A Fe-N/FeS@C composite prepared via mechanical activation and pyrolysis for sulfite activation to degrade organic contaminants: Single atomic irons anchored into carbon matrix with encapsulated FeS nanoparticles

A novel structural composite (Fe-N/FeS@C) was synthesized via a self-doping strategy by the pyrolysis of a solid mixture of lignosulfonate and ferric nitrate pretreated by mechanical activation. The as-synthesized composite contained 7.5 wt% of single atomic iron moieties and 16.0 wt% of encapsulated FeS nanoparticles , with a high surface area of 1095.81 m 2 g –1 and plentiful micropores and mesopores (0.4–10 nm). The Fe-N/FeS@C composite was confirmed to possess outstanding catalytic activity for sulfite activation in the degradation of methylene blue , with a high reaction rate constant (0.77 g g –1 min –1 ) and long cycling stability (83.1% of initial activity retention after 10 recycles). Moreover, the composite exhibited satisfactory activity recovery via a simple rinsing process with ethanol and HCl , and performed well in a wide pH range of 3–11. Density functional theory calculation results further demonstrated that the reaction energy of the rate-determining step, in which SO 3 •− was produced from SO 3 2– on atomic Fe moieties, was lower than that of the formation of FeSO 3 + by Fe 3+ and SO 3 2– . The excellent performance of Fe-N/FeS@C was mainly attributed to the combination of the high mass loading of active sites of atomic Fe moieties anchored into carbon matrix with encapsulated FeS nanoparticles. This study provides new insight into the development of environment-friendly, stable and atomic dispersion iron composite for sulfite activation to efficiently remove organic pollutants .