Artificial Surface Dirts for etergency Studies wit Surfaces J
WESLEY E. SHELBERG, JAAIES L. 1IACIZIN, AYVDROSS K. FULLER U . S. iP‘aval Radiological Defense Laboratory, San Francisco 24, Calif. URFACE dirts are extrinsic deposit’s n-hich accumulate on clean surfaces. They are formed on outside structures and machinery by a variety of pollutants, such as industrial wastes, greases and oils, smokes, salt sprays, and smogs. I n the course of radiological defense studies on the ability of solut,ions to remove radioact,ive contaminants from surfaces, i t vias necessary to determine the ability of mildly agitated solutions to remove orthodox outdoor surface dirts from immersed painted surfaces. A new laboratory scale detergency test was devised to compare paint cleaning solutions. This test, involved immersion of painted surfaces coated n-ith a standardized, artificial surface dirt. Tvio artificial surface dirts were developed for use in the test; one closely simulated a typical urban surface dirt, the other a typical shipboard surface dirt. The term “dirt” will denote the sample obtained when a surface dirt is collected by brushing, sweeping, or solvent action. The term “synthetic dirt” will apply t o a laboratory compounded substance which may b e applied to test surfaces as an artificial surface dirt. This paper describes the formation and properties of outdoor surface dirts; the preparation of synthetic dirta, and their conversion to artificial surface dirts on painted test surfaces; and a detergencj. test which unequivocally measures the ability of cleaning soIutions to remove the artificial surface dirts from test surfaces. SURFACE DIRT FORMATION
.
Urban surface dirts are formed by direct contact between outdoor objects and bulk liquid and solid pollutants and by deposition of smogs and other atmospheric aerosols. Their formation from bulk pollutants requires litt’le description since the soiling of streets, lower portions of structures, vehicles, and machinery is familiar. Factors involved in t.he deposition and nature of outdoor surface dirts from bulk pollutants are physical and chemical surface characteristics; surface orientation or configuration] exposure time, geographical location (climate and industrial environment); and physical and chemical properties of the pollutant. These factors also apply to surface dirts formed from atmospheric aerosols. Surface dirts resulting from smogs and ot,her atmospheric aerosols merit consideration since they are xvidespread because of the tremendous outdoor areas exposed to the polluted atmospheres of many of our industrialized regions. These surface dirts may be considered as end products of a formation scheme to which the following processes contribute: 1. Atmospheric pollut,ion with industrial and natural wastes, formation of solid or liquid aerosols 2. Chemical and physical modification of the aerosol pollutants in the atmosphere 3. Deposition of the atmosphcric pollutants as surface dirts 4. Aging and weathering of the surface dirts
Atmospheric pollution comprises solids, liquids, and gases and arises from natural dusts and many human activit,ies-e.g., chemical operations, factories, metal Forks, garbage dumps and home incinerators, and mining. Many unfortunate localities have multiple sources of severe pollution. The character of air pollution in a locality is affected by prevalent industries ( 7 ) . The Pittsburgh, Pa., region has contributed to the atmosphere
much soot from coal combustion and metallic oxides from iron and zinc production ( 7 ) . The Los Angeles region, where coal consumption and smoke are relatively unimportant, contributes dusts, fumes, and liquid aerosols predominantly on heavy smog days ( 9 ) . Atmospheric pollutants may comprise aerosols which have undergone spontaneous modification or modification by thc chemical and physical conditions of the atmosphere, as well as unmodified aerosols. Some natural dusts probably exemplify the unmodified aerosols. There are many examples of physical and chemical modifications. St. Clair ( I S ) has stated the well known fact, that smokes, fogs, and other dispersions of liquids or solids in gases are unstable and undergo spontaneous flocculation with lapse of time. I n certain cases, flocculation is rapid; the rate for most industrial smokes and fumes can be measured only some minutes after their release. hicCabe and coworkers ( 9 ) reported that solid particles are capable of acting as nuclei of hygroscopic aerosols and thesc. particles, as well as atmospheric and toxic gases such as sulfiii, dioxide, tend t o dissolve in the condensed moisture. Johnstone ( 5 , 9) pointed out that an increase in relative humidity causes an increase in the partide size of a hygroscopic aerosol and stabilizes the aerosol. Chemical reactions may occur between the component parts of a n aerosol or between the aerosol and t.he atmosphere. Aldchydes may be converted to acids, olefins to polymers or alcoholq, and acids t o esters. Sulfur dioxide dissolved in a hygroscopic aerosol is readily oxidized to sulfuric acid ( 9 ) . hlcCabe and coworlrers ( 9 )reported the presence of both liquid and solid material on cascade impactor slides exposed in Los Angeles; immediately following rain, only solids were collected. The deposition, thickness, and chemical composition of surface dirts from atmospheric pollutants are detcrmined by factors to which are also related the surface dirt weathering phenomena. Substratum. Rough or porous eurfaces may retain more surface dirt per unit area than similar smooth ones because of the increased surface area. Surfaces wettable with water or oil are expected to react more readily than nonwettable ones with moist or oily air pollutants. Slightly oily, greasy, or moist surfaces are expected to react more readily than clean, dry ones with dry aerosols. Werner, Shelberg, and Gevantman ( 1 8 ) showed that horizontal paint surfaces supporting an artificial, oil-containing surface dirt retained more siliceous dust than clean ones vhen the surfaces were exposed identically t o a dusty atmosphere. Substratum components frequently become incorporated in surface dirts. Thus, chalking of paints renders them more wettable and increases the complexity of their surface dirts by incorporating exposed fillers and extenders in them. Metallic corrosion also ahers surface dirt compositions, and water soluble corrosion products, as in the case of zinc, may be washed from surfaces along with the surface dirts they supported and modified. Substratum Orientation and Configuration. Surface position, slope, or angle determine the degree and force with which a surface is contacted by a n atmospheric aerosol and its transporting medium. For surfaces which are collecting surface dirts, an increased degree of contact should provide an increased rat,e of surface dirt deposition. Orientation influenccs thc effectiveness of the surface dirt weathering and removal by the elements. The degree of exposure to sunlight is important to weathering, especially with painted surfaces since it promotes chalking. Figure 1 demonstrates that identical, painted surfaces which are exposed
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December 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
together but a t different orientations in an outdoor industrial atmosphere may collect varying amounts of surface dirt. I n some instances, vertical surfaces have been observed to collect more surface dirt than similar horizontal ones exposed in the same location. Substratum Exposure Period. The amount of surface dirt collected by a surface should increase with exposure time, vided there is continued atmospheric pollution and little sur?;: dirt removal by the elements. It is conceivable t h a t surface dirts on some surfaces might fluctuate about a limiting value, corresponding to a balance between deposition and removal by the erosive effects of wind, rain, ice, or snow. Geographical Location of Substratum. Geographical location determines the climatic conditions that will affect surface dirt removal and weathering and will determine the nature of the polluting atmosphere. Atmospheric Pollutant. The character of a surface dirt is determined by the physical and chemical nature of its aerosol precursor. Aerosol particle size may influence the extent and rate of deposition; it is known that small, dry particles may adhere more tenaciously to surfaces than larger ones ( 1 2 ) . The corrosiveness of an air pollutant may influence the composition of a surface dirt. Metallic corrosion and paint destruction introduce components in surface dirts and, when corrosion products are removed by the elements, they may take surface dirt with them. The degree of moistness or oiliness of an air pollutant influences the adhesiveness of its consequent surface dirt. The authors have observed that when completely dry, synthetic dirts were converted into aerosols and permitted to settle on painted surfaces, the surface dirts did not adhere as tenaciously as those formed when the dirt aerosol was moist or oily. Topside surface dirts are formed on Naval vessels by direct contact of surfaces, usually Navy gray paint ( 1 7 ) , with bulk pollutants and by deposition of smoke aerosols from the stacks. Bulk pollutants are derived from human occupation, oily machinery, sea water evaporation, paint chipping, and ship repair. Stack smokes may be modified by air oxidation and moisture before deposition. Previously considered factors-surface chalking and condition, surface orientation and configuration, exposure period (determined by the frequency of washing and painting), and smoke particle size-will determine the deposition and nature of shipboard surface dirts. SURFACE DIRT PROPERTIES
Urban Surface Dirts. Sanders and Lambert ( I & ) have described physical and chemical analyses on those portions of street sweepings from six large Eastern cities that passed through a 200mesh sieve. Their averaged data are summarized in Table I and provide a clue to the properties of some urban surface d i r k formed from bulk pollutants. Sweepings from painted or metallic structures were not examined; these might have contained chalking or corrosion products. Sweepings from outdoor areas and structures of factories (such as metal works) and chemical plants (such as oil refineries) might provide different data from those reported by Sanders and Lambert; inorganic percentages might be increased and varied for the former situation while organics (ether solubles) might be increased and varied for the latter. There is little information on the collection and examination of surface dirts formed from air pollutants. Samples exposed on roofs in industrial areas have appeared particulate and microscopically discontinuous (Figure 1). Data on the physical and chemical properties of air pollution provide clues to the properties of their consequent surface dirts; however, McCabe and coworkers (8, 9) have pointed out that procurement of representative snzog samples is a major problem. For example, smog samples collected with cascade impactors or Greenburg-Smith impingers may not simulate the original smog a t all because of reactions in the collecting apparatus; chemical composition as well as particle size may be altered. Although cascade impactor slides do provide samples for examination, they cannot be considered truly representative surface dirts. Furthermore, such samples do not reflect the effects of aging, corrosion, orientation, and elemental weathering.
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TABLE I. SUMMARIZED SANDERS AND LAMBERT DATAFOR URBAN DIRTSAND FOR THEIRURBANSYNTHETIC DIRT(14) Natural Dirtse, Components Water soluble Ether soluble Moisture Total oarbon Ash SiOz, total RaOs,total , CaO, total MgO total Ca0,'wster soluble MgO, water soluble Nitrogen
pH of 10% slurry Particle size distribution, s 20
% 14.4 8.3 2.31, 26.1 53.6 24.5 10.4 7.2 1.8
Synthetic Dirt,
%
0.2 1.9c
9.8 5.1 8.0 25.8 40.9 21.9 7.5 4.8 1.9 0.3 0.2 1.3
7.1
8.9
0.5
68 16 8
4 2 2
4 Averaged d a t a from Pittsburgh, Detroit, Cleveland, Buffalo, St. Louis' and Boston dirts. b Averaged d a t a from Detroit, Cleveland, a n d Boston dirts. e Averaged Sanders a n d Lambert d a t a from Detroit a n d Boston dirts d Composite sample of dirts from cities listed in 5 ,
McCabe and collaborators (9) examined Los Angeles smog samples collected with a modified cascade impactor. During dense smogs, 1 or 2 cu. ft. of air provided satisfactory slides for microscopic examination; in excess of that quantity, the aerosol built up until the particles coalesced and lost form, Owing t o agglomeration during collection, particles were larger than before impaction; approximate sizes for impacted particles ranged from 0.1 to 1.5 y. Solids, some crystalline, were found on all slides and a preponderance of the liquid phase was noted during dense smogs. Particles did not coalesce or evaporate on standing for several days, and sulfate ion was positively identified in the liquid portions. Magill ( 1 1 ) has listed a number of substances known to be present in the Los Angeles air. These include dust particles (50 to 1000 per cc.; Smith-Greenburg impinger), aldehydes (0 to 0.5 p.p.m. calculated by volume of formaldehyde), sulfur oxides (0 to 0.4 p.p.m. calculated by volume of sulfur dioxide), nitrogen oxides (0 to 0.1 p.p.m. calculated by volume of nitrogen dioxide), and halogens (0.15 to 0.3 p.p.m. calculated b y weight of chlorine in dry air). Thus, on the basis of these data, surface dirts from Los Angeles smog might comprise solid particles, some crystalline and liquid droplets; they might contain organic acids from aldehyde oxidation and sulfuric or other inorganic acids; they probably consist of particles ranging down from 1.5 p , Shipboard Surface Dirts. The importance of shipboard surface dirts in detergency and radiological decontamination studies and the absence of any literature describing them necessitated their collection and detailed analysis. Naval vessels, such as destroyers and cruisers, are frequently washed and generally clean, but surface dirts still accumulate on machinery, in corners, in air intakes, on turret tops, and other areas. Some survive washing because of the adhesiveness of their oily constituents. Pioncommissioned ships, inactive for long periods, have been observed to collect considerable land dust and soil. Surface dirts were collected by sweeping and brushing the soil collecting and the horizontal areas of the torpedo and main decks of a destroyer. The destroyer was especially selected on the basis of typical, extended, and varied sea duty and was boarded for dirt collection immediately after its arrival a t a Naval shipyard and before the activities of shipyard maintenance crews. Those portions of the torpedo and main deck sweepings which passed through a 20-mesh sieve were investigated individually. These sweepings revealed small paint chips, rust, rope lint, and asbestos fibers from pipe insulation, and were magnetic due t o the presence of small iron particles and magnetite.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Clean Unexposed Surface Figure 1.
Horizontal Surface
Vertical Surface
1.8 M g . / a q . inch of gurface dirt
0.8 Mg./sq. inch of surface dirt
Effect of Surface Orientation on Deposition of Air Pollutants o n Navj Gray Paint
Particle size distributions (Table 11) were similar for the dried dirts from the torpedo and main deck. Size distributions were determined by drying samples of the minus 20-mesh (840 p and smaller) dirts overnight a t 110" C., sieving them with Tyler sieves, then drying the separated portions and weighing them.
TABLE 11. D.4TA
FOR
DESTROYER DIRTBB N D SHIPBOARD
SYKTHETIC DIRT
Synthetic Dirt, Torpedo Deck Dirt,
%
Dried Components Total carbon Hydrogen Residue after ignition h-itrogen Zinc Chloride Total iron Elementary iron Silica Ether extractables K a t e r extractables pH of 10% slurry Particle size distribution, 1.1
