Formatted Title
Efficient Degradation of Tetrachloroethylene by Modified Fenton's Reaction Using Calcium Peroxide as an Oxidizer: A Bench-Scale Study
Background/Objectives
The present study presents a different approach for the degradation of chlorinated ethenes in groundwater. Remediation of tetrachloroethylene (PCE), a common chlorinated ethene, can be a difficult challenge to overtake, since PCE is harder to oxidize than other common organic contaminants, such as hydrocarbons. That is due to the fact that the carbon structure of PCE is already electron deficient, which makes it harder to take away extra electrons from it. The result is that a relatively large concentration of common oxidants, such as sodium persulfate, are needed to be able to achieve remediation goals. As an alternative to oxidation, a common procedure used nowadays to treat PCE in groundwater is to promote bioremediation, by means of injecting organic amendments, such as emulsified vegetable oils (EVO), molasses, sodium lactate and other organic substances that can act as electron donors. However, these biostimulation procedures are dependent on 1) the existence of the right kind of bacteria to promote full dehalogenation (mainly Dehalococcoides sp.); and 2) proper environmental conditions for these bacteria to thrive, in cases where bioaugmentation is being proposed. Herein, we propose the use of GEOAMBIENTE S.A.’s PeroxyGEO®, for the degradation of PCE in groundwater, as an alternative for common Fenton-like reactions, in which hydrogen peroxide is used as the oxidizer, activated by iron species. PeroxyGEO® is composed of calcium peroxide, which is tipically used either as an oxygen releasing compound or as the alkaline activator for persulfate treatments. The use of calcium peroxide, which acts as a slow releaser of hydrogen peroxide, mitigates common problems associated with the short half-life of hydrogen peroxide in the subsurface. By doing so, it is possible to take advantage of the powerful oxidation potential of the hydroxyl radicals generated by this modified process, which may be an efficient and cost-effective alternative to treat PCE contaminated groundwaters. Since Fenton processes are not selective, this can very likely be used to treat other kinds of contaminants as well.
Approach/Activities
Bench-scale studies were conducted with artificially produced contaminated groundwater, containing about 5339.73 ± 314.56 µg/L of PCE. Three independent variables were studied in this project: 1) the pH of the solution, which was adjusted using phosphate buffers, with values ranging from 5.37 to 8.63; 2) the amount of PeroxyGEO® used, in concentrations ranging from 0.75 to 3.55 g/L; and 3) the amount of iron(III)/EDTA used as the activator for the Fenton system, with concentrations ranging from 12.8 mg/L to 67.2 mg/L. The reaction time was 6 hours, after which, the reaction was quenched with the addiction of excess ascorbic acid. Afterwards, 10 mL of the quenched reaction solution was transferred to 20 mL vials, which were analyzed by a gas chromatograph coupled to a mass spectrometer. A rotatable central composite design was constructed, with triplicate runs in the center of the hypersphere, which allowed for the creation of a quadratic statistical model. In total, 20 chromatographic analyses were performed (17 to stablish the model and three to obtain the initial concentrations of PCE).
Results/Lessons Learned
Very promising degradation results were obtained with these experiments. Most of the experiments showed degradation of more than 90% of the PCE in six hours, with 100% occurring in one of the experiments (pH = 6, PeroxyGEO = 2,12 g/L and iron/EDTA = 25 mg/L). Consistently, the conditions in which the degradation were the poorest were the ones were pH was significantly more or less than neutral (smaller than 6 and higher than 8). The first statistical model showed that, in this experimental domain, the most important variables for the degradation of PCE were the pH (p = 0.001), iron squared (p = 0.059) and pH squared (p = 0.077). The fact that the calcium peroxide concentration was not shown to be statistically significant suggests that for the whole experimental domain studied, calcium peroxide was present in a more than enough concentration to degrade all of the PCE – if the other important variables are in their optimal values, of course (in other words, it is extremely important that a buffer solution is applied in situ with the oxidant system). The same observation is valid for the iron/EDTA concentration, although the quadratic term was still somewhat statistically significant at these concentrations. Although this study is in its early stages, it can be argued that using PeroxyGEO®-based Fenton processes may be an interesting alternative to rapidly degrade PCE and possibly other contaminants in groundwater, but it is extremely important that the whole oxidizing solution, when applied in situ, is prepared with some kind of pH buffering system, to keep the pH optimized.