Lanthanum Carbonate-Based Adsorbents for Low-Level Phosphate Treatment

Abstract

Phosphorus (P) pollution is a widespread environmental concern worldwide. Excessive phosphate discharge can trigger eutrophication, negatively affecting the aquatic ecosystems, economic development and human health. The low threshold of 0.02 mg P/L for inducing eutrophication in water bodies challenges conventional technologies to effectively remove phosphate. Adsorption technology is widely recognized for its high capacity for low-level phosphate removal. However, the reduced adsorption capacity of adsorbents at low phosphate concentrations results in high costs, posing an impediment to the control of eutrophication. This project aims to develop lanthanum (La) carbonate-based adsorbents suitable for low-level phosphate removal. Indeed, La carbonate exhibited an excellent adsorption capacity at low phosphate concentrations. Exploration of the adsorption mechanism revealed the importance of the ion exchange capacity of the adsorbent for low-level phosphate removal. To further enhance the adsorption capacity and reduce costs, the Fe modification strategy was proposed. It was demonstrated that the conversion of Fe(II) to Fe(III) during the synthesis process could transfer electrons to the adjacent La active site, which optimized its electronic structure and enhanced electron transfer between phosphate and La, thereby facilitating phosphate adsorption. Additionally, Fe modification also enhanced the adsorbent's resistance to dissolved organic matter in real water due to the weaker coordination ability of Fe than La. Further, magnetic La/Fe carbonate was prepared using inexpensive magnetite as the carrier and Fe source, exhibiting the high adsorption capacity, excellent separability and low cost. The study also demonstrated that metal ligands species and amount and surface-active metal sites coverage caused a decrease in adsorption capacity of La-based adsorbents. The study clarifies the primary mechanism involved in La carbonate-based adsorbents-bound phosphate at low levels, reveals the advantages of the Fe modification strategy for phosphate removal, and provides a possible way to develop adsorbents for low-level phosphate removal.