Environ. Sci. Technol. 1993, 27, 1169-1 175
Removal of Organic Dye (Direct Blue) from Synthetic Wastewater by Adsorptive Bubble Separation Techniques Jang-Yeun Horng and Shang-Da Huang'
Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China C.I. Direct Blue 1, an anionic dye, was removed from synthetic wastewater by ion flotation of Direct Bluehexadecyltrimethylammonium complex. Over 98 % of Direct Blue was removed from the solution in 5 min. A stoichiometric amount of surfactant was found to be most effective for Direct Blue removal. The separation efficiency increased with increasing rate of air flow and decreased with increasing concentration of NaN03. The efficiencyof separation of Direct Blue by solvent sublation is similar to that by ion flotation. Direct Blue was also removed by adsorbing colloid flotation with Fe(OH)3 floc. Sodium lauryl sulfate was used as the collector and frother, and 99.8% of Direct Blue was removed in 3 min. The separation efficiency decreased with increasing ionic strength of the solution. Use of AI(II1) ion as activator compensates somewhat for the deleterious effect of neutral salt on the adsorbing colloid flotation of Direct Blue. Both ion flotation and adsorbing colloid flotation may find application in the removal of organic dye from wastewater.
Introduction The major environmental problem of colorants is the removal of dyes from effluents (I). Untreated effluents from dyestuff production and dyeing mills may be highly colored and, thus, particularly objectionable if discharged into open waters. The concentration of dye may be much less than 1ppm, but the dye is visible even at such small concentrations; the transparency of streams would also be reduced. Because dyes absorb sunlight, plants in drainage stream may perish; thus, the ecosystem of streams can be seriously affected (2). Possible chronic risks of colorants and their intermediates are carcinogenicity and to a lesser extent sensitization and allergies (I). The potential toxicity of some azo dyes has long been known. Disazo dyes based on benzidine are known to be carcinogenic (1, 3). Many papers on the relation between structure and carcinogenicity of azo dyes have been published ( I ) . Direct dyes are the compounds able to dye cellulose fibers (cotton, viscose, etc.) without the aid of mordants. Direct dyes constitute about 17 % of all dyes used for dyeing textiles and about 30% of the dyes used for dyeing cellulose fibers (4). A direct dye (sodium salt of a sulfonic acid) is anionic and soluble in water. According to its structure (Figure 1))Direct Blue 1is a diazo dye based on benzidine. Methods of decolorization have become important. In principle, decoloration is possible with one or more of the following methods: adsorption, coagulation, biodegradation, chemical degradation, and photodegradation (I).For adsorption, activated charcoal, silica gel, bauxite, peat, wood, cellulose derivatives, and ion-exchange resins have been used, but these processes are in most cases not economically feasible (I). Application of adsorption on ~~~
* To whom correspondence should be addressed. 0013-936X/93/0927-1169$04.00/0
0 1993 American Chemical Society
S03No
Na03S
Figure 1. Structural formula of Direct Blue 1.
granular carbon for color removal from wastewater is economically attractive, as the spent carbon can be thermally reactivated (5). The coagulation process effectively decolorizes insoluble dyes, such as disperse dyes, but does not work well for soluble dyes (2). The biological process does not effectively decolorize commercial dyes, because most commercial dyes are toxic to organisms used in the process (2). Chemical destruction by oxidation with chlorine or ozone is effective, but the oxidant requirements are high and also expensive. The effectiveness of decolorization by the oxidation process is reduced by impurities in the wastewater. It may increase the amount of exhausted chlorine or ozone in the water and hence the treatment cost. Chlorine treatment works well with monoazo and anthraquinone anionic dye but unsatisfactorily with disperse and direct azo dyes (I1. Photochemical degradation in aqueous solution is likely to progress slowly as synthetic dyes are, in principle, designed to possess high stability to light. Accordingly,a new method to treat dye wastewater is highly desirable. The objective of our work was to evaluate adsorptive bubble separation techniques, such as ion flotation, solvent sublation, and adsorbing colloid flotation, as possible treatment alternatives for dye wastewater. The principles of separation using adsorptive bubble separation techniques are based on differences in surface activity. Surface-active material, which may be ionic, molecular, colloidal, or macroparticulate in size, is selectively adsorbed at the surface of bubbles rising through the liquid. During the foam flotation process, these bubbles form a foam at the top of the solution; sometimes addition of a frother is required to maintain a stable foam. This foam is relatively rich in adsorbed material, so that enrichment or separation results. A substance which is not surface-active itself may be made effectively surfaceactive through union with or adsorption on a surface-active material. This effect can occur either through the formation of a complex or precipitate with another compound or through electrostatic attraction by the surfactant layer adsorbed on the particulate surface. For instance, adsorbing colloid flotation (coprecipitate flotation) involves the addition of a coagulant (suchas alum or ferric chloride) to produce a floc. The dissolved species, such as metal ion, is adsorbed onto the floc particle and/or coprecipitated with it. A surfactant is then added, adsorbs onto the floc particle, and renders it hydrophobic (surface active), and the floc (with adsorbed species) is removed by air flotation. The surface-inactive component (such as a metal ion) so removed is termed the colligend,and the surfactant added is called the collector. Ion flotation involves the reaction Environ. Scl. Technol., Vol. 27, No. 6, 1993 1189
of surface-inactive ions with a surfactant collector to yield a surface-active insoluble precipitate or scum which is then removed in the foam. Solvent sublation operates in much the same fashion except that the surface-active material is transferred to an immiscible liquid layer floating on the top of the liquid from which the surface-active material is extracted (6). Adsorptivebubble separation techniques have been used to separate or to enrich minerals, surfactants, proteins, enzymes, microorganisms, and various metallic and nonmetallic ions (6). The extensive literature in this field has been reviewed by Lemlich (6-IO),Somasundaran (11,12), Grieves (13-15), Sebba (16),Mizuike (17, 18),Clarke and Wilson (19, 201, Caballero et al. (21),and Huang (22). Recent reviews with emphasis on analytical applications of foam flotation are given by Hiraide and Mizuike (17, 18) Caballero et al. (211, and Huang (22). When dealing with dilute waste, flotation techniques appear to possess distinct advantages (23): small requirements of energies, high efficiency of removal, small requirements of space (important if land costs are high), flexibility of application to various pollutants at various scales, production of small volumes of sludge highly enriched with the contaminant, and moderate cost. The chemical costs and capital costs of treatment of wastewater containing heavy metals by adsorbing colloid flotation were compared with those of lime precipitation (23). The economics appear to favor adsorbing colloid flotation by a substantial margin. Although adsorptive bubble separation techniques have been applied in various fields, the appIication of this technique for removal of dyes has been little investigated. Most part of the studies are the solvent sublations of dyes from aqueous solutions. Caragay, Karger, and Lee separated methyl orange from rhodamine B by solvent sublation and a cationic surfactant at a pH at which the methyl orange was anionic, whereas the rhodamine B was zwitterionic (24, 25). Karger, Prinfold, and Palmer investigated the solvent sublation of a methyl orangehexadecyltrimethylammonium ion pair (26). Womack, Lichter, and Wilson (27)reported the solvent sublation of two dye-surfactant ion complexes, methylene blue-tetradecyl sulfate and methyl orange-hexadecyltrimethylammonium ion. We reported on the solvent sublation of a cationic dye, Magenta, with sodium lauryl sulfate (28), and two anionic dyes, C.I. Direct Red 1 and Acid Red, with hexadecyltrimethylammonium ion (29, 30). Other work of particular interest includes the solvent sublation of various organic pollutants from aqueous systems, such as alkyl phthalate, volatile chlorinated organics, dichlorobenzenes, nitrophenols, polynuclear aromatics, and chlorinated pesticides by Wilson and coworkers (20,31-36). Grieves et al. studied the removal of phenol by solvent extraction, solvent sublation, and foam fractionation (37). Ion flotation technique seems never to have been applied for dye removal. The applicability of adsorbing colloid flotation as a technique to treat dyes in wastewater has been investigated for dyes of only three kinds-Magenta, Direct Red, and Acid Red-and by our group. Previous work (28) on the adsorbing colloid flotation of Magenta (a cationic dye) with ferric hydroxide and sodium lauryl sulfate (NLS) showed that even though the rate of separation of the floc (with adsorbed dye) was large, particulates, which readily settled in a small amount at
the bottom of the separation column, were not removed by the foam. Furthermore, a relatively large concentration of surfactant (200 ppm) was required to remove the floc from the solution. These drawbacks were not found in the adsorbing colloid flotation of Direct Red (29)and Acid Red (30);all the flocs were removed from the solution by the foam, and the concentration of surfactant needed for an effective separation was much smaller (40ppm). It is thus worthwhile to test adsorbing colloid flotation on wastewater containing other dyes to assess whether this technique is generally useful to treat such wastewater. In our investigation of ion flotation and solvent sublation of C.I.Direct Blue 1 from synthetic wastewater, sodium nitrate was used to prepare samples of wastewater of varied ionic strength, and hexadecyltrimethylammonium bromide (HTA) was used as the collector. Direct Blue was removed from the solution in 5 min to an extent 98% by ion flotation and 96 % by solvent sublation. We removed Acid Blue by adsorbing colloid flotation. Ferric hydroxide was the coprecipitant. Sodium lauryl sulfate (NLS) was the collector and frother. The rate of removal was great; 99.8% of Direct Blue was removed by adsorbing colloid flotation in 3 min. Both ion flotation and adsorbing colloid flotation may find application in the removal of organic dye from wastewater. As the proposed methods rely on the addition of surfactant to the wastewater, the cost and recovery of surfactant and its potential impact on downstream treatment process or after discharge are of great concern. One of the major operating expenses of these operations is the cost of surfactant. The surfactant HTA for ion flotation of Direct Blue is more expensive than NLS for adsorbing colloid flotation, but the latter process produces sludge which may present disposal problems. Solidification (23,28)of flotation wastes is possible by the addition of a mixture of CaO-SiOz. The resulting solid has a load resistance 29 Kg cm-2. The product is used as landfill. Work on the ultimate disposal of these sludges or, preferably, on their recycle will probably be carried out in connection with work on the recovery of surfactants from them (23),because sludge characteristics are affected by the procedures used to displace surfactant from the sludge. Slapik investigated recovery of surfactant from the collapsed foamate by displacement of NLS from Fe(OH)3floc surface with hydroxide ion (39). This process permits recycle of the surfactant and causes the floating, hydrophobic sludge to become hydrophilic and sink. Huang recovered dodecylbenzenesulfonate from adsorbing colloid flotation foamates of batch type (40).Further work is needed to demonstrate this feasibility of surfactant recovery in continuous-flow pilot-scale apparatus (23). Other factors that demand great concern are the concentration of surfactant in the inflowing wastewater and in the effluent (in a continuous flow process), as it may have an impact on downstream treatments. Slapik et al. removed lead from wastewater by adsorbing colloid flotation in a continuous-flow pilot plant (23, 39); they reduce the concentration of NLS in the column effluent from 15.2 mg/L to 5.95 mg/L (influent flow rate of 7.5 L/min) by increasing the depth of the liquid pool above the air dispersion head from 2.5 to 60 cm. This process allows effective stripping of the surfactant from the effluent and directly recycles it into the foam, making possible decreased Concentration of NLS in the inflow without loss
of a satisfactory foam. Evidently very substantially decreased surfactant requirements result from stripping of the surfactant from a deep pool at the bottom of the column (23). Experimental Section
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The system for ion flotation and adsorbing colloid flotation was similar to that described earlier (28-30). A soft glass column 90 cm long and of inside diameter 3.5 cm was used. There was a side arm with a rubber septum near the bottom to inject the collector. The bottom of the column was closed with a rubber stopper with holes for a gas sparger and a stopcock to take samples and to drain the column. The gas sparger was a commercially available gas dispersion tube. A lipped side arm near the top of the column served as a foam oulet. The solvent sublation system was similar to that for adsorbing colloid flotation, as described above, except that the column was only 60 cm long. Compressed air was generated from an air pump. The rate of flow of air was adjusted with a Hoke needle value with micrometer control and measured with a soap film flowmeter. The air was purified by passage through glass wool to remove particulates, through Ascarite to remove carbon dioxide, and through distilled water for controlled rehumidification. : 0 Reagent-grade sodium lauryl sulfate (NLS, Wako Pure Chemical Industry, Japan) and hexadecyltrimethylammonium bromide (HTA, Aldrich, 95%) were used as collectors without purification. Reagent-grade C.I. Direct Blue 1 (Tokyo Chemical Industry, Japan) and sodium nitrate were used to prepare samples of wastewater of varied ionic strength. For the solvent sublation runs, HTA was added to the sample solution to form the dye-surfactant complex; the pH of the solution was adjusted, the solution was poured into the separation column, paraffin oil (10mL) was added immediately, and the timer was started. The rate of air flow was adjusted before the sample solution was poured into the column. The procedure for ion flotation was similar except that no paraffin oil was required; the dyesurfactant complex was carried by the foam through the foam outlet. For the adsorbing colloid flotation runs, ferric nitrate was added to the sample solution, and the pH was adjusted to produce the floc. Direct Blue was adsorbed on or coprecipitated with the floc. NLS was added and adsorbed on the surface of the floc to render the floc hydrophobic, and the solution was then poured into the column for separation. The pH of the solution was measured with a pH meter (Radiometer pHM83 Autocal). All runs were made with test solution (250 mL) that contained Direct Blue (25ppm). The experiments were performed under room temperature. Duplicate runs were performed for each set of conditions. The concentrations of dye in the sample solutions were determined using a UVIVis spectrophotometer (Jasco 505). Spectrophotometric experiments revealed a peak shift and change in absorbance for Direct Blue in the aqueous phase when HTA was added. The change in absorbance depended on the molar ratio of HTA to Direct Blue. For the ion flotation and solvent sublation runs, a portion (4 mL) of the sample solution was mixed with acetone (2 mL) to dissolve the dye-surfactant precipitate. The absorbance
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Figure 2. Effect of pH on Ion flotation of Direct Blue. Molar ratio of HTA to dye was 1.5, rate of air flow was 150 mL/min, duration of air flow was 10 and 5 min from top down. 100
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Figure 3. Effect of rate of air flow on ion flotation. pH = 4.0, [HTA]/ [dye] = 1.5, duration of air flow is 10 and 5 min from top down.
was measured and compared with the calibration curve that was obtained by treating the standard solutions (with the same dye to HTA mole ratio as the sample solution) by the same procedure as used for sample solutions (assuming the molar ratio of dye to HTA does not greatly vary during solvent sublation and ion flotation). For the adsorbing colloid flotation runs, a sample (5 mL) was withdrawn and the pH of the solution was adjusted to 11. Direct Blue was desorbed almost completely from the Fe(OH13 floc which was removed by centrifugation. The absorbance of the supernatant was measured at 628 nm and compared with the calibration curve obtained by measurement of the absorbance of the standard aqueous solution at pH 11. The addition of NLS to the solution had no effect on the absorption at 628 nm.
Results and Discussion Removal of Direct Blue by Ion Flotation. The effect of pH on the ion flotation of the Direct Blue-HTA complex is shown in Figure 2. The mole ratio of HTA to dye was 1.5 for these runs. Over 85% of Direct Blue was removed from the solution by ion flotation in 10 min at pH 3-6. The rate of separation and separation efficiencydecreased significantly at pH 9, probably due to interference with the Direct Blue-HTA complex formation by the added base. The effect of rate of gas flow is shown in Figure 3. The rate of separation increased somewhat with increasing rate of gas flow. Too great a rate even caused the rate of Environ. Sci. Technol., Vol. 27, No. 6, 1993
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Figure 4. Effect of HTA dose on ion flotation. pH = 4.0, rate of air flow = 200 mL/min, duration of air flow = 5 min.
Flgure 5. Effect of concentrationof NaN03on ion flotation. pH = 4.0, [HTA]/[dye] = 2.0, rate of air flow = 200 mL/mln.
separation to decrease somewhat, probably due to the increased mean radius of bubbles that we observed with increasing rates of air flow. Similar phenomena were observed by other investigators (23,32,41-44). Valsaraj and Springer removed pentachlorophenol from aqueous acidic solutions by solvent sublation (41). In their column, the mean bubble diameter was 0.05 cm at a rate 1.2 mL/s of air flow compared with 0.15 cm at 2.8 mL/s. Smaller bubbles are more efficient than larger bubbles for ion flotation and solvent sublation; they rise more slowly, so they have a greater duration of contact with the solution; they have a larger ratio of surface to volume than larger bubbles. These two combined effects increase the extent of mass transfer of the colligend from the aqueous phase to small air bubbles (32, 42). Furthermore, air flow can also affect mixing behavior. A large rate of air flow causes much axial dispersion, which has also been found to impair the performance of the sublation process (23, 41,431. The effect of HTA concentration on the ion flotation of Direct Blue is shown in Figure 4. It was found that a 2:l mole ratio of surfactant to dye gave the fastest rate of separation and the lowest residual dye concentration, with 98 % of Direct Blue being removed in 5 min. At a smaller concentration of surfactant, the rate of separation is smaller and the level of residual dye is greater, presumably because of the incomplete formation of a dye-surfactant complex. At a greater concentration of surfactant, the rate of separation of Direct Blue is smaller, presumably due to the competition of the bubble surface by the excess surfactant ion with the dye-surfactant complex. Optimum separation with surfactant dosage at the stoichiometric amount was also observed for the solvent sublation of Acid Red-HTA (30),Direct Red-HTA (291,and Magenta-NLS (28). This finding was quite different from the solvent sublation results for methylene blue and methyl orange studied by Wilson et al. (27) and Karger et al. (261,who found that the rate of removal of methylene blue and methyl orange increased with increasing concentration of surfactant, when it was much in excessof the stoichiometric amount. This contradiction is probably due to the difference in the formation constant of varied complexes of dye and surfactant and may also be due to the altered molar ratio of the surfactant and dye in the complexes. The effect of NaN03 on the ion flotation of Direct BlueHTA appears in Figure 5. The presence of neutral salt decreases the separation efficiency of ion fractionation and ion flotation due to the competition for collector between the colligend and the ions of salt (1-11). Wilson
et al. (25) observed an inhibiting effect of neutral salts (NaN03, KC1, and NaHZP04) on the solvent sublation of both methylene blue-tetradecyl sulfate and methyl orange-HTA dye-surfactant complexes. The separation efficiency of Direct Blue-HTA by ion flotation decreased only somewhat with the addition of neutral salt, probably due to the strong interaction between Direct Blue and HTA, such that the formation of the Direct Blue-HTA complex was insignificantly affected by the presence of neutral salt. Over 86 % of Direct Blue was removed by ion flotation from solution containing 0.5 equiv/L of NaN03. Removal of Direct Blue by Solvent Sublation. Solvent sublation of the Direct Blue-HTA complex operates in a way similar to that occurring in ion flotation except that the surface-active material is transferred to the paraffin oil on the top of the aqueous solution. The Direct Blue-HTA complex is slight soluble in paraffin oil, so that part of the complex is carried out of the separation column by the foam. The foam produced in the solvent sublation process is somewhat less stable than that formed in the ion flotation process. The separation efficiency of Direct Blue removed by solvent sublation is similar to that by ion flotation; 96 5% of Direct Blue was removed by solvent sublation in 5 min. There is almost no benefit resulting from the addition of paraffin oil to the separation system; it is advisable to remove Direct Blue-HTA complex from solution by ion flotation rather than by solvent sublation. Removal of Direct Blue by Adsorbing Colloid Flotation. Experiments were performed using the adsorbing colloid flotation technique to remove Direct Blue from aqueous solution. The effect of pH on the adsorbing colloid flotation of Direct Blue with iron(II1) hydroxide floc appears in Figure 6. The anionic surfactant sodium lauryl sulfate (NLS) was used as the collector and frother. The rate of removal was rapid at a pH range of 4-6; over 99% of Direct Blue was removed in 5 min. The optimum pH for separation was 5.0; 99.8% of Direct Blue was removed in 3 min. The removal of the floc and Direct Blue was poor at pH 7 and 8, presumably due to the decrease in surface potential of the floc with increasing pH, such that the anionic surfactant was not effectively adsorbed on the surface of the floc to render it hydrophobic. The effect of surfactant dosage on separation efficiency is shown in Figure 7. NLS concentration (40 ppm) was sufficient for an effective separation. Increases in surfactant dosage had little effect on the separation efficiency.
1172 Envlron. Sci. Technol., Vol. 27,
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Figure 9. Effect of iron dose on adsorbing colloid flotatlon. [NLS] = 40 ppm, rate of air flow = 120 mL/mln, pH = 5.0, duration of alr flow is 10, 3, and 1 min from top down. 100 T . 6 ,
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Figure 8. Effect of rate of air flow on adsorbing colloid flotatlon. [Fe(III)] = 100 ppm, [NLS] = 40 ppm, pH = 5.0, duration of air flow is 10, 5, 3, and 1 min from top down.
The effect of rate of air flow on the efficiency of adsorbing colloid flotation of Direct Blue appears in Figure 8. The rate of separation increased somewhat with increasing rate of flow, as expected. A total of 98.6% of Direct Blue was removed in 1 min with an air flow rate of 150mL/min, and 99.5 % of Direct Blue was removed in 10 min with an air flow rate of 30 mL/min. The effect of iron(II1) dosage appears in Figure 9. It takes 80 ppm of iron or more for an effective separation. The separation of Direct Blue was incomplete at iron
Flgure 10. Effect of NaN03on adsorbing colloid flotatlon. [Fe(III)] = 80 ppm, [NLS] = 40 ppm, pH = 5.0, rate of air flow = 120 mL/mln, duration of air flow Is 10, 5, and 3 min from top down.
dosages of 60 or 70 ppm, presumably due to incomplete coprecipitation of Direct Blue with an insufficient amount of ferric hydroxide floc. The effect of neutral salt (NaN03) on the separation efficiency of the adsorbing colloid flotation of Direct Blue appears in Figure 10. The separation efficiencydecreased with increasing ionic strength of the solution, presumably due to a decrease of the surface potential of the positively charged floc by the adsorption of the anion (nitrate ion) in the solution, such that the surface potential of the floc was no longer positive enough for a sufficient amount of anionic surfactant to be adsorbed. Only 66% of Direct Blue was removed in 10 min from a solution containing 0.5 equiv/L of NaN03. The inhibition effect of neutral salts on adsorbing colloid flotation of various heavy metal ions (45-49) and dyes (28-30)with Fe(OH13 floc and NLS can be compensated with the aid of aluminum ion as the activators. We attempted to use this technique for the removal of Direct Blue from solution containing 0.5 equiv/L of NaN03 by adsorbing colloid flotation. The results appear in Figure 11. Effective separation with 99.2% removal of Direct Blue was achieved in 10 min when 20 ppm of aluminum(111) was added. The effect of aluminum ion as the activator results from increased surface potential of the floc by the adsorption of aluminum(II1) species on the Fe(OH)3 floc (or by forming a mixed precipitate), such that sufficiently negatively charged surfactant (NLS) is Environ. Sci. Technol., Vol. 27, No. 6, 1993 1173
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Literature Cited
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Figure 11. Effect of AI(II1) dose on adsorbing colloid flotation of Direct Blue from solution containing 0.5 equlv/L of NaN03. [Fe(III)] = 80 ppm, [NLS] = 40 ppm, rate of air flow = 120 mL/min, pH = 5.0, duration of air flow is 10, 5, and 3 min from top down.
adsorbed onto the surface of the floc, thus rendering the surface of the floc hydrophobic. Hence, separation is effective. Conclusion
Direct Blue was removed effectively from wastewater by ion flotation with a cationic surfactant, hexadecyltrimethylammonium (HTA) bromide; over 98 % of Direct Blue was removed from the solution in 5 min. A stoichiometric amount of surfactant (2 mol of surfactant to 1 mol of dye) was found to be most effective for removal of Direct Blue. The separation efficiency increased with increased rate of air flow and decreased with increased concentration of NaN03. The efficiency of separation of Direct Blue by solvent sublation is similar to that resulting from ion flotation. However, addition of paraffin oil to the separation system results in no benefits to the process. It is thus advisable to remove Direct Blue-HTA complex from the solution by ion flotation rather than by solvent sublation. Adsorbing colloid flotation of Direct Blue with Fe(OH)3 flocand NLS (sodium lauryl sulfate) is effective with99.8 5% removal of Direct Blue in 3 min. The efficiency of separation decreased with increased concentration of neutral salt (NaNOs),presumably due to decreased surface potential of the positively charged floc by the absorption of anion (nitrate ion) in the solution; the surface potential of the floc was no longer positive enough for adsorption of a sufficient anionic surfactant. The deleterious effect of neutral salt on the adsorbing colloid flotation of Direct Blue is compensated by use of aluminum ion as activator, which increases the surface potential of the floc as a result of the adsorption of Al(II1) species. We have presented preliminary experimental data on a laboratory scale to determine the potential application of adsorptive bubble separation techniques on treatment of wastewater for dyes. Both ion flotation and adsorbing colloid flotation were effective to remove dye from wastewater. However, to transfer this technique to continuous processes on an industrial scale, many tests should be made for those continuous processes on a statistical basis. Acknowledgments
We thank the National Science Council of the Republic of China for a grant (NSC 79-0421-M007-02). 1174
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(48) Huang, S.-D.; Huang, Ma-K.;Gua, J.-Y.; Wu, T.-P.; Huang, J.-Y. In 6th International Symposium on Surfactants in Solution; New Delhi, India, Aug 18-22, 1986. (49) Huang, S.-D.; Wang, T.-F. In Sixth International Conference, Chemistry for Protection of the Environment; Torino, Italy, Sept 15-18, 1987. Received for review September 9, 1992. Revised manuscript received February 8, 1993. Accepted February 25, 1993.
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