Highly-effective phosphate removal from aqueous solutions by calcined nano-porous palygorskite matrix with embedded lanthanum hydroxide

Efficient phosphate removal from wastewater is critical for the safety of natural water bodies against eutrophication and for the replenishment of unrenewable phosphorus resources. In order to enhance phosphate capture efficiency and exploit raw clay for high value-added products, HPAL-LaOH was fabricated by embedding lanthanum hydroxide onto calcined nano-porous palygorskite clay through a simple-green hydrothermal method where the host palygorskite with cross-linked networks providing abundant binding sites for La(OH) ₃ incorporation, allowing the accessibility of phosphate for the adequate crystallization of lanthanum phosphate without triggering pore blockage. Excellent phosphate adsorption capacity (109.63 mg/g) was achieved by HPAL-LaOH, over 13 times higher than that of raw palygorskite, also much higher than commercial pure La(OH) ₃ (69.64 mg/g) in batch runs. Interestingly, for solutions below 100 mg/L in a wide pH range of 3–11, almost complete phosphate sequestration (< 0.01 mg/L) was achieved. The presence of high level competing anions (sulfate, nitrate, bicarbonate and chloride) merely exhibited an insignificant effect. Notably, HPAL-LaOH demonstrated satisfactory recyclability, settleability and excellent stability with negligible lanthanum leaching even under ultrasonic challenge. Mechanism analysis revealed that the impregnated La(OH) ₃ exerted specific phosphate adsorption where inner-sphere complexation by ligand-exchange played a major role. As compared to several La-based adsorbents developed recently for phosphate sequestration, HPAL-LaOH exhibited great competiveness in terms of adsorption capacity and La usage. The results indicated the potential utilization of HPAL-LaOH as a highly cost-effective adsorbent for phosphate removal from wastewater.