Fe electrocoagulation technology for effective removal of molybdate from water: Main influencing factors, response surface optimization, and mechanistic analysis

Excessive molybdate (MoO42-) in drinking water poses a potential hazard to human health. Herein, for the first time, Fe electrocoagulation (EC) was proposed for the removal of Mo(VI) from water. The effects of main aquatic and electrochemical factors on Mo(VI) removal were investigated. Results showed that, among the electrolytes (NaCl, Na2SO4, and NaNO3), NaNO3 obviously inhibited the anodic dissolution and Mo(VI) removal, and caused high energy consumption. NaCl was favorable for the reduction of energy consumption whereas its high concentrations inhibited Mo(VI) removal. Via using the response surface method with Box-Behnken design, the optimal process parameters for the maximum Mo(VI) removal were obtained as follows: pH(i), 5.5-7.0; CD, 3.5-4.0 A/m(2); ED, 0.5-0.9 cm. The analysis of variance demonstrated that initial pH, current density, and electrode distance had highly significant impacts on Mo(VI) removal (p-value < 0.01). The quadratic polynomial model was developed using the Mo(VI) removal efficiency as the response value. The Mo(VI) adsorption experiments demonstrated the effectiveness of adsorption during the Fe EC process. Results of flocs characterization by SEM-EDS, XPS, XRD, and FTIR showed that the main crystalline phases in the flocs included lepidocrocite (gamma-FeOOH) and magnetite (Fe3O4), and no reduction reactions of Mo(VI) were observed. The mechanisms of Mo(VI) removal mainly involved flocculation, adsorption (electrostatic attraction and inner-sphere complexation), and co-precipitation. Continuous flow tests (172 h) using simulated groundwater showed an average Mo(VI) removal efficiency of 83.4 %. This study indicates that Fe EC is a promising water treatment technology for Mo(VI) removal.