Curious Behavior of Fe³⁺-As³⁺ Chemical Interactions and Nucleation of Clusters in Aqueous Medium

The simultaneous presence of Fe³⁺ and As³⁺ ions in groundwater (higher ppb or lower ppm level concentrations at circumneutral pH) as well as in acid mine drainages (AMDs)/industrial wastewater (up to few thousand ppm concentration at strongly acidic pH) are quite common. Therefore, understanding the chemical interactions prevalent between Fe³⁺ and As³⁺ ions in aqueous medium leading to nucleation of ionic clusters/solids, followed by aggregation and growth, is of great environmental significance. In the present work, we attempt to probe the nucleation process of Fe³⁺-As³⁺ clusters in solutions of various concentrations and pHs (from AMD to groundwater-like) using a combination of experimental and theoretical techniques. Interestingly, our study reveals nucleation of primary FeAs clusters in nearly all of them independent of concentration or pH. Theoretical studies employed density functional theory (DFT) to predict the primary clusters as stable Fe₄As₄ units. The surprising resemblance of these clusters with known Fe³⁺-As³⁺ minerals at the local level was observed experimentally, which provides an important clue about solid-phase growth from a range of Fe³⁺-As³⁺ solutions. Our experimental findings are further supported by a stepwise reaction mechanism established from detailed DFT studies.