BACKGROUND
The Fenton's Reaction has been known since 1894 and is currently one of the most powerful oxidizing reactions available. The reaction involves hydrogen peroxide and a ferrous iron catalyst (Eqn. 1). The peroxide is broken down into a hydroxide ion and a hydroxyl free radical. The hydroxyl free radical is the primary oxidizing species and can be used to oxidize and break apart organic molecules.
It is well known that organic compounds can be easily oxidized. One primary advantage of the Fenton's Reaction is that it does not produce further organic compounds or inorganic solids such as permanganate and dichromate, since there is no carbon in the peroxide. This makes the Fenton's Reaction more appealing than a biological process, if the goal is removal of organic compounds. However, there are organic species that show resistance to oxidation by the Fenton's Reaction. Small chlorinated alkanes, n-paraffins, and short-chain carboxylic acids, compounds that are typical oxidation products of larger molecules, seem to resist further fragmentation by the Fenton's Reaction.
The mechanism of reaction with respect to hydrogen peroxide is very complex and may change with conditions of the reaction. Generally, though, the reaction follows a mechanism similar to the one listed below, (Eqns. 2 and 3).
If the reaction is carried to completion, then ultimately the organic molecules break down into CO2 and water, which are the normal end products of a combustion reaction. Also similar to a regular combustion reaction, organic destruction by the Fenton's Reagent is highly exothermic. Unlike combustion, Fenton's Reaction is associated with foaming, often very heavy and thick in the early parts of the reaction, especially for large compounds with high amounts of carbon.
THE PROBLEM
The T-1 and T-2 tanks at Oak Ridge National Laboratories (ORNL) contain radioactive and transuranic waste, along with a high concentration of an organic ion-exchange resin. Before the waste can be sent to the Melton Valley Storage Tanks (MVST) for storage and eventual final disposal at the Nevada Test Site (NTS), the organic resin must be separated from the sludge or destroyed. Since various leaching tests have not been successful in separating the resin from the transuranics, we have been testing the Fenton's Reaction as a means of destroying the resin. The first major problem encountered with this was that the reaction did not seem to proceed to complete destruction of the resin. Foaming was a secondary problem resulting from the initial reaction. Also, in reviewing the literature, we found that the majority of the work that has been done with this reaction appears to be soil remediation. Therefore we had to find ways to apply the data we found to our specific problem, since we were dealing with a radioactive-contaminant sludge, not soil.
APPLICATION
In this particular case, we used the Fenton's Reaction to oxidize the organic ion-exchange resin. The resin was in a slurrywith several ionic metals, iron included, so the only thing that had to be added was the peroxide. As the resin broke down, it created long chain organics, similar to soap, which created high levels of foam. After the resin was destroyed, the long chain organics were broken down further. Ultimately, in one way or another, nearly all the organic carbon was oxidized into carbon dioxide and evaporated off as this gas.
From the tests done, we found that during the time that there is still resin being destroyed, a temperature range anywhere between 60 and 90 degrees Celsius is acceptable. As stated before, the oxidation process is highly exothermic, therefore a reaction temperature in the 60-degree range is much easier to control than a higher temperature. However, after the resin is destroyed the only organics left are long-chain carbon compounds; a temperature in the 60s will not be sufficient. Instead the temperature should be maintained in the 80s range to effectively destroy the remaining organic compounds. This means that the reaction must be watched more closely as the heat released during the reaction has the potential to raise the temperature too high in a short period of time, causing the liquid to boil.
Another important consideration is the pH. A lower pH, in the range of 3 to 4, yields the best reaction rate. The pH tends to drop during the initial part of the reaction, therefore a strong base such as sodium hydroxide must be used for pH control. However, after the resin has been destroyed and only long chain carbon compounds remain, the pH tends to move to a more basic region, sometimes approaching a pH of 6 or higher. Allowing the pH to drift naturally does not seem to hinder the reaction, but adjusting it back to around 4 with a strong acid such as nitric acid is advisable. The reaction may appear to be continuing at a higher pH, but measuring the changes in peroxide concentration shows that the reaction being observed is actually the acidic peroxide reacting with the more basic solution.
Unfortunately, a heat balance could not be performed on the system under study. The container was a one liter beaker placed on a hot plate. There was no heat jacket and no measurements were taken of the heat, except for temperature readings. There is no way to determine exactly how much heat was gained or lost due to the plate, the reaction, the off gas, or any other factors. The closest one can get to analyzing the heat of reaction is by looking at some of the sections of data from the batch addition tests where readings were taken in very short spans, within ten to twenty seconds of each other (see Figure 1, below). Plotting the temperature and pH against time, starting with the addition of peroxide and ending when the peroxide was apparently used up, shows a very noticeable spike in both values. Of even more interest is the fact that both the temperature tended to consistently drop slightly as soon as the peroxide is added, before taking a quick rise within the first fifteen seconds. This is probably due to the fact that the peroxide is at room temperature, and is much cooler than the liquid in the beaker. It should also be mentioned that these close observations were only done on the solution after the resin was solubilized.
Analytical results for soluble total organic carbon (TOC) have been received for samples taken during the treatment process. The TOC initially increases, up to 22,000 mg/L, as the ion exchange resins are solubilized to long-chain organic sulfonates and amines. After most of the resin beads have disappeared, the TOC decreases as the long-chain organics are oxidized to carbon dioxide and water. The TOC concentration at the end of the treatment is about 1000 mg/L, which is similar to the TOC for the supernates currently in the MVSTs. The final product of the treatment is a solution of ammonium sulfate and inorganic sludge.
Oxidation of the resin using Fenton's Reagent appears to be a promising method for treating the T-1 and T-2 tank slurries to meet the Waste Acceptance Criteria for transfer to the MVSTs.