Mechanistic insights into the roles of Ni and Co in accelerating pyrite formation for efficient sulfide sequestration

Pyrite (FeS 2 ) formation has been proposed as an effective strategy for achieving permanent sulfide immobilization in contaminated aquatic environments, but the slow nucleation rate at ambient temperatures limits its practical application. Although Ni 2+ / Co 2+ are known to accelerate the formation of FeS 2 and Ni exhibits stronger catalytic effects, the underlying acceleration mechanisms still remain ambiguous. This work systematically investigated the catalytic action of Ni and Co by combining kinetic experiments, aqueous speciation analysis, solid-phase characterization, and density functional theory (DFT) calculations. The results demonstrate that FeS 2 forms via the reaction between FeS species derived from mackinawite (FeS (m) ) and polysulfide (S n 2− ), with pentasulfide (S 5 2− ) being identified as the dominant reactive species. Compared with pure FeS, Ni/Co doping reduced the apparent reaction order and activation energy, and increased the selectivity to FeS 2 , as revealed by quantitative kinetic analysis. Both metals are incorporated into mackinawite through a three-step pathway: adsorption as M(H 2 O) 6 2+ , ligand exchange to M(H 2 O) 5 (HS) + , and final substitution into (Fe,M)S solid solutions. The electrons released during incorporation drive the ring-opening of elemental sulfur (S 8 ) to generate reactive S 5 2− . DFT calculations indicate that both Ni and Co doping lower the density of states (DOS) of FeS and shift Fe d-band centers toward the Fermi level, thereby enhancing S 5 2− adsorption. However, Co doping induces excessive binding stability of S 5 2− , which hinders subsequent transformation and raises the reaction barrier. Our findings fundamentally clarify the catalytic mechanisms of Ni and Co, providing guidance for optimizing rapid sulfide immobilization strategies. • Ni/Co doping into FeS through a common pathway: M(H 2 O) 6 2+ → M(H 2 O) 5 (HS) + → (Fe,M)S. • S 5 2− is the key and advantageous intermediate for the formation of FeS 2 . • The incorporation improved the selectivity for the formation of FeS 2 . • Co doping induces overly strong adsorption that hinders intermediate further transformation.