Biosorption of Sr2+ was observed under equilibrium conditions in 2 L bench scale anaerobic bioreactors (2 L). The initial Sr2+ concentration in the

experiments was varied between 75 and 1000 mg/L, while the sulfate reduc­ing bacteria (SRB) cell density was kept constant at 1 mg/L. The suspen­sions were agitated for 3 hours and then samples were withdrawn for residual Sr2+ concentration analysis.

Sr2+ concentration was measured in the medium, and a desorption process was conducted to determine the amount sorbed on cells. Based on the amounts in the medium and the amount recovered from cells, it was observed that adsorption on the cells followed the Langmuir model. This suggests that Sr2+ removal occurred until equilibrium was reached, as opposed to precipitation reactions where the data cannot be fitted within a simple Langmuir model.

Straightforward precipitation follows a multi-layer model including surface precipitates. The SRB cells were then demonstrated conclusively to adsorb Sr2+ sorbent. The sorption capacity of the cells (qmax) was relatively high (444 mg/g), a value much higher than values obtained from other cell types (Shaukat et al., 2005; Dabbagh et al., 2007, Chegrouche et al., 2009) and purified cultures of sulfate reducing bacteria (Vijayaraghavan and Yun, 2008).

The solution to the classical Langmuir model is shown in Equation 15.6 and the Freundlich isotherm in Equation 15.7 with the optimum values obtained for the sulfate reducing consortium shown in Table 15.4.

where: q = sorption uptake (MM-1), qmax = maximum sorbate uptake (MM-1), b = coefficient related to the affinity between the sorbent and sorbate, Ceq = equilibrium concentration of the sorbate remaining in the solution (ML-3), k = constant corresponding to the binding capacity and n = coefficient related to the affinity between the sorbent and sorbate.