This study presents the external mass transfer effects on the reduction of hexavalent chromium (Cr(VI)) using calcium alginate immobilized Bacillus sp. in a re-circulated packed bed batch reactor (RPBR). The effect of flow rate on the reduction Cr(VI) was studied. Theoretically calculated rate constants for various flow rates were analyzed using external film diffusion models and compared with experimental values. The external mass transfer coefficients for the bioconversion of Cr(VI) were also investigated. The external mass transfer effect was correlated with a model of the type J(D) = K Re-(1-n). The model was tested with various K values and the mass transfer correlation J(D) = 5.7 Re-0.70 was found to predict the experimental data accurately. The proposed model would be useful for the design of industrial reactor and scale up. (C) 2009 Elsevier Ltd. All rights reserved.