Cadmium Ion Biosorption by the Thermophilic Bacteria Geobacillus stearothermophilus and G. thermocatenulatus

Abstract

ABSTRACT This study reports surface complexation models (SCMs) for quantifying metal ion adsorption by thermophilic microorganisms. In initial cadmium ion toxicity tests, members of the genus Geobacillus displayed the highest tolerance to CdCl 2 (as high as 400 to 3,200 μM). The thermophilic, gram-positive bacteria Geobacillus stearothermophilus and G. thermocatenulatus were selected for further electrophoretic mobility, potentiometric titration, and Cd 2+ adsorption experiments to characterize Cd 2+ complexation by functional groups within and on the cell wall. Distinct one-site SCMs described the extent of cadmium ion adsorption by both studied Geobacillus sp. strains over a range of pH values and metal/bacteria concentration ratios. The results indicate that a functional group with a deprotonation constant pK value of approximately 3.8 accounts for 66% and 80% of all titratable sites for G. thermocatenulatus and G. stearothermophilus , respectively, and is dominant in Cd 2+ adsorption reactions. The results suggest a different type of functional group may be involved in cadmium biosorption for both thermophilic strains investigated here, compared to previous reports for mesophilic bacteria.