ZARITZKY Noemi Elisabet
congresos y reuniones científicas
Modelling Cr(VI) reduction by a combined carbon-activated sludge system
Rio de Janeiro, Brasil
Congreso; 2nd Mercosur Congress on Chemical Engineering,4th Mercosur Congress on Process Systems Engineering; 2005
Abstract. The chromium compounds are usually released by effluent discharge from steelworks, chromium electroplating, leather tanning and chemical manufacturing. Chromium is usually encountered in the environment as Cr(III) and Cr(VI). Each of the above oxidation states has very different biological and chemical properties; Cr(VI) is soluble and toxic, while Cr(III) is more stable and presents lower  toxicity. The most commonly used technology for treatment of heavy metals in wastewaters is chemical precipitation. In the case of chromium, Cr(III) but not Cr(VI) may be removed from water as an insoluble chromium hydroxide. Recently, several Cr(VI) reducing bacterial species have been identified; these bacteria reduce the toxic hexavalent chromium to the less toxic and less mobile state Cr(III) utilizing a wide range of substrate at near neutral pH. In addition, the reduction of Cr(VI) to Cr(III) using activated sludges under aerobic conditions has also been reported. The combined carbon-activated sludge process has been proposed as an alternative to protect the biomass against toxic substances in wastewaters; however, the information about the effect of powdered activated carbon (PAC) addition in activated sludge reactors for the treatment of liquid waste containing heavy metals is limited. The objectives of the present study were: a) to evaluate the influence of powdered activated carbon addition on the removal of hexavalent chromium by activated sludge micro-organisms in batch reactors, b) to propose a mathematical model that interprets the experimental results. The activated sludge used in this study was obtained from a laboratory scale completely mixed, continuous-flow activated sludge reactor. Cr(VI) (as K2Cr2O7) concentrations of 10, 25, 50, 75 y 100 mgCr/L and 1 to 8 g/L PAC were tested. Different experiments were performed: i) Cr(VI) removal using PAC (Case 1); ii) Cr(VI) removal using activated sludge (Case2); iii) Cr(VI) removal using the combined carbon-activated sludge system (Case3). A first order kinetics with respect to Cr(VI) and PAC respectively was proposed to model the removal of Cr(VI) in Case1. A Monod based mathematical model was used to describe the kinetics of Cr(VI) removal in Case 2. Cr(VI) removal in the combined carbon-activated sludge system (Case 3) was faster than both Cr(VI) removal using PAC or activated sludge individually. Experimental results showed that the removal of Cr(VI) using the carbon-activated sludge system was adequately described by combining the kinetic equations proposed for Cases 1 and 2.