Phosphate sequestration by lanthanum-layered rare earth hydroxides through multiple mechanisms while avoiding the attenuation effect from sediment particles in lake water

Abstract

Lanthanum-based adsorbents have been used extensively to capture phosphate from wastewater. However, the attenuation effect that arises from the coexistence of sediment and humic acid is the major drawback in practical applications. The Lanthanum-layered rare earth hydroxides (LRHs)-Cl (La-LRH-Cl) was synthesized and achieved high elemental phosphorus (P) adsorption capacity (138.9 mg-P g^-1) along with a fast adsorption rate (k₂ = 0.0031 g mg^-1·min^-1) over a wide pH range while avoiding the attenuation effect that arises from the coexistence of sediment and humic acid in lake water. The La-LRH-Cl effectively captured phosphate through multiple interactions, such as the ion exchange of Cl^- and phosphate, the memory effect of LRH and the inner-sphere complexation of La-P. Moreover, physical models demonstrated that the adsorption of phosphate onto La-LRH-Cl was a monolayer endothermic process, during which PO₄^3- interacted by multi-docking via parallel orientation at 293 K and multi-ionic interactions through pure non-parallel orientation at 303 K. Hence, 1000 L of 11.08 mg-P L^-1 of the acquired lake water was decontaminated by 30 g of La-LRH-Cl to 0.09 mg-P L^-1 within 7 days. In addition, over ~12,125 BV of an industrial effluent containing 3.26 mg-P L^-1 was treated to below USEPA's discharge limit in fixed-bed tests. It was found that the memory effect of LRH was responsible for the stable performance and reusability. Therefore, more focus should be placed on the collective role of La and LRH layered structure as a means of preventing the attenuation effect in the real water matrix.

Keywords: Adsorption; Attenuation effect; Humic acid; Phosphate; Sediment.