In underground repository environments, various types of engineered barriers are installed to hinder the mobility of radionuclides. Cement admixtures, especially used to improve workability for concrete, are composed of fairly high organic molecules and have a dispersing effect through bonding with the C-S-H of the concrete. Previous studies have shown that complex-forming organics like EDTA, NTA, and ISA have a significant effect on the mobility of radionuclides, but the studies on the behavior and stability of combined complexes in hydrated cement are lacking. So, we selected a commonly used polycarboxylic-ester (PCE) type cement admixture and stable Co as a surrogate of Co-60 to perform desorption experiments from hydrated cement containing the admixture. Radioactive Co is known to be a common contaminant in nuclear fission and medical facilities and considered to exist as a relatively stable phase in repositories. In addition, the evaluation of cobalt can be a standard of safety issue for other radionuclides with the presence of cement admixture in repository. In this study, cement samples were prepared at water/cement ratio of 0.55 and cured for 28 days at 23-25°C and at least 80% of humidity with varying cement admixtures of 0.0, 0.1, and 2.0wt%. To evaluate the stability of cobalt in the weathered cement, a 0.001 M HCl solution was used to simulate cement weathering conditions on a hot plate at 60°C for 1 day using a solid/liquid ratio of 1:100. Degree of weathering was confirmed using XRD analysis. The adsorption experiments were performed by adding 0.0042 mmol of cobalt (CoCl2, Sigma-Aldrich, anhydrous ≥ 98.0%) to the weathered cement for 3 days using a platform shaker at 200 rpm, and the supernatant was separated using a syringe filter (<0.20 um) before ICP-MS analysis to determine the amount of Co adsorption. Cobalt desorption was tested for the Co-adsorbed cement using 0.019 mmol of calcium (Ca(NO3)2·4H2O, Sigma-Aldrich, 99%) for 3 hours to 14 days. The results showed that adsorbed cobalt with and without cement admixture was stably bound to cement, and did not increase any noticeable Co release by 2.0wt% PCE admixture. However, additional experiments using varying contents of PCE and other admixtures should be conducted to provide a standard for assessing the safety of cement admixtures in repositories.

The intermediate product resulting from the radical degradation experiment of PCE and the atomic charge gained through Gaussian03W were compared against each other. The result was that the ratio of PCE radical degradation was almost 98% or higher after the 9 hr point in reaction time. The reaction speed constant was 0.16 hr-1 and it followed the first reaction. We could see that at each location of the PCE molecule, dechlorination happened at a point where the negative atomic charge was the greatest. Moreover, the intermediate product of PCE radical degradation that was confirmed in the experiment and literature coincided exactly with the intermediate product in the atomic charge calculation. Therefore, when the atomic charge is calculated, the radical degradation pathway of the organic chlorine compound could be forecast.

Research trend and basic knowledge on the biological degradation of PCE (tetrachloroethylene) were reviewed. At first, anaerobic PCE degradation pathway was introduced, and microorganisms related with biological PCE dechlorination under both anaerobic and aerobic conditions were shown. PCE degradation is readily occurred in anaerobic conditions. Anaerobic PCE degradation was carried out by replacing the strongly combined chloride ion with hydrogen ion. The replacement occurs by electrons from electron donors such as H₂, acetate, lactate, and methanol. The best electron donor for PCE degradation is hydrogen. H₂ produced by the fermentation of carbon source can be used by microorganisms involved in acetogenesis, methanogenesis, sulfidogenesis, dehalorespiration, and iron reduction. These organisms can compete one another in natural ecosystem. Due to the competition, H₂ is sustained at low level. At this low level of H₂, the dechlorination of PCE can be maintained, but is inhibited by highly toxic VC accumulation in anaerobic PCE degradation. Since Dehalococcoides ethenogenes strain 195 can convert PCE into ethene which doesn"t have any toxicity and can further be used as a carbon source, it is very useful for field application. Pseudomonas stutzeri OX1 was able to utilize o-xylene and toluene as a carbon source using toluene-o-xylene monooxygenase for the first time in aerobic coditions. However, it has not further been proven. Based on these basic knowledge of biological PCE degradation, various field applications have been carried out and evaluated. Dehalococcoides related with PCE dechlorination were examined in contaminated sites. In a recent study on a wetland having both anaerobic and aerobic conditions, the effects of soil depth and plant species, and seasonal differences were researched as well as the sequential degradation of PCE. Formate was suggested as an alternative organic compound to efficiently neutralize the PCE degradation product, hydrochloric acid. To improve the PCE biodegradation effectively, the following research directions were proposed: (1) development of aerobically PCE degrading microorganism, (2) development of aerobic/anaerobic hybrid system (3) development of a system capable of maintaining consistent PCE degradation rate regardless of seasonal changes.

This paper presents applicability of Fenton oxidation to perchloroethylene(PCE) contaminated soil. The initial concentration of PCE was 187mg/kg and Fenton oxidation conditions were 1.0M H2O2 and 0.5M Fe2+. More than 97% of PCE decomposition and 98% of dechlorination were obtained within 5 hrs. It was found that the decomposition of PCE by Fenton oxidation was followed pseudo first order and its reaction coefficient was 0.78 hr-1. GC-MS and GC-ECD analysis of reaction intermediates confirmed only the presence of trichloroacetic acid(i.e., 1.0% of initial PCE concentration). Under Fenton oxidation conditions, it was proposed that PCE was decomposed not simultaneously but one by one.

The applicability of in situ biobarrier or microbial filter technology for the remediation of groundwater contaminated with chlorinated solvent was investigated through column study. In this study, the effect of packing materials on the reductive dechlorination of PCE was investigated using Canadian peat, Pahokee peat, peat moss and vermicompost (or worm casting) as a biobarrier medium. Optimal conditions previously determined from a batch microcosm study was applied in this column study. Lactate/benzoate was amended as electron donors to stimulate reductive dechlorination of PCE. Hydraulic conductivity was approximately 6×10-5－8×10-5 cm/sec and no difference was found among the packing materials. The transport and dispersion coefficients determined from the curve-fitting of the breakthrough curves of Br- using CXTFIT 2.1 showed no difference between single-region and two-region models. The reductive dechlorination of PCE was efficiently occurred in all columns. Among the columns, especially the column packed with vermicompost exhibited the highest reductive dechlorination efficiency. The results of this study showed the promising potential of in situ biobarrier technology using peat and vermicompost for the remediation of groundwater contaminated with chlorinated solvents.

A new method based on solid phase microextraction(SPME), coupled with GC/FID, has been developed for the determination of PCE and TCE in water samples. The experimental parameters affecting the SPME process (i.e, kinds of fibers, extraction time, desorption time, extraction temperature, volume ratio of sample to headspace, salt addition, and magnetic stirring) were optimized. The coefficients of determination (R2) for PCE and TCE were 0.9951 and 0.9831, respectively when analytes concentration ranges from 10 to 300㎍/L. The relative standard deviations were 3.4 and 2.1% for concentration of 10㎍/L(n=5), respectively. The detection limits of PCE and TCE were 0.5 and 1.3㎍/L, respectively.