Comparative experimental and mathematical analysis on removal of dye using raw rice husk, rice husk charcoal and activated rice husk charcoal: batch, fixed-bed column, and mathematical modeling

A methodology involving rice husk (RH), rice husk charcoal (RHC), and activated rice husk charcoal (ARHC) as an effective adsorbent to eliminate malachite green dye from aqueous solution using adsorption process by varying the pH, initial dye concentration (mg/L), contact time, temperature, and adsorbent doses (g/L). The surface characteristics were observed under scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) spectroscopy was used to identify the chemical bonds present on the adsorbent surface. Data obtained from adsorption system at equilibrium condition were fitted to isotherm models such as Langmuir and Freundlich model where it was found that unlike raw RH and RHC equilibrium, data for ARHC fitted best to Langmuir model with 1000 mg/g as maximum adsorption capacity at 303 K, indicating homogeneous monolayer adsorption by the mesopores. The adsorption kinetics for all three adsorbents were observed to fit the pseudosecond-order model better. The negative value of ΔH and ΔS obtained from the thermodynamic study indicates that the process is exothermic and involves an associative mechanism respectively. Boyd plot was used to understand the mass transfer mechanism. Moreover, fixed-bed column studies were performed using ARHC as the adsorbent. At different experimental conditions such as inlet flow rate (Q), initial dye concentration (C0), bed height (Lb), and pH, it was observed that the adsorption capacity (kd*) of MG dye on ARHC in a fixed bed continuous flow in a column mode, ranged between 0.008 and 0.177 (g/g). Present investigation and comparison with other reported adsorbents concluded that RH, RHC, and ARHC can be used as efficient and environment-friendly adsorbents for the removal of dissolved malachite green from contaminated water. However, ARHC was proved to be the best-suited candidate among the three adsorbents.