A combined study on Vallisneria spiralis and lanthanum modified bentonite to immobilize arsenic in sediments

The transition from macrophyte-dominated to algae-dominated lake systems enhances arsenic release from sediments

Declining macrophytes in eutrophic lakes are altering material cycling in sediments. However, the transformation of arsenic (As) in response to these changes remains poorly understood. In this study, high-resolution dialysis was used to measure dissolved As in sediments from macrophyte-dominated (MD) and algae-dominated (AD) zones across different seasons. The relationship between sedimentary As fractionation and environmental variations was analyzed, and the As transformation process was explored. Results showed that the shift from macrophyte- to algae-dominated zones enhanced As release in sediments. Dissolved As in pore water of AD peaked at 120.36 μg/L in summer, exhibiting the highest release intensity, while MD showed a notable As release profile in spring (34.92 μg/L). In spring, decomposition and acidification of macrophyte residues, along with organic matter (OM) complexation, promoted the release of adsorbed As in MD. In contrast, reduction and dissolution of iron (Fe) oxides, along with competition for adsorption sites by dissolved phosphorus (P), drove As release in AD during summer. The high humification and low redox potential in MD sediments in summer promoted As-S co-precipitation, leading to As sequestration instead of release, this contrasts with the common view that warmer temperatures favor As release from sediments. The conversion from macrophytes to algae in eutrophic lakes may exacerbate the risk of As release, warranting further investigation.

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.

Tungsten migration and transformation characteristics in lake sediments under changing habitats from algae to macrophytes

Tungsten (W), a toxic and hazardous pollutant, poses substantial risks to both aquatic life and human health. However, the available understanding of the migration properties of W in lake sediments under various habitats is still limited. This study was designed to evaluate variations in the concentrations of soluble W, manganese (Mn), and iron (Fe) in the summer season by applying a high-resolution Peeper sampling device. According to the results, soluble W concentrations and release fluxes were higher in the pore water of sediments in algae-dominated lake areas than in areas dominated by aquatic plants. This result indicates that the competition for adsorption between algae-derived dissolved organic matter and W, as well as the reductive dissolution caused by dissolved organic matter on Fe (III)/Mn (IV) (hydroxyl) oxides, contributes to the release of W from lake sediments. W uptake by aquatic plants and in-situ formation of Fe (III)/Mn (IV) (hydroxyl) oxides might be the primary factor that controls W release from lake sediments. Aquatic plants can effectively control W release from sediments. The findings of this work provide a scientific basis for the effective control of W release from shallow lake sediments.

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.

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.

Simultaneous immobilization of sediment internal phosphorus, arsenic and tungsten by lanthanum carbonate capping

In this study, lanthanum carbonate (LC) was selected as a capping agent to examine its effectiveness in immobilizing sediment internal phosphorus (P), arsenic (As) and tungsten (W). With a 180-day incubation experiment, it was determined that LC capping efficiently reduced the concentrations of soluble reactive P (SRP), soluble As and soluble W in pore water, with the highest reduction rate of 83.39%, 56.21% and 68.52%, respectively. The primary mechanisms involved in the adsorption of P, As and W by LC were precipitation reactions and ligand exchange. Additionally, P, As and W were immobilized by LC capping through the transformation of fractions from mobile and less stable forms to more stable forms. Furthermore, LC capping led to an increase in the Eh value, which promoted the oxidation of soluble Fe (Ⅱ) and soluble Mn. The significantly positive correlation and synchronized variations observed between SRP, soluble As, soluble W, and soluble Fe (II) indicated that the effects of LC on Fe redox played a crucial role in immobilizing sediment internal P, As and W. However, the oxidation of Mn, promoted by LC, played a more significant role in immobilizing sediment internal As than P and W. These effects resulted in LC capping achieving the highest reduction of SRP, soluble As and soluble W flux at 145.22, 22.19, and 0.58 μg m-2d-1. It is of note that LC capping did not lead to an elevated release hazard of Co, Ni, Cu, and Pb, barring Cd. Besides, LC capping did not modify the entire microbial communities in the sediment, but altered the proportional representation of specific microorganisms. Generally, LC has potential as a capping agent capable of simultaneously immobilizing sediment internal P, As and W.

Degradation of algae promotes the release of arsenic from sediments under high-sulfate conditions

Sulfate concentrations in eutrophic waters continue to increase; however, the transformations of arsenic (As) in sediments under these conditions are unclear. In this study, we constructed a series of microcosms to investigate the effect of algal degradation on As transformations in sediments with high sulfate concentrations. The results showed that both the elevated sulfate levels and algal degradation enhanced the release of As from sediments to the overlying water, and degradation of algal in the presence of elevated sulfate levels could further contribute to As release. Sulfate competed with arsenate for adsorption in the sediments, leading to As desorption, while algal degradation created a strongly anaerobic environment, leading to the loss of the redox layer in the surface sediments. With high sulfate, algal degradation enhanced sulfate reduction, and sulfur caused the formation of thioarsenates, which may cause re-dissolution of the arsenides, enhancing As mobility by changing the As speciation. The results of sedimentary As speciation analysis indicated that elevated sulfur levels and algal degradation led to a shift of As from Fe2O3/oxyhydroxide-bound state to specifically adsorbed state at the sediment water interface. This study indicated that algal degradation increases the risk of As pollution in sulfate-enriched eutrophic waters.