||Improving removal of natural organic matter (NOM) during drinking water treatment is important in order to minimize the formation of disinfection by-products (DBPs). The research presented in this thesis investigates modification of iron oxide-coated sand (IOCS) with organic cations as a means to improve NOM removal by adsorption. Several types of IOCS prepared with different methods were compared and one type of IOCS prepared by the “sludge coating” method was found to possess relative better physical/chemical stability and NOM adsorption capacity and was selected. Modifying the selected IOCS with three quaternary ammonium compounds (QAC), including tetrapropylammonium chloride (TPA), hexadecyltrimethylammonium bromide (HDTMA) and hexadecylbenzyldimethylammonium chloride (HDBDMA) was examined under different conditions. Modification of IOCS with HDTMA at high pH with an addition of 0.2% H2O2 appears to be the best condition for producing modified IOCS with most significantly enhanced NOM adsorption. Evaluation of NOM adsorption on this HDTMA-modified IOCS was extensively conducted in both laboratory batch and column adsorption tests. The HDTMA-modified IOCS displayed faster initial NOM adsorption rate and substantially higher NOM adsorption capacity than the unmodified IOCS at a wide pH range, with the enhancement being more pronounced at higher pH conditions. The modification also decreased the interference from common anions (e.g., phosphate ions) in water on NOM adsorption onto IOCS. Compared to the unmodified IOCS, the HDTMA-modified IOCS preferentially removed the more hydrophobic and larger size fractions of NOM molecules and resulted in enhanced reduction of trihalomethane formation potential (THMFP). The longer contact time at a lower filtration rate in the column operation increased the breakthrough capacity of the HDTMA-modified IOCS. The exhausted NOM adsorption capacity of the modified IOCS can be recovered (with slight decreases) through in-situ regeneration with NaOH solution and/or through ex-situ re-modification with HDTMA solution. The mechanisms contributing to the enhanced NOM adsorption on the HDTMA-modified IOCS were explored with surface characterization techniques, in combination with the adsorption test results. The electrostatic interactions that were thought to be brought from the positively-charged, surface-coated HDTMA seemed to be unimportant, likely because the outwards-pointing tail groups of the surface-coated HDTMA hindered the interaction. Improved hydrophobic interactions followed by ligand exchange are believed to be the dominant mechanisms governing the enhanced NOM adsorption on the HDTMA-modified IOCS. Force analysis by atomic force microscopy (AFM) was first successfully applied to explore the functioning mechanisms between NOM and IOCS. It provided direct evidence to demonstrate the pull-on force changes and the enhanced hydrophobic interactions brought from the modification of IOCS with HDTMA.