Insight into the Role of Tannins in the Enhancement of Sulfidated Zerovalent Iron toward Trichloroethylene Degradation

Abstract

The application of sulfidated zerovalent iron (S-ZVI) has been extensively reported, while the easy passivation of S-ZVI has been overlooked. Therefore, tannins (TAs) with phenolic hydroxyl groups that are beneficial to oxidation resistance were employed to enhance the performance of S-ZVI (TA@S-ZVI). TA@S-ZVI could degrade 91.3% of trichloroethylene (TCE) to cis-dichloroethylene (cis-DCE) (46%), ethene (31%), and ethane (6.2%) mainly via hydrolysis, which was higher than S-ZVI (80.1%) mainly degraded via β-elimination. However, compared with TA@S-ZVI, S-ZVI could sufficiently degrade TCE according to the less accumulation of DCE, indicating that S-ZVI presented higher reactivity on the intermediates (i.e., DCE). TA@S-ZVI (18.2%) consumed more Fe0 than S-ZVI (10.2%) in the dechlorination. TA could gather electrons and facilitate the formation of Fe3O4, which promoted the electrons transfer. The aqueous chlorines were more than absorbed chlorines in TA@S-ZVI because TAs enhanced the absorption capacity of TA@S-ZVI. TAs were conductive for ZVI to remain reactive under oxic conditions by reducing Fe(III) and inhibiting the oxidation of Fe(II). Besides, the complexation between TAs and Fe(III) could prevent the passivation of S-ZVI. Overall, this study showed an efficient modification technology by TAs to enhance the reactivity of S-ZVI.