The effect of acid pre-treatment on the biosorption of chromium(III) by Sphaerotilus natans from industrial wastewater

Living cells of a strain of Sphaerotilus natans are employed to remove Cr(III) from acid wastewater. Batch experiments carried out at starting acid conditions (pH 3.0–3.5) show that the pH progressively increases but the removing activity starts only when conditions closed to neutrality are reached. Studies carried out either at initial acid conditions or at standard conditions for this micro-organism (pH 7.0) confirm that the biomass is able to grow also on acid medium, although the lag phase is longer than the one observed at standard conditions. This strain shows its maximum ability to remove Cr(III) at a biomass concentration of about 0.4 g l−1. Tests carried out at both lower and higher biomass levels show lower yields, while the time necessary to reach the maximum removal considerably increases. Biomass previously adapted to acid conditions ensures a specific uptake of this metal of 120 mg g−1, which is much higher than that reported in the literature for other micro-organisms. Continuous tests in CSTR confirm the possibility of developing a biological treatment process for the continuous removal of Cr(III) from acid solutions.