Residence time effects on molybdenum adsorption on soils: Elucidation by multi-reaction modeling and xanes analysis

To investigate the influence of residence time on molybdenum [Mo(VI)] adsorption behavior in soil environments, kinetic batch experiments coupled with X-ray near-edge structure (XANES) spectroscopy were performed for a neutral-pH soil (Webster loam) and two acidic soils (Mahan sand and Windsor sand) at different time scales (1 day–1 year). Batch-type experiments indicated that retention of Mo(VI) was rate limited and typical biphasic for soils. Initial rapid retention was followed by a continued slow retention with increasing aging time for Mahan and Windsor soils. In contrast, the reaction for Webster soil was nearly complete after 8 h, reflecting difference in soil properties. XANES analysis for Webster soil confirmed that most of Mo was bound to montmorillonite during long-term reaction time, whereas kaolinite constitutes a very important host phase for Mahan and Windsor soils. Sequential extraction results indicated that the percentages of Fe/Al oxide and residual fractions increased at the advanced time periods for Mahan and Windsor soils. The goodness-of-fit of numerical modeling results indicated that a simple version of multi-reaction model (MRM) with equilibrium and kinetic sorption sites was capable of describing Mo(VI) retention data for Webster loam. However, for Windsor and Mahan soils, an additional irreversible sorption site was required to simulate Mo(VI) retention over time. Although each site from MRM model cannot be unequivocally clarified from each other by either XANES analysis or sequential extraction results, their finding provided evidence of surface irreversible reactions at long residence times