Anion effects on the electrochemical regeneration of Ce(IV) in nitric acid used for etching chromium

The anion impurities such as SO42−, Cl−, and Cr2O72− commonly present in the spent (hazardous) Cr-etch solutions from color filter manufacturing processes may influence the solutions' regeneration by the electrooxidation of Ce(III) to Ce(IV). This study, therefore, investigated the effects of these anions on Ce(III)/Ce(IV) redox reactions at glassy carbon in HNO3. In cyclic voltammetric tests, the presence of SO42− decreased the formal potential but increased the peak potential separation (ΔEp) of Ce(III)/Ce(IV) couple, and lowered the peak current for Ce(IV) reduction whereas Cl− did not change the formal potential and ΔEp, but the peaks for Cl− and Ce(III) oxidation partially overlapped. Cr2O72− slightly lowered the peak current for Ce(III) oxidation but significantly decreased that for Ce(IV) reduction. The Tafel slope for Ce(III) oxidation was ∼65 mV decade−1 in the absence of anion impurities. Increasing SO42−, Cl−, or Cr2O72− in solution raised the Tafel slope. The Ce(III)/Ce(IV) equilibrium potential decreased with the increase of SO42− or Cl− but was hardly influenced by Cr2O72− addition. These observations from individual anion species together well explained the anions' co-effect (kinetic hindrance) on the Ce(III) oxidation in HNO3, revealing that these anions are unfavorable for the electrooxidation of Ce(III) in the spent Cr-etch solutions.