Interactions between Organic Model Compounds and Ion Exchange Resins

Abstract

Ion exchange (IEX) can successfully remove natural organic matter (NOM) from surface water. However, the removal mechanism is not well understood due to the complexity and variability of NOM in real source waters as well as the influence of multiple parameters on the removal behaviour. For example, this includes the physicochemical properties of the NOM and IEX resin, and the presence of competing anions. Model compounds with a range of physical and chemical characteristics were therefore used to determine the mechanisms of NOM removal by IEX resins. Fifteen model compounds were selected to evaluate the influence of hydrophobicity, size and charge of organic molecules on the removal by ion exchange, both individually and in mixtures. Three different resins, comprising polystyrene and polyacrylic resin of macroporous and gellular structure, showed that charge density (CD) was the most important characteristic that controlled the removal, with CD of >5 meq mg DOC-1 resulting in high removal (89%). Size exclusion of compounds with high MW (8 kDa) was evident. The hydrophobicity of the resin and model compound was particularly important for removal of neutral molecules such as resorcinol, which was best removed by the more hydrophobic polystyrene resin. Relationships were identified that provided explanations of the interactions observed between NOM and IEX resin in real waters.

Revealing the effect of anion exchange resin conditioning on the pH and natural organic matter model compounds removal mechanisms

“…Other measured anions were below the detection limit, excluding the possibility that ion exchange was responsible for the measured changes. The release of Clions for NaCl conditioned SBA resins was reported by Rahmani et al [55] and MacKeown et al [18], and can be deduced from the results of Finkbeiner et al as well [28]. This phenomenon could be explained by the concept of Donnan exclusion.…”

“…π-π interactions and H-bonds between the carboxyl groups of HA and the functional group of the resin) rather than ion exchange. Due to the hydrophobic nature of HA, desolvation from the solution and adsorption by the resin (interactions between the π-electron systems of the styrenic backbone of the resin and humic acid) results in a net negative free energy change which is a driving force for HA removal by (entropy assisted) adsorption [28,30]. In the FB form (pH 7.6), a lower removal was obtained.…”

“…In the work of Finkbeiner et al, hydrophobic effects and π − π stacking were proposed as main removal mechanisms for resorcinol, also a surrogate for hydrophobic LMWN, from a synthetic water at neutral pH, where the strongest interaction was found for styrenic resins. They studied three SBA, a MP styrenic and acrylic as well as a gel type acrylic resin, of which the styrenic resin achieved the highest resorcinol removal, explained by the stronger interactions with this backbone [28]. For tannic acid, a model compound for HS, the same authors concluded that the removal was principally driven by ion exchange, but also hydrophobic interactions contributed to the removal which was demonstrated the efficiency of the styrenic compared to acrylic resins [28].…”

“…They studied three SBA, a MP styrenic and acrylic as well as a gel type acrylic resin, of which the styrenic resin achieved the highest resorcinol removal, explained by the stronger interactions with this backbone [28]. For tannic acid, a model compound for HS, the same authors concluded that the removal was principally driven by ion exchange, but also hydrophobic interactions contributed to the removal which was demonstrated the efficiency of the styrenic compared to acrylic resins [28].…”

“…Over the past years, researchers are focusing on understanding and further optimizing the removal mechanisms of NOM (fractions) by AER. Synthetic water mixtures containing model compounds in ultrapure water facilitate the identification of interactions between these molecules and the resins, since other influencing factors such as the water matrix are eliminated [28][29][30]. Purschel states that possible removal mechanisms of starch molecules, representatives for the polysaccharide BP, from a synthetic water at neutral and acidic pH by strong base AER (SBA) are ion-dipole and/or van der Waals interactions.…”