The reaction of amorphous iron sulfide with Mo(VI) under different pH conditions

Iron sulfide (FeS) is an important scavenger for hexavalent molybdate (Mo(VI)) in an anoxic environment; it plays a crucial role in the mobilization and transformation of Mo(VI), although the underlying reaction mechanisms between Mo(VI) and FeS remain unclear. This study investigates the Mo(VI) reaction kinetics with the amorphous FeS over a pH range 5.0-9.0 and Mo's chemical form on the FeS surface. It is found that the Mo(VI) reaction kinetics with FeS follow a pseudo first-order model, and the reaction rate constant (k_obs) increases with a decrease in the pH value. The k_obs at pH 5.0 is 0.027 min^-1, which is about 38 times higher than that at pH 9.0. The rapid Mo(VI) removal under acidic conditions is due to quick Mo(VI) transformation into stable MoS₂ and thiomolybdate (Mo^VO_xS_y). The amount of MoS₂ formed on the surface of FeS increases with a decrease in the pH value. Under neutral and alkaline conditions, Mo(VI) is not transformed into MoS₂ by FeS because the precipitation of iron oxy-hydroxide passivates the active sites of FeS. The study also investigates the effect of the initial dosage of FeS (20-200 mg L^-1) and Mo(VI) (10-50 mg L^-1) on the reaction kinetics of Mo(VI) with FeS. The results provides important information on the environmental fate of Mo(VI) in the anoxic environment containing amorphous FeS.