Restricted reaction of layered double hydroxide nanoparticles with phosphate in a confined microsphere space

Abstract

Over the past decades, considerable efforts have been made to find useful solutions for phosphate pollution control. The state transition of nanomaterials from freely dispersed to encapsulated provides a realizable route for their application in phosphate elimination. The separation convenience offered by encapsulation has been widely recognized, however, the unique binding mode of nanostructures and phosphate in the confined space remains unclear, limiting its further development. Here, carboxymethyl cellulose (CMC) microspheres were used as hosts to deploy layered double hydroxide (LDH) nanoparticles. On this basis, we described an attempt to explore the adsorption behavior of LDH and phosphate in the microsphere space. Compared to their freely dispersed analogues, LDH particles exhibited higher structural stability, wider pH adaptability, and better phosphate selectivity when spatially confined in the CMC microsphere. Nevertheless, the kinetic process was severely inhibited by three orders of magnitude. Besides, the saturated phosphate adsorption capacity was also reduced to 74.6 % of the freely dispersed system. A combinative characterization revealed that the highly electronegative CMC host not only causes electrostatic repulsion to phosphate, but also extracts the electron density of the metal center of LDH, weakening its ability to act as a Lewis acid site for phosphate binding. Meanwhile, the microsphere encapsulation also hinders the ion exchange function of interlayer anions and phosphate. This study offers an objective insight into the reaction of LDH and phosphate in the confined microsphere space, which may contribute to the advanced design of encapsulation strategies for nanoparticles.

Keywords: Carboxymethyl cellulose microsphere; Confined space; Layered double hydroxide; Nanoparticle encapsulation; Phosphate.