Equilibrium partitioning behaviors and remobilization of trace metals in the sediment profiles in the tributaries of the Three Gorges Reservoir, China

In situ high-resolution insights into the dynamics of arsenic (As) species and heavy metals across the sediment-water interface in a deep karst reservoir

Arsenic (As) and heavy metal contamination in aquatic systems pose critical environmental challenges, particularly in reservoirs. This study utilized dual-sided high-resolution diffusive gradients in thin films (DGT) probes on-site to investigate the spatial distribution and mobility of As species and heavy metals (Cd, Cr, Cu, Ni, Pb, Sb, and Zn) in the Hongfeng Reservoir, a deep karst reservoir in southwest China. Results revealed that As mobility was primarily governed by redox-sensitive processes, including the reduction of As(V) to As(III) and the reductive dissolution of Fe/Mn oxides. As(III) dominated porewater under reducing conditions, while As(V) was prevalent in overlying water under oxidative environments. Sulfate reduction significantly influenced As mobility, and competitive adsorption with P enhanced As release in eutrophic conditions. Heavy metals exhibited distinct spatial profiles and inter-element correlations, shaped by redox variability. Flux analysis identified sediments as sources for As, Fe, Mn, P, and S, and as sinks for most heavy metals. As(III) fluxes in the North Central reflected strong reducing conditions, while As(V) fluxes in the South Central highlighted localized oxidative processes. These findings offer valuable insights into geochemical processes in karst reservoirs, aiding in the understanding of contaminant dynamics and providing guidance for managing sediment pollution and protecting water quality.

Tracing sources-oriented ecological risks of metal(loid)s in sediments of anthropogenically-affected coastal ecosystem from northeast bay of Bengal

The current study focused on thirty-nine locations in the four islands (i.e., St. Martin, Moheskhali, Kutubdia, and Sonadia) and beach (Innani Beach) along the northeast Bay of Bengal to quantify sources-orientated ecological risks of metal(loid)s. The mean concentrations of As, Mn, Cr, Cd, and Pb are 4.8, 8.7, 1.6, 1.1, and 2 times higher than average shale volume (ASV) values. Key findings revealed that Mn, Cr, Cd, Pb, and As exceed safe levels, particularly on St. Martin and Moheshkhali islands, where tourism and coal mining intensify contamination. Ecological indexes showed moderate to considerable contamination levels, suggesting diverse impacts on aquatic life. Positive matrix factorization (PMF) model-based Nemerow integrated risk index (NIRI) indicated that mixed and coal mining sources posed a moderate risk for 10.26 % and 5.13 % of sediment samples, respectively. This paper serves as a model-based plan for mitigating pollution risks of metal(oid)s in coastal sediments on the northeast coast.

Arsenic mobilization across the sediment-water interface of the Three Gorges Reservoir as a function of water depth using DGT and HR-Peepers, a preliminary study

The artificial regulation of the Three Gorges Reservoir (TGR) creates large water level fluctuation zones (WLFZ) that may change the behavior of metals and metalloid in sediment, particularly redox sensitive elements. Mobilization of As, Fe and Mn across the sediment-water interface (SWI) in the TGR as a function of different water depth (periodically and permanently submerged sediments, respectively) was in situ determined by diffusive gradients in thin films (DGT) and high-resolution dialysis technique (HR-Peeper), respectively. The results showed that the mobilization of As was significantly affected by Fe/Mn especially Mn, across the SWI. Duo to the oxic-anoxic transitional state in near bottom water, the reduced Fe and Mn in sediment pore water could be oxidized and precipitated again, leading to the co-precipitation of As with Fe/Mn oxides (hydroxides). Consequently, concentrations of As, Fe and Mn in labile phases and pore water were generally low across the SWI, then they sharply increased at a few centimeters below the SWI. Considering different water depth, various trends were found in labile phase, whereas concentrations of As, Fe and Mn in pore water in permanently submerged sediments were significantly higher than those in periodically submerged sediments. The dry-re-wetting alternation processes in the WLFZ may play vital roles in the resupply capacity of sediments as it was found that periodically submerged sediments with longer re-wetting time had higher Fe/Mn resupply capacity than those with shorter re-wetting times and permanently submerged sediments.

Principles, applications, and limitations of diffusive gradients in thin films induced fluxed in soils and sediments

The diffusive gradients in thin films (DGT) technique serves as a passive sampling method, inducing analyte transport and concentration. Its application is widespread in assessing labile components of metals, organic matter, and nutrients across various environmental media such as water, sediments, and saturated soils. The DGT devices effectively reduce the porewater concentration through irreversible binding of solutes, consequently promoting the release of labile species from the soil/sediment solid phase. However, the precise quantification of simultaneous adsorption and desorption of labile species using DGT devices alone remains a challenge. To address this challenge, the DGT-Induced Fluxes in Soils and Sediments (DIFS) model was developed. This model simulates analyte kinetics in solid phases, solutions, and binding resins by incorporating factors such as soil properties, resupply parameters, and kinetic principles. While the DIFS model has been iteratively improved to increase its accuracy in portraying kinetic behavior in soil/sediment, researchers' incomplete comprehension of it still results in unrealistic fitting outcomes and an oversight of the profound implications posed by kinetic parameters during implementation. This review provides a comprehensive overview of the optimization and utilization of DIFS models, encompassing fundamental concepts behind DGT devices and DIFS models, the kinetic interpretation of DIFS parameters, and instances where the model has been applied to study soils and sediments. It also highlights preexisting limitations of the DIFS model and offers suggestions for more precise modeling in real-world environments.

Remobilization characteristics and diffusion kinetic processes of sediment zinc (Zn) in a tidal reach of the Pearl River Estuary, South China

Exploration of the remobilization mechanism of trace metals in estuarine sediments remain challenging because of dynamic hydrochemical conditions. This study integrated a chemical sequential extraction procedure (BCR), the diffusive gradient in thin films (DGT) and high-resolution dialysis techniques, and Visual MINTEQ ver.3.1 to identify the seasonal mobilization characteristics of sediment Zn within a tidal reach, South China. The mobility of sediment Zn based on the BCR procedure contradicted the results of DGT analysis. In summer, reductive dissolution of Fe/Mn oxides was the key driver of sediment Zn remobilization; during winter, cation exchange reactions facilitated the mobilization of Zn in the brackish water zone. The time-dependence ratios of DGT-labile Zn and dissolved Zn concentrations (mean: 0.34-0.81) indicated the sediment solid phase had partially sustained capacity to resupply Zn to the porewater in both seasons. Sediments generally functioned as a source of Zn in the freshwater zone with organically complexed Zn being diffusively released into the water column at rates of 0.3-15.5 μg·m^-2·d^-1. In the brackish water zone, the dominant Zn species were transformed into free Zn ions and Zn-inorganic complexes and migrated into sediment, with respective influxes of 18.9-70.7 μg·m^-2·d^-1 and 18.9-68.3 μg·m^-2·d^-1, which shifted to a sink of Zn.