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Point of Zero Charge: Role in Pyromorphite Formation and Bioaccessibility of Lead and Arsenic in Phosphate-Amended Soils
Citation:
Karna, R., M. Noerpel, T. Luxton, AND K. Scheckel. Point of Zero Charge: Role in Pyromorphite Formation and Bioaccessibility of Lead and Arsenic in Phosphate-Amended Soils. Soil Systems. MDPI, Basel, Switzerland, 2(2):22, (2018). https://doi.org/10.3390/soilsystems2020022
Impact/Purpose:
The pH at which the sorbent surface charge takes a zero value is defined as point of zero charge (pHPZC). At this pH, the sum charge of the positive surface sites is equal to that of the negative ones, resulting in a high state of entropy and disequilibrium. The knowledge of pHPZC helps to hypothesize on the ionization of functional groups and their interaction with metal species in solution. At solution pHs higher than pHPZC, sorbent surface is negatively charged and could interact with positively charged metal species while at pHs lower than pHPZC, solid surface is positively charged and could interact with negative species [19]. We theorized that driving the pH of a soil system to a state of disequilibrium may enhance contaminant immobilization. An essential aspect of in-situ immobilization of Pb via phosphate amendments is to lower the risk (bioavailability) of Pb in soil. Bioavailability can be assessed with animal (in-vivo) and simple chemical extraction assays (in-vitro). In this study, EPA Method 1340 was utilized as the in vitro bioaccessibility assay (IVBA), which includes pH 1.5 as the standard pH for extracting in vitro bioaccessible Pb and As in order to account the maximum absorption during fasting condition. As stated in Method 1340, the assay at pH 1.5 is not suitable for phosphate amended soils, which overestimates Pb bioaccessibility in amended soils. However, several reports have found less variability at pH 2.5 compared to pH 1.5. The difference in Pb extractability at pH 1.5 versus pH 2.5 for phosphate amended soils may be due to change in phosphate chemistry. Below pH 2.12, phosphate prefers to be H3PO4, and above pH 2.12 phosphate prefers H2PO4−1. A slight shift in extraction pH can have a profound effect on phosphate chemistry and extractability. Therefore, IVBA using pH 2.5 rather than 1.5 has potential to more accurately measure efficacy of phosphate soil amendments to reduce bioaccessible Pb [6,24,25]. This study conducted the IVBA extraction at both pH 1.5 and pH 2.5 following the standard IVBA procedure. The objective of this study was to compare the combined effect of pH with respect to PZC and different rates of phosphate application on the formation and stability of pyromorphite over time, and on Pb and As bioaccessibility as impacted by speciation changes. Attempts have been made to fine-tune and understand the limitations of Pb conversion to pyromorphite in phosphate amended soils.
Description:
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 > pHPZC and higher at pH pHPZC 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.
URLs/Downloads:
DOI: Point of Zero Charge: Role in Pyromorphite Formation and Bioaccessibility of Lead and Arsenic in Phosphate-Amended Soils
Free access through PubMed Central
https://www.mdpi.com/2571-8789/2/2/22