Adsorptive removal of phosphate and nitrate anions from aqueous solutions using ammonium-functionalized mesoporous silica

Adsorption of nitrate and monovalent phosphate anions from aqueous solutions on ammonium-functionalized mesoporous MCM-48 silica was investigated. The adsorbent was prepared via a post-synthesis grafting method, using aminopropyltriethoxysilane, followed by acidification in HCI solution to convert the attached surface amino groups to ammonium moieties. The adsorbent was determined to be effective for the removal of both anions. The effects of pH, temperature, initial concentration of anions, and adsorbent loading on both anions adsorption were examined. At ambient temperature, the removal of nitrate was maximum at pH < 8, whereas phosphate removal was maximized at 4 < pH < 6. At a given initial concentration, the percentage anions removal increased as the adsorbent loading increased. For instance, maximum removals of 71% and 88% were obtained for nitrate and phosphate solutions, respectively, using an adsorbent loading of 10 g/L. The results also showed that the adsorption capacity decreased as the temperature increased. The adsorption isotherms were approached by the standard adsorption model equations. Based on a model discrimination study, only the Freundlich model resulted in physicochemically sound adsorption enthalpies and entropies for both anions. Desorption of both anions was rapidly achieved within 10 min, using 0.01 M NaOH. Regeneration tests showed that the adsorbent retained its capacity after five adsorption-desorption cycles.