Influence of pH on Removal of Heavy Metallic Cations by Fly Ash in Aqueous Solution

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Abstract
Coal fly ash was used to study adsorption of several cations (Cu2+, Ni2+, Zn2+, Cd2+, Pb2+, Cr(III)) within various experimental conditions: dry or wet fly ash, constant or not constant pH, metallic ion/fly ash mass ratio varying from 0.005 to 0.05. It has been shown that alkalinity of fly ash, increasing pH to 11 for a fly ash concentration equal to 20 g l−1, leads to higher removal capacity, due partly to bulk solution precipitation. Wet fly ash gives smaller adsorption capacity resulting from lost alkalinity and dissolution of alumina sites at pH 11. Removal order established in non-constant pH condition is: Pb > Cu > Ni > Zn. Experiments carried out at several constant pH levels indicate that sorption capacities for metallic ions increase from 10% or 40% to 100% when pH varies from 1 to 10. The removal order is: Pb > Cr > Cu > Ni > Zn > Cd. Removal observed vs pH was discussed taking the hydrolysis property of the metallic ions into account. It was concluded that two groups of metallic cations should be distinguished: hydrolysing (Cu2+, Pb2+) and non-hydrolysing (Ni2+, Zn2+) Lead, nickel, zinc and cadmium are certainly adsorbed in their free ionic form, even when both hydroxide species and free ionic forms are responsible for the removal of copper and chromium. However, removal affinity order separates easily hydrolyzable ions from less hydrolyzable ions.

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