Removal of 4-nitrophenol from aqueous solution by natural low-cost adsorbents

Abstract

7-25Use of adsorption isotherms, kinetics and thermodynamics for the removal of 4-nitrophenol (4-NP) from aqueous solutions by zeolite and bentonite has been investigated in a lab-scale batch study. Various parameters, such as adsorbent dosage, initial concentration of 4-NP, pH, operating temperature, and agitation time have been optimised for the maximum removal of 4-NP. With the optimized conditions (180 rpm agitation rate, 240 min agitation time, 7 pH, 298 K, 100 mg/L initial adsorbate concentration, and 5 g/L & 20 g/L for zeolite & bentonite dosages respectively), the removal percentages of 4-NP for natural zeolite and bentonite are found to be 51% and 68% respectively. The adsorption mechanisms and characteristic parameters for the present process are analyzed by four two-parameter (Langmuir, Freundlich, Temkin and Dubinin-Radushkevich), five three-parameter (Redlich–Peterson, Toth, Sips, Radke-Prausnitz and Khan) isotherm models, and six kinetic models (Lagergren’s pseudo-first order, Ho and McKay’s pseudo-second order, Elovich, intraparticle diffusion, Bangham and modified Freundlich) using the non-linear regression methodology. Among the two-parameter isotherms, the experimental data yield excellent fits with Langmuir and Freundlich isotherm equations for both adsorbents. Three-parameter isotherms show higher determination coefficients (R2 > 0.999) and lower residual and standard errors than two-parameter isotherms. Of the three-parameter isotherms, Toth and Khan isotherms are the best to describe the experimental data of the 4-NP-zeolite system. Results of the kinetic studies show that the best-fitted kinetic models are the modified Freundlich, the Elovich and the Ho and McKay’s pseudo-second order for both 4-NP-zeolite and 4-NP-bentonite systems. The results also indicate that three-parameter isotherms have no significant correlations for the 4-NP-bentonite system. Adsorption thermodynamics of 4-NP [equilibrium constant, standard free energy changes, standard enthalpy and entropy changes] on zeolite and bentonite are also studied at various temperatures of 298, 318 and 338 K. The values of enthalpy changes and entropy changes are found to be –4.43 and –7.77 kJ/mol, and 3.43 and −9.76 kJ/mol.K for zeolite and bentonite respectively. The thermodynamic analysis indicates that the present adsorption systems are found to be exothermic, and the adsorption of 4-NP on zeolite and bentonite is spontaneous and physical in nature. Findings of this study clearly show that these materials could be used as efficient adsorbents for the removal of 4-NP from aqueous solutions