Regulating the FeS assembly pattern of sulfidated zero-valent iron: All-in-one interface modulation with activated carbon.

Specifically designing the heterogeneous interface in sulfidated zero-valent iron (S-ZVI) has been an effective, yet usually overlooked method to improve the decontamination ability. However, the mechanism behind FeS assembly remains elusive and the lack of modulating strategies that can essentially tune the applicability of S-ZVI further imposes difficulties in creating better-performing S-ZVI with heterogeneous interface. In this study, by introducing powdered activated carbon (PAC) during S-ZVI preparation, S-ZVI/PAC microparticles were prepared to modulate the assembly pattern of FeS for the applicability and reactivity of the material. S-ZVI/PAC showed robust performance in Cr(VI) sequestration, with 11.16 and 1.78 fold increase in Cr(VI) reactivity compared to ZVI and S-ZVI, respectively. This was attributed to the fact that the introduced PAC could acquire FeS to enhance the electron transfer capacity matching its adsorption threshold, thus helping to accommodate the transfer of the reduction center to PAC in S-ZVI/PAC. In optimizing the FeS allocation between ZVI and PAC, the chemical assembly of FeS on S-ZVI was superior to physical adsorption. Critically, we found that isolated FeS in the prepared solution was physically adsorbed by the PAC, allowing chemically assembled FeS on the S-ZVI. This was achieved by controlling the addition sequence of NaS and PAC, as it effectively controlled the release rate and content of Fe(II) in the preparation solution. S-ZVI/PAC was demonstrated to be extremely effective in simulated wastewater and electrokinetics-permeable reactive barrier (EK-PRB) treatments. Introducing PAC enriches the diversity of sulfidation mechanisms and may realize the universality of the S-ZVI/PAC application scenarios. This study provides a new interface optimization strategy for S-ZVI targeted design towards environmental applications.