Arsenate adsorption onto hematite nanoparticles under alkaline conditions: effects of aging

Arsenate adsorption onto freshly synthesized hematite nanoparticles was carried out under highly alkaline conditions (~pH 10) at room temperature (21 °C). Dynamic light scattering measurements of hydrated hematite colloids ranged from 43 to 106 nm (~96 %). The measured zeta potential was 28.1 mV (±5.85) suggesting that the hematite nanoparticles were moderately stable. X-ray diffraction and Raman spectroscopy data showed that hematite was stable under the conditions tested, with no crystal modification evident at the completion of the experiment (9 days). An additional band position at ~826 cm⁻¹ in the Raman spectra represented arsenate adsorbed onto hematite. The pH of the slurry dropped from ~10 to ~8 during the experiment; this was coincident with a drop in the aqueous concentration of arsenic (from ~121 to ~92 mg/L) as determined via inductively coupled plasma mass spectrometry (ICP-MS). ICP-MS analyses on the solid samples indicated a significant amount of arsenic partitioned to the solid phase during aging, corroborating the results of aqueous analyses. X-ray absorption spectroscopic analyses revealed that the bonding environment remained the same irrespective of the pH and the amount of arsenate adsorbed. Arsenate adsorbed onto hematite through a strong inner-sphere bidentate-mononuclear complex both before (0 days) and after (9 days) aging. These results are valuable for understanding the fate of potential contaminants in alkaline mine tailings environments where 2-line ferrihydrite frequently transforms to hematite rather than goethite.