Abstract
Eucalyptus camdulensis biochar (Ec-bio) was used to adsorb crystal violet (CV) and methylene blue (MB) dyes, which was optimized and further evaluated using different isotherm and kinetic models. Microscopy and spectroscopy techniques showed the interactions of the dyes with the surface functional groups of the Ec-bio, resulting in the removal of the dyes from aqueous solution. Both dyes were immediately uptaken, with equilibrium reached in 60 min, with a higher sorption efficiency of CV compared to MB. Thermodynamic parameters showed endothermic adsorption and the nonspontaneous adsorption of both dyes onto the Ec-bio. Both the adsorption capacity and percentage removal increased with the increasing solution pH from 2.0 to 4.0 and to 10 for CV and MB. An increase in adsorption capacity was observed upon increasing the initial concentrations, with a corresponding decrease in the percentage removal. The pseudo-second-order (PSO) and Elovich kinetic models (nonlinear approach) were a good fit to the data of both dyes, confirming a chemisorptive adsorption process. The Langmuir isotherm fitted well to the CV data, supporting its monolayer adsorption onto the Ec-bio, while the Freundlich isotherm was a good fit to the MB dye data, suggesting the surface heterogeneity of the Ec-bio. The Dubinin-Radushkevich isotherm was a good fit to the adsorption CV data compared with the MB dye, suggesting the physisorption of both dyes onto the Ec-bio due to its mean free energy of adsorption of <8 kJ mol(-1).