a b s t r a c tEffect of norfloxacin (Nor) on the sorption of 1,3-dinitrobenzene (1,3-DNB), and PAHs (naphthalene (NAPH), phenanthrene (PHEN) and pyrene (PYR)) to K + -montmorillonite was studied. Nor suppressed 1,3-DNB sorption due to their competition for the same sorption sites. 1,3-DNB was sorbed on K + -montmorillonite surface via cation-polar interaction and n-electron donor-acceptor interaction. Nor also was sorbed on these sites through cation exchange, cation bridging and/or surface complexation. Nor increased three PAHs sorption on montmorillonite and the enhanced magnitude was positively correlated with the -donor strength of three PAHs. The enhanced sorption of PAHs by Nor was primarily attributed to -interaction between -electron-depleted quinoline ring of Nor and -electron-rich PAHs. Compared with cation (Nor + ) and anion (Nor − ), zwitterion (Nor ± ) of Nor increased PHEN and PYR sorption more pronounced due to additional cation-interaction between the sorbed Nor ± and PAHs. 1 H NMR spectrum provided direct evidence for -and cation-complexation between PAHs and Nor + in solution by ring-current-induced upfield chemical shifts of amino group and methylene group of Nor + .