Chloramine Complexities

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Science  02 May 2008:
Vol. 320, Issue 5876, pp. 586
DOI: 10.1126/science.320.5876.586a

Chloramine is a comparatively recent weapon in the ongoing battle to eliminate harmful microorganisms from drinking water supplies. Though its disinfecting properties are straightforward, the concomitant generation of ammonia as a byproduct can give rise to a complex web of downstream chemistry that remains an active area of study. One important reaction is microbial nitrification, or oxidation of the ammonia to nitrite and nitrate, which also lowers the water's pH by acid production. Zhang et al. have systematically explored the efficiency of nitrification in plumbing pipes of differing compositions—polyvinyl chloride (PVC), copper, lead, and brass—at various pH and phosphate levels. They found that relative to PVC, copper inhibited nitrifier growth, whereas lead enhanced it (probably through reductive cycling of nitrate back to ammonia via lead corrosion). Brass initially resisted nitrification activity, but then shifted its behavior after ∼120 days, as the efficiency of copper leaching from the alloy diminished. A perhaps counterintuitive consequence of this reaction web is that PVC pipes may ultimately cause more metal ion leaching into the water stream than copper pipes, as the acid byproducts of nitrification degrade brass valves and faucets. — JSY

Environ. Sci. Technol. 42, 10.1021/es702483d (2008).

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