Insights into the contrasting effects of sulfidation on dechlorination of chlorinated aliphatic hydrocarbons by zero-valent iron

• Published in: Water research (Oxford) Vol. 255; p. 121494
• Main Authors: Sun, Yuankui, Zheng, Kaiwei, Du, Xueying, Qin, Hejie, Guan, Xiaohong
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 15-05-2024
• Subjects: Atomic hydrogen; Electron transfer; Linear free energy relationships; Selectivity; Solvent kinetic isotope effect; Atomic hydrogen; Electron transfer; Selectivity; Solvent kinetic isotope effect; Linear free energy relationships
• DOI: 10.1016/j.watres.2024.121494
• EISSN: 1879-2448
• ISSN: 0043-1354

•The broad-spectrum reactivity of sulfidated ZVI toward chlorinated compounds was evaluated.•Sulfidation favors direct electron transfer and thus the reduction of high chlorinated compounds.•Sulfidation hinders H* mediated reaction and thus the reduction of low chlorinated compounds.•Sulfidation can alter the products distribution during the dechlorination process by ZVI.
Contrasting effects of sulfidation on contaminants reduction by zero-valent iron (ZVI) has been reported in literature but the underlying mechanisms remain unclear. Here, under well-controlled conditions, we compared the performance of ZVI and sulfidated ZVI (S-ZVI) toward a series of chlorinated compounds. Results revealed that, although S-ZVI was more reactive than ZVI toward hexachloroethane, pentachloroethane, tetrachloroethylene, and trichloroethene, sulfidation hindered the dechlorination of the other ten tested chlorinated aliphatics by a factor of 1.5–125. Moreover, S-ZVI may lead to an accumulation of toxic partially-dechlorinated products. Analogous to its effects on ZVI reactivity, sulfidation also exerted positive, negligible, or negative effects on the electron efficiency of ZVI. Solvent kinetic isotope effect analysis suggested that direct electron transfer rather than reaction with atomic hydrogen was the dominant reduction mechanism in S-ZVI system. Hence, the sulfidation enhancing effects could be expected only when direct electron transfer is the preferred reduction route for target contaminants. Furthermore, linear free energy relationships analysis indicated one-electron reduction potential could be used to predict the transformation of chlorinated ethanes by S-ZVI, whereas for chlorinated ethenes, their adsorption properties on S-ZVI determined the dechlorination process. All these findings may offer guidance for the decision-making regarding the application of S-ZVI.
[Display omitted]