Phosphorus removal from water using La-based absorbents: Insights into the impacts of crystal structure, surface area, and crystallinity on the removal efficiency and mechanisms

Abstract

Removal of nutrients and pollutants from water using structured adsorbents is of great interest. Lanthanum (La) oxide/hydroxides and their derivative composites are promising platforms for phosphate (P) removal from wastewater. However, various key factors including crystal structure, specific surface area (SBET) and crystallinity restrict their P removal performance. In this study, LaOCl, a novel adsorbent with tetragonal crystal, and La2O3, a traditional adsorbent with hexagonal crystal, were prepared at different calcination temperatures and tested as P adsorbents. Due to different crystal structures, LaOCl and La2O3 exhibit significant differences in P adsorption. LaOCl crystal is more stable than La2O3 in solution. The main LaOCl crystal phase is not changed after P adsorption, while La2O3 transformed into La(OH)3 due to proton capturing form H2O. The Cl− on LaOCl can be used as a ligand for P ions exchange. It is different from OH− exchange which increases the pH sharply. The isoelectric points (IEP) of LaOCl is slightly higher than La2O3. All of these factors, caused by the different structures of LaOCl, keep its P adsorption capacity and rate high in acidic conditions. The Langmuir maximum P adsorption capacity of LaOCl is 164.52 mg/g, which is 2.3 times greater than La2O3. The LaOCl adsorbent exhibited a faster adsorption rate than La2O3. It is able to reduce the H2PO4− concentration from 50 mg/L (pH ∼ 5.0) to < 0.002 mgP/L in 180 min, while the residual P concentration is about 11.27 mgP/g by La2O3 in the same time. The P adsorption capacity was found positively correlated with crystallinity at pH 3.0 but with SBET only in pH 5.0 ∼ 9.0 range for LaOCl. However, SBET and crystallinity are not the main factors influencing the P adsorption capacity of La2O3. Following P adsorption, both adsorbents were verified for P desorption. The results showed the amount of P desorbed was small, thus the adsorbents have the ability to lock P in natural water. Overall, this study provides essential perspectives on the effects of crystal structure, surface area, and crystallinity on P adsorption and the scientific basis for developing new and functionalized La-based adsorbents.