Vapor Density of Soil-Applied Dieldrin as Related to Soil-Water Content,Temperature, and Dieldrin Concentration1

W. F. SPENCER, M. M. CLIATH, AND W. J. FARMER2

ABSTRACTVapor densities of dieldrin (principal constituent HEOD-

hexachloro-epoxyoctahydro-endo, exo-dimethanonaphthalene) indieldrin-soil mixtures increased with temperature and diel-drin concentration but were not affected by soil-water contentuntil the water content decreased below that equivalent of onemolecular layer of water. Vapor densities dropped to very lowvalues when the water content fell below this level, but in-creased again as water was added to the dry soil, indicatingthat the drying effect is reversible. When more than a mono-molecular layer of water was present in the soil, vapor densityincreased with increasing soil dieldrin (HEOD) concentrationuntil a saturation vapor density equal to that of HEOD withoutsoil [54, 202, and 676 ng HEOD/liter at 20, 30, and 40C,respectively (9 ) ] was reached at approximately 25 ppmHEOD. This implies that surface applications of dieldrin andprobably other similar chlorinated hydrocarbon insecticideswill volatilize as rapidly from mineral soils as from the purematerials until the concentration at the surface falls to rela-tively low levels.

The data indicate that loss of water, per se, is not requiredfor significant rates of volatilization to occur from soils or othersurfaces on which water can successfully compete for adsorp-tion sites.

Additional Key Words for Indexing: insecticide residues,volatilization, pesticides.

DIELDRIN is one of the more persistent chlorinatedhydrocarbon insecticides and often becomes a soil

residue problem. It is important to understand the soil andenvironmental factors that affect this persistence in orderto develop better means of enhancing its dissipation fromsoils. Considerable evidence indicates that volatilizationfrom the soil surface may be an important pathway for lossof dieldrin and other such persistent insecticides (4, 7) .Acree, Bowman, and coworkers (1, 2, 3) reported thatloss of water contributed to insecticide volatilization by anapparent "codistillation" process. Spencer and Cliath (9)reported that the vapor densities associated with solid-phase dieldrin (HEOD) and dieldrin-soil mixtures mea-sured by a gas saturation technique were three to 12 timesgreater than predicted from published vapor pressurevalues (8). Their measured vapor densities were 54, 202,and 676 ng HEOD/liter equivalent to an apparent vaporpressure of 2.6 X 1O6, 1.0 X 1CH5, and 3.47 x 10'5

mmHg at 20, 30, and 40C, respectively. The vapor densityof dry HEOD was the same as that of HEOD plus water

1 Contribution from the Southwest Branch, Soil & Water Con-servation Research Division, ARS, USDA, and the CaliforniaAgr. Exp. Sta., Riverside, Calif. Presented before Div. S-l andS-2, Soil Science Society of America, Nov. 13, 1968, New Or-leans, Louisiana. Received Jan. 27, 1969. Approved March19, 1969.

2 Soil Scientist, Chemist, USDA, and Asst. Chemist, UCR,Riverside, Calif.

and the vapor density of HEOD in moist soil at 100 ppmwas the same as that of HEOD without soil; however, at10 ppm the vapor density was reduced approximately80%. The heat of vaporization of HEOD, either with orwithout soil, was 23.6 K cal/mole.

The objective of the present work was to pinpoint thefactors affecting volatilization and movement of chlorinatedhydrocarbon compounds, such as dieldrin, in soils. Thispaper presents data relating solid-phase dieldrin concen-tration in soil to vapor density as affected by the soil-water content and temperature, and discusses the impli-cation of the results to volatilization from soils and todiffusion through soils.

METHODS AND MATERIALSVapor density of soil-applied dieldrin as affected by dieldrin

concentration, soil-water content, and temperature was mea-sured by a gas saturation method. The details of the apparatusand procedures have been previously described (9) . Briefly, inthe gas saturation method a current of inert gas is passedthrough or over the material at a sufficiently slow rate to insureequilibrium vapor saturation. In our studies the vapor densityof recrystallized dieldrin, 99% HEOD (1,2,3,4,10,10-hexa-chloro-6,7-epoxy-l,4,4a,5,6,7,8,8a-octahydro-l,4-endo,exo-5,8-dimethanonaphthalene), was determined by measuring theamount of HEOD in a stream of nitrogen gas slowly movingthrough a column of soil containing various concentrations ofHEOD. The HEOD was removed from the nitrogen gas streamin gas-washing bottles containing hexane. The HEOD contentof the hexane was determined with a gas-liquid chromatographequipped with an electron-capture detector.

The Gila silt loam used in these studies is a desert soil con-taining 18.4% clay—predominately montmorillonite—0.6%organic matter, with a surface area of approximately 90 mVgand an exchange capacity of 18 meq/100 g. Dieldrin in acetonewas added to moist, autoclaved soil with an atomizer. The soilwas aerated with moist air to remove the acetone, then adjustedto the desired water content by adding a predetermined amountof water with an atomizer, or as a weighed amount of finelyground ice at temperatures below freezing. After packing thecolumns with approximately 900 g of treated soil, they wereincubated at 30C for 30 days before measurements were initi-ated. Nitrogen gas flow rates of from 3 to 6 ml/min were gen-erally used to provide a total flow through the saturator offrom 10 to 80 liters. In all measurements with soils at differentwater contents, the humidity of the nitrogen carrier gas wascontrolled in equilibrium with the moisture content of the soils.This resulted in no net loss, or gain, of water from the soil col-umns during the periods of measurement. Vapor density at 20,30, and 40C was determined on each column made up at a par-ticular HEOD concentration and/or water content. From threeto nine measurement? were made on each set of columns at eachtemperature, which resulted in an average coefficient of varia-tion in vapor density of 6%.

RESULTS AND DISCUSSIONDesorption isotherms relating vapor density and HEOD

concentration in Gila silt loam at water contents of 10%or greater are shown in Fig. 1. At all temperatures, vapordensity increased rapidly as concentration increased until

509

510 SOIL SCI. SOC. AMER. PROC., VOL. 33, 1969

25 50 75 100 HEOOHEOO IN SOIL, ppm ONLY

Fig. 1—Vapor density of HEOD (dieldrin) in Gila silt loamat water contents of 10% or greater as affected by tempera-ture and concentration of HEOD.

a saturation vapor density, equal to that of HEOD withoutsoil, was reached at concentrations near 25 ppm HEOD.Vapor density of soil-applied HEOD increased with tem-perature in the same manner as HEOD without soil. Thiscorroborates the earlier finding by Spencer and Cliath (9)that heats of vaporization for HEOD in soil and HEODonly are similar.

Figures 2 and 3 show the effect of soil-water content onvapor density of HEOD in Gila silt loam at 100 ppm and10 ppm HEOD, respectively. At 100 ppm HEOD the soil-water content had essentially no effect on vapor density,until the soil was dried to a water content approaching 1molecular layer of water. When the water content droppedonly slightly below this to 2.1%, the vapor density de-creased markedly. The vertical line in Fig. 2 and 3 at 0.028g water/g soil indicates the calculated amount of waterequivalent to a monomolecular layer, assuming a surfacearea of approximately 90 m2/g. In Gila silt loam 17%water is equivalent to field capacity, 10% water is equiva-lent to approximately 2 atm matrix suction, and 3.94%water to 94% relative humidity or 90 atm suction. Thus thesoil is extremely dry before the vapor density decreasesappreciably. The vapor density approached a concentrationnear the lower limit of measurement when the soil wasair dried to 1.6% water. However, this drying effect isreversible. Upon rewetting the air-dry soil vapor densityincreased to its original maximum value. For example, at100 ppm HEOD and 40C vapor density of HEOD in theair-dry soil was 1.8 ng/liter. When water was added, to17%, the vapor density immediately increased to 710 ngHEOD/liter.

At 10 ppm HEOD the vapor density begins to decreaseat a slightly higher moisture content than at 100 ppmHEOD, as illustrated by lower vapor density at 3.9 than

'200-1

11

' i3 T>'02

0 n

,''U' HEOD

ONLY

' 30°C * HEODONLY

0.04 006 008 010 012 0.14 0.16 0.18SOIL WATER CONTENT, g/q

Fig. 2—Effect of soil-water content on vapor density of HEOD(dieldrin) in Gila silt loam at 100 ppm HEOD.

150

QOUJ

zO 50OLO

ESTIMATED 40°C

30°C

0.04 0.06 008 010 0.12SOIL WATER CONTENT, q/g

0.14 0.16

Fig. 3—Effect of soil-water content on vapor density of HEOD(dieldrin) in Gila silt loam at 10 ppm HEOD. Vapor den-sity at 40C and 10% water content estimated from otherHEOD concentrations at 10% water.

O 20 40 60 80 100HEOD IN SOIL, ppm

Fig. 4—Relative vapor density of HEOD (dieldrin) vs con-centration of HEOD in Gila silt loam as affected by soil-water content. Vapor density of HEOD without soil equals1.0.

SPENCER ET AL.: VAPOR DENSITY OF SOIL-APPLIED DIELDRIN 511

at 10% water in Fig. 3. This is probably due to the inter-action between moisture and HEOD in competition foradsorption sites on the soil.

The great difference in vapor density between wet anddry soils could conceivably result in significant aerial trans-fer of chlorinated hydrocarbon insecticides from relativelywet treated areas to dry untreated fields.

Figure 4 combines all variables studied—HEOD con-centration, water content, and temperature into a general-ized picture of the relationship between concentration inthe soil and vapor density. Relative vapor densities or theratio of the vapor density of soil-applied HEOD to vapordensity of HEOD without soil was calculated using all dataobtained at temperatures of 30 and 40C. These relation-ships should hold at any temperature for this particularsoil. Vapor density increases linearly with concentrationuntil essentially a saturated vapor density is reached. Thecurve for 10 or 17% water would apply to most field situa-tions, particularly during the growth of a crop. The right-hand curve in Fig. 4 was obtained at 3.94% water, or 90atm matrix suction. As the soil continues to dry below thislevel to air dryness, the curves will approach the X axis,or negligible vapor density. At 2.1% water, equivalentto 50% relative humidity in this soil, the relative vapordensity was just above the horizontal line even at 100ppm HEOD.

The very marked effect of small amounts of water onvapor density of dieldrin in the dry soil range would explaingreater volatilization of chlorinated hydrocarbons from wetthan from dry soils. Apparently, water increases the vapordensity in this very dry range due to competition for ad-sorption sites on the soil. When sufficient water is presentto cover the surface, the dieldrin volatility approaches thatof the pure material. There is no evidence from our datato indicate that the evaporation of water, per se, increasesvapor density by a "codistillation" process. The actualamount of dieldrin volatilized during a given period, a weekfor example, would be related to the time it takes to drythe soil sufficiently to reduce the vapor density to an insig-nificantly low value. Competition between water and thechlorinated hydrocarbons for adsorption sites would alsooccur on other surfaces and the effect of humidity on theirvolatility from surfaces such as glass, metal, or skin couldbe explained on the basis of the higher humidity furnishingwater molecules to displace the hydrocarbons from thesolid surfaces. The fact that water also had no effect onvapor density of HEOD without soil (9) is additional evi-dence to indicate that water loss does not increase vapordensity.

The fact that the vapor density and heat of vaporizationof HEOD applied to a slightly moist soil at 25 ppm, orgreater, is the same as HEOD without soil indicates thatthe adsorption forces between HEOD and soil are quiteweak and probably the HEOD is present as globules oradsorbed at the air-water interface. Since HEOD is a rela-

tively nonpolar molecule it would not be expected to bestrongly adsorbed by soil mineral surfaces. Therefore, sur-face applications of dieldrin and probably other similarchlorinated hydrocarbon insecticides will volatilize asrapidly from mineral soils as from the pure materials untilthe concentration at the surface falls below approximately25 ppm. The high vapor density, until relatively low con-centration levels are reached, indicates that dissipationcurves for dieldrin from moist mineral soils would be simi-lar to those postulated by Gunther and Blinn (6) for thedisappearance of insecticides on and in plant tissues. Intheir models, relatively high rates of loss by volatilizationare predicted soon after application of the materials to theplant surfaces. The loss from soils would probably besimilar, but on a somewhat different time scale.

Diffusion of a volatile compound through soil can beeither in the vapor or nonvapor phase. The apparent vapordiffusion coefficient is proportional to the slope of thevapor density-concentration curve. Thus, the data pre-sented in Fig, 1 and 4 aid in the calculation of the apparentvapor diffusion coefficient. The linear relationship betweenvapor density and concentration of HEOD in soil meansthat the ratio of vapor density to soil concentration is aconstant, therefore the apparent vapor diffusion coefficientis constant over this concentration range (5). Differentsoils and different insecticides will result in different slopesof lines relating vapor density to concentration in soils, andit would appear to be worthwhile to determine these rela-tionships for other soils, particularly those higher in organicmatter, and for other insecticides.