Biosorption and desorption studies of chromium (III) by free and immobilized Rhizobium (BJV r 12) cell biomass

Experiments with free cell biomass (cells + exopolysaccharides) of Rhizobium BJVr 12(mungbean isolate) showed that amount of Cr3+ ion sorbed is influenced by the amount of biomass to Cr3+ concentration ratio and time of contact. A ratio of 0.5 g fresh biomass to 10.0 mi 5.03 ppm Cr3+ sorbed 0.0275 mg Cr equivalent to an uptake of 2.86 mg Cr g-(1) dry biomass and 1.0 g: 10.0 mi sorbed 0.0366 mg Cr equivalent to an uptake of 1.9 mg Cr g-(1) biomass. Immobilized cell biomass in ceramic beads and in aquacel (a porous cellulose carrier with a charged surface) were more efficient than free cell biomass in adsorbing Cr(III). A reduction of 49.7% of Cr(III) for free cells, 95.6% for cells immobilized in ceramic beads and 94.6% for cells in aquacel was achieved after 48 hours under shaken conditions. Sorption capacities of immobilized cell biomass in ceramic beads and aquacel ranged from 5.01 to 5.06 mg Cr g(-1) dry cell biomass. The biosorption of Cr3+ follows generally the Langmuir and Freundlich models of adsorption at low Cr3+ concentrations. The Langmuir constant for immobilized cells in ceramic beads are: Q(0), 0.065 mmol Cr g(-1) biomass; b (affinity constant), -6941 mmol(-1) Cr and for cells in aquacel Q, 0.07 mmol Cr g(-1) biomass; b, -6941 mmol Cr g(-1) Cr. The Freundlich constants are: K, 0.071 mmol Cr g(-1) biomass; n, 0.13 g(-1) biomass l(-1) and for aquacel: K, 0.074 mmol g(-1) biomass; n, 0.13 g(-1) biomass. Biotraps made up of immobilized cells in ceramic beads and aquacel were tested for adsorbing Cr(III) using two different flow rates: 0.5 ml/min and 1.5 ml/min. A significantly higher amount of Cr(III) was adsorbed at the lower flow rate of 0.5 ml/min. Biosorption of Cr3+ is competitive. The treatment of a waste water sample containing 6.03 ppm Cr3+ and other cations with the biomass reduced the Cr3+ concentration to that much lower than for the test solution containing only Cr. Recovery of biosorbed Cr(III) was by treatment at a different pH using dilute HCl solution. Recovery was higher for cells imbibed in ceramic beads than aquacel. Percentage recoveries for cells in aquacel are 46.4% at pH 1.0, 33.0% at pH 3.0 and 6.6% at pH 6.0-7.0. For cells in ceramic beads, percentage recoveries are: 93.1% at pH 1.0, 75.6% at pH 3.0 and 16.4% at pH 6.0-7.0. Biosorption of Cr3+ by cells immobilized in ceramic beads is reversible but only partially for cells in aquacel.