Effects of combined flocculant - Lanthanum modified bentonite treatment on aquatic macroinvertebrate fauna

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.

Risk of Collapse in Water Quality in the Guandu River (Rio de Janeiro, Brazil)

The Guandu River, one of the main rivers in the state of Rio de Janeiro, provides water for more than nine million people in the metropolitan region. However, the Guandu has suffered from massive domestic and industrial pollution for more than two decades, leading to high levels of dissolved total phosphorus, cyanobacteria, and enteric bacteria observed during the summers of 2020 and 2021. The use of Phoslock, a palliative compound, was not effective in mitigating the levels of phosphorus in the Guandu River. Furthermore, potable water driven from the river had levels of 2-MIB/geosmin and a mud smell/taste. With all these problems, several solutions are proposed for improving the Guandu River water quality, including establishment of (i) sewage treatment plants (STPs), (ii) strict water quality monitoring, (iii) environmental recovery (e.g., reforestation), and (iv) permanent protected areas. The objective of this paper is to verify the poor water quality in the Guandu and the ineffectiveness and undesired effects of Phoslock.

Synergistic Effects and Ecological Responses of Combined In Situ Passivation and Macrophytes toward the Water Quality of a Macrophytes-Dominated Eutrophic Lake

Combined use of in situ passivation and macrophytes is a valuable technology that exerts remarkable effects on aquatic systems. However, the effectiveness and ecological functions of this combined technology for macrophytes-dominated eutrophic (MDE) lakes with organophosphorus-controlled internal phosphorus (P) loading were poorly understood. In this study, aquatic simulation experiments were performed to study the combination of La-modified materials (LMM; La-modified bentonite (LMB), and La/Al co-modified attapulgite (LAA)) with macrophytes (Myriophyllum verticillatum L. (MVL), Hydrilla verticillata (Linn. f.) royle (HVR), and Ceratophyllum demersum L. (CDL)) for the control of P mobility in the water column, and to investigate the passivator effects on the physiological characteristics of macrophytes. The mineralization of organophosphates (BD−Po, HCl−Po, and Res−Po) is an important factor for maintaining high internal P loadings and overlying water P concentrations in the experiments. Compared with individual treatment groups, the reduction of internal P release flux and porewater SRP concentrations was more obvious in the combined treatments. Moreover, the redox-sensitive P forms transformation is more pronounced in the surface sediments. In the LAA+M group, internal P release flux was reduced by 55% and 55% compared with individual passivators and macrophytes retreatment groups, respectively. In contrast, the LMB+M group decreased by 16% and 46%, respectively. Simultaneously, LMM had less effect on macrophytes traits compared with individual macrophytes group and enhanced the absorption of phosphate by macrophytes. The phosphate content of macrophytes in the LAA+M and LMB+M groups increased by 24% and 11%, respectively, in comparison with the individual macrophytes group. Results concluded that the combination of passivator and macrophytes enhanced the effect of ecological restoration and exerts a synergistic effect on internal P pollution with macrophytes.

Effect of juvenile omni-benthivorous fish (Carassius carassius) disturbance on the efficiency of lanthanum-modified bentonite (LMB) for eutrophication control: a mesocosm study

Lanthanum-modified bentonite (LMB) is widely used for eutrophication control and has demonstrated good efficiency in some eutrophic lakes. However, the efficiency of LMB on eutrophication control in some eutrophic lakes, where the structure of food webs is mainly dominated by omni-benthivorous fish, remains ambiguous. Omni-benthivorous fish usually disturbs sediment and promotes the release of internal nutrients, the effect of which on the efficacy of LMB remains to be studied. Thus, a 30-day mesocosm experiment was conducted to determine whether omni-benthivorous fish disturbance and LMB would cause antagonistic responses. LMB significantly reduced dissolved P concentration in overlying water, converting mobile P to bound P in the surface layer of sediment in the absence of crucian carp (Carassius carassius). However, there were significantly negative interaction effects between LMB and crucian carp. Although LMB still effectively reduced the total dissolved phosphorus (TDP) and soluble reactive phosphorus (SRP) concentrations of overlying water in the presence of crucian carp, it had limited efficacy on inhibiting the increased concentrations of suspended solids, particulate nutrients, and chlorophyll a (Chl a) due to crucian carp disturbance. Furthermore, the crucian carp disturbance also increased the risk of mobile P releasing from surface sediment, whether with or without LMB application. The results indicated that the efficacy of LMB was insufficient to offset the negative effect of omni-benthivorous fish disturbance on eutrophication control. Hence, the omni-benthivorous fish also need to be considered for eutrophication control in shallow eutrophic lakes. Some measures need to be taken to control the biomass of omni-benthivorous fish.

Emerging lanthanum (III)-containing materials for phosphate removal from water: A review towards future developments

The last two decades have seen a rise in the development of lanthanum (III)-containing materials (LM) for controlling phosphate in the aquatic environment. >70 papers have been published on this topic in the peer-reviewed literature, but mechanisms of phosphate removal by LM as well as potential environmental impacts of LM remain unclear. In this review, we summarize peer-reviewed scientific articles on the development and use of 80 different types of LM in terms of prospective benefits, potential ecological impacts, and research needs. We find that the main benefits of LM for phosphate removal are their ability to strongly bind phosphate under diverse environmental conditions (e.g., over a wide pH range, in the presence of diverse aqueous constituents). The maximum phosphate uptake capacity of LM correlates primarily with the La content of LM, whereas reaction kinetics are influenced by LM formulation and ambient environmental conditions (e.g., pH, presence of co-existing ions, ligands, organic matter). Increased La solubilization can occur under some environmental conditions, including at moderately acidic pH values (i.e., < 4.5-5.6), highly saline conditions, and in the presence of organic matter. At the same time, dissolved La will likely undergo hydrolysis, bind to organic matter, and combine with phosphate to precipitate rhabdophane (LaPO₄·H₂O), all of which reduce the bioavailability of La in aquatic environments. Overall, LM use presents a low risk of adverse effects in water with pH > 7 and moderate-to-high bicarbonate alkalinity, although caution should be applied when considering LM use in aquatic systems with acidic pH values and low bicarbonate alkalinity. Moving forward, we recommend additional research dedicated to understanding La release from LM under diverse environmental conditions as well as long-term exposures on ecological organisms, particularly primary producers and benthic organisms. Further, site-specific monitoring could be useful for evaluating potential impacts of LM on both biotic and abiotic systems post-application.

Lanthanum modified bentonite behaviour and efficiency in adsorbing phosphate in saline waters

Lanthanum-modified bentonite (LMB, commercially called Phoslock®) has been widely applied in freshwater systems to manage eutrophication. Little is known, however, about its behaviour and efficiency in binding filterable reactive phosphorus (FRP) in saline environments. We assessed if LMB would adsorb phosphate over a range of salinities (0-32 ppth) comparing the behaviour in seawater salts and equivalent concentrations of NaCl. Lanthanum release from the bentonite matrix was measured and the La species prevailing in saline environments were evaluated through chemical equilibrium modelling. We demonstrated that LMB was able to adsorb FRP in all the salinities tested. Filterable lanthanum (FLa) concentrations were similarly low (<5 μgL^-1) at all seawater salinities but considerably elevated, on occasion >2000 times greater in equivalent NaCl salinities. Mineralogical analysis indicates that La present in the clay interlayer was (partially) replaced by Na/Ca/Mg present in the seawater and a possible secondary P-reactive phase was formed, such as kozoite (LaCO₃OH) or lanthanite (La₂(CO₃)₃·8H₂O) that may be physically dissociated from the LMB. Geochemical modelling also indicates that most FLa dissociated from LMB would be precipitated as a carbonate complex. In light of the identification of reactive intermediate phases, further studies including ecotoxicologial assays are required to assess any deleterious effects from the application of LMB to saline waters.

Lanthanum in Water, Sediment, Macrophytes and chironomid larvae following application of Lanthanum modified bentonite to lake Rauwbraken (The Netherlands)

Lanthanum Modified Bentonite (LMB; Phoslock®) is used to mitigate eutrophication by binding phosphate released from sediments. This study investigated the fate of lanthanum (La) from LMB in water, sediment, macrophytes, and chironomid larvae in Lake Rauwbraken (The Netherlands). Before the LMB application, water column filterable La (FLa) was 0.02 µg L^-1, total La (TLa) was 0.22 µg L^-1. In sediment the total La ranged 0.03-1.86 g m^-2. The day after the application the maximum FLa concentration in the water column was 44 µg L^-1, TLa was 528 µg L^-1, exceeding the Dutch Maximum Permissible Concentrations (MPC) of 10.1 µg L^-1 by three to fourfold. TLa declined below the MPC after 15 days, FLa after 75 days. After ten years, FLa was 0.4 µg L^-1 and TLa was 0.7 µg L^-1. Over the post-application years, FLa and TLa showed statistically significant downward trends. While the LMB settled homogeneously on sediment, after 3 years it redistributed to 0.2-5.4 g La m^-2 within shallow zones, and 30.7 g m^-2 to 40.0 g La m^-2 in deeper zones. In the upper 20 cm of sediment, La concentrations were 7-6702 mg kg ^-1 dry weight (DW) compared to 0.5-7.0 mg kg^-1 before application. Pre-application anaerobic sediment release of FLa was 0.006 mg m^-2 day^-1. Three months after the application it was 1.02 mg m^-2 day^-1. Three years later it was 0.063 mg m^-2 day^-1. Before application La in plants was 0.8-5.1 mg La kg^-1 DW, post-application values were up to 2925 mg La kg^-1 DW. In chironomid larvae, La increased from 1.7 µg g^-1 DW before application to 1421 µg g^-1 DW after one month, 3 years later it was 277 µg g^-1 DW. Filtration experiments indicate FLa is not truly dissolved free La³⁺ cations.

A new method to overall immobilization of phosphorus in sediments through combined application of capping and oxidizing agents

A new method has been developed for improving the overall immobilization efficiency of phosphorus (P) in sediment. A capping agent (lanthanum modified bentonite, LMB) was sprinkled on the sediment surface to prevent the release of P in the top sediment layer. Meanwhile, an oxidizing agent (calcium nitrate, CN) was injected into the sediment layer (~5 cm) to immobilize labile P in deep sediment layers. High-resolution sampling techniques, including diffusive gradients in thin films (DGT) and high-resolution dialysis (HR-Peeper) were employed to investigate the fine-scale changes of labile and/or soluble nitrogen, P, sulfide and iron in sediments, respectively. The results showed that the combined application of LMB and CN had significant advantages over the individual treatments. The average concentrations of soluble reactive phosphorus (SRP) (0.01 mg/L) in the overlying water after a 68-day incubation were only 10%, 21% and 4% for the CK, LMB and CN treatments, respectively. Furthermore, the immobilization effect caused by the combined treatment reached from the sediment-water interface to a depth of 60 mm in the sediment, and the effective depth was much >20 mm caused by LMB treatment. The concentrations of SRP in the sediment profile were also lower than those of the other treatments. The results of this work indicate that the combined application of capping and oxidizing agents is a promising method to control water eutrophication by preventing the release of P from both the top and deep sediment layers.

Ecotoxicity screening of novel phosphorus adsorbents used for lake restoration

Short-term standardized laboratory tests were carried out for evaluating acute and chronic toxicological effects of novel phosphorus (P) adsorbents on Raphidocelis subcapitata (algal growth rate inhibition) and on Daphnia magna (immobilization, with direct and indirect exposure to adsorbents, and uptake-depuration tests). Four P adsorbents were tested: two magnetic (HQ and Fe₃O₄) and two non magnetic (CFH-12^® and Phoslock^®). For the case of the algal growth inhibition test, the EC₅₀ was 1.5 and 0.42 g L^-1 for HQ and CFH-12^®, respectively, and no inhibition patterns were observed neither for Fe₃O₄ nor for Phoslock^®. When organisms were exposed to a direct contact, in the D. magna immobilization test, no statistically significant differences were found in the EC₅₀ values among the four studied adsorbents. The huge difference between direct and indirect contact experiments suggests that toxicity is mainly physically mediated. The uptake-depuration test evidenced a much faster uptake and depuration rates for Phoslock^®, which was precisely the adsorbent with the highest particle size. In a realistic worst-case scenario using data from Honda lake (Almería, Spain), where lake restoration is carried out by a adding a single large dose to bind surplus P in the lake, the predicted environmental concentrations for all adsorbents were lower than EC₅₀ for all adsorbents and they were found to exceed a provisional limit value for ecotoxicity after a short-term exposure. All in all, since neither accumulation nor longer term effects of P adsorbents in the pelagic phase is expected, this risk may however, on a case-to-case basis, be acceptable.

A simple method to improve the adsorption properties of drinking water treatment residue by lanthanum modification

The loading of La can substantially enhance the adsorption capability of drinking water treatment residue (DWTR) for better recycling. Normally, the modification was based on incubation of DWTR and La solution at a certain ratio, following by solid-liquid separation and drying processes. This study attempted to simplify La loading procedures by adopting high ratio of DWTR and La solution to eliminate the solid-liquid separation, aiming to promote the potential actual production. According to the results of the short- (2 d) and long-term (30 d) P adsorption tests, the N₂ gas sorption and desorption analysis, the X-ray photoelectron spectroscopy analysis, and the metal fractionation, the substantial enhanced adsorption capability of the modified DWTR was maintained and the La loading mechanisms to DWTR changed little after eliminating solid-liquid separation processes during modification; typically, La loading increased the initial P adsorption rates from 1.00 (raw DWTR) to 6.08 and 6.03 mg g^-1 d^-1 for the modified DWTR with and without the separation processes. Furthermore, the DWTR before and after modification had little unfavorable effect on the survival of snail Bellamya aeruginosa, while eliminating the separation processes tended to reduce the bioavailability of Al, Fe, and La in the modified DWTR. These results demonstrated that solid-liquid separation was not the key step for DWTR modification and that the developed simple modification method was feasible for La loading to DWTR, promoting the beneficial recycling in environmental remediation.

Do magnetic phosphorus adsorbents used for lake restoration impact on zooplankton community?

Magnetic microparticles (MPs) have been recently proposed as innovative and promising dissolved inorganic phosphorus (DIP) adsorbents. However, before using them in a whole-lake restoration project, it is essential to assess their toxicological effects (direct and indirect) on aquatic biota. In the present study we hypothesized that zooplankton community is affected by MPs used for lake restoration. To test our hypothesis we designed a microcosms experiment (n = 15) containing lake water and surface sediment from a hypertrophic lake. Temporal changes (70 days) on physico-chemical conditions and on zooplankton structure (rotifers, copepods and branchiopods) were monitored under different scenarios. In particular, three different treatments were considered: no addition of MPs (control) and MPs addition (1.4 g MPs L^-1) on the surface water layer (T-W) and on the sediment (T-S). After 24 h of contact time, MPs were removed with a magnetic rake. A total of 15 zooplankton species (12 rotifers, 1 branchiopod and 2 copepods) were recorded and a high abundance of zooplankton was registered during the experiment for all treatments. No significant differences (RM-ANOVA test; p > 0.05) in total abundance, species richness and species diversity among treatments were found. The absence of any effect of MPs on zooplankton can be explained because MPs did not significantly alter any of its physico-chemical (e.g. temperature, pH, O₂) or biological (e.g. food quantity and quality) drivers. These results confirm the suitability of MPs as a promising tool for removing DIP in eutrophic aquatic ecosystems.

Lanthanum-modified drinking water treatment residue for initial rapid and long-term equilibrium phosphorus immobilization to control eutrophication

This study presents an approach for developing inactivating materials to achieve an initial rapid and a long-term equilibrium P immobilization to control eutrophication based on drinking water treatment residue (DWTR), which is a byproduct of potable water production. By taking advantage of the long-term equilibrium P adsorption by DWTR, the La chemical properties, and the previous success of using La-modified bentonite clay (Phoslock^®), we used DWTR as a La carrier with different ratios to develop the specific materials. The La loading mechanisms, the potentially toxic effect of La-modified DWTR on snail Bellamya aeruginosa (within 120 d), and the short- and long-term (within 80 d) P immobilization characteristics of the modified DWTR were investigated to understand the performance of the developed materials. The results showed that La loading into DWTR was based on ligand exchanges and the formation of new particles; DWTR loaded with <5% La had no toxicity against the snail. Most importantly, the loading of 5% La to DWTR substantially enhanced the rapid immobilization capacity of DWTR, achieving an initial rapid and a long-term equilibrium P adsorption in aqueous solutions. This study promotes the beneficial recycling of DWTR and results in a win-win situation for lake restoration.

Synergistic adsorption of phosphorus by iron in lanthanum modified bentonite (Phoslock®): New insight into sediment phosphorus immobilization

Iron redox cycle plays a primary role in controlling the mobility of P in sediments. It is crucial to better understand how lanthanum (La) modified bentonite (LMB, Phoslock^®), an increasingly employed capping agent, immobilizes P from sediments by altering Fe redox-coupled P cycling. Batch adsorption experiments found that LMB effectively adsorbed Fe(II) with a capacity of 8.51 mg g^-1. Fe(II)-preloaded LMB effectively retained P during a 518-hour equilibration, while up to 16.7% of adsorbed P was release-sensitive in LMB without Fe(II) preloading. A 60-day incubation experiment was performed using sediment cores, with an LMB amendment dosage of up to 200 LMB/P_mob (w/w, P_mob denotes the amount of mobile P in the surface 40 mm sediment layer). The concentrations of pore water soluble reactive P (SRP) and labile P were measured by high resolution dialysis (HR-Peeper) and by diffusive gradient in thin films (DGT), respectively, at a vertical millimeter scale. They stratified into static layers with extremely low concentration distribution in the top 16-22 mm sediments (mean SRP ≤ 0.28 mg L^-1 and mean DGT-labile P ≤ 0.051 mg L^-1) and active layers with decreased upward diffusion potential (≤5.85 for SRP and ≤12.7 for DGT-labile P) below the static layer, when the applied dosage reached 60 LMB/P_mob. The LMB amendment reduced the pore water Fe and DGT-labile Fe in sediments, while considerable amounts of Fe and Fe-bound P existed in the LMB binding layer (25% of the total P in 200 LMB/P_mob treatment). These findings show that the adsorption of Fe by LMB plays a significant role in the stabilization of LMB-bound P, possibly by adsorbing release-sensitive P initially bound to the rhabdophane surface. LMB adsorbed Fe and P were not released until the redox potential decreased to extremely reductive conditions (-150 mV to -300 mV), possibly due to the re-adsorption of Fe and P by LMB. This study reveals synergistic effects of Fe adsorption and provides new insight into the immobilization mechanisms of P by LMB application.