Wednesday, November 7, 2007 - 10:00 AM
326-3
Low-Level Colorimetric Determination of Arsenic in the Presence of Phosphorus in Soil Solutions.
Arsenic (As) is a human carcinogen, posing serious human health risk at concentrations as low as 10 ppb in drinking-water. Elevated As concentrations in former agricultural soils with a prolonged history of arsenical pesticide applications, which are currently used for residential development, may increase the probability of human exposure to toxic As concentrations. Accurate, rapid, and sensitive measurements of As is required during screening and monitoring of As concentrations in soil solutions. Phosphate and arsenate are chemically similar and both can be found in soil solutions; however, P concentrations typically exceed those of As by at least one order of magnitude, interfering with colorimetric As determination. We propose a modification of the malachite green method originally developed for P by Van Veldhoven and Mannaerts (1987) (V&M) by the addition of ascorbic acid (AA) in the acidic molybdate reagent. Malachite green and polyvinyl alcohol were added to the mixed reagent and the absorbance of the complex was measured at 610 nm. The AA concentrations had a significant effect on the absorbance of mixed P + As samples. We identified an optimum AA concentration where there is no absorbance difference between P and As up to a 1:10 molar As:P concentration range at 610 nm. Arsenic is measured by the difference between the V&M method, which measures only P, and the modified V&M method that measures both P and As at 610 nm. The proposed method is sensitive, measuring low As concentrations (2.5 ppb As), as well, as rapid, using a 96-well microtiter plate reader. The proposed method was successfully tested using water-extracts and total digests of 14 As-contaminated soil samples. Excellent (80-120 %) reproducibility was obtained for As concentrations measured by our colorimetric method and by the Inductively Coupled Plasma Mass Spectrometry (ICP-MS).
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