Facile synthesis of rod-like alpha-FeOOH nanoparticles adsorbent and its mechanism of sorption of Pb(II) and indigo carmine in batch operation

Abstract

Nanoparticles (NPs) of alpha-FeOOH were synthesized by hydrothermal assisted method. X-Ray Diffraction (XRD) result showed that single-crystalline orthorhombic alpha-FeOOH was formed, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) images showed that the synthesized alpha-FeOOH nanoparticles were rod-like in shape, while Dynamic Light Scattering (DLS) study gave average nanoparticles sizes of 58.24 nm. The mechanism of batch adsorption of Pb(II) and IDC onto alpha-FeOOH NPs adsorbent were studied at various conditions of solution pH, concentration, adsorbent dose, temperature and time of agitation. The optimum adsorption capacity of Pb(II) and IDC at pH 10 and 4 were estimated as 287.98 and 18.99 mg/g at 313 and 318 K respectively. The Langmuir isotherm model fitted well with regression coefficient, R-2 = 0.9898 and 0.9966 for Pb (II) and IDC respectively. The pseudo-second-order kinetics best described the sorption processes of Pb(II) and IDC respectively, with R-2 = 0.9974 and 0.9861. However, intraparticle diffusion and pore filling mechanism were found to control the adsorption rates. The change in Gibbs free energy (Delta G degrees) obtained were negative indicating spontaneous adsorption processes, while positive values of Delta H (+107.08 and +102.15 KJ/mol) and Delta S (+0.361 and +0.341 KJ/mol) for Pb(II) and IDC respectively, showed endothermic and randomness at the NPs of alpha-FeOOH-aqueous interface during the adsorption processes. The activation energy, Ea, were found to be +3.307 and +6.552 KJ/mol for adsorption of Pb(II) and IDC respectively, showing that adsorption onto alpha-FeOOH NPs adsorbent were physical processes with external diffusion as the rate determining step.