Transport of metals in subsurface soils and their remediation.

Abstract

Batch and miscible displacement experiments were performed to investigate the sorption and transport behavior of cadmium, nickel, and strontium, and potential use of calcium and fulvic acid to facilitate desorption and removal of cadmium and nickel in subsurface soils. Sorption of cadmium, nickel, and strontium in the soil was quite rapid, essentially completed within two hours. Sorption isotherms for cadmium, nickel, and strontium exhibited strong nonlinearity and it was well described by the use of the Freundlich equation. The sorption of cadmium was greater than nickel and strontium. Electrostatic sorption was major contributor to the sorption of these metals. The breakthrough curves of cadmium, nickel, and strontium were asymmetrical and had significant tailing. The results of the simulations obtained by using models incorporating both nonlinear, and rate-limited sorption and flow-interruption experiments indicated that nonlinear sorption was the major cause of nonideal transport, with rate-limited sorption a minor component. The transport of metals was significantly affected by the presence of another metal. The transport of cadmium and nickel was enhanced in the mixture of cadmium and nickel due to competition for adsorption sites. Adsorption of cadmium, nickel, and strontium was significantly inhibited by the presence of calcium in the solutions. The transport of cadmium through the soil column was enhanced due to the competition between calcium and cadmium for exchange sites on the surface of soil particles. The cadmium adsorbed to the soil was removed by injecting different concentration of calcium, and the removal of cadmium also increased significantly with increasing concentration of calcium. Chloride was found to promote the desorption of cadmium in comparison with nitrate due to greater affinity of the former for cadmium to form series of less charged or uncharged species. The positions of the cadmium elution peaks by different concentration of calcium were successfully estimated by sorption isotherm of calcium on the soil.