Immobilization of Lead in Soil Influenced by Soluble Phosphate and Calcium: Lead Speciation Evidence

We investigated the effect of soluble Ca on the speciation of Pb in phosphate‐amended soils. A calcareous soil was amended in sequence with soluble phosphate (0 and 299 mg kg−1 P), calcium nitrate (0, 200, and 400 mg kg−1 Ca), and/or lead nitrate (0 and 1000 mg kg−1 Pb) and incubated for 50 d. Extractable Ca was relatively similar across all samples, with a minor reduction in Ca release for treatments amended with P possibly from the formation of Ca‐phosphate minerals. Olsen‐P extractability from P‐added treatments was highest in the non–Pb‐amended soils and was about 40% lower when Pb was added as a result of Pb‐phosphate formation. In the absence of P, diethylene triamine pentaacetic acid (DTPA)‐Pb extractability increased with increasing Ca amendment; however, in the P‐amended treatments, DTPA‐Pb decreased with increasing Ca addition. X‐ray absorption spectroscopy results of the Pb spiked soils indicate that adsorbed Pb is the primary phase (up to 92%) in non–P‐amended soils with minor distribution to hydrocerussite and Pb‐phosphate phases. In the P‐amended treatments, Pb speciation shifted to Pb‐phosphate [chloropyromorphite and Pb3(PO4)2] (42–48%) and adsorbed Pb (44–50%). As Ca concentration increased in the P‐amended soils, Pb‐phosphate speciation moved from combined chloropyromorphite and Pb3(PO4)2 (0 mg kg−1 Ca) to exclusively chloropyromorphite. The study demonstrates that soluble Ca enhances Pb immobilization with P amendments.