A stable rare earth-based layered double hydroxide embedded chitosan hydrogel beads for enhanced phosphate removal from aqueous solution: Performance, mechanisms and applicability

As one of the attractive phosphate adsorption materials, layered double hydroxide (LDH) is frequently used in the form of nanomaterials, which makes LDH suffer from weak recyclability and agglomeration. Powder immobilization is applicable for separation, though usually sacrifices the adsorption capacity. In this investigation, Two types of rare earth-based layered double hydroxide/chitosan (CS) hydrogel beads were fabricated, namely LaCa-LDH/CS and CeCa-LDH/CS. The results demonstrate that the combination of CS and LDH exhibits superior adsorption performance than individual LDH, primarily due to the incorporation of -NH2 and the uniform dispersion of LDH in CS. The maximum adsorption capacities for phosphate on LaCa-LDH/CS and CeCa-LDH/CS are 149.5 and 174.6 mg P/g at 200 mg P/L and pH 5, respectively, surpassing the powder form of LaCa-LDH (107.9 mg P/g), CeCa-LDH (105.0 mg P/g), and pure CS beads (10.2 mg P/g). Furthermore, both LaCa-LDH/CS and CeCa-LDH/CS exhibit steady phosphate removal performance at a wide pH range, and the adsorption capacities only experience a decrease of 20.0 % and 28.4 % from pH 3–7, outperforming previous reports. The co-existing anions effect experiments proved excellent selectivity of LDH/CS to phosphate. The stable binding of phosphate on LDH/CS is confirmed through a long-term adsorption stability test lasting for 10 days. Moreover, two LDH/CS could remove 96 % phosphate from natural water. After five cycles, the adsorption capacities of two LDH/CS were maintained at over 95 %. The primary adsorption mechanism involves electrostatic attraction as pH < pHpzc and ligand exchange as pH > pHpzc. Ion exchange and hydrogen bonding also contribute to a certain extent.