Molecular sieving of alkyl sulfate anions on strong basic gel-type anion-exchange resins

The effort of today's society towards green, wasteless, and economic production technologies brings new challenges concerning the efficiency of chemical processes. Electromembrane separations are considered as one of the possible candidates in tackling the challenges mentioned above. They offer a competitive alternative to industrially employed standard separation processes regarding cost and control. However, their susceptibility to fouling, especially when the processed solutions are of biological origin, hinders their full application potential. Here, we experimentally determine the group of potential foulants by indirect measurement of the internal characteristic dimensions of a strongly basic anion-exchange resin. Such a resin is used in both ion-exchange beds and heterogeneous ion-exchange membranes. We determine the characteristic dimensions by running a set of electrochemical experiments with a homologous series of alkyl sulfates having a molecular weight in the range between 134 and 288 g/mol. The alkyl sulfates represent potential resin (membrane) foulants. Our results show that the alkyl sulfates can be divided into two groups. While the short alkyl sulfates are exchanged by the studied anion resin particle, the long ones virtually block the resin and hinder any ion exchange or transfer in the electric field. The molecules having dimensions corresponding to the length of the alkyl sulfates of the second group or longer are the potential foulants of the studied and related anion-exchange systems. The alkyl sulfate of the critical length has a distance between sulfur and outermost carbon atoms of around 1.18 nm as predicted from its structure in ChemDraw. We showed that the alkyl sulfates of the first group facilitate water splitting reaction, which indicates the water splitting is the primary mechanism of the observed overlimiting current. The electroconvective motion possibly contributing to the overlimiting current at anion-exchange systems is mostly suppressed.

Automated summary

Electromembrane separations offer a competitive alternative to standard separation processes but are hindered by fouling. Short alkyl sulfates can be exchanged by anion resin while long ones block the resin, hindering ion exchange. The distance between sulfur and outermost carbon atoms determines the potential foulants of anion-exchange systems.