Absorption of chlorophenols on granular activated carbon

Graduation date: 1994Presentation date: 1993-05-21Studies were undertaken of the adsorption of chlorinated phenols from aqueous solution on granular activated carbon (Filtrasorb-400, 30x40 mesh). Single-component equilibrium adsorption data on the eight compounds in two concentration ranges at pH 7.0 fit the Langmuir equation better than the Freundlich equation. The adsorptive capacities at pH 7.0 increase from pentachiorophenol to trichiorophenols and are fairly constant from trichiorophenols to monochlorophenols. Equilibrium studies were performed at various temperatures at pH 7.0. The adsorption process was found to be exothermic for pentachiorophenol and 2,4,6-trichiorophenol, and endothermic for 2,4-dichlorophenol and 4-chlorophenol.\ud Equilibrium measurements were also conducted for 2,4,5-trichiorophenol, 2,4-dichlorophenol, and 4-chlorophenol over a wide pH range. A surface complexation model\ud was proposed to describe the effect of pH on adsorption equilibria of chlorophenols on\ud activated carbon. Activated carbon surface functional sites are divided into acidic groups and basic groups with which molecular and ionized forms of chiorophenols interact,\ud respectively, to form two neutral surface complexes, of which the complex of basic\ud groups is more substantial and more stable than that of acidic groups. The simulations of the model are in excellent agreement with the experimental data. Batch kinetics studies were conducted of the adsorption of chlorinated phenols on granular activated carbon. The external film diffusion model, linear-driving-force approximation, and surface reaction kinetics model were employed to fit the adsorption kinetics data of chlorophenols. The results show that the surface reaction model best describes both the short-term and long-term kinetics, while the external film diffusion model describes the short-term kinetics data very well and the linear-driving-force approximation improved its performance for the long-term kinetics. The mass transfer coefficient was found to increase from more chlorinated compounds to less chlorinated compounds. Two-component adsorption kinetics experiments revealed that the adsorption\ud of chlorophenols on activated carbon is to some extent an irreversible process and non-ideal competition between two components exists. Multicomponent adsorption equilibria of chiorophenols on granular activated\ud carbon was investigated in the micromolar equilibrium concentration range. The Langmuir\ud competitive and Ideal Adsorbed Solution (IAS) models were tested for their performances on the three binary systems of pentachlorophenol/2,4,6-trichlorophenol, 2,4,6-trichlorophenol/2,4-dichlorophenol, and 2,4-dichlorophenol/4-chlorophenol, and the tertiary system of 2,4,6-trichlorophenol/2,4-dichlorophenol/4-chlorophenol, and found to fail to predict the two-component adsorption equilibria of the former two binary systems and the tertiary system. A new prediction method and a modification of the IAS model, both based on thermodynamic considerations, were proposed. The required parameters are the single-component Langmuir isotherm constants and initial concentrations for each\ud component The proposed new method and the modification of the IAS model were found\ud to significantly improve the accuracy of the predictions of two-component adsorption\ud equilibria of chlorophenols. This new method also performs much better than the\ud Langmuir competitive and IAS models in the three-component system. Studies were also conducted on the desorption equilibria and kinetics of chlorophenols. The results indicate that the Langmuir equation fits both adsorption and desorption equilibrium data better than the Freundlich equation and the degree of the irreversibility of adsorption increases from 4-chlorophenol to pentachlorophenol. A linear-driving-force desorption rate equation was proposed, which describes the desorption kinetic data very well. The desorption mass transfer coefficients were found to increase from pentachiorophenol to 4-chlorophenol