Enhanced adsorption of methylene blue by mixed-phase bismuth ferrite prepared by non-aqueous sol-gel route

Abstract

We report here a non-aqueous sol-gel synthesis of bismuth ferrite (BFO) particles in THF, DMF, and DMSO solvents, their subsequent utilization in adsorption of methylene blue, and a comparison of their adsorption efficiencies. BFO is attractive for adsorption because of its excellent dispersion in water. XRD data shows that all the solvents yield a mixture of a primary rhombohedral and a secondary orthorhombic phases with varying fractions. The DMSO yields the most impure samples having the highest fraction of secondary phase. However, the adsorption efficiencies of the samples order as DMF < THF < DMSO, with DMF showing 26.28 %, THF 51.63 %, and DMSO 96.54 %. A higher refractive index and donicity of DMSO leads to better solvation of the sol and incorporation of sulfur in the metal-oxide network during the sol-gel reaction. DMSO aids in columnar growth of orthorhombic phase, which, due to sulfur incorporation, forms a more open and less crystalline phase, which are the factors responsible for DMSO samples showing enhanced adsorption. Analysis of adsorption kinetics and isotherm on the DMSO samples confirms diffusive regimes pertaining to film-diffusion and intra-particle diffusion with the diffusive kinetics becoming faster at higher dye concentration. Fitting to Langmuir isotherm yields maximum monolayer coverage of 7.14 mg of dye per g of the adsorbent. The DMSO samples also show excellent adsorption at high pH conditions.