Kinetic and thermodynamic modeling for the removal of Direct Red-31 and Direct Orange-26 dyes from aqueous solutions by rice husk

The present study deals with the utilization of an inexpensive agro-industrial waste rice husk for the removal of Direct Red-31 and Direct Orange-26 from aqueous solutions in a batch mode. Experiments were carried out as function of pH, biosorbent dose, particle size of biosorbent, initial dyes concentration, contact time and temperature. The equilibrium biosorption data were analyzed by Langmuir, Freundlich, Temkin, Doubinin–Radushevich (D–R) and Harkins–Jura isotherm models. The results indicated that the Langmuir model provided the best correlation of the experimental data for both dyes. The biosorption kinetic data were modeled using the pseudo-first-order, pseudo-second-order and intra-particle diffusion kinetic equations. It was observed that the pseudo-second-order kinetic equation could explain the biosorption kinetics of dyes on rice husk. Thermodynamic parameters such as enthalpy change (ΔH°), entropy change (ΔS°) and free energy change (ΔG°) were also investigated. Free energy change showed that biosorption of Direct Red-31 and Direct Orange-26 was spontaneous at all studied temperatures (30–70 °C). Surface adsorption of both the dyes at specific binding sites was confirmed through and Fourier Transform infrared spectroscopic (FT-IR) analysis. The changes in surface morphology of rice husk before and after adsorption was investigated through scanning electron microscopy (SEM). It was implied that rice husk may be suitable as an adsorbent for removal of direct dyes from aqueous solutions.

Research Highlights
► We used low cost rice husk for the removal of direct dyes from aqueous solution.
► pH of the medium strongly affected the removal of direct dyes.
► Langmuir model provided the best correlation of the experimental data dyes.
► Thermodynamics showed that removal of dyes was spontaneous at studied temperature.
► Results indicated that optimum removal of dyes depends on process parameters.