Nickel Removal by Adsorption/Photoelectrochemistry on ZnS Prepared by Chemical Bath Deposition

Nickel is a natural element used in a variety of mineral forms, and its mobilization is regulated by the physicochemical qualities of the soil. Its release into the environment is linked to the industries, oil and coal-burning power stations, and waste incinerators, and is therefore a source of air, water, and soil pollutions. In order to reduce these effects, many materials are used as photocatalysts under sunlight. Among the candidates, the blende ZnS prepared by chemical bath deposition (CBD) is a wide band gap semiconductor with a direct optical transition of 3.92 eV obtained from the diffuse reflectance. The X-ray diffraction pattern showed narrow peaks with a well crystallization. It was studied electrochemically to assess its photocatalytic properties for the nickel deposition. The electrochemical impedance spectroscopy (EIS) confirmed the semiconductor behavior under UV illumination. At pH similar to 7, its conduction band (-1.18 V scE ), determined from the capacitance(-2)-potential (C-2-E) plot derives from Zn2+: 4 s orbital. The compound works as n-type semiconductor and is located below the level of Ni2+ (-0.59 V-SCE), yielding a spontaneous Ni2+ reduction to elemental state. The efficiency of Ni deposition was dependent on the ZnS amount of and pH; these parameters were studied in order to find optimal conditions for the Ni2+ photoreduction. The results display a high removal efficiency of Ni2+ (100 mg L-1) after 1-h adsorption/2-h solar light and the best ZnS dose was obtained for 75 mg/100 mL where 99% of nickel was photoelectrodeposited at natural pH. The kinetic of Ni2+ photoreduction obeys a first-order model with a half-photocatalytic life of 89 min.