Transport and retention of Molybdenum(VI) on iron oxide-coated sand: A modified multi reaction model

Abstract In this investigation, batch and miscible displacement experiments were performed to investigate the pH-dependent effect on retention and transport of Mo(VI) on iron oxide-coated sand (IOCS). Measured results indicated that Mo(VI) sorption was strongly nonlinear and kinetically controlled, where the amount of Mo(VI) sorbed and its reaction rates increased with decreasing solution pH. Mo(VI) breakthrough curves (BTCs) obtained at varying pHs were asymmetrical in shape and exhibited different degree of tailing, where mobility of Mo(VI) was lowest under acidic condition (pH 4.0) and highest under alkaline condition (pH 7.5). In this study, based on extended Freundlich equation, we proposed a modified multi-reaction model (M-MRM) including a pH term, which was utilized to describe pH-dependent sorption and transport process. From the goodness-of-fit of modeling results, the M-MRM model provided a good prediction for Mo(VI) retention and transport datasets on IOCS. In comparison to numerical surface complexation models (SCMs), the proposed M-MRM is relatively less complex and requires fewer parameters. Moreover, it is simpler to integrate reaction transport equations into kinetic models to simulate non-equilibrium transport behavior. Our work provided a unified kinetic model to describe pH-dependent sorption and transport in IOCS.