Adsorption of Humic Substances on Activated Carbon from Aqueous Solutions and Their Effect on the Removal of Cr(III) Ions M. A. Ferro-Garcı ´a, J. Rivera-Utrilla,* I. Bautista-Toledo, and C. Moreno-Castilla Departamento de Quı ´mica Inorga ´ nica, Grupo de Investigacio ´ n en Carbones, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain Received May 29, 1997. In Final Form: January 13, 1998 The adsorption of different compounds such as gallic, tannic, and humic acids on an activated carbon at 298 K has been studied. The capacity of this carbon to adsorb gallic acid is much higher than for the other two acids, which has been explained on the basis of both their molecular size and ionization degree. The influence of dissolved Cr(NO3)3 on the adsorption of these acids has been investigated. The capacity of the carbon to adsorb gallic and tannic acids in the presence of Cr(III) is slightly higher than in the absence of this metal. The variation of the surface properties of the carbon, both porous texture and charge, with the adsorption of these humic substances also has been tested. The porous texture changes have been studied from the N2 adsorption isotherms and the changes in the carbon surface charge from the pH drift tests. The results indicate that the adsorption of these acids on the activated carbon brings about, to a greater or lesser extent, both pore blockage and an increase in the negative surface charge of the carbon. Finally, changes in the amount of Cr(III) adsorbed on the carbon as a function of the concentration of each of these three acids have been studied. A large decrease in the Cr(III) uptake is observed when these acids are present at low concentrations due to the pore blockage effect of the acid adsorbed on the carbon surface. When the acid concentration increases, the Cr(III) uptake also increases due to interactions of the Cr(III) cations with the negatively charged unbound functional groups of the adsorbed acid. Introduction Humic substances are naturally occurring organic materials formed by the breakdown of animal and vegetable matter in the environment. They are, therefore, present in most surface water used for drinking water. These products of decaying vegetation are complex and are usually aromatic and acidic in nature, and gallic acid (3,4,5-trihydroxybenzoic acid) is considered to be repre- sentative of the kinds of compounds that are present in this decaying. Therefore, the actual organic matter in surface waters consists essentially of polymerized products such as fulvic, tannic, and humic acids. Thus, humic substances are high-molecular-weight, polyelectrolytic macromolecules. Molecular weights range from a few hundred for fulvic acid to tens of thousands for humic acid. 1-3 Special attention has been given to humic substances since about 1970, following the discovery of trihalo- methanes in water supplies. It is now generally believed that these suspected carcinogens can be formed in the presence of humic substances during the disinfection of raw municipal drinking water by chlorination. The formation of trihalomethanes can be, therefore, reduced by removing as much of the humic material as possible prior to chlorination. 4 The classical drinking water treatment processes of sand filtration, settling, and coagulation remove between 20 and 50% of the humic substances present in water. Humic acids are generally preferentially removed, leaving the smaller, more highly charged tannic and fulvic acids in solution. 5,6 Granular activated carbon is used as a final polishing step in drinking water treatment. Activated carbon filters are generally used for removing compounds that are not always present in the water at high concentrations (algae toxins, pesticides, tastes and odors, and industrial mi- cropollutants). However, due to its nature, activated carbon also removes other constituents, such as dissolved humic substances. The surface of the activated carbon and, therefore, its adsorption properties will change as different species are adsorbed, and highly charged species such as humic compounds are of particular interest since they can drastically change the surface properties, in general, and surface charge, in particular. 7-9 Humic substances may also affect the adsorption of chemical species, mainly metal ions, on activated carbons by forming complexes with these ions. 4 The binding of metal ions by humic substances is one of the most important environmental qualities of these substances. This binding can occur as chelation between a carboxyl group and a phenolic hydroxyl group, as chelation between two carboxyl groups, or as complexation with a carboxyl group. * Corresponding author. (1) Choudry, G. G. Humic SubstancessStructural, Photophysical, Photochemical and Free Radical Aspects of Interactions with Environ- mental Chemicals; Gordon and Breach: New York, 1984. (2) Hedges, J. I. In Humic Substances and Their Role in the Environment; Frimmel, F. H., Christman, R. F., Eds.; Wiley: New York, 1988. (3) Bruchet, A.; Anselme, C.; Duquet, J. P.; Mallevialle, J. In Aquatic Humic Substances: Influence on Fate and Treatment of Pollutants; Suffet, I. H., McCarthy, P., Eds.; American Chemical Society: Wash- ington, DC, 1989. (4) Manahan, S. E. Fundamentals of Environmental Chemistry; Lewis: MI, 1993. (5) Collins, M. R.; Amy, G. L.; Steelink, C. Environ. Sci. Technol. 1986, 20, 1028. (6) Amy, G. L.; Sierka, R. A.; Bedessem, J.; Price, D.; Tan, L. J. Am. Water Works Assoc. 1992, 84, 67. (7) La France, P.; Mazet, M. J. Am. Water Works Assoc. 1989, 4, 155. (8) La France, P.; Mazet, M. Water Res. 1985, 19, 1059. (9) Newcombe, G.; Drikas, M. Water Res. 1993, 27, 161. 1880 Langmuir 1998, 14, 1880-1886 S0743-7463(97)00565-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/12/1998