Potential of waste pumice and surface modified pumice for hexavalent chromium removal: Characterization, equilibrium, thermodynamic and kinetic study

The sorption potential of natural (NP) and surface modified pumice using MgCl2 (MGMP) as an abundant and low cost geo-material for the removal of Cr(VI) ion was investigated. The influence of contact time, solution pH, initial metal concentration, amount of absorbents and solution temperature was studied. Natural and modified adsorbents were characterized by means of XRD, XRF, SEM and FTIR technologies. Maximum sorption was observed at pH 1 and 100 mg/L metal concentration. Equilibrium data were accurately fitted onto Langmuir, Freundlich and Temkin isotherms, showing the heterogeneous nature of the adsorbents; maximum sorption capacity according to the Langmuir isotherm were 87.72 mg/g and 105.43 mg/g for NP and MGMP, respectively, showing a high sorption potential if compared to adsorbents used for Cr(VI) removal. Intra-particle model demonstrated that film diffusion was the rate-limiting step instead of intra-particle diffusion, as confirmed from the analysis of pseudo-second order rate constants, showing an absence of limitation due to pore diffusion. Relevance of pumice was confirmed since high regeneration yields were obtained, 94.3% in acidic conditions (1 M HCl) for spent non-modified pumice and 91.3% in alkaline conditions (4 M NaOH) for spent modified pumice.