FeS was evaluated as heterogeneous activator for peroxydisulfate (PS) with trichloroethene (TCE) chosen as model substance representing organic water pollutants prone to fast oxidation by sulfate radicals. The TCE degradation followed in most cases pseudo-first-order kinetics with a FeS-normalized maximum rate constant of k’_TCE = 140 M⁻¹min⁻¹ (with c_FeS,0 = 0.28 mM and c_PS,0 = 5 mM).

Sulfate radicals were determined as predominant radical species formed during PS activation by FeS by evaluation of the kinetic isotope effect for oxidation of cyclohexanes C₆H₁₂ vs. C₆D₁₂ (k_C6H12: k_C6D12 = 2.22). Monitoring of sulfur and iron species during the course of the reaction revealed that sulfide is rapidly oxidized, whereas Fe(II)_solid is rather stable over time.

The present study describes in detail the influence of the pH value on the reaction and the long-term performance of FeS as activator for PS with an optimum pH value of 5. On the basis of the radical yield from PS of 1.6 mols SO₄⁻ per mol S₂O₈²⁻ and the activation energy of E_A = (31 ± 1) kJ mol⁻¹ for the heterogeneous radical generation, a surface-assisted homolytic bond cleavage without simultaneous electron transfer is proposed as activation mechanism, which is contrary to the currently prevailing opinion.