Controlling eutrophication by combined bloom precipitation and sediment phosphorus inactivation

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.

Oxidation effects on Microcystis aeruginosa inactivation through various reactive oxygen species: Degradation efficiency, mechanisms, and physiological properties

The study investigated the inactivation of Microcystis aeruginosa using a combined approach involving thermally activated peroxyacetic acid (Heat/PAA) and thermally activated persulfate (Heat/PDS). The Heat/PDS algal inactivation process conforms to first-order reaction kinetics. Both hydroxyl radical (•OH) and sulfate radical (SO4-•) significantly impact the disruption of cell integrity, with SO4-• assuming a predominant role. PAA appears to activate organic radicals (RO•), hydroxyl (•OH), and a minimal amount of singlet oxygen (1O2). A thorough analysis underscores persulfate's superior ability to disrupt algal cell membranes. Additionally, SO4-• can convert small-molecule proteins into aromatic hydrocarbons, accelerating cell lysis. PAA can accelerate cell death by diffusing into the cell membrane and triggering advanced oxidative reactions within the cell. This study validates the effectiveness of the thermally activated persulfate process and the thermally activated peroxyacetic acid as strategies for algae inactivation.

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.

Control of phosphorus release from sediment by iron/aluminum co-modified zeolite: efficiency, mechanism, and response of microbial communities in sediment

The efficiency of iron/aluminum co-modified zeolite (FeAl-Z) covering and amendment for controlling the internal loading of phosphorus (P) from sediment to the overlying water (OW) and its controlling mechanism were explored. The response of the composition of sedimentary microbial communities in sediment and their function to the FeAl-Z capping and amendment was also examined. FeAl-Z showed good removal performance for phosphate in aqueous solution. The maximum phosphate adsorption quantity for FeAl-Z at pH 7 attained 11.2 mg P/g. The release of sediment endogenous phosphorus to OW can be successfully restrained by the FeAl-Z covering and amendment, and the suppression ability of FeAl-Z covering was stronger than that of FeAl-Z amendment. Under the capping or amendment condition, FeAl-Z can effectively inactivate the labile phosphorus measured by diffusion gradient in thin film (DGT-LP) in the overlying water and surface sediment. The added FeAl-Z transformed redox-sensitive phosphorus (BD-P) to metal oxide-bound phosphorus (NaOH-IP) and residual phosphorus (Res-P) in sediment, which increased the stability of inorganic phosphorus in the sediment. The passivation of soluble reactive phosphorus (SRP) and DGT-LP in the surface sediment by FeAl-Z significantly contributed to the inhibition of sediment endogenous phosphorus release to OW by the FeAl-Z capping, and the passivation of SRP, DGT-LP and mobile phosphorus in the surface sediment played a pivotal role in the control of sediment internal phosphorus release by the FeAl-Z amendment. The FeAl-Z amendment and capping did not increase the liberation risk of Fe from sediment, and the microorganisms in the sediments under the conditions of FeAl-Z amendment and covering still can perform good ecological functions. Results of this research demonstrate that FeAl-Z capping has high application potential in the control of phosphorus transfer from sediment to OW.

Response mechanism of sediment endogenous phosphorus release to functional microorganisms and its cyanobacterial growth and disappearance effects

Endogenous phosphorus (P) release from lake sediments is an important factor in the eutrophication of overlying waters, as P is the limiting nutrient salt affecting cyanobacterial growth. Microorganisms are also key to the evolution of cyanobacterial growth and disappearance, as they can influence the release of endogenous P. Meanwhile, endogenous phosphorus can also have an impact on microbial structure. However, there is a lack of studies on the response mechanisms between endogenous P release and microorganisms, as well as the exploration of endogenous P release on the whole cyanobacterial growth and disappearance evolution process. In this study, metagenome sequencing was used to characterize the microbial community structure at different times and to explain the P cycle from the perspective of functional genes. The results showed that the number of sediment microorganisms (genes) gradually increased with the P release capacity, and the outbreak with the strongest P release capacity possessed the most abundant microorganisms (genes). Proteobacteria with P solubilizing functions were consistently the most abundant phylum in all four periods and were positively correlated with P release potential assessment factors EPC0, EPC0F, and NAP. Functional genes affect the P cycle by acting primarily on inorganic P solubilization, organic P mineralization, and P transport. These P-functional genes are mainly found in Proteobacteria, Acidobacteria, Chloroflexi, and Actinobacteria microorganisms. In addition, the P form in the sediments was dominated by IP, with the highest concentration (704.86 mg/kg) occurring during the dormant period. Sediments from this period acted as a strong P "sink", creating a precondition for cyanobacterial recovery and outbreaks to provide a source of P. The results of this study can provide a theoretical basis for controlling endogenous P release at the microscopic level of cyanobacterial growth and disappearance.

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.

Reversible and Turn-On Fluorescence Detection of Phosphate in Aqueous Solution and Living Cell Imaging by Supramolecular Metallacycles with AIE-Active Ligands

Luminescent supramolecular metallacycles have attracted great interest as a new promising class of sensing substrates. In this work, two tetraphenylethene (TPE)-based diimidazole and dipyrazole ligands with the aggregation-induced emission (AIE) feature were designed for the construction of supramolecular tetragonal metallacycles 1-4 with two 90° mononuclear [(bpy)M]2+ or dinuclear [(bpy)2M2]4+ acceptors (bpy = 2,2'-dipyridine; M = Pd, Pt), in which the fluorescence can be quenched to an "off" state due to the ligand-to-metal charge transfer (LMCT). Metallacycle 1 was utilized as a fluorescence sensor for phosphate (PO43-) detection in aqueous solution by means of disassembly, leading to the release of the ligand. Additionally, the metallacycle can be regenerated through self-assembly via the introduction of Pd(II) acceptors. PO43- was detected using TPE-based metallacycles over a wide concentration range, with a detection limit as low as 2.1 × 10-8 M. Furthermore, sensor 1 also presented the semiquantitative visual detection ability for PO43- in the test paper mode via fluorescence changes. The aforementioned studies not only enhance the current research on fluorescent materials but also offer a strategy for the creation of stimuli-responsive supramolecular coordination complexes.

Isolation and optimisation of polyphosphate accumulating bacteria for bio-treatment of phosphate from industrial wastewater

Phosphorus in wastewater influents is a global issue. Controlling eutrophic water is crucial. Biological phosphorus removal is an economically and environmentally sustainable method for removing phosphorus from wastewater. This study aims to isolate and improve the capacity of aerobic phosphorus-removing bacteria to reduce excessive phosphate concentrations in the environment. Only three out of fourteen bacterial isolates demonstrated the highest phosphate removal efficiency using Toluidine blue-O. Klebsiella pneumoniae 6A, Klebsiella quasipneumoniae 6R, and Enterobacter mori 8R were isolated from activated sludge and identified by 16srRNA. In a single-factor experiment, the effect of incubation periods, phosphate concentrations, carbon sources, sodium acetate concentrations, temperature, pH, and irradiation dosages were studied. Seventy-two hours of incubation, 55 mg/L PO4, sodium acetate as the carbon source, 30°C and pH 7 resulted in maximum phosphorus removal. After optimising the parameters, the removal efficiency of Klebsiella pneumoniae 6A, Klebsiella quasipneumoniae 6R, and Enterobacter mori 8R increased from 73.5% to 85.1%, 79.1% to 98.1%, and 80.6% to 91.9%, respectively. Gamma irradiation showed significant results only in Klebsiella pneumoniae 6A where 100 Gy increased the phosphorous removal efficiency from 85.1% to 100%. Immobilised mixed culture of the three strains adapted better to 100 mg/L Phosphorus than pure cells. Therefore, this technique holds great new promise for phosphorus-contaminated sites bioremediation.

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.

Combined amendment and capping of sediment with ferrihydrite and magnetite to control internal phosphorus release

This study developed novel active capping systems with recycling convenience using ferrihydrite (Fh) combined with magnetite (Mag), and investigated the effectiveness and mechanism for the restriction of endogenous phosphorus movement from sediment into overlying water (OW) by the combined use of Fh and Mag. The Fh/Mag combined amendment effectively hindered endogenous phosphorus release from sediment to OW in dissolved oxygen (DO)-deficit environment, and the immobilization of diffusion gradient in thin film-labile phosphorus (LP_DGT) and mobile phosphorus in the sediment played a key role in the control of endogenous phosphorus liberation by the Fh/Mag combined amendment. Combined capping sediment with Fh and Mag effectively hindered endogenous phosphorus release from sediment to OW in anoxic environment, and the inactivation of LP_DGT in the upper sediment played a key part in the control of sediment phosphorus release by the Fh/Mag mixture capping. The stability of phosphorus immobilized by the Fh/Mag combined covering layer was related to its construction way, and the majority (around 90%) of P immobilized to the Fh/Mag mixture covering layer had low risk of release in common pH (5-9) and DO-deficit environments. The Fh/Mag mixture amendment or capping did not increase the risk of sediment iron release, and it also did not produce a large impact on the diversity and richness of bacterial community in the sediment. The combined utilization of Fh and Mag as a composite amendment or capping material to prevent the internal phosphorus from being moved to OW can make full use of their respective advantages. The Fh/Mag mixture capping wrapped by permeable fabric has high potential to reduce the risk of endogenous phosphorus from sediment into OW due to its advantages of high internal phosphorus release suppression efficiency, environmental friendliness, application convenience and sustainability.

Sudden eutrophication of an aluminum sulphate treated lake due to abrupt increase of internal phosphorus loading after three decades of mesotrophy

Aluminum salts are widely used to immobilize phosphorus (P) in lakes suffering from internal loading. However, longevity of treatments varies among lakes; some lakes eutrophy faster than others. We conducted biogeochemical investigations of sediments of a closed artificial Lake Barleber, Germany that was successfully remediated with aluminum sulfate in 1986. The lake became mesotrophic for almost 30 years; a rather rapid re-eutrophication took place in 2016 leading to massive cyanobacterial blooms. We quantified internal loading from sediment and analyzed two environmental factors that might have contributed to the sudden shift in trophic state. Increase in lake P concentration started in 2016, reaching 0.3 mg L^-1, and remained elevated into the spring of 2018. Reducible P fraction in the sediment was 37 - 58% of total P, indicating a high potential for mobilization of benthic P during anoxia. Estimated P release from sediments for 2017 was approximately 600 kg for the whole lake. This is consistent with sediment incubation results; higher temperature (20°C) and anoxia contributed to release of P (27.9 ± 7.1 mg m^-2 d^-1, 0.94 ± 0.23 mmol m^-2 d^-1) to the lake, triggering re-eutrophication. Loss of aluminum P adsorption capacity together with anoxia and high water temperatures (organic matter mineralization) are major drivers of re-eutrophication. Accordingly, treated lakes at some time require a repeated aluminum treatment for sustaining acceptable water quality and we recommend regular sediment monitoring in treated lakes. This is crucial given the effects of climate warming on duration of stratification in lakes which may result in the need for treatment of many lakes.

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.

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.

Phosphorus recovery as struvite and hydroxyapatite from the liquid fraction of municipal sewage sludge with limited magnesium addition

Phosphorus (P) is an essential element to produce feed and fertilizers but also a nonrenewable resource. Both the predicted exhaustion of phosphatic rocks and the risk of eutrophication lead to an increasing necessity for P recovery methodologies to be applied in municipal wastewater treatment plants (WWTPs). One of the most promising solutions involves the precipitation of P-based minerals reusable as slow-release fertilizers. In this study, P recovery as struvite and hydroxyapatite from a municipal WWTP digestate liquid fraction (centrate) was investigated at varying pH (8-10), reagent typologies (MgCl₂ , NaOH, Ca(OH)₂ , and CaCl₂ ), and concentrations under limiting magnesium doses through liquid- and solid-phase analyses and thermodynamical modeling. A maximum P recovery of 87.3% was achieved at pH 9 by adding NaOH and MgCl₂ at a dose of 656 mg/L (the higher tested). According to these data, it was estimated that 92.0 tons/year of struvite and 33.2 tons/year of hydroxyapatite could be recovered from the WWTP centrate with a cost for reagent consumption being almost 50% of the mean P market value. An increase in P precipitation was observed while comparing experiments with the same pH values but with a higher Mg²⁺ dose. Ca²⁺ addition led to extensive P precipitation but mainly as amorphous phases that interfere with struvite formation.

Response of internal phosphorus loading from dredged and inactivated sediment under repeated resuspension in a eutrophic shallow lake

In this study, a laboratory sediment resuspension simulation system (RSS) was used to investigate the effect of wind-induced (5.3 and 8.7 m/s) repeated sediment resuspension on internal phosphorus (P) in sediment treated by dredging and La-modified clay (LMC) based inactivation in a shallow lake. The results indicated that the dredged sediment had a better capability to resist repeated wind disturbance than the LMC-inactivated sediment. The concentration of suspended solids (SS) in the inactivated treatment (70.7 mg/L) was 1.7 times that in the dredged treatment (41.7 mg/L) under moderate wind disturbance, and was similar for the two treatments under strong wind disturbance. Nevertheless, dredging performed better than inactivation in reducing 44 % total phosphorus (TP) in overlying water (43 % reduction by inactivation) and 31 % mobile P in sediment (27 % reduction by inactivation) under moderate wind disturbance (p < 0.01) compared with control treatment. Inactivation performed better in reducing 57 % P in porewater (52 % reduction by dredging) and 81 % P flux (13 % reduction by dredging) (p < 0.01) compared with control treatment. Surprisingly, under strong wind disturbance, LMC inactivation could still reduce 18 % P in porewater and 75 % P flux (p < 0.01), whereas dredging increased 25 % P in porewater and 13 % P flux compared with control treatment (p < 0.01). LMC inactivation can increase the sediment P adsorption capacity and decrease the equilibrium P concentration (EPC₀) when compared with control treatment. The contrasting control effects of the two methods were probably due to the different P buffer mechanisms for the two treated sediment. The wind disturbance-induced sediment P release was controlled by the inactivation of Fe and co-inactivation of Fe and La at the surface of dredged and LMC-inactivated sediments, respectively. The results of this study indicated inactivation can be a better method to control sediment internal P loading with repeated strong wind disturbances in eutrophic lakes.

Oxygen micro-nanobubbles for mitigating eutrophication induced sediment pollution in freshwater bodies

Sediment hypoxia is a growing problem and has negative ecological impacts on the aquatic ecosystem. Hypoxia can disturb the biodiversity and biogeochemical cycles of both phosphorus (P) and nitrogen (N) in water columns and sediments. Anthropogenic eutrophication and internal nutrient release from lakebed sediment accelerate hypoxia to form a dead zone. Thus, sediment hypoxia mitigation is necessary for ecological restoration and sustainable development. Conventional aeration practices to control sediment hypoxia, are not effective due to high cost, sediment disturbance and less sustainability. Owing to high solubility and stability, micro-nanobubbles (MNBs) offer several advantages over conventional water and wastewater treatment practices. Clay loaded oxygen micro-nanobubbles (OMNBs) can be delivered into deep water sediment by gravity and settling. Nanobubble technology provides a promising route for cost-effective oxygen delivery in large natural water systems. OMNBs also have the immense potential to manipulate biochemical pathways and microbial processes for remediating sediment pollution in natural waters. This review article aims to analyze recent trends employing OMNBs loaded materials to mitigate sediment hypoxia and subsequent pollution. The first part of the review highlights various minerals/materials used for the delivery of OMNBs into benthic sediments of freshwater bodies. Release of OMNBs at hypoxic sediment water interphase (SWI) can provide significant dissolved oxygen (DO) to remediate hypoxia induced sediment pollution Second part of the manuscript unveils the impacts of OMNBs on sediment pollutants (e.g., methylmercury, arsenic, and greenhouse gases) remediation and microbial processes for improved biogeochemical cycles. The review article will facilitate environmental engineers and ecologists to control sediment pollution along with ecological restoration.

Estimation of nutrient sources and fate in groundwater near a large weir-regulated river using multiple isotopes and microbial signatures

The excessive input of nutrients into groundwater can accelerate eutrophication in associated surface water systems. This study combined hydrogeochemistry, multi isotope tracers, and microbiological data to estimate nutrient sources and the effects of groundwater-surface water interactions on the spatiotemporal variation of nutrients in groundwater connected to a large weir-regulated river in South Korea. δ11B and δ15N-NO3- values, in combination with a Bayesian mixing model, revealed that manure and sewage contributed 40 % and 25 % respectively to groundwater nitrate, and 42 % and 27 % to nitrate in surface water during the wet season. In the dry season, the source apportionment was similar for groundwater while the sewage contribution increased to 52 % of nitrate in river water. River water displayed a high correlation between NO3- concentration and cyanobacteria (Microcystis and Prochlorococcus) in the wet season. The mixing model using multiple isotopes indicated that manure-derived nutrients delivered with increased contributions of groundwater to the river during the wet season governed the occurrence of cyanobacterial blooms in the river. We postulate that the integrated approach using multi-isotopic and microbiological data is highly effective for evaluating nutrient sources and for delineating hydrological interactions between groundwater and surface water, as well as for investigating surface water quality including eutrophication in riverine and other surface water systems.

Metagenomics reveals biogeochemical processes carried out by sediment microbial communities in a shallow eutrophic freshwater lake

Introduction

As the largest shallow freshwater lake in the North China Plain, Baiyangdian lake is essential for maintaining ecosystem functioning in this highly populated region. Sediments are considered to record the impacts of human activities.

Methods

The abundance, diversity and metabolic pathways of microbial communities in sediments were studied by metagenomic approach to reveal patterns and mechanism of C, N, P and S cycling under the threat of lake eutrophication.

Results

Many genera, with plural genes encoding key enzymes involved in genes, belonging to Proteobacteria and Actinobacteria which were the most main phylum in bacterial community of Baiyangdian sediment were involved in C, N, S, P cycling processes, such as Nocardioides (Actinobacteria), Thiobacillus, Nitrosomonas, Rhodoplanes and Sulfuricaulis (Proteobacteria).For instance, the abundance of Nocardioides were positively correlated to TN, EC, SOC and N/P ratio in pathways of phytase, regulation of phosphate starvation, dissimilatory sulfate reduction and oxidation, assimilatory sulfate reduction, assimilatory nitrate reduction and reductive tricarboxylic acid (rTCA) cycle. Many key genes in C, N, P, S cycling were closely related to the reductive citrate cycle. A complete while weaker sulfur cycle between SO₄ ^2- and HS^- might occur in Baiyangdian lake sediments compared to C fixation and N cycling. In addition, dissimilatory nitrate reduction to ammonia was determined to co-occur with denitrification. Methanogenesis was the main pathway of methane metabolism and the reductive citrate cycle was accounted for the highest proportion of C fixation processes. The abundance of pathways of assimilatory nitrate reduction, denitrification and dissimilatory nitrate reduction of nitrogen cycling in sediments with higher TN content was higher than those with lower TN content. Besides, Nocardioides with plural genes encoding key enzymes involved in nasAB and nirBD gene were involved in these pathways.

Discussion

Nocardioides involved in the processes of assimilatory nitrate reduction, denitrification and dissimilatory nitrate reduction of nitrogen cycling may have important effects on nitrogen transformation.

Long-term study of ecological restoration in a typical shallow urban lake

We investigated the long-term effects (6 years) of sediment improvement and submerged plant restoration of a subtropical shallow urban lake, Hangzhou West Lake China. To reveal the lake ecosystems variations, we analyzed the sediment properties, submerged macrophyte characteristics, sediment microorganisms, and benthic macroinvertebrate communities from 2015 to 2020. The ecological restoration project decreased sediment TP and OM, increased submerged macrophyte biomass and sediment microbial diversity, and improved the benthic macroinvertebrate communities in the restored area. The sediment TP decreased from 2.94 mg/g in 2015 to 1.33 mg/g in 2020. The sediment OM of the restored area decreased from 27.44 % in 2015 to 8.08 % in 2020. Principal component analysis (PCA) confirmed that the restoration improved the sediment conditions, making it suitable for the growth of submerged macrophytes, and then sped up the restoration and reconstruction of the lake ecosystem. These results have significant implications on the ecological management of shallow lakes.

Decrease in macrofauna density increases the sediment phosphorus release and maintains the high phosphorus level of water column in Lake Taihu: A case study on Grandidierella taihuensis

Internal phosphorus (P) loading can increase the P level in the water column and further sustains cyanobacterial blooms. This study focused on the role of benthic fauna bioturbation in affecting the sediment P release and the P level of water column in a eutrophic lake, Lake Taihu. The macrofauna density decreased from 4766.56 ± 10541.80 ind/m² in 2007 to 345 ± 447.63 ind/m² in 2020 due to the frequent bottom-water hypoxia in Lake Taihu. The reduced macrofauna density majorly resulted from Grandidierella taihuensis, Limnodrilus hoffmeisteri, and Tanypus chinensis larvae, and their total density decreased by approximately 97% in 2020 compared to 2007. G. taihuensis, one of the major benthic faunas, was further used as a representative to investigate the effects of bioturbation on sediment P release using high-resolution sampling and imaging techniques. The results show that G. taihuensis can increase the O₂ penetration depth by more than 20 mm through bio-irrigation, and causes the redox conditions in burrows and surrounding sediments to change dramatically within a few minutes due to the intermittent ventilation. Subsequent oxidation of the soluble Fe(II) led to the formation of Fe-oxide bound P in the surface sediments, thereby increasing the P retention in the sediments. When the G. taihuensis density was 1563 ind/m² at the sampling site, approximately 0.12 g m^-2 yr^-1 P can be retained in sediments. As previous studies have shown that L. hoffmeisteri and T. chinensis played a similar role in increasing the P retention in sediments through their bioturbation activities, the sharp decline in benthic fauna density and burrowing activities in Lake Taihu should be an important reason for maintaining the high P level in the water column by decreasing the P retention in sediments.

The impact of iron coagulant on the behavior and biochemistry of freshwater mussels Anodonta cygnea and Unio tumidus during lake restoration

Iron (Fe) treatment is one of the most commonly used methods to restore eutrophic lakes and reservoirs. The Fe-based coagulants dosage results in an almost immediate improvement in water quality at a relatively low cost. However, the effects of the application of coagulants are not always predictable, and the scale of the risks is not fully understood. The dosage of coagulants changes the chemical and physical properties of water, thereby affecting aquatic biocenoses. In this study, several laboratory experiments were conducted to evaluate the effects of Fe-based coagulant dosage on two bivalves species: Anodonta anatina and Unio tumidus. Their ability to efficiently filter water and reduce seston makes them a key component of aquatic ecosystems in terms of maintaining proper ecological health and stable functioning. Behavioral response, biochemical parameters, and body chemistry changes in mussels exposed to different doses of coagulant were surveyed. A dose-dependent reduction in filtration activity of both species was observed. As early as 10 g Fe m² (which is a moderate dose used in lakes restoration), mussels of both species almost completely reduced their filtration activity and remained with closed valves for several subsequent days. Significant Fe accumulation in muscles of bivalves exposed to coagulant was also observed. This was particularly the case when very high doses of coagulant were applied. Then, the iron content in leg muscles of both species increased over fourfold. At the same time, a decrease in muscles calcium and phosphorus content was observed. No symptoms of oxidative stress (TBARS, H₂O₂) after mussels exposure to coagulants were found. The results suggest that the application of Fe-based coagulant for water ecosystem restoration may be a threat to the mussels population. These findings are significant for decisions on the selection of restoration methods for a specific lake.

Effect of sediment burial depth on the control of sedimentary phosphorus release by iron/aluminum co-modified calcite and strategy for overcoming the negative effect of sediment burial

After placing an active capping material on surface sediments, the capping layer will be buried by the newly formed sediment. In this research, the influence of sediment burial depth on the performance of iron/aluminum co-modified calcite (FeAlCAL) to suppress sedimentary phosphorus (P) release into overlaying water (OL-water) was studied. Furthermore, in order to find out the strategy for overcoming the negative effect of sediment burial, the efficiencies and mechanisms of three different FeAlCAL treatments (one-time FeAlCAL capping with 3 cm sediment burial, multiple FeAlCAL capping with 1 cm sediment burial, and amendment of top 3 cm sediment with FeAlCAL) in the inhibition of sediment P release were contrastively studied. The results showed that with the increase of sediment burial depth, the efficiency of FeAlCAL to block the release of sediment P into OL-water gradually decreased until the FeAlCAL lost the ability to hinder sediment-P release. In contrast to the one-time FeAlCAL capping in the presence of 3 cm sediment burial, the multiple FeAlCAL capping in the presence of 1 cm sediment burial and amendment of top 3 cm sediment with FeAlCAL both effectively prevented the release of P from sediment into OL-water. All results of this work suggest that although sediment burial can negatively affect the ability of FeAlCAL in the inhibition of sediment P release into OL-water and the negative effect becomes stronger as the sediment burial depth increases, the transformation of the application mode of FeAlCAL from one-time capping to multiple capping or from capping to amendment can overcome the negative influence of sediment burial.

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.

Implementation of P-Reactive Layer for Improving Urban Water Quality: Kinetic Studies, Dimensioning and Economic Analysis

Urbanization and climate change affecting water quality are the most critical problems that humanity has to encounter globally. Undoubtedly, urban water bodies are heavily contaminated by phosphorus (P). This study aims to identify the mechanisms and efficiency of the P sorption process for selected reactive materials (Autoclaved Aerated Concrete (AAC), Filtralite® Nature P, lightweight expanded clay aggregate (Leca®), limestone, opoka, and zeolite) with surface water as adsorbate and dimension of P-reactive reactive layer supported with economic analysis. Four kinetic models were used to know the sorption mechanism: pseudo-first order, pseudo-second order, Elovich, and intra-particle diffusion model. Calculating the P-reactive layer was based on dimensioning rain retention spaces standards. The pseudo-second model provided the best description of the adsorption kinetics of most materials. The sorption properties obtained after 72 h showed the reduction of 83, 81, 59, 53, 37, and 36% for AAC, opoka, Filtralite® Nature P; limestone, Leca®, and zeolite, respectively. Depending on the volume, the P-reactive layer can remove 29–77 or 61–163 g of P-PO4. The unit cost of removing P-PO4 by the P-reactive layer range from 49.57 to 85.53 €/P-PO4 g. For these reasons, reactive materials seem to be an effective way of removing P from the urban water environment worldwide from both environmental and economic points of view.

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.

Effects of Light-Emitting Diode Illumination on Sediment Surface Biological Activities and Releases of Nutrients and Metals to Overlying Water in Eutrophic Lake Microcosms

The release of nutrients and metals from the sediment to the overlying water induced by oxygen depletion is an important issue in eutrophic aquatic systems. Effects of light-emitting diode (LED) illumination on oxygen conditions and release of nutrients and metals from the sediment were examined by comparing with those effects of aeration in microcosms using water and sediment of Lake Taihu, China. Periphyton with filamentous algae developed on the sediment surface in the LED (blue wavelength) treatment. Dissolved oxygen became rapidly saturated and gradually supersaturated in the aeration and LED treatments, respectively, but remained low in the control. A thicker oxic layer developed on the sediment for the LED than aeration but was poorly developed with a blackened surface in the control. Invertebrate burrows were distributed deeper and the bacterial community was more dominated by aerobic species in the LED, indicating deeper penetration of oxygen into the sediment. Nutrients (e.g., N and P) and some metals (e.g., Hg, As, and Mn) in water were lower for the LED and aeration than in the control; nutrients and other solutes that increased electric conductivity (e.g., Ca, Mg) were lower for the LED than aeration. These results suggest that LED can effectively oxygenate the bottom water by stimulating algal photosynthesis and benthic invertebrate activity, resulting in greater retention of nutrients and metals in/on sediment compared to aeration.

Phosphorous removal and recovery from urban wastewater: Current practices and new directions

Phosphate rocks are an irreplaceable resource to produce fertilizers, but their availability will not be enough to meet the increasing demands of agriculture for food production. At the same time, the accumulation of phosphorous discharged by municipal wastewater treatment plants (WWTPs) is one of the main causes of eutrophication. In a perspective of circular economy, WWTPs play a key role in phosphorous management. Indeed, phosphorus removal and recovery from WWTPs can both reduce the occurrence of eutrophication and contribute to meeting the demand for phosphorus-based fertilizers. Phosphorous removal and recovery are interconnected phases in WWTP with the former generally involved in the mainstream treatment, while the latter on the side streams. Indeed, by reducing phosphorus concentration in the WWTP side streams, a further improvement of the overall phosphorus removal from the WWTP influent can be obtained. Many studies and patents have been recently focused on treatments and processes aimed at the removal and recovery of phosphorous from wastewater and sewage sludge. Notably, new advances on biological and material sciences are constantly put at the service of conventional or unconventional wastewater treatments to increase the phosphorous removal efficiency and/or reduce the treatment costs. Similarly, many studies have been devoted to the development of processes aimed at the recovery of phosphorus from wastewaters and sludge to produce fertilizers, and a wide range of recovery percentages is reported as a function of the different technologies applied (from 10-25% up to 70-90% of the phosphorous in the WWTP influent). In view of forthcoming and inevitable regulations on phosphorous removal and recovery from WWTP streams, this review summarizes the main recent advances in this field to provide the scientific and technical community with an updated and useful tool for choosing the best strategy to adopt during the design or upgrading of WWTPs.

Magnetic zirconium-based Prussian blue analog nanozyme: enhanced peroxidase-mimicking activity and colorimetric sensing of phosphate ion

A magnetic zirconium hexacyanoferrate-based Prussian blue analog (MB@ZrHCF) nanozyme was synthesized using dopamine (DA) reduction-assisted method and employed for colorimetric PO₄^3- sensing. The MB@ZrHCF exhibits enhanced peroxidase-mimicking activity and ultrafast catalytic rate via the color reaction of 3,3',5,5'-tetramethylbenzidine (TMB) oxidized by hydrogen peroxide (H₂O₂). The catalytic reaction mechanism of MB@ZrHCF catalyzing H₂O₂ to produce hydroxyl radical (∙OH) was studied. Then, MB@ZrHCF was successfully applied to the detection of H₂O₂. Additionally, the catalytic activity of the nanocomposite is inhibited due to the steric hindrance effect from the coordination of PO₄^3- and Zr(IV) node. Based on this, the MB@ZrHCF nanozyme can be used to detect PO₄^3- in two linear ranges (10-100 µM and 100-200 µM) with a limit of detection of 2.25 µM. The proposed colorimetric sensor possesses excellent selectivity and reliability for PO₄^3- sensing, which can be successfully applied to detect PO₄^3- in sea and tap water samples.

Impacts of the Floc and Sink technique on the phytoplankton community: A morpho-functional approach in eutrophic reservoir water

The Floc and Sink technique promotes, through the application of coagulants and clays, the removal of phosphorus and algal biomass from the water column by flocculation and sedimentation. Although it is a promising technique for mitigating harmful cyanobacterial blooms, little is known about the impacts on other phytoplankton species as well as it is not known how species with mechanisms of resistance to sedimentation respond to the application of these products. In this study, a laboratory experiment was carried out with water from a eutrophic reservoir to assess the impact of applying aluminum-based coagulants, aluminum sulfate and polyaluminium chloride, and chitosan, alone and combined with lanthanum modified bentonite and natural bentonite on the phytoplankton community, in a functional approach based on morphology (Morphology-Based Functional Groups - MBFG, Kruk et al., 2010 and Reynolds et al., 2014), with an emphasis on the characteristics that provide resistance to sedimentation. We tested two hypotheses: phytoplankton species with adaptive mechanisms that provide buoyancy to cells are more resistant to the removal from the water column by coagulants and clays; and coagulants based on metals and modified clays are more efficient in sedimentation of microalgal cells compared to natural products, regardless of the presence of an adaptive mechanism of resistance to sedimentation. Our results showed that aluminum sulfate and polyaluminium chloride alone or combined with lanthanum modified bentonite and natural bentonite effectively sedimented the cells, regardless of the presence of buoyancy mechanisms. In contrast, the natural coagulant chitosan alone or combined with lanthanum modified bentonite and natural bentonite removed only those species that were small-celled or small colonial without specialized structures or with the presence of spines, arms and siliceous exoskeleton. In the case of species with aerotopes and flagella, the removal was not effective and still caused an increase in algal biomass due to the formation of suspended cell aggregates on the surface of the water column of the experimental units. Therefore, we concluded that the Floc and Sink technique has an impact on the phytoplankton community because it removes from the water column species that are not the target of coagulants and clays, but that are considered important sources of energy in freshwater trophic webs. This result differs according to the type of product used as well as it is related to the morphological adaptations that favor the buoyancy of cells in the water column. Species with aerotopes and flagella are more resistant to sedimentation and may make the use of products applied in the Floc and Sink technique unfeasible. Nevertheless, these results represent only an immediate effect of the technique on the phytoplankton community, thus requiring a longer time scale evaluation to determine the algae that can effectively recover water column. Therefore, we also emphasize that algae of nutritional value can recover over time and make the long-term application of the technique acceptable.

Influence of temperature and pH on phosphate removal efficiency of different sorbents used in lake restoration

Phosphorus sorbents (PS) are viewed as a powerful tool to manage eutrophication. Here, we tested three commercially available PS - lanthanum-modified bentonite (LMB), aluminium-modified zeolite (AMZ) and aluminium salts (Al) on their capacity to chemically inactivate soluble reactive phosphorus (SRP) at six different temperatures (5 to 35 °C) and five pH values (6 to 10). We also evaluated if the SRP bound at a neutral pH would be released if pH increases to pH 10. Results showed that temperature affected the SRP binding behavior differently for each PS. For instance, the highest SRP binding capacities of LMB, AMZ and Al were 14.0, 29.9 and 251.1 mg P g^-1 at 30 °C, 35 °C and 30 °C, respectively; and the lowest was at 35 °C for LMB, 25 °C for AMZ and 20 °C for Al (6.3, 4.0 and 205.2 mg P g^-1, respectively). The pH also affected the SRP binding differently. When pH increased from pH 6 to pH 10, LMB and Al decreased their binding capacity from 10.0 to 4.9 and from 571.7 mg P g^-1 to 21.3 mg P g^-1, respectively. The SRP adsorption capacity of AMZ was similar at pH 7 and 10 (6.3 and 6.2 mg P g^-1). We observed that in high pH, LMB did not release the SRP precipitated. In contrast, AMZ and Al desorbed around 39%, and 71% of the SRP adsorbed when pH changed from 7 to 10. Abiotic factors such as pH should be considered when selecting the most promising material in lake restoration.

A Summer of Cyanobacterial Blooms in Belgian Waterbodies: Microcystin Quantification and Molecular Characterizations

In the context of increasing occurrences of toxic cyanobacterial blooms worldwide, their monitoring in Belgium is currently performed by regional environmental agencies (in two of three regions) using different protocols and is restricted to some selected recreational ponds and lakes. Therefore, a global assessment based on the comparison of existing datasets is not possible. For this study, 79 water samples from a monitoring of five lakes in Wallonia and occasional blooms in Flanders and Brussels, including a canal, were analyzed. A Liquid Chromatography with tandem mass spectrometry (LC-MS/MS) method allowed to detect and quantify eight microcystin congeners. The mcyE gene was detected using PCR, while dominant cyanobacterial species were identified using 16S RNA amplification and direct sequencing. The cyanobacterial diversity for two water samples was characterized with amplicon sequencing. Microcystins were detected above limit of quantification (LOQ) in 68 water samples, and the World Health Organization (WHO) recommended guideline value for microcystins in recreational water (24 µg L⁻¹) was surpassed in 18 samples. The microcystin concentrations ranged from 0.11 µg L⁻¹ to 2798.81 µg L⁻¹ total microcystin. For 45 samples, the dominance of the genera Microcystis sp., Dolichospermum sp., Aphanizomenon sp., Cyanobium/Synechococcus sp., Planktothrix sp., Romeria sp., Cyanodictyon sp., and Phormidium sp. was shown. Moreover, the mcyE gene was detected in 75.71% of all the water samples.

Effective orthophosphate removal from surface water using hydrogen-oxidizing bacteria: Moving towards applicability

Effective orthophosphate removal strategies are needed to counteract eutrophication and guarantee water quality. Previously, we established that hydrogen-oxidizing bacteria (HOB) have the ability to remove orthophosphate from artificial surface water. In the present study, we expand the application of the HOB orthophosphate removal strategy (1) to treat artificial surface water with low initial orthophosphate concentrations, (2) to treat real surface water and real wastewater effluent, and (3) to remove orthophosphate continuously. For synthetic surface water, irrespective of the initial concentration of 0.7, 0.5, 0.3, and 0.1 mg PO₄^3--P/L, ultra-low concentrations (0.0058 ± 0.0028 mg PO₄^3--P/L) were obtained. When artificial surface water was replaced by real surface water, without added nutrients or other chemicals, it was shown that over 90% orthophosphate could be removed within 30 min of operation in a batch configuration (0.031 ± 0.023 mg PO₄^3--P/L). In continuous operation, orthophosphate removal from surface water left an average concentration of 0.040 ± 0.036 for 60 days, and the lowest orthophosphate concentration measured was 0.013 mg PO₄^3-/L. Simultaneously, nitrate was continuously removed for 60 days below 0.1 mg/L. The ability to remove orthophosphate even under nitrogen limiting conditions might be related to the ability of HOB to fix nitrogen. This study brings valuable insights into the potential use of HOB biofilms for nutrient remediation and recovery.

Research on application of a new bottom trap technology to catch particles rich in nutrients in a large shallow lake

The resuspension and sedimentation of particulate matter and the release of nutrients from sediment are important factors affecting the eutrophication of shallow lakes. The capture and removal of particles rich in nitrogen, phosphorus, and other nutrients at the bottom of lakes is of great significance for improving the management and eutrophication status of lakes. This study investigated the feasibility of applying lake bottom trap technology in seven different locations in Lake Chaohu, which is the fifth largest freshwater lake in China. The results showed that the trap in the western part of Lake Chaohu had the highest sedimentation rate and could capture most of the nutrients. The sedimentation rates were higher in spring and summer than in autumn and winter. The bottom trap effectively collected and preserved chlorophyll a, organic matter, total nitrogen, and total phosphorus. The trap per meter length (15-20 m wide) could catch 20.7-27.6 m³ of particles rich in nutrients with a water content of 50-70%, organic matter content of 281.9-375.8 kg, total nitrogen content of 24.5-32.6 kg, and total phosphorus content of 10.5-14 kg. The proposed bottom trap had little impact on the benthic organism system of the lake. The bottom trap technology used in this study could solve the problem of nitrogen, phosphorus, and algae accumulation in lakes and reservoirs, broaden the utility of lake hydrodynamics in environmental pollution control, and provide new ideas and strategies for the control and management of cumulative pollution in shallow lakes and reservoirs.

Drinking water treatment residual as a ballast to sink Microcystis cyanobacteria and inactivate phosphorus in tropical lake water

The combination of a low dose of coagulant with a ballast that can inactive phosphorus (P) in lake sediment-a technique known as "flock and lock"-is one method for restoration of eutrophic lakes. The effectiveness of a drinking water treatment residual (DWTR) as a ballast in flock and lock was assessed using assays of eutrophic lake water from Thailand dominated by Microcystis aeruginosa cyanobacteria colonies by measuring changes in chlorophyll-a, pH, and zeta potential. P sorption isotherms were developed from long-term batch equilibrium experiments; desorption of nutrients and metals was assessed via leaching experiments; and morphological changes to cellular structure were assessed using scanning electron microscopy. Results showed that combining DWTR with a low dose of aluminum sulfate (0.6-4.0 mg Al/L) effectively sank 74-96% of Microcystis, with DWTR dose (50-400 mg/L), initial chlorophyll-a concentration (92-976 µg/L), pH (7.4-9.3), and alkalinity (99-108 ppm CaCO₃) identified as factors significantly associated with sinking efficacy. P sorption capacity of the DWTR (7.12 mg/g) was significantly higher than a local soil (0.33 mg/g), enabling the DWTR to inactivate P in lake sediment. Desorption of Al, Fe, Ca and N from the DWTR was estimated to contribute to a marginal increase in concentrations of those compounds in the water column of a small shallow lake (1.2, 0.66, 53.4, and 0.07 µg/L, respectively) following a simulated application. Therefore, pre-treated DWTRs may be a viable alternative ballast in the flock and lock approach to lake restoration, supplementing or replacing modified local soils or lanthanum modified clays.

Controlling Eutrophication via Surface Aerators in Irregular-Shaped Urban Ponds

Surface aerators have often been introduced in urban ponds for esthetics, but their roles in remediating water quality are less understood. Effects of surface aerators on controlling eutrophication were examined in two urban ponds, in which anaerobic odors and cyanobacterial blooms had occurred and several aerators had been installed. In one of the ponds, a dramatic improvement in dissolved oxygen (DO) (from 1.8 to 8.1 mg L−1) and total phosphorus (TP) (from 1.6 to 0.4 mg L−1) was evident based on the comparison before and after the aeration. Although cyano-bloom did not occur, phytoplankton was dominated by cyanobacteria Microcystis species in both periods. Chlorophyll a (Chl-a) increased (from 29 to 51 μg L−1) and water transparency decreased (from 81 to 27 cm) after the aeration. In the other pond with an irregular shape, water quality was monitored two years after the installation to examine seasonal variation in trophic state and its spatial variation associated with aerator distribution. The water was mixed vertically well for the whole pond, as indicated by small surface-to-bottom differences in temperature. DO decreased in summer after rainfall but was always >5.7 mg L−1. Total nitrogen (TN) and TP were 0.8–2.3 and 0.03–0.07 mg L−1, respectively, and no cyano-blooms were observed across sites throughout the year. Phytoplankton was dominated by green algae and diatom species, which may be favored by the lower phosphorus level of the pond. Chl-a was higher and transparency was lower in the north side, which had more aerators and less shade from trees and buildings. These results suggest that surface aerators increased DO by vertical and horizontal mixing of water, reduced phosphorus release from sediment, and prevented cyano-bloom occurrence, but they did not improve Chl-a level and transparency. Rather, aeration can promote algal growth, and thus, additional purifying measures such as filtration and contact oxidation are required to further improve the trophic state of these ponds.

Phosphorus removal from sediments by Potamogeton crispus: New high-resolution in-situ evidence for rhizosphere assimilation and oxidization-induced retention

Macrophytes are usually chosen for phytoremediation tools to remove P in eutrophic aquatic ecosystems, but the lack of test methods hinders the understanding of removal mechanism and application. In this study, we used the novel technologies combined of Diffusive gradients in thin films (DGT), Planar optode (PO), and Non-invasive micro-test technology (NMT) to explore P dynamics in water-sediment continuum and rhizosphere of Potamogeton crispus over time. Results of the high-resolution in situ measurement showed that labile P(LP_DGT) fluxes at the surficial sediment significantly decreased from approximate 120, 140, and 200 pg/ (cm²•sec) via 30 days incubation period to 17, 40, and 56 pg/(cm²•sec) via that of 15 days. Obvious synchronous increase of LP_DGT was not detected in overlying water, suggesting the intense assimilation of dissolve reactive P via root over time. PO measurement indicated that O₂ concentration around the rhizosphere remarkably increased and radially diffused into deeper sediment until 100% saturation along with the root stretch downwards. NMT detection of roots showed the obvious O₂ inflow into root tissue with the uppermost flux of 30 pmol/(cm²•sec) from surroundings via aerenchyma on different treatment conditions. Different from previous reports, gradually saturating O₂ concentrations around the rhizosphere was principally driven by O₂ penetration through interspace attributing to root stretch downward rather than root O₂ leakage. Increased O₂ concentrations in deep sediment over time finally induced the oxidization of labile Fe(II) into Fe(III) bound P and local P immobilization.

The Effects of Ferric Sulfate (Fe2(SO4)3) on the Removal of Cyanobacteria and Cyanotoxins: A Mesocosm Experiment

Cyanobacterial blooms are a global concern. Chemical coagulants are used in water treatment to remove contaminants from the water column and could potentially be used in lakes and reservoirs. The aims of this study was to: 1) assess the efficiency of ferric sulfate (Fe₂(SO₄)₃) coagulant in removing harmful cyanobacterial cells from lake water with cyanobacterial blooms on a short time scale, 2) determine whether some species of cyanobacteria can be selectively removed, and 3) determine the differential impact of coagulants on intra- and extra-cellular toxins. Our main results are: (i) more than 96% and 51% of total cyanobacterial cells were removed in mesocosms with applied doses of 35 mgFe/L and 20 mgFe/L, respectively. Significant differences in removing total cyanobacterial cells and several dominant cyanobacteria species were observed between the two applied doses; (ii) twelve microcystins, anatotoxin-a (ANA-a), cylindrospermopsin (CYN), anabaenopeptin A (APA) and anabaenopeptin B (APB) were identified. Ferric sulfate effectively removed the total intracellular microcystins (greater than 97% for both applied doses). Significant removal of extracellular toxins was not observed after coagulation with both doses. Indeed, the occasional increase in extracellular toxin concentration may be related to cells lysis during the coagulation process. No significant differential impact of dosages on intra- and extra-cellular toxin removal was observed which could be relevant to source water applications where optimal dosing is difficult to achieve.

Recyclable self-floating A-GUN-coated foam as effective visible-light-driven photocatalyst for inactivation of Microcystis aeruginosa

In this work, a recyclable self-floating A-GUN-coated (Ag/AgCl@g-C₃N₄@UIO-66(NH₂)-coated) foam was fabricated for effective inactivation of Microcystis aeruginosa (M. aeruginosa) under visible light. The floating photocatalyst was able to inactivate 98% of M. aeruginosa within 180 min under the visible-light irrigation, and the floating photocatalyst exhibited a stable performance in various conditions. Moreover, the inactivation efficiency can still maintain nearly 92% after five times recycle experiments, showing excellent photocatalytic stability. Furthermore, effects of A-GUN/SMF floating catalyst on the physiological properties, cellular organics, and algal functional groups of M. aeruginosa were studied. The floating photocatalyst can not only make full use of excellent photocatalytic activities of A-GUN nanocomposite, but also promote contact between catalyst and algae, and realize the effective recovery of the photocatalyst. Finally, possible photocatalytic inactivation mechanisms of algae were obtained, which provides references for removing cyanobacteria blooms in real water bodies.

Effects of Lanthanum Modified Bentonite and Polyaluminium Chloride on the Environmental Variables in the Water and Sediment Phosphorus Form in Lake Yanglan, China

The application of lanthanum modified bentonite (Phoslock®) and polyaluminium chloride (PAC) is popular in the restoration of European temperate lakes; however, the effects of the application on the concentrations of phosphorus (P) in both the water and the sediments have been poorly evaluated to date. We studied the effects of the application of Phoslock® + PAC on the concentrations of total phosphorus (TP), particulate phosphorus (PP), soluble reactive phosphorus (SRP), total suspended solids (TSS) and chlorophyll a (Chla) in the water, and different P forms in the sediments, in an isolated part of Lake Yanglan. The results showed that the concentrations of TP, PP, SRP, TSS and Chla decreased significantly after the addition of Phoslock® + PAC. Moreover, the concentrations of labile-P, reductant-soluble-P and organic-P in the sediments were also significantly decreased after the Phoslock® + PAC application. However, the concentrations of both the stable apatite-P and residual-P in the sediments after application of Phoslock® + PAC were much higher than the pre-addition values, while the concentrations of metal-oxide-P did not differ significantly between the pre- and post- application conditions. Our findings imply that the combined application of Phoslock® and PAC can be used in the restoration of subtropical shallow lakes, to reduce the concentrations of P in the water and suppress the release of P from the sediments.

Response of Magnetite/Lanthanum hydroxide composite on cyanobacterial bloom

Magnetite/lanthanum hydroxide composite (MLC-10) was applied in simulate natural water, sediment and cyanobacteria (WSC) system to evaluate its effect on cyanobacterial bloom in this study. According to the results, the addition of MLC-10 showed a good performance on inhibition of cyanobacterial bloom in systems. The cyanobacteria density of WSC-0.5 and WSC-1.0 (adding 0.5 g and 1.0 g MLC-10) at 30 day was 99.39% and 99.84% less than that in WSC-C (adding no MLC-10 in WSC system), respectively. The addition of MLC-10 could form a phosphorus-binding layer that adsorbed soluble reactive phosphate (SRP) in overlying water, improved the release of internal phosphorus (P) from sediment to pore water then blocked SRP release from pore water to overlying water, especially in WSC-0.5 and WSC-1.0. The results may be due to the high adsorption capacity of MLC-10 to phosphorus. Additionally, oxidative stress and oxidative damage of cyanobacteria were observed after exposing to MLC-10, and oxidative damage degree increased with the elevated amount of MLC-10. MLC-10 addition showed a slight effect on microbial community of surface sediment. Phosphorus limitation, cell damage and limited cells' floating performance were the possible mechanisms of cyanobacterial bloom controlling by MLC-10. Based on these results, MLC-10 could be used as a potential P-inactive material for cyanobacterial bloom controlling.

'Floc and Sink' Technique Removes Cyanobacteria and Microcystins from Tropical Reservoir Water

Combining coagulants with ballast (natural soil or modified clay) to remove cyanobacteria from the water column is a promising tool to mitigate nuisance blooms. Nevertheless, the possible effects of this technique on different toxin-producing cyanobacteria species have not been thoroughly investigated. This laboratory study evaluated the potential effects of the "Floc and Sink" technique on releasing microcystins (MC) from the precipitated biomass. A combined treatment of polyaluminium chloride (PAC) with lanthanum modified bentonite (LMB) and/or local red soil (LRS) was applied to the bloom material (mainly Dolichospermum circinalis and Microcystis aeruginosa) of a tropical reservoir. Intra and extracellular MC and biomass removal were evaluated. PAC alone was not efficient to remove the biomass, while PAC + LMB + LRS was the most efficient and removed 4.3-7.5 times more biomass than other treatments. Intracellular MC concentrations ranged between 12 and 2.180 µg L^-1 independent from the biomass. PAC treatment increased extracellular MC concentrations from 3.5 to 6 times. However, when combined with ballast, extracellular MC was up to 4.2 times lower in the top of the test tubes. Nevertheless, PAC + LRS and PAC + LMB + LRS treatments showed extracellular MC concentration eight times higher than controls in the bottom. Our results showed that Floc and Sink appears to be more promising in removing cyanobacteria and extracellular MC from the water column than a sole coagulant (PAC).

The role of rhizofiltration and allelopathy on the removal of cyanobacteria in a continuous flow system

A continuous flow filtration system was designed to identify and quantify the removal mechanisms of Cyanobacteria (Microcystis aeruginosa) by hydroponic biofilters of Phalaris arundinacea compared to synthetic filters. The filtration units were continuously fed under plug-flow conditions with Microcystis grown in photobioreactors. Microcystis cells decreased at the two flow rates studied (1.2 ± 0.2 and 54 ± 3 cm³ min-¹) and results suggested physical and chemical/biological removal mechanisms were involved. Physical interception and deposition was the main removal mechanism with packing density of the media driving the extent of cell removal at high flow, whilst physical and chemical/biological mechanisms were involved at low flow. At low flow, the biofilters decreased Microcystis cell numbers by 70% compared to the controls. The decrease in cell numbers in the biofilters was accompanied by a chlorotic process (loss of green colour), suggesting oxidative processes by the release of allelochemicals from the biofilters.

Biogeochemistry of Mediterranean Wetlands: A Review about the Effects of Water-Level Fluctuations on Phosphorus Cycling and Greenhouse Gas Emissions

Although Mediterranean wetlands are characterized by extreme natural water level fluctuations in response to irregular precipitation patterns, global climate change is expected to amplify this pattern by shortening precipitation seasons and increasing the incidence of summer droughts in this area. As a consequence, a part of the lake sediment will be exposed to air-drying in dry years when the water table becomes low. This periodic sediment exposure to dry/wet cycles will likely affect biogeochemical processes. Unexpectedly, to date, few studies are focused on assessing the effects of water level fluctuations on the biogeochemistry of these ecosystems. In this review, we investigate the potential impacts of water level fluctuations on phosphorus dynamics and on greenhouse gases emissions in Mediterranean wetlands. Major drivers of global change, and specially water level fluctuations, will lead to the degradation of water quality in Mediterranean wetlands by increasing the availability of phosphorus concentration in the water column upon rewetting of dry sediment. CO2 fluxes are likely to be enhanced during desiccation, while inundation is likely to decrease cumulative CO2 emissions, as well as N2O emissions, although increasing CH4 emissions. However, there exists a complete gap of knowledge about the net effect of water level fluctuations induced by global change on greenhouse gases emission. Accordingly, further research is needed to assess whether the periodic exposure to dry–wet cycles, considering the extent and frequency of the cycles, will amplify the role of these especial ecosystems as a source of these gases and thereby act as a feedback mechanism for global warming. To conclude, it is pertinent to consider that a better understanding about the effect of water level fluctuations on the biogeochemistry of Mediterranean wetlands will help to predict how other freshwater ecosystems will respond.

The value of novel ecosystems: Disclosing the ecological quality of quarry lakes

Intense sand and gravel mining has created numerous man-made lakes around the world in the past century. These small quarry lakes (1-50 ha) are usually hydrologically isolated, often deep (6-40 m) and stratify during summer and in cold winters. Due to their small size, these deep man-made lakes are usually not included in the regular monitoring campaigns, e.g. as required for the European Water Framework Directive (WFD). Therefore, not much is known about the ecological functioning of these novel ecosystems. During two summers, we determined the macrophyte diversity and measured a range of physico-chemical and biological parameters in 51 quarry lakes in the catchment area of the rivers Meuse and Rhine. We compared the results of this campaign to the chemical and macrophyte sampling as performed for the WFD in the immediate surrounding shallow standing waters. Alpha (local) and beta diversity (regional), and local contribution to beta diversity were calculated for the whole region of which beta diversity was further partitioned into a true species replacement and richness difference component. Quarry lakes contain higher water quality reflected by lower nutrient and chlorophyll-a concentration compared with shallow water bodies. Additionally, quarry lakes contribute significantly to the regional macrophyte diversity pool by harboring distinctly different macrophyte communities (beta diversity - replacement). Specifically quarry lakes with a total phosphorus concentration in the water column below 35 μg P/l contribute most to beta diversity among quarry lakes. Novel ecosystems such as deep quarry lakes are often perceived as less valuable ecosystems, with strong implications regarding their management. Our results show that quarry lakes are in general of better chemical and biological quality compared with shallow standing waters. We therefore call for a more integrated assessment of the quality of quarry lakes and corresponding management strategy of these waters by water managers.

The combined effects of macrophytes (Vallisneria denseserrulata) and a lanthanum-modified bentonite on water quality of shallow eutrophic lakes: A mesocosm study

Establishment of submerged macrophyte beds and application of chemical phosphorus inactivation are common lake restoration methods for reducing internal phosphorus loading. The two methods operate via different mechanisms and may potentially supplement each other, especially when internal phosphorous loading is continuously high. However, their combined effects have so far not been elucidated. Here, we investigated the combined impact of the submerged macrophyte Vallisneria denseserrulata and a lanthanum-modified bentonite (Phoslock®) on water quality in a 12-week mesocosm experiment. The combined treatment led to stronger improvement of water quality and a more pronounced reduction of porewater soluble reactive phosphorus than each of the two measures. In the combined treatment, total porewater soluble reactive phosphorus in the top 10 cm sediment layers decreased by 78% compared with the control group without Phoslock® and submerged macrophytes. Besides, in the upper 0-1 cm sediment layer, mobile phosphorus was transformed into recalcitrant forms (e.g. the proportion of HCl-P increased to 64%), while in the deeper layers, (hydr)oxides-bound phosphorus species increased 17-28%. Phoslock®, however, reduced the clonal growth of V. denseserrulata by 35% of biomass (dry weight) and 27% of plant density. Our study indicated that Phoslock® and submerged macrophytes may complement each other in the early stage of lake restoration following external nutrient loading reduction in eutrophic lakes, potentially accelerating the restoration process, especially in those lakes where the internal phosphorus loading is high.

Effect of algae on phosphorus immobilization by lanthanum-modified zeolite

Phosphorus-inactivating agents (PIAs) as geoengineering tools in lakes have been investigated extensively, but PIA resuspension in the photic layer occurs frequently in shallow lakes and little is known about the influence of algae on PIA performance. Our results proved that algae increased the dissolved oxygen, pH and dissolved organic carbon concentration substantially. In the absence of sediment, lanthanum modified zeolite (LMZ) as a representative PIA and algae could deplete dissolved inorganic phosphorus (DIP) from water but the former was faster than the latter. When LMZ and algae coexisted, the amount of phosphorus that was captured by LMZ was 3.1 times greater than that taken up by algae. An increase in pH or dissolved organic carbon increased the zero-equilibrium phosphorus concentration (EPC₀) of the sediment but LMZ addition could lower the EPC₀ and reduce the risk of phosphorus release during the algal blooming season. In the presence of sediment, LMZ reduced the DIP concentration more rapidly and yielded a lower final DIP concentration compared with algae. In conclusion, the influence of algae on the performance of LMZ by (i) taking up DIP to reduce the availability of DIP and convert DIP into a releasable phosphorus form and (ii) increasing the pH and dissolved organic carbon concentration to hinder the adsorption ability of DIP were recognized. The LMZ performed well, even in the presence of algae.

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.