Influence of Water and Temperature on Adsorption of Parathion by Soils1

BRUNO YARON AND SARINA SALTZMAN2

ABSTRACTTemperature and water effect on the adsorption of parathion

(O, O-diethyl O-p-nitrophenyl phosphorothioate) by soils wasstudied using 14C-tagged parathion. The parathion was appliedin aqueous solutions and in hexane over a concentration rangeof 1-10/tg/ml. It was found that the parathion is adsorbed bysoils from aqueous solution without any appreciable thermiceffect. In a dry soil-hexane-parathion system the slightly polarparathion molecules efficiently compete with the apolar hexanemolecules for adsorption sites. In hydrated soil-hexane-parathionsystems there was no parathion adsorption. Generally as the soilwater content increases, parathion adsorption decreases. Thewater-temperature composite effect in a soil-hexane-parathionsystem was also studied. The rise of temperature in the soil-hydrated parathion-hexane system favors the endothermicprocess of water desorption, resulting in greater free adsorptionsites, and consequently, in increasing parathion adsorption.

Additional Index Words: soil pesticide residues, escapingtendency, competitive adsorption.

PRECIPITATION and temperature are very important cli-matic factors in determining soil-pesticide relationships.

The study of their effect on soil-pesticide interactions maypromote an understanding of the nature of pesticide adsorp-tion and give information pertinent to the forecasting ofpesticide transport into soil and groundwater, as well aspermit practical conclusions on the application scheduling.Such information is of particular interest in arid and semi-arid regions, where, under irrigation, large fluctuations insoil water content and temperature occur over short periodsof time.

One of the most extensively used organophosphorus in-secticides for the control of a very wide range of pests isparathion (O,0-diethyl-O-p-nitrophenyl phosphorothioate).Despite its widespread use in agricultural practice, little isknown on soil-parathion interaction. One of the first studieson parathion adsorption by soils was carried out by Kingand McCarthy (1966, 1968). Using very dilute aqueoussolutions of the chemical (< 1 ppm), they found thatparathion adsorption was a function of the clay content ofthe soils, and could be adequately described by the Freund-lich adsorption equation. Swoboda and Thomas (1968)found that three different soils exhibited linear adsorptivecapacities for parathion, at initial aqueous solution concen-trations of up to 16.5 ppm. Saltzman and Yaron (1971)studied parathion adsorption from dilute aqueous solutions(1-10 ppm) on a broad spectrum of soils; they indicatedthe influence of soil mineralogy and organic matter con-tent on the adsorption process.

1 Contribution from the Agricultural Research Organization,Volcani Center, Bet Dagan, Israel. 1971 Series no. 2095-E.This research has been financed in part by a grant made by theUS Department of Agriculture, ARS, authorized by PublicLaw 480. Received Feb. 2, 1972. Approved April 4, 1972.2 Senior Scientist and Scientist, respectively, Institute of Soilsand Water, Agr. Res. Org., Volcani Center, Bet Dagan, Israel.

However, these data on parathion adsorption by soils arevalid only for highly hydrated systems; there is little infor-mation on parathion adsorption as influenced by water con-tent and temperature fluctuations. Some indications aregiven in the bioassay studies of Lichtenstein and Schultz(1959), who observed that parathion was more persistentin dry than in moist soil, and in the bioassay study of Har-ris (1959), who noted an increasing toxicity of parathionwith increasing soil moisture content. Information on theinfluence of water and temperature on adsorption-desorp-tion of another two weakly-polar insecticides, such as lin-dane and dieldrin, were recently reported by Mills andBiggar (1969a, 1969b), Spencer et al. (1969), and Spencerand Cliath (1970).

The objectives of this study were: (i) to investigate theinfluence of temperature on parathion adsorption by soilcolloids, by considering the temperature-solubility effecton adsorption; (ii) to study the influence of water on ad-sorption by comparing the competitivity of parathion withwater and a nonpolar solvent for adsorption sites; and (iii)to study the composite effect of water and temperature onthe adsorption process.

MATERIALS AND METHODSSampling of three soils widely found in Israel, having differ-

ent soil mineralogy and texture, were taken from the upperlayers only; analytical data on the soils are given in Table 1.The soil samples were ground to a particle size which couldpass through a 60-mesh sieve, oven-dried, and stored in a vac-uum dessicator over P2O5.

In all the experiments, pure parathion (Analabs Inc., NorthHaven, Conn.) and 14C-labeled parathion (Amersham Radio-chemical Centre) were used. The purity of the material waschecked periodically by gas chromatography; no decompositionproducts were detected during the experiments. For counting14C activity, a Packard 3003 Tri-carb liquid scintillation spec-trometer was used. The scintillation liquid consisted of: 50 gnaphthalene, 7 g PPO (2.5 diphenyloxazole), and 0.05 gPOPOP [2,2-p-phenylenebis (5-phenyloxazole)] brought to 1liter with dioxane. A high speed centrifuge was used for sampleseparation at constant temperature.

The adsorption equilibria were studied by batch equilibriumexperiments using aqueous and hexane solutions of parathion,at concentrations ranging between 1-10 ppm. The soil-to-solu-tion ratio was 1:20. Preliminary kinetic measurements showedthat equilibrium is quickly reached, so that an equilibrationtime of 60 min was arbitrarily selected. After equilibration ina constant temperature bath, the suspensions were separated bycentrifugation, since a preliminary test showed that filter paper

Table 1—Composition of soils used in experiments

Soil

Har BarqanCalcareous reddish-brown alluvial grumsol

NetanyaSandy clay loam hamra

MivtahimSandy regosol

ClayOrganicmatter CEC PH

——— % —— meq/lOOg

56

14

6

1.9

0.7

0.3

63

8

4

7.8

6.3

8.0

SurfaceareamVg

410

90

39

Predominantclay

Montmorillonite

Kaolinite +Montmorillonite

Montmorillonite

583

584 SOIL SCI. SOC. AMER. PROC., VOL. 36, 1972

Table 2—Soil water content at different relative humidities

SoilHar Barqan

Calcareous reddish-brown alluvialgrumusol

NetanyaSandy clay loam

Mlvtahlm~" Sandy'regosol

15%

5.22

0.70

0.46

Relative32%

7.77

1.07

0.66

humidity51%

9.80

1.62

0.90

98%

17.44

3.18

1.71

retained parathion. During centrifugation the samples were keptat the same temperature as during the batch equilibrium experi-ments. Concentrations of parathion in the solution were deter-mined and the amount adsorbed calculated by difference.

To study the effect of temperature on adsorption, batchequilibrium experiments were carried out at 10, 30, and SOC.The solubility correction factor was determined for these tem-peratures by adding parathion in excess to water or hexane inglass-stoppered bottles (10 mg to 50 ml water, and 100 mg to50 ml hexane). The bottles were placed in the constant-temper-ature bath and shaken for 1 hour. Samples were then filteredthrough a fritted glass filter and parathion concentration in thefiltrate determined.

Soil samples were also equilibrated in dessicators over satu-rated salt solutions to different relative humidities (15%, 32%,51%, 98%). The soil water content at these relative humiditiesis listed in Table 2.

Hexane was chosen as solvent for the humidity effect studiesafter preliminary experiments on parathion adsorption by soils,employing a wide range of organic solvents including: meth-anol, ethanol, acetone, chloroform, ethyl acetate, dioxane, ben-zene, and hexane. Parathion was adsorbed by dry soils onlyfrom benzene and hexane solutions, and in greater amountsfrom hexane solutions.

RESULTS AND DISCUSSIONTemperature Effect

It is generally known that the adsorption process is re-lated to the liberation of thermal energy. Kipling (1965),discussing the case of solute adsorption on solid surfaces,showed that the decrease in adsorption with increase intemperature may correspond not only to a weakening ofthe attractive forces between the solute and the solid sur-face, but also to an increase in solubility of the solute inthe solvent. That is, if the solute is considered as distributedbetween the adsorbed layer and the solution in a partitionequilibrium, this equilibrium will be displaced in favor ofthe solution, when the temperature rises. It is suggested thata parameter to be used in defining the activity, or the escap-ing tendency of the solute, is the "reduced concentration."By reduced concentration is understood the equilibriumconcentration of the solution divided by the solubility ofthe solute in the solvent, at a specific temperature. Conse-quently, from "reduced" or "corrected" adsorption iso-therms at different temperatures, it may be possible todistinguish between the influence of temperature on theadsorption energy and the solubility effect.

In Fig. 1 are presented "corrected" adsorption isothermsfor soil-water-parathion systems. It may be noted that foreach soil, the adsorption isotherms for different tempera-tures overlap, showing that parathion is adsorbed by soilswithout any appreciable thermic effect. Therefore it is pre-sumed that only weak adsorptive forces are implicated inthe adsorption process. Mills and Biggar (1969a) studyingthe influence of temperature for lindane adsorption by soilssimilarly found that "the adsorption process which is indi-cated to be exothermic by the normal isotherms is at least

120

100

80

60

40

20

0100

80

60

40

20

0

— MIVTAHIM9

HAR-BARQAN

NETANYAC/Co

O10 °C

• 30°CA50°C

1 2C/Co

Fig. 1—Corrected adsorption isotherms of parathion fromwater at 10, 30, and SOC.

partially so, because of the solubility-temperature interac-tion."

For practical purposes it is more useful to consider the"real" effect of temperature on adsorption than the "cor-rected isotherms." In the normal adsorption isothermsshown in Fig. 2, in all the three soils, a decrease may beobserved in parathion adsorption with increase in tempera-ture. From the linear adsorption isotherms at SOC and SOC,the differences in adsorption at these temperatures seem tobe quite small. The adsorption at 10C is considerably great-er, especially at the higher solution concentrations. Since theparathion solubility at 10C is only 4.3 ppm, the adsorptionisotherms may not be valid measures of the amount ad-sorbed above this value.

Water Effect

Since the water content of the soil in arid and semiaridzones under irrigation fluctuates greatly, it is evident thatpesticide adsorption is occurring under conditions wherethe adsorption sites are initially, totally, or partially occu-pied by water. Therefore, soil-organic compound inter-action involves, besides the adsorption of the organic com-pounds on the free surfaces, the interactions with water inthe adsorbed layer or displacement of water from the sur-face (Greenland, 1970). In a recent review on clay-organiccomplexes, Mortland (1970) considers that the water con-tent of the system is one of the principal factors in deter-mining the nature of the interaction between nonionic,polar, organic molecules and clays. He points out that theadsorption mechanism in aqueous suspensions is quite dif-ferent from that which occurs in dry systems.

In the present study, parathion adsorption from a non-polar organic solvent and from aqueous solutions by dry,partially hydrated and hydrated soils was considered.

Parathion Adsorption in Dry Systems—Data on adsorp-

YARON & SALTZMAN: ADSORPTION OF PARATHION BY SOILS 585

i I

a,glOOr—

I I

3 4C jjg/ml

O10°C• 30°CA50°C

Fig. 2—Normal adsorption isotherms of parathion from waterat 10, 30, and 50C.

tion of parathion from hexane solutions by dry soils at 30Care presented in Fig. 3. The adsorption isotherms are linear,for Netanya and Har Barqan soils, and is exponential inthe case of the Mivtahim soil. In the range of solutionconcentrations used, the experimental soils showed a highaffinity for parathion. Mivtahim soil adsorbed 83%, Ne-tanya soil 90%, and Har Barqan soil 100% of the para-thion in the initial solutions. This may be explained by thefact that in dry soil-hexane-parathion systems, the slightlypolar parathion molecules compete efficiently with apolarhexane molecules for adsorption sites. Since in a prelimin-ary experiment it was observed that the parathion was notabsorbed on an organic material from hexane solutions, itmay be presumed that the differences in the adsorptive ca-pacity of the soils is due only to their mineral colloid con-tent. Distribution coefficients (/ig adsorbed/g adsorbent atan equilibrium concentration of 1 /xg/ml) for Netanya andMivtahim soils were 220 and 110, respectively, the differ-ence being reasonably proportional to the difference in theclay content of these soils. For Har Barqan soil, in therange of solution concentration used, all parathion is dis-tributed in the solid phase.

Parathion Adsorption in Partially Hydrated and HydratedSystems—Figure 4 shows parathion adsorption from hexanesolution by the three soils as a function of the initial soilwater content. On increasing the relative humidity from 15to 32%, for example, the amount of parathion adsorbedby the Har Barqan soil decreased by 24%, in Netanya soilby 16%, and in Mivtahim soil by 15%. Generally, as thesoil water content increases, parathion adsorption decreases.

The relative parathion adsorption at different soil watercontents was noted A/A0 where A is the amount adsorbedby the partially hydrated soil and A0 the amount adsorbedby the dry soil. The relative free soil surface was notedSA/SA0 where SA is the surface area of the partially hy-drated soil and SA0 the surface area of the dry, soil. Whenplotting A/A0 vs. SA/SA0 at the same soil humidities(Fig. 5), a rather good correlation is obtained (/- = 0.6).This indicates that in partially hydrated systems parathionmolecules do not remove the strongly adsorbed water, sothat parathion adsorption occurs mainly on water-free sur-

200

160

120

80

'>*

> I I I

'I' >' -' ' /

~'V °^ —,1 A HAR-BARQAN

40 T*- • NETANYA -UI4f O MIVTAHIMP i l l

0 2 4 6 8C jjg/ml

Fig. 3—Adsorption isotherms of parathion from hexane ondry soils (30C).

Fig. 4—Adsorption isotherms of parathion from hexane onpartially hydrated soils (30C).

80

60

40

20

f

I I I IA HAR-BARQAN A ,

_ • NETANYAO MIVTAHIM 9

~ A

O

s~ A

0k I I I I

—— p

-

—

-

20 80 10040 60sA/sAor/.)Fig. 5—Parathion adsorption from hexane as a function of the

free surface area.(Ao = adsorbed on dry soil; A = adsorbed on par-tially hydrated soil [percentage of initial solution];SA0 — soil surface area; and SA = free surface area

at different water contents).

586 SOIL SCI. SOC. AMER. PROC., VOL. 36, 1972

O 20 40 60 80—I RELATIVE HUMIDITY (%)

010°C• 30°CA50°C

20 40 60 80 10RELATIVE HUMIDITY Cl.)

Fig. 6—Composite effect of water and temperature on soilparathion adsorption from hexane solutions.

faces. Similar results were obtained by Spencer et al. (1969,1970) for lindane and dieldrin adsorption. It was foundthat the vapor density of these insecticides is high whenmore than a monomolecular layer of water was present insoil and decreased markedly when the soil water contentdropped below this value.

In hydrated soil-hexane-parathion systems, i.e. soils equil-ibrated at 98% relative humidity, there was no parathionadsorption. However, as was previously shown (Fig. 2),parathion is adsorbed in such systems from aqueous solu-tions. Thus, it appears that different mechanisms governparathion adsorption in aqueous soil suspensions and in hy-drated soil systems in which the pesticide is dissolved in anonpolar organic solvent. It may be presumed that in soil-water-parathion systems, the solvent is preferentially ad-sorbed, but small amounts of parathion diffuse through thewater pelicules, and as they approach the colloid surfacesthey are adsorbed. It was shown that in such cases, an ion-dipole or coordination interaction occurs between the ad-sorbed non-ionic, polar molecule and clay, either directlyor through a water bridge (Mortland, 1970). In hydratedsoil-hexane systems, the liquid phase is actually composedof two immiscible liquids, the parathion is distributed onlyin the organic solvent, and cannot approach the adsorptivesurfaces.

Water-Temperature Composite EffectThe composite effect of water and temperature on para-

thion adsorption by soils was studied in soil-hexane-para-thion systems, at different humidities (corresponding to O,15, 32, 50, and 98% relative humidity), and at differenttemperatures (10, 30, SOC). Data are presented in Fig. 6where parathion adsorption (% of initial concentration) isplotted against the relative humidity.

It may be observed that the effect of temperature onparathion adsorption from hexane solutions is quite differ-ent from that in adsorption from aqueous solutions. In

aqueous soil suspensions, parathion adsorption decreases asthe temperature increases; in soil-hexane-parathion systems,the adsorption equilibria are rather similar at IOC and 30C,while adsorption increases strongly at 50C. It is worth not-ing that in the case of hexane solutions, in the range ofconcentrations used, it is not a question of "reduced con-centration" since, as preliminary experiments showed, tem-perature variations do not influence parathion solubility inhexane, even at a parathion concentration 200 times thatused in this experiment.

In the soil-hexane-parathion system at SOC the initialwater content has much less influence on adsorption thanat IOC and 30C. It is presumed that this is mainly due toan increase in mobility of adsorbed water. The rise of tem-perature favors the endothermic process of water desorp-tion, resulting in greater free surfaces, and consequently, inincreasing parathion adsorption. For Netanya and Mivtahimsandy soils, for instance, when the water content is increasedfrom zero to a monolayer, parathion adsorption at SOC de-creases only 16 and 24%, respectively, while at IOC and30C the decrease is 100%. For the clay soil of Har Bar-qan, the decrease in adsorption with increase in water con-tent at SOC is more pronounced (63%), showing a strongerretention of water at this temperature.