(20c) Adsorptive Removal of Phosphate and Nitrate Anions From Aqueous Solutions Using Ammonium-Functionalized Mesoporous Silica Materials: Effects of Experimental Conditions in the Batch & Continuous Modes

In recent years, the problem of disposing of wastewaters containing nutrient species such as phosphates, nitrate and ammonium has become increasingly acute due to the tightening of the environmental regulations. Nitrate and phosphate anions are particularly deleterious as both are implicated in aquatic eutrophication. Adsorption using mesoporous functional materials is promising for the removal of such pollutants. Indeed, the use of mesostructured adsorbents offers a number of advantages such as high surface areas, open pore structures, high capacity and selectivity. The present work is devoted to the study of the adsorption process for phosphate (H₂PO₄^-) and nitrate (NO₃^-) aqueous solutions over three mesoporous silica materials (MCM-41; MCM-48 and SBA-15) modified with organic ammonium functional groups. The objectives pursued are: i) synthesis and characterization of the adsorbents, and ii) experimental optimization of the operating conditions in the batch and continuous modes of operation. The adsorbents were prepared via a post-synthesis grafting method. They were then characterized using N2 adsorption and the BET analysis, powder X-ray diffraction and CHN-elemental analysis. The adsorption tests were first performed batchwise in 100-mL stirred Erlenmeyer flasks. The effects of temperature, adsorbent loading, anions initial concentration and the presence of competitive species on nitrate and phosphate adsorption were investigated. Results showed that adsorption capacity decreased with increasing temperature. It reached 43 and 46 mg/g at the lowest temperature investigated (5 °C) for nitrate and phosphate anions, respectively. Regeneration tests showed that the adsorbents retained their capacity after five adsorption?desorption cycles. As for the continuous mode of operation, results showed that breakthrough times and breakpoints decreased with increasing flow rates and inlet anion concentration. Moreover, increasing the bed depth increased the breakpoint and breakthrough times. Regeneration tests performed for phosphate anions showed that the column recovered its complete performances with no significant change in the breakpoint and exhaustion points even after three adsorption-elution-regeneration cycles.