Sorption of As to Biogenic Iron Oxides.

Arsenic (As) is a widespread and problematic pollutant that can be derived from natural or anthropogenic sources. Iron (oxyhydr)oxides readily sorb arsenic and thus play critical roles in arsenic cycling in terrestrial environments; however, little is known about the affinity and mechanism of As(V/III) sorption by biogenic iron (oxyhydr)oxides formed in natural hydrological environments. To investigate this, we conducted sorption isotherm and kinetics experiments to compare As(V) and As(III) sorption to synthetic 2-line ferrihydrite and those harvested from circumneutral pH waters. Inductively coupled plasma mass spectrometry (ICP-MS) was used to
quantify both As(V) and As(III), whereas X-ray adsorption spectroscopy (XAS) was utilized to obtain As and Fe spectra for As(V) and As(III) sorbed to environmental and laboratory Fe(III) biominerals. All environmental Fe(III) biominerals were determined to be predominately composed of 2-line ferrihydrite but have a surface area normalized affinity for As that is greater or equivalent to synthetic 2-line ferrihydrite. Inner sphere complexation of As(V/III) to environmental Fe(III) biomineral is suggested from the As EXAFS data. Whereas the extent of sorption was similar for As(III) on all minerals, As(V) sorption to environmental Fe(III) biominerals was approximately three times higher than what was observed for synthetic 2-line ferrihydrite. We speculate these differences in relative sorption behavior are
attributable to differences in the quantity of corner sharing sorption sites in synthetic and environmental oxides. This study helps to improve our understanding of the sorption reactivity of biogenic iron (oxyhydr)oxides for environmentally relevant As species (V and III) and is critical to understanding As cycling in natural systems while also having the potential to improve As remediation systems.