Sulfate radical (SO4•−) advanced oxidation processes (SR-AOPs) are efficient for degrading a broad spectrum of contaminants. This study demonstrates that the existence of environmentally relevant concentrations of nitrite (NO2−) can lead to the formation of N-nitrosodimethylamine (NDMA), a probable human carcinogen, when heat activated peroxydisulfate (heat/PDS) is applied to address contaminants with dimethylamine moieties, such as tetracyclines. NO2− effectively competes with tetracyclines for SO4•− at a high second-order reaction rate constant of 8.8 × 108 M−1s−1, thus suppressing the degradation. Simultaneously, SO4•− reacts with NO2− to form the nitrogen dioxide radical (NO2•) which rapidly dimerizes to a potent nitrosating agent named dinitrogen tetroxide (N2O4). N2O4, in turn, attacks the dimethylamine moiety in tetracyclines via nucleophilic substitution, contributing to the degradation of parent compounds and generating an active intermediate that quickly decomposes to NDMA, along with the release of R+ and NO3−. The released R+ can be further oxidized by SO4•− to form phenolic intermediates which combine with NO2• to generate nitrated products. When 5 μM oxytetracycline (OTC) was treated with 0.5 mM PDS in the presence of 20 μM NO2− at 60 °C, the highest formation of NDMA was 0.045 μM in 1 h. NDMA formation was also observed for other compounds with dimethylamine moieties, such as methylene blue, phenylurea herbicides, etc., with the molar yield ranging from 0.07 to 3.53 %, negatively related to the R-N bond dissociation energy of the precursors. These findings suggest that NDMA can be commonly generated when SO4•− is applied to the degradation of dimethylamine pollutants in wastewater treatment or groundwater remediation where NO2− ubiquitously exists, which may pose a serious threat to the aquatic environment.

中文翻译：

---

亚硝酸盐导致二甲胺化合物在硫酸根自由基氧化过程中形成 N-亚硝基二甲胺


硫酸根自由基 （SO4•−） 高级氧化过程 （SR-AOP） 可有效降解多种污染物。本研究表明，当热活化过氧化物二硫酸盐 （heat/PDS） 处理带有二甲胺部分的污染物（如四环素）时，环境相关浓度的亚硝酸盐 （NO2−） 的存在会导致 N-亚硝基二甲胺 （NDMA） 的形成，NDMA 是一种可能的人类致癌物。NO2− 在高二级反应速率常数 8.8 × 108 M−1s−1 下与四环素类有效竞争 SO4•−，从而抑制降解。同时，SO4•− 与 NO2− 反应形成二氧化氮自由基 （NO2•），该自由基迅速二聚化成一种名为四氧化二氮 （N2O4） 的强效亚硝化剂。反过来，N2O4 通过亲核取代攻击四环素中的二甲胺部分，促进母体化合物的降解并产生活性中间体，该中间体迅速分解为 NDMA，同时释放 R+ 和 NO3−。释放的 R+ 可被 SO4•− 进一步氧化，形成酚类中间体，与 NO2• 结合生成硝化产物。当在 60 °C 下用 0.5 mM PDS 和 20 μM NO2− 处理 5 μM 土霉素 （OTC） 时，1 小时内 NDMA 的最高形成为 0.045 μM。对于其他具有二甲胺基团的化合物，如亚甲蓝、苯脲除草剂等，也观察到 NDMA 的形成，摩尔产率为 0.07 至 3.53 %， 与前驱体的 R-N 键解离能呈负相关。 这些发现表明，当 SO4•− 应用于废水处理或地下水修复中二甲胺污染物的降解时，通常会产生 NDMA，其中 NO2− 普遍存在，这可能对水生环境构成严重威胁。