The kinetics of sulfur oxidation by T. thiooxidans has been studied in a batch well-mixed reactor and in shaker¯asks. A mathematical model is proposed, which considers the attachment of the cells onto the sulfur particles' surface following Freundlich isotherm, growth of the attached bacteria, and growth inhibition by sulfates accumulation. Best-®t values of the model parameters have been calculated from the experimental data. Results show that the addition of dimethyl-dichloro-silane in the aerated reactor to prevent the formation of foam reduces the maximum speci®c growth rate of attached bacteria, probably because of the resulting changes in surface properties of the sulfur particles. The other model parameters are not signi®cantly affected. The formation of clusters of sulfur particles has been observed at an initial sulfur concentration of 5% . This phenomenon reduces the rate of sulfur conversion due to the reduction of the total surface area of the particles, and the model therefore overestimates the formation of sulfates. At lower initial sulfur concentration, the phenomenon has not been observed and the model simulations are then satisfactory. List of symbols aProjected surface area of one cell (10 À12 m 2 acell) A Total surface area of the sulfur particles at time t m 2 A 0 Initial total surface area of the sulfur particles m 2 C SO 2À 4 Sulfate concentration in solution kg/m 3 C i Inhibitory sulfate concentration in solution kg/m 3 K c Equilibrium constant for cell adsorption to sulfur particles m 3 acellule K i Inhibition constant m 3 akg m S Mass of sulfur in the reaction medium at time t (kg) m S 0 Initial mass of sulfur in the reaction medium (kg) m SO 2À 4 Mass of sulfates in solution at time t (kg) N Number of sulfur particles in the reaction medium r Equivalent radius of the sulfur particles at time t (m) r 0 Initial equivalent radius of the sulfur particles (60 Â 10 À6 m) t Incubation time (days) V Reaction volume m 3 X l Free cells concentration at time t cells/m 3 X lo Initial free cells concentration cells/m 3 X f Concentration of cells attached to the sulfur particles at time t cells/m 2 X f max Maximum concentration of cells attached to the sulfur particles at time tr cells/m 2 Y Sulfur to cells proportionality constant (cells/kg sulfur converted) lSpeci®c growth rate d À1 l m Maximum speci®c growth rate d À1 hFraction of the sulfur particles' surface occupied by cells at time t q S Volumetric mass of sulfur particles 1960 kg/m 3

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An automated bench-scale countercurrent biosorption system (CBS) has been designed for the removal of metals from aqueous effluents. The system has been tested with activated sludge microorganisms as a biosorbent and lead and copper as model metals. Nearly 5 1 of a lead nitrate solution at 100 mg I -t of lead have been treated down to a final concentration of 0.1 mg I l (99.9% removal) by using 4.8 g of dry biosorbent. Under similar conditions, copper chloride solutions at 100 mg 1-1 of copper were treated down to a final concentration of 35-45 mg 1-l representing 60% removal. The advantage of the CBS is to maximize metal concentration in the biosorbent, from which the metal may thereby be recovered if desired. In addition, the CBS minimizes metal concentration in the treated effluent, which is the first objective of the treatment.

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