Changes of biochemical properties and heavy metal bioavailability in soil treated with natural liming materials

This study evaluated the efficiency of naturally occurring lime-based waste materials (oyster shells, eggshells, and mussel shells) on immobilization of selected heavy metals (Cd and Pb) and a metalloid (As) in a contaminated agricultural soil. A 30-day incubation experiment was performed using soil mixture with natural liming materials or calcite (CaCO3) at 0, 1, 3, 5, and 10 wt %. Soil biochemical properties including pH, electrical conductivity (EC), exchangeable cations, organic matter (OM), total nitrogen (TN), microbial populations, and enzyme activities were determined to ensure the changes in soil quality during incubation. The results showed that the application of natural liming materials led to an increase in soil pH similar to that of CaCO3. Soil concentrations of Cd, Pb, and As extracted with 0.1 or 1 M HCl, and diethylene triamine pentacetic acid (DTPA) were decreased significantly after adding liming materials, accompanied by increased microbial population and enzyme activities of dehydrogenase, phosphatase, beta-glucosidase, and arylsulfatase. Additionally, eggshells and mussel shells induced significant increases in OM and TN in the soil. Application of natural liming materials offers a cost-effective way to immobilize heavy metals and metalloids in soils.