Evaluating the mobility and labile of As and Sb using diffusive gradients in thin-films (DGT) in the sediments of Nansi Lake, China

Application of high-resolution techniques in the assessment of the mobility of Cr, Mo, and W at the sediment-water interface of Nansi Lake, China

There are few studies on the simultaneous behavior of chromium (Cr), molybdenum (Mo), and tungsten (W) belonging to group VIB of the periodic table. Herein, based on high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) technology, the vertical distribution characteristics of DGT-labile and soluble Cr, Mo, and W in two lakes of Nansi Lake (Weishan Lake and Dushan Lake) were analyzed. In addition, the net diffusion fluxes and R-value (CDGT/Csol) were used to evaluate the mobility and release risk of metals at the sediment-water interface. The results showed that the DGT-labile concentrations of the three metal elements (Cr, Mo, and W) in Weishan Lake were higher than those in Dushan Lake, both in overlying water and sediment. This is mainly due to the dredging of the Dushan Lake area, which can permanently remove the polluted sediment in the lake. Meanwhile, the exogenous input is relatively high near the tourist area of Weishan Island. The net diffusion fluxes indicate that the W has a potential release risk of diffusion to the overlying water in Dushan Lake. The release of Cr, Mo, and W is thought to be related to the reductive dissolution of Fe/Mn (hydr)oxides based on Pearson correlation coefficients. The R-values of Cr and W indicate that Cr and W belong to the partial continuity case. The R-value of Mo was lower than the minimum value, meaning that Mo belongs to the single diffusion type and it is difficult for Mo sediments to supply pore water.

Arsenic migration at the sediment-water interface of anthropogenically polluted Lake Yangzong, Southwest China

The lability and controlling factors of arsenic (As) at the sediment-water interface (SWI) are crucial for understanding As behaviors and fates in As-contaminated areas. In this study, we combined high-resolution (5 mm) sampling using diffusive gradients in thin films (DGT) and equilibrium dialysis sampling (HR-Peeper), sequential extraction (BCR), fluorescence signatures, and fluorescence excitation-emission matrices (EEMs)-parallel factor analysis (PARAFAC) to explore the complex mechanisms of As migration in a typical artificially polluted lake, Lake Yangzong (YZ). The study results showed that a high proportion of the reactive As fractions in sediments can resupply pore water in soluble forms during the change from the dry season (winter, oxidizing period) to the rainy season (summer, reductive period). In dry season, the copresence of Fe oxide-As and organic matter (OM)-As complexes was related to the high dissolved As concentration in pore water and limited exchange between the pore water and overlying water. In the rainy season, with the change in redox conditions, the reduction of Fe-Mn oxides and OM degradation by microorganisms resulted in As deposition and exchange with the overlying water. Partial least squares path modelling (PLS-PM) indicated that OM affected the redox and As migration processes through degradation. Based on comprehensive analyses of the As, Fe, Mn, S and OM levels at the SWI, we suggest that the complexation and desorption of dissolved organic matter and Fe oxides play an important role in As cycling. Our findings shed new light on the cascading drivers of As migration and OM features in seasonal lakes and constitute a valuable reference for scenarios with similar conditions.

Remobilization of legacy arsenic from sediment in a large subarctic waterbody impacted by gold mining

Arsenic contamination from mining poses an environmental challenge due to the mobility of this redox-sensitive element. This study evaluated arsenic mobility in sediments of Yellowknife Bay (Canada), a large subarctic water body impacted by gold mining during the 20th century. Short-term measurements of arsenic flux from sediment, arsenic profiling of the water column and sediment porewater, and mass balance modelling were conducted to assess the importance of sediment as an arsenic source. Sediment arsenic fluxes were highly variable throughout Yellowknife Bay and ranged from - 65-1520 µg m^-2 day^-1. Elevated fluxes measured near the mine site were among the highest published for well-oxygenated lakes. Redox boundaries were typically 2-3 cm below the sediment surface as indicated by porewater profiles of iron, manganese, and arsenic, with arsenic maxima of 65-3220 µg L^-1 predominately as arsenite. Sediment arsenic flux was positively related to its solid-phase concentration. Modelling indicated sediment was a principal source of arsenic to the water column. Adsorption and precipitation processes in the oxidizing environment of near-surface sediments did not effectively attenuate arsenic remobilized from contaminated sediments. Internal recycling of legacy arsenic between sediment and surface water will impede a return to background conditions in Yellowknife Bay for decades.

Simultaneous measurement of labile As (III) and As (V) in soils combining DGT and HPLC-ICP-MS

The speciation of arsenic (As) determines its biogeochemistry and ecotoxicity in soils. However, the approach to in situ monitor labile As (III) and As (V) in soils still requires more exploration. In this study, we developed a method for simultaneously obtaining in-situ data on labile As (III) and As (V) in soils using diffusive gradients in thin films (DGT) and high performance liquid chromatography-inductively coupled plasma mass spectrometry. The Fe₂O₃∙xH₂O DGT sampler exhibited rapid and simultaneous accumulation of As (III) and As (V) in solutions within 90 min. The high efficiency of simultaneous elution of As (III) (~84 %) and As (V) (~97 %) was achieved using 0.8 % H₃PO₄ as eluent at 90 °C for 80 min. The method detection limits for As (III) and As (V) were 0.01 and 0.005 μg/L, respectively. This method was applied to reveal the labile As (III) and As (V) in soils in the water level fluctuation zones of the Three Gorges Reservoir, which is the largest reservoir in China. The concentrations of As (III) and As (V) measured by DGT varied with different sampling sites, ranging from 0.01 μg/L to 1.20 μg/L and from 0.01 μg/L to 0.26 μg/L, respectively. The labile As (III) exhibited the higher resupply rate from soil solid phase to soil solution than labile As (V). This study helps to achieve simultaneous in-situ quantification of labile As (III) and As (V) in soils, and will improve the understanding of As mobilization and ecotoxicity in soils.

Evaluating the release risk of potentially toxic elements from sediments in the New Zhuzhao River Estuary of Nansi Lake, using high-resolution technology and sequential extraction

Potentially toxic elements (PTEs) re-release from sediment is an essential process in the sediment-water interface (SWI), especially for the influent river estuary as an important accumulation site. In this study, the diffusive gradient in thin films (DGT), high-resolution dialysis (HR-peeper) technique, and BCR sequential extraction were employed to evaluate the release risk of PTEs (As, Cu, Pb, Zn, Cd) in the New Zhuzhao River Estuary of Nansi Lake. Results showed that Cd existed primarily in the non-residual fraction (accounting for 59.87%), and the residual fractions of As, Cu, Pb, and Zn accounted for a greater proportion (12.65 to 33.07%). The mobility of Cd was the highest with a risk assessment code of 33.53% reaching the medium risk category. The resupply capacity calculated by C_DGT/C_Dis showed that As was the largest, with an average value of 0.43, indicating the strongest release capacity of As from the sediment to pore water. Furthermore, the diffusive fluxes using DGT and HR-peeper showed that As possesses a much higher potential to release upward overlying water than other elements.

Partitioning behavior and ecological risk of arsenic and antimony in the sediment-porewater profile system in the Three Gorges Reservoir, China

Arsenic and antimony are widely distributed toxic metalloids in aquatic environments. However, their partitioning behaviors in the sediment profile remain not well understood. Here, partitioning behaviors, diffusive fluxes, as well as the ecological risks of As and Sb in the sediment-porewater profile system in the tributaries of the Three Gorges Reservoir (TGR) were investigated. As and Sb showed markedly different spatial variations in the longitudinal profiles of both porewater and sediment samples. Specifically, the concentration of As showed an accumulation trend with depth, while that of Sb showed a relatively complicated trend. Further, As showed lower sediment-porewater partitioning coefficient (K_d) values, suggesting that it had a relatively lower sediment affinity and a higher mobility than Sb. Its residual fraction (30%-60%) was also lower than that of Sb. This phenomenon could be attributed to the chemical fractions of the trace metals and the pH value of the sediments. Furthermore, the K_d values corresponding to As were influenced by both the residual fraction (r = 0.338, p < 0.05) and the exchangeable fraction (r = -0.643, p < 0.01), while those corresponding to Sb were only influenced by pH. Additionally, even though these two trace metals showed low ecological and mobility risks, the diffusive fluxes at the sediment-water interface suggested that the sediment acted as a source of As and a sink for Sb relative to the overlying water. This study indicated that As and Sb had different partitioning behaviors and release risks in the sediment-porewater profile system, enhanced the understanding the transport and fate of As and Sb in the aquatic environment.

Does sand mining affect the remobilization of copper and zinc in sediments? - A case study of the Jialing River (China)

It is generally accepted that the sand mining industry causes severe destruction in river basin environments. In this study, six sediment cores were collected, and sequential extraction was applied in conjunction with the diffusive gradients in the thin films (DGT) technique to explore the effect of sand mining on the remobilization of Cu and Zn in the sediments. The results showed that Cu and Zn were mainly bound in the residual fraction in the sediments. C_DGT-Cu/Zn in the sediments presented obvious increasing trends at the bottom (-9 to -12 cm) at the four sites that experienced sand mining and a decreasing trend at the sites with no sand mining disturbance. Cu and Zn also tended to be transported from the sediments to the overlying water at the four sand mining sites. A correlation analysis found that F1 and F3 correlated well with C_DGT-Cu/Zn, indicating that the water/exchangeable fraction and oxidized fraction were the main fractions that led to increases in DGT-labile Cu and Zn in the sediments. Further analysis found that the introduction of oxygen (O₂) was the main reason for the simultaneous release of sulfur (S), Cu and Zn in the sediments, as indicated by the "dark area" of AgI gel appearing at the same position as the "hot spot area" of Chelex gel. Two main sand mining effects on the release of Cu and Zn were hypothesized: (1) intense sand disturbance leads to the transfer of the water/exchangeable fraction (F1) to the DGT-labile fraction and (2) O₂ introduction promotes the reaction of stable sulfide (F3), thus transferring it to the DGT-labile fraction. The above results indicated that the sand mining industry should be paid much attention in the Jialing River, as it can obviously cause labile Cu and Zn release into the water.

Insights into the fate of antimony (Sb) in contaminated soils: Ageing influence on Sb mobility, bioavailability, bioaccessibility and speciation

The effect of long-term ageing (up to 700 days) on the mobility, potential bioavailability and bioaccessibility of antimony (Sb) was investigated in two soils (S1: pH 8.2; S2: pH 4.9) spiked with two Sb concentrations (100 and 1000 mg·kg^-1). The Sb mobility decreased with ageing as highlighted by sequential extraction, while its residual fraction significantly increased. The concentration of Sb (C_DGT), as determined by diffusive gradients in thin films (DGT), showed a reduction in potential contaminant bioavailability during ageing. The DGT analysis also showed that Sb-C_DGT after 700 days ageing was significantly higher in S1-1000 compared to S2-1000, suggesting soil pH plays a key role in Sb potential bioavailability. In-vitro tests also revealed that Sb bioaccessibility (and Hazard Quotient) decreased over time. Linear combination fitting of Sb K-edge XANES derivative spectra showed, as a general trend, an increase in Sb(V) sorption to inorganic oxides with ageing as well as Sb(V) bound to organic matter (e.g. up to 27 and 37% respectively for S2-100). The results indicated that ageing can alleviate Sb ecotoxicity in soil and that the effectiveness of such processes can be increased at acidic pH. However, substantial risks due to Sb mobility, potential bioavailability and bioaccessibility remained in contaminated soils even after 700 days ageing.

Transport and speciation of uranium in groundwater-surface water systems impacted by legacy milling operations

Growing worldwide concern over uranium contamination of groundwater resources has placed an emphasis on understanding uranium transport dynamics and potential toxicity in groundwater-surface water systems. In this study, we utilized novel in-situ sampling methods to establish the location and magnitude of contaminated groundwater entry into a receiving surface water environment, and to investigate the speciation and potential bioavailability of uranium in groundwater and surface water. Streambed temperature mapping successfully identified the location of groundwater entry to the Little Wind River, downgradient from the former Riverton uranium mill site, Wyoming, USA. Diffusive equilibrium in thin-film (DET) samplers further constrained the groundwater plume and established sediment pore water solute concentrations and patterns. In this system, evidence is presented for attenuation of uranium-rich groundwater in the shallow sediments where surface water and groundwater interaction occurs. Surface water grab and DET sampling successfully detected an increase in river uranium concentrations where the groundwater plume enters the Little Wind River; however, concentrations remained below environmental guideline levels. Uranium speciation was investigated using diffusive gradients in thin-film (DGT) samplers and geochemical speciation modelling. Together, these investigations indicate uranium may have limited bioavailability to organisms in the Little Wind River and, possibly, in other similar sites in the western U.S.A. This could be due to ion competition effects or the presence of non- or partially labile uranium complexes. Development of methods to establish the location of contaminated (uranium) groundwater entry to surface water environments, and the potential effects on ecosystems, is crucial to develop both site-specific and general conceptual models of uranium behavior and potential toxicity in affected ground and surface water environments.

A study on the occurrence of black water in reservoirs in Eucalyptus Plantation region

Tianbao reservoir in southern China (surrounded by Eucalyptus plantation) serves as a source of drinking water for the inhabitants. However, the reservoir water experiences black water (BW) of which the cause remains unclear. In this study, field observation and simulated laboratory experiment were conducted to understand the cause of the BW. The diffusive gradient in thin-film (DGT) device monitored the spatial changes in concentration of iron (Fe²⁺), manganese (Mn²⁺), sulfide (S^2-), and dissolved organic carbon (DOC) at the SWI. The planar optode (PO) showed that hypoxia contributed immensely to the high positive fluxes Fe²⁺, Mn²⁺, and S^2- measured, which co-precipitated to form black materials (FeS and MnS) at the SWI. The co-precipitation between Fe-S and Mn-S was supported by their significant positive correlation (Fe-S: r > 0.05, p < 0.05, Mn-S: r > 0.2, p < 0.05). Significant reduction (p < 0.05) in tannins concentration from November (strong thermal stratification) to December (weak thermal stratification) indicated that Fe²⁺ and tannins reacted during the mixing of reservoir water in December due to weak stratification. The simulated experiment confirmed that fresh Eucalyptus leaves produces a significant (p < 0.05) amount of tannins during hypoxia and reacts with Fe²⁺ to produce black water. A high positive correlation (r > 0.8) between Fe²⁺ and DOC demonstrated that Fe²⁺ and DOC combined and contributed to the reservoir water blackening. The study provides a better understanding on the impact of Eucalyptus plantation on water quality and provide guidance for scientific planting of Eucalyptus plantation in reservoir basins in southern China to ensure safe drinking water.

High resolution evidence of iron-phosphorus-sulfur mobility at hypoxic sediment water interface: An insight to phosphorus remobilization using DGT-induced fluxes in sediments model

The deterioration of reservoirs in southern China due to the kinetics of Iron (Fe), Phosphorus (P) and sulphide (S) at the sediment-water interface (SWI) is a major problem that needs urgent attention. Studies on the biogeochemistry of Fe, P, and S using high-resolution profile techniques in reservoirs in this region are limited. The diffusive gradient in thin films (DGT) technique, high-resolution dialysis, DGT-computer imaging densitometry (CID), DGT-induced fluxes in sediments (DIFS) and planar optode (PO) device were used to describe the dynamics Fe-P-S in SWI during hypoxia. The results showed the release of Fe-P-S in SWI was due to sulfate reduction and iron reduction influenced greatly by hypoxia. Positive apparent fluxes were recorded indicating that the sediments release Fe-P-S to the overlying water. High positive correlations (r² > 0.7) for DGT-labile Fe and DGT-labile P in sediments revealed that iron-bound P controlled the release of P at SWI during reductive dissolution. The low correlation between DGT-labile Fe and DGT-labile S (r² < 0.4) disclosed the combative nature between sulfate reduction and iron reduction process. The low correlation occurred because of the co-precipitation between Fe and S, forming black materials such as monosulfide (FeS) and pyrite (FeS₂) in a hypoxic environment. The DIFS model showed the resupply ability (R-values) of P in sediments belonged to the partially sustained case with a steady state case of resupply at TB3 (Tc = 1088s, Kd = 1005.61 cm³/g R = 0.72, K_-1 = 0.19 day^-1) and TB4 (Tc = 712 s, Kd = 712.53 cm³/g, R = 0.78, K_-1 = 0.46 day^-1). The resupply rate belonged to the non-steady state case at TB1 (Tc = 10,990 s, Kd = 396.3 cm³/g, R = 0.35, K_-1 = 0.07 day^-1) and TB2 (Tc = 6097 s, Kd = 578.5 cm³/g, R = 0.45, K_-1 = 0.10 day^-1). The DGT-CID-PO-DIFS provided a deep insight on the mechanism of Fe-P-S and remobilization of P at SWI leading to Blackwater events and eutrophication.