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Removal of copper, lead, and cadmium ions by microflotation
Removal of Copper, Lead, and Cadmium Ions by Microflotation z JAMES B. M E L V I L L E AND EGON MATIJEVIC Institute of Colloid and Surface Science and Department of Chemistry, Clarkson College of Technology, Potsdam, New York 13676, and Unilever Research Laboratory, Port Sunlight, Wirral, Cheshire L62 4XN, England Received August 25, 1975; accepted March 22, 1976 Removal of copper, lead, and cadmium ions from aqueous solution by microflotation has been studied as a function of p H in the presence and in the absence of aluminum nitrate and aluminum sulfate, respectively. Laurie acid and ethanol were used as the collector/frother mixture. Efficient removal occurred only when precipitation of a metal hydroxide took place. Residual levels of copper as low as 0.03 ppm were achieved. In the absence of aluminum salts the three metal ions exhibited different behavior but the presence of aluminum salts dominated all systems eliminating these differences. Using a synthetic "sea water" it was shown that precipitating ions could be separated from nonprecipitafing ions.
INTRODUCTION
tion between ion, colloid, and precipitate flotation becomes artificial. This work describes the removal of copper, lead, and cadmium ions from aqueous solutions by microflotation. The latter process (3, 4) is characterized by the use of small bubbles (40-60 pm), and by addition of a suitable collector-frother solution to control bubble size, to promote bubble attachment, and to supply a low, stable foam. Furthermore, the presence of a hydrolyzable metal salt is considered essential as the separations take place over the pH ranges where metal hydroxides precipitate. With aluminum salts as additives efficient removals of materials as diverse as bacteria (3, 5), colloidal silica (6), latex (7), neutral color (8), humic acid (9), and proteins (10) have been reported. Employing the same technique, copper, lead, and cadmium solutes could be separated in the presence and in the absence of aluminum salts if the pH was properly adjusted to cause the precipitation of hydroxides. Rubin (11-14) has applied essentially the same procedure to remove traces of iron, copper, zinc, and lead
Removal of matter, either dissolved or dispersed in a liquid, may be achieved by a variety of so-called adsorptive bubble separation techniques. Several suggestions have been made regarding classification of these procedures (1, 2) based upon considerations such as gas flow rate, whether or not a foam is formed, or whether the material to be separated is dissolved, colloidal, or coarse particulate. However, owing to the complexity of many systems it is often not possible to distinguish between the various categories. Moreover, confusing nomenclature has further tended to obscure rather than clarify the differentiation of the processes involving bubbles as a means of phase separation. This inadequacy is most noticeable when one considers removal of ions, because in the vast majority of cases (with the exception of foam fractionation of soluble complexes) a precipitate is formed prior to bubble attachment. Thus, a distinc1 Supported by a grant from Unilever Research Laboratory, Port Sunlight. 94
Journal oJ Colloid and Interface Science, Vol. 57, No. 1, October 1976
Copyright O 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.
MICROFLOTATION ions from aqueous solution in the absence of aluminum ions. Probably because aluminum ions were not present he designated the process as ion and/or precipitate flotation rather than microflotation. However, we feel that it is more meaningful to regard the technique as microflotation, whatever the solid material floated, especially in view of the aforementioned considerations. This work represents a systematic study of the removal of metal ions by microflotation with special reference to metal hydroxide precipitation domains. I t also relates the removal and the separation efficiency to the colloid stability of the precipitated systems in the absence and in the presence of aluminum salts. Furthermore, the selective separation of hydrolyzable metal ions from solutions of nonhydrolyzable electrolytes, having the composition of synthetic sea water, is demonstrated. Thus, the technique could be used to separate potentially toxic ions from natural waters with little or no detriment to the latter. The mechanism of the microflotation process is discussed. The authors believe that separation of metal ions by microflotation either in water purification or metal recovery is technically feasible and the process might prove economical if a cheaper substitute for ethanol, used to generate microbubbles, could be found. EXPERIMENTAL
1. Materials All chemicals were of the highest purity grade commercially available and were used without further purification. All experiments were conducted with doubly distilled water, the second distillation being carried out in an all-Pyrex still.
2. Microflotation Procedure A detailed description of the microftotation apparatus was published elsewhere (2, 3). The cell consisted of a 600 ml Btichner funnel with a fine sintered glass frit through which humidified nitrogen was introduced at a controlled rate
95
of 30 ml/min (0.38 ml of N~ at STP/cm2/min). All experiments were conducted using a lauric acid collector at a concentration of 1.25 )< 10.4 M and a frother dosage of 2.5 ml/liter of absolute ethanol of the sample; the collector was dissolved in the frother and simultaneously added into the cell. Photographic measurement indicated that the resulting average bubble diameter was about 50 ~m (15). The following procedure was used in all microflotation experiments. A precalculated amount of doubly distilled water which would yield a final volume of 400 ml was introduced into the cell. The requisite amounts of cupric, lead, or cadmium ions, added as nitrate solutions, were pipetted into the cell. In experiments where aluminum salts were also used, either aluminum nitrate or aluminum sulfate solutions were introduced along with the other metal salts. The contents of the cell were mixed thoroughly and a l0 ml sample was withdrawn for analysis. The p H of the solution was adjusted with NaOH and nitrogen was allowed to bubble through the cell for 10 min at a rate of 30 ml/min in order to agitate the contents. The p H was then recorded using a Beckman Research p H meter with a combination glass electrode. One milliliter of collector/frother solution containing 1 mg of lauric acid/ml of ethanol was introduced into the cell and the time was started. Changes in p H during microflotation were recorded at different time intervals. Usually, after 5 min of flotation, samples were removed for analysis. In some cases, sampling was also done earlier and later.
3. Precipitation Experiments To study the precipitation phenomena in systems used in microflotation, duplicate solutions of metal salts were prepared and the p H was adjusted with NaOH. These contained no collector or frother. The samples of different p H were then allowed to stand for 18 hr. At the end of this period they were observed visually by means of a high intensity light beam and the nature of any precipitated solid
Journal of Colloid and Interface Science, VoL 57, No. 1, October 1976
96
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FIG. 1. (a) Percentage removal of copper from solutions containing 1.25 X 10-4 M Cu(NO3)2 ((3) and 1 X 10-3 M Cu(NO3), (A) after 5 rain microflotation as a function of initial pH. Microflotation of copper from solutions containing 1.25 X 10-4 M Cu(NO3)~ as a function of final pH (El). (b) Removal of copper from solutions containing 1.25 X 10-4 M Cu(NO3)2 after 5 min microflotation (©) and after 18 hr by precipitation (V). Shaded area represents coagulated dispersions. was noted. In addition, Rayleigh ratios were determined using a Brice-Phoenix Series 2000 light scattering photometer to measure the intensity of the transmitted beam at 0°C and the scattered intensity at 45 °C. The contents of the tubes were then shaken and Rayleigh ratios were again measured. Finally the samples were centrifuged for 30 rain at 4000 rpm using an International bench centrifuge and the supernatant liquids were taken for analysis.
4. Analyses for Copper, Lead, Cadmium, and Aluminum Analyses for metal ions were performed by means of a Perkin-Elmer model 290B atomic absorption spectrometer. For copper, lead, and cadmium an oxidizing air-acetylene flame was used and for aluminum analysis it was necessary to employ a nitrous oxide-acetylene flame with a high temperature burner head. Detection limits were 0.05 p p m for Cu 2+, 0.2 ppm for Pb 2+, and 0.07 ppm for Cd ~+. RESULTS
1. Copper (a) M icroflotation in the absence of aluminum salts. Figure la shows the removal by micro-
flotation of copper from aqueous solutions of Cu(NO3)~ as a function of pH. Two curves are given for the same system ([Cu 2+] = 1.25 )< 10-4 M or 8 ppm) with p H values taken before the addition of the collector/frother solution (circles) and after 5 min of microflotation (squares). I t is evident that the final p H was considerably lower in all systems at p H < 10. All data to follow will refer to p H measurements made before collector/frother addition since, as will be shown later, these are more meaningful in terms of copper hydroxide precipitate formation. The percent removal of copper by microflotation from solutions containing 1 X 10-3 M Cu(NO3)2 (63 ppm) is given by triangles. I t is obvious that flotation of copper is very sensitive to p H ; below p H 6.5 no removal can be detected and, at the highest p H values, the efficiency of separation becomes smaller. Clearly the p H effect must be related to the precipitation of copper ions as hydroxides. This is illustrated in Fig. lb, which gives the percentage of copper removed by precipitation and centrifugation from aqueous 1.25 X 104 M Cu(NO~)2 solutions as a function of pH. The superimposed flotation curve shows that at p H < 8 the foam separation is closely related to precipitation. At higher p H values precipitation is complete in all cases, but the removal by flotation varies. The precipitation curve refers to the removal of solute metal ions from solution into precipitated solid, regardless of the nature of the latter. Some of the solid phase appeared as stable colloidal dispersions, which did not settle over extended periods of time. In these cases high speed centrifugation was needed prior to determining the removal of ions from the solute state. Other systems, indicated by shading on the abscissa (Fig. lb), gave flocs which settled. Both types of systems make up the precipitation diagram. Obviously, whereas flotation results in the removal of metal ions from the solution in a very short time, precipitation--unless accompanied by flocculation and rapid settling--will produce
Journal of Colloid and Interface Science, Vol. 57, No. 1, October 1976
MICROFLOTATION colloidal dispersions which are stable over very long times. The amount of copper separated by microflotation is also dependent on time. In cases where precipitation after 18 hr resulted in complete removal and microflotation after 5 min did not, separation could be made total by lengthening the flotation time as illustrated in Table I. I t is noteworthy that at the lowest p H the removal by microflotation was never complete, but neither was it by precipitation. Similar results have been attained with the other systems discussed below.
(b) Microflotation in the presence of aluminum salts. I t was shown earlier (3-10) that addition of aluminum salts and adjusting p H led to significant enhancements in microflotation efficiency for a variety of colloidal dispersions. Thus, mixed solutions of copper and aluminum nitrates were tested following the same procedures described above. Figure 2a shows the effect of 1.1 X 10-3 M Al(NO3)3 on the microflotation of 1.25 X 10-4 M Cu(NO3)~ as a function of pH. The two solid curves refer to the percent removal of copper and aluminum ions, respectively, from the mixture. For comparison purposes Fig. 2b
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shows the separation curves for the two metals when the microflotation experiments were carried out with individual solutions of each salt at the same concentration as used in the mixed system. Whereas the separation curve for the aluminum was not significantly affected by the presence of copper, the opposite was not true. The dotted line in Fig. 2a shows the effect of a higher aluminum concentration on the removal of copper by microflotation. Effective fotation was seen over a slightly wider p H range than for the lower aluminum nitrate concentration. Also the flotation peak or maximum seen at p H 7.5 was eliminated. Figure 2c shows the percentage of copper and aluminum ions which precipitate from cJ I
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FIo. 2. (a) Removal of copper (O) and aluminum (O) from solutions containing 1.25 X 10-4 M Cu(NO3)2 and 1.1 X 10-3 M AI(NO~)3 by microflotation for 5 min. Dashed line indicates removal of copper from solutions containing 1.25 X 10-4 M Cu(NO3)~ and 6.25 X 10-3 M Al(NO3)3. (b) Removal of copper (O) and aluminum ( • ) by 5 rain microflotation for 5 rain from solutions containing 1.25 X 10-4 M Cu(NO3)2 and 1.1 X 10-3 M Al(NO3)3, respectively. (c) Removal of copper (O) and aluminum ( • ) after 18 hr by precipitating from solutions containing 1.25 X 10-4 M Cu(NO3)~ and 1.1 X 10-a M A1(NO3)3. Journal of Colloid and Interface Science, V o l . 57, N o . 1, O c t o b e r 1 9 7 6
98
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FIG. 3. (a) Removal of copper (O) and aluminum ($) by microflotation (5 min) from solutionscontaining 1.25 X 10-4 M Cu(NO3)l and 5 X 10-4 M A12(SO4)3. Microflotation of copper (I-1)from solutions containing 1.25 X 10-4 M Cu(NO3)2 and 1 X 10-3 M A12(SO4)3. Dotted line indicates removalof copperfrom 1.25X10-4 M Cu(NOa)2 solutions in the absence of aluminum sulfate. (b) Removal of copper (O) and aluminum ($) by precipitation (18 hr) from solutions containing 1.25 X 10-* M Cu(NO3)i and 5 X 10-4 M A12(SO4)3. Shaded area represents coagulated dispersions. solutions containing 1.25 X 10-4 M Cu(NO~)2 and 1.1 X 10-3 M AI(NO3)3 after standing for 18 hr. The range over which copper precipitated was essentially unchanged; above pH 7 effectively all of the copper was separated from solution, whilst aluminum hydroxide precipitated over the pH range 6-10. Figure 3 illustrates the effect of aluminum sulfate on microflotation of copper from aqueous solutions as a function of pH. Open and closed circles in Fig. 3a denote the removal of copper and aluminum ions, respectively, as a function of pH from solutions containing a mixture of 1.25)< 10-4 M Cu (NO8)2 and 5 )< 10-* M AI~(SO,)3. The efficient separation for the two metals was sensitive to pH, with complete aluminum removal taking place over the pH range 5.5-8.5 and complete removal of copper over the range pH 8.3-9.5. Again, for comparison, removal of copper in the absence of aluminum salts from a solution of the same concentration of copper nitrate is also included (dotted line). The presence of A12(SO4)a greatly enhanced the microflotation of copper. Doubling the amount of A12(SO,), added to Journal of Colloid and Interface Science,
the solutions of 1.25 X 10-4 M Cu(NO3)2 broadened somewhat the pH range over which the complete separation of copper took place, as illustrated by squares. Figure 3b shows the removal of copper and aluminum by precipitation from solutions containing a mixture of 1.25 X 10-4 M Cu(NO3)2 and 5 )< 10-4 M A12(SO,)~. All colloidal precipitates formed were unstable. A comparison of microflotation and precipitation curves shows that the removal of aluminum by both techniques took place over an identical pH range, whereas copper separation boundaries agreed only at low pH.
2. Lead (a) Microflotation in the absence of aluminum salts. Microflotation of lead from solutions containing 1.25 X 10-4 M Pb(NO3)2 (triangles), shown in Fig. 4, was very poor with only slight separation in the pH range 9.510.7. On the other hand, removal of lead by precipitation from similar solutions, after 18 hr (squares), occurred over a broad pH range (6.8-11.6). Thus, the behavior of this system was different from that observed with copper salt solutions. IOC
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FIG.4. Removalof lead from 1.25 X 10-4 M Pb (N03) solutions by microflotation for 5 min (A) and by precipitation after 18 hr (rN).
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(b) Microflotation in the presence of aluminum salts. Addition of 1.1 X 10-a M aluminum nitrate to 1.25 ;< 10-4 M Pb(NO3)2 solutions dramatically enhanced the removal of lead b y microflotation (Fig. 5a). Varying amounts of lead were separated over the p H range 6.8-11.5 with complete removal taking place at p H 9-9.7. Whereas aluminum had such a distinct effect on microflotation of lead the converse was n o t true. Precipitation data for the corresponding systems after standing for 18 hr are shown in Fig. 5b. Again microflotation occurred at those values of p H at which precipitation took place, but complete separation by microflotation did not always coincide with complete removal by precipitation. Substituting aluminum sulfate for aluminum nitrate under otherwise identical conditions somewhat enhanced the microflotation efficiency of lead (Fig. 6a). Microflotation of aluminum occurred over the same p H range as in the absence of lead. As in earlier cases precipitation and microflotation separation domains were in good agreement (Fig. 6b).
3. Cadmium (a) Microflotation in the absence of aluminum salts. Microflotation from solutions containing IO(
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FIG. 6. (a) Removal of lead (O) and aluminum ($) by microflotation (5 rain) from solutions containing 1.25 X 10.4 M Pb(NO~)2 and 5 X 10.4 M A12(SO4)3. (b) Removal of lead (O) and aluminum (O) by precipitation (18 hr) from solutions containing 1.25 X 10.4 M Pb(NO3)2 and 5 X 10.4 M A12(SO4)3. 1.25 X 10.4 M Cd(NO3)2 (Fig. 7, circles) shows that between p H 6 and p H 9.2 about 12% separation was achieved and that at p H 9.2 the removal efficiency rose sharply. However at no p H was removal complete after 5 min microflotation. The sharp rise in separation at p H --~9 coincided with the onset of precipitation (Fig. 7, squares). Precipitation resulted in complete removal of cadmium from solution at p H > 10.4.
(b) Microflotation in the presence of aluminum salts. The effect of aluminum nitrate and aluminum sulfate on microflotation and precipitation of cadmium are illustrated in Figs. 8 and 9, respectively. In all cases the concentration of Cd(NO3)2 was kept at 1.25 X 10.4 M. The concentration of aluminum ions was --~1 ;< 10-3 M. As with other ions, aluminum salts enhanced the microflotation of cadmimn, but the opposite was not true. The relationship between the precipitation and microflotation domains is analogous to the previously described systems.
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FIG. 5. (a) Removal of lead (©) and aluminum ($) by microflotation (5 rain) from solutions containing 1.25 X 10.4 M Pb(N03)2 and 1.1 X 10.3 M AI(NO~)~. (b) Removal of lead (©) and aluminum (O) by precipitation (18 hr) from solutions containing 1.25 M 10.4 M Pb(NO3)2 and 1.1 N 10--3 M AI(NO~)3.
4. The Effect of Indifferent Electrolytes To investigate the effects of other electrolytes on the microflotation of copper ions, and particularly to establish whether these
J o u r n a l of Colloid and Interface Science, V o l . 57, N o . 1, O c t o b e r 1 9 7 6
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(5 min) (©) and by precipitation (18 hr) (V1). ions can be selectively removed from a solution of nonprecipitating electrolytes, a synthetic "sea water" consisting of 0.47 M NaC1, 5 X 10-3 M K2SO4, 1 )< 10-2 M CaC12, and 5.4 >( 10-2 M MgCl2 was prepared. Into this was incorporated 1.25 X 10-4 M Cu(.NO3)~ and 5 X 10-4 M A1~(SO4)3. The removals of various ions by microflotation for 5 rain as a function of pH are listed in Table II. It is clear that the nonprecipitating ions (Ca 2+ and Mg 2+) remained in solution while copper and aluminum ions were separated.
The nature of the precipitated solid was shown to have an important bearing upon the rate of microflotation. The two important parameters affecting this rate are the collision efficiency between microbubbles and particles and their sticking probability once having collided. Particle-bubble adhesion in these systems was aided by the use of laurate ions as a surface active collector. Thus, one would expect microflotation to be most effective when large flocs are present to optimize collision efficiency and when the particles do not have high negative charges which may reduce adsorption of collector ions. This was indeed found to be the case in all systems studied. Often, at lower pH, stable dispersions were produced and 5 min microfotation was insufficient to cause complete removal. At very high pH all systems displayed incomplete separation after 5 rain microflotation, even though precipitation was complete, due to decreased collector adsorption with increasing negative charge. In all cases where separation by microflotation was lower than that by precipitation, but where precipitation took place within the duration of the flotation experiment, comparable removals could be achieved by increasing flotation times to about 20 rain.
DISCUSSION
The results obtained illustrate that in all cases microflotation efficiency is closely related to precipitation of the ions to be removed. The superiority of the microflotation technique to separation by precipitation and settling is also clearly demonstrated. Although removal of copper, lead, and cadmium from aqueous solution can be achieved by precipitation alone, the times are much longer than by microfotation. Indeed, where stable precipitates are formed, settling without centrifugation was found to be incomplete even after 18 hr or more. The same systems could be separated by microflotation within minutes.
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FIG. 8. (a) Removal of cadmium (O) and aluminum ( • ) by microflotation (5 rain) from solutions containing 1.25 X 10-4 M Cd(NO3)2 and 1.1 X 10-8 M AI(NO3)8. (b) Removal of cadmium (O) and aluminum ( • ) by precipitation (18 hr) from solutions containing 1.25 X 10-4 M Cd(NO3)2 and 1.1 X 10- 3 M AI(NO,)3.
Journal o/Colloid and l~erface Science, Vol. 57, No. 1, October 1976
MICROFLOTATION Thus, on the basis of the experiments presented here it may be concluded that an ion can be removed by microflotation if it can be precipitated. Of particular importance are the results obtained using the synthetic sea water as these demonstrate that separation of precipitating and nonprecipitating ions can be achieved and that toxic metal ions could be separated from systems containing other indifferent ions, including natural water, without affecting the contents of the indifferent ions. The three ions studied here Fcopper(II), lead(II), and cadmium-] exhibited considerable differences in microflotation in the absence of aluminum ions, but these differences were eliminated by the presence of the latter. I t is thus of interest to discuss these systems individually. Precipitation phenomena of copper from solutions containing 1.25 X 10-4 M Cu(NO3)2 were in good agreement with those reported earlier (16). I t is likely for this concentration of copper that the solid phase formed at pH <,--,10.3 was a mixture of the basic nitrate, Cu(OH)I.~(NO3)0.5 and the hydroxide Cu(OH)2 (16). Above the p H 10.3 the solid phase was probably Cu(OH)2. Rayleigh ratios of samples, taken after precipitation had IOC
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FIG. 9. (a) Removal of cadmium (©) and aluminum ( • ) by microflotation (5 min) from solutions containing 1.25 X 10-4 M Cd(NO3)2 and 5 X 10-4 M A1~(SO4)3. (b) Removal of cadmium (O) and aluminum (0) by precipitation (18 hr) from solutions containing 1.25 X 10-4 M and 5 X 10-4M AI~(SO,)a.
101 TABLE II
Microflotation from Solutions of 1.25 X 10-4 M Cu(NO3)~ and 5 X 10-4 M A12(SO4)3in Synthetic Sea Water pH
6.6 7.2 7.4 8.2 9.3
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Removal
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95 94 95 79 80
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occurred, showed a minimum at p H 9.7 which coincided with the small minimum seen in the microflotation curve. This is possibly indicative of the formation of smaller particles and hence a slower flotation rate. The particles have been found to be negatively charged at p H > ~5(16). Further evidence that precipitates were not positively charged follows from the observation that no adhesion to the glass occurred. It is perhaps surprising, therefore, that over the p H range 6.5-8.7, where perfectly stable colloidal precipitates were produced, the laurate anion served as a very efficient collector. Indeed the exact correspondence of the precipitation and microflotation curves over this p H range indicates that essentially all of the copper that was precipitated was removed. The behavior of lead regarding microflotation from 1.25 X 10-4 M Pb(NO3)2 solutions was totally different to that of copper and offered some interesting points which contrast somewhat with earlier findings. Separation of lead by microffotation occurred over the narrow p H range 9.5 10.7 and then only to a small extent. This seemed to be due to a considerable amount of lead remaining in solution and interacting with the laurate ions, as evidenced by the lack of formation of a foam at all p H values below about 11, when PbO2- ions become abundant. However, in all systems over the p H range 6.8-11.5 the presence of precipitates was observed using a high intensity light beam. Thus, although sufficient lead remained in solution to cancel the efficiency of the laurate ions, at least part of the lead had
Journal of Colloid and Interface S~ience, Vol. 57, No. 1, October 1976
102
MELVILLE AND MATIJEVI(~
been removed from the solute state. This is in contrast to Rubin's (2, 14) statement that at 2 X 10-4 M Pb 2+ there was no evidence of precipitate flotation of insoluble hydroxide at any pH. However, Rubin's findings that at pH 8-9 flotation was effectively complete with a collector to metal ion ratio of 1 (the ratio also used throughout this work) and, moreover, that at these pH values flotation was essentially independent of collector ratio seem to be indicative more of "precipitate flotation" than "ion flotation." Also the precipitates we observed were very finely dispersed and difficult to detect without good illumination. Clearly if the equilibrium constants used by Rubin and taken from Latimer (17) and Fuerstenau and Atak (18) are correct then lead hydroxide should not precipitate at the concentration of Pb(NO3)2 (1.25 X 10- 4 M) used in this work. Notwithstanding, Fig. 4 confirmed that after 18 hr precipitation occurred over the pH range 6.8-11.7 and moreover, that it was complete at pH 10; at this pH maximum flotation was found. Lest there be any doubt over the accuracy of the lead concentrations the stock lead nitrate solutions were analyzed gravimetrically and found to be as supposed. This work would indicate that lead hydroxide or a basic lead salt (such as basic nitrate) is less soluble than indicated in the literature. The discrepancy may arise from the fact that the precipitate formed is in a very highly dispersed state which is not readily detected. Apparently, it is the state of lead in the system which accounted for the difference in the precipitation and the flotation behavior shown in Fig. 4. This is best illustrated by the following experiment. The pH of a 400 ml solution of 1.25 X 10-4 M Pb(NO3)2 was adjusted to 9.6, the sample was allowed to stand in a stoppered vessel for 18 hr, and then the microflotation was performed in the usual manner. Removal was found to be 83°/o, compared with 10aTo, if microflotation was carried out with the same system after 5 rain. The former value is in good agreement with the 85% removal by precipitation shown in Fig. 4. Apparently the rate determining step in microflotation from
1.25 )< 10-4 M Pb(NOz)2 solutions is not related to bubble/particle collision or interaction but rather to the slow rate of precipitation of the solid phase. This is further substantiated by the fact that froth formed in systems foated after 18 hr of aging. Ferguson, Hinkle, and Watson (19) from their foam separation studies on Pb(II) and Cd(II) concluded that adsorption of lead on glassware could lead to errors in so called "ion flotation" experiments. Obviously no such effect could account for the results observed in this work, because adsorption would have been reflected in apparent flotation where there was actually none. It was, however, noticeable that below about pH 8.5, particles, if allowed to stand for long periods of time, tended to adhere to the walls of the containers and it was felt desirable to raise the pH of such samples with NaOH before analysis. The only systems in which microflotation was achieved in the absence of precipitation were solutions of 1.25 X 10-4 M Cd(NOa)2 below about pH 9.2. This was undoubtedly due to ion flotation, i.e., flotation of the metal ion collector complex and was much less efficient than flotation of the hydrous cadmium oxide (precipitate flotation), the onset of which was seen at pH 9.2. The addition of aluminum ions dominated all of the systems studied and in their presence all three metal ions behaved similarly. At the same time the behavior of aluminum sulfate and aluminum nitrate was not appreciably affected by other ions. Perhaps this is not too surprising as the aluminum ions were in approximately a tenfold excess. Generally the effect of the aluminum ions was to enhance microflotation by broadening the pH domain at which precipitation of the copper, lead, or cadmium occurred. Aluminum sulfate gave greater benefit to microflotation than did the nitrate salt as the colloidal precipitates produced in the presence of sulfate ions were always unstable at all pH values. Also, aluminum salts increased the rate at which solid phase was formed from lead solutions, thereby enhancing microflotation. Specifically, sulfate ions probably caused
Journal of Colloid and Intfrfac¢ Selene#, Vol. 57, No. 1. October 1976
103
MICROFLOTATION precipitation of lead sulfate or of basic lead sulfate. The role of the collector in these experiments merits some discussion. Addition of lauric acid (1.25 X 10-4 M) caused the p H to decrease in all experiments except those at high pH, due to the dissociation of the lauric acid. Although such decreases in p H during microflotation have been noted (e.g., Ref. (9)) little has been said about their significance. The dissociation constant of lauric acid is 10-5 (20). This means that at p H 5 lauric acid is only 500-/o dissociated, the remainder being essentially insoluble, while at p H 3 or less more than 990-/o acid precipitates. Under these conditions the collector would be effectively absent from the system and this could explain why, for example, silica was found not to float with a lauric acid collector (9) at low pH, despite being in a coagulated state. In the majority of experiments presented in this work the p H was sufficiently high for complete dissociation of lauric acid to have taken place. However, at the lower p H values ( < 5 ) in the presence of aluminum sulfate incomplete dissociation was reflected in rather low surface foams, and, had the domains of interest occurred at still lower pH, then the use of an alternative anionic collector (e.g., sodium dodecyl sulfate) would have been necessary. Finally, it remains to mention the sensitivity of the microflotation technique. By taking a 100-ml sample from a flotation experiment using copper nitrate and aluminum nitrate and concentrating by evaporation the residual copper was shown to be 0.03 ppm. This value is well below levels found in m a n y drinking water supplies (21). Similar experiments were not performed for cadmium and lead but removal to the limit of detection showed that lead levels were reduced to <0.2 p p m and cadmium to <0.07 ppm. ACKNOWLEDGMENT One of us (J.B.M.) is grateful to Unilever Research Laboratory, Port Sunlight, England, for a leave of absence. We also acknowledge the assistance of Professor E. A. Cassell in the early stages of this work.
REFERENCES 1. KARGER, B. L., GRIEVES, R. B., LE~LICH, R., ROBIN, A. J., A N D SEBBA, F., Separ. Sci. 2, 401 (1967). 2. ROBIN,A. J., J. Amer. Water Works Assoc. 60, 832 (1968). 3. RVBIN, A. J. AND CASSELL, E. A., Proc. 14th Southern Water Resources and Pollution Control Conf. 14, 222 (1965).
4. ROBIN, A. J., CASSELL, E. A., HENDERSON, O., JOHNSON,J. D., ANDLAMB,J. C., Biotechnol. Biol. 8, 135 (1966). 5. RtlBIN, A. J. AND LACKEY,J. Amer. Water Works Assoc. 60, 1156 (1968). 6. MANGRAVITE, F. J., JR., CASSELL, E. A., AND MATIJEVI~:, E., J. Colloid Interface Sci. 39, 357
(1972). 7. CASSELL, E. A., MATIJEV1C, E., MANGRAVITE,F. J., JR., BUZZELL, T., M., AND BLABAC, S. B., AIChE J. 17, 1486 (1971). 8. CASSELL,E. A., ROBIN, A. J., LA FEVER, M. B., AND MATIJEVId, E., Proc. 23rd Purdue Ind. Waste Conf. 23, 966 (1968). 9. MANGRAVITE,F. J., JR., BUZZELL,T. D., CASSELL, E. A., MATIJEVIC, E., AND SAXTON, G. B., J. Amer. Water Works Assoc. 67, 88 (1975).
10. KOISHI,M. ANDMATIJEVld, E. to appear. 11. ROBIN, A. J., J. Amer. Water Works Assoc. 60, 832 (1968). 12. ROBIN, A. J. ANDJOHNSON,J. D., Anal. Chem. 39, 298 (1967). 13. ROBIN, A. J., JOHNSON, J. D., AND LAMB, J. C., Ind. Eng. Chem. Process Design Develop. 5, 368
(1966). 14. ROBIN, A. J., in "Adsorptive Bubble Separation Techniques," (R. Lemlich, Ed.), Academic Press, New York, 1974. 15. CASSELL,E. A., KAOFMAN,K. M., ANDMATIJEVIC, E., Water Res. 9, 1017 (1975). 16. McFADYEN,P. ANDMATIJEVIC, E., J. [norg. Nucl. Chem. 35, 1883 (1973). 17. LATIMER,W. M., "Oxidation Potentials," p. 151. Prentice-Hall, Inc., Englewood Cliffs, N.J., 1952. 18. FOERSTENAU,M. C. ANDATAK,S., Trans. Met. Soc. Amer. Inst. Mining Met. Eng. 232, 24 (1965). 19. FERGUSON,B. B., HINKLE, C., ANDWILSON, D. J., Separ. Sci. 2, 125 (1974). 20. YOUNG,S. L. ANDMATIJEVIC,E., ANDMEITES, L., J. Phys. Chem. 78, 2626 (1974).
21. Committee on Water Quality Goals, A.W.W.A. Pt 2, 60, Sept., 1973.
Journal of Colloid and Interface Science, Vol. 57. No. 1, October 1976
INTRODUCTION
tion between ion, colloid, and precipitate flotation becomes artificial. This work describes the removal of copper, lead, and cadmium ions from aqueous solutions by microflotation. The latter process (3, 4) is characterized by the use of small bubbles (40-60 pm), and by addition of a suitable collector-frother solution to control bubble size, to promote bubble attachment, and to supply a low, stable foam. Furthermore, the presence of a hydrolyzable metal salt is considered essential as the separations take place over the pH ranges where metal hydroxides precipitate. With aluminum salts as additives efficient removals of materials as diverse as bacteria (3, 5), colloidal silica (6), latex (7), neutral color (8), humic acid (9), and proteins (10) have been reported. Employing the same technique, copper, lead, and cadmium solutes could be separated in the presence and in the absence of aluminum salts if the pH was properly adjusted to cause the precipitation of hydroxides. Rubin (11-14) has applied essentially the same procedure to remove traces of iron, copper, zinc, and lead
Removal of matter, either dissolved or dispersed in a liquid, may be achieved by a variety of so-called adsorptive bubble separation techniques. Several suggestions have been made regarding classification of these procedures (1, 2) based upon considerations such as gas flow rate, whether or not a foam is formed, or whether the material to be separated is dissolved, colloidal, or coarse particulate. However, owing to the complexity of many systems it is often not possible to distinguish between the various categories. Moreover, confusing nomenclature has further tended to obscure rather than clarify the differentiation of the processes involving bubbles as a means of phase separation. This inadequacy is most noticeable when one considers removal of ions, because in the vast majority of cases (with the exception of foam fractionation of soluble complexes) a precipitate is formed prior to bubble attachment. Thus, a distinc1 Supported by a grant from Unilever Research Laboratory, Port Sunlight. 94
Journal oJ Colloid and Interface Science, Vol. 57, No. 1, October 1976
Copyright O 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.
MICROFLOTATION ions from aqueous solution in the absence of aluminum ions. Probably because aluminum ions were not present he designated the process as ion and/or precipitate flotation rather than microflotation. However, we feel that it is more meaningful to regard the technique as microflotation, whatever the solid material floated, especially in view of the aforementioned considerations. This work represents a systematic study of the removal of metal ions by microflotation with special reference to metal hydroxide precipitation domains. I t also relates the removal and the separation efficiency to the colloid stability of the precipitated systems in the absence and in the presence of aluminum salts. Furthermore, the selective separation of hydrolyzable metal ions from solutions of nonhydrolyzable electrolytes, having the composition of synthetic sea water, is demonstrated. Thus, the technique could be used to separate potentially toxic ions from natural waters with little or no detriment to the latter. The mechanism of the microflotation process is discussed. The authors believe that separation of metal ions by microflotation either in water purification or metal recovery is technically feasible and the process might prove economical if a cheaper substitute for ethanol, used to generate microbubbles, could be found. EXPERIMENTAL
1. Materials All chemicals were of the highest purity grade commercially available and were used without further purification. All experiments were conducted with doubly distilled water, the second distillation being carried out in an all-Pyrex still.
2. Microflotation Procedure A detailed description of the microftotation apparatus was published elsewhere (2, 3). The cell consisted of a 600 ml Btichner funnel with a fine sintered glass frit through which humidified nitrogen was introduced at a controlled rate
95
of 30 ml/min (0.38 ml of N~ at STP/cm2/min). All experiments were conducted using a lauric acid collector at a concentration of 1.25 )< 10.4 M and a frother dosage of 2.5 ml/liter of absolute ethanol of the sample; the collector was dissolved in the frother and simultaneously added into the cell. Photographic measurement indicated that the resulting average bubble diameter was about 50 ~m (15). The following procedure was used in all microflotation experiments. A precalculated amount of doubly distilled water which would yield a final volume of 400 ml was introduced into the cell. The requisite amounts of cupric, lead, or cadmium ions, added as nitrate solutions, were pipetted into the cell. In experiments where aluminum salts were also used, either aluminum nitrate or aluminum sulfate solutions were introduced along with the other metal salts. The contents of the cell were mixed thoroughly and a l0 ml sample was withdrawn for analysis. The p H of the solution was adjusted with NaOH and nitrogen was allowed to bubble through the cell for 10 min at a rate of 30 ml/min in order to agitate the contents. The p H was then recorded using a Beckman Research p H meter with a combination glass electrode. One milliliter of collector/frother solution containing 1 mg of lauric acid/ml of ethanol was introduced into the cell and the time was started. Changes in p H during microflotation were recorded at different time intervals. Usually, after 5 min of flotation, samples were removed for analysis. In some cases, sampling was also done earlier and later.
3. Precipitation Experiments To study the precipitation phenomena in systems used in microflotation, duplicate solutions of metal salts were prepared and the p H was adjusted with NaOH. These contained no collector or frother. The samples of different p H were then allowed to stand for 18 hr. At the end of this period they were observed visually by means of a high intensity light beam and the nature of any precipitated solid
Journal of Colloid and Interface Science, VoL 57, No. 1, October 1976
96
MELVILLE AND MATIJEVIC
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FIG. 1. (a) Percentage removal of copper from solutions containing 1.25 X 10-4 M Cu(NO3)2 ((3) and 1 X 10-3 M Cu(NO3), (A) after 5 rain microflotation as a function of initial pH. Microflotation of copper from solutions containing 1.25 X 10-4 M Cu(NO3)~ as a function of final pH (El). (b) Removal of copper from solutions containing 1.25 X 10-4 M Cu(NO3)2 after 5 min microflotation (©) and after 18 hr by precipitation (V). Shaded area represents coagulated dispersions. was noted. In addition, Rayleigh ratios were determined using a Brice-Phoenix Series 2000 light scattering photometer to measure the intensity of the transmitted beam at 0°C and the scattered intensity at 45 °C. The contents of the tubes were then shaken and Rayleigh ratios were again measured. Finally the samples were centrifuged for 30 rain at 4000 rpm using an International bench centrifuge and the supernatant liquids were taken for analysis.
4. Analyses for Copper, Lead, Cadmium, and Aluminum Analyses for metal ions were performed by means of a Perkin-Elmer model 290B atomic absorption spectrometer. For copper, lead, and cadmium an oxidizing air-acetylene flame was used and for aluminum analysis it was necessary to employ a nitrous oxide-acetylene flame with a high temperature burner head. Detection limits were 0.05 p p m for Cu 2+, 0.2 ppm for Pb 2+, and 0.07 ppm for Cd ~+. RESULTS
1. Copper (a) M icroflotation in the absence of aluminum salts. Figure la shows the removal by micro-
flotation of copper from aqueous solutions of Cu(NO3)~ as a function of pH. Two curves are given for the same system ([Cu 2+] = 1.25 )< 10-4 M or 8 ppm) with p H values taken before the addition of the collector/frother solution (circles) and after 5 min of microflotation (squares). I t is evident that the final p H was considerably lower in all systems at p H < 10. All data to follow will refer to p H measurements made before collector/frother addition since, as will be shown later, these are more meaningful in terms of copper hydroxide precipitate formation. The percent removal of copper by microflotation from solutions containing 1 X 10-3 M Cu(NO3)2 (63 ppm) is given by triangles. I t is obvious that flotation of copper is very sensitive to p H ; below p H 6.5 no removal can be detected and, at the highest p H values, the efficiency of separation becomes smaller. Clearly the p H effect must be related to the precipitation of copper ions as hydroxides. This is illustrated in Fig. lb, which gives the percentage of copper removed by precipitation and centrifugation from aqueous 1.25 X 104 M Cu(NO~)2 solutions as a function of pH. The superimposed flotation curve shows that at p H < 8 the foam separation is closely related to precipitation. At higher p H values precipitation is complete in all cases, but the removal by flotation varies. The precipitation curve refers to the removal of solute metal ions from solution into precipitated solid, regardless of the nature of the latter. Some of the solid phase appeared as stable colloidal dispersions, which did not settle over extended periods of time. In these cases high speed centrifugation was needed prior to determining the removal of ions from the solute state. Other systems, indicated by shading on the abscissa (Fig. lb), gave flocs which settled. Both types of systems make up the precipitation diagram. Obviously, whereas flotation results in the removal of metal ions from the solution in a very short time, precipitation--unless accompanied by flocculation and rapid settling--will produce
Journal of Colloid and Interface Science, Vol. 57, No. 1, October 1976
MICROFLOTATION colloidal dispersions which are stable over very long times. The amount of copper separated by microflotation is also dependent on time. In cases where precipitation after 18 hr resulted in complete removal and microflotation after 5 min did not, separation could be made total by lengthening the flotation time as illustrated in Table I. I t is noteworthy that at the lowest p H the removal by microflotation was never complete, but neither was it by precipitation. Similar results have been attained with the other systems discussed below.
(b) Microflotation in the presence of aluminum salts. I t was shown earlier (3-10) that addition of aluminum salts and adjusting p H led to significant enhancements in microflotation efficiency for a variety of colloidal dispersions. Thus, mixed solutions of copper and aluminum nitrates were tested following the same procedures described above. Figure 2a shows the effect of 1.1 X 10-3 M Al(NO3)3 on the microflotation of 1.25 X 10-4 M Cu(NO3)~ as a function of pH. The two solid curves refer to the percent removal of copper and aluminum ions, respectively, from the mixture. For comparison purposes Fig. 2b
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shows the separation curves for the two metals when the microflotation experiments were carried out with individual solutions of each salt at the same concentration as used in the mixed system. Whereas the separation curve for the aluminum was not significantly affected by the presence of copper, the opposite was not true. The dotted line in Fig. 2a shows the effect of a higher aluminum concentration on the removal of copper by microflotation. Effective fotation was seen over a slightly wider p H range than for the lower aluminum nitrate concentration. Also the flotation peak or maximum seen at p H 7.5 was eliminated. Figure 2c shows the percentage of copper and aluminum ions which precipitate from cJ I
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FIo. 2. (a) Removal of copper (O) and aluminum (O) from solutions containing 1.25 X 10-4 M Cu(NO3)2 and 1.1 X 10-3 M AI(NO~)3 by microflotation for 5 min. Dashed line indicates removal of copper from solutions containing 1.25 X 10-4 M Cu(NO3)~ and 6.25 X 10-3 M Al(NO3)3. (b) Removal of copper (O) and aluminum ( • ) by 5 rain microflotation for 5 rain from solutions containing 1.25 X 10-4 M Cu(NO3)2 and 1.1 X 10-3 M Al(NO3)3, respectively. (c) Removal of copper (O) and aluminum ( • ) after 18 hr by precipitating from solutions containing 1.25 X 10-4 M Cu(NO3)~ and 1.1 X 10-a M A1(NO3)3. Journal of Colloid and Interface Science, V o l . 57, N o . 1, O c t o b e r 1 9 7 6
98
MELVILLE AND MATIJEVIC
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FIG. 3. (a) Removal of copper (O) and aluminum ($) by microflotation (5 min) from solutionscontaining 1.25 X 10-4 M Cu(NO3)l and 5 X 10-4 M A12(SO4)3. Microflotation of copper (I-1)from solutions containing 1.25 X 10-4 M Cu(NO3)2 and 1 X 10-3 M A12(SO4)3. Dotted line indicates removalof copperfrom 1.25X10-4 M Cu(NOa)2 solutions in the absence of aluminum sulfate. (b) Removal of copper (O) and aluminum ($) by precipitation (18 hr) from solutions containing 1.25 X 10-* M Cu(NO3)i and 5 X 10-4 M A12(SO4)3. Shaded area represents coagulated dispersions. solutions containing 1.25 X 10-4 M Cu(NO~)2 and 1.1 X 10-3 M AI(NO3)3 after standing for 18 hr. The range over which copper precipitated was essentially unchanged; above pH 7 effectively all of the copper was separated from solution, whilst aluminum hydroxide precipitated over the pH range 6-10. Figure 3 illustrates the effect of aluminum sulfate on microflotation of copper from aqueous solutions as a function of pH. Open and closed circles in Fig. 3a denote the removal of copper and aluminum ions, respectively, as a function of pH from solutions containing a mixture of 1.25)< 10-4 M Cu (NO8)2 and 5 )< 10-* M AI~(SO,)3. The efficient separation for the two metals was sensitive to pH, with complete aluminum removal taking place over the pH range 5.5-8.5 and complete removal of copper over the range pH 8.3-9.5. Again, for comparison, removal of copper in the absence of aluminum salts from a solution of the same concentration of copper nitrate is also included (dotted line). The presence of A12(SO4)a greatly enhanced the microflotation of copper. Doubling the amount of A12(SO,), added to Journal of Colloid and Interface Science,
the solutions of 1.25 X 10-4 M Cu(NO3)2 broadened somewhat the pH range over which the complete separation of copper took place, as illustrated by squares. Figure 3b shows the removal of copper and aluminum by precipitation from solutions containing a mixture of 1.25 X 10-4 M Cu(NO3)2 and 5 )< 10-4 M A12(SO,)~. All colloidal precipitates formed were unstable. A comparison of microflotation and precipitation curves shows that the removal of aluminum by both techniques took place over an identical pH range, whereas copper separation boundaries agreed only at low pH.
2. Lead (a) Microflotation in the absence of aluminum salts. Microflotation of lead from solutions containing 1.25 X 10-4 M Pb(NO3)2 (triangles), shown in Fig. 4, was very poor with only slight separation in the pH range 9.510.7. On the other hand, removal of lead by precipitation from similar solutions, after 18 hr (squares), occurred over a broad pH range (6.8-11.6). Thus, the behavior of this system was different from that observed with copper salt solutions. IOC
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V o l . 5 7 , N o . 1, O c t o b e r 1 9 7 6
MICROFLOTATION
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(b) Microflotation in the presence of aluminum salts. Addition of 1.1 X 10-a M aluminum nitrate to 1.25 ;< 10-4 M Pb(NO3)2 solutions dramatically enhanced the removal of lead b y microflotation (Fig. 5a). Varying amounts of lead were separated over the p H range 6.8-11.5 with complete removal taking place at p H 9-9.7. Whereas aluminum had such a distinct effect on microflotation of lead the converse was n o t true. Precipitation data for the corresponding systems after standing for 18 hr are shown in Fig. 5b. Again microflotation occurred at those values of p H at which precipitation took place, but complete separation by microflotation did not always coincide with complete removal by precipitation. Substituting aluminum sulfate for aluminum nitrate under otherwise identical conditions somewhat enhanced the microflotation efficiency of lead (Fig. 6a). Microflotation of aluminum occurred over the same p H range as in the absence of lead. As in earlier cases precipitation and microflotation separation domains were in good agreement (Fig. 6b).
3. Cadmium (a) Microflotation in the absence of aluminum salts. Microflotation from solutions containing IO(
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FIG. 6. (a) Removal of lead (O) and aluminum ($) by microflotation (5 rain) from solutions containing 1.25 X 10.4 M Pb(NO~)2 and 5 X 10.4 M A12(SO4)3. (b) Removal of lead (O) and aluminum (O) by precipitation (18 hr) from solutions containing 1.25 X 10.4 M Pb(NO3)2 and 5 X 10.4 M A12(SO4)3. 1.25 X 10.4 M Cd(NO3)2 (Fig. 7, circles) shows that between p H 6 and p H 9.2 about 12% separation was achieved and that at p H 9.2 the removal efficiency rose sharply. However at no p H was removal complete after 5 min microflotation. The sharp rise in separation at p H --~9 coincided with the onset of precipitation (Fig. 7, squares). Precipitation resulted in complete removal of cadmium from solution at p H > 10.4.
(b) Microflotation in the presence of aluminum salts. The effect of aluminum nitrate and aluminum sulfate on microflotation and precipitation of cadmium are illustrated in Figs. 8 and 9, respectively. In all cases the concentration of Cd(NO3)2 was kept at 1.25 X 10.4 M. The concentration of aluminum ions was --~1 ;< 10-3 M. As with other ions, aluminum salts enhanced the microflotation of cadmimn, but the opposite was not true. The relationship between the precipitation and microflotation domains is analogous to the previously described systems.
75
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FIG. 5. (a) Removal of lead (©) and aluminum ($) by microflotation (5 rain) from solutions containing 1.25 X 10.4 M Pb(N03)2 and 1.1 X 10.3 M AI(NO~)~. (b) Removal of lead (©) and aluminum (O) by precipitation (18 hr) from solutions containing 1.25 M 10.4 M Pb(NO3)2 and 1.1 N 10--3 M AI(NO~)3.
4. The Effect of Indifferent Electrolytes To investigate the effects of other electrolytes on the microflotation of copper ions, and particularly to establish whether these
J o u r n a l of Colloid and Interface Science, V o l . 57, N o . 1, O c t o b e r 1 9 7 6
100
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(5 min) (©) and by precipitation (18 hr) (V1). ions can be selectively removed from a solution of nonprecipitating electrolytes, a synthetic "sea water" consisting of 0.47 M NaC1, 5 X 10-3 M K2SO4, 1 )< 10-2 M CaC12, and 5.4 >( 10-2 M MgCl2 was prepared. Into this was incorporated 1.25 X 10-4 M Cu(.NO3)~ and 5 X 10-4 M A1~(SO4)3. The removals of various ions by microflotation for 5 rain as a function of pH are listed in Table II. It is clear that the nonprecipitating ions (Ca 2+ and Mg 2+) remained in solution while copper and aluminum ions were separated.
The nature of the precipitated solid was shown to have an important bearing upon the rate of microflotation. The two important parameters affecting this rate are the collision efficiency between microbubbles and particles and their sticking probability once having collided. Particle-bubble adhesion in these systems was aided by the use of laurate ions as a surface active collector. Thus, one would expect microflotation to be most effective when large flocs are present to optimize collision efficiency and when the particles do not have high negative charges which may reduce adsorption of collector ions. This was indeed found to be the case in all systems studied. Often, at lower pH, stable dispersions were produced and 5 min microfotation was insufficient to cause complete removal. At very high pH all systems displayed incomplete separation after 5 rain microflotation, even though precipitation was complete, due to decreased collector adsorption with increasing negative charge. In all cases where separation by microflotation was lower than that by precipitation, but where precipitation took place within the duration of the flotation experiment, comparable removals could be achieved by increasing flotation times to about 20 rain.
DISCUSSION
The results obtained illustrate that in all cases microflotation efficiency is closely related to precipitation of the ions to be removed. The superiority of the microflotation technique to separation by precipitation and settling is also clearly demonstrated. Although removal of copper, lead, and cadmium from aqueous solution can be achieved by precipitation alone, the times are much longer than by microfotation. Indeed, where stable precipitates are formed, settling without centrifugation was found to be incomplete even after 18 hr or more. The same systems could be separated by microflotation within minutes.
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8
I0
12
FIG. 8. (a) Removal of cadmium (O) and aluminum ( • ) by microflotation (5 rain) from solutions containing 1.25 X 10-4 M Cd(NO3)2 and 1.1 X 10-8 M AI(NO3)8. (b) Removal of cadmium (O) and aluminum ( • ) by precipitation (18 hr) from solutions containing 1.25 X 10-4 M Cd(NO3)2 and 1.1 X 10- 3 M AI(NO,)3.
Journal o/Colloid and l~erface Science, Vol. 57, No. 1, October 1976
MICROFLOTATION Thus, on the basis of the experiments presented here it may be concluded that an ion can be removed by microflotation if it can be precipitated. Of particular importance are the results obtained using the synthetic sea water as these demonstrate that separation of precipitating and nonprecipitating ions can be achieved and that toxic metal ions could be separated from systems containing other indifferent ions, including natural water, without affecting the contents of the indifferent ions. The three ions studied here Fcopper(II), lead(II), and cadmium-] exhibited considerable differences in microflotation in the absence of aluminum ions, but these differences were eliminated by the presence of the latter. I t is thus of interest to discuss these systems individually. Precipitation phenomena of copper from solutions containing 1.25 X 10-4 M Cu(NO3)2 were in good agreement with those reported earlier (16). I t is likely for this concentration of copper that the solid phase formed at pH <,--,10.3 was a mixture of the basic nitrate, Cu(OH)I.~(NO3)0.5 and the hydroxide Cu(OH)2 (16). Above the p H 10.3 the solid phase was probably Cu(OH)2. Rayleigh ratios of samples, taken after precipitation had IOC
"
E C (18 hr )
~AI'I ca"l ¢Y
25
o 4
6
8
I0
12
14 4
~llhqlI/~i/il¢ll~,l, 6
8
I0
12
14.
pH
FIG. 9. (a) Removal of cadmium (©) and aluminum ( • ) by microflotation (5 min) from solutions containing 1.25 X 10-4 M Cd(NO3)2 and 5 X 10-4 M A1~(SO4)3. (b) Removal of cadmium (O) and aluminum (0) by precipitation (18 hr) from solutions containing 1.25 X 10-4 M and 5 X 10-4M AI~(SO,)a.
101 TABLE II
Microflotation from Solutions of 1.25 X 10-4 M Cu(NO3)~ and 5 X 10-4 M A12(SO4)3in Synthetic Sea Water pH
6.6 7.2 7.4 8.2 9.3
•
Removal
Cu 2+
AlS+
Ca2+
Mg2+
92 90 93.5 78 78
95 94 95 79 80
0 0 0 0 0
0 0 0 0 0
occurred, showed a minimum at p H 9.7 which coincided with the small minimum seen in the microflotation curve. This is possibly indicative of the formation of smaller particles and hence a slower flotation rate. The particles have been found to be negatively charged at p H > ~5(16). Further evidence that precipitates were not positively charged follows from the observation that no adhesion to the glass occurred. It is perhaps surprising, therefore, that over the p H range 6.5-8.7, where perfectly stable colloidal precipitates were produced, the laurate anion served as a very efficient collector. Indeed the exact correspondence of the precipitation and microflotation curves over this p H range indicates that essentially all of the copper that was precipitated was removed. The behavior of lead regarding microflotation from 1.25 X 10-4 M Pb(NO3)2 solutions was totally different to that of copper and offered some interesting points which contrast somewhat with earlier findings. Separation of lead by microffotation occurred over the narrow p H range 9.5 10.7 and then only to a small extent. This seemed to be due to a considerable amount of lead remaining in solution and interacting with the laurate ions, as evidenced by the lack of formation of a foam at all p H values below about 11, when PbO2- ions become abundant. However, in all systems over the p H range 6.8-11.5 the presence of precipitates was observed using a high intensity light beam. Thus, although sufficient lead remained in solution to cancel the efficiency of the laurate ions, at least part of the lead had
Journal of Colloid and Interface S~ience, Vol. 57, No. 1, October 1976
102
MELVILLE AND MATIJEVI(~
been removed from the solute state. This is in contrast to Rubin's (2, 14) statement that at 2 X 10-4 M Pb 2+ there was no evidence of precipitate flotation of insoluble hydroxide at any pH. However, Rubin's findings that at pH 8-9 flotation was effectively complete with a collector to metal ion ratio of 1 (the ratio also used throughout this work) and, moreover, that at these pH values flotation was essentially independent of collector ratio seem to be indicative more of "precipitate flotation" than "ion flotation." Also the precipitates we observed were very finely dispersed and difficult to detect without good illumination. Clearly if the equilibrium constants used by Rubin and taken from Latimer (17) and Fuerstenau and Atak (18) are correct then lead hydroxide should not precipitate at the concentration of Pb(NO3)2 (1.25 X 10- 4 M) used in this work. Notwithstanding, Fig. 4 confirmed that after 18 hr precipitation occurred over the pH range 6.8-11.7 and moreover, that it was complete at pH 10; at this pH maximum flotation was found. Lest there be any doubt over the accuracy of the lead concentrations the stock lead nitrate solutions were analyzed gravimetrically and found to be as supposed. This work would indicate that lead hydroxide or a basic lead salt (such as basic nitrate) is less soluble than indicated in the literature. The discrepancy may arise from the fact that the precipitate formed is in a very highly dispersed state which is not readily detected. Apparently, it is the state of lead in the system which accounted for the difference in the precipitation and the flotation behavior shown in Fig. 4. This is best illustrated by the following experiment. The pH of a 400 ml solution of 1.25 X 10-4 M Pb(NO3)2 was adjusted to 9.6, the sample was allowed to stand in a stoppered vessel for 18 hr, and then the microflotation was performed in the usual manner. Removal was found to be 83°/o, compared with 10aTo, if microflotation was carried out with the same system after 5 rain. The former value is in good agreement with the 85% removal by precipitation shown in Fig. 4. Apparently the rate determining step in microflotation from
1.25 )< 10-4 M Pb(NOz)2 solutions is not related to bubble/particle collision or interaction but rather to the slow rate of precipitation of the solid phase. This is further substantiated by the fact that froth formed in systems foated after 18 hr of aging. Ferguson, Hinkle, and Watson (19) from their foam separation studies on Pb(II) and Cd(II) concluded that adsorption of lead on glassware could lead to errors in so called "ion flotation" experiments. Obviously no such effect could account for the results observed in this work, because adsorption would have been reflected in apparent flotation where there was actually none. It was, however, noticeable that below about pH 8.5, particles, if allowed to stand for long periods of time, tended to adhere to the walls of the containers and it was felt desirable to raise the pH of such samples with NaOH before analysis. The only systems in which microflotation was achieved in the absence of precipitation were solutions of 1.25 X 10-4 M Cd(NOa)2 below about pH 9.2. This was undoubtedly due to ion flotation, i.e., flotation of the metal ion collector complex and was much less efficient than flotation of the hydrous cadmium oxide (precipitate flotation), the onset of which was seen at pH 9.2. The addition of aluminum ions dominated all of the systems studied and in their presence all three metal ions behaved similarly. At the same time the behavior of aluminum sulfate and aluminum nitrate was not appreciably affected by other ions. Perhaps this is not too surprising as the aluminum ions were in approximately a tenfold excess. Generally the effect of the aluminum ions was to enhance microflotation by broadening the pH domain at which precipitation of the copper, lead, or cadmium occurred. Aluminum sulfate gave greater benefit to microflotation than did the nitrate salt as the colloidal precipitates produced in the presence of sulfate ions were always unstable at all pH values. Also, aluminum salts increased the rate at which solid phase was formed from lead solutions, thereby enhancing microflotation. Specifically, sulfate ions probably caused
Journal of Colloid and Intfrfac¢ Selene#, Vol. 57, No. 1. October 1976
103
MICROFLOTATION precipitation of lead sulfate or of basic lead sulfate. The role of the collector in these experiments merits some discussion. Addition of lauric acid (1.25 X 10-4 M) caused the p H to decrease in all experiments except those at high pH, due to the dissociation of the lauric acid. Although such decreases in p H during microflotation have been noted (e.g., Ref. (9)) little has been said about their significance. The dissociation constant of lauric acid is 10-5 (20). This means that at p H 5 lauric acid is only 500-/o dissociated, the remainder being essentially insoluble, while at p H 3 or less more than 990-/o acid precipitates. Under these conditions the collector would be effectively absent from the system and this could explain why, for example, silica was found not to float with a lauric acid collector (9) at low pH, despite being in a coagulated state. In the majority of experiments presented in this work the p H was sufficiently high for complete dissociation of lauric acid to have taken place. However, at the lower p H values ( < 5 ) in the presence of aluminum sulfate incomplete dissociation was reflected in rather low surface foams, and, had the domains of interest occurred at still lower pH, then the use of an alternative anionic collector (e.g., sodium dodecyl sulfate) would have been necessary. Finally, it remains to mention the sensitivity of the microflotation technique. By taking a 100-ml sample from a flotation experiment using copper nitrate and aluminum nitrate and concentrating by evaporation the residual copper was shown to be 0.03 ppm. This value is well below levels found in m a n y drinking water supplies (21). Similar experiments were not performed for cadmium and lead but removal to the limit of detection showed that lead levels were reduced to <0.2 p p m and cadmium to <0.07 ppm. ACKNOWLEDGMENT One of us (J.B.M.) is grateful to Unilever Research Laboratory, Port Sunlight, England, for a leave of absence. We also acknowledge the assistance of Professor E. A. Cassell in the early stages of this work.
REFERENCES 1. KARGER, B. L., GRIEVES, R. B., LE~LICH, R., ROBIN, A. J., A N D SEBBA, F., Separ. Sci. 2, 401 (1967). 2. ROBIN,A. J., J. Amer. Water Works Assoc. 60, 832 (1968). 3. RVBIN, A. J. AND CASSELL, E. A., Proc. 14th Southern Water Resources and Pollution Control Conf. 14, 222 (1965).
4. ROBIN, A. J., CASSELL, E. A., HENDERSON, O., JOHNSON,J. D., ANDLAMB,J. C., Biotechnol. Biol. 8, 135 (1966). 5. RtlBIN, A. J. AND LACKEY,J. Amer. Water Works Assoc. 60, 1156 (1968). 6. MANGRAVITE, F. J., JR., CASSELL, E. A., AND MATIJEVI~:, E., J. Colloid Interface Sci. 39, 357
(1972). 7. CASSELL, E. A., MATIJEV1C, E., MANGRAVITE,F. J., JR., BUZZELL, T., M., AND BLABAC, S. B., AIChE J. 17, 1486 (1971). 8. CASSELL,E. A., ROBIN, A. J., LA FEVER, M. B., AND MATIJEVId, E., Proc. 23rd Purdue Ind. Waste Conf. 23, 966 (1968). 9. MANGRAVITE,F. J., JR., BUZZELL,T. D., CASSELL, E. A., MATIJEVIC, E., AND SAXTON, G. B., J. Amer. Water Works Assoc. 67, 88 (1975).
10. KOISHI,M. ANDMATIJEVld, E. to appear. 11. ROBIN, A. J., J. Amer. Water Works Assoc. 60, 832 (1968). 12. ROBIN, A. J. ANDJOHNSON,J. D., Anal. Chem. 39, 298 (1967). 13. ROBIN, A. J., JOHNSON, J. D., AND LAMB, J. C., Ind. Eng. Chem. Process Design Develop. 5, 368
(1966). 14. ROBIN, A. J., in "Adsorptive Bubble Separation Techniques," (R. Lemlich, Ed.), Academic Press, New York, 1974. 15. CASSELL,E. A., KAOFMAN,K. M., ANDMATIJEVIC, E., Water Res. 9, 1017 (1975). 16. McFADYEN,P. ANDMATIJEVIC, E., J. [norg. Nucl. Chem. 35, 1883 (1973). 17. LATIMER,W. M., "Oxidation Potentials," p. 151. Prentice-Hall, Inc., Englewood Cliffs, N.J., 1952. 18. FOERSTENAU,M. C. ANDATAK,S., Trans. Met. Soc. Amer. Inst. Mining Met. Eng. 232, 24 (1965). 19. FERGUSON,B. B., HINKLE, C., ANDWILSON, D. J., Separ. Sci. 2, 125 (1974). 20. YOUNG,S. L. ANDMATIJEVIC,E., ANDMEITES, L., J. Phys. Chem. 78, 2626 (1974).
21. Committee on Water Quality Goals, A.W.W.A. Pt 2, 60, Sept., 1973.
Journal of Colloid and Interface Science, Vol. 57. No. 1, October 1976