TECHNICAL FIELD

The present disclosure broadly relates to methods for the remediation of environmental per- and polyfluoroalkyl substances in soil and aquifers.

BACKGROUND

Per- and polyfluoroalkyl substances (collectively PFAS) are a group of chemicals that includes low molecular weight surfactants such as perfluoroalkanoic acids, perfluoroalkanesulfonic acids, and perfluoroethers. As detection capabilities for these materials has improved, they have been found in the environment and have come under regulatory oversight in recent years.

To address the issue, much work has focused on their source including finding alternative materials and minimizing or eliminating their presence in products and waste streams. Other activity involves environmental remediation including digging up affected sites followed by incineration, filtration, oxidation and sonolysis (e.g., see Ross et al., Remediation 2018, 28(2), 101-126). A recent publication describes a bacteria strain that appears to degrade perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) (e.g., see S. Huang, et al., Environmental Science and Technology 2019, 53(19), 11410-11419). While these approaches have shown some promise ex situ, they may not be practical in situ solutions.

SUMMARY

The present disclosure provides an in situ method capable of quickly and efficiently recovering fluorinated surfactant from soil and or aquifers. Once recovered, they can be recycled or disposed of in an appropriate manner.

In one aspect, the present disclosure provides a method of reducing a concentration of a fluorinated surfactant in an aquifer, the method comprising sequential steps: a) introducing a first composition into the aquifer, wherein the first composition comprises water and a nonfluorinated surfactant having a critical micelle concentration of at least 0.1 percent by weight at 23 °C; and b) recovering a second composition comprising at least a portion of the water and mixed micelles from the aquifer, wherein the mixed micelles comprise at least a portion of the nonfluorinated surfactant and at least a portion of the fluorinated surfactant.

In another aspect, the present disclosure provides a method of reducing a concentration of a fluorinated surfactant in soil, the method comprising sequential steps: a) contacting a first composition with the soil, wherein the first composition comprises water and a nonfluorinated surfactant having a critical micelle concentration of at least 0.1 percent by weight at 23 °C; and b) recovering a second composition comprising at least a portion of the water and mixed micelles from the soil, wherein the mixed micelles comprise at least a portion of the nonfluorinated surfactant and at least a portion of the fluorinated surfactant.

In yet another aspect, the present disclosure provides a composition comprising water and mixed micelles comprising a nonfluorinated surfactant having a critical micelle concentration of at least 0.1 percent by weight at 23 °C and a fluorinated surfactant, wherein the fluorinated surfactant has a critical micelle concentration, wherein the mixed micelles have a concentration that is less than the critical micelle concentration of the fluorinated surfactant, and wherein the concentration of the mixed micelles is less than the critical micelle concentration of the nonfluorinated surfactant.

As used herein:

The term "aquifer" means a body of permeable rock which can contain or transmit groundwater.

The phrase "essentially free of " means containing less than 0.01 weight percent; for example, containing less than 0.01 weight percent of, less than 0.001 weight percent of, less than 1 part per million by weight, or even completely free of.

The term "micelle" refers to an organized assembly of surfactant molecules and include structures that are spherical, ellipsoidal, disks, rod-like, or worm-like, as well lipid bilayer structures such as liposomes and vesicles.

The term "mixed micelle" refers to a micelle formed of at least two different types of surfactants.

The term "surface soil" refers to the upper layer of earth in which plants grow, a black or dark brown material typically consisting of clay, rock particles, and optionally a mixture of organic remains.

The term "subsoil" refers to the stratum of weathered material that underlies the surface soil.

The term "soil", when used without further modification, refers collectively to surface soil and subsoil.

Features and advantages of the present disclosure will be further understood upon consideration of the detailed description as well as the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of Surface Tension vs. Surfactant Concentration for MEGA9 and APFO.

FIG. 2 is a plot of Surface Tension vs. Surfactant Concentration for 15-S-7 and APFO.

It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the disclosure. The figures may not be drawn to scale.

DETAILED DESCRIPTION

Method according to the present disclosure are useful for reducing the concentration of fluorinated surfactant(s) in soil and/or an aquifer. In embodiments involving removal of fluorinated surfactants from aquifers, the method comprises at least two sequentially performed sequential steps. The steps may be performed with intervening steps or, more typically, consecutively.

Embodiments involving removal of fluorinated surfactants from soil involve analogous steps, except that the material treated is soil instead of an aquifer.

The first step involves introducing a first composition into the aquifer. Typically, this would be done by pressurized injection, although any method capable of accomplishing the task may be used. The first composition comprises water and a nonfluorinated surfactant having a critical micelle concentration of at least 0.1 percent by weight at 23 °C.

A micelle is an aggregate (or supramolecular assembly) of surfactant molecules dispersed in a liquid colloid. Micelles generally have dynamic structures, wherein surfactant molecules can migrate back and forth between the micelles and the liquid medium in which the surfactant molecules exist.

A typical micelle in aqueous solution forms an aggregate with the hydrophilic (polar) region of the nonfluorinated surfactant molecules in contact with the surrounding aqueous medium, while sequestering the hydrophobic (nonpolar) tail region in the micelle center. This type of micelle is known as a normal-phase micelle. Inverse micelles have the polar head groups at the center with the tails extending out. The micelles may comprise stmctures that are spherical, ellipsoidal, disks, rod-like, or worm-like, as well a lipid bilayer stmctures such as liposomes and vesicles. The shape and size of micelles may depend on the molecular geometry of its nonfluorinated surfactant molecules and solution conditions such as nonfluorinated surfactant concentration, temperature, pH, and ionic strength. The process of forming micelles is known as micellization. Micelles form when the concentration of surfactant molecules rises above a critical concentration value, known as the critical micelle concentration (CMC) at a specified temperature.

The CMC can be determined by a variety of experimental techniques, each one best suited for a particular concentration range. One common technique utilizes a force tensiometer to measure the surface tension of a concentration series. With pure surfactants, the SFT is linearly dependent on the logarithm of the concentration over a large range. Above the CMC, the surface tension is extensively independent of the concentration. The CMC results from the intersection between the regression straight line of the linearly dependent region and the straight line passing through the plateau.

CMC values can be determined, for example, by plotting surface tension vs. concentration as described by Mukerjee, P.; Mysels, K. J. in "Critical Micelle Concentrations of Aqueous Surfactant Systems"; NIST National Institute of Standards and Technology: Washington D.C. USA, 1971; Vol. NSRDS-NBS 36, p.8. An exemplary useful force tensiometer for determining CMC values is marketed by Kriiss GmbH, Hamburg, Germany, as model K100.

In some embodiments, the concentration of the mixed micelles may be at least 0.1 percent by weight, at least 0.2 percent by weight, at least 0.3 percent by weight, at least 0.4 percent by weight, at least 0.5 percent by weight, at least 0.75 percent by weight, at least 1 percent by weight, at least 1.5 percent by weight, at least 2 percent by weight, at least 2.5 percent by weight, at least 3.0 percent, at least 5 percent, at least 10 percent, at least 20 percent, ant least 30 percent, at least 40 percent, at least 50 percent, at least 60 percent, at least 70 percent, at least 80 percent, at least 90 percent, or even at least 100 percent by weight of the second composition.

Advantageously, under comparable conditions (e.g., same liquid vehicle, pressure, and temperature) mixed micelles incorporating the nonfluorinated surfactant and fluorinated surfactant can form at concentrations of the nonfluorinated surfactant and fluorinated surfactant below those required of the fluorinated surfactant and the nonfluorinated surfactant individually. Put another way, the concentrations of fluorinated surfactant and non-fluorinated surfactant required to form mixed micelles is lower than the CMC of the fluorinated surfactant alone under comparable conditions, which leads to enhanced recovery of low concentration fluorinated surfactants as compared to using the liquid vehicle alone.

In order to avoid introducing unnecessary chemicals into aquifer and/or soil during remediation, it is often preferable that first composition be essentially free of halogenated organic compounds and/or essentially free of water-soluble and/or water-miscible solvent; however, this is not a requirement. For example, small amounts of low toxicity cosolvents such as ethanol or glycerol may be tolerated in some instances.

In some embodiments, the nonfluorinated surfactant is non-ionic or anionic. Combinations of non-ionic surfactants, combinations of anionic surfactants, and combinations of anionic surfactants and non-ionic surfactants may also be used.

Exemplary nonfluorinated surfactants include bisalkyl sulfosuccinates such as dihexyl sulfo succinates (e.g., as a sodium salt available as Aerosol MA from Solvay, Brussells, Belgium) and bis(2 -ethylhexyl) sulfosuccinates (e.g., as a sodium salt available as Aerosol OT from Solvay), C ₇ to C ₁₁ alkanoyl-A-alkylglucamides (e.g., nonanoyl-N-methylglucamide available as MEGA-9 from Avanti Polar Lipids, Alabaster, Alabama), C ₈ to C ₁₂ alkyl polyglycosides (e.g., caprylyl/capryl polyglucosides available as Plantaren 810UP from BASF, Ludwigshafen, Germany), C ₈ to C ₁₆ alkyl sulfates (e.g., sodium lauryl sulfate), C ₈ to C ₁₆ alkyl benzenesulfonates (e.g., sodium dodecylbenzenesulfonate), and combinations thereof.

The concentration of nonfluorinated surfactant is typically greater than the CMC, preferably 1.1 to 10 times the CMC, more preferably 2 to 5 times the CMC, and more preferably 3 to 4 times the CMC, however this is not a requirement. In some embodiments, the nonfluorinated surfactant is represented by the formula wherein R ¹ represents a linear alkyl group having from 8 to 12 (e.g., 8, 9, 10, 11, or 12) carbon atoms. MEGA-9 is an example wherein R ¹ is a linear alkyl group having 8 carbon atoms. In some embodiments, the nonfluorinated surfactant may comprise a combination of nonfluorinated surfactants with different values of R ¹ .

The second step involves recovering a second composition comprising at least a portion of the water and mixed micelles from the aquifer, wherein the mixed micelles comprise at least a portion of the nonfluorinated surfactant and at least a portion of the fluorinated surfactant.

In some embodiments, the fluorinated surfactant comprises a perfluoroalkyl group having from 4 to 10 carbon atoms (i.e., 4, 5, 6, 7, 8, 9, or 10 carbon atoms). Examples of such fluorinated surfactants include perfluorooctanesulfonic acid (PFOS), perfluoro-n-octanoic acid (PFOA), perfluoroheptanoic acid (PFHpA), perfluorohexanoic acid (PFHxA), perfluorohexanesulfonate (PFHxS), 6:2 fluorotelomer- sulfonic acid, perfluoropentanoic acid (PFPeA), perfluorobutanoic acid (PFBA), perfluorobutanesulfonic acid (PFBS), perfluorononanoic acid (PFNA), perfluorodecanoic acid (PFDA),and certain fluorotelomer sulfonic acids, salts of the foregoing acids (e.g., Na ⁺ , K ⁺ , quaternary ammonium, and NH ₄ ⁺ salts); ethoxylated fluorotelomers (e.g., Zonyl FSO);.fluororalkylsulfonamides and their salts (e.g., Na ⁺ , K ⁺ , quaternary ammonium, and NH ₄ ⁺ salts); alkoxylated fluoroalkylsulfonamides; and nonionic polymeric fluorochemical surfactants.

In some embodiments, the fluorinated surfactants can be represented by the formula R _f X-Y-Z wherein

R _f is a fluorinated group with C ₁ -C ₁₂ alkyl, aryl, or alkaryl;

X is an optional bivalent linking group such as a saturated or unsaturated alkyl, aryl, alkaryl, heteroalkyl, heteroaryl, heteroalkaryl;

Y is a hydrophilic group such as carboxyl, sulfate, sulfonate, phosphate, phosphonate, quatemized amine, quatemized phosphine and polyalkylene oxide; and

Z is H, monovalent cation, monovalent anion, or a saturated or unsaturated alkyl, aryl, or alkaryl group. When X and Z are unsaturated, oligomers and polymers can be created through conventional means.

While methods according to the present disclosure may be used to reduce the concentration of a single fluorinated compound, it is contemplated that they may also be used to simultaneously reduce the concentration of multiple (e.g., at least 2, at least 4, or even at least 4) fluorinated surfactants as described above. In that case, mixed micelles may include at least one nonfluorinated surfactant and at least 2 fluorinated surfactants.

The first composition may be introduced into an aquifer and/or soil at a different location than where the second composition is recovered, however, this is not a requirement. Typical aquifers comprise at least one of a limestone or sandstone geological formation, although this is not a requirement. These porous rock formations permit the first composition to penetrate the aquifer and desorb the fluorinated surfactant and incorporate it into the mixed micelles.

Preferably, the first composition may be introduced by pressurized injection into the aquifer and/or soil, although any method capable of introducing the first compound into the aquifer may be used. Methods such as pressurized injection are well known and within the capabilities of those skilled in the art. In some embodiments, the first composition may be introduced into the aquifer as a foam (e.g., formed by high speed mixing to entrain air prior to injection). Foams can be used to increase the viscosity of the first composition, which improves sweep efficiency. Typically, viscosity of the first composition prior to injection will be less than or equal to 10 centipoise (10 mPa-sec), although this is not a requirement.

Alternatively or in addition, the first composition may further comprise one or more water- soluble thickeners such as, for example, polyacrylamide, polyvinyl alcohol, polyacrylic acid, and/or polysaccharides to increase viscosity. Examples of suitable polysaccharides may include: starches such as arrowroot, com starch, katakuri starch, potato starch, sago, tapioca, and derivatives thereof; vegetable gums such as alginin, guar gum, locust bean gum, and xanthan gum; and sugar polymers such as agar, carboxymethyl cellulose, pectin, and carrageenan.

The second composition may be recovered from the aquifer by pumping from a well bore into the aquifer. Such methods are well known and within the capabilities of those skilled in the art.

In some embodiments, the first composition may further comprise a base that can provide an alkaline pH to the first composition. Adjusting the pH to at least 8, at least 9, or even at least 10 may help limit adsorption of the nonfluorinated surfactant onto the rock formations in the aquifer. Exemplary bases include alkali metal hydroxides, alkali metal carbonates, alkali metal bicarbonates, alkali metal acetates.

An analogous process may be accomplished in situ with soil by drilling shallower injection and recovery well bores and using the same general technique. Again, foaming prior to injection may be desirable in some cases. Less desirably, soil may also be remediated ex situ by digging it up and mixing it with the first composition, then draining of the second composition in either a batchwise or continuous process.

Objects and advantages of this disclosure are further illustrated by the following non-limiting examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this disclosure.

EXAMPLES

Unless otherwise noted, all parts, percentages, ratios, etc. in the Examples and the rest of the specification are by weight.

A Kriiss K100 Force Tensiometer from Kriiss GmbH, Hamburg, Germany, equipped with a Wilhelmy plate and running the Surfactant Characteristics add-in module was used to determine the critical micelle concentration (CMC). The tensiometer was equipped with a Metrohm dosimeter for automatic dosing of the prepared surfactant solution.

The method used for determining CMC by plotting surface tension vs. concentration can be found in Mukeijee, P. and Mysels, K. J . in "Critical Micelle Concentrations of Aqueous Surfactant Systems"; NIST National Institute of Standards and Technology: Washington D.C. US A, 1971; Vol. NSRDS-NBS 36, p.8.

Table 1, below reports various materials used in the Examples,

TABLE 1

Surfactants that were delivered in water were used as received. Aerosol MA 80 comes as an 80% solution in IPA/water. Therefore, MA was dried using a rotary evaporator with a bath temperature of 60 °C.

General Procedure for CMC Determination

Dosing solutions of HC surfactants (which are non-fluorinated) and FC Surfactants (which are at least partially fluorinated) were prepared at the concentrations indicated in “HC Concentration” and “FC Concentration” in Tables 2 and 3. The dosing solutions were allowed to stir for one hour before use. Each dosing solution was connected to the dosimeter and 30 mL of the dosing solution was run through the tensiometer tubing prior to starting the experiment. Distilled water (50 mL) was placed in the receiver flask equipped with a magnetic stirbar. The Wilhelmy plate was washed with acetone followed by deionized water, and then it was heated with a propane torch until orange in color. The plate was allowed to cool to ambient conditions prior to starting the experiment. Dosing was carried out using a linear factor (LF) of 1.0 and a dosing factor (DF) of 1.0 mL (i.e., 1 mL was delivered per dose). These values were entered into the CMC determination program of the tensiometer software. The experiment was then started. The experiment was run until the receiver flask was full (i.e., addition of 70 mL of surfactant solution) or until the CMC was achieved. If a minimum existed in the CMC curve, the minimum surface tension value was used to determine the CMC.

When dosing a single surfactant, the concentration of that surfactant is plotted along the x-axis and the CMC can be read directly off the axis at the minimum of the curve.

When dosing a mixture of surfactants that exhibits synergy, the x-axis is plotting the concentration of the fluorochemical. This concentration of the fluorochemical can be read directly from the graph, as above. The hydrocarbon concentration can be determined by the following equation;

C _F = (LF x DF x N x C _I )/(50 mL + LF x DF x N) wherein,

C _F is the concentration (in ppm) of the hydrocarbon surfactant at the minimum of the mixed CMC curve;

LF is the linear factor;

DF is the dosing factor;

N is the number of dosing steps needed to obtain the minimum in the curve; and C _I is the concentration of the dosing solution in ppm.

When the mixture does not show synergy, there are two CMCs (i.e., two changes in slope) equating to the CMCs of the individual surfactants.

Results are reported in Tables 2 and 3. In Model Example MEI in Table 2, 50 mL of 0.25 wt. % MEGA9 was placed in the receiver flask in order to obtain the entire CMC trace. In Model Example ME6 in Table 2, 50mL of 2 wt. % NaCl (in deionized water) was placed in the receiver flask instead of just deionized water. In Examples Model ME17 to E30 in Table 2 and Comparative Examples CE6 to CE11 in Table 3, the linear factor was 1.0 and the dosing factor was 0.1 mL (i.e., 0.1 mL delivered per dose).

TABLE 2

TABLE 3

FIG. 1 is a plot of Surface Tension vs. Surfactant Concentration for MEGA9 and APFO. It shows evidence of mixed Micelles.

FIG. 2 is a plot of Surface Tension vs. Surfactant Concentration for 15-S-7 and APFO. It does not show evidence mixed micelles.

While FIG. 1 exhibits only one minimum for the mixture of APFO and MEGA9, indicating mixed micelles and FIG. 2 exhibits two minima for the mixture of Tergitol 15-S-7 and APFO, indicating the lack of mixed micelles (i.e., micelles consisting of Tergitol 15-S-7 and another set of micelles consisting of APFO) this is not conclusion without additional proof.

Diffusion experiments were carried out by ¹ H and ¹⁹ F NMR (DOSY) (i.e., nuclear magnetic resonance diffusion ordered spectroscopy) to determine the presence or lack of mixed micelles. This was determined by finding the diffusion constant of each individual surfactant and then the diffusion constant of the individual surfactants in the mixed solution.

If in the mixed surfactant solution, the log (i.e., log ₁₀ ) of the diffusion constant is the same or nearly the same for the two surfactants, then that means the two surfactants are moving in solution at the same rate and are therefore exist in the same micelle. If the two log values are different then the two surfactants are traveling at different rates and are not in mixed micelles but are forming micelles consisting of the hydrocarbon surfactant only and micelles of the fluorocarbon surfactant only.

For the APFO/MEGA9 solution, the logs of the diffusion constants were found to be -9.722 and - 9.753, respectively. This indicates that mixed micelles were formed. For the APFO/Tergitol 15-S-7 solution, the log diffusion constants were found to be -9.455 and -9.824, respectively. This indicates that these two species did not exist in mixed micelles. Several examples of the combinations that indicate mixed micelles by NMR are listed in Table 4, below.

TABLE 4 Several examples of the combinations that did not indicate mixed micelles by NMR are listed in Table 5, below.

TABLE 5

Any cited references, patents, and patent applications in this application that are incorporated by reference, are incorporated in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in this application shall control. The preceding description, given in order to enable one of ordinary skill in the art to practice the claimed disclosure, is not to be construed as limiting the scope of the disclosure, which is defined by the claims and all equivalents thereto.