Phosphorylation triggered growth of metal phosphate on halloysite and sepiolite nanoparticles: preparation, entrapment in chitosan hydrogels and application as recyclable scavengers

The rational design of porous, biomass-derived eco-friendly adsorbents instead of costly synthetic, petroleum-based resins stands as a promising approach to resolving the thorny issue of sewage treatment (for example, by supplying safe water for drinking and irrigation). Herein we explore a straightforward transformation of abundant clay and polysaccharides into highly-reactive water-cleaning adsorbents. Surface-functionalization of sepiolite and halloysite by phosphoryl chloride triggered metal dissolution from the framework (inside of the lumen for halloysite and on the external surface for sepiolite) and the concomitant growth of well-truncated crystalline metal phosphates. These unprecedented nanomaterials exhibited outstanding adsorption ability, outperforming a set of phosphorylated solids (graphene, mesoporous SBA-15 and cellulose nanocrystals) but unfortunately failed in recycling. However, their entrapment on a chitosan backbone afforded porous self-standing microspheres where the resulting hydrogels displayed good adsorption for both anionic Congo red and cationic Malachite green dyes and their mixture. The importance of shaping the material framework as porous hydrogels was demonstrated through a comparison with shrinking chitosan-clay xerogel and powder analogues. The resulting beads can be easily recovered from the water medium and reused several times. This strategy seems attractive for scavenging persistent chemicals from aqueous media.