Combined use of diffusive gradients in thin film, high-resolution dialysis technique and traditional methods to assess pollution and bioavailability of sediment metals of lake wetlands in Taihu Lake Basin

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.

Diffusion kinetic processes and release risks of trace metals in plateau lacustrine sediments

The ecological risk posed by trace metals in the plateau lacustrine sediments of China has attracted worldwide attentions. A better understanding of the kinetic diffusion processes and bioavailability of these metals in plateau lakes is needed. Using the diffusive gradient in thin films (DGT) and Rhizon, concentrations of Mn, Mo, Ni, Cr, and Co in the sediments, labile fractions, and interstitial water of Lake Fuxian were comprehensively analyzed. According to the DGT-induced fluxes in sediments (DIFS) model, fully sustained and unsustained resupplies are possible ways in which metals are released from solids to the solution. Moreover, the resupply characteristics of metals varied at different depths in the sediments and at different sites in the lake. Based on the DIFS model, the effective concentrations (CE) of the trace metals were calculated and all except Cr showed good linear relationships with the DGT-labile concentrations, indicating that the CE values were valuable for predicting metal bioavailability. According to the CE values, the metal contamination released from the sediments was relatively low based on the Monte Carlo simulation. This study provides a comprehensive solution for studying the environmental behavior and potential ecological risks of toxic metals in sedimentary environment.

Spatiotemporal Changes in Trace Metal Bioavailability in the Sediment Pore water of a Constructed Wetland Using Passive Pore water Samplers

Sediments in aquatic systems often act as a major sink for contaminants. Diffusive gradient in thin films (DGTs) and in situ equilibrium dialysis samplers (peepers) are two major in situ pore water sampling devices that overcome the problems associated with conventional pore water sampling methods. In the present study, DGTs and peepers were used to study the spatial and seasonal effects (cool months, October-February; warm months, May-September) on metal bioavailability in the H-02 constructed wetland and the sink versus source role of the sediments by calculating the metal resupply capacity. Data showed similar seasonal trends in metal concentrations using passive samplers, peepers, and DGTs. Pooled Cu and Zn concentrations measured using DGTs were lower in warm months (1.67 ± 1.50 and 2.62 ± 0.68 μg L-1 , respectively, p < 0.001) versus in cool months (2.12 ± 0.65 and 5.58 ± 1.33 μg L-1 , respectively, p < 0.001; mean ± 95% confidence interval). Sulfate (SO4 2- ) concentrations were significantly (p = 0.0139) lower in warm months (averaged at 0.22 ± 0.05 mg L-1 ) compared to in cool months (0.16 ± 0.05 mg L-1 ). The increase in SO4 2- concentration is an indicator of the lower activity of sulfate-reducing bacteria, which need SO4 2- during anaerobic respiration, in which SO4 2- is reduced to sulfide (S2- ) that forms insoluble salts with Cu and Zn, which could partially explain the higher bioavailability of these metals in the cool season. Metal resupply capacity of the sediments was mostly <0.2 for Cu and Zn. Taken together, the H0-2 wetland sediments mostly acted as a sink to both Cu and Zn over the course of the present study. Environ Toxicol Chem 2023;42:2726-2736. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Heavy Metals in Surface Sediment of Plateau Lakes in Tibet, China: Occurrence, Risk Assessment, and Potential Sources

Tibetan Plateau lakes have high ecological value and play a crucial role in maintaining ecological balance. This research aimed to study the pollution characteristics, ecological risk, and potential sources of eight heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) in the surface sediments of 12 Tibetan Plateau lakes. The results of the toxicity risk index (TRI) showed that only Gongzhu Tso (28.09) and La' ang Tso (20.25) had heavy metals that could pose a very high risk of toxicity to aquatic organisms. Hg posed the highest potential ecological risk to aquatic organisms. Based on the results of multiple analyses, we inferred that the contents of Cr, Cu, Hg, and Ni in sediments of Tibetan lakes were influenced by industrial and agricultural development; Cd, Pb, and Zn were influenced by transport and atmospheric transport; and As was derived from geothermal activity and rock weathering.

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.

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.

Multiple geochemical and microbial processes regulated by redox and organic matter control the vertical heterogeneity of As and Cd in paddy soil

The heterogeneity of arsenic (As) and cadmium (Cd) in paddy soils seriously hinders the assessment of contamination status and prediction of rice uptake. Their vertical patterns across different environmental conditions and the underlying mechanisms remain largely unexplored. In this study, maximum vertical differences of bioavailable As and Cd within 0-30 cm depth in paddy soils were 4.1-fold and four orders of magnitude, respectively. The vertical patterns of As and Cd followed the vertical redox gradient in long-term reduced paddies, but were shaped by the vertical pH gradient derived from acidic wastewater irrigation in partly oxidized soils. Iron(III)- and sulfate-reducing bacteria played key roles in the formation of vertical pH gradient and the immobilization of As and Cd by iron (hydr)oxides and sulfides under varied redox conditions. Soil redox and organic matter determined the transition between these two mechanisms via regulating microbial iron(III) and sulfate reduction processes. The work proposes that soil vertical As and Cd patterns directly affect the accumulation of As and Cd in different rice cultivars with different vertical root patterns. This is the first study elucidating the controlling mechanisms governing the vertical As and Cd patterns in paddy fields, providing important references to identify, manage and remediate contaminated paddy fields.

Sink or source? Insights into the behavior of copper and zinc in the sediment porewater of a constructed wetland by peepers

The H-02 free water surface constructed wetland has been applied to remove heavy metals, mainly copper (Cu) and zinc (Zn), from wastewater on the Savannah River Site (Aiken, SC, USA). More and more studies focus on the metal behavior between the sediment and the overlying water, which directly reflects the stability of metals after sedimentation in constructed wetlands. This study focused on the biogeochemical pathways in metal bioavailability and remobilization in the sediment after metals were removed from the overlying water. The dialysis sampling device (peeper) was used to collect porewater samples from eight depths in the sediment for the measurement of Cu, Zn, dissolved organic carbon (DOC), and major anions (sulfate and chloride). Surface water samples were also collected for the measurement of Cu, Zn, DOC, and anions. Different temporal trends were observed for dissolved Cu between the surface and bottom waters, but not for dissolved Zn. There were no obvious changes in porewater metal concentrations with increasing depths in the sediment. Sediment served as a sink for Cu as only 3% of porewater samples showed higher labile Cu concentrations than the surface water during the entire year, and these samples were collected below the sediment-water interface. However, sediment served as a source for Zn in summer and winter as 32% of porewater samples showed higher labile Zn concentrations than the surface water, and these samples were collected at all sediment depths. We think the seasonal changes in the behaviors of Cu and Zn are primarily controlled by the sulfur dynamics and the metal removal processes in the constructed wetland, as well as the different complexing chemistry between Cu and Zn. Also, our study supports that peeper is a powerful tool for studying the biogeochemistry of metals in the sediment.

Review on strategies of close-to-natural wetland restoration and a brief case plan for a typical wetland in northern China

Wetlands play an important role in sustaining ecosystems on the earth, which regulate water resources, adjust local climate and produce food for human beings, etc. However, wetlands are facing huge challenges due to human activities and other natural evolution, such as area shrinkage, function weakening and biodiversity decrease, and so on, therefore, some wetlands need to be urgently restored. In this study, the main technology components of close-to-natural restoration of wetlands were summarized. The ecological water requirement and water resource allocation can be optimized for the water balance between social, economy and ecology, which is a key prerequisite for maintaining wetland ecosystem. The pollution of wetland sediments and soils can be assessed by various indicators to provide the scientific basis for natural restoration of wetland base, and suitable strategies should be taken according to the actual conditions of wetland bases. The hydrological connectivity in wetlands and with related water system can be numerically simulated to make the optimal plan for improvement of hydrological connectivity. The ecological restoration of wetlands with the synergetic function of plants, animals and microorganisms was summarized, to improve the quality of wetland water environment and maintain the ecosystem stability. Based on the wetland close-to-natural restoration strategies, a brief ecological restoration plan for a typical wetland, Zaozhadian Wetland, near Xiong'an New Area in the north China was proposed from water resource guarantee, base pollution management, hydrological connectivity improvement and biological restoration. The close-to-natural restoration shows more effective, sustainable and long-lasting and thus a practical prospect.

Kinetic characteristics of mobile Mo associated with Mn, Fe and S redox geochemistry in estuarine sediments

Estuarine sediments are crucial repositories and incubators of molybdenum (Mo) during its transport from rivers to the ocean. Here, Mo mobility and related processes in estuarine sediments were explored using high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques. Better correlations were observed between dissolved Mn and Mo than between dissolved Fe and Mo, implying that Mn geochemistry plays a key role in dissolved Mo mobility via molybdate adsorption onto abundant Mn oxides and its substantial release upon intense Mn reduction. As a result, oxic intertidal sediments functioned as Mo sinks, and anoxic subtidal sediments functioned as Mo sources. The opposite vertical distributions between DGT-Labile S and DGT-Labile Mo indicated that the availability of labile Mo can be blocked by aqueous sulfide. However, the corresponding high concentrations of DGT-Labile S and dissolved Mo at subtidal sites demonstrated that the abundant dissolved Mo remobilized via Mn reduction was not effectively solidified by sulfide. Simulation with the DIFS model further verified that redox conditions and induced physicochemical processes are crucial factors controlling Mo mobility, with relatively low dissolved Mo concentrations but an adequate and steady resupply capacity of the bioavailable molybdate in intertidal sediments.

Spatial distribution and level of contamination of potentially toxic elements in sediments and soils of a biological reserve wetland, northern Amazon region of Ecuador

This study quantifies the degree of pollution and assess the ecological risk of As, Ba, Cd, Co, Cr, Cu, Mo, Ni, Pb, V and Zn in sediments and soils of the Limoncocha Biological Reserve (Ecuador), identified as a Ramsar site with high ecological and socioeconomic value. The hydrologic system of the Reserve is mainly formed by two rivers that drain into the Limoncocha lagoon, which occupies only five percent of the protected area but support a high anthropic influence. Local statistical baseline of studied potentially toxic elements is established using cumulative frequency method, and Al is selected as reference element due to the good correlation with the studied elements. The grade of pollution and the potential ecological risk are evaluated applying three individual (Contamination Factor, Geo-accumulation Index and Enrichment Factor) and six integrated (Degree and modified degree of contamination, Pollution Load Index, Nemerow and modified Nemerow pollution indexes and potential ecological risk index) indices. Results analysis are based on the combined application of traditional statistics, multivariate data analysis and self organizing maps. Outcomes suggest to classify sediments and soils as "moderate contamination and enrichment" due mainly to the concentrations of Cu (66.4-110 mg/kg) and Cd (0.0262-0.808 mg/kg), derived from domestic wastewaters and agricultural activities, and in a lesser extent due to Mo (0.822-4.37 mg/kg), Ni (10.3-25.8 mg/kg), Co (7.27-24.8 mg/kg) and V (60.3-178 mg/kg), derived from oil field drilling activities. The distributions of As (0.328-8.83 mg/kg), Ba (143-1100 mg/kg), Pb (7.20-26.5 mg/kg), Zn (60.1-276 mg/kg) and Cr (10.1-48.6 mg/kg) are heterogeneous in the studied sampling sites. Sediments located next to the pier and at the central area of the Limoncocha lagoon, show moderate potential risk and according to sediment quality guidelines, the calculated mean Effect Range Median quotient (mERMq) classify the sites as medium-low priority risk level. A three-level classification of a mean quotient based on soil quality and soil potential uses is proposed. Soil sites with high anthropogenic activities show low to moderate potential ecological risk being classified as poor soil quality sites but with all potential soil uses allowed according to the legal limits for land uses in Ecuador.

Distribution, risk and bioavailability of metals in sediments of Lake Yamdrok Basin on the Tibetan Plateau, China

Total contents of metals in soil and sediments on the Tibetan Plateau of China have been widely analyzed, but existing information is insufficient to effectively evaluate metal ecological risk because of a lack of metal bioavailability data. In this study, distribution, potential risk, mobility and bioavailability of metals in sediments of Lake Yamdrok Basin in Tibet of China were explored by combined use of total digestion, sequential extraction and the diffusive gradient in thin-films (DGT). Average concentrations of Cr, Ni, Cu, Zn, As, Cd and Pb in surface sediments were 31.25, 30.31, 22.00, 45.04, 31.32, 0.13 and 13.39 mg/kg, respectively. Higher levels of metals were found near the inflowing rivers. Residual form was dominant in Cr, Ni, Zn, Cd and Pb, and reducible form was dominant in As and Cd. Metals in surface sediments showed a low enrichment degree overall, but Cd and As had higher ecological risk levels than the other metals. Furthermore, there was a larger average proportion of exchangeable form of As (20.4%) and Cd (9.0%) than the other metals (1.7%-3.3%), implying their higher mobility and release risk. Average DGT-labile concentrations of Cr, Ni, Cu, Zn, As, Cd and Pb were 0.5, 4.5, 0.7, 25.1, 60.0, 0.22 and 1.0 µg/L, respectively. The DGT-labile As was significantly correlated with extractable As forms (p< 0.01), suggesting that extractable As in sediments acts as a "mobile pool" for bioavailable As. These results suggest potential risks of As and Cd, especially As, deserve further attention in Lake Yamdrok Basin.

Chemical fractionation and risk assessment of surface sediments in Luhun Reservoir, Luoyang city, China

To understand the potential risks of heavy metals, including their bioavailability and toxicity, 15 surface sediment samples were collected from Luhun Reservoir in Luoyang city, China. Total concentrations and chemical fractions of Cd, Cr, Cu, Ni, Pb, and Zn were analyzed. Various rating methods were used to evaluate the degree, risk, and toxicity of the heavy metal pollution. Results showed that Cd and Pb were preferentially associated with exchangeable (55.77-69.76%) and reducible (53.54-69.43%) fractions, respectively, and therefore exhibited high potential availability. Cr (57.14-86.56%) and Ni (32.21-72.77%) occurred primarily in the residual fraction. Metal concentrations in the effective fraction of the sediment decreased in the order: Cd (96.32%) > Pb (91.61%) > Cu (64.54%) > Zn (57.23%) > Ni (41.51%) > Cr (21.68%). Risk assessment indicated that the risk for Cd is extremely high (62.96%); Cu, Pb, and Zn are ranked as medium risk. Based on the potential ecological risk index, these metals (especially Cd) showed higher potential risk near the dam region. Toxic unit values (2.89-6.05) in more than 60% of sediment sites exceeded a value of 4, and Pb had a relatively higher contribution (1.06-2.65). Cd and Pb are the main contaminants in sediments of Luhun Reservoir and should be paid more attention in the future.

Chemical fractions, diffusion flux and risk assessment of potentially toxic elements in sediments of Baiyangdian Lake, China

The pollution of potentially toxic elements (PTEs) in Baiyangdian Lake (BYDL), the largest shallow lake in northern China, has been focused on since the construction of the Xiong'an New Area. However, research on the bioavailability and diffusive flux of PTEs in BYDL sediments has been still limited. Herein, sediment samples were collected from BYDL to systematically evaluate the pollution risk, bioavailability, and diffusion flux of PTEs using multiple methods, including the pollution indexes, risk assessment code, bioavailable metal index, the sequential extraction, and diffusive gradients in thin-films (DGT). The results showed that the average concentrations of PTEs (except for Cd) were similar to the local background. The spatial distribution of PTEs showed that their contents were higher in northern sediments than in southern sediments, and risk assessment results suggest that Cd is the priority pollutant in the BYDL. Most PTEs in the sediments were mainly present in the residue fractions; however, Cd was mainly present in the non-residue fraction. Further analysis of the Cd content and chemical fraction showed that Cd was not only abundant in the northern sediments, but also that the non-residual fraction of Cd was significantly higher than in the southern sediments. The diffusive fluxes of PTEs in the northern sediments were also investigated in comparison with their chemical fractions. Results suggest that Cd has the potential to diffuse from the sediment into the overlying water. Additionally, upon combining the DGT and chemical fractions analyses, it was found that the PTEs which mainly in non-residual fraction tend to diffuse upwards into the overlying water. But, the release tendency of PTEs does not fully depend on their non-residual content. Overall, PTEs did not significantly contaminate BYDL sediments; nevertheless, the potential ecological risk of Cd should be considered.

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.