| Abstract
| - The mechanisms of hexavalent chromium [Cr(VI)] co-removal with copper [Cu(II)] during homogeneous precipitationwere studied with batch tests using a synthetic solutioncontaining Cr(VI) and Cu(II). Metal precipitation was inducedby adding Na2CO3 stepwise to different pH, and therespective removals of Cu(II) and Cr(VI) were measured.At the same time, the relative quantities of Cu(II) andCr(VI) in the precipitates were also analyzed to establishtheir stoichiometric relationship. The results indicated that,in a solution containing 150 mg/L Cu(II) and 60 mg/LCr(VI), the initial co-removal of Cr(VI) with Cu(II) began atpH 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 CuCrO4·2Cu(OH)2]. Thereafter, the remaining soluble copperstarted to react with carbonate in a heterogeneousenvironment to form the negatively charged basic coppercarbonate precipitates [CuCO3·Cu(OH)2], which subsequently adsorbed additional Cr(VI) (or HCrO4-) at pH5.2−6.2. The maximum Cr(VI) co-removal took place at pH6.2. Between the two mechanisms, coprecipitationaccounted for about 29% of the total chromium's co-removal while the remaining 71% was attributed to surfaceadsorption, mainly through electrostatic attraction andligand exchange. When the solution pH was increased tobeyond 7.5, a surface charge reversal took place on thebasic copper carbonate solids, and this led to someCr(VI) desorption. Thus, the extent of Cr(VI) adsorption ishighly pH dependent.
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