Article pubs.acs.org/est
Novel Speciation Method Based on Diffusive Gradients in Thin-Films for in Situ Measurement of CrVI in Aquatic Systems Yue Pan,† Dong-Xing Guan,† Di Zhao,† Jun Luo,*,† Hao Zhang,‡ William Davison,‡ and Lena Q. Ma†,§ †
State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing, Jiangsu 210023, China ‡ Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, United Kingdom § Soil and Water Science Department, University of Florida, Gainesville, Florida 32611, United States S Supporting Information *
ABSTRACT: Hexavalent chromium (CrVI) is much more toxic and mobile than the trivalent species (CrIII) and consequently, in situ monitoring of CrVI can improve the understanding of Cr biogeochemistry and toxicity in ecosystems. The passive diffusive gradients in thin-films (DGT) technique is a powerful tool for determining metal(loid) speciation, but a binding phase that absorbs only one specific species of Cr is needed. N-Methyl-D-glucamine (NMDG) functional resin was incorporated into the DGT binding phase for selective measurement of CrVI. This NMDG-DGT sampler exhibited a theoretically linear accumulation of CrVI, with negligible accumulation ( 8 with NaOH (for CrVI measurement) and pH < 2 with HNO3 (for total Cr measurement). Upon return to the laboratory, the samples were filtered (0.45 μm membrane) and stored in closed polypropylene vessels at 4 °C until analysis. Deployment in a CrVI-Contaminated Stream. To assess the capability of NMDG-DGT for measuring CrVI in a contaminated aquatic environment, it was deployed in a stream located in the west Henan province of China (SI Figure S2). The stream receives surface runoff originating from an abandoned chromium slag treatment plant situated about 380 m upstream. Parameters regarding stream variables (temperature, pH, DOC, and major dissolved elements) are summarized in SI Table S3. Generally, the depth at the sampling sites was 0.2−0.4 m and the flow velocity 0.2−0.8 m s−1 during the sampling period. This flow velocity was high enough to justify the diffusive boundary layer being ignored when the simple DGT eq 2 is used to calculate concentration.35,36 DGT devices were deployed along the stream in situ for ∼24 h in October 2014 (in triplicate). The sites were chosen according to their distance from the point source as well as considering the input of other streams of potentially different sources (Figure S2, SI). DGT samplers together with button dataloggers were suspended in the stream at about 5 cm below the surface (Figure S3, SI). The settings of button dataloggers and collection and pretreatment of water samples from active sampling were the same as for lake and river water deployments.
(2)
Ce(Vg + Ve) fe
(3)
where Ce is the concentration of the analyte in the eluent, Vg is the volume of the gel, and fe is the elution efficiency. Characteristics of DGT Performance in the Laboratory. Selectivity for CrVI. The selective measurement of CrVI using NMDG-DGT was examined by employing DGT devices containing NMDG binding gels in 6 L of 0.01 mol L−1 NaNO3 solution spiked with 50 μg L−1 CrVI or CrIII. Triplicate devices were removed at 2, 4, 6, 8, 12, 24, 48, and 72 h. To confirm the stability of Cr species, subsamples of the deployment solution were taken at each time and analyzed for Cr using ICP-MS and for CrVI using the DPC method. NMDG-DGT devices were deployed in synthetic freshwater spiked with different proportions of both CrVI and CrIII for 4 h to further evaluate their selective performance. The concentration of CrVI in synthetic freshwater (See in SI for the composition of synthetic freshwater) was 50 μg L−1 while the concentration of CrIII was set at 50 and 500 μg L−1. Chromium accumulated in the binding gels was eluted using 1 mL of 1 mol L−1 HNO3 (see Results and Discussion for elution efficiency validation). Effect of pH and Ionic Strength. The effect of pH on DGT performance was investigated by deploying triplicate NMDGDGT devices for 4 h in well-stirred solution of 2 L of 50 μg L−1 CrVI and 0.01 mol L−1 NaNO3 at pH 3−12. The pH was adjusted using 1 mol L−1 HNO3 or NaOH. Similarly, to test the effect of ionic strength, NMDG-DGT devices were deployed in triplicate for 4 h in solutions containing 50 μg L−1 CrVI and different concentrations of NaNO3 ranging from 1 to 100 mmol L−1 at pH 6.5 ± 0.5. The reason for choosing NO3− to represent the ionic strength are presented in the SI. Capacity of NMDG-DGT. Adequate adsorption capacity is necessary if DGT samplers are to be deployed long-term or in highly polluted environments. The capacity of NMDG-DGT for CrVI was determined by deploying the NMDG-DGT devices for 4 h in 2 L of well-stirred synthetic solutions containing CrVI at various concentrations (0.1 to 240 mg L−1) and 0.01 mol L−1 NaNO3. To verify the capacity of the NMDG-DGT sampler in a real aquatic environment, DGT devices were deployed in contaminated groundwater with CrVI concentrations ranging from 2.2 to 315 mg L−1. Details of the sampling procedure and physicochemical parameters of the groundwater are in SI. Applications of NMDG-DGT in Environmental Waters. Deployment in Natural Lake and River Waters. To test the reproducibility of NMDG-DGT measurement in field environments, two local natural waters (Lake Taihu and Jiuxiang River, a tributary of Yangtze River, Jiangsu, China) were selected as the field sites. Details of the physicochemical parameters of the waters are in SI Table S2. Six DGT devices were assembled into a hexahedral unit according to Zheng et al.35 The hexahedral multidevice, together with button dataloggers (Maxim Integrated Products) set to record temperature every 1 h (for calibrating the temperature-depended diffusion coefficient), was placed in natural waters for 72 h. On retrieval, DGT samplers were thoroughly rinsed on-site with Milli-Q water and stored in sealed plastic bags before removing the NMDG gels. No biofouling were visible on the filter membrane of the DGT devices. To validate the DGT measurement, a conventional
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RESULTS AND DISCUSSION Kinetics of Binding and Elution Efficiency. The uptake efficiency of CrVI by NMDG binding gel after 2 h’s exposure in 50 μg L−1 CrVI or CrIII solution was more than 95% while less than 4.8% of CrIII was absorbed (Figure 1A). After immersing NMDG gels in Cr solutions, the uptake of CrVI initially increased linearly and almost reached 70% of its final adsorption after 30 min. The mass measured after 2 h was nearly the same as that measured at 24 h. Therefore, NMDG binding gels can adsorb CrVI rapidly with high selectivity, demonstrating their potential for DGT measurement of CrVI. Accurate calculation of CrVI concentrations in the environment from DGT deployments depends on consistent and quantitative elution of Cr from the binding gel. Previous studies showed that HNO3 was effective in removing CrVI from NMDG resins for resin regeneration.30 In this study, 1 mL of 0.5, 1, 2, and 3 mol L−1 HNO3 were used to investigate the elution efficiency. The obtained elution efficiency was 71.2% using 1 mol L−1 HNO3 compared to 64.6% using 0.5 mol L−1 HNO3. Similar elution efficiencies (63%−85%) of oxyanions (V, As, Se, Sb) from precipitated ferrihydrite gels using 1 mol L−1 HNO3 were reported by Luo et al.37 As increasing the acid concentration (2 or 3 mol L−1) did not further enhance the recovery (SI Table S4), 1 mL of 1 mol L−1 HNO3 was used to elute CrVI from the NMDG binding gels. Diffusion Coefficient in the Diffusive Gel. The masses of CrVI diffused through the diffusive gel showed a good linear relationship (r2 = 0.999) against time (Figure S4, SI). The calculated diffusion coefficient Dcell, (8.82 ± 0.42) × 10−6 cm2 s−1, for CrVI at 25 °C was ∼79% of the measured value (Dwater) in water,38 similar to the proportions of Dcell versus Dwater of C
DOI: 10.1021/acs.est.5b03742 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
(Table 1). The ratio (R) of the DGT-measured to solution concentration of CrVI was 0.97 and 1.02 in solutions where the ratio of CrIII to CrVI was 1 and 10, respectively. Table 1. Comparison of CrVI Measurement by NMDG-DGT (CDGT) and DPC Colorimetry (CDPC) at Different Proportions of CrVI and CrIII in Synthetic Freshwater
a
(mol/mol)a
CDGT (μg L−1)
CDPC (μg L−1)
CDGT/CDPC
1:1 1:10
50.8 54.1
52.4 53.0
0.97 1.02
Concentration of CrVI was 50 μg L−1
Effect of pH and Ionic Strength. The effect of pH and ionic strength on the NMDG-DGT measurement is shown in Figure 2. Good agreement was obtained between the measured
Figure 1. Masses of Cr accumulated by (A) NMDG gels soaked in solutions containing 0.01 mol L−1 NaNO3, 50 μg L−1 CrVI or CrIII for 0.5 min to 24 h, (B) NMDG-DGT deployed in separate solutions containing either 50 μg L−1 CrVI or 50 μg L−1 CrIII and 0.01 mol L−1 NaNO3 at pH 6.5 for 2−72 h, (C) NMDG-DGT deployed for 4 h in synthetic solutions containing 0.1−240 mg L−1 CrVI, 0.01 mol L−1 NaNO3 at pH 6.5 and CrVI-contaminated groundwater. The solid lines in (B) and (C) are the theoretical response calculated using eq 2, DDGT was used for the calculation of the theoretical response (solid lines) in figures B and C. Error bars are calculated from the standard deviations of replicates (n = 3). Figure 2. Effect of pH (A) and ionic strength (B) on the ratio of DGT measured concentrations of CrVI, CDGT, to deployment solution concentrations, Csoln. Error bars are calculated from the standard deviations of replicates (n = 3). The solid and dotted horizontal lines represent target values of 1.0 ± 0.1.
other oxyanions (P, As, Se, and Sb).37,39,40 To further verify the diffusion coefficient, DDGT, (8.18 ± 0.28) × 10−6 cm2 s−1, at 25 °C was also obtained by deploying DGT devices in CrVI containing water. The difference between the values from the two approaches was acceptably less than 8%. The Dcell value for CrVI is 35% larger than the value for CrIII which is 5.74 × 10−6 cm2 s−1 at 25 °C,34 indicating a larger flux of CrVI into NMDGDGT samplers. Validation of Selective Measurement of CrVI Using NMDG-DGT. Selective accumulation of CrVI in NMDG-DGT samplers is a prerequisite of its performance. As shown in Figure 1B, CrVI accumulated linearly with a theoretically predicted slope while there was little CrIII measurable in the binding layer ( 20%) of CrVI measured by the DPC method in streamwater at site 1 reflected the scatter of active sampling and showed that a single spot sample might not reflect the true contamination level in waters. As expected at high pH (7.91−8.35, Table S3, SI) CrVI constituted ≥95% of total Cr for all sites. Farmer et al.47 reported that carboxylate groups associated with highconcentrations of DOC not only reduce CrVI, but also form soluble CrIII-humic complexes. However, concentrations of DOC in the stream were low (3.75−8.55 mg L−1, Table S3, SI). Only about 2 μg L−1 of total Cr was found in the other two influents, at the confluence of sites 1 and 2 (Figure S2, SI), indicating that no other artificial sources contributing to the Cr level in the stream. Changes in concentrations of CrVI measured by NMDG-DGT and the conventional active sampling with DPC method along the stream are shown in Figure 3. The
Table 2. Concentration of Cr (μg L−1) in Natural Waters of the Five Studied Sitesa waters Lake Taihu
Jiuxiang River
CDGT
CDPC
Ctotal Cr
120°11′26.41″E 31°30′47.41″N 120°11′39.59″E 31°28′34.79″N 120°11′16.66″E 31°26′9.92″N 120°11′14.39″E 31°24′40.21″N
0.16 ± 0.02
ndb
1.17
0.28 ± 0.05
ndb
0.96
0.17 ± 0.03
ndb
0.98
0.21 ± 0.04
ndb
1.05
118°56′39.11″E 32°6′52.85″N
0.62 ± 0.08
ndb
1.68
locations
Figure 3. Concentrations of CrVI in the CrVI-polluted stream, located in the west Henan province of China, measured by NMDG-DGT and a conventional active sampling technique with analysis by the diphenylcarbazide (DPC) spectrophotometric method. Error bars of NMDG-DGT and DPC measurement represent the standard deviations of three replicates and two replicates, respectively; where error bars do not show, they are smaller than the symbols. Normally, the relative standard deviations are
