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Adsorption of cadmium(II) onto goethite and kaolinite in the presence of benzene carboxylic acids
Colloids and Surfaces A: Physicochemical and Engineering Aspects 146 (1999) 243 – 251
Adsorption of cadmium(II) onto goethite and kaolinite in the presence of benzene carboxylic acids Michael J. Angove *, John D. Wells, Bruce B. Johnson La Trobe Uni6ersity, Bendigo, PO Box 199, Bendigo, Victoria 3552, Australia Received 29 June 1998; accepted 15 September 1998
Abstract The effect of benzene carboxylic acids on the adsorption of Cd(II) (5 ×10 − 5 M) by goethite and kaolinite has been studied in 0.005 M NaNO3 at 25°C. The concentrations of phthalic (benzene-1,2-dicarboxylic acid), hemimellitic (1,2,3), trimellitic (1,2,4), trimesic (1,3,5), pyromellitic (1,2,4,5) and mellitic (1,2,3,4,5,6) acids varied from 2.5 × 10 − 5 to 1 ×10 − 3 M. Mellitic acid complexes Cd(II) strongly above about pH 3, but the other acids only at higher pH, phthalic acid forming the weakest complexes. Phthalic, trimesic and mellitic acids adsorbed strongly to goethite at pH 3, but adsorption decreased at higher pH; however, mellitic acid was still about 50% adsorbed at pH 9, by which the other two were almost entirely in solution. At 10 − 3 M all the acids enhanced the adsorption of Cd(II) to goethite, the higher members of the series being the most effective. The higher members of the series suppressed Cd(II) adsorption onto kaolinite, but phthalic and trimesic acids caused slight enhancement. The effects of mellitic acid on Cd(II) adsorption depended strongly on its concentration. The maximum enhancement of Cd(II) adsorption onto goethite was at 10 − 4 M. The greatest suppression of Cd(II) adsorption onto kaolinite was at 10 − 3 M, and at 2.5 × 10 − 5 M mellitic acid enhanced Cd(II) adsorption onto kaolinite at intermediate pH. The results are interpreted in terms of complexation between metal and ligand (acid), metal and substrate, ligand and substrate, and the formation of ternary surface complexes in which the ligand acts as a bridge between the metal and the surface. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Cadmium; Kaolinite; Goethite; Carboxylic acid; Ligand; Complexation
1. Introduction In 1972 James and Healy [1 – 3] introduced the idea that complexation of metal ions by hydroxide ligands is an important aspect of their adsorption onto mineral surfaces. More recently interest has * Corresponding author. Tel.: +61-35-4447222; fax: + 6135-4447476.
developed in the effects of organic ligands on the adsorption of heavy metals. This has been prompted in part by the recognition that natural aquatic and soil systems contain a wide range of complexing ligands, from simple inorganic and organic species to macromolecular humic acids. In this paper we report the results of a study of the effects of benzene carboxylic acids on the adsorption of cadmium(II) onto goethite and
0927-7757/99/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 8 ) 0 0 7 9 9 - 7
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kaolinite. The lower members of the benzene carboxylic acid series may be regarded as analogues of aromatic carboxylic acid moieties of fulvic and humic acids. The behaviour of the higher members of the series helps to elucidate the processes which are occurring in the system. Adsorption of Cd(II) onto goethite and kaolinite has been the subject of several recent studies [4–12]. Cd(II) adsorption onto goethite is typical of heavy metal adsorption onto oxide surfaces in that the adsorption edges are sigmoid, and most of the metal is removed in a relatively narrow pH range. By contrast, Cd(II) adsorbs significantly to kaolinite at much lower pH, and under many conditions there is a step in the adsorption edge around pH 5 [4 – 6,12]. We have argued [4–6] that at low pH Cd(II) adsorbs to the faces of the kaolinite crystals which carry permanent negative charge because of isomorphous substitution. Above about pH 5 Cd(II) also adsorbs on variable-charge edge sites, which are similar to those of oxides and hydrous oxides. Complexing ligands can affect the adsorption of metal ions in several ways. First, soluble ligands may complete with surface sites for metal ions. By holding the metal ions in solution in the form of stable complexes the ligands can reduce the adsorption of metal to a substrate. For example, Ali and Dzombak [13] explained the suppression of calcium adsorption onto goethite by chelidamic acid in terms of the formation of non-sorbing complexes. Second, ligands may compete with metal ions for surface sites. A ligand which binds to the substrate may deny adsorption sites to metal ions, and hence diminish the extent to which the metal adsorbs. A third possibility is that the ligands may complex simultaneously with surface sites and metal ions, thereby binding the metal ions indirectly to the substrate. Ali and Dzombak [13] found that chelidamic acid enhanced the sorption of Cu(II) to goethite: in this case the strong complex between copper and the ligand formed at lower pH, where the ligand itself bound strongly to the surface. Lamy et al. [14] ascribed the enhancement of Cd(II) uptake by goethite in the presence of oxalic acid to adsorption of
Cd(II) via oxalic acid bridges. In each of these cases the effect of the complexing ligand on the adsorption of metal ions depends not only on the strengths of the interactions between the three principal species—metal ion, ligand, and surface sites—but also on the concentrations of these species. For example, suppose at low concentration a metal is partly adsorbed, occupying a small fraction of the available surface sites. If we now add a small amount of a ligand which binds strongly to both metal and substrate more metal will be adsorbed, held indirectly to the surface by the ligands. On the other hand if the ligand concentration becomes much higher than that needed to saturate the surface sites, so that most of the ligand is in solution, the greater part of the metal may be held in solution as soluble complexes, and hence the adsorption lessened. Finally we note that because anionic ligands tend to adsorb to mineral surfaces more strongly at low pH, whereas heavy metals adsorb more at high pH, the same ligand may enhance adsorption of a particular metal under some conditions but suppress it under others. Humic substances, and their effect on metal adsorption, have been received much attention in the recent past [15–18]. Some workers, Harding and Healy [17] for example, have studied the effect of organic materials on cation adsorption by following the fate of the cation. Others have attempted a detailed description of the interaction between organics and metals in solution [18], or have concentrated on the adsorption of humic materials in systems free from heavy metal ions [19–21]. Yet another approach has been to study low molecular weight analogues of humic acids, such as simple organic acids adsorbing to alumina [22] or goethite [23–25]. In this study we have investigated the effects of benzene carboxylic acids on the uptake of Cd(II) by goethite and kaolinite at 25°C. The acids investigated were phthalic (benzene-1,2-dicarboxylic acid), hemimellitic (1,2,3-tri), trimellitic (1,2,4-tri), trimesic (1,3,5-tri) pyromellitic (1,2,4,5-tetra), and mellitic (1,2,3,4,5,6-hexa) acids. We have also examined the complexation of Cd(II) in solution by the benzene carboxylic acids, and the adsorption of some of the acids to
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the substrates, to give a better basis for interpreting the effects of the acids on Cd(II) adsorption.
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3. Results
3.1. Cd(II) -carboxylate complexation 2. Experimental
2.1. Preparation and characterisation of substrates The kaolinite and goethite samples have been described previously [5,8]. The BET surface areas were 14.7 m2 g − 1 (kaolinite) and 49.6 m2 g − 1 (goethite). XRD analysis showed that both the goethite and kaolinite samples were contaminantfree.
2.2. Speciation experiments Complexation of Cd(II) by benzene carboxylic acids was studied by potentiometric titrations. H + and free Cd2 + were monitored by use of pH and cadmium-selective electrodes (Orion) connected to an Orion 720A voltmeter. The total Cd(II) concentration was 5× 10 − 5 M, and the carboxylic acid concentration either1 ×10 − 4 or 1× 10 − 3 M. The carboxylic acids (Aldrich Chemicals) had a purity of 99%. The apparatus was the same as that used for adsorption experiments.
The effect of the addition of benzene carboxylic acids on the concentration of free Cd2 + was investigated using 5×10 − 5 M Cd(II) and two carboxylate concentrations, 1× 10 − 4 M and 1× 10 − 3 M. The concentration of Cd2 + as a function of pH in the presence of each of the acids is shown in Fig. 1. Reducing the acid concentration to 1× 10 − 4 M had little impact on free Cd2 + (results not shown). Fig. 1 indicates that mellitic acid forms a very strong complex with Cd(II) between pH 3 and 5, with complexation decreasing for the other acids in the order, hemimellitic (1,2,3)\pyromellitic (1,2,4,5)] trimellitic (1,2,4)] trimesic (1,3,5)\ phthalic (1,2). Free Cd2 + concentration without the addition of acid is also shown in Fig. 1; the results reflect the formation of Cd-hydroxy complexes at high pH.
3.2. Carboxylic acid adsorption Adsorption edges for phthalic, trimesic and mellitic acids were measured on both goethite and
2.3. Adsorption edges Adsorption edge experiments were conducted as described by Angove et al. [5]. The supporting electrolyte was 0.005 M KNO3. Initial Cd(II) concentrations were 5× 10 − 5 M. Initial concentrations of the benzene carboxylic acids varied from 2.5×10 − 5 M to 1×10 − 3 M. Temperature was controlled by passing water from a constant temperature water bath around a jacketed reaction vessel. Samples taken during experiments were filtered through 0.22 mm polycarbonate filters (Poretics) before analysis. Residual Cd(II) was measured by flame atomic absorption spectrometry (Varian SpectrAA10). Organic acids were analysed by HPLC (Shimadzu LC-10AI, Vydac c 302IC4.6 anion exchange column, 0.005 M H2SO4 mobile phase). A photo diode array detector (Shimadzu SPD-M10AVP) was used at 230 nm.
Fig. 1. Percent free Cd2 + in the presence of various benzene carboxylic acids at 25°C. [acid] = 1 ×10 − 3 M, [Cd(II)]=5× 10 − 5 M.
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The results for Cd(II) adsorption onto kaolinite in the presence of 1 × 10 − 3 M benzene carboxylic acids, given in Fig. 4, show a very different trend. Here adsorption of Cd(II) was suppressed over much of the pH range with the effect of the acids decreasing in the order mellitic (1,2,3,4,5,6)\pyromellitic (1,2,4,5): hemimellitic (1,2,3)\ trimellitic (1,2,4). Phthalic acid, and to a lesser extent trimesic acid, caused slight enhancement of Cd(II) adsorption over much of the pH range.
3.4. Effect of organic acid concentration on Cd(II) adsorption
Fig. 2. 1 ×10 − 4 M acid adsorption onto goethite (closed symbols) and kaolinite (open symbols) at 25°C. () phthalic acid, () trimesic acid, ( ) mellitic acid.
kaolinite at an initial acid concentration of 1× 10 − 4 M. Fig. 2 indicates that more of each organic acid adsorbs on the goethite surface than on kaolinite, particularly at pH values below 6. Maximum adsorption onto goethite appears at about pH 3 for the three acids, while for kaolinite adsorption varies from about 10% at pH 3, with a maximum of ca.18% around pH 5 for mellitic and trimesic acids. The other feature of note is that much more mellitic acid remains adsorbed to goethite at pH values above 6 than either phthalic or trimesic acid.
The effects of benzene carboxylic acid concentration on Cd(II) adsorption onto both goethite and kaolinite are shown in Figs. 5 and 6 for the Cd-phthalic acid systems and Figs. 7 and 8 for the Cd-mellitic acid systems. Figs. 5 and 6 show that phthalic acid enhances adsorption onto goethite to a small extent, but has little effect on Cd(II) adsorption onto kaolinite irrespective of the organic acid concentration. For goethite, mellitic
3.3. Effect of benzene carboxylic acids on Cd(II) adsorption Fig. 3 shows adsorption edges for Cd(II) onto goethite in the presence of 1 × 10 − 3 M of each benzene carboxylic acid. The addition of the carboxylic acid shifted Cd(II) adsorption to lower pH compared with the carboxylic acid-free system. The enhancement in adsorption at a given pH depended on the acid, with the order being mellitic (1,2,3,4,5,6)\pyromellitic (1,2,4,5): trimesic (1,3,5)\trimellitic (1,2,4)\hemimellitic (1,2,3):phthalic (1,2).
Fig. 3. Cd(II) adsorption onto goethite in the presence of various benzene carboxylic acids at 25°C. Initial Cd(II) concentration was 5 ×10 − 5 M and acid concentration 1 ×10 − 3 M. () Cd(II) only, () phthalic acid, ( ) trimesic acid, (") trimellitic acid, () hemimellitic acid, () pyromellitic acid, ( ) mellitic acid.
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trimesic\ phthalic, providing an indication of the relative strengths of the complexes formed. For phthalic acid below pH 8 most of the Cd(II) was present in solution as free Cd2 + .
4.2. Carboxylic acid adsorption
Fig. 4. Cd(II) adsorption onto kaolinite in the presence of various benzene carboxylic acids at 25°C. Initial Cd(II) concentration was 5 ×10 − 5 M and acid concentration 1 ×10 − 3 M. () Cd(II) only, () phthalic acid, ( ) trimesic acid, (") trimellitic acid, () hemimellitic acid, () pyromellitic acid, ( ) mellitic acid.
acid enhances Cd(II) adsorption at all concentrations, with the greatest effect at a mellitic acid concentration of 1×10 − 4 M. For the kaolinite system, almost complete suppression of Cd(II) adsorption was found over the whole pH range when the mellitic acid concentration was 1× 10 − 3 M. Lower acid concentrations caused some enhancement of Cd(II) uptake by kaolinite at lower pH, but adsorption was still suppressed at higher pH values.
Fig. 2 shows that the benzene carboxylic acids have a higher affinity for the goethite surface than kaolinite. The adsorption edges for phthalic acid onto goethite found in this study are similar in shape and occur over a similar pH range to those found by Gu et al. [20] for phthalate adsorption onto iron oxide and by Ali and Dzombak [23] for phthalate adsorption onto goethite. The amount of phthalic or mellitic acid adsorbed to kaolinite was only about one fifth of that adsorbed to goethite. Zachara et al. [16] reported a similar trend for humic acid, with kaolinite adsorbing about half the humic acid that was adsorbed by goethite. Adsorption of polar organic materials to mineral surfaces has been attributed to interactions with hydroxyl species at the surface [23,24]. The lesser adsorption onto kaolinite compared with
4. Discussion
4.1. Cd(II) -benzene carboxylic acid complexation The data in Fig. 1 indicate that solution complexes between Cd(II) and mellitic acid form from pH 3, with virtually no free Cd2 + evident above 5. Results for the other acids studied show similar results, with the pH for the onset of Cd(II)-acid formation shifted to higher pH in the order mellitic \hemimellitic\ pyromellitic ]trimellitic:
Fig. 5. Cd(II) adsorption onto goethite in the presence of various concentrations of phthalic acid. Initial Cd(II) concentration was 5×10 − 5 M. () no added phthalic acid, ( ) 5 ×10 − 5 M acid, ( ) 1× 10 − 4 M, () 1×10 − 3 M ().
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Fig. 6. Cd(II) adsorption onto kaolinite in the presence of various concentrations of phthalic acid. Initial Cd(II) concentration was 5×10 − 5 M. () no added phthalic acid, ( ) 5 ×10 − 5 M acid, ( ) 1× 10 − 4 M, () 1× 10 − 3 M.
goethite may be due, at least in part, to the fact that the siloxane face of the kaolinite crystals is not hydroxylated, and therefore not available for organic adsorption. Fig. 2 also shows that the adsorption maxima for both substrates appear at or below pH 5, with adsorption decreasing to near zero at higher pH values, a feature that is typical of anion adsorption to a variety of substrates [27]. However, the results also indicate, especially for goethite, that the more substituted benzene carboxylic acids remain adsorbed to higher pH values.
and observed significant enhancement of adsorption at low pH. Similarly, Zachara et al. [16] found that Co(II) adsorption onto goethite and other minerals was enhanced by the presence of humic acids. At high ligand concentrations (1× 10 − 3 M) Cd(II) adsorption onto kaolinite (Fig. 4) was somewhat enhanced by phthalic and trimesic acids but was generally suppressed by the other ligands. Phthalic and trimesic acids show the weakest complexation of Cd(II) (Fig. 1). These results agree with previous studies of organicmetal-kaolinite systems, in which the effect of the organic ligand depended on factors such as the nature and concentration of the metal or ligand, and the ligand-metal ratio. Haas and Horowitz [26] found that while alignic and humic acids enhanced Cd(II) adsorption onto kaolinite, its uptake was diminished when EDTA was added. Zachara et al. [16] found that Co(II) adsorption was enhanced by the presence of humic material, while Frimmel and Huber [15] found that sorption of Cu(II) was suppressed, and that of Cd(II) and Pb(II) enhanced, in humic-kaolinite systems. Haas and Horowitz [26] concluded that, at mod-
4.3. Cd(II) adsorption in the presence of benzene carboxylic acids The enhancement of Cd(II) adsorption onto goethite in the presence of carboxylic acids (Fig. 3) generally reflects the findings of other workers who have studied a variety of organic-metal-mineral systems. Lamy et al. [14], for example, found that Cd adsorption increased when oxalic acid was added to a Cd-goethite system. Ali and Dzombak [13] studied the Cu(II)-goethite system in the presence of phthalic and chelidamic acids
Fig. 7. Cd(II) adsorption onto goethite in the presence of various concentrations of mellitic acid. Initial Cd(II) concentration was 5 ×10 − 5 M. () no added mellitic acid, ( ) 5 ×10 − 5 M acid, ( ) 1× 10 − 4 M, () 1×10 − 3 M.
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Fig. 8. Cd(II) adsorption onto kaolinite in the presence of various concentrations of mellitic acid. Initial Cd(II) concentration was 5 ×10 − 5 M. () no added mellitic acid, ( ) 5 ×10 − 5 M acid, ( ) 1× 10 − 4 M, () 1× 10 − 3 M.
erate to low ligand concentrations there was potential for adsorption to be enhanced where the ligand adsorbed to the substrate. Where the ligand was non-sorbing, as was the case for EDTA, adsorption was suppressed. The pH range for Cd-mellitic acid complex formation (Fig. 1) coincides with the pH at which Cd(II) adsorption begins in the presence of mellitic acid. In addition, from Fig. 2 we know that mellitic acid adsorbs strongly to goethite at low pH. It is therefore likely that the enhancement of Cd(II) adsorption onto goethite at low pH in the presence of mellitic involves the formation of a complex between Cd and adsorbed mellitic acid. Because similar results were also found for the other organic acids we conclude that adsorption via a ligand bridge is the most probable explanation for the increased uptake of Cd(II) by goethite in the presence of the six carboxylic acids studied. Mechanisms of this type have been suggested in the past [13 – 16,26]. The difference in the degree of enhancement appears to be related to the complexing strength of the particular ligand. Fig. 1 indicates that phthalic acid forms the weakest complexes with Cd(II), and may therefore be ex-
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pected to enhance Cd(II) adsorption the least. Fig. 3 shows this to be the case. However, the order of enhancement does not exactly follow the order of complexing strength, suggesting that another factor is also important. The ligands which have the weakest effect are phthalic and hemimellitic acid, both of which have carboxyl groups on adjacent carbons. This may inhibit the simultaneous interaction of the carboxylate groups with Cd(II) and the surface. The effect of benzene carboxylic acids on Cd(II) adsorption onto kaolinite can also be explained in terms of complexation and ligand adsorption. Fig. 2 shows that these ligands do not adsorb strongly to kaolinite. We note that the results shown in Fig. 4 are for ligand concentrations of 1 × 10 − 3 M, far in excess of that required to saturate the kaolinite surface. Because only a small fraction of the ligand is adsorbed to the kaolinite surface, most of the complexed Cd(II) will be held in solution and thus prevented from adsorbing. The order of suppression is again related to the order of complexing strength, with ligands that complex Cd(II) most strongly suppressing adsorption the most.
4.4. Effect of benzene carboxylic acid concentration Figs. 5–8 show the effect of varying the concentrations of phthalic and mellitic acid on Cd(II) adsorption. These two acids were chosen for more detailed study because phthalic was the weakest and mellitic acid the strongest complexer of Cd(II). Figs. 5 and 6 show that phthalic acid had little effect on Cd(II) uptake on either substrate, irrespective of the acid concentration. The fact that there is little enhancement in the goethite system is consistent with our observations that Cd(II)-phthalate complexes do not form to a significant extent until pH\ 8 (Fig. 1), which is beyond the pH at which Cd(II) adsorption normally occurs. In addition, phthalate begins to desorb from the mineral surfaces at pH values above 7. Thus, Cd(II) uptake in the phthalic acid systems is likely to involve adsorption of the cation directly to the goethite surface, with the ligand bridge mechanism playing, at most, a mi-
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nor role. The effect of phthalate on the Cd(II)kaolinite system is even smaller because less phthalate adsorbs to the kaolinite surface. Mellitic acid, on the other hand, strongly effects Cd(II) adsorption onto both substrates at all acid concentrations (Figs. 7 and 8), because mellitic acid complexes dominate Cd(II) speciation at pH\ 3. The maximum enhancement of Cd(II) adsorption onto goethite occurs at a mellitic acid concentration of 1× 10 − 4 M. From the mellitic acid adsorption edge (Fig. 2) we know that most of the acid is adsorbed to the goethite surface over the pH range from B3 to 8. Hence Cd(II)-mellitic acid complexation must occur almost exclusively at the goethite surface, with almost no competition from the small fraction of mellitic acid in solution. When the mellitic acid concentration is increased to 1×10 − 3 M, competition for Cd(II) complexation occurs between acid bound to the surface and that free in solution. Thus decreased adsorption of Cd(II) is found above pH 3.5 where solution complexes form (Fig. 1). At mellitic acid concentrations below 1 ×10 − 4 M virtually all of the ligand is adsorbed and Cd(II) uptake is probably dominated by complexation to adsorbed ligand. However, metal uptake directly to free goethite surface is also possible, so at the lower mellitic acid concentrations less enhancement is observed due to competition between surface bound mellitic acid and bare hydroxyl surface sites for free Cd(II). The effect of mellitic acid on Cd(II) uptake by kaolinite (Fig. 8) is also due to competition. At lower acid concentrations we see enhanced adsorption at low pH, while at the high concentration adsorption is almost completely suppressed. Fig. 2 indicates the amount of mellitic acid that adsorbs to the kaolinite surface is small ( : 2× 10 − 5 M) compared with goethite (:1× 10 − 4 M). Hence, when the initial concentration of mellitic acid is 1× 10 − 3 M most of the Cd(II) is complexed by mellitic acid in solution and strong suppression results. At mellitic acid concentrations where a significant fraction of the acid can adsorb (5 1 × 10 − 4 M) enhancement occurs through surface complexation of Cd(II) by adsorbed ligand. If, as suggested earlier, mellitic
acid does not adsorb to the unhydroxylated siloxane face of kaolinite, then the permanentnegatively charged sites on that face will also be available to adsorb free Cd(II). Hence at low mellitic acid concentrations and at pH values below 7, adsorption of Cd(II) is enhanced through a combination of complexation by adsorbed mellitic acid and interaction with permanent change sites. At high pH (\ 7) Fig. 2 shows that very little mellitic acid remains adsorbed with the result that suppression of adsorption occurs through competition for Cd(II) between surface sites and mellitic acid in solution.
5. Conclusion Cd(II) adsorption onto goethite is enhanced for all the benzene carboxylic acids studied, over a wide range of concentrations. However, high concentrations of some acids, especially mellitic acid, inhibit adsorption of Cd(II) onto kaolinite. Our results suggest that the effect of benzene carboxylic acids on the adsorption of Cd(II) onto goethite and kaolinite depends on extent to which they adsorb to the substrates, their ability to complex Cd(II), and the position of carboxyl groups around the benzene ring. Carboxylic acid adsorption is strongest at lower pH, with the adsorption maximum occurring below pH 3 for goethite and at about pH 5 for kaolinite; about five time as much acid is adsorbed to goethite compared with kaolinite. Acids that form strong complexes tend to have the greatest impact on Cd(II) uptake. Where adsorption is enhanced by the complexer, the formation of a surface complex via adsorbed ligand is likely. Suppression of adsorption occurs where there is a large solution concentration of the complexing ligand and relatively little adsorbed to the surface.
Acknowledgements Financial support was provided by the Australian Research Council Small Grants Scheme. MJA is the recipient of an Australian Postgradu-
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ate Award with stipend. Stephen Johnson (AMPC, Melbourne University) is gratefully acknowledged for determining the surface areas of the goethite sample.
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Adsorption of cadmium(II) onto goethite and kaolinite in the presence of benzene carboxylic acids Michael J. Angove *, John D. Wells, Bruce B. Johnson La Trobe Uni6ersity, Bendigo, PO Box 199, Bendigo, Victoria 3552, Australia Received 29 June 1998; accepted 15 September 1998
Abstract The effect of benzene carboxylic acids on the adsorption of Cd(II) (5 ×10 − 5 M) by goethite and kaolinite has been studied in 0.005 M NaNO3 at 25°C. The concentrations of phthalic (benzene-1,2-dicarboxylic acid), hemimellitic (1,2,3), trimellitic (1,2,4), trimesic (1,3,5), pyromellitic (1,2,4,5) and mellitic (1,2,3,4,5,6) acids varied from 2.5 × 10 − 5 to 1 ×10 − 3 M. Mellitic acid complexes Cd(II) strongly above about pH 3, but the other acids only at higher pH, phthalic acid forming the weakest complexes. Phthalic, trimesic and mellitic acids adsorbed strongly to goethite at pH 3, but adsorption decreased at higher pH; however, mellitic acid was still about 50% adsorbed at pH 9, by which the other two were almost entirely in solution. At 10 − 3 M all the acids enhanced the adsorption of Cd(II) to goethite, the higher members of the series being the most effective. The higher members of the series suppressed Cd(II) adsorption onto kaolinite, but phthalic and trimesic acids caused slight enhancement. The effects of mellitic acid on Cd(II) adsorption depended strongly on its concentration. The maximum enhancement of Cd(II) adsorption onto goethite was at 10 − 4 M. The greatest suppression of Cd(II) adsorption onto kaolinite was at 10 − 3 M, and at 2.5 × 10 − 5 M mellitic acid enhanced Cd(II) adsorption onto kaolinite at intermediate pH. The results are interpreted in terms of complexation between metal and ligand (acid), metal and substrate, ligand and substrate, and the formation of ternary surface complexes in which the ligand acts as a bridge between the metal and the surface. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Cadmium; Kaolinite; Goethite; Carboxylic acid; Ligand; Complexation
1. Introduction In 1972 James and Healy [1 – 3] introduced the idea that complexation of metal ions by hydroxide ligands is an important aspect of their adsorption onto mineral surfaces. More recently interest has * Corresponding author. Tel.: +61-35-4447222; fax: + 6135-4447476.
developed in the effects of organic ligands on the adsorption of heavy metals. This has been prompted in part by the recognition that natural aquatic and soil systems contain a wide range of complexing ligands, from simple inorganic and organic species to macromolecular humic acids. In this paper we report the results of a study of the effects of benzene carboxylic acids on the adsorption of cadmium(II) onto goethite and
0927-7757/99/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 8 ) 0 0 7 9 9 - 7
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kaolinite. The lower members of the benzene carboxylic acid series may be regarded as analogues of aromatic carboxylic acid moieties of fulvic and humic acids. The behaviour of the higher members of the series helps to elucidate the processes which are occurring in the system. Adsorption of Cd(II) onto goethite and kaolinite has been the subject of several recent studies [4–12]. Cd(II) adsorption onto goethite is typical of heavy metal adsorption onto oxide surfaces in that the adsorption edges are sigmoid, and most of the metal is removed in a relatively narrow pH range. By contrast, Cd(II) adsorbs significantly to kaolinite at much lower pH, and under many conditions there is a step in the adsorption edge around pH 5 [4 – 6,12]. We have argued [4–6] that at low pH Cd(II) adsorbs to the faces of the kaolinite crystals which carry permanent negative charge because of isomorphous substitution. Above about pH 5 Cd(II) also adsorbs on variable-charge edge sites, which are similar to those of oxides and hydrous oxides. Complexing ligands can affect the adsorption of metal ions in several ways. First, soluble ligands may complete with surface sites for metal ions. By holding the metal ions in solution in the form of stable complexes the ligands can reduce the adsorption of metal to a substrate. For example, Ali and Dzombak [13] explained the suppression of calcium adsorption onto goethite by chelidamic acid in terms of the formation of non-sorbing complexes. Second, ligands may compete with metal ions for surface sites. A ligand which binds to the substrate may deny adsorption sites to metal ions, and hence diminish the extent to which the metal adsorbs. A third possibility is that the ligands may complex simultaneously with surface sites and metal ions, thereby binding the metal ions indirectly to the substrate. Ali and Dzombak [13] found that chelidamic acid enhanced the sorption of Cu(II) to goethite: in this case the strong complex between copper and the ligand formed at lower pH, where the ligand itself bound strongly to the surface. Lamy et al. [14] ascribed the enhancement of Cd(II) uptake by goethite in the presence of oxalic acid to adsorption of
Cd(II) via oxalic acid bridges. In each of these cases the effect of the complexing ligand on the adsorption of metal ions depends not only on the strengths of the interactions between the three principal species—metal ion, ligand, and surface sites—but also on the concentrations of these species. For example, suppose at low concentration a metal is partly adsorbed, occupying a small fraction of the available surface sites. If we now add a small amount of a ligand which binds strongly to both metal and substrate more metal will be adsorbed, held indirectly to the surface by the ligands. On the other hand if the ligand concentration becomes much higher than that needed to saturate the surface sites, so that most of the ligand is in solution, the greater part of the metal may be held in solution as soluble complexes, and hence the adsorption lessened. Finally we note that because anionic ligands tend to adsorb to mineral surfaces more strongly at low pH, whereas heavy metals adsorb more at high pH, the same ligand may enhance adsorption of a particular metal under some conditions but suppress it under others. Humic substances, and their effect on metal adsorption, have been received much attention in the recent past [15–18]. Some workers, Harding and Healy [17] for example, have studied the effect of organic materials on cation adsorption by following the fate of the cation. Others have attempted a detailed description of the interaction between organics and metals in solution [18], or have concentrated on the adsorption of humic materials in systems free from heavy metal ions [19–21]. Yet another approach has been to study low molecular weight analogues of humic acids, such as simple organic acids adsorbing to alumina [22] or goethite [23–25]. In this study we have investigated the effects of benzene carboxylic acids on the uptake of Cd(II) by goethite and kaolinite at 25°C. The acids investigated were phthalic (benzene-1,2-dicarboxylic acid), hemimellitic (1,2,3-tri), trimellitic (1,2,4-tri), trimesic (1,3,5-tri) pyromellitic (1,2,4,5-tetra), and mellitic (1,2,3,4,5,6-hexa) acids. We have also examined the complexation of Cd(II) in solution by the benzene carboxylic acids, and the adsorption of some of the acids to
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the substrates, to give a better basis for interpreting the effects of the acids on Cd(II) adsorption.
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3. Results
3.1. Cd(II) -carboxylate complexation 2. Experimental
2.1. Preparation and characterisation of substrates The kaolinite and goethite samples have been described previously [5,8]. The BET surface areas were 14.7 m2 g − 1 (kaolinite) and 49.6 m2 g − 1 (goethite). XRD analysis showed that both the goethite and kaolinite samples were contaminantfree.
2.2. Speciation experiments Complexation of Cd(II) by benzene carboxylic acids was studied by potentiometric titrations. H + and free Cd2 + were monitored by use of pH and cadmium-selective electrodes (Orion) connected to an Orion 720A voltmeter. The total Cd(II) concentration was 5× 10 − 5 M, and the carboxylic acid concentration either1 ×10 − 4 or 1× 10 − 3 M. The carboxylic acids (Aldrich Chemicals) had a purity of 99%. The apparatus was the same as that used for adsorption experiments.
The effect of the addition of benzene carboxylic acids on the concentration of free Cd2 + was investigated using 5×10 − 5 M Cd(II) and two carboxylate concentrations, 1× 10 − 4 M and 1× 10 − 3 M. The concentration of Cd2 + as a function of pH in the presence of each of the acids is shown in Fig. 1. Reducing the acid concentration to 1× 10 − 4 M had little impact on free Cd2 + (results not shown). Fig. 1 indicates that mellitic acid forms a very strong complex with Cd(II) between pH 3 and 5, with complexation decreasing for the other acids in the order, hemimellitic (1,2,3)\pyromellitic (1,2,4,5)] trimellitic (1,2,4)] trimesic (1,3,5)\ phthalic (1,2). Free Cd2 + concentration without the addition of acid is also shown in Fig. 1; the results reflect the formation of Cd-hydroxy complexes at high pH.
3.2. Carboxylic acid adsorption Adsorption edges for phthalic, trimesic and mellitic acids were measured on both goethite and
2.3. Adsorption edges Adsorption edge experiments were conducted as described by Angove et al. [5]. The supporting electrolyte was 0.005 M KNO3. Initial Cd(II) concentrations were 5× 10 − 5 M. Initial concentrations of the benzene carboxylic acids varied from 2.5×10 − 5 M to 1×10 − 3 M. Temperature was controlled by passing water from a constant temperature water bath around a jacketed reaction vessel. Samples taken during experiments were filtered through 0.22 mm polycarbonate filters (Poretics) before analysis. Residual Cd(II) was measured by flame atomic absorption spectrometry (Varian SpectrAA10). Organic acids were analysed by HPLC (Shimadzu LC-10AI, Vydac c 302IC4.6 anion exchange column, 0.005 M H2SO4 mobile phase). A photo diode array detector (Shimadzu SPD-M10AVP) was used at 230 nm.
Fig. 1. Percent free Cd2 + in the presence of various benzene carboxylic acids at 25°C. [acid] = 1 ×10 − 3 M, [Cd(II)]=5× 10 − 5 M.
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The results for Cd(II) adsorption onto kaolinite in the presence of 1 × 10 − 3 M benzene carboxylic acids, given in Fig. 4, show a very different trend. Here adsorption of Cd(II) was suppressed over much of the pH range with the effect of the acids decreasing in the order mellitic (1,2,3,4,5,6)\pyromellitic (1,2,4,5): hemimellitic (1,2,3)\ trimellitic (1,2,4). Phthalic acid, and to a lesser extent trimesic acid, caused slight enhancement of Cd(II) adsorption over much of the pH range.
3.4. Effect of organic acid concentration on Cd(II) adsorption
Fig. 2. 1 ×10 − 4 M acid adsorption onto goethite (closed symbols) and kaolinite (open symbols) at 25°C. () phthalic acid, () trimesic acid, ( ) mellitic acid.
kaolinite at an initial acid concentration of 1× 10 − 4 M. Fig. 2 indicates that more of each organic acid adsorbs on the goethite surface than on kaolinite, particularly at pH values below 6. Maximum adsorption onto goethite appears at about pH 3 for the three acids, while for kaolinite adsorption varies from about 10% at pH 3, with a maximum of ca.18% around pH 5 for mellitic and trimesic acids. The other feature of note is that much more mellitic acid remains adsorbed to goethite at pH values above 6 than either phthalic or trimesic acid.
The effects of benzene carboxylic acid concentration on Cd(II) adsorption onto both goethite and kaolinite are shown in Figs. 5 and 6 for the Cd-phthalic acid systems and Figs. 7 and 8 for the Cd-mellitic acid systems. Figs. 5 and 6 show that phthalic acid enhances adsorption onto goethite to a small extent, but has little effect on Cd(II) adsorption onto kaolinite irrespective of the organic acid concentration. For goethite, mellitic
3.3. Effect of benzene carboxylic acids on Cd(II) adsorption Fig. 3 shows adsorption edges for Cd(II) onto goethite in the presence of 1 × 10 − 3 M of each benzene carboxylic acid. The addition of the carboxylic acid shifted Cd(II) adsorption to lower pH compared with the carboxylic acid-free system. The enhancement in adsorption at a given pH depended on the acid, with the order being mellitic (1,2,3,4,5,6)\pyromellitic (1,2,4,5): trimesic (1,3,5)\trimellitic (1,2,4)\hemimellitic (1,2,3):phthalic (1,2).
Fig. 3. Cd(II) adsorption onto goethite in the presence of various benzene carboxylic acids at 25°C. Initial Cd(II) concentration was 5 ×10 − 5 M and acid concentration 1 ×10 − 3 M. () Cd(II) only, () phthalic acid, ( ) trimesic acid, (") trimellitic acid, () hemimellitic acid, () pyromellitic acid, ( ) mellitic acid.
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trimesic\ phthalic, providing an indication of the relative strengths of the complexes formed. For phthalic acid below pH 8 most of the Cd(II) was present in solution as free Cd2 + .
4.2. Carboxylic acid adsorption
Fig. 4. Cd(II) adsorption onto kaolinite in the presence of various benzene carboxylic acids at 25°C. Initial Cd(II) concentration was 5 ×10 − 5 M and acid concentration 1 ×10 − 3 M. () Cd(II) only, () phthalic acid, ( ) trimesic acid, (") trimellitic acid, () hemimellitic acid, () pyromellitic acid, ( ) mellitic acid.
acid enhances Cd(II) adsorption at all concentrations, with the greatest effect at a mellitic acid concentration of 1×10 − 4 M. For the kaolinite system, almost complete suppression of Cd(II) adsorption was found over the whole pH range when the mellitic acid concentration was 1× 10 − 3 M. Lower acid concentrations caused some enhancement of Cd(II) uptake by kaolinite at lower pH, but adsorption was still suppressed at higher pH values.
Fig. 2 shows that the benzene carboxylic acids have a higher affinity for the goethite surface than kaolinite. The adsorption edges for phthalic acid onto goethite found in this study are similar in shape and occur over a similar pH range to those found by Gu et al. [20] for phthalate adsorption onto iron oxide and by Ali and Dzombak [23] for phthalate adsorption onto goethite. The amount of phthalic or mellitic acid adsorbed to kaolinite was only about one fifth of that adsorbed to goethite. Zachara et al. [16] reported a similar trend for humic acid, with kaolinite adsorbing about half the humic acid that was adsorbed by goethite. Adsorption of polar organic materials to mineral surfaces has been attributed to interactions with hydroxyl species at the surface [23,24]. The lesser adsorption onto kaolinite compared with
4. Discussion
4.1. Cd(II) -benzene carboxylic acid complexation The data in Fig. 1 indicate that solution complexes between Cd(II) and mellitic acid form from pH 3, with virtually no free Cd2 + evident above 5. Results for the other acids studied show similar results, with the pH for the onset of Cd(II)-acid formation shifted to higher pH in the order mellitic \hemimellitic\ pyromellitic ]trimellitic:
Fig. 5. Cd(II) adsorption onto goethite in the presence of various concentrations of phthalic acid. Initial Cd(II) concentration was 5×10 − 5 M. () no added phthalic acid, ( ) 5 ×10 − 5 M acid, ( ) 1× 10 − 4 M, () 1×10 − 3 M ().
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Fig. 6. Cd(II) adsorption onto kaolinite in the presence of various concentrations of phthalic acid. Initial Cd(II) concentration was 5×10 − 5 M. () no added phthalic acid, ( ) 5 ×10 − 5 M acid, ( ) 1× 10 − 4 M, () 1× 10 − 3 M.
goethite may be due, at least in part, to the fact that the siloxane face of the kaolinite crystals is not hydroxylated, and therefore not available for organic adsorption. Fig. 2 also shows that the adsorption maxima for both substrates appear at or below pH 5, with adsorption decreasing to near zero at higher pH values, a feature that is typical of anion adsorption to a variety of substrates [27]. However, the results also indicate, especially for goethite, that the more substituted benzene carboxylic acids remain adsorbed to higher pH values.
and observed significant enhancement of adsorption at low pH. Similarly, Zachara et al. [16] found that Co(II) adsorption onto goethite and other minerals was enhanced by the presence of humic acids. At high ligand concentrations (1× 10 − 3 M) Cd(II) adsorption onto kaolinite (Fig. 4) was somewhat enhanced by phthalic and trimesic acids but was generally suppressed by the other ligands. Phthalic and trimesic acids show the weakest complexation of Cd(II) (Fig. 1). These results agree with previous studies of organicmetal-kaolinite systems, in which the effect of the organic ligand depended on factors such as the nature and concentration of the metal or ligand, and the ligand-metal ratio. Haas and Horowitz [26] found that while alignic and humic acids enhanced Cd(II) adsorption onto kaolinite, its uptake was diminished when EDTA was added. Zachara et al. [16] found that Co(II) adsorption was enhanced by the presence of humic material, while Frimmel and Huber [15] found that sorption of Cu(II) was suppressed, and that of Cd(II) and Pb(II) enhanced, in humic-kaolinite systems. Haas and Horowitz [26] concluded that, at mod-
4.3. Cd(II) adsorption in the presence of benzene carboxylic acids The enhancement of Cd(II) adsorption onto goethite in the presence of carboxylic acids (Fig. 3) generally reflects the findings of other workers who have studied a variety of organic-metal-mineral systems. Lamy et al. [14], for example, found that Cd adsorption increased when oxalic acid was added to a Cd-goethite system. Ali and Dzombak [13] studied the Cu(II)-goethite system in the presence of phthalic and chelidamic acids
Fig. 7. Cd(II) adsorption onto goethite in the presence of various concentrations of mellitic acid. Initial Cd(II) concentration was 5 ×10 − 5 M. () no added mellitic acid, ( ) 5 ×10 − 5 M acid, ( ) 1× 10 − 4 M, () 1×10 − 3 M.
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Fig. 8. Cd(II) adsorption onto kaolinite in the presence of various concentrations of mellitic acid. Initial Cd(II) concentration was 5 ×10 − 5 M. () no added mellitic acid, ( ) 5 ×10 − 5 M acid, ( ) 1× 10 − 4 M, () 1× 10 − 3 M.
erate to low ligand concentrations there was potential for adsorption to be enhanced where the ligand adsorbed to the substrate. Where the ligand was non-sorbing, as was the case for EDTA, adsorption was suppressed. The pH range for Cd-mellitic acid complex formation (Fig. 1) coincides with the pH at which Cd(II) adsorption begins in the presence of mellitic acid. In addition, from Fig. 2 we know that mellitic acid adsorbs strongly to goethite at low pH. It is therefore likely that the enhancement of Cd(II) adsorption onto goethite at low pH in the presence of mellitic involves the formation of a complex between Cd and adsorbed mellitic acid. Because similar results were also found for the other organic acids we conclude that adsorption via a ligand bridge is the most probable explanation for the increased uptake of Cd(II) by goethite in the presence of the six carboxylic acids studied. Mechanisms of this type have been suggested in the past [13 – 16,26]. The difference in the degree of enhancement appears to be related to the complexing strength of the particular ligand. Fig. 1 indicates that phthalic acid forms the weakest complexes with Cd(II), and may therefore be ex-
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pected to enhance Cd(II) adsorption the least. Fig. 3 shows this to be the case. However, the order of enhancement does not exactly follow the order of complexing strength, suggesting that another factor is also important. The ligands which have the weakest effect are phthalic and hemimellitic acid, both of which have carboxyl groups on adjacent carbons. This may inhibit the simultaneous interaction of the carboxylate groups with Cd(II) and the surface. The effect of benzene carboxylic acids on Cd(II) adsorption onto kaolinite can also be explained in terms of complexation and ligand adsorption. Fig. 2 shows that these ligands do not adsorb strongly to kaolinite. We note that the results shown in Fig. 4 are for ligand concentrations of 1 × 10 − 3 M, far in excess of that required to saturate the kaolinite surface. Because only a small fraction of the ligand is adsorbed to the kaolinite surface, most of the complexed Cd(II) will be held in solution and thus prevented from adsorbing. The order of suppression is again related to the order of complexing strength, with ligands that complex Cd(II) most strongly suppressing adsorption the most.
4.4. Effect of benzene carboxylic acid concentration Figs. 5–8 show the effect of varying the concentrations of phthalic and mellitic acid on Cd(II) adsorption. These two acids were chosen for more detailed study because phthalic was the weakest and mellitic acid the strongest complexer of Cd(II). Figs. 5 and 6 show that phthalic acid had little effect on Cd(II) uptake on either substrate, irrespective of the acid concentration. The fact that there is little enhancement in the goethite system is consistent with our observations that Cd(II)-phthalate complexes do not form to a significant extent until pH\ 8 (Fig. 1), which is beyond the pH at which Cd(II) adsorption normally occurs. In addition, phthalate begins to desorb from the mineral surfaces at pH values above 7. Thus, Cd(II) uptake in the phthalic acid systems is likely to involve adsorption of the cation directly to the goethite surface, with the ligand bridge mechanism playing, at most, a mi-
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nor role. The effect of phthalate on the Cd(II)kaolinite system is even smaller because less phthalate adsorbs to the kaolinite surface. Mellitic acid, on the other hand, strongly effects Cd(II) adsorption onto both substrates at all acid concentrations (Figs. 7 and 8), because mellitic acid complexes dominate Cd(II) speciation at pH\ 3. The maximum enhancement of Cd(II) adsorption onto goethite occurs at a mellitic acid concentration of 1× 10 − 4 M. From the mellitic acid adsorption edge (Fig. 2) we know that most of the acid is adsorbed to the goethite surface over the pH range from B3 to 8. Hence Cd(II)-mellitic acid complexation must occur almost exclusively at the goethite surface, with almost no competition from the small fraction of mellitic acid in solution. When the mellitic acid concentration is increased to 1×10 − 3 M, competition for Cd(II) complexation occurs between acid bound to the surface and that free in solution. Thus decreased adsorption of Cd(II) is found above pH 3.5 where solution complexes form (Fig. 1). At mellitic acid concentrations below 1 ×10 − 4 M virtually all of the ligand is adsorbed and Cd(II) uptake is probably dominated by complexation to adsorbed ligand. However, metal uptake directly to free goethite surface is also possible, so at the lower mellitic acid concentrations less enhancement is observed due to competition between surface bound mellitic acid and bare hydroxyl surface sites for free Cd(II). The effect of mellitic acid on Cd(II) uptake by kaolinite (Fig. 8) is also due to competition. At lower acid concentrations we see enhanced adsorption at low pH, while at the high concentration adsorption is almost completely suppressed. Fig. 2 indicates the amount of mellitic acid that adsorbs to the kaolinite surface is small ( : 2× 10 − 5 M) compared with goethite (:1× 10 − 4 M). Hence, when the initial concentration of mellitic acid is 1× 10 − 3 M most of the Cd(II) is complexed by mellitic acid in solution and strong suppression results. At mellitic acid concentrations where a significant fraction of the acid can adsorb (5 1 × 10 − 4 M) enhancement occurs through surface complexation of Cd(II) by adsorbed ligand. If, as suggested earlier, mellitic
acid does not adsorb to the unhydroxylated siloxane face of kaolinite, then the permanentnegatively charged sites on that face will also be available to adsorb free Cd(II). Hence at low mellitic acid concentrations and at pH values below 7, adsorption of Cd(II) is enhanced through a combination of complexation by adsorbed mellitic acid and interaction with permanent change sites. At high pH (\ 7) Fig. 2 shows that very little mellitic acid remains adsorbed with the result that suppression of adsorption occurs through competition for Cd(II) between surface sites and mellitic acid in solution.
5. Conclusion Cd(II) adsorption onto goethite is enhanced for all the benzene carboxylic acids studied, over a wide range of concentrations. However, high concentrations of some acids, especially mellitic acid, inhibit adsorption of Cd(II) onto kaolinite. Our results suggest that the effect of benzene carboxylic acids on the adsorption of Cd(II) onto goethite and kaolinite depends on extent to which they adsorb to the substrates, their ability to complex Cd(II), and the position of carboxyl groups around the benzene ring. Carboxylic acid adsorption is strongest at lower pH, with the adsorption maximum occurring below pH 3 for goethite and at about pH 5 for kaolinite; about five time as much acid is adsorbed to goethite compared with kaolinite. Acids that form strong complexes tend to have the greatest impact on Cd(II) uptake. Where adsorption is enhanced by the complexer, the formation of a surface complex via adsorbed ligand is likely. Suppression of adsorption occurs where there is a large solution concentration of the complexing ligand and relatively little adsorbed to the surface.
Acknowledgements Financial support was provided by the Australian Research Council Small Grants Scheme. MJA is the recipient of an Australian Postgradu-
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ate Award with stipend. Stephen Johnson (AMPC, Melbourne University) is gratefully acknowledged for determining the surface areas of the goethite sample.
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