Cation exchange during subsurface iron removal

Subsurface iron removal (SIR), or in-situ iron removal, is an established treatment technology to remove soluble iron (Fe2+) from groundwater. Besides the adsorptive-catalytic oxidation theory, it has also been proposed that the injection of O2-rich water onsets the exchange of adsorbed Fe2+ with other cations, such as Ca2+ and Na+. In sand column experiments with synthetic and natural groundwater it was found that cation exchange (Na+–Fe2+) occurs during the injection-abstraction cycles of subsurface iron removal. The Fe2+ 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 O2 concentrations (0.55 mM) showed increased Fe removal efficacy; the operational parameter V/Vi (abstraction volume with [Fe]<2 μM divided by the injection volume) increased from an average 7 to 16, indicating that not the exchangeable Fe2+ on the soil material is the limiting factor during injection, but it is the supply of O2 to the available Fe2+.

► Subsurface iron removal (SIR) removes soluble iron (Fe2+) from groundwater.
► Besides Fe2+ oxidation, the injection of O2-rich water onsets the exchange of adsorbed Fe2+ with other cations.
► Sand column experiments confirmed that Fe2+ retardation can be achieved in the absence of O2, at high Na+ concentrations.
► Field results with injection of elevated O2 concentrations (0.55 mM) showed increased Fe removal efficacies.