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==Hydrothermal Alkaline Treatment (HALT)==  
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==Thermal In Situ Sustainable Remediation to Enhance Biotic and Abiotic Reactions and Accelerate Remediation Timeframes==  
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Hydrothermal alkaline treatment (HALT) is a thermochemical processing technology effective at destroying and defluorinating [[Wikipedia: Halogenation | halogenated]] organic compounds such as [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | per- and polyfluoroalkyl substances (PFAS)]]. HALT is highly effective at destroying and defluorinating all types of PFAS that have been evaluated. The HALT technology enables end-to-end treatment and destruction of PFAS from a variety of matrices when integrated into a suitable treatment train.
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Thermal In Situ Sustainable Remediation (TISR<sup>®</sup>; US Pat. Nos. 10,384,246 and 10,688,545) is a cost-effective and adaptable low temperature thermal remediation technology that accelerates contaminant treatment. It achieves this by enhancing biotic and abiotic contaminant degradation rates through a modest increase in subsurface temperature. TISR<sup>®</sup> is suitable for a wide range of hydrogeologic settings. It also offers reduced operational costs, a lower carbon footprint, and possibly shorter remedial timeframes than more conventional methods.
 
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'''Related Article(s):'''
 
'''Related Article(s):'''
  
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*[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
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*[[Thermal Remediation]]
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*[[PFAS Ex Situ Water Treatment]]
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*[[Thermal Remediation - Electrical Resistance Heating | Electrical Resistance Heating]]
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*[[PFAS Sources]]
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*[[Thermal Conduction Heating (TCH)]]
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*[[PFAS Transport and Fate]]
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*[[Thermal Remediation - Combined Remedies | Combined Remedies]]
  
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'''Contributors:'''  
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'''Contributors:''' Jonah Munholland, Julia Vidonish, and Craig Divine
  
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*Dr. Brian Pinkard
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'''Key Resources:'''
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*Dr. Timothy Strathmann
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*Thermal In Situ Sustainable Remediation to Enhance Biotic and Abiotic Reactions and Accelerate Remediation Timeframes, Final Report. ESTCP Project ER20-5028<ref>Divine, C., Munholland, J., Zardouzian, E., Justicia-Leon, S., Aspinall, J.V., Randhawa, D., Falta, R., Freedman, D., Becker, M., 2025. Thermal In Situ Sustainable Remediation to Enhance Biotic and Abiotic Reactions and Accelerate Remediation Timeframes, Final Report. Environmental Security Technology Certification Program (ESTCP), Project ER20-5028. [https://serdp-estcp.mil/projects/details/bab4bc87-b4bc-42df-ad0e-11ea0adf27dc/er20-5028-project-overview Project Website]&nbsp; [[Media: ER20-5028 FR DevineEtAl2025.pdf | Final Report.pdf]]</ref>.
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*Dr. Shilai Hao
 
  
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'''Key Resource(s):'''
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*Advances in Low-Temperature Thermal Remediation. Munholland ''et al.'', 2024<ref name="MunhollandEtAl2024">Munholland, J, Rosso, D., Randhawa, D., Devine, C., Pennington, A., 2024. Advances in Low-Temperature Thermal Remediation. In: García-Rincón, J., Gatsios, E., Lenhard, R.J., Atekwana, E.A., Naidu, R. (eds), Advances in the Characterisation and Remediation of Sites Contaminated with Petroleum Hydrocarbons, Chapter 18 pp. 623-653. Environmental Contamination Remediation and Management series, Springer Nature. [https://doi.org/10.1007/978-3-031-34447-3_18 doi: 10.1007/978-3-031-34447-3_18]&nbsp; [[Media: MunhollandEtAl2024.pdf | Open Access Chapter]]</ref>.
  
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*Hydrothermal Technologies for On-Site Destruction of Site Investigation Wastes Impacted by PFAS<ref name="Strathmann2023">Strathmann, T.J., 2023. Hydrothermal Technologies for On-Site Destruction of Site Investigation Wastes Impacted by PFAS. Project number ER18-1501, Strategic Environmental Research and Development Program (SERDP).</ref>
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*A Design Tool for Solar Thermal Remediation Using Borehole Heat Exchangers. Ornelles, Falta, and Divine, 2023<ref name="Ornelles">Ornelles, A.D., Falta, R.W., Divine, C.E., 2023. A Design Tool for Solar Thermal Remediation Using Borehole Heat Exchangers. Ground Water, 61(2), pp. 245-254. [https://doi.org/10.1111/gwat.13265 doi: 10.1111/gwat.13265]&nbsp; [https://ngwa.onlinelibrary.wiley.com/doi/epdf/10.1111/gwat.13265 Free download from NGWA]</ref>.
  
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*Hydrothermal Alkaline Treatment for Destruction of Per- and Polyfluoroalkyl Substances in Aqueous Film-Forming Foam<ref name="HaoEtAl2021">Hao, S., Choi, Y.J., Wu, B., Higgins, C.P., Deeb, R., Strathmann, T.J., 2021. Hydrothermal Alkaline Treatment for Destruction of Per- and Polyfluoroalkyl Substances in Aqueous Film-Forming Foam. Environmental Science and Technology, 55(5), pp. 3283-3295.&nbsp; [https://doi.org/10.1021/acs.est.0c06906 doi: 10.1021/acs.est.0c06906]</ref>
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*Low-Temperature Thermal Remediation: Gaining Traction as a Green Remedial Alternative. Horst, ''et al.'', 2018<ref name="HorstEtAl2018">Horst, J., Flanders, C., Klemmer, M., Randhawa, D.S., Rosso, D., 2018. Low-Temperature Thermal Remediation: Gaining Traction as a Green Remedial Alternative. Groundwater Monitoring and Remediation, 38(3), pp. 18-27. [https://doi.org/10.1111/gwmr.12295 doi: 10.1111/gwmr.12295]</ref>.
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*[[Media: PinkardEtAl-2024a.pdf | Degradation and Defluorination of Ultra Short-, Short-, and Long-Chain PFASs in High Total Dissolved Solids Solutions by Hydrothermal Alkaline Treatment ─ Closing the Fluorine Mass Balance]]<ref name="PinkardEtAl2024a">Pinkard, B., Smith, S.M., Vorarath, P., Smrz, T., Schmick, S., Dressel, L., Bryan, C., Czerski, M., de Marne, A., Halevi, A., Thomsen, C., Woodruff, C., 2024. Degradation and Defluorination of Ultra Short-, Short-, and Long-Chain PFASs in High Total Dissolved Solids Solutions by Hydrothermal Alkaline Treatment─Closing the Fluorine Mass Balance. ACS ES&T Engineering, 4(11), pp. 2810-2818.&nbsp; [https://doi.org/10.1021/acsestengg.4c00378 doi: 10.1021/acsestengg.4c00378]&nbsp; [[Media: PinkardEtAl-2024a.pdf | Open Access Report.pdf]]</ref>
 
  
 
==Introduction==
 
==Introduction==
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[[File:PinkardFig1.png|thumb|300px|Figure 1. HALT refers to the subcritical water region on the pressure–temperature phase diagram of water]]
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[[File: MunhollandFig1.png | thumb | 500 px | Figure 1: Components and layout of a TISR<sup>®</sup> system]]
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Hydrothermal alkaline treatment (HALT) is a thermochemical processing technology effective at destroying and defluorinating halogenated organic compounds such as per- and polyfluoroalkyl substances (PFAS). HALT is also known as “[[Wikipedia: Hydrolysis#Alkaline_hydrolysis | alkaline hydrolysis]],” and is very similar to processing technologies such as [[Wikipedia: Hydrothermal liquefaction | hydrothermal liquefaction (HTL)]] which have been developed and investigated for organic waste-to-energy applications.
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TISR<sup>®</sup> utilizes [[Wikipedia: Downhole heat exchanger | borehole heat exchangers (BHEs)]] to heat target treatment areas in the subsurface. This is achieved through a combination of solar collectors, which harness solar radiation as heat, and the utilization of other heat sources such as waste heat or hybrid heating with heat pumps. That heat is then transferred to the subsurface via a closed loop circulation system connected to the BHEs (Figure 1). By incorporating these additional heat sources, TISR<sup>®</sup> ensures a more robust and constant heat supply, enabling customization of the system to address specific site challenges and facilitate efficient heat transfer.
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HALT processing subjects PFAS in an aqueous solution to high pressure, high temperature, and high pH conditions. The required operating conditions are dependent on the specific target PFAS being destroyed, as [[Wikipedia: Perfluoroalkyl carboxylic acids | perfluorocarboxylic acids (PFCAs)]] such as trifluoroacetic acid (TFA) can be destroyed under mild conditions (e.g., P ~ 2 MPa, T ~ 200 °C, pH ~ 13)<ref name="AustinEtAl2024">Austin, C., Purohit, A., Thomsen, C., Pinkard, B.R., Strathmann, T.J., Novosselov, I.V., 2024. Hydrothermal Destruction and Defluorination of Trifluoroacetic Acid (TFA). Environmental Science and Technology, 58(18), pp. 8076-8085.&nbsp; [https://doi.org/10.1021/acs.est.3c09404 doi: 10.1021/acs.est.3c09404]</ref>, whereas [[Wikipedia: Perfluorosulfonic acids | perfluorosulfonic acids (PFSAs)]] such as [[Wikipedia: Perfluorobutanesulfonic acid | perfluorobutanesulfonic acid (PFBS)]] require more aggressive processing conditions (e.g., P ~ 25 MPa, T ~ 350 °C, pH ~ 14.7) [5] (Figure 1) . HALT is capable of facilitating complete “mineralization” of PFAS, defined as the conversion of organic fluorine to dissolved inorganic fluoride. The treatment time for HALT is relatively shorter (<2 hours) compared to most other PFAS destructive technologies. For instance, treatment of two-fold diluted [[Wikipedia: Firefighting foam | aqueous film-forming foams (AFFFs)]] using HALT in batch mode achieved nearly complete defluorination in just 30 minutes under conditions of 350 °C and 5 M NaOH<ref name="HaoEtAl2021"/>. PFCAs can be destroyed with even faster kinetics at milder conditions; for example, >90% destruction and defluorination of [[Wikipedia: Trifluoroacetic acid | trifluoroacetic acid (TFA)]] was achieved within 40 min at 200 °C<ref name="AustinEtAl2024"/>. Kinetic rate constants for individual PFAS compounds in HALT environments have been proposed in several studies<ref name="AustinEtAl2024"/><ref name="WuEtAl2019">Wu, B., Hao, S., Choi, Y.J., Higgins, C.P., Deeb, R., Strathmann, T.J., 2019. Rapid Destruction and Defluorination of Perfluorooctanesulfonate by Alkaline Hydrothermal Reaction. Environmental Science and Technology Letters, 6(10), pp. 630-636.&nbsp; [https://doi.org/10.1021/acs.estlett.9b00506 doi: 10.1021/acs.estlett.9b00506]</ref>. Several studies have also investigated the fluorine mass balance during HALT processing, showing near-stoichiometric conversion of organic fluorine to inorganic fluoride under optimal conditions<ref name="PinkardEtAl2024a"/>.
 
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From a practical perspective, HALT is best suited for destroying PFAS in concentrated liquids such as liquid concentrate streams produced as byproducts of other water treatment processes (e.g., [[PFAS Treatment by Anion Exchange | regenerable ion exchange]], foam fractionation). Previous publications demonstrate that complex sample matrices, including high concentrations of inorganic salts (e.g., 83 g/L chloride) and dissolved organic carbon (e.g., 13 g/L), do not inhibit the degradation rate of PFAS compared to a clean matrix, such as groundwater<ref name="HaoEtAl2022">Hao, S., Choi, Y.J,. Deeb, R.A., Strathmann, T.J., Higgins, C.P., 2022. Application of Hydrothermal Alkaline Treatment for Destruction of Per- and Polyfluoroalkyl Substances in Contaminated Groundwater and Soil. Environmental Science and Technology, 56(10), pp. 6647-6657.&nbsp; [https://doi.org/10.1021/acs.est.2c00654 doi: 10.1021/acs.est.2c00654]</ref><ref name="HaoEtAl2023">Hao, S., Reardon, P.N., Choi, Y.J., Zhang, C., Sanchez, J.M., Higgins, C.P., Strathmann, T.J., 2023. Hydrothermal Alkaline Treatment (HALT) of Foam Fractionation Concentrate Derived from PFAS-Contaminated Groundwater. Environmental Science and Technology 57(44), pp. 17154-17165.&nbsp; [https://doi.org/10.1021/acs.est.3c05140 doi: 10.1021/acs.est.3c05140]</ref>. Moreover, several field demonstrations of HALT have been performed successfully, and the technology is scalable for commercialization.
 
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==Reaction Mechanisms and Treatment Efficacy==
 
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[[File:PinkardFig2.png|thumb|300px|Figure 2. Representative classes of PFAS structures among 148 PFASs demonstrated complete degradation during HALT<ref name="HaoEtAl2021"/>]]
 
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[[File:PinkardFig3.png|thumb|300px|Figure 3. The degradation of representative classes of PFAS during HALT of 1-to-1000 diluted AFFF under conditions of 1 M NaOH, 350 °C, and a reaction time of 60 minutes<ref name="HaoEtAl2021"/>.]]
 
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Laboratory scale batch experiments have shown that the full suite of PFAS detected in aqueous film-forming foams (AFFFs) through targeted [[Wikipedia: Liquid chromatography–mass spectrometry | LC-MS/MS and LC-HRMS]] suspect screening analysis are degraded and defluorinated by HALT<ref name="HaoEtAl2021"/>. Figure 2 presents representative classes of PFAS structures among 148 PFAS demonstrating complete degradation during HALT. Figure 3 illustrates the degradation during HALT of representative classes of PFAS detected in an AFFF. The extent of destruction for all PFAS is highly temperature dependent, but results show that some subclasses of PFAS degrade in the absence of alkali amendments (e.g., PFCAs)<ref name="AustinEtAl2024"/>, whereas other subclasses require strong alkali in addition to near-critical reaction temperatures (e.g., PFSAs)<ref name="Strathmann2023"/><ref name="HaoEtAl2021"/><ref name="WuEtAl2019"/>.  This is attributed to different mechanisms that initiate the destruction of the individual PFAS subclasses. Degradation of PFCAs is initiated by thermally driven decarboxylation reactions<ref name="AustinEtAl2024"/>, whereas PFSA degradation, in the temperature range of HALT reactors, is proposed to be initiated via attack by the strong nucleophile OH-.<ref name="HaoEtAl2021"/> 
 
  
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A mechanistic understanding of the HALT process for PFAS destruction needs further evaluation to optimize the process and reduce the consumption of chemicals and energy. While the studies of neat compounds are relatively straightforward, one of the major challenges is to address the effect of co-contaminants and apply the process to real-world operating scenarios. Recent laboratory studies with batch reactors conducted at the Colorado School of Mines (CSM) have extended the application of HALT for the destruction of PFAS in a variety of contaminated matrices, including groundwater and soils<ref name="HaoEtAl2022"/> and foam fractionation-derived liquid concentrate<ref name="HaoEtAl2023"/>. Apparent rates for the transformation of individual PFAS have been found to be largely insensitive to the type of media<ref name="HaoEtAl2023"/>, but there is a need to account for any reactions with the media that consume OH· (e.g., OH· reactions with silica-containing soil minerals)<ref name="HaoEtAl2022"/> Notably, while alkali is not required to degrade PFCAs, it is still necessary to convert the organically bound fluorine to inorganic F<sup>-</sup>. Austin ''et. al.''<ref name="AustinEtAl2024"/> showed that TFA, a C<sub>1</sub> PFCA, degrades at similar rates in the absence and presence of NaOH, but mineralization to F<sup>-</sup> and CO<sub>3</sub><sup>2-</sup> only occurs when NaOH is added; otherwise fluoroform (CHF<sub>3</sub>) is the terminal product when no NaOH is added to the reaction solution.
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The heating of the subsurface through TISR<sup>®</sup> accelerates various processes such as biodegradation, abiotic reactions, and contaminant desorption and dissolution. The technology is also suitable for diverse hydrogeologic environments, including sites with low permeability zones, due to the relatively uniform thermal conductivity of geologic materials, which is unaffected by grain size<ref name="MunhollandEtAl2024"/>. This versatility allows TISR<sup>®</sup> to integrate with conventional remediation technologies, enhancing their performance.
  
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HALT can also be applied to destroy other fluorinated compounds, for example, hydrofluorocarbon (HFC) refrigerants. HFC refrigerants are known to decompose into PFAS such as TFA in the atmosphere and thereby subsequently appear in concerning concentrations in rainwater. By themselves, HFCs are resistant to thermal degradation; however, in the presence of alkali (e.g., NaOH), alkaline hydrolysis can occur at T < 150˚C<ref name="AustinEtAl2024"/>.
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One of the key advantages of TISR<sup>®</sup> is its ability to accelerate treatment processes while minimizing energy consumption. By harnessing renewable and sustainable energy sources such as solar radiation and effectively utilizing waste heat or hybrid heating with heat pumps, TISR<sup>®</sup> delivers an efficient, cost-effective remediation solution. Compatibility with multiple conventional remediation technologies makes it an effective option for addressing a variety of contamination challenges.
  
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==State of the Art==
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==Scientific Principles==
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[[File:PinkardFig4.png | thumb |300px| Figure 4: HALT field demonstration at Fairbanks International Airport (FAI) in August 2023]]
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[[File: MunhollandFig2.png | thumb | 500 px | Figure 2: Effect of temperature on hydrolysis rates for VOCs]]
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Recently, several field demonstrations of pilot-scale HALT systems were performed by commercial HALT provider Aquagga, Inc. These have focused on treating PFAS-rich liquids, including industrial wastewater at a 3M Company facility (April 2024)<ref name="PinkardEtAl2024b">Pinkard, B.R., Smith, S.M., Bryan, C., 2024. PFAS Degradation and Defluorination of High TDS Wastewater via Continuous Hydrothermal Alkaline Treatment (HALT). In: (Proceedings of the) 85th Annual International Water Conference (IWC 2024), Volume 1, pp. 359-374. Engineers Society of Western Pennsylvania. ISBN: 979-8-3313-1299-2</ref>, foam fractionate from a fire training pit in Fairbanks, AK (August 2023), foam fractionate from groundwater at Beale Air Force Base, CA (May 2024), and AFFF (May 2024). For all field demonstrations, a containerized HALT system was mobilized to the site and operated for up to several weeks. The systems were typically operated at a throughput between 5 and 10 gallons per hour (gph). Since 2019, HALT has progressed from small-scale batch reactors to successful field demonstration of pilot-scale systems. This technology maturation attests to strong technical and regulatory tailwinds. Effort is still needed to demonstrate the technology at full scale and in complex treatment scenarios. Long-term operation of the systems will allow for further optimization of the systems and provide data on the applicability of HALT for the treatment of industrial and environmental PFAS-contaminated waste streams.
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During the TISR<sup>®</sup> heating process, thermal conduction and advection play a crucial role in boosting physical, biological, and chemical processes within the treatment zones, leading to enhanced attenuation, degradation, and removal of contaminants from the subsurface<ref>Dettmer, K., 2002. A Discussion of the Effects of Thermal Remediation Treatments on Microbial Degradation Processes. U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Technology Innovation Office, Washington, DC. [[Media: dettmer2002.pdf | free report]]&nbsp; [https://www.clu-in.org/download/techdrct/dettmer.pdf free download from clu-in.org]</ref><ref name="HorstEtAl2018"/>. TISR<sup>®</sup> systems are typically designed to enhance the biological activity and performance of other active technologies, such as [[Wikipedia: Air sparging | air sparge]]/[[Soil Vapor Extraction (SVE) | soil vapor extraction]] (AS/SVE) or [[Chemical Oxidation (In Situ - ISCO) | chemical oxidation]], by increasing the subsurface temperature by approximately 10°C to 20°C. The target groundwater temperature is often set to reach up to about 35°C, which is the optimal temperature for promoting the activity of mesophilic microbial communities responsible for biodegradation of certain contaminants. In cases where the objective is to maximize abiotic hydrolysis mechanisms, such as for some chlorinated alkanes, TISR<sup>®</sup> systems can be designed to reach a final groundwater temperature of up to 60°C.
  
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Pilot-scale HALT systems are typically continuous flow tubular reactor systems, consisting of a single high-temperature, high-pressure fluid path. In commercial HALT systems offered by Aquagga, Inc., chemical dosing for pH adjustment is achieved via an automated chemical dosing and mixing system. The high pH feedstock is then introduced to the high-pressure reactor via a high-pressure metering pump. Pressure is controlled via a back-pressure device downstream of the high-temperature reactor zone. The pressurized reactants are brought to reaction temperatures via a recuperative heat exchanger followed by electric resistive heaters. The reactor vessel contains the reactants at the necessary temperature and pressure and for a sufficient residence time to facilitate the destruction reactions. The product stream is then cooled through a recuperative heat exchanger, before being throttled to ambient pressure through the back-pressure device. Pressure transducers, flow meters, and thermocouples are used to monitor the reactor operations at various points in the system. All reactor components are typically housed within a shipping container, for ease of system transport and to provide secondary chemical containment.
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The effectiveness of TISR<sup>®</sup> in enhancing treatment performance has been demonstrated through several mechanisms, which are discussed below:
  
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==Practical Applications==
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*Primary dechlorinating bacteria, such as [[Wikipedia: Dehalococcoides | Dehalococcoides]] and [[Wikipedia: Dehalogenimonas | Dehalogenimonas]], are known to be [[Wikipedia: Mesophile | mesophilic]]. Therefore, increasing the temperature can significantly enhance microbial activity and biodegradation by multiple orders of magnitude<ref>Macbeth, T., Truex, M., Powell, T., Michalsen, M., 2012. Combining Low-Energy Electrical Resistance Heating with Biotic and Abiotic Reactions for Treatment of Chlorinated Solvent DNAPL Source Areas. [https://serdp-estcp.mil/ Environmental Security Technology Certification Program (ESTCP)] [https://serdp-estcp.mil/projects/details/877a3b41-6bed-4f4a-b43a-87fbcd9b7f0b/er-200719-project-overview Project ER-200719 webpage]&nbsp; [[Media: Macbeth2012_ER-200719-FR.pdf | free download]]&nbsp; [https://www.youtube.com/watch?v=-EbwKbs8scE Project Summary Presentation]</ref>.
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[[File:PinkardFig5.png | thumb |300px| Figure 5: An on-site HALT pilot demonstration at a 3M Company wastewater treatment facility]]
 
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The ideal use case for HALT is treating PFAS-rich liquid matrices. PFAS concentrations are high enough for HALT to be directly applicable primarily in the cases of AFFF treatment or industrial process water treatment. In the majority of use cases, it is more practical to apply a separation and concentration technology prior to HALT, to reduce the volume of liquid requiring HALT treatment while increasing PFAS concentrations in that liquid. These concentration technologies may include regenerable sorbents, membranes, or foam fractionation, all of which produce a liquid byproduct amenable for HALT.
 
  
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==='''Destruction of PFAS in Ion Exchange Regeneration Brine'''===
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*Reduced iron minerals, particularly iron sulfides, can participate in the abiotic degradation of chlorinated ethenes and other contaminants. The rates of these reactions generally depend on temperature, following the [[Wikipedia: Arrhenius equation | Arrhenius equation]].
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One of the most promising applications of HALT is for treating PFAS-rich ion exchange (IX) regeneration brines, either in site remediation applications (e.g., groundwater treatment<ref name="Pinkard2024">Pinkard, B.R., 2024. Hydrothermal Alkaline Treatment for a Closed-Loop, On-Site PFAS Treatment Solution. Project Number ER23-8400, Environmental Security Technology Certification Program (ESTCP).&nbsp; [https://serdp-estcp.mil/projects/details/a4c6918a-fe3b-43d2-95cb-fa3dfa3a50a2 Project Website]</ref>) or industrial wastewater treatment applications<ref name="PinkardEtAl2024a"/>. IX capture and regeneration involve sorbing PFAS to an IX resin, followed by chemical desorption of PFAS from the resin, typically with a solvent and/or salt wash solution. The IX regeneration technology is commercially mature and available from several vendors.  
 
  
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A treatment train of IX followed by HALT shows promise for several reasons. One reason is that the HALT process is highly compatible with the liquid matrix produced through the IX regeneration. Typically, IX regeneration brine (a.k.a. “still bottoms”) contains high levels of dissolved solids such as sodium chloride, which can cause practical processing challenges with other liquid treatment technologies. However, high levels of TDS do not appear to cause processing challenges with HALT<ref name="PinkardEtAl2024a"/>. Another reason is that IX regeneration brines often contain ultra short- and short-chain PFAS, which are amenable to destructive treatment with HALT.
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*[[Wikipedia: Hydrolysis | Hydrolysis]] rates for certain compounds exhibit high temperature dependence. For example, a 10°C increase can lead to a 50% reduction in the half-life of [[Wikipedia: 1,1,1-Trichloroethane | 1,1,1-trichloroethane (1,1,1-TCA)]]<ref>Suthersan, S., Horst, J., Klemmer, M., Malone, D., 2012. Temperature-Activated Auto-Decomposition Reactions: An Under-Utilized In Situ Remediation Solution. Groundwater Monitoring and Remediation, 32(3), pp. 34-40. [https://doi.org/10.1111/j.1745-6592.2012.01407.x doi: 10.1111/j.1745-6592.2012.01407.x]</ref>. Figure 2 presents the calculated temperature-dependent hydrolysis rates for 13 common [[Wikipedia: Volatile organic compound | volatile organic compounds (VOCs)]] at a pH of 7.
  
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In 2022, commercial HALT provider Aquagga performed a bench study in partnership with the 3M Company, demonstrating PFAS destruction performance for HALT processing of a synthetic IX regeneration brine<ref name="PinkardEtAl2024a"/>. Seven treatment conditions were tested, and fluorine mass balance closure was demonstrated for most conditions using a range of analytical techniques. In 2024, Aquagga performed an on-site demonstration in partnership with the 3M Company treating IX regeneration brine produced from active wastewater treatment activities<ref name="PinkardEtAl2024b"/>.
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*[[Wikipedia: Henry's law | Henry's Law]] constant, [[Wikipedia: Vapor pressure | vapor pressure]], solubility, and viscosity values are also temperature dependent. For instance, the Henry's Law constant values for many VOCs, including [[Wikipedia: Tetrachloroethylene | tetrachloroethene (PCE)]], have been observed to double between 8 and 24°C<ref>Chen, F., Freedman, D.L., Falta, R.W., Murdoch, L.C., 2012. Henry's law constants of chlorinated solvents at elevated temperatures. Chemosphere, 86(2), pp.156-165. [https://doi.org/10.1016/j.chemosphere.2011.10.004 doi: 10.1016/j.chemosphere.2011.10.004]</ref>.
  
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==Using Peeper Data at a Sediment Site==
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*Rates of other processes that often limit the pace of remediation, such as dissolution, desorption, and [[Matrix Diffusion | back diffusion]], are positively correlated with temperature. This results in an enhanced bioavailability of contaminant mass that may otherwise be inaccessible.
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Peeper data can be used to enable site specific decision making in a variety of ways. Some of the most common uses for peepers and peeper data are discussed below.
 
  
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'''Nature and Extent:''' Multiple peepers deployed in sediment can help delineate areas of increased metal availability. Peepers are especially helpful for sites that are comprised of coarse, relatively inert materials that may not be conducive to traditional bulk sediment sampling. Because much of the inorganics present in these types of sediments may be associated with the porewater phase rather than the solid phase, peepers can provide a more representative measurement of C<small><sub>0</sub></small>. Additionally, at sites where tidal pumping or groundwater flux may be influencing the nature and extent of inorganics, peepers can provide a distinct advantage to bulk sediment sampling or other point-in-time measurements, as peepers can provide an average measurement that integrates the variability in the hydrodynamic and chemical conditions over time.
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These various mechanisms collectively contribute to the improved performance of other active treatments when temperature is moderately increased by utilization of TISR®.
  
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'''Sources and Fate:''' A considerable advantage to using peepers is that C<small><sub>0</sub></small> results are expressed as concentration in units of mass per volume (e.g., mg/L), providing a common unit of measurement to compare across multiple media. For example, synchronous measurements of C<small><sub>0</sub></small> using peepers deployed in both surface water and sediment can elucidate the potential flux of inorganics from sediment to surface water. Paired measurements of both C<small><sub>0</sub></small> and bulk metals in sediment can also allow site specific sediment-porewater partition coefficients to be calculated. These values can be useful in understanding and predicting contaminant fate, especially in situations where the potential dissolution of metals from sediment are critical to predict, such as when sediment is dredged.
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==Implementation and Practical Considerations==
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[[File: MunhollandFig3.png | thumb | 500 px | Figure 3: Transformation of 1,1-DCE and VC following installation of a TISR<sup>®</sup> system]]
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TISR<sup>®</sup> has proven successful in reducing contaminant concentrations across diverse hydrogeologic settings, including those with low and complex hydraulic conductivity conditions. Figure 3 shows a sharp initial decrease in [[Wikipedia: 1,1-Dichloroethylene | 1,1-dichloroethene (1,1-DCE)]] concentrations due to enhanced biodegradation, and then a temporary small increase followed by subsequent decreases in 1,1-DCE concentrations to below detection limits, possibly the result of temperature-enhanced desorption or dissolution of 1,1-DCE from the aquifer. There is also a temporary increase and then decrease in the concentrations of [[Wikipedia: Vinyl chloride | vinyl chloride (VC)]] (a biodegradation product) to very low levels.  
  
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'''Direct Toxicity to Aquatic Life:''' Peepers are frequently used to understand the potential direct toxicity to aquatic life, such as benthic invertebrates and fish. A C<small><sub>0</sub></small> measurement obtained from a peeper deployed in sediment (''in situ'') or surface water (''ex situ''), can be compared to toxicological benchmarks for aquatic life to understand the potential toxicity to aquatic life and to set remediation goals<ref name="USEPA2017"/>. C<small><sub>0</sub></small> measurements can also be incorporated in more sophisticated approaches, such as the Biotic Ligand Model<ref>Santore, C.R., Toll, E.J., DeForest, K.D., Croteau, K., Baldwin, A., Bergquist, B., McPeek, K., Tobiason, K., and Judd, L.N., 2022. Refining our understanding of metal bioavailability in sediments using information from porewater: Application of a multi-metal BLM as an extension of the Equilibrium Partitioning Sediment Benchmarks. Integrated Environmental Assessment and Management, 18(5), pp. 1335–1347. [https://doi.org/10.1002/ieam.4572 doi: 10.1002/ieam.4572]</ref> to understand the potential for toxicity or the need to conduct toxicological testing or ecological evaluations.
+
The TISR<sup>®</sup> system construction utilizes off-the-shelf modular components, promoting cost-efficiency, reusability, and a reduced carbon footprint. Waste heat can supplement or replace the solar setup, and remote monitoring through dashboard apps reduces operation and maintenance costs. TISR® projects typically achieve an average capital cost payback within 1 to 3 years due to accelerated rates of remediation.
  
−
'''Bioaccumulation of Inorganics by Aquatic Life:''' Peepers can also be used to understand site specific relationship between C<small><sub>0</sub></small> and concentrations of inorganics in aquatic life. For example, measuring C<small><sub>0</sub></small> in sediment from which organisms are collected and analyzed can enable the estimation of a site-specific uptake factor. This C<small><sub>0</sub></small>-to-organism uptake factor (or model) can then be applied for a variety of uses, including predicting the concentration of inorganics in other organisms, or estimating a sediment C<small><sub>0</sub></small> value that would be safe for consumption by wildlife or humans. Because several decades of research have found that the correlation between C<small><sub>0</sub></small> measurements and bioavailability is usually better than the correlation between measurements of chemicals in bulk sediment and bioavailability, C<small><sub>0</sub></small>-to-organism uptake factors are likely to be more accurate than uptake factors based on bulk sediment testing.
+
When implementing TISR<sup>®</sup>, [[Wikipedia: Solar irradiance | solar insolation]] and groundwater velocity must be considered for optimal equipment sizing and cycling. Depending on remedial goals and site history and conditions, the TISR<sup>®</sup> system and target temperatures may be designed to enhance specific remedial mechanisms (e.g. hydrolysis over bioremediation). Recently Ornelles ''et al.''<ref name="Ornelles"/> developed an easy to use Excel-based model that can be used to optimize TISR<sup>®</sup> design and predict subsurface temperature changes over time. Frequent monitoring of the temperature and microbial community composition relative to targets is useful for evaluating treatment effectiveness and optimizing system performance. The system and target temperatures can be customized to enhance specific remedial mechanisms based on site conditions and goals.  
  
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'''Evaluating Sediment Remediation Efficacy:''' Passive sampling has been used widely to evaluate the efficacy of remedial actions such as active amendments, thin layer placements, and capping to reduce the availability of contaminants at sediment sites. A particularly powerful approach is to compare baseline (pre-remedy) C<small><sub>0</sub></small> in sediment to C<small><sub>0</sub></small> in sediment after the sediment remedy has been applied. Peepers can be used in this context for inorganics, allowing the sediment remedy’s success to be evaluated and monitored in laboratory benchtop remedy evaluations, pilot scale remedy evaluations, and full-scale remediation monitoring.
+
==Summary==
 +
TISR<sup>®</sup> is a cost-effective and adaptable technology that can be customized to address specific site challenges. It can be used as a standalone solution or be integrated with existing remediation strategies to enhance their effectiveness. TISR<sup>®</sup> is particularly effective in challenging hydrogeological conditions and can be optimized to suit specific treatment mechanisms. By shortening the active remediation timeframe, TISR<sup>®</sup> reduces lifecycle costs and minimizes the carbon footprint associated with the remediation process.
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==References==
 
==References==
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==See Also==
 
==See Also==
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*[https://vimeo.com/809180171/c276c1873a Peeper Deployment Video]
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[https://vimeo.com/1019489395 TISR<sup>®</sup> Explanation Video]
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*[https://vimeo.com/811073634/303edf2693 Peeper Retrieval Video]
 
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*[https://vimeo.com/811328715/aea3073540 Peeper Processing Video]
 
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*[https://sepub-prod-0001-124733793621-us-gov-west-1.s3.us-gov-west-1.amazonaws.com/s3fs-public/2024-09/ER20-5261%20Fact%20Sheet.pdf?VersionId=malAixSQQM3mWCRiaVaxY8wLdI0jE1PX Fact Sheet]
 

Latest revision as of 19:44, 13 October 2026

Thermal In Situ Sustainable Remediation to Enhance Biotic and Abiotic Reactions and Accelerate Remediation Timeframes

Thermal In Situ Sustainable Remediation (TISR®; US Pat. Nos. 10,384,246 and 10,688,545) is a cost-effective and adaptable low temperature thermal remediation technology that accelerates contaminant treatment. It achieves this by enhancing biotic and abiotic contaminant degradation rates through a modest increase in subsurface temperature. TISR® is suitable for a wide range of hydrogeologic settings. It also offers reduced operational costs, a lower carbon footprint, and possibly shorter remedial timeframes than more conventional methods.

Related Article(s):

Contributors: Jonah Munholland, Julia Vidonish, and Craig Divine

Key Resources:

  • Thermal In Situ Sustainable Remediation to Enhance Biotic and Abiotic Reactions and Accelerate Remediation Timeframes, Final Report. ESTCP Project ER20-5028[1].
  • Advances in Low-Temperature Thermal Remediation. Munholland et al., 2024[2].
  • A Design Tool for Solar Thermal Remediation Using Borehole Heat Exchangers. Ornelles, Falta, and Divine, 2023[3].
  • Low-Temperature Thermal Remediation: Gaining Traction as a Green Remedial Alternative. Horst, et al., 2018[4].

Introduction

File:MunhollandFig1.png
Figure 1: Components and layout of a TISR® system

TISR® utilizes borehole heat exchangers (BHEs) to heat target treatment areas in the subsurface. This is achieved through a combination of solar collectors, which harness solar radiation as heat, and the utilization of other heat sources such as waste heat or hybrid heating with heat pumps. That heat is then transferred to the subsurface via a closed loop circulation system connected to the BHEs (Figure 1). By incorporating these additional heat sources, TISR® ensures a more robust and constant heat supply, enabling customization of the system to address specific site challenges and facilitate efficient heat transfer.

The heating of the subsurface through TISR® accelerates various processes such as biodegradation, abiotic reactions, and contaminant desorption and dissolution. The technology is also suitable for diverse hydrogeologic environments, including sites with low permeability zones, due to the relatively uniform thermal conductivity of geologic materials, which is unaffected by grain size[2]. This versatility allows TISR® to integrate with conventional remediation technologies, enhancing their performance.

One of the key advantages of TISR® is its ability to accelerate treatment processes while minimizing energy consumption. By harnessing renewable and sustainable energy sources such as solar radiation and effectively utilizing waste heat or hybrid heating with heat pumps, TISR® delivers an efficient, cost-effective remediation solution. Compatibility with multiple conventional remediation technologies makes it an effective option for addressing a variety of contamination challenges.

Scientific Principles

File:MunhollandFig2.png
Figure 2: Effect of temperature on hydrolysis rates for VOCs

During the TISR® heating process, thermal conduction and advection play a crucial role in boosting physical, biological, and chemical processes within the treatment zones, leading to enhanced attenuation, degradation, and removal of contaminants from the subsurface[5][4]. TISR® systems are typically designed to enhance the biological activity and performance of other active technologies, such as air sparge/ soil vapor extraction (AS/SVE) or chemical oxidation, by increasing the subsurface temperature by approximately 10°C to 20°C. The target groundwater temperature is often set to reach up to about 35°C, which is the optimal temperature for promoting the activity of mesophilic microbial communities responsible for biodegradation of certain contaminants. In cases where the objective is to maximize abiotic hydrolysis mechanisms, such as for some chlorinated alkanes, TISR® systems can be designed to reach a final groundwater temperature of up to 60°C.

The effectiveness of TISR® in enhancing treatment performance has been demonstrated through several mechanisms, which are discussed below:

  • Primary dechlorinating bacteria, such as Dehalococcoides and Dehalogenimonas, are known to be mesophilic. Therefore, increasing the temperature can significantly enhance microbial activity and biodegradation by multiple orders of magnitude[6].
  • Reduced iron minerals, particularly iron sulfides, can participate in the abiotic degradation of chlorinated ethenes and other contaminants. The rates of these reactions generally depend on temperature, following the Arrhenius equation.
  • Rates of other processes that often limit the pace of remediation, such as dissolution, desorption, and back diffusion, are positively correlated with temperature. This results in an enhanced bioavailability of contaminant mass that may otherwise be inaccessible.

These various mechanisms collectively contribute to the improved performance of other active treatments when temperature is moderately increased by utilization of TISR®.

Implementation and Practical Considerations

File:MunhollandFig3.png
Figure 3: Transformation of 1,1-DCE and VC following installation of a TISR® system

TISR® has proven successful in reducing contaminant concentrations across diverse hydrogeologic settings, including those with low and complex hydraulic conductivity conditions. Figure 3 shows a sharp initial decrease in 1,1-dichloroethene (1,1-DCE) concentrations due to enhanced biodegradation, and then a temporary small increase followed by subsequent decreases in 1,1-DCE concentrations to below detection limits, possibly the result of temperature-enhanced desorption or dissolution of 1,1-DCE from the aquifer. There is also a temporary increase and then decrease in the concentrations of vinyl chloride (VC) (a biodegradation product) to very low levels.

The TISR® system construction utilizes off-the-shelf modular components, promoting cost-efficiency, reusability, and a reduced carbon footprint. Waste heat can supplement or replace the solar setup, and remote monitoring through dashboard apps reduces operation and maintenance costs. TISR® projects typically achieve an average capital cost payback within 1 to 3 years due to accelerated rates of remediation.

When implementing TISR®, solar insolation and groundwater velocity must be considered for optimal equipment sizing and cycling. Depending on remedial goals and site history and conditions, the TISR® system and target temperatures may be designed to enhance specific remedial mechanisms (e.g. hydrolysis over bioremediation). Recently Ornelles et al.[3] developed an easy to use Excel-based model that can be used to optimize TISR® design and predict subsurface temperature changes over time. Frequent monitoring of the temperature and microbial community composition relative to targets is useful for evaluating treatment effectiveness and optimizing system performance. The system and target temperatures can be customized to enhance specific remedial mechanisms based on site conditions and goals.

Summary

TISR® is a cost-effective and adaptable technology that can be customized to address specific site challenges. It can be used as a standalone solution or be integrated with existing remediation strategies to enhance their effectiveness. TISR® is particularly effective in challenging hydrogeological conditions and can be optimized to suit specific treatment mechanisms. By shortening the active remediation timeframe, TISR® reduces lifecycle costs and minimizes the carbon footprint associated with the remediation process.

References

  1. ^ Divine, C., Munholland, J., Zardouzian, E., Justicia-Leon, S., Aspinall, J.V., Randhawa, D., Falta, R., Freedman, D., Becker, M., 2025. Thermal In Situ Sustainable Remediation to Enhance Biotic and Abiotic Reactions and Accelerate Remediation Timeframes, Final Report. Environmental Security Technology Certification Program (ESTCP), Project ER20-5028. Project Website  Final Report.pdf
  2. ^ 2.0 2.1 Munholland, J, Rosso, D., Randhawa, D., Devine, C., Pennington, A., 2024. Advances in Low-Temperature Thermal Remediation. In: García-Rincón, J., Gatsios, E., Lenhard, R.J., Atekwana, E.A., Naidu, R. (eds), Advances in the Characterisation and Remediation of Sites Contaminated with Petroleum Hydrocarbons, Chapter 18 pp. 623-653. Environmental Contamination Remediation and Management series, Springer Nature. doi: 10.1007/978-3-031-34447-3_18  Open Access Chapter
  3. ^ 3.0 3.1 Ornelles, A.D., Falta, R.W., Divine, C.E., 2023. A Design Tool for Solar Thermal Remediation Using Borehole Heat Exchangers. Ground Water, 61(2), pp. 245-254. doi: 10.1111/gwat.13265  Free download from NGWA
  4. ^ 4.0 4.1 Horst, J., Flanders, C., Klemmer, M., Randhawa, D.S., Rosso, D., 2018. Low-Temperature Thermal Remediation: Gaining Traction as a Green Remedial Alternative. Groundwater Monitoring and Remediation, 38(3), pp. 18-27. doi: 10.1111/gwmr.12295
  5. ^ Dettmer, K., 2002. A Discussion of the Effects of Thermal Remediation Treatments on Microbial Degradation Processes. U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Technology Innovation Office, Washington, DC. free report  free download from clu-in.org
  6. ^ Macbeth, T., Truex, M., Powell, T., Michalsen, M., 2012. Combining Low-Energy Electrical Resistance Heating with Biotic and Abiotic Reactions for Treatment of Chlorinated Solvent DNAPL Source Areas. Environmental Security Technology Certification Program (ESTCP) Project ER-200719 webpage  free download  Project Summary Presentation
  7. ^ Suthersan, S., Horst, J., Klemmer, M., Malone, D., 2012. Temperature-Activated Auto-Decomposition Reactions: An Under-Utilized In Situ Remediation Solution. Groundwater Monitoring and Remediation, 32(3), pp. 34-40. doi: 10.1111/j.1745-6592.2012.01407.x
  8. ^ Chen, F., Freedman, D.L., Falta, R.W., Murdoch, L.C., 2012. Henry's law constants of chlorinated solvents at elevated temperatures. Chemosphere, 86(2), pp.156-165. doi: 10.1016/j.chemosphere.2011.10.004

See Also

TISR® Explanation Video

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