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Biochar Facilitated Dechlorination of Chlorinated Ethylenes in Aqueous Solution

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

Chlorinated ethylenes (CEs) have been widely used as cleaning agents for clothes and electronics for a long period. The widespread manufacture, transportation use, and inappropriate disposal of the solvents have inevitably caused worldwide contamination of soil and groundwater. Because of the potentially carcinogenicity of CEs, remediation technologies have been developed and applied for environmental restoration. Nanoscale zero-valent iron (nZVI) and sulfidated ZVI (SZVI) are state-of-the-art remediation reductant materials; nevertheless, the high cost and tedious processes for production have limited the application of nZVI and SZVI on large scales. One of the promising alternatives to ZVI materials is iron(II)-containing compounds due to their low redox potential and compatibility with natural biogeochemical reduction, and importantly, lower production costs than nZVI and SZVI. Layered iron(II)-iron(III) hydroxide (green rust, GR) is one of these iron(II)-containing compounds that exist in natural suboxic environments. Green rust (GR) has exhibited a high reactivity on reduction of both organic and inorganic pollutants such as carbon tetrachloride, hexahydro-1,3,5-trinitro-1,3,5-triazine, disinfection byproducts, nitrate, chromate, and cupric ions. However, fast dechlorination of CEs by GR has not been observed, which is ascribed to a substantial reaction barrier between CE and GR. Hence, a catalyst is needed in order to break this reaction barrier. Biochar may present one such possible catalyst. Biochar, produced by pyrolysis of biomass in an oxygen-limited atmosphere, present unique physical and chemical properties such as high specific surface area, high porosity, polarity, and aromaticity. As a result, biochar has been used as adsorbent for removal of both organic and inorganic contaminants. In addition, biochar has shown high catalytic activity to facilitate redox transformation of variable contaminants such as nitroaromatics and halogenated phenols and aliphatics. This activity is attributed to redox-active species and electron-conducting structures of biochar serving as an electron mediator. The review part of the Ph.D. thesis has documented that biochar potentially has the capability of facilitating dechlorination of CE by GR, by acting as an active ³bridge´, which may not only connect hydrophilic GR and hydrophobic CE but also mediate electron transfer from GR to adsorbed CE. Particularly, the properties of biochar can be readily manipulated by applying different pyrolysis conditions, and both GR and biochar are low-cost and easy-to-manufacture and have insignificant adverse side effects. Hence, it is feasible to use biochar to enhance dechlorination of CE by GR for remediation of CEs in groundwater. This Ph.D. project has focused on facilitating reductive dechlorination of CEs by GR using biochars. The thesis work comprises three separate studies: i) Fast dechlorination of CEs by GR in presence of bone char. It is demonstrated that bone char exhibits the capability of eliminating the kinetic hindrance of GR to reduce CEs, thereby significantly facilitating reduction of Ces by GR. Acetylene is the major product of reduction of tetrachloroethylene (PCE), trichloroethylene (TCE), cis-dichloroethylene (cDCE), and trans-chloroethylene (tDCE), whereas GR alone does not show reduction of CEs at all. It was observed that GR could directly reduce bone char, which implies that the catalyzed dechlorination may be attributed to an electron mediating process where reducible species on bone char first accept electrons from GR and subsequently the reduced species reduce CE to acetylene. Reduction of PCE and TCE by the GR and bone char mixture performed in contaminated groundwater showed a rate comparable to that conducted in lab-spiked solutions. This study opens a new perspective for both in situ remediation and sustaining natural attenuation of CEs. ii) Investigation of mechanisms of bone char-catalyzed TCE dechlorination by GR. In this study, we prepared bone chars at a range of pyrolysis temperatures between 450 and 1050 °C, and tested their catalytic activity for TCE reduction by GR. The bone char pyrolyzed at 950 °C showed the highest reactivity, while no reaction was seen for bone char pyrolyzed at 450 °C. This reactivity was then compared with structural and compositional properties of the bone chars, and correlated with their electrochemical properties. This analysis demonstrated that both electron-accepting groups and electronconducting structures govern the catalytic activity of the bone chars. In addition, the interaction of GR with biochar and adsorption of TCE to biochar have also been proposed as essential elements for the TCE-bone char-GR reaction process. iii) Explore which biochar properties matter the biochar catalyzed dechlorination. Biochars, obtained by pyrolysis of a broad diversity of feedstocks at 950 °C, were tested for the catalytic activity towards TCE dechlorination by GR, following the same method as in the above study. Besides, we investigated adsorption of TCE, structure, extent of graphitization, elemental composition, and chemical speciation of these biochars. Multiple regression of the surface catalytic activity of biochar on all the biochar properties corroborated that electron-accepting moieties and conductive graphitic structures are the critical and general requirements for biochars to express catalytic activity. In addition, strong adsorption affinity to TCE and interaction with GR were pointed out to additionally enhance the dechlorination reactivity. This s
OriginalsprogEngelsk
ForlagDepartment of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen
StatusUdgivet - 2019

ID: 237415408