For both initial pH values, the pH of the diluate increased as the ED process progressed, while the pH of the concentrate first decreased then increased (Figure ). The increase in diluate pH was due to the dominant H + transport; toward the end of the experiment, the rate of increase in pH was lower as H + ions were depleted. The initial decrease in concentrate pH was due to the predominant transport of strongly acidic sulfate species, supporting a high H + concentration. The following increase in pH was due to the uptake of H + by H2PO4 -as the transport of such weakly acidic species became more significant. For practical purposes, the increase in diluate pH leads to the neutralization of most environmentally hazardous acidity in the PIWAA.