Adsorptive removal of nitrate and phosphate anions from aqueous solutions using functionalised SBA-15: Effects of the organic functional group

Adsorption of nitrate and monovalent phosphate anions from aqueous solutions on mono, di- and tri-ammonium-functionalised mesoporous SBA-15 silica was investigated. The adsorbents were prepared via a post-synthesis grafting method, using either 3-aminopropyltrimethoxysilane (N-silane) or [1-(2-aminoethyl)-3-aminopropyl]trimethoxysilane (NN-silane) or 1-[3-(trimethoxysilyl)-propyl]-diethylenetriamine (NNN-silane), followed by acidification in HCl solution to convert the attached surface amino groups to positively charged ammonium moieties. The nominal loading of amino moieties on the SBA-15 surface was varied from 5% to 20% as organoalkoxysilane/silica molar ratio. The adsorption experiments were conducted batchwise at room temperature. Results showed that adsorption capacity increased with increasing the concentration of monoammonium groups on the SBA-15 adsorbent. Nitrate adsorption capacity increased from 0.34 to 0.66 mmol NO3-/g adsorbent while phosphate adsorption capacity increased from 0.34 to 0.63 mmol H2PO4-/g adsorbent when the molar ratio organoalkoxysilane/silica was varied from 5% to 20%, respectively. Also, for the same organoalkoxysilane/silica molar ratio of 10%, the adsorption capacity increased with the increase of the number of protonated amines in the functional groups. Therefore, maximum adsorption capacities of 0.80, 1.16 and 1.38 mmol NO3-/g adsorbent and 0.72, 0.82 and 1.17 mmol H2PO4-/g adsorbent were obtained using mono-, di- and triammonium functionalised SBA-15 adsorbents, respectively.