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  • Coupling-oxidation process promoted ring-opening degradation of 2-mecapto-5-methyl-1,3,4-thiadizaole in wastewater.

Coupling-oxidation process promoted ring-opening degradation of 2-mecapto-5-methyl-1,3,4-thiadizaole in wastewater.

Water research (2020-09-12)
Chen Zhong, He Zhao, Qingzhen Han, Hongbin Cao, Feng Duan, Junyi Shen, Yongbing Xie, Wanqian Guo, Shanshan Sun
ABSTRACT

As an important raw material and intermediate of widely used antibiotics cefazolin and cefazedone, 2-mecapto-5-methyl-1,3,4-thiadizaole (MMTD) in antibiotic wastewater is hardly decyclized during wastewater treatment, posing great risk to the environment. This work proposed a green "coupling-oxidation" process to enhance ring-opening of MMTD during antibiotic wastewater treatment. In particular, the significant role of humic substances (HS) as pre-coupling reagent was emphasized. Real HS and different model HS, especially quinones, not only efficiently pre-coupled MMTD (>95%) but also promoted the MMTD removal by MnO2 (from 72.4% to 92.4%). Mass spectrometric analysis indicated that MMTD pre-coupled to HS would be oxidized with ring opening to environmental-friendly sulfonated HS, while direct oxidation of MMTD produced MMTD dimers or sulfonated MMTD that would not undergo ring opening. Theoretical calculations indicated that pre-coupling to HS enabled the ring-opening oxidation by increasing the nucleophilicity and decreasing ring-opening barrier of thiadiazole. Based on the density function theory (DFT), the global nucleophilicity index (Nu) followed the trend of HS-MMTD > MMTD dimer > sulfonated MMTD, while the ring-opening barrier followed the trend of HS-MMTD (274 kJ/mol) < first ring of MMTD dimers (286 kJ/mol) < MMTD (338 kJ/mol). Theoretical calculations further confirmed that the cross-coupled HS-MMTD intermediate was more likely to be decyclized than intermediates from direct oxidation. Moreover, nitrogen, acetaldehyde group, sulfate and CO2 were the products of thiadiazole ring degradation. Pre-coupling of MMTD with HS provides a new idea and strategy in developing a green and sustainable scheme for wastewater treatment.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
4-Methylcatechol, ≥95%
Sigma-Aldrich
Chlorohydroquinone, technical grade, 85%
Sigma-Aldrich
2,3-Dihydroxybenzoic acid, 99%
Sigma-Aldrich
1,4-Bis(acryloyl)piperazine, BioXtra, suitable for electrophoresis, ≥99.0% (TLC)