Point of Zero Charge: Role in Pyromorphite Formation and Bioaccessibility of Lead and Arsenic in Phosphate-Amended Soils

Soluble lead (Pb) can be immobilized in pure systems as pyromorphite through the addition of phosphorus (P) sources; however, uncertainties remain in natural systems. Knowledge of point zero charge (PZC) is important to predict the ionization of functional groups and their interaction with metal species in solution. This study utilized Pb- and As-contaminated soils to determine the combined effect of pH with respect to PZC and different rates of P-application on pyromorphite formation as well as Pb and arsenic (As) bioaccessibility as impacted by speciation changes. Solution chemistry analysis along with synchrotron-based Pb- and As-speciation as well as bioaccessibility treatment effect ratios (TERs) were conducted. Results indicated no significant effect of PZC on pyromorphite formation in P-amended soils; however, the TERPb appeared significantly lower at pH > pH(PZC) and higher at pH < pH(PZC) (alpha = 0.05). In contrast, the TERAs was significantly higher at pH > pH(PZC) compared to the other two treatments for the tested soils. The lack of conversion of soil Pb to pyromorphite may be attributed to several reasons including the presence of highly stable minerals, such as plumbojarosite, limiting soluble Pb availability to react with phosphates, high Fe and S content in IKS, high organic matter in BO, and high Ca content in NW.