Point of Zero Charge: Role in Pyromorphite Formation and Stability in Lead Contaminated Soils.

Soluble lead (Pb) is immobilized in pure systems as pyromorphite by adding sources of phosphorus (P), but doubts still remain about the effectiveness of this approach in natural soil systems. The net surface charge of sorbents play a crucial role in sorption processes. The pH at which the sorbent surface charge takes a zero value is defined as point of zero charge (pzc). The knowledge of pzc helps to predict the ionization of functional groups and their interaction with metal species in solution. We hypothesize that protonation states of phosphate; especially di-protonated phosphate may shift the pH just below pzc, at which more pyromorphite formation with greater stability may occur. The objective of this study is to measure the effect of pzc on pyromorphite formation and stability. The study will be conducted using a completely randomized design with two-way treatment structure of pH at 3 different levels (pH = pHpzc, pH < pHpzc, and pH > pHpzc), and phosphate at three different levels (low, medium, high). Each treatment combination will have 3 replicates, along with controls. The desired pH range will be determined via soil pzc determination, and will be monitored and controlled periodically. Sampling will be done at 0-day, 1-week, 1-month, 3-month, and 6-month time points. Solution chemistry analyses along with advanced molecular spectroscopy will be used to determine Pb solubility, and their speciation in soil at different treatment combinations. The results from this study will be used to understand the chemistry involved with pzc, and its influence on pyromorphite formation and stability in natural systems. This may help the design of remediation strategies for stabilizing Pb via P application in Pb contaminated sites and reduce doubts about inducing pyromorphite formation.