Phosphate-induced lead immobilization from different lead minerals in soils under varying pH conditions

Abstract

This study investigated phosphate-induced lead immobilization from different Pb minerals in soils under varying pHs. Four soils were used, including one Pb-contaminated soil (NC-Soil) and three soils spiked with litharge (PbO), cerrusite (PbCO₃), or anglesite (PbSO₄), referred to as PbO-soil, PbCO₃-soil, and PbSO₄-soil, respectively. The soils were equilibrated with KCl and Ca(H₂PO₄)₂·H₂O under pH of 3–7. At low pH (3 and 5), Pb solubility followed PbO-soil > PbCO₃-soil > PbSO₄-soil; while at pH = 7, it was PbSO₄-soil > PbO-soil > PbCO₃-soil. Phosphate decreased Pb dissolution time from >180 to <60 min and reduced soluble Pb by 67–100%. This was mostly via transformation of Pb minerals into chloropyromorphite [Pb₅(PO₄)₃Cl]. Our results indicated that P addition can effectively transform various Pb minerals into insoluble chloropyromorphite in soils. This transformation was more significant at acidic condition (e.g., pH ≤ 5). Among the three Pb minerals tested, PbSO₄ was the most effectively immobilized by P, followed by PbO and PbCO₃. This study clearly demonstrated the importance of the form of Pb contamination and soil pH in determining the effectiveness of Pb immobilization in soils.

Introduction

Both laboratory experiments and field tests have demonstrated that phosphorus (P) is effective in immobilizing lead (Pb) in contaminated soils and waters via transformation of soluble Pb into sparingly soluble pyromorphite-like minerals [Pb₅(PO₄)₃(Cl, F, OH)] (Ma et al., 1995, Ryan et al., 2001, Cao et al., 2002). Under a well-mixed system, the formation of pyromorphite from soluble P and Pb is rapid and soluble Pb can be rapidly decreased when adequate P is present (Zhang and Ryan, 1999a). A potential limitation of in situ Pb immobilization is the limited solubility of Pb minerals in contaminated soils.

In Pb-contaminated soils, Pb is present in a variety of physical (dust, chips, and chunks) and chemical (oxides, carbonates, hydroxides, and sulfates) forms (Royer et al., 1992). Common Pb minerals found in contaminated soils include Pb carbonates, Pb oxides, and Pb sulfate (Nedwed and Clifford, 1997). These minerals have been identified as major lead phases in contaminated soils from several industrial sites (Van Benschoten et al. 1998). Hessling et al. (1990) also found lead carbonate, basic lead carbonate, lead sulfate and lead oxides as major lead compounds in several automobile battery reclamation sites. Cerrusite (PbCO₃) and hydrocerrusite (Pb₂CO₃(OH)₂) were also identified in Pb-contaminated soils at shooting range sites (Cao et al., 2003).

Because Pb species in soils vary widely and depend upon the sources of Pb contamination and environmental conditions (Davis and Hotha, 1998), knowledge of the solubility of primary Pb solid phases and their reaction behavior with phosphate becomes important in assessing the efficiency of P-induced Pb immobilization in soils. Solubility of Pb minerals is mainly determined by their thermodynamic stability (Lindsay, 1979). However, in a dynamic system like a soil, the kinetics of a reaction must be taken into account in order to determine the likelihood and the extent of a reaction occurring for a given condition and time scale. For example, in a kinetic study by Gasser et al. (1996), they found, under simulated gastric conditions, Pb release from materials impacted by smelter and mine waste was related to Pb speciation, with stable Pb forms resulting in slow rates of Pb dissolution.

In general, metal solubility in soils depends on pH, liquid-to-soil ratio, metal type, and contact time. Of these parameters, soil pH and metal type are the most important limiting factors determining metal solubility. The pH determines the equilibrium solubility (concentration) achievable, while the metal determines its potential solubility at a given pH. Using different lead minerals to study the formation of pyromorphite in aqueous systems in the presence of synthetic apatite under varying pH conditions, Zhang and Ryan, 1999a, Zhang and Ryan, 1999b found that at low pH (≤4), the dissolution rate of cerrusite is rapid and the conversion of cerrusite to pyromorphite is quick (Zhang and Ryan, 1999a). High pH (>5) is favorable for dissolution of galena (PbS) and transformation of galena to pyromorphite (Zhang and Ryan, 1999b). To date, the effects of solubility of Pb minerals on pyromorphite formation at different pHs in a soil matrix system has not been investigated using soluble P. Understanding the reaction kinetics of P–Pb in Pb-contaminated soils dominated with different Pb-minerals will provide additional information essential for selection of in situ remediation technology for different soils. It will also be useful to optimize the treatment application rate as well as design parameters for a more effective in-situ remediation technology.

This study was conducted using a soil spiked with different Pb minerals to evaluate (1) dissolution behavior of Pb in soils at pH ranging from 3 to 7 and (2) their effects on Pb immobilization in the presence of phosphate. The Pb minerals included litharge (PbO), cerrusite (PbCO₃), and anglesite (PbSO₄), which are commonly present in Pb-contaminated soils. Information obtained from this study should be helpful to determine the applicability of P for remediating soils contaminated with PbO, PbCO₃, and/or PbSO₄ as the primary lead phases.