Capability of cation exchange technology to remove proven N-nitrosodimethylamine precursors

N-nitrosodimethylamine（NDMA） precursors consist of a positively charged dimethylamine group and a non-polar moiety, which inspired us to develop a targeted cation exchange technology to remove NDMA precursors. In this study, we tested the removal of two representative NDMA precursors, dimethylamine（DMA） and ranitidine（RNTD）, by strong acidic cation exchange resin. The results showed that pH greatly affected the exchange efficiency, with high removal（DMA 〉 78% and RNTD 〉 94%） observed at pH 〈 pk_a-1 when the molar ratio of exchange capacity to precursor was 4. The exchange order was obtained as follows： Ca~（2＋）〉 Mg~（2＋）〉 RNTD~＋〉 K~＋〉 DMA~＋〉 NH_4~＋〉 Na~＋. The partition coefficient of DMA~＋to Na~＋was 1.41 ± 0.26, while that of RNTD~＋to Na~＋was 12.1 ± 1.9. The pseudo second-order equation fitted the cation exchange kinetics well. Bivalent inorganic cations such as Ca~（2＋）were found to have a notable effect on NA precursor removal in softening column test. Besides DMA and RNTD, cation exchange process also worked well for removing other 7 model NDMA precursors. Overall, NDMA precursor removal can be an added benefit of making use of cation exchange water softening processes.