BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to the extraction of chemical pollutants from the soil, and, more particularly, to the selection of specific surfactants to extract specif- ic pollutants from a given soil.

2. Description of Related Art

The clean-up of all the hazardous waste sites in Amer¬ ica is an enormous task. Not only is the number of sites growing each year, but also the clean-up of the sites is difficult both technologically and economically. The num¬ ber of CERCLA (Comprehensive Environmental Resource Conser¬ vation Liability Act) Superfund sites on the National Pri¬ ority List is large (1,236 at the present time) and growing each day. Increasing legal restrictions on clean-up prac¬ tices make it even more difficult to comply with responsi¬ bilities. Landfills no longer accept hazardous wastes. Soil aeration is prohibited in most populated regions. Pump and treat methods have been ineffective in many cases. Rules against burying and discarding wastes have prolifer¬ ated. Treatment plans and schedules must be submitted to the EPA, the local agencies, and to the public for approv¬ al, which can impose more constraints on remediation choic¬ es. In response to increased environmental concern, numer¬ ous innovative clean-up technologies have been proposed by both public agencies and private companies. Table I below

shows extraction and destruction techniques which have been proposed for polychlorinated biphenyls (PCBs). Extraction and destruction of toxic chemicals are accomplished through chemical, thermal, or mechanical means. A cost effective combination of both extraction and destruction is desired. Effectiveness and cost considerations are high on the list of priorities, so any proposed clean-up approach must be more effective and cost less than existing technologies.

Table I Treatment Methods.

Extraction Techniques Destruction Techniques Solvents Electron beam Steam heating Solvent dechlorination Surfactants Biore ediation Supercritical fluids Incineration Pump & treat

The difficulty in cleaning contaminated soils today is that, depending on the contaminant, acceptable contamina¬ tion limits are in the range of parts per million or less. Most of the technologies currently used in soil remediation are either not capable of meeting these standards or are not feasible for economic reasons.

Very high concentration sludges may be effectively decontaminated by thermal destruction techniques such as incineration, or plasma torch, or electron beam irradia¬ tion. But low level contamination spread out over hundreds of thousands of cubic yards of soil requires other, less costly methods. Here, chemical or biological treatment technologies would take longer, but would cost orders of magnitude less than incineration. Although bioremediation is difficult to engineer, both bioremediation and soil washing impose the least harm to the environment. These latter techniques are also potentially the least costly.

Current technology can be used to enhance these techniques so they can reduce contamination to the regulated levels.

Like all remediation techniques, surfactant treatment also has its advantages and disadvantages. It is inexpen- sive, non-toxic, and removes and concentrates pollutants before destruction. However, although extraction takes place, there is no destruction of the contaminant, the in situ extraction process can be slow, and removal of large contaminant concentrations can be impeded unless large scale earthmoving, grinding, and mixing operations are per¬ formed. Many experiments rely on solubility to predict performance, only to find that upon application to a real site, solubilization of pollutants varies dramatically with soil conditions. Thus, some surfactants are not as effec- tive in the field as expected based on lab analysis. Other problems which emerge in field tests are enough to stop pursuits with this technology.

The distribution of contaminants in soils depends upon the porosity of the soil and the actual mineral content of the soil. Contaminants adhere to soil constituents with varying strengths. Presently available are a variety of practical and empirically-derived tools which provide yard¬ stick measurements to surfactant performance. Some of these tools include hydrophilic-lipophilic balance (HLB) values, solubility results, previous experimental results, etc. The problem with these tools is that they all require numerous experimental tests of a variety of surfactants, most of which are chemical formulations of unknown (to the user) compounds. The use of physical models helps to pro- vide a more realistic and quantifiable understanding of surfactant action. A coherent evaluation of the chemistry and physics of adhesion can reduce, if not eliminate, poor surfactant selections.

Thus, a need remains for a predictable method of de- termining the appropriate approach to removing contaminants in the soil using surfactants.

SUMMARY OF THE INVENTION

In accordance with the invention, a method is provided for selecting the appropriate surfactant, or surfactants, for the removal of a given contaminant from a specific soil. There are three aspects to surfactant selection: characterization of the soil, contact angle measurements to determine the surface energies of pollutants on soils, and estimation of the chemical nature of the surfactant which would provide effective removal.

Application of the method of the invention character¬ izes the polar and non-polar contributions of the surfac¬ tants needed to extract the particular contaminant from the soil. The structure of the surfactant is dictated from these polar and non-polar forces. Selection of surfactants can be predicted from the actual chemistry of the soil-pol¬ lutant system, thus reducing time and effort spent on nu¬ merous experimental trials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram depicting the solubility and formation of micelles;

FIG. 2 is a cross-sectional view, depicting a drop of liquid on a solid surface, and showing the contact angle; and

FIGS. 3a and 3b are schematic diagrams depicting the significance of contact angles in oil removal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Solubilization of contaminants is a multi-stage physi¬ cal process. FIG. 1 schematically depicts the roll-back mechanism for removal of oil 10 which is adhered to rock 12. The oil 10 is surrounded by surfactant molecules 14 oriented so that the lipophilic end 14a lines up towards the oil 10, while the hydrophilic end 14b is surrounded by

water (which surrounds the assembly shown in FIG. 1, but which is not depicted) . The surfactant 14 acts to initiate drop formation, followed by necking and eventual oil remov¬ al and the formation of micelles. The present invention is directed to the initial re¬ lease rather than the formation of micelles in solution. In particular, the present invention is directed to the selection of one or more appropriate surfactants to remove contaminant(s) from a given soil. There are three aspects to surfactant selection: char¬ acterization of the soil, contact angle measurements to de¬ termine the surface energies of pollutants on soils, and estimation of the chemical nature of the surfactant which would provide effective removal. Specifically, a full characterization of the contami¬ nated site soil mineralogy is required. Then, measurements of the contact angle between the contaminants of interest and three types of surfaces are necessary. ^" These measure¬ ments provide the surface tension contributions to the in- teraction energy between the contaminant and the soil. To determine the adhesive force between surfactants and con¬ taminant, interfacial energy measurements between surfac¬ tant (polar and non-polar groups) and test surfaces must be made. The test surfaces may be the same as those for de- termining the interaction energy between the contaminant and the soil or they may be different. The important as¬ pect is to obtain well-defined values.

Interfacial tension between surfactants and the soil can be computed to assist in evaluating whether selected surfactants may adhere to soils. Surfactant non-polar and polar surface tension contributions (based on surfactant chemical structures) are evaluated in terms of the total free energy of interaction between the contaminant and sur¬ factant versus contaminant and soil to predict the perfor- mance of existing and novel surfactant molecules.

Application of the method of the invention will char¬ acterize the polar and non-polar contributions of the sur-

factants needed to extract the particular contaminant from the soil. The structure of the surfactant will be dictated from these polar and non-polar forces. Selection of sur¬ factants can be predicted from the actual chemistry of the soil-pollutant system, thus reducing time and effort spent on numerous experimental trials.

Interactions of surfactants with clays, silica, and other soil minerals can be predicted from the teachings of this invention. The surfactant selection methodology is described be¬ low. It includes contributions from surface chemistry .and applies them to surfactant performance for the remediation of soils. The method of the invention provides for both the determination of the surface adhesive forces between the contaminant of interest and the soils in which the con¬ taminant(s) lie, and the determination of the surface adhe¬ sive forces between the surfactant and contaminant and be¬ tween the surfactant and soils.

Having selected the surfactant (or combination of surfactants) based on the teachings herein, any of the well-known in situ and on-site reactor soil remediation processes and apparatus may be employed to treat the contaminated soil. These extraction processes and appara¬ tus are well-known and do not form a part of this inven- tion, which is directed to the selection of the surfac- tant(s) used to treat the contaminated soil.

I. Soil Characterization.

The first step is to determine the mineral character of the soil. Mineralogy is needed to determine the adhe¬ sive energy between the soil and the pollutant. The adhe¬ sive energy is the force which must be overcome by the sur¬ factant to remove the contaminant.

The principal tool for this work, X-ray diffraction, can be found in typical mineralogy laboratories. Typical¬ ly, minerals are identified, together with how their com¬ position varies with particle size. For example, there are

coarse, medium, and fine grained silicas; however, in soils, clays are typically finer grained than silicas.

For example, a Siemens D-500 diffractometer, driven by a DEC Microvax computer and equipped with a copper anode X- ray tube operating at 40 KV and 30 mA, is suitably employed in soil analysis. A graphite diffracted-beam monochromator is positioned between the sample and the detector. Analy¬ sis is done grinding the soil sample to pass a 350 mesh screen and then recording the diffraction pattern. The patterns are identified through a JCPDS (Joint Committee on Powder Diffraction Standards) Powder Diffraction File stored on computer. Search is both automatic using Siemens software and manual by visual inspection. Each constituent mineral phase present in quantities greater than 5% by vol- ume can be identified.

A sample from a PCB-contaminated site was taken and dispersed in distilled water. Two fractions of soil were taken based on particle size which were, roughly, the clay (fine) fraction and the coarser fraction. The fine frac- tion was deposited on a glass slide using a conventional technique for clay mineral analysis. The coarse fraction was packed in a bulk sample container. The diffraction patterns for both were similar, indicating that both con¬ tain essentially the same minerals, but not necessarily in the same proportions.

The dominant mineral was quartz, with minor amounts of other silicates. This mineralogy, listed in Table II, is typical of a soil derived from glacial sediments and reflects the igneous rock types in Canada, the source of these sediments.

Table II. Mineral Analysis.

Mineral Relative Abundance Mineral Type

Quartz major tektosilicate

Chlorite minor clay mineral

clay mineral tektosilicate inosilicate

II. Determination of Contaminant-to-Soil Adhesion.

Determining the interfacial polar and non-polar inter¬ actions between solids and liquids is known; see, e.g., C.J. Van Oss et al, "Interfacial Lifshitz - van der Waals and Polar Interactions in Macroscopic Systems", Chemical Reviews, Vol. 88, No. 6, pp. 927-941 (1988). Further, de¬ termining the acid-base interactions between clay minerals and human serum albumin in aqueous media through a series of contact angle measurements is also known; see, e.g., P.M. Costanzo et al, "Determination of the acid-base char¬ acteristics of clay mineral surfaces by contact angle mea¬ surements", Journal of Adhesion Science and Technology . Vol. 4, No. 4, pp. 267-275 (1990). The inventors have discovered that the foregoing pro¬ cedures can be applied to the treatment of contaminated soil with surfactants.

A. Contact Angle Measurements.

Young's equation, which defines the surface tension (γ _SL) between a solid (S) and liquid (L) , can be computed from measuring the contact angle (FIG. 2), given the sur¬ face tension between the solid and air (γ _SA ) and the liquid and air (y^) :

FIG. 2 shows the contact angle φ which a drop 20 of liquid makes with a surface 22. Contact angle provides a measure of wettability. As shown in FIG. 3a, in a water-oil-silica system (the water is not shown in the drawing, but surrounds the assembly),

oil 10 does not spread on (wet) the substrate 12, but will form a finite contact angle in water. FIG. 3b shows that the surfactant solution (again, not shown) in place of wa¬ ter reduces the surface tension between the substrate 12 and the oil 10, enough to pull the oil into solution. Dur¬ ing soil washing, the surfactant bath will spontaneously displace the oil from the substrate when the contact angle is 180°; if the contact angle is less than 180° but more than 90°, the contaminant will not be displaced spontane- ously but might be removed by hydraulic currents in the bath. When the contact angle is less than 90°, at least part of the contaminant will remain attached to the sub¬ strate. In oil-water systems, hydrogen bonding plays a significant role in the oil-water surface tension and the interaction with the substrate, thus the separation of po¬ lar and non-polar contributions is needed.

In order to calculate the free energy of adhesion of the contaminant with the solid substrate (or soil), one must obtain independent surface tension values which are divided into non-polar (Lifshitz-van der Waals: LW) and po¬ lar (Lewis acid-base: AB) surface tension values. A total of three values are determined for each contaminant and soil mineral type. One of the three values is based upon the non-polar dispersion forces of interaction defined by Lifshitz-van der Waals theory and the other two values are based upon two polar forces of interaction defined by the electron donor and electron acceptor definitions of Lewis acid-base theory.

The total surface tension of a given material is the sum of its polar and non-polar components:

γ = γ ^« * + γ^ ^B - (2) where, γ ^1* " = Lifshitz-van der Waals contribution γ ^AB = 2(γ ^→" γ-)' ⁵ = Lewis acid-base contribution γ ^* = Lewis acid surface tension contribution (electron acceptor)

γ ^~ = Lewis base surface tension contribution (electron donor) .

Experimentalists use total surface tension as one of their yardsticks in predicting solubilization. Additional valuable information can be obtained from the division of surface tension into its three chemically significant com¬ ponents, Lifshitz-van der Waals and the positive and nega¬ tive Lewis acid-base components. The three separate ener- gies are related in the following equation for interfacial tension between two substances (γ ₁₂ ):

( ^ y-.-) ^h - (γ _* ^* v _* -) ^H - (γ γ ₂ ^* )"] (3)

The significance of this equation is the acid-base in¬ teraction parameters. Not only are the acid-base interac¬ tions between different molecules given, but also the acid- base interaction with itself. The total surface tension of liquids can be measured or found in published tables. If measured, three different surfaces are employed, such as a polytetrafluoroethylene material for the non-polar component and polymethylmethac- rylate for the polar (Lewis base) component) . There are no reliable solid surfaces with a large polar (Lewis acid) component. Thus, the Lewis acid component of the liquid must be computed from measurements on another surface with a different Lewis base value, such as polystyrene.

The desired surfaces will be in either a solid, smooth crystal form or prepared in a pressed cake with a smooth surface which can be reliably reproduced. All surfaces must have known γ ^1* ", γ" ^" , and γ ^~ values.

Once the total surface tension is known, the γ _L ^rκ can be found by one of two methods. One method is that of Lif- shitz as described by D.B. Hough et al, "The Calculation of Hamaker Constants from Lifshitz Theory with Applications to Wetting Theory", Advances in Colloid and Interface Science,

Vol. 14, pp. 3-41 (1980), where the dispersion forces be¬ tween bulk materials is found from the dielectric of the materials in question,, the refractive index, etc.

Another method is by measuring contact angles of the (partly polar) liquid on a solid non-polar substrate, such as polytetrafluoroethylene or other fluoroethylene polymer, knowing the value of γg ^1* " and using the equation:

_Yl .(l + cosø) = 2(γ _s ^I*w rι. ^I,w ) . (4)

The γ _L ^LM of a strictly apolar liquid can be found, by contact angle measurements with an apolar surface material using the equation:

1 + cosø = 2(γ _s ^r*w /γχ.)' ^a (5) where,

Unlike apolar interactions, polar interactions are essentially asymmetrical and can only be satisfactorily treated by taking that asymmetry into account, dividing up the polar component γ ^AB of the surface tension into elec¬ tron acceptor γ ^" * ^* and electron donor γ ^~ parameters. The Young-Dupre equation can be expressed as

γ _L (l + cosø) = -ΔG _SL ^τoτ (6) where,

is the total free energy of interaction between a solid and a liquid. The polar and non-polar components of the free energy of interaction are:

ΔG _SX ^B = γ _SI . ^AB - γ _s ^AB - γ _L ^AB (8) ΔG^™ = γ _SL ^LW - γ^" - γ^ ^w (9)

YSL ¹ " = [(Ys^)" ¹ - (γ-, ¹ -")' ³ ] ² (io)

SX- ^AB = γ _s ^AB + γ _L ^AB - 2[(γ _s * _Yx .-) +

From Eqn. (6) and taking into account Eqns. (7)-(ll):

Y .(l + cosø) = 2[(γ ₃ - ^t*w Y^" ) ^* * + (γ _s - _γι ) ^*> +

(γ _s - Yx. ^* ) ^*1 ]. (12)

Thus, by contact angle measurement with three differ¬ ent liquids (of which two must be polar) with known Y _L ^L , Yx." ^" / and Y, ^~ values, using Eqn. (12) three times, then the Ys ¹ "* ⁷ , Ys-- _z Ys ^~ of any solid can be determined. Similarly, by contact angle measurement of a liquid on various solids (of which two must be polar), the γ _L ^L , γ _L ⁺ , and γ _L ^~ can be determined. Thus, with the surface tension parameters mea- sured, the free energy of interaction can be calculated us¬ ing Eqns. (7)-(9). The goal thus is to select a surfactant solution that will take the contact angle towards 180° and lift the contaminant off the soil matrix.

B. Example of Contact Anαle Measurement.

The determination of total surface tension and the non-polar component of surface tension of Aroclor 1248, which is a PCB contained in some hydraulic fluid, is rela¬ tively straight-forward. The measurement of the Lewis acid-base parameters of the polar surface tension component proved somewhat more difficult. Two independent experi¬ ments were made. The pendant drop method was used to de¬ termine the total surface tension of the Aroclor 1248. The shape of the drop results from the interplay between the gravitational force, which is pulling on the drop and ex¬ tending it, and the surface tension, which tends to make the drop spherical. The method is a conventional one, and is described in texts relating to the physical chemistry of surfaces (see, e.g., Adamson, Physical Chemistry of Surfac- e_s, 4th Ed., Section II-9A, Wiley-Interscience) . Two drops were photographed and measured. One of the calculations is reproduced below.

The size and shape of the drop are determined by two parameters: one is the -width (in cm) of the drop at the widest point (d _β ) and the other is the width of the drop measured at a distance d _e from the bottom of the drop. These values determine a parameter, 1/H, describing the shape of the drop:

γ = (Δpgd _β ² )/H.

The average value for the two drops was 42.8 mJ/m ² .

The Lifshitz-van der Waals component of the surface tension, γ ¹ " , was determined from contact angle measure¬ ments on a smooth fluoroethylene polymer surface. Several drops were measured and multiple measurements of each drop were made. The average contact angle was found to be 81.3°. The Young's equation for an apolar solid is:

(1 + cos ) γ^° ^τ = 2(γ _s ^x*w γ^") (13)

and Y _s ^1*1 " = 17.9 mJ/m ² . The Aroclor 1248 has γ _L ^LW = 34 mJ/m ² and, from

it is clear that the polar component of the surface tension γ ^AB is 8.9 mJ/m ² .

The Lewis acid-base parameters are related to the po¬ lar component, γ ^AB , by:

γ ^AB = 2(γ- γ ^~ ) .

The complete Young's equation for apolar materials is

(1 + cosø)γ _x ^τoτ = 2(γ _s ^I - ^κ Y ^) ¹⁵ + (γ _s ^* Y.-) ^*5 + (γ/- γ _s ^~ ) (14)

14

Two polar substrates were used to estimate the polar surface tension parameters; polystyrene (PS) and polymeth- ylmethacrylate (PMMA) . Their surface tension components are set forth in Table III below.

Table III. Surface Tension Components,

Surface Tension Component

Y ^" " Y ^~

Polystyrene is less useful than PMMA because it has a small γ-; both are monopolar substances. This monopolarity makes it possible to solve for γ ^" " and γ ^~ for Aroclor 1248 given the value of γ ^AB . The average contact angle of Aro¬ clor 1248 on PMMA is 23.8°, giving Y _J ^*" = 11.4 mJ/m ² . From the value of γ _L ^AB of 8.9 mJ/m ² yields γ^ ^" = 1.7 mJ/m ² . The average contact angle on polystyrene was 17.9° and a simi¬ lar calculation was done. The results of these calcula¬ tions based on the contact angles of Aroclor 1248 on these two substrates are set forth in Table IV below.

Table IV. Polar Surface Tension Values for Aroclor 1248.

Given the limited measurements that can be made of Aroclor 1248 liquid on well-characterized solid substrates,

there is some uncertainty associated with these values, but that is inherent in studies of the surface tension of li¬ quids. It is much easier, and the results are more cer¬ tain, to measure the surface tension components of solid surfaces because a variety of different liquids may be used.

A second attempt to estimate the polar component of the surface tension was made using commercially-available PARAFILM, which is a non-polar paraffin material with y ^T °' ^:c = Y ^* ™ = 25.5 mJ/m ² . The average contact angle of Aroclor 1248 on this material was 51.2°, giving y _FCB .' ^T °' ^:r = 38.6 and Y _P C _B ^AB = 4.3 mJ/m ² . This is a smaller value for the AB component, which is more in line with the expected proper¬ ties for this material. It should be noted, however, that use of these values with the contact angles for Aroclor 1248 on PMMA yielded a small γ ^~*" but a very large γ ^~ (about 40 mJ/m ² ). This is not consistent with the low solubility of PCB.

As a final and independent analysis of the surface tension components of Aroclor 1248, the solubility in water of this material may be utilized. Some assumptions were made about Aroclor 1248, namely, that it is a monopolar liquid with a γ ^"4" = 0 and γ ^x * ^w = 42.9 mJ/m ² . The value of the solubility used was 1 ppm. From polymer solubility studies, it is known that the interfacial free energy is related to the solubility by the following relation:

AG ₁₂₁ = -kT ln(l/S), (15)

where Ξ is the solubility in moles/liter (M) , k is the Boltzmann constant, and T is the temperature in Kelvin. Summarizing the contact angle data which are the most reliable:

Summary of PCB experiments: _PCB = 20.4 on PMMA _aS o«n _B t»d.« _aβ = 51.9 on PMMA

and given the values for PMMA, γ^" = 42.0, γ* = 0.0, γ ^~ = 16.7.

It is reasonable to assume that Aroclor 1248 is non- polar (γ ^τoτ = γ ¹ -" = 43.6 mJ/m ² , slightly larger than the pendent drop measurements indicated, then γ"*" = 0 mJ/m ² ).

The solubility (S) is approximately 1 ppm, the molecu¬ lar weight (MW) is approximately 360, the contactable sur¬ face area (Sc) is approximately 0.8 nm ² (estimated from twice the Sc value for glucose) . Substituting in the val- ues of S (1 ppm or 2.78xl0 ^"e M) , k (l.38xl0 ^~23 J/K) and T (300K) in Eqn. (15) gives ΔG-_ _2L = -64.7 ergs/cm ² or mJ/m ² . From ΔG-. _2;L = -2γ _l2 , a value of γ _X2 = 32.3 mJ/m ² re¬ sults.

Using

γ ₁₂ = [ ( ) ¹ *

"a - - ι «a

^" * T a-Cer J w a. -fc. « 3r ) ( pcb [ ι

^{"" '} \ Y jDC=tD Y EL "fc. β XT ) J

and entering the values which are known for water and those which have been derived for Aroclor 1248 yields

γ _x2 = [(43.4) - (21.8) ^* =j ² + 2[(0 X Y _POto -) ^H + (25.5 X 25.5)^ - (Y _PC:to ^* X 25.5^ - (O x 25.5) ]

From the solubility data, it is independently known that γ-_ ₂ = 35.75 mJ/m ² , so this allows solving for the val¬ ue of γ _PCB _ (= 3.5 mJ/m ² ). The following Table IV lists values of γ _PCS ^" for different values of the solubility.

Table IV. Interfacial Tension between Aroclor 1248 and Water (γ ₁₂ ) and Lewis Base Component of Aroclor 1248 (γ _PCB ^~ ) Versus Solubility.

Solubility Xl2- ϊpcb— 0.1 ppm 38.2 2.7 1.0 ppm 32.3 4.9 10.0 ppm 26.5 7.8

For a smaller value of Sc (0.6 nm ² ), and S = 1 ppm, then γ _ia = 43.1 and γ _PCB ^" = 1.3 mJ/m ² . Thus, the best estimate for the surface tension com¬ ponents of Aroclor 1248 from a combination of contact angle measurements and solubility is given in Table V, below.

Table V. Surface Tension Components for Aroclor 1248.

43.4 mJ/m ² 43.4 mJ/m ²

III. Determination of the Interfacial Tension between Contaminant and Surfactant and the Criteria for Surfactant Selection.

The next part of the method of the invention is to determine the interfacial energy of the surfactant and the pollutant of interest. The interfacial tension between two liquids is measured by a variety of approaches, such as

hanging drop, spinning drop, and drop weight method. By making use of

Y _i2 = [(Yχ ^LW )' - (Y^)*] ² + 2[( _Yl - ) * + (γ ₂ ⁺ Y _z ^~ ) ^{h ~} (Yx ⁺ Y _z ^~ ) + (Yα.- Y^) ^H ) (16)

one can obtain y^™, γ _x ^"*" , and y _^ ^~ once the interfacial ten¬ sion γ ₁₂ between this liquid and three other completely characterized liquids are known. Surfactant polar (γ" ^" and γ ^~ ) and non-polar (γ ^1- ™) sur¬ face tension components are then listed based upon chemical structure. This methodology, shown below, reveals the three surface tension components required for both the po¬ lar (hydrophilic) and the non-polar (lipophilic) parts of the surfactant molecule. Thus, if the surface tension val¬ ues for N polar groups and M non-polar groups are known, then estimates for N*M surfactant combinations can be made. This gives one the ability to fine-tune surface tension re¬ quirements and to design surfactants for contaminant remov- al.

One Surfactant HYDROPHILE - LIPOPHILE

Hydrophilic Group + Lipophilic Group

And six surface tension measurements: three for: (Y _H ^LW ₍ γ _H ^* , γ _H ^~ ) and three for: ( y^™ , γ _L ", Yχ. ^~ )

Gives

(YH- ^L / YH- , Yι.-r. ^" ) for the surfactant.

The AG _p/m ^τoτ between the contaminant, or pollutant (p) , and soil, or mineral ( ) , is computed from Eqn. (7) or each combination. The ΔG _s/ϊ= ^':ro'r between the surfactant (s) and pollutant can also be determined.

When

A TOT ■. A c TOT

then the contaminant prefers to stick to the surfactant rather than the soil. Additionally, surfactants selected should hold to the criteria

ΔG _s/E , ^τoτ > ΔG _β/In ^τoτ

so that the surfactant will not preferentially stick to the soil, thus interfering in the extraction process.

A. Example for Interfacial Measurements. The relevant surface tension values for water are: γ

= 72.8, γ ^LW = 21.8, γ ^* = 25.5, and γ ^" = 25.5 mJ/m ² and for hexane are: γ = 18.4, γ ₁₂ = 18.4, γ" ^1" = 0, and γ ^~ = 0. The values for quartz are: y ^' '™ = 39.9, γ ^"*" = 0, and γ ^~ = 25 mJ/m ² . The values for Aroclor 1248 are given in Table V above. The interfacial tension is given from Eqn. (16) above by:

Y ₁₂ = [(Yp. _* ¹ "")' ¹ - (Y _wat = _r ^LW ) ^% ] ² + 2[(γ _pcto - _Y]poto -)^

^"■" ( Y ater (water ) ^— I YfjcJ-j Ywoter I + ( Yjaesto Ywater ) J

The Dupre equation gives the free energy of adhesion:

Table VI below sets forth the adhesion energy of

Aroclor 1248 to quartz in the presence of (a) water and (b) hexane.

Table VI. Adhesion Energy of Aroclor 1248 to

Quartz in the Presence of Water and Hexane.

Parameter Value Type 32.3 mJ/m ² Interfacial tension between between Aroclor 1248 (1) and water (2)

ΔG- -35.5 Free energy of adhesion be¬ tween Aroclor 1248 (2) and quartz (1) in the presence of water (3)

ΔG.; - 9.3 Free energy of adhesion be¬ tween Aroclor 1248 (2) and quartz (1) in the presence of hexane (3)

In Table VI, one can see that there is a substantial adhesion energy between the Aroclor 1248 and quartz, a com¬ mon constituent in soils, in the presence of water. If the water were replaced by hexane, the adhesion energy is sub¬ stantially reduced, but is still negative, which means that the Aroclor will still bind to the quartz.

Performing the same type of calculations with a suit¬ able surfactant would reveal whether that surfactant created a positive adhesion energy, by which would mean that the Aroclor (or contaminant) would no longer bind to the quartz. That surfactant could then be useful in treating a soil polluted with the contaminant.

IV. Improvement of Surfactant Action by the Addition of a Co-Surfactant. Subsequent to characterization of the polar and non- polar contributions to interfacial tensions in the soil- contaminant system, it is possible to improve the adhesion

between surfactant and pollutant by seeding the soil with an oil-soluble co-surfactant.

The surface tension components which have been deter¬ mined by contact angle measurements give parameters for the co-surfactant. For example, if the contaminant is largely apolar, (large γ ^~ component), then a desirable combination of surfactant and oil-soluble co-surfactant would be a sur¬ factant which is largely basic in nature and a co-surfac¬ tant which is largely acidic in nature. In order to have effective removal, the surfactant-co-surfactant pair must be chosen in such a way that the soil-contaminant contact angle goes to 180° when the surfactant solution is added to the contaminant soil, thus lifting the contaminant off the soil completely. The co-surfactant provides an additional set of parameters whereby this might be accomplished.

INDUSTRIAL APPLICABILITY

The method of the invention is expected to find use in the extraction of chemical pollutants from contaminated soils.