Preparation and Chemical Modification of Rice Husk Char for the Removal of a Toxic Dye (Orange G) from Aqueous Medium

Abstract

Abstract The rice husk char (RHC) was prepared by keeping a known amount of the rice husk in furnace at 400°C. The product was modified with KOH and labeled as KOH modified rice husk char (KMRHC) which was used as an adsorbent for the removal of toxic dye, Orange G (OG) from aqueous media. Variation in the experimental conditions (agitation time, dye concentration, adsorbent dose, pH and temperature) play significant role in the adsorption process. The maximum adsorption capacity of OG on KMRHC was investigated as 38.8 mg/g at pH=4 using initial dye concentrations of 80 mg/L containing 2 g/L of the adsorbent dose with agitation speed of 250 rpm at 303 K. The % adsorption of dye was inspected as 96%. Thermodynamics studies of the adsorption of OG on KMRHC indicated that the value of ΔG and ΔH were negative which revealed that the adsorption process is exothermic and spontaneous process. The negative value of ΔS suggested that randomness decreases at the interface of adsorbent–adsorbate during the adsorption process. The kinetics study indicated that the experimental data of the adsorption process best fits to pseudo-second order kinetic model. The equilibrium data was tested on Langmuir, Freundlich and Temkin adsorption isotherm models. It was inspected that data follows all the three isotherm models (R2>0.91). However, the values of correlation coefficients (R2) indicated that the data is best fit to the Langmuir isotherm model (R2>0.99) which suggest for chemi-sorption process. The effect of temperature (303–343 K) shows that by varying the temperature the adsorption process is significantly affected. The general trend indicates that adsorption efficiency is higher at lower temperature as compared to higher temperature. This trend also suggests that the adsorption coefficient (K), rate of adsorption, and hence the spontaneity of adsorption process also decreases with raising the temperature.