Cathodic stripping voltammetric analysis of arsenic species in environmental water samples

Voltammetric determination of inorganic arsenic in mildly acidified (pH 4.7) groundwaters from Mexico and India

Routine monitoring of inorganic arsenic in groundwater using sensitive, reliable, easy-to-use and affordable analytical methods is integral to identifying sources, and delivering appropriate remediation solutions, to the widespread global issue of arsenic pollution. Voltammetry has many advantages over other analytical techniques, but the low electroactivity of arsenic(V) requires the use of either reducing agents or relatively strong acidic conditions, which both complicate the analytical procedures, and require more complex material handling by skilled operators. Here, we present the voltammetric determination of total inorganic arsenic in conditions of near-neutral pH using a new commercially available 25 μm diameter gold microwire (called the Gold Wirebond), which is described here for the first time. The method is based on the addition of low concentrations of permanganate (10 μM MnO4-) which fulfils two roles: (1) to ensure that all inorganic arsenic is present as arsenate by chemically oxidising arsenite to arsenate and, (2) to provide a source of manganese allowing the sensitive detection of arsenate by anodic stripping voltammetry at a gold electrode. Tests were carried out in synthetic solutions of various pH (ranging from 4.7 to 9) in presence/absence of chloride. The best response was obtained in 0.25 M chloride-containing acetate buffer resulting in analytical parameters (limit of detection of 0.28 μg L-1 for 10 s deposition time, linear range up to 20 μg L-1 and a sensitivity of 63.5 nA ppb-1. s-1) better than those obtained in acidic conditions. We used this new method to measure arsenic concentrations in contrasting groundwaters: the reducing, arsenite-rich groundwaters of India (West Bengal and Bihar regions) and the oxidising, arsenate-rich groundwaters of Mexico (Guanajuato region). Very good agreement was obtained in all groundwaters with arsenic concentrations measured by inductively coupled plasma-mass spectrometry (slope = +1.029, R2 = 0.99). The voltammetric method is sensitive, faster than other voltammetric techniques for detection of arsenic (typically 10 min per sample including triplicate measurements and 2 standard additions), easier to implement than previous methods (no acidic conditions, no chemical reduction required, reproducible sensor, can be used by non-voltammetric experts) and could enable cheaper groundwater surveying campaigns with in-the-field analysis for quick data reporting, even in remote communities.

A critical review of on-site inorganic arsenic screening methods

Approximately 94 to 220 million people worldwide are at risk of drinking well water containing arsenic > 10 µg/L, the WHO guideline value. To identify non-compliant domestic wells, assess health risks and reduce exposure, accurate and rapid on-site inorganic arsenic screening methods are desirable because all domestic wells worldwide need to be tested. Here, the principles, advantages and limitations of commonly used colorimetry, electrochemistry, and biosensing methods are critically reviewed, with the performance compared with laboratory-based benchmark methods. Most commercial kits are based on the classic Gutzeit reaction. Despite being semi-quantitative, the more recent and more expensive products display improved and acceptable accuracy and shorter testing time (∼10 min). Carried out by trained professionals, electrochemical methods are also feasible for on-site analysis, although miniaturization is desirable yet challenging. Biosensing using whole bacterial cells or bio-engineered materials such as aptamers is promising, if incorporated with function specific nanomaterials and biomaterials. Since arsenic is frequently found as arsenite in reducing groundwater and subject to oxidation during sampling, transportation and storage, on-site separation and sample preservation are feasible but the specific methods should be chosen based on sample matrix and tested before use. To eliminate arsenic exposure among hundreds of millions of mostly rural residents worldwide, we call for concerted efforts in research community and regulatory authority to develop accurate, rapid, and affordable tests for on-site screening and monitoring of arsenic in drinking water. Access to affordable testing will benefit people who are socioeconomically disadvantaged.

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.

Determination of total arsenic using a novel Zn-ferrite binding gel for DGT techniques: Application to the redox speciation of arsenic in river sediments

A new laboratory-made Zn-ferrite (ZnFe2O4) binding gel is fully tested using Diffusive Gradient in Thin films (DGT) probes to measure total As [including inorganic As(III) and As(V), as well as MonoMethyl Arsenic Acid (MMAA(V)) and DiMethyl Arsenic Acid (DMAA(V))] in river waters and sediment pore waters. The synthesis of the binding gel is easy, cheap and its insertion into the acrylamide gel is not problematic. An important series of triplicate tests have been carried out to validate the use of the Zn-ferrite binding gel in routine for several environmental matrixes studies, in order to test: (i) the effect of pH on the accumulation efficiency of inorganic As species; (ii) the reproducibility of the results; (iii) the accumulation efficiency of As species; (iv) the effects of the ionic strength and possible competitive anions; and (v) the uptake and the elution efficiency of As species after accumulation in the binding gel. All experimental conditions have been reproduced using two other existing binding gels for comparison: ferrihydrite and Metsorb® HMRP 50. We clearly demonstrate that the Zn-ferrite binding gel is at least as good as the two other binding gels, especially for pH values higher than 8. In addition, by taking into consideration the diffusion rates of As(III) and As(V) in the gel, combining the 3-mercaptopropyl [accumulating only As(III)] with the Zn-ferrite binding gels allows for performing speciation studies. An environmental study along the Marque River finally illustrates the ability of the new binding gel to be used for field studies.

A stripping chronopotentiometric (SCP) method with a gold film electrode for determining inorganic arsenic species in seawater

An electrochemical method based on stripping chronopotentiometry (SCP) with a gold film electrode has been developed for determining arsenic in seawater. The detection limits were 0.053 ppb (0.71 nM) and 0.022 ppb (0.29 nM) for total inorganic As (As(T)) and As(III) after deposition times of 60 and 150 s, respectively. Compared to other stripping chronopotentiometric methods that use a gold macroelectrode to perform measurements of arsenic in seawater, the procedure described here exhibits better sensitivity and a fourfold shorter deposition time. Among the SCP methods, our procedure had proven its ability to analyse arsenic(III) in seawater. It therefore allows the concentrations of the various arsenic inorganic species in seawater--i.e. As(T), As(III) and As(V)--to be analysed. The proposed method is reliable, inexpensive and compact. It was successfully applied to the study of arsenic speciation along the salinity gradient of the Penzé estuary (NW France).