Optimum pH for Cr6+ Co-removal with Mixed Cu2+, Zn2+, and Ni2+ Precipitation

It is advisable to co-remove Cr(VI) with available Cu(II), Zn(II), and Ni(II) since they coexist in most plating wastewater. Previous studies showed that coprecipitation and adsorption are the main mechanisms contributable to Cr(VI) co-removal with Cu(II) precipitation, and both are highly pH dependent. This study presents the effect of pH on Cr(VI) co-removal with mixed metal precipitation in batch tests and also in a continuous compact system. Batch tests indicate that a maximum of 46.8 mg L^-1 Cr(VI) was co-removed with the precipitation of Cu(II), Zn(II), and Ni(II), each 150 mg L^-1, at pH of 7.0-7.3. However, co-removal of Cr(VI) decreased significantly with further pH increasing. Therefore in the continuous system, a two-stage nucleated precipitation technology was designed with the first stage being operated at around pH 7.2 to obtain maximum Cr(VI) co-removal and the second stage at around pH 9.2 to achieve further Cr(VI) co-removal with Zn(II) and Ni(II) precipitation.