Removal of malachite green and toluidine blue dyes from aqueous solution using a clay-biochar composite of bentonite and sweet sorghum bagasse

Abstract

A clay-biochar composite was prepared through slow pyrolysis of a mixture of bentonite clay and sweet sorghum bagasse. The adsorption of cationic dyes, namely, toluidine blue (TB) and malachite green (MG), by the clay-biochar composite was investigated to determine the adsorption mechanism and its sorption potential. Bentonite clay, sweet sorghum biochar and the bentonite-biochar composite were characterized through X-ray fluorescence (XRF), scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) analyses. For the adsorption study, the effects of exposure time, initial dye concentration and temperature were investigated. The findings clearly illustrated the complex physico-chemical properties of the clay-biochar composite encompassing distinct features of bentonite clay and biochar, therefore confirming successful preparation. The adsorption of MG was found to occur on a heterogeneous surface as predicted by the Freundlich isotherm model, while the adsorption of TB occurred mostly at a monolayer surface described by the Langmuir isotherm model. The adsorption equilibrium data was best described by the pseudo-second order kinetic model for all adsorbents. The estimated adsorption capacity of the clay-biochar composite (12.1255 mg/g for MG and 9.9356 mg/g for TB) suggests improved adsorption capacity of the biochar after incorporation of clay. The thermodynamic study revealed that the adsorption of dyes was mostly a spontaneous and exothermic process. The adsorbent investigated in this study showed good potential for the removal of cationic dyes from aqueous solution and could be considered for the remediation of water polluted by industrial effluents