Dechlorination Of Trichloroethylene Research Articles

Abiotic dechlorination in rock matrices impacted by long-term exposure to TCE

Field and laboratory tests were performed to evaluate the abiotic reaction of trichloroethene (TCE) in sedimentary rock matrices. Hydraulically conductive fractures, and the rock directly adjacent to the hydraulically conductive fractures, within a historically contaminated TCE bedrock aquifer were used as the basis for this study. These results were compared to previous work using rock that had not been exposed to TCE (Schaefer et al., 2013) to assess the impact of long-term TCE exposure on the abiotic dechlorination reaction, as the longevity of these reactions after long-term exposure to TCE was hitherto unknown. Results showed that potential abiotic TCE degradation products, including ethane, ethene, and acetylene, were present in the conductive fractures. Using minimally disturbed slices of rock core at and near the fracture faces, laboratory testing on the rocks confirmed that abiotic dechlorination reactions between the rock matrix and TCE were occurring. Abiotic daughter products measured in the laboratory under controlled conditions were consistent with those measured in the conductive fractures, except that propane also was observed as a daughter product. TCE degradation measured in the laboratory was well described by a first order rate constant through the 118-d study. Observed bulk first-order TCE degradation rate constants within the rock matrix were 1.3×10−8s−1. These results clearly show that abiotic dechlorination of TCE is occurring within the rock matrix, despite decades of exposure to TCE. Furthermore, these observed rates of TCE dechlorination are expected to have a substantial impact on TCE migration and uptake/release from rock matrices.

Effect of different types of calcium sulfate on the reactivity of cement/Fe(II) system in dechlorination of trichloroethylene

Several studies have been carried out to find out the reactive species in cement/Fe(II) system for the treatment of ground water contaminated with a number of chlorinated organic pollutants such as trichloroethylene (TCE). The previous studies suggest that cement hydration minerals associated with sulfate (SO42-) could be one of the reactive species responsible for dechlorination reaction. Calcium sulfate which is a primary source of sulfate in cement has not been studied in detail. Present study deals with the effect of different types of calcium sulfate on the reactivity of cement/Fe(II) system in dechlorination of TCE. To evaluate the effect of different types of calcium sulfate on reactivity of cement/Fe(II) system, different types of calcium sulfate were mixed with clinker powder to get three different types of cements. TCE reduction experiments were carried out in the presence of Fe(II) using these cements. The results showed that cement with anhydrate and hemihydrate forms of calcium sulfate showed improved TCE reduction kinetics compared to cement with calcium sulfate dihydrate. Different analytical tools such as thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy were used to differentiate between the types of calcium sulfate and understand the minerals generated at the end of TCE reduction experiments. Further studies revealed that the dissolution behavior of calcium sulfate in individual cement was different and had an influence on the formation of ettringite, which was suspected as a reactive mineral.

Aerobic degradation of trichloroethylene by co-metabolism using phenol and gasoline as growth substrates.

Trichloroethylene (TCE) is a common groundwater contaminant of toxic and carcinogenic concern. Aerobic co-metabolic processes are the predominant pathways for TCE complete degradation. In this study, Pseudomonas fluorescens was studied as the active microorganism to degrade TCE under aerobic condition by co-metabolic degradation using phenol and gasoline as growth substrates. Operating conditions influencing TCE degradation efficiency were optimized. TCE co-metabolic degradation rate reached the maximum of 80% under the optimized conditions of degradation time of 3 days, initial OD600 of microorganism culture of 0.14 (1.26 × 107 cell/mL), initial phenol concentration of 100 mg/L, initial TCE concentration of 0.1 mg/L, pH of 6.0, and salinity of 0.1%. The modified transformation capacity and transformation yield were 20 μg (TCE)/mg (biomass) and 5.1 μg (TCE)/mg (phenol), respectively. Addition of nutrient broth promoted TCE degradation with phenol as growth substrate. It was revealed that catechol 1,2-dioxygenase played an important role in TCE co-metabolism. The dechlorination of TCE was complete, and less chlorinated products were not detected at the end of the experiment. TCE could also be co-metabolized in the presence of gasoline; however, the degradation rate was not high (28%). When phenol was introduced into the system of TCE and gasoline, TCE and gasoline could be removed at substantial rates (up to 59% and 69%, respectively). This study provides a promising approach for the removal of combined pollution of TCE and gasoline.

Inhibition of iron (III) minerals and acidification on the reductive dechlorination of trichloroethylene

Reductive dechlorination of chlorinated ethenes is inhibited by acidification and by the presence of Fe (III) as a competitive electron acceptor. Synergism between both factors on dechlorination is predicted as reductive dissolution of Fe (III) minerals is facilitated by acidification. This study was set-up to assess this synergism for two common aquifer Fe (III) minerals, goethite and ferrihydrite. Anaerobic microbial dechlorination of trichloroethylene (TCE) by KB-1 culture and formate as electron donor was investigated in anaerobic batch containers at different solution pH values (6.2–7.2) in sand coated with these Fe minerals and a sand only as control. In the absence of Fe, lowering substrate pH from 7.2 to 6.2 increased the time for 90% TCE degradation from 14±1d to 42±4d. At pH 7.2, goethite did not affect TCE degradation time while ferrihydrite increased the degradation time to 19±1d compared to the no Fe control. At pH 6.2, 90% degradation was at 78±1 (ferrihydrite) or 131±1d (goethite). Ferrous iron production in ferrihydrite treatment increased between pH 7.2 and 6.5 but decreased by further lowering pH to 6.2, likely due to reduced microbial activity. This study confirms that TCE is increasingly inhibited by the combined effect of acidification and bioavailable Fe (III), however no evidence was found for synergistic inhibition since Fe reduction did not increase as pH decreases. To the best of our knowledge, this is the first study where effect of pH and Fe (III) reduction on TCE was simultaneously tested. Acid Fe-rich aquifers need sufficient buffering and alkalinity to ensure swift degradation of chlorinated ethenes.

Carbon and chlorine isotope fractionation during Fenton-like degradation of trichloroethene

Dual isotope approach has been proposed as a viable tool for characterizing and assessing in situ contaminant transformation, however, little data is currently available on its applicability to chlorinated ethenes. This study determined carbon and chlorine isotope fractionation during Fenton-like degradation of trichloroethene (TCE). Carbon and chlorine isotope enrichment factors were εC=−2.9±0.3‰ and εCl=−0.9±0.1‰, respectively. An observed small secondary chlorine isotope effect (AKIECl=1.001) was consistent with an initial transformation by adding hydroxyl radicals (OH) to CC bonds without cleavage of CCl bonds. The relative change in carbon and chlorine isotope ratios (Δ=Δδ13C/Δδ37Cl) was calculated to be 3.1±0.2, approximately equal to the ratio of chlorine and carbon isotope enrichment factors (εC/εCl=3.2). The similarity of the Δ (or εC/εCl) values between Fenton-like degradation and microbial reductive dechlorination of TCE was observed, indicating that application of solely dual isotope approach may be limited in distinguishing the two transformation pathways.

Control of trichloroethylene plume migration using a biobarrier system: a field-scale study.

The objective of this field-scale study was to evaluate the effectiveness of controlling trichloroethylene (TCE) plume migration using the polycolloid substrate (PS) biobarrier. The developed PS (containing soybean oil, lactate and surfactants) could release substrate to enhance the TCE dechlorination. In this study, a biobarrier comprising PS injection wells was installed. Injection wells were installed at 5-m intervals, and approximately 15 L of PS was injected into each well. Results show that TCE concentrations in the injection wells dropped from an average of 87 μg/L to below 1 μg/L after 35 days of PS injection. The total organic carbon concentrations in the injection wells increased from an average of 2.1-543 mg/L after 30 days of PS injection. The dissolved oxygen (DO) concentrations and oxidation-reduction potential (ORP) values dropped from an average of 1.6 mg/L to below 0.1 mg/L and from 124 mv to -14 mv after 20 days of injection, respectively. The DO and ORP remained in anaerobic conditions during the remaining 100 days of the operational period. TCE degradation by-products were observed in groundwater samples during the operational period. This reveals that the addition of PS could effectively enhance the reductive dechlorinating of TCE.

Assessment of potential positive effects of nZVI surface modification and concentration levels on TCE dechlorination in the presence of competing strong oxidants, using an experimental design

Nanoscale zero-valent iron (nZVI) particles are efficient for the remediation of aquifers polluted by trichloroethylene (TCE). But for on-site applications, their reactivity can be affected by the presence of common inorganic co-pollutants, which are equally reduced by nZVI particles. The aim of this study was to assess the potential positive effects of nZVI surface modification and concentration level on TCE removal in the concomitant presence of two strong oxidants, i.e., Cr(VI) and NO3−. A design of experiments, testing four factors (i.e. nZVI concentration, nZVI surface modification, Cr(VI) concentration and NO3− concentration), was used to select the best trials for the identification of the main effects of the factors and of the factors interactions. The effects of these factors were studied by measuring the following responses: TCE removal rates at different times, degradation kinetic rates, and the transformation products formed. As expected, TCE degradation was delayed or inhibited in most of the experiments, due to the presence of inorganics. The negative effects of co-pollutants can be palliated by combining surface modification with a slight increase in nZVI concentration. Encouragingly, complete TCE removal was achieved for some given experimental conditions. Noteworthily, nZVI surface modification was found to promote the efficient degradation of TCE. When degradation occurred, TCE was mainly transformed into innocuous non-chlorinated transformation products, while hazardous chlorinated transformation products accounted for a small percentage of the mass-balance.

Application of polycolloid-releasing substrate to remediate trichloroethylene-contaminated groundwater: A pilot-scale study

The objectives of this pilot-scale study were to (1) evaluate the effectiveness of bioremediation of trichloroethylene (TCE)-contaminated groundwater with the supplement of slow polycolloid-releasing substrate (SPRS) (contained vegetable oil, cane molasses, surfactants) under reductive dechlorinating conditions, (2) apply gene analyses to confirm the existence of TCE-dechlorinating genes, and (3) apply the real-time polymerase chain reaction (PCR) to evaluate the variations in TCE-dechlorinating bacteria (Dehalococcoides spp.). Approximately 350L of SPRS solution was supplied into an injection well (IW) and groundwater samples were collected and analyzed from IW and monitor wells periodically. Results show that the SPRS caused a rapid increase of the total organic carbon concentration (up to 5794mg/L), and reductive dechlorination of TCE was significantly enhanced. TCE dechlorination byproducts were observed and up to 99% of TCE removal (initial TCE concentration=1872μg/L) was observed after 50 days of operation. The population of Dehalococcoides spp. increased from 4.6×101 to 3.41×107cells/L after 20 days of operation. DNA sequencing results show that there were 31 bacterial species verified, which might be related to TCE biodegradation. Results demonstrate that the microbial analysis and real-time PCR are useful tools to evaluate the effectiveness of TCE reductive dechlorination.

Variability in the carbon isotope fractionation of trichloroethene on its reductive dechlorination by vitamin B12.

Stable carbon isotope fractionation through the reductive dechlorination of trichloroethylene by vitamin B12 was determined to assess the possibility of using stable carbon isotope analysis to determine the efficacy of remediation of trichloroethylene using vitamin B12. We elucidated the effects of environmental conditions, including the pH, reaction temperature, and vitamin B12 concentration, on the carbon isotope enrichment factor (ε). The ε values were relatively insensitive to the reaction temperature and vitamin B12 concentration, ranging from -15.7‰ to -16.2‰, with a mean of -15.9 ± 0.2‰, at different temperatures and vitamin B12 concentrations. Such a reproducible ε value could be particularly useful for estimating the extent of degradation in reactions in which a mass balance is difficult to achieve. However, changing the initial solution pH from 6.5 to 9.0 caused a notable change in the ε values, from -14.0‰ to -18.0‰. Reactions were investigated by calculating the apparent kinetic isotope effects for carbon, which, at 1.029-1.037, were smaller than the kinetic isotope effect values previously found for C-Cl bond cleavage. This indicates that a reaction other than the elimination of chloride may be a competitive degradation pathway. The dominant degradation pathway may be different for different initial solution pH values, and this will clearly influence carbon isotope fractionation. Therefore, if the ε value varies with reaction conditions, such as the solution pH, the calculations should take into account the actual environmental conditions that affect the rate limiting pathways.

Quantitative and functional dynamics of Dehalococcoides spp. and its tceA and vcrA genes under TCE exposure.

This study aimed at monitoring the dynamics of phylogenetic and catabolic genes of a dechlorinating enrichment culture before, during, and after complete dechlorination of chlorinated compounds. More specifically, the effect of 40 μM trichloroethene (TCE) and 5.6 mM lactate on the gene abundance and activity of an enrichment culture was investigated for 40 days. Although tceA and vcrA gene copy numbers were relatively stable in DNA extracts over time, tceA and vcrA mRNA abundances were upregulated from undetectable levels to 2.96 × and 6.33 × 10⁴ transcripts/mL, respectively, only after exposure to TCE and lactate. While tceA gene transcripts decreased over time with TCE dechlorination, the vcrA gene was expressed steadily even when the concentration of vinyl chloride was at undetectable levels. In addition, ratios between catabolic and phylogenetic genes indicated that tceA and vcrA gene carrying organisms dechlorinated TCE and its produced daughter products, while vcrA gene was mainly responsible for the dechlorination of the lower VC concentrations in a later stage of degradation.

Trichloroethylene dechlorination by copper-contained zero-valent iron slurry

Zero valent iron technology has been widely used for treating contaminated wastewater these years. However, it always results in inefficiency in the processes of drying and storage due to oxidation and passivation. This could be avoided by in situ synthesized zero valent iron slurry in an emergency if it possesses the same performance as zero valent iron. In this study, iron slurry was synthesized and directly used for dechlorinating trichloroethylene to measure its degradation efficiencies and properties. Results show that 2%(mass ratio) copper-contained zero-valent iron slurry exhibits the optimal performance compared with the other iron slurries. Batch experiments indicate that factors such as the concentration of trichloroethylene, pH, dissolved oxygen and equilibrium to a certain extent affect the reduction of trichloroethylene by 2%(mass ratio) copper-contained zero-valent iron slurry. Persistent, high-efficiency degradation performance could last 7 cycles. These demonstrate that the application of copper-contained zero-valent iron slurry in treating trichloroethylene-contained wastewater is realistic.

Application of Emulsified Substrate Biobarrier to Remediate TCE-Contaminated Groundwater: Pilot-Scale Study

Groundwater at many existing and former industrial areas and disposal sites is contaminated by halogenated organic compounds that were released into the environment. One cost-effective approach for the remediation of the chlorinated solvent contaminated aquifers is the installation of biobarriers within aquifers. The objective of this study was to assess the potential of using an in situ biobarrier system containing the emulsified substrate (ES) to bioremediate and contain the trichloroethylene (TCE) plume. The ES contained soybean oil, lactate, biodegradable surfactant, and nutrients, and thus it was able to provide carbon sources for the enhancement of the reductive dechlorination of TCE. A field-scale study was operated at a TCE-contaminated site, and 150 L of ES solution was pressure-injected into three remediation wells located in the upgradient area of the plume to form a biobarrier. Results show that the total organic carbon (TOC) went up from 12 mg/L (before ES injection) to 9,800 mg/L after 45 days of ES injection, and slowly declined to 1,500 mg/L after 230 days of operation. The increased TOC caused the complete consumption of groundwater dissolved oxygen in the injection wells (dropped from 0.6 to 0 mg/L) after 5 days of the ES injection. Results also show that the TCE concentrations dropped from 140 to below 1 μg/L after 10 days of ES injection in the biobarrier. The TCE degradation byproducts [e.g., 1,1-dichloroethene (1,1-DCE), vinyl chloride (VC)] were also observed in the injection and downgradient monitor wells. Results of microbial analyses show that various TCE-degrading bacteria exist in the subsurface, including Ralstonia sp., Clostridium sp., uncultured Burkholderiales bacterium, Hydrogenophaga sp., Acidovorax sp., Hydrocarboniphaga sp., uncultured Curvibacter sp., Pseudomonas sp., Comamonas sp., Aquabacterium sp., and Variovorax strains. Results indicate that the injected ES enhanced the anaerobic dechlorination of TCE, and the developed biobarrier could contain the TCE plume effectively. The biobarrier has the potential to be developed into an environmentally and economically acceptable remedial system.

Kinetics of dechlorination by Dehalococcoides mccartyi using different carbon sources

Stimulated anaerobic dechlorination is generally considered a valuable step for the remediation of aquifers polluted with chlorinated ethenes (CEs). Correct simulation and prediction of this process in situ, however, require good knowledge of the associated biological reactions. The aim of this study was to evaluate the dechlorination reaction in an aquifer contaminated with trichloroethene (TCE) and its daughter products, discharging into the Zenne River. Different carbon sources were used in batch cultures and these were related to the dechlorination reaction, together with the monitored biomarkers. Appropriate kinetic formulations were assessed. Reductive dechlorination of TCE took place only when external carbon sources were added to microcosms, and occurred concomitant with a pronounced increase in the Dehalococcoides mccartyi cell count as determined by 16S rRNA gene-targeted qPCR. This indicates that native dechlorinating bacteria are present in the aquifer of the Zenne site and that the oligotrophic nature of the aquifer prevents a complete degradation to ethene. The type of carbon source, the cell number of D. mccartyi or the reductive dehalogenase genes, however, did not unequivocally explain the observed differences in degradation rates or the extent of dechlorination. Neither first-order, Michaelis-Menten nor Monod kinetics could perfectly simulate the dechlorination reactions in TCE spiked microcosms. A sensitivity analysis indicated that the inclusion of donor limitation would not significantly enhance the simulations without a clear process understanding. Results point to the role of the supporting microbial community but it remains to be verified how the complexity of the microbial (inter)actions should be represented in a model framework.

Integration of stable carbon isotope, microbial community, dissolved hydrogen gas, and 2HH2O tracer data to assess bioaugmentation for chlorinated ethene degradation in fractured rocks

An in situ bioaugmentation (BA) experiment was conducted to understand processes controlling microbial dechlorination of trichloroethene (TCE) in groundwater at the Naval Air Warfare Center (NAWC), West Trenton, NJ. In the BA experiment, an electron donor (emulsified vegetable oil and sodium lactate) and a chloro-respiring microbial consortium were injected into a well in fractured mudstone of Triassic age. Water enriched in ²H was also injected as a tracer of the BA solution, to monitor advective transport processes. The changes in concentration and the δ¹³C of TCE, cis-dichloroethene (cis-DCE), and vinyl chloride (VC); the δ²H of water; changes in the abundance of the microbial communities; and the concentration of dissolved H₂ gas compared to pre- test conditions, provided multiple lines of evidence that enhanced biodegradation occurred in the injection well and in two downgradient wells. For those wells where the biodegradation was stimulated intensively, the sum of the molar chlorinated ethene (CE) concentrations in post-BA water was higher than that of the sum of the pre-BA background molar CE concentrations. The concentration ratios of TCE/(cis-DCE+VC) indicated that the increase in molar CE concentration may result from additional TCE mobilized from the rock matrix in response to the oil injection or due to desorption/diffusion. The stable carbon isotope mass-balance calculations show that the weighted average ¹³C isotope of the CEs was enriched for around a year compared to the background value in a two year monitoring period, an effective indication that dechlorination of VC was occurring. Insights gained from this study can be applied to efforts to use BA in other fractured rock systems. The study demonstrates that a BA approach can substantially enhance in situ bioremediation not only in fractures connected to the injection well, but also in the rock matrix around the well due to processes such as diffusion and desorption. Because the effect of the BA was intensive only in wells where an amendment was distributed during injection, it is necessary to adequately distribute the amendments throughout the fractured rock to achieve substantial bioremediation. The slowdown in BA effect after a year is due to some extend to the decrease abundant of appropriate microbes, but more likely the decreased concentration of electron donor.

Effects of Varying Growth Conditions on Stable Carbon Isotope Fractionation of Trichloroethene (TCE) by tceA-containing Dehalococcoides mccartyi strains

To quantify in situ bioremediation using compound specific isotope analysis (CSIA), isotope fractionation data obtained from the field is interpreted according to laboratory-derived enrichment factors. Although previous studies that have quantified dynamic isotopic shifts during the reductive dechlorination of trichloroethene (TCE) indicate that fractionation factors can be highly variable from culture-to-culture and site-to-site, the effects of growth condition on the isotope fractionation during reductive dechlorination have not been previously examined. Here, carbon isotope fractionation by Dehalococcoides mccartyi 195 (Dhc195) maintained under a variety of growth conditions was examined. Enrichment factors quantified when Dhc195 was subjected to four suboptimal growth conditions, including decreased temperature (-13.3 ± 0.9‰), trace vitamin B12 availability (-12.7 ± 1.0‰), limited fixed nitrogen (-14.4 ± 0.8‰), and elevated vinyl chloride exposure (-12.5 ± 0.4‰), indicate that the fractionation is similar across a range of tested conditions. The TCE enrichment factors for two syntrophic cocultures, Dhc195 with Desulfovibrio vulgaris Hildenborough (-13.0 ± 2.0‰) and Dhc195 with Syntrophomonas wolfei (-10.4 ± 1.2‰ and -13.3 ± 1.0‰), were also similar to a control experiment. In order to test the stability of enrichment factors in microbial communities, the isotope fractionation was quantified for Dhc-containing groundwater communities before and after two-year enrichment periods under different growth conditions. Although these enrichment factors (-8.9 ± 0.4‰, -6.8 ± 0.8‰, -8.7 ± 1.3‰, -9.4 ± 0.7‰, and -7.2 ± 0.3‰) were predominantly outside the range of values quantified for the isolate and cocultures, all tested enrichment conditions within the communities produced nearly similar fractionations. Enrichment factors were not significantly affected by changes in any of the tested growth conditions for the pure cultures, cocultures or the mixed communities, indicating that despite a variety of temperature, nutrient, and cofactor-limiting conditions, stable carbon isotope fractionations remain consistent for given Dehalococcoides cultures.

Successful operation of continuous reactors at short retention times results in high-density, fast-rate Dehalococcoides dechlorinating cultures

The discovery of Dehalococcoides mccartyi reducing perchloroethene and trichloroethene (TCE) to ethene was a key landmark for bioremediation applications at contaminated sites. D. mccartyi-containing cultures are typically grown in batch-fed reactors. On the other hand, continuous cultivation of these microorganisms has been described only at long hydraulic retention times (HRTs). We report the cultivation of a representative D. mccartyi-containing culture in continuous stirred-tank reactors (CSTRs) at a short, 3-d HRT, using TCE as the electron acceptor. We successfully operated 3-d HRT CSTRs for up to 120 days and observed sustained dechlorination of TCE at influent concentrations of 1 and 2 mM TCE to ≥ 97 % ethene, coupled to the production of 10(12) D. mccartyi cells Lculture (-1). These outcomes were possible in part by using a medium with low bicarbonate concentrations (5 mM) to minimize the excessive proliferation of microorganisms that use bicarbonate as an electron acceptor and compete with D. mccartyi for H2. The maximum conversion rates for the CSTR-produced culture were 0.13 ± 0.016, 0.06 ± 0.018, and 0.02 ± 0.007 mmol Cl(-) Lculture (-1) h(-1), respectively, for TCE, cis-dichloroethene, and vinyl chloride. The CSTR operation described here provides the fastest laboratory cultivation rate of high-cell density Dehalococcoides cultures reported in the literature to date. This cultivation method provides a fundamental scientific platform for potential future operations of such a system at larger scales.

Reactivity of different cement minerals in presence of Fe(II) for reducing trichloroethylene

Ferrous iron Fe (II) in combination with Portland cement is effective in dechlorinating trichloroethylene (TCE). However, there is no clear evidence about the component in cement responsible for TCE dechlorination. In present study different cement hydration minerals, such as ettringite (AFt) and monosulfate (AFm) were synthesized separately in laboratory. The TCE dechlorination ability of these minerals in combination with Fe(II) was investigated. It was found that these minerals in pure form do not have TCE dechlorination capacity. Further α-hematite (α-Fe2O3) that is suspected reactive mineral in cement/Fe(II) was investigated. It was found that when extra pure α-Fe2O3 along with CaO/Fe(II) was used for TCE did not show any reduction potential. This result was contradictory to earlier researchers, who used α-Fe2O3/CaO/Fe(II) for dechlorination of TCE. Thus, the α-Fe2O3 (Bayferrox-110 M) used by earlier researchers was investigated and it was found that it had some other impurities present in it. These impurities were suspected to play significant role in dechlorination of TCE. Further detailed studies were carried out and α-Fe2O3 was synthesized by following manufacturing procedure given for α-Fe2O3 (Bayferrox-110 M). When such α-Fe2O3 was used for TCE reduction, it showed improved reactivity. Detailed investigations showed that the α-Fe2O3 not in pure form but in combination with other impurities has reduction capacity for TCE.

합성 Calcium Sulfoaluminate계 시멘트 수화물의 Trichloroethylene (TCE) 제거능

고화제로 사용되는 포틀랜드 시멘트와 2가철을 이용하여 염소계 유기화합물을 탈염소화 시키는 기술은 많은 연구로부터 알려져 왔다. 하지만 TCE (trichloroethylene)의 탈염소화에 관계하는 명확한 유효물질 및 기작 규명은 요원한 상태이다. 많은 연구에서 ettringite와 monosulfate와 같은 시멘트 수화물질이 TCE의 탈염소화 반응에 관여할 가능성을 보고한 바 있다. 따라서 본 연구에서는 TCE의 탈염소화 반응에 유효물질로 예상 되어왔던 ettringite와 monosulfate를 별도로 합성하여, TCE 제거 실험을 수행함으로서 기존 시멘트/2가철을 사용한 TCE 분해제거에 관계하는 명확한 유효물질 및 기작을 규명 하고자 하였다. 실험결과, <TEX>$Fe^{3+}$</TEX> 및 <TEX>$Al^{3+}$</TEX>로 구성된 ettringite, monosulfate 합성에 성공하였고, 이후 2가철을 첨가하여 TCE 분해실험에서 합성물의 분해능을 확인하였지만 뚜렷한 제거가 나타나지 않았다. 이는 기존문헌의 예상과는 상반된 결과를 나타냈으며, 실험결과를 통해 시멘트/2가철을 사용한 TCE 제거 실험의 분해기작을 예상할 수 있었다. 예상 되는 분해기작으로는 ettringite 및 monosulfate 외의 시멘트 수화물에 의한 것과 C3A외의 시멘트 클링커 화합물 그리고 시멘트 내 trace metal에 의한 영향일 것으로 추측하였다. Portland cement used as binding material in combination of ferrous iron for reductive dechlorination of chlorinated organics is already widely studied topic by several researchers. However there is no clear evidence about the component solely responsible in cement for trichloroethylene (TCE) dechlorination. Many researchers suspect that the ettringite, monosulfate phases associated with hydration of cement are responsible active agents for TCE dechlorination. This study deals with synthesizing different pure crystalline minerals like ettringite and monosulfate phases of cement hydration and check individual phase's TCE dechlorinating capacity in combination with ferrous iron. The results indicated that the synthesized minerals showed no reduction capacity for TCE. The findings in the present study is significant as it shows that ettringite and monosulfate phases which were suspected minerals by previous researchers for TCE dechlorination are not reactive. Hence it is suspected that some other mineral or mineral form in cement phase could be responsible for TCE degradation.

Combined calcium sulfoaluminate and ordinary portland cement/Fe(II) system for enhanced dechlorination of trichloroethylene

The trichloroethylene (TCE) reduction capacity of four different types of cements, ordinary Portland cement (OPC), two types of calcium aluminate cements (CAC) with higher Al2O3 (UAC-50), and higher Fe2O3 (ISTRA-40), and calcium sulfoaluminate cement (CSA), were evaluated in presence of Fe(II). The OPC and CSA cements individually showed dechlorination capacity for TCE, following pseudo-first-order kinetics, but both types of CAC cements did not show reduction capacity. Further combined systems of CSA and OPC were found to enhance the rate of dechlorination compared to OPC or CSA used alone. The cement mixtures were optimized to get fastest reduction kinetics. The iron oxide and sulfate originally present in cement during manufacturing were found to play a significant role together. The end products of the dechlorination reaction were acetylene, ethylene and ethane suggesting β-elimination as the major pathway. The X-ray diffraction (XRD), scanning electron microscope with energy dispersive spectroscopy (SEM–EDS), analyzes were used to identify the solids that were produced during TCE reduction experiments. Ettringite crystals were abundant in all the reactive cement samples. However the formation of ettringite was not seen in nonreactive cement samples. It is speculated that Fe–ettringite associated with Fe(II) could be one of the reactive species in TCE reduction.

New and effective multi-element α-hematite systems for reduction of trichloroethylene

The reactivity of different α-hematite (α-Fe2O3) systems for dechlorination of trichloroethylene (TCE) in the presence of Fe(II) and CaO was investigated. Initially different experiments were conducted to investigate the reactivity of pure and doped α-Fe2O3. It was found that the presence of elements such as Si, Cu, and Mn in α-Fe2O3 had a significant effect on TCE reduction potential of α-Fe2O3; however, the reduction potential was less than that of α-Fe2O3 (Bayferrox®-110 M, used in a previous study). Further studies were carried out and α-Fe2O3 was synthesized in a manner similar to that of Bayferrox®-110 M. This synthetic α-Fe2O3 showed improved reactivity and was found to follow pseudo-first-order kinetics when used in TCE reduction experiments. The preliminary end products analysis showed that TCE degradation was probably via β-elimination pathway. Detailed investigations of α-Fe2O3 systems were carried out using X-ray diffraction, X-ray fluorescence, and scanning electron microscopy with energy-dispersive spectrometry. The results demonstrated that the TCE reduction capacity of α-Fe2O3 was strongly dependent on the other elements present in iron powder used to synthesize α-Fe2O3. It was suspected that these multi-elements in α-Fe2O3 helped to improve its conduction property. Current findings suggest that α-Fe2O3 not in the pure but combined with other elements could be thought as a potential system for TCE reduction.