Growth Mechanism of Tooeleite-like Minerals, a Potential Mineral Sink for As(III)

Developing a strategy to trap As3+ within naturally occurring minerals is promising for water decontamination. Contextually, tooeleite, a mineral containing As3+ and Fe3+ in a sulfate medium, could be a suitable candidate. But, tooeleite is typically known to form at high concentrations of both Fe3+ and As3+ in strongly acidic pH (∼2.5) (acid mine drainage (AMD)-like conditions), which reduced its applicability to this narrow regime only. Herein, the nucleation and growth of tooeleite mineral has been probed at the molecular level, which follows the nonclassical nucleation pathway, deviating from classical nucleation theory. Importantly, contrary to the above-mentioned narrow regime, primary FeAsS clusters of an average diameter of 0.78 nm, nearly identical to tooeleite locally, have been found within a wide range of Fe/As solutions in situ, even at much lower concentrations (10 ppm of Fe concentration, Fe3+/As3+ = 1.5) and higher pH (pH ∼6), where bulk tooeleite formation is unexpected. Furthermore, when subjected to AMD-like conditions, these extracted clusters readily convert into an extended tooeleite-like bulk crystalline phase. This observation that tooeleite-like clusters can originate and remain stable under conditions like ≥10 ppm Fe, Fe3+/As3+ = 1.5, and pH ∼2 to 6 naturally opens up the possibility of mineralization as a permanent removal technique for As3+ from contaminated water.