Regeneration of Ce(IV) in simulated spent Cr-etching solutions using an undivided cell

Abstract

This study investigated regeneration of Ce(IV) from Ce(III) oxidation at 0.05–0.5 A cm −2 in 4 M HNO 3 with/without anion impurities (SO 4 2− (0.01–0.2 M), Cl − (0.01–0.08 M), and/or Cr 2O 7 2− (0.005–0.016 M)) in an undivided cell. Both Ce(IV) yield and current efficiency (CE) were significantly lower in 0.1 M than in 1–4 M HNO 3 and different on anode materials (in order Pt > IrO 2/Ti > glassy carbon). The apparent rate constants for Ce(III) oxidation on the Pt anode ( k 1) and for Ce(IV) reduction on a stainless steel cathode ( k 2) were (0.40–1.80) × 10 −4 and (0.08–1.01) × 10 −4 s −1, respectively, corresponding to the apparent mass transfer coefficients of (2.0–9.0) × 10 −3 and (0.4–5.1) × 10 −3 cm s −1, respectively. For Ce(III) oxidation at 0.3 A cm −2 in 4 M HNO 3 containing multi-impurity (0.025 M SO 4 2− + 0.08 M Cl − + 0.016 M Cr 2O 7 2−), the k 1 was lowered by one order of magnitude although the k 2 remained unchanged, and both Ce(IV) yield and CE were reduced by ∼89%. The decrease of Ce(IV) yield and CE by the uni-impurity was in order Cl − > Cr 2O 7 2− > SO 4 2−. The obtained parameters are useful to design undivided batch reactors for the Ce(IV) electro-regeneration in spent Cr-etching solutions.