Kinetics of arsenate reduction by dissol

The mobility and toxicity of arsenic in soil and aquatic environments are significantly affected by its chemical speciation, which is influenced by reducing conditions. This study aimed to investigate the kinetics and reaction pathways of arsenate reduction by hydrogen sulfide, a potent reducing agent often present in anaerobic systems. Through a series of experiments, it was established that the reduction of arsenate by hydrogen sulfide occurs rapidly and conforms to a second-order kinetic model with a rate constant of 3.2 × 10² M⁻¹ h⁻¹, exhibiting more than a 300-fold increase at pH 4 compared to pH 7. Notably, arsenite was not the direct product of this reduction; instead, transient arsenic-sulfide complexes were formed, including a trimeric species that remains in solution for several days before dissociating and leading to the eventual production of dissolved arsenite. Moreover, the precipitation of orpiment was only significant at high sulfide to arsenic ratios. These findings suggest that models predicting arsenic behavior in contaminated systems need to incorporate the abiotic reduction of arsenate by sulfide, particularly under mildly acidic conditions, while also accounting for the dynamics of arsenic-sulfide complexes.