not only react with SRP due to its high Al and Fe content, but also stimulate the migration of active Ca²⁺ to the capping layer, thus precipitating as Ca-bound P (Ca-P). Clean soil capping also contributed to the restoration of macrophytes during the growing season. However, the effect of controlling internal nutrient loading only lasted for one year under in-situ conditions, after which the sediment properties returned to pre-capping conditions. Our results highlight that clean Ca-poor soil is a promising capping material and further research is needed to extend the longevity of this geoengineering technology.

A model for predicting reduction in mobile phosphorus of lake sediment by aluminum drinking water treatment residuals

Drinking water treatment residual (DWTR) derived from flocculation and sedimentation of raw water using aluminum coagulants is a valuable environmental remediation byproduct capable of inactivating phosphorus (P). However, no generalizable model exists in the literature to describe reduction of releasable (mobile) P in lake sediment as a result of DWTR addition. The reduction of mobile P (sum of labile P and reductant soluble P) was investigated in over 100 sub-samples using five sediment samples from two lakes and three DWTRs from different water treatment plants. A consistent relationship was determined across a range of mobile P contents (0.23 g/m²/cm to 0.92 g/m²/cm, or 15.8 to 186.1 µg/g DW) and DWTRs. The relationship was best described as a function of the mobile P content of the sediment and the oxalate-extractable aluminum content of the DWTR. An empirical model was developed to predict the immediate reduction in mobile P following the addition of DWTR containing aluminum. This model was validated using two additional lake sediments and one additional DWTR (R² = 0.995). Thus, the immediate inactivation of P in lake sediment following DWTR addition can be predicted with this model, which can be used with internal P loading or other water quality goals to determine an appropriate DWTR dose. Further recommendations were made about dosing DWTRs for lake restoration, allowing practitioners to use DWTR to inactivate P in lake sediment without conducting individual sorption experiments.

Sediment phosphorus immobilization with the addition of calcium/aluminum and lanthanum/calcium/aluminum composite materials under wide ranges of pH and redox conditions

Aquatic environment factors often influence and regulate the direction of phosphorus (P) flow at the sediment-water interface (SWI). High pH and low DO, common in eutrophic lakes, would induce large releases of P from sediment, and thus cause the negative effect on the efficiency of some P-passivators. Hence, the development of P passivators that could function over a wide range of pH condition and redox state in the overlaying water with reduced undesirable side effects is critical for the eutrophic lake remediation. In the present study, a calcium (Ca)/aluminum (Al) composite (CA) and a lanthanum (La)/Ca/Al composite (LCA) were prepared for P immobilization in lake sediments, using calcium and lanthanum coprecipitated with aluminum. CA and LCA were shown to have good P sorption performance at pH 4-11, particularly at pH 8-11. Furthermore, CA and LCA have an ability to correct the pH of water that deviates from neutral. The maximum P adsorption (Q_max) of sediment amended by 4 % CA and 4 % LCA increased by 83 % and 103 %, and their equilibrium P concentration (EPC₀) decreased by 76 % and 88 %, respectively. Under various pH and DO conditions, the P concentration in overlying water was significantly decreased by CA and LCA amendment, and their addition could effectively counteract the P release from sediments induced by high pH and low DO. The mechanisms of P immobilization in amended sediments under various pH and DO levels are primarily the conversion of reactive P to stable P. The P immobilization performance of CA and LCA could cope with a wide range of pH and redox conditions in eutrophic lakes, and they would help to correct extreme pH values, thus they are expected to be a new generation of commercial P-passivators.

Enhanced phosphate removal from water by hydrated neodymium oxide-based nanocomposite: Performance, mechanism, and validation

Phosphorus (P) control has been recognized as an imperative task to mitigate water eutrophication and settle the imminent shortage of P resources. Despite intensive effort put into this matter, it is still generally challenging for the current methods to remove and even potentially recover phosphorus (as phosphate) from complicated water matrices. To this end, we proposed a novel nanocomposite via coupling polystyrene anion exchanger (PsAX) with hydrated neodymium oxide (HNdO) nanoparticle for selective removal of phosphate. The developed nanocomposite, i.e., HNdO-PsAX, exhibited quite stable and efficient phosphate adsorption over a wide pH range of 3.0-10.0 with the maximum adsorption capacity as 85.4 mg P/g. It also showed satisfied anti-interference against various competing substances; notably, HNdO-PsAX obviously outperformed Phoslock, a commercial lanthanum-based adsorbent exclusively for phosphate sequestration, particularly under the interference of bicarbonate and humic acid, which were admitted as the paralyzing factors for Phoslock. The superior affinity of HNdO-PsAX towards phosphate, driven by the specific Nd-P inner-sphere complexation as evidenced by XPS, FT-IR, and the lattice evolution of HNdO nanoparticle, renders the nanocomposite eminently suitable for sequestrating trace phosphate. Fixed-bed treatment validated that HNdO-PsAX was capable of treating ∼11,800 bed volume of a simulated wastewater (from 2.0 to below 0.5 mg P/L), approximately 12 times higher than that of the previously reported Fe-based nanocomposite (HFO-PsAX, ∼ 900 BV); also, a satisfactory outcome in treating authentic municipal wastewater by HNdO-PsAX and the feasibility of regenerating the exhausted one by a binary NaOH-NaCl solution were recognized. This work provides a new potion of enhanced phosphorous control for surface water and wastewater.

Locked up Inside the Vessels: Rare Earth Elements Are Transferred and Stored in the Conductive Tissues of the Accumulating Fern Dryopteris erythrosora

Rare earth elements (REEs) are strategic metals strongly involved in low-carbon energy conversion. However, these emerging contaminants are increasingly disseminated into ecosystems, raising concern regarding their toxicity. REE-accumulating plants are crucial subjects to better understand REE transfer to the trophic chain but are also promising phytoremediation tools. In this analysis, we deciphered REE accumulation sites in the REE-accumulating fern Dryopteris erythrosora by synchrotron X-ray μfluorescence (μXRF). This technique allows a high-resolution and in situ analysis of fresh samples or frozen-hydrated cross sections of different organs of the plant. In the sporophyte, REEs were translocated from the roots to the fronds by the xylem sap and were stored within the xylem conductive system. The comparison of REE distribution and accumulation levels in the healthy and necrotic parts of the frond shed light on the differential mobility between light and heavy REEs. Furthermore, the comparison emphasized that necrotized areas were not the main REE-accumulating sites. Finally, the absence of cell-to-cell mobility of REEs in the gametophyte suggested the absence of REE-compatible transporters in photosynthetic tissues. These results provide valuable knowledge on the physiology of REE-accumulating ferns to understand the REE cycle in biological systems and the expansion of phytotechnologies for REE-enriched or REE-contaminated soils.

In situ remediation mechanism of internal nitrogen and phosphorus regeneration and release in shallow eutrophic lakes by combining multiple remediation techniques

Large anthropogenic inputs of N and P alter the nutrient cycle and exacerbate global eutrophication problems in aquatic ecosystems. This study in Lake Datong, China, investigates the remediation mechanism of multiple remediation technique combinations (dredging, adsorbent amendment, and planting aquatic vegetation) on sediment N and P loads based on two high-resolution sampling techniques (HR-Peeper and DGT) and P sequential extraction procedures. The results showed that high temperature and low dissolved oxygen considerably enhanced pore water dissolved reactive P (DRP) and NH₄⁺ concentrations attributable to abundant Fe-P and organic matter content in the sediment. Fe reduction is critical for regulating pore water DRP release and promoting N removal. Overall, for Lake Datong, combining multiple remediation techniques is more effective in controlling sediment P loads (pore water DRP, P fluxes, forms of P, and labile P), from a long-term perspective, than a single remediation. Lanthanum-modified bentonite (LMB) inactivation treatment can transfer mobile P in the surface sediment into more refractory forms over time, thereby reducing the risk of sediment labile P release. However, it is difficult to effectively remediate internal P loads owing to inappropriate dredging depths and low biomass of aquatic vegetation. Future lake restoration practices should optimize the selection of different remediation technique combinations based on internal N and P pollution characteristics, while reducing external wastewater input. These results are important for understanding the remediation mechanisms of internal N and P and provide suggestions for sediment management of shallow eutrophic lakes.

Adsorption properties and mechanism research of phosphorus with different molecular structures from aqueous solutions by La-modified biochar

In order to explore the adsorption characteristics of phosphorus from molecules with different molecular structures and varying number of phosphate groups on metal-modified biochar, walnut shell biochar was modified with LaCl₃ to prepare lanthanum-loaded biochar (BC-La). Adsorption of four polar components, namely phytic acid (IHP), adenosine-5'-disodium triphosphate (5-ATP), hydroxyethylidene diphosphonic acid (HEDP), and sodium pyrophosphate (PP), was studied. The adsorption properties and mechanism of phosphorus sorption by BC-La were analyzed by SEM-EDS and FTIR for the different structures. The results showed that the maximum adsorption capacity of BC-La for IHP, 5-ATP, HEDP, and PP was 85.85, 9.04, 15.80, and 14.45 mg/g, respectively. The adsorption capacity was positively correlated with the polarity of organic phosphorus. The adsorption behavior conformed to the quasi second-order kinetic fitting equation, and the increase of temperature was conducive to the removal of all four phosphorus pollutants. BC-La adsorbs IHP and HEDP mainly through electrostatic attraction. The adsorption of 5-ATP and PP is dominated by complexation. The La-modified biochar has broad prospects in water remediation, which can provide a theoretical basis for removal of different forms of phosphorus pollutants and prevention and control of water eutrophication.

Phosphate adsorption by riverborne clay sediments in a southern-Italy Mediterranean reservoir: Insights from a "natural geo-engineering" experiment

This paper reports data from a southern-Italy reservoir (Lake Occhito) characterized by a strong riverborne sediment transport. Main hydrochemical, trophic, and nutrient variables were measured (over a twelve-month period) in both lake and tributaries. Lacustrine sediments were subjected to mineralogical characterization and to phosphorus fractioning, while a 6-day long batch experiment was carried out to evaluate the lake sediment orthophosphate adsorption capacity. A set of algal growth potential tests was also undertaken on the lake and its tributaries. Results highlight the presence of a strong gradient in nutrient availability among the inflows. Most of the nutrient loads were from the main tributary (20.3 t P a^-1, ~83 %), that showed the highest trophic potential (average: 56.8 mg L^-1) and was nitrogen/phosphorus co-limited. The other inflows were phosphorus limited and characterized by a higher sediment transport. The lake showed the lowest nutrient concentrations (average total phosphorus: 21 μg P L^-1) and was strongly phosphorus limited. Clays were the principal minerals in the lake sediments (~51 %), while the main phosphorus fraction was apatite (~78 %). The batch experiment demonstrated the capability of the lake sediments to reduce orthophosphate concentrations in phosphorus-rich waters (initial orthophosphate: 320 μg P L^-1; ~80 % reduction). The lake sediment orthophosphate kinetics of abatement was similar to that of a commercially available phosphorus sorbent (lanthanum modified bentonite), although the stability of phosphorus binding was higher for the commercial product. Theoretical average in-lake total phosphorus, chlorophyll-a, and transparency values, estimated through Vollenweider models, were approximately double of the average values measured in the lake. Therefore, the massive presence of riverborne clay sediments seems to markedly reduce the in-lake orthophosphate concentrations (and light penetration), inducing an overall lowering of the lake trophic state, as if the lake ecosystem were permanently subjected to a geo-engineering phosphorus sorbent treatment.

Water quality status response to multiple anthropogenic activities in urban river

Water quality evaluation and degrading factors identification are crucial for predicting water quality evolution trends in an urban river. However, under the coupling of multiple factors, these targets face great challenges. The water quality status response to multiple anthropogenic activities in an urban river was evaluated and predicted based on comprehensive assessment methods and random forest (RF) model. We found that the distribution of each physicochemical parameter exhibits an obvious spatial clustering. The mean pollution level and trophic status of the urban river are medium pollution (water quality index = 59.79; Nemerow's pollution index = 2.00) and light eutrophication (trophic level index = 57.30). The water quality status is sensitive to anthropogenic activities, showing the following order of TLI and NPI values: residential district > industrial district > agricultural district and downtown > suburbs > countryside. According to the redundancy analysis, constructed land (F = 15.90, p < 0.01) and domestic sewage (F = 14.20, p < 0.01) evinced as the crucial factors that aggravated the water quality pollution level. Based on the simulation results of the RF model (variation explained = 94.91%; R² = 0.978), improving domestic sewage treatment standards is the most effective measure to improve the water quality (increased by 40.3-49.3%) in residential and industrial districts. While in a suburban district, improving the domestic sewage collection rate has more effectively (23%) than those in the residential and industrial districts. Conclusively, reducing exogenous pollution input and improving domestic sewage treatment standards are vital to urban river restoration. Clinical trial registration Not applicable.

Phosphorus control and dredging decrease methane emissions from shallow lakes

Freshwater ecosystems are an important source of the greenhouse gas methane (CH₄), and their emissions are expected to increase due to eutrophication. Two commonly applied management techniques to reduce eutrophication are the addition of phosphate-binding lanthanum modified bentonite (LMB, trademark Phoslock©) and dredging, but their effect on CH₄ emissions is still poorly understood. Here, this study researched how LMB and dredging affected CH₄ emissions using a full-factorial mesocosm design monitored for 18 months. The effect was tested by measuring diffusive and ebullitive CH₄ fluxes, plant community composition, methanogen and methanotroph activity and community composition, and a range of physicochemical water and sediment variables. LMB addition decreased total CH₄ emissions, while dredging showed a trend towards decreasing CH₄ emissions. Total CH₄ emissions in all mesocosms were much higher in the summer of the second year, likely because of higher algal decomposition and organic matter availability. First, LMB addition lowered CH₄ emissions by decreasing P-availability, which reduced coverage of the floating fern Azolla filiculoides, and thereby prevented anoxia and decreased surface water NH₄⁺ concentrations, lowering CH₄ production rates. Second, dredging decreased CH₄ emissions in the first summer, possibly it removed the methanogenic community, and in the second year by preventing autumn and winter die-off of the rooted macrophyte Potamogeton cripsus. Finally, methanogen community composition was related to surface water NH₄⁺ and O₂, and porewater total phosphorus, while methanotroph community composition was related to organic matter content. To conclude, LMB addition and dredging not only improve water quality, but also decrease CH₄ emissions, mitigating climate change.

Rare earth elements in oysters and mussels collected from the Chinese coast: Bioaccumulation and human health risks

Rare earth elements (REEs) are increasingly used in various industries worldwide, resulting in their release into aquatic ecosystems. We evaluated the distribution and bioaccumulation of 14 REEs in marine sediments and biotas along the Chinese coasts. The total concentration of REEs (ΣREEs) in sediments was 41.65-170.94 mg/kg. The concentrations of ΣREEs were 1.97-4.77 and 0.62-4.96 mg/kg dry mass (DM) for oysters and mussels. The concentration of total light REEs (ΣLREEs) was higher than the concentration of total heavy REEs (ΣHREEs) at all samples. The bioaccumulation factor (BAF) of ΣLREEs was higher than ΣHREEs and BAF of ΣREE was 0.34-1.49 and 0.25-1.10 for oysters and mussels. The positive correlation between sediments and biotas was higher in mussels than oysters, showing a good potential for being environmental indicators for REEs. The risk of REEs to humans via oysters and mussels consumption could be negligible based on the estimated daily intake.

Compounds to mitigate cyanobacterial blooms affect growth and toxicity of Microcystis aeruginosa

Numerous products and techniques are used to combat harmful cyanobacterial blooms in lakes. In this study, we tested nine products, the phosphate binders Phoslock® and Aqual-P^TM, the coagulant chitosan, the phosphorus binder and coagulant aluminum salts (aluminum sulphate and sodium aluminate), the copper-based algicides SeClear, Captain® XTR and CuSO₄·5H₂O, the antibiotic Streptomycin and the oxidant hydrogen peroxide (H₂O₂) on their efficiency to manage the cyanobacterium Microcystis aeruginosa (M. aeruginosa). To this end, 7 days of laboratory experiments were conducted and effects were determined on chlorophyll-a, photosystem II efficiency (PSII), soluble reactive phosphorus (SRP) and intracellular and extracellular microcystin (MC) concentrations. The algicides, chitosan and H₂O₂ were the most powerful in reducing cyanobacteria biomass. Biomass reductions compared to the controls yielded: Chitosan (99.8%) > Hydrogen peroxide (99.6%) > Captain XTR (98.2%) > SeClear (98.1%) > CuSO₄·5H₂O (97.8%) > Streptomycin (86.6%) > Phoslock® (42.6%) > Aqual-P^TM (28.4%) > alum (5.5%). Compounds that caused the largest reductions in biomass also strongly lowered photosystem II efficiency, while the other compounds (Phoslock®, Aqual-P^TM, aluminum salts) had no effect on PSII, but strongly reduced SRP. Intracellular MC concentration followed the biomass patterns, extracellular MC was generally lower at higher doses of algicides, chitosan and H₂O₂ after one week. Recovery of PSII was observed in most algicides and chitosan, but not at the highest doses of SeClear and in all streptomycin treatments. Our results revealed that M. aeruginosa can be killed rapidly using several compounds, that in some treatments already signs of recovery occurred within one week. P fixatives are efficient in reducing SRP, and thus acting via resource suppression, which potentially may provide an addition to fast-acting algicides that kill most of the cells, but allow rapid regrowth as sufficient nutrients remain.

Response of sediment phosphorus partitioning to lanthanum-modified clay amendment and porewater chemistry in a small eutrophic lake

Sustained eutrophication of the aquatic environment by the remobilization of legacy phosphorus (P) stored in soils and sediments is a prevailing issue worldwide. Fluxes of P from the sediments to the water column, referred to as internal P loading, often delays the recovery of water quality following a reduction in external P loads. Here, we report on the vertical distribution and geochemistry of P, lanthanum (La), iron (Fe) and carbon (C) in the culturally eutrophied Lake Bromont. This lake underwent remediation treatment using La modified bentonite (LMB) commercially available as Phoslock™. We investigated the effectiveness of LMB in decreasing soluble reactive phosphorus (SRP) availability in sediments and in reducing dissolved fluxes of P across the sediment-water interface. Sediment cores were retrieved before and after LMB treatment at three sites representing bottom sediment, sediment influenced by lakeside housing and finally littoral sediment influenced by the lake inflow. Sequential extractions were used to assess changes in P speciation. Depth profiles of dissolved porewater concentrations were obtained after LMB treatment at each site. Results indicate that SRP extracted from the sediments decreased at all sites, while total extracted P (P_TOT) bound to redox-sensitive metal oxides increased. ³¹P NMR data on P extract reveals that 20-43% of total solid-phase P is in the form of organic P (P_org) susceptible to be released via microbial degradation. Geochemical modelling of porewater data provides evidence that LaPO_4(s) mineral phases, such as rhabdophane and/or monazite, are likely forming. However, results also suggest that La³⁺ binding by dissolved organic carbon (DOC) hinders La-phosphate precipitation. We rely on thermodynamic modelling to suggest that high Fe²⁺ would bind to DOC instead of La³⁺, therefore promoting P sequestrations by LMB under anoxic conditions.

A biological and chemical approach to restoring water quality: A case study in an urban eutrophic pond

Efforts to improve water quality of eutrophic ponds often involve implementing changes to watershed management practices to reduce external nutrient loads. While this is required for long-term recovery and prevention, eutrophic conditions are often sustained through the recycling of internal nutrients already present within the waterbody. In particular, internal phosphorus bound to organic material and adsorbed to sediment has the potential to delay lake recovery for decades. Thus, pond and watershed management techniques are needed that not only reduce external nutrient loading but also mitigate the effects of internal nutrients already present. Therefore, our objective was to demonstrate a biological and chemical approach to remove and sequester nutrients present and entering an urban retention pond. A novel biological and chemical management technique was designed by constructing a 37 m² (6.1 m × 6.1 m) floating treatment wetland coupled with a slow-release lanthanum composite inserted inside an airlift pump. The floating treatment wetland promoted microbial denitrification and plant uptake of nitrogen and phosphorus, while the airlift pump slowly released lanthanum to the water column over the growing season to reduce soluble reactive phosphorus. The design was tested at the microcosm and field scales, where nitrate-N and phosphate-P removal from the water column was significant (α = 0.05) at the microcosm scale and observed at the field scale. Two seasons of field sampling showed both nitrate-N and phosphate-P concentrations were reduced from 50 μg L^-1 in 2020 to <10 μg L^-1 in 2021. Load calculations of incoming nitrate-N and phosphate-P entering the retention pond from the surrounding watershed indicate the presented biological-chemical treatment is sustainable and will minimize the effects of nutrient loading from nonpoint source pollution.

Temporal and spatial variation in the efficiency of a Floc & Sink technique for controlling cyanobacterial blooms in a tropical reservoir

One of the main symptoms of eutrophication is the proliferation of phytoplankton biomass, including nuisance cyanobacteria. Reduction of the external nutrient load is essential to control eutrophication, and in-lake interventions are suggested for mitigating cyanobacterial blooms to accelerate ecosystem recovery. Floc & Sink (F&S) is one such intervention technique that consists of applying a low dose of coagulants in combination with ballasts for removing cyanobacteria biomass. It is especially suitable for deep lakes with an external nutrient load that is higher than the internal load and suffers from perennial cyanobacterial bloom events. Studies showing the efficacy of the F&S technique have been published, but those testing its variation in efficacy with changes in the environmental conditions are still scarce. Therefore, we evaluated the efficiency of the F&S technique to remove cyanobacteria from water samples collected monthly from two different sites in a deep tropical reservoir (Funil Reservoir, Brazil) in the laboratory. We tested the efficacy of two coagulants, chitosan (CHI) and poly-aluminum chloride (PAC), alone and in combination with lanthanum-modified bentonite (LMB) in settling phytoplankton biomass. We hypothesized that: ⅰ) the combined treatments are more effective in removing the algal biomass and ⅱ) the efficiency of F&S treatments varies spatially and monthly due to changes in environmental conditions. The combined treatments (PAC + LMB or CHI + LMB) removed up to seven times more biomass than single treatments (PAC, CHI, or LMB). Only the treatments CHI and LMB + CHI differed in efficiency between the sites, although all treatments showed significant variation in efficiency over the months at both the sampling sites. The combined treatments exhibited lower removal efficacy during the warm-rainy months (October-March) than during the mild-cold dry months (April-September). At high pH (pH > 10), the efficiency of the CHI and LMB + CHI treatments decreased. CHI had lower removal efficiency when single-cell cyanobacteria were abundant, while the combined treatments were equally efficient regardless of the morphology of the cyanobacteria. Hence, the combination of PAC as a coagulant with a ballast LMB is the most effective technique to precipitate cyanobacteria under the conditions that are encountered around the year in this tropical reservoir.

Effect of application mode (capping and amendment) on the control of cadmium release from sediment by apatite/calcite mixture and its phosphorus release risk

In this research, the influence of application mode (capping and amendment) on the control of cadmium (Cd) liberation from sediment by apatite/calcite mixture and its phosphorus release risk were investigated. The results showed that calcite addition had a limited effect on the speciation of Cd in sediment, but apatite addition had a significant impact on the fractionation of Cd in sediment. Apatite amendment could effectively immobilize the most readily mobilized Cd by transferring the acid-soluble fraction to the reducible and residual fractions. Apatite addition also could effectively reduce the concentration of toxicity characteristic leaching procedure (TCLP)-leachable Cd in sediment, and apatite had a much higher reduction efficiency of TCLP-leachable Cd than calcite. Apatite/calcite mixture capping could reduce the risk of Cd liberation from sediment into the overlying water, and the controlling efficiency of apatite/calcite mixture capping was higher than that of apatite/calcite mixture amendment. The effect of apatite/calcite mixture addition on the concentration of reactive soluble phosphorus (SRP) in the overlying water was limited. The introduction of calcite into the apatite capping layer could lower the risk of phosphorus release from apatite to the overlying water as compared to single apatite capping. However, the apatite/calcite mixture capping layer still had a relatively high risk of phosphorus liberation into the overlying water. Results of this work suggest that apatite/calcite mixture has a high potential to be used as a capping material to control Cd release from sediment from the perspective of controlling efficiency and application convenience.

Combining lanthanum-modified bentonite (LMB) and submerged macrophytes alleviates water quality deterioration in the presence of omni-benthivorous fish

Bioturbation by omni-benthivorous fish often causes sediment resuspension and internal nutrient loading, which boosts phytoplankton growth and may lead to a shift of clear water lakes to a turbid state. Removal of large-sized omni-benthivorous individuals is a lake restoration measure that may revert lakes from a turbid to a clear water state, yet the rapid reproduction of small omni-benthivorous fish in tropical and subtropical shallow lakes may impede such lake recovery. In lake restoration, also a combination of lanthanum-modified bentonite (LMB) and planting submerged macrophytes has been used that may synergistically improve lake water quality. How the combined effect works in the presence of small omni-benthivorous fish has not been studied, which is needed given the high abundances of small omni-benthivorous fish in (sub)tropical lakes. We conducted a two-by-two factorial mesocosm experiment with and without the submerged macrophytes Vallisneria natans and with and without LMB, all in the presence of small crucian carp. At the end of the experiment, turbidity in the V. natans, LMB and combined LMB + V. natans treatments had decreased by 0.8%, 30.3% and 30.9%, respectively, compared with the controls. In addition, the nitrogen (N) and phosphorus (P) release from the sediment in the combined LMB + V. natans treatments had decreased substantially, by 97.4% and 94.3%, respectively, compared with the control. These N and P fluxes were also significantly lower in the combined LMB + V. natans treatments than in the sole LMB treatment (88.1% and 82.3%) or the V. natans treatment (93.2% and 90.3%). Cyanobacteria in the overlying water in the combined LMB + V. natans treatments significantly decreased by 84.1%, 63.5% and 37.0%, respectively, compared with the control and the sole LMB and V. natans treatments. Our results show that LMB and submerged macrophytes complement each other in effectively improving the water quality, even in the presence of small omni-benthivorous fish.

Towards climate-robust water quality management: Testing the efficacy of different eutrophication control measures during a heatwave in an urban canal

Harmful algal blooms are symptomatic of eutrophication and lead to deterioration of water quality and ecosystem services. Extreme climatic events could enhance eutrophication resulting in more severe nuisance algal blooms, while they also may hamper current restoration efforts aimed to reduce nutrient loads. Evaluation of restoration measures on their efficacy under climate change is essential for effective water management. We conducted a two-month mesocosm experiment in a hypertrophic urban canal focussing on the reduction of sediment phosphorus (P)-release. We tested the efficacy of four interventions, measuring phytoplankton biomass, nutrients in water and sediment. The measures included sediment dredging, water column aeration and application of P-sorbents (lanthanum-modified bentonite - Phoslock® and iron-lime sludge, a by-product from drinking water production). An extreme heatwave (with the highest daily maximum air temperature up to 40.7 °C) was recorded in the middle of our experiment. This extreme heatwave was used for the evaluation of heatwave-induced impacts. Dredging and lanthanum modified bentonite exhibited the largest efficacy in reducing phytoplankton and cyanobacteria biomass and improving water clarity, followed by iron-lime sludge, whereas aeration did not show an effect. The heatwave negatively impacted all four measures, with increased nutrient releases and consequently increased phytoplankton biomass and decreased water clarity compared to the pre-heatwave phase. We propose a conceptual model suggesting that the heatwave locks nutrients within the biological P loop, which is the exchange between labile P and organic P, while the P fraction in the chemical P loop will be decreased. As a consequence, the efficacy of chemical agents targeting P-reduction by chemical binding will be hampered by heatwaves. Our study indicates that current restoration measures might be challenged in a future with more frequent and intense heatwaves.

Production and transformation of organic matter driven by algal blooms in a shallow lake: Role of sediments

The generation of organic matter (OM) occurs synchronously with phytoplankton growth. Characterization of the generated particulate and dissolved OM during algal blooms in eutrophic lakes is crucial for better understanding the carbon cycle but remains limited. We speculate that sediments play a critical role in the biogeochemical transformation of OM derived from algal blooms in shallow lakes. In this study, changes in OM quantity and quality and the concentrations of biogenic elements (nutrients and metals) during algal blooms, were studied in situ in a shallow eutrophic lake (Lake Chaohu, China). Two enclosure treatments in the presence and absence of sediments were compared, and the cause-effect relationships among sediment, nutrients, metals, phytoplankton, particulate OM (POM), and dissolved OM (DOM) were revealed by a partial least square-path model (PLS-PM). The results showed that the changes in nutrients and metals concentrations over time were consistent with that of chlorophyll a (Chl a), and at the end of the treatment, the concentrations of Chl a, nutrients, and metals in Treatment S (with sediments) were approximately 3-5 times of those in Treatment N (without sediments). The high concentration of Chl a in Treatment S resulted in a high quantity of POM, which showed low molecular weight, low humification, and was enriched in protein-like components (∼ 70%). For DOM, the quantity increased after the decrease in POM, and DOM quality showed a significantly higher abundance of humic-like components and a higher molecular weight than POM did. The PLS-PM results showed that the significant positive effects of sediment on nutrients, metals, phytoplankton, POM, and DOM were 0.28, 0.37, 0.28, 0.25, and 0.25, respectively, suggesting that sediment had an important role in the biogeochemical cycles of these substances. The significant negative relationship between POM and DOM (-0.62) and the distinct difference in POM and DOM quality implied the efficient transformation of the freshly generated OM to those with a higher molecular weight, higher humification, and potentially refractory. Our results depicted the quick biogeochemical transformation of nutrients, metals, and the potential formation of refractory organic carbon in water column, as driven by the couple of the algae pump with the microbial carbon pump.

Oxygen Nanobubbles for Lake Restoration—Where Are We at? A Review of a New-Generation Approach to Managing Lake Eutrophication

Nutrient enrichment of lakes from anthropogenic activities is a significant and increasing issue globally, impairing the health, biodiversity and service provisioning from lakes, with impacts on cultural, recreational, economic and aesthetic values. Internal nutrient loads from lakebed sediment releases are a primary cause of lake eutrophication and have necessitated geoengineering methods to mitigate releases and speed up recovery from eutrophication. Our objective in this review was to evaluate the use of oxygen nanobubbles as a geoengineering technology to remediate low oxygen conditions at the lake sediment/water interface, as a precursor to alleviating eutrophication linked to high internal nutrient loads. Oxygen nanobubbles (NBs) are bubbles < 1000 nm formed at the interface of solid surfaces and aqueous solutions. These bubbles have higher density than water, persist for longer and facilitate greater oxygen solubility than larger bubbles. Methods have been developed to enable NB formation at the surface of carrier materials, which are then used in conjunction with modified local soils (MLSs), to ‘floc, lock and oxygenate’ to strip nutrients from the water column, locking them in lakebed sediments and oxygenating the sediments to prevent re-release of nutrients. Most studies of NBs for lake restoration have thus far only demonstrated their potential for this purpose, using short-term, small-scale core incubations conducted mainly in laboratory settings. Work is required to (1) address scalability, including procurement and cost, (2) extend laboratory incubation studies to large outdoor enclosures and pond/lake trials, (3) examine longevity of the effects in the natural environment, including potential for MLSs to smother benthos and/or have toxic effects, and (4) extend to a range of lake environments and MLS types. Legal, cultural and social acceptance of the technology is another prerequisite of applications in the natural environment and requires individualised analysis. Until these issues are addressed in a systematic way that addresses scalability and recommends suitable carrier materials and MLSs, NBs may continue to remain largely untried as a geoengineering method to address lake eutrophication.

Responses of Different Submerged Macrophytes to the Application of Lanthanum-Modified Bentonite (LMB): A Mesocosm Study

Lanthanum-modified bentonite (LMB) has remarkable efficacy on eutrophication control, but the reduced bioavailable phosphorus and formed anaerobic horizon from LMB may be harmful to submerged macrophytes. We conducted this study to explore the influence of LMB on Hydrilla verticillata and Vallisneria natans in mixed-species plantings. The concentrations of TP, TDP, SRP, and TDN in the LMB treatments were lower than the Control, but the Chl a concentration in the HLMB treatment (850 g m−2) was higher than the Control by 63%. There were no differences of V. natans growth among the treatments. For H. verticillata, its biomass, RGR, height, branch number, root number, and length in the LLMB treatment (425 g m−2) were lower than the Control by 48%, 22%, 13%, 34%, 33%, and 8%, respectively. In addition, the biomass of H. verticillata was 62%, the RGR was 32%, the height was 19%, the branch number was 52%, the root length was 40%, and the root number was 54% lower in the HLMB treatment than those in the Control. In summary, LMB had negative effects on submerged macrophytes with underdeveloped roots. Submerged macrophytes with more developed roots are preferred when using combined biological–chemical methods for water restoration.

Control of Endogenous Phosphorus Release at the Sediment–Water Interface by Lanthanum-Modified Fly Ash

This study optimizes the modification and granulation of fly ash to make it more stable at the sediment–water interface. Through laboratory simulations, the modified fly ash pellets were optimally granulated to cover the sediment–water interface, and its control effect and mechanism were evaluated. The results showed that the phosphorus adsorption effect of lanthanum-modified fly ash was 34% and 40% higher compared with those of acid-modified and alkali-modified fly ash, respectively, with the phosphorus adsorption effect reaching 85%. The best dosing ratio was about 0.3 g/L. Adsorption was affected by pH and was more effective under weak alkalinity, close to the Langmuir adsorption model, which was consistent with the unimolecular layer adsorption characteristics and the presence of chemisorption and physical adsorption. The saturation adsorption amount of phosphate by lanthanum-modified fly ash was 8.89 mg/g. The optimized granulation conditions for lanthanum-modified fly ash pellets were a fly ash/montmorillonite ratio of 7:3, a roasting temperature of 900 °C, a roasting time of 4 h, and a particle size of 3 mm. After 20 days, the orthophosphate removal rate was more than 60% higher than that of the control group, with a total phosphorus removal rate of 43%. After covering for 60 days, active phosphorus in the surface layer of the sediment was gradually transformed into a stable phosphorus form, with calcium phosphorus accounting for 70% of the total inorganic phosphorus. The ability of the sediment to release phosphorus to the overlying water body was also significantly weakened. Meanwhile, the total phosphorus removal rate in the overlying water at the sediment–water interface reached more than 40%, and orthophosphate removal reached more than 60%, indicating an obvious phosphorus control effect. Transmission electron microscopy analysis showed that lanthanum was present at locations enriched with elemental phosphorus and was adsorbed onto the material surface. Therefore, lanthanum-modified fly ash pellets are a promising in situ phosphorus control agent with good endogenous phosphorus pollution control abilities in eutrophic water bodies.

Mechanisms of orthophosphate removal from water by lanthanum carbonate and other lanthanum-containing materials

Removing phosphorus (P) from water and wastewater is essential for preventing eutrophication and protecting environmental quality. Lanthanum [La(III)]-containing materials can effectively and selectively remove orthophosphate (PO₄) from aqueous systems, but there remains a need to better understand the underlying mechanism of PO₄ removal. Our objectives were to 1) identify the mechanism of PO₄ removal by La-containing materials and 2) evaluate the ability of a new material, La₂(CO₃)_3(s), to remove PO₄ from different aqueous matrices, including municipal wastewater. We determined the dominant mechanism of PO₄ removal by comparing geochemical simulations with equilibrium data from batch experiments and analyzing reaction products by X-ray diffraction and scanning transmission electron microscopy with energy dispersive spectroscopy. Geochemical simulations of aqueous systems containing PO₄ and La-containing materials predicted that PO₄ removal occurs via precipitation of poorly soluble LaPO_4(s). Results from batch experiments agreed with those obtained from geochemical simulations, and mineralogical characterization of the reaction products were consistent with PO₄ removal occurring primarily by precipitation of LaPO_4(s). Between pH 1.5 and 12.9, La₂(CO₃)_3(s) selectively removed PO₄ over other anions from different aqueous matrices, including treated wastewater. However, the rate of PO₄ removal decreased with increasing solution pH. In comparison to other solids, such as La(OH)_3(s), La₂(CO₃)_3(s) exhibits a relatively low solubility, particularly under slightly acidic conditions. Consequently, release of La³⁺ into the environment can be minimized when La₂(CO₃)_3(s) is deployed for PO₄ sequestration.

Particle size-related vertical redistribution of phosphorus (P)-inactivating materials induced by resuspension shaped P immobilization in lake sediment profile

Lake geoengineering with phosphorus (P)-inactivating materials to reduce sediment P loading is often used for eutrophication control. The redistribution of materials in sediment, especially those induced by resuspension, is reportedly a common phenomenon during practical applications, which may interfere with the pollution control. Notably, a recent study by the authors initially found that the heterogeneous properties of materials and sediments varied the P immobilization in different sized sediments which exhibited diverse movement characteristics. Therefore, this study hypothesizes a particle size-related vertical redistribution of materials in the sediment profile induced by resuspension, which shapes sediment P immobilization at different depths. Based on two differently sized materials, lanthanum (La)-modified bentonite clay (Phoslock) and drinking water treatment residue (DWTR), this study found a weakened reduction of mobile P and bioavailable P pool by both DWTR and Phoslock in surface sediment after resuspension. As the depth decreased from >12 to surface 0-1 cm, the remaining mobile P increased from 7.11%-10.8% to 11.0%-17.8% of the total P in the sediment with Phoslock and from 1.66%-4.73% to 9.70%-20.7% of the total P in the sediment with DWTR; meanwhile, bioavailable P pool reduction proportions decreased from 48.6%-72.3% to 3.23%-45.1% for Phoslock and from 51.5%-71.4% to 4.94%-25.2% for DWTR. Further analysis verified the hypothesis of this study; importantly, the redistributions of the potential target P (including mobile and bioavailable P) for immobilization were regulated by relatively small sediments (e.g., <8 μm fraction), which tended to become enriched in surface sediment after resuspension, while relatively large materials (e.g., >63 μm fraction) regulated their redistributions and were more likely to be buried at the bottom of the sediments. Accordingly, to design appropriate strategies for lake geoengineering, relatively small materials (e.g., <8 μm) targeting to immobilize both mobile and bioavailable P are typically recommended to be developed for restoration of lakes with frequent sediment resuspension.