nZVI based nanocomposites: Role of noble metal and clay support on chemisorptive removal of Cr(VI)

The reductive removal of Cr(VI) is investigated with zero-valent iron, Fe–Ni bimetallic nanoparticles and Fe–Ni bimetallic-montmorillonite nanocomposites. XRD and TEM studies reveal generation of active sites on nanocomposites possessing increased surface area.The removal of Cr(VI) follows pseudo-second order rate model with 2 g L−1 composite loading with sorption capacity (qe) in the range of 30–50 mg g−1 for the composites. Employing adsorption isotherms like Langmuir, Freundlich, Redlich–Peterson, Dubinin–Radushkevich (D–R), Temkin and Flory–Huggins (F–H), it is observed that adsorption process essentially follows pseudo-multilayer exothermic chemisorption process with free energy of adsorption (DG°) in the range of −10 to −15 KJ mol−1. Pore diffusion is predominant as compared to film diffusion process; evaluated from intra-particle diffusion models, augurs well for stronger ionic interactions between Cr(VI) ions and adsorbents. The improved efficiency of composites may be attributed to the large number of available surface Fe0 atoms that significantly contributes towards reduction of adsorbed Cr(VI) on the surface. XPS measurements of composites after last cycle clearly establish the fact that formation of surface hydroxides mediates efficient flow of electron from bulk to Cr(VI) suggesting their potential usage for continuous removal capabilities.