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 Coprecipitation

摘要

Seven different treatment/storage conditions were investigated for the preservation of the native As(Ⅲ)/ As(Ⅴ) found in 10 drinking water supplies from across the United States. These 10 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 analyses 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 and 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 nontreated 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 0.2μm filter, the resulting arsenic concentration (As(Ⅲ) + As(Ⅴ)) was relatively constant (the largest LLS slope was -1.4 x 10~(-2) (ng As g water~(-1)) day~(-1)). The AcOH treatment eliminated the formation of the Fe/As precipitate observed in the nontreated 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~(-1)) day~(-1) and -1.0 x 10~(-1) (ng As(Ⅲ) g water~(-1)) day~(-1). 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 nontreated sample. The LLS slopes were less than -7.5 x 10~(-3) (ng As(Ⅲ) g water~(-1)) day~(-1) 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 ℃ storage temperature minimized the rate of conversion relative to 20 ℃ storage.