A Drop Impact Sampler Lung Cheng" and Warren G. Cross Pittsburgh Mining and Safety Research Center, Bureau of Mines, U S . Department of the Interior, Pittsburgh, Pennsylvania 15213
The drop impact sampler developed by the Bureau of Mines is based on the stain technique for measuring airborne drops. The stain technique requires a calibration curve to relate stain and drop diameters at a known impact velocity, usually the terminal value. However, physical constraints limit sampler location, making it difficult to attain the terminal value and thereby introducing complications; extrapolation of a calibration curve based on the spherical quiescent model to large drops results in an erroneously large diameter. The new sampler eliminates these difficulties and in addition makes it possible to determine rates of depositions and spatial concentrations. The sampler can measure drop diameters ranging from 0.005 to 2.5 mm and is suitable for water sprays, raindrops, and carry-over drops such as from cooling towers. Laboratory tests have shown that the sampler is reliable, rugged, lightweight, and easy to use. Introduction Knowledge of drop velocity, drop size, and concentration distribution is important in addressing such problems as the scavenging of airborne particles by raindrops (I), elimination of drop carry-over from cooling towers ( 2 ) , and dust suppression by water sprays ( 3 , 4 ) .For a specific mining operation such as a belt transfer station, the spray nozzle for optimum dust suppression due to impact coverage can be selected ( 4 ) on the basis of geometry, desired degree of coverage, and local line pressure, if the distributions of drop size, concentration, and drop velocity are known. While drop velocity for drops several hundred micrometers in diameter can be estimated for most situations, most nozzle manufacturers do not provide the necessary information on drop size and concentration. Laboratory methods for measuring drop size and drop concentration and velocity distributions are available but are complex and expensive. Measurements of drop size and concentration distributions are especially needed to improve performance and technology through a better understanding of the processes involved. Portable drop samplers based upon hot-wire and stain techniques have been developed but are somewhat fragile. This paper describes a rugged, lightweight, simple mechanical drop sampler suitable for use in an underground mine. This sampler is based on the stain method of measuring drop size and drop velocity carresponding to gravitational sedimentation. The sampler will be useful for drops ranging from 0.005 to 2.5 mm in diameter. Theory When a drop impacts a target surface, it flattens and leaves an imprint or stain on the target. Microscopic examination of the target gives the size and concentration distributions. For water drops impacting an inert solid target surface, the drop and stain diameters, D and S, are related by ( 4 , 5 ) = ((1 - cos 8)2(2
+ cos e)
4 sin3 0
where 0 is the mean contact angle of the advancing and receding value, u is the surface tension of water against air, pw is the density of water, V is the impact velocity, and C is the ratio of the experimental to theoretical stain diameter. Equation 1 is used for drops whose diameters are less than 0.12 mm with negligible impact velocity, and it is based on a spherical quiescent model. Equation 2 is used for drop sizes larger than 0.3 mm and impact velocities greater than 150 cm/s, and it is based on a dynamic spreading model wherein the incident drop spreads into a flat disk ( 4 ) .The value of C in eq 2 depends on the nature of the liquid, the nature of the target, and the impact velocity; it must be determined experimentally. However, even if one assumes that C is unity, eq 2 predicts drop size to within 16%for water drops in most practical situations (0.2-1.4 mm diameter, 100-2500 cm/s velocity). For drop diameters between 0.12 and 0.3 mm and velocities equal to or less than the terminal values, eq 1 and 2 must be interpolated. The value of the drop velocity V can be estimated for most practical situations but can be conveniently related to the fraction x of the terminal drop velocity Vt, where x = V/V,. The equation of motion of a drop falling under the action of gravity is dV/dt
+ x2g-g
=0
(3)
where t is the time and g is the gravitational acceleration. Since V = dh/dt, where h is the vertical distance, eq 3 becomes dh = Vt2x dx/[(1 - x2)g]
(4)
Integration of eq 4 for zero initial height and initial vertical velocity gives
vt2 (1- x 2 ) h = --ln (5) 2g Figure 1 gives the terminal velocities for drops of various diameters (6). Figure 2 gives the height of fall required for different values of x , and Figure 3 relates the drop and stain diameters for different values of x ; thus, rendering the x value to be identical in these figures determines the drop diameter. For clarification of the determination of drop size, the curves in Figure 2 are equivalent to a functional equation of Fl(D,x) = h , and those in Figure 3, Fa(D,x) = S. The F1 and F2 are independent. By solving the F1 and F2 simultaneously, one can obtain each value of D and x with each measured value of h and S , whereas solving them simultaneously is equivalent to applying cut-and-try on the curves of Figures 2 and 3. Drop Impact Sampler The drop impact sampler (Figures 4 and 5) is a mechanical device used to produce drop stains from which to measure drop size and concentration. The impact head (a) consists of a shutter made of a movable orifice plate (c) and a stationary orifice plate (d) with orifice diameters of 0.952 and 1.27 cm, respectively, and an impact plate (e) that holds a pyrolytic graphite planchet (f) purchased from Ernest F. Fullam, Inc., Latham, NY 12110, by means of a restraining plate (g). The planchet, 2.54 cm in diameter and coated with a thin layer of magnesium oxide, provides a target
This article not subject to U.S. Copyright. Published 1981 American Chemical Society
Volume 15, Number 4, April 1981
459
1000
so0 600 500 400
2 Y
100
>
eo
Z
50 40
Y
30
20kL I O06 08 I
2
3 4 5 6 8 1 OROP DIAMETER , mm
2
3
4 5 6
surface for incident drops. All of these parts are enclosed in a 20-cm-diameter cylindrical container with a top cover (h). The handle (here, 86 cm long) contains a rechargeable 200-cm3air storage tank (9,a needle valve 6). a recharge port (k), a pressure gage (I), a pressure regulator (m). a three-way valve (n), a flow-control valve (4,and a spring-loaded single-acting air cylinder (p) that connects the moving plate through a plunger. The shutter, normally in a closed position, opens and then closes as the sliding plate (c) moves across the stationary plate (d). The opening area is 0.712 cm'; the sliding speeds determine the shutter openings (cm' s) which are calibrated against various dial settings on the turning knob of the flow-control valve (0) a t a fixed air pressure of 2.81 kglcm' (Figure 6). The user selects the location of the sampler head relative to the drop source such as to obtain a value of x from a known height, h , and selects a dial setting on the flow-control valve such as to obtain a convenient stain concentration on the planchet. The three-way valve is turned to operate the shutter and then to exhaust the air from the air cylinder. The stained planchet is removed for microscopic observation. Counting the drop stains on the planchet and knowing the shutter opening, one can calculate the rate of deposition. Knowing the rate of deposition and the impact velocity from Figures 1 and 2, one can calculate the spatial concentration. Measuring individual stain diameters and using the given relation between the stain diameter and drop diameter (Firmre 3), one can calculate the size distrihution. Results
DROP D I A M E T E R , m m
F ~ ~ U2. I OFalling height fw various drop diameters: x = percentage of
terminal velocity.
A spray nozzle with a narrow (30°)spray angle was tested in the laboratory a t a water pressure of 5.6 kgIcm2 (80 IbIin.2) and a flow rate of 1.32 Llmin (0.35 gallmin). The nozzle was positioned as shown in Figure 7. Because of the narrow spray angle, a single fall height ( h )of 132.1 cm was selected. Five equally spaced sampling locations with two samples per each location were taken on the level plane along one longitudinal axis of the sprayed area. No. 9 setting of the flow-controlvalve (Figure 6 ) allowed an ample number of drops to pass through the shutter without severe coincidence. Figure 8 shows a micrograph of a typical stain sample. Stains were counted and measured with a microscope equipped with a micrometer eyepiece. Table I lists the results. The rate of deposition and the volume concentration of drops a t the level plane are also shown in Table I. Using the average value of the rate of volume deposition, 2.62 X IO-? cmR/(cm2 s), and the total wetted area on the level plane, which was
600t
010
ooe
006
STAIN OIAMETER ,mm
Flaun3. Drop diameter vs. stain diameter at variws impact velociile~ given by percentage ( x ) of terminal velocity. 460
Environmental Science 8 Technology
Figure 4. Photograph of the prototype drop impact sampler
RESTRAINING P L A T E
I PRESSURE GAGE
0 IMPACT HEAD
#
b HANDLE
n
C M O V A B L E ORIFICE P L A T E U S T A T I O N A R Y OR1IfICE P L A T 1 l M P I C T PLATE f G R k P H I T E PLANCHCT
STOWGE TANK I NEEDLE VALVE
n 3-WA" VALVE
1 RECHARGE PORT
D SINGLE-ACTING AIR CYLINDER
.
TOP C O Y E R
b
I
PRESSURE REGULATOR
0 FLOW-REGULATING
VALVE
-
Figure 5. Schematic drawing of the drop impact sampler.
measured to he 10 200 cm2, we estimate the water flow rate to he 1.60 L/min, which agrees fairly well w i t h the measured value o f 1.32 L/min. F r o m the data in Table I, the mean volume diameter is calculated to he 0.221 mm; the size dispersion is plotted as shown in Figure 9, which approximates t o a normal distribution. Therefore, this test should give a size of 0.221 0.003 mm a t a confidence level of 95%. swir.1 SP'W
join1 001211
Flgure 7. Samplin a water spray. The swivel joint permits a last pcsitioningto anain = a.where @ is nozzle angle and 2a is the spray angle: thus h s are falling heights relatingto sampling locations inside the sampling region for nozzles of a wide spray angle.
1
.I
0 2 4 6 8 10 12 14 16 I8 20 22 24 D l b L SETTING ON FLOW-RbTE-REGULATING VALVE
Flgurb 8. lnverw of shutter opening vs. dial sening on a turning knob of lhe llow-rate-regulating valve. A pressure regulator is set at 2.81 kg/cm2 (40 Ib/in.2).
a
b
Flgure 8. Micrograph of drop stains on a Min MgO coated graphite planchet: (a) the micrograph; (b) an illustration. Volume 15. Number 4. April 1981 481
DROP DIAMETER, mm
Figure 9. A probability plot of size distribution.
E
Discussion The previous stain techniques, for example those of Engelmann ( I ) , require a calibration curve that relates the stain and drop diameters. This means that the incident drop must impact the target surface a t a known velocity, usually the terminal velocity. However, physical constraints may limit the sampler location and thereby introduce complications. For example, a 0.60-mm diameter drop requires a falling height of 200 cm (6.6 ft), as shown in Figure 2, to attain a fully developed terminal velocity ( x = 99.9%), while a 2.0-mm diameter drop attains only 78% of the terminal velocity at the same height and requires 1400 cm (46 ft) to reach its fully developed value. Neglect of the nonterminal effect for the 2.0-mm drop would lead to an apparent drop diameter 25% larger than the true diameter. The calibration curves in Figure 3 can be used to correct for nonterminal velocities at a known falling height. Extrapolation of a calibration curve based on the spherical quiescent model in eq 1to large drops results in an erroneously large drop diameter. For example, extrapolation of eq 1would lead to an apparent drop diameter 50% larger than the true diameter for a 0.40-mm drop and 85% larger for a 0.60-mm drop. The use of a graphite substrate thinly coated with magnesium oxide film provides a distinctive stain boundary. Any stains superimposed by coincident drops can be easily identified, thus eliminating the coincidence loss. With the new technique, the effect on impact velocity due to air entrainment by fast-moving drops and the aerodynamic disturbance caused by the impact head is not taken into consideration. Therefore, this technique can be applied to a drop population that maintains its falling trajectory, without any visualized draft caused by air movement. ij
3
L
P i E Q$.!?E
"
0 0 0 0 0 0 0 0 0 0 0
m o m o m o m o m o m O r r N c u m m b f m m
0 0 ~ 0 0 0 0 0 0 0 0
Q
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c 462
Environmental Science & Technology
Summary The present sampler eliminates the difficulties encountered previously with the stain technique in .determining size distribution. In addition to achieving accurate sizing, the present sampler is able to determine the rate of deposition and the spatial concentration at the measuring location. The sampler can measure drop sizes ranging from 0.005 to 2.5 mm in diameter, which makes it suitable for water sprays, raindrops, and drop carryover from cooling towers. Laboratory testing has shown that the device is reliable, rugged, lightweight, and easy to use.
Acknowledgment We thank Dr. Welby G. Courtney for his valuable suggestions in editing the manuscript. Literature Cited (1) Engelmann, R. J. “The Hartford Raindrop Sampler and Selected
Spectra”; AEC Res. and Dev. Rep. HW-73119, 1962. (2) Martin, A.; Barber, F. R. Atmos. Enuiron. 1974,8,325. (3) Cheng, L. Ind. Eng. Chem. Process Des. Deu. 1973,12, 221. (4) Cheng, L. Ind. Eng. Chem. Process Des. Deu. 1977,16,192. ( 5 ) Cheng, L. Enuiron. Sci. Technol. 1977,11, 192. (6) Gunn, R.; Kinzer, G. D. J. Meteorol. 1949,6,243. Received for review July 2,1979. Accepted November 21,1980.
Preparative Isolation of Aquatic Humic Substances Earl M. Thurman* and Ronald L. Malcolm U.S. Geological Survey, Box 25046, MS 407, Denver Federal Center, Denver, Colorado 80225
A useful procedure has been developed which utilizes adsorption chromatography followed by size-exclusion chromatography, hydrogen saturation by ion exchange, and lypholization to obtain low-ash aqueous humic substances. The preparative concentration of aquatic humic substances is done by multiple reconcentration procedures even though initial concentrations of aqueous humus may be less than 25 pg/L. The procedure yields concentration factors of 25 000 times for both humic and fulvic acid in water.
Introduction Aquatic humic substances are polar, straw-colored, organic acids that are derived from soil humus and terrestrial and aquatic plants. They generally comprise one-third to one-half of the dissolved organic carbon (DOC) in water. The role of humic substances in water chemistry is receiving increasing attention because humic substances are known to complex trace metals ( I ) ; they are a source of methyl groups for the production of chlorinated methanes in water treatment (2); and they are implicated in the complexation or solubilization of pesticides and hydrocarbons in the aqueous environment ( 1 , 3 ) . For researchers interested in the role of humic substances in water chemistry, it is often necessary to isolate preparative amounts of aqueous humus. Because the concentration of humic substances in water rarely exceeds 5 m g L DOC, very large volumes of water (1000 L) must be processed in order to obtain gram quantities of humic substances. Although humic substances have been isolated from water by various methods including precipitation ( 4 , 5 ) ,ultrafiltration (6),solvent extraction (7), and freeze-drying ( 8 ) ,there were no simple analytical techniques to remove and concentrate humic substances from large volumes of water until the recent development of macroporous resins for adsorption chromatography (9-13). Since the development of these resins, the theory of adsorption chromatography onto macroporous resins and the usefulness of these resins for removing trace organic solutes from water have been thoroughly examined (10, 14-25). Methods of resin adsorption for isolation of aquatic humic substances are superior to the above-mentioned procedures (4-8), because they are quantitative by dissolved organic carbon analysis, they do not use organic solvents in preparation but isolate humus directly from water, they are simple and rapid for large volumes, and they separate the humic material from the inorganic substances in water. A disadvantage of these methods is the possible incorporation of organic matter from the adsorbent; therefore, careful resin cleanup is required. This report presents the use of macroporous resins to obtain preparative quantities of low-ash, aquatic humic substances by adsorption chromatography, ion exchange, and lyophili-
zation. This method is novel, not in the use of macroporous resins but in the use of multiple cycles of adsorption and desorption to concentrate aquatic humus from water and to separate it from inorganic solutes and nonacidic organic components. With multiple recycling of eluates, preparative amounts of humic substances from groundwater are concentrated with original concentrations of aquatic humus of less than 50 pg/L DOC.
Experimental Procedures Resin Cleaning. Amberlite XAD-8 resin (40-60 mesh which was obtained in cubic-foot quantities from Rohm and Haas) was extracted in a 4-L beaker with 0.1 N NaOH. Fines were decanted off after each daily rinsing of NaOH for 5 successive days. The DOC of the rinse dropped off exponentially from 1000 m g L to 10 m g L during this period. Next, the resin was Soxhlet-extracted sequentially for 24 h with methanol, diethyl ether, acetonitrile, and methanol and stored in methanol until used. Before column packing, methanol was rinsed from the resin with distilled water by using a slurry technique in a large beaker. The resin is packed as a methanol-water slurry and then rinsed with distilled water until free of methanol (less than 2 mg/L DOC), requiring -50 bed volumes of water. The packed column is now rinsed 3 times, alternating from 0.1 N NaOH to 0.1 N HC1; this removes impurities which may otherwise be incorporated into the sample. It is paramount that this rinse precede sample application. Glenco glass columns (3500 series) with lo-, 1-,and 0.25-L volumes were used for column chromatography, and samples were pumped with either a Cole-Palmer Masterflex pump or a Pharmacia peristaltic P-3 pump. Water was filtered through Millipore stainless-steel, 142-mm, plate filters fitted with Ag filters (Selas Flotronics). Procedure. The procedure is outlined in Table I, and an example is included for clarity. The procedure begins with collection of the water sample in glass bottles with volumes of 4,20, and 48 L, depending on the concentration of humic substances in the water and on the amount of aquatic humus required. The sample is then filtered through a silver-membrane filter (0.45 micrometer) to remove suspended matter (26).After filtration, the pH of the sample is lowered to 2.0 with concentrated HC1 and pumped through a column of XAD-8 resin a t 15 bed volumes per hour. The size of the XAD-8 column is chosen such that a solute with a k’ (column-capacity factor) of 100 is 50% retained by the column. The k’ is related to column size by the following equation: VEL = 2Vo(1+ k’), where VEL is the volume of sample that is applied to the column a t 50% retention, V Ois the void volume of the column (60%of the bed volume), and k’ is the columncapacity factor. The hydrophobic acids (12) adsorbed onto XAD-8 are
This article not subject to U.S. Copyright. Published 1981 American Chemical Society
Volume 15, Number 4, April 1981 463
