Effects of NH+4 on Ce(IV) electro-regeneration in simulated and real spent TFT-LCD Cr-etching solutions

This investigation studies the electro-regeneration of Ce(IV) from Ce(III) in 4 M HNO3 in the presence/absence of NH4+ and real spent thin-film transistor liquid-crystal display (TFT-LCD) Cr-etching solutions. On Pt, at 2 A and 70 °C for 100 min, the Ce(IV) yield and apparent rate constant of Ce(III) oxidation in 4 M HNO3 without NH4+ were 100% and 5.54 × 10−4 s−1, respectively (and the activation energy was 13.1 kJ mol−1). Cyclic voltammetric and electrolytic measurements consistently support the noticeable inhibition by NH4+ of Ce(III) oxidation and lowering of the Ce(IV) yield, respectively. The apparent diffusion coefficients for 0.2 and 0.02 M Ce(III) oxidation in 4 M HNO3 that contained 0–0.6 M NH4+ were (0.38–0.25) × 10−5 and (1.6–0.9) × 10−5 cm2 s−1, respectively. Because of combined effects of NH4+ and anion impurities, the 100 min Ce(IV) yield of a real spent TFT-LCD Cr-etching solution (with [NH4+]/[Ce(III)] = 0.74 M/0.39 M) was 82%, lower than that of 4 M HNO3 without NH4+, but higher than those of 4 M HNO3 that contained anion impurities with/without 0.4 M NH4+.

► Ce(IV) regeneration is tested in simulated/real spent TFT-LCD Cr-etching solutions.
► The Ce(IV) yield reaches 100% in the absence of NH4+.
► NH4+ shows detrimental effects on the Ce(IV) electro-regeneration.
► The kinetic/mass transfer parameters for Ce(III) electro-oxidation are presented.
►  The regenerated solutions may be used for recycling purposes.