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 where at pHs above pHpzc, the sorbent surface is negatively charged while at pHs lower than pHpzc, the sorbent surface is positively charged. The purpose of this study is to measure the effect of pH, and the different rate of phosphate application, and their combined effect on the formation and stability of pyromorphite over time.

The study was conducted using a completely randomized design with a 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 had 3 replicates, along with controls. The desired pH range was determined via soil pzc determination, and was monitored and controlled periodically. Sampling was done at 0-day, 1-week, 1-month, 3-month, and 6-month time points. Solution chemistry analyses along with advanced molecular spectroscopy was used to determine Pb solubility, and their speciation in soil at different treatment combinations. The findings from this study indicated that there was no significant difference in total dissolved Pb in amended soils compared to control soil, however IVBA test showed reduction in bioaccessible Pb at >pH_PZC. Less arsenic was mobilized at higher pH which is not usually expected. 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.