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

Abstract

• Nanoparticles of α-FeOOH was synthesized and characterized using XRD, FTIR, SEM, TEM and DLS methods. • The Rod-like α-FeOOH Nanoparticles was used for removal of Pb(II) and Indigo carmine from aqueous solution in batch adsorption mode. • The optimum adsorption of Pb(II) occurred at pH 10 in 180 min agitation with initial concentration of 800 mg/L. • The optimum adsorption of indigo carmine occurred at pH 4 in 150 min agitation with initial concentration of 50 mg/L. • The adsorption capacity of α-FeOOH Nanoparticles for Pb(II) was 287.98 mg/g while indigo carmine was 18.99 mg/g. • Pseudo-second order kinetics and intraparticle diffusin models fitted best and the adsorption processes were spontaneous and endothermic in nature. Nanoparticles (NPs) of α-FeOOH were synthesized by hydrothermal assisted method. X-Ray Diffraction (XRD) result showed that single-crystalline orthorhombic α-FeOOH was formed, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) images showed that the synthesized α-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 α-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 (ΔG°) obtained were negative indicating spontaneous adsorption processes, while positive values of ΔH (+107.08 and +102.15 KJ/mol) and ΔS (+0.361 and +0.341 KJ/mol) for Pb(II) and IDC respectively, showed endothermic and randomness at the NPs of α-FeOOH-aqueous interface during the adsorption processes. The activation energy, E a , were found to be +3.307 and +6.552 KJ/mol for adsorption of Pb(II) and IDC respectively, showing that adsorption onto α-FeOOH NPs adsorbent were physical processes with external diffusion as the rate determining step.