AU2021107454A4

AU2021107454A4 - In-situ Soil Immobilisation utilising Powdered Activated Carbon (PAC) with High-Shear Mixing with an Enclosed Integrated Spreader System.

Info

Publication number: AU2021107454A4
Application number: AU2021107454A
Authority: AU
Inventors: Fred Lunsmann; Andrew Thomas
Original assignee: Enviropacific Services Ltd
Current assignee: Enviropacific Services Ltd
Priority date: 2021-08-24
Filing date: 2021-08-25
Publication date: 2021-12-23
Anticipated expiration: 2029-08-25
Also published as: AU2022221463A1

Abstract

The present invention relates to a method of remediating an in situ horizon of Per- or PolyFluoroAlkyl Substance (PFAS)-contaminated soil through the use of a Powdered Activated Carbon (PAC) reagent and a soil stabilizer, whereby a horizon of contaminated soil is mixed with the PAC so as to provide a means for immobilisation of at least a portion of the corresponding PFAS contamination. rio rli i Ih

Description

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IN-SITU PER- AND POLYFLUOROALKYL SUBSTANCE IMMOBILISATION USING POWDERED ACTIVATED CARBON AND HIGH-SHEAR MIXING WITH AN ENCLOSED INTEGRATED SPREADER SYSTEM. TECHNICAL FIELD

[0001] The present invention relates generally to the field of the contamination of land with Per- or PolyFluoroAlkyl Substances and more particularly methods of remediation through the use of Powdered Activated Carbon.

BACKGROUND

[0002] PFAS (Per- and PolyFluoroAlkyl Substances) are a class of synthetic chemicals used in a range of industries globally. The class in particular includes PerFluoroOctanoic Acid (PFOA) and PerFluoroOctaneSulfonic acid (PFOS) which have been widely used as surfactants because they lower the surface tension of water more than hydrocarbon-based surfactants.

[0003] One application favouring the use of PFAS, and PFOA/PFOS in particular, includes the manufacture of firefighting foams for liquid fuel fires. Even though fires only arise occasionally, the foams are frequently deployed as part of commissioning, testing, and training. The PFAS component of such foams can remain in the upper soil horizons, be transported downgradient via surface drainage, or be transported vertically toward the groundwater horizon with each consecutive rain event. Once mobilised in the groundwater, PFAS is difficult to remove and can be transported great distances, to resurface in river systems and groundwater bores in particular, such as the drinking water bores used by remote communities.

[0004] PFAS compounds do not break down in the environment and can accumulate over time. With a growing body of literature evidencing adverse health and environmental effects upon exposure to PFAS, this class of chemicals is gradually being phased out particularly in first world countries. Nevertheless, PFAS chemicals continue to have a range of industrial applications, continue to by synthesised abroad, and persist at existing contamination sites.

[0005] The remediation of contaminated land is an important step in mitigating PFAS exposure. A conventional approach to the treatment of PFAS-contaminated soil involves immobilising the leachable PFAS compounds with a sequestration reagent such as Powdered Activated Carbon (PAC). The immobilised PFAS compounds remain in the soil, however their activity is significantly reduced whereby they are less prone to leaching into, for example, water ways. In an alternative, the contaminated material can be stored onsite in lined repositories but not without significant cost.

[0006] Treatment of PFAS soil contamination using PAC has conventionally been performed ex-situ by excavating the contaminated soil and transporting it to a treatment facility where the PAC is applied, and then transporting the treated soil back to the excavation site for reinstatement (so-called ex situ treatment). More particularly, the conventional approach requires excavation, loading, transport, stockpiling, treatment, re stockpiling, re-loading, re-transporting, spreading and compaction of the contaminated soil volumes.

[0007] These activities are associated with significant machinery requirements, project cost, time, and the risk of dispersing contaminated soils to other areas, including by airborne dust and from water run-off. The latter can be particularly problematic as a water treatment plant may be required onsite to meet the prevailing standards. The conventional approach is also one of batching and is not adapted for continuous processing.

[0008] Furthermore, many PFAS-contaminated sites are characterised by the PFAS being concentrated within the upper soil horizons and spread over large areas. The material handling involved with conventional ex-situ treatment is not particularly well adapted to such sites.

[0009] Impermeable capping layers are often used in combination with ex situ treatment. In particular, a physical cap can be emplaced over PFAS-contaminated soils to limit erosion (whether by traffic or natural processes) and to limit transportation of PFAS by wind or water. Conventional caps include asphalt, concrete, compacted clay, and geomembranes, which all suffer the drawback of cost as well as the need for engineered drainage in particular due to their impermeability and the corresponding increase in natural ground level.

[0010] The present invention originates out of a desire to ameliorate some of the deficiencies in the prior art, and in particular to provide a means of in situ PFAS treatment that is adapted to high volume projects.

DISCLOSURE OF THE INVENTION

[0011] In a first aspect, the present invention relates to a method of remediating an in situ horizon of Per- or PolyFluoroAlkyl Substance (PFAS)-contaminated soil through the use of a Powdered Activated Carbon (PAC) reagent and a soil stabilizer; the horizon of soil having a lineal extent and a depth; the soil stabilizer being provided with a means of propulsion in a forward direction along the lineal extent of the horizon and consisting of: a drum or rotor for mixing the PFAS-contaminated soil with the PAC, the drum or rotor having a rotational axis and a plurality of elements for ripping the PFAS-contaminated soil disposed about the drum or rotor at predetermined radii from the rotational axis, a container or chute for holding a predetermined quantity of the PAC, and a means of releasing the PAC from the container or chute and spreading the PAC substantially uniformly onto the horizon substantially forward of the drum or rotor; the method consisting of the steps of: lowering the drum or rotor into at least an upper portion of the horizon, and while releasing and spreading the PAC onto the horizon, and with the drum or rotor rotating, propelling the soil stabilizer in the forward direction, whereby the at least upper portion of the horizon is mixed with the PAC so as to provide a means for immobilisation of at least a portion of the corresponding PFAS contamination.

DESCRIPTION OF FIGURES

[0012] Embodiments of the present invention will now be described in relation to figures, wherein

Figure 1 is a cutaway drawing of the soil stabilizer of the preferred embodiment whilst in operation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0013] With reference to Figure 1, according to a preferred embodiment of the invention there is provided a method of remediating an in situ horizon of Per- or PolyFluoroAlkyl Substance (PFAS)-contaminated soil 1 through the use of a Powdered Activated Carbon (PAC) reagent 2 and a soil stabilizer 3. The horizon of soil 1 has a lineal extent and a depth.

[0014] The soil stabilizer 3 is provided with a means of propulsion in a forward direction along the lineal extent of the horizon 1, and consists of: a drum or rotor 4 for mixing the PFAS-contaminated soil 1 with the PAC 2, the drum or rotor 4 having a rotational axis and a plurality of elements 5 for ripping the PFAS-contaminated soil 1 disposed about the drum or rotor 4 at predetermined radii from the rotational axis, a container or chute 6 for holding a predetermined quantity of the PAC 2, and a means 7 of releasing the PAC 2 from the container or chute 6 and spreading the PAC 2 substantially uniformly onto the horizon 1 substantially forward of the drum or rotor 4.

[0015] The method consists of the steps of: lowering the drum or rotor 4 into at least an upper portion of the horizon 1, and while releasing and spreading the PAC 2 onto the horizon 1, and with the drum or rotor 4 rotating, propelling the soil stabilizer 3 in the forward direction, whereby the at least upper portion of the horizon 1 is mixed with the PAC 2 so as to provide a means for immobilisation of at least a portion of the corresponding PFAS contamination.

[0016] In the preferred embodiment, the elements 5 for ripping the PFAS-contaminated soil consist of picks, chisels, or blades that are adapted and configured on the drum or rotor 4 to excavate a soil or bituminous or cementitious material consisting of a substantially continuous solid phase.

[0017] Thereby advantageously, the soil stabilizer 3 is provided as re-purposed in-situ pavement stabilisation machinery which can provide the high shear rates desirable under the present invention, and can perform well in the moist to wet clays, sandy soils, and clay-sand blends that are desirable in applications of the present invention.

[0018] Preferably, the PAC 2 is mixed with the PFAS-contaminated soil 1 in a ratio of 2t of PAC 2 for every 100t of PFAS-contaminated soil 1. It will be appreciated that the person skilled in the art can vary the ratio of PAC 2 according to the extent of PFAS contamination and other characteristics such as the activity of the PAC 2. Preferably, the PAC 2 has an iodine number of 900 according to ASTM D4607.

[0019] In the preferred embodiment, the soil stabilizer machinery 3 is provided with a conveying unit 8 to continuously release the PAC 2 from the container or chute 6, and an auger unit 9 to continuously spread the PAC 2 substantially uniformly along an axis parallel to the rotational axis of the drum or rotor 4.

[0020] In the preferred embodiment, the soil stabilizer machinery 3 is provided with a means to continuously feed the PAC 2 into the container or chute 6 while the soil stabilizer machinery 3 is being propelled in the forward direction.

[0021] Preferably, the PAC 2 is delivered directly to the soil stabilizer machinery 3 from the supplier in a pneumatic tanker to allow pneumatic delivery into the container or chute 6 to avoid rehandling and associated dust issues. In an alternative embodiment, the PAC 2 is stored in onsite bulk storage such as a silo and then delivered to the soil stabilizer machinery 3 in the pneumatic tanker. Where haulage distances from the supplier are too great, conventional bulka-bag transport/stowage is provided. The bulka-bags are unloaded into the onsite bulk storage with the addition of moisture for dust suppression.

[0022] In the preferred embodiment the means of propulsion consists of a tractor, however in other embodiments the means of propulsion is integrated with the soil stabilizer machinery.

[0023] In another embodiment, the method is further provided with the step of releasing and spreading a powdered bentonite along with the PAC 2 onto the horizon 1. Accordingly the at least upper portion of the horizon 1 when mixed with the PAC 2 and bentonite is provided with a reduced permeability arising from an occupation of soil pore space by the bentonite and/or from intermingled lens of contiguous bentonite.

[0024] Preferably, the powdered bentonite is mixed with the PAC 2 before feeding the mixture into the container or chute 6. Preferably, the bentonite is added in a proportion of 3 7% by weight of soil and more preferably about 5%.

[0025] The reduced permeability effectively provides an in situ capping layer whereby a leaching potential of the corresponding PFAS is reduced. This process is particularly advantageous where the need for a separate capping layer as provided through a separate process is otherwise too costly.

[0026] A covering layer of low permeability soil will in some circumstances be advantageous for protecting the underlying soil from erosion, traffic, and the effects of seasonal drying and wetting. The latter in particular may induce cracking, forming a pathway for water migration.

[0027] In another embodiment, the method is further provided with the step of releasing and spreading a cementitious binder along with the PAC 2 onto the horizon 1. Accordingly, the at least upper portion of the horizon when mixed with the PAC 2 and the cementitious binder is provided with an increased soil strength upon cure for resisting erosion and dust generation. It will be appreciated that the use of cementitious binder also reduces the permeability.

[0028] Preferably, the cementitious binder consists of hydrated lime and/or cement, optionally with the addition of pozzolan. More preferably, the cementitious binder consists of about 50/50 mixture of hydrated lime and cement in an overall proportion of 3-5% by weight of soil and most preferably about 4%. Preferably, the strength gains of the selected cementitious binder are controlled to substantially occur over 4 to 48 hours following application to allow time for soil compaction processes.

[0029] Accordingly there is provided a means of in situ treatment of PFAS contaminated soil 1 with PAC 2 to immobilise the PFAS, and/or with bentonite to reduce a permeability hence leachability of the PFAS contaminated soil, and/or with a cementitious binder to increase the durability of the soil. The PFAS can accordingly be both chemically and physically contained.

[0030] In preferred applications the depth of the horizon of soil 1 is less than 500mm, corresponding to the preferred radius of the drum or rotor 4 of about 500mm. It will be appreciated that the radius of the drum or rotor 4 can be adapted to suit the prevailing digging conditions of a particular site.

[0031] In an alternative embodiment of the present invention, where the depth of the horizon of soil 1 is substantially greater than 500mm or substantially greater than the radius of the drum or rotor 4, there is provided a modified method wherein the method of remediating the horizon of PFAS contaminated soil is repeated in multiple passes along the lineal extent of soil however with intermediate windrow re-handle.

[0032] In a first step of the modified method, a predetermined depth of the horizon 1 is windrowed to at least one side of a predetermined path of the soil stabilizer machinery 3, with a remainder of the horizon 1 being left in situ along the path. In a second step, the method of remediating the horizon of PFAS contaminated soil is then performed along the remainder of the horizon 1. In a third step, a predetermined volume of the windrow(s) is returned to the path of the soil stabilizer machinery 3. In a fourth step, the method of remediating the horizon of PFAS contaminated soil is performed along the returned soil.

[0033] The third and fourth steps are repeated as many times as is necessary to exhaust the windrow(s). For example, given a 1.2m deep horizon 1, 0.8m of depth can be windrowed and the remaining in situ 0.4m remediated, then 0.4m of depth returned from the windrow and remediated, then the final 0.4m of depth returned from the windrow and remediated.

[0034] The modified method may also be adapted to the treatment of stockpiles. In lieu of a windrowing approach, the stockpiled material is re-handled onto the path with each pass of the soil stabilizer machinery 3. Preferably the stockpile is shaped with a broad profile having multiple predetermined paths of the soil stabilizer machinery 3 across its width.

[0035] Accordingly there are provided methods that significantly reduce project costs and treatment time when compared with the prior art, with reference to ex situ treatment methods in particular. Furthermore, the present invention minimises the risk of dispersing contaminated soil to other areas. The method is amenable to significantly greater volumetric rates of material processed, and is particularly amenable to wet and clayey soils which would otherwise require pre-treatment or drying. Machinery requirements are minimised.

[0036] While the invention has been described with reference to preferred embodiments above, it will be appreciated by those skilled in the art that it is not limited to those embodiments, but may be embodied in many other forms, variations and modifications other than those specifically described. The invention includes all such variation and modifications. The invention also includes all of the steps, features, components and/or devices referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

[0037] In this specification, unless the context clearly indicates otherwise, the word "comprising" is not intended to have the exclusive meaning of the word such as "consisting only of", but rather has the non-exclusive meaning, in the sense of "including at least". The same applies, with corresponding grammatical changes, to other forms of the word such as "comprise", etc.

[0038] Other definitions for selected terms used herein may be found within the detailed description of the invention and apply throughout. Unless otherwise defined, all other scientific and technical terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the invention belongs.

[0039] Any promises made in the present document should be understood to relate to some embodiments of the invention, and are not intended to be promises made about the invention in all embodiments. Where there are promises that are deemed to apply to all embodiments of the invention, the applicant/patentee reserves the right to later delete them from the description and they do not rely on these promises for the acceptance or subsequent grant of a patent in any country.

Claims

1. A method of remediating an in situ horizon of Per- or PolyFluoroAlkyl Substance (PFAS)-contaminated soil through the use of a Powdered Activated Carbon (PAC) reagent and a soil stabilizer machinery; the horizon of soil having a lineal extent and a depth; the soil stabilizer machinery being provided with a means of propulsion in a forward direction along the lineal extent of the horizon and consisting of: a drum or rotor for mixing the PFAS-contaminated soil with the PAC, the drum or rotor having a rotational axis and a plurality of elements for ripping the PFAS contaminated soil disposed about the drum or rotor, a container or chute for holding a predetermined quantity of the PAC, and a means of releasing the PAC from the container or chute and spreading the PAC substantially uniformly onto the horizon substantially forward of the drum or rotor; the method consisting of the steps of: lowering the drum or rotor into at least an upper portion of the horizon, and while releasing and spreading the PAC onto the horizon, and with the drum or rotor rotating, propelling the soil stabilizer machinery in the forward direction, whereby the at least upper portion of the horizon is mixed with the PAC so as to provide a means for immobilisation of at least a portion of the corresponding PFAS contamination.

2. A method according to Claim 1 wherein the elements for ripping the PFAS contaminated soil consist of picks, chisels, or blades that are adapted and configured on the drum or rotor to excavate a soil or bituminous or cementitious material consisting of a substantially continuous solid phase.

3. A method according to Claim 1 wherein the soil stabilizer machinery is provided as re-purposed in-situ pavement stabilisation machinery.

4. A method according to Claim 1 wherein the soil stabilizer machinery is provided with a conveying unit to continuously release the PAC from the container or chute, and an auger unit to continuously spread the PAC substantially uniformly along an axis parallel to the rotational axis of the drum.

5. A method according to Claim 1 wherein the soil stabilizer machinery further includes a storage reservoir to store the PAC and is provided with a means to continuously feed the PAC from the storage reservoir into the container or chute while the soil stabilizer machinery is being propelled in the forward direction.

6. A method according to Claim 1 wherein there is further provided the step of releasing and spreading a powdered bentonite along with the PAC onto the horizon whereby the at least upper portion of the horizon when mixed with the PAC and the bentonite is provided with a reduced permeability.

7. A method according to Claim 1 wherein there is further provided the step of releasing and spreading a cementitious binder along with the PAC onto the horizon whereby the at least upper portion of the horizon when mixed with the PAC and the cementitious binder is provided with an increased soil strength after curing.

8. A method according to Claim 1 wherein the depth of the horizon of soil is less than 500mm.

9. A method according to Claim 1, further comprising the steps of: windrowing a predetermined depth of the horizon to at least one side of a predetermined path of the soil stabilizer machinery, with a remainder of the horizon being left in situ along the path,

remediating the remaining horizon of PFAS contaminated soil,

returning a predetermined volume of the windrowed horizon of soil to the path of the soil stabilizer machinery, and

remediating the returned horizon of PFAS contaminated soil.