Effects of Free chlorine, Chloramines, and Phosphate on Dissolution of PbO2 and the Reductive Dissolution Mechanism

摘要

PbO_2 is a corrosion product that develops on lead pipes that have been in contact withrnfree chlorine present as a residual disinfectant. When the residual disinfectant is switched fromrnfree chlorine to monochloramine, the accelerated reductive dissolution of PbO_2 can causernelevated lead concentrations in tap water. The objectives of this study were to investigate therneffects of chlorine disinfectants and orthophosphate on PbO_2 dissolution rates, gain insights intornPbO_2 dissolution mechanisms, and evaluate the effect of chlorine disinfectants andrnorthophosphate on lead release from pipe scales containing PbO_2. The effects of free chlorine,rnmonochloramine, and orthophosphate on the dissolution rates of plattnerite were quantified inrncompletely-mixed continuous-flow reactors at relevant pH and dissolved inorganic carbonrnconditions. PbO_2 and hydrocerussite were developed in the scales of new lead pipes, and leadrnrelease from the pipe scales was examined in the presence and absence of chlorine disinfectantsrnand orthophosphate. PbO_2 dissolution rates decreased in the following order: no disinfectant >rnmonochloramine > chlorine, which was consistent with the trend in the redox potential caused byrnmonochloramine and chlorine. Addition of orthophosphate lowered net dissolution rate of PbO_2.rnThe effects of free chlorine and orthophosphate on lead release from pipe scales containing PbO_2rnare consistent with the effects of free chlorine and orthophosphate on PbO_2 dissolution rates.