Formatted Title
Combined Remedy: Non-ZVI Chemical Reduction with Colloidal Carbon
Background/Objectives
Activated carbon (AC) sequestration of contaminants has become a go-to strategy for in situ remediation of petroleum sites for over 20 years. It is often combined with other amendments to add treatment components, e.g., biological or chemical oxidation. More recently, AC has been applied to chlorinated solvent sites with chemical reduction additives that are based on zero valent iron (ZVI). In both cases there is an ongoing industry and regulatory debate as to which process dominates, sequestration or the treatment chemistries.
Cascade, an industry provider of colloidal carbon chemistries, through its internal R&D program, has developed a patent pending “ColloidalChem +ISCR” technology that combines a novel chemical reduction using sodium dithionite with activated carbon that has distinct advantages for in situ treatment of PCE and TCE when compared with conventional approaches.
An exploratory laboratory research program was focused on coupling potential contaminant destruction reactions with the effective adsorption properties of AC. These experiments were run for seven days and generally focused on the destruction of 5 mg/L of either PCE or TCE in water. Reactions were monitored by GC/MS, and intermediates were quantified. The formation of ethene and ethane products were evaluated by GC/FID.
This work has resulted in discovery of a new chemical remediation process whereby activated carbon surfaces facilitate chemical reduction of the carbon-chlorine bonds of VOCs such as chlorinated ethenes. This new chemistry allows enhancement of activated carbon treatment in many scenarios, including the in situ treatment of PCE and TCE with colloidal forms of activated carbon.
Approach/Activities
If any in situ solvent projects are implemented in 2024, Cascade will present project overviews and results, if available
Results/Lessons Learned
Thorough reactivity studies have provided unequivocal evidence of the destruction of PCE and TCE by sodium dithionite/AC mixtures with minimal generation of the toxic daughter products, cis-1,2-dichloroethene and vinyl chloride. Analysis of dissolved gases confirmed the formation of ethene and ethane in reaction mixtures. The mechanism of these reactions is under investigation, and early reactivity data coupled with carbon-free control experiments emphasize the importance of the AC surface in these transformations. Compared with earlier literature reports of the use of dithionite for TCE remediation on mineral surfaces, this new carbon-based reaction is much faster and more reliable. Compared with conventional ISCR based on iron metal particles (ZVI), the use of the water-soluble dithionite provides a distinct injection application advantage where injection of solid metal particles is challenging. The potential of this new ISCR technology will be discussed, along with currently unanswered questions, ongoing topics of study, and plans for field implementation and commercialization.