Improved Diffusive Gradients in Thin Films (DGT) Measurement of Total Dissolved Inorganic Arsenic in Waters and Soils Using a Hydrous Zirconium Oxide Binding Layer

Seasonal variations in spatial distribution, mobilization kinetic and toxicity risk of arsenic in sediments of Lake Taihu, China

This study investigated seasonal variations in spatial distribution, mobilization kinetic and toxicity risk of arsenic (As) in sediments of three representative ecological lakes in Lake Taihu. Results suggested that the bioavailability and mobility of As in sediments depended on the lake ecological types and seasonal changes. At the algal-type zones and macrophyte-type zones, elevated As concentrations were observed in April and July, while these occurred at the transition areas in July and October. The diffusion flux of soluble As ranged from 0.03 to 3.03 ng/cm2/d, indicating sediments acted as a source of As. Reductive dissolution of As-bearing iron/manganese-oxides was the key driver of sediment As remobilization. However, labile S(-II) caused by the degradations of algae and macrophytes buffered sediment As release at the algal-type and macrophyte-type zones. Furthermore, the resupply ratio was less than 1 at three ecological lakes, indicating the resupply As capacity of sediment solid phase was partially sustained case. The risk quotient values were higher than 1 at the algal-type zones and transition areas in July, thereby, the adverse effects of As should not be ignored. This suggested that it is urgently need to be specifically monitored and managed for As contamination in sediments across multi-ecological lakes.

Coastal aquaculture regulates phosphorus cycling in estuarine wetlands: Mobilization, kinetic resupply, and source-sink process

Estuarine mangrove wetlands have gradually declined owing to the growing construction of aquaculture ponds. How the speciation, transition, and migration of phosphorus (P) adaptively change in the sediments of this pond-wetland ecosystem remains unclear. In this study, we used high-resolution devices to explore the contrasting P behaviors associated with the redox cycles of Fe-Mn-S-As in estuarine and pond sediments. The results showed that the construction of aquaculture ponds increased the content or percentage of the silt, organic carbon, and P fractions in sediments. Dissolved organic P (DOP) concentrations in pore water were fluctuant with depths, accounting for only 18±15% and 20±11% of total dissolved P (TDP) in estuarine and pond sediment, respectively. Furthermore, DOP was less strongly correlated with other P species, including Fe, Mn, and sulfide. The coupling of dissolved reactive P (DRP) and TDP with Fe and sulfide confirmed that P mobility is regulated by Fe redox cycling in estuarine sediments, whereas Fe(III) reduction and sulfate reduction co-regulate P remobilization in pond sediments. The apparent diffusion flux revealed all sediments acting as sources for TDP (0.04-0.1 mg m^-2 d^-1) to the overlying water, while mangrove sediments were sources of DOP, and pond sediments were major sources of DRP. The DIFS model overestimated the P kinetic resupply ability, which was evaluated using DRP rather than TDP. This study improves our understanding of P cycling and budget in aquaculture pond-mangrove ecosystems and has important implications for understanding water eutrophication more effectively.

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.

Evaluation of the distribution and mobility of labile phosphorus in sediment profiles of Lake Nansi, the largest eutrophic freshwater lake in northern China

Understanding various biogeochemical processes, especially in eutrophic sediments, necessitates fine-scale phosphorus (P) measurements in pore waters. To the best of our knowledge, the fine-scale distributions of P across the sediment profiles of Lake Nansi have rarely been investigated. Herein we evaluated the dynamic distributions of labile P and Fe across the sediment-water interface (SWI) of Lake Nansi at two-dimensional (2D) and sub-millimeter resolution, using well-established colorimetric diffusive gradients in thin films (DGT) methodology. The concentrations of labile P in all investigated sediment profiles exhibited strong spatial variations, ranging from 0 to 1.50 mg/L with a considerable number of hotspots. Lake Nanyang (0.55 ± 0.21 mg/L) had the highest mean concentration of labile P, followed by Lake Dushan (0.38 ± 0.19 mg/L), Lake Weishan (0.28 ± 0.21 mg/L), and Lake Zhaoyang (0.18 ± 0.09 mg/L). The highest concentrations of labile P were always detected in Lake Dushan, which had been subjected to excessive exogenous P pollution. The co-distributions of labile P and Fe in the majority of the sediment of Lake Nansi confirmed highly positive correlations (P < 0.01), suggesting that the mobility of labile P throughout the SWI was likely governed by iron redox processes. The apparent diffusion fluxes of P across the SWI ranged from -7.7 to 33.6 μg/m²·d, with a mean value of 5.26 ± 7.80 μg/m²·d. Positive apparent fluxes for labile P were recorded in most sediment cores, demonstrating the strong upward mobility of P from the sediment to the overlying water. Our results provided accurate and extensive information regarding the micro-distribution and dynamic exchange of labile P across the SWI. This allows for a better understanding of eutrophication processes and the implementation of P management strategies in Lake Nansi.

Arsenic extraction from seriously contaminated paddy soils with ferrihydrite-loaded sand columns

Reductive dissolution of iron oxides in flooded paddy soils is the most important cause of arsenic (As) release into soil aqueous solution and thus entry into rice. From the perspective of soil cleanup, however, As release under flooded condition could facilitate labile As removal. In this study, a porous column pre-loaded with ferrihydrite (Fh) was constructed, and its efficiency of soil As extraction was investigated using a purpose-designed mesocosm coupled with diffusive gradients in thin films (DGT) for in situ visualization. With Fh-column deployed in aqueous solution, >90% removal of As(III) was achieved within 5 days at initial As (100 mg L^-1) of two orders of magnitude higher than in most paddy soil solutions (1-1538 μg L^-1). By applying Fh-column in a seriously contaminated paddy soil (102 mg As kg^-1), porewater As showed stepwise decreases from 2727 μg L^-1 to 129-1455 μg L^-1 at a distance-dependent manner over four intermittent extractions during 91 days. Soil DGT-As exhibited similar spatiotemporal changes to porewater As. After four extractions, 17.8% of total soil As was removed by Fh-column in a 10 cm radius range on average and ∼1/3 of As bound to amorphous and crystalline Fe/Al oxides was depleted, which accounted for 88.7% of decline in total soil As. With the post-extracted soil, a 48% lower As accumulation in rice seedlings and a 65% decline in bulk soil DGT-As were attained. This study provides a conceptual foundation for rapid removal of high soluble As by Fh-columns from flooded soils, improving seriously As-contaminated paddies to sustainable resources for safe food production.

In Situ Simultaneous Analysis of Nitrogen and Phosphorus Migration in Urban Black Odorous Runoff

The extremely serious urban runoff eutrophication and black odorous phenomenon pose a significant threat to the lake aquatic ecosystem, resulting in a significantly increased frequency, magnitude, and duration of algal blooms in lakes. However, few investigations focus on small tributaries of the lakes, despite the ubiquity and potential local importance of these runoffs. Thus, the labile sediments NH₄⁺-N, NO₃^--N, PO₄^3-, Fe²⁺, and S^2- in black odorous runoff at Wuxi were overall analyzed at high resolution using diffusive gradients in thin films (DGT). The variations in labile N, P, Fe, and S distribution profiles at different sampling sites indicated high heterogeneity in sediments. The concentrations of labile P, Fe, and S showed synchronous variation from the sediment-water interface (SWI) up to -20 mm along sediment profiles. Moreover, there existed a significant positive correlation among labile P, Fe, and S concentrations (p < 0.05), which might represent typical odor compounds' FeS and H₂S synchronous release process in urban runoff. Furthermore, the apparent diffusion fluxes of labile P, Fe, and S across the SWI were all released upward, while fluxes of NH₄⁺-N and NO₃^--N release downward, indicating the sediments act as source and sink of P and N, respectively. Sediments' potential for endogenous P and N fractions release results in the black-odorous water, and sediment finally abouchement the Taihu, which intensifies further lake eutrophication phenomenon.

Simultaneous evaluation of kinetic release of labile arsenic and phosphorus in agricultural soils using cerium oxide-based DGT

The adsorption/desorption of arsenic (As) in agricultural soils is of utmost importance for the evaluation of its kinetic release and potential of entering the food chain by uptake of crops. However, the mobility of As in soils is closely related to the migration behavior of soil phosphorus (P) due to their chemical similarity. Here, the distribution and desorption kinetics of As and P in four different types of farmland soils were simultaneously estimated by cerium oxide-based diffusive gradients in thin films technique (CeO₂-DGT) coupled with dynamic model of DGT induced fluxes in soils (DIFS). CeO₂-DGT was deployed in the soils over 400 h to investigate the interactions between As and P for their migration behaviors. The accumulated masses of As in the DGT devices showed reverse orders with those of P among the four soils, indicating their competitive adsorption on soil solids. The distribution coefficients (K_dl) for the labile As and P derived from the DIFS model were mutually exclusive. Clay in the soil reduced the pool size of the labile As by increasing the irreversible adsorption of As on soil particles. The adsorption rate constants of As were much smaller than P but their desorption rate constants were comparable. Among the four soils, the soil with the highest soil labile As/P molar ratio measured by DGT showed the largest potential of As phytotoxicity. Both As and P could reach the equilibrium of resupply within 0.7- 18 min under DGT depletion, and significant negative correlation was observed between the desorption rate (k_b) of As and clay content in the soils.

In situ fractionation and redox speciation of arsenic in soda lakes of Nhecolândia (Pantanal, Brazil) using the diffusive gradients in thin films (DGT) technique

In situ fractionation and redox speciation of As in three different saline-alkaline lakes (green, black and crystalline lakes) in the Pantanal of Nhecolândia (Brazil) were performed by using Diffusive Gradients in Thin films (DGT). The results indicated that As is present mainly in dissolved form. Total As concentration was similar when using different filter membranes, demonstrating that the species adsorbed by DGT devices were <10 kDa. Higher concentrations of labile total As were observed in the center of the lakes, indicating that the nature of the organic matter influences the formation of As complexes. Total As concentrations determined by using ZrO₂ DGT were consistent with As concentration in ultrafiltered water samples collected in the black lake. However, part of the data about As(III) obtained in grab samples contrasted with DGT results. The differences observed may indicate that alterations in the species occur during the storage period before analysis by ultrafiltration. As(III) concentrations measured by DGT in the black and crystalline lakes were 1-3 μg L^-1 and 4-7 μg L^-1, respectively, accounting for only 4%-8% of the total DGT inorganic As. In the green lake, As(III) concentrations were significantly higher at the center (217 μg L^-1). Both the phytoplankton community and the dissolved organic carbon influence the As speciation and bioavailability in the lakes of Nhecolândia. The DGT approach used in the present work was able to perform As speciation and demonstrates that in situ sampling analytical techniques are essential in understanding As speciation and its behavior in complex natural aquatic systems.

Effects of manganese, iron and sulfur geochemistry on arsenic migration in the estuarine sediment of a small river in Xiamen, Southeast China

The geochemistry of iron (Fe), manganese (Mn) and sulfur (S) and their effects on arsenic (As) mobility in the mudflats of small river estuaries remain unclear. Here, diffusive gradient in thin films (DGT) and high-resolution dialysis (HR-Peeper) techniques combined with a sequential extraction procedure (BCR) were employed to investigate As, Fe, Mn and S geochemistry in the mudflat of the Jiuxi River estuary, Southeast China. Grain size analysis indicated that fine-grained particles were likely to be deposited in the estuarine intertidal zone and coastal area. DGT and HR-Peeper results revealed that in the estuary and coastal area, the dissolved As in sediment in summer was controlled by Mn geochemistry, which includes not only the release of As through Mn/Fe reduction but also the stabilization of dissolved As in pore water. This stabilization of dissolved As may due to the formation of As-Mn-OM complexes. In winter, the significant positive correlations between DGT-Fe, DGT-Mn, DGT-As and DGT-S indicated that sulfate reduction was the start of As mobilization in sediment in winter. In both the estuary and the coastal area, the easily reducible Fe, Mn and As contents in intertidal sediment were higher than those in the subtidal zone. Combined with the As flux across the sediment-overlying water interface (SWI), these phenomena suggested that As in subtidal sediment diffused into overlying water and that As in overlying water tended to accumulate in the intertidal sediment. The total organic carbon content (TOC) and DGT results in the lower reach, estuary and coastal areas indicated that organic matter is the controlling factor of Fe/Mn reduction, sulfate reduction and As mobilization. The BCR test results showed higher reactive fraction contents of Fe, Mn and As in winter sediment, which threaten the overlying water quality.

In situ arsenic speciation at the soil/water interface of saline-alkaline lakes of the Pantanal, Brazil: A DGT-based approach

Arsenic (As) is a naturally occurring element in the Earth's crust, exhibiting toxicity towards a wide range of living organisms. Its properties and environmental dynamics are strongly regulated by its speciation, and the species As(III) and As(V) are the most commonly found in environmental systems. Recently, high concentrations of As were found in saline-alkaline lakes of the Pantanal (Brazil), which is the largest wetland area in the world. Therefore, we evaluated As contamination and its redox speciation (As(III) and As(V)) at the soil/water interface of biogeochemically distinct saline-alkaline lakes of Pantanal wetlands (Brazil). Both conventional sampling and in situ diffusive gradients in thin films (DGT) technique were employed. Zirconium oxide and 3-mercaptopropyl were used as ligand phases in DGT to selectively bind As species. High concentrations of total dissolved As in a shallow water table were found (<2337.5 μg L^-1), whereas levels in soils were up to 2.4 μg g^-1. Distinct scenarios were observed when comparing speciation analysis through spot sampling and DGT. Considering spot sampling, As(V) was the main species detected, whereas As(III) was only detected in only a few samples (<4.2 μg L^-1). Conversely, results obtained by DGT showed that labile As(III) dominated arsenic speciation at the soil/water interface with levels up to 203.0 μg L^-1. Coupling DGT data and DGT induced fluxes in sediments and soils model allowed obtaining kinetic data, showing that the soil barely participated in the arsenic dynamics on the shore of the lakes, and that this participation depends on the evapoconcentration process occurring in the region. Therefore, soil acts like a nonreactive matrix depending on the natural concentration process. In addition, our results reinforced the different geochemical characteristics of the studied saline-alkaline lakes and highlights the importance of robust passive sampling techniques in the context of metal/metalloid speciation in environmental analysis.

Investigation of the combined use of capping and oxidizing agents in the immobilization of arsenic in sediments

The combined use of capping (lanthanum modified bentonite; LMB) and oxidizing (calcium nitrate; CN) agents was investigated to immobilize arsenic (As) in sediments. The vertical changes in labile As and dissolved As were measured using diffusive gradients in thin films (DGT) and Rhizon devices. The results showed that the combined application of LMB and CN had the optimal effect on the immobilization of both DGT-labile As and dissolved As, compared to single treatments using LMB or CN. After 60 days of incubation, the maximum reduction efficiencies of DGT-labile As at sediment depths were 76.4%, 70.8%, and 44.9% of those treated with LMB + CN, CN, and LMB, respectively. After 32 days of incubation, the average concentrations of dissolved total As throughout the depths decreased from 7.71 μg/L after the control treatment without any amendments to 5.25, 4.03, and 3.15 μg/L after the addition of LMB, CN, and LMB + CN, respectively. The larger part of exchangeable As at sediment depths was converted into the reducible As mainly bound Fe/Mn oxide-hydroxides after combining LMB and CN. Due to the As(III) existing mainly in the form of electrically neutral H₃AsO₃ in sediments, it is hard to adsorb As(III) for the LMB and iron/manganese oxide-hydroxides formed by the oxidation effect of calcium nitrate. Thus, the single or combined LMB and CN use had much weaker effect on the immobilization of As(III) compared with As(V). The results of current study indicated that the combined use of LMB and CN could be a promising method to control the potential release of As from the sediment to the overlying water. However, this method needs further improvement to achieve a better immobilization effect on As(III) in sediments.

Simultaneous determination of arsenic and uranium by the diffusive gradients in thin films technique using Lewatit FO 36: Optimization of elution protocol

The sorption ability of Lewatit FO 36-DGT resin gel, which has been developed for arsenic determination, towards uranium was tested by batch experiments within this study for the first time. Since the uptake efficiency of uranium was 99.0 ± 0.4% and the maximum uptake capacity was not achieved even at the U spike of 1250 μg in the solution, the Lewatit FO 36 resin seems to be a suitable binding phase for DGT resin gels for the determination of uranium. The resin gel also does not display any significant sorption selectivity in favour of one element over another. A novel protocol for simultaneous elution of arsenic and uranium from Lewatit FO 36 resin gel was therefore proposed in this study. The elution efficiencies of 90.3 ± 3.9% and 85.2 ± 3.1% for As and U, respectively, were obtained using 5 mL of 1 M NaOH at 70 °C for 24 h. The comparison with the original elution protocol using microwave-assisted elution by 0.25 M NaOH and 0.17 M NaCl at 130 °C for 16 min indicates, that the novel elution protocol provides good results in the performance of arsenic elution and, in addition, allows simultaneous elution of uranium. Moreover, the elimination of NaCl from the elution process allows a fast and simple analysis of both elements using ICP-MS, and therefore, the Lewatit FO 36-DGT technique can become more commonplace among laboratories without the need to modify the analytical method as proposed in the original study.

Determination of Labile Cadmium in Soils Using a New Sodium Alginate-Polyglutamic Acid-Diffusive Gradient in Thin Films

Sodium alginate-polyglutamic acid was used to develop a new diffusive gradient in thin films (SA-PGA-DGT) device, which was proven to be suitable for the investigation of labile Cd in soil. The adsorption capacity of Cd was calculated to be approximately 16.8 μg/cm² , which was hardly affected by factors including pH (5-9), ionic strength (0.1-100 mM), and the presence of other metals (Pb, Cu, Ni, and Cr). The SA-PGA gel has dense and uneven pores with large specific surface area, which ensures the adsorption of Cd by functional groups of the gel. A kinetics study indicated that the adsorption rate of Cd by the binding gel can be described as a pseudo-second-order reaction. Deployment of the SA-PGA-DGT in the soils of Tang Gu (located in Binhai New District, Tianjin, China) showed a strong positive linear correlation between Cd measured by the device and exchangeable Cd measured by the Tessier method (R = 0.73, p < 0.01). Cadmium determined by the SA-PGA-DGT device was less affected by soil properties. This new SA-PGA-DGT has obvious advantages over other methods in respect of the labile Cd analysis in soil. The innovative novel device expands the variety of existing DGT technologies and can be utilized to monitor the level of labile Cd in soil effectively. Environ Toxicol Chem 2021;40:1559-1569. © 2021 SETAC.

Development and evaluation of a DGT sampler using functionalised cross-linked polyethyleimine for the monitoring of arsenic and selenium in mine impacted wetlands

Arsenic and selenium are both carcinogenic and their presence in fresh water has attracted the development of robust and accurate monitoring techniques. A new diffusive gradients in thin-films (DGT) sampler was developed and evaluated for the in situ measurements of arsenic and selenium. The binding layer was made from a mixture of sulphonated and phosphonated cross-linked polyethylenimine (SCPEI and PCPEI, respectively). The optimum ratio of a SCPEI and PCPEI resin mixture was determined. The DGT sampler was calibrated under laboratory conditions to determine the influence of sample turbulence, concentration and pH. The optimised DGT passive sampler was field deployed in a mine impacted dam for 12 days. Binding layer optimisation shows that the polymers had to be mixed in a specific ratio of 80% sulphonated and 20% phosphonated per 0.8 g of the resin mixture, in the loose polymer form. Embedding the resin mixture in agarose gel reduced the uptake of both arsenic and selenium dramatically. At sample pH 3.0 and 5.0, the DGT sampler did not show significant differences in uptake of the two elements during the 15 day deployment. The passive sampler had limited adsorption capacity and was found better suited for dilute solutions, with concentrations below 0.5 mg L^-1 of the target metals. This effect was more pronounced when exposed to dam water which had competing cations. Cations may have reduced the capacity by binding to the PEI backbone via the large number of amine groups. Nonetheless, these cations did not show linear uptake.

Evaluation of Drinking Water Quality in Schools in a District Area in Hanoi, Vietnam

BACKGROUND

Drinking water quality affects directly human health. Assessment and prevention of water-borne diseases are crucial for primary prevention, especially for children.

OBJECTIVE

The main aim of this study was to investigate the quality of drinking water from tap water in preschools and primary schools in a district area in Hanoi City, Vietnam.

METHODS

A cross-sectional study was performed from August to October 2019. Water samples from tap water of 154 schools in a district area of Hanoi were collected to determine the quality of drinking water. From each school, at least 2 bottles of water samples were collected on the basis of a standard operating procedure (SOP). Each water sample was analyzed for microbial and physicochemical parameters, including Color, Taste and Odor, Turbidity, pH, Nitrite, Nitrate, Ammonium, Total Iron, Permanganate, Chloride, Hardness, Total Manganese, Sulfate, Arsenic, Coliform, and E.coli, by analytical methods. The obtained values of each parameter were compared with the standard values set by WHO and National Technical Regulation on Domestic Water Quality of Vietnam.

RESULTS

All of the schools employed community water system as a main source for drinking water. The results showed that all tested samples were found to be within the standards for some physicochemical properties, including Color, Taste and Odor, Hardness, Chloride, Total Iron (Fe²⁺ và Fe³⁺), Total Manganese (Mn), Nitrate (NO₃ ^-), Sulfate (SO₄ ^2-), and Total Arsenic (As). On the other hand, some samples did not meet the allowable limits of the national standard, due to pH (3.9%), Turbidity (0.6%), Nitrite (3.2%), Permanganate (6.5%), and Ammonium (5.8%). Furthermore, the microbial data revealed that the substandard water samples from municipal water systems were contaminated by Coliform (9.7%) and/or E.coli (7.8%).

CONCLUSIONS AND RECOMMENDATIONS

Contaminants such as bacterial and chemical agents in to drinking water could be occurred during transport, storage and handling before using by the consumer without regular surveillance. A periodic treatment procedure and monitoring system to keep the level of microbial and chemical contamination of drinking water in schools under control should be performed.

Using rice as a remediating plant to deplete bioavailable arsenic from paddy soils

Rice plant is efficient in arsenic (As) accumulation due to enhanced soil As release under flooded condition and its effective As uptake. Therefore, rice plant can be used to remove bioavailable As from paddy soil. In this study, the depleting dynamics of soil As with rice growth was mapped two-dimensionally with the zirconium-oxide diffusive gradients in thin films (DGT) technique. Further, the key biochemistry promoting soil As bioavailability was studied to better understand the underlying processes. Results revealed that the average DGT-As encompassing the root zone decreased steeply from 331 in the seedling stage to 136 in the heading & flowering stage and further to 118 μg l^-1 at harvest, which was 26% lower than that of the control at 160 μg l^-1. During this process, rhizosphere porewater As developed a dynamic profile similar to Fe and dissolved organic carbon, with the diversity of arrA gene peaking at heading & flowering stage. The data support soil As release from microbial reduction of Fe hydroxides fueled by root exudation as carbon source. Arsenic was mainly accumulated in the roots, accounting for up to 95% of total As in rice plants. Removal of rice roots resulted in ~19% lower DGT-As in post-harvest soil compared to without removing the roots. As a result, a sharp decline in As accumulation in rice plants was obtained in the second planting after removing one crop of rice roots. The results highlight that rice, as a paddy-adapted plant, is effective in As uptake in the roots, and thereby removing rice roots efficiently depletes bioavailable As from paddy soils.

Anoxia remediation and internal loading modulation in eutrophic lakes using geoengineering method based on oxygen nanobubbles

Benthic anoxia and internal P release, widely occurring in eutrophic lakes, are major factors threatening the health of aquatic ecosystems. In this paper, we experimentally evaluated the efficacy of a new type of "flock-lock" geoengineering method based on oxygen nanobubble technology to remediate sediment anoxia and reduce the internal P release in waters with and without algal blooms. Oxygen-carrying materials (OCM) modified from natural zeolites were used as capping agents and an oxygen-locking layer consists of OCM and the oxidized sediment was formed between anoxic sediment and overlying water. The synergy of diffusion and retention of oxygen in this layer contributes to both the increase of DO and reversal of anoxic conditions. By capping with OCM, the DO in overlying water improved instantly from around 1.5 mg/L to 3.5-4 mg/L and 5-6 mg/L in the systems with algal blooms and without algal blooms, respectively, and maintained throughout the incubation period. The oxygen penetration depth in the sediment can be significantly enhanced from around 0 cm to 3 cm and form an oxygen-locking layer at the end of the experiment by capping with OCM. The labile P was effectively retained via the re-oxidation of ferrous iron in this layer compared with the obvious release of labile P and Fe in control. More importantly, the oxygen depletion and labile P increase at the sediment-water interface caused by the decomposition of the deposited algal biomass can be substantially eliminated after capping with OCM. The study shed insights on the sustainable modulation of sediment anoxia and internal loading in eutrophic waters.

Ceria oxide nanoparticle-based diffusive gradients in thin films for in situ measurement of dissolved reactive phosphorus in waters and sewage sludge

A passive sampling method based on diffusive gradients in thin films (DGT) using ceria oxide (CeO₂) binding gel was developed for in situ measurement of dissolved reactive phosphorus (DRP). CeO₂-based DGT showed excellent uptake performance for DRP, and the uptake mass was consistent with the predication by DGT equation. pH (4.2~9.4) and ionic strength (0.01~500 mM) had no effects on the uptake of DRP. Filed deployment of CeO₂-DGT in reservoir water and seawater showed that the measureable concentrations of DRP were comparable to those obtained by grab sampling. CeO₂-DGT was deployed in sewage sludge, and results showed the ratios (R_S) between the concentration (C_DGT) by CeO₂-DGT and the concentration (C_S) obtained by a traditional centrifugation method ranged from 0.23 to 0.58. This result indicated that sludge solid phase was a potential pool of DRP in sludge solution, and the DRP released from sludge solid phase could compensate partly the consumption of DRP at the interface of DGT device during the deployment. The ratios R_S had positive correlation with the content of Fe (r = 0.847, p < 0.01) but were reversed with the level of Ca (r = - 0.879, p < 0.01) in sewage sludge. The proposed method provided a powerful tool for in situ measurement of DRP in natural waters and for release behavior of DRP in sludge.

Development and evaluation of the iron oxide-hydroxide based resin gel for the diffusive gradient in thin films technique

An ion-exchange resin Lewatit FO 36 was used for the preparation of a new resin gel for the diffusive gradient in thin films technique (DGT). The DGT method was optimized for the accumulation of four bioavailable arsenic species (As^III, As^V, monomethylarsonic acid, dimethylarsinic acid) in the aquatic environment. The total sorption capacity of Lewatit FO 36 resin gel was 535 μg As disc^-1. The microwave-assisted extraction in the presence of NaCl (10 g L^-1) and NaOH (10 g L^-1) was used for the isolation of arsenic species from the Lewatit FO 36 resin gel. The elution efficiency of arsenic was 98.4 ± 2.0%. Arsenic was determined by the optimized electrothermal atomic absorption spectrometry (ET-AAS) method using palladium modifier, pre-atomization cool-down step and tungsten carbides coating of graphite tube. The Lewatit FO 36 resin gel provides accurate results (c_DGT/c_SOL ratio 0.86-1.00) in the pH range 4-8. No significant influence of experimental conditions was observed in the presence of chlorides (0-0.5 mol L^-1) and humic acid (0-100 mg L^-1). Only a very high concentration of phosphates (10 mg L^-1) caused a slight decrease in the diffusion coefficients of MMA and As^V species (8.4% and 12.4%, respectively). The presence of iron (0-1 mg L^-1) caused a decrease in the diffusion coefficients, but with regard to the common concentrations of iron (less than 0.3 mg L^-1), the negative effect was considered not significant for As^III and DMA in natural water. The DGT-ET-AAS method was applied for the determination of bioavailable arsenic species in the spiked river water samples and also in-situ in the water reservoir. The new resin gel was characterized by a homogeneous gel structure with excellent reproducibility (< 5% variation of results between batches) and high sorption capacity which suggests its possible long-term application (up to 286 days in the environment with the arsenic concentration of 100 μg L^-1).

A novel Mg(OH)2 binding layer-based DGT technique for measuring phosphorus in water and sediment

Diffusive gradients in thin films (DGT) have gained wide attention for in situ measurement of reactive phosphorus species (PO4) in natural water, sediments and potentially soils. In this study, a novel Mg(OH)2 binding gel was formed using magnesium hydroxide obtained by in situ hydration of calcined magnesium oxide. Laboratory scale experiments showed that the novel Mg(OH)2 gel had a homogeneous dispersion of fine particles of Mg(OH)2 with a particle size of 2-5 μm. With 10 mL of 2.0 mol L-1 NaOH as the eluting agent, the optimal elution efficiency of PO4 on the Mg(OH)2 gel was 72 ± 5%. There were linear relationships between the accumulated PO4 mass and the applied PO4 concentration (0.1 to 20 mg P per L), time (0 to 24 h) and temperature (22 to 40 °C). The capacity of the Mg(OH)2 binding layer was determined to be 99.5 μg P per disc. Tests in synthetic seawater, Chaohu Lake and Yihai Pond confirmed that Mg(OH)2-DGT was able to accurately measure phosphorus up to 10 days. This was indicated by the good agreements between the concentrations measured by DGT (CDGT) technology and by an ex situ chemical method in solution (Csoln), with a CDGT/Csoln ratio between 0.91 and 1.09.

Arsenic removal from flooded paddy soil with spontaneous hygrophyte markedly attenuates rice grain arsenic

China ranks the top in global annual rice output. However, extensive mining and smelting has led to elevated arsenic (As) in paddy soils, potentially imperiling local population health and sustainable rice production in the country. Under flooded condition, reductive As mobilization generally occurs, providing a unique advantage for soil As removal. In this study, we explore the depletion magnitude of labile As from paddy soils through cycling of flooding-drainage with three distinct co-strategies, i.e. (1) no soil disturbance with spontaneously established hygrophyte plants, (2) selective fertilization to enhance soil As release, and (3) soil ploughing following each drainage. After 151 days of flooding with periodic drainage, diffusive gradients in thin film (DGT)-labile As through 0-14 cm soil profile with hygrophyte plants growing decreased from initial 292 μg l^-1 to well below the required threshold level (57-77 μg l^-1) for safe rice production. Correspondingly, an average of 22.9% of total soil As was removed, with up to 76.7% of As bound to amorphous Fe hydroxides being stripped in this treatment. In the following rice cultivation, inorganic As in the polished rice from the naturally vegetated treatment (0.15 mg kg^-1) fell successfully below the Chinese food safety standard (0.2 mg kg^-1). The results highlight that As removal from paddy soils with native hygrophyte under shallow flooded condition can decrease soil bioavailable As specifically to safe levels within a relatively short period, and thus provides a novel and quite cost-effective pathway securing rice production.

Biochar-assisted phytoextraction of arsenic in soil using Pteris vittata L

The alkaline nature of biochar provides a potential for soil arsenic (As) mobilization and, hence, enhancing efficiency of As phytoextraction by combining with As hyperaccumulator. To testify the feasibility and potential risk of the above strategy, biochar effect on As transfer in a paddy soil and accumulation in P. vittata was investigated in a pot experiment. By leaching soil (total As concentration 141.17 mg/kg) with simulated acid rain (pH 4.2), As the concentration in leaching eluate increased proportionally with increasing biochar ratio. Coincident with elevated soil As mobility, apparent enhancement in As uptake and translocation in P. vittata was determined with 1-5% biochar amendment after 40 days of plant growth. Furthermore, diffusive gradients in thin film (DGT) technique were employed to characterize any potential risk in vertical downward migration of As at 2-mm resolution. A significantly increasing profile of DGT-As ranging from on average 20 μg/L in CK to 50-100 μg/L in 1-3% biochar treatments was recorded over 0-60 mm depth, with 25-71% lower labile As in the rhizosphere than non-rhizosphere zone with few exceptions. As compared to Chinese quality standard for groundwater (Class IV 50 μg/L), biochar ratio at ≤ 1% was suggested for local water safety while actual application should take the physicochemical characteristic of tested soil into account. Our results demonstrated the biochar-assisted P. vittata phytoremediation can serve as an emerging pathway to enhance efficiency of soil As phytoextraction. The combination of DGT techniques and greenhouse assay provided a powerful tool for evaluating the gradient distribution of heavy metal in rhizosphere and accessing corresponding ecological risk at more precise scale.

Removal of labile arsenic from flooded paddy soils with a novel extractive column loaded with quartz-supported nanoscale zero-valent iron

Efficient removal of labile arsenic (As) from paddy soil is a fundamental pathway mitigating As accumulation in rice from a long-term perspective. In this study, a porous and pencil-shaped column prepacked with quartz-supported nanoscale zero-valent iron (NZVI) was designed to extract elevated porewater As from paddy soil under flooded condition. With fine quartz as supporting medium in the core layer, only 0.07% out-migration of the loaded NZVI occurred in arsenite As(III) solution. At pH 5-9, removal of aqueous As(III) with NZVI-column was 73-78%, while silicic acid and phosphate at their environmentally realistic concentrations exhibited 27-30% and 14-17% inhibition on As(III) extraction, respectively. For two paddy soils with slight (S-As) and moderate (M-As) As contamination, four cycles of intermittent extraction with NZVI-column induced steady and marked decrease in porewater As. By the end of four successive extractions, profiles of DGT-labile As in S-As and M-As soils decreased by 22% and 29% on average with simultaneous decline of the most available fraction of soil As (including soluble and exchangeable fraction) by 26% and 17%, respectively. For the post-extracted two soils, As accumulation of rice seedlings declined by 29-57% than those in control. These results identify the effectiveness of NZVI-column in extracting elevated labile As from paddy soils with the aid of flooding. Targeting fast removal of high porewater As, column-extraction could serve as the first step in "remediation train" of paddy soils with relatively high As to shorten cleanup time by rendering much lowered soil As burden for the following phytoextraction and other measures.

Passive sampler measurements of inorganic arsenic species in environmental waters: A comparison between 3-mercapto-silica, ferrihydrite, Metsorb®, zinc ferrite, and zirconium dioxide binding gels

The performances of five Diffusive Gradients in Thin Films (DGT) binding gels, namely 3-mercapto-functionalized silica (3MP), ferrihydrite (Fh), Metsorb^®, zinc ferrite (ZnFe₂O₄)_, and Zirconium oxide (ZrO₂)_, were evaluated for in situ determination of As speciation in water and sediments. A combination of batch experiments at various pH (without addition of buffers) and in the presence of reduced species (Mn²⁺, Fe²⁺ and HS^-),time-series experiments in oxic waters, and in situ deployment in anoxic river sediments has permitted to evaluate the potential interferences among the binding gels. Firstly, the efficiency of each DGT binding gel dedicated to total As (i.e., Fh, Metsorb®, ZnFe₂O₄ and ZrO₂) or As(III) (i.e., 3MP) determination confirms that the determination of As species is possible in oxic freshwater and seawater over 96 h for a wide range of pH (5-9). Secondly, concerning the deployment in river sediment, high HCO₃^- concentrations have a little negative effect only on the DGT performances of the iron(III)-binding gels (i.e, Fh and ZnFe₂O₄). Thirdly, the presence of sulfides does not show any effect on the DGT uptake of As, but strongly affects the elution factor parameter. Discrepancies in elution between the different binding gels potentially result in precipitation of orpiment, especially in 1 mol L^-1 HNO₃. A correction of the classical elution factor derived from batch experiments was applied to provide more representative results. Finally, this study shows the difficulties to determine As speciation in anoxic sediments, and suggests that corrections of the elution factor may be required as a function of the species present in the deployment matrices.

Development of cerium oxide-based diffusive gradients in thin films technique for in-situ measurement of dissolved inorganic arsenic in waters

A diffusive gradients in thin films (DGT) method using a new high-capacity cerium oxide (CeO₂) binding gel, for the first time, was developed for measuring dissolved inorganic arsenic in freshwater and seawater. The capacities of the new CeO₂ binding gel were 682 μg and 375 μg for As^III and As^V, respectively. The masses of As^III and As^V accumulated by CeO₂-DGT device increased linearly with time and agreed well with the theoretical value calculated by DGT equation. The arsenic accumulation by CeO₂-DGT was independent of pH (4.05-9.04) and ionic strength (0.1-750 mM), and common anions including CO₃^2-, SO₄^2-, Cl^- and PO₄^3- had no obvious interference. CeO₂-DGT showed excellent long-term deployment performance in freshwater and synthetic seawater. Field trials with CeO₂-DGT achieved successfully the time-weighted-average concentrations of total inorganic arsenic in reservoir water (1.38 ± 0.09 μg/L) and coastal seawater (0.45 ± 0.06 μg/L). The results were comparable to those measured by grab sampling. The proposed method was reliable and robust for in-situ measurements of dissolved inorganic arsenic in environmental waters.

Enhanced DGT capability for measurements of multiple types of analytes using synergistic effects among different binding agents

There is a requirement for simultaneous measurements of diverse analytes to reveal their biogeochemical couplings in the environment. In this study, we first realized simultaneous measurements of three types of analytes, including sulfide [S(-II)], metallic cations, and oxyanions by diffusive gradients in thin films (DGT) using a new mixed ZrO-CA binding gel. The ZrO-CA gel exhibited faster absorption rates for oxyanions than previously reported ZrO-Chelex gel, and faster absorption rates for cations when the gel was saturated with S(-II). It implies that there were synergistic effects among the three binding agents which should facilitate the DGT uptake. The technique's performance was validated under laboratory conditions in mixtures of dissolved compounds. It was shown to be independent of the solution's pH (5-9) and ionic strength (from 1 to 3 mM to 750 mM). The capacities of the ZrO-CA DGTs without or with [S(-II)] saturation to measure As(III), As(V), Cd(II), Fe(II), and P(V) individually were significantly greater than those of other DGTs except for Zr-oxide based DGTs. The measurements in sediment revealed similar distributions of three types of analytes in the vertical profile and confirmed the feasibility and advantage of the ZrO-CA DGT. This study provides a new perspective to enhance the DGT capability in measurement of multiple analytes through utilizing the synergistic effects among different binding agents.

Influence of algal blooms decay on arsenic dynamics at the sediment-water interface of a shallow lake

Algal blooms decay affects the mobility, bioavailability, and toxicity of arsenic (As) in aquatic environments. The decaying process alters aquatic redox conditions and also introduces ample organic matter, conditions favorable for microbial reduction to simulate As release and As species transformation. This study investigated the role of algae (Microcystis aeruginosa (MA)) blooms decay on the dynamics of labile As and elements associated with its release, such as iron (Fe), manganese (Mn) and sulfide (S), at the sediment-water surface (SWI) of a shallow lake (Lake Taihu) in China, applying mesocosm incubation. A combined AgI/ZrO-Chelex diffusive gradients in thin films (DGT) technology was used to examine labile As, Fe, Mn, and S variations in sediments during none-MA, low-MA, and high-MA treatments. High-MA blooms decay generated strong anoxic conditions, which are favorable for the release of labile As, Fe, Mn, and S from sediment. Labile S produced from the reduction of sulfate caused by decaying MA was likely removed by labile Fe through the formation of Fe-sulfides under reducing conditions. Furthermore, this condition facilitated arsenate (As(V)) reduction to arsenite (As(III)), which was weakly adsorbed by Fe-sulfides and thus mobilized to the aqueous phase. An increase in the sediment resupply of solids As to pore water along with MA blooms decay was also revealed by the DGT-induced fluxes in sediment model, coupled with the average desorption rate constant increasing from 0.27 × 10^-6 s^-1 to 3.41 × 10^-6 s^-1, and the average response time decreasing from 7859 s to 1538 s.

In situ arsenic speciation and the release kinetics in coastal sediments: A case study in Daya Bay, South China Sea

In-situ study on arsenic speciation and the release kinetics in marine sediments was scarce. In this study, the distributions of labile As and their speciation in coastal sediments of Daya Bay were obtained by separate diffusive gradients in thin films (DGT) probes. Results showed that the DGT-labile As(V) was the main speciation in surface sediments (from -20 to 0 mm) with a concentration range of 0.07-3.05 μg·L^-1, while the labile As(III) was the main speciation in deep layers of sediments (from -100 to -20 mm). In coastal areas, mariculture farms was the most dominated contributor to As(V) contamination in surface sediments. Both the apparent diffusion flux estimation and the DGT induced flux in sediments (DIFS) simulation indicated that As(V) contamination in surface sediments of mariculture, harbor and petrochemical areas suffered the potential risk of As(V) release into the overlying water from sediments. DIFS modeling also found that the sediments of mariculture farms were the main sediment As pools. Linear regression analysis indicated that the mobility of As mainly attributed to the As(V) in sediments.

Simultaneous measurements of arsenic and sulfide using diffusive gradients in thin films technique (DGT)

Diffusive gradients in thin films technique (DGT) is a dynamically passive sampling technique which has been applied increasingly to the environmental monitoring field. In the preliminary period, the DGT with zirconium hydroxide-silver iodide as the binding phase (ZrO-AgI DGT) has been developed for the determination of sulfide (S(II)). On this basis, this paper developed its determination method for inorganic arsenite (As(III)) to further realize the simultaneous and high-resolution measurements of labile inorganic As and S(II) in sediments. ZrO-AgI binding gel had a strong ability in adsorbing and fixing As(III), showing a linear increase in the initial 12.5 min. After saturation of S(II) on ZrO-AgI binding gel, the adsorption rate and adsorption capacity of As(III) reduced by 8 and 14%, respectively. A stable elution rate (89.1 ± 2.2%) was obtained by extraction of As(III) on the binding gel using a mixture solution of 1.0 M NaOH and 1.0 M H₂O₂ (1:1). The DGT capacity of As(III) determined by the ZrO-AgI DGT was 23.6 μg cm^-2. DGT uptakes of As(III) were independent of pH (4.0-9.0) and ionic strength (0.01-100 mM), and they did not interfere with each other during the uptake process. Simultaneous measurements of labile As and S(II) in four sediment cores of Taihu Lake (China) with ZrO-AgI DGT showed that they had similarly vertical distributions in the top 16-mm layer in one core and in the whole profile up to the 35 mm depth in two cores. It likely reflected a simultaneous release of As and S(II) in sediments by synchronous reduction of As-hosted oxidized iron and sulfate, respectively. The simultaneous decreases of labile As and S(II) from their co-precipitation (e.g., As₂S₃) were not obvious in deeper sediment layer through the measurement with ZrO-AgI DGT.

Assessment of Cr pollution in tributary sediment cores in the Three Gorges Reservoir combining geochemical baseline and in situ DGT

The mobility and transfer of trace metals in sediments are vital to understanding trace metals environmental behavior in water environment. However, as a predominant aquatic carcinogen, an effective method for assessing the release and deposition for Cr at the sediment-water interface (SWI) is still not clearly understood. Here we established a comprehensive methodology to evaluate the release risk of Cr at the SWI combining regional geochemical baseline (RGB) and diffusive gradients in thin films (DGT). Sediment cores and water samples were collected in the two tributaries and mainstream of the Three Gorges Reservoir, which is the world's largest man-made hydroelectric station. Results showed that the calculated Cr carcinogenic risks in surface water did not exceed US EPA maximum recommended level. The RGB of Cr (85.53 ± 14.44 mg/kg) were calculated and the differentials between Cr concentration and RGB in surface sediments showed the average anthropogenic contribution rate was 6.03% and the upstream of Meixi River (MX-S) and mainstream were influenced by anthropogenic activities. The net diffusive flux using DGT showed that Cr in the midstream of Caotang River and MX had the potential to move upwards into the overlying water. Furthermore, combining the results of differential (Cr concentration vs. RGB) and the net flux, MX-S was the only site with a risk of Cr release. To the best of our knowledge, this study is the first attempt to combine RGB and DGT to scientifically assess metal release at SWI and provided a new perspective to comprehensively assess metal pollution in water environment.

In situ sampling and speciation method for measuring dissolved phosphite at ultratrace concentrations in the natural environment

Phosphite (P^+III) is of emerging chemical interest due to its importance within the global phosphorus cycle. Yet, to date, precise/accurate measurements of P^+III are still lacking due to the inherent analytical challenges linked to its instability/ease of oxidation and ultra-trace concentration. Here, we present the first in-situ sampling and speciation analysis method, for dissolved P^+III, using the diffusive-gradients-in-thin-films (DGT) technique, combined with capillary-column-configured-dual-ion-chromatography (CC-DIC). Method optimization of the DGT elution regime, to simultaneously maximize desorption efficiency and CC-DIC sensitivity, along with the characterization of diffusion coefficients for P^+III, were undertaken before full method validation. Laboratory-performance testing confirmed DGT-P^+III acquisition to be independent of pH (3.0-10.0) and ionic strength (0-500 mM). The capacity for P^+III was 45.8 μg cm^-2, while neither P^+V (up to 10 mg L^-1) nor As^+V (up to 1 mg L^-1) impacted the DGT-P^+III measurement. This novel method's functionality stems from the herein confirmed speciation preservation and double pre-concentration of P^+III, resulting in quantification limits as low as 7.44 ng L^-1 for a 3-day deployment. Applications of this method in various terrestrial/aquatic environments were demonstrated and simultaneous profiles of P^+III and P^+V across a sediment-water interface were captured at mm resolution in two contrasting redox-mesocosm systems.

Physiographical variability in arsenic dynamics in Bangladeshi soils

Rice plants grown on soils with elevated arsenic have been shown to have increased arsenic content in their grains. To gain a better understanding of the likelihood of high grain arsenic in rice grown in different soils, it is important to understand the factors affecting the bioavailability and mobility of arsenic. Paddy soils from six different physiographic regions of Bangladesh were collected, and diffusive gradients in thin-films (DGT) were used to assess the porewater and solid phase arsenic. While significant differences were identified in total soil arsenic (1.4-9.8mg/kg), porewater arsenic (^AsC_soln) (5.6-64.7μg/l), labile arsenic (^AsC_DGT) (6.3-77.6μg/l), and solid phase pool of arsenic (^AsK_d) (52-1057l/kg), importantly arsenic resupply capacity was not different between the physiographic regions. All soils had a high ratio of DGT to porewater arsenic (~1), this in conjunction with the porewater arsenic values and the high ^AsK_d values suggesting a large solid phase pool of arsenic capable of contributing towards the resupply/transport of the labile pool of arsenic in the soil porewater. This indicates that there is less difference in soil arsenic availability than might be predicted based solely on total soil arsenic content between the physiographic regions.

Assessing potential release tendency of As, Mo and W in the tributary sediments of the Three Gorges Reservoir, China

As the largest man-made reservoir in China, the Three Gorges Reservoir (TGR) has significant influence on national drinking water safety. The geochemical behavior of trace elements at the sediment-water interface (SWI) is still unknown. The mobilization characteristics of trace elements (As, Mo and W)-determined by diffusive gradients in thin films (DGT)-were studied to quantitatively calculate the release trends in the SWI in three typical tributaries and the mainstream of the TGR in the summer. The results showed that concentrations of DGT-labile As, Mo and W in the overlying water and sediment cores showed significant variations in the ranges of 0.05-50.90, 0.30-1.63 and 0.01-0.42μgL^-1, respectively. The apparent net diffusive fluxes were significantly positive in most sampling sites (77.8% for As, 88.8% for Mo and 66.6% for W), suggesting that the sediment was the source of these three elements. It was noteworthy that the maximum net diffusive fluxes of As and W were found in the upstream of Meixi tributary, which may be attributed to anthropogenic activities. In addition, As, Mo and W may be incorporated in Fe and Mn oxyhydroxides and these three elements simultaneously remobilized with Fe and Mn.

First observation of labile arsenic stratification in aluminum sulfate-amended sediments using high resolution Zr-oxide DGT

Arsenic contamination in sediments has received increasing attention because it may be released to the water and threaten aquatic organisms. In this study, aluminum sulfate (ALS) was used to immobilize As in sediments through dosage-series and time-series experiments. Diffusive gradients in thin films (DGT) was used to obtain labile As at a vertically 2.0mm resolution. Our results indicated that a "static" layer with extremely low labile As concentration (minimally 0.13mgL^-1) with weak variation (<30% RSD) formed within the top 12mm sediment layer at the dosage of 6-12ALS/As_mobile (kmolmol^-1, As_mobile means the total mobile As in top 40mm sediment) and on days 30-80 after amendment at the dosage of 9 ALS/As_mobile. The maximum labile As decreased from 1.83 to 0.99μgL^-1 and from 1.96 to 1.20μgL^-1 in the dosage-series (3-12 ALS/As_mobile) and time-series (10-80days) experiments, respectively, while the depths showing the maximal concentrations moved deeper from 22 to 34mm and 20 to 32mm in the sediments. It implied a reduced upward diffusion potential of labile As to the static layer in deeper sediments. Both distribution coefficient for As between sediment solid pool and pore water (K_d) and the adsorption rate constant (k₁) consistently increased, reflecting that As release from sediment solid became increasingly difficult with the progress of ALS immobilization. The results of this millimeter-scale investigation showed that ALS could efficiently immobilize As in sediments under simulated conditions.

Assessing remobilization characteristics of arsenic (As) in tributary sediment cores in the largest reservoir, China

The environmental impact of the Three Gorges Reservoir (TGR) in China has raised widespread concern especially in relation to metal pollution. The diffusive gradient in thin films (DGT) technology was applied to investigate arsenic (As) remobilization in sediment cores collected from the main stream and a tributary in the TGR during July 2015. The results showed that the horizontal and vertical distributions of C_DGT-As varied among the four sampling sites. For the same DGT probe, the horizontal distributions of C_DGT-As (0-6mm, 6-12mm, 12-18mm) exhibited similarity in the overlying water and different tendencies in the sediment profiles; the vertical variations of C_DGT-As showed different mobilization tendencies. Moreover, the mobility patterns of As in the sediment profile showed the diffusion potential of As from the deep sediment to the surface sediment and overlying water were in the order of MX-2< MX-1< CJ < MX-3. In addition, similar distribution characteristics and correlation analysis showed that the mechanisms of As mobility were associated with Fe and Mn. The results also showed that sulphide and As were simultaneous remobilization in the tributary sediment core in the TGR.

Characterization of arsenic availability in dry and flooded soils using sequential extraction and diffusive gradients in thin films (DGT) techniques

Much attention has been paid to the availability of arsenic (As) in soils, while few studies were carried out on the comparison between the dry and flooded soils using different methods. In this study, chemical sequential extraction and diffusive gradients in thin films (DGT) techniques were employed to study the availability of As in soils amended with a range of exogenous As followed by one-year incubation under dry and flooded conditions, respectively. The results showed that the proportions of available solid As pools, including non-specifically adsorbed As (F1) and specifically adsorbed As (F2), had consistent increases with the increase of As amendment. The concentration of DGT-measured As (C _DGT) and dissolved As in soil solution (C _SOL) also increased up to 2573 (dry) and 1823 (flooded) times and 4067 (dry) and 3105 (flooded) times of the control, respectively, while their ratios (R) showing the extent of C _SOL sustained from solid resupply decreased from 0.17 to 0.10 (dry) and 0.35 to 0.21 (flooded). Modelling with DGT-induced fluxes in soils (DIFS) further showed an increase of T _c (the characteristic time to reach equilibrium from DGT perturbation) and decreases of desorption and adsorption rate constants (k ₁ and k _-1) with the increase of As amendment, reflecting a decrease in kinetic exchange rate of As between available solid As pool and soil solution. The flooded soils had greater values of R, k _-1 and k ₁ and lower value of T _c in comparison with the dry soils, indicating a greater availability of As under the flooded condition.

Assessment of Availability and Human Health Risk Posed by Arsenic Contaminated Well Waters from Timis-Bega Area, Romania

Mobilization of As from geological materials into ground and drinking water sources may represent an important threat to human health. The objective of this study was to assess the As concentration and availability in underground water used as drinking water sources. Water samples were collected from public and private wells in Timis-Bega area of Pannonian Basin, West Romania. Total-dissolved As measured after "classical" filtration of water samples was in the range of 0.10-168 μg L^-1, thus exceeding the guideline value in majority of the samples. The aim of this study was also to assess the "truly dissolved" concentrations of As considered as available concentrations, in well waters, after passive sampling by Diffusive Gradients in Thin-films (DGT). The results showed that over 70% of total-dissolved As is in available forms. The obtained data were used to evaluate the risks of using the wells as drinking water source. Hazard quotients for ingestion and dermal pathways and hazard index (HI) for exposure to As were calculated. The HI values > 1 found that majority of samples indicated a health risk for local residents.

Zr oxide-based coloration technique for two-dimensional imaging of labile Cr(VI) using diffusive gradients in thin films

A novel Zr oxide diffusive gradients in thin films (DGT)-based measurement technique for high-resolution imaging of labile Cr (VI) is introduced in this study. The method is based on the diphenylcarbazide coloration technique for Cr (VI) combined with computer-imaging densitometry, which provides a relation between the accumulated mass of Cr (VI) and the grayscale intensity. The Zr oxide gels show good performance in reflecting the accurate measurement of Cr (VI), independent of the effects of pH, ionic strength, and PO4(3-) concentration. Settling of the Zr-oxide gel was identified as a simple and effective method to suppress leaching of the claret-colored compound out of the gel. In addition, the optimal volume of added coloration reagent was 125 times that of the binding gel and 30min was selected as the optimal time for the chromogenic reaction. The relationship between the accumulated Cr (VI) and grayscale intensity was analyzed under the optimized conditions. The Zr oxide DGT technique could also obviously reflect the heterogeneity of sediment. Consequently, Zr oxide DGT-based coloration is certified to be a robust and suitable tool for providing effective and high-resolution information on bioavailable Cr (VI).

In situ speciation of dissolved inorganic antimony in surface waters and sediment porewaters: development of a thiol-based diffusive gradients in thin films technique for Sb(III)

Antimony is a priority environmental contaminant typically present as either the trivalent (Sb(III)) or the pentavalent (Sb(V)) oxidation state in aquatic systems. Both the toxicity and mobility of antimony are affected by its speciation, and thus the accurate measurement of antimony speciation is essential for investigating the behaviour of this contaminant in aquatic systems. Here we present a diffusive gradients in thin films (DGT) technique, which utilises a binding layer containing a thiol-based adsorbent (3-mercaptopropyl functionalised silica gel), for the selective measurement of Sb(III) in surface waters and sediment porewaters. We also evaluated the Metsorb DGT technique, which has been previously reported to accurately measure Sb(V), for its ability to accumulate Sb(III) and thus allow the measurement of total inorganic antimony. Both the mercapto-silica and Metsorb DGT techniques showed a high affinity for Sb(III), with uptake efficiencies >97%. Elution efficiencies of 86.9 ± 2.6% and 88.1 ± 1.2% were obtained for mercapto-silica and Metsorb, respectively, with 1 mol L(-1) H2O2 in 1 mol L(-1) NaOH. The accumulation of Sb(III) by these DGT techniques was linear with time (R(2) > 0.99) and unaffected by pH (4.07-8.05), ionic strength (0.001-1.0 mol L(-1) NaCl), bicarbonate (1-15 mmol L(-1)), and an artificial seawater matrix (pH 8.34; salinity 34.8). Finally, the mercapto-silica DGT technique was applied to measure porewater concentrations of Sb(III) and As(III) in a contaminated freshwater sediment at high resolution.

Simultaneous Measurements of Eight Oxyanions Using High-Capacity Diffusive Gradients in Thin Films (Zr-Oxide DGT) with a High-Efficiency Elution Procedure

A zirconium oxide binding gel-based diffusive gradients in thin films (Zr-oxide DGT) was developed for simultaneous measurements of P(V), As(V), Cr(VI), Mo(VI), Sb(V), Se(VI), V(V), and W(VI). All of the oxyanions were rapidly bound to Zr-oxide gel with differences in binding affinity. The eight bound oxyanions were successfully recovered by one-step elution using a mild reagent of 0.2 M NaOH-0.5 M H2O2 by overcoming the problems in analyses of the oxyanions. The optimized elution time was reduced to 3-5 h from 24-48 h required by other DGTs. DGT uptakes of all the oxyanions were independent of pH (4.42-8.45) and ionic strength (0.1-500 mM). The DGT capacities for six oxyanions detected in multioxyanion solution were only 0.19 to 0.35 times of those detected in single-oxyanion solution, reflecting a strong competition among the oxyanions during DGT uptake. Except for Se(VI) in seawater, Zr-oxide DGT accurately measured all of the oxyanions in synthetic freshwater and seawater, with the capacities ∼29 to >2397 times and ∼7.5 to 232 times those of two commonly used DGTs (Metsorb and precipitated ferrihydrite (PF) DGTs) in freshwater and seawater, respectively. Measurements by Zr-oxide DGT in contaminated sediments were in agreement with only two oxyanions with the two commonly used DGTs; the two DGTs accumulated less or no mass of other oxyanions. This study demonstrates significant advantage of Zr-oxide DGT over the other DGTs in simultaneous measurements of the eight oxyanions due to the former's high capacity and a wide tolerance to environmental interferences, together with a high efficiency in elution.

In-situ characterization and assessment of arsenic mobility in lake sediments

In-situ characterization and assessment of arsenic (As) mobility in sediments was scarce. In this study, the distributions of labile As at a vertical resolution of 2 mm were obtained in the sediments of a large Lake Taihu through in-situ measurements using a Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) technique. The DGT-labile As, interpreted as DGT flux (FDGT), exhibited three different patterns in the lake, with all the patterns generally showing an increasing mobility followed by a decreasing mobility with sediment depth. The mobility of As could be characterized by the average FDGT (0.06-1.27 pg cm(-2) s(-1)) in the top 10 mm surface sediments, the maximal FDGT (FDGT-M, 0.14-2.44 pg cm(-2) s(-1)) in the end of the initial increasing phase of FDGT, and the diffusion length (ΔL, 28-66 mm) from the depth showing the FDGT-M to the sediment-water interface. The upward mobilization of labile As from the deep sediments to the surface sediments and overlying water became evident when FDGT-M > 1.7 pg cm(-2) s(-1) or ΔL < 41 mm. The results, for the first time, showed a prospect in in-situ risk assessment of the pollution of sediment As. It was suggested that the increasing mobility of As in the upper sediments was controlled by the reduction of As(V) and the reductive dissolution of Fe(III) (hydr)oxides, while the decreasing mobility in the deep sediments was attributed to immobilization of As(III) by secondary Fe(II)-bearing minerals.

In situ high-resolution evaluation of labile arsenic and mercury in sediment of a large shallow lake

The precise evaluation of arsenic (As) and mercury (Hg) bioavailability in sediment is crucial to controlling As and Hg contamination, but traditional ex situ measurements hamper comprehensive analysis of labile As and Hg in sediment. In this study, we characterized in situ labile As and Hg in sediment of Lake Hongze using the zirconium (Zr) oxide diffusive gradients in thin films (DGT) technique and 3-mercaptopropyl functionalized silica gel DGT, respectively. The concentrations of DGT-labile As and Hg in the sediment profiles were found to exhibit considerable variation, ranging from 0.15 to 4.15 μg L(-1) for As and from 0.04 to 1.35 μg L(-1) for Hg. As and Hg flux values, calculated based on the concentration gradients measured from the DGT profiles for both the overlying water and sediment close to the sediment-water interface, were used to determine the contamination status of As and Hg. Flux values of As and Hg were between -0.066 and 0.067 ng cm(-2)d(-1) and between -0.0187 and 0.0181 ng cm(-2)d(-1), respectively. The GNU's Not Unix R (GNU R) programming language was used to identify outliers of As and Hg at various depths at the sampling sites. The results indicate that the sites with the most outliers were all located in the regions that were seriously affected by contaminants from the Huai River. The DGT-labile As and Hg concentrations in the 0-30 mm layer were found to be significantly correlated with concentrations of labile As and Hg, total dissolved As and Hg, and total As and Hg in the overlying water, as indicated by ex situ measurements. Results show that DGT is a reliable and high-resolution technique that can be used for in situ monitoring of the labile fractions of As and Hg in sediment in fresh water bodies.