Adsorption of Humic Substances on Activated Carbon from Aqueous Solutions and Their Effect on the Removal of Cr(III) Ions

Transcription

1 1880 Langmuir 1998, 14, Adsorption of Humic Substances on Activated Carbon from Aqueous Solutions and Their Effect on the Removal of Cr(III) Ions M. A. Ferro-García, J. Rivera-Utrilla,* I. Bautista-Toledo, and C. Moreno-Castilla Departamento de Química Inorgánica, Grupo de Investigación en Carbones, Facultad de Ciencias, Universidad de Granada, Granada, Spain Received May 29, 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(NO 3) 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 N 2 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 representative 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 trihalomethanes 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 * Corresponding author. (1) Choudry, G. G. Humic SubstancessStructural, Photophysical, Photochemical and Free Radical Aspects of Interactions with Environmental Chemicals; Gordon and Breach: New York, (2) Hedges, J. I. In Humic Substances and Their Role in the Environment; Frimmel, F. H., Christman, R. F., Eds.; Wiley: New York, (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: Washington, DC, 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 micropollutants). 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. (4) Manahan, S. E. Fundamentals of Environmental Chemistry; Lewis: MI, (5) Collins, M. R.; Amy, G. L.; Steelink, C. Environ. Sci. Technol. 1986, 20, (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, (9) Newcombe, G.; Drikas, M. Water Res. 1993, 27, 161. S (97) CCC: $ American Chemical Society Published on Web 03/12/1998

2 Adsorption of Humic Substances on Activated C Langmuir, Vol. 14, No. 7, In a previous paper, 10 the adsorption process of Cr ions on activated carbons was studied and the presence of humic acid in solutions was found to decrease the Cr(III) uptake mainly when humic acid is in low concentrations. As a hypothesis, this decrease was explained as due to a blockage effect of the adsorbed humic acid on the surface of the carbon. The aim of the present paper is to gain insight into the effect of humic compounds on the adsorption of Cr(III) by activated carbons, studying the changes of the activated carbon surface (charge and porous texture) owing to the adsorption of humic substances such as gallic, tannic, and humic acids. The capacity of the activated carbon to adsorb these acids both in the absence and presence of Cr(III) will be also studied. Experimental Section An activated carbon supplied by Merck (sample M) was used in this study. This carbon was characterized by N 2 adsorption at 77 K. From the data obtained the apparent surface area (S N2 ) was calculated by applying the BET equation. Mercury porosimetry up to 4200 kg cm -2 was carried out in a Quantachrome Autoscan 60 porosimeter. The mercury and water densities of this sample were also determined. The ph of the point of zero charge, ph PZC, of the activated carbon was given in a previous paper 10 and was equal to 7.5. Other characteristics of this carbon such as ash content and elemental analysis were assessed. The values obtained were ash content, 1.7%; C, 82.1%; H, 0.9%; N, 1.0%; and O (by difference), 16%. Details of the experimental procedure followed in all these experiments are given elsewhere. 10 The particle size used in all experiments was between 0.15 and 0.25 mm. The adsorption of the different compounds, gallic acid (GA) [(HO) 3C 6H 2CO 2H], tannic acid (TA) [C 76H 52O 46], and humic acid (HA), on the activated carbon was carried out at 298 K. For this purpose, aqueous solutions with different initial known concentrations were used. Adsorption isotherms were determined using stoppered flasks containing 0.2 g of carbon/100 cm 3 of solution. They were kept in a thermostat shaker bath at 298 K for 7 days, and then the equilibrium concentration was determined spectrophotometrically at the maximum absorbance wavelength (gallic acid, nm; tannic acid, 275 nm; humic acid, 260 nm). The samples obtained after saturating the activated carbon M with GA, TA, and HA will be denominated in the text as M-GA, M-TA, and M-HA, respectively. The adsorption isotherms of the above acids were also obtained in the presence of Cr(III) ions from Cr(NO 3) 3 9H 2O. The concentration of Cr ions in the solutions was 40 mg L -1, which was sufficient to saturate the adsorption capacity of the activated carbon, according to the results reported in a previous paper. 10 The chromium concentration of the solutions was determined by atomic absorption spectrophotometry. All adsoption isotherms were determined without adding any buffer to control the ph to prevent introduction of any new electrolyte into the system. However, in the concentration measurements, the molar absorptivity changes with ph and the presence of Cr(III) were taken into account. The ph drift tests of the four samples (M, M-GA, M-TA, and M-HA) were carried out as follows: 50 cm 3 of 0.01 M NaCl solution was placed in a jacketed titration vessel (298 K). N 2 was bubbled through the solution to stabilize the ph by preventing the dissolution of CO 2. The ph was adjusted to a value between 3 and 9 by addition of HCl or NaOH. The activated carbon (0.15 g) was added to the solution, and after 3 h, the final ph was measured and plotted against the initial ph. As reported elsewhere, 11,12 the ph at which the curve crossed the line, ph initial ) ph final, was taken as the point of zero charge, ph PZC. (10) Bautista-Toledo, I.; Rivera-Utrilla, J.; Ferro-García, M. A.; Moreno Castilla, C. Carbon 1994, 32, 93. (11) Sontheimer, H.; Crittenden, J. C.; Summers, R. S. Activated Carbon for Water Treatment. DVGW Forschungsstelle Engler-Bunte Institut, Karlsruhe, (12) Newcombe, G.; Hayes, R.; Drikas, M. Colloids Surf. A 1993, 78, 65. Table 1. Characteristics of the Activated Carbon Sample a sample ph pzc m 2 g -1 cm 3 g -1 cm 3 g -1 S N2, V 2, V 3, V H2O, cm 3 g -1 M a ph pzc) ph of the point of zero charge. S N2 ) Surface area as determined by N 2 adsorption. V 2 ) pore volume contained in pores in the diameter range nm, as determined by mercury porosimetry. V 3 ) pore volume contained in pores greater than 50.0 nm in diameter, as determined by mercury porsimetry. V H2O ) pore volume accessible to water, as determined from the densities of the carbon obtained by using both mercury and water Table 2. Results Obtained from the Langmuir Equation Applied to the Adsorption Isotherms of GA, TA, and HA on Activated Carbon M, Both in the Absence and Presence of Cr (III) Ions adsorbate X m,mg g -1 B,L mg -1 BX m,l g -1 GA TA HA GA + Cr(III) TA + Cr(III) To study the textural changes produced on the activated carbon surface after adsorption of the different acids, the N 2 adsorption isotherms on the saturated carbon (samples M-GA, M-TA, and M-HA) at 77 K were obtained. The effects of GA, TA, and HA on Cr(III) uptake by the activated carbon were tested as follows: solutions containing different concentrations of these acids, between 5 and 100 mg L -1, activated carbon (0.1 g), and Cr(III) ions (concentration ) 40 mg L -1 ) were prepared and kept in a thermostated shaker bath at 298 K for 7 days, and then the Cr(III) concentration was measured, and the amounts of Cr adsorbed by the carbon were calculated and plotted as a function of the acid concentration. Electrophoretic measurements of the above systems both in the presence and absence of Cr(III) were made with a Zeta-Sizer IIc (from Malvern Instruments) designed to make rapid and accurate measurements of colloid electrophoretic mobility. All experiments were carried out at 298 K in a cylindrical cell. Experimental details of these experiments are given elsewhere. 13 Results and Discussion Table 1 summarizes some of the characteristics of the activated carbon used in this study. This is a carbon with a high surface area and high values of V 2, V 3, and V H2O. The ph PZC value, equal to 7.5, indicates that activated carbon M has a zero charge for ph ) 7.5; therefore, it shows a positive charge density on its surface for a solution ph < 7.5 and a negative one for a solution ph > 7.5. Adsorption isotherms of GA, TA, and HA on the activated carbon M are depicted in Figure 1. These isotherms belong to type L of the Giles classification, 14 which indicates that as more sites in the substrate are filled, it becomes increasingly difficult for the solute molecules to find an available vacant site. This could be either because the adsorbate molecules are more likely to be adsorbed flat or because there is no strong competition from the solvent. All experimental points of the isotherms were fitted by the Langmuir equation. From it, the adsorption capacity, X m, the constant, B, and the adsorbent-adsorbate relative affinity in the adsorption process, X m B, were obtained. All these data are listed in Table 2. The adsorption capacity of this carbon increases in the order HA < TA, GA, reaching a value of mg g -1 for GA. This change (13) López Ramón, M. V.; Moreno-Castilla, C.; Rivera-Utrilla, J.; Hidalgo-Alvarez, R. Carbon 1993, 31, 815. (14) Giles, C. H.; MacEwan, T. H.; Nakhwa, S. N.; Smith, D. J. Chem. Soc. 1960, 786, 3973.

3 1882 Langmuir, Vol. 14, No. 7, 1998 Ferro-García etal. Figure 1. Adsorption isotherms of GA (]), TA (4), and HA (O) at 298 K on activated carbon M. in X m is mainly due to the molecular size of these acids. Thus, GA, with a molecular weight of 170, has the lowest molecular size, whereas HA, with a molecular weight ranging from 500 to , has a diameter 15 between 1.2 and 4 nm. Therefore, since adsorption on a porous substance such as activated carbon may take place in pores of a diameter similar to that of the adsorbing species, the micropores of the activated carbon (less than 2 nm in diameter) should mostly be inaccessible to HA. The effect of the molecular size of the humic substance on adsorption by the activated carbon has been studied by Summers and Roberts. 16 The adsorption capacities of a wide range of molecular size fractions of humic substances display an inverse dependence on molecular size, when expressed on an adsorbent mass basis. Nevertheless, normalizing the amount adsorbed on the basis of the available adsorbent surface area accounts for the sizeexclusion behavior displayed by the molecular size fractions, resulting in convergence of their adsorption capacities. Another factor favoring the adsorption process of GA against TA and HA is the higher degree of ionization per unit weight of the former. Thus, at the ph values used to carry out the isotherms (lower than 7.5), the surface charge of the carbon was positive and, therefore, the adsorption process would be favored with the ionization degree of the adsorbates. The greater B value found for GA compared with those for TA and HA (Table 2) indicates stronger interactions between GA and the carbon surface than for the other two acids. These stronger interactions in the case of GA could be due to both its adsorption in the micropores of the activated carbon and its higher degree of ionization. The adsorption isotherms of GA and TA in the presence of Cr(III) (40 mg L -1 ) added to the solutions as Cr(NO 3 ) 3 9H 2 O were also obtained. It was not possible to obtain the adsorption isotherm of HA due to the precipitation of this compound for a concentration higher than 150 mg L -1 because of the solution acidity. The corresponding adsorption isotherms are depicted in Figure 2, and the (15) Gjessing, E. T. Physical and Chemical Characteristics of Aquatic Humus; Ann Arbor Science: Ann Arbor, MI, (16) Summers, R. S.; Roberts, P. V. J. Colloid Interface Sci. 1988, 122, 382. data determined from the Langmuir equation applied to these isotherms are also included in Table 2. The capacity of this carbon to adsorb GA and TA in the presence of Cr(III) is slightly higher than in the absence of this metal. The large increase in B corresponding to the adsorption of GA in the presence of Cr(III) is noteworthy, which indicates that, in this case, the adsorbent-adsorbate interactions and relative affinity in this adsorption process are very high. The effects of salts on activated carbon adsorption of organic compounds have been studied by many researchers They found that increasing the concentrations of salts generally enhances the adsorptive capacity of adsorbents for anionic organic molecules, while nonionized (uncharged) organic compounds are relatively unaffected by changes in salt concentration. Weber 20 observed that activated carbon had a much higher affinity for humic acid when adsorbed from tap water than from distilled water. The precise nature of the interactions responsible for this effect is a matter of speculation. The mechanisms proposed to explain the effects of salts on the adsorption of organic anions can be divided into three groups: (i) interactions in solution between salts and organic compounds to produce a change in the distribution of organic species present, influencing, therefore, the extent of adsorption; (ii) interactions between salts and adsorbed organics, resulting in alteration of the packing, spacing, or alignment of adsorbed molecules; and (iii) interactions between salts and the adsorbent to neutralize repulsive forces between adsorbate and adsorbent and perhaps to create favorable adsorption sites. 19 There are different publications concerning the effects of salts on the configurations of complex organic molecules which influence adsorption. 21,22 Several investigators have (17) Randtke, S. J.; Jepsen, C. P. J. Am. Water Works Assoc. 1981, 73, 411. (18) Ishizaki, C.; Cookson, J. T. In Chemistry of Water Supply, Treatment and Distribution; Rubin, A. J., Ed.; Ann Arbor Science: Ann Arbor, MI, (19) Randtke, S. J.; Jepsen, C. P. J. Am. Water Works Assoc. 1982, 3, 84. (20) Weber, W. J., Jr. In Activated Carbon Adsorption of Organics from the Aqueous Phase; Suffe, I. H., McGuire, M. J., Eds.; Ann Arbor Science: Ann Arbor, MI, 1980; Vol. 1.

4 Adsorption of Humic Substances on Activated C Langmuir, Vol. 14, No. 7, Figure 2. Adsorption isotherms of GA (]) and TA (4) at 298 K on activated carbon M in the presence of Cr(III). Figure 3. Adsorption isotherms of N 2 at 77 K on samples M (O), M-GA (]), M-TA (4), and M-HA (0). discussed the Fuoss effect, which is described as the coiling of a macromolecule as a result of the neutralization of its negatively charged functional groups, rendering the molecule both smaller and more hydrophobic. This effect would be expected to enhance adsorption of complex molecules as the molecules become smaller and more hydrophobic. Humic substances can interact with cations, 21,23 having a significant Fuoss effect. The formation of complex compounds between Cr(III) and GA, TA, and HA was studied in the same experimental conditions used for the isotherms depicted in Figure 2. Increasing amounts of GA, TA, and HA were added to solutions containing 40 mg L -1 of Cr(III), and their maximum absorbance wavelengths were determined. This (21) Schnitzer, M.; Khan, S. U. Humic Substances in the Environment; Marcel Decker: New York, (22) Ong, H. L.; Bisque, R. E. Soil Sci. 1968, 106, 220. (23) Stevenson, F. J. Soil Sci. 1977, 123, 10. wavelength was a constant value (GA ) nm, TA ) 275 nm, and HA ) 260 nm), up to a given amount of acid added (GA ) 300 mg L -1,TA) 150 mg L -1, and HA ) 100 mg L -1 ). Nevertheless, for higher amounts of the corresponding acids, the maximum absorbance wavelengths decreased, implying that some complex compounds were formed between Cr(III) and the corresponding acid. On the other hand, as the ionized functional groups of these acids are neutralized by complexation, the molecules become more hydrophobic, which enhances their adsorption on the carbon. This aspect will be discussed further in reference to data on the influence of GA, TA, and HA on the adsorption of Cr(III). As mentioned in the Introduction, one of the objectives of this work is to study the changes in the activated carbon surface properties, both porous texture and charge, after the adsorption of GA, TA, and HA. The surface characteristic changes were mainly studied from the N 2 adsorp-

5 1884 Langmuir, Vol. 14, No. 7, 1998 Ferro-García etal. Figure 4. Determination of the ph pzc of samples M (O), M-GA (]), M-TA (4), and M-HA (0) using the ph drift method. 11,12 Table 3. Characteristics of the Activated Carbon Samples Obained from the N 2 Isotherm at 77 K by Applying the BET Equation and r-method sample S N2,m 2 g -1 W 0,cm 3 g -1 S me, m 2 g -1 M M-GA M-TA M-HA tion data. Figure 3 shows the N 2 adsorption isotherms obtained for the fresh activated carbon M and the exhausted samples M-GA, M-TA, and M-HA. The N 2 adsorption isotherms corresponding to M, M-TA, and M-HA are practically coincident; however, in the case of M-GA, the N 2 adsorption isotherm is lowered to around 50%. To determine the surface area accessible to N 2 (S N2 ) the BET equation was used. The value of the mesopore surface, S me, was obtained from the slope of the linear part of the R plots and the micropore volume, W 0, from the intercept of this linear region. The R plots were constructed using the standard isotherm obtained on a heat-treated carbon from olive stones. 28 All these results are given in Table 3. The data in Table 3 indicate that the GA adsorbed on the M caused a considerable degree of pore blockage, which limits the accessibility of the N 2 molecules in such a manner that the total surface area, the micropore volume, and the mesopore surface were halved by the adsorption of GA. When HA was adsorbed, the pore blockage observed was at the mesopore level; thus, the value of S me for sample M-HA was slightly lower than that corresponding to sample M (fresh carbon). The greater degree of pore blockage when adsorbing GA should be due to the larger amounts of this acid adsorbed on the carbon and to its easier accessibility to the carbon s narrower porous structure because of its smaller size. (24) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic: London, (25) Sing, K. S. W. In Surface Area Determination; Everett, D. H., Otewill, R. H., Eds.; Butterworth: London, (26) Lecloux, A.; Pirard, J. P. J. Colloid Interface Sci. 1979, 70, 265. (27) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (28) Rodríguez-Reinoso, F.; Martín-Martínez, J. M.; Prado-Burguete, C.; McEnaney, B. J. Phys. Chem. 1987, 91, 515. Figure 5. (a) Relative Cr(III) uptake as a function of the GA (]), TA (4), HA (0) concentration. (b) Magnification of the low concentration range of acids. With regard to the changes in the carbon surface charge caused by the adsorption of the three acids, Figure 4 shows the ph drift tests 11,12 for the fresh carbon sample and for the saturated ones. The ph PZC of the carbon decreases when the acids are adsorbed on its surface; this decrease is much greater for GA, which has a value of 0.8, whereas that for M is 7.5. These results indicate that the negative

6 Adsorption of Humic Substances on Activated C Langmuir, Vol. 14, No. 7, Figure 6. Electrophoretic mobility of the activated carbon as a function of the acid concentration both in the absence of Cr(III) (GA, ]; TA, 4; HA, 0) and in the presence of Cr(III) (GA, [; TA, 2; HA, 9). surface charge of the carbon was enhanced when adsorbing these acids, and this increase varied in the order M-HA < M-TA, M-GA. Again, GA shows the highest effect due to its large ionization degree per unit weight and the larger amounts absorbed. The increase in the negative charge of a carbon could be of capital importance when it is used as an adsorbent in drinking water treatment, since this could enhance its adsorption capacity for positively charged pollutants such as metal cations. To check this, and according to the changes of the activated carbon surface owing to the adsorption of the acids mentioned above (Table 3 and Figure 4) in the present paper, the effect of GA, TA, and HA on the adsorption of Cr(III) on M has been studied. Figure 5 shows the change in the relative amount of Cr(III) adsorbed as a function of the amount of GA, TA, and HA added to the solution. X is the amount of chromium adsorbed per gram of carbon when each of the above acids is present in the solution at the indicated concentration and X m the adsorption capacity of the activated carbon for Cr(III) in the absence of these acids. As shown in the magnification of the low concentration range of acids in Figure 5, there is a large decrease in the X/X m value when these acids are present at very low concentrations (less than 3 mg L -1 ); this could be due to the pore blockage effect of the acid adsorbed on the surface of the carbon mentioned above. In the three cases, when the acid concentration increases, X/X m also increases due to the interactions Cr- (III) with the negatively charged unbound functional groups of the adsorbed acids. Morris and Newcombe 29 found that the negative surface charge of activated carbons increased with an increased surface concentration of adsorbed humic matter, although the degree of ionization of the adsorbed material was found to decrease with increasing surface concentration. This decrease was interpreted as an effect of the concentration of negative charge on the surface which would shift the ionization equilibrium of the adsorbed humic material compared with that in dilute solution. (29) Morris, G.; Newcombe, G. J. Colloid Interface Sci. 1993, 159, 413. Electrophoretic mobility experiments were performed in order to determine the surface charge of the carbon in solutions of GA, TA, and HA in the concentration range used in the experiments corresponding to Figure 5. Figure 6 shows the electrophoretic mobility of the particles as a function of the amount of acid added to solutions both in the presence and absence of Cr(III) and in the same experimental conditions used to carry out the experiments of Figure 5. In the absence of Cr(III), the charge density of the carbon particles is negative, and except for HA, this negative charge increases with the concentration of acid. When Cr(III) is present in the solutions, the charge density became positive as a result of the adsorption of this cation. With the exception of the GA solution, this charge decreases with the amount of acid. One must, however, bear in mind that microelectrophoresis only measures the potential at the plane of shear of the surface of the carbon particle, which corresponds to the external surface of the carbon rather than the internal surface corresponding to the micropores. Moreover, carbon particles with adsorbed compounds have a shear plane at a greater distance from the surface than fresh particles. Thus, whereas the ph drift measurements give the bulk surface charge, the electrophoretic mobility measurements only examine the external surface charge. The negative-charged unbound functional groups of the adsorbed acid, which in turn are responsible for the adsorption of Cr(III) by the carbon in the presence of GA, TA, and HA, are affected by the conformation of the adsorbed molecules. This conformation could be dependent on the surface concentration since reorientation is known to take place with an increase in surface coverage. 30,31 Newcombe found 32 that a change in orientation of adsorbed humic substances took place for a surface concentration of 4.5 mg of DOC g -1 (DOC ) dissolved organic carbon). The orientation of the adsorbed molecules at that low concentration led to ion-pair formation between carboxyl groups of the humic material and positive groups (30) Ash, S. G. In Colloid Science; Everett, D. H., Ed.; Chemical Society: London, 1973; Vol. 1. (31) Tjipangandjara, K. T.; Somasundaran, P. Colloids Surf. 1991, 55, 245. (32) Newcombe, G. J. Colloid Interface Sci. 1994, 164, 452.

7 1886 Langmuir, Vol. 14, No. 7, 1998 Ferro-García etal. on the surface. At higher surface concentrations, the reorientation of adsorbed molecules leads to greater electrostatic interaction between charged groups, possibly due to an increase in the proportion of the adsorbed molecules in tails extending into solution. This last orientation would favor the adsorption of Cr(III), explaining the results shown in Figure 5. Conclusions The capacity of the activated carbon used to adsorb humic substances increases in the order HA < TA, GA, which could be explained on the basis of their molecular size and degree of ionization. When the adsorption of GA and TA was carried out in the presence of Cr(III), the adsorption capacity was slightly higher than in the absence of this metal. The adsorbent-adsorbate interactions for GA in the presence of Cr(III) are very strong. When the activated carbon was saturated with GA, TA, and HA, its surface properties such as porous texture and charge were affected. Thus, both pore blockage and an increase in the negative surface charge of the carbon were detected. The degree of both effects was greater in the case of GA. The relative amount of Cr(III) adsorbed as a function of the amount of GA, TA, and HA present in the solution was tested. A large decrease in the amount of Cr(III) adsorbed is observed for very low concentrations of these acids, which could be due to the above blockage. When the acid concentration increased, the amount of Cr adsorbed was enhanced, owing to the interactions of the cations Cr(III) with the negatively charged unbound functional groups of the adsorbed acids. Acknowledgment. We thank DGICYT, Project No. PB , for financial support. LA970565H