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

Abstract

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 TER Pb appeared significantly lower at pH > pH PZC and higher at pH < pH PZC (α = 0.05). In contrast, the TER As 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.

In Vitro, in Vivo, and Spectroscopic Assessment of Lead Exposure Reduction via Ingestion and Inhalation Pathways Using Phosphate and Iron Amendments

“…In the study of anglesite immobilization using Ca­(H 2 PO 4 ) 2 ·H 2 O and HA, Cao et al and Zhang and Ryan reported a rapid dissolution of anglesite in aqueous systems, with the detection of chloropyromorphite. However, more complex interactions may occur in natural soil system, limiting Pb immobilization in situ (e.g., transient increases in soluble Pb, incomplete formation, or dissolution of Pb phosphates due to inadequate Pb dissolution, and competition from high cation concentrations with phosphate for sorption sites). − Higher phosphate application rates may be necessary in future studies in Broken Hill soil to compensate for competition posed by cations (e.g., Al, Ca, Fe, Mn, and Zn).…”

“…However, more complex interactions may occur in natural soil system, limiting Pb immobilization in situ (e.g. transient increases in soluble Pb, incomplete formation or dissolution of Pb-phosphates due to inadequate Pb dissolution, competition from high cation concentrations with phosphate for sorption sites) [50][51][52] . Higher phosphate application rates may be necessary in future studies in Broken Hill soil to compensate for competition posed by cations (e.g.…”

Long-Term in Situ Reduction in Soil Lead Bioavailability Measured in a Mouse Model

“…Additional studies involving repeated sampling of remediated sites will be needed to characterize the time dependence of such changes. Differences in inorganic and organic components of soils that affect formation of insoluble Pb–P species and the efficacy of soil remediation procedures also require additional study. , Determination of soil Pb RBA in the mouse gives a direct measure of the efficacy of treatment that can be coupled with determination of Pb speciation in treated soil to provide insights into the design of more effective soil-specific remediation processes.…”

Bioavailable soil Pb minimized by in situ transformation to plumbojarosite

“…In some cases, P treatment may have a minimal effect on soil Pb bioavailability. A bench scale study ( 34 ) with a soil used in the current study evaluated Pb bioaccessibility as a function of soil suspension pH relative to the soil’s point of zero charge and three P amendment levels. The best-case scenario found that P treatment produced a modest 35% reduction in Pb bioaccessibility relative to the untreated (U) sample.…”