(519a) Recovery of Inorganic Phosphorus Via Metal-Exchanged ZSM-5

Interest is growing in the development of separation processes for efficient removal and regeneration of P from wastewaters. Dissolved P, including phosphate (PO₄^3−), hydrogen phosphate (HPO₄^2−) and dihydrogen phosphate (H₂PO₄^−), is a primary source of water contamination originating from industrial wastewater or agricultural runoffs. The presence of high levels of P in ponds, rivers and lakes can lead to eutrophication, threatening the existence of fish and other aquatic life. On the other side, P is broadly used as fertilizers in agriculture to promote the growth of crops, whose demand escalates as the population continues to grow. The world’s supply of P today, however, relies on depleting feedstocks such as phosphate rocks. It thus becomes imperative, not only for mitigation of pollution, but also for sustainable production of fertilizers, to develop efficient and cost-effective methods for separation of P from aqueous solutions.

The most common method of phosphorus separation is chemical precipitation, typically in the form of struvite. Although feasible for implementation, this method consumes valuable feedstocks such as ammonia and/or magnesium salts. Meanwhile, struvite precipitation requires operation under high-pH conditions and precise control over the molar ratio of ammonium (NH₄⁺), magnesium (Mg²⁺), and phosphates, which add challenges to the process design and incur additional costs for scale-up applications. The use of other cations, such as Al³⁺ and Fe³⁺ that are capable of precipitating phosphate under more robust conditions, however, generates products undesired for agricultural purposes. In light of the challenges present in chemical precipitation, efforts have been devoted to the development of new separation processes, such as using sorbents for capture of the phosphate anions.

Zeolites have attained great attention as ion-exchange materials due to their chemical and structural stability, tunable and selective adsorption properties, employment of naturally abundant elements, and environmental compatibility. Natural zeolites such as clinoptilolite and gibbsite have previously been reported for capture and release of phosphates, but their limited capacity and specificity have inspired the exploration of artificial zeolitic materials as more robust sorbents. Here we report on the use of copper-substituted Zeolite Socony Mobil-5 (Cu-ZSM-5) as sorbents for efficient recovery of P. It has been demonstrated that phosphate (inorganic P) can be captured from synthetic solutions and released in brine solutions. By tailoring the synthesis of the zeolites, the capacity and efficiency of P capture/release were systematically studied. The captured inorganic P was released from the adsorbents when they were added to regenerative solutions and exhibited high-performance, stability, and recyclability after multiple successive treatments. Variations in Cu content and alterations to the Cu oxidation state in the adsorbents have enabled us to determine the active site for capture and to elucidate a potential mechanism for inorganic P capture and release. Fast capture and release of phosphate anions are demonstrated with >90% efficiency of recovery using synthetic solutions of Na₂HPO₄. The zeolite sorbents are also found to be recyclable and sustain high recovery efficiency after multiple capture-release cycles.