Potential of homogeneous persulfate activation as a strategy for the oxidative treatment of tetramethylammonium hydroxide: Superiority of sulfate radical over hydroxyl radical in the oxidation of methylammoniums

Abstract

Tetramethylammonium hydroxide (TMAH) as the etching and chelating agent exhibits the extreme recalcitrance to advanced oxidation processes utilizing hydroxyl radical (OH). This study showed that based on the relative position of PDS/heat and PDS/UV against H2O2/UV in terms of TMAH degradation efficiency, sulfate radical (SO4−) was superior to OH in oxidizing TMAH. PDS/heat (at 60 °C) and PDS/UV at the initial pH = 7 achieved TMAH degradation efficiency of ∼ 80–90 % within 1 h whereas H2O2/UV barely eliminated TMAH, which contrasted with H2O2/UV decomposing phenol faster than the activated PDS. The high susceptibility of TMAH (in which four methyl groups bonded to nitrogen removed the lone pair of electrons) to oxidation by SO4− aligned with the pH–dependent efficiency of PDS/heat for the treatment of methylamines. PDS/heat effectively degraded methylammoniums (lacking nitrogen lone pair available for electron transfer due to amine protonation) under non–alkaline conditions, whereas acidification caused the efficiency loss of ∼ 90 % for H2O2/UV. Methylammoniums as radical scavengers retarded benzoic acid degradation by PDS/heat to a more pronounced extent than counterparts with deuterated N–methyl groups, which implied that hydrogen–atom abstraction primarily contributed to the SO4−–induced oxidation of methylammoniums. Estimation of the bimolecular reaction rate constants using laser flash photolysis and electron paramagnetic resonance spectroscopic detection of TMAH–derived carbon–centered radical supported the superiority of SO4− over OH in TMAH oxidation. These findings suggested the potential of homogeneous persulfate activation in the pH-insensitive oxidative treatment of alkylamines and quaternary ammonium cations structurally analogous to TMAH.