Sorption of Chromium Ions from Aqueous Solution onto Chemically Activated Carbons Developed from Maize Cobs

Abstract

Chemically activated carbons were prepared from maize cobs, using phosphoric acid of variable concentration. The textural parameters of the activated carbons were determined from the nitrogen adsorption isotherms measured at 77 K. The chemistry of the carbon surface was determined by measuring the surface pH, the pHPZC and the concentration of the carbon - oxygen groups of the acid type on the carbon surface. Kinetics of Cr(VI) sorption/reduction was investigated at 303 K. Two processes were investigated in terms of kinetics and equilibrium namely; Cr(VI) removal and chromium sorption were studied at various initial pH (1-7). Removal of Cr(VI) shows a maximum at pH 2.5. At pH<2.5, sorption decreases because of the proton competition with evolved Cr(III) for ion exchange sites. The decrease of sorption at pH>2.5 is due to proton insufficiency and to the decrease of the extent of Cr(VI) reduction. The chemistry of the surface of activated carbon is an important factor in determining its adsorption capacity from aqueous solutions particularly when the sorption process involves ion exchange.