Environ. Sci. Technol. 1992, 26, 1036-1040
Surface Tension of Wastewater Samples Measured by the Drop Volume Method Rok Gunde,' Myrtle Dawes,+and Stanley Hartland Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland
Markus Koch Amt fur Gewasserschutz und Wasserbau des Kantons Zurich, 8090 Zurich, Switzerland
rn A systematic study of the application of the drop volume technique to the measurement of the surface tension of wastewater samples taken at the second clarifier outlet of sewage treatment plants in the Zurich area is reported. Use of a computer-controlled drop volume instrument allows us to obtain reproducible surface tension data for samples containing surface-active solutes in widely varying concentrations. It is shown that biodegradation must be inhibited (by sodium azide) between sampling and measurement. Two measuring procedures (standard and equilibrating) capable of yielding reproducible surface tension data are suggested.
presented by Adamson (4). In this work the drop volume method is used and, therefore, will be described to the extent required for the purpose of this paper. This method previously has not been used for surface tension measurement of wastewater. The method consists of determining the volume, V, of the largest drop hanging at the end of a vertical capillary of radius r which can be held by the surface tension 6. In the static case (i.e., absence of kinetic energy and diffusion processes affecting the surface tension), mechanical equilibrium is expressed as ApgV,, = 27rn sin CP + .rrr2Ap (1)
Introduction Surface tension measurements of treated wastewaters by the Du Nouy ring method (1)and by a drop spreading technique (2) have been reported. Often severe problems affecting the reproducibility of surface tension measurementa of wastewater by means of (commercially available) ring tensiometers render such data practically meaningless. The Department of Water Pollution Control of the Kanton Zurich (Switzerland) routinely measures the surface tension of wastewater in urban areas for control of contamination by surface-active agents (e.g., detergents). The surface tension of water is sensitively lowered by the presence of surface-active agents. The Swiss Government prescribes that surface tension of the second clarifier outlet of wastewater treatment plants must not fall below 60 mN/m (compared to 72.7 mN/m for pure water) (3). In this paper we will report on surface tension measurements of wastewater from a variety of wastewater plants in the area of Ziirich, carried out by the drop volume method (4). This method has been used frequently for the measurement of surface tension of solutions of pure surfactants (5, 6). In the present paper the method will be shown to yield reproducible and accurate values for surface tension of wastewaters, provided certain rules for measurement procedure are respected. It had earlier proved useful for the surface tension measurement of fog samples collected in the area of Ziirich (7). The procedure is simple and allows reproducible measurements even if the surface tension is dependent on the kinetics of diffusion of surface-active agents between the surface and the bulk of the aqueous phase. However, in order to achieve acceptable reproducibility, stabilization of wastewater samples between sampling and measurement is found necessary. In the form suggested in this paper, the drop volume technique is shown to meet the requirements of simplicity and reliability needed for routine determination of surface tension of wastewaters.
where Ap is the pressure drop between the liquid and gas phases and 9 is the inclination of the liquid-gas interface to the horizontal. Both of these are measured at the nozzle tip. Ap denotes the density difference of the liquid and vapor phases, and g stands for the acceleration due to gravity (g = 981.652 cm/sz for Zurich). Though V,, can be measured directly, both 9 and Ap are difficult to determine. By increasing the drop volume (slow addition of liquid through the capillary), the maximum volume of the drop at which a drop of volume Vf falls off is reached, leaving behind a portion (V- - Vf) on the tip. The force balance may then be empirically expressed as VgAp = 21rruf (1') with f denoting a correction factor introduced by Harkins and Brown (8) which corrects the directly measured volume Vf to V., Some unportant advantages of this technique are surface tension measurements in presence of a vapor phase in equilibrium with the liquid phase, formation of fresh surface during the growth of each drop, and simple repetition of the experiment, The technique also allows the possibility of the direct observation of diffusion effects between the bulk and the surface during growth of drops. This is expressed by the systematic dependence of the apparent surface tension on drop growth speed and drop formation time. The experimental apparatus is schematized in Figure 1, including the following essential points: (i) Drops are generated at the lower tip of a vertical capillary ( r 1.7 mm) inserted in the outlet of a gas-tight precision syringe (Hamilton Series 1000). The smallest measurable displacement of the piston, which is driven by a dc motor, is 0.25 pm. Depending on the inner diameter of the syringe, liquid in the syringe may be transferred to the tip of the capillary in small increments (typically pL). A wide range of speeds for piston displacements (typical range 0.08-0.8 pm/s) is possible. This is necessary if the kinetic energy effects of the liquid flow in the capillary are to be kept negligibly small and/or if the surface tension measurements in the presence of kinetic effects of diffusion between the bulk and the surface are needed. Determination of drop volume Vf (typical volume size 70 pL) is simply achieved by counting (by the encoder) the number of increments required for formation of a maxi-
Experimental Procedures Surface Tension Measurements. A review of methods for surface tension measurement of liquid phases is Present address: Department of Engineering and Chemical Technology, Imperial College London, South Kensington, London SW7, U.K. 1036
Environ. Sci. Technol., Voi. 26, No. 5, 1992
0013-936X/92/0926-1036$03.00/0
@ 1992 American Chemical Society
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Flgure 3. Effect of drop formation time on steady-state surface tension of 4 % sodium azide solutlons. (a) Sewage piant in Kusnacht. (b) Sewage plant in Adliswil. Sample collected 3 days after the lncklent. The clusters of measurement points at drop formation times (DFT) of -410, -870, and -1300 s (a) and -500, -990, and -1440 s (b) represent data obtained by the standard measuring technique for moderately (a) and strongly (b) contamlnated samples. These flgures may m e as an illustration for the qualitative relatkm between standard and equilibrating measuring technlques.
Figure 3, panels a and b, which relate surface tension and drop formation time (typical drop formation times for Environ. Scl. Technol., Vol. 26, No. 5, 1992
1037
Table I. Sewage Plant i n Kusnacht, Switzerland; 24-h Composite Sample from J a n 23, 1990 NaN, concn, wt %'
densitv. g/cm5'
surface tension,"Zb mN/m
drop formn time,".b s
no* Of drops steady total
Steady-State Surface Tension (20 "C) of Fresh NaN3 Solutions 4
1.023
3
1.017
2 1 0
1.010 1.004 0.998
67.9 63.5 61.6 66.9 63.6 61.4 63.0 63.9 68.4
(0.2) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.2)
414 (28) 869 (16) 1293 (19) 384 (12) 778 (7) 1240 (12) 1313 (18) 1289 (23) 1258 (73)
6 7 8 8 6 10 10 12 13
16 18 20 18 15 20 20 20 20
Steady-State Surface Tension (20 "C) of Old NaN3 Solutionsc 4 3 2
1.023 1.017 1.010 1.004 0.998
1 0
60.4 61.5 64.9 64.4 70.6
(0.2) (0.0) (0.2) (0.2) (0.2)
1252 (31) 1214 (18) 1288 (17) 1237 (21) 1267 (43)
10 9 9 14 25
25 25 25 25 25
Mean value for drops taken with steady-state experiments. *Values in parentheses are standard deviations over steady-state experiments. Sample age was between 2 and 3 weeks. Table 11. Sewage Plant in Adliswil, Switzerland, Steady-State Surface Tension (20 "C) of 4% NaN, Solutions of 24-h Composite Samples from March 1990 sample (days after incident)
density, g/cm3
3
1.022
10
1.022
24
1.022
surface tension," mN/m 49.8 47.6 47.0 64.9 62.6 61.9 72.0 72.1
(0.1) (0.1) (0.1) (0.1) (0.0) (0.1) (0.1) (0.1)
drop formn time," s
no. of drops steady total
505 (10) 986 (30) 1441 (40) 680 (12) 1248 (20) 1815 (14) 750 (41) 1411 (48)
23 21 25 17 29 15 20 20
40 31 40 29 40 31 30 35
Compare footnotes in Table I.
these samples are shown in Tables I and 11). The figures show that the surface tension values in the steady state depend on the measurement scheme and generally differ from the equilibrium-state value. This observation is the reason for the use of a second measuring technique (equilibrating technique) described below. (2) Drop formation with a self-adapting waiting time schedule. This technique allows the instantaneous surface tension to closely approximate the equilibrium-state value with respect to adsorption of surface-active solutes in the liquid under investigation. As experience has shown, attainment of adsorption equilibrium (aging of drop surface) requires widely varying times. In the equilibrating technique the drop volume V ,is determined from the number of increments required to produce the maximum drop volume, but the time sequence is chosen as follows. In step 1, a drop of preselected volume V1 is ejected and allow to stand for the time interval tl (typically 1h). If within this time the drop detaches due to aging of the surface (decrease of surface tension), the experiment is repeated with an equal volume V , dispensed until the drop formation time exceeds the preselected t , value. After that, both V I and t , are incremented by AV and At, with AV preselected and At estimated from AVlAt of the preceding experiment. In this way a sequence of experiments is created for which AVlAt converges toward negligibly small values and V converges to V,. The equilibrating technique is expected to yield surface tension values characteristic of 1038
Environ. Sci. Technol., Vol. 26, No. 5, 1992
an equilibrium state and, therefore, should be considered as the most meaningful estimate. In practice, conditions often occur where the standard technique produces surface tension data approaching equilibrium data in much shorter time and may be used satisfactorily. Auxilliary Procedures and Calibration. The sample densities were measured by a Paar DMA-35 density meter calibrated with high-purity water at the measurement temperature. Wastewater samples investigated in this study typically have densities of 0.998-0.999 g/cm3 at 20 "C. The temperature of the gas phase in the measurement cell and the liquid was measured by means of resistance thermometers and found to remain constant within 0.2 " C near its preselected value. The speed of expulsion of liquid from the syringe (speed of piston) was chosen such that the Reynolds number of the flow in the capillary remains far below 0.01. This should warrant negligible fluid dynamic effects on drop volume V,. The radii of capillaries were measured with a digital micrometer. Since capillary radius is a critical quantity for the drop volume technique, the radii measurements were tested by determination of the surface tension of high-purity water and ethanol (Fluka 02860). Sampling and Sample Treatment. The surface tension of pure samples investigated in this work was observed to increase rapidly between sampling and measurement toward the value of pure water. This is probably due to the activity of microorganisms continuing to degrade the surface-activeagents present in the samples. In this work, the following procedure minimizes biological degradation during the time between sampling and measurement: (i) After the composite sampling (9),the sample bottles ( 1L) were tightly capped, chilled to near 0-0.2 "C in an ice bath, and stored in a refrigerated room (at 4 "C). (ii) Some time before measurement, sodium azide (Fluka 71289) was added at 0 "C to form final solutions of 4% (w/w). Sodium azide in this concentration was found to inhibit efficiently biodegradation at temperatures 520 "C. The effect of sodium azide on the surface tension of wastewater samples has been studied using aqueous solutions containing 0, 1,2,3, and 4% (w/w) sodium azide. At 4% NaN, the surface tension is -0.8 mN/m above that of pure water. (iii) Immediately before measurement, inhibited samples were brought to measurement temperature in the thermostat of the drop volume tensiometer. All measurements reported in this paper were made at 20.0 f 0.2 "C. Software for the Control of Measurements. Both the standard and the equilibrating techniques require extended software packages which were developed for MS-DOS and Hewlett-Packard BASIC environments. The software allows a choice of widely varying measurement schemes on either an interactive or a self-adapting basis and nearly automatic measurements. N
Results A set of surface tension measurements were made on 24-h composite samples taken at the outlet of the second clarifier of nine sewage treatment plants in the Zurich metropolitan area during 1990. The influence of sample stabilization with sodium azide and of measuring technique on surface tension are presented for the plant in Kusnacht for normal operating conditions (Table I). Results from stabilized 24-h composite samples from the plant in Adliswil collected 3,10, and 24 days after a serious breakdown of the normal function (by unknown cause) are given in
Table 111. Surface Tensions (20 "C) of 4% NaN, Solutions of 24-h Composite Samples for Six Different Sewage Plants in the Area of Zurich, Switzerland
sewage
density. g/cms'
Bulach'
1.023
WeiachC
1.023
Fallandend
1.023
Zurich-Glatte
1.023
Dubendorf"
1.022
OpfikonKlotenc
1.022
surface tension," dyn/cm
drop formn time," s
71.4 (0.2) 68.5 (0.3) 66.8 (0.3) 71.9 (0.0) 68.6 (0.1) 66.6 (0.4) 72.1 (0.2) 70.9 (0.2) 70.4 (0.6) 72.3 (0.0) 70.8 (0.2) 68.9 (0.1) 57.8 (0.0) 53.1 (0.1) 53.2 (0.3) 65.0 (0.1) 60.0 (0.1) 59.0 (0.8)
698 (58) 2758 (62) b 830 (18) 3124 (38) b 697 (53) 2774 (71) b 807 (41) 3104 (46) b 653 (56) 2351 (21) b 639 (12) 2360 (11) b
no' Of drops steady total 23 16 14 12
15 11 14 14 20 13 10 10 13 13 15 13 15 16
30 30 37 30 30 33 21 21 30 20 20 28 30 35 32 30 35 35
" Compare footnotes in Table I. Equilibrating measurement, drop formation times in general >4000 s. July 19, 1990. July 12, 1990. e July 9, 1990. Table 11. All data in Tables I and I1 were obtained by the standard measuring technique. Information on the drop formation times obtained by this procedure and on the root-mean-square (rms) deviation for surface tensions and drop formation times is also presented. The data in Table I document the influence of the concentration of sodium azide. Surface tension measurements made on 24-h composite stabilized samples from six sewage treatment plants in widely different industrial and urban environments are presented in Table 111. Surface tension values were obtained by both the standard and the equilibrating techniques. For the data listed in Tables 1-111 it should be noted that (i) standard technique measurement schemes, with time interval t 2 varying by a factor 2-4, were used in order to arrive at estimates of the equilibrating time and to compare the surface tension values associated with significantly different states with respect to the drop formation times, and (ii) for each time scheme surface tension is reproducible within the rms value of a single measurement.
Discussion The measurements on samples from the plant in Kusnacht (Table I and Figure 2) illustrate that reliable inhibition (at 20 "C)of the biological degradation of surfactants requires a 3-4% (w/w) concentration of sodium azide. At lower concentrations the degradation occurs at a rate such that the increase of the surface tension is clearly recognizable. A t 3-4% sodium azide concentration, degradation remains blocked over 2-3 weeks, as far as can be detected by surface tension measurement. The efficiency of sodium azide as an inhibitor has also been proved in bacterial growth experiments with the same sample from the plant in Kiisnacht on violet-red bile agar. At sodium azide concentrations of 13% (w/w) bacterial growth stops completely. (Analogous observations may be made for samples collected from all other plants investigated in this work.) For the samples stabilized with sodium azide, the dependence of surface tension on drop formation time (time schedule) is clearly recognizable. Though measurements with each time schedule yield surface tension values with
a rms of 0.1-0.3 mN/m reproducibly, these stationary-state values differ significantly for different drop formation times. Generally only values with equal (or similar) drop formation time should be compared. The breakdown of the sewage plant in Adliswil and its recovery to normal operation is clearly expressed by the surface tension of the stabilized samples taken 3, 10, and 24 days after the breakdown (Table 11). Surface tension (standard measuring technique) of the 3-day sample was rather low (45 mN/m), indicating strong contamination by surface-active agents. It increased to -62 mN/m within 1week and to 72 mN/m within 24 days after the incident. The surface tension values of the 3-day sample are only slightly dependent on the drop formation time in the interval of 900-1400 s (Figure 3b), suggesting that the corresponding stationary-state values approximate the equilibrium-state value very closely. Hence, for the case of strongly contaminated, by surfactants, samples, the choice of drop formation times around lo00 s yields nearly equilibrated surface tension data. The 10-day sample still indicates considerable contamination by surface-active solutes (62 mN/m). Near-equilibrium surface tension of these samples is reproduced by the standard technique only after drop formation times reach > M O O s. At short drop formation times (typically 600 s) the data differ considerably (-3 mN/m). The data observed 24 days after incident (72 mN/m) indicate low surfactant contamination. In this case the doubling of drop formation time (750 to 1500 s) yields practically equal surface tension values, which are equal also to the equilibrium value. Two standard technique measurements on each sample from the six randomly selected plants operating on widely different wastewater inputs with drop formation times of -800 and -2500 s were made (Table 111). Subsequently, the equilibrating technique measurements were carried out for each of the samples. For the strongly contaminated samples (plants in Dubendorf and Opfikon-Kloten; both failed to satisfy the legal lower limit) the same behavior as for the plant in Adliswil is found, that is, surface tension values measured by the standard technique experiment with long drop formation times approximate closely the equilibrium value. For the remaining four plants, the data indicate medium to modest contamination by surfactants (surface tension in the range 67-72 mN/m). The standard technique yields a systematic decrease of surface tension (2-3 mN/m) with increasing drop formation time. Apparently the discrepancy is largest if equilibrium values lie slightly below 70 mN/m. For highly contaminated samples, the surface tension values, determined by the standard technique using drop formation times of 2000-3000 s, nearly approximate the equilibration technique values, whereas for modestly contaminated samples (surface tension 66-71 mN/m), the two techniques lead to larger discrepancies. This is an observation of practical importance. At high-contamination levels, the surface of the water containing surfaceactive molecules reaches a state near equilibrium within the time interval of drop formation times usually applied with the standard technique. At low-contaminant concentrations the creation of an equilibrium surface structure is slower, implying that measurements with standardized time schedules yield values that differ more widely from equilibrium values.
Conclusions On the basis of the present investigation, the following rules for surface tension measurements by the drop volume Environ. Scl. Technol., Vol. 26, No. 5, 1992
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Environ. Sci. Technol. 1992, 26, 1040-1047
method of samples from the second clarifier outlet may be proposed: (1)Samples should be chilled to near 0 "C and stabilized with 4 % (w/w) sodium azide immediately after sampling. This minimizes the biodegradation of surface-active pollutants over 2-3 days. (2) Either the standard measuring technique (defined time schedule for drop formation) or the equilibrating measuring technique (self-adapting time schedule) can be used to obtain reproducible surface tension values. (3) Surface tension data reported for experiments carried out either by the standard or by the equilibrating measuring technique should include the information on the drop formation times. (4) In practice, a preliminary standard technique experiment with 15-20-min drop formation time should be made (step A). If a surface tension value of 565 mN/m is obtained in step A, a second standard experiment with tripled drop formation time should be carried out, which then will produce a near-equilibrium value. If a value above 71 mN/m is obtained in step A, no further action is required for pollution control purposes. If a value betwen 65 and 71 mN/m is found in step A, then a further experiment with tripled drop formation time should be made in order to decide whether an equilibrating experiment is required. The latter is required if the surface tension of the former experiment is within 2 mN/m of the legal limit (60 mN/m for urban wastewater treatment in Switzerland). (5) Automatization of both measuring techniques is essential in applications to polluted wastewaters. The procedures might also be used for the determination of biodegradation kinetics of surface-active solutes in aqueous media.
Acknowledgments We thank the members of the mechanical workshop (J. Hostettler, B. Jorg, R. Mader) and the electronic workshop (M. Wohlwend) in our laboratory for valuable assistance and the co-workers of the Department of Water Pollution Control of Kanton Zurich for delivery of samples. Registry No. Water, 7732-18-5; sodium azide, 26628-22-8.
Literature Cited (1) Sridhar, M. K. C.; Reddy, C. R. Environ. Pollut., Ser. B 1984,7,49-69. (2) Hardy, C. D.; Baylor, E. P. J . Geophys. Res. 1975,80, 2696-2699. (3) Amendment on wastewater disposal. Bundesamt fiir Umwelt, Wald und Landschaft (BUWAL): Bern, Revision 1990. (4) Adamson, A. W. Physical Chemistry of Surfaces, 3rd ed.; Wiley & Sons: New York, 1976; Chapter 1. ( 5 ) Mukerjee, P.; Handa, T. J . Phys. Chem. 1981, 85, 2298-2303. ( 6 ) Van den Bogaert, R.; Joos, P. J . Phys. Chem. 1979,83, 2244-2248. (7) Capel, P. D.; Gunde, R.; Zurcher, R.; Giger, W. Environ. Sci. Technol. 1990,24, 722-727. (8) Harkins, W. D.; Brown, F. E. J. Am. Chem. SOC.1914,41, 499-524. (9) Norris, J. E. In Principles of Environmental Sampling; Keith, L. H., Ed.; American Chemical Society: Washington, DC, 1988; Chapter 16.
Received for review June 20,1991.Revised manuscript received October 30, 1991. Accepted January 22,1992. W e thank the student-exchange administration of the Swiss Federal Institute of Technology (ETH, Zurich) for financial support (M.D.).
Behavior of Heavy Metals in the Combustion Gases of Urban Waste Incinerators Miguei A. Fernhdez, Lluis Marfiner, MercP Segarra, Jos6 C. Garcia, and Ferran Espieil" Departamento de Ingenieia Qdmica y Metalurgia, Universldad de Barcelona, Avenida Marti i Franquds 1, 08028 Barcelona, Spain
rn The behavior of each of the principal heavy metals present in fly ash from urban waste incinerators is described on the basis of the standard free energies of formation of the oxide and the chloride from the metal, oxygen, and hydrochloric acid, as well as the vapor pressure of the chlorides at working temperatures. The thermodynamic parameters are correlated with the three types of behavior in the volatilization process. The solubility in acid aqueous medium is also related to the oxide and chloride thermodynamic parameters. A correlation has been established between the amount of each heavy metal in the fly ash and the standard free energy of vaporization of its chloride, both in the samples used and in the data on content found in the literature. Finally, kinetic modeling studies have been used to show that the neutralization of the fly ash alkalinity takes place by a diffusion mechanism through a solid matrix. Introduction The treatment of urban waste by incineration makes it possible to obtain energy from this waste while reducing 1040
Environ. Sci. Technol., Vol. 26, No. 5, 1992
its volume by a large amount. In addition, the dumping of combustion ash is much less conflictive than the dumping of nonincinerated waste. These attractive aspects of the process have extended its application and, consequently, augmented the interest in studies on its environmental implications. The presence of heavy metals in urban waste combustion gases has been a subject of particular interest. The incineration of solid waste contributes significantly to the presence of Cd, Zn, and Sb and possibly Ag, I n , and Sn in urban area aerosols (1,2). Mercury is capable of passing through the gas treatment system, generating emission levels of the order of 0.05-1 mg/Nm3 (3). A review of the available literature leads to the conclusion that the release of heavy metals has been clearly established, both in the combustion of fossil fuels in general and in the burning of urban waste (4-7). Another basic aspect of the burning of organic matter of any origin is the presence of SOz, HC1, and oxides of nitrogen in the combustion gases. The hydrochloric acid present in urban waste combustion gases comes from the burning of plastics-especially polyvinyl chloride-and its concentration in the gases increases along
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