Switching from alum (aluminum sulfate) to polyaluminum chloride (PACl) as coagulant has triggered problems with lead at the tap, for several utilities that were previously in compliance with the US EPA lead action level. This seemingly innocuous change in coagulant type results in a higher chloride to sulfate ratio of finished water, since PACl adds chloride to water whereas alum adds sulfate. Bench scale testing revealed that PACl-treated water (high Cl-:SO4 -2), typically increases lead leaching from leaded brass by a factor of 1.2-2.7 times, compared to alum-treated water (low Cl-:SO4-2 ratio). In the case of 50:50 lead:tin solder connected to copper pipe, and when a phosphate inhibitor was added to the water, a much more dramatic difference of 40 times was noted after two weeks. The mechanism driving galvanic corrosion is the lowering of pH near the solder surface in a water of low buffering capacity, which in turn increases lead solubility. Local pH on the surface of galvanic solder dropped from the neutral range to as low as 3.4 after exposure to PACl-treated water, and to as low as 4.4 after exposure to alum-treated water.
展开▼
