Treatment of solutions containing trivalent chromium by electrodialysis

Trivalent chromium is present at high concentration in effluents from the tanning process in the leather industry. The chromium recycling process must be able to separate chromium from other cations (mainly Na+ but also Ca2+, Mg2+) because their presence has a negative effect on the quality of the tanned leather. Moreover, in order to recycle chromium, a concentration step is necessary to increase the concentration of Cr(III) in the concentrate stream. In this paper, a two-step Cr(III) separation–concentration process is proposed. In the first step, a cation-exchange membrane, Nafion®117, modified by electrodeposition of polyethylenimine (PEI) was used to separate successfully Cr(III) from NaCl solutions. In the second step, conventional electrodialysis with Nafion® 117, Nafion® 324 or CMX membranes permits to increase the concentration of Cr(III) in the solutions produced in the separation stage.

Fig. 14. Effect of pH on chromium current efficiency in mixture H+/Cr(III) with CMX membrane. 0.06 eq dm–3. The result is shown in Fig. 14. It is seen that the current efficiency increases when pH increases. This is due to the competition between hydrogen and chromium ions. For pH 2, the current efficiency is identical for chromium and hydrogen ions. The concentration ratio is 0.01 to 0.09 eq dm –3 ( average value). The permselectivity H+/Cr3+ is about 9. 3.2.5. Membrane comparison CMX and desorbed Nafion® 117 membranes exhibit nearly the same current efficiencies at It is possible to reduce sodium ion concentration to a low level compared to chromium concentration with an acceptable current efficiency using Nafion® 117 membranes modified by PEI electrodeposition. Sodium is preferred by the membranes due to the presence of an electrostatic positive film at the membrane surface. The separation is followed by increasing the chromium concentration. This is possible with the three cation-exchange membranes used in this study. Because of the possible precipitation of chromium hydroxide at the anodic face of the cation-exchange membrane, phenomena emphasized by local pH variations due to the hydrogen ion local concentration variation, the pH control in the dilute compartment is crucial. This pH may be adjust to increase the current efficiency of the both chromium and sodium ions, and avoid the chromium precipitation.