Technical Applications Manager, ISCO Technologies
Dr. Smith earned his Ph.D. in Civil Engineering from Washington State University and has published more than 50 technical presentations internationally. His research work has been published in several journals, including Environmental Science and Technology, Journal of Environmental Engineering and Environmental Toxicology and Chemistry.
Establishing Contact with Oxidative and Reductive Chemical Technologies: Fundamentals and New Concepts
Successful implementation of chemical oxidation and reduction technologies requires the selection of the proper technology and establishing contact between sufficient mass of reagents for the mass of contamination. Oxidative and reductive technologies are typically selected based on a variety of site specific factors including the contaminant characteristics, the remediation time frame, site specific limitations or constraints, costs, and the selected method of establishing contact. The methods of establishing contact between the reactive reagents and the contamination for source zones or soil treatment have fallen in three categories: 1) Mobile aqueous phase reagents are injected to treat relatively immobile contamination on soil; 2) The reagents and contaminated soils are blended together using physical mixing to establish contact; and 3) Aqueous phase contaminants migrate to and contact immobile reagents. This presentation will focus on the importance of combining a remedial technology with the method of establishing contact.
Injection strategies with aqueous phase reagents involve displacing a given pore volume to establish contact with the contamination. These strategies use injection locations that vary from fixed injection wells to temporary injection through specialized tooling. Injected reagents tend to follow preferential pathways in the subsurface and have had historic success as these are often the same pathways through which the contamination originally migrated. Injection strategies tend to have limited success in treating contamination that is not sufficiently contacted during the injection event either because it is not within the preferential flow path, because of difficulties in injecting sufficient reagents or because of displacement of aqueous phase contaminants. Oxidative injected technologies include activated persulfate, permanganate, ozone, and hydrogen peroxide. Reductive injected technologies include zero valent iron, ISCR reagents and under certain conditions activated persulfate and hydrogen peroxide.
Soil mixing overcomes many of the limitations of injection strategies. The physical blending process homogenizes the heterogeneity in both the soil and contamination and directly establishes contact between the reagents and contamination. A variety of reagents can be soil mixed including both oxidative and reductive solid or liquid amendments. In addition, soil mixing allows for the application of greater reagent mass which is important for the treatment of highly contaminated sites. However, soil mixing can compromise the soil structure. A newer concept in soil mixing is to combine alkaline activated sodium persulfate with in situ stabilization (ISS) reagents such as Portland cement which allows for better control over typical ISS parameters such as hydraulic conductivity and compressive soil strength. This allows for the potential combined remedy of combining ISCO and ISS.
The final typical concept in establishing contact is where mobile aqueous phase contaminants migrate to immobile reagents, such as in a permeable reactive barrier (PRB). PRBs are classically considered to be a trench format but this concept can take a variety of forms including the injection of solid or solid slurries in transects or into lower permeable soils to await back diffusion of contaminants. Historically, these reagents have been reductive technologies such as zero valent iron (ZVI) or ZVI based technologies such as EHC. More recent concepts introduce immobile oxidative technologies that can be used to treat aqueous phase contaminants. This is especially critical to treat contaminants requiring the oxidative pathway, such as 1,4-dioxane or petroleum hydrocarbons.