Reductive dechlorination of trichloroethylene (TCE) in competition with Fe and Mn oxides – observed dynamics in H2-dependent terminal electron accepting processes

<p>The determination of hydrogen (H₂) concentration together with the products of microbial reduction reactions in a trichloroethylene dechlorinating system is conducted to delineate the ongoing predominant terminal electron accepting processes (TEAP). Formate was used as electron donor and synthetic Fe minerals or environmental samples were used as the substrate. Iron(III) and Mn(IV) reduction limited microbial dechlorination by the mixed anaerobic culture by decreasing the level of H₂ in the system. The H₂ measurements indicated that the H₂ concentration at which different TEAPs occur can overlap and thus these TEAPs can therefore occur concurrently rather than exclusively. Difference in Fe(III) bioavailability and hence, Fe(III) reduction partially explain this wide range. The distinction between dechlorination and other microbial reduction processes based on H₂ threshold values is not feasible under such conditions, though there appears to be a relation between the rates of H₂ consuming process and the observed H₂ level.</p

Role of Chemical Characteristics of an Aquifer on the Biotic Reductive Dechlorination of Trichloroethylene

Chlorinated solvents, such as trichloroethene (TCE), are major groundwater contaminants that often persist as dense nonaqueous phase liquids (DNAPLs) in subsurface environments. Microbial reductive dechlorination as a polishing step after physical-chemical treatments is a promising approach for the remediation of TCE-DNAPL source zones and dechlorination activity at or near the source zones can enhance DNAPL dissolution. The physicochemical properties of the aquifer can affect the microbial dechlorination activity and, hence, the TCE DNAPL dissolution. This dissertation explores the influence of geochemical factors on TCE dechlorination, more in particular the competition between microbial reduction of iron (III) minerals and dechlorination processes. Thermodynamic considerations as well as observed threshold hydrogen concentration show that iron (Fe) reducers can compete with anaerobic TCE dechlorinators. This competition can impede the enrichment of dechlorinating populations and inhibit anaerobic reductive dechlorination. All studies were performed by inoculating matrices with a mixed dechlorinating culture (KB-1) that also contain Fe (III) reducers in order to limit the influence of the composition of the microbial communities on the microbial processes; all studies used formate as electron donor. In a first part, the influence of the quantity and mineralogy of Fe (III) minerals on TCE dechlorination was investigated. A matrix extraction procedure for testing bioavailable Fe (III) in sediments was also calibrated. Experiments were performed in batches with either 1) variable amounts of ferrihydrite or 2) with 14 different Fe (III) minerals coated onto or mixed in with quartz sand at constant total Fe and at a stoichiometric excess Fe (III) over electron donor. At constant total Fe in the sand, TCE dechlorination time varied with Fe mineralogy between 8 days (no Fe added) to > 120 days (Fe-containing bentonite). Poorly crystalline Fe (III) minerals increased the dechlorination time whereas crystalline Fe (III) minerals such as goethite or hematite had no effect. The TCE dechlorination time increased with increasing total reduced Fe and with increasing surface area of the Fe (III) minerals. Extractable Fe determined based on Fe (III) reduction using NH2OH.HCl predicted the competitive inhibition of TCE degradation in these model systems. This study shows that Fe mineralogy rather that total Fe content determines the competitive inhibition of TCE dechlorination.In a second part, the H2 pressure was measured during dechlorination in presence and absence of Fe (III) to determine critical H2 for the different terminal electron accepting reactions (TEAP) in the system. Different natural and synthetic Fe containing matrices were used and compared. Iron (III) and Mn (IV) reduction limited microbial dechlorination by decreasing the level of H2 in the system and subsequently resulted in a H2 limited condition. The H2 measurements indicated that a wide concentration range of H2 is possiblefor different TEAPs in these systems and that these TEAPs can therefore occur concurrently rather than exclusively. Differences in Fe (III) bioavailability and, hence, Fe (III) reduction partially explain this wide range. The distinction between dechlorination and other microbial reduction processes based on H2 concentrations threshold values is not feasible under such conditions. Dechlorination leads to acidification and slows down dechlorination. In addition, acidification facilitates Fe (III) reduction because of increased Fe (III) solubility. In a third part, it was investigated if acidification and Fe (III) reduction have a synergistic inhibition on reductive dechlorination. Two common aquifer Fe (III) minerals, goethite and ferrihydrite and sand only as control were studied in batch bottles at different solution pH values (6.2 - 7.2). In the absence of Fe, lowering matrix pH between 7.2 to 6.2 increased the time for TCE degradation. At pH 7.2, goethite did not affect TCE degradation time while ferrihydrite increased it. Iron reduction in ferrihydrite 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 reduction 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 decreased. A fourth part of this work examined to what extent Fe (III) minerals affect dechlorination in environmental samples. Seventeen environmental matrices with contrasting properties (pH 3.5-11; total Fe 0.1-87 g kg-1, total Fe (II) 0.05 - 2 % of total Fe) were inoculated in unbuffered media with 1 mM TCE and 9 mM formate, sufficient for complete TCE dechlorination. The time for 90% conversion of TCE to cis-DCE ranged 5 - >100 days with only partial or no dechlorination past cis-DCE due to the lack of electron equivalents. Reduction of Fe (III), SO42-and NO3- was detected in almost all treatments. The TCE and cis-DCE dechlorination was completely inhibited below pH 4.3, above pH 10.5 and in matrices where Fe reduction was highest. No cis-DCE degradation was observed below pH 5.9. The TCE dechlorination time and the inhibition of cis-DCE dechlorination increased with increasing concentrations of available Fe (III), determined with citrate bicarbonate extraction for matrices between pH 4.3 - 10, however statistical effects were small (R2<0.30). Weak Fe extraction methods such as the citrate bicarbonate ascorbate extraction determines bioavailable Fe (III) in the matrix and predict the stoichiometric requirements of electron donor to overcome inhibition of dechlorination.The fifth part of this study assessed the role of Fe (III) minerals on the bio-enhanced dissolution of a TCE DNAPL. Columns were set up as 1-D diffusion-cells consisting of a lower DNAPL layer, a layer with an aquifer matrix and an upper water layer that is regularly refreshed with formate containing medium. The matrices were either inert sand, inert sand coated with 2-line ferrihydrite (HFO) and two environmental Fe containing matrices. In none of the diffusion cells, vinyl chloride or ethene was detected while dissolved and extractable Fe (II) increased strongly during 60 days incubation. The cis-DCE concentration peaked at 4.0 cm from the DNAPL (inert sand) while it was at 3.4 cm (sand + HFO), 1.7 cm and 2.5 cm (environmental samples). The TCE concentration gradients near the DNAPL indicate that the DNAPL dissolution rate was larger than that in an abiotic cell by factors 1.3 (inert sand), 1.0 (sand + HFO) and 2.2 (both environmental samples). This results show that high bioavailable Fe (III) in the HFO reduces the TCE degradation by competitive Fe (III) reduction, yielding lower bioenhanced dissolution. However, Fe (III) reduction in the environmental samples was not affecting bioenhanced dissolution which was even larger than in inert sand. It is speculated that physical factors, e.g., micro-niches in the environmental samples, protect microorganisms from toxic concentrations of TCE. In conclusion, this work demonstrated that Fe (III) reduction can limit the overall dechlorination rate and bioenhanced dissolution. Amorphous Fe (III) oxyhydroxides are readily reduced and largely inhibit dechlorination whereas crystalline Fe (III) oxyhydroxides have weak or no effects. Effects of Fe on dechlorination past cis-DCE are larger than on TCE and this deserves further study. The occurrence of multiple terminal electron acceptors increase the electron donor consumption and result in a cis-DCE or VC stall due to the electron donor limitation. Addition of surplus electron donor without calculations can overstimulate methanogens and also can increase the bioremediation costs. Thus, a stoichiometric calculation of electrons equivalents is necessary to match the Fe (III) reduction. Citrate bicarbonate ascorbate extraction is recommended as a simple matrix Fe extraction assay in order to approximate the electron equivalents necessary for depleting the bioavailable Fe fraction. Acid pH conditions can severely limit the dechlorination and iron reduction can acts as an additive stress factor in the acid systems limiting the microbial dechlorination capacity.Table of Contents Acknowledgements i Summary iii Samenvatting vii List of Abbreviations xi Chapters 1. General Introduction and thesis objectives 1 2. Inhibition of microbial trichloroethylene dechorination by Fe (III) reduction depends on Fe mineralogy: a batch study using the bioaugmentation culture KB-1 23 3. Reductive dechlorination of trichloroethylene (TCE) in competition with Fe and Mn oxides – observed dynamics in H2-dependent terminal electron accepting processes 45 4. Combined inhibition of iron (III) minerals and acidification on the reductive dechlorination of trichloroethylene 59 5. Abiotic factors affecting microbial reductive dechlorination of trichloroethylene (TCE) in inoculated anaerobic environmental matrices: a comparative study 73 6. Bioenhanced dissolution of dense non-aqueous phase of trichloroethylene under iron reducing conditions 89 7. General conclusions and research perspectives 105 8. References 115 9. List of publications 131 10. Appendices 133nrpages: 173status: publishe