REMOVAL OF TRIVALENT AND HEXAVALENT CHROMIUM BY SEAWEED BIOSORBENT

Protonated or Ca-form Sargassum seaweed biomass bound up to 40 mg/g of Cr(Ill) by ion exchange at pH 4. An ion-exchange model assuming that the only species taken up by the biomass was Cr(OH)(2+) successfully f itted the experimental biosorption data for Cr(lll). The maximum uptak e of Cr(VI) by protonated Sargassum biomass at pH 2 was explained by s imultaneous anion exchange and Cr(VI) to Cr(lll) reduction. At pH <2.0 , the reduction of Cr(VI) to Cr(III) dominated the equilibrium behavio r of the batch systems, which was explained by the dependence of the r eduction potential of HCrI4- ions on the pH. At pH >2.0, the removal o f Cr(VI) was linked to the depletion of protons in equilibrium batch s ystems via an anion-exchange reaction. The optimum pH for Cr(VI) remov al by sorption lies in the region where the two mechanisms overlap, wh ich for Sargassum biomass is in the vicinity of pH 2. The existence of the optimum pH for the removal of Cr(VI) may be explained by taking i nto account (a) the desorption of Cr(lll)from biomass at low pH and (b )the effect of pH on the reduction potential of Cr(VI) in aqueous solu tions. Seventy percent of Cr(VI) bound to the seaweed at pH 2 can be d esorbed with 0.2 M H2SO4 via reduction to Cr(lll).