Surface Properties of Petroleum Refinery Waste Oil Emulsions Richard G. Luthy’ Department of Civil Engineering, Carnegie-Mellon University, Pittsburgh, Pa. 152 13
Robert E. Selleck Division of Sanitary Engineering, University of California, Berkeley, Calif. 94720
Terry R. Galloway Chemistry and Materials Science Department, Lawrence Livermore Laboratory, Livermore, Calif. 94550
The surface properties of oil droplets in refinery wastewater were determined largely by water quality rather than by source of oil. Distilled water suspensions of crude oils and waste oils recovered from a refinery API separator behaved very much in accord with theory qualitatively, having H+ and OH- as potential-determining species. However, surface properties of crude oils and separator float oil differed markedly when suspended in wastewater owing to the presence of wastewater anionic surfactants; these surfactants could not be displaced easily with cationic surfactants. PDADMA cationic polyelectrolytes possessing high molecular weight and being relatively highly charged were effective waste oil droplet coagulants. Charge neutralization was an important factor in achieving good flocculation; however, efficient polymer bridging between oil droplets was also dependent upon polymer charge density. H
As of January 1976 there were 256 operating petroleum refineries in the United States with a combined daily capacity of 2.4 mil m3/day (15 mil bbl/day) (1).The amount of oil and grease in wastewaters from these refineries has been strictly regulated by recent legislation ( 2 ) .The performance of refinery wastewater oil removal process is influenced by oil emulsion stability; the purpose of this work was to investigate the surface properties of refinery wastewater oil emulsions which might affect oil emulsion stability and ease of coagulation as well as to investigate organic coagulant aids which might be used reliably to remove the waste oil from API separator effluents. The latter was considered to be important from a practical viewpoint because if dependable organic coagulants can be employed in lieu of the iron or alum coagulants commonly used today, then the inorganic content of the scum collected from oil separation processes would be reduced appreciably, making it possible to recover the waste oil more easily and economically. The oil emulsions investigated in this study ranged in strength from about 100 to 700 mg/L oil; the subject matter concerns stabilized oil-in-water emulsions which show little tendency for breaking. Little previous work has been published on surface chemical properties of refinery wastewater oil emulsions. “Emulsion Science” ( 3 )gives a comprehensive discussion of physical and chemical properties of emulsions; deBruyn and Agar ( 4 ) and Davies and Rideal ( 5 ) provide background for application of principles of surface chemistry; and Churchill and Kaufman (6) and Luthy et al. (7) summarize petroleum refining experience for processing oily wastewaters.
Analytical M e t h o d s The crude oil samples used in this study were line composites of a light Arabian and a heavy Southern California crude oil. Samples of oil float and wastewater effluent were obtained from an API separator facility as well as from an
in-plant gross oil separator of an operating petroleum refinery. Oil droplet electrokinetic mobility measurements were made with a zeta-meter with 8-10 individual measurements being averaged for each data point. Oil-water interfacial tension was measured with a ring-pull type tensiometer by placing a sample of oil over an aliquot of a water sample after the ring of the tensiometer had been lowered into the water layer. The interface was allowed to age for 1 h, and the measurement was made with standard correction factors applied. Jar flocculation tests were performed on 500-mL samples with the samples being stirred rapidly for 30 s following pH adjustment and the addition of the destabilizing agents. The samples were then flocculated for 15 min followed by 30 min of quiescent standing. Residual turbidity was then determined on samples collected 4 cm from the surface with a turbidimeter. Wastewater samples were analyzed for anionic surface active agents by the methylene blue method, and results were reported as methylene blue active substances (MBAS) (8). The total oil present in the samples was determined by extracting with carbon tetrachloride and then measuring infrared absorbency (9).Dissolved oil was determined similarly following filtration through precoated Whatman No. 40 filter paper. Suspended oil was computed by taking the difference between the total and dissolved oil concentrations. Wastewater suspended solids were determined by filtering the samples through a Gooch filter crucible, washing with chloroform, and then drying to constant weight a t 103-105 “C (9).
Clean Water-Oil Suspensions The presence of even very low levels of anionic surfactants in clean water or wastewater is sufficient to impair the use of cationic surfactants as coagulant aids. However, the action of high charge density cationic polyelectrolytes can lower and reverse electrokinetic mobilities and allow for waste oil flocculation and separation. These results are discussed first with regard to clean water experiments to investigate waste oil emulsion surface properties under well-defined conditions; results of tests with refinery wastewater follow this presentation. Emulsions of the crude oils and the API separator oil float were prepared in distilled water solutions with NaC1. The emulsions were then stirred for 1h to obtain equilibrium following pH adjustment with HC1 or NaOH. The electrokinetic mobilities of the oil droplets were then measured with the zetameter. Representative results of these experiments are shown in Figures 1 and 2. All of the emulsions tested exhibited similar electrokinetic properties. The charges on the droplets were positive at the lower pH values and negative a t the high pH levels with a charge reversal occurring at a pH of approximately 5.0 regardless of the ionic strength of the suspension. The electrokinetic mobilities decreased with increasing ionic strength, reflecting the suppression of the electrical double layer by the Volume 11, Number 13, December 1977
1211
I
I
I
I
I
I
I
I
I
1
x , x e x
I H+ I H+ I
ZH'
Z-
Z&
The surface described by Equation 1 will have a positive charge a t a low pH, a negative charge a t high pH, and a zero net charge at some intermediate pH. When the surface charge responds to the H+ and OH- ions in the manner shown in Equation 1, a linear relationship between surface potential, $0, and surface charge is expected for small potentials (pH near pH,,,) by the following expression:
'y-41
!.
Hence, negative slopes are observed in the vicinity of p H = PHpzc
B -5
W
h I
I
I
I
I
2
3
4
5
,
6
I
I
I
7
8
9
i 0
II
PH
Figure 1. Electrokinetic oil droplet mobility of heavy Southern California crude in distilled water
+4
t
where the symbol ( U H + ) ~represents ~, the activity of H+ a t the point of zero net surface charge (pzc), R is the gas constant, F the Faraday constant, and T the absolute temperature of the water. The results of the electrokinetic mobility experiment reflect oil droplet potentials within the plane of slippage between the droplets and the water, rather than the surface potentials themselves. Such potentials are called zeta potentials j-, and they are related approximately to the electrokinetic mobilities me by the equation
{ = m e -4-T P t
I
2
3
4
5
6
7
8
9
IO
II
PH
Figure 2. Electrokinetic oil droplet mobility of API separator waste float oil emulsified in distilled water
monovalent ions present in solution. The changes in the electrokinetic mobilities with p H were constant within the vicinity of the charge reversal. These observations tend to support the conclusions of other investigators (6, IO),Le., the surface potentials on crude oils emulsified in clean water are determined by surface active agents indigenous to the oil, by the H+ and OH- ions present in the water phase, and by salt ions present in the water which may suppress the electrical double layers. The surfactants indigenous to the oils are reported to be asphaltenes and resins, with the asphaltenes being amphoteric (Z *) and the resins being weak dibasic acids (HzA). Representing the oil droplet surface by the symbol x, it may be seen that: 1212
Environmental Science & Technology
where p is the absolute water viscosity, and t the dielectric constant. The relationship between {and $0 is influenced by the ionic strength of the water suspension and has been the subject of many discussions ( 3 , 4 ) .The position of the shearing plane is not known, but generally the magnitude of ( is less than $0 or $ 6 , the Stern potential. Measurements of oil-water interfacial tension were made on the same oil samples employed for the electrokinetic mobility measurements with the interfaces being allowed to equilibrate for 1h prior to making the determinations. Figure 3 shows the results of these experiments plotted against water pH. A comparison of this figure with Figures 1 and 2 shows that the interfacial tensions increased with decreasing oil droplet mobility, with maximum tensions occurring a t a pH of approximately 5.0, the point of zero net charge on the oil droplets. This too is in accord with theory when H+ and OHare the surface potential-determining ions ( 4 ) .
dy -dpH
2.3RT F
"O
These equations hold for dilute solutions a t constant ionic strength where 7 is the interfacial tension, and I-, the adsorption density of specie i at chemical potential p,. The change of interfacial tension with pH is predicted to be proportional to the surface charge UO, and the maximum tension is attained when uo = 0. Organic cationic surfactants were added to the oil emulsions at a constant pH of 8.0 to determine if the highly negative
+3 0
+eo
tl0
E
P 9
r
0
P
"B \
3
2
p f
-10
Temp.: 2 3 - Z F
C
-3,$
tI
I
I
I
I
I
I
I
I
I
I
2
3
4
5
6
7
8
9
IO
I
0 II
PH
1
I
I
2
3
4
l l 5 6 Dose, mJ
/e
l
l
7
l
8
9
l 1
l 0
Figure 4. Electrokinetic oil droplet mobility of light Arabian crude in clean water dosed with cationic surfactants
Figure 3. Interfacial tension variation with pH for oil against distilled water of varying salt concentration
Table I . Cationic Surfactants Investigated Name
Dodecyltrimethylammonium chloride Hexadecyltri methylammonium chloride Du Pont retarder LAN Dialkyl quaternary ammonium chloride Dialkyl quaternary ammonium chloride Aliphatic diamine
Formula or
Mol
description
Wt
H25C12N(CH3)3C1
263.9
H,,C 6 N (CH ,),CI
320.0
AI kyltrimethylammonium bromide (H ,,C, 2) ,N (CH J2CI
=447
= (H,,C,,),N(CH,),CI
-573
r
-330
R-N-C-C-N
1
electrokinetic droplet mobilities usually encount.ered a t that p H could be reversed. The concepts underlying this approach are based on the equation (11):
where ra is the density of the organic ion adsorbed a t the interface, ri the radius of the adsorbed ion, Ci the bulk water concentration of the ion, and the terms (-zF+&) and ( - N 4 ) represent the free energies of electrostatic action and adsorption per mole of surfactant, respectively. The term for electrostatic action represents the product of the valence t of the organic ion, Faraday's constant F , and the surface potential $8 of the oil droplet a t the Stern layer. This free energy term will be positive for electrostatic attraction and negative
for electrostatic repulsion. The term for the free energy of adsorption refers to the product of the number N of -CH2groups contained in the aliphatic tail of the surfactant and the free energy of adsorption 4 per mole of the -CH2groups. This term is positive for adsorption of organic ions on oil droplets, and even an anionic surfactant may be adsorpted a t the oil-water interface if the free energy of adsorption is greater than the free energy of electrostatic repulsion. The cationic surfactants used in these studies were primarily the quaternary ammonium compounds listed in Table I because these compounds are strongly ionized. The results of these studies (shown in Figure 4)indicated that the charge on the oil droplets could be reversed easily with relatively modest doses of the cationic surfactants ( < 2 mg/L). The oil droplets refused to coalesce even when the electrokinetic mobilities were reduced to zero, either by pH adjustment or by the addition of cationic surfactants. This may have resulted from the formation of tough interfacial films of the type reported by Reisberg and Doscher (12).
Wastewater Oil Suspensions The surface properties of waste oils contained in the effluents of oil refinery separators were also determined, and the results differed grossly from these found for the same types of oil emulsified in distilled water. The electrokinetic mobilities of oil droplets suspended in an API separator effluent and an in-plant gross oil separator are shown in Figure 5 . No significant charge reversal was evidenced a t p H values as low as 2.0. Similar trends of gradually increasing negative electrokinetic mobility with pH with no charge reversals were observed for emulsions of crude oils and API separator float oil prepared in both filtered and unfiltered wastewater effluent (data not shown). These studies indicated that the wastewater oil emulsions had a lower (pH),,, than clean water suspensions, and that the wastewater suspended solids did not affect the oil droplet mobilities grossly. I t appeared that the oil droplet surface properties were being influenced significantly by the surfactants present in the wastewater. Volume 11, Number 13, December 1977
1213
c
+ 1.0 I-' f0.5
I I I I I a- LOP Plant Gross Oil Seporatw efflwnf W&ic
I
I
I
IJ
I
conductonce: 1 4 0 0 ~ m h o a/an
0- API Sepaatar effluent spaciflc
\
conductance~4100xmb/cm
i -1.0
.t
-15
1
2
3
4
5
6
7
8
9
1
0
l
l
PH
-20
3
2
Temp.: 23- 25OC
Figure 7. Interfacial tension of API separator float oil against refinery API separator effluent
-25
-3.u I
I
I
2
3
I 4
I
I 6
5
I
I
7
8
IO
9
1 I
PH
Figure 5. Electrokinetic mobility of in situ waste oil droplets in gravity seoarator effluents I
I
1
+I 0
'
l
l
1
\
k
+O,,t
I
DstilledWa+!r: IO-% NaCl! 840 fiumhos/un; 0 0 mg/L MBAS
Dlsfilled Water
,#mhcs/cm 0.1 mg/e H2g12S04No(0 26mg/l MBASI
P API Separator Effluent'4600
fimhos/cm' IlOmp/.C MBAS
Temp = 23-25OC
3 .O I
I
I
2
3
tensions of the droplets in the wastewater were very low; becoming almost nkgligible a t pH values less than 4 or greater than 11,and that the maximum interfacial tension had shifted upward. The filtered and unfiltered wastewater effluent samples gave similar interfacial tensions indicating that the wastewater suspended solids were not a primary cause of surface tension lowering. Theoretical interpretations may be invoked for the differences between distilled water and the wastewater suspension results. The downward shift in the pzc may be explained on the propensity for small concentrations of organic surfactants to have a pronounced effect on oil-water interfacial properties. For example, consider the heavy Southern California crude M NaCl (Figure 1).The surface oil a t pH = pH,,, in charge ($) resulting from the addition of 0.1 mg/L (3.5 X M/L) sodium lauryl sulfate a t these conditions (Figure 6) may be estimated from an adsorption equation developed by Davies and Rideal(5) for long-chain ions a t the oil-water interface.
I 4
I 5
I 6
I 7
I 8
I
I
I
9
IO
II
(BlIB2)C exp
(-""f,
Nd)
PH
Flgure 6. Electrokinetic oil droplet mobility of heavy Southern California crude in waters containing various levels of MBAS
A few experiments were then conducted to evaluate the effects of anionic surfactants on the electrokinetic mobilities of the oil droplets. Emulsions of a heavy crude oil were prepared in distilled water in the manner described previously for the clean water experiments, except in this case 100 kg/L of sodium lauryl sulfate were added to each experimental run. The results are compared with a control sample with no sodium lauryl sulfate added as well as results typical of API separator effluent in Figure 6. A dose of 100 kg/L of sodium lauryl sulfate was sufficient to produce electrokinetic responses similar to those observed in the API separator effluent. T o test this further, an emulsion of the crude oil was prepared in tap water with no surfactant added. The tap water contained slight traces of anionic surfactants (MBAS = 28 pg/L), and these were sufficient to alter the electrokinetic properties of the crude oil droplets significantly as shown in Figure 6. Only very slight contamination of the water with anionic surfactants was sufficient to alter the oil droplet surface characteristics significantly from those hypothesized by past investigators for ideal conditions (Equation 1). The interfacial tensions of waste oil droplets suspended in API separator effluent also differed grossly from those observed for oils suspended in distilled water. Comparison of some typical results shown in Figure 7 with the studies presented in Figures 3, 5 , and 6 indicates that the interfacial 1214
Environmental Science & Technology
where n. equals the number of adsorbed molecules per cm2 of surface, c is the bulk concentration of surfactant (M/L), and no the maximum possible adsorption density. B1 and B2 are constants, for lauryl sulfate B1IB2 = 2 lo1* L/M/cm2; 4 is the specific adsorption term which is on the order of -810 cal/mol per -CHz-group. This equation is Langmurian in form, but not in application because $ is a function of n . 4 may be calculated from the Guoy-Chapman equation (12).
where e is the dielectric constant for water, and ii the number of ion pairs per cm3 in the bulk solution. Solving Equations 8 and 9 simultaneously gives a surface potential of about -140 mV. This is in agreement with the data of Figure 6 where a mobility of about -1.5 is observed at pH = 5 , which corresponds to a zeta potential of about -20 mV. The variation appears to be predictable a t least in direction, although the observed zeta potential was less than calculated from theory owing in part to the inherent difference in 4 and zeta potential. The upward shift in pH of maximum y may be explained as follows: d y = r H + d p H + t TOH-dpOH- -k r A n - d p A n -
(10)
where An- represents an adsorbed surface active organic anion. For strongly ionized surfactants
Table II. Commercial Polyelectrolytes Investigated Type
and
Trade name
Nonionic WC 2690
+2 0
I
i
I
4
D i i l k d Water 0.0 mg/P MBAS
+1.0
I I
0
1
/
16% NoU, DistilledWo(er,
Description
Calgon Corp.
(12) where UH represents the portion of the change due to H+ and OH-. y is then maximum for UH = 0, Le., for pH = PHIEP,not p H = pH,,,. The distinction between zero point of charge (zpc) and isoelectric point (IEP) is made according t o Parks (13).The zpc is the p H a t which the surface charge from all sources is zero; whereas IEP is a zpc arising from interaction of H+, OH-, the surface, and water alone. The specific adsorption of anionic surfactants would be expected to shift the zpc to a more acid value and to the extent that Hf binding is affected by the specially sorbable anion the IEP would rise (13, 14). The organic cationic surfactants listed in Table I were added to emulsions of API separator float oil prepared in clean and dirty waters a t a constant pH of 8.0 to determine the effects of surface contamination on the charge reversal properties of the cationic surfactants. The emulsions were allowed to equilibrate for 1h. Typical results for the cationic surfactant dodecyltrimethylammonium chloride are shown in Figure 8. A charge reversal was obtained at a dose of less than 5 mg/L of the surfactant when the separator float oil was emulsified in distilled water. Doses as high as 25 mg/L did not reverse the droplet charges for emulsions prepared in distilled water with 1.0 mg/L of the anionic surfactant sodium lauryl sulfate added, or in the API separator effluent. The results obtained for the other cationic surfactants listed in Table I were similar. The data indicated that even slight anionic surfactant contamination can impair the charge reversal capabilities of a cationic surfactant grossly; that is, a cationic surfactant apparently will not displace anionic surfactants previously adsorbed a t an oil-water interface.
Manufacturer
Cationic WT 2575
WT 2635
Dry solid, high molecular weight, 5-10 lo6, completely nonionic polyacrylamide Liquid copolymer, slightly Calgon cationic, 20% PDADMA,a Corp. molecular weight 2 l o 6 Liquid copolymer, moderately Calgon cationic, 50% PDADMA, Corp. molecular weight 2 l o 6 Calgon Liquid copolymer, strongly cationic, 7 5 % PDADMA, Corp. molecular weight 1-2 1O6 Calgon Liquid copolymer, very strongly cationic, 95 YO Corp. PDADMA, molecular weight 1-2 106 Calgon Liquid copolymer, very strongly cationic, 100% Corp. PDADMA, molecular weight 3-4.105 American Liquid cationic polyamide, Cyanamid molecular weight 3-5 IO4
-
-
WT 2640
WT 2860
-
CAT FLOC
MAGNIFLOC 521-C WC-3 1
Tretolite Corp.
Liquid, polyacrylamine, molecular weight 2-3
lo4
aPDADMA: polydiallyldimethylammoniumcompound.
Polyelectrolyte Coagulation of Oil Emulsions in Wastewater Electrokinetic mobility and jar flocculation tests were performed using several types of polyelectrolytes. The polyelectrolytes investigated were selected on the basis of functionality and the availability of information concerning their composition. The general characteristics of the nonionic and cationic polyelectrolytes studied are listed in Table 11. Two classes of anionic polymers were tested: hydrolyzed polyacrylamide and polystyrene sulfonate, for which the characteristics are given in ref. 7 .The cationic polymers represented selections from polyamines and polydiallyldimethylammonium (PDADMA) compounds. The cationic polydiallyldimethylammonium polyelectrolytes used in the experiments represented a homologous series of copolymers which possessed approximately the same high molecular weight but variable positive charge depending on the degree of replacement of the acrylamide groups with polydiallyldimethylammonium groups (+ charges). The general structure of these polymers was as follows:
-CH,-CH
-
1 I
-40
t 0
r
0H:bO
5
IO
Dose, rq /e
15
20
25
Flgure 8. Electrokinetic droplet mobility of API separator waste float oil immersed in clean and dirty waters dosed with cationic surfactant
Volume 11, Number 13, December 1977
1215
+IO
' 1
+o 5
6
..# $
0
-05
t
f -1.0
a
B
g
-I5
f
1 0
5
10
15
20
25
Dose, m g / f
Figure 9. Electrokinetic mobility of in situ waste oil droplets in API separator effluent dosed with cationic polydiallyldimethylammonium polyelectrolytes with charge density varying from 0 to 100%
with the degree of charge density being defined as the ratio of the number of charged groups N to the total number of groups present (N M). The charge densities used in this work were varied from 0 to 100%. The effects of the cationic PDADMA polyelectrolytes on the electrokinetic mobility of the waste oil droplets suspended in the effluent of an API separator are shown in Figure 9. As might be expected, the dose of polyelectrolyte required to reverse the charge on the oil droplets decreased with increasing polyelectrolyte charge density. For example, the 95 and 100%charge density polymers required a dose somewhere between 10 and 15 mg/L to reverse the droplet mobilities, whereas the slightly charged 20% PDADMA polyelectrolyte did not reverse the mobilities with doses as great as 25 mg/L. These experiments indicated that the PDADMA cationic polyelectrolytes could reverse oil droplet charges at doses much less than those required for the cationic surfactants. The construction of polymer bridges has been proposed as an important mechanistic step in flocculation kinetics with polymeric coagulants (15,16). However, Kasper (17)suggests a variation in these mechanisms whereby flocculation of charged particles by polyelectrolytes of opposite charge involves the formation of charged polyion patches on the oppositely charged surface. Kasper found that with high molecular weight polymers and/or high ionic strengths, polymer dose was independent of molecular weight. To study polyelectrolyte flocculation of oil droplets, jar tests were performed on composite samples of API separator effluent at an initial pH of 8.0-8.2 employing various doses of the polyelectrolytes listed in Table 11; alum was used for comparative purposes. Results for the PDADMA polyelectrolytes and alum are shown in Figure 10 for two different samples of API separator effluent. The general quality characteristics of the effluent samples are presented in Table 111. The more highly charged PDADMA polyelectrolytes produce minimum residual turbidities in the separator effluent at doses equivalent to those necessary to reverse the electrokinetic mobilities of the waste oil droplets (Figure 9). This was also true for the alum (electrokinetic data not shown). The doses necessary to produce minimum residual turbidities appear to be independent of the separator effluent oil concentration or turbidity, an observation which was later supported by pilot plant flocculation data (7). This result is at variance with laboratory investigations on polyelectrolyte flocculation of clay suspensions. However, relatively massive doses of polyelectrolyte had to be employed in this work to achieve good oil droplet flocculation, and losses
Figure 10. Results of jar tests using alum and PDADMA copolymer coagulants with API separator effluent samples. Percentages refer to degree of cationic charge density
Table 111. API Separator Effluents Used in Jar Tests
+
1216
Environmental Science 8, Technology
Analysls
Oil, mg/L
Suspended solids, mg/L Initial, pH MBAS, mg/L Turbidity, JTU (after pH adjustment)
Sample 1
Sample 2
256 49 9.5
670 164 9.8
4.2 36
4.7
66
of polyelectrolyte to the droplets would be less noticeable than in the case of clayey turbidity flocculation. For example, the ratio of polyelectrolyte dose to suspended phase surface area was on the order of 200 times greater for this work (50-11diameter droplets, 200-700 mg/I,, 15 mg/L polymer dose) as compared to the ratios commonly encountered in clay suspensions [25-50 mg/L ( I 8 ) , surface area = 20 m2/g (19), 1 mg/L polymer dose ( 1 8 ) ] .Also a flocculation process more dependent on electric double-layer compression than polyelectrolyte bridging would tend to be less dependent on the concentration of the suspended phase. The PDADMA cationic polyelectrolyte with a 75% charge density outperformed all of the polyelectrolytes listed in Table I1 a t a pH of 8.0-8.2. (The nonionic and anionic polymers performed poorly, and the results of those experiments are not shown in Figure 10). Evidently, the 75% charged PDADMA polyelectrolyte possessed a charge density sufficient to neutralize the charges on the surfaces of the oil droplets but not so great as to interfere with polymer bridging mechanisms.
Summary Crude oils and float oil recovered from a refinery API separator, with pH and ionic strength adjusted with the addition of monovalent inorganic ions, exhibited similar surface characteristics when suspended in distilled water. The surface charges appeared to depend on surfactants contained in the oil phase and on H+ and OH- ions in the aqueous phase with a zero zeta potential being achieved at a pH of approximately 5.0 regardless of the source of the oil or the ionic strength of the water. The nature of the responses of the surfaces to pH change appeared to follow ideal theory closely for conditions where the H+ and OH- ions are the potential-determining species. The crude oils and separator float oil exhibited almost identical surface properties when emulsified in filtered or unfiltered API separator effluent water, and these properties differed grossly from those observed in distilled water. A
change in pH did not change the surface charges dramatically until very low pH values were attained (pH < 2-3); the interfacial tensions were far lower than those observed for the distilled water emulsions. Very low concentrations of anionic surfactants contained in the wastewater may account for these effects, and those surfactants were not readily displaced with cationic surfactants. Anionic and nonionic polyelectrolytes did not flocculate the oil droplets effectively, but relatively highly charged cationic polyelectrolytes did perform well in wastewaters with best flocculation being achieved at the point of zero net charge on the oil droplets. A 75% charged PDADMA cationic polyelectrolyte performed by far the best of all the polyelectrolytes tested. Discussion and Conclusions The surface properties of oil emulsions in a refinery API separator effluent appeared to be dependent on the anionic surfactants contained in the water phase, and not on the source of the oil. The particular effluent tested commonly contained appreciable concentrations of anionic surfactants, but the interfaces of fresh crude oils would readily adsorb an anionic surfactant present at very low concentration (100 pg/L) and even good quality tapwater contained sufficient anionic surfactants to influence the surface properties of the crude oil grossly. Consequently, it appears impractical to try to eliminate contact between the waste oil and anionic surfactants in refining operations for purposes related to wastewater oil removal. The anionic surfactants inhibit oil removal from waste streams by increasing the negative charges on the oil droplets at all pH levels and by occupying specific adsorption sites a t the oil water interface. The latter effect negated the use of cationic surfactants as surface active counter ions. The dose of a cationic polyelectrolyte necessary to achieve good coagulation appeared to be relatively insensitive to oil concentration ranging from 100 to 700 mg/L, implying that some type of electrical double layer compression-adsorption flocculation mechanism was significant in the process because bulk polyelectrolyte concentration would tend to be less dependent on the oil concentration in this case. However, polyelectrolyte bridging cannot be entirely discounted as a causative factor in oil droplet flocculation, because if it was not important then the degree of removals achieved ought to have been equal for the most highly charged cationic polyelectrolytes tested. This proved not to be the case; polyelectrolyte performance was highly dependent on the polymer’s charge density. The data indicate that neutralization of the oil droplet charges was a significant factor in achieving good flocculation and that a lesser charged cationic polyelectrolyte was a more effective oil coagulant than the highest charged polyelectrolytes, all other factors being held constant including the mo-
lecular weights of the polyelectrolytes. Perhaps the higher charge density caused portions of the polyelectrolyte to adsorb too quickly on the same droplet, making these polyelectrolytes less effective for interdroplet bridging. Experimental data not presented in this paper demonstrated that a cationic polyelectrolyte can be used reliably in a wastewater treatment facility with good removals being achieved under a wide variety of input water quality conditions (7). Literature Cited (1) Cantrell, A., and Staff, Oil Gas J., 74 (13), 129 (1976).
(2) “Development Document for Effluent Limitation Guidelines and New Source Performance Standards for Petroleum Refinery Point Source Category”, EPA, EPA-440/1-75-014-a, Washington, D.C., Apr. 1974. (3) Sherman, P., Ed., “Emulsion Science”, especially Chap. 2, “The Theory of Stability of Emulsions”, J. A. Kitchener and P. R. Mussellwhite, Academic Press, New York, N.Y., 1968. (4) deBruyn, P. L., Agar, G. E., “Surface Chemistry of Flotation”, in “Froth Flotation”. 50th anniversarv vol. D. W. Fuerstenau, Ed., pp 91-138, AIME, New York, N.Y., i962. (5) Davies. J. T., Rideal, E. K., “Interfacial Phenomena”, Academic Press, New York, N.Y., 1961. (6) Churchill, R. J., Kaufman, W. J., “Waste Processing Related Surface Chemistrv of Oil Refinerv Wastewaters”. SERL Rem No. 73-3, Sanitary Engineering Research Lab, University of California, Berkeley, Calif., Aug. 1973. (7) Luthy, R. G., Selleck, R. E., Galloway, T. R., “Removal of Emulsified Oil with Organic Coagulants and Dissolved Air Flotation”, presented at 49th Annual Conf., Water Pollution Control Fed., Minneapolis, Minn., Oct. 1976; J. Water Pollut. Control Fed., in press. (8) “Standard Methods”, APHA, 13th ed., 1970. (9) American Petroleum Institute, “Manual of Disposal of Refinery Wastes, Methods for Sampling and Analysis of Refinery Wastes”, Washington, D.C., 1958. (10) Bartell, F. F., Niederhauser, D. O., API Drilling Prod. Pract., 57 (1949). (11) Fuersternau, D. W., Healy, T. W., “Principles of Mineral Flotation”, in “Adsorptive Bubble Separation Techniques”, R. Lemlich, Ed., pp 91-131, Academic Press, New York, N.Y., 1972. (12) Reisberg, J., Doscher, T. M., Prod. Mon., 43 (Nov. 1956). (13) Parks, G. A,, “Equilibrium Concepts in Natural Water Systems”, Advances in Chemistry Series, No. 67, American Chemical Society, Washington, D.C., 1967. (14) Stumm, W., Morgan, J. J., “Aquatic Chemistry”, Wiley-Interscience, New York, N.Y., 1970. (15) LaMer, V. K., Healy, T. W., Reo. Pure Appl. Chern., 13, 112 (1968). (16) O’Melia, C. R., “Coagulation and Flocculation”, in “Physicochemical Processes for Water Quality Control”, W. J. Weber, Jr., Ed., Wiley-Interscience, New York, N.Y., 1972. (17) Kasper, R. D., PhD thesis, California Institute of Technology, 1971. (18) Hesphanol, I., Selleck, R. E., “The Role of Polyelectrolytes in Flocculation Kinetics”, Rep. 75-2, Sanitary Engineering Research Lab, Univ. of California, Berkeley, Calif., 1975. (19) Perloff, UT.H., Baron, W., “Soil Mechanics”, Ronald Press, New York, N.Y., 1976. Received for recieu: August 26, 1976. Accepted September 6, 1977. Work supported in p a r t by a research grant from S H E L L Oil Co. Manufacturing Complex, Martinez, Calif.
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