Vacuum microbalance studies of lead deposits from natural waters

Abstract

Studies have been made of the solubility of lead In water and its deposition in the form of the hydroxide, carbonate and basic carbonate at different pH levels. Removal of soluble lead by precipitation as phosphate was also examined. The microstructure and surface activity of the precipitated and deposited lead compounds was determined by gravimetric B.E.T. nitrogen gas sorption and themoanalytical techniques, supported by X-ray diffractometry. Generally concentrations of lead in tap water are determined from the solubility products of the pipe-scale deposits, so that the solubility of lead decreases as the pH increases from 6 to 9. Thermal analysis and X-ray diffractometry results show that the scale consists of basic lead carbonate, Pb 3(CO 3) 2(OH) 2, at higher pH and normal lead carbonate, PbCO 3, at lower pH. Lead hydroxide, carbonate and basic carbonate form less porous and more compact deposits, which afford protection of the lead pipe interiors against further lead dissolution. Where pH adjustment alone proves insufficient at reducing plumbosolvency, phosphate dosing is used. This depends on the low solubility of the lead phosphates, particularly PbHPO 4. However, the precipitated lead phosphates are too finely-divided to form stable coatings on the pipe linings, even after a longer time.