High-Resolution Simultaneous Measurements of Dissolved Reactive

Jun 26, 2012 - For deployment in solutions, piston-type DGT holders with 2 cm diameter exposure windows open to the solution were used. A flat probe w...
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High-Resolution Simultaneous Measurements of Dissolved Reactive Phosphorus and Dissolved Sulfide: The First Observation of Their Simultaneous Release in Sediments Shiming Ding,*,† Qin Sun,‡ Di Xu,† Fei Jia,‡ Xiang He,† and Chaosheng Zhang§ †

State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China ‡ College of Environmental Science and Engineering, Hohai University, Nanjing, China § GIS Centre, Ryan Institute and School of Geography and Archaeology, National University of Ireland, Galway, Ireland S Supporting Information *

ABSTRACT: The reassessments of environmental processes in sediments rely upon capturing the heterogeneous features of elements at a small scale and at the same location. In this study, a diffusive gradients in thin films (DGT) technique was developed for the high-resolution simultaneous measurements of dissolved reactive phosphorus (DRP) and dissolved sulfide. A new binding gel was used in this DGT technique, which was prepared by incorporating AgI particles into the zirconium oxide binding gel previously used in the DGT measurement of DRP. The concentrations of the DRP and sulfide loaded into the binding gel were determined by a routine procedure and a computerimaging densitometry (CID) technique, respectively. The performance of this DGT technique was tested under laboratory conditions and applied to in situ measurements in sediments of a shallow lake. Simultaneous release of DRP and dissolved sulfide was observed in a sulfide microniche with a diameter of ∼3 mm and in locally aggregated zones with a length over 1 cm, which was attributed to the simultaneous reductions of Fe(III) and sulfate and the associated release of Fe-bound P in the zones of the reactive organic matter in sediments. The good performance of this technique implies that there is a great potential for the development of new DGT techniques capable of simultaneous measurements of more analytes.

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INTRODUCTION The heterogeneous biogeochemistry of sediments has been long recognized,1 even though it has received particular attention recently.2,3 This recognition is highlighted particularly by the distinct heterogeneity observations surveyed from the solute distributions at two dimensions (2D) and at different scales.3−10 Considering the heterogeneous nature of sediment, it has become evident that the reassessment of element diagenesis should be performed because this process maybe more complex than previously assumed. Both phosphorus (P) and sulfur (S) are essential elements for living organisms, and they have major environmental impacts on the aquatic ecosystem. Previous studies have focused on the remobilization process of P in sediments and its negative influence on water quality.11 This process was found to be indirectly related to sulfate reduction in some sediments through the formation of insoluble iron sulfide compounds (e.g., pyrite) and the associated release of P binding with iron oxides.12,13 Despite this hypothesis, there is a general lack of detailed information about their relationship, particularly when their dissolved forms in sediments, such as dissolved reactive phosphorus (DRP) and sulfide (H2S + HS− + S2−, defining as © 2012 American Chemical Society

S(-II) here), have been found to display large degrees of heterogeneity in distribution.4,6,8−10,14−16 The reassessments of element diagenesis and related processes rely upon the development of analytical techniques to capture their heterogeneous features in sediment profiles. These techniques must satisfy several requirements in performance, including high spatial resolution (millimeters to submillimeters), two dimensions, rapid response (within days), and simultaneous measurements of analytes. The final requirement is that the technique must achieve chemical distribution information of different analytes at exactly the same location. This criterion is critical to investigate the relationships between small-scale processes, such as those occurring in microniches.3 Several techniques have been developed to meet these demands, including planar optodes,7,17 diffusive gradients in thin films (DGT)5,18 and diffusive equilibrium in thin films (DET).19 The DGT normally measures a solute through the Received: Revised: Accepted: Published: 8297

March 24, 2012 June 18, 2012 June 26, 2012 June 26, 2012 dx.doi.org/10.1021/es301134h | Environ. Sci. Technol. 2012, 46, 8297−8304

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flux of the solute to diffuse a well-defined gel layer. This technique appears to be particularly promising because it can measure a wide range of analytes.20,21 Its measurements can also be used to assess the local supply of a solute from pore water and solid phase in sediments in response to a DGT probe perturbation.20 The DGT technique has been applied to high-resolution 2D measurements of S(-II) with the use of computer-imaging densitometry techniques (CID),8,16,22 and it has been used in the high-resolution 2D measurements of P in combination with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses.14 Simultaneous measurements of P and S(-II) have been performed using DGT or combined DGT/ DET, with P measured with DET6 and S(-II) measured with DGT qualitatively.15 Recently, Ding et al.4 developed a Zr oxide DGT technique for the 2D measurement of DRP at a high resolution in combination with a routine procedure consisting of 2D slicing, elution, and microcolorimetric determination. This technique uses a zirconium oxide-impregnated binding gel (Zr oxide gel) with a white, opaque surface,23 which likely allowed for the quantification of grayscale density changes using CID, if simultaneously measuring S(-II) with DGT. The objective of this study was to establish a novel DGT technique for high-resolution simultaneous measurements of DRP and dissolved S(-II) based on the Zr oxide DGT technique. The Zr oxide binding gel was modified for simultaneous binding of DRP and dissolved S(-II) prior to the use in the DGT assembly. The performance of this DGT technique was tested under laboratory conditions and was applied to in situ measurements in sediments of a shallow lake.

Zr oxide added in the stock gel solution was reduced by half, which was the same as in this study. The distributions of Zr oxide, AgI, or Ag2S on the various gels with or without S(-II) exposure were checked by scanning electron microscopy and energy dispersive spectrometry (SEM-EDS). The gels were placed on a gel dryer for 2 h at 60 °C under a vacuum. Surface morphology of the dried gels was checked using a ZEISS EV018 scanning electron microscopy (ZEISS EV018, Germany) with an attached energy dispersive spectrometer (Bruker, Germany). Two different assemblies were used for the DGT deployment. For deployment in solutions, piston-type DGT holders with 2 cm diameter exposure windows open to the solution were used. A flat probe with a 1.8 cm × 16 cm (wide × length) exposure window was used for the deployment in the sediments. In both types of DGT assemblies, a binding gel (ZrO−AgI surface up) was placed on the bottom of the holder, which was covered sequentially by a diffusion gel and a 0.13 mm cellulose nitrate filter membrane (Whatman, 0.45 μm pore size). The thickness of the diffusive gel in the piston-type DGT unit was 0.80 mm, while a 0.40 mm thick diffusive gel was used in the sediment probes. The DGT assemblies were deoxygenated with nitrogen for at least 16 h and kept in the dark prior to their deployments in the solutions or in the sediments. Optimization of the ZrO−AgI Binding Gel. The ZrO− AgI binding gels incorporating different amounts of AgI were scanned in a flat-bed scanner (Canon 5600F) at a resolution of 150 dpi, corresponding to a resolution of 0.169 mm × 0.169 mm. The grayscale intensity of the scanned images was analyzed with ImageJ 1.42 (downloaded from http://rsb.info. nih.gov/ij). The binding gels were then assembled with the diffusive gel using the piston-type DGT holders. The DGT units were exposed to a S(-II) solution (Na2S·9H2O, Sinopharm Chemical Reagent) containing 0.03 M NaNO3 and ∼15 mg L−1 S(-II) for 24 h to allow for the uptake of S(-II) by the DGT to reach saturation. After retrieval of the DGT assemblies, the binding gels were removed from the units. The grayscale intensity of each binding gel was achieved according to the procedure mentioned earlier. The ZrO−AgI binding gels with and without S(-II) saturation were cut into discs with 1.1 cm diameters. The kinetics of DRP bindings to the gels were examined by immersing the gel discs in 5 mL of a KH2PO4 solution containing 1 mg P L−1 and 0.03 M NaNO3 at pH 7.0 for different time intervals ranging from 5 min to 2 h. The elution efficiencies for the gels were investigated by immersing the gel discs in various KH2PO4 solutions with different concentrations for 24 h, followed by elution of the gel discs with 1 mol L−1 NaOH. The DRP concentrations in the solutions before and after the gel immersions were detected using the molybdenum blue method.24 Calibration Procedure for Sulfide Measurements. Calibration of the S(-II) measurement was performed by studying the relationships between the grayscale intensities of the optimized ZrO−AgI binding gel and its accumulated masses of S(-II) after the DGT deployments in the S(-II) solutions with different concentrations (0 to ∼6.5 μg L−1 S(II)). Briefly, each S(-II) solution (containing 0.03 mol L−1 NaNO3) was added into a 500 mL glass jar. The piston-type DGT units assembled with the ZrO−AgI binding gel were placed in each jar. All of the jars were tightly sealed and shaken for 24 h at a speed of 180 rpm by placing them in a

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MATERIALS AND METHODS Preparation of the Diffusive Gel, ZrO−AgI Binding Gel and DGT Sampler. The diffusive gel was prepared with 15% acrylamide and 0.3% agarose-derived cross linker following published procedures.18 Two procedures were investigated for the preparation of the binding gel in advance, and one procedure was selected for further examination (see the Supporting Information for the details). In this procedure, a 4 mL stock gel solution (composed of 28.5% acrylamide (w/v) and 1.5% methacrylamide (w/v)) was mixed with 1 g of halfdried Zr oxide (with 50 ± 5% water content) and 0, 0.05, 0.1, 0.15, 0.2, 0.3, and 0.4 g of AgI (Sinopharm Chemical Reagent), respectively. The mixture was thoroughly ground in an agate mortar, followed by further dispersion in an ultrasonic disruptor. The mixture was left to stand for 5 min to remove the large particles settled down at the bottom, and then, 3.0 μL of tetramethylethylenediamine (TEMED) catalyst and 75 μL of freshly prepared ammonium persulfate initiator (10% w/v) was added. For treatment with the addition of 0.4 g of AgI, the added amounts were reduced to 2.0 and 50 μL, respectively, to prevent the rapid polymerizations of the gels. After mixing, the solutions were immediately cast between the glass plates separated by 0.40 mm plastic spacers. The glass plate assembly was placed in a refrigerator at 3 ± 1 °C for 0.5 h to allow the zirconium hydroxide and AgI particles to settle by gravity to one side of the gel and was then transferred to an incubator at 15 ± 1 °C to allow the gel to polymerize for 0.5 h. The gel sheet removed from the glass plates was soaked in deionized water for at least 24 h (the water was replaced 2−3 times) and stored in deionized water prior to use. The Zr oxide binding gel was also prepared for comparison according to a procedure previously reported.4 The amount of 8298

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Figure 1. Optimization of the ZrO−AgI binding gels for DGT use. (A) Uptake of DRP by different binding gels as a function of time. The numbers show amounts of AgI (dry weight) added in the 4 mL stock gel solution prior to gel cast. The “s” in brackets shows that the gels took up sulfide to the level of saturation prior to the uptake of DRP. (B) Changes in the grayscale density on the surfaces of the different binding gels with and without the saturation uptake of sulfide.

developed ZrO−AgI DGT technique in Meiliang Bay of Lake Taihu, the third largest freshwater lake in China. This bay has been polluted by sewage discharges since the 1980s, resulting in water eutrophication and associated algal blooms.25 The coordinates of the sampling location were 31°30′57.7″ N, 120°11′20.7″ E; this site has a representative situation where both water and sediment properties of Meiliang Bay are present. Several DGT probes containing the ZrO−AgI binding gels were deoxygenated with nitrogen for 16 h and transported to their sampling sites by placing them in a container filled with deoxygenated 0.03 M NaNO3. The probes were inserted across the sediment−water interface of the site using a releasing device. After deployment for 48 h, the DGT probes were retrieved and brought to the laboratory in a dark container. The grayscale intensity of each binding gel was measured by CID. Two special zones containing elevated concentrations of dissolved S(-II) on two of the binding gels were selected for further 2D measurement of DRP using a routine procedure recently developed by the authors.4 The gels were cut along two dimensions, producing square arrays with each gel square having a size of 0.45 mm × 0.45 mm. The DRP in each gel square was eluted with 40 μL of 1 mol L−1 NaOH. The concentrations of the DRP in the elution solutions were determined by a microcolorimetric method using 384-microwell plates.26 Both the 2D spatial distribution of dissolved S(II) and DRP concentrations were plotted using the software Origin 8.0 (OriginLab Corporation, USA). Calculations. The accumulation mass of P (M) in the binding gel is calculated according to eq 1 when it was eluted using a known volume of eluting solution (Ve).18

reciprocating shaker. After retrieval of the binding gels, the grayscale intensities of the binding surfaces in the gels were measured by CID, as described earlier. The accumulated masses of S(-II) in the gels were then measured using a purge-and-trap method, followed by colorimetry as described by Teasdale et al.16 Their relationships were fitted using an exponential equation. Validation of Simultaneous Measurements with the DGT. The simultaneous measurements of DRP and S(-II) with the DGT were examined by varying the thickness of the diffusive layer (sum of the diffusive gel and filter membrane) used for assembling the DGTs and the concentrations of DRP and S(-II) in the solutions used for the DGT exposures. The DGT uptakes with the varying thicknesses of the diffusive layer were investigated by deploying the ZrO−AgI DGT units for 3 h in 4 L well-stirred solutions (pH 9.0) containing 0.03 mol L−1 NaNO3, 1.12 mg L−1 P, and 1.33 mg L−1 S(-II) at 24 °C. The DGT uptakes with the varying concentrations of DRP and S(II) were investigated by deploying the ZrO−AgI DGT units for 4 h in 4 L well-stirred solutions containing DRP and S(-II) at 22 °C. The concentrations of DRP and S(-II) ranged from 1 to 10 mg L−1 P and 0.3 to 2 mg L−1 S(-II), respectively. The thickness of the diffusion gel used was 0.67 mm. To minimize the loss of S(-II) through oxidation and volatilization, the S(-II) solutions were prepared with deionized water purged with high-purity nitrogen. Their pH values were adjusted at 9.0 ± 0.2 using diluted HCl and NaOH. Additionally, the headspace in each of the deployment vessels was purged with high-purity nitrogen during the DGT deployments. After retrieval of the DGT units from the solutions, the grayscale intensity of each ZrO−AgI binding gel was measured using CID, from which the accumulation masses of S(-II) were calculated using the calibration equation. Phosphorus accumulated in each gel was eluted with 20 mL of 1 mol L−1 NaOH and detected using the molybdenum blue method.24 The performance of the ZrO−AgI DGT technique in the measurement of P was also characterized in detail following the procedure previously described,23 including the effects of pH and ionic strength and the capacities of the ZrO−AgI binding gels for DGT response (see the Supporting Information). Field Application. Simultaneous sampling of the DRP and dissolved S(-II) in the sediments were performed using the

M=

Ce(Vg + Ve) fe

(1)

Vg is the volume of the gel, and fe is the ratio of the eluted and accumulated analytes. The accumulation mass of S(-II) was calculated using the exponential equation established from the calibration procedure. The concentrations of the DRP or dissolved S(-II) measured by the DGT (CDGT) were calculated using the following equation: 8299

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Figure 2. SEM images of the binding gels. (A) Front image of the Zr oxide binding gel previously reported,4 (B) and (C) front images of the ZrO− AgI binding gels without and with the saturation uptake of sulfide, respectively, and (D) and (E) cross section of the ZrO−AgI binding gel and its corresponding EDS analytical results showing accumulation of binding agents toward the gel surface due to settling during gel casting.

C DGT =

M Δg DAt

As the densities of AgI(s) are larger than that of the amorphous Zr oxide, some of these solids would preferentially settle to the surface during the gel cast. This effect can be viewed from the SEM images of the ZrO−AgI binding gels, in which the AgI(s) particles or the formed Ag2S(s) particles after S(-II) exposure were distributed evenly on their surfaces (Figure 2). Analysis with EDS also showed that the enrichment of Ag was much greater than Zr at the surface region. Therefore, the preferential settling of AgI(s) or the formed Ag2S(s) at the gel surface would partly hinder the binding sites of the Zr oxide, resulting in the slight decreases of DRP bindings as observed. There was no obvious difference in the binding dynamics of DRP for the ZrO−AgI binding gels when the amount of AgI(s) added was different. The average binding rates of DRP for the first 20 min were 82, 79, and 76 ng cm−2 min−1 for the Zroxide binding gel and the ZrO−AgI binding gels without and with S(II) saturation, respectively. All of these values were much higher than the flux of 4 ng cm−2 min−1 observed through the diffusive layer when performing the DGT measurement in a phosphate solution with a concentration of 1 mg L−1 and using a 0.93 mm diffusive layer at 25 °C. These results demonstrate that the decreases in the binding dynamics of DRP for the ZrO−AgI binding gels should not affect their use in the DGT measurements. The measurement of dissolved S(-II) with the DGT-CID requires the quantification of grayscale density change on the gel surface caused by the transition of AgI(s) to Ag2S(s) during the deployment period. A large change of the grayscale density in response to the S(-II) uptake should enable the gel to be more sensitive in the measurement of S(-II). Correspondingly, better results can be achieved if the gel has a low background grayscale density and a high grayscale density after the

(2)

where Δg is the thickness of the diffusive layer, D is the diffusion coefficient of the phosphate or S(-II) in the diffusive layer, t is the deployment time, and M is the corresponding accumulated mass of P or S(-II) over the deployment time.18 The values of D for H2PO4−(aq) and HS−(aq) have been reported elsewhere.27,28

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RESULTS AND DISCUSSION Optimization of the ZrO−AgI Binding Gel. Measurement with the DGT should allow the binding gel to act as an infinite sink for the dissolved analyte; that is, the concentration of the dissolved analyte should remain effectively zero at the interface between the binding and diffusive gels over the deployment period. Specifically, this requires that the binding gel takes up the analyte rapidly and irreversibly. Previous studies show that the Zr oxide binding gel has a good performance in binding DRP and satisfies this requirement.23 However, this feature may have altered when AgI(s) was added into the gel. Therefore, the binding dynamics of the Zr oxide binding gel as well as different ZrO−AgI binding gels were investigated by immersing the gels in stirred phosphate solutions with a concentration of 1 mg L−1 (Figure 1A). The results showed that the uptake of DRP by each gel increased linearly with time for the first 20 min. The addition of AgI(s) slightly decreased the binding amounts of DRP for the first 40 min but had no effects until 120 min. A larger decrease was observed over the experimental period when the ZrO−AgI binding gels were saturated with S(-II) prior to the uptakes of DRP. 8300

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saturation uptake of S(-II). The background and final grayscale densities after the saturation uptake of S(-II) were investigated using CID with various binding gels (Figure1B). The background grayscale density of the Zr oxide gel was 41 initially, and then, it increases slightly to 44 when the addition of AgI(s) reached 0.15 g and remained steady after that. The grayscale density for the gel with S(-II) saturation successively increased as the added amount of AgI(s) increased to 0.3 g; with higher amounts of AgI(s), a steady state with a maximum of 220 was reached. The addition of 0.3 g of AgI(s) had a better result, as it only increased the background a little but also achieved the greatest and most stable grayscale density after the saturation uptake of S(-II). Furthermore, the binding dynamics of this gel for DRP was comparable to other ZrO−AgI binding gels as mentioned earlier (Figure1A); therefore, it was selected as the binding gel for the DGT to use. Calibration of the S(-II) measurement with the ZrO−AgI binding gel selected showed that the grayscale density had a nonlinear increase with the accumulation mass of S(-II) in the gel, reflected by an initially fast increase when the accumulation was less than ∼10 μg cm−2, a slow increase when less than 30 μg cm−2, and a steady stage after that (Figure 3). It

demonstrates that the sensitivity of the DGT-CID technique decreases with increasing S(-II) accumulation in the binding gel. Similar changes have been observed on other binding gels.8,16,22,29 The relationships between the S(-II) accumulations in the ZrO−AgI binding gel and the corresponding changes in grayscale density can be described using an exponential equation as displayed in Figure 3. Their good fitting (R2 = 0.98) implied that this binding gel can be used for measurement of S(-II) at a spatial resolution of 0.169 mm × 0.169 mm used in this study. The standard deviation (SD) of the grayscale density of a series of blank binding gels was 1.42. A limit of detection for S(-II) measurement, calculated from the sum of the blank grayscale density (49) and three times the SD, was 0.08 μg cm−2. This value was converted to a blank concentration of 1.6 μg L−1 assuming a deployment time of 24 h at 25 °C with a 0.8 mm thick diffusive gel and a 0.13 mm filter. Validation of Simultaneous Measurements with the DGT. Simultaneous measurements of DRP and dissolved S(-II) using the ZrO−AgI DGT technique was validated through investigating the DGT-accumulated masses of PO43−−P and S(-II) with the changes in thickness of the diffusive layers in the DGT units and the concentrations of DRP and S(-II) in the solutions. To avoid the errors caused by the loss of S(-II) in the solutions, the temporal lengths for the DGT deployments were less than 5 h. After the deployments, both of the measured masses of PO43−−P and S(-II) increased linearly with the reciprocal of thickness of the diffusive layer (Figure 4A). Additionally, the experimental data agreed well with the theoretical prediction calculated using eq 2. The linear relationships were also observed for the accumulation mass of PO43−−P or S(-II) with their concentrations, and the experimental data also agreed well with the theoretical prediction (Figure 4B). Therefore, the results validated the use of the ZrO−AgI DGT technique for the simultaneous measurements of DRP and dissolved S(-II) in solutions. Furthermore, a supplementary examination showed that the performance of the ZrO−AgI DGT in the measurement of DRP was comparable to that of the Zr oxide DGT, including similar recovery rate (95%) of DRP from elution of the binding gels using 1 mol L−1 NaOH, similar DGT response with pH, ionic strength, and deployment time (Supporting Information, Figure S1). It further demonstrated that addition of AgI in the

Figure 3. Calibration curve for sulfide measurement. Plot of the grayscale intensities (0−255) versus the accumulation masses of sulfide in the ZrO−AgI binding gel deployed in the sulfide solutions.

Figure 4. Relationships of DGT accumulation masses of DRP or sulfide with (A) the reciprocal of the diffusive layer thickness and (B) concentrations of DRP or sulfide in the solutions. Values are means ± SD of three replicate analyses showing as vertical bars. 8301

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Figure 5. Two-dimensional concentration distribution images of the dissolved sulfide and DRP (CDGT) from the in situ deployments of the ZrO− AgI DGT probes in two of the sediment profiles taken in Lake Taihu. The left images show color changes on the binding gels after retrieval. The spatial resolutions are 0.169 mm × 0.169 mm and 0.45 mm × 0.45 mm for the images of dissolved sulfide and DRP, respectively.

hotspot. It seems that both the DRP and dissolved S(-II) had a trend to enrich in the sulfide spot zone. The simultaneous elevations in the concentrations of DRP and S(-II) were much more pronounced in Profile 2 (Figure 5). In this profile, highly enriched dissolved S(-II) was observed in two zones. One zone had an elongated shape from the middle to the bottom of the profile with a concentration maximum of ∼100 μg L−1 at a depth of 15−19 mm and a width of 5−7 mm. Another zone located in the middle of the profile and extending to the right boundary had a concentration maximum of ∼80 μg L−1. A similar enrichment pattern of DRP was observed in the same two zones, with the enrichments mostly striking in the zone to a large extent corresponding to the sulfide maximum observed in the first zone. The elevated concentration of DRP distributed in the locally aggregated patch and did not exhibit a pattern of a gradual change similar to that of dissolved S(-II). The former should be more realistic because that the sensitivity of the DGT-CID technique used for determination of dissolved

Zr oxide binding gel had no effect on the DGT measurement of P. Field Application. As a pilot study, the newly established DGT technique was used in the 2D simultaneous measurement of DRP and dissolved S(-II) in sediments. The concentrations of the DRP and S(-II) loaded into the binding gel were determined using a routine procedure and CID technique, respectively. Two special zones were selected from two sediment profiles (Profiles 1 and 2) to investigate the relationship between the DRP and dissolved S(-II) distributions (Figure 5). A dark spot in Profile 1 with a diameter of ∼3 mm was observed, which was located at the vertical position between 7 and 10 mm and horizontal position between 8 and 11 mm. The concentration of S(-II) reached a maximum of ∼0.2 mg L−1 in the center of the spot. It was unexpected that a distinct enrichment of DRP appeared in the sulfide spot zone. High concentrations of DRP (∼0.9 mg L−1) aggregated in an irregular shape in this zone, which were in line with the sulfide 8302

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the simultaneous measurement of Fe using DGT. Further results have revealed that these coincident processes can occur on a centimeter scale, likely implying that there is a wide range of sedimentary conditions in favor of their occurrence. Better understanding of these coincident processes by considering kinetic effects is still required.

S(-II) decreases with increasing S(-II) accumulation by DGT as mentioned earlier (Figure 3). Previous studies that have used DGT measurement have observed localized dark spots of elevated sulfide in lacustrine sediments with diameters of ∼1 mm or greater.8,22,30 These elevated sulfide spots were attributed to a supply of highly localized sulfate reductions in discrete aggregates of reactive organic matter with associated high populations of sulfate reducing bacteria.3,30 The simultaneous measurements of S(-II) and metals with the DGT also showed localized release of Fe and other trace metals at the site of the sulfide spot.30,31 Their coincident release strongly suggests the coexistence of iron- and sulfide-reducing bacteria in the same localized zone, which would enable both Fe(III) and sulfate to act as electron acceptors for oxidations of reactive organic matter.30,31 These findings provide a possible mechanism responsible for the simultaneous release of P at the site of the sulfide spot as observed in Profile 1 of this study. Specifically, the release of P is likely from Fe-bound P and is a result of the localized reductions of Fe(III) oxides. This hypothesis is consistent with the previous recognition that Fe plays a central role in controlling the remobilization of P in sediments.32−34 Interestingly, the authors also correspondingly observed coincident distributions of DRP and dissolved ferrous Fe in sediment pore waters of Meiliang Bay, Lake Taihu at a millimeter scale,26 which further supports this hypothesis. The relatively large-scale overlap of the elevated concentration distributions of DRP and dissolved S(-II) in Profile 2 further demonstrates that the simultaneous release of DRP and sulfide should not be limited to the microniche as observed previously and can extend to a centimeter scale. In fact, Postma and Jakobsen35 revealed that the simultaneous reduction of sulfate and iron oxides can occur thermodynamically under a wide range of environmental conditions in natural sediments due to the presence of multiple iron oxide stabilities. This finding emphasizes the necessity of reassessing the diagenetic processes for key elements in the sediments that differentiate from the traditional tertiary electron accepting processes,36 while high-resolution simultaneous measurements can provide details to help explore this field as performed in this study. Generally Appraisal. Previous DGT techniques were established mostly based on the uses of various binding gels impregnated into single binding agents for the measurement of an analyte or a type of analyte. However, the DGT measurements of different analytes using a binding gel simultaneously impregnated into more binding agents have seldom been reported.15,37 To our knowledge, this study is the first time that quantitative DGT measurements of different types of analytes have been performed at a small scale and at the 2D level based on the use of a mixed binding gel impregnated with two binding agents. The good performance of this novel DGT technique implies that there is great potential in development of new DGT techniques capable of simultaneous measurements of more analytes. The ability to take simultaneous measurements is critical to reassess element diagenesis and related environment processes, while also taking into account the heterogeneous nature of sediments. The simultaneous release of DRP and dissolved sulfide has not been reported previously but has been related to the simultaneous release of both dissolved Fe and sulfide recently observed in the microniche of the sediments.30,31 The mechanisms behind their coincidence need to be further examined in combination with other measurements, especially

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ASSOCIATED CONTENT

S Supporting Information *

The performance of the ZrO−AgI DGT technique in the measurement of DRP, including the effects of pH and ionic strength and the capacities of the ZrO−AgI binding gels for DGT response. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-25-86882207; fax: 86-25-86882207; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was sponsored by the National Scientific Foundation of China (21177134, 41001334), the Project of Knowledge Innovation for the third period, CAS (KZCX2-YWJS304), and the Nanjing Institute of Geography and Limnology, CAS (NIGLAS2010KXJ01).

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REFERENCES

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