LACTIC ACID REMOBILIZATION OF LEAD APATITE AND INCREASED BIOAVAILABILITY OF PB IN CONTAMINATED SOILS

A lead elevated concentration typically caused by human activity is one of the common environmental issues resulting in serious health defects. In‑situ Pb immobilization is one of the methods now routinely applied for the reclamation of lead contaminated soils (Morin et al, 2001). This method is based on the principle that aqueous phosphate added to soil pore solutions forms a very stable mineral pyromorphite (Pb apatite) Pb₁₀(PO₄)₆C_l2. Bioavailability of aqueous Pb is thus minimized due to very low solubility and high thermodynamic stability of pyromorphite but biogeochemichal behavior of lead is significantly controlled by its chemical speciation.

Three main types of organic ligands however, increases Pb apatite solubility at low pH. These types of organic ligands typically occurring in natural soil environments include ethylene diamine tetra-acetic acid (EDTA), low molecular weight organic acids (LMWOAs), and humic substances (HSs). LMWOAs resulting from microbial activity affect soil Pb primarily by lowering soil pH and by complexation of cations. Elevated Pb soil concentration due to LMWOAs activity can be used as the first evidence of possible additional increased Pb concentration from the same contaminated soils if Pb-EDTA chelates are present, as they also cause enhanced desorption from soils, elevated Pb toxicity by binding toxic Pb²⁺ cations, and increased Pb translocation to plant shoots (Shahid et al, 2012). Similar observations have been made in case of Pb arsenolites, with EDTA solubilizing Pg arsenolite more actively, then LMWOAs did (Bajda, 2011).

Results of Pb apatite solubility in lactic acid, as an example of LMOWs, across various pH ranges, are presented to illustrate increased mobility with a lower pH. Mostly surface-controlled dissolution Pb apatite dissolution is overall consistent with PREEQC modeling. Differences between solubility of Pb apatite in lactic acid as compared to EDTA are consistent with binding constants used in MINTEQ modeling. Measurable dissolution of Pb apatite surfaces at the presence of lactic acid suggests possibility of Pb remobilization through efficient ligand-promoted organic complexation and lowering of the pH by LMOWs. This may affect long-term efficiency of in-situ Pb immobilization procedures applied to contaminated soils.