Time-resolved Study of Early-stage Formation of Iron Oxyhydroxide Nanoparticles in Simulated Acid Mine Drainage (AMD) Solutions

Early stage development of ferric iron oxyhydroxide (FeOOH) nanoparticles from bicarbonate-neutralized Fe₂(SO₄)₃ solutions was investigated using time-resolved UV-Vis spectroscopy and quick extended X-ray absorption fine structure (Q-EXAFS) spectroscopy. Two neutralization ratio ( h =[HCO₃^-]/[Fe³⁺]) of 1 and 2 were used. The neutralized systems changed visually from transparent solutions after the base addition to blurry and then opaque suspensions at extended reaction time. For the h=1 solution, its absorbance measured at 600 nm (due to scattering because of no absorption bands at this wavelength) increased quickly within initial 10 min, slightly between 10 - 26 min, and explosively afterwards. These three stages have been previously ascribed to hydrolysis and initial condensation, condensation, and aggregation processes, respectively. For the h=2 solution, only the two later evolution stages were recorded. UV-Vis spectra of the h=1 solution show that three absorption bands, likely resulting from Fe³⁺-sulfate species, gradually disappeared with the increase of reaction time and were not visible at about 10 min. Meanwhile, an electron pair transition (EPT) band centered at 485 nm emerged and gradually grew in its amplitude, indicating particles of a FeOOH phase being formed. For h=2, the three bands were not observed, but the EPT band at 485 nm was pronounced in the first collected spectrum and also grew with time, indicating that FeOOH particles were already formed from the very beginning. In addition, in both of the h=1 and 2 solutions the bands around 370 - 400 nm shifted to higher wavelength, suggesting that FeOOH particle size increased with time. Time-resolved Q-EXAFS spectra were collected every 2 s and were averaged over 15 consecutive scans, resulting in a time resolution of 30 s. For the h=1 solution, the EXAFS radial structural functions (RSF) initially had no or very weak peaks at positions corresponding to edge- and corner-sharing Fe shells. After 10 min of reaction time, the two Fe shells became obvious. This corroborates with the UV-Vis analysis that Fe-sulfate clusters were present at the beginning and FeOOH particles emerged later. For the h=2 solution, RSF shows high amplitudes of the two Fe shells in the first EXAFS spectrum. The amplitudes significantly grew with time, consistent with increasing Fe coordination numbers obtained by EXAFS shell-by-shell fitting. The EXAFS fitting also shows that the second Fe-Fe distance decreased from 3.41 to 3.36 Å with time while the first Fe-Fe distance was almost constant at 3.03 Å. X-ray diffraction identified the final precipitates as schwertmannite, a FeOOH phase containing structural sulfate.