Total Oxidant Demand Testing
Proper design of a field-scale implementation of in situ chemical oxidation (ISCO) requires data on target contaminant levels and quantitative estimates of other oxidant sinks. If all of the oxidant sinks are not properly accounted for, the amount of oxidant that needs to be injected will be underestimated, and the ISCO effort will likely fail. Besides the target contaminant, the oxidant sinks can be reduced minerals and naturally occurring organic materials (NOM). The amount of reduced minerals that will deplete oxidant depends upon the present oxidation-reduction potential (ORP) of the subsurface environment, as well as the chemical composition of the soil matrix (percentage of iron, for example). Rough estimates of the oxidant demand for reduced minerals can be made based on soil description and semi-qualitative descriptions of the ORP state of the aquifer (for example, iron- or sulfate-reducing conditions). However, this type of estimate can easily be in error by as much as an order of magnitude and cause the ISCO to fail or lead to the over-injection of oxidants. The other major oxidant sink is NOM. Obviously, not all NOM will be amenable to oxidation, and the level of oxidation of NOM depends upon the oxidant. Therefore, a simple analytical measurement such as total organic carbon may not accurately estimate the oxidant required for NOM.
Redox Tech has developed a simple colorimetric technique to estimate the total oxidant demand (TOD) of the aquifer or soil material. The colorimetric technique uses varying ratios of oxidant mass to soil mass prepared in separate vials. If the oxidant is permanganate, no color indicator is necessary because of the strong purple color from permanganate. If the oxidant is persulfate or peroxide, a starch-iodide indicator is necessary. The varying ratios of oxidant/soil mixtures are allowed to react, and the color is observed. The TOD can be narrowed down to the mixture ratio where color remains and the mixture ratio where color is depleted. Based upon numerous TOD tests, the TOD can be as little as 0.05 grams of oxidant per kilogram of saturated soil (for carbonate aquifers, for example) or as high as 15 grams of oxidant per kilogram of saturated soil (for organic-rich sediments under sulfate-reducing conditions, for example). Below is a figure that shows sample containers after a TOD test was completed. Redox Tech provides TOD testing as a service to all clients.
Figure 1. Sample Containers after a TOD Test with Permanganate
Figure 1. Sample Containers after a TOD Test with Permanganate