Mechanisms of 2,4,6-trichlorophenol removal by activated persulfate with hexagonal Fe7S8 sheets and enhancing removal by oxidizing porous media

Pyrrhotite (Fe7S8) is a natural mineral with multiple valence states of iron and an efficient peroxydisulfate (PS) activator. However, the activation mechanism of PS and the performance of 2,4,6-trichlorophenol (TCP) removal by the Fe7S8/PS system are not clear. Herein, the hexagonal Fe7S8 sheets were fabricated using the facile one-step solvothermal method, which could enhance PS activation and TCP degradation. The Fe7S8/PS system removed 94.3% of the TCP within 60 min with an apparent rate constant of 0.052 +/- 0.003 min-1. The dissolution of Fe7S8 during the Fe7S8/PS activation facilitated the regeneration of Fe3+/Fe2+ redox cycles. Free radical species analysis confirmed the presence of sulfate radical (SO4 center dot-) and hydroxyl radical (center dot OH). The fundamental involvement of the free radicals was observed in two stages. The first stage (0-5 min) was dominated by the contribution of center dot OH, whereas, in 2nd stage (5-60 min), the contribution of SO4 center dot-exceeded that of center dot OH. Toxicity prediction studies revealed that degradation intermediates had lower toxicity than TCP. In the reaction column filled with 15 g of quartz sand (QS), the removal efficiency on TCP decreased from 100% to 0% over a period of 30 min to 80 h. In addition, after 155 h, the TCP removal efficiencies of column experiments loaded with 15, 18, and 20 g of QS and Fe7S8 were 33.2%, 44.6%, and 50.2%, respectively. These findings suggested the Fe7S8/PS system showed high potential for removing TCP from groundwater.