Phosphate Removal by Anion Binding on Functionalized Nanoporous Sorbents

Phosphate was captured from aqueous solutions by cationic metal EDA complexes anchored inside mesoporous silica MCM-41 supports (Cu(II) EDA-SAMMS and Fe(III) EDA-SAMMS). Fe EDA-SAMMS was more effective at capturing phosphate than the Cu EDA-SAMMS and was further studied for matrix effects (e.g., pH, ionic strength, and competing anions) and sorption performance (e.g., capacity and rate). The adsorption of phosphate was highly pH dependent it increased with increasing pH from 1.0 to 6.5, and decreased above pH 6.5. The adsorption was affected by high ionic strength (0.1 M of NaCl). In the presence of 1000-fold molar excess of chloride and nitrate anions, phosphate removal by Fe-EDA-SAMMS was not affected. Slight, moderate and large impacts were seen with bicarbonate, sulfate, and citrate anions, respectively. The phosphate adsorption data on Fe-EDA-SAMMS agreed well with the Langmuir model with the estimated maximum capacity of 43.3 mg/g. The material displayed rapid sorption rate (99% of phosphate removal within 1 min) and lowering the phosphate content to similar to 10 mu g/L of phosphorus, which is lower than the EPA's established freshwater contaminant level for phosphorus (20 mu g/L).