Full length ArticleSynergistic effects of microbial anaerobic dechlorination of perchloroethene and nano zero-valent iron (nZVI) – A lysimeter experiment
Graphical abstract
Introduction
Chlorinated hydrocarbons such as perchloroethene (PCE) are hazardous environmental pollutants which can enter the groundwater. PCE is found globally at urban contaminated sites due to its heavy use in the dry-cleaning and the metal processing industry [1,2]. Nanoscaled zero-valent iron particles (nZVI) are able to degrade PCE by reductive dechlorination to ethene and ethane [[3], [4], [5]]. In addition to the desired degradation reaction with PCE, nZVI also reacts with water. Under anaerobic conditions Fe0 is oxidized and H2 is formed [6]. Depending on the type of nZVI applied, a high percentage of available electron equivalents (e− eq) is dissipated by this anaerobic corrosion [7]. Moreover nZVI is not very mobile in the underground due to its high adsorption to the aquifer matrix and sedimentation enhanced by aggregation [8,9]. Thus, an excessive amount of nZVI is necessary to achieve a satisfying remediation outcome resulting in increased remediation costs.
Bacterial biodegradation of chlorinated ethenes has been observed at several contaminated sites [[10], [11], [12]]. Different aerobic and anaerobic pathways have been described for the degradation of higher and lower chlorinated ethenes. Dehalococcoides spp. is known for its ability to dechlorinate PCE completely to ethene. In the metabolic pathway termed dehalorespiration, H2 is used as an electron donor and PCE as an electron acceptor for growth [13,14]. In this process PCE is dechlorinated stepwise to trichloroethene (TCE), cis-dichloroethene (cis-DCE), vinyl chloride (VC) and ethene as the fully dechlorinated end-product. The lack of H2 and/or Dehalococcoides spp. at contaminated sites can lead to a lengthy persistence of PCE or the accumulation of the hazardous metabolites cis-DCE or VC. Other anaerobic microbial processes, such as hydrogenotrophic methanogenesis, also use H2 as an electron donor, thus Dehalococcoides spp. must compete for the H2 present [15]. Furthermore, unsuitable environmental parameters, such as a low pH, dissolved O2 or high redox values can negatively affect Dehalococcoides spp. [[16], [17], [18], [19]].
The combination of chemical degradation via nZVI and biological degradation via Dehalococcoides spp. has the potential to overcome the weaknesses of either method alone [20]. nZVI could act as a possible H2 donor for anaerobic bacterial PCE dechlorination and could create favorable conditions for Dehalococcoides spp., such as a low redox-potential or the depletion of O2, and degrade hazardous metabolites. However, laboratory studies investigating the effects of nZVI on microorganisms are inconsistent, since both stimulation and inhibition of Dehaloccocoides spp. by nZVI have been shown in several studies [[20], [21], [22], [23], [24], [25]]. Exposure to nZVI can lead to a significant down-regulation of genes encoding reductive dehalogenases. Uncoated nZVI has been shown to attach to bacterial cells [26]. However, the proximity of nZVI and Dehalococcoides spp. in batch experiments may not be representative of larger spatial distances under field conditions. A lysimeter experiment can thus help to gain a better insight into the interplay between nZVI and Dehalococcoides spp. under field-like, but nevertheless controlled conditions.
The aim of this lysimeter study was therefore to investigate if nZVI can stimulate PCE degradation by a PCE-degrading bacterial consortium under controlled field-like conditions. Apart from controlling the mass and position of the PCE source the lysimeter study facilitated calculation of a mass balance for PCE and its metabolites. In the lysimeter chambers an oxygen-free aquifer was established followed by a controlled injection of PCE. One of the chambers served as an untreated control, while in the other three treatments with nZVI, a dechlorinating culture and molasses, as well as a combination of bacteria and nZVI, were investigated. Following the injection, concentrations of PCE its metabolites and relevant environmental parameters were monitored for over 350 days.
Section snippets
Materials and chemicals
Nanofer Star (NANO IRON s.r.o., Židlochovice, Czech Republic) was used for the chemical PCE degradation. Prior to use, the nZVI was stored in an argon-flushed glovebox to minimize O2 contact. nZVI solutions were produced by dispersing 50 g in 200 ml O2-free water for 3 min using an Ultra-Turrax (ULTRA-TURRAX T 18 basic, IKA®-Werke GmbH & Co. KG), idle speed: 15,600 rpm, immediately before use. nZVI solutions were produced in an argon-flushed glove tent. The KB-1 culture (SiREM, Guelph, Ontario,
Dehalococcoides spp. in KB1 and COMB
Significant amounts of Dehalococcoides spp. could be detected in the samples taken downstream of the treatment zone (DS30 and DS60) of chamber COMB and KB1 (results shown in Supplementary Material).
Total cumulative efflux-loads of chloroethenes, ethene and ethane
Chamber CONT had the highest cumulative efflux-load of PCE (223 μmol) followed by NZVI (135 mmol), KB1 (47 mmol) and COMB (29 mmol) (Fig. 2). For CONT, the amount discharged during the experimental period was 23% of the injected PCE, indicating that the mass of PCE in each lysimeter chamber was
Conditions
The conditions in the chambers enabled microbial PCE dechlorination to occur by the injected bacterial culture. The pH, temperature and ORP could be adjusted to a suitable range for dehalogenating bacteria (see Supplementary Material for detailed data). Different dehalogenating bacteria have been reported to dechlorinate in a pH range of 6.5–8.0 and at 10–35 °C [14,16,[30], [31], [32]]. Microbial reductive dehalogenation is most effective in the ORP range of SO42− reduction (-50 to −180 mV) and
Conclusion
Under field-like and controlled conditions it was shown that the combination of nZVI and a microbial culture containing Dehalococcoides spp. stimulated the degradation of PCE and production of fully dechlorinated end-products (ethene and ethane). In particular, when the addition of the H2 source (molasses) was stopped, the dechlorination process was maintained if nZVI was present. A potential stimulation of CH4 production by aged nZVI remains to be further investigated. Overall, this study
Acknowledgements
The project BIANO was funded by the Austrian Federal Ministry of Sustainability and Tourism (BMNT); Management by Kommunalkredit Public Consulting (KPC) (Proj.No.: B420003).
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