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PRESERVATION OF AS(III) AND AS(V) IN DRINKING WATER SUPPLY SAMPLES FROM ACROSS THE UNITED STATES USING EDTA AND ACETIC ACID AS A MEANS OF MINIMIZING IRON-ARSENIC CO-PRECIPITATION
Citation:
Gallagher, P A., C A. Schwegel, A N. Parks, B. M. Gamble, L J. Wymer, AND J T. Creed. PRESERVATION OF AS(III) AND AS(V) IN DRINKING WATER SUPPLY SAMPLES FROM ACROSS THE UNITED STATES USING EDTA AND ACETIC ACID AS A MEANS OF MINIMIZING IRON-ARSENIC CO-PRECIPITATION. ENVIRONMENTAL SCIENCE AND TECHNOLOGY 38(10):2919-2927, (2004).
Impact/Purpose:
To develop an in-field preservation strategy for As(III) and As(V) in iron rich drinking waters which will allow samples to be shipped to the laboratory for arsenic speciation analysis. The overall goal is to improve treatment decision making and in turn minimize exposure to arsenic resulting from the consumption of drinking water.
Description:
This paper evaluates seven different treatment/storage conditions for the preservation of the native As(III)/As(V) found in ten drinking water supplies from across the US. These ten waters were chosen because they have different As(III)/As(V) distributions; six of these waters contained enough iron to produce an iron precipitate during shipment. The waters were treated and stored under specific conditions and analyzed periodically over a span of approximately 75 days. Linear least squares (LLS) was used to estimate the change in As(III) and As(V) over the study period. Point estimates for the first and last analysis days and 95% confidence bounds were calculated from the LLS. The difference in the point estimates for the first and last day were then evaluated with respect to drinking water treatment decision making.
Three primary treatments were evaluated: EDTA/AcOH-treatment, AcOH treatment, as well as no treatment. The effect of temperature was explored for all treatments while the effect of aeration was evaluated for only the EDTA/AcOH treated samples. The non-treated samples experienced a 0-40% reduction in the native arsenic concentration due to the formation of Fe/As precipitates. The Fe/As precipitates were resolubilized and shown to contain elevated concentrations of As(V) relative to the native distribution. Once this Fe/As precipitate was removed from solution using a 0.45 and 0.2 m filter, the resulting arsenic concentration (As(III) + As(V)) was relatively constant (the largest LLS slope was -1.4 ng x 10-2 As/g water/day). The AcOH treatment eliminated the formation of the Fe/As precipitate observed in the non-treated samples. However, two of the AcOH water samples produced analytically significant changes in the As(III) concentration. The LLS slopes for these two waters were -5.7 x 10-2 ng As(III)/g water/day and -1.0 x 10-1 ng As(III)/g water/day. This corresponds to a -4.3 ng/g and a -7.8 ng/g change in the As(III) concentration over the study period, which is a 10% shift in the native distribution. The third and final treatment was EDTA/AcOH. This treatment eliminated the Fe/As precipitate that formed in the non-treated sample. The LLS slopes were less than -7.5x10-3 ng As(III)/g water/day for the above mentioned waters, corresponding to a 0.6 ng/g change over the study period. One of the EDTA/AcOH treated waters did indicate that using the 5 C storage temperature minimized the rate of conversion relative to 20 C storage.