Reevaluation of Phosphate as a Means of Retarding Lead Transport from Sandy Firing Ranges

Abstract

Phosphate has become an accepted remediation strategy to immobilize lead on firing ranges. In some cases, however, phosphate treatment has been reported to increase lead concentrations in field water leaching tests. The present study evaluated the influence of phosphate (sodium phosphate, PO4(aq), or particulate hydroxyapatite, HA) and lead (lead nitrate, Pb(aq), or particulate lead oxide, PbO(s)) sources on the physical, chemical, and relative transport properties of their reaction products. Relative transport behavior of the product of each of the four possible combinations was investigated with settling columns assessing potential surface water transport and sand columns assessing potential ground water transport. Pyromorphite (HYP) was the only product formed when PO4(aq) and Pb(aq) were combined. The HYP formed under these conditions was the least mobile form of lead and phosphate examined in this work. Although additional products were formed when PO4(aq) was combined with PbO(s), lead transport was significantly reduced compared to the PbO control. On the other hand, HA was much less effective at controlling the transport of Pb(aq) in sand columns, particularly at low pH. In addition, the presence of HA increased the mobility of PbO at pH 7.2 relative to the control. It is recommended that the accepted practice of using HA in sandy firing range soils, under low to neutral pH conditions, be reconsidered.