Early Stage Formation of Iron Oxyhydroxides during Neutralization of Simulated Acid Mine Drainage Solutions

The phases and stability of ferric iron products formed early during neutralization of acid mine drainage waters remain largely unknown. In this work, we used in situ and time-resolved quick-scanning X-ray absorption spectroscopy and X-ray diffraction to study products formed between 4 min and 1 h after ferric iron sulfate solutions were partially neutralized by addition of NaHCO₃ ([HCO₃^–]/[Fe³⁺] < 3). When [HCO₃^–]/[Fe³⁺] = 0.5 and 0.6 (initial pH ∼ 2.1 and 2.2, respectively), the only large species formed were sulfate-complexed ferrihydrite-like molecular clusters that were stable throughout the duration of the experiment. When [HCO₃^–]/[Fe³⁺] = 1 (initial pH ∼ 2.5), ferrihydrite-like molecular clusters formed initially, but most later converted to schwertmannite. In contrast, when [HCO₃^–]/[Fe³⁺] = 2 (initial pH ∼ 2.7), schwertmannite and larger ferrihydrite particles formed immediately upon neutralization. However, the ferrihydrite particles subsequently converted to schwertmannite. The schwertmannite particles formed under both conditions aggregated extensively with increasing time. This work provides new insight into the formation, stability and reactivity of some early products that may form during the neutralization of natural acid mine drainage