Calcite and calcium oxalate sequestration of heavy metals

Heavy metals like cadmium, lead and zinc pose a significant threat to human health and environment. Several factors such as pH, EH, organic matter and clay content of the soil affect the bioavailability of such heavy metals in the environment. The presence of several naturally occurring minerals such as calcite (calcium carbonate, CaCO3) and calcium oxalate (CaC 2O4.) in the environment significantly influence the fate and transport of some of the heavy metal cations. Sequestration of heavy metals such as cadmium (Cd) and zinc (Zn) from solution by calcite has been clearly demonstrated in the literature. However, studies on heavy metal sequestration by calcite and calcium oxalate from a multi-metal environmental that represents real world situations are rare. In this contribution, column flow studies of Cd and Zn sequestration by calcite exposed to influent solutions saturated or non-saturated with calcite and containing either 1 mg/L of Cd, 1 mg/L of Zn or combined 1 mg/L of Zn and Cd, followed by desorption of the sequestered metals were conducted. Complementary scanning electron microscopy (SEM) coupled with electron dispersive x-ray spectroscopy (EDS) data were generated. Kinetic studies show that reaction rates of Cd and Zn with calcite are governed by a simple rate law with reaction orders of less than 1 (0.02--0.07) indicating at least mathematically, the occurrence of reactions that went to completion if the reaction orders did not change. When the influent solution contains a single cation, the rate of Zn removal from solution by calcite and calcium oxalate is greater than Cd removal rate. However in a multi-cation environment, cadmium removal rate was greater than zinc removal rate. MINTEQA2 a geochemical equilibrium speciation model was used to compute the equilibrium between the various species in the cation-calcite environment. Complimentary desorption studies and surface SEM/EDS analysis indicate that the removal of Cd and Zn from solution by calcite and calcium oxalate is probably due to precipitation/complexation reaction. The SEM and EDS results appear to confirm the presence of a precipitate on the mineral surface in the case of the influent solution containing Zn. The current research also examines the effect of citrate, a commonly present urinary tract species on calcium oxalate dissolution. The dissolution studies indicate that citrate solution is capable of dissolving sodium oxalate at high pH. The dissolution of calcium oxalate results in the release of heavy metals that were previously sequestered within the mineral. Results show that a greater percentage of zinc was removed than cadmium, from calcium oxalate due to its dissolution by citrate.