Speciation, Complexation of Antimonite, and Its pH Dependence in Aqueous Solution: Insights from First-Principles...

Antimony(III) (Sb(III)) contamination in aqueous environments is an increasingly important concern, but its atomic-level hydration structure and complexation mechanisms remain poorly understood. In this work, first principles molecular dynamics (FPMD) simulations were applied to investigate the solvation structure, acidity, and cation complexation behavior of Sb(III) in aqueous solution. Our results reveal that aqueous antimonite primarily occurs as Sb(OH)₃(H₂O), adopting a 4-fold coordination, with a computed pK_a of about 10.2. Sb(III) is found to form stable inner-sphere complexes with common environmental cations, following an ionic-potential-dependent stability trend: Na⁺ < Ca²⁺ < Mg²⁺ < Al³⁺. Despite the neutral character of Sb(OH)₃(H₂O), its complexation stability is comparable to or even exceeds that of the anionic Sb(OH)₆^- (Sb(V)). Furthermore, cation complexation largely increased the acidity of the directly complexed hydroxyl groups of Sb(III), decreasing the pK_a to 7.9 and 6.2 upon complexation with Mg²⁺ and Al³⁺, respectively. This pK_a reduction suggests that Sb(III) deprotonation may occur in Mg²⁺/Al³⁺-rich environments under circumneutral pH conditions. These molecular-level findings provide insights into Sb(III) speciation and thermodynamic stability in aqueous systems, which may serve as a basis for future modeling of antimony mobility and geochemical cycling in natural environments.