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Environmental Health Perspectives Vol. 20, pp. 55-70, 1977

Chemodynamics: Transport and Behavior of Chemicals in the Environment A Problem in Environmental Health by V. H. Freed,* C. T. Chiou,* and R. Haquet

In the manufacture and use of the several thousand chemicals employed by technological societies, portions of these chemicals escape or are intentionally introduced into the environment. The behavior, fate, and to some extent the effects produced by these chemicals are a result of a complex interaction of the properties of the chemical with the various processes governing transport, degradation, sequestration, and uptake by organisms. In addition, such processes as adsorption, evaporation, partitioning, and degradation are influenced by ambient conditions of temperature, air movement, moisture, presence of other chemicals, and the concentration and properties of the subject chemicals. These influence the level and extent of exposure to these chemicals that man might receive. Study of the physicochemical properties of compounds in relation to these various processes has provided a basis for better understanding of the quantitative behavior. Such information is useful in development of predictive models on behavior and fate of the chemicals in relation to human exposure. Beyond this, it provides information that could be used to devise procedures of manufacture, use, and disposal that would minimize environmental contami- nation. Some of the physical principles involved in chemodynamics are presented in this review.

Introduction The use of these chemicals has provided society with better health and a higher standard of living. During the past three decades there has been However, the proliferation of chemical usage has nearly an exponential increase in the amount and elicited concern over the long range effect of these number of chemicals used by modern society (1-3). materials on man and his environment. The reason This situation is a result of the productivity of the for this concern is the realization that many chemi- synthetic chemist in responding to man's desire for cals have become widespread environmental con- a higher standard of living. Thus, we have a con- taminants (1, 8-12). Thus, it is apparent that our tinuing increase in the production and use of drugs, knowledge of chemical behavior has not kept pace additives, , and industrial chemicals with the technology required to produce chemicals. (1, 2, 4-6). Two major categories of environmental pollu- The types of classes of chemicals used range tants are pesticides and industrial chemicals (2, 13). from simple aliphatic organic chemicals to complex Pesticides have received the most attention, since polynuclear compounds to high molecular weight they are used on food crops and are applied to large polymers. Also included are a variety of inorganic areas of the environment (14-17). The ubiquitous substances of widely different properties. Tables I nature of chlorinated hydrocarbon res- and 2 list the production of some representatives of idues was a major factor in demonstrating the im- some of the classes of chemicals currently in use. portant role of behavior of a chemical in the envi- ronment (10, 12, 14, 15, 17-22). However, pes- *Environmental Health Sciences Center, Oregon State Uni- versity, Corvallis, Oregon 97331. ticides are not unique chemicals which predispose tCriteria and Standards Division, Office of Water Supply, them to become environmental contaminants. It Environmental Protection Agency, Washington, D. C. 20460. has been amply demonstrated that many of the in-

October 1977 55 dustrial chemicals such as polychlorobiphenyls and ing environmental exposure. With the appropriate organohalogens have also become widely distrib- understanding, methods of manufacture, handling, uted (4, 5, 11-13, 15, 21, 23). Thus, there is poten- use and disposal can be devised to abate the prob- tial for widespread dissemination of residues of any lem. Similarly, a knowledge of transport through chemical. food chains or by physical means permits the ap- The basis of concern of chemicals in the envi- propriate avoidance reaction. A number of exam- ronment is whether or not the substance will effect ples could be cited to illustrate how a prior under- the health and welfare of man. The effect may be a standing of the chemodynamics of a compound direct one, producing a lesion or deterioration of could have been utilized to avoid problems such as health; it may be indirect, producing a less healthful with DDT, PCBs, mercury, vinyl chloride, and environment or restricting food production or other various heavy metals. activities of man; or it may be a remote effect caus- If one were to consider the path followed by a ing a deterior-ation of environmental quality. It is, of chemical from its release or escape to the site of course, important to prevent these consequences if action in an organism or population of organisms, it we are to protect the health and welfare of man. It would be found that there are at least four major would seem the prudent course of action to prevent steps, namely: interaction of the chemical with the health hazards in preference to having to attempt to environment during its transport to the boundary of ameliorate the effects of such exposure. However, the organism; interaction with the boundary of the until there is an understanding of the transport and organism; passage through the boundary; and intra- behavior of chemicals in the environment, it is dif- cellular action of the chemical. These four steps ficult to devise the appropriate methods of prevent- give a means of partially relating the properties of

Table 1. Production of various types of chemicals.

Annual production, Clalss Representative compounds (millions of pounds) Example of uses Inorganic 19.7 x 10: Water purification 28.6 x 10: Fertilizer Titanium dioxide 1.4 x 10: Pigment Orgtnohalogen Ethylene dibromide 315.5 Gasoline additive Carbon tetratchloride 996.7 Specialty solvent Grain fumigant Hexachlorobutadiene 8.0 Solvent Heat transfer liquid Vinyl chloride 5088.0 Chemical intermediate OrgatnophospholrOLus Tri(2-chloroethyl) phosphaite 29.4 Flame retardant for plastics Synthetic lubricants Acyclic phosphorodithioaite 68.0 Lube oil additive Lube oil additives [zinc di(butylhexyl)- phosphorocithioate] Origtnosilicones Silicone fluids 74.7 Release agents [] Antifoaming agents Polish and cosmetic ingiredients -contatining Dioxaine 14 Specialty solvent Inhibitor in chlorinated solvents Nitrogen-Contatining (Ethylenedinitr-ilo) tetraacetic 64.3 Agriculture, detergents pulp . tetrasodiLIm sallt Pulp and paper processing Ethyleneimine 5.0 Chemical intermediate Plastics Polyethylene 7.63 x 10' Platstic Detergents Dodecylbenzenesulfonic icid. 364.1 Detergents sodiLIm N N-Dimethyldodecylamine 36.1 DeterLgent oxide Hydrocarbons 8937.1 Chemical intermediate

56 Environmental Health Perspectives Table 2. Pesticide production volumes." Table 3. Processes involved in chemical action. Production, Steps Processes Class Common name 106 lb A. 1.' Interaction with the environment Adsorption of surfaces Methyl 45 Vaporization 50 Photochemical breakdown 45 Autochemical breakdown 35 Dissolution 25 Partitioning Parathion 15 10 Interaction with barrier of organisms Adsorption 10 Destructive reactions 8 Rejection 8 Intracellular transport Adsorption 8 Metabolic binding Herbicide Atrazine 90 2,4-D 45 Partitioning MSMA-DSMA 35 Reaction with critical site Adsorption chlorate 30 Reaction Trifluralin 25 Propachlor 23 Chloramben 20 Alachlor 20 CDAA 10 Escape of Chemicals 2,4,5-T 6 Fungicide PCP and 46' In the production and use of any chemical it is Dithiocarbamates 40" nearly impossible to avoid some transfer of material TCP and salts 20 into the environment (1, 2, 8, 9, 24-26). In theory, Captan 18 even in the use of a "closed" system there may be PCNB 3 to Dodine 3 some opportunity for a chemical (or chemicals) escape during transfer in the system. In manufac- "From Lawless et a]. (7). "Active ingredient. turing and use of chemicals under normal circum- 'Includes use as herbicide, desiccant, molluscicide, and for stances, the systems are at best only partially termite control. closed, thus affording opportunities for chemicals "Includes CDEC, Ditane M-45, Ditane S-31, Ferbam, to escape through air and water effluent from the Maneb, Metham, Nabam, Niacide, Polyram, , Zineb, manufacturing handling and fabrication, or and Ziram. plant, use to some other sites (4, 6, 16). Having escaped into the environment, the chemical may be trans- ported by water or air to afford an exposure to man the chemical to the ultimate action on the biota. It is and other organisms in the environment (1, 10, 15, with the first step, that the chemodynamics is 22, 23, 27). primarily concerned with and it is the step at which The chemicals used as starting materials or in- preventive action can be taken. At each of the four termediates in manufacture may end up as residues steps, a number of different processes or reactions in air through vaporization (14, 15, 24, 28-30), in can occur. Certain of these processes or reactions surface and underground water through contact and are common in several of the steps. These process- leaching (18, 31-34), by sewage effluents (1, 4), or in es or reactions are dependent on both the proper- soil through adsorption (19, 23, 29, 35-37). All of ties of the chemical as well as the property of the these points of release are potential sources for particular element of the system with which it may widespread contamination. In light of the large be interacting. On this basis then, the physical scale production and use of chemicals, we therefore chemical properties of the compound permits some need a critical evaluation of the uses and properties prediction of probable behavior and an estimate of of these chemicals in assessing their hazard to man whether the exposure received by man is likely to and the environment (1, 16, 38). produce serious consequences. Table 3 attempts to An evaluation of the hazard from a chemical's identify the different processes that may be as- use involves consideration of a number of factors sociated with the four different steps described (1, 4, 16, 22, 31, 39-43). These include the amount above. and nature of chemical being used, the manner in From the foregoing it can be seen that there are which it is used, the extent of transport and altera- some general processes or reactions common to tion in the environment, and the biological effect of two or more of the steps. residues on organisms including man.

October 1977 57 Over the years, a considerable body of informa- had pointed to the fact that there is a correlation tion has built up relating physical chemical proper- between the partition coefficient and the sorption of ties and reactivity to the behavior and fate of chem- the chemical by the soil organic matter (10, 34, 36). icals in the environment (10, 16, 18, 25, 44-47). Soil and sediments in water are quite complex From this information, it is possible to develop systems consisting of clays, accompanying organic models of the transport of these chemicals and vari- matter, and living organisms. While the initial reac- ous facets of theiir behavior. tion with the chemical may be physical in nature, The soil becomes the residence for a large portion this heterogeneous system may promote chemical of the chemical that finds its way into the environ- reactions through through intervention of ment (17, 19, 34, 35, 48). What happens after the enzymes from the living organism (12, 17, 22, 26, chemical reaches the soil is of considerable impor- 53). These reactions usually result in the reduction tance in determining whether the substance will of the biological activity of the compound, or its subsequently find its way into air or water, how sequestration, the rates of which depend both on long it will persist, and its bioavailability (14, 15, 22, the soil or sediment system and the properties of the 27, 31, 36, 49, 50). The interaction between the compound. Some of the common reactions include chemical and the various soil constituents (sorp- hydrolysis, oxidation, reduction, and conjugation tion) is quite important. It was noted very early that (9, 14, 17, 38, 54, 55). The rate at which these reac- with nonpolar organic substances there is an in- tions, or decompositions, occur may give an indica- verse relationship between water and the tion of the persistence of the chemical in the aque- extent to which the chemical is absorbed (38, 49, ous or other components of the environment. There 51). Subsequent studies of the thermodynamics of is evidence that the raite of hydrolysis of certain the sorption process suggested that the strength of classes of organics in laborator-y buffer- systems give binding for such chemicals may be related to the a good indication of the likely behavior of the com- latent heat of solution (16, 19-21, 41, 46, 52). Others pound in the natur-al system (3, 12, 56-58). C70 ~~~~I 2,4,5,2'4:5 - PCB

DOT *0 DDE 0 D C')en 4,4'-PCB 0 Dich/ofenthion 0 -j Ch/orpyrlfos Ro*Ronnel Dial/for 0.cO 0 0 Methyl Chlorpyr/fos 41_ *Dlphen.yl Ether 1-S \orothion -V 04 Dicapthon Naphthalene 0 p -Dichlbrobenzene 0 lodobenrene ; 3 Malathion \ PhosmPhomet02,-Det 2 # D # Chloroben"er, 0 N;Carbon Tetrachlorlde 0 Saicylic acid c TetrachloroethyleneyeeBenzene\e alylcai Fluorobenzene e Chloroform 2o2k Nitrobenzenet * Benzo/c acid Phonylacetic acid%e Phenoxyacetic acid e 10 lo-2 l0o- I 10 102 103 104 105 106 Solubility In Water (p~moles/l ), Log Scale FIGURE 1. Relation between watter solubility and partition coefficient of varying compounds. 58 October 1977 Likewise, a knowledge of the and 0 an latent heat of vaporization allows estimate of the 0 10s I rate at which the chemical may evaporate or ex- change across a water/air and air/soil interface (14, -j an poten- 21, 27, 28). This provides indication of the - .*2,4,2;4'-PCB tial of the chemical for aerial contamination and _ 10 .slo Hexach/orobenzene transport. I- The accumulation of certain compounds in living organisms (biomagnification) has led to problems \ ipheny/ for populations of organisms. Many of the com- C x pDichoro- pounds that do this have a low water solubility and .6 102 DiphenrlEE er *1 benzene a high solubility in nonpolar solvents (lipophilic). The relation between water solubility and partition 0 ._ retrach/oro coefficient for a number of compounds is seen in c~ Carbon Tetrach/orlde o Figure 1 and Table 4. The partition coefficient of 0) 0- ,-2 -i*12 1]3 4 C IV * I IV IV IV Iv Table 4. and partition coefficients 0 of various compounds. 0 Solubility in Water ( p.moles/l ), Log Scale m Solubility in Log (n-octanol/H20 FIGURE 2. Water solubility . bioconcentration. Data of Neely water, ppm partition et al. (47). Compound (temp, °C) coefficient) Benzene 1710 (20)k 2.13' between an aqueouLs phase and a solvent phase can Toluene 470 (16) " 2.69' utilize a variety of organic solvents. The most de- Fluorobenzene 1540 (30)" 2.27 " sirable solvent, however, is one that more nearly Chlorobenzene 448 (30) " 2.84 " resembles the adipose tissue of the living organism. Bromobenzene 446 (30) " 2.99 " Such solvents as and corn oil have been lodobenzene 340 (30) " 3.25 ' p-Dichlorobenzene 79 (25)" 3.38" used but while they may satisfactorily represent Naphthalene 30 " 3.37' some system the solvent failed to meet the criteria Diphenyl ether 21 (25)" 4.20" of representation and convenience is ni-octanol (25, Tetrachloroethylene 400 (25)" 2.60" Chloroform 7950 (25)" 1.97 42). Accordingly, the ni-octanol/water partition Carbon tetr,achloride 800" 2.64" coefficient is the more widely reported. p,p'-DDT 0.0031 "-0.0034' 6.19 However, no singular physicochemical property p,p'-DDE 0.040 (20) 5.69' is adequate to describe and predict the probable 2700 (18) " 1.87 " 1800 (20)" 2.26 behavioi and fate of a chemical in the environment 16600 (20)" 1.41 and its likely method of transport. Rather, as has Phenoxyacetic acid 12000 (10) " 1.26 " been pointed out, one needs a profile of these vari- 2.4-D 890 (25)" 2.81X ous char-acteristics in order to estimate the trans- 2,4.,52',5'-PCB 0.010 (24) 6.11 port and fate of the chemicals in the environment 2.4.5,2',4',5'-PC B 0.00095 (24) 6.72 4.4'-PC B 0.062 (20) 5.58 (/, 4, 11, 22, 23, 56). Such a profile of some Phosalone 2.12 (20) 4.30 used as pesticides is found in Methyl 4.76 (20) 4.31 Table 5. "Data of Hodgman (59). The specific properties which are required to "Data of Fujita et al. (60). predict such behavior are: water solubility, heats of 'Data of Leo et al. (42). solution, ionization constants, vapor pressures, "Data of Kenaga (10). "Data of Hansch and Fujita (6/). rates of hydrolysis, and partition coefficients. The fData of Hansch and Anderson (62). first three properties determine the degree and 01ata of Macy (63). strength to which a chemical is adsorbed on sur- "Data of Bowman et al. (44). faces such as soil or river sediments. This behavior 'Data of Biggar et al. (18). will determine the extent to which a 'Data of O'Brien (64). obviously "Data of Arnold et al. (88). chemical is transported by the aqueous environ- ment. With chemicals subject to hydrolysis, this such compound provides a good measure of their property will govern the persistence of chemicals in tendency to accumulate in living organisms (10, 47, both the physical and biological environment. 53) (see Fig. 2). This coupled with a refractoriness Vapor pressure and heat of vaporization serve as a towatd reaction gives an indication not only of ac- measure of the rate of vaporization and tendency of cumulation but transmission from one trophic level volatilization across air/chemical as well as to another. The determination of the partitioning air/water and air/soil interfaces. Finally, partition October 1977 59 Table 5. Physical properties ofsome compounds.

Vapor pressure Compound Structure Mp, °C Bp, °C/mm Hg mm Hg (t, °C)

Parathion 6.1" 113/0.05" C2H0,P,0 NO 157-162/0.6""e 3.78 x l0-5 (20)f

Methyl parathion CH30"* 1NO 35-36" 109/0.05" 9.7 x 10-6 (20)d CH3O

Cl Dicapthon CH301,. 52-53("' 3.59 x 10-6 (20)b ,P.0 N02 51-529 2.38 x 10-5 (30)b CH30

G3O,II 6 x 10-6 (20)aI P.O-~)-N02 95/0.01la Fenitrothion 030CH- CH3 140-145/0.4c 5.4 x 10-5 (20)"

Cl 108/0.01 a Dichlofenthion 120-123/0.2" C2H5O

Cl CH3O )xc 41" 97/0.01a 5.29 x 10-5 (20)b Ronnel CH30 35-37i 1.86 x 10-4 (30)b Cl

44.5-45.5b 3.37 x 10-5 (20)b Methyl chlorpyrifos cH30J 45.6-46.51 1.47 x 10-4 (30)b

C Chlorpyrifos 42-43.5n 1.87 x 10-5 (25)0 41.5-43a 8.87 x 10-5 (35)0 cps N0 c 7.82 x 10-5 (30)b

60 Environmental Health Perspectives kcal kcal mg/kg solubility OctanolH2O Hydrolysis LD-,,,. mole ppm (t, °C) coefficient Half-life pH t,°C mole Oral, Dermal. rabbit

15.5 11.9 (20)" 6,430" 26.8 days 7.4 37.5" 16.31 13 (M)' 21 (rat)' 11(40)" 130 days 7.4 20" 3.6(F)' 6.8 (F)"

21 25(20)" 175 days 1-5 20.0" 14 (M)' 67 (rat)' 77 (40)" 11 hr 1-5 70.0" 24 (F)'

33.4" 6.25 (20)" 3,790" 5.5 days 7.4 37.5" 17.17 400 (M)" >2000 (guineat pig)' 35"1 29.0 days 7.4 20" 330 (F)Y

22 30" 2,380" 11.2 hr 6.0 (1:4 /H20) 70" 250' 3000 (mice)' 250-500' 1300 (rat)'

0.245 (25)' 137,000" 48.0 hr 6.0 (1:4 ethanol/ H20) 70" 270' 6000"

22.2b 1.08 (20)b 75,300b 10.2 hr 6.0 (1:4 ethanol/H20) 70" 1500' 2000 (rat)' 40'' 10.4 hr 6.0 (1:4 ethanol/H20) 70" 1740k 1500'

26.0b 4.76 (20)b 20,200b 12.5 days 7.4 20b 16.22 941"' 4.0 (23-25)' 9,300" 2.6 days 7.4 37.5b

28.4 0.4 (23)0 66,600" 13.4 days 7.4 37.5" 14.21 163 (M)' 2000' 2P 128,700b 53.0 days 7.4 20" 135 (F)'

October 1977 61 Table 5. Physical properties of some organophosphate compounds (continued).

Vapor pressure Compound Structure Mp, °C Bp, 'C/mm Hg mm Hg (t, 'C)

Cl Leptophos 70.2-70.6c 2.3 x 10-8 (20)b @P.0>8itBr 71.5-72.0q 1.7 x 10-7 (30)b CH3OC

0 S 72-72.70 4.52 x 10-7 (30)b 71.9' 9.38 x 10-7 (40)b CH30 .eP.S.C A1 Dialifor C2H S CH2CI°0 ,P.S. CH-.C 67-69s 6.2 x 10-8 (30)b 3.45 x 10-7 (40)b C252" CI C H 0 C 45-48r Phosalone 45-47a

0 II 0130lS CH2.C.0C2Hs 2.85b 120/0.2a 1 25 x 10-6 (20)" Malathion ,,P.S.CH.C.0C2H5 156-157/0.7c,d 4 x 10-5 (30)d,l

CH3OcH3Osq 0bl P 43.5-45.8c 107/0.Oa 8.5 x 10-6 (20)a 030 .0.CH2.C.NH.CH3 42-46b

0430 \0 0 99-103/0.03' 2.2 x 103 (20)a P.O.C=CH.C.OCH3 106 107/Ik 5.7 x 10-3 (29)d CH30 CH3

0130 ON ,P.CH. CC13 8344c 100/0. 1C 7.8 x 10-6 (20)c Trichlorfon 7880k CH30

aData of Mel'nikov (6S). 5NIEHS data (66). cData of Spencer (67). "Data of von Rumker and Horay (68). (72). iData from Farm Chemicals Handbook (73). JData of Hammer (74). kData of Burchfield and Johnson Collier and Dieter (80). "Data of Velsicol Chemical Corp. (81). rData of Stauffer Chemical Co. (82). 'Data

62 Environmental Health Perspectives AHr,,p,' H2 Octanol/H20 AHhYd, kcal solubility, pa2r Hydrolysis kcal LD., mg/kg mole ppm (t, "C) coefficient Half-life pH t,°C mole Oral, rat Dermal, rabbit

35.3b 0.0047 (20)b) 2.02 x 106b 19.Ohr 6.0 (1:4 ethanol/H20) 70b 42-53c > 10,000 (rat)" 0.03 (25)" 90M >8009

13.8b 25 (25)" 677"b 1.1 hr 7.4 37.5b 19.27 230 (M)' >3160r 7.1 hr 7.4 20.0"b 299 (F)C

32.4 0.18" 49,300b 1.8 hr 7.4 37.5b 21.19 5-70" 145C 14.0 hr 7.4 20.0"b 50"'

10" 20, 1oob 120' 1000" 2.15 (20)b 135-170(F)" 390(rat)g

17 145 (20)" 781b 1.3day 7.4 37.5b 21.58 1375' 4100' 300 (30)d 10.5 days 7.4 20.0b 2800"

22 39,W000 0.508b 0.8 hr 9 70" 250-265" 400 (rat)" 50,000 (30)d 21 hr 2 70' 245"

16.6 Higha 30-35 days 7a 5.0-6.8c 4.7-33.8e Miscibled 3 days 9q 3.7-12.0" 16-34"

25.7 154,000 (25)C 630(M)c >2000 (rat) 120,000 (26)U 560 (F)

eData from Shell Chemical Co. (69). 'Data of Bright et al. (70). "Data of Martin (71). hData of Ruzicka et al. (75). 'Data of Kenaga (76). mData of Thomson (77). "Data of Kenaga (78). "Data of Brust (79). "Data of of Hercules Inc. (83). 'Data of American Cyanamid Co. (84). uData of Zweig (85).

October 1977 63 coefficients indicate the propensity for uptake and biota (biosphere). A rough estimate of the masses of storage of chemicals in biological systems. Com- the various phases yields the following: atmosphere pounds of low water solubility are nearly always 5.3 x 10I8 kg, soil to 6-in. depth 1.1 x 1017 kg, water extensively adsorbed on surfaces. In biological sys- 1.3 x 1021 kg, animals 2.0 x 1013 kg, and plants 1.1 tems, the adsorption and storage of chemicals is a x 10'5 kg. When a chemical is released into the function of a compound's lipophilicity as well as its environment it will be distributed (partitioned) be- water solubility. So partition coefficients, or tween the various phases with the concentration in lipophilic to hydrophilic ratios, will determine any phase being a function of the properties of both whether a compound will be stored and biomag- the chemical and the phase (9, 34, 41). Applying the nified. simple law of Boltzmann distribution to this com- Table 6 attempts to summarize what has been plex system we have the following equation: demonstrated in a number of publications on the relation of physicochemical properties to environ- Nij = Noe -eikT (1) mental behavior. If we assume that Nij = Nji, Nij = 0 when i = j and No = N1 + N2 + N3 + N, + N5, where No is the Table 6. number of molecules of the chemical initially intro- Physical chemical data Related to duced and N1, . . ., N5 are the number of molecules Solubility in water Leaching, degree of adsorption, mobil- in respective phases of the environment. In the Eq. ity in environment (1), E represents the energy barrier between two Latent heat of solution Adsorption, leaching, vaporization phases, k represents the Boltzmann constant, and T from surfaces To understand Partition coefficient Bioaccumulation potential, adsorption represents the absolute temperature. by organic matter the behavior of a chemical in the above five compo- Hydrolysis Persistence in environment and biota nents one must have a detailed knowledge of the Ionization Route and mechanism or adsorption or physical and chemical properties of the chemical. uptake, persistence, interaction with other molecular species Vapor Pressure Atmospheric mobility, rate of vaporiza- tion

Thus, such data provide not only a basis of predict- ing the transport and possible compartments of ac- cumulation, but would also give an indication of persistence and fate of the chemical. Quantitative Behavior of Chemicals in the Environment Chemicals used by man represent a wide variety of classes of compounds. They are grouped accord- ing to the purpose for which they are used, e.g. pesticides for insect, weed, or fungus control, plas- tics, pharmaceuticals, detergents, etc. One of the main characteristics of many of the compounds is that are low molecular weight or- they generally FIGURE 3. Man's exposure to a chemical through cycling of res- ganic compounds with very low water solubility; idues in the environment. however, some inorganic compounds and or- ganometallics are also used (2, 26). In order to answer the question as to what hap- pens to a chemical when it is introduced into the environment, one must have an understanding of Evaporation and Transport the nature of the environment itself. The fundamen- To illustrate the evaporation of liquids, pesticide tal phases of the environment are: land (litho- spray application will be used. The application of sphere), water (hydrosphere), air (atmosphere), and sprays for involves forcing the carrier

64 Environmental Health Perspectives liquid under pressure through the orifices of ap- is large in relation to the volume of the drop (4.18 x propriately spaced and arranged nozzles. Almost 10-6 cm3). Obviously, if a drop is yet smaller, it regardless of the type of nozzle, fluids, e.g., water, presents a larger surface area in relation to its vol- oil, tend to break up into droplets as they leave the ume for evaporation. The relationship of evapora- orifice. The droplet size is not uniform, rather any tion to drop size is shown in Eq. (3): given nozzle and orifice will tend to produce a spec- trum of droplet sizes on either side of the median. A variety of forces and properties such as surface P = P,e 2yVIrAT (3) tension, , pressure of the fluid, orifice size and geometry, shear effects, etc., influence the This equation indicates that as the radius gets small- droplet size spectrum. er, the vapor pressure increases, and hence the rate The rate of fall of a droplet or any particle is of evaporation increases. governed by Eq. (2): During the fall of the droplet, evaporation will take place. This decreases the size of the droplet V = 2r2AXpg/9gr (2) allowing even more extended drift.The rate of evaporation increases with temperature and, of where V is terminal velocity, r is radius of the drop- let, Ap is the difference between the of the course, reduced humidity which further exacer- particle and the density of air, g is the acceleration bates the problem. due to gravity, and -q is the viscosity of air (1.8 x Once a chemical is introduced in the environ- 10-4 poise). Obviously then, with a given liquid, the ment, its entry into and transport through the at- size of the droplet becomes important in the rate at mosphere will depend on several factors such as the which it falls to the target area. The smaller the vapor pressure and the heat of vaporization of the droplet, the slower this terminal velocity will be. If chemical, the partition coefficient between the at- there is a wind to displace the droplet from a purely mosphere and any other phase, and the air flow vertical line of fall, the droplet is said to "drift." It mass which will transport any chemical dispersed in thus must follow a longer path from the nozzle to the atmospheric phase. the target surface. The degree of displacement that The vapor pressure of the chemical will play a a particle can experience is illustrated in Table 7. major role in the atmospheric transport since it re-

Table 7. Drift of naturally occurring water droplets.

Drift (at wind speed 10 mph) Description Droplet size, ,um Height 5 ft Height 50 ft

Fog 10 14.5 miles Mist 100 75 ft 750 ft Avg. particle size of mist under Niagara Falls 217.3 21.5 ft 210 ft Drizzle 300 8 ft 4 in. 83 ft Light rain 590 2 ft 2 in. 21.5 ft Moderate rain 800 1 ft 3 in. 12.0 ft

Every substance has a tendency to change from lated to the proportional amount of chemical in the solid to liquid to vapor due to the motion of the gas phase (14, 28, 32). The vapor pressure P of a molecules making up that substance. The tendency pesticide is related to the temperature T by the of a liquid to pass into gaseous state or vaporize is well-known Clausius-Clapeyron equation (4), indicated by its vapor pressure and is materially influenced by the amount of energy required to d(ln P) _ AH accomplish this procedure. The liquid droplet (4) emerging from the spary nozzle now presents a dT RT2 large surface from which the various components of the droplet can evaporate. Thus, for example, the where R is the gas constant and AH is the heat of area of a particle of 200 gm diameter (a not uncom- vaporization. The quantity AH regulates the mon size for sprays) is 0.001256 cm2. However, this amount of chemical converted into vapor phases at October 1977 65 a given heat flux. The vapor pressure of organic soil surface other factors which may control the chemicals vary over a wide range from the highly release of the chemical may include temperature, volatile substances such as fluorocarbons, initial concentration of the chemical, moisture, chloroform, and vinyl chloride through the moder- and pH. ately volatile compounds like parathion, to the low volatile materials such as DDT, PCBs, and poly- Solution Behavior mers (1, 23, 48). An expression relating vapor pres- sure to quantity evaporating is found in the Lang- The major factors contributing to the partition- muir equation (5): ing of a chemical into the aquatic environment are its water solubility (17, 18, 33, 34) and the latent Q = fiP (M/2rRT)- (5) heat of solution (16, 41, 46). Many organic com- where /3 is an evaporation constant of a chemical pounds in use today evidence a hydrophobic under a given atmospheric condition. character having water solubilities in the parts per In the atmospheric phase, the kinetic motion of million (ppm) or even parts per billion (ppb) range. molecules as well as eddy current will cause their This makes exact determinations of their sol- distribution. The molecules will distribute vertically ubilities quite difficult. of a concentration The alkalinity or acidity of the solution is thought like a column gases having gra- to influence the stability and the solubility of certain dient. The heavier molecules will have the highest pesticides. For example, the solubility of triazine concentration at the bottom and the lighter ones on molecules (a class of pesticides) usually increases the top, and the pressure P (or concentration) of a with lowering pH and is attributed to protonation of chemical will be governed by the barometric for- nitrogen with the formation of cationic species. The mula (6): presence of salts in an of pesticide may cause ion-association or ion-pair formation. P = P0e-mYxRT (6) As would be expected, temperature significantly where x is the height at which pressure is sought, P0 influences the behavior of organic chemicals in is the pressure at some reference point, and m is the aqueous solution. Though the exact mechanisms of mass of the gaseous molecule. the solubilization of some of the sparingly soluble The presence of suspended dust or aerosol parti- chemicals is not known, solubility usually increases cles may result in sorption of some of the vapors with temperature. The question still remains and consequently will increase the partition func- whether or not they form ideal solutions. However, tion of the chemical between the atmospheric phase by substituting the solubility of the compound at and other elements of the environment. The sorbed two temperatures in the Van't Hoff equation, an chemical may then be transported some distance enthalpy of solution value AH can be obtained. The with the particulate. This may explain the finding of enthalpy value may be used as an approximate pesticides where they have never been used (13, 15, index of the tendency of a chemical to transport to 27, 30). Since the vaporization of a chemical from a the aqueous phase from the solid state, or dissolve. surface is related to its diffusion in the air, air cur- The XH values thus derived for a solution approxi- rents can increase the rate of vaporization. mate the heat of adsorption for a physical-type ad- Many chemicals in an aqueous system will sorption. evaporate simultaneously with the water; in other Thiolcarbamate herbicides show reversal in the words, codistill (44). Vapor loss of a pesticide from effect of temperature on solubility; solubility de- the soil system, for example, is accelerated by the creases with an increase in temperature. It has been presence of moisture. This has been shown with suggested that this behavior is due to hydrogen- such diverse materials as 2,4-D , thiolcarba- bond formation between water and the thiolcarba- mates, triazines, N-phenylcarbamates, and the or- mates. ganochlorine insecticide (24, 46, 51). Other factors which may contribute to the trans- Although vapor pressure of a chemical to a great port and the persistence of chemicals in an aquatic extent determines the entry of the chemical in the environment are the presence of clay or soil parti- atmosphere, caution must be exercised in interpret- cles or proteins and lipids, and the effects of ul- ing the data. The vapor pressure of a chemical can traviolet or other high-energy radiations causing a give a good estimate of air transport as long as the decomposition of the molecule. The presence in chemical is in the free state or is evaporating from water of soil particles of any nature (soil, clay, an inert surface. However, when the chemical is sand, or biocolloid) will reduce the concentration of bound to surface, the vapor pressure cannot be dissolved chemical by adsorption. The extent of ad- used as an index for vapor transport. It must be sorption will depend on the nature of the suspended pointed out that while studying the vapor loss of a particles and the temperature. 66 Environmental Health Perspectives Soil Behavior in most instances an exothermic process. UsLually a lowering in temperature means an increase in the The two major processes controlling the behavior adsorption. In general, for neutral adsorption of of chemicals in a soil matrix are adsorption and neutral organic compounds, the heat of adsorption leaching-diffusion (32-34, 38, 48). A change in the is in the range of only a few kilocalories per mole, moisture content of the soil-chemical system and of indicating a physical type adsorption or in some temperature will greatly influence both processes. cases weak hydrogen bonding between adsorbate A soil matrix represents a heterogeneous mixture and the surface. Formation of a chemical bond or of various constituents, namely organic matter, chemisorption has rarely been observed in neutral sand, clays, and inorganic salts. These sometimes pesticide-soil system (20, 37). For most of the present a large surface area with a number of sites neutral organic molecules the adsorption is of the on which adsorption can occur. Although the bulk physical type, in which there is first the formation of the adsorption may be from solution, adsorption of a monolayer on the surface followed by a build- to a certain extent also occurs from the chemicals up of multilayers. Hence, by analogy to the adsorp- present in the vapor state. tion of gases on solids, the heat of adsorption for The equilibrium in a chemical-soil system can be aqueous solution of a pesticide should be in the represented by Eq. (7): range of heat of solution. P(H20)x. + S(H.,O),, = P(H2,0)ZS (7) Transport in Soil where P and S represent the compound and soil matrix, respectively. With sufficient water present, Another important process which controls the both the chemical molecule and the soil matrix will transport of a chemical in a soil matrix is its move- be in the hydrated form. The symbols x, y, and z ment with water, a process termed leaching (12, 33, denote the hydration numbers of the chemical, the 34, 52). The leaching may take place in three direc- soil, and the complex, respectively. The equilib- tions: downward, since this is the usual direction of rium constant KE for the reaction is: water movement, but the lateral movement of the chemical in the soil with water and even upward KE = [P(H20)ZS] movement are sometimes significant. The upward [P(H20)X] [S(H20)] (8) movement, which is a result of mass transfer of water upward under the influence of evaporation Here the quantities in brackets represent the ac- fiom the surface, may concentrate a chemical at the tivities of the compound. An exact determination of soil surface, thus effectively removing it from the KE is difficult since an estimate of the exact volume root zone. occupied by the adsorbed species is nearly unattain- The movement of water downward in soil is able. Usually the adsorption data for a soil-chemical thought to be in the form of film and is produced by system are represented with a Freundlich isotherm: combined effects of capillary and gravitational forces. The chemicals are usually applied to the sur- xl/n = KC" (9) face of the soil; as water arriving at the surface where xlnz is the amount of chemical sorbed per penetrates, it encounters the chemical, dissolving weight of the absorbent, C is the equilibrium con- and carrying the chemical with it as it percolates centration of the chemical, and K and n are con- through the soil. stants. For a dilute solution of many compounds, The displacement of the chemical under rapid the value of n approaches unity. The constant n percolation of water is predominantly with the bulk throws much light on the nature of the adsorption, of the water solution. Counteracting this downward whereas K represents the extent of adsorption and movement is the tendency of isodiametric diffusion is related to the free-energy changes in the adsorp- of the chemical in solution. Where the water perco- tion. lation is rapid, the bulk movement of the chemical The adsorption also depends upon the nature of will be in the direction of water flow, but as water the chemical under investigation. Inorganic salts percolation becomes slower and slower, diffusion and organic cations adsorb on the clay portion of becomes a greater factor in determining the dis- the soil through an exchange reaction. Most neutral tribution of chemical. In other words, there is a organic molecules follow a physical type adsorp- dynamic equilibrium between the free chemical and tion, and the amount of chemical sorbed in many the chemical in the adsorbed phase as the chemical cases follows an inverse relation to its solubility (35, is carried through the soil profile by the movement 36). and the adsorbed phase. As a consequence, a chem- Adsorption of chemicals from aqueous solution is ical which is tightly adsorbed should be leached October 1977 67 slowly, and vice versa (37). Thus, the AH of solu- perienced. However, prudence would indicate that, tion gives an indication of a chemical's mobility in in so far as possible, further reduction or prevention leaching. of this exposure is to be sought rather than attempt- In practice the important factors controlling ing to deal with the health problem that may arise leaching are water solubility of the chemical, ad- from it. sorption, soil type, and moisture and percolation A body of knowledge is emerging and indicates a velocity. A highly soluble chemical having a low relationship between the properties of the chemical enthalpy of adsorption will be leached soon because and the transport and behavior in the environment. of its tendency to go into solution. As a conse- In many instances quantitative data and usable quence the amount of chemical carried in the soil mathematical relationship has been developed. On will be proportional to the amount of water avail- this basis it is felt that we are rapidly approaching able to dissolve the chemical. Temperature will the time where transport, behavior, and persistence play an important role in the leaching since it affects of a chemical in the environment can be predicted the solubility. from the properties of the chemical, method of use, The behavior of chemicals described in the and rate of degradation. Such predictions will be foregoing has dealt primarily with the different useful in evaluating possible exposure to man and components of subsystem of the total environment. the level at which this exposure may occur. Also, it What is desired is a more comprehensive picture of would serve as a basis fort designing systems of the movement and behavior in the total environ- handling, use, and disposal to minimize health and ment system taking into account the interactions of environimiental impacts. the different subsystems. Perhaps the most inten- The indirect impact of these chemicals on man is sive attempt to model behavior of a pesticide in the another area meriting attention. The alteration of total environment has been done with DDT. Harri- biologically mediated geochemical cycles or mod- son et al. (86), provided one of the early detailed ification of biota diversity indices can be of conse- models of the behavior of this chemical in an at- quence to man's health and well being. There is tempt to estimate transport through various trophic reason to believe that chemicals in the environment levels and possible persistence of the chemical. acting singly or in concert may bring about altera- Woodwell (30), in assessing a global model for tions of these factors with a consequence to man. transport and persistence of DDT, found the con- It would seem, therefore, that the area of trans- centration much lower in certain compartments port, behavior, and persistence of chemicals in the than would be predicted on the basis of the model environment, i.e. chemodynamics, warrants further utilizing the parameters available to him. The most attention and research. recent attempt to give a quantitative description or Further research on the relationship between the model of the circulation of DDT was done by properties of the chemical, characteristics of en- Kramer (87). He pointed out that rates of degrada- vironmental compartments (e.g., soil, air, and tion, transfer, and adsorption by plankton are criti- water) the transport, behavior, and persistence of cal factors in such a model. Recent monitoring data the material in relation to exposure to man is neces- would appear to suggest the rate of disappearance sary. Concomitantly, efforts should go forward to of DDT may be more rapid in the general environ- develop appropriate models of this behavior so as to ment than had been previously estimated. Others be able to assess exposure levels and devise the have given some attention to modeling behavior of appropriate protective measures. pesticides other than the organochlorines in the at- Development of a program for inventorying the mosphere, soil, and water. As yet, insufficient data chemicals in the environment, both as to kinds and has been accumulated to facilitate further develop- quantities, would lead to the knowledge necessary ment of these models. for development of the appropriate monitoring pro- grams. Assessment of direct, latent, and indirect effects Summary and Conclusions of environmental chemicals on man and the envi- ronmental processes of consequence to man's Quantities of chemicals in ever growing numbers health as these chemicals move from source to ul- are being manufactured and used and in the process timate sink is the ultimate goal. escape into the environment. As a result of this environmental contamination, man is being ex- The authors would like to acknowledge the invaluable techni- posed to low quantities of many of these chemicals cal assistance of Erika Saunders, Rodger Kohnert, and David Schmedding in the preparation of this report. either directly or through his food chain. In many Much of the work reported in this paper was supported by instances, little is known about the long-term con- grants ES 00040 and ES 00210 from the National Institute of sequences of such exposure at the low levels ex- Environmental Health Sciences. 68 Environmental Health Perspectives This material is drawn from a Background Document pre- 19. Freed, V. H., and Haque, R. Adsorption, movement, and pared by the author for the NIEHS Second Task Force for distribution of pesticides in soils. In: Pesticide Formula- Research Planning in Environmental Health Science. The Re- tions. W. van Valkenburg Ed., Dekker, New York, 1973, port of the Task Force is an independent and collective report p. 441. which has been published by the Government Printing Office 20. Haque, R., and Coshow, W. R. Adsorption of isocil and under the title, "Human Health and Environment-Some Re- bromacil from aqueous solution onto some mineral surfaces. search Needs." 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