Environ. Sci. Technol. 1987, 21, 863-869
(17) Mitsue, T. B.; Saha, B. C.; Ueda, S. J . Appl. Biochem. 1979, 1, 410. (18) Miah, M. N. N.; Ueda, S. Die Starke 1977, 29, 235. (19) Robyt, J. F.; French, D. J . Biol. Chem. 1970, 245, 3917. (20) Tabata, S.; Ide, T.; Umemura, T.; Torii, D. Biochim. Biophys. Acta 1984, 797, 231. (21) Bull, A. T.; Chesters, C. G. C. Adu. Enzymol. Relat. Areas Mol. Biol. 1966, 28, 325. (22) Krisch, K. In The Enzymes; 3rd ed.; Boyer, P. D., Ed.; Academic: New York, 1971; Vol. V, pp 43-69. (23) Abraham, E. P.; Fawcett, P. In Methods in Enzymology; Hash, J. H., Ed.; Academic: New York, 1975; Vol. 43, pp 728-731.
De Haan, H.; de Boer, T. Water Res. 1978, 12, 1035. De Haan, H.; de Boer, T. Arch. Hydrobiol. 1979, 85, 30. Sweet, M. S.; Perdue, E. M. Environ. Sci. Technol. 1982, 16, 692.
Colclough, C. A,; Johnson, J. D.; Christman, R. F.; Millington, D. s.In Water Chlorination: Environmental Impact and Health Effects; Jolley, R. L., Ed.; Ann Arbor Science: Ann Arbor, MI, 1983; Vol. 4, Book 1, p 220. Dische, Z. In Methods of Biochemical Analysis; Glick, D., Ed.; Interscience: New York, 1955; Vol. 11, p 320. Keleti, G.; Lederer, W. H. Handbook of Micro-Methodsfor the Biological Sciences; Van Nostrand Reinhold: New York, 1974; p 19. Keleti, G.; Luderitz, 0.;Mlynarcik, D.; Sedlak, J. Eur. J. Biochem. 1971,20, 237. Warburg, 0.; Christian, W. Biochem. 2. 1941, 310, 384. Pazur, J. H.; Ando, T. J. Bid. Chem. 1960, 235, 297. Pazur, J. H.; Kleppe, K. J. Biol. Chem. 1962, 237, 1002. Abdullah, M.; Fleming, I. D.; Taylor, P. M.; Whelan, W. J. Biochem. J. 1963,89, 35P.
Received for review June 27, 1986. Revised manuscript received January 29,1987. Accepted April 30, 1987. This research was supported by NSF Grant CEE 83-12997 with the use of facilities at NIEHS, Research Triangle Park, NC, and Research Triangle Laboratories, Research Triangle Park, NC.
Adsorption and Desorption of Metals on Ferrihydrite: Reversibility of the Reaction and Sorption Properties of the Regenerated Solid Matthew F. Schultr, Mark M. Benjamin,” and John F. Ferguson Department of Civil Engineering, University of Washington, Seattle, Washington 98 195
rn The feasibility of recycling ferrihydrite in a metal adsorption process was investigated. In this process, metal ions are removed from dilute solution by sorption onto ferrihydrite and are then desorbed into a more concentrated solution a t lower pH. The ferrihydrite thus becomes available for reuse in subsequent sorption operations. Copper, lead, nickel, zinc, cadmium, and chromium(II1) can be quantitatively sorbed onto ferrihydrite at pH 9.5. Lowering the pH to 4.5 substantially desorbs the metals. However, for all metals except cadmium, a measurable fraction of the bound metal is not easily desorbed. This fraction increases with increasing pH and duration of the high-pH stage and increases more or less continuously in sequential cycles. The retention of metals in the solid does not interfere with sorption in subsequent cycles, within the range of concentrations investigated. This included retention of up to 0.7 mol of Cr(III)/mol of Fe in the adsorbent. This process has the potential to provide the advantages of sorption processes, including low residual soluble metal concentrations, moderate pH requirements, and simultaneous removal of several metals, without excessive sludge production.
Introduction Adsorption is a process by which dissolved substances are removed from solution to the surface of a particle in contact with that solution. Adsorption of organic compounds and some inorganic compounds onto activated carbon has been applied widely as a unit operation in the treatment of drinking water and some waste waters (e.g., 1 , 2 ) . By contrast, oxides of iron, aluminum, and silicon have been shown to be strong adsorbents for metal ions and certain metal oxyanions in laboratory studies ( 3 4 , but these properties have not been exploited in practice to a significant extent. Rather, in situations where removal of metals is required, precipitation by raising the solution pH has been the most widely used technology. Since the base used in these processes is frequently lime, such socalled precipitation processes are usually coprecipitation in actuality, with much of the solid phase often consisting of calcium carbonate. 0013-936X/87/0921-0863$01.50/0
A particularly frustrating aspect of both adsorption and coprecipitation is that large quantities of relatively inert chemicals may have to be added to the waste stream to cause a small quantity of pollutant to partition into the insoluble phase, and hence, large quantities of mostly inert sludge may have to be disposed. With the cost of sludge disposal ever increasing, especially for sludges containing toxic metals that may leach out after disposal, the quantities and qualities of this sludge have become increasingly significant and may dominate the decision as to which type of process is used. The investigation described here involved the laboratory-scale treatment of simulated plating wastes. I t was undertaken to evaluate the potential of a process that has good metal removal efficiency while producing much less sludge than coprecipitation or conventional adsorption processes. Specifically, adsorption is used to concentrate the metals onto a solid surface, from which they are then desorbed into a much smaller volume for recovery or treatment by a conventional process such as precipitation. The adsorbent is then recycled to recover more metal from the original, dilute waste stream. The potential advantages of such a process are the relatively small amount of inert material that would have to be disposed of with the sludge, the possible recovery and reuse of metals, the savings in chemical costs since adsorption can usually be carried out a t a lower pH than coprecipitation, and the ability to adjust removal efficiencies to almost any desired level by varying the conditions during the adsorption step. Since the concentrated metal stream could be treated by precipitation, the process would have the flexibility of adsorption processes, redundancy in the potential to treat the entire waste by precipitation in the event of an upset, and the advantage of low sludge production. A possible problem with this approach is that changes in the solid may interfere with either the desorption or subsequent adsorption steps. Adsorption processes for metal removal have been studied extensively in model systems. The patterns observed when metal oxyhydroxide adsorbents (e.g., oxyhydroxides of Fe, Al, or Si) are used are similar regardless
0 1987 American Chemical Society
Environ. Sci. Technol., Vol. 21, No. 9, 1987 863
of the identity of the solid, and the controlling parameters have been discussed by several authors (6-9). By contrast, the reverse process, desorption, is relatively unstudied. In some studies, desorption has been incomplete in the time frame of the experiments. Hodgsen (12) distinguished rapidly and slowly reversible (he called the latter “irreversible”) bound cobalt on montmorillonite. Throughout this paper we use the term “slowly reversible sorption” to refer to metals that remain bound to the sorbent during the desorption period. While somewhat cumbersome, we feel this term is more accurate than “irreversible sorption”, which implies an unproven permanence of the situation. Farrah and Pickering (13)observed slowly reversible adsorption of lead, cadmium, copper, and nickel on kaolinite, illite, and montmorillonite when desorbing with nitric acid and several other extracting agents. Gadde and Laitenen (14) reported “unrecoverable” fractions of M lead a t pH 2 after adsorption a t pH 6 onto ferrihydrite, far exceeding the fraction originally adsorbed a t low pH. However, completely reversible adsorption has also been observed by several workers. Kinniburgh (15) reported complete pH reversibility when 2 X lo4 M calcium sorbed onto 2.33 X M aluminum hydroxide gel. Gadde and Laitenen (14) found the same with M lead, zinc, and cadmium on 6.25 X loM4 M hydrous manganese oxide. Thus, a change occurs during the adsorption step in some, but not all, systems causing metal ions to be retained when the solution conditions are altered to a range where sorption is not normally observed. In light of this, the specific objectives chosen for this research were to evaluate the adsorption of various metals under simulated treatment conditions, to evaluate their desorbability via pH adjustment, to characterize the extent of and effects of slow desorption if it was observed, and to evaluate the potential for reuse of the “regenerated” adsorbent during several adsorption/desorption cycles.
Methods and Materials Choice of Adsorbent. Although the concentration of adsorbent required for a given metal removal efficiency varies from system to system, the binding strength of metals onto most oxyhydroxides increases in the order Cd INi and Zn ICo < Cu IP b 5 Cr with few exceptions. On a mass basis, amorphous iron oxyhydroxides (often called ferrihydrite) are among the strongest adsorbents, are reasonably inexpensive, and are insoluble over a wide pH range. Their surface properties have also been studied extensively. For these reasons, freshly precipitated ferrihydrite was chosen as the model adsorbent for study. Adsorption Experiments. Adsorption experiments consisted of precipitation and aging of the adsorbent, addition of metals, pH adjustment, equilibration, sampling, and analysis. The adsorbent (ferrihydrite) was prepared by adding stock Fe(N03)3solution (1.0 M, acidified) to a covered glass beaker containing deionized water and sufficient NaN03 to produce a final ionic strength of 0.1 M. While the solution was mixed with a Teflon-coated magnetic stir bar, 1.0 N NaOH was added dropwise to precipitate ferrihydrite. The suspension was then aged at pH 8.0 f 0.2 for 2-4 h. C02-free nitrogen was bubbled into the solution throughout the adsorbent preparation. Fresh ferrihydrite prepared in this way has a point of zero charge (PZC) of 7.9-8.1, a specific surface area of approximately 260 m2/g of Fe, and 9.8 surface sites/nm2 (10).
Adsorbates were zinc(II), cadmium(II), copper(II), lead(II), nickel(II), and chromium(III), added as acidified, concentrated solutions of the nitrate salts. All metals, 864
Environ. Sci. Technol., Vol. 21, No. 9, 1987
radioactive tracers, and ligands, if used, were combined before being added to the adsorbent slurry. In most experiments, 10-40-mL aliquots of the slurry were pipetted into glass centrifuge tubes purged with C0,-free nitrogen gas. Incremental pH adjustments to the bulk slurry were made between samples by addition of nitric acid or sodium hydroxide. The tubes were capped and placed in an end-over-end roller or stirred with small magnetic stir bars. Other experiments were conducted with the entire volume of slurry in the beaker without subsampling; nitrogen purging and stirring continued in these cases. Reaction times were 1-3 h. The minimum time of 1h was sufficient for adsorption, and the extra time resulted in no significant changes in removal. In the experiments described as “coprecipitation”, the procedure was the same, except the metals were added to the solution before the ferrihydrite was precipitated. After aging, the pH was measured within 0.03 unit, and two samples of the slurry were collected, one of which was centrifuged at 2600g for 3 min. Samples of the supernatant liquid and of the unaltered slurry were acidified, the concentration of adsorbate in each sample was measured, and fractional adsorption was calculated. Cadmium and zinc were analyzed by crystal scintillation counting with logCdand 65Zntracers. Chromium, nickel, copper, lead, and iron were measured by flame atomic absorption spectrophotometry. Desorption Experiments. Desorption experiments were “adsorption experiments in reverse” in most regards. A batch of adsorbent and adsorbate was first aged 1-3 h a t a predetermined pH to adsorb the metal(s) of interest. Nitric acid was then added. A pH-adsorption edge was generated by adjusting pH, equilibrating 10-30-mL aliquots for 1-3 h in centrifuge tubes on an end-over-end roller rotating a t approximately 2 rpm, and analyzing for the fraction sorbed as in the adsorption experiments. Desorption kinetics were analyzed by collecting samples from a single batch of slurry a t various times after the pH adjustment. Cycling Experiments. Cycling experiments involved alternating adsorption and desorption steps. These experiments started with the usual procedure for adsorption experiments. After adsorption was measured, the pH was lowered with nitric acid, and sorption was measured again after 1-2 h. The pH was then raised with sodium hydroxide, and the process was repeated. The pH values and reaction times were constant for each experiment. When large batches of solution were used, sampling and analysis were as described above. In small-volume experiments, the supernatant and slurry samples were returned to the system after nondestructive analysis to prevent volume depletion. In these cases, the supernatant liquid and slurry were not acidified prior to analysis.
Results and Discussion Adsorption a n d Desorption i n Noncomplexing Systems. The adsorption of several cations was evaluated initially to provide a comparison with previous work and a base line for evaluating subsequent desorption. When the data are plotted as fractional adsorption vs. pH, the characteristic S-shaped curve frequently called a pH-adsorption edge results. The adsorption edge for zinc, shown in Figure 1, is typical. When conditions of high pH favoring nearly complete adsorption are followed by a low-pH treatment, the pHadsorption edge is not necessarily reproduced. Figure 1 also shows a desorption edge for M zinc on ferrihydrite. Slowly reversible adsorption of about 5 X lo-’ M zinc accounts for the difference in the edges at pH less than
~~
90
1
Table I. Desorption of Several Cations at pH 4.5, Each M Ferrihydrite, Present at lov5M MetalT, from following Adsorption or Coprecipitation a t pH 9.0 for 2 ha
I r 3 M Fe
Adsorption, pH 4.5 cation
% adsorption
chromium(II1) lead(I1) nickel(I1) copper (11) zinc(I1) cadmium(I1)
86 2Sb 0 Ob
0 0 % bound, elapsed time
cation 4
7
6
5
8
9
PH
Flgure 1. Adsorption and desorption of Zn onto ferrihydrite. Desorption was initiated after holding the system at pH 9.0 for 2 h.
60min
21.5 h
120min
Adsorption, pH S.O/Desorption, pH 4.5 chromium(II1) 94 94 94 lead(I1) 48 48 47 nickel(I1) 20 17 14 12 copper(I1) 13 13 9 11 10 zinc(I1) 8 6 3 cadmium(I1)
94 41 0 0
4 0
Coprecipitation, pH S.O/Desorption, pH 4.5 chromium(II1) 97 97 97 lead(I1) 66 65 64 37 34 33 nickel(I1) 38 35 31 copper(I1) zinc(I1) 37 33 29 2 9 6 cadmium(I1)
z I-
p
30min
40
97 58 0
9 23 0
“All cations were present in a single solution. Adsorption at pH 4.5 is listed for comparison. bFrom ref 4.
1(
n
10
20
30
Table 11. Desorption of Zn from 10” M Ferrihydrite Using Various Aging Regimesa
0
40
50
60
70
80
90
100 110 120 1 7 h r
Time (Minutes)
Flgure 2. Kinetics of desorption of Zn from ferrihydrlte. Desorption was at pH 4.5 after a 2-h adsorption step at pH 9.0.
about 6. The “flatness” of the desorption curve at low pH serves to validate the measurements and indicates that slow reversibility is significant over a broad, low-pH range. Desorption kinetics a t pH 4.5 of and 10+ M zinc from ferrihydrite (Figure 2) indicate that most of the reversibly bound sorbate desorbs within 20-30 min; these times are comparable to those necessary for adsorption on ferrihydrite (11). Some of the metal still sorbed after 30 min may eventually desorb, but a fraction is retained at least 17 h and may be bound permanently. Furthermore, while the absolute amount of slowly released zinc increases with increasing total zinc in the system, the fraction of the adsorbed metal that slowly desorbs decreases. Desorption experiments with a variety of adsorbate ions and under a variety of sorption conditions were conducted next to evaluate the generality of the phenomenon and to provide insight into the changes taking place. Table I summarizes sorption results for six cations a t pH 4.5 following three different treatments. The first set of values is from a simple adsorption experiment and provides base-line data. The second set is for desorption following adsorption a t pH 9.0, and the third set is for desorption following coprecipitation a t pH 9.0. The fractions of the metals remaining bound after 2 h at pH 4.5 depend on the system’s history. This fraction is greatest after coprecipitation at pH 9.0, less after adsorption a t pH 9.0, and still less in the absence of a prior sorption step for nickel, copper, zinc, lead, and possibly chromium. (The trend was observed for chromium, but the high base-line adsorption of chromium a t low pH makes it difficult to determine if
aging time, h ferrihydrite with total adsorbent % remaining group alone M zinc aging sorbed
A B C D E F G H I J
0 0 0 0 1 2 5 5 0 44
1 2 3 7 1 1 1 2 44 0
1 2 3 7 2 3 6
I 44 44
9f2 15 f 1 17 f 4 24 f 2 7f3 6f2 4f2 7k2 43 f 1 O f 1
a Values of percent sorbed are averages and standard deviations of six samples (A-H) or two samples (I and J). Desorption pH ranged from 4.0 to 5.3 for each group. A-H: [Zn], = 5 X 10“ M; aging pH 10.0. I and J: [Zn], = M; aging pH 9.0.
the differences among the three rystems were significant.) Little cadmium remained bound following any of the sorption sequences. The effects of aging the solid for various times with and without a metal sorbate present were evaluated next, and the results are presented in Table 11. Samples A-D indicate that slowly reversible adsorption increases the longer the system is held a t alkaline pH. Samples A and E-G all have Zn/Fe contact times of 1h but differ in the length of time the solid was aged at high pH in the absence of Zn. Slowly reversible sorption was about equal in these four samples. By contrast, in the B/E, C/F, D/H, and I / J sets, the two members of each pair had equal total aging times, but one member had a longer Zn/Fe contact time than the other. In each case, longer Zn/Fe contact led to an increase in slowly reversible sorption. Thus, the proc leading to slow reversibility depends on contact betwe 1 Environ. Sci. Technol., Vol. 21, No. 9, 1987
865
100
Table 111. Desorption of lo-’ M Zinc from M Ferrihydrite following Various Aging Time and pH” sample
A B C D
E F G H
I J K
adsorption adsorption desorption % bound after time, h PH PH desorption 1 1
1 1 1 1 2 2 2 2 190
8.9 8.0 7.0 8.9 8.0 7.0 9.0 11.5 4.5 5.0 4.5
4.95 4.95 4.95 4.15 4.15 4.15 4.5 4.5
15 15 6 8 10 2 10 21 0 1 2
“The sorbate was present during aging. Desorption time was 1
h.
the metal and the solid; it is not simply a change that occurs in the solid structure and in the absence of adsorbate that affects subsequent sorption reactions. The slowly reversible quantity of bound zinc was also investigated as a function of the pH of the adsorption and desorption steps (Table 111). For the ranges investigated the effects of these variables were considerably less dramatic than that of aging time. The sets of experiments A/B/C, D/E/F, and G/H suggest that one must increase the aging pH by a t least 2 units to generate a significant increase in the fraction of the adsorbed zinc that is slowly released. Decreasing the desorption pH from 4.95 to 4.15 might have had a slight effect on the slowly reversible metal in these systems, but subsequent experiments failed to confirm this trend. Despite the relatively low values of slowly reversible sorption in many of the systems studied, the absolute values were significant when compared with systems aged only a t low pH with the metal present (no alkaline adsorption step), even when the low-pH aging lasted several days (Table 111). The presence of slowly reversible bound adsorbate in the solid poses an interesting question with regard to adsorbate ions in solution; specifically, if the soluble ions approach the surface, are they “aware” of the slowly reversible fraction? That is, does dissolved metal behave as if the bound fraction were sorbed (i.e., contributing to the sorption density) or as if it were not there? To investigate this issue, an adsorption edge was established for a 5 X M Zn solution between pH 3 and pH 9 (Figure 3, curve a), after which the suspension was kept a t pH 9.0 for 72 h. A desorption edge from pH 9 to pH 3 was then established, allowing 2 h for desorption equilibration (Figure 3, curve b). At pH less than about 5.0, the Zn remaining with the solid represented 40% of the total or 2 X lo4 M. Thus, curve b represents a summation of 2 X 10* M slowly reversible bound Zn plus variable sorption of the remaining 3 x 10-6 M “freely reversible” zinc. By subtracting 2 X lo4 M from the adsorbed zinc a t each pH, the fractional adsorption of only the freely reversible zinc can be computed. The adsorption edge for this fraction is also shown in M Zn was added to Figure 3 (curve c). Finally, 2 X the solution to bring the concentration of freely reversible Zn back to approximately its original value (5 X lo4), and another adsorption edge was established (Figure 3, curve d). This curve could then be manipulated in the same way as the previous one to compute adsorption of the 5 X lo4 M freely reversible zinc (curve e) so that the data could be compared directly with the initial curve for adsorption on fresh Zn-free ferrihydrite. At low pH, there is apparent negative adsorption in curve e, since none of the freely 866
.~
90
Environ. Scl. Technol., Vol. 21,NO. 9, 1987
80
0
f
70 60
0,
9 so +-
2
40
0 w .
30 20
10 0 7
6
8
9
PH
Flgure 3. (a, A) Adsorption of 5 X lo-’ M Zn on fresh, Zn-free ferrihydrite. (b, 0) Desorption after aging system a at pH 9.0 for 72 h. (c, 0)Calculated desorption of the easily exchangeable Zn (3 X lo-’ M) in system b. (d, 0) Adsorption after adding 2 X lo-‘ M Zn to system b (percent adsorption based on total Zn, 7 X M). (e, Q) Calculated adsorption of the easily exchangeable Zn in system d. See text for details of calculations for c and e.
exchangeable Zn adsorbs and some of the previously sorbed Zn desorbs. Other than this, the three curves representing sorption of freely exchangeable Zn (curves a, c, and e) are essentially coincident. Since the soluble Zn behaves almost identically in the presence or absence of bound zinc retained from the prior sorption step, the Zn in solution apparently does not “recognize”slowly reversibly bound Zn as adsorbed Zn. In other words, slowly reversibly bound Zn does not contribute to the activity of reversibly sorbed Zn. This has the effect of increasing the apparent sorptive binding strength and/or capacity when adsorption is evaluated in terms of the total Zn in the system. It was shown earlier that aging ferrihydrite at pH 9-10 in the absence of Zn does not alter subsequent sorption of Zn. This conclusion can now be extended to say that aging ferrihydrite at pH 9-10 in the presence of Zn leads to slowly reversible sorption of some of that zinc but does not alter the sorption properties of the solid for zinc in a subsequent process. Thus, to summarize the results of Tables I1 and I11 and Figure 3, slowly reversible adsorption requires an alkaline aging step during which both ferrihydrite and a metal adsorbate are present. For zinc, the slowly released fraction increases with duration of the aging step for at least 7 h and possibly much longer. Increasing the pH of the aging solution between 7 and 11 increases the slowly released fraction, but the effect is not dramatic for short aging times (12 h). Varying the pH of the desorption step between 4.0 and 5.3 (data not shown) does not affect the slowly reversible fraction. Bound Zn that is not released during a low-pH desorption step does not contribute to the activity of adsorbed Zn with respect to subsequent equilibrium partitioning of dissolved Zn. Possible Causes of Slowly Reversible Adsorption. Formation of a slowly dissolving metal hydroxide surface precipitate or a mixed-metal solid (16-18), creation of and adsorption to sites higher in bonding energy than those initially available ( 4 ) , and diffusion limitations are all possible explanations for slowly reversible adsorption. The absence of enhanced adsorption when the solid is aged with no adsorbate present argues against the creation of high-energy sites, a t least in the sense that such sites are created via rearrangement of bonds in the solid and are available for sorption of metals entering the system subsequently. Similarly, the possibility that slowly re-
M Ferrihydrite as pH is Alternated between
Table IV. Adsorption Changes for lo-' M Zinc, Cadmium, and Nickel on Low and High Values'
% adsorption for cycle
concn, metal
PH
1
2
M zinc
4.0 10.0 5.0 9.0 9.0 4.5
0 99 0 100 96 3
99 9 100 93 19
M cadmium M nickel
I
3
4
5
15 99
22 99 8 100 94 25
19 99 6 100 95 17
I 100 92 21
6
94
Reaction times were 2 h for the zinc and cadmium and 1h for the nickel. Cycling began at low pH for zinc and cadmium and at high pH M, but site coverage is probably low enough to have no impact. for nickel. Nickel was present with five other metals each at
versible adsorption is a result of slow dissolution of a relatively pure surface precipitate seems unlikely in view of the results presented in Figure 3. The chemical activity of such a phase would be constant or nearly so (equal to 1.0 for the pure solid), and in such a case fractional removal of zinc should increase dramatically with increasing total zinc concentration in the system. This is the reverse of the order shown in Figure 3. Furthermore, the likelihood of precipitation of a zinc hydrous oxide solid in a system several orders of magnitude subsaturated seems remote at best, and Benjamin and Bloom (19) have argued on the basis of other evidence that precipitation of such a solid does not occur in these systems a t concentrations of adsorbate less than M. There are a number of other plausible explanations, which include formation of a dilute, mixed hydrous oxide of adsorbate plus iron, slow diffusion of adsorbate into or out of the solid, slow restructuring of the solid in a process that depends on the presence of adsorbate, or some combination of these. Adsorption Cycling between High and Low pH. In the proposed waste treatment process, adsorbent may be recycled and exposed to alternating low- and high-pH solutions. This process was simulated to assess changes in adsorption behavior as the system is cycled between pH extremes. It has been shown that the slowly released fraction of several cations increases when they are present during high-pH aging of ferrihydrite. Therefore, changes in this fraction during cycling experiments were of particular interest. The effects of cycling on adsorption of M zinc, M cadmium, and M nickel are presented in Table IV. The adsorption of all three metals a t high pH is constant and nearly complete for all cycles. The increased adsorption a t low pH of zinc and nickel and the constant low-pH sorption of cadmium as cycling progresses are consistent with the trends in slowly reversible adsorption with the aging time noted earlier (Table 11). Apparently, the low-pH exposure interrupts the high-pH reaction but does not reverse it, so that cycling produces "stepwise" increases in slowly reversible adsorption. Thus, the slowly reversible adsorption of 20% of loy5M zinc after a total of 6-8-h exposure at pH 10.0 (cycles 4 and 5) is comparable to the 24% slowly reversible adsorption obtained after a continuous 7-h exposure (sample D, Table 11). Slowly reversible cadmium adsorption reaches its relatively low and constant value early in the aging process in either system. It was shown earlier that small concentrations of slowly released Zn are not sensed as being adsorbed when other Zn ions in solution approach the surface. However, if ferrihydrite is indeed recycled repeatedly as in the proposed treatment process, slowly reversible sorption of some ions may cause them to build up to large concentrations in the adsorbent phase. If these ions provide new, stronger binding sites, a net increase in overall affinity for other
metal ions may result; conversely, if they provide weak binding sites, a decrease in adsorption of other cations is expected. The possibility that the slowly reversible bound metal occupies adsorption sites of the iron without providing new ones for other metals to bind to is a limiting case of the second alternative. The slowly reversibly bound metal may also act indirectly to strengthen or weaken the bond between the ferrihydrite and other adsorbates. To investigate these possibilities, a series of experiments was run in which the total chromium in the system was gradually increased to a relatively high level, while the concentrations of other ions were held constant. In this way, the effects of site blockage and the possible precipitation of chromium on the behavior of the other ions could be monitored. Chromium was chosen because it is the most strongly sorbed and the most insoluble of the ions studied and was therefore expected to have the greatest effect. In these experiments, a suspension containing M ferrihydrite and M of each of four metals was cycled between pH 4.5 and pH 9.0, with lo4 M chromium added between each cycle. A control with no chromium was also run. The results of all the experiments are shown in Table V. Calculations indicate that Cr(OHI3was 2-3 orders of magnitude supersaturated in all the systems in which it was present at pH 9.0 and undersaturated by a comparable amount a t pH 4.5. Leckie et al. (19) reported 80% adsorption of 5 X 10" M Cr at pH 4.5, so both adsorbed and precipitated Cr may be affecting the sorption of the other ions in some of the systems. While there may have been an increase in the affinity of the solid for zinc, cadmium, and copper at pH 9.0 in the Cr-supplemented system compared to the Cr-free system, the nearly complete adsorption of these metals in both systems makes the comparison difficult. What is clear is that sorption or precipitation of chromium did not interfere with the removal of the other metals. A comparison of the data at pH 4.5 reveals patterns that are moderately consistent for a given metal but inconsistent among metals. That is, sorption of nickel and zinc was generally less in the system with chromium than in the one without it, that of copper was greater, and that of cadmium was about the same in both systems. After the six cycles were completed, the suspensions were left at the low-pH condition overnight, and then their sorption behavior was investigated between pH 3.5 and pH 5.5. The adsorption of all the metals except cadmium was significantly enhanced in the system with chromium (Figure 4). The flatness of the chromium adsorption edge under these conditions suggests that much of the chromium present in the system after the overnight exposure is not rapidly exchangeable. Thus, it appears that the chromium has been incorporated into the ferrihydrite matrix and either has enhanced the affinity of the ferrihydrite for the other metals or is providing strong binding sites itself. The results indicate that a lower desorption Environ. Sci. Technol., Vol. 21, No. 9, 1987
867
Table V. Adsorption of Cations at 10" M on Absence of Chromium(II1)"
M Ferrihydrite with Cycling between pH 4.5 and pH 9.0, in the Presence and Chromium-Free System
M
metal, zinc cadmium nickel copper
PH
1
2
3
9.0 4.5 9.0 4.5 9.0 4.5 9.0 4.5
98 6 98 0 96 3 99 12
100 10 98 1 93 19 98 9
97 18 96 6 92 21 98 12
% adsorption for cycle 4 5 98 26 98 9 94 25 98 13
99 13 98 9 95 17 99 13
overnight
6 99
15 97 5 94 24 100 12
Chromium-Supplemented System % adsorption for cycle ([Cr(III)], X104 M)
M
PH
1 (1.4)
2 (2.7)
3 (3.7)
99 3 99 2 93
chromium (var concn)
9.0 4.5 9.0 4.5 9.0 4.5 9.0 4.5 9.0 4.5
100 0 99 0 99 5 100 17 92 61
100 5 100 7 97 11 100 24 96 47
metal, zinc cadmium nickel copper
0 100 12 88 58
4 (5.1) 98 12 3 100 0 100 27 95 50
5 (5.8)
6 (7.0)
98 12 100 4 98 21 99 34 98 50
98
overnight (7.0) 19
100 1
97 34 100
34 97 70
"Reaction times were 1 h. The overnight exposure was for 14 h starting at pH 5.5, which increased to pH 5.8 for the chromium-deficient svstem and decreased to aH 4.8 for the chromium-suaalemented svstem. .
n.. ,
80
=
70
& w
60
s
2
I o v
=
, ,,
0.1M
7 x 10-4M Chromium Present
~ o - ~cUM ~ o - ~~i M
1
50
z b-
E
40
OL 0 w
30
/
Chromium Absent
20
10 A
.
4
,
A
.
5
n
b
A
6
3
4
"
5
6
PH
Figure 4. Adsorption of several metals on ferrihydrite in the presence and absence of slowly reversible adsorbed Cr(II1).
pH would be needed to maximize release of sorbed Cu and Zn. More importantly, under no experimental condition did adsorption of the metals diminish a t the high-pH condition. In other words, no result suggested that the proposed reuse of the solid was in any way problematic. To summarize these experiments, cycling of ferrihydrite and metal adsorbate ions between low and high pH alternately interrupts and then resumes the process leading to slowly reversible adsorption. The low-pH exposure does not reverse the process, a t least for the time frames investigated. Thus, slowly reversible adsorption of zinc and nickel increases with cycling on the basis of the exposure time a t the high-pH condition. The slowly reversible fraction of cadmium adsorption, which does not increase with high-pH aging, also does not increase with cycling. A significant increase in chromium adsorption density has variable effects on the slowly reversible fraction of other adsorbed metals, but in no case does it inhibit their adsorption a t alkaline pH. I t is not clear at this time if a 868
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maximum, equilibrium quantity of slowly reversible adsorbed metal may eventually be attained. S u m m a r y and Conclusions
Cation adsorption on freshly precipitated ferrihydrite may be partially irreversible in time frames of a few hours, typical of industrial treatment operations. Forming the ferrihydrite in the presence of metal (coprecipitation) and aging the ferrihydrite while adsorbate is bound increase the slowly reversible fraction of copper, lead, nickel, and zinc, and possibly chromium, but not cadmium. During cycling of adsorbate/ferrihydrite systems between low and high pH, the slowly reversible fraction of the metal increases with aging time a t high pH. During low-pH periods, this process is interrupted but not reversed. After desorption of bound metals, ferrihydrite adsorbent retains its ability to bind metal ions and may be recycled. Buildup of slowly reversible bound chromium does not interfere with adsorption of other metals. The effects of buildup of other adsorbates were not investigated. The results support the technical feasibility of a metal removal process in which solid hydrous oxide adsorbent is regenerated and recycled. Though the work here has used ferrihydrite, other hydrous oxides may prove equally or more useful in particular applications. More work remains to test the process fully and to evaluate its economic potential. Registry No. Cu, 7440-50-8; Pb, 7439-92-1; Ni, 7440-02-0; Zn, 7440-66-6; FeS(OH)903,39473-89-7; Cr, 7440-47-3; Cd, 7440-43-9.
Literature Cited (1) Huang, C. P.; Fu, P. L. K. J.-Water Pollut. Control Fed. 1984,56, 233-242. (2) Randtke, S. J.; Snoeyink, V. L. J.-Am. Water Works ASSOC.1983. 75. 406-413. (3) Hansmann, D. D.; Anderson, M. A. Environ. Sci. TechnoE. 1985, 19, 544-551.
Environ. Sci. Technol. i987, 21, 869-875
Hodgsen, J. F. Soil Sei. SOC.Am. Proc. 1960,24,165-168. Farrah, H.; Pickering, W. F. Water, Air, Soil Pollut. 1978, 9,491-498. Gadde, R. R.; Laitenen, H. A. Anal. Chem. 1974, 46, 2022-2026. Kinniburgh, D. G.; Syers, J. K.; Jackson, M. L. Soil Sci. SOC.Am. Proc. 1975,39,464-470. Corey, R. B. In Adsorption of Inorganics at Solid-Liquid Interfaces;Anderson, M. A., Rubin, A. J., Eds.; Ann Arbor Science: Ann Arbor, MI, 1981; pp 161-182. Farley, K. J.; Dzombak, D. A,; Morel, F. M. M. J. Colloid Interface Sci. 1985, 106, 226-242. Davis, J. A.; Fuller, C. C.; Cook, A. D. Geochim. Cosmochim. Acta, in press. Leckie, J. 0.;Appleton, A. R.; Ball, N. B.; Hayes, K. F.; Honeyman, B. D. Electric Power Research Institute: Palo Alto, CA, in press.
Leckie, J. 0.; Benjamin, M. M.; Hayes, K. F.; Kaufman, G.; Altmann, S. Final Report EPRI-RP-910-1;Electric Power Research Institute: Palo Alto, CA, 1980. Dugger, D. L.; Stanton, J. H.; Irby, B. N.; McConnelI, B. L.; Cummings, W. W.; Maatman, R. W. J. Phys. Chem. 1964, 68, 757. Kinniburgh, D. G.; Jackson, M. L. In Adsorption of Inorganics at Solid-Liquid Interfaces;Anderson, M. A., Rubin, A. J., Eds.; Ann Arbor Science: Ann Arbor, MI, 1981; pp 91-160. Schindler, P. W. In Adsorption of Inorganics at SolidLiquid Interfaces;Anderson, M. A,, Rubin, A. J., Eds.; Ann Arbor Science: Ann Arbor, MI, 1981; pp 1-49. James, R. 0.;Healy, T. W. J . Colloid Interface Sei. 1972, 40, 53. Benjamin, M. M.; Leckie, J. 0. Environ. Sci. Technol. 1981, 15, 1050-1057. Davis, J. A.; Leckie, J. 0. J. Colloid Interface Sci. 1978, 67,90-107.
Schultz, M. F. MSE Thesis, University of Washington, Seattle, WA, 1985.
Received for review July 24, 1986. Revised manuscript received February 2, 1987. Accepted April 1, 1987.
Experimental System for Investigating Vapor-Particle Partitioning of Trace Organic Pollutantst Wllllam T. Foremantis and Terry F. Bidleman*itglI Department of Chemistry and Marine Science Program and Belle W. Baruch Institute for Marine Biology and Coastal Research, University of South Carolina, Columbia, South Carolina 29208
rn An experimental system was designed to equilibrate urban air particulate matter on a filter with controlled vapor concentrations of semivolatile organic compounds (SOC) at 20 "C under simulated high-volume air sampling conditions. Vapor-particle distributions (V/P) for organochlorine pesticides and three- to four-ring polycyclic aromatic hydrocarbons were estimated from laboratory measurements of the apparent partition coefficient A[TSP]/F, where A and F a r e the adsorbent- and filterretained SOC concentrations (ng/m3) and [TSP] is the total suspended particle concentration (pg/m3). Laboratory measured A[TSP]/F correlated well with the subcooled liquid-phase vapor pressures ( p o L )of the SOC tested but not with their solid-phase vapor pressures. Comparisons of field and laboratory A[TSP]/F are made, and implications of pol-dependent partitioning to the atmospheric chemistry of SOC are discussed.
Introduction Semivolatile organic compounds (SOC), including three to four ring polycyclic aromatic hydrocarbons (PAH), pesticides, polychlorinated biphenyls (PCB), dibenzo-pdioxins (PCDD), and dibenzofurans (PCDF), are present in air in gaseous and particulate forms. Knowledge of SOC vapor/particle distribution (V/P) is important to understanding the atmospheric transport of these pollutants, because V/P influences the process by which the contaminant returns to the earth and the atmospheric residence time. V/P is also an important consideration in developing This is Contribution No. 684 of the Belle W. Baruch Institute.
*Departmentof Chemistry.
f Present address: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309. "Marine Science Program and Belle W. Baruch Institute for Marine Biology and Coastal Research.
0013-936X/87/0921-0869$01.50/0
sampling methods and designing pollution control systems. Factors influencing V/P in ambient air can be investigated with conventional high-volume (hi-vol) sampling. Air is pulled through a glass-fiber filter (F) to collect particles and then through an adsorbent trap (A) to collect the vapors. The apparent V / P is operationally estimated by the adsorbent-to-filter retained ratio (A/F). Values of A/F for organochlorines (OC) (1) and PAH (2,3)were determined from hi-vol field experiments in several cities. Results of these studies showed that A/F was related to the average sampling temperature (T,kelvin) and the total suspended particle concentration (TSP, pg/m3) by log (A[TSP]/F) = m / T b (1) where A is the adsorbent-retained SOC and F is the filter-retained SOC, both in nanograms per cubic meter. A[TSP]/F has units of nanograms of SOC per cubic meter of air inanograms of SOC per microgram of particles = vapor concentration in air + concentration on particles =
+
CA/CP.
The strong dependence of A/F on temperature suggested a general relationship based on SOC volatility. Plots of field A[TSP]/F at 20 "C vs. OC and PAH vapor pressures revealed that A/F partitioning is controlled largely by the subcooled liquid-phase vapor pressure boL) rather than the solid-phase vapor pressure @Os) ( I , 4 ) . How closely A/F represents the true V/P in the atmosphere remains uncertain. Sampling times of 12 h or more are usually necessary to collect enough SOC for analysis. During the collection period, SOC concentrations and especially ambient temperatures are likely to change, resulting in blow-off losses from or adsorption gains to the particle load on the filter (4-10). Other factors that may affect A/F are variations in particulate matter, including size distribution, surface area, and content of carbonaceous material. The influence of relative humidity on A/F is also unknown. Additional considerations include the possibility
0 1987 American Chemical Society
Environ. Sci. Technol., Vol. 21, No. 9, 1987 869
