pH-controlled precipitation of cobalt and molybdenum from industrial waste effluents of a cobalt electrodeposition process

This study was carried out in attempt to investigate how to efficiently treat effluents generated from a novel electrodeposition of nanocrystalline Co-based alloys using pH-controlled precipitation of cobalt and molybdenum from Co-Mo aqueous solutions. The most significant factors studied included the effects of pH, cobalt and molybdenum concentrations, reaction temperature and reaction time on the precipitation process and kinetics. The chemical and physical characteristics of the precipitated materials were investigated by scanning electronic microscope (SEM), X-ray diffraction (XRD), atomic absorption spectrometer (AAS) and energy dispersive X-ray analysis (EDX). The results showed that pH had a substantial effect on the reaction between cobalt and molybdenum. The initial concentration of molybdenum had a significant impact on the pH at which precipitation began. Co-precipitation of molybdate and cobalt occurred when increasing the pH to 4. The precipitate was initially present as cobalt molybdate oxide hydrate and subsequently formed sodium cobalt molybdenum oxide. Most of the molybdenum was present in the solid phase when the pH reached 5. With increasing pH to 7 or higher, the molybdenum redissolved. Results of precipitate-washing indicated that the molybdenum was chemically tied to the precipitate produced at the low pH, but physically entrapped in the precipitates as liquid at the high pH. Cobalt precipitation increased with increasing solution pH. At pH 12, more than 99.9% of the cobalt was present in the solid phase as cobalt hydroxide. Reaction temperature had a significant positive impact on both cobalt and molybdenum precipitation. The precipitation rates increased with increasing temperature.