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[ EN ]



[0001] The present disclosure relates to environmental remediation. In particular, to removal of perfluoroalkyl and polyfluoroalkyl substances from soil.


[0002] Per perfluoroalkyl and polyfluoroalkyl substances (PFAS) are contaminants of concern. There are about 3,000 types of these compounds in the environment. They are soluble, highly resistant to biotic and abiotic degradation, and can withstand extremely high temperatures before breaking down.

[0003] PFAS compounds have been widely used in consumer products and industrial products and processes. The PFAS characteristics that make them beneficial for these applications can also prevent them from readily degrading and challenging to remediate. The unique characteristics of PFAS compounds render many remediation techniques that are effective on other contaminants ineffective to remediate PFAS compounds. Bioremediation is mostly ineffective for treating PFAS-contaminated soil. Additionally, limited success has been found by applying soil vapor extraction and other common hydrocarbon remediation techniques. Furthermore, it is generally accepted that very high temperatures, above 600 degrees Celsius for example, are necessary to effectively remediate PFAS compounds.

Although most PFAS compounds boil at temperatures in the range of 76 degrees Celsius to 218 degrees Celsius, there is very poor removal even at 225 degrees Celsius applied over multiple days.

[0004] Accordingly, there is a need for an effective PFAS remediation technique in soil.


[0005] The disclosed methods and systems may be used to remediate soil containing

PFAS compounds and organic carbon. Illustrative embodiments include volatilizing these compounds from a soil matrix, which may contain relatively high concentrations of total organic carbon (TOC), or amounts generally found in United States soil, typically about .25% by weight. Illustrative embodiments also include soil remediation in which the percent of TOCs is initially less than .25%. TOC is reduced by heating the soil at a sufficient temperature and for a sufficient duration to reduce surface effects between the PFAS compounds and the organic carbon to permit evaporation of the PFAS compounds from the soil. In an illustrative embodiments soil is treated in-situ at a temperature in the range of above about 225 degrees Celsius and below about 440 degrees Celsius, and in a further illustrative embodiment at a temperature in the range of about 300 degrees Celsius to about 400 degrees Celsius. The invention includes various combinations of temperature levels and duration of heating to remediate PFAS compounds.


[0006] The detailed description refers to the accompanying Figure, which depicts an illustrative embodiment.

[0007] The Figure is a flow chart of an illustrative PFAS remediation method.


[0008] The descriptions provided herein may have been simplified to illustrate aspects that are relevant for an understanding of the systems and methods described herein while eliminating, for the purpose of clarity, other aspects that may be found in typical systems and methods. Those of ordinary skill may recognize that other elements or operations may be desirable or necessary to implement the systems and methods described herein. Because such elements and operations are well known in the art, and because they do not facilitate a better understanding of the present disclosure, a discussion of such elements and operations may not be provided herein. This disclosure is deemed to inherently include all such elements, variations, and modifications to the described aspects that could be implemented by those of ordinary skill in the art.

[0009] A method of remediation of soil containing PFAS compounds is disclosed, and a system in which the method is implemented. The Figure provides a flow chart of an illustrative PFAS remediation method. The method is carried out using thermal remediation techniques. In step 102 heater wells are installed. In step 104 total organic carbon (TOC) is reduced by heating the soil at a sufficient temperature and for a sufficient duration to reduce surface effects between the PFAS compounds and the organic carbon by breaking down the organic carbon. The surface effects may be, for example, electrostatic forces and van der Walls forces. This permits evaporation of the PFAS compounds from the soil. In an illustrative embodiment, the soil is heated to reduce TOC to less than about 0.15% by weight. In a further embodiment, soil is heated at a temperature and for a duration sufficient to reduce total organic carbon to less than about 0.10% by weight. In yet another embodiment soil is heated at a temperature and for a duration sufficient to reduce total organic carbon to less than about 0.05% by weight. An illustrative range of TOC reduction is to about 0.01% to about 0.15%.

[00010] Temperature ranges for remediating PFAS compounds according to illustrative embodiments of the method include, for example, about 225 degrees Celsius to below 440 degrees Celsius; about 300 degrees Celsius to about 400 degrees Celsius; about 320 degrees Celsius to about 380 degrees Celsius, and about 330 degrees Celsius to about 350 degrees Celsius.

[00011] Heating duration for remediating PFAS compounds according to illustrative embodiments of the method include about one day to about 30 days, about five days to about 25 days, and about two days to about 10 days.

[00012] Heating may be performed by various thermal remediation techniques. In an illustrative embodiment, heating is performed by conductive heat transfer using heater wells installed into the soil. One or more heater wells are installed to create a temperature gradient in the soil to establish conductive heat transfer through the soil from the heater wells. When the temperature of the heater well is higher than the surrounding soil, heat energy will flow to the soil. Details of an illustrative conductive heat transfer method and system are provided below.

[00013] It has been accepted in the industry that conventional soil remediation heating techniques could not be applied because of the level of heat necessary to remediate PFAS compounds. Disclosed embodiments of PFAS remediation target the TOC reduction to more readily release the PFAS compounds from carbon compounds, thus facilitating use of various thermal remediation techniques.

[00014] Importantly, heat is selectively modulated to balance the energy input to the system with heat losses, such as to the atmosphere. The heat must also be regulated to sufficiently reduce TOC while maintaining temperatures at levels that will not damage the heater wells. The timing and amount of heat transferred to the soil is regulated based on

factors such as types and concentrations of PFAS compounds, for example. Accordingly, the temperature may be monitored throughout the soil matrix and also at the heater wells.

Adjustments are made to balance the TOC reduction required with the temperature limits of the heater wells

[00015] Heat sensors provide temperature level information, which can be accessed and monitored manually, or configured to provide input to the heater wells to increase or decrease power to the heater wells to regulate the temperature. .

[00016] In step 106 evaporated PFAS compounds can be captured once released from the soil pursuant to the application of heat. For example, a vapor recovery system can be used to capture the evaporated PFAS. In step 108 PFAS can be removed from the vapor stream by cooling the vapor stream and condensing the PFAS. In steps 110, 112 steam may be produced in the process or added to the process to allow for capture of the PFAS in a condensed and concentrated aqueous solution where the PFAS are then destroyed in a continuous process using electro-oxidation techniques such as advanced electrochemical oxidation or an electrical discharge plasma reactor as provided in step 114.

[00017] The combination of heating duration and temperature level can be adjusted for the particular PFAS being remediated. Embodiments of the remediation method include the various combinations of any of the illustrative temperature ranges and time ranges. For example, the most common PFAS compounds, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), may be remediated to concentrations of less than 1 microgram per kilogram by applying temperatures in the range of 350 to 400 degrees Celsius to the soil.

[00018] The method can be performed in-situ or soil can be removed and treated ex-situ.

[00019] In an illustrative thermal conduction heating method, heater wells are installed in in the soil to be remediated, distributed in an array according to factors, such as for example, contaminant concentration and location, soil characteristics and heater well characteristics. Heater wells may be installed at regular intervals, or varying intervals depending, for example, on the necessary energy to be imparted into the soil and the temperatures needed to adequately remediate the contaminants· Heater wells generally comprise heating components or elements disposed within steel pipes. The heat may either move the contaminants in the soil toward the heater wells or destroy the contaminants in-situ. For example, sufficient levels of heat can be produced to boil the water in the soil and vaporize the contaminants· Contaminants that migrate toward the heater wells can be collected and directed through the wells or piping to the surface, where they can be removed or treated. PFAS compounds may be removed from the vapor stream by passing the heated vapors through a condensing system such as a heat exchanger or scrubber where the PFAS compounds are concentrated into an aqueous solution for treatment.

[00020] In an illustrative embodiment, a flexible, generally, helical heater is implemented to uniquely achieve PFAS remediation in soils by effectively bringing the soil to the necessary temperature or temperatures. The term“helical” or“helix” as used to describe the flexible heater shall include coils that are true helixes and those that are not necessarily a true mathematical helix. The flexible helical heater includes an electrical resistance heating wire coiled about a current return wire that fits inside a small diameter metal casing. The electrical resistance heating wire may be comprised, for example, of a nickel-chromium alloy such as NiChrome®, a copper-nickel alloy such as Cuprothal® or an iron-chromium- aluminum ferritic allow such as Kanthal®. Heating soils to temperatures in the range of 225 to 440 degrees Celsius, for example, requires relatively tight spacing for heater wells, which may drive up the price and complexity of remediation. The flexible helical heaters can fit into smaller diameter metal casings than traditional heaters, which may significantly reduce the cost and complexity of the PFAS remediation. In an illustrative embodiment, specific zones along the length of a heater well that contain more PFAS or TO than other zones are targeted with higher temperature by adjusting the coil spacing. C. In a further embodiment, heat loss is compensated for by adjusting coil spacing to put more heat in at the top and bottom of a casing to compensate for heat loss.

[00021] PFAS remediation may be affected by site characteristics, types of PFAS compounds being remediated due to varying chemical and physical characteristics, source of the PFAS, whether there are multiple types of PFAS compounds, nature of the release pathways, the make-up of other contaminants and materials in the soil. The disclosed remediation technique can take into consideration these variables and the number of carbon bonds and the alkyl functional group of the PFAS being remediated. In general, the larger the molecule, the higher the boiling point, but this general relationship can vary. The disclosed methods may include combinations of the remediation techniques.

[00022] Various embodiments of the invention have been described, each having a different combination of elements. The invention is not limited to the specific embodiments disclosed, and may include different combinations of the elements disclosed, omission of some elements or the replacement of elements by the equivalents of such structures.

[00023] While illustrative embodiments have been described, additional advantages and modifications will occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to specific details shown and described herein. Accordingly, it is intended that the invention not be limited to the specific illustrative embodiments, but be interpreted within the full spirit and scope of the appended claims and their equivalents.