Dissolved organic matter and Fe/Mn enhance the combination and transformation of As in Lake Chaohu Basin

Effects of Oxygenation Resuspension on DOM Composition and Its Role in Reducing Dissolved Manganese in Drinking Water Reservoirs

Anaerobic conditions in source water sediments are a key driver of manganese (Mn) release in drinking water systems. Enhancing sediment oxidation can inhibit Mn release, but the mechanisms of Mn speciation under varying oxidative conditions remain unclear. This study examined sediment exposure to oxygenated water layers at controlled dissolved oxygen levels (0, 2, 5, 7 mg L-1) through laboratory simulations. Results showed Mn release is negatively correlated with DO (R2 = 0.93, p = 0.034), with oxygen driving reactions between dissolved organic matter (C2 and C3 components) and forming functional groups (-OH, -COOH) that remove Mn through adsorption or complexation (C2: R2 = 0.57, p < 0.001; C3: R2 = 0.53, p < 0.001). Field studies in six reservoirs identified operational thresholds for sediment resuspension to mitigate Mn risks (compensation threshold: 17.4 μg L-1; risk threshold: China: 95.5 μg L-1; WHO: 70.8 μg L-1). These findings clarify Mn-organic matter interactions and can provide practical guidance for Mn and algae removal in source water systems.

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.

Immobilization or mobilization of heavy metal(loid)s in lake sediment-water interface: Roles of coupled transformation between iron (oxyhydr)oxides and natural organic matter

Iron (Fe) (oxyhydr)oxides and natural organic matter (NOM) are active substances ubiquitously found in sediments. Their coupled transformation plays a crucial role in the fate and release risk of heavy metal(loid)s (HMs) in lake sediments. Therefore, it is essential to systematically obtain relevant knowledge to elucidate their potential mechanism, and whether HMs provide immobilization or mobilization effect in this ternary system. In this review, we summarized (1) the bidirectional effect between Fe (oxyhydr)oxides and NOM, including preservation, decomposition, electron transfer, adsorption, reactive oxygen species production, and crystal transformation; (2) the potential roles of coupled transformation between Fe and NOM in the environmental behavior of HMs from kinetic and thermodynamic processes; (3) the primary factors affecting the remediation of sediments HMs; (4) the challenges and future development of sediment HM control based on the coupled effect between Fe and NOM from theoretical and practical perspectives. Overall, this review focused on the biogeochemical coupling cycle of Fe, NOM, and HMs, with the goal of providing guidance for HMs contamination and risk control in lake sediment.

Arsenic distribution characteristics and release mechanisms in aquaculture lake sediments

It is well known that aquaculture can alter the microenvironments of lakes at sediment-water interface (SWI). However, the main mechanisms underlying the effects of aquaculture activities on arsenic (As) transformations are still unclear. In this context, the present study aims to investigate the variations in the sediment As contents in Yangcheng Lake, as well as to assess its chemical transformations, release fluxes, and release mechanisms. The results showed substantial spatial differences in the dissolved As concentrations in the sediment pore water. The As release fluxes at the SWI ranged from 1.32 to 112.09 μg/L, with an average value of 33.68 μg/L. In addition, the highest As fluxes were observed in the aquaculture areas. The transformation of crystalline hydrous Fe oxide-bound As to adsorbed-As in the aquaculture lake sediments increased the ability of As release. The Partial least squares path modeling results demonstrated the great contributions of organic matter (OM) to the As transformations by influencing the sediment microbial communities and Fe/Mn minerals. The changes in the As fractionation and competing adsorption increased the dissolved As concentrations in the 0-10 mm surface sediment. Non-specifically and specifically adsorbed As were the major sources of dissolved As in the sediments. Specifically, microbial reduction of As[V] and dissolution of Fe oxides increased the dissolved As concentrations at the SWI (20 to -20 mm). The results of the current study highlight the positive enhancement effects of aquaculture on As release from sediments.