Effect on pH of heavy metals retention by kaolinite

Abstract

Heavy metals are naturally present in soils but in high concentrations they can be hazardous to the environment and human health. To understand the adsorption of the metals, Batch tests were conducted to measure the adsorption of 1, 2 and 3 mM/L concentrations of cadmium, lead and zinc onto kaolinite over periods of 0.1, 1, 2, 4, 8, 12 and 24 hours and to measure the change in hydrogen ion concentration. The kaolinite was found to contain iron oxides which increased the point of zero charge of the kaolinite to pH 4.7 and the pH 7 CEC to 17.8 meq/100 g. The clay retained a maximum of 10 μM/g of lead, 8.4 μM/g of zinc and 6.0 μM/g of cadmium. Similar adsorption behaviours were found for Pb and Zn, while Cd shows a different trend. The pH of the solution highly influences the adsorption of Cd but it has little effect in the adsorption of the other two metals. Among the three metals Pb was present the slowest adsorption at 0.1 hrs. From the kaolinite reactive spaces, the aluminium hydroxyl, silicon oxide and iron oxide sites are probably involved in the release or retention of hydrogen ions. The ratio of hydrogen ions released to metal adsorbed is presented and the trend for each metal is examined with their respective equations. In addition, the desorption for the 1 mM/L concentrations of the three metals is examined. Adsorption isotherms are used to help analyze the adsorption data.