Superfund Research Program
Strategies for Quantitative and Rapid Measurements of Arsenic in Water
Arsenic contamination of groundwater has been documented in nearly 20 countries, including the United States, as the result of either natural geologic processes or anthropogenic releases from mining, industrial and agricultural activities. Because exposure to arsenic in drinking water has been associated with the development of skin, bladder, kidney and lung cancers and noncarcinogenic effects such as diabetes, peripheral neuropathy, and cardiovascular diseases, public health intervention strategies to reduce arsenic exposure are critical. Many drinking water sources need to be tested - in Bangladesh alone there are millions of tubewells that must be checked. Some of the existing field test kits are not reliable at arsenic concentrations below 100 ug/L. SBRP-funded researchers at the University of Kentucky, City University of New York and Columbia University are taking varied approaches to address the challenge of developing and validating reliable field-applicable analytical tools that are sensitive, fast, portable and easy to use by untrained personnel.
Dr. Sylvia Daunert at the University of Kentucky SBRP works with a team of researchers that has developed a set of whole cell bacterial biosensors for testing groundwater. The researchers genetically engineered a nonpathogenic laboratory strain of Escherichia coli to produce specific fluorescent proteins upon recognition of the target chemical. While other bacterial sensors detect only arsenite and their sensitivity is hampered by high levels of background noise, the newly developed biosensor detects both arsenite [As(III)] and arsenate [As(V)] in the range of 5-100 ug of As/L.
To optimize the biosensors for field testing, the research team prepared "dipsticks" by drying sensor cells on a paper matrix. When the paper strips are placed in a tube with 1 milliliter of an aqueous sample for 30 minutes, then taken out and treated with a substrate, blue spots appear, the intensity of which is proportional to the arsenite concentration. These biosensors have been employed in the determination of arsenic contamination in field samples from Bangladesh. The sensors could cost as little as 2 cents to produce and field costs would also be low as neither highly trained personnel nor costly analytical equipment are needed. Potential issues with application of the biosensor system include the vulnerability of the bacteria to toxic chemicals that may be in the groundwater and current restrictions on use of genetically modified bacteria outside of the laboratory.
Researchers working with Dr. Yan Zheng at Queens College, City University of New York, with support from the Columbia University SBRP have developed a sensitive electrochemical method for quantification of inorganic arsenic. The scientists have optimized a Differential Pulse Cathodic Stripping Voltammetry (DPCSV) methodology to detect arsenic at levels below 1 ug/L; differentiate between arsenite and arsenate; and function in both the laboratory and the field. The method involves using copper and selenium to concentrate As(III) on a mercury electrode. The As(III) is then stripped from the electrode by application of potential and the concentration of the metal is determined by the resulting current. As(V) is quantified by reduction to As(III) using sodium meta-bisulfate/sodium thiosulfate reagent, and calculating the difference of As(III) concentrations in samples with and without reduced As(V).
Results for the electrochemical method compare favorably with high resolution inductively coupled plasma mass spectrometry (ICP-MS), graphite furnace atomic adsorption spectrometry, and inductively coupled atomic fluorescence spectrometry. The researchers have demonstrated the on-site analysis capability by analysis of groundwater performed at the Vineland, New Jersey Superfund site. They have been encouraged by EPA's inorganic methods program manager to submit it for consideration as an EPA standard method.
In collaboration with Dr. Alexander van Geen, who heads Columbia University's earth-science activities supported by SBRP, scientists in Dr. Zheng's lab have also modified a classic colorimetric method used to assay dissolved arsenic, significantly reducing the both the detection limit (from ~20 ug/L to ~2 ug/L) and reaction time (from > 1hour to 10 minutes). The method can be used to determine the proportion of As(III) and As(V) in a sample. Modifications to the method include optimization of sample pre-treatment and color reagents. Achieving this low detection limit presently requires a double-beam spectrophotometer with 4 decimal point absorbance recording capability, however.
The optimized method was tested in the field in Bangladesh analyzing a representative set of samples. The colorimetry results obtained in the field were consistent with HR ICP-MS analyses. A major advantage of this colorimetric method compared to others is that no toxic arsine gas is generated during the analysis. Reagent costs are approximately 1 cent per sample. Dr. van Geen's group is developing a test kit for arsenic on the basis of this method with a company. U.S. and international patents are pending.
Dr. van Geen's team has also developed and successfully tested a novel and inexpensive groundwater sampling device that allows sampling of groundwater before a well is installed. The device is built around a 10-gauge 18"-long needle to sample groundwater ahead of the sediment disturbed by the drilling process and a 100-mL evacuated centrifuge tube capped with a silicone stopper. Through a sliding mechanism, the needle pierces the silicone stopper when the device reaches unperturbed groundwater and groundwater fills the centrifuge tube. This device could be particularly important in Bangladesh where although most villagers live near low-arsenic groundwater, they do not know it because of the prohibitive cost of repeated drilling to install a well.
These analytical advances are important not only in Bangladesh, but also in the United States where 2.5 million people get their drinking water from sources containing more than 25 ug/L arsenic. Accurate analysis of arsenic concentrations in drinking water is critical for the establishment of public health mitigation strategies and assessment of remediation efforts. SBRP-funded researchers are making significant contributions to the development of rapid and reliable field tests to accurately measure environmentally relevant concentrations of arsenic in drinking water.
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To learn more about this research, please refer to the following sources:
- Stocker J, Balluch D, Gsell M, Harms H, Feliciano JS, Daunert S, Malik KA, van der Meer JR. 2003. Development of a set of simple bacterial biosensors for quantitative and rapid measurements of arsenite and arsenate in potable water. Environ Sci Technol 37(20):4743-4750. PMID:14594387
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