Efficient phosphate removal by dendrite-like halloysite-zinc oxide nanocomposites prepared via noncovalent hybridization

Abstract The efficient capture of phosphate from contaminated water is crucial for aquatic ecosystem protection and drinking water utilities, but designing nanohybrid materials with high phosphate-adsorption capacity remains challenging. This study reports on a way to tackle this challenge, by creating a dendrite-like nanocomposite through the functionalization of tubular halloysite (Hal) clay mineral with ZnO nanoparticles. Specifically, ZnO nanoparticles were coated on the surfaces of Hal, thereby by providing adsorption sites capable of removing phosphate. Furthermore, dendrite-like ZnO-Hal can grasp phosphate firmly because of the strong binding of ZnO nanoparticles to phosphate. Thus, the ZnO coating with its high adsorption affinity provides a new benchmark for the scavenging of phosphate in water. The adsorption capacity of ZnO-Hal exhibits an exceptionally high phosphate capacity of 97.3 mg P·g−1 under the equilibrium phosphate concentration of 346 mg P·L−1, which is superior to most other reported phosphate adsorbents. After normalizing to the content of ZnO in the nanocomposite, the phosphate adsorption capacity of ZnO-Hal is higher than that of the pure ZnO particles. The equilibrium adsorption isotherm and kinetic data of ZnO-Hal were fitted well with Freundlich and pseudo-second-order models, respectively. Moreover, good adsorption performance of ZnO-Hal was confirmed in the presence of competitive anions at molar ratios of 1:10 or 1:100 (phosphate-to-anion ratio), both in the acidic and neutral pH ranges, and in domestic wastewater as well. The adsorption mechanism was investigated; those results revealed that phosphate was removed by the formation of a zinc phosphate mineral hopeite on the Hal surfaces. This work, therefore, provides a novel material for removing phosphate from water for environmental remediation purposes.