Optimization of cadmium biosorption by Shewanella putrefaciens using a Box-Behnken design.

Microbial adsorption of heavy metals has been attracted more interest in the recent years. However, there are very few studies in investigating the biosorption of heavy metals by Shewanella putrefaciens, which is a metal reducing bacterium. Firstly, the effects of contact time, pH value, temperature, biomass dosage and initial cadmium concentration on the cadmium adsorption by Shewanella putrefaciens were studied by single factor experiments. Then, the response surface methodology (RSM) based on Box-Behnken design was used to optimize the cadmium adsorption by Shewanella putrefaciens. The results showed that the empirical model was suitable for experimental data, and the maximum cadmium removal efficiency by Shewanella putrefaciens was 86.54% under the optimum conditions of contact time 4.0 days, pH value 5, initial cadmium concentration of 20 mg/L, which was further verified by experiments. In addition, scanning electron microscope - Energy Dispersive Spectrometer (SEM-EDS) analysis showed that the bacteria were seriously deformed, and a "bamboo" shape was observed on the surface which consisted of cadmium according to the EDS analysis. Fourier transform infrared spectroscopy (FT-IR) analysis was used to evaluate the possible functional groups involving in interaction between cells and metal ions. The results showed that the distribution of cadmium on the cell surface was related to the carboxyl, amide, hydroxyl and phosphoric acid groups of Shewanella putrefaciens. These studies can provide a comprehensive understanding of the process and mechanism of microbial removal of heavy metals, and theoretical support for the follow-up practice of using biological adsorbents to remediate heavy metal contaminated soil.