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Organohalide-respiring bacteria are generally sensitive to pH. Reductive dechlorination of tetrachloroethene (PCE) to trichloroethene (TCE) and then to cis-1,2-dichloroethene(cDCE) can occur at a pH as low as 5.5. However, rates of cDCE reduction to vinyl chloride (VC) and then to ethene are reduced below a pH of 6.0. For efficient dechlorination to non-toxic end products, aquifer pH should be maintained above 6.0 during enhanced reductive dechlorination.


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CONTRIBUTOR(S): Dr. Robert Borden, P.E.


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Introduction

Enhanced reductive dechlorination (ERD) is commonly used to treat chlorinated solvents and related contaminants in groundwater by providing a fermentable organic substrate to serve as both an electron donor and a carbon source to stimulate microbially mediated reductive dechlorination[4][5][6]. During ERD, the pH may decline as hydrochloric acid (HCl) is produced during reductive dechlorination[7] [8] and carbonic acid and organic acids are produced by substrate fermentation. However, dechlorinating bacteria appear to be particularly sensitive to pH changes with dechlorination of cDCE and VC to ethene completely inhibited at a pH of 5.5[9][10][11][2], resulting in a significant decline in degradation rates[12][13].

Effect of pH on Bioremediation Processes

Many biological processes are sensitive to pH. Most microorganisms important for subsurface bioremediation function most efficiently in near neutral conditions[14]. Low pH can interfere with pH homeostasis or increase the solubility of toxic metals[15]. Microorganisms can expend cellular energy to maintain homeostasis, or conditions in the cytoplasm and periplasm may change in response to external changes in pH[16]. Some anaerobes have adapted to low pH conditions through alterations in carbon and electron flow, cellular morphology, membrane structure, and protein synthesis[14].

There is some evidence that organohalide-respiring bacteria are particularly sensitive to changes in pH. Pure cultures and consortia of organohalide-respiring microorganisms show highest dechlorination rates at circumneutral pH[2]. Reduction of cDCE to VC to ethene is primarily carried out by strains of Dehalococcoides mccartyi (Dhc)[17]. Growth and dechlorination by Dhc occurs between pH 6 and 8, with highest activity measured between pH 6.9 and 7.5[17]

Table 1 shows the range and optimum pH for growth of bacteria that reduce PCE to TCE and cDCE. Zhuang and Pavlostathis (1995)[18] found that neutral pH was optimum for reductive dechlorination by a methanogenic mixed culture capable of dechlorinating PCE to VC. Vainberg et al. (2006)[10] reported an optimum pH of 6.0-6.8 for dechlorination of PCE by the SDC-9™ bioaugmentation culture. Dechlorination of PCE and TCE to cDCE can occur at pH down to 5.5[19].


Table 1. Range and optimum pH for growth of pure cultures reducing PCE

Organism pH Range pH Optimum Reference
Dehalobacter restrictus PER- K23 6.5 to 8.0 6.8 to 7.6 Holliger et al., 1998[20]
Sulfurospirillum multivorans 5.5 to 8.0 7.0 to 7.5 Neumann et al., 1994[21]; Scholz-Muramatsu et al., 1995Scholz-Muramatsu et al., 1995 [22]; Yang, 2017a[2]
Desulfitobacterium dehalogenans JW/IU-DC-1 6.0 to 9.0 7.5 Utkin et al., 1994[23]
Desulfitobacterium sp. PCE-1 7.2 to 7.8 7.2 Gerritse et al., 1996[24]
Desulfitobacterium dichloroeliminans DCA1 7.2 to 7.8 Fogel et al., 2009[25]
Desulfuromonus chloroethenica TT4B 6.5 to 7.4 7.4 Krumholz el al., 1996[26];Krumholz, 1997[27]
Desulfomonile tiedjei DCB-1 6.5 to 7.8 6.8 to 7.0 DeWeerd et al., 1990[28]
Desulfuromonas michiganensis sp. Nov 6.8 to 8.0 7.0 to 7.5 Sung et al., 2003[29]
Geobacter lovleyi SZ 6.5 to 7.2 Sung et al., 2006[30]
(adapted from Damborský, 1999; Yang et al. 2017a[2])


References

  1. ^ Li, Y.X., 2012. Adaptation of a Dechlorinating Culture, KB-1, to Acidic Environments. M.S. Thesis, University of Toronto, Toronto, ON, Canada. Report.pdf
  2. ^ 2.0 2.1 2.2 2.3 2.4 Yang, Y., Cápiro, N.L., Marcet, T.F., Yan, J., Pennell, K.D. and Loffler, F.E., 2017. Organohalide respiration with chlorinated ethenes under low pH conditions. Environmental Science & Technology, 51(15), pp.8579-8588. doi: 10.1021/acs.est.7b01510
  3. ^ Yang, Y., Cápiro, N.L., Yan, J., Marcet, T.F., Pennell, K.D. and Löffler, F.E., 2017. Resilience and recovery of Dehalococcoides mccartyi following low pH exposure. FEMS microbiology ecology, 93(12), p.fix130. doi: 10.1093/femsec/fix130. Report.pdf
  4. ^ AFCEE, NFESC, ESTCP, 2004. Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents. Parsons Corporation. Air Force Center for Environmental Excellence, Naval Facilities Engineering Service Center, and Environmental Security Technology Certification Program. Report PDF
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  14. ^ 14.0 14.1 Lowe, S.E., Jain, M.K. and Zeikus, J.G., 1993. Biology, ecology, and biotechnological applications of anaerobic bacteria adapted to environmental stresses in temperature, pH, salinity, or substrates. Microbiological reviews, 57(2), pp.451-509
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  17. ^ 17.0 17.1 Löffler, F.E., Yan, J., Ritalahti, K.M., Adrian, L., Edwards, E.A., Konstantinidis, K.T., Müller, J.A., Fullerton, H., Zinder, S.H. and Spormann, A.M., 2013. Dehalococcoides mccartyi gen. nov., sp. nov., obligately organohalide-respiring anaerobic bacteria relevant to halogen cycling and bioremediation, belong to a novel bacterial class, Dehalococcoidia classis nov., order Dehalococcoidales ord. nov. and family Dehalococcoidaceae fam. nov., within the phylum Chloroflexi. International journal of systematic and evolutionary Microbiology, 63(2), pp.625-635. doi: 10.1099/ijs.0.034926-0 [media: 2013-Loffler-Dehalococcoides_mccartyi_gen._nov....pdf| Report.pdf]
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  21. ^ Neumann, A., Scholz-Muramatsu, H. and Diekert, G., 1994. Tetrachloroethene metabolism of Dehalospirillum multivorans. Archives of Microbiology, 162(4), pp.295-301. ISSN: 0302-8933, 1432-072X doi: 10.1007/BF00301854
  22. ^ Scholz-Muramatsu, H., Neumann, A., Meßmer, M., Moore, E. and Diekert, G., 1995. Isolation and characterization of Dehalospirillum multivorans gen. nov., sp. nov., a tetrachloroethene-utilizing, strictly anaerobic bacterium. Archives of Microbiology, 163(1), pp.48-56. doi: 10.1007/BF00262203
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  29. ^ Sung, Y., Ritalahti, K.M., Sanford, R.A., Urbance, J.W., Flynn, S.J., Tiedje, J.M. and Löffler, F.E., 2003. Characterization of two tetrachloroethene-reducing, acetate-oxidizing anaerobic bacteria and their description as Desulfuromonas michiganensis sp. nov. Applied and Environmental Microbiology, 69(5), pp.2964-2974. doi 10.1128/AEM.69.5.2964–2974.2003. Report.pdf
  30. ^ Sung, Y., Fletcher, K.E., Ritalahti, K.M., Apkarian, R.P., Ramos-Hernández, N., Sanford, R.A., Mesbah, N.M. and Löffler, F.E., 2006. Geobacter lovleyi sp. nov. strain SZ, a novel metal-reducing and tetrachloroethene-dechlorinating bacterium. Applied and Environmental Microbiology, 72(4), pp.2775-2782. doi: 10.1128/AEM.72.4.2775-2782.2006
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