Cr(VI) Generation and Stability in Drinking Water
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The current US Environmental Protection Agency (USEPA) maximum contaminant level (MCL) for total chromium of 100 ppb is under revision to consider a specific level for Cr(VI), which has a proposed MCL of 10 ppb in California. Cr(VI) is a suspected carcinogen, and interconverts with the other most commonly found chromium species, Cr(III). To regulate and further understand the behavior of Cr(VI) in water systems, appropriate sample preservation methods are essential for accurate measurements. The ammonia buffer (recommended by EPA) was proven to be the most effective preservation when a holding time of 14 days is considered. Apart from proper Cr(VI) preservation, sampling at an appropriate site is important for determining the public's exposure to Cr(VI). The proposed MCL for Cr(VI) in the state of California will be monitored at the entry point of distribution systems. To the extent that Cr(VI) is formed in the distribution system or in water contacting plumbing, measurements at the treatment plant might not reflect consumer exposure at the tap. Cr(VI) can be released to drinking water from Cr present in stainless steel alloys. At the maximum residual disinfectant level (MRDL), Cr(VI) formation decreased in the order chlorine dioxide > chlorine > chloramine. Less Cr(VI) was released from stainless steel at lower pH in the presence of chlorine, but the opposite trend was observed for chlorine dioxide. Stainless steels with a higher chromium content tended to release more Cr(VI).
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