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

Global meta-analysis deciphering ecological restoration performance of dredging: Divergent variabilities of pollutants and hydrobiontes

Global "Sustainable Development Goals" propose ambitious targets to protect water resource and provide clean water, whereas comprehensive understanding of restoration performance and ecological mechanisms are lacking for dredging adopted for purifying polluted waterbodies and maintaining navigation channels. Here, we conducted a global meta-analysis to estimate ecological restoration consequence of dredging as pollution mitigation and navigation channel maintenance measures using a dataset compiled from 191 articles covering 696 studies and 84 environmental and ecological parameters (e.g., pollutants and hydrobiontes). We confirm that dredging shows negative influences on 77.50% pollutants in the BA model (before dredging vs. after dredging) and 84.21% pollutants in the CI model (control vs. impact) as well as on sediment nutrient fluxes. Additionally, 57.14% attributes (i.e., richness, diversity, biomass, and density) of hydrobiontes in the BA model and 89.47% attributes of hydrobiontes in the CI model responded negatively to dredging. As a result, 76.32% of the pollutants and 61.11% of the hydrobiont attributes responded uniformly to dredging in the BA and CI models. Our findings emphasize that dredging generally decreases pollutants and mitigates algal blooms, controlling phosphorus is easier than controlling nitrogen by dredging, and attributes (i.e., richness, diversity, and biomass) of hydrobiontes (i.e., zooplankton, phytoplankton, and zoobenthos) are density-dependent in dredging-disturbed environments. Our findings broaden our knowledge on ecological restoration performance of dredging as a mitigation measure in global aquatic ecosystems, and these findings might be helpful to use and optimize dredging to efficiently and sustainably purify polluted aquatic ecosystems.

Phosphorus loading in impaired wetlands: The role of temperature and simulated dredging under controlled experimental conditions

Restoring wetlands can increase biodiversity and water quality in both wetlands and connecting ecosystems; however, failing to consider past land uses can result in negative water quality effects. Prior agricultural practices at a coastal wetland restoration site in West Michigan (USA) have caused high legacy phosphorus (P) concentrations in sediments, as well as high P levels in the water column. Sediment dredging is being considered as a restoration tool to control internal phosphorus loading (IPL). Additionally, climate warming is increasing both mean temperature and temperature extremes, which can affect IPL. The influence of both sediment dredging and climate warming on P dynamics was examined in this wetland complex by incubating sediment cores in a controlled laboratory experiment. Both ponds exhibited hypereutrophic conditions, although mean water column and sediment total phosphorus (TP) concentrations were ∼ 2.5 and ∼ 8 times higher, respectively, in the north pond compared to the south pond. Unexpectedly, sediment cores with the top 10 cm removed (simulated dredging) had slightly higher mean soluble reactive phosphorus (SRP) release rates in both ponds (north ∼22 mg m-2 d-1, south ∼1.9 mg m-2 d-1) compared to undredged cores (north ∼18.6 mg m-2 d-1, south ∼1.5 mg m-2 d-1). Equilibrium P concentrations suggest sediments are a source of SRP to the overlying water column regardless of dredging treatment. Increases to both average temperature and simulated heatwaves increased P release and P accumulation in core tubes in the less enriched south pond but not in the north pond. Our results emphasize the need for precursory research before implementing management actions and show that dredging alone might not be the appropriate management technique for this restoration site. Successful management of internal and external P loading is crucial to reducing eutrophication worldwide, leading to healthier freshwater ecosystems and enhancing the beneficial ecosystem services they provide.

The reaction of the macroinvertebrate Gammarus pulex to lanthanum-modified bentonite (LMB)

Recently, lanthanum-modified bentonite (LMB) has been widely used for internal phosphorus control in eutrophication management. Generally, LMB application results in the formation of LMB layer, which can affect the surface sediment environment and may affect organisms living on the sediment surface. To explore it, we studied the effects of LMB on the macroinvertebrate Gammarus pulex. In a 3-day habitat choice trial, compared with the pebbles covered with sand, bentonite or LMB, G. pulex preferred the uncovered pebbles. In a 4-day acute toxicity trial, one death was found in the 500, 1000 and 2000 g m-2 LMB treatments, respectively. In a 7-day feeding trial, the feeding rate of G. pulex in the Low, Middle and High-LMB treatments was lower than that in the control by 19 %, 59 % and 55 %, respectively. In a 21-day survival trial, the survival rate of G. pulex in the control was significantly higher than in Low, Middle and High-LMB treatments regardless of the presence of sediment. Our results show that G. pulex preferred a substrate environment with gap and shelter. There was no acute toxicity of LMB to G. pulex, but LMB covered the leaves, thus reducing its feeding rate on leaves. In addition, with food restrictions and unwelcome changes in the environment by LMB, intraspecific competition and cannibalism of G. pulex might be enhanced, resulting in a reduced survival rate of G. pulex under LMB application. Hence, our study indicates that extended experimental studies and in situ monitoring of macroinvertebrates after LMB applications are needed.

Control of cyanobacterial blooms with iron addition can favor stress-tolerant toxic species

The control of internal phosphorus (P) load by in-lake measures has been the subject of decades of research. Although iron (Fe) is effective in precipitating P, it has been less tested due to its redox sensitivity. The effectiveness of Fe in controlling P availability and sinking cyanobacterial blooms contrasts to its function as a nutrient for phytoplankton growth. Both roles of Fe were tested in enclosures placed for 36 days in a shallow lake with a perennial cyanobacterial bloom, and in laboratory experiments with Fe-deficient Raphidiopsis raciborskii. Based on total P (TP) of lake water, we applied two doses of FeCl3, corresponding to 30:1 (16 mg Fe L-1) and 90:1 (47 mg Fe L-1) (Fe:P molar), to cause P precipitation, and flocculation and sinking of cyanobacterial populations. Three enclosures per treatment and three without FeCl3 additions (control) were used. The 90:1 treatment sank the main cyanobacterial biomass (50-fold) dominated by Planktothrix agardhii, with a significant decrease in turbidity, chlorophyll a and TP, without lasting decrease in pH, and achieved mesotrophic-like conditions. However, signs of recovery of R. raciborskii were detected between days 3 and 14. In laboratory experiments, Fe-deficient R. raciborskii MVCC19 grown under nitrate availability (+N) and N2-fixation (-N) were exposed to five FeCl3 concentrations from 0 to 17.9 (10:1) mg Fe L-1 (Fe:P molar). A remarkable tolerance to high Fe was found at concentrations 7-fold higher than culture medium and decreased under -N. Also R. raciborskii stood low Fe levels in +N and exhibited higher Fe requirements under N2-fixation. The increase in trichome length suggests resistance to the stressor, with shorter trichomes in -N. Therefore, effective management of R. raciborskii requires additional control of N in lakes. Our results point out the double role of Fe applications in which stress-tolerant species may become dominant under the reoligotrophication scenario.

External nitrogen influxes hinder the efficacy of lanthanum-modified bentonite (LMB) on phosphorus and algae control in shallow lakes

Regulating internal and external phosphorus (P) holds a predominant position in eutrophication management of lakes and other water bodies, with less emphasis on controlling nitrogen (N) due to the presence of N2-fixing cyanobacteria. Nonetheless, external N influxes may stimulate the proliferation of non-N2-fixing cyanobacteria, thereby fostering cyanobacteria blooms during summer seasons. To elucidate the significance of N regulation, a two-factor orthogonal experiment was performed to study the influences of external N input on the efficacy of lanthanum-modified bentonite (LMB), a sediment capping material for P immobilization. At the experimentation ends, the total suspended solids (TSS), organic suspended solids (OSS) concentrations and optical attenuation coefficient (Kd) in the LMB + N treatment were 7.34, 8.65 and 5.20 times higher, respectively, compared to the LMB treatment. The total nitrogen (TN), total phosphorus (TP) and soluble reactive phosphorus (SRP) concentrations in the LMB + N treatment were 3.02, 1.30 and 0.60 times higher, respectively, than those in the LMB treatment. However, TP and SRP in the LMB + N treatment were 46.98% and 54.93% lower, respectively, compared to N treatment. The chlorophyll a (Chl a) concentration of algae in the LMB + N treatment was observed to be 2.86 times higher compared to the LMB treatment, and 17.13% lower compared to the N treatment. The biomass of cyanobacteria accounted for more than 95% of algae in the LMB + N treatment and N treatment. Furthermore, the photosynthetic performance of algae in the N treatment increased significantly, compared to the LMB + N treatment. Our results indicated that external N influxes significantly reduce the efficacy of LMB to control P and algae. Thus, the implementation of more stringent N control policies holds great significance in the eutrophication control.

Combining effects of submerged macrophytes and lanthanum-modified bentonite on sediment enzyme activity: Evidence from mesocosm study

Lanthanum-modified bentonite (LMB) combined with submerged macrophytes (SM) has been a conventional means of eutrophication management in lakes in recent years, and is one of the most important methods for P removal. However, trends in nutrients and sediment enzymes at the water-sediment interface during this process have not been systematically assessed, and there are still some gaps in how abiotic properties drive changes in enzyme activity. Here, we show changes in aquatic environmental conditions under the action of different ratios of modified bentonite (0, 10%, 20%, and 30%) in combination with SM (Vallisneria natans, Potamogeton lucens, and Hydrilla verticillate) and quantify their effects on sediment enzyme activities. The results showed that the nutrient cycling at the water-sediment interface was facilitated by the combined effect of SM and LMB, which effectively reduced the overlying water nutrient concentration, increased the sediment enzyme activity and enhanced the N cycling process. Partial least squares structural equation model (PLS-SEM) showed that sediment parameters strongly influenced changes in enzyme activity, with NO3-N as the main controlling factors. Our study fills in the process of changing environmental conditions in lake water under geoengineered materials combined with macrophyte measures, especially the changes in biological properties enzyme activities, which contributes to a clearer understanding of nutrient fluxes during the management of eutrophication in lakes.

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

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

Recent advances of application of bentonite-based composites in the environmental remediation

Bentonite-based composites have been widely utilized in the removal of various pollutants due to low cost, environmentally friendly, ease-to-operate, whereas the recent advances concerning the application of bentonite-based composites in environmental remediation were not available. Herein, the modification (i.e., acid/alkaline washing, thermal treatment and hybrids) of bentonite was firstly reviewed; Then the recent advances of adsorption of environmental concomitants (e.g., organic (dyes, microplastics, phenolic and other organics) and inorganic pollutants (heavy metals, radionuclides and other inorganic pollutants)) on various bentonite-based composites were summarized in details. Meanwhile, the effect of environmental factors and interaction mechanism between bentonite-based composites and contaminants were also investigated. Finally, the conclusions and prospective of bentonite-based composites in the environmental remediation were proposed. It is demonstrated that various bentonite-based composites exhibited the high adsorption/degradation capacity towards environmental pollutants under the specific conditions. The interaction mechanism involved the mineralization, physical/chemical adsorption, co-precipitation and complexation. This review highlights the effect of different functionalization of bentonite-based composites on their adsorption capacity and interaction mechanism, which is expected to be helpful to environmental scientists for applying bentonite-based composites into practical environmental remediation.

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.

Fe(II)Cl2 amendment suppresses pond methane emissions by stimulating iron-dependent anaerobic oxidation of methane

Aquatic ecosystems are large contributors to global methane (CH4) emissions. Eutrophication significantly enhances CH4-production as it stimulates methanogenesis. Mitigation measures aimed at reducing eutrophication, such as the addition of metal salts to immobilize phosphate (PO43-), are now common practice. However, the effects of such remedies on methanogenic and methanotrophic communities-and therefore on CH4-cycling-remain largely unexplored. Here, we demonstrate that Fe(II)Cl2 addition, used as PO43- binder, differentially affected microbial CH4 cycling-processes in field experiments and batch incubations. In the field experiments, carried out in enclosures in a eutrophic pond, Fe(II)Cl2 application lowered in-situ CH4 emissions by lowering net CH4-production, while sediment aerobic CH4-oxidation rates-as found in batch incubations of sediment from the enclosures-did not differ from control. In Fe(II)Cl2-treated sediments, a decrease in net CH4-production rates could be attributed to the stimulation of iron-dependent anaerobic CH4-oxidation (Fe-AOM). In batch incubations, anaerobic CH4-oxidation and Fe(II)-production started immediately after CH4 addition, indicating Fe-AOM, likely enabled by favorable indigenous iron cycling conditions and the present methanotroph community in the pond sediment. 16S rRNA sequencing data confirmed the presence of anaerobic CH4-oxidizing archaea and both iron-reducing and iron-oxidizing bacteria in the tested sediments. Thus, besides combatting eutrophication, Fe(II)Cl2 application can mitigate CH4 emissions by reducing microbial net CH4-production and stimulating Fe-AOM.

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

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

Combining lanthanum-modified bentonite and calcium peroxide to enhance phosphorus removal from lake sediments

Lanthanum-modified bentonite (LMB) and calcium peroxide (CP) are known for their effective removal phosphorus (P) capacities. The present study aims to investigate the effects of the combined use of LMB and CP（LMB + CP）on the sediment P, dissolved organic matter (DOM) and iron (Fe) concentrations through a 90-day incubation experiment. The combined treatment showed strong removal effects on sediment P and DOM. Indeed, the SRP and DOM concentrations in the 0-10 cm sediment layer decreased following the combined application of LMB and CP by 40.67 and 28.95%, respectively, compared to those of the control group (CK). In contrast, the HCl-P in the 0-5 cm sediment layer increased following the combined treatment by 13.28%. In addition, compared with the single application of LMB, the LMB + CP treatment significantly reduced the soluble Fe (Ⅱ) in the sediment pore water and promoted the oxidation of Fe. Therefore, LMB + CP can enhance the removal of internal P from sediments. The DOM removal and Fe oxidation in sediment pore waters are beneficial for enhancing the adsorption of P by LMB. On the other hand, the single and combined applications of LMB and CP increased the richness of the sediment microbial communities while exhibiting slight effects on their diversity. According to the results of this study, the combined use of LMB and oxidizing materials represents a novel method for treating lakes with high internal phosphorus and DOM loads in sediments.

Urban lake water quality responses to elevated road salt concentrations

Road salt runoff from de-icing applications is increasingly impacting water quality around the globe. Excess salt (especially chloride) concentrations can negatively impact the biological, chemical, and physical properties of freshwater ecosystems. Though road salt pollution is a prevalent issue affecting many northern temperate lakes, there are few studies on how freshwater salinization interacts with other ecological stressors such as eutrophication. We investigated how chloride from road deicers influences water quality in an urban lake. We sampled a tributary and lake receiving large amounts of road salt runoff from a nearby highway in Grand Rapids, Michigan over a 20-month period. Chloride concentrations in the deepest part of the lake consistently exceeded the US EPA chloride chronic toxicity threshold of 230 mg/L, at times reaching up to 331 mg/L. These high chloride concentrations appear to be responsible for preventing part of the lake from complete mixing, and causing hypoxia in the deepest regions of the lake. Total phosphorus concentrations near the surface averaged 35 μg/L but exceeded 7500 μg/L in the deepest part of the lake, which occupies 3-5 % of total lake volume. Phosphorus release rates from the sediments were low and unlikely to be a current source of the high phosphorus concentrations. Rather, both phosphorus and chloride likely have been accumulating in the hypolimnion over a relatively long period of time. Lake management actions will require control of both internal and external phosphorus and chloride sources in the future. We recommend that phosphorus be addressed first to avoid the extremely high phosphorus concentrations from reaching the photic zone and stimulating algal blooms, which would occur if salt was removed first and the halocline broke down. Our findings and recommendations are applicable to other lakes facing similar issues.