Lead Stabilization in Soil Amended with Lime Waste: An Extended X-ray Absorption Fine Structure (EXAFS) Investigation

To determine Pb species in soils following the immobilization process, sequential extraction has been used despite the possibility of overestimating Pb species from unintended reactions during chemical extraction. Meanwhile, the application of extended X-ray absorption fine structure (EXAFS) has been shown to provide a more precise result than chemical extraction. In this study, the immobilization of Pb in contaminated soils treated with liming materials such as oyster shell (OS) or eggshell (ES) was evaluated with thermodynamic modelling and EXAFS analysis. Thermodynamic modelling by visual MINTEQ predicted the precipitation of <TEX>$Pb(OH)_2$</TEX> in OS and ES treated soils. In particular, the values of saturation index (SI) for <TEX>$Pb(OH)_2$</TEX> in OS (SI=0.286) and ES (SI=0.453) treated soils were greater than in the control soil (SI=0.281). Linear combination fitting (LCF) analysis confirmed the presence of <TEX>$C_{12}H_{10}O_{14}Pb_3$</TEX> (lead citrate, 44.7%) by citric acid from plant root, Pb-gibbsite (Pb adsorbed gibbsite, 26.4%), and Pb-kaolinite (Pb adsorbed kaolinite, 20.3%) in the control soil. On the other hand, <TEX>$Pb(OH)_2$</TEX> (16.8%), Pb-gibbsite (39.3%), and Pb-kaolinite (25.6%) were observed in the OS treated soil and <TEX>$Pb(OH)_2$</TEX> (55.2%) and Pb-gibbsite (33.8%) were also confirmed in the ES treated soil. Our results indicate that the treatment with OS and ES immobilizes Pb by adsorption of Pb onto the soil minerals as a result of the increase in soil negative charge and the formation of stable <TEX>$Pb(OH)_2$</TEX> under high pH condition of soils.