Chlorinated solvents and 1,4-dioxane mixtures are primary contaminants of concern at Department of Defense (DoD) sites. 1,4-Dioxane occurrence in large groundwater contamination plumes represents a liability due to potential adverse health risks and remediation costs. Properties of 1,4-dioxane, including its high water solubility, resistance to adsorption, and low volatilization, have resulted in extensive groundwater contaminant plumes that are often as large as, or larger, than those associated with chlorinated solvents.

The project's objective was to develop advanced in situ chemical oxidation (ISCO) as a viable technology for 1,4-dioxane by enhancing the solubility, stability, activation, and transportability of strong oxidants (e.g., O₃ and activated persulfate), which enables in situ treatment of groundwater plumes. This goal has been achieved through the 1) co-injection of oxidants with chemical delivery agents that facilitate the transport and stability of the oxidant, 2) co-injection of oxidant mixtures that stabilize and activate oxidation, and 3) in situ activation of oxidants. The specific objectives of this project included:

1. Develop the ability to remediate 1,4-dioxane using in situ oxidation combined with facilitated transport, or delivery, of the oxidants within the subsurface;
2. Investigate oxidant activation methods and use of delivery agents to stabilize commonly used advanced oxidation process (AOP) reagents, such as O₃, persulfate, and oxidant mixtures, through complexation and activation;
3. Investigate the ability of delivery agents to increase the specificity of oxidant reactivity for 1,4-dioxane, and to enhance the stability and longevity of the oxidant;
4. Investigate the effectiveness of facilitated transport of oxidants complexed with delivery agents within sediments;

5. Investigate delayed activation or timed-release of oxidants from complexation.

   

This project developed ISCO as a viable technology for 1,4-dioxane groundwater plume remediation by enhancing the solubility, stability, activation, and transportability of strong oxidants (e.g., ozone or O₃), which facilitates the in situ treatment of groundwater plumes. This goal was achieved through the co-injection of oxidants with chemical agents that facilitate the transport of the oxidant. In case of ozone-based AOP, O₃ forms an inclusion complex or partitions into the delivery agent cyclodextrin, which increases the amount of oxidant delivered. Also, facilitated transport increased specificity of oxidation for 1,4-dioxane and co-contaminant chlorinated solvents as they also partition into the cyclodextrin. Additional methods for stabilizing and activating oxidants were developed by injecting mixtures of oxidants, such as persulfate, and using in situ aquifer materials and solid forms of iron (siderite, iron filings) to support delayed activation of injected oxidants to allow ISCO reagents to transport into contaminated groundwater systems.

During initial screening tests, the project examined polyamidoamine (PAMAM) dendrimers, Tween 80, and EDTA, which were all not effective for oxidant complexation. However, a potential for dendrimer application was found for extracting various ionic contaminants, such as nitrate and copper, from soil and groundwater. Importantly, cyclodextrin complexation with O₃ was proven to increase O₃ stability by more than a factor of 30. Multiple spectroscopic methods including UV and fluorescence for direct and competitive complexation, of O₃, 1,4-dioxane, and chlorinated co-contaminants (1,1,1-trichloroethane [TCA] and trichloroethene [TCE]) with HPβCD and γ-CD, produced comparable binding constant results although method applicability and accuracy can vary with guest compound analysis properties. Aqueous O₃, delivered with and without HPβCD, was capable of degrading 1,4-dioxane, TCE, and TCA to nondetectable concentrations, and the rate constants increased with addition of co-contaminants and salts whereas bicarbonate (i.e., radical scavenger) did not inhibit kinetics for 1,4-dioxane removal in HPβCD solutions. 1,4-Dioxane was successfully degraded by the siderite-activated binary H₂O₂-persulfate system, and the impact of radical scavengers was evaluated. A perozone activated persulfate (PAP) mixture was highly effective at transforming 1,4-dioxane, TCE, and TCA individually and in mixtures, and the degradation process was continuous for at least 13 days. The PAP application, such as the OxyZone® formulation, was demonstrated in both pulse and continuous injections into 1,4-dioxane contaminated plume experimental systems, which indicated oxidation potential extended long after the end of injection, thereby extending the oxidant solution’s range of influence. Also, iron filing were demonstrated for activating persulfate and 1,4-dioxane treatment within a permeable reactive barrier, and the results show that >99% removal would typically occur for bed lengths of about 3 to 4 meters.  

This research developed multiple, novel, cost effective, in situ treatment alternatives for 1,4-dioxane-contaminated source zones and groundwater plumes. It also advanced the understanding of processes controlling the complexation of oxidants and the impact of complexation, or other transport-related processes, on oxidation reactions. Three oxidant delivery methods were shown effective for 1,4-dioxane treatment, namely (1) co-injecting oxidants, (2) using complexing agents to facilitate reagent transport, and (3) exploiting aquifer materials to delay activation of oxidants. All three enhance longevity and transportability of oxidants result enhanced AOP treatment by reaching farther into the 1,4-dioxane contaminated aquifer. These findings suggest the potential for each of the 3 oxidant delivery methods to support sustained oxidation of recalcitrant organic contaminants that are difficult to treat using standard methods. These novel approaches enhance contaminated site remediation, and they could also reduce cleanup cost while increasing cleanup efficiency, which supports the effective management of DoD sites contaminated with 1,4-dioxane.

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