Sorption potential of rice husk for the removal of 2,4-dichlorophenol from aqueous solutions: Kinetic and thermodynamic investigations

The sorption potential of chemically and thermally treated rice husk (RHT) for the removal of 2,4-dichlorophenol (DCP) from aqueous solutions has been investigated. Sorption of DCP by rice husk was observed over a wide pH range of 1–10. The effect of contact time between liquid and solid phases, sorbent dose, pH, concentration of sorbate and temperature on the sorption of DCP onto rice husk has been studied. The pore area and average pore diameter of RHT by BET method are calculated to be 17 ± 0.6 m2 g−1 and 51.3 ± 1.5 nm, respectively. Maximum sorption (98 ± 1.2%) was achieved for RHT from 6.1 × 10−5 mol dm−3 of sorbate solution using 0.1 g of rice husk for 10 min agitation time at pH 6 and 303 K, which is comparable to activated carbon commercial (ACC) 96.6 ± 1.2%, but significantly higher than chemically treated rice husk (RHCT) 65 ± 1.6% and rice husk untreated (RHUT) 41 ± 2.3%. The sorption data obtained at optimized conditions was subjected to Freundlich, Langmuir and Dubinin–Radushkevich (D–R) isotherms. Sorption intensity 1/n (0.31 ± 0.01) and sorption capacity multilayer Cm (12.0 ± 1.6 mmol g−1) have been evaluated using Freundlich sorption isotherm, whereas the values of sorption capacity monolayer Q (0.96 ± 0.03 mmol g−1) and binding energy, b, (4.5 ± 1.0) × 104 dm3 mol−1 have been estimated by Langmuir isotherm. The Langmuir constant, b, was also used to calculate the dimensionless factor, RL, in the concentration range (0.6–6.1) × 10−4 mol dm−3, suggesting greater sorption at low concentration. D–R sorption isotherm was employed to calculate sorption capacity Xm (2.5 ± 0.07 mmolg−1) and sorption energy E (14.7 ± 0.13 kJ mol−1). Lagergren and Morris–Weber equations were employed to study kinetics of sorption process using 0.2 g of RHT, 25 cm3 of 0.61 × 10−4 mol dm−3 sorbate concentration at pH 6, giving values of first-order rate constant, k, and rate constant of intraparticle transport, Rid, (0.48 ± 0.04 min−1 and 6.8 ± 0.8 nmolg−1 min−1/2min−1/2, respectively) at 0.61 × 10−4 mol dm−3 solution concentration of DCP, 0.1 g RHT, pH 6 and 2–10 min of agitation time. For thermodynamic studies, sorption potential was examined over temperature range 283–323 K by employing 6.1 × 10−4 mol dm−3 solution concentration of DCP, 0.1 g RHT at pH 6 and 10 min of agitation time and values of ΔH (−25 ± 1 kJ mol−1), ΔS (−61 ± 4 J mol−1 K−1) and ΔG303K (−7.1 ± 0.09 kJ mol−1) were computed. The negative values of enthalpy, entropy, and free energy suggest that the sorption is exothermic, stable, and spontaneous in nature.