Formatted Title
Design Considerations and Field Techniques for In Situ Injection of Modified Clay for PFAS Remediation
Background/Objectives
Use of common adsorbents for the remediation of per- and polyfluorinated substances (PFAS) in situ has generally been limited to liquid activated carbon (LAC), also known as colloidal activated carbon (CAC), and biochar (BC), or conventional pump and treat systems. A significant issue with LAC/CAC/BC installations for remediation of contaminants is the mobility of the product after the energy of injection ceases. Most of these low-energy applications are not adequate to capture the total contaminant mass present due to limits in total effective sorption capacity and mobility of the product in the subsurface pore space. Further, conventional emplacement techniques of LAC/CAC/BC are ineffective for optimal distribution within certain overburden and regolith mediums.
Until recently, due to mesh size and chemical composition, practitioners believed that modified clay was not deployable in situ without the use of conventional soil mixing or civil construction techniques. However, overburden injection of an organically modified clay has been demonstrated using direct-push technology (DPT) and high-solids slurry batching and injection equipment.
Approach/Activities
The modified clay used in this demonstration was for the remediation of PFAS. The modified clay selected was FLUORO-SORB, which is manufactured by applying an organic chemical modifier to bentonite clay. The resultant product has high sorption kinetics, significant sorption capacity, can be effective across a wide range of PFAS concentrations, and, if necessary, is compatible for co-mixing with many other common site remediation reactants. In most cases, this combination functions without detrimental interaction or competitive adsorption for PFAS contaminants.
These statements have been verified by independent university laboratory testing where FLUORO-SORB was comparatively assessed with ion exchange resin (IX), GAC, and biochar. Additionally, competitive adsorption was tested with co-contaminants such as chlorinated volatile organic compounds (CVOCs) and petroleum hydrocarbons (PHCs). Relevant sorption and kinetics data will be discussed.
Results/Lessons Learned
Field deployments of modified clay were conducted in demonstration projects in Kentucky, USA and Alberta, Canada to prove the injectability of the technology in source (grid) and transmissive zone settings, respectively. These demonstrations verified the injectability and distribution of the modified clay as effective in different geologies and site deployment usages. Various slurry designs were also tested in the Kentucky example, examining increasingly dense and higher solids mixes to mimic site situations where significant product mass would be matched to significant PFAS mass. The slurry designs and specifications will be discussed from bench-scale evaluation to field deployment, and the lessons learned from varying the ratios of product and carrier fluid (water).
A discussion of quantified high density, remedial design characterization (RDC) activities necessary to construct this concise design and plan for injection success in the field will be included, plus a comparison of conventional and recently improved techniques for high-solids, true slurry injection. This includes the use of adaptable overburden remediation injection units (high energy/high flow trailer systems) combined with unique downhole tooling and field installation protocols to allow precise and proficient installation of modified clay injectate in a variety of unconsolidated and consolidated geologic settings to remediate PFAS.