Ion exchange for the removal of natural organic matter

Because of their aliphatic and carboxylic acid structures, the polar components of natural organic matter (NOM), both charged and neutral, can give high yields of disinfection by-products (DBPs). For removal of charged organic species, ion exchange is better performing than coagulation with inorganic salts such as alum, as it very efficiently removes essentially all of the charged material, while alum preferentially removes only the larger molecules in these fractions. The influence of resin structure on performance is reviewed. There is an increase in NOM removal for quaternary ammonium resins of higher water content, with the more open structures allowing easier entry of the larger NOM species. Polar groups and the degree of crosslinking affect the water content of the resin. The environment around the nitrogen is evidently critical, with hydroxyethyl or the absence of two methyl groups on the N (as with a secondary amino group) being beneficial in NOM removal. Having a pendant OH group close to the quaternary nitrogen is a distinct advantage as the resins remove more NOM than would be expected from their water content: an appropriate balance of polar and non-polar regions in the resin structure appears to be required. Having a higher charge density than quaternary ammonium groups, protonated secondary amino sites have a greater affinity for hydrophilic counter ions. There is scope for the synthesis of improved resin systems, both strong and weak base. Thus chitosan, an amino polysaccharide, can function as a soluble cationic coagulant for NOM, as can polyethyleneimine. It is postulated that they act via hydrogen bonding, as a charge mechanism is unlikely because the polymers are only partially ionised at neutral pH. Crosslinked versions could therefore be effective in removing NOM. Crown Copyright (C) 2004 Published by Elsevier B.V. All rights reserved.