Sorption behaviour of heavy metals in sand-bentonite-coal ash mixes for use as a liner material in landfills

Abstract

The heavy metals are among the potential groups present in the landfill leachate in addition to various organic and inorganic contaminants and their content often exceeds the permissible limits. These heavy metals in leachate are an environmental threat unless contained or adsorbed by an impermeable liner. The barrier material/liner for landfills is generally fabricated by mixing bentonite in locally available soil to meet out the hydraulic conductivity requirement in absence of clay in the vicinity. The sorption characteristics have an important bearing to evaluate the adsorption behaviour and migration of contaminants in order to restrict the mobility of contaminants within the liner to prevent contamination of geo-environment. This study includes the sorption behaviour of landfill liner fabricated by mixing sand, bentonite and coal ash in varying proportions using Cd, Zn, Ni and Pb as the target metals including potting of adsorption isotherms. Double distilled water was used as the permeating fluid in the sorption study. With a change in percentage of sand/bentonite/coal ash in the mix, the constituent material dominating the adsorption properties differed leading to an altogether different adsorption behaviour of the soil mix. The addition of bentonite was found to be beneficial to both sand and coal ash in improving the sorption behaviour. Langmuir Isotherm was found to be best fit to the experimental data. The adsorption capacity of the formulated soil mixes with sand–bentonite and coal ash–bentonite continued to increase up to the limiting percentages considered in this study i.e. 40 and 20% for sand–bentonite and coal ash–bentonite mixes, respectively, although coal ash in itself does not prove to be a good adsorbent.