Molecular investigation on the binding of Cd(II) by the binary mixtures of montmorillonite with two bacterial species

Bacteria–phyllosilicate complexes are commonly found in natural environments and are capable of immobilizing trace metals. However, the molecular binding mechanisms of heavy metals to these complex aggregates still remain poorly understood. This study investigated Cd adsorption on Gram-positive Bacillus subtilis, Gram-negative Pseudomonas putida and their binary mixtures with montmorillonite using surface complexation model, Cd K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and isothermal titration calorimetry (ITC). We have shown that larger amounts of Cd are adsorbed by B. subtilis than by P. putida at pH<∼6, and Cd sorption that binding to phosphate groups plays a more important role in P. putida than in B. subtilis. This remind us that we should consider the microbe species when predict the biochemical behavior of trace metals in microbe-bearing environments. The observed Cd adsorption on the binary bacteria–clay composites was more than that predicted based on the component additivity approach. When taking bacteria–clay (1:1 mass ratio) as a representative example, an approximately 68%:32% metal distribution between the bacterial and mineral fraction was found. Both the EXAFS and ITC fits showed that the binding stoichiometry for Cd-carboxyl/phosphate was smaller in the binary mixtures than that in pure bacteria. We proposed that the significant deviations were possibly due to the physical-chemical interaction between the composite fractions that might reduce the agglomeration of the clay grains, increase the negative surface charges, and provide additional bridging of metals ions between bacterial cells and clays.