CHEMICAL OXIDATION PROCESSES
ETEC has utilized many different chemical oxidation processes via direct injection and groundwater recirculation technologies:
CHEMICAL OXIDATION - BIOREMEDIATION
In Situ Chemical Oxidation (ISCO) has been shown to be a very effective remedial alternative for a broad-range of petroleum, PAH, and chlorinated solvent contaminants. ETEC improves upon the typical slug injection of chemical oxidants by utilizing groundwater recirculation. The groundwater recirculation approach enables us to deliver the appropriate mass of chemical oxidants/catalysts to the subsurface and to achieve a greater degree of contact with the contaminants than what would be possible with a typical slug injection.
Selecting the most appropriate chemical oxidant for a particular contaminant is as critical as selecting the right delivery system. ETEC has utilized many different chemical oxidation processes in combination with groundwater recirculation. Before implementation of these methods, ETEC recommends conducting bench-scale tests with impacted media from the site. Many factors affect chemical oxidation (ex. type of catalyst, concentrations of oxidant, natural organic material, etc.), and ETEC can conduct thorough bench-scale testing to evaluate different concentrations of oxidants and metal catalysts that will provide you the most cost-effective approach prior to application at the site.
The most common chemical oxidant used is hydrogen peroxide, which requires the use of a metal catalyst to generate hydroxyl radicals capable of oxidizing organic compounds. Typical catalysts are ferrous/ferric iron or a chelated metal compound. The hydroxyl radicals generated from this method have a very short half-life in the subsurface because they are highly reactive with all organic material and carbonates. This characteristic makes this method more amenable to source zone or hot spot treatment, and not so favorable for remediating large areas due to its limited transportability.
This oxidant has characteristics that are very favorable for in situ remediation. Specifically, it is highly soluble and more stable in the subsurface than hydroxyl radicals, providing a high degree of transportability from the injection points. A metal catalyst or heat is required to create the sulfate radicals capable of oxidizing a wide-range of organic contaminants. Its low cost and high solubility allows a user to deliver the appropriate total mass of persulfate to the subsurface without incurring significant cost or labor in the field.
Combination of Hydrogen Peroxide and Persulfate
Academic research and ETECs experience have shown that using hydrogen peroxide in combination with persulfate provides the most effective chemical oxidation method. This method still requires the use of a metal catalyst to generate the hydroxyl and sulfate radicals. Initially, the metal catalyst solution is injected into a well that is then followed by a persulfate solution, and then a peroxide solution. This approach delivers the catalyst ahead of the oxidants so the reaction will occur where the iron solution is. The persulfate is injected ahead of the peroxide because the heat produced from the peroxide reaction will increase the reactivity of the persulfate. Since persulfate is more stable it will linger in the subsurface until the rapidly reacting peroxide is injected. This method has been shown to effectively oxidize PAHs, some PCBs, chlorinated solvents, and petroleum hydrocarbons.
This oxidant has been shown to be the most effective with chlorinated ethenes, and less effective on aromatic compounds. Permanganate oxidizes contaminants via an electron transfer reaction, and not via a free radical like Fenton’s. It is the most stable oxidant and can reside in groundwater for long periods of time if no reaction occurs. This ability allows it to penetrate tighter soil types (i.e. silt or clay) where other oxidants fail to achieve contact. When permanganate reacts with a contaminant it produces manganese dioxide that is insoluble in water and forms precipitants in the porespace. This can significantly reduce the effective porosity in the saturated zone, possibly limiting future injections.