UNDERSTANDING BLENDED CORROSION CONTROL CHEMICALS FOR CONTROLLING LEAD RELEASE AND WATER QUALITY IN DRINKING WATER

Abstract

This thesis provided a comprehensive review of the occurrence, measurement, and characterization of colloidal lead in drinking water. Colloids have been underestimated in conventional drinking water monitoring, where metals are defined as soluble based on their ability to pass through a 0.45 μm filter. Field studies show that colloidal lead can represent a significant proportion of total lead concentrations. This work also explored various approaches to controlling iron and lead release, and manganese sequestration using blends of orthophosphate and one of three model polyphosphates -- tripoly-, trimeta- and hexametaphosphate— , or orthophosphate and sodium silicate. Polyphosphate structure was a significant factor for determining their behavior with lead, iron, and manganese. Blends with linear polyphosphates provided greater capacity for chelating metal ions than cyclophosphates. Longer chain lengths increased lead but decreased iron release. This was attributed to the steric constraints of cyclophosphates inhibiting their interactions with metals. Unlike cyclophosphates, linear polyphosphates also appeared to remain in solution rather than adsorb to mineral surfaces, resulting in greater metals release. Sequesterants may increase manganese mobility by forming soluble metal-complexes or via their mobilization with iron colloids. Here, polyphosphates or silicate sequestered 2.3-7.4 and 3 times more manganese than orthophosphate, respectively. Orthophosphate-silicate was the most effective blend for reducing iron corrosion. Moreover, field flow fractionation (A4F) data were consistent with the mobilization of manganese via adsorption to suspended iron colloids. Small colloids present unique challenges for maintaining drinking water quality. Techniques used for sampling and colloids characterization can shape corrosion control decisions. The use of sequestrants entails considerable risk and may be counterproductive to minimizing consumer exposure to lead and manganese. The dispersive properties of sequestrants resulted in the mobilization of manganese via its attachment to iron colloids or maintain dissolved metals in solution.