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

This study reveals the existence of FeAs clusters (primarilyFe(4)As(4), having a tooeleite-like local structure)in a nearly groundwater-like situation, i.e., far from the acid minedrainage condition, consequently entrapping As3+ from suchsolutions within tooeleite-like cages.The simultaneous presence of Fe3+ and As3+ ions in groundwater (higher ppb or lowerppm level concentrationsat circumneutral pH) as well as in acid mine drainages (AMDs)/industrialwastewater (up to few thousand ppm concentration at strongly acidicpH) are quite common. Therefore, understanding the chemical interactionsprevalent between Fe3+ and As3+ ions in aqueousmedium leading to nucleation of ionic clusters/solids, followed byaggregation and growth, is of great environmental significance. Inthe present work, we attempt to probe the nucleation process of Fe3+-As3+ clusters in solutions of various concentrationsand pHs (from AMD to groundwater-like) using a combination of experimentaland theoretical techniques. Interestingly, our study reveals nucleationof primary FeAs clusters in nearly all of them independent of concentrationor 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 observedexperimentally, which provides an important clue about solid-phasegrowth from a range of Fe3+-As3+ solutions.Our experimental findings are further supported by a stepwise reactionmechanism established from detailed DFT studies.