Effect 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-pi 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 pi-donor strength of three PAHs. The enhanced sorption of PAHs by Nor was primarily attributed to pi-pi interaction between pi-electron-depleted quinoline ring of Nor and pi-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-pi interaction between the sorbed Nor(+/-) and PAHs. H-1 NMR spectrum provided direct evidence for pi-pi and cation-pi complexation between PAHs and Nor(+) in solution by ring-current-induced upheld chemical shifts of amino group and methylene group of Nor(+). (C) 2011 Elsevier B.V. All rights reserved.