Synthesis of quaternary ammonium-functionalized silica gel through grafting of dimethyl dodecyl [3-(trimethoxysilyl)propyl]ammonium chloride for nitrate removal in batch and column studies

In this study, quaternary ammonium functionalized-silica gel was synthesized for nitrate removal using dimethyl dodecyl[3-(trimethoxysilyl)propyl] ammonium chloride (DMDAC) as a quaternary ammonium silane coupling agent, which was grafted to silica gel to obtain the DMDAC-silica gel. Batch experiments showed that the DMDAC-silica gel could effectively remove nitrate in the pH range of 4 to 10. Kinetic and equilibrium experiments showed that nitrate removal by the DMDAC-silica gel reached equilibrium in 60 min with a maximum nitrate removal capacity of 56.4 mg/g. Fixed-bed column experiments (bed depth = 10–30 cm; inner diameter of column = 2.5 cm; flow rate = 0.06–0.18 L/h) in nitrate solution demonstrated the dynamic behaviors of nitrate removal by the DMDAC-silica gel under various experimental conditions of flow rate, bed depth, and influent nitrate concentration. The DMDAC-silica gels could be successfully used for nitrate removal and regenerated and reused with 10% NaCl solution in the column. In groundwater (nitrate = 140 mg/L, chloride = 63 mg/L, sulfate = 440 mg/L, and bicarbonate = 20–300 mg/L), the nitrate BTCs appeared behind the sulfate BTCs with α S N values of 5.7–11.3, indicating that the DMDAC-silica gel could selectively remove nitrate over sulfate in dynamic flow conditions.