Management of eutrophication in Lake De Kuil (The Netherlands) using combined flocculant - Lanthanum modified bentonite treatment

Mitigating harmful cyanobacterial blooms in drinking water reservoirs through in-situ sediment resuspension

Mitigating harmful cyanobacterial blooms is a global challenge, particularly crucial for safeguarding source water. Given the limitations of current technologies for application in drinking water reservoirs, we propose an innovative strategy based on in-situ sediment resuspension (SR). This method's effectiveness in cyanobacterial control and its potential impacts on water quality were assessed through laboratory culture experiments and further validated via field applications in five drinking water reservoirs. The results revealed that SR could significantly mitigate cyanobacterial growth, evidenced by the treated sets (removal rate: 3.82×106 cells L-1d-1) compared to the control set (growth rate: 2.22×107 cells L-1d-1) according to the laboratory experiments. The underlying mechanisms identified included underwater light reduction (2.38× increase in extinction coefficient) and flocculation and entrainment of cells by resuspended particles (30 % reduction per operation). Additional contributions were noted in the reduction of bioavailable phosphate and remediation of anaerobic sediment characterized by increased redox potential. This facilitated the oxidation of iron, which in turn promoted the co-precipitation of phosphate (removal rate: 46 μg L-1d-1) and inhibited its release from the sediment. The SR operation, devoid of importing extra substances, represents a safe and economical technology for controlling harmful cyanobacteria in drinking water reservoirs.

The use of coagulant and natural soil to control cyanobacterial blooms in a tropical reservoir

Cyanobacterial blooms have been a growing problem in water bodies and attracted attention from researcher and water companies worldwide. Different treatment methods have been researched and applied either inside water treatment plants or directly into reservoirs. We tested a combination of coagulants, polyaluminium chloride (PAC) and iron(III) chloride (FeCl3), and ballasts, luvisol (LUV) and planosol (PLAN), known as the 'Floc and Sink' technique, to remove positively buoyant cyanobacteria from a tropical reservoir water. Response Surface Methodology (RSM) based on Central Composite Design (CCD) was used to optimize the two reaction variables - coagulant dosage (x1) and ballast dosage (x2) to remove the response variables: chlorophyll-a, turbidity, true color, and organic matter. Results showed that the combination of LUV with PAC effectively reduced the concentration of the response variables, while PLAN was ineffective in removing cyanobacteria when combined to PAC or FeCl3. Furthermore, FeCl3 presented poorer floc formation and lower removal efficiency compared to PAC. This study may contribute to the theoretical and practical knowledge of the algal biomass removal for mitigating eutrophication trough different dosages of coagulants and ballasts.

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.

Coagulation as an effective method for cyanobacterial bloom control: A review

Eutrophication, the over-enrichment with nutrients, for example, nitrogen and phosphorus, of ponds, reservoirs and lakes, is an urgent water quality issue. The most notorious symptom of eutrophication is a massive proliferation of cyanobacteria, which cause aquatic organism death, impair ecosystem and harm human health. The method considered to be most effective to counteract eutrophication is to reduce external nutrient inputs. However, merely controlling external nutrient load is insufficient to mitigate eutrophication. Consequently, a rapid diminishing of cyanobacterial blooms is relied on in-lake intervention, which may encompass a great variety of different approaches. Coagulation/flocculation is the most used and important water purification unit. Since cyanobacterial cells generally carry negative charges, coagulants are added to water to neutralize the negative charges on the surface of cyanobacteria, causing them to destabilize and precipitate. Most of cyanobacteria and their metabolites can be removed simultaneously. However, when cyanobacterial density is high, sticky secretions distribute outside cells because of the small size of cyanobacteria. The sticky secretions are easily to form complex colloids with coagulants, making it difficult for cyanobacteria to destabilize and resulting in unsatisfactory treatment effects of coagulation on cyanobacteria. Therefore, various coagulants and coagulation methods were developed. In this paper, the focus is on the coagulation of cyanobacteria as a promising tool to manage eutrophication. Basic principles, applications, pros and cons of chemical, physical and biological coagulation are reviewed. In addition, the application of coagulation in water treatment is discussed. It is the aim of this review article to provide a significant reference for large-scale governance of cyanobacterial blooms. PRACTITIONER POINTS: Flocculation was a promising tool for controlling cyanobacteria blooms. Basic principles of four kinds of flocculation methods were elucidated. Flocculant was important in the flocculation process.

What makes a cyanobacterial bloom disappear? A review of the abiotic and biotic cyanobacterial bloom loss factors

Cyanobacterial blooms present substantial challenges to managers and threaten ecological and public health. Although the majority of cyanobacterial bloom research and management focuses on factors that control bloom initiation, duration, toxicity, and geographical extent, relatively little research focuses on the role of loss processes in blooms and how these processes are regulated. Here, we define a loss process in terms of population dynamics as any process that removes cells from a population, thereby decelerating or reducing the development and extent of blooms. We review abiotic (e.g., hydraulic flushing and oxidative stress/UV light) and biotic factors (e.g., allelopathic compounds, infections, grazing, and resting cells/programmed cell death) known to govern bloom loss. We found that the dominant loss processes depend on several system specific factors including cyanobacterial genera-specific traits, in situ physicochemical conditions, and the microbial, phytoplankton, and consumer community composition. We also address loss processes in the context of bloom management and discuss perspectives and challenges in predicting how a changing climate may directly and indirectly affect loss processes on blooms. A deeper understanding of bloom loss processes and their underlying mechanisms may help to mitigate the negative consequences of cyanobacterial blooms and improve current management strategies.

Effects of different chemical agents on changes in sediment phosphorus composition and the response of sediment microbial community

Controlling the release of sediment phosphorus (P) using chemical agents is a promising method for controlling internal P in eutrophic lakes. However, mineral P formation and changes in the organic P composition after sediment amendment with P-inactivation agents remain poorly understood. Furthermore, little is known about the changes in the sediment microbial community composition after remediation. Here, various ratios of poly aluminum chloride (PAC) and lanthanum-modified bentonite (LMB) were added to nutrient-rich sediments and incubated. Sequential P extraction, solution/solid-state ³¹P nuclear magnetic resonance (NMR), and microbial analyses were periodically performed on the inactivated sediments. The results indicate that PAC and LMB effectively reduced sediment iron-bound P and organic P, respectively, markedly increasing the content of aluminum- and calcium-bound P in the sediment, respectively. Solid-state ³¹P NMR results confirmed the formation of rhabdophane (LaPO₄. nH₂O) in the LMB-amended sediment. Solution ³¹P NMR results showed that PAC preferentially reduced the organic P fractions of pyrophosphate, whereas LMB efficiently reduced the organic P fractions of orthophosphate, monoesters, and diesters in the sediment. Compared with the control sediment, PAC addition can cause short-term negative effects on sediment microbes at high doses, whereas LMB addition can increase bacterial diversity or richness in the sediment. These results provide a deeper understanding of the differences between PAC and LMB in internal sediment P control.

Effect of capping mode on control of phosphorus release from sediment by lanthanum hydroxide

The use of in situ active capping to control phosphorus release from sediment has attracted more and more attentions in recent years. It is important to identify the effect of capping mode on the control of phosphorus release from sediment by the in situ active capping method. In this study, the impact of capping mode on the restraint of phosphorus migration from sediment into overlying water (OW) by lanthanum hydroxide (LH) was studied. Under no suspended particulate matter (SPM) deposition condition, LH capping effectively restrained the liberation of endogenous phosphorus into OW during anoxia, and the inactivation of diffusive gradient in thin film-unstable phosphorus (UP_DGT) and mobile phosphorus (P_Mobile) in the topmost sediment served as a significant role in the restraint of endogenous phosphorus migration into OW by LH capping. Under no SPM deposition, although the transformation of capping mode from the single high dose capping to the multiple smaller doses capping had a certain negative impact on the restraint efficiency of endogenous phosphorus liberation to OW by LH in the early period of application, it increased the stability of phosphorus in the static layer in the later period of application. Under SPM deposition condition, LH capping had the capability to mitigate the risk of endogenous phosphorus liberation into OW under anoxia conditions, and the inactivation of UP_DGT and P_Mobile in the topmost sediment was a significant mechanism for the control of sediment phosphorus liberation into OW by LH capping. Under SPM deposition condition, the change in the covering mode from the one-time high dose covering to the multiple smaller doses covering decreased the efficiency of LH to limit the endogenous phosphorus transport into OW in the early period of application, but it increased the performance of LH to restrain the sedimentary P liberation during the later period of application. The results of this work suggest that the multiple LH capping is a promising approach for controlling the internal phosphorus loading in freshwater bodies where SPM deposition often occurs in the long run.

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.

Removal efficacy and mechanism of nitrogen and phosphorus by biological aluminum-based P-inactivation agent (BA-PIA)

In this study, aluminum-based P-inactivation agent (Al-PIA) was used as a high-efficiency microbial carrier, and the biological Al-PIA (BA-PIA) was prepared by artificial aeration. Laboratory static experiments were conducted to study the effect of BA-PIA on reducing nitrogen and phosphorus contents in water. Physicochemical characterization and isotope tracing method were applied to analyze the removal mechanism of nitrogen and phosphorus. High-throughput techniques were used to analyze the characteristic bacterial genus in the BA-PIA system. The nitrogen and phosphorus removal experiment was conducted for 30 days, and the removal rates of NH₄⁺-N, TN and TP by BA-PIA were 81.87%, 66.08% and 87.97%, respectively. The nitrogen removal pathways of BA-PIA were as follows: the nitrification reaction accounted for 59.0% (of which denitrification reaction accounted for 56.4%), microbial assimilation accounted for 18.1%, and the unreacted part accounted for 22.9%. The characteristic bacteria in the BA-PIA system were Streptomyces, Nocardioides, Saccharopolyspora, Nitrosomonas, and Marinobacter. The loading of microorganisms only changed the surface physical properties of Al-PIA (such as specific surface area, pore volume and pore size), without changing its surface chemical properties. The removal mechanism of nitrogen by BA-PIA is the conversion of NH₄⁺-N into NO₂^--N and NO₃^--N by nitrifying bacteria, which are then reduced to nitrogen-containing gas by aerobic denitrifying bacteria. The phosphorus removal mechanism is that metal compounds (such as Al) on the surface of BA-PIA fix phosphorus through chemisorption processes, such as ligand exchange. Therefore, BA-PIA overcomes the deficiency of Al-PIA with only phosphorus removal ability, and has better application prospects.

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.

Response of sediment microbial communities to different levels of PAC contamination and exposure time

Coagulants such as polyaluminium chloride (PAC) are widely used for removing phosphorus from eutrophic water, but its application for water treatment can potentially harm the environment. In this study, a four-timepoint exposure experiment was performed at week 1, 3, 7 and 10 to investigate how microbial communities in lake sediments respond to different concentrations of PAC (RS (raw lake water with nothing added), Low, Medium and High). The results showed that, while PAC can efficiently decrease the amount of C, N and P in lake water, the presence of residual aluminum and aluminum precipitates can greatly affect the microbial communities in lake sediments. In particular, different concentrations of PAC and exposure time affected the microbial diversity and structure of lake sediments, with changes being especially obvious at high concentration of PAC after 10 weeks of exposure. Moreover, the use of PAC significantly increased the relative abundances of Gammaproteobacteria and Competibacter, while reducing those of Thermodesulfovibrionia, Vicinamibacterales, and BSV26 in time- and concentration-dependent manners. Network analysis further showed strong correlations between differential bacterial species of PAC in high concentration at 10 weeks, which further suggested that PAC treatment changed the complex structure of microbiota in lake sediment. Finally, correlation analysis indicated a close connection between water parameters and differential species induced by PAC treatment. Overall, PAC contamination changed the microbial communities at different taxonomy levels and influenced the functional pathways to potentiate the P removal, and the results offered interesting insights into the use of PAC in water treatment and its impact on biogeochemical cycling. These results indicated that more attention need to be paid to the potential impact of chemical phosphorus removing reagents on the environment, including eutrophic water.

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

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

Phosphate removal and recovery by lanthanum-based adsorbents: A review for current advances

Controlling eutrophication and recovering phosphate from water bodies are hot issues in the 21st century. Adsorption is considered to be the best method for phosphate removal because of its high adsorption efficiency and fast removal rate. Among the many adsorbents, lanthanum (La)-based adsorbents have been paid more and more attention due to their strong affinity to phosphorus. This paper reviews research of phosphate adsorption on La-based adsorbents in different La forms, including lanthanum oxide/hydroxide, lanthanum mixed metal oxide/hydroxide, lanthanum carbonate, La³⁺, La-based metal-organic framework (La-MOF) and La-MOF derivatives. The La-based adsorbents can be loaded on many carriers, such as carbon material, clay minerals, porous silica, polymers, industrial wastes, and others. We find that lanthanum oxide/hydroxide and La³⁺ adsorbents are mostly studied, while those in the forms of lanthanum carbonate, La-MOF, and La-MOF derivatives are relatively few. The kinetic process of most phosphate adsorption is pseudo-second-order and the isotherm process is in accordance with the Langmuir model. The cost of La-based and other traditional adsorbents was compared. The adsorption mechanisms are categorized as electrostatic attraction, ligand exchange, Lewis acid-base interaction, ion exchange and surface precipitation. Besides, regeneration methods of La-based adsorbents are mainly acid, alkali, and salt-alkali. In addition, the La-based adsorbents after absorbing phosphate can be directly used as a slow-release fertilizer. This review provides a basis for the research on phosphate adsorption by La-based adsorbents. It should be carried out to further develop La-based materials with high adsorption capacity and good regeneration ability. Meanwhile, studies have been conducted on the reuse of phosphate after desorption, which needs more attention in future research.

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.

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

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

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.

Three kinds of active thin-layer capping materials for reducing the phosphorus load in eutrophic water body: comparison in dynamic experiment

In this article, dynamic simulation experiments have studied the effects of three capping materials, quartz sand (QS), aluminum-based phosphorus-locking agent (Al-PIA), and lanthanum-modified bentonite (LMB) in reducing phosphorus load in eutrophic water bodies. The changes of various forms of phosphorus in Al-PIA and sediment before and after the test were analyzed, and the mechanism of phosphorus migration and transformation in different capping systems was described. The dynamic simulation test lasted 95 days. The results showed that when the initial concentration of total phosphorus (TP) was 3.55 mg/L, the capping strength was 2 kg/m2 and the hydraulic retention time of water circulation was 0.5 days, indicating that the average reduction rates of TP by LMB, Al-PIA and QS systems were 74.66%, 69.54%, and 3.64%, respectively, compared with the control system. The analysis of variance showed that there were significant differences (P < 0.05) in the TP concentration of the overlying water between the LMB, Al-PIA capping system, and the control system. Lanthanum ions in LMB can fix phosphorus. Al-PIA reduces the phosphorus concentration in water by means of ion exchange, adsorption, complexation, etc. LMB and Al-PIA promoted the migration of phosphorus in sediment. Among them, the phosphorus fixed by Al-PIA was mainly in the form of non-apatite inorganic phosphorus (NAIP) in inorganic phosphorus (IP), which can be seen; Al-PIA can effectively reduce the phosphorus load of eutrophic water.

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.

Water quality evaluation and dissolved organic matter characterization of a tropical hypereutrophic reservoir and its streams treated with Phoslock® and microbial bioremediation Enzilimp®

Worldwide, freshwater environments are impacted by inputs of nutrients and dissolved organic matter from human activities. Yet, the recovery of aquatic systems is usually focused only on nutrient management. In our work, we presented the case of an urban and hypereutrophic environment (Pampulha reservoir, Belo Horizonte, Brazil) that receives discharges from several streams and was treated with lanthanum modified bentonite (Phoslock®) and microbial bioremediation (Enzilimp®). Our goals were to evaluate whether the treatment could improve the water quality and characterize the spatiotemporal variation of dissolved organic matter sources and indices according to absorbance and fluorescence measurements from the reservoir and streams post-application months (2018). In our results, the reservoir showed a relative decrease in its phosphorus concentration compared to data from before the treatment. On the other hand, carbon concentrations reached expressive values in the post-application months following a similar pattern found in the streams. Our data showed that the reservoir's high resistance in its hypereutrophic condition was related to the elevated loading of external inputs coming from the streams. The parallel factor analysis (PARAFAC) identified four main carbon sources, two of them being potential tracers of organic pollution in the Pampulha reservoir and watershed, together with absorbance and fluorescence indices. Our findings suggest that carbon parameters can be essential tools to provide adequate monitoring and optimization of water recovery attempts in complex, polluted environments.

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.

Effectiveness and mechanism of aluminum/iron co-modified calcite capping and amendment for controlling phosphorus release from sediments

The effectiveness and mechanism of aluminum/iron co-modified calcite (Al/Fe-CA) for the control of phosphorus (P) liberation from sediments was investigated. The results showed that Al/Fe-CA possessed good sorption performance for phosphate, and the maximum phosphate sorption capacity for Al/Fe-CA could reach 27.0 mg/g. The major mechanisms involved the surface adsorption of phosphate on calcite, the precipitation between phosphate and Ca²⁺ leached from calcite, and the ligand exchange between Al/Fe-bound hydroxyl groups and phosphate to form the Al-O-P and Fe-O-P inner-sphere complexes. The re-releasing risk of Al/Fe-CA-bound P under the circumstances of normal pH (5-9) and reducing environment was very low. Al/Fe-CA addition could significantly reduce the risk of P releasing from sediment to overlying water (OL-water), and the inactivation of mobile P, reactive soluble P (SRP) and diffusive gradient in thin-films (DGT)-labile P in sediment by Al/Fe-CA had a great part in the suppression of sediment-P liberation to OL-water by the Al/Fe-CA amendment. Al/Fe-CA capping and fabric-wrapped Al/Fe-CA capping both could greatly reduce the risk of P releasing from sediment into OL-water, and the formation of a static layer with low concentrations of SRP and DGT-labile P in the upper sediment was the key to sustaining a high P controlling efficiency. When the applied mode of Al/Fe-CA varied from capping to amendment, although the inactivation efficiency of DGT-labile P in the overlying water and upper sediment by Al/Fe-CA would decrease to a certain degree, the inactivation efficiency of DGT-labile P in the lower sediment by Al/Fe-CA would increase. Results of this study suggest that Al/Fe-CA has the high potential to be used as an active capping or amendment material for the management of internal P loading in surface 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.

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.

Phosphorus immobilization in water and sediment using iron-based materials: A review

As an essential element for life, phosphorus (P) is very important for organisms. However, excessive P in water and sediment can cause eutrophication, which poses a potential risk to drinking water safety and the sustainability of aquatic ecosystems. Therefore, effective phosphorus-control in water and sediment is the key strategy to control eutrophication. Iron-based materials exhibit high efficiency for P immobilization due to their strong affinity with P, low cost, easy availability, and environmentally friendliness. They are promising materials for controlling P in application. This work comprehensively summarizes the recent advances on P immobilization in water and sediment by different iron-based materials, including iron (hydr)oxides, iron salts, zero-valent iron and iron-loaded materials. This review is focused on the mechanism of the processes and how they are impacted by major influencing factors. The combination of iron-containing materials with other assisting materials is a good strategy to enhance P-fixation efficiency and selectivity. Finally, the current challenges and prospects of P-control technologies based on iron-containing materials are proposed. This review provides a systemic theoretical and experimental foundation for P-immobilization in water and sediment using iron-based materials.

Removal of cyanobacteria from a water supply reservoir by sedimentation using flocculants and suspended solids as ballast: Case of Legedadi Reservoir (Ethiopia)

The massive growth of potentially toxic cyanobacteria in water supply reservoirs, such as Legedadi Reservoir (Ethiopia), poses a huge burden to water purification units and represents a serious threat to public health. In this study, we evaluated the efficiency of the flocculants/coagulants chitosan, Moringa oleifera seed (MOS), and poly-aluminium chloride (PAC) in settling cyanobacterial species present in the Legedadi Reservoir. We also tested whether coagulant-treated reservoir water promotes cyanobacteria growth. Our data showed that suspended solids in the turbid reservoir acted as ballast, thereby enhancing settling and hence the removal of cyanobacterial species coagulated with chitosan, Moringa oleifera seed, or their combination. Compared to other coagulants, MOS of 30 mg/L concentration, with the removal efficiency of 93.6%, was the most effective in removing cyanobacterial species without causing cell lysis. Contrary to our expectation, PAC was the least effective coagulant. Moreover, reservoir water treated with MOS alone or MOS combined with chitosan did not support any growth of cyanobacteria during the first two weeks of the experiment. Our data indicate that the efficacy of a flocculant/coagulant in the removal of cyanobacteria is influenced by the uniqueness of individual lakes/reservoirs, implying that mitigation methods should consider the unique characteristic of the lake/reservoir.

Ionic strength and polyelectrolyte molecular weight effects on floc formation and growth in Taylor-Couette flows

Polyelectrolyte-driven flocculation of suspended particulate in solution is an important process in a variety of industrial processes such as drinking water treatment and composite material synthesis. Flocculation depends on a wide variety of physicochemical and hydrodynamic properties, which affect floc size, growth rate, and floc morphology. Floc formation and growth behavior is explored here using two different molecular weights of a cationic polyacrylamide flocculant and anisotropic Na-bentonite clay particles under a variety of solution ionic strengths. A Taylor-Couette cell with radial injection capabilities was used to study the effects of solution ionic strength and polyelectrolyte molecular weight on floc size, growth rate, and floc morphology during the flocculation process with a constant global velocity gradient. The floc size generally decreased with increasing ionic strength whereas the floc growth rate initially increased then decreased. This likely occurred due to charge screening effects, where increased bentonite aggregate size and a less expanded polyelectrolyte conformation at higher ionic strengths results in a decreased ability for the polyelectrolyte to bridge multiple bentonite aggregates. The densification of bentonite aggregates at higher ionic strengths resulted in floc morphologies that were more resistant to shear-induced breakage. With the exceptions of optimal dose concentration and dispersion coefficients, there were no clear differences in the floc growth rate behaviors for the two molecular weights studied. This work contributes to an improved understanding of the physicochemical complexities of polyelectrolyte-driven flocculation that can inform dosing requirements for more efficient industrial operations.

Low Dose Coagulant and Local Soil Ballast Effectively Remove Cyanobacteria (Microcystis) from Tropical Lake Water without Cell Damage

The combination of a low dose of coagulant with a ballast, also known as “flock and sink,” has been proposed as a lake restoration and cyanobacteria bloom management strategy. The effectiveness of this technique using aluminum sulfate (alum) as a coagulant and a local soil (LS) from Thailand as a ballast in eutrophic water dominated by positively buoyant Microcystis colonies collected from a tropical lake was investigated by measuring changes in chlorophyll-a (chl-a), pH, and zeta potential. Cell integrity was also evaluated using scanning electron microscopy. Results showed that alum alone could reduce chl-a (up to 60% to 83%) at doses (higher than 3 to 6 mg Al/L) dependent on the initial pH (7.6 to 8.2) and initial chl-a concentration (138 to 615 µg/L) of the lake water but resulted in morphological changes to cellular structure and generally required a dose that reduced pH to <7. LS ballast alone was able to reduce chl-a concentrations (up to 26% at highest dose of 400 mg/L) and caused no significant changes to pH or zeta potential. Combining a low dose of alum (2 mg Al/L) with some amount of LS ballast (50 to 400 mg/L) created an interaction effect that resulted in 81 to 88% reduction in chl-a without changes to zeta potential or morphological changes to cellular structure. Flock and sink may serve a niche role in lake restoration when positively buoyant cyanobacteria are present in the water column during time of treatment. This research showed that an 800% increase in ballast dose resulted in about an 8% reduction in chl-a when combined with 2 mg Al/L of alum. Therefore, it is recommended that ballast dose should be determined by considering its phosphorus sorption capacity and the potentially releasable phosphorus in the lake sediment in order to realize long-term reductions in sediment nutrient release.

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.

Chitosan as a Coagulant to Remove Cyanobacteria Can Cause Microcystin Release

Chitosan has been tested as a coagulant to remove cyanobacterial nuisance. While its coagulation efficiency is well studied, little is known about its effect on the viability of the cyanobacterial cells. This study aimed to test eight strains of the most frequent bloom-forming cyanobacterium, Microcystis aeruginosa, exposed to a realistic concentration range of chitosan used in lake restoration management (0 to 8 mg chitosan L^-1). We found that after 1 h of contact with chitosan, in seven of the eight strains tested, photosystem II efficiency was decreased, and after 24 h, all the strains tested were affected. EC₅₀ values varied from 0.47 to > 8 mg chitosan L^-1 between the strains, which might be related to the amount of extracellular polymeric substances. Nucleic acid staining (Sytox-Green^®) illustrated the loss of membrane integrity in all the strains tested, and subsequent leakage of pigments was observed, as well as the release of intracellular microcystin. Our results indicate that strain variability hampers generalization about species response to chitosan exposure. Hence, when used as a coagulant to manage cyanobacterial nuisance, chitosan should be first tested on the natural site-specific biota on cyanobacteria removal efficiency, as well as on cell integrity aspects.

Removal of Positively Buoyant Planktothrix rubescens in Lake Restoration

The combination of a low-dose coagulant (polyaluminium chloride—‘Floc’) and a ballast able to bind phosphate (lanthanum modified bentonite, LMB—‘Sink/Lock’) have been used successfully to manage cyanobacterial blooms and eutrophication. In a recent ‘Floc and Lock’ intervention in Lake de Kuil (the Netherlands), cyanobacterial chlorophyll-a was reduced by 90% but, surprisingly, after one week elevated cyanobacterial concentrations were observed again that faded away during following weeks. Hence, to better understand why and how to avoid an increase in cyanobacterial concentration, experiments with collected cyanobacteria from Lakes De Kuil and Rauwbraken were performed. We showed that the Planktothrix rubescens from Lake de Kuil could initially be precipitated using a coagulant and ballast but, after one day, most of the filaments resurfaced again, even using a higher ballast dose. By contrast, the P. rubescens from Lake Rauwbraken remained precipitated after the Floc and Sink/Lock treatment. We highlight the need to test selected measures for each lake as the same technique with similar species (P. rubescens) yielded different results. Moreover, we show that damaging the cells first with hydrogen peroxide before adding the coagulant and ballast (a ‘Kill, Floc and Lock/Sink’ approach) could be promising to keep P. rubescens precipitated.

Exploring How Cyanobacterial Traits Affect Nutrient Loading Thresholds in Shallow Lakes: A Modelling Approach

Globally, many shallow lakes have shifted from a clear macrophyte-dominated state to a turbid phytoplankton-dominated state due to eutrophication. Such shifts are often accompanied by toxic cyanobacterial blooms, with specialized traits including buoyancy regulation and nitrogen fixation. Previous work has focused on how these traits contribute to cyanobacterial competitiveness. Yet, little is known on how these traits affect the value of nutrient loading thresholds of shallow lakes. These thresholds are defined as the nutrient loading at which lakes shift water quality state. Here, we used a modelling approach to estimate the effects of traits on nutrient loading thresholds. We incorporated cyanobacterial traits in the process-based ecosystem model PCLake+, known for its ability to determine nutrient loading thresholds. Four scenarios were simulated, including cyanobacteria without traits, with buoyancy regulation, with nitrogen fixation, and with both traits. Nutrient loading thresholds were obtained under N-limited, P-limited, and colimited conditions. Results show that cyanobacterial traits can impede lake restoration actions aimed at removing cyanobacterial blooms via nutrient loading reduction. However, these traits hardly affect the nutrient loading thresholds for clear lakes experiencing eutrophication. Our results provide references for nutrient loading thresholds and draw attention to cyanobacterial traits during the remediation of eutrophic water bodies.

Interactions of phosphate and dissolved organic carbon with lanthanum modified bentonite: Implications for the inactivation of phosphorus in lakes

Lanthanum-modified bentonite (LMB) is a widely used phosphorus-inactivating agent in lakes. However, dissolved organic carbon (DOC) exists ubiquitously in lakes, and its influence on phosphate binding is still not adequately understood. Our results showed that both phosphate and DOC can be adsorbed by LMB. The Langmuir adsorption maxima of phosphate and DOC were 9.06 mg P/g and 5.31 mg C/g, respectively, generating a C/P molar ratio ∼1.5. When phosphate and DOC coexisted at this ratio, the adsorption of phosphate was not influenced by DOC and vice versa. However, the phosphate capture by LMB was significantly reduced by raising the ratio above ∼9, and the reduction was increased with increasing the ratio. Once adsorbed by LMB, phosphate was essentially not desorbed by DOC, while adsorbed DOC can be mostly liberated by phosphate. It is deemed that phosphate can interact preferentially with La on LMB. However, DOC can still be adsorbed by LMB, even after LMB was saturated with phosphate, which was attributed to (i) the high coordination capacity of La; (ii) the interaction of DOC with the hydroxyl group(s) of the adsorbed phosphate via hydrogen bonding; and (iii) the interaction of DOC with the La sites unoccupied by phosphate. We proposed that LMB can be applied in the season (time) when the DOC/P ratio in lakes is low enough to facilitate the adsorption of phosphate, which will no longer be released into water, even after the C/P ratio is raised later.

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.

Composition of dissolved organic matter controls interactions with La and Al ions: Implications for phosphorus immobilization in eutrophic lakes

Applications of aluminium (Al) salt or lanthanum (La) modified bentonite (LMB) have become popular methodologies for immobilizing phosphorus (P) in eutrophic lakes. The presence of humic substances, has been shown to inhibit this form of treatment due to the complexation with La/Al. However, the effects of other dissolved organic matter (DOM), especially that derived from phytoplankton (the dominant source in eutrophic lakes) are unknown. In this study, the interaction with La/Al of Suwannee River Standard Humic Acid Standard II (SRHA) and algae-derived DOM (ADOM) were investigated and compared. Differed to SRHA which was dominated by polyphenol-like component (76.8%, C1-SRHA), majority in ADOM were protein-like substance, including 41.9% tryptophan-like component (C2-ADOM) and 21.0% tyrosine-like component (C3-ADOM). Two reactions of complexation and coprecipitation were observed between SRHA/ADOM and La/Al. Complexation dominated at low metal inputs less than 10 μM and coprecipitation was the main reaction at higher metal inputs. For ADOM, the tryptophan-like component (C2-ADOM) was the important component to react with metal. The reaction rate for C2-ADOM with La were about two-third of that for C1-SRHA, indicating that the influence of C2-ADOM was significant during the P immobilization by La/Al-based treatment in eutrophic lakes. The P removal data in the presence of ADOM confirmed the significant inhibition of ADOM. In addition, based on the composition of coprecipitates and relatively biodegradable character of tryptophan-like substances (C2-ADOM), the coprecipitation of ADOM was assumed to reduce the stability of precipitated P in eutrophic lakes. The release of P from the potential biodegradation of the coprecipitates and thus the possible decline of the performance of P immobilization by La/Al-based treatments is an important work in the future.

Human health risk associated with the management of phosphorus in freshwaters using lanthanum and aluminium

The use of geo-engineering materials to manage phosphorus in lakes has increased in recent years with aluminium and lanthanum based materials being most commonly applied. Hence the potential impact of the use of these compounds on human health is receiving growing interest. This review seeks to understand, evaluate and compare potential unintended consequences on human health and ecotoxicological risks associated with the use of lanthanum- and aluminium-based materials to modify chemical and ecological conditions in water bodies. In addition to their therapeutic use for the reduction of intestinal phosphate absorption in patients with impaired renal function, the phosphate binding capacity of aluminium and lanthanum also led to the development of materials used for water treatment. Although lanthanum and aluminium share physicochemical similarities and have many common applications, their uptake and kinetics within the human body and living organisms importantly differ from each other which is reflected in a different toxicity profile. Whilst a causal role in the development of neurological pathologies, skeletal lesions, hematopoietic disorders and respiratory effects has unequivocally been demonstrated with increased exposure to aluminium, studies until now have failed to find such a clear association after exposure to lanthanum although caution is warranted. Our review indicates that lanthanum and aluminium have a distinctly different profile with respect to their potential effects on human health. Regular monitoring of both aluminium and lanthanum concentrations in lanthanum-/aluminium-treated water by the responsible authorities is recommended to avoid acute accidental or chronic low level accumulation.

Decomposition of cyanobacterial bloom contributes to the formation and distribution of iron-bound phosphorus (Fe-P): Insight for cycling mechanism of internal phosphorus loading

Lake eutrophication and the resulting cyanobacterial blooms have become a global water environment problem. These eutrophic lakes usually have relatively high internal phosphorus loading such as Fe-P to support the formation of cyanobacterial blooms. In order to reveal the mechanisms and processes of phosphorus cycling in lake sediments, in this study, Lake Chaohu was selected as the research area, and the effects of cyanobacterial bloom decomposition on the horizontal distribution pattern of Fe-P was studied by field investigation and laboratory simulations. According to the phosphorus fractions in the sediments, Lake Chaohu can be divided into three lake areas, and the Fe-P content in western Chaohu is the highest (908.6 ± 54.9 mg kg^-1). The contents and proportions of Fe-P were significantly positively correlated with cyanobacterial pigments in sediments, but they negatively correlated with undegraded chl-a, especially when the Fe-P content was <400 mg kg^-1. Based on these statistical analyses, we proposed a hypothesis that the settled cyanobacterial organic matters (COM) could promote the formation of Fe-P. This hypothesis was proved by the simulation experiments of adding COM to the oligotrophic lakeshore clay. The results suggested that the content and proportion of Fe-P in sediments were significantly increased by the COM addition, and also, they were significantly positively correlated with the decomposition of the COM. The formation processes of Fe-P were further confirmed by the analysis of Fourier transform infrared (FT-IR) spectra. Microbial community analysis suggested that the bacterial species including FeOB and genus Pseudomonas might play an important role in the formation of Fe-P. This study suggested that the settled COM could enhance the eutrophication of sediments through a positive feedback cycle. Therefore, it is necessary to carry out bloom removal and sediment dredging simultaneously, and only then the cyanobacterial bloom can be effectively controlled.

In situ polymer flocculation and growth in Taylor-Couette flows

Flocculation of small particulates suspended in solution is a key process in many industries, including drinking water treatment. The particles are aggregated during mixing to form larger aggregates, known as flocs, through use of a polyelectrolyte flocculant. The flocculation of these particulates in water treatment, however, are subject to a wide spatial variation of hydrodynamic flow states, which has consequences for floc size, growth rate, and microstructure. Floc assembly dynamics are explored here using a commercially available cationic polyacrylamide, commonly used in water treatment, and anisotropic Na-bentonite clay particles under a variety of hydrodynamic mixing conditions. A Taylor-Couette cell with the unique ability to radially inject fluid into the rotating annulus was used to study how specific hydrodynamic flow fields affect assembly and structure of these materials during the flocculation process. Faster floc growth rates and decreased floc fractal dimensions were observed for higher order flow states, indicating improved mass transfer of the polymer flocculant and breakage at the edges of the flocs (shear rounding), respectively. This work sheds more light on the complexities of polymer-induced flocculation, towards improving dosing and efficiency of large-scale operations.

Effects of polyaluminum chloride and lanthanum-modified bentonite on the growth rates of three Cylindrospermopsis raciborskii strains

In tropical and subtropical lakes, eutrophication often leads to nuisance blooms of Cylindrospermopsis raciborskii. In laboratory experiments, we tested the combined effects of flocculant polyaluminum chloride (PAC) and lanthanum-modified bentonite (LMB) on the sinking and growth rates of three C. raciborskii strains. We tested the hypothesis that the combination of PAC and LMB would (1) effectively sink C. raciborskii in a test tube experiment and (2) impair C. raciborskii growth, irrespective of the biomass of the inoculum (bloom) and the strain in the growth experiment. We tested the recommended (LMB1) and a three-times higher dose of LMB (LMB3). The combined addition of PAC and LMB enhanced the sedimentation of all C. raciborskii strains. Moreover, both the PAC and LMB doses decreased the phosphate concentration. PAC and LMB1 decreased the growth rate of all strains, but the efficacy depended on the biomass and strain. The combined addition of PAC and LMB3 inhibited the growth of all strains independently of the biomass and strain. We conclude that a low dose of PAC in combination with the recommended dose of LMB decreases C. raciborskii blooms and that the efficiency of the technique depends on the biomass of the bloom. A higher dose of LMB is needed to obtain a more efficient control of C. raciborskii blooms.

Phosphorus fractionation in sediments of tropical semiarid reservoirs

The reduction of external loads of phosphorus (P) is the main action to control eutrophication in lakes. However, the elimination of these sources is not always sufficient for lakes' recovery, because the P accumulated in the sediment can be released for decades after it has accumulated. Thus, to restore a lake, it is also necessary to reduce its internal P loading. For this, it is essential to know the composition of P in the sediment to evaluate the potential P availability. In this study, the forms of P in the sediment of two reservoirs in a semiarid tropical region were investigated. The forms of P were determined by fractionation, sorting out the forms in loosely sorbed P (P-Water), reductant-soluble P (P-BD), metal oxide-bound P (P-NaOH), P bound to organic matter (P-Humic), calcium-bound P (P-HCl) and residual P (P-residual). The sediment was collected in September 2016 and sliced in situ. The total amount of P forms in the sediment varied from 5mgkg^-1 to 349mgkg^-1 in the Gargalheiras Reservoir and from 12mgkg^-1 to 371mgkg^-1 in the Cruzeta Reservoir. Despite some variation in amounts at different depths, the general range of fractions in Gargalheiras and Cruzeta was: P-BD>P-NaOH>P-HCl>P-water>P-Residual>P-Humic. In both reservoirs, the predominant form was BD, followed by NaOH and HCl. The first two forms are available and released easily, making them bioavailable for eutrophication processes and thus phytoplankton growth.

Assessment of possible solid-phase phosphate sorbents to mitigate eutrophication: Influence of pH and anoxia

Managing eutrophication remains a challenge to water managers. Currently, the manipulation of biogeochemical processes (i.e., geo-engineering) by using phosphorus-adsorptive techniques has been recognized as an appropriate tool to manage the problem. The first step in finding potential mitigating materials is conducting a sequence of upscaling studies that commence with controlled laboratory experiments. Here, the abilities of 10 possible solid-phase-sorbents (SPS) to adsorb P were examined. Four materials adsorbed P, and two of these materials were modified, i.e., a lanthanum-modified-bentonite (LMB) and an aluminum-modified-zeolite (AMZ), and had the highest adsorption capacities of 11.4 and 8.9mgPg^-1, respectively. Two natural materials, a red soil (RS) and a bauxite (BAU), were less efficient with adsorption capacities of 2.9 and 3.4mgPg^-1, respectively. Elemental composition was not related to P adsorption. Since SPS might be affected by pH and redox status, we also tested these materials at pH values of 6, 7, 8 and 9 and under anoxic condition. All tested materials experienced decreased adsorption capacities under anoxic condition, with maximum adsorptions of 5.3mgPg^-1 for LMB, 5.9mgPg^-1 for AMZ, 0.2mgPg^-1 for RS and 0.2mgPg^-1 for BAU. All materials were able to adsorb P across the range of pH values that were tested. The maximum adsorption capacities of LMB and RS were highest at pH6, AMZ was higher at a pH of 9 and BAU at a pH of 8. Thus, pH influenced P adsorption differently. Given the effects of pH and anoxia, other abiotic variables should also be considered. Considering the criteria that classify a useful SPS (i.e., effective, easy to produce, cheap and safe), only the two modified materials that were tested seem to be suitable for upscaling to enclosure studies with anoxic sediments.

Bio-accumulation of lanthanum from lanthanum modified bentonite treatments in lake restoration

Lanthanum (La) modified bentonite (LMB) is one of the available mitigating agents used for the reduction of the phosphorus (P) recycling in eutrophic lakes. The potential toxicity of the La from LMB to aquatic organisms is a matter of concern. In this study the accumulation of La was investigated in the macrophyte Elodea nuttallii, in chironomid larvae and in several fish species during periods up to five years following in situ LMB applications. The application of LMB increased the La concentration of exposed plants and animals. During the first growing season following LMB applications, the La content of E. nuttallii increased 78 fold (3.98-310.68 μg La g^-1 DW) to 127 fold (2.46-311.44 μg La g^-1). During the second growing season following application, the La content decreased but was still raised compared to plants that had not been exposed. The La content of chironomids was doubled in the two years following LMB application, although the increase was not significant. Raised La concentrations in fish liver, bone, muscle and skin were observed two and five years following to LMB application. Liver tissues showed the highest La increase, ranging from 6 fold (0.046-0.285 μg La g^-1 DW) to ∼20 fold (0.080-1.886 μg La g^-1, and 0.122-2.109 μg La g^-1) two years following application and from 6 fold (0.046-0.262 μg La g^-1) to 13 fold (0.013-0.167 μg La g^-1) after five years in pelagic and littoral fish. The La content of the liver from Anguilla anguilla (eel) had increased 94 fold (0.034-3.176 μg La g^-1) two years and 133 fold (0.034-4.538 μg La g^-1) five years following LMB application. No acute and chronic effects of La accumulation were observed and human health risks are considered negligible. We advocate the long-term study of effects of La accumulation following future LMB applications.

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

A low dose flocculant (FeCl₃), combined with lanthanum modified bentonite (LMB) as phosphate-binding agent, has been applied for eutrophication management in Lake De Kuil (The Netherlands). After the treatment, the state of the lake shifted from hypertrophic to mesotrophic. Although macroinvertebrate fauna is important for lake ecosystems, the knowledge of its response to this lake restoration method is fragmented and scarce. Because insight in the macroinvertebrate fauna response is important to assess future applications, pre and post application macroinvertebrate assemblages were identified in Lake De Kuil. The research was accompanied by a microcosm experiment in which the effects of LMB, FeCl₃ and LMB + FeCl₃ were studied on macroinvertebrate communities. Results show the reduction of macroinvertebrate numbers and taxa during the first month following the application. The number of Gastropoda was strikingly reduced one month after the application. One year after the application, the macroinvertebrate numbers and taxa exceeded the pre-application situation and Gastropoda and Oligochaeta prospered. The effects one month after the treatment are most likely due to the combination of physical impacts of the use of bentonite and chemical impacts of the use of FeCl₃, while effects after one year are likely attributed to the shift in trophic state of the lake.

Chitosan as coagulant on cyanobacteria in lake restoration management may cause rapid cell lysis

Combining coagulant and ballast to remove cyanobacteria from the water column is a promising restoration technique to mitigate cyanobacterial nuisance in surface waters. The organic, biodegradable polymer chitosan has been promoted as a coagulant and is viewed as non-toxic. In this study, we show that chitosan may rapidly compromise membrane integrity and kill certain cyanobacteria leading to release of cell contents in the water. A strain of Cylindrospermopsis raciborskii and one strain of Planktothrix agardhii were most sensitive. A 1.3 h exposure to a low dose of 0.5 mg l^-1 chitosan already almost completely killed these cultures resulting in release of cell contents. After 24 h, reductions in PSII efficiencies of all cyanobacteria tested were observed. EC50 values varied from around 0.5 mg l^-1 chitosan for the two sensitive strains, via about 5 mg l^-1 chitosan for an Aphanizomenon flos-aquae strain, a toxic P. agardhii strain and two Anabaena cylindrica cultures, to more than 8 mg l^-1 chitosan for a Microcystis aeruginosa strain and another A. flos-aquae strain. Differences in sensitivity to chitosan might be related to polymeric substances that surround cyanobacteria. Rapid lysis of toxic strains is likely and when chitosan flocking and sinking of cyanobacteria is considered in lake restoration, flocculation efficacy studies should be complemented with investigation on the effects of chitosan on the cyanobacteria assemblage being targeted.