Abstract In this study, five different quaternary ammonium functionalized-mesoporous silica SBA-15 adsorbents with different alkyl chain lengths (C1Q-, C4Q-, C8Q-, C12Q-, and C18Q-SBA-15) were synthesized. This study focused on the effect of alkyl chain length of quaternary ammonium silane coupling agent grafted to SBA-15 on the selective nitrate removal from aqueous solutions. Fourier-transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) spectra confirmed that quaternary ammonium functional groups were successfully grafted onto the surface of SBA-15. In the FTIR spectra of C8Q-, C12Q-, and C18Q-SBA-15, peaks of CH2 asymmetric and CH2 symmetric stretching appeared at 2922–2944 cm−1 and 2852–2856 cm−1, respectively, which were getting stronger with an increase of alkyl chain length. In the N 1s spectra of the quaternized SBA-15, new peaks of NH4NO3 appeared at 405.8–406.2 eV after removal experiments, providing evidence of nitrate removal. The observed 13C solid-NMR spectra of the quaternized SBA-15 demonstrated that the quaternization occurred successfully on the adsorbents. Batch experiments demonstrated that the quaternized SBA-15 could effectively remove nitrate in the pH range from 4 to 10. The nitrate removal by the quaternized SBA-15 was fast process with equilibrium time of 10 min for C1Q- and C4Q-SBA-15 and of 30 min for C8Q-, C12Q-, and C18Q-SBA-15. The maximum nitrate removal capacities (mg/g) were in the order of C8Q-SBA-15 (136.4) > C4Q-SBA-15 (129.9) > C1Q-SBA-15 (89.4) > C12Q-SBA-15 (71.9) > C18Q-SBA-15 (30.7). In the presence of competing anions (chloride, phosphate, bicarbonate, and sulfate), C8Q-and C12Q-SBA-15 were more effective for nitrate removal than C1Q- and C4Q-SBA-15. In ground water (pH = 7.6, chloride = 47.7 mg/L, nitrate = 153.8 mg/L, sulfate = 445.4 mg/L; bicarbonate = 20.2 mg/L), C8Q- and C12Q-SBA-15 had nitrate-to-sulfate separation factors ( α S N ) of >3.7, whereas C1Q- and C4Q-SBA-15 had the α S N values of