A Usable Kinetic Model of Suspended Ion Exchange Processes for Natural Organic Matter Removal

The suspended ion exchange (SIEX) process is an emerging technology for the removal of dissolved natural organic matter (NOM). To facilitate the SIEX selection, design, and operation, a usable mechanistic model is proposed. The simple kinetic model unifies the Lagergren equation and the Glueckauf model and has analytical solutions for common SIEX processes. It is shown, through comparison with experimental data, that the proposed model describes SIEX effectively. The plug-flow and the mixed-flow processes, such as SIX and MIEX, treat the removable NOM well. At (removable NOM) removal efficiency above 50%, though, both SIEX processes increasingly run into poor resin capacity utilization due to the process thermodynamic constraint and, to a lesser extent, the kinetic effect. The choice of chloride-form or bicarbonate-form resin has little effect on the adsorption distribution constant of removable NOM. Replacing chloride with bicarbonate as the counterion, however, increases the nonremovable NOM fraction, i.e., causes an additional amount of NOM unable to access the adsorption sites. The result of counterion binding suggests that the nonremovable NOM fraction is just a mixture of the NOM compounds that have insufficient affinity to displace resin counterion from adsorption sites and raises question on the hypothesis in the literature that attributes the nonremovable NOM to the uncharged NOM species only.