Bacteria-Modified Red Mud for Adsorption of Cadmium Ions from Aqueous Solutions

Cadmium is a toxic metal widely used in industry and it enters the environment from electroplating, smelting, alloy manufacturing, pigments, plastics, cadmium-nickel batteries, fertilizers, pesticides, mining, pigments and dyes, textile operations, and refining industries. Considering the harmful effects of cadmium, it is necessary to remove cadmium from liquid wastes at least to the limit accepted by regulations. The present study describes removal of cadmium from aqueous solutions using bacteria-modified red mud waste material. Batch adsorption experiments have been performed as a function of pH, contact time, temperature, and adsorbent dosage. The optimum results were obtained at pH 4.0, contact time of 60 min, temperature of 30 degrees C, and an adsorbent dose of 1 mg/mL. The adsorption data was correlated with Langmuir and Freundlich adsorption models. The maximum adsorption capacity obtained from Langmuir adsorption model was 83.034 mg/g. The kinetic processes of cadmium adsorption on bacteria-modified red mud were described by applying pseudo-first-order and pseudo-second-order rate equations. The kinetic data for the adsorption process obeyed pseudo-second-order rate equations. Various thermodynamic parameters, such as Delta G degrees, Delta H degrees, and Delta S degrees were calculated and the negative value of AG obtained indicate that the process was spontaneous, and the positive value of Delta H degrees confirms the reaction to be endothermic, and the positive value of Delta S degrees shows that the decrease in the degree of freedom of the adsorbed species.