The application of Diffusive Gradients in Thin Films (DGT) for improved understanding of metal behaviour at marine disposal sites

Contrasting fate and binding behavior of Mn and Cu with dissolved organic matter during in situ remediation using multicomponent capping in malodorous black water

The common use of peroxides in the remediation of malodorous black water may lead to the activation of heavy metals in sediment when eliminating black and odorous substances. The mechanisms of heavy metal interactions with dissolved organic matter (DOM) in response to in situ capping have not been elucidated, but this information could guide the optimization of capping materials. We developed a capping material consisting of hydrothermally carbonized sediment (HCS), hydrated magnesium carbonate (HMC) and sodium percarbonate (SPC) and used microcosm experiments to investigate the dynamics of Mn and Cu at the sediment-water interface in malodorous black water. The results showed that HCS, HMC and SPC contributed multiple functions of mechanical protection, chemical isolation and oxygen provision to the new caps. HMC promoted the conversion of Mn/Cu into carbonate minerals. The optimal mass proportions were 25 % HCS, 60 % HMC and 15 % SPC based on the mixture design. In situ capping altered the fate and transformation of metals in the sediment-overlying water profile in the short term through Mn immobilization and Cu activation. The complexation of Cu(II) ions was significantly stronger than that of Mn(II) ions. In situ capping had a significant effect on the order of complexation of different fluorescent DOM molecules with Mn(II)/Cu(II) ions: microbial byproducts and fulvic acid-like components were preferentially complexed with Cu(II) ions after capping, while phenolic and humic acid-like components preferentially interacted with Mn(II) ions. Humic-like components bound to Cu were affected the most by capping treatment, whereas protein-like components were relatively weakly affected. Our study provides valuable knowledge on the impact of in situ capping on DOM-metal complexes.

Passive Sampling-Based versus Conventional-Based Metrics for Evaluating Remediation Efficacy at Contaminated Sediment Sites: A Review

Passive sampling devices (PSDs) are increasingly used at contaminated sites to improve the characterization of contaminant transport and assessment of ecological and human health risk at sediment sites and to evaluate the effectiveness of remedial actions. The use of PSDs after full-scale remediation remains limited, however, in favor of evaluation based on conventional metrics, such as bulk sediment concentrations or bioaccumulation. This review has three overall aims: (1) identify sites where PSDs have been used to support cleanup efforts, (2) assess how PSD-derived remedial end points compare to conventional metrics, and (3) perform broad semiquantitative and selective quantitative concurrence analyses to evaluate the magnitude of agreement between metrics. Contaminated sediment remedies evaluated included capping, in situ amendment, dredging and monitored natural recovery (MNR). We identify and discuss 102 sites globally where PSDs were used to determine remedial efficacy resulting in over 130 peer-reviewed scientific publications and numerous technical reports and conference proceedings. The most common conventional metrics assessed alongside PSDs in the peer-reviewed literature were bioaccumulation (39%), bulk sediments (40%), toxicity (14%), porewater grab samples (16%), and water column grab samples (16%), while about 25% of studies used PSDs as the sole metric. In a semiquantitative concurrence analysis, the PSD-based metrics agreed with conventional metrics in about 68% of remedy assessments. A more quantitative analysis of reductions in bioaccumulation after remediation (i.e., remediation was successful) showed that decreases in uptake into PSDs agreed with decreases in bioaccumulation (within a factor of 2) 61% of the time. Given the relatively good agreement between conventional and PSD-based metrics, we propose several practices and areas for further study to enhance the utilization of PSDs throughout the remediation of contaminated sediment sites.

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.

Potential pollution assessment of labile trace metals in Xixi River estuary sediments in Xiamen, China

The release of trace metals caused by industrial effluents and anthropogenic activities has been recorded in the Xixi River estuary, southern China. However, a thorough understanding of the behavior of trace heavy metals in Xixi River sediments is lacking. A total of 12 sediment cores were collected in June and December in the upper estuary section and mouth of the estuary. Here, an in situ high-resolution sampling technique, namely, diffusive gradients in thin films (DGT), was employed to acquire profiles of trace element concentrations and the release of bioavailable metals from sediments in different seasons. A three-step Community Bureau of Reference (BCR) sequential extraction method was used to explore the chemical speciation of trace metals in different seasons and to thereby assess the release potential of trace elements in sediments. The BCR sequential extraction results showed that the trace metals Fe, Mn, Co and Pb were mainly in the residual fraction, which rarely influences living organisms. The total mobile fractions (F1 + F2 + F3) of all trace metals were higher in winter than in summer, suggesting that accumulation occurred from summer to winter. DGT measurements showed that the intensity of sulfate reduction was higher in summer than in winter because of the high temperatures and high organic matter in summer. The intensity of sulfate and Mn(III/IV) reduction increased from the upper estuary section to the lower estuary. Fe(III) reduction decreased in summer but increased slowly in winter. The Pearson correlation results showed that the release of DGT-labile Co in pore water was related to Mn(III/IV) reduction, while the release of DGT-labile Pb was basically not controlled by the Fe-Mn-S redox transition. Abnormally high DGT-labile Pb concentrations were observed at the sampling station (XR3) closest to the estuary in winter, which might have been caused by the high Pb content in the local micro-sediments.

Dual diffusive gradients in the thin films (DGT) probes provide insights into speciation and mobility of sediment chromium (Cr) from the Xizhi River basin, South China

Investigation of the speciation and remobilization mechanisms of chromium (Cr) in sediment is essential for accurate estimation of its ecological risks in aquatic systems. In this work, a three-step chemical extraction procedure and diffusive gradient in thin films (DGT) technique were combined to investigate the geochemical speciation, mobility potentials, and release characteristics of sediment Cr. The geochemical speciation of sediment Cr decreased in the following order: oxidizable > reducible > residual > acid-soluble fraction. Dissociation of OM-bound Cr(III) and oxidation by Mn oxides contributed to higher labile Cr(III) and Cr(VI) levels in winter, with the labile Cr(III) being the dominant species and accounting for 48.1%65.5% of the total concentration of labile Cr; whereas, reductive dissolution of Mn oxides was responsible for the remobilization of labile Cr(VI) in summer, leading to a shift in dominant Cr species to Cr(VI) (48.9%65.7%) due to rapid precipitation of Cr(III). Sediment acted as a major sink for labile Cr(VI) in two sampling campaigns. For labile Cr(III), sediment converted from source in winter to sink in summer. The diffusive release of labile Cr(III) deserves preferential concern due to its potential to be re-oxidized to more toxic Cr(VI) under the oxic conditions of river water in winter.

Seasonal variations of cadmium (Cd) speciation and mobility in sediments from the Xizhi River basin, South China, based on passive sampling techniques and a thermodynamic chemical equilibrium model

Understanding the speciation and mobilization mechanisms of potentially toxic metals in sediments is critical to aquatic ecosystem health and contamination remediation in urban rivers. In this study, chemical sequential extraction, a thermodynamic chemical equilibrium model (Visual MINTEQ ver. 3.1), diffusive gradient in thin films (DGT), and high-resolution dialysis (HR-Peeper) techniques were integrated to identify seasonal variations in cadmium (Cd) mobility in sulfidized sediments. Acid-soluble Cd was the dominant geochemical fraction in sediments, followed by residual, oxidizable, and reducible Cd. The DGT-labile Cd concentration was associated with various geochemical processes and was independent of the total concentration and geochemical fractionation of Cd in sediments. Sulfate reduction facilitated the formation of insoluble CdS and induced low Cd concentrations in sediment porewater. Sulfide oxidation was principally responsible for lowered porewater pH and elevated Cd concentrations in summer. Strongly acidic conditions promoted release of sediment Cd but might reduce the binding efficiency of Chelex resin gel for dissolved Cd, leading to underestimation of the mobility of sediment Cd. Sediments generally functioned as a sink for Cd in winter and shifted to acting as a source in summer, releasing Cd into the overlying water mainly as Cd-S complexes with high potential to migrate downstream.

Mobilization mechanisms and toxicity risk of sediment trace metals (Cu, Zn, Ni, and Pb) based on diffusive gradients in thin films: A case study in the Xizhi River basin, South China

Identifying the mobilization mechanisms and predicting the potential toxicity risk of metals in sediment are essential to contamination remediation in river basins. In this study, a sequential extraction procedure and diffusive gradients in thin film (DGT) were employed to investigate the mobilization mechanisms, release characteristics, and potential toxicity of sediment metals (Cu, Zn, Ni, and Pb). Acid-soluble and reducible fractions were the dominant geochemical species of Cu, Zn, Ni, and Pb in sediments, indicating high mobility potentials for these metals under reducing conditions. In summer, the sediment acted as a source of water-column metals due to mineralization of organic matter and reductive dissolution of iron/manganese oxides in surface sediments, and the formation of metal sulfide precipitates markedly lowered DGT-labile metal concentrations with depth, while localized sulfide oxidation was responsible for fluctuating labile metal concentrations. Stable distribution patterns of labile metals resulted from the weak reducing conditions of sediment in winter, when the sediment shifted to a metal sink. The interstitial water criteria toxicity unit (IWCTU), calculated from DGT measurements, indicated no and low-to-moderate toxic risk of sediments in summer and winter seasons, respectively, and Pb was the major contributor to the predicted toxic effects in the soft interstitial water.

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 novel systematic, risk based approach to support the designation of aquatic disposal sites

Humans rely fundamentally on the marine environment, which is at the same time subject to an increasingly broad range of anthropogenic pressures, leading to growing concerns and the need for effective management for marine protection. One activity is the dredging of ports and harbours which is necessary to maintain safe channels for commercial and recreational navigation. Regulatory authorities in developing countries have few resources to conduct full EIAs for determining dredged material disposal sites but are required to do so under international obligations. The Tool in this paper provides an effective, pragmatic, transparent, consistent, and robust approach to protect the environment whilst using limited technical and scientific resources through a risk based approach to defining need, characterising and designating disposal sites at sea for dredged material. Whilst this approach for dredged material disposal sites was developed for use in UK, this process is equally applicable to other waste types, worldwide.

Determining cadmium bioavailability in sediment profiles using diffusive gradients in thin films

Cadmium (Cd) uptake by plants or benthic organisms largely depends on its bioavailability in sediments, so it is necessary to understand Cd bioavailability for determining its ecological risks in riverine sediments. Pore water is easily disturbed during sample collection, indicating that there was a shortage of traditional methods for investigating Cd bioavailability. Here, sediment cores were collected from rivers, after which sequential extraction and diffusive gradients in thin films (DGT) method were employed to determine Cd potential bioavailability in the sediments and pore water. We found that Cd concentrations measured by DGT were lower than that in pore water profiles, and Cd distribution in various fractions changed remarkably. Pearson correlation analysis showed significant positive correlations between Cd concentrations measured by DGT and total Cd concentrations (r² = 0.76), exchangeable and weak acid soluble fraction (r² = 0.68), ferromanganese fraction (r² = 0.72) and bound organic matter or oxidizable fraction (r² = 0.54). However, the correlation was relatively low between Cd concentrations measured by DGT and that in pore water profiles (r² = 0.26). These results demonstrated that DGT method could provide more accurate information of Cd bioavailability in sediment profiles than traditional methods.

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

The geochemical behavior of trace metals at the sediment/water interface in Taihu Lake, the third-largest fresh water lake in China, has been widely explored. However, information on metals in lake wetlands of the basin is lacking. Here, diffusive gradients in thin film (DGT), high-resolution dialysis technique (HR-Peeper) and traditional methods were jointly used to study the occurrence characteristics, pollution degree, bioavailability, and mobility of sediment metals in the northern lake wetlands of Jiaxing City in Taihu Lake Basin. The contents of Cr, Ni, Cu, Zn, As, Cd and Pb were 101, 52.8, 62.3, 184, 10.3, 0.4, and 39.8 mg/kg, respectively. The metals in the sediments were in an overall low enrichment level. The main form of Cd was acid-soluble (F1), and the other metals mainly existed in residual (F4) or oxidable (F3) forms. The mean DGT-labile contents (C_DGT) of Cr, Ni, Cu, Zn, As, Cd and Pb were 1.3, 1.2, 9.3, 6.7, 13.4, 0.7, and 0.8 μg/L, respectively. C_DGT-Cu and C_DGT-As were significantly and positively related to the Cu and As contents in pore water (C_sol). C_DGT-Cr, C_DGT-Cd, C_DGT-Pb, and C_DGT-Cu were significantly and positively related to C_F1-Cr, C_F1-Cd, C_F1-Pb, and C_F3-Cu, respectively. The stability of Cd was the worst with a mean risk assessment code of 40%, indicating a high risk of remobilization in the sediment. The remobilization risks of other metals were low or moderate. The C_DGT/C_sol ratio of Cd was also the largest, with a mean of 0.99, suggesting that the Cd resupplying ability from sediment solid to pore water was strong.

In situ simulation of thin-layer dredging effects on sediment metal release across the sediment-water interface

Dredging is widely applied to remediate contaminated sediments in aquatic ecosystems. However, the efficiency of thin-layer dredging for metal pollution control remains uncertain and even controversial. This study conducted an in-situ simulation experiment in Lake Taihu to investigate dredging effects on sediment metal release based on metal fractions, diffusion flux and kinetics parameters of metal resupply, using diffusive gradient in thin films (DGT), multi-microelectrode, and European Community Bureau of Reference (BCR) sequential extraction scheme. Results indicated that the exchange fluxes of metals did not necessarily correspond to total sediment metal concentrations or the contents of different sequentially-extracted metal fractions; there were appreciable decreases in Ni, Cd, Cu and Zn in terms of total sediment metal concentrations and metal fractions, whereas the bioavailability and release fluxes of labile Ni, Cu and Zn (but not Cd) were all notably promoted (by 136, 128 and 149%, respectively) in dredged area compared to those in un-dredged sediments. Further analysis on the kinetics of metal resupply by DGT technique and DGT-induced fluxes in sediments model (DIFS) showed higher concentrations of labile metals, with a larger resupply ability from sediments after dredging. Therefore, thin-layer dredging had the possibility to increase metal release from sediments to the water column. This was attributed to the remobilization of metal sulfides in anoxic deep sediments, as oxidation increased after dredging due to the introduction of oxygenated water, causing subsequent dissolution of sulfide-bound metals. In conclusion, dredging may not mitigate metal contamination, although it can reduce the total pollution load. Our findings indicated dual effects of dredging and provided new insights into the remobilization mechanism of metal release induced by dredging.

A cyclodextrin polymer membrane-based passive sampler for measuring triclocarban, triclosan and methyl triclosan in rivers

In recent years, extensive attention has been paid to the passive sampling technology of diffusive gradients in thin films (DGT) due to its growing application in the measurement of a widening variety of compounds. Within any DGT device, the binding phase is a key component, and seeking novel binding phases is an issue worth studying. Cyclodextrin polymer, as a green and eco-friendly material, may be a good choice for measuring organic chemicals. In this study, a novel DGT sampler with cyclodextrin polymer membrane (CDPM) as the binding phase was developed for measuring the concentrations of triclosan, triclocarban and methyl triclosan. Firstly, the type and content of cyclodextrin used in CDPM was optimized, and a series of tests showed that CDPM had good hydrophilicity, thermal stability, fast uptake rate and sufficient uptake capacity, thus CDPM was determined to be suitable for use as the binding phase of DGT sampler. Moreover, the sampling rates of this DGT sampler were not influenced by ionic strength and dissolved organic matter, making it feasible for in situ monitoring of compounds in the field. Hence, we deployed the developed DGT sampler in the Qinhuai and Jiuxiang Rivers to measure the concentrations of three compounds. We also collected water samples and processed them with the solid phase extraction (SPE) method. Results indicated that there was no significant difference between the DGT-measured and the SPE-measured concentrations for each compound, which confirmed the reliability of this DGT sampler for monitoring the concentrations of compounds in natural waters.

Insights into mixed contaminants interactions and its implication for heavy metals and metalloids mobility, bioavailability and risk assessment

Mobility of heavy metals at contaminated sites is mainly influenced by the soil physicochemical properties and environmental conditions, therefore assessing heavy metals (HMs) and metalloids fractionation can provide insights into their potential risk and the mechanisms that regulate bioavailability. A 12-months mesocosms experiment was setup to investigate the effect of physicochemical factors (pH, moisture, and temperature) and weathering (time) on HMs and metalloids fractionation in three different multi-contaminated soil matrices (low, medium, and high contamination) collected from a soil treatment facility located in the United Kingdom, and two rural contaminated soil samples. The study demonstrates that even though Pb and Zn were found associated with the exchangeable fraction in the soil with the highest contamination (total average Pb 3400 mg/kg, and total average Zn 2100 mg/kg in Soil C), neither the condition applied nor the weathering caused an increase in their mobility. Although it was expected that lower pH (4.5) would favours the dissociation of HMs and metalloids, no significant differences were observed, potentially due to the initial alkaline pH of the genuine-contaminated soil samples. The results show that even though total concentration of Pb, Cu, and Zn exceed the soil standards and guideline values, HMs were predominantly associated with the non-exchangeable fraction, while only 5% were dissolved in the pore water fraction (potentially bioavailable). In addition, the mobility and bioavailability of HMs remained constant over the 12 months monitoring, suggesting that these soils pose negligible risk to the environment.

Field and laboratory evaluation of DGT for predicting metal bioaccumulation and toxicity in the freshwater bivalve Hyridella australis exposed to contaminated sediments

The diffusive gradients in thin films (DGT) technique has shown to be a useful tool for predicting metal bioavailability and toxicity in sediments, however, links between DGT measurements and biological responses have often relied on laboratory-based exposures and further field evaluations are required. In this study, DGT probes were deployed in metal-contaminated (Cd, Pb, Zn) sediments to evaluate relationships between bioaccumulation by the freshwater bivalve Hyridella australis and DGT-metal fluxes under both laboratory and field conditions. The DGT-metal flux measured across the sediment/water interface (±1 cm) was useful for predicting significant cadmium and zinc bioaccumulation, irrespective of the type of sediment and exposure. A greater DGT-Zn flux measured in the field was consistent with significantly higher zinc bioaccumulation, highlighting the importance of performing metal bioavailability assessments in situ. In addition, DGT fluxes were useful for predicting the potential risk of sub-lethal toxicity (i.e., lipid peroxidation and lysosomal membrane damage). Due to its ability to account for multiple metal exposures, DGT better predicted bioaccumulation and toxicity than particulate metal concentrations in sediments. These results provide further evidence supporting the applicability of the DGT technique as a monitoring tool for sediment quality assessment.

Assessing bioavailability of complex chemical mixtures in contaminated soils: Progress made and research needs

Understanding the distribution, behaviour and interactions of complex chemical mixtures is key for providing the evidence necessary to make informed decisions and implement robust remediation strategies. Much of the current risk assessment frameworks applied to manage land contamination are based on total contaminant concentrations and the exposure assessments embedded within them do not explicitly address the partitioning and bioavailability of chemical mixtures. These oversights may contribute to an overestimation of both the eco-toxicological effects of the fractions and the mobility of contaminants. In turn, this may limit the efficacy of risk frameworks to inform targeted and proportionate remediation strategies. In this review we analyse the science surrounding bioavailability, its regulatory inclusion and the challenges of incorporating bioavailability in decision making process. While a number of physical and chemical techniques have proven to be valuable tools for estimating bioavailability of organic and inorganic contaminants in soils, doubts have been cast on its implementation into risk management soil frameworks mainly due to a general disagreement on the interchangeable use of bioavailability and bioaccessibility, and the associated methods which are still not standardised. This review focuses on the role of biotic and abiotic factors affecting bioavailability along with soil physicochemical properties and contaminant composition. We also included advantages and disadvantages of different extraction techniques and their implications for bioavailability quantitative estimation. In order to move forward the integration of bioavailability into site-specific risk assessments we should (1) account for soil and contaminant physicochemical characteristics and their effect on bioavailability; (2) evaluate receptor's potential exposure and uptake based on mild-extraction; (3) adopt a combined approach where chemical-techniques are used along with biological methods; (4) consider a simplified and cost-effective methodology to apply at regulatory and industry setting; (5) use single-contaminant exposure assessments to inform and predict complex chemical mixture behaviour and bioavailability.

Assessment of potential bioavailability of heavy metals in the sediments of land-freshwater interfaces by diffusive gradients in thin films

It is important to understand the potential bioavailability of heavy metals in the sediments of land-freshwater interfaces (between terrestrial and aquatic ecosystems). Therefore, we evaluated the pollution of Cr, Ni, Cu, Zn, As, and Cd in land-freshwater interface sediments/soils according to total concentrations, and used sequential extraction method to measure different fractions of Cd. Then, the diffusive gradients in thin films (DGT) technique was employed to study the potential bioavailability of metals. Results showed that the concentrations of exchangeable and weak acid soluble fraction and oxidizable fraction were higher in ecotone area, and the values of reducible fraction and residual fraction were higher in deep water area. There existed significant positive correlations between [Cd]-DGT (concentration of Cd measured by DGT) and EXC-Cd (exchangeable and weak acid soluble fraction of Cd) (r² = 0.65), but the significant negative correlation was found between [Cd]-DGT and RES-Cd (Residual fraction of Cd) (r² = 0.52). DGT technique is a feasible method to measure potential bioavailability of heavy metals for risk assessment in the sediments/soils of land-freshwater interfaces.