Factors affecting co-removal of chromium through copper precipitation

The phenomenon of hexavalent chromium [Cr(VI)] co-removal during copper [Cu(II)] precipitation and the extent of co-removal is significantly pH-depend had been reported by the authors. To optimize the co-removal, this paper examines other various factors affecting the extent of co-removal in batch tests using a synthetic solution containing both Cr(VI) and Cu(II). Metal precipitation was induced by dosing with either sodium carbonate (Na2CO3) or potassium hydroxide (KOH) solutions in different manners (slow drip-wise dosing versus fast slug dosing) to reach two target pHs (6.5 and 10.0). The experimental results indicate that the co-removal of Cr(VI) during Cu(II) precipitation depends significantly on the alkali dosing method, the reaction time allowed and the initial Cu(II) concentration. However, the co-removal is not affected by the type of alkali employed for the precipitation. With an initial 60 mg/L Cr(VI) and 150 mg/L Cu(II), the maximum co-removal of Cr(VI) is 38.2 mg/L which occurs at pH around 6.5 under drip-wise alkali dosing technique. A similar co-removal can also be achieved by instantaneous slug dosing, but a sufficient time must be allowed to reach reaction equilibrium.