Adsorption of molybdate by synthetic hematite under alkaline conditions: Effects of aging

► XRD, RS, and SEM revealed that hematite were stable at 75°C and pH 10 for 7day. ► No change in crystal morphology or reduction in surface area was evident. ► Hematite with adsorbed Mo was similar in size/shape to pure hematite. ► ICP-MS analyses indicate that hematite retains adsorbed Mo after aging. Hematite is a common primary/secondary mineral in mine drainage and mine waste settings that can adsorb dissolved metals and metalloids. This study explored the ability of synthetic hematite to retain one such contaminant, molybdate, on its surfaces under highly alkaline (pH=∼10) conditions. X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM), and specific surface area (BET) analyses show that synthetic hematite particles are stable and able to adsorb molybdate. Raman spectra show that the hematite efficiently adsorbs molybdate and retains it on its surfaces via strong inner-sphere surface complexation. Inductively coupled plasma-mass spectrometry (ICP-MS) data indicate that hematite aged (7 and 9days) at high and room temperatures (75 and 25°C) retains adsorbed molybdate and that molybdate sorption increases with aging. SEM images show that aged hematite particles with adsorbed molybdate are similar in size and shape to pure hematite and exhibit no significant reduction in surface area. These findings are valuable for understanding the fate of Mo in mine wastes and mill tailings environments where the 2-line ferrihydrite to which it is adsorbed can transform to hematite.