INFLUENCE OF PH ON REMOVAL OF HEAVY METALLIC CATIONS BY FLY-ASH IN AQUEOUS-SOLUTION

Coal fly ash was used to study adsorption of several cations (Cu2+', N i2+, 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 alkalin ity of fly ash, increasing pH to 11 for a fly ash concentration equal to 20 g 1(-1), leads to higher removal capacity, due partly to bulk so lution precipitation. Wet fly ash gives smaller adsorption capacity re sulting 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 indica te that sorption capacities for metallic ions increase from 10%. or 40 % to 100% when pH varies from 1 to 10. The removal order is : ph > Cr > Cu > Ni > Zn > Cd. Removal observed ns pH was discussed taking the h ydrolysis property of the metallic ions into account. It was concluded that two groups of metallic cations should be distinguished: hydrolys ing (CU2+, Pb2+) and non-hydrolysing (Ni2', Zn2+) Lead, nickel, zinc a nd cadmium are certainly adsorbed in their free ionic form, even when both hydroxide species and free ionic farms are responsible for the re moval of copper and chromium. However, removal affinity order separate s easily hydrolyzable ions from less hydrolyzable ions.