(DGT) Measurement of Total Dissolved Inorganic Arsenic in Waters

Feb 17, 2014 - Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, College of. Environment, Hoha...
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Improved Diffusive Gradients in Thin Films (DGT) Measurement of Total Dissolved Inorganic Arsenic in Waters and Soils Using a Hydrous Zirconium Oxide Binding Layer Qin Sun,† Jing Chen,† Hao Zhang,‡ Shiming Ding,*,§ Zhu Li,⊥ Paul N. Williams,∥ Hao Cheng,‡ Chao Han,§ Longhua Wu,⊥ and Chaosheng Zhang@ †

Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, Nanjing 210098, China ‡ Lancaster Environment Center (LEC), Lancaster University, Lancaster LA1 4YQ, United Kingdom § State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China ∥ Institute for Global Food Security, School of Biological Sciences, Queen’s University Belfast, Belfast BT9 7BL, United Kingdom ⊥ Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China @ GIS Centre, Ryan Institute and School of Geography and Archaeology, National University of Ireland, Galway, Ireland S Supporting Information *

ABSTRACT: A high-capacity diffusive gradients in thin films (DGT) technique has been developed for measurement of total dissolved inorganic arsenic (As) using a long shelf life binding gel layer containing hydrous zirconium oxide (Zr-oxide). Both As(III) and As(V) were rapidly accumulated in the Zr-oxide gel and could be quantitatively recovered by elution using 1.0 M NaOH for freshwater or a mixture of 1.0 M NaOH and 1.0 M H2O2 for seawater. DGT uptake of As(III) and As(V) increased linearly with deployment time and was independent of pH (2.0−9.1), ionic strength (0.01−750 mM), the coexistence of phosphate (0.25−10 mg P L−1), and the aging of the Zr-oxide gel up to 24 months after production. The capacities of the Zr-oxide DGT were 159 μg As(III) and 434 μg As(V) per device for freshwater and 94 μg As(III) and 152 μg As(V) per device for seawater. These values were 5−29 times and 3−19 times more than those reported for the commonly used ferrihydrite and Metsorb DGTs, respectively. Deployments of the Zr-oxide DGT in As-spiked synthetic seawater provided accurate measurements of total dissolved inorganic As over the 96 h deployment, whereas ferrihydrite and Metsorb DGTs only measured the concentrations accurately up to 24 and 48 h, respectively. Deployments in soils showed that the Zr-oxide DGT was a reliable and robust tool, even for soil samples heavily polluted with As. In contrast, As in these soils was underestimated by ferrihydrite and Metsorb DGTs due to insufficient effective capacities, which were likely suppressed by the competing effects of phosphate. iffusive gradients in thin films (DGT) is a passive sampling technique used worldwide for measurements of a broad range of labile metals and metalloids in waters, sediments, and soils.1,2 A typical DGT device consists of a binding layer overlaid by a diffusive layer. The binding layer is usually a hydrogel containing an analyte-specific adsorbent. The analyte (in dissolved form) diffuses through the diffusive layer and then is bound in the binding gel layer. A concentration gradient is rapidly established within the diffusive layer and maintained during the period of DGT deployment. The timeaveraged concentration of the solute in the solution is quantified using Fick’s first law.3 There is potential for the adsorbent to become saturated with the increasing accumulation of the analyte in the binding gel, resulting in an inaccurate measurement of the analyte when the

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© 2014 American Chemical Society

standard DGT equation is used.3 Various laboratory tests have shown that the effective capacity of a binding gel layer may be significantly affected by a range of factors, including pH, ionic strength, aging effects of the binding gels, competitive ions, and organic ligands.4−8 Correspondingly, a number of studies have reported that DGT may underestimate target analytes in waters, soils, and sediments due to insufficient capacity of the binding gels.4−8 In those experiments, effects associated with capacity limitation were experienced using typical deployment durations of 1 to 2 days, so extension of the measurement to embrace geochemical processes occurring on a longer timescale Received: December 11, 2013 Accepted: February 17, 2014 Published: February 17, 2014 3060

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equipped with a 3-mercaptopropyl-functionalized silica gel was recently developed for selective measurement of inorganic As(III).27 The in situ speciation of inorganic As has been achieved by using it in combination with the Metsorb DGT.27 Recently, Ding and co-workers have well developed a new DGT technique for measurement of phosphorus using a binding layer impregnated with hydrous zirconium oxide (Zroxide).4,11 The present study investigates for the first time the detailed performance characteristics of the DGT equipped with a Zr-oxide binding layer for the measurement of total dissolved inorganic As, since the Zr-oxide has also been shown to exhibit a good performance in binding inorganic arsenic through its negatively charged hydroxyl groups.31 A series of validation experiments were conducted to evaluate the performance of the Zr-oxide DGT in various solutions, and its performance in natural waters and soils was compared with DGTs equipped with slurry ferrihydrite and Metsorb binding layers.

is currently a challenge. Liu et al. were able to overcome similar sampling limitations with successive deployments of a specialized DGT with a very specific affinity and high capacity for methyl mercury.9 This enabled the bioavailability of methyl mercury to be successfully monitored and predicted during the entire growth period of rice, by successive deployments at twoweek intervals.9 Unfortunately, even this approach is not possible for other metal(loids) because the binding gel capacity restricts sampling to very short deployments. Capacity limitations are also a consideration for high resolution DGT applications, where small geochemically reactive zones, termed microniches, mobilize high concentrations of trace elements.10 To date, high-resolution DGTs are limited to only a few binding gel types with a limited element specificity and capacity.11−14 Arsenic (As) is a notorious toxin that has been ranked by the International Agency for Research on Cancer (IARC) as a group 1 carcinogen.15 Due to a number of naturally occurring processes and increasing anthropogenic releases, dangerous As concentrations in natural waters are now a worldwide problem and have been referred to as a 20th−21st century calamity.16 More recently, considerable concerns have been raised about As biogeochemistry in paddy soils and As accumulation in rice plants, because rice is the primary food for billons of people17 and has been shown to accumulate As more efficiently than other cereal crops.18 Concentrations of As vary greatly in the terrestrial environment, ranging from less than 0.5 μg L−1 to more than 5000 mg L−1 in natural waters and 1.5 mg kg−1 to 8000 mg kg−1 in soils.19 Concentrations of As in flooded soil solutions are often much greater than those in aerobic soils due to the release of iron oxide-bound As species and the reduction of strongly adsorbed arsenate to more weakly adsorbed arsenite.20,21 Arsenic exists mostly in inorganic forms but is also present to a lesser extent in organic forms.19,22 Because inorganic forms of As are more toxic and more common than its organic forms,23 accurate measurement of inorganic arsenic is a priority in understanding its toxicity, transport, and risk assessment. The DGT technique has been developed for measurements of dissolved inorganic As in waters, soils, and sediments. Zhang et al. and Fitz et al. first reported the uses of the slurry ferrihydrite DGT in measurements of total dissolved inorganic As in sediments and soils, respectively.24,25 Panther et al. reported a detailed investigation of this type of DGT and showed that it was a reliable technique for measuring As in freshwater.26 Bennett et al. found it underestimated both inorganic As(III) and As(V) in As-spiked seawater, due to a relatively low capacity and selectivity of the adsorbent for binding inorganic As ions.27 A precipitated ferrihydrite gel, with a greater capacity than the slurry ferrihydrite gel, was also developed to enable DGT measurement of As at high spatial resolution. The gel worked well only within a limited period (38 days) after its production because of the progressive conversion of ferrihydrite into goethite or hematite.28 DGT equipped with another type of binding gel layer containing titanium dioxide, named Metsorb, was reported to be capable of measuring As in seawater.29 Its capacity for As(III) is only 37% of that reported for the slurry ferrihydrite DGT, which likely limits its application, especially in flooded soils and sediments where inorganic As(III) is the dominant species. 19,30 Furthermore, there has been no demonstration that this type of binding gel is capable of high-resolution measurement at the submillimeter scale. A new high capacity DGT variant,

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EXPERIMENTAL SECTION Reagents, Materials, and Solutions. The chemicals used in this work were of analytical reagent grade and supplied by SCR Company Ltd., China, unless stated otherwise. Synthetic freshwater and seawater were prepared according to Langmuir32 and Grasshoff et al33 (Table S1 of the Supporting Information). Arsenic solutions for experiments were prepared using separate As(III) and As(V) stock solutions. The details of solution preparation together with an examination of As(III) stability in solutions are presented in Figure S1 of the Supporting Information. Arsenic Analysis. Concentrations of inorganic As(III) and As(V) in solution samples were analyzed using a hydride generation coupled with AFS (HG-AFS) (AF-610D, Beijing Rayleigh Analytical Instrument Corporation, China). The detailed procedures for the analysis are presented in the Supporting Information. Inorganic As(III) and As(V) measured using HG-AFS agreed well with measurements made by ICPMS (Table S2 and Figure S2 of the Supporting Information). Gel Preparation, DGT Assembly, and Calculation. The diffusive gel was prepared with 15% acrylamide and 0.3% agarose-derived cross-linker following a published procedure.34 The Zr-oxide, slurry ferrihydrite, and Metsorb binding gels were prepared according to Ding et al.,4 Zhang et al.,35 and Bennett et al.,29 respectively. The piston-type DGT holder with a 2 cm diameter exposure window was supplied by DGT Research Limited (Lancaster, U.K.) and was used for deployment in test solutions, waters, and soils. In the DGT assembly, a binding gel was covered sequentially by a diffusion gel and a filter membrane (0.13 mm thickness, Whatman, 0.45 μm pore size). The thickness of the diffusive gel used was 0.80 mm. The thicknesses of the Zr-oxide, ferrihydrite, and Metsorb binding gels used were 0.40 mm. The DGT-measured concentration of each arsenic species in solutions was calculated based on the well-established DGT equation3 using the diffusion coefficients (D) measured in this study. For seawater deployments, a value of 0.9 × D was used.3,36 Performance Test of the Zr-Oxide DGT. Binding Kinetics. The dynamics of inorganic As(III) and As(V) bindings to the Zr-oxide gel were investigated by exposing gel discs (2.5 cm diameter) to 10 mL of solutions at pH 7.0, containing either As(III) or As(V) at 50 μg L−1 and 0.01 M NaNO3. The solutions were immediately shaken at room temperature for various time intervals ranging from 2.5 to 120 min. 3061

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Zr-oxide DGT units were deployed at 24 °C for 25, 72, and 96 h, respectively. The accumulated As in the gel was eluted using 1.8 mL of 1.0 M NaOH. The synthetic seawater was spiked with equivalent amounts of As(III) and As(V) at a total concentration of approximately 200 μg L−1. The Zr-oxide DGT units were deployed at 24 °C for up to 96 h. Slurry ferrihydrite and Metsorb DGTs were simultaneously deployed for comparison. Deployment in Soils. Twenty nine soil samples were used for the test, with 28 of them collected from smelting-impacted areas with various heavy metals (e.g., Pb, Cu, Zn, and Cd) in Hunan Province, China. Total contents of As in these soils varied from 7.3 to 703 mg kg−1. One soil sample containing very high As (2886 mg kg−1) was collected from a mining area in Guangdong Province, China. The physical and chemical properties of all the soil samples are summarized in Table S3 of the Supporting Information. The soil samples were air-dried at room temperature, sieved using a 2 mm mesh sieve, and mixed homogeneously. They were incubated at approximately 60% of the maximum water holding capacity (MWHC) for 48 h and then 80% of the MWHC for 24 h to achieve a sufficient moisture level prior to DGT deployment. The deployment time of 24 h was typical of that used by others,37 and the temperature was maintained at 20 ± 1 °C. The slurry ferrihydrite and Metsorb DGTs were deployed simultaneously for comparison.

Elution Efficiencies. Elution efficiencies were investigated by exposing the Zr-oxide gels to 10 mL of 0.01 M NaNO3 solutions or synthetic seawater containing either As(III) or As(V) at concentrations ranging from 10 to approximately 7000 μg L−1. The solutions were shaken at room temperature for 24 h to ensure maximum uptake. After 24 h, the gels were retrieved and rinsed with deionized water. Each gel was then eluted using 1.8 mL of 1.0 M NaOH or a mixed solution containing 1.0 M NaOH and 1.0 M H2O2 (1.0 M NaOH/1.0 M H2O2). Time Dependence of DGT Uptake. The time dependence of As(III) and As(V) uptake by Zr-oxide DGT was investigated by deploying DGT units in 10 L of 0.01 M NaNO3 solution at pH 6.0 containing separate solutions of As(III) and As(V) at 50 μg L−1. The deployment time varied from 4 to 24 h. The solutions were well-mixed using a magnetic stirrer during the experiment. At each retrieval time, three duplicate DGT devices were removed and rinsed with deionized water. The As(III) or As(V) accumulated in the binding gel was eluted with 1.8 mL of 1.0 M NaOH. Effects of pH, Ionic Strength, and Phosphate on DGT Uptake. The effect of pH was investigated by deploying the DGT units for 4 h in 2 L of 0.01 M NaNO3 solutions containing approximately 100 μg L−1 As(III) or As(V). The pH of the solutions was preadjusted to different values (2.0−9.1). The effect of ionic strength was tested by deploying the DGT units for 4 h in 2 L of solutions at pH 6.0 containing 50 μg L−1 of As (III) or As(V) and different concentrations of NaNO3, ranging from 0.01 to 750 mM. The effect of phosphate was investigated by deploying the DGT units for 24 h at pH 6.0 in 2 L of 0.01 M NaNO3 solutions containing approximately 100 μg L−1 of As (III) or As(V) and different concentrations of PO43− (KH2PO4), ranging from 0.25 to 10 mg P L−1. Capacity and Aging Effect. The DGT units were deployed in well-mixed 0.01 M NaNO3 solutions at pH 6.0 or synthetic seawater at pH 8.2 containing different concentrations of As(III) for 11 h and As(V) (in separate solutions) for 8 h or containing fixed concentrations of single As(III) or As(V) for different time intervals ranging from 2 to 24 h. To examine the possible aging effect of the Zr-oxide gel on the performance of the DGT, the DGT units were assembled with the prepared freshly Zr-oxide gels and with aging for periods of 3, 6, 10, 14, and 24 months, respectively, from their production by storing the gels in a refrigerator (2−4 °C). The assembled DGT units were deployed for 8 h in 2 L of 0.01 M NaNO3 solutions containing approximately 1.0 mg L−1 As(III) or As(V). Measurement of Diffusion Coefficients. The diffusion coefficients of As(III) and As(V) in diffusive gel (Dcell) were measured using a diffusion cell, as described previously,36 with the details presented in the Supporting Information. The effective diffusion coefficients (DDGT) for As(III) and As(V) were also determined from mass accumulation of DGT over time experiments. The measured coefficients at a given temperature were corrected to 25 °C, according to Zhang et al.3 Application of the Zr-Oxide DGT. Deployment in Waters. The Zr-oxide DGT was used to measure total dissolved inorganic As in synthetic freshwater and seawater and in an unfiltered natural freshwater collected from Lake Taihu, China. The composition of the natural freshwater is summarized in Table S1 of the Supporting Information. The natural and synthetic freshwater samples were spiked with As(III) and As(V) at different ratios, while the total concentrations of inorganic As were kept at 50 μg L−1. The

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RESULTS AND DISCUSSION DGT Blanks and Method Detection Limit. The mass of As in the blank Zr-oxide gel was determined by deploying 15 Zr-oxide DGT devices in 10 L of 0.01 M NaNO3 solution for 24 h. After retrieval, the Zr-oxide gel was eluted with 1.8 mL of 1.0 M NaOH. The average mass of As in the blank gels was measured at 1.83 ± 0.71 ng per device, which were slightly higher than those reported in blank ferrihydrite and Metsorb gels (0.58−1.15 ng per device).38,39 The method detection limit (MDL), calculated as three times the standard deviation of 15 blank masses, was converted to a concentration of 0.08 μg L−1, assuming a deployment time of 24 h at 25 °C with a 0.80 mm thick diffusive gel and a 0.13 mm filter membrane. The MDL was comparable to those of blank ferrihydrite and Metsorb gels (0.02−0.16 μg L−1).28,29,38,39 The value was slightly lower than the minimum concentration of As in river, lake, and estuarine waters (0.1 μg L−1) and much lower than the minimum concentrations of As in groundwaters (34.5b

As(V)

434

As(III) As(V)

94 152

87c, 40b, 31.5a, 30d, 15e 20a 11.5a, 21.8b

MetsorbDGT 8.5a 82a 7.5a 52a

MercaptoSilica DGT 77.5a − >129a −

Bennett et al. (2011).27 bPrice et al. (2013).39 cLuo et al. (2010).28 Panther et al. (2008).26 eHuynh et al. (2012).37

d

seawater relative to those in freshwater, as reported for other DGT variants,27,39 have been attributed to competition by major anions for binding sites on the adsorbents7 and a high pH value in seawater.45 The measured capacities of the Zr-oxide DGT for As(III) were 7 and 19 times (freshwater) and 5 and 13 times (seawater) greater than that of slurry ferrihydrite and Metsorb DGTs, respectively. Its capacity for measurement of As(V) (freshwater) was 5−29 times higher than that of ferrihydritebased DGTs (i.e., 11−14 times that of the slurry ferrihydrite DGT, 5 times that of the precipitated ferrihydrite DGT, and 29 times that of a DGT equipped with a mixed binding layer containing slurry ferrihydrite). It was 5 times that of the Metsorb DGTs for freshwater, while for seawater, its capacity was 13 and 3 times that of the slurry ferrihydrite and Metsorb DGTs, respectively. The capacity of the Zr-oxide DGT for As(III) was lower than that of a Mercapto-Silica DGT for selective measurement of As(III) in seawater, but it was twice that of the Mercapto-Silica DGT for freshwater. 3064

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Deployment in Waters. The performance of the Zr-oxide DGT was first tested in natural and synthetic freshwaters spiked with inorganic As(III) and As(V) at two different ratios (4:1 and 1:4) (Table S4 of the Supporting Information). A deployment time of 96 h was used to reflect the possible effect of competing ions on the effective capacity of the DGT.7 Since there is generally a mixture of inorganic As(III) and As(V) in natural environments, the DGT-measured concentration represents total inorganic As. Accordingly, as for previous measurements using the slurry ferrihydrite and Metsorb DGTs, the average diffusion coefficients of inorganic As(III) and As(V) were used for the DGT calculation.29,46 The DGT-measured concentrations of total inorganic As corresponded well to those directly measured in waters for all the deployments. Their ratios (0.85−1.17) were within an acceptable experimental uncertainty considered for an in situ sampling method (0.80−1.20, Table S4 of the Supporting Information).29 Seawater, with its high concentrations of competing ions, is a challenging medium for DGT. The Zr-oxide DGT-measured concentrations of total As corresponded to those directly measured in seawaters for all the deployments, with the measured-to-predicted ratios varying from 0.95 to 1.08 (Figure 2). Accurate measurements were achieved at 24 h for the slurry

Deployment in Soils. The Zr-oxide DGT was evaluated together with the slurry ferrihydrite and Metsorb DGTs using 29 soils. The total concentration of As in the soil sample from Guangdong (2886 mg kg−1) was much greater than those from Hunan (7.3−703 mg kg−1). Such high concentrations of As have been reported for other As-contaminated soils and sludges.19,23 The measured masses of total As in the Zr-oxide DGT, when below 10 μg per device, corresponded well to those in the other two types of DGTs for the 27 soils collected from Hunan Province (Figure 3). However, when the masses of

Figure 3. Relationships between uptake of total As from soils by ferrihydrite and Metsorb DGTs and uptake by Zr-oxide DGT. The line shows the 1:1 relationship. Values are means ± SD of three replicate analyses.

accumulated As were 23 μg per device for the remaining soil of Hunan Province and 78 μg per device for the soil of Guangdong Province, the masses of As accumulated in the ferrihydrite DGT were only 82% and 66%, respectively, of those in the Zr-oxide DGT. The mass of As accumulated in the Metsorb DGT was only 86% of that in the Zr-oxide DGT for the Guangdong soil, but these two measurements agreed for all Hunan soils. These results show that while ferrihydrite and Metsorb DGTs can be expected to work well for most As concentrations in soils, they may underestimate the As in very contaminated soils. Moreover, the soil samples tested had been air-dried prior to DGT deployment. It has been well-recognized that concentration of inorganic As(III) in soil solution are likely to be much higher (around 10 times) when the soil is under continuously flooded conditions.20,21 Accordingly, As would be underestimated by these two types of DGT in more soil samples if the soils were anoxic prior to DGT measurement. As arsenates are the dominant species under oxic soil conditions;23 the uptake of As by the DGT should be mainly inorganic As(V). The theoretically predicted masses of As for the two problem soils can be estimated from the measurement by the Zr-oxide DGTs to be 23 and 78 μg per device. These two values are very close to the capacities of the ferrihydrite and Metsorb DGTs for measurement of As(V) in freshwater (∼30 and 82 μg per device, Table 2). Underestimation of the two soil As reflected that the effective capacities of the two DGTs for measurement of As(V) were lowered due to interferences of the two soil matrixes. The lowering of the effective capacity is unlikely to be due to the competing effects of Cl−, SO42−, and HCO3−, as these ions are found in the matrix of seawater.7,27 However, PO43−, which has been neglected in previous studies,7,27 lowers the DGT capacity. In accordance with Figure S6, PO43− at concentration levels greater than 0.25 and

Figure 2. Dependence of uptake of total As by different DGTs on deployment time in synthetic seawater. The line is the theoretical response calculated using the DGT equation. Values are means ± SD of three replicate analyses.

ferrihydrite DGT and up to 48 h for the Metsorb DGT, but for longer deployment times the measured-to-predicted ratios were all below 0.80. Therefore, the Zr-oxide DGT still functions properly in seawater at deployment times when the other two DGT devices begin to fail. This performance was attributed to the high capacities of the Zr-oxide DGT in measurements of As(III) and As(V) in seawater. The capacities for As(III) and As(V) were 6 and 9 times, respectively, the total mass of As predicted to accumulate (17 μg per device) at 96 h. In contrast, the calculated accumulation mass of As(III) at 48 h (5.2 μg per device) approached the capacity of the Metsorb DGT for measurement of As(III) in seawater (7.5 μg per device, Table 2). For the slurry ferrihydrite DGT, the calculated accumulation masses of As(III) and As(V) at 24 h (2.6 and 1.7 μg per device) were much less than their capacities in measurements of As(III) and As(V) in seawater (20 and 11.5 μg per device, respectively; Table 2). The apparent saturation of the ferrihydrite gel may be attributed to a low selectivity toward oxyanionic species, which has been reported previously in experiments comparing it to Metsorb.7,27 3065

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1.0 mg P L−1 lowers the uptake of inorganic As(V) by the ferrihydrite and Metsorb DGTs, respectively, when they are deployed for 24 h. Soil solutions for the two soils were isolated by centrifugation. The concentrations of DRP were 0.42 and 1.41 mg P L−1 for the Hunan and Guangdong soils, respectively, consistent with DGT uptake of As being lowered because of the presence of phosphate. This result suggests that ferrihydrite and Metsorb DGTs should be used cautiously for measurements of total dissolved inorganic As in soils and sediments where concentrations of DRP up to several mg P L−1 have often been reported.4,47−49 The Zr-oxide DGT is a more reliable technique because it can tolerate a wider concentration range of DRP in soils or sediments. Further detailed investigation is required to assess the competing effects of PO43− on measurement of inorganic As using DGT.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Drs. Laiqing Lou and Pengjie Hu for providing soil samples. This study was sponsored by the National Natural Scientific Foundation of China (Grants 41322011, 21177134, and 41001334), the Nanjing Institute of Geography and Limnology (Grant NIGLAS2010KXJ01), and K. C. Wong Education Foundation, Hong Kong.

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CONCLUSIONS The developed Zr-oxide DGT offers promising improvements over the previously used DGTs in measurements of total dissolved inorganic As, primarily because of its very high capacity for both inorganic As(III) and As(V). This capability enables the Zr-oxide DGT to be applied to a broad range of environments, such as very contaminated soils and As-enriched seawaters as observed in this study. It is also possible to extend the deployment time to weeks, enabling successive monitoring of As bioavailability in soils/sediments over long time periods, such as complete crop cycles.9 Furthermore, this advantage of high capacity facilitates the use of nondestructive on-site methods (e.g., field portable X-ray fluorescence) to detect As accumulated in the DGT devices, because the analytical errors of these methods can be significantly reduced by having a high accumulated mass.50 The Zr-oxide gel could be stored under low temperature (∼4 °C) for a long period (at least 2 years) prior to DGT use, thereby simplifying the practical use of DGT. The Zr-oxide DGT was less prone to the competing effect of phosphate on the measurement of As, which can be a significant factor influencing the robustness of ferrihydrite and Metsorb DGTs when deployed in soils and sediments. Previous studies have shown that the Zr-oxide DGT has notable advantages in high-resolution measurements of DRP, enabling simultaneous measurements of DRP with dissolved sulfide and ferrous iron.4,5,11,12,51 There is a great potential for incorporation of As within this suite of determinands, allowing investigation of the As biochemistry cycle. These developments will overcome the limitations of the short shelf life of the precipitated ferrihydrite gel and the low capacities of existing DGT assemblies equipped with mixed binding gels for simultaneous measurements of As and metals.28,37 Future work should also focus on the combined uses of the Zr-oxide and the Mercapto-Silica DGTs for in situ differentiating inorganic As speciation between As(III) and As(V).27 Their high capacities may significantly improve the robustness of this method.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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