Oceanouaahv”. 2nd ed.. J. P. Riley- and G. Skirrow, Eds., Academic Press, I&qork, & N.Y., 1975. (16) Stumm, W.. Brauner, Ph. A,, “Chemical Speciation”, ibid. (17) Quintin, M., Compt. Rend., 204,968 (1937). (18) Spivakovskii, V. B., Makovskaya, G. V.,Russ. J Inorg. Chem., 13,815 (1968). (19) Vuceta, J., Morgan, J. J., Limnol. Oceanogr, 22 (4), 742 (1977). (20) Mesmer, R. E., Baes, C. F., Jr., “The Hydrolysis of Cations: A Critical Review of Hydrolytic Species and Their Stability Constants in Aqueous Solution”, Oak Ridge National Lab, ORNL-NSFEATC-3, P a r t 111, 1974.
(21) Jenne, E. A., “Controls of Mn, Fe, Co, Ni, Cu, and Zn Concentrations in Soil and Water: The Significant Role of Hydrous Mn and Fe Oxides”, Adu. Chem. Ser., No. 73,337 (1968). (22) Morgan, J. J., Stumm, W., J . Colloid Interface Sci., 19, 347 (1964). (23) Murray, J. W., PhD thesis, Massachusetts Institute of Technology, Woods Hole Oceanographic Institution, 1973. (24) Sholkovitz, E. R., Geochim. Cosmochim. Acta, 14,831 (1976).
Receiced for review January 12,1978. Accepted June 19,1978. Work supported by the National Institute of Enuironmental Health Sciences, Grant 57’01 ES0004 and by E P A Grant R-801069.
Effect of Adsorbed Complexing Ligands on Trace Metal Uptake by Hydrous Oxides James A. Davis” and James 0. Leckie Environmental Engineering and Science, Department of Civil Engineering, Stanford University, Stanford, Calif. 94305
w The roles of complexing ligands and pH in affecting trace metal adsorption a t the sediment/water interface are determined in model experimental systems. The adsorption behavior of complexing ligands must be considered to determine the overall effect on Cu(I1) or Ag(1) uptake on amorphous iron oxide. In some cases metal uptake is increased by the presence of adsorbed ligands a t the surface. Other ligands form nonadsorbing complexes in solution and compete with the surface for coordination of metal ions. The results suggest that the distribution of trace metals in natural aqueous systems may be controlled by surface binding on colloidal particles coated with humic compounds rather than reactions with simple oxide surface sites. Numerous studies of river, estuarine, and marine sediments have shown that many trace metals (e.g., Cu, Pb, Hg, Cd, Zn, Ag) are concentrated in sedimentary material in natural aquatic systems (1-8). Removal of many trace elements is known to be related to adsorption or other surface association phenomena with hydrous metal oxides, clays, or detrital organic matter (9-11). The distribution among phases varies with the identity of the trace metal, the solid phases present, and the chemical environment. With the current awareness of trace metals as pollutants in natural waters, there has been much study devoted to the processes by which they are transported, entrapped, and released a t the sediment/water interface. Figure 1 illustrates some of the major reaction pathways for metal removal from water with eventual deposition in the sediments. A major fraction of heavy metals is transported by association with suspended particulates in freshwater and estuarine systems. Since equilibrium calculations generally show apparent undersaturation with respect to known heavy metal solid phases, some type of interfacial phenomena is usually invoked to account for the observed distribution. The trace metal-particulate association may include: adsorption of metal ions a t oxide surface sites, ion exchange within clay minerals, binding by organically coated particulate matter or organic colloidal material, or adsorption of a metal-ligand complex. The interactions represented in Figure 1 emphasize the role of natural organic matter in affecting the distribution of trace metals, because a significant fraction of the metals in the water and sediment phases is often associated with organic matter (12-17). The form in which metals enter the sediment phase Present address, Swiss Federal Institute of Technology (EAWAG), CH-8600 Dubendorf/ZH, Switzerland.
is important since it will determine the rate and extent to which redistribution occurs within the sediment (9) and affects the toxicity to deposit-feeding animals (8, 18) in the surface sediments. In addition, humic compounds have been shown to reduce the toxicity of copper to a unicellular alga (19).
Significant concentrations of natural and synthetic complexing ligands are present in many aquatic systems. Most experimental studies of the adsorption behavior of trace metals, however, have been conducted in simple electrolyte systems in the absence of complexing ligands. Consequently, the significance of complexing ions and chelating macromolecules, e.g., humic compounds, in altering metal adsorption is unknown. MacNaughton and James (20) and Vuceta (21) have investigated the effects of a few ligands (chloride, citric acid, EDTA) in systems where the ligand did not adsorb. These authors found that metal adsorption (Hg, Pb, Cu) is decreased in a manner that suggests competition between the ligand and oxide surface for complexation of the metal ion. In these cases, metal adsorption was dramatically decreased by the presence of a complexing ligand in the system a t appropriate concentrations. While other studies have examined nonadsorbing ligands, we have made a detailed analysis of the adsorption behavior of several inorganic and organic complexing ligands and the effects of these ligands on trace metal uptake by amorphous iron oxide. Little is known about the adsorptive characteristics of oxide surfaces modified by adsorbed organic compounds, although recent studies (22-24) have shown that metal ions are strongly adsorbed by colloidal humic material. While our experimental systems are simple in comparison to natural waters, we have identified the roles of complexing ligands and pH in controlling the adsorption properties of oxide surfaces. We will show that the adsorption behavior of complexing ligands is an important factor in determining the overall effect on trace metal uptake by oxide surfaces. For several ligands the effect cannot be explained by simple competition between complexation with the ligand in solution and adsorption on the surface. In fact, trace metal adsorption is enhanced in some cases. After presenting results from our model experimental systems, we discuss the significance of these findings for trace metal distribution in natural waters. Experimental
Amorphous iron oxide was prepared in batch for each adsorption experiment with double-distilled water a t 25 “C under a circulating purified nitrogen atmosphere. Sodium
00 13-936) 7.5 no thiosulfate or metal-ligand complex is adsorbed and the ligand functions as a simple complexing agent in solution, competing with the surface for coordination of Ag(I), similar to the previous results for chloride and cyanide. At a higher thiosulfate concentration (Figure 4), the observed effects are even greater. At low pH more thiosulfate is present a t the surface to bind Ag(I), and a t higher pH, more complexing ligand is present in solution to decrease metal adsorption. Adsorption Systems with Organic Complexing Ligands. Experimental results for inorganic systems show that adsorption behavior of a complexing ligand must be considered before its effect on trace metal adsorption can be fully understood. While the silver/thiosulfate/iron oxide system is of interest conceptually, a study of the effects of organic complexing ligands with chemical structures similar to naturally occurring organic compounds can be applied more directly to natural waters. 1312
Environmental Science & Technology
Adsorption of salicylic acid and protocatechuic acid (PCCA) by Fe(OH)s(am) as a function of pH is shown in Figure 8. PCCA is very strongly adsorbed over a large pH range. By comparison with adsorption behavior of other aromatic compounds with varying substituent groups (28), it can be inferred that the adjacent phenolic groups are involved
cool
salicylic acid in mid-pH region
cooI
OH protocatechuic acid (PCC.4, in mid-pH region
in the surface bonding of PCCA. Salicylic acid is more weakly adsorbed, with a pH dependence similar to that observed for sulfate. Despite the large amount of organic material present a t the surface in the pH range 4-6, Cu(I1) uptake by Fe(OH)s(am)is not significantly affected by the addition of these ligands to the system or by M added sulfate (Figure 9). Since the organic ligands have functional groups similar to that of humic materials (37,38),one might expect that these compounds would be useful model ligands for a study of the effects of natural organic matter on trace metal distribution. However, it will be shown later that the orientation of an adsorbed ligand is also an important factor in determining the overall effect on metal uptake by hydrous oxides. Adsorption of glutamic acid, picolinic acid, and 2,3-pyrazinedicarboxylic acid (2,3-PDCA) is illustrated in Figures 10
-----
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Figure 11. Adsorption of picolinic acid and 2,3-pyrazinedicarboxylic acid (2,3-PDCA) on amorphous iron oxide as a function of pH
Figure 12. Adsorption of copper(l1) on amorphous iron oxide as a function of pH in presence of picolinic acid, glutamic acid, and 2,3PDCA
and 11. Uptake ofthese weak acids by Fe(OH)3(am)is similar to the other anions studied, with the exception of PCCA
GLUTAMIC ACID --0oC-(CHZ)2-CH-C00I NH;
glutmiic Licid i n mid-pH region
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(Figure 8). Maximum adsorption for picolinic acid occurs at a p H approximately equal t o the pK, of the carboxylic acid group, in agreement with the correlation observed by Hingston et al. (39).The adsorption edge of 2,3-PDCA is comparatively steep with respect to pH, suggesting that surface bonding of t h e adsorbed molecule involves both carboxylic acid groups. Figure 12 shows effects of these ligands on Cu(I1) adsorption by Fe(OH),i(am).Cu(I1) uptake is increased by glutamic acid and 2,3-PDCA, but picolinic acid effectively prevents copper(1I) removal by complexation in solution. Although the exact stereochemical arrangement of these molecules a t the surface is unknown. it is apparent that the presence of adsorbed 2,3-PDCA and glutamic acid enhances the binding strength of the colloid surface for Cu(I1). For 2,3-PDCA this might occur via (1) direct Complexation of Cu(I1) by the adsorbed organic compound, or (2) by formation of a mixedligand surface complex involving adsorbed 2,3-PDCA and an adjacent oxide surface site. Glutamic acid surface bonding probably involves the terminal carboxylic group (Figure 13), since experiments with glycine showed that the zwitterion group forms very weak bonds with the surface. Consequently, the zwitterion group is still available for binding metal ions, and Cu(I1) uptake may be enhanced by surface reactions of copper-glutamate complexes, or alternatively, by complexation of Cu(I1) with adsorbed glutamate. These interactive processes are similar t o those discussed for the silver/thiosulfate/iron oxide system. Ag(1) adsorption on Fe(OH):i(am) is also enhanced by the presence of glutamic acid (28). The difference between the effects of adsorbed 2,3-PDCA and picolinic acid on Cu(I1) uptake (Figure 12) is striking, since the adsorption behavior of these ligands is comparable (Figure 11)and each ligand has similar functional groups for
Figure 13. Proposed surface complexes formed by adsorbed glutamate, picolinate, and 2,3-PDCA ions
metal complexation. Surface bonding of picolinic acid probably involves donor electrons of the carboxyl group and nitrogen heteroatom (Figure 13). As a result a n adsorbed picolinate ion cannot function as a complexing ligand for metals, since the coordinating groups are unavailable. Cu(I1) adsorption is decreased by complexation with the picolinate remaining in solution (Figure 12). Although the exact mechanism of surface bonding remains unknown, it is apparent that Cu(I1)-glutamate and Cu(I1)2,3-PDCA complexes are present a t the surface. Since adsorbed glutamate and 2,3-PDCA possibly have functional groups directed toward solution (Figure 131,these ligands may serve as complexing agenbs a t the surface. Enhancement of trace metal uptake is possible on other oxides as well. Figure 14 demonstrates a n increase in Ag(1) adsorption on tu-quartz caused by addition of ethylenediamine. Bourg and Schindler ( 4 6 ) have shown that Cu(en) complexes are also adsorbed by silica. The results of studies with other trace metal/complexing ligand/hydrous oxide heterogeneous systems are available elsewhere (28,33). Trace Metal Distribution in Complex Systems. While other reports have examined the reactions of metal ions with oxide surfaces in simple electrolyte solutions (30-32), we have studied changes in metal uptake that result when oxide surfaces are modified by adsorbed complexing ligands. Although the experimental systems are simple in comparison to natural waters, it is now possible to categorize the roles of certain types of complexing ligands and their probable significance in Volume 12, Number 12, November 1978
1313
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Figure 14. Adsorption of silver(1) on cu-quartz as a function of pH in presence of ethylenediamine
controlling the distribution of trace metals in natural aquatic systems. Our experimental results with nonadsorbing ligands are in agreement with the conclusions of similar previous studies (20, 21). Complexing ligands that are not adsorbed, e.g., chloride, decrease trace metal adsorption. If one considers the oxide surface as a ligand for metal binding (31),then chloride ions simply compete with the surface for coordination of the metal ion. The magnitude of the effect depends on the relative intensities of the metal-ligand and metal-surface bonds and the ligand concentration. Trace concentrations of cyanide (8 X M) prevented adsorption of Ag(1) by Fe(OH)3(am); however, high concentrations of chloride (-0.1 M) are necessary for the same effect because of the weaker Ag(1)chloride bonds. When the major coordinating functional groups of an adsorbed complexing ligand are involved in surface bonding, the ligand will decrease or have a minor effect on metal adsorption, e.g., picolinic acid, protocatechuic acid, salicylic acid. Nevertheless, the adsorption behavior of the ligand is important from the viewpoint of metal speciation and distribution, since the concentration of ligand remaining in solution will be controlled by its surface reactions. The magnitude of the effect on metal uptake again depends on the relative intensities of metal binding by the complexing ligand and oxide surface. Cu(I1) uptake was significantly decreased by the presence of dilute picolinic acid (4 X I O p 5 M), despite partial sorption of the ligand. On the other hand, salicylic acid ( lop4 M) had a relatively minor effect. Trace metal adsorption can be significantly enhanced when a ligand is adsorbed with a strongly complexing functiona! group directed outward toward solution, e.g., thiosulfate, glutamic acid, 2,3-PDCA, ethylenediamine. The experimental results provide convincing evidence that metal-ligand complexes can adsorb, or alternatively, that trace metals can be complexed by adsorbed ligands. At the present time there are no analytical methods capable of distinguishing between these two mechanisms. Adsorption of ligands and metal-ligand complexes is an important consideration in the partitioning of trace metals by oxide surfaces. The functional groups of an adsorbed ligand may serve as “new adsorption sites” for trace metals a t the surface or may simply stabilize adjacent surface sites. The “new adsorption site” may be more reactive than an isolated oxide surface site. In this sense the presence of an adsorbing complexing ligand in an oxide suspension can change the chemical microenvironment a t the surface (adsorbed thiosulfate, glutamate, or 2,3-PDCA ions). The effects on trace metal adsorption varied widely among the ligands studied. Nonetheless, in each case, the type and magnitude of effect 1314
Environmental Science & Technology
can be understood in terms of changes in solution speciation and/or the chemical microenvironment a t the oxide/water interface. Trace Metal Partitioning in Natural Waters. The chemical composition of natural aquatic systems is derived from weathering, atmospheric fallout, degradation products of plant and animal tissue, and anthropogenic inputs. Consequently, there are many natural and synthetic complexing agents present. The molecules and electron donor groups selected for this study are representative of the range of complexing agents found in natural aquatic systems. The major fraction of dissolved organic matter in natural waters is composed of humic compounds which are chemically similar to fulvic acids found in soils and sediments (12,37,38). Numerous field studies have indicated the importance of natural organic matter with regard to trace metal speciation in seawater, freshwater, and soil solutions (17,40-43). Recent determinations of trace metal-natural organic matter stability constants (44,451 have confirmed that humic substances are significant complexing agents in natural waters (12).Despite this fact, progress has been slow in elucidating the chemical structure and properties of these compounds due to their complexity. However, the functional groups of humic and fulvic acids are predominantly carboxylic, phenolic, and alcohol groups, and these functional groups are important with respect to metal coordination (37,38,44). The role of natural organic matter in promoting or inhibiting trace metal adsorption is not well understood. The uptake of humic matter by clays has been studied (47),but adsorption on oxide surfaces has only been examined recently (48).The large size and low solubility of humic compounds suggest an affinity for the solid/water interface, and there is some evidence that suspended particulate matter in natural waters is coated with a film of adsorbed organic compounds (49).Thus, the physical and chemical microenvironment at solid/water interfaces will differ substantially from that of oxide colloids dispersed in simple electrolyte solutions. “Adsorption sites” for trace metal ions may be dominated by the functional groups of adsorbed organic compounds. Although the significance of humic compounds in metal speciation is frequently discussed, its importance as a surface coating on colloidal particulate matter in natural waters is often ignored. The results of our study suggest that trace metal adsorption may be enhanced by natural organic matter. Metal ions may have a larger affinity for coordination with the chelating groups of adsorbed humic or fulvic acids than with oxide surface sites. The strong binding of metal ions, particularly Cu(II), by humic colloids has already been demonstrated (22-24). Alternatively, metals could be complexed in solution and subsequently transported to the sediment/water interface as humic compounds are adsorbed. Thus, natural organic matter could be an important contributing factor to the observed association of trace metals with aquatic sediments ( 2 4 , 8). Some investigators have attributed the association directly to humic compounds (14-16,50). Summary By studying the adsorption behavior of ligands in combination with their effects on trace metal uptake, we have clarified the mechanisms by which metal adsorption can be altered by complexing ligands. Some metal-ligand complexes are strongly bound by oxide surfaces, while others form nonadsorbing complexes in solution. Thus, several interactive processes may influence the equilibrium distribution and speciation of metal ions in complex heterogeneous systems. Additional experimental work is needed to determine the role of organic matter in affecting the distribution of metals in natural waters. Our results suggest that particulate matter
coated with adsorbed humic compounds could contribute significantly to trace metal partitioning by aquatic sediments. Literature Cited (1) Turekian, K. K., Scott, M. R., Enuiron. Sei. Technol., 1, 940
(1967). (2) Harding, S. C., Brown, H. S., Enuiron. Geol., 1, 181 (1975). (3) Pita, F. W., Hyne, N. J., Water Res., 9,701 (1975). (4) Catanzaro, E. J., Enuiron. Sci. Technol., 10,386 (1976). (5) Ramamoorthy, S., Rust, B. R., Can. J . Earth Sci., 13, 530 (1976). (6) Lewis, D. M., Geochim. Cosmochim. Acta, 41,1557 (1977). (7) Taylor, D., Estuarine Coastal Mar. Sci., 2,417 (1974). ( 8 ) Bloom, H., Ayling, G. M., Enuiron. Geol., 2 , 3 (1977). (9) Leckie, J. O., James, R. O., in “Aqueous-Environmental Chemistry of Metals”, A. J. Rubin, Ed., Chap. 1,Ann Arbor Science, Ann Arbor, Mich., 1975. (10) Parks, G. A., in “Chemical Oceanography”,Riley and Skirrow, Eds., 2nd ed., Academic Press, New York, N.Y., 1975. (11) Jenne, E. A., in “Symposium on Molybdenum in the Environment”, W. Chappel and K. Petersen, Eds., Vol 2, Chap. 5, Marcel Dekker, New York, N.Y., 1977. (12) Reuter, J. H., Perdue, E. M., Geochim. Cosmochim. Acta, 41, 325 (1977). (13) Alexander, J. E., Corcoran, E. F., Limnol. Oceanogr., 12, 236 (1967). (14)-Lindberg, S. E., H a r r i s , R. C., Enuiron. Sei. Technol., 8,459 (1974). (15) Nissenbaum, A,, Swaine, D. J., Geochim. Cosmochim. Acta, 40, 809 (1976). (16) Gardiner, J., Water Res., 8,23 (1974). (17) Florence, T. M., ibid., 11,681 (1977). (18) Luoma. S.N.. Jenne. E. A.. Proceedings. Trace Substances in EnvirGnmental Health-X, D. D. Hemphii, Ed., pp 343-51, Univ. of Missouri, Columbia, Mu., 1976. (19) Sunda, W. G., Lewis, J.A.M., Limnol. Oceanogr., in press (1978). (20) MacNaughton, M. G., James, R. O., J . Colloid Int Sei., 47,431 (1974). (21) Vuceta. J.. PhD thesis. California Institute of Technolorv. Pasadena, Calif., 1976. (22) Green, J . B., Manahan. S. E.. Can J C h e m . 55.3248 (1977). (23) Guy, R. D., Chakrabarti, C. L., Schramm, L. L.; ibid., 53, 661 (1975). (24) Rashid, M. A., Chem. Geol., 13,115 (1974). ”
