The mechanisms of hexavalent chromium [Cr(VI)] co-removal with copper [Cu(II)] during homogeneous precipitation were studied with batch tests using a synthetic solution containing Cr(VI) and Cu(II). Metal precipitation was induced by adding Na2CO3 stepwise to different pH, and the respective removals of Cu(II) and Cr(VI) were measured. At the same time, the relative quantities of Cu(II) and Cr(VI) in the precipitates were also analyzed to establish their stoichiometric relationship. The results indicated that, in a solution containing 150 mg/L Cu(II) and 60 mg/L Cr(VI), the initial co-removal of Cr(VII with Cu(II) began at pH 5.0 and completed at pH 6.2. At pH 5.0-5.2, coprecipitation took place through the formation of copper-chromium-bearing solids [such as CuCrO4 and/or CuCrO4 x 2Cu(OH)2]. Thereafter, the remaining soluble copper started to react with carbonate in a heterogeneous environment to form the negatively charged basic copper carbonate precipitates [CuCO3 x Cu(OH)2], which subsequently adsorbed additional Cr(VI) (or HCrO4-) at pH 5.2-6.2. The maximum Cr(VI) co-removal took place at pH 6.2. Between the two mechanisms, co-precipitation accounted for about 29% of the total chromium's co-removal while the remaining 71% was attributed to surface adsorption, mainly through electrostatic attraction and ligand exchange. When the solution pH was increased to beyond 7.5, a surface charge reversal took place on the basic copper carbonate solids, and this led to some Cr(VI) desorption. Thus, the extent of Cr(VI) adsorption is highly pH dependent.

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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.