Calixpyridinium–Polyoxometalate Nanoparticle Assemblies for the Removal of Organic Dyes from Wastewater

In this work, three organic–inorganic hybrid calixpyridinium–polyoxometalate nanoparticle assemblies were constructed by the strong electrostatic interactions of calixpyridinium with phosphotungstic acid (PTA, H3PW12O40), silicotungstic acid (STA, H4SiW12O40), and phosphomolybdic acid (PMA, H3PMo12O40). Interestingly, the three assemblies were all irregular, spherical nanoparticles but with quite different sizes. The smallest calixpyridinium–PTA nanoparticle assembly had the most excellent removal effect on organic dyes, such as crystal violet (CV), eriochrome black T (EBT), and congo red (CR) in the presence of H2O2 and light. This effect became worse and worse when using a larger and larger nanoparticle assembly from calixpyridinium–STA to calixpyridinium–PMA under the same conditions. In addition, the solid phosphoric calixpyridinium–PTA and calixpyridinium–PMA assemblies had a selective adsorption capacity for cationic organic dyes, such as CV, methylene blue (MB), and rhodamine B (RhB), under acidic conditions. This study not only gave a structure–function relationship of calixpyridinium–polyoxometalate organic–inorganic hybrid nanoparticle assemblies but also supplied an alternative green and convenient method for the removal of organic dyes from wastewater.