Organic acid-induced release of lead from pyromorphite and its relevance to reclamation of Pb-contaminated soils

The long-term stability of pyromorphite [Pb5(PO4)3Cl] (PY) in root–soil interface (or rhizosphere) where production of organic acids from biological activities takes place is not fully understood. We conducted a 1-year long laboratory batch dissolution experiment to elucidate the release of Pb (and P) from PY by four commonly occurring low molecular weight organic acids (LMWOA) in rhizosphere: acetic, citric, malic and oxalic acid. Mean maximum amount of Pb in milliQ (mQ) water (1.8 μM) was lower than in solutions from LMWOA alone or in combination with each other (i.e., mixed acid). However, there was no significant difference in the amount of Pb (and P) in solution in all treatments including mQ water after 6 months. Among the 100 μM LMWOA, mean of five highest soluble Pb (μM Pb in solution) followed the order: oxalic acid (17.6) > citric (6.2) > malic (5.6) > acetic acid (3.0 μM Pb). Mixed acid solution had a maximum amount of 14.0 μM Pb. We calculated a range of solubility product constant (Ksp) of PY in this study from 8.6 × 10−54 (mQ) to 7.0 × 10−45(oxalic acid); these values are within the range of PY Ksp reported elsewhere. Despite the low Ksp values, LMWOA-induced released Pb from PY are in concentrations higher than both Canadian and international drinking water and agricultural water use quality standards. This suggests that soil organic acids such as in rhizosphere can potentially liberate Pb from PY in contaminated soils.