INTERFERENCES Common alloying elements such as aluminum, zinc, and low concentrations (
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Analytical chemists have studied adsorption of foreign ions on hydrous oxides from the viewpoint of contamination of precipitates as well as the removal of radioactive ell Present address, Department of Chemistry, University of Florida, Gainesville, Fla. 32611.
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ements from solution (1-4). In general, a dramatic increase in adsorption is observed with increasing p H and hydrogen ions are released into the solution. The adsorption and, in turn, the release of hydrogen ions is dependent on the surface area of hydrous oxide. In fact, it has been suggested (5-7) that adsorption of zinc ions be used as a measure of the surface area of oxides such as MnOp, ZrOn, and SiOp. Although the displacement of hydrogen ion from the surface is an indication of specific ion exchange adsorption, the number of hydrogen ions released per heavy metal ion adsorbed cannot be taken directly as a measure of the number of binding sites to which the heavy metal is attached. For example, four hydrogen ions are released from a silica sample for each copper(I1) ion adsorbed from an ammonium acetate solution ( 5 ) ,presumably because of the involvement of copper amine complexes in the adsorption process. (1) I. M. Kolthoff and B. Moskovitz, J. Pbys. Chem., 41, 629 (1937). (2) I. M. Kolthoff and L. B. Overholser. J. Pbys. Cbem., 43, 767, 909 (1939). (3) M. H. Kurbatov, G. B. Wood, and J. D. Kurbatov, J. Phys. Cham., 55, 1170(1951). (4) J. Korkisch, "Modern Methods for the Separation of Rarer Metal Ions," Pergamon Press, New York, N.Y.. 1969. (5) A. Kozawa. J. Electrochem. Soc.. 106, 552 (1959). (6) A. Kozawa, J. horg. Nucl. Chem., 21, 315 (1961). (7) A. Kozawa, S. C. Paterniti, and T. A. Reilly. in "Oxide-Electrolyte Interfaces," R. s. Alwitt, Ed., The Electrochemical Society Inc., Princeton, N.J., 1973, pp 72-90.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
Hydrous oxides have long been recognized as playing an important role in binding trace metals in soils and sediments. In an excellent review article, Jenne (8)states, "The general mode of occurrence of the hydrous oxides in soils and recent sediments as partial coatings on the silicate minerals rather than as discrete, well-crystallized minerals, allows the oxides to exert chemical activity which is far out of proportion to their activity." The hydrous oxides of iron and'manganese are important in the aquatic environment because changes in oxidation potential, acidity, and complexing agents can bring about the formation and dissolution of colloidal hydrous oxides. The size of colloidal hydrous iron oxide particles varies with pH, and can be as small as 100 A (9). In sea water, adsorption on hydrous manganese(1V) and iron(II1) oxides is recognized as the most important mechanism in controlling the concentrations of heavy metals (10). Goldberg ( 1 1 ) has stressed the importance of iron oxide as a catalyst for the oxidation of manganese(I1) in sea water by dissolved oxygen and the role of the mixed hydrous oxide system as hosts for a number of heavy metal ions. The p H sensitivity and reversible nature of the adsorption of heavy metals are of great environmental significance. Relatively large seasonal fluctuations of p H can occur in poorly buffered natural water systems. Lee (12) in an excellent review of the role of hydrous oxides in the environmental transport of heavy metals, has surveyed the literature of this subject, and emphasized the importance of redox chemistry, pH, presence or absence of organic and inorganic complexing agents in solution. He concluded that "while there is no doubt that hydrous metal oxides are important sinks and modes of transport for heavy metals in the environment, the quantitative magnitude of this role is not known for a variety of natural water conditions." Although soils are generally buffered within a moderately narrow p H range, extremely acid situations can be encountered (13) for example in the oxidation of pyrites by dissolved oxygen in mine runoff liquids, to produce high concentrations of soluble iron which can then be precipitated when the acid is neutralized. Aquatic organisms may encounter toxic metals in high concentrations adsorbed on particles in suspension or in sediments. Plant roots are known to exude organic substances that act as complexing agents (14) to aid in the absorption of trace metal nutrients and a t the same time expose the growing plant to toxic metals. The present research is intended to focus upon the mechanism of adsorption of heavy metals upon hydrous oxides. Although the concentrations of hydrous oxides and of heavy metal ions are higher than normally would be encountered in natural aquatic systems, the results should establish guidelines for future environmental research, and also be of interest in analytical separations involving hydrous oxides. In a recent communication (15), we presented the results of a study of the adsorption of lead(I1) on amorphous hy-
(8) E. A. Jenne in "Trace lnorganics in Water," Advan. Chem. Ser., 7, No. 73, American Chemical Society, Washington, D.C., 1968,Chapter 21. (9)J. J. Morgan and W. Stumm in "Advances in Water Pollution Research," Voi. 1, Proceedings of the Second International Conference on Water Pollution Research, Tokyo, 1964, Pergamon Press, Elmsford, N.Y., 1965,pp 103-131. (10)K. E. Krauskopf, Geochim. Cosmochim. Acta, 9, l(1956). ( 1 1 ) E. G. Goldberg. in "Oceanography," Publ. No. 67, AAS. Washington, D.C., 1961,p 583. (12)G. Fred Lee, Proceedings of International Conference on Heavy Metals in the Aquatic Environment, Vanderbiit University, 1973,in press. (13)R. S. Campbell, 0. T. Lind, W. T. Geiling, and G. L. Harp in "Symposium on Acid Mine Drainage," Mellon Institute, Pittsburgh, Pa., 1965. (14)R. R. Gadde and H. A. Laitinen, Environ. Lett., 5,91 (1973). (15)R . R . Gadde and H. A. Laitinen, Environ. Lett., 5, 223 (1973).
drous ferric oxide (HFO). As much as 0.28 mole of lead ion could be adsorbed per mole of iron(II1) a t pH 6. At lower pH values, the amount of adsorption decreased markedly, and a substantial amount of lead ion was released by changing the p H from 6 to 4. Accompanying the adsorption of each lead ion was the release of 1.6 hydrogen ions a t pH 6 and 1.2 hydrogen ions a t p H 5 . Inasniuch as the point of zero charge (PZC) of colloidal HFO is about 8.5 ( 1 6 ) , we can regard the adsorption as a specific replacement of weakly acidic protons by aquated Pb2+ or PbOH+ rather than as a generalized counter ion process such as could occur on a colloid of opposite sign. In the present paper, we describe adsorption studies of other heavy metal-hydrous oxide systems. The hydrous oxides of iron(II1) and manganese(1V) (HMO) were studied most extensively, both because of their importance in environmental systems and because of their ease of preparation and stability over periods of a t least several days. Hydrous aluminum oxide (HAO) varied with time in its adsorption behavior, and only a limited study was made of it. Lead, cadmium, and thallium were chosen for study because of their toxicities, and zinc was included because of its importance as a plant nutrient a t low concentrations and toxicity a t high concentrations.
EXPERIMENTAL Hydrous manganese oxide (HMO) was prepared by slowly adding manganese(I1) nitrate solution to alkaline permanganate solution. The amounts of manganese(I1) nitrate, potassium permanganate, and sodium hydroxide mixed were in the mole ratio 3:2:4. By using these ratios, Morgan and Stumm (17) found the product to have the composition MnO, where x varied from 1.90 to 1.95. Before using this HMO in adsorption studies, it was filtered, washed with and redispersed in distilled water, adjusted to pH 6, made up to the desired volume and aged for 16 to 20 hours. X-Ray studies showed that the HMO prepared in this way is amorphous in nature. A known aliquot of the HMO suspension was used in the individual experiments. The amount of HMO in the data reported below is expressed as the moles of Mn present in the aliquot. Hydrous oxides of iron and aluminum were prepared by titrating the nitrate solutions to pH 6 with NaOH (15). Hydrous ferric oxide (HFO) was then washed and aged as described under HMO. Hydrous aluminum oxide (HAO) was not easily filtered or centrifuged, so the washing step was eliminated. The amount of NaN03 left in H A 0 amounted to approximately 0.02M in the final solution used for the adsorption study. As in the case of HMO, the amounts of HFO and H A 0 were calculated from the initial amounts of Fe and AI used in making these hydrous oxides. For HFO and HMO, the amount of H + released during adsorption was determined by the following procedure. The pH of the suspension was adjusted to the desired value with "03. the heavy metal ion was added, and the pH was periodically readjusted with NaOH until equilibrium had been reached, generally within 20 minutes. All p H values reported are equilibrium values. All the chemicals used were of reagent grade. Triple distilled water with the second distillation from alkaline permanganate was used in making up all the solutions. Ferric nitrate solution was standardized by titration with KMn04 after reduction with stannous chloride (18). Manganese(I1) nitrate solution was standardized by potentiometric titration with KMn04 in the presence of large excess of pyrophosphate (19). Aluminum nitrate concentration was calculated directly from the weight of AI(N03)j. H20. In obtaining the adsorption data, known amounts of hydrous oxide and the metal ion of interest were equilibrated for three hours at a predetermined pH. The metal ion concentration in solution a t the end of the equilibration period was determined by pulse polarography. The amount of metal ion adsorbed was then computed from the amount initially added and the amount left at G. A. Parks, Chem. Rev., 65,177 (1965). J. J. Morgan and W. Stumm. J. Colloid. Sci., 19,347 (1964). I. M. Kolthoff, E. B. Sandeil, E. J. Meehan, and S. Brukenstein, "Quantitative Chemical Analysis," The Macmillan Company, New York, N.Y.. 1965,pp 832-834. (19)J. J. Lingane and R . Karpius. lnd. Eng. Chem., Anal. Ed., 18, 191 (1946).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
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Figure 1. Effect of pH on the adsorption of heavy metal ions on hydrous manganese oxide (HMO) HMO 0 436 mmole Heavy metal ion 0 1 mmole Solution volume 100 ml
Figure 3. Effect of pH on the adsorption of lead on hydrous oxides Lead 0 1 mmole Solution volume 100 ml 0 Hydrous manganese oxide, 0 436 mmole 0 Hydrous ferric oxide, 0 625 mmole V Hydrous aluminum oxide 0 625 mmole
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Figure 2. Effect of pH on the adsorption of heavy metal ions on hydrous ferric oxide (HFO) HFO: 0.625 mmole. Heavy metal ion: 0.1 mmole. Solution volume: 100 ml
Figure 4. Effect of the heavy metal ion concentration at equilibrium on its adsorption on hydrous manganese oxide
equilibrium. Further details of the procedure can be found in our earlier communication ( 1 5 ) . With HFO adsorption, equilibrium was nearly reached in about 10 minutes but, in the case of HMO, more than 30 minutes was needed. RESULTS Effect of pH. On hydrous manganese(1V) oxide, the adsorption of lead, cadmium and zinc was favored with increased pH (Figure 1).At any pH in the range 2 to 8, the extent of adsorption lay in the order P b > Zn > Cd. Using 0.1 mmole of Pb2+ for 0.436 mmole of HMO, virtually 100% adsorption, corresponding to a molar ratio of Pb:Mn = 0.23, was observed a t p H >4.5. Appreciable adsorption occurred even a t pH 2. With zinc and cadmium, a nearly linear increase of adsorption was observed over the pH range 2 to 6. Thallium showed a similar behavior in the pH range 4 to 6, but a t pH 4 similar to that of cadmium and zinc is also probably due to specific adsorption rather than simple counter-ion type adsorption. Specific adsorption of Ni2+, Cu2+, and Co2+ on manganous manganite was reported earlier by Murray e t al. (21). The apparent increase in thallium adsorption on hydrous manganese oxide a t pH
