Removal of phosphate from natural waters using innovative modified bentonites

Abstract

A modified inorganic bentonite (Zenith/Fe) based on natural bentonite with the code name Zenith-N, was prepared by embedding Fe ions in the interlayer space of bentonite. Bench-scale batch experiments were performed to examine its efficiency as an adsorbent for phosphate removal from natural waters. Phosphate uptake was evaluated vs. pH using adsorption kinetics and adsorption isotherms. The effects of temperature and salinity (fresh, brackish and marine waters) on phosphate-uptake capacity were also investigated. Comparative experiments were carried out for unmodified bentonite (Zenith-N) and a commercial product Phoslock (lanthanum modified clay). The results showed that phosphate-uptake capacity of Zenith/Fe [11.15mg/g, at pH=7] was improved by ~350% compared to unmodified bentonite (Zenith-N) [4.12mg/g, at pH=7] and was comparable to the phosphate-uptake capacity of Phoslock [11.60mg/g]. The optimal pH for phosphate adsorption ranged between 6 and 7. However, phosphate adsorption by Zenith/Fe and Phoslock appeared to be less pH-dependent than natural bentonite. Adsorption kinetics showed that over 80% of phosphate was removed from water within 1h. The resulting isotherms also fit well with Langmuir and Freundlich isotherm models. The increase in adsorption capacity of Zenith/Fe at higher temperatures was caused by the enlargement of pore size and/or activation of the adsorbent surface. Finally, the phosphate adsorption capacity followed the order: fresh water<brackish<marine water.