Effects of elevated sulfate in eutrophic waters on the internal phosphate release under oxic conditions across the sediment-water interface

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.

Cable bacteria: widespread filamentous electroactive microorganisms protecting environments

Cable bacteria have been identified and detected worldwide since their discovery in marine sediments in Aarhus Bay, Denmark. Their activity can account for the majority of oxygen consumption and sulfide depletion in sediments, and they induce sulfate accumulation, pH excursions, and the generation of electric fields. In addition, they can affect the fluxes of other elements such as calcium, iron, manganese, nitrogen, and phosphorous. Recent developments in our understanding of the impact of cable bacteria on element cycling have revealed their positive contributions to mitigating environmental problems, such as recovering self-purification capacity, enhancing petroleum hydrocarbon degradation, alleviating phosphorus eutrophication, delaying euxinia, and reducing methane emission. We highlight recent research outcomes on their distribution, state-of-the-art findings on their physiological characteristics, and ecological contributions.

Anion-exchange facilitated selective extraction of sulfate and phosphate by overcoming the Hofmeister bias

Selective recognition and removal of sulfate and phosphates from aqueous media in the presence of highly competing anions is very demanding because of their biological and environmental implications. In this paper, we present the anion recognition approach for the selective and efficient extraction of sulfate by nitrophenyl-functionalized tris-urea receptors (L₁-L₂) from highly competitive aqueous media with an equivalent concentration of nitrate and other anions. Tetrabutylammonium hydroxide has been used for the first time as a phase transfer anionic extractant for sulfate-exchange from the aqueous phase to the organic phase (dichloromethane) containing a tris-urea receptor (L₁-L₃). The sulfate extraction efficacy of L₂ (≈84-90%) was observed to be higher than those of L₁ (≈76-82%) and L₃ (≈68-75%) in competitive extraction experiments. In contrast, an analogous nitrophenyl-functionalized tris-thiourea receptor (L₄) has been recognized for the selective and efficient extraction of phosphates from aqueous media in the presence of several competing anions including sulfate and nitrate, with ≈85-92% extraction efficiency. In this case, tetrabutylammonium acetate has been used as a phase transfer anionic extractant for phosphate exchange between the two immiscible phases. Due to the higher acidity of tris-thiourea -NH groups in comparison to the analogous tris-urea, tetrabutylammonium hydroxide could deprotonate a hydrogen bond donating -NH group of the thiourea receptor and phosphate extraction was observed to be inefficient in such a case. Several liquid-liquid extraction (LLE) experiments have been carried out to establish the selective removal of sulfate and phosphates by the tripodal receptors from competitive aqueous media having different combinations of two or more anions. The LLE products obtained from organic phases were characterized by NMR (¹H, ¹³C, ³¹P, and ¹⁹F) spectroscopy to affirm the oxoanion selectivity of the receptors and purity of the complexes. The tripodal receptors can easily be recycled for successive extraction cycles by simply washing the LLE products (oxoanion complexes) with a methanol-water (1 : 1, v/v) solvent system followed by filtration.

Phosphorus immobilisation in sediment by using iron rich by-product as affected by water pH and sulphate concentrations

Iron (Fe) rich by-product from drinking water treatment plants can be added to rivers and lakes to immobilise phosphorus (P) in sediment and lower eutrophication risks. This study was set up to investigate the P immobilisation efficiency of an Fe rich by-product as affected by the pH and sulphate (SO₄) concentration in the overlying water. Both factors are known to inhibit long-term P immobilisation under anoxic conditions. A static sediment-water incubation was conducted at varying buffered water pH values (6, 7 and 8) and different initial SO₄ concentrations (0-170 mg SO₄ L^-1) with or without Fe rich by-product amendment to the sediment. In the unamended sediment, the P release to the overlying water was highest, and up to 6 mg P L^-1, at lowest water pH due to higher reductive dissolution of Fe(III) oxyhydroxides. The Fe rich by-product amendment to the sediment largely reduced P release from sediment by factors 50-160 depending on pH, with slightly lowest immobilisation at highest pH 8, likely because of pH dependent P sorption. The total sulphur (S) concentrations in the overlying water reduced during incubation. The P release in unamended sediments increased from 2.7 mg L^-1 to 4.2 mg L^-1 with higher initial SO₄ concentrations, suggesting sulphide formation during incubation and FeS precipitation that facilitates release of P. However, no such SO₄ effects were found where Fe rich by-product was applied that lowered P release to <0.1 mg L^-1 illustrating high stability of immobilised P in amended sediments. This study suggests that Fe rich by-product is efficient for P immobilisation but that loss of Fe in low pH water may lower its long-term effect.

Joint role of land cover types and microbial processing on molecular composition of dissolved organic matter in inland lakes

Anthropogenic activities have greatly changed the land use and land cover (LULC) and further influenced the chemical properties and amount of DOM transported into aquatic systems, meanwhile, microbial processing is also critical to DOM molecular composition in freshwaters. However, how they jointly shape DOM's chemical composition and chemodiversity in lakes is poorly understood. Here we examined DOM characteristics for seven inland lakes with three different land cover conditions (forest-dominated, cropland-dominated, and urban-dominated). Results indicated that DOM in cropland-dominated and forest-dominated lakes exhibited more characteristics of terrestrial organic matter, while urban-dominated lakes had more allochthonous organic matter driven by relatively high nutrient input. Human activities extended terrestrial DOM input to lakes and intensified the amount of heteroatomic organic molecules containing nitrogen and sulfur in lakes, with cropland contributing more N-containing compounds and urban contributing more S-containing compounds. Differential bacterial community composition appeared in the three types of land cover lakes, while strong co-occurrence/exclusion patterns between specific microbes and molecular formula groups revealed the key DOM metabolism functions of these bacteria. Matrix correlations based on Mantel tests confirmed that watershed landcover status was a dominating factor for DOM sources and molecular composition in mountainous lakes through direct input of terrestrial organic matter, and microbial processing was not the key factor for DOM molecular formula. Our findings help to assess the influence of human activities and microbial processing in the transfer and transformation of DOM in environmental waters.

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

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