Reagent-free degradation of Tetramethylammonium Hydroxide (TMAH) in water by atmospheric air and nitrogen plasma

초록

Tetramethylammonium hydroxide (TMAH) is a widely used photoresist developer and etchant in semiconductor manufacturing; however, its strong toxicity and poor biodegradability make it a critical pollutant, especially in semiconductor process wastewater. This study demonstrates a reagent-free advanced oxidation process (AOP) using atmospheric-pressure air and nitrogen plasma jet for efficient degradation of TMAH. Under optimized plasma conditions, air and nitrogen plasmas achieved 83.0% and 84.1% TMAH removal, with corresponding TOC reductions of 74.1% and 75.5%, respectively. Degradation followed pseudo-first-order kinetics with rate constants of 0.0267 min−1 (air) and 0.0298 min−1 (N2), comparable to oxidant-assisted AOPs. Optical emission spectroscopy revealed the presence of strong OH (309 nm) and O (777 nm) emissions, while nitrogen plasma emission was dominated by N2 second-positive system (SPS) (337–357 nm) and CN (388 nm), indicating distinct reactive oxygen and nitrogen species (ROS/RNS) environments. Air plasma generated higher O3 (24–61 μM) and converted NOₓ mainly to NO3− whereas N2 plasma produced higher aqueous •OH and H2O2 concentrations and favored NH4+ formation with modest NO3− accumulation. Scavenger experiments confirmed that TMAH degradation is dominated by •OH and H2O2 in air and N2 plasma, respectively. Both plasmas induced pronounced self-acidification of the TMAH solution (initial pH 10.8, final pH ∼ 1.8–2.6), which stabilized ROS/RNS and sustained stepwise demethylation. Toxicity assays showed that plasma-treated solutions were non-toxic and supported normal growth, whereas untreated TMAH inhibited germination. These findings establish atmospheric-pressure plasma treatment as an efficient, chemical-free AOP for semiconductor process wastewater, demonstrating competitive kinetics, efficient mineralization, and controlled nitrogen transformation. © 2026 Elsevier B.V.

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