Nitrophenols removal from aqueous solutions by activated carbon – temperature effect of adsorption kinetics and equilibrium

Abstract The temperature dependences of adsorption equilibrium and kinetics were measured and analyzed for 2-, 3-, 4-nitrophenol removed from aqueous solutions by activated carbon. The structural and surface characteristics of the applied adsorbent were estimated by nitrogen adsorption/desorption and potentiometric titration. The adsorption equilibrium measurements were carried out at 278, 298 and 318 K. High sorption capacities were obtained, reaching even about 11, 5 and 6 mmol/g for 2-, 3- and 4-nitrophenol, respectively. The adsorption kinetics were measured at 278, 288, 298, 308, 318 and 328 K. An increase in the adsorption rate and amount was observed with increasing temperature. For analysis of the equilibrium experimental data the General Langmuir isotherm was applied, in the case of kinetic data various equations and models including first-, second-, mixed-order, their fractal analogus, and multi-exponential equations, and diffusion models were used. The thermodynamic analysis of adsorption isotherms was conducted. The surface functional groups of activated carbon and the nitrophenol adsorption mechanism, were studied by using XPS technique and the thermal analysis, moreover, the thermal stability of adsorbate-adsorbent systems was determined. The thermal analysis and XPS revealed weaker interactions of 2-nitrophenol molecules with carbon surface as a result of formation of the intramolecular hydrogen bonds, on the contrary to 3- and 4-nitrophenol which could stronger interact with the adsorbent surface groups.