Adsorption of Zn(II), Cd(II) and Pb(II) onto Fly Ash

Heavy metals like Zn(II), Cd(II) and Pb(II) are accumulated in food chain and persistent in nature. The removal these toxic heavy metals from wastewater is seems to be utmost important. The potential of fly ash as adsorbent for the removal of Zn(II), Cd(II) and Pb(II) ions from aqueous solutions was thoroughly investigated. The effects of relevant parameters like pH, adsorbent concentration and ionic strength on the adsorption capacity were examined. The equilibrium was achieved within 1 hr of contact time for Cd(II) and Pb(II) respectively and for Zn(II) it was 2 hr. The adsorption data were better fitted by Langmuir isotherm for Zn(II) and the Freuindlich isotherm for Cd(II) and Pb(II) respectively. The maximum adsorption capacities in terms of monolayer adsorption were found to be 13.85, 34.12 and 94.52 mg/g for Zn(II), Cd(II) and Pb(II) respectively at optimum condition. The kinetics of the adsorption process was also studied and values of adsorption rate constants including intraparticle diffusion were calculated. Mass transfer coefficients for the each process were also determined. The effective diffusivity was found to be 10(-10) m(2)/s order for all metal ions. The sorption energy as calculated from Dubinin-Raduskevich (DR) isotherm were found to be 8.895, 11.785 and 11.166 kJ/mol for Zn(II), Cd(II) and Pb(II) respectively, indicating the adsorption process is chemisorptions in nature for all the metal ions studied. The negative value of Delta G degrees indicated the spontaneity of the adsorption process. The adsorption process was exothermic for Cd(II) whereas for Zn(II) and Pb(II) it was endothermic in nature.