Effects of Phosphate Rock on Sequential Chemical Extraction of Lead in Contaminated Soils

Lead contamination is of great concern because of its adverse effects on human health, especially children. This research evaluated the effects of phosphate rock on chemical associations of Pb in eight Pb-contaminated soils using a sequential extraction procedure. The chemical fractions are operationally defined by an extraction sequence in the order of increasing ability to dissolve Pb of lower solubilities. Additionally, more soluble forms of Pb are considered to be potentially more bioavailable than the less soluble forms. Lead in these soils was primarily associated with the carbonate and Fe-Mn oxide fractions (63-85%). Up to 21% of the Pb in these soils was associated with either the organic or the residual fraction and <11 % was associated with the water-soluble and the exchangeable fractions. Phosphate rocks effectively converted Pb from the water soluble, exchangeable, carbonate, Fe-Mn oxide, and organic fractions (collectively the nonresidual fraction) to the residual fraction, thus reducing Pb solubility and presumably bioavailability. Lead precipitation as a fluoropyromorphite-like mineral in these contaminated soils was suggested as the primary mechanism for reduced Pb solubility and Pb reduction in the nonresidual fraction. The effective conversion of Pb from potentially available fractions to the residual fraction suggests that phosphate rock has potential for in-situ immobilization in Pb contaminated soils. T EAD IS A HEAVY METAL that is toxic to humans and .L animals. Its extensive use and widespread disposal Soil and Water Science Dep., Univ. of Florida, Gainesville FL 326110290. Received 1 July 1996. *Corresponding author ([email protected]. ufl.edu). Published in J. Environ. Qual. 26:788-794 (1997). in the environment have resulted in numerous Pb-contaminated soils (Turjoman and Fuller, 1987). The health concern with Pb-contaminated soils arises mostly from plant contamination by soil particles, and soil and dust ingestion by humans, especially children, and grazing animals (McBride, 1994). Therefore it is important to minimize adverse Pb impacts on the environment. Reducing Pb solubility and bioavailability in contaminated soils without removing Pb from the soil is one of such measures that could reduce Pb impacts on the environment. The combination of containment and in-situ Pb immobilization is a promising technology to remediate Pbcontaminated soils (Czupyrna et aI., 1989). The geochemical behavior of Pb indicates that phosphate, when present in sufficient amounts, reduces Pb solubility (Ma et aI., 1993, 1994a,b, 1995; Nriagu, 1974; Ruby et aI., 1994). Thus phosphate minerals have the potential to immobilize Pb in contaminated soils. Several insoluble Pb orthophosphate minerals may form after P reaction with Pb-contaminated soils depending on the reaction conditions, such as pH and presence of other anions and cations (Ma et aI., 1993, 1994a,b, 1995). Both phosphate rocks and hydroxyapatite have been used as the primary P sources in these studies and both materials effectively reduced Pb solubility. Ma et aI. (1995) have shown that phosphate rock (primarily CalO(P04)6F2) effectively immobilized Pb Abbreviations: OC, Occidental Chemical Corp.; PR, phosphate rock. MA & RAO: EFFECTS OF LEAD IMMOBILIZATION BY PHOSPHATE ROCK 789 from aqueous solutions, with Pb immobilization ranging from 39 to 100%. The primary mechanism of Pb immobilization is via dissolution of phosphate rock and subsequent precipitation of a fluoropyromorphite-like mineral (PblO(P04)6F2), although precipitation of Pb as hydrocerussite also occurred in some instances. Moreover, the potential of using Florida phosphate rock to immobilize aqueous Pb from Pb-contaminated soils was demonstrated (Ma et al., 1995). Florida phosphate rock effectively immobilized 22 to 100% aqueous Pb from 13 Pb-contaminated soils. In these studies, the effectiveness of phosphate rock to immobilize aqueous Pb was assessed based on reduction in aqueous Pb concentrations (Ma et al., 1995). Although aqueous Pb concentrations are critical for evaluating Pb bioavailability in contaminated soils, knowledge of Pb distribution in different geochemical forms will provide more detailed information on the effectiveness of Pb immobilization by phosphate rock. Geochemical forms of trace metals in soil affect their solubilities, therefore directly influence their potential bioavailability (Xian, 1987, 1989). Thus, assessing environmental impacts of trace metals in soils by aqueous concentrations of trace metals is incomplete. In particular, it may be helpful to obtain information on the degree of bioavailability of trace metals (Scokart et al., 1987). Sequential extraction using various leaching agents is commonly used to help assess bioavailability of trace metals in soils. Numerous schemes for soils and sediments have been described (Gupta and Chen, 1975; Miller and McFee, 1983; Tessier and Campbell, 1988; Tessier et al., 1979; Welte et al., 1983). The extraction scheme is based on operationally defined fractions: water soluble, exchangeable, carbonate, Fe-Mn oxides, organic, and residual fractions (Tessier et al., 1979). It must be borne in mind that the extractions are not entirely specific and there will be overlap between the fractions. Despite these uncertainties, extraction procedures provide semiquantitative evidence regarding the forms of trace metals, and indirectly their bioavailability (Harrison and Wilson, 1982). A major pathway for Pb exposure by small children is via direct soil ingestion, and an important question is the bioavailability of Pb from various sources. Lead in soil is present in different chemical or physical forms and thus its bioavailability may vary (Freeman et al., 1992). Epidemiological studies of children residing in communities contaminated by Pb from smelter or urban sources frequently have higher blood levels than children living in areas contaminated with Pb from mining wastes, even when soil Pb concentrations are similar (Steele et al., 1990). It seems that differences in Pb source and thus Pb species rather than the total amount of Pb may help to account for the discrepancy. In this case, sequential extraction may be useful to indirectly assess Pb bioavailability in soils. Assuming the nonresidual Pb (sum of water soluble, exchangeable, carbonate, Fe-Mn oxides, and organic fractions) is more bioavailable than the residual Pb, then the effectiveness of in-situ remediation of Pb-contaminated soils can be assessed using a fractionation scheme, with more effective treatment converting greater amounts of Pb from the nonresidual to the residual fraction or from more to less bioavailable forms, i.e., from the water soluble and exchangeable to carbonate, Fe-Mn oxide, or organic fractions. The scheme of Tessier et al. (1979) , one of the most widely used, was selected to evaluate the efficacy of decontamination treatments (Pardo et al., 1990; Rauret et al., 1988; Spevackova and Kucera, 1989). The objectives of this study were to use sequential extraction to determine Pb distribution in eight Pb-contaminated soils and to evaluate the effectiveness of using phosphate rocks to immobilize Pb from Pb-contaminated soils. MATERIALS AND METHOD