Isolation and quantification of cadmium removal mechanisms in batch reactors inoculated by sulphate reducing bacteria: biosorption versus bioprecipitation.

Biosorbing properties of sulphate reducing bacteria were tested to distinguish the amount of cadmium removed by bioprecipitation from that bound onto biomass surface (biosorption). Experimental results of cadmium abatement in batch growth tests (bioprecipitation tests) were then compared with metabolism-independent binding properties of SRB cell wall surface (biosorption tests performed with dead biomass). Experimental results showed that SRB inoculum removed 59 + or - 5% of sulphates in 21 days even in presence of cadmium (0-36 mmol L(-1)), while non-monotonous kinetic effects were observed for increasing Cd concentrations. Comparison between bioprecipitation and biosorption tests denoted a significant contribution of biosorption (77%) in total Cd removal (0.40 + or - 0.01 mmol g(-1)). Characterisation of bacterial acid-base surface properties by potentiometric titrations and mechanistic modelling denoted that carboxylic, phosphate and amino groups of cell wall are the main responsible of metal removal by biosorption mechanism.