current research S Y M P O S I U M O N COLLOID A N D SURFACE CHEMISTRY IN AIR A N D WATER POLLUTION
Joint with the Divisions of Petroleum Chemistry and Water, Air, and Waste Chemistry, this symposium was presented at 156th Meeting, ACS. Atlantic City, N. J.. September 1968. Those presiding were: C. R. O’Melia N a t i o n a l Center for Atmospheric Research, Boulder, Colo. C. M. Hidy S c h o o l of Public Heaith, University of N o r t h Carolina, Chapel Hill, N. C. The first three articles i n this issue, pages 551-67, were given at this symposium. Other papers from this symposium will appear in subsequent issues of the journal.
Control of the Bacterial Content of Water with Synthetic Polymeric Flocculants J. K. Dixon and M. W. Zielyk Torrington Branch, University of Connecticut, Torrington, Conn. 06790
In a study of synthetic polymers for flocculation of bacteria in water, filtration rates, electrophoretic mobility, and light transmission were measured to evaluate flocculation. E. coli were studied at bacterial concentrations of 50 to 2700 mg. per liter and at pH 4 to 9 with nonionic, anionic, and cationic polymers as flocculants. Nonionic and anionic polyacrylamides were studied with molecular weights from 7000 to 3,000,000, without evidence of flocculation at polymer concentrations of 0.1 to 50 mg. per liter. Cationic polyethyleneimines with molecular weights from 300 to 112,000 were effective at concentrations from 0.1 to 80 mg. per liter, but molecular weight had no effect on the amount of polymer required to initiate flocculation. Low molecular weight polymers at concentrations even in excess of 800 mg. per liter caused flocculation, but above about 10 mg. per liter the high molecular weight polymers caused redispersal of the bacteria. The results for the polymer-bacteria systems are similar to those obtained for flocculation of some inorganic and biocolloid s) stems. The cationic polyethyleneimines should be of practical value for quality control of water because of their ability to flocculate and settle solids of various types at low dosage levels.
polysaccharide polymers produced by the E. coli during their growth, had shown these polymers to be effective flocculating agents (Busch, 1966). The use of polymers for flocculating inorganic solids-for example, phosphate slimes, clays, amorphous silica, crystalline silica. and inorganic halides-has been studied in detail by a number of investigators (Black, Birkner, et u/., 1965; La Mer, 1966; Matijevic and Janauer. 1966). The value of synthetic polymers in practical applications, notably sewage plant operations, has been amply demonstrated on a large scale by a number of industrial concerns, but some important properties of their synthetic polymers have often been somewhat obscure and the conditions of application not as completely specified as desired for understanding their performance. For this reason one of the present authors (J.K.D.) studied the action of a series of well-characterized polymers of ethyleneimine for flocculation of crystalline silica under well-defined conditions of flocculation (Dixon, LaMer, et a / . , 1967a). The present investigation of the flocculation of E. coli with cationic, nonionic, and anionic polymers extends this work on silica, and demonstrates that the two systems behave in similar manner in many respects, despite the differences in naturr: of the solid surfaces involved. Experinzentul
T
he removal of low concentrations of bacteria by flocculation with synthetic polymers was studied. Cationic polymers were known to be effective in flocculating E. coli and settling from their dispersions in water (Teiiney and Stumm, 1965) and it was suggested that such polymers might be of practical importance when used in a controlled manner in activated sludge sewage operations. Other studies of the use of anionic and nonionic polymers, and of polyamino acid and
Filtration Rate. The value of filtration rates as a measure of flocculation of phosphate slimes and clays has been demonstrated (Smellie and La Mer, 1958), and this method was used recently to study the flocculation of crystalline silica with polyethyleneimine polymers of different molecular weights (Dixon, La Mer, et a/., 1967b). In the present work, techniques of this last study were followed to control pH, effect of time of stirring, concentration of polymer added, etc. The E. coli were grown on a glucose-phosphate buffer Volume 3, Number 6, June 1969 551
nutrient system, giving about 200 mg. per liter of bacteria. These were treated with polymer with no dilution, or after dilution to 100 or 50 mg. per liter, the buffer maintaining the pH at about 6.2 to 6.7. After polymer addition, pH adjustment, and stirring for 10 minutes, 50 ml. was filtered through a 9.6-sq. cm. Millipore HA filter, with 0.45-micron pore diameter and the time to filter was obtained with a 74-cm. of Hg head across the filter. The E. coli have a size of 1.0 to 4.0 microns and, when filtered through Millipore filters of 0.8- and 1.2-micron pore diameter, the filtrates were not completely clear, as was the case for the 0.45-micron filter. It was observed, after much lack of reproducibility, that the rate of filtration through 0.8- and 1.2-micron paper was greatly dependent on the direction of flow during filtration, the papers having different pore sizes and shapes on the two sides. This two-sidedness was not found with the 0.45-micron filter and hence results given below are only for this type. The filtration rates were measured by the time taken to filter 50 ml. of bacterial dispersion. When attempts were made to find the time to refilter 50 ml. of filtrate, the results were not reproducible, depending on whether or not the bacteria were sucked dry on the filter, and probably other factors not identified. This required that filtration time be used as a measure of flocculation. The standard deviation of the time to filter 50 ml. at pH 4,6.3, and 9, a t dilutions of 1 to 5, 1 to 2, and no dilution, averaged 15 on 20 different batches of E. coli. With the high molecular weight polymers (21M, 14M, and 20M, Table I), concentrations higher than 15 to 25 mg. per liter, in the absence of E. coli, resulted in very long filtration times because of blockage of the Millipore filters by the polymers (Dixon, La Mer, et al. 1967b). These polymers reacted with undiluted nutrient at pH 7, but not pH 4 or 9, at concentrations above 10 to 25 mg. per liter, forming a precipitate which was presumed to be polymer-phosphate. Fivefold dilution of the nutrient prevented formation of this precipitate at all pH's and all polymer concentrations. To avoid both complications the higher polymers were not studied by the filtration method at any concentration of polymer above about 10 mg. per liter, but use of the transmission method was permissible as long as the higher polymers were not tested at pH 7 with undiluted E. coli above 10 mg. per liter. Electrophoretic Mobility. The electrophoretic mobilities of E. coli-polymer dispersions were measured in a Briggs-type cell (Briggs, 1940) following more recent improvements and recommendations (Black and Smith, 1962). A modified Briggs cell was actually used (Rock and Burbank, 1966). Conductance of bacterial dispersions was measured with a suitable Wheatstone bridge. Electrophoretic mobilities were measured on
z,
Table I. Description of Polyethyleneimine Polymers Molecular Weights as Measured by DesigVapor osPolymer nation Viscosity mometry Manufacturer 450-750 0 8M 770 265 Montrek 6 970 530 1050-1350 1 .O M Montrek 12 1,260 670 1650-1950 Montrek 18 1 . 3M 1,400 392 ... 111-23 1.4M 1,850 ... 3,000-5,000 Montrek 1117 1 . 9 M 3,400 . . . 10,000-15,000 Montrek 1612 3 . 4 M 14,200 . . . 40,000-60,000 14 M Montrek 600 111-42 20M 20,100 ... ... Montrek 1120 21 M 20,600 . . . 50,000-100,000 552 Environmental Science & Technology
E. coli dispersions, after dilution with distilled water from 200 to 100 or 50 mg. per liter. The bacteria were not easily observed at 200X to 400X magnification, compared with silica or blood cells, because of their small size and the fact that their refractive index is close to that of water. Mobility, filtration, and transmission measurements were usually made on the same dispersions. Dilution with nutrient in place of water gave equivalent results. Light Transmission and Light Scattering. Light scattering was determined in a Brice-Phoenix light scattering apparatus (instrument 2075). The scattering, always at 90°, before or after 1 to 2 or 1 to 5 dilution, was measured as described (Phoenix Instrument Co., 1968). At about 200, 100, and 50 mg. per liter of E. coli the transmissions were 45,25, and 11 respectively, with an accuracy of about & 1 The ratio of the intensity of light scattered at 90" to the intensity of the incident beam varied from 30 to 75 X The nutrient for the E. coli originally contained ten times the Mg ion concentration recorded below, but gave a precipitate when the pH was raised to 8 to 9. This increased light scattering substantially. Calculations based on solubility products showed that the precipitate might be either a calcium or a magnesium phosphate. It seemed desirable to carry out flocculation experiments in which there was no chance of interference by such a precipitate; hence the Mg content of the nutrient was decreased by a factor of 10 to that recorded below, after which no precipitation occurred at any pH. In another procedure for using light transmission to follow flocculation, samples of E . coli-polymer were stirred for 10 minutes at the desired pH and then allowed to settle for 1 hour in a 50-ml. graduated cylinder. Then the top 10 ml. was removed with a pipet and its transmission measured in a Bausch and Lomb Spectronic 20. This permitted evaluation of transmission for all polymers at pH 4 and 9 above 15 to 25 mg. per liter. This could not be done by the filtration method for the high polymers because of blockage of the filter medium by these polymers.
z.
z,
Analytical Procedures The chemical oxygen demand of the E. coli was determined by the standard procedure (American Public Health Association, 1965). The E. coli content of a batch was found by filtration and weighing of the bacteria retained on a 0.45-micron Millipore filter after drying at 104 "C.(Engelbrecht and McKinney, 1956). The glucose content of E. coli batches was determined by the anthrone method (Scott and Melvin, 1953). Preparation of E. coli Samples (Gunsalis and Stanier 1960; Roberts, Abelson, et al., 1963; Thimann, 1963). Escherichia coli were prepared from a pure strain of E. coli B. by growth in a nutrient consisting of 0.025M KH?P04,0.025M KJ3P04, and (NH4)$04 1000, NasSOl 100, MgCL.6HgO 16, CaCl?.2H?O 2 , FeC13.6 H 2 02, N a H C 0 3420, and glucose 1300 mg. per liter. The cultures were grown for about 16 hours under sterile conditions of operation for about 16 hours at 35" &2"C., in 21-liter bottles stoppered with cotton. Batches were used for several days, care being taken to prevent contamination with other bacteria. Microscopic observation of gramstained bacteria from each batch indicated the presence of nothing but coliforms. Twenty-two batches were prepared in which the average bacterial solids content was 244 mg. per liter, with a standard deviation of 49. Batches were made in which sterile air was blown through the batches, but the solids content did not increase as much as desired, since generation of organic acids
exhausted the phosphate buffer and the pH dropped below 6.0, 000. Conductometric analyses had shown less than 1 % of carboxyl groups and the molecular weights had been deterstopping the bacterial growth. By doubling the phosphate buffer and the glucose it was possible to increase the bacterial mined by calculation from viscosity measurements in 1 N sodium chloride, using an established relationship between yield to 400 mg. per liter before the batch reached pH less than 5, when growth again stopped. One large batch was grown viscosity and light-scattering molecular weights. Our own viswith the magnesium content and with a very high glucose cosity measurements gave calculated values for molecular weights in reasonable agreement with those reported to US by concentration of 5000 mg. per liter in a large well-aerated sterile fermenter in order to obtain 640 mg. per liter of E. coli American Cyanamid. Samples with molecular weights of at pH 4.7. A large portion of this batch was centrifuged to con7000, 12,000, and 3,000,000 are referred to hereafter as AI, A4, centrate the E . coli, which were then reconstituted with clear and AS, respectively. ANIONICPOLYACRYLAMIDE POLYMERS. Anionic polymers supernatant to 2725 mg. per liter, in the hope that this very high concentration of E. coli might be flocculated with nonwere obtained by hydrolysis of polyacrylamide samples with dilute 15 mole ionic and anionic polymers. of 0.1N NaOH. Hydrolysis with 0.1 % polyacrylamide solutions at 100 "C. for 5 hours converted 15 % of The average E. coli batch had the following characteristics and standard deviation (cJ):pH 6.3; concentration 244 mg. the amide groups to carboxyl, making the polymer anionic t o this extent. Samples AI, A4, and AS were hydrolyzed and are per liter, u 49; chemical oxygen demand 1362, u 127; time to designated as A1-15, A4-15, and AS-15, respectively. filter 100 ml. 1260 seconds, u 250; glucose consumed 97 to 9Sz;turbidity 90°/Oo of 50 to 100 X lo-*; and transmission 2 In describing the results with the polymers, concentration of to 4. polymer of any kind added to the bacterial dispersion is given Polymer Preparations and Samples. POLYETHYLENEIMINE. as milligrams per liter, and the concentration of E. coli to The Dow Chemical Co. provided samples of ethyleneimine which the polymer was added as milligrams per liter. monomer and its polymers. The latter included Montrek 6, Flocculation of E. coli with Nonionic Polyacrylamide. 12, 18, 1117. 1612, 600, and 1120 (reported molecular weights Attempts were made to flocculate E. coli, without dilution, at around 200 mg. per liter, with nonionic polyacrylamides A l , shown in Table I). Montrek 6, 12, 18, and 1117 are believed to A4, and AS (120,000 to 3,000,000 molecular weight), at pH be not crosslinked, but 1612,600, and 1120 are (Dow Chemical Co., 1966). Although no details are reported for the procedures 4 and 6.5. The electrophoretic velocity was not altered appreused for determining the properties of these polymers, light ciably from 0.2 to 50 p.p.m. of polymer. At 40 mg. per liter scattering, gel permeation, and vapor osmometry methods have and pH 4, 6.2, and 9.0, polymers A l , A4, and AS showed no been used with success (Dick, 1968). Samples are referred to by flocculating action as measured by filtration rate up to 10 their viscosity molecular weights shown in Table I-for examp.p.m. At a concentration of about 5 p.p.m. of polymer the ple, Montrek 1612 is referred to as 3.4 M, meaning 3400 filtration rate decreased rapidly, but it was demonstrated that molecular weight. this was due to the action of the polymer alone on the Millipore Monomeric ethyleneimine was freshly distilled and polyfilter, since the same decrease in filtration rate was observed in merized using hydrochloric acid as catalyst. Further details on the absence of E. coli. polymer preparation and determination of molecular weight Since results at 100 and 200 mg. per liter failed to show any by viscosity have been given (Dixon, La Mer, et al., 1967a). flocculation for the nonionic polymers, and such polymers had A 3 0 z by weight solution of ethyleneimine in water was failed to flocculate silica at 400 mg. per liter, but did flocculate polymerized using a mole ratio of ethyleneimine to HCl of at concentrations of 5000 mg. per liter or higher (Kane, La 1000 to 1 by heating at 100 "C. The molecular weight of this Mer, et al., 1964a), it was thought that higher concentrations polymer 111-23, calculated from viscosity measurements, was of E. coli might be destabilized by polymer. The E. coli batch 1400. A 10% by weight solution of this polymer in water was which had been concentrated to 2725 mg. per liter by centrifucrosslinked by heating with epichlorohydrin a t 70 "C. for a time gation was treated with polyacrylamide AS at pH 4.7, but somewhat short of that required for gelation. The mole ratio of no flocculation was observed. In contrast. the cationic polyepichlorohydrin to ethyleneimine was 0.1 8. The viscosity relaethyleneimine caused good flocculation at 5 to 100 mg. per tive to water had increased to 2.310. Using the relation (Jones, liter on this concentrated E. coli. At 200 mg. per liter of E. coli 1-angsjoen er al., 1944), polymer AS showed no effect on light scattering at any pH up to a polymer concentration of 200 mg. per liter. Molecular weight = (nrel. - 1) X 1.54 x 104 Flocculation of E. coli with Anionic Polyacrylamide. The it is calculated that the molecular weight had increased from flocculating action on E. coli was studied for polyacrylamide 1400 for 111-23 to 20,100 for crosslinked polymer 111-42. It is which had been hydrolyzed to form 15 of anionic groups in believed that 3.4 M, 14 M, and 21 M are also crosslinked polythe chain, with molecular weights of 120,000 to 3,000,000. ethyleneimine polymers. At 40 to 2725 mg. per liter and pH 4.7 and 6.4, and with polySince 0.8 M , 1.0 M , 1.3 M, and 111-23 were free of inorganic mer at 0.1 to 100 mg. per liter, there was no observable effect material, it was possible to determine their (low) molecular on filtration rate. At 100 mg. per liter of E. coli and pH 9, weights by vapor pressure osmometry. These results were obthe very high polymer showed no effect on the light transtained at the Galbraith Laboratories in Knoxville, Tenn., and mission at 0.25 to 5 mg. per liter. Under the same wide range of are reported in Table I. conditions the electrophoretic mobility was not changed from The molecular weights found by viscosity and osmometry 0.5 to 50 mg. per liter. It is concluded, as in the case of the nonare not expected to agree, but indicate the general range for ionic polyacrylamides, that the anionic polymers were not able this number, differentiating between the low and high polyto flocculate the E. coli, probably because of their failure to be mers. Molecular weights cannot be determined for the high adsorbed, and thus they were unable to reduce or reverse the polymers by osmometry. The viscosity results shown in negative charge sufficiently to permit flocculation to occur. column 3 are used below for the sake of consistency. Flocculation with Cationic Polymers. As flocculation occurs POLYACRYLAMIDE NONIONICPOLYMERS. The American the transmission after settling should increase and the time Cyanamid Co. provided a series of well-characterized polyto filter 50 ml. should decrease. The results obtained by these mers of acrylamide with molecular weights of 7000 to 3,000,two methods are compared in Figure 1 to illustrate this point
z
Volume 3, Number 6 , June 1969 553
and to show the effect of the molecular weight of the polymer and the p H on flocculation. Both methods show that the flocculation was initiated at p H 4 by polymer concentrations of about 0.1 mg. per liter for the polymers with molecular weights of 1.9 M and 21M, but give no evidence of flocculation by polymer 0.6M at any concentration. Polymer 21M was the most efficient, increasing the transmission and decreasing the filtration time more than the lower polymer. Filtration measurements for polymer 21M at concentrations above 10 mg. per liter are not shown because the polymer blocks the filter above that value. Data are also plotted in Figure 1 for pH 8, this p H being chosen to avoid precipitation encountered under some conditions at pH 7 and 9. At p H 8 the E. coli surface should be less protonated and its charge more negative than at pH 4, thus requiring a higher concentration of polymer to reverse the charge and initiate flocculation. This was the case, since the concentration needed to initiate flocculation increased from about 0.1 mg. per liter at pH 4 to 1 mg. per liter at pH 9, and the optimum concentration, where transmission and filtration rate were greatest, from 1 to 10 mg. per liter. In the earlier work on flocculation of E. coli with synthetic polymers (Tenney and Stumm, 1965). flocculation occurred only with cationic polymers. One of the cationic polymers, presumably a polyethyleneimine of unspecified molecular weight, was studied in detail. The amount of polymer required for flocculation was somewhat higher (10 to 200 mg. per liter) than found in this work, but both studies are in general agreement. At high concentrations of polymer, adsorption may impart a high positive charge to the surface; consequently strong electrostatic repulsion may cause the bacteria in the flocs to separate-Le., redisperse. Or, high surface coverage by the polymer may prevent bridging by the polymer from one particle to the other, thus preventing flocculation, as suggested by Smellie and La Mer (1958). The high polymer (21M) caused redispersion of the bacteria at p H 4 and 8, as measured by both transmission and filtration. At p H 8 both methods showed that up to 200 mg. per liter the low polymer (1.9M) did not redisperse the flocs-that is, did not return the transmission or filtration time to the same values as for the untreated disper-
sion. At pH 4 the transmission results indicated that redispersion occurred but filtration results did not. The results with low molecular weight polymers are considered below in more detail. The transmission results in Figure 1 were obtained on dispersions which had been settled for 1 hour. Transmission measurements were made on similar dispersions immediately after adding polymer and stirring for 10 minutes. The results were similar, giving curves of transmission cs. milligrams per liter of polymer having broad maxima as in Figure 1, at the same concentrations of polymer. The polymers with molecular weights below 1400 did not change the transmission suficiently to be measurable, even with the sensitive Brice-Phoenix instrument. The procedure using settling was preferred because the changes in transmission were greater. There was no change in light scattering as polymer was added at p H 4, 6, or 9, even though one could see the flocs of E. coli after appropriate additions of polymer. Apparently the relationship between particle size and number of particles is so complicated that no change in scattering could be observed with the sensitive instrumental methods employed. Figure 2 shows the effect of the amount of polymer added on the filter time at concentrations of E. coli ranging from 70 to 2725 mg. per liter. The lower concentrations were prepared by diluting production batches with water and the 640 and 2725 mg. per liter batches as described in detail above. The results indicate that the concentration of polymer required to initiate flocculation increased as the concentration of E . coli increased, possibly according to a Langmuir type of adsorption isotherm mechanism found by Black, Birkner, er al. (1965) for clay. There are no data on equilibrium concentrations of polymer to support this speculation. Redispersion of the bacteria is evident in Figure 2. The concentration of polymer required to initiate and produce significant E. coli flocculation was estimated from a large number of experiments at pH 4 , 6 to 7?and 8 to 9 for polymers ranging from 800 to 21,000 over a change in E. coli concentration from 50 to 3000 mg. per liter (Figure 3). Points for all molecular weights are not differentiated, because there was no significant difference among them. Transmission and filtration
-
200
BA-
Am
-AI
\\ 0
w
v)
J
2 IOC Figure 1. Effect of polymer molecular weight and pH on floc- K w culation of E. coli PH 0 4.0
x 4.0 0 4 0 8.0 A 8.0
Polymer
‘.
5
LL
21 M 1.9 M 0.8 M 21 M 1 9 M
554 Environmental Science & Technology
2
0
.
A
1
0
0.I
1.0 MG./L. O F POLYMER
.. pH- 8
I
20.6M
IO. ADDED
I
100
1 1000
Figure 2. Effect of E. coli concentration on filter time of 50 ml. PH
Poljmcr
E. coli, Mg.'L.
A 4.0 x 4.0
21 M 21 M 21 M 21 M 21jM 21 M
70 117 155 170 640 2725
04.0
1.0
1
0.1 MG./L. OF POLYMER ADDED I
I
1.0
IO
04.0 rn 4 . 7
A 4.1 l
a
Figure 3. Effect of E. coli concentration on amount of polymer to initiate flocculation PH 0 4.0
10
100 CONCENTRATION OF
1000 E. coli, MG./L.
results were used, and, where both were available, the two methods gave concordant results. Some of the single points have been lumped together and an average value has been plotted. The molecular weight of the polymer had no significant effect on the ability to initiate flocculation. A lower concentration of polymer was required to start flocculation with a given concentration of E. coli at p H 4 compared with p H 6 to 9, but there was little change from p H 6 to 9. Figure 4 illustrates the effect of low polymers on filtration times for two polymers at two concentrations of E. coli. At p H 4 the addition of polymer affects the filtration time slightly, but, as soon as the p H is raised to 7 or 9, the reduction in filter time brought about by polymer addition is much more significant. As the E. coli are diluted to, say, 50 mg. per liter, the
6-1 A 9 X 8
Mol. Wt. of Polymers
1.3 1.3 1.9 1.9 -
21 21 21 21
M M M
M
effect of polymer treatment becomes smaller than shown in Figure 4 and at p H 4 changes in filter time are of doubtful significance. Polymers with molecular weights above about 2000 produce more striking changes at even p H 4 with only 50 mg. per liter of bacteria. Transmission measurements were made under a variety of conditions with and without settling, the results being in agreement with those just cited from filtration time studies. At p H 4 the concentrations of H+, Mg+2, and monomer units in the polymer (at 1 mg. per liter) were 1 X M , respectively. At this pH, lo+, 0.8 x and 2.3 X therefore, the polymer has to compete with a higher concentration of H+ ion for adsorption sites on the surface of the bacteria and apparently is not able to d o so effectively when the molecular weight is low, and there is little effective flocculaVolume 3, Number 6 , June 1969
555
\
170 MG./L.
\ \
\
I-
a
100
Figure 4. Effect of pH on flocculation with low molecular weight polymers PH
A 4.0 0 7.0 9.0
x 4.0
+ 7.0
Polymer (Table I) 1.9 M 1.9 M 1.9 M 1.3 M 1.3 M
E. coli Concn., Mg.iL. 117 117 117 170
0
tion. If the low polymer is not adsorbed sufficiently and since it has a small over-all length, there may be little bridging by the polymer from one particle to another and flocculation is not effective nor easily measured. The high polymers are probably more strongly adsorbed, possibly because of multipoint adsorption (Healy and La Mer, 1962), and are able to displace the hydrogen ions on the surface, bridging is greater, and flocculation is more readily observed. Figure 4 shows that the polymers with molecular weights of 1300 and 1900 at pH 4 produce a minimum filtration time at 0.1 to 1.0 mg. per liter of polymer, which is brought about by flocculation, followed by an increase in filtration time above 1.0 mg. per liter, which is evidence of redispersion of the Aocs. At this pH the hydrogen ion concentration is apparently high enough to contribute to the performance of the cationic polymer and thus aids in redispersing the flocs. At pH 7 to 9 this is no longer the case, Figure 4 showing that up to 50 mg. per liter of these low polymers do not increase the filtration to nearly the value for that with no polymer which is necessary for good redispersion. Transmission measurements also confirm this finding that redispersion fails to take place when the molecular weight of the polymer is below about 1900 and the pH is greater than 4 to 6. The higher polymers with molecular weights of 3400 or greater produced redispersion at all pH’s from 4 to 9. E. coli showed negative mobilities of from -3 to -6g/sec.i volt/cm. Addition of higher molecular weight cationic polymers decreased the negative “mobility” and finally reversed it, until it was as much as + 2 to f4. This is the general behavior in other systems-for example, crystalline silica (Kane, La Mer, et al., 1964a; Dixon, La Mer, et a/., 1967a; La Mer, 1966) and dispersions of clay, etc. (Crook and Pollio, 1965 ; Hannah, Cohen, et al., 1967; Pressman, 1967). With low molecular weight polymers the “mobility” reached only & O to 1. and then only at higher polymer concentrations. The concentration range of polymer required to change the chargz was about the same as that at which the filtration time was at or near its minimum. This behavior was exhibited by the E . coli as they were treated with the cationic polymers (Figure 5). The top of Figure 5 shows the change in filter time of 100 mg. per liter of E. coli when treated with polymer 21 M at pH 4,
+
556 Environmental Science & Technology
0.1 MG./L.
170
I.o
IO
IO
OF POLYMER ADDEO
6.2, and 9. The optimum concentrations for filtration were 2, 10, and 10 mg. per liter, respectively. E. coli at 100 mg. per liter which had been treated with the same polymer and at the same pH’s showed that the electrophoretic mobility changed from (-) to (f) at 1, 3, and 5 mg. per liter, in the same order as that for the filtration time, and the concentrations were about the same. Figure 5 also includes a filtration curve for polymer 1.4 M at pH 6. This shows a minimum in the filtration time at 10 to 20 mg. per liter, whereas the mobility reached a zero value at the same concentration. A polymer with a molecular weight of 800 contains 870/43 or 19 monomer units in its chain. The length of each monomer unit is 2.5 A., the over-all length then being 48 A. per polymer molecule, compared with 40,000 A. for the length of the E. coli, 100,000 A. for the length of the flagellae on the E. coli, and an average distance of separation of the bacteria of 300,000 A. The length of the polymer might be an important factor in controlling the amount of multipoint adsorption on the surface of the E. coli, a possibility which has been discussed (Healy and La Mer, 1962). Multipoint adsorption would be less for the low polymers, which would decrease the strength of bonding to the surface and the amount of adsorption. This would in turn account for the observed fact that the low polymers do not reverse the charge on the bacteria as well as the high ones. To obtain redispersion of the E. coli at higher polymer concentrations the adsorption of the polymer must be high, with surface coverages from 0.1 to 1.0 (Stumm and O’Melia, 1968), and apparently such coverage is not attained with the polymers of molecular weights below 1000 to 2000. The results with E. coli and polyethyleneimines are similar to those observed with crystalline silica (Dixon, La Mer, et a/., 1967a). Measurements of adsorption would undoubtedly help to clarify these matters. The high molecular weight polymers which bring about charge reversal are made by crosslinking the low polymers with a difunctional agent such as epichlorohydrin. Thus, polymer 111-23 (Table I) was converted to polymer 111-42 by heating with epichlorohydrin. This process might have altered the configuration at the nitrogen atoms in the polymer chain, perhaps quaternizing the nitrogen, so that the charge on the nitrogen was altered and adsorption markedly changed. The
Figure 5. Effect of polymer on electrophoretic mobility and filtration time of 50 ml. E. coli, 100 mg. per liter
PH
-
-
n 4 n
100 W -I W
A 6.2
.9 0 x 6 2
MG./L. OF POLYMER ADDED
net erect would have been the same as that ascribed to multipoint adsorption. Stumm and O’Melia recently (1968) summarized data from the literature on the flocculation of a number of different kinds of solid dispersions by various types of agents as part of an excellent discussion on the stoichiometry of coagulation. They showed that hydrolyzed ions of AI(III), Fe(III), and Th(IV) produced flocculation of negatively charged silver halide sols at 1 0 P to mole per liter and polydiallyldimethyl ammonium ions (PDADMA) were effective as low as l O P to 10-8 mole per liter, the amount depending on pH, amount of solid, type of solid, etc. Anionic and nonionic agents were usually less effective, requiring 10-6 to lop3 mole per liter for silver halide sols, bacteria, and silica. Additional information may be added to that of Stumm and O’Melia(Tenney and for example, E. coli with polethyleneimine, Stumm, 1965, and this paper); algae with polyethyleneimine, (Tenney, Echelberger, et al., 1968); clays with PDADMA, to 10-9 (Black, Birkner, et al., 1965; Hannah, Cohen, (Rubin and et al., 1967); E. coli with Al(III), 10-e to Hanna, 1968) ; river water solids with polyamine polymers, (Pressman, 1967); manganese dioxide with PDADMA, lop8 (Posselt, Reides, et ai., 1968); insoluble organic and inorganic materials with polyvinylimidazolines, 10-8 (Crook and Pollio, 1965); and other dispersions with commercial, high molecular weight cationic polymers. Although there is a range of concentration of as much as 10 to 100 in which flocculation occurs, depending on the polymer, the surface area of the solid, pH, etc., the general conclusion is that 10-9 to lo-’ M flocculant will usually be effective, or say roughly 1 mg. per liter, when the solids content of the dispersion is 10 to 1000 mg. per liter. Bearing in mind the above figures, about 1 mg. per liter of polymer flocculates many dispersed solids at concentrations up to 10 to 1000 mg. per liter. At a polymer cost of $1.00 per pound, the treatment of 1,000,000 gallons per day (8.35 M M pounds) would require 8.35 pounds of polymer per day, or $8.35 per day. The flocculation, settling, and filtration of the dispersed solid might then be greatly improved by the polymer and the cost justified.
1000
Polyethyleneimine Polymer 21 M i1 M 21 M 14M
Acknowledgment
The authors acknowledge with thanks the generous supply of polymers and help of the Dow Chemical Co. and the American Cyanamid C o ; and the excellent support of Phillip Oles, Susan Slate, and Robert Picard, who performed many of the experiments with care and skill. Helpful discussions with Werner Stumm are gratefully acknowledged. Literature Cited
American Public Health Association, New York, “Standard Methods for the Examination of Water and Wastewater,” 12th ed., pp. 280, 510, 1965. Black, A. P., Birkner, F. B., Morgan, J. J., J. Am. Water Works Assoc. 57, 1547 (1965). Black, A. P., Smith, A. L., J . Am. Water Works Assoc. 54,926 (1962). Black, A. P., Smith, A. L., J . Am. Water Works Assoc. 57,485 (1965). Black, A. P., Smith, A. L., J . Arn. Water Works Assoc. 58,445 (1966). Briggs, D. R., Anal. Chem. 12, 703 (1940). Busch, P. L., “Chemical Interactions in the Aggregation of Bacteria,” Ph.D. thesis, Harvard University, Cambridge, Mass., 1966. Crook, E. H., Pollio, F. X., “Removal of Soluble Organic and Insoluble Organic and Inorganic Materials by Flocculation,” 26th Annual Meeting, International Water Conference, Pittsburgh. Pa., Oct. 20-22, 1965. Dick. C. R.. Dow Chemical Co.. Midland. Mich.: Drivate communication, 1968. Dixon, J. K., La Mer, V. K., Li, C.. Messinger, S., Linford, H. B., J . Colloid Interfuce Sei. 23. 465 (1967a). Dixon, J. K., La Mer, V . K., Li, C., Messinger, S., Linford, H. B., J . Water Pollution Control Fed. 39,647 (1967b). Dow Chemical Co., Midland. Mich., Tech. Bull., “Montrek@ Polyethyleneimine Products,” 1966. Engelbrecht, R. S., McKinney, R. E., Sewage Ind. Wastes 28, 1321 (1956). Gunsalis, I. C., Stanier, R . Y . , “Bacteria-Treatise on Structure and Function.” Vol. I. Cham 1-4. Academic Press. New York, 1960. Hannah, S. A., Cohen, J. M., Robeck, G. G., J . Am. Water Works Assoc. 59. 1149 (1967). Healy, T. W., La Mer, V.’K., j , Phys. Chem. 66, 1835 (1962). Jones, G. D., Langsjoen, A,, Neumann, M., Zomlefer, J., J . Org. Chein. 9, 125 (1944). I
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Kane, J. C., La Mer, V. K., Linford, H . B., J . Ain. Cliein. Soc. 86, 3450 (1964a). Kane, J. C., La Mer, V. K., Linford, H. B.. J . Phys. Cliein. 68,2273 (1964b). La Mer, V. K., Discussions Furuduy Soc. 1966, No. 42, 248. Matijevik, E., Janauer, G. E., J. Colloid Interface Sci. 21, 197 (1966. Phoenix Instrument Co., Philadelphia, Pa., technical bulletin, 1968. Posselt, H. S., Reidies, A. H.. Weber, W. J., J . Ain. Wuter Works Assoc. 60, 48 (1968). Pressman, M., J . Am. Water Worlcs Assoc. 59, 169 (1 967). Roberts, R . B., Abelson, P. H., Cowie, D. B., Bolton, E. T., Britten, R. J., “Studies of the Biosynthesis of E . coli,” Carnegie Institution of Washington, Pub. 607 (1963). Rock, R . M.. Burbank. N. C.. J . Am. Wuter Works Assoc. 58. 676 (1966): Rubin, A. J., Hanna, G. P., J. ENVIRON. Scr. TECHNOL. 2, 358 (1968). Scott, T. A,, Melvin, E. H., A n d . Clieni. 25, 1656 (1953)
Smellie, R. H., La Mer. V. K., J . ColloidSci. 13, 589 (1958). Stumm, W., O’Melia, C . R., J . Ani. Water Works Assoc. 60, 514 (1968). Tenney, M . W., Echelberger, W. F., Schuessler, R . G . , Bretthauer, R. K., Division of Water, Air and Waste Chemistry, 155th Meeting, ACS, San Francisco, Calif., April 5 , 1968. Tennev. M. M., Stumm, W., J . Water Pollution Control Fed: 37, 1370 (1 965). Thimann, K. V . , “The Life of Bacteria,” 2nd ed., Macmillan, New York, 1963. Receiced,fi)r reciew FebrLiurjs 5 , 1968. Accepted Noceinber 29, 1968. Dicision of‘ Colloid und Surfiice Clzeinistry, 156th Meeting, ACS, Atlantic City, N . J . , September 1968. Work S L I P ported in part 1 3 ~fiinds ~ procided b~ the United States Depurtment of‘the Interior us uuthorized under the Water Resources Reseurcli Act of‘ 1964, Public Law 88-379, and administered by the Institute of’ Wuter Resources ut the Unicersity qf ‘Connecticut.
Interaction of Airborne Particles with Gases Benjamin M. Smith’ and Jack Wagman National Air Pollution Control Administration, Cincinnati, Ohio 45227
Birney R. Fish Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
A promising method for making sorption measurements in aerocolloidal systems is described. Initial studies are being conducted on the interaction of sulfur dioxide with metal oxide aerosols. The latter are generated by exploding metal wires in air, resulting in particles with diameters from about 0.01 to 0.1 micron. These are mixed with S3j-labeled SO2 at controlled conditions in an 820-liter cylindrical chamber Samples for analysis are withdrawn through diffusion tubes lined with leadperoxide-coated lead foil and followed by membrane filters. Diffusion tube analysis of several S02-aerosol mixtures indicated that this is a possible method of distinguishing between chemisorption and physical adsorption. In measurements that included an adsorption isotherm for SO2 on iron oxide aerosol, preferential chemisorption on iron oxide and aluminum oxide was observed at low SO2concentrations and was followed by multilayered physical adsorption at higher concentrations.
E
xperimental evidence suggests the possible influence of particulate matter upon the toxicity of irritant gases in respiratory systems. The first such evidence was presented by Dautrebande (1939), who suggested that adsorption of a gas on inert aerosol particles increased the amount of gas reaching the lungs. Dautrebande, Shaver, et a/. (1951) observed a marked increase in the irritation produced in human subjects exposed to sulfur dioxide (SO?), formaldehyde, and other gaseous pol-
‘Present address, Department of Physics, Gainesville College, Oakwood, Ga. 30566 558 Environmental Science & Technolog?
lutants when exposures occurred in the presence of various aerosols. Goetz (1958) has proposed a physicochemical explanation of the physiologic effects of combinations of gases and aerosols. Very briefly, the theory recognizes the degree of sorption of the gas by the aerosol particle, the rate of desorption of the sorbed gas from the particle, the degree of chemical reaction which may occur, and the toxicity of the new chemical compound if such reaction occurs. This theory is sufficiently general to be consistent with the findings of Dautrebande (1939), Amdur (1961), LaBelle, Long, et al. (1955), and others, and can explain both synergism and antagonism between gas and particle. A special type of synergism may occur as a result of differences in the aerodynamic behavior of gases and particles. For example, it can be demonstrated in principle that a gas such as SO, can penetrate more deeply into the lungs when it is adsorbed on particles of respirable size. This report describes a study of gas-particle interactions that is being carried out as a joint effort of the National Air Pollution Control Administration (NAPCA) and the Oak Ridge National Laboratory (ORNL). The importance of this study stems not only from its bearing on the question of synergism in the physiological effects of pollutants, but also from the significance of such interactions in photochemical and other chemical transformations in the atmosphere. In the type of experiments usually carried out to study adsorption at solid-gas interfaces, the solid phase is in the form of bulk powder, film, o r filament, and great care is taken to clean the surfaces by outgassing or heating before they are exposed to the gas phase. Such experiments bear little relation to real conditions in the atmosphere, where the solid phase is in aerosol form and in contact with a complex mixture of gases. Consequently, the emphasis in this work is on reactions of dilute gases with particles in the dispersed or aerosol state. The experimental procedures include generation of submicron-
