Environ. Sci. Techno/. 1995, 29, 1802- 1807
Adsorbing Colloid Flotation of Heavy Metal Ions from Aqueous Solutions at Large Ionic Strength S H A N G - D A HUANG,* HOLLY H O . Y U N N - M I K G L1, AND C H E N G - S H I IJ N L I I\j Department of Chemistry, National Tsing Hua University, flsirzchic. Traitvan 30043, Republic of China
~~~~
The electrolyte tolerance of adsorbing colloid flotation of heavy metal ions (Cd, Pb, Zn, Cr, and Cu) from wastewater with floc of iron(ll1) hydroxide is improved significantly when a mixture of sodium oleate and sodium dodecyl sulfate (SDS) is used as the collector and frother, relative to the tolerance with SDS alone. The electrolyte tolerance is also improved using Mg(ll) as the activator. The applicability of adsorbing colloid flotation to remove heavy metals from wastewater is thus greatly extended.
1802 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 7.1995
Introduction Foam separation techniques are of interest due to their ability to remove trace metals from industrial effluents. The literature is reviewed by Lemlich (1-3,Somasundaran (6, 3 , Grieves (8-10), Sebba (I]), Mizuike (12, 13), Clarke and Wilson ( 1 4 , 1 3 ,Caballero et al. (161, and Huang (17). These techniques are based on the fact that surface-active materials tend to concentrate at gas-liquid interfaces.When air is bubbled through the solution, the surface-active material adsorbs on the rising bubbles, which then separate it from the solution. The substance to be removed (i.e., colligend) if not surface-active can be made so through union with or adsorption of a surfactant. For instance, adsorbing colloid flotation involves the addition of a coagulant to produce the floc, which adsorbs the dissolved heavy metal ion. A surfactant (the collector) is then added and adsorbs onto the floc, rendering it hydrophobic, and the floc (with adsorbed metal ions) is removed by flotation. Foam separations appear to possess distinct advantages for treating dilute wastes: small energy requirements, low residual concentrations of metals, rapid operation, small space requirements, flexibility of application to various metals at various scales, production of small volumes of sludge highly enriched with the contaminant, and moderate cost (14,15). The chemical and capital costs of treatment by adsorbing colloid flotation were compared with those of lime precipitation (14, 18);the results favored adsorbing colloid flotation significantly. Foam separation has one disadvantage: separation efficiency decreases with increasing ionic strength (14, 15, 17-23). Because industrial wastewater is typically a complex mixture, this drawback restricts the applicability of foam separations for wastewater treatment and is a major reason why these techniques are not used much for wastewater treatment despite their advantages. Several studies of the removal of heavymetal(s) by foam separation have been carried out by Wilson et al. (14, 15, 18, 23-29). These mainly concern adsorbing colloid flotation of heavy metal hydroxides with iron(II1) or aluminum(II1) hydroxides as the coprecipitant. Fe(II1) is most commonly used in adsorbing colloid flotation to remove metal ions from aqueous solution (14-29). An anionic surfactant, such as sodium dodecyl sulfate (SDS), is generally used as collector. Work by Kim and Zeitlin (30),Wilson and co-workers (14),and Huang et al. ( 19,Z O ) indicates that the actual coprecipitation is an important mechanism in the scavenging of an ion from solution by iron(II1)hydroxide floc. SDS is physically adsorbed on the floc surface through Coulombic interactions (14, 31, 32). In addition to SDS, which is the most commonly used collector for adsorbing colloid flotation, carboxylic acids and their salts are collectors of other types commonly used in mineral flotation. Fuerstenau (33)used sodium oleate as a collector for flotation of hematite; adsorption of oleate on hematite involved a chemical mechanism [formation of iron(II1)oleate at the surface],and oleate anions absorbed on hematite even at a pH 2 units above the point of zero charge (PZC) of hematite. Peck et al. (34)provided infrared spectral evidence of the formation of iron(II1)oleate on the floc surface.
0013-936X/95/0929-1802$09,00/0
Z 1995 American Chemical Society
Sodium oleate (or a mixture of SDS and sodium oleate) was used as the collector for adsorbing colloid flotation of various heavy metal ions by many investigators (35-44). However, the benefits of this mixed collector (SDS/oleate) to compensate for the deleterious effect of increasing ionic strength on adsorbing colloid flotation seem to have been explored only recently (32); we found that effective separation of Cu(I1)was achieved over a greater range of pH (7.510.0) from wastewater with much greater ionic strength (Na2S040.8 M or NaN03 1.6M) when a mixture of SDS and sodium oleate was used as frother and collector in contrast to the use of SDS alone. We showed that the electrolyte tolerance of adsorbing colloid flotation of various heavy metal ions with iron(II1) hydroxide and SDS can be improved somewhat with the aid of activators such as AI(II1) and Zn(1I) ions (19-22,45). In recent work (3.21,we found that the less toxic metal ion Mg(I1) could also be used as an activator to remove Cu(1I) from wastewater at large ionic strength. Here, we demonstrate further the effect of Mg(I1) as an activator on adsorbing colloid flotation of other toxic heavy metal ions [Cd(II),Pb(II), Zn(II), Cr(VI)]from solutions of large ionic strength; SDS is used as collector. We also show that the electrolyte tolerance of adsorbing colloid flotation of heavy metal ions (Cd,Pb, Zn, Cr) is improved significantlywhen a mixture of SDS and sodium oleate is used as frother and collector compared with the use of SDS alone. Because the separation efficiency of foam flotation generally decreases with increasing ionic strength of the aqueous solution, this work will be helpful to define the capabilities and limitations of adsorbing colloid flotation to treat heavymetal wastewater.
Experimental Section The batch foam flotation system used was similar to that described earlier (21,22,32). A glass column (length60 cm and inside diameter 3.5 cm) was used for flotation. 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 (pore size 25-50 pm) was a commercially available gas dispersion tube. A lipped side arm near the top of the column served as the foam outlet. Compressed air was generated from an air pump. The rate of flow of air was adjusted with a needle valve (Hoke) 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 to control humidity. Sodium dodecyl sulfate (SDS)and sodium oleate (laboratory grade) were used as frother and collector. Cd(N03)2*4H20,Zn(N03)2.GH20,Pb(N0312, K2Cr207, Cu(N03)2-3H20, Fe(N03)3.9H20,FeS04*7H20, Mg(N0&.GH20, NaOH, NaN03, and Na2S04(reagent grade) were used for sample preparation. The initial concentrations of heavy metal ion in the synthetic wastewaters were Cd(II),10ppm, and Pb(II),Zn(II),C r w ) , and Cu(II),50 ppm, respectively. The ionic strength of the syntheticwastewater was adjusted with NaN03 or Na2S04. The dosage of Fe(II1)was 100 ppm for runs with Pb, Zn, and Cu and was 30 ppm for runs with Cd. Floc foam flotation of C r w ) was performed by reducing CrW) (50 ppm) with Fe(I1) (175 ppml. The solution was stirred for 10 min to allow the reaction to proceed to completion before treatment by foam flotation. All experi-
ments were run using 250 mL of solution. The airtlow rate was maintained at about 85 mllmin. The duration of flotation was 10 min for all runs. Measurements of pH were made with a digital pH meter (Radiometer PHM 82). Concentrations of heavy metals were determined with an atomic absorption spectrophotometer (Varian Spectra AA-20). The 9-potentials of particles were measured with a 9-meter (Zeta-Meter,Inc.) consisting of a cell across which a potential was applied to cause charged particles to move. The period required for a colloid particle to pass a certain distance was measured. Ten to twenty particles were tracked. The average velocity of particles is calculated at a known applied voltage to determine the 5-potential. It is difficult to measure the 9-potentialsof the flocs in solutions at large ionic strength; the values of 5-potential are only used qualitatively. The electrolyte tolerance for flotation system is defined as the largest concentration of NaN03 in a solution from which heavy metal is removed effectively by foam separation, such that the residual heavy metal concentrations are smaller than specified levels (Cd,0.1 ppm; Cr, 0.5 ppm; Pb, Zn; and Cu, 1 pprn).
Results and Discussion Adsorbing Colloid Flotation of Cd(I1) with SDS and Mg(1I). The effect of pH and variation of ionic strength (adjustedby adding NaN03 salt) on the separation efficiency is shown in Figure 1. The separation efficiency decreases with increasing ionic strength, in large part to the decreased 5-potential of the floc at large ionic strength (14,19-22,32, 45). Effective separation with residual cadmium levels less than 0.1 ppm was achieved provided that the concentration of NaN03 in the solution was no greater than 0.005 M in the pH range 8.5-9.0. Correlation of the 5-potential as a function of pH and Mg(I1) for Cd(I1)lferric hydroxide with residual Cd after flotation with SDS is shown in Figure 2. The concentration of NaN03 is 0.01 M. Without Mg(II),the residual cadmium concentrations after flotation for 10 min are greater than 0.1 ppm in the pH range 8.5-11.8. The residual cadmium concentrations increase and the 5-potentials of the floc decrease with increasing pH of the solution. The residual cadmium level was greater than 0.1 ppm at pH less than 9.0 due to the incomplete coprecipitation of cadmium ion with the floc affected by NaN03 in the solution. The coprecipitationof cadmium ion with the iron(II1)hydroxide floc is more efficient at greater pH, but the 5-potentials of the floc become negative at pH values greater than 9.5, such that the anionic surfactant (SDS) can no longer physically adsorb on the surface of the floc to render the surface of the floc hydrophobic, which results in poor separation. When Mg(I1)(loppm) was added, the residual cadmium levels decreased with increasing pH of the solution. Effective separation with residual cadmium levels less than 0.1 ppm was achieved in the pH range 10.5-10.8. In this range, coprecipitation of cadmium with the floc of iron(II1) hydroxide is complete, and the e-potential of the floc remained positive due to adsorption of Mg(I1)species (Mg2+ and M@H+ ions). The 5-potentials of the floc at pH 10.5 and pH 10.8 are larger than that at pH 9.5 and pH 10.0 due to adsorption of Mg(I1) species on the surface of the floc being enhanced at pH 10.5-10.8, when Mg(I1) ion hydrolyzes to Mg(OH)+(31);adsorption of hydrolyzed Mg(I1)ion VOL. 29, NO. 7,1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
1803
TABLE 1
Residual Cd (ppm) Showing Effects of Ionic Strength, pH, and Mg(ll) on Adsorbing Colloid Flotation of Cd(ll) with SDSa Ms(W (ppm) NaN03(M)
pH
5
10
0.01 0.01 0.02
10.5 10.8 10.8 10.8 10.8 10.8 10.8 10.8 10.8 10.8 10.8 10.9 11.0
0.02 0.04
0.04
0.03 0.05
0.08 0.10 0.20
0.30 0.50 1.oo 1.oo 1.oo
0.03 0.05
0.07 0.05
0.07 0.08 0.08
1.23
0.06 0.09 0.06 0.06 0.05
0.10
20
0.10
0.04
0.16 0.13
15
0.07 0.07 0.13 0.11 0.11
0.10 0.15 0.13 0.55
"Cd(ll1 = 10 pprn; Fe(ll1) = 30 ppm; SDS = 90 ppm. 0
-
0 '
e
'=e
FIGURE 1. Effects of pH and ionic strength on adsorbing colloid flotation of Cd(ll). Cd(ll) = 10 ppm; Fe(1ll) = 30 ppm; SDS = 90 ppm.
\
22
2 4
8C
'I.
85 ~
90
9 5
1CO
'05
113
PH
FIGURE 2. Correlation of 5
0.46 1.31
'Cd(ll) = 10 ppm; Fe(lll) = 30 pprn; SDS = 90 ppm. bAverage value
loo
150
0.07 0.12
0.83
80
65
0.29 0.16 0.12 0.54
0.33
* standard deviation, n = 3.
TABLE 3
TABLE 4
Residual Pb (ppm) Showing Effects of pH and Ionic Strength on Adsorbing Colloid Flotation of Pb(ll) with SDS (No Activator)B
Residual Pb (ppm) Showing Effects of Ionic Strength, pH, and Mg(ll) on Adsorbing Colloid Flotation of Pb(II)with 505"
NaNO3(M)
6.5
7.0
0 0.05 0.10 0.20 0.30 0.40 0.50
0.32
0.22 0.44 0.55 0.65 1.76 2.41 25
a
0.78 0.97 2.08 3.71 2.00
PH 7.5
0.19
8.0
0.27
8.5
NaNO3(M) pH
0.3
0.24
0.46 2.56 >5
3.11
>5 0.4
Pb(ll) = 50 ppm; Fe(lll) = 100 pprn; SDS = 60 ppm.
oleate is chemically adsorbed on the surface of iron(II1) hydroxide floc (31, 33, 34, 47). The flotation behavior is insignificantly affected by surface charge of the floc when chemically adsorbed surfactants were used as collectors (31);hence, we can achieve effective separation at high pH and high ionic strength. SDS was used as the frother to maintain a stable foam. SDS may also be adsorbed on the floc surface through hydrophobic interaction with oleate after sufficient oleate becomes chemisorbed on the floc surface (31). Adsorbing Colloid Flotation of Pb(I1) with SDS and Mg(I1). The effects of pH and ionic strength on separation efficiency are shown in Table 3. Effective separation with residual lead concentration less than 1ppm was achieved provided that the solution contained NaN03 no greater than 0.2 M. The effects of NaN03, pH, and Mg(1I) dosage on separation are shown in Table 4. The residual lead concentration was less than 1ppm, with added Mg(I1) (20 ppm) as activator, provided that the solution contained NaN03 no greater than 0.4 M. A comparison of Tables 3 and 4 shows the advantage of adding Mg(I1). Adsorbing Colloid Flotation of Pb(I1) with SDS and Sodium Oleate. The effects of pH and ionic strength on separation efficiency, with sodium oleate (10 ppm) and SDS (60 ppm) as collector and frother, are shown in Table 5 . Effective separation with residual lead concentration less than 1 ppm was achieved from a solution containing NaN03 (1.5 M) at pH 8.5-9.0. This is at much higher ionic strength than was achievable without sodium oleate.
0.5 a
0
6.5 7.0 7.5 8.0 8.5 6.5 7.0 7.5 8.0 8.5 9.0 7.0 7.5
5
Mg(ll)(ppm) 10 20
2.08 1.67 1.85 1.76 0.54 0.45 2.56 0.44 0.58 0.56 3.11 > 5 0.49 0.57 >5 >5 3.71 2.03 2.41 0.72 >5 0.84 >5 4.48 25 25 25 >5 >5 5' 15 3.70 >5 >5
50
loo
1.40 0.43 0.58 0.52 0.47 0.41 >5 2.18 0.65 0.83 25 >5 >5 >5 5' 5' 1.62 1.17
Pb(ll) = 50 pprn; Fe(lll) = 100 ppm; SDS = 60 ppm.
TABLE 5
Residual Pb (ppm) Showing Effects of pH and Ionic Strength on Adsorbing Colloid Flotation of Pb(ll) with SDS and Oleate PH NaNOa (M)
0.5 0.6 0.7 0.8 1 .o 1.3 1.5
6.5
1.88
7.0
0.82 0.65 1.34 1.21 1.36 2.68
7.5
8.0
8.5
9.0
0.57 0.63 1.16 1.14 1.24
0.58 0.84 1.10
0.73 0.88
0.88
a Pb(ll) = 50 ppm; Fe(lll) = 100 ppm; SDS = 60 ppm; sodium oleate = 10 pprn.
Adsorbing Colloid Flotation of Zn(1I) with SDS and Mg(I1). The effects of pH and ionic strength on separation efficiency are shown in Table 6. Effective separation with residual zinc concentration less than 1 ppm was achieved at pH 8.5-9.0. The separation efficiency was poor at pH less than 7.5 due to incomplete coprecipitation of Zn(I1) with the floc of iron(II1) hydroxide. The separation was worse at pH larger than 9.5 due to the surface potential of the floc being no longer positive enough for adsorption of the anionic surfactant. The residual zinc concentration VOL. 29. NO. 7,1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
= 1805
TABLE 6
TABLE 8
Residual Zn (ppm) Showing Effects of pH and Ionic Strength on Adsorbing Colloid Flotation of Zn(ll) with SDS (No Activator)a
Residual Zn (ppm) Showing Effects of Ionic Strength, pH, and Oleate Concentration on Adsorbing Colloid Flotation of Zn(ll) with SDS and Oleatea
PH NaN03 (M)
0 0.01 0.02 0.05 0.08 0.10
2.24 i- 0.1EIb 5.52 3Z 1 .98h >IO >IO ..IO ,IO
0.55 rt 0.12b 0.79 i 0.02b 1.58 It 0.42b 1.56 It 0.35h 2.48 i- 0.07b '10
.'Zn(ll) = 50 pprn; Fe(lll)= 100 ppm; SDS = 40 i standard deviation, n = 3.
sodium oleate (ppm)
4.0
8.5
8.0
0.21 It 0.0Ib 0.38 i- 0.01 0.48 i- 0.01 > 10 10 .> 10
NaN03(M)
pH
0
0.1 0.2 0.5 0.6
8.5 8.5 8.5 9.0 9.5 10.0 10.5 8.0 8.5 9.0 9.5 10.0 10.5 8.0 8.5 9.0 9.5 10.0
>IO 210 >IO >IO >IO 210
ppm. Average value
1.o
TABLE 7
Residual Zn (ppm) Showing Effects of Ionic Strength and Mg(ll) on Adsorbing Colloid Flotation of Zn(ll) with SDSa
1.5
Mdll) (ppm) NaN03(M)
0
0.1 0.2 0.3 0.4
>IO -10
'10 >IO
100
50
0.66 >IO '10 >IO
t
0.20b
0.50 It 0.08b 1.31 3Z 0.58b 3.97 1.42b >IO
210 >IO >lo >10 >IO >IO >IO '10 >IO '10 >10
20
10
was less than 1 pprn provided that the solution contained NaNO:>no greater than 0.02 M. The effect of Mg(I1)and ionic strength on separation are shown in Table 7. The residual zinc level was less than 1 ppm from the solution containing NaN03 (0.1 M) and Mg(IIj (50 ppm) at pH 9.5. The electrolyte tolerance of adsorbing colloid flotation of Zn(I1) was improved somewhat using Mg(I1) as an activator. Adsorbing Colloid Flotation of Zn(I1) with SDS and Sodium Oleate. The effects of sodium oleate dosage, ionic strength, and pH on separation efficiency are shown in Table 8. The separation was effective with residual levels of zinc less than 1 ppm from a solution containing NaNO:, at a large concentration (1.5 M) over a broad range of pH 18.5- 10.0). The separation was also effectivefrom a solution containing Na2S04(1.0 M) at pH 9.0 when sodium oleate (30 ppm) and SDS (40 ppm) were added as collector and frother. Floc Foam Flotation of Cr(vr) with Fe(II), SDS, and Mg(I1). In these experiments, CrW)was reduced to Cr(II1) with Fe(I1). The resulting floc [containing Cr(OHI3, Fe(OH):i,and the coprecipitated chromium species] was then removed using SDS as collector and frother. The effects of ionic strength and pH on separation are shown in Table 9. Effective separation with residual chromium levels less than 0.5 ppm was achieved at pH 5.0-6.0 provided that the solution contained NaNO3 at concentration less than 0.1 M: increasing ionic strength of the solution generally appeared to decrease removal. The effects of Mg(II), NaNOI. and pH on separation were studied. Effective separation with a residual chromium level less than 0.5 ppm was achieved from a solution containing 0.3 M NaN03 at pH 5.0 provided that a high concentration of Mg(I1) (200 ppm) was added as activator. As this system had to run at ENVIRONMENTAL SCIENCE & TECHNOLOGY VOL 29 NO 7 1995
30
1.89 1.47 2.44 1.36 2.14 3.32 1.34 1.66 1.78 0.84 0.32 0.37 0.46 1.49 >IO 3.97 >10 1.54 0.97 0.68 0.19 0.19 0.21 5.00 >IO 7.38 5.21 3.03 >IO 0.68 0.64 >IO 1.96 0.37 0.33 >IO 0.57 0.34 0.25 'IO 0.26
40
50
3.90 0.60 0.35 0.23
2.90 0.74 0.28 0.22
Zn(ll) = 50 ppm; Fe(llll = 100 ppm; SDS = 40 ppm.
+
'Znill) = 50 ppm; Fe(lll) = 100 pprn; SDS = 40 ppm, pH = 9.5. Average value i standard deviation, n = 3.
1806
a
>10
5
~~
TABLE 9
Residual Cr (ppm) Showing Effects of pH and Ionic Strength on Floc Foam Flotation of Cr(v1) with Fe(ll) and SDS (No Activatorp PH
NaNOs(M)
5.0
5.5
6.0
6.5
7.0
0 0.05 0.10 0.20 0.30
0.09 0.09 0.16 1.06 >5
0.04 0.07 0.10 0.83 '5
0.02 0.04 0.09 0.69 >5
0.03 0.09 0.81 '5 >5
0.04 3.94 '5 '5 >5
"Cr(VI) = 50 pprn; Fe(ll) = 175 ppm; SDS = 60 ppm.
lower pH [Cr(III)hydroxide hydrolyzed and redissolved at high pH], adsorption of Mg(I1) on the surface of the floc is inefficient: therefore, Mg(I1) is an inefficient activator for this system. Floc Foam Flotation of CrW) with Fe(II), SDS, and Sodium Oleate. The electrolyte tolerance of floc foam flotation of Cr(VI) with Fe(I1) was significantly improved with a mixed surfactant (a mixture of sodium oleate and SDS) as collector and frother. The effects of surfactant concentration and pH on separation efficiencyof chromium from a solution containing NaN03 (0.8 M) were studied. Effective separation with residual chromium levels less than 0.5 ppm was achieved from a solution containing NaN03 (0.8 M) over the range of pH 7.0-8.0 using collector 1 [sodium oleate (20 ppm) and SDS (20 pprn)] and over the range of pH 5.5-9.0 using collector 2 [sodium oleate (20 ppm) and SDS (40 ppm)]. Effective separation achieved over a broader range of pH using more SDS was mainly due to maintaining a stable foam over a broader range of pH. Floc foam flotation of Cr(VI) with Fe(I1)was also effective with residual chromium levels less than 0.5 ppm from a solution containing NaN03 (1.6 M) at pH 6.5-7.0, using sodium oleate (20 ppm) and SDS (40 ppm) as collector and frother.
TABLE 10
Electrolyte Tolerance (IUaNO3, M) Showing Effects of Collector and Activator on Electrolyte Tolerance of Adsorbing Colloid Flotation of Heavy Metals collector and activator
SDS
+ Mg(ll)
SDS
Cd Pb Zn Cr cua
0.005
0.5
0.1
0.2 0.08 0.1 0.4
0.4 0.1 0.3 0.8
1.5 1.5 1.6
a
SDS
+ sodium olaate
heavy metal
1.6
For details, see ref 32.
Electrolyte Tolerance of Adsorbing Colloid Flotation of Heavy Metals. The electrolyte tolerance of adsorbing colloid flotation of various heavy metals using various collectors and an activator (Mg) is summarized in Table 10. The greatest concentration of NaN03 we tested for the synthetic wastewater was 1.5 or 1.6 M, as few wastewaters contain inert salt with concentration exceeding these levels. The residual levels of heavy metals were less than the specified levels; for instance, the residual zinc level was 0.23 ppm after flotation for 10 min with sodium oleate (40 ppm) and SDS (40 ppm) from a solution containing NaN03 (1.5 M) at pH 9.5. Foam separations of Pb, Zn, Cr, and Cu were effective from the solution containing NaN03 at a large concentration (1.5-1.6 M) with the mixed surfactant (sodium oleate and SDS) as collector and frother. The electrolyte tolerance of adsorbing colloid flotation of Cd(11) was also improved (from 0.005 to 0.1 M NaN03) with this mixed surfactant compared to the use of SDS alone. The electrolyte tolerance of Cd(I1) was improved further (0.5 M NaN03) with Mg(I1) (10 ppm) as the activator. The electrolyte tolerances of other heavy metals were also improved somewhat with Mg(I1) as the activator. The electrolyte tolerance of adsorbing colloid flotation of heavy metals (Cd, Pb, Zn, Cr, Cu) was improved either with Mg(I1)as an activator to increase the surface potential of the floc (to allow the physical adsorption of an anionic surfactant, SDS) or with a mixed surfactant (sodium oleate and SDS) as collector and frother, such that the surface of the floc was rendered hydrophobic through chemisorption of oleate ion even though the surface potential of the floc was negative in the solution containing a high concentration of NaN03. This work eventually eliminated the greatest drawback of adsorbing colloid flotation. The applicability of adsorbing colloid flotation for removal of heavy metals from wastewater is thus greatly extended.
Acknowledgments We thank the National Science Council of the Republic of China for support (Grants NSC-84-2621-M007-001ZA and NSC-83-0208-M007-067).
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Received f o r review October 3, 1994. Revised manuscript received March 22, 1995. Accepted March 27, 1995.@
ES940617E @
Abstract published in Advance ACS Abstracts, May 1, 1995.
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