Sustainable Upcycling of Ferrous Sulfate Waste into Functional Fe7S8 via COS-Mediated Controlled Sulfidation.

The global accumulation and improper disposal of industrial ferrous sulfate (FeSO4) waste present significant environmental and resource management challenges, including soil acidification, groundwater contamination, iron loss, and unrecovered sulfate sulfur. Existing disposal and recovery routes mainly produce low-value iron oxides, require intensive reagent input or high energy consumption, and generate additional waste burdens. Here, we propose a novel carbonyl sulfide (COS)-mediated gas-solid strategy for upcycling FeSO4 into a calculated pyrrhotite (Fe7S8) phase fraction of 96.6 wt.%, realizing high-value conversion of both iron and sulfur. Structural characterizations demonstrate a stepwise conversion from FeSO4 to Fe3O4, followed by controlled sulfidation to Fe7S8, during which the random stacking of newly formed Fe7S8 particles generates textural interparticle voids and reconstructs the pore structure. Density functional theory calculations show that COS exhibits the most negative adsorption Gibbs free energy (-9.58 eV) and, relative to S and H2S, the largest comparative sulfur-incorporation barrier under a common-final-state framework, indicating stronger interfacial stabilization and greater kinetic resistance to rapid sulfur transfer. Kinetic analysis indicates that COS governs the reaction (order = 1.10), whereas CO acts as a secondary promoter (order = 0.60), lowering the apparent activation energy from 188.9 to 129.6 kJ/mol. Practical evaluations with real FeSO4 waste validated the successful upcycling into Fe7S8, which exhibited efficient gaseous mercury removal performance from simulated flue gas. Life-cycle and preliminary cost analyses further indicate the potential environmental and resource-utilization advantages of this cross-industry FeSO4 valorization strategy under the adopted assumptions.