Keywords: in-situ remediation, PFAS, bioremediation, colloidal solids
Summary:Remediation amendments are used to decontaminate soil and groundwater containing toxic soil and groundwater contaminants such as per- and polyfluorinated alkyl substances (PFAS), halogenated hydrocarbons, and petroleum hydrocarbons. Effective in-situ remediation is predicated on addressing several challenges including: 1) Delivering the remediation amendments to the contaminated soil and groundwater. This is usually accomplished by diluting the amendments in water and injecting the mixture into the ground using permanent or temporary injection wells. Best outcomes are usually achieved when the amendments are injected at low pressures that do not induce fracturing or channeling in the soil matrix. Low pressure applications require either water soluble products or solids/liquids that are small enough (typically ~ 1 um or less) to fit within and pass through the soil pore throats. 2) The most reactive amendments are ineffective if they disappear before the contaminants are eliminated from the soil and groundwater matrix. Often the most reactive products are consumed quickly and provide insufficient persistence. This issue is commonly used when using Fenton’s reagent and NZVI. This problem can be addressed by using engineered solid phase amendments that persist for several years in the sub-surface. 3) The amendment should sufficient reactivity with the targeted contaminants. The amendments should: a. Directly react with and convert the contaminants to innocuous substances. Examples are chemical reduction and oxidation. b. Create an environment that promotes contaminant degradation. Examples include anaerobic and aerobic bioremediation. c. Immobilize the contaminants and eliminating exposure risk in wells and other receptors. This is accomplished by using activated carbon and other adsorbents. In many circumstances, colloidal solid remediation amendments provide the best combination of reactivity, persistence, and sub-surface mobility. We will describe research, development, and implementation of several recently developed colloidal remediation products. These include: 1) Colloidal activated carbon that is used to immobilize groundwater contaminants including PFAS, chlorinated hydrocarbons, pesticides, and polycyclic aromatic hydrocarbons. These products contain micron sized activated carbon particles that are suspended in water using polymeric dispersants. 2) Sulfidated zero valent iron that is used to promote the direct abiotic reaction of chlorinated hydrocarbons to innocuous gases such as ethane. This material contains a surface layer of reduced iron sulfide that passivates the ZVI from unproductive hydrolysis reactions that consume the product and compromise its persistence. Sulfidation overcomes the longevity issues that had prevented the widespread commercial implementation of nanocrystalline zero valent iron (NZVI). 3) Colloidal fermentable remediation amendments that are used promote the biodegradation of groundwater contaminants. This product consists of solid phase, sub-micrometer, plant based organic compounds that ferment to produce soluble short chain fatty acids and molecular hydrogen. These particles degrade more slowly than traditional bioremediation amendments and support the metabolism of anaerobic bacteria such as dehalococcoides ethenogenes that degrade chlorinated hydrocarbons.