Removal of nickel and cadmium from aqueous solutions by sewage sludge ash: study in single and binary systems

The sewage sludge ash can potentially be used for the removal of metal ions from wastewater due to its similar chemical composition to that of fly ash. The aim of this work was to investigate the adsorptive characteristics of this material including specific surface area and pH of zero point of charge (pHZPC) and to assess the possibility of removing nickel and cadmium from aqueous solutions by this sorbent. The effects of agitation time, pH, initial metal ion concentration and temperature on the removal of these metals were studied. In order to study the sorption isotherm, two equilibrium models, the Langmuir and Freundlich isotherms, were analyzed. The effect of solution pH on the adsorption onto sewage sludge ash was studied in the pH range from 2 to 8. The adsorption was endothermic and the computation of the parameters, ΔH°, ΔS° and ΔG°, indicated that the interactions were thermodynamically favourable. Experiments with Ni and Cd adsorption measured together showed that Cd severely interfered with Ni adsorption to sewage sludge ash and vice versa under the conditions of the two coexisted ions adsorption.