The mechanisms of hexavalent chromium [Cr(VI)] co-removal with copper [Cu(II)], nickel [Ni(II)] and zinc [Zn(II)] during homogeneous precipitation are studied with batch tests under different experimental designs. Synthetic solutions containing either single- or multiple-metal with the presence or absence of Cr(VI) were employed. Metal precipitation was induced by adding Na₂CO₃ stepwise to different pH, and the respective removal of each metal was measured. At the same time, the compositions of the precipitates and the changes of their zeta potentials as well as particle size distributions were also determined....[ Read more ]

The mechanisms of hexavalent chromium [Cr(VI)] co-removal with copper [Cu(II)], nickel [Ni(II)] and zinc [Zn(II)] during homogeneous precipitation are studied with batch tests under different experimental designs. Synthetic solutions containing either single- or multiple-metal with the presence or absence of Cr(VI) were employed. Metal precipitation was induced by adding Na₂CO₃ stepwise to different pH, and the respective removal of each metal was measured. At the same time, the compositions of the precipitates and the changes of their zeta potentials as well as particle size distributions were also determined.

The results indicate that, in a synthetic solution containing 150-mg/L Cu and 60-mg/L Cr(VI), co-removal of Cr(VI) during Cu(II) precipitation is most significant in the pH range of 5.0 to 6.2. However, if Cu is replaced by Ni(II) or Zn(II), the extents of co-removal were much less over a broad pH range of up to 10.0. There are two mechanisms involved in the chromium's co-removal with copper. The first involves direct interaction of Cu²⁺ and CrO₄^2- [in the formation of copper chromate precipitates, either as CuCrO₄ or as CuCrO₄⋅2Cu(OH)₂]. The newly produced copper chromate crystallites are able to induce an early formation of basic copper carbonate precipitates [mainly as CuCO₃⋅Cu(OH)₂] at pH around 5.2 and higher, which is one pH unit lower than that required to initiate copper carbonate precipitation for a pure copper solution. The second mechanism involves the adsorption of soluble chromate ions, either as HCrO₄^- or CrO₄^2- on the surface of basic copper carbonate precipitates through a combination of electrostatic attraction and ligand exchange. The extent of adsorption is highly pH dependent. At pH 7.5 and below, copper carbonate adsorbent is found to carry positive surface charges, as indicated by its positive zeta potential. At a higher pH, a charge reversal takes place. However, a significant portion of the adsorbed chromium is not desorbed, presumably through the mechanisms of chemisorption, ligand exchange, and/or mechanical occlusion or entrapment. The first mechanism (direct Cu-Cr interaction) accounts for approximately one third of the total Cr(VI) co-removal, and this mechanism plays a major role in initiating the first step of Cr(VI) co-removal (at a low pH of 5.0 to 5.2). The second mechanism (adsorption) is responsible for the remaining two thirds of co-removal. The maximum adsorption takes place at pH approximately 6.2.

For a Cr(VI) concentration of 60 mg/L, the maximum chromium co-removal during precipitation of a mixed Cu(II), Ni(II), Zn(II) solution (each metal 150 mg/L) with Na₂CO₃ dosing can reach as high as 78%, which occurs at pH of around 7.3. This level of co-removal is higher than the sum of the Cr co-removal with each single metal under the same operating condition.

Key Words: Heavy metal removal, hexavalent chromium co-removal, copper, nickel, zinc, mechanism, precipitation, co-precipitation, adsorption, electrostatic attraction, ligand exchange, and zeta potential.