Cation exchange during subsurface iron removal

Subsurface iron removal (SIR), or in-situ iron removal, is an established treatment technology to remove soluble iron (Fe 2+) from groundwater. Besides the adsorptive-catalytic oxidation theory, it has also been proposed that the injection of O 2-rich water onsets the exchange of adsorbed Fe 2+ with other cations, such as Ca 2+ and Na +. In sand column experiments with synthetic and natural groundwater it was found that cation exchange (Na +–Fe 2+) occurs during the injection-abstraction cycles of subsurface iron removal. The Fe 2+ exchange increased at higher Na + concentration in the injection water, but decreased in the presence of other cations in the groundwater. Field results with injection of elevated O 2 concentrations (0.55 mM) showed increased Fe removal efficacy; the operational parameter V/Vi (abstraction volume with [Fe]