European Journal of Soil Biology 43 (2007) 276e282 http://www.elsevier.com/locate/ejsobi Original article Electron donor and pH relationships for biologically enhanced dissolution of chlorinated solvent DNAPL in groundwater Perry L. McCarty*, Min-Ying Chu, Peter K. Kitanidis Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA Available online 26 March 2007 Abstract Biologically enhanced dissolution offers a method to speed removal of chlorinated solvent dense non-aqueous-phase liquid (DNAPL) sources such as tetrachloroethene (PCE) and trichoroethene (TCE) from aquifers. Bioremediation is accomplished by adding an electron donor to the source zone where fermentation to intermediates leading to acetic acid and hydrogen results. The hydrogen and possibly acetic acid are used by dehalogenating bacteria to convert PCE and TCE to ethene and hydrochloric acid. Reductive dehalogenation is thus an acid forming process, and sufficient alkalinity must be present to maintain a near neutral pH. The bicarbonate alkalinity required to maintain pH above 6.5 is a function of the electron donor: 800 mg/L of bicarbonate al- kalinity is sufficient to achieve about 1.2 mM TCE dechlorination with glucose, 1.7 mM with lactate, and a much higher 3.3 mM with formate. Laboratory studies indicate that in mixed culture, formate can be used as an electron donor for complete conversion to ethene, contrary to pure cultures studies indicating it cannot. Various strategies can be used to add electron donor to an aquifer for DNAPL dehalogenation while minimizing pH problems and excessive electron donor usage, including use of injection-extraction wells, dual recirculation wells, and nested injection-extraction wells. Ó 2007 Elsevier Masson SAS. All rights reserved. Keywords: DNAPL; Dehalogenation; Tetrachloroethene; Trichloroethene; pH; Alkalinity; Bioremediation; Electron donor; Formate; Glucose; Lactate; Hydrogen; Groundwater 1. Introduction indicated that near-saturation concentrations of chlori- nated solvents can be biodegraded by specialized Tetrachloroethene (PCE) and trichloroethene (TCE) anaerobic microorganisms that use the chlorinated sol- are the most frequently found and costly to control vents as electron acceptors in energy metabolism organic contaminants in groundwater. Chlorinated [13,14]. These organisms require an electron donor, solvent spills migrate downward to form dense non- which may be present in the aquifer, leading to natural aqueous phase liquids (DNAPLs), which constitute attenuation, or most often, must be added as part of an sources of contamination to groundwater that may engineered bioremediation scheme. Efforts based upon last for decades, if not centuries. Recent research has this research are now being directed towards use of biodegradation to reduce the life span of chlorinated solvent DNAPLs. * Corresponding author. Fax: þ1 650 725 3164. Among the advantages of chlorinated solvent E-mail address: [email protected] (P.L. McCarty). DNAPL biodegradation [13] are: (1) it can result in 1164-5563/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ejsobi.2007.03.004 P.L. McCarty et al. / European Journal of Soil Biology 43 (2007) 276e282 277 enhanced rates of solvent dissolution, especially for significant problem, necessitating a high buffer or PCE; (2) the high chlorinated solvent concentrations neutralization capacity to prevent adverse pH condi- near the DNAPL and their degradation products as tions [1]. well are toxic to microorganisms, such as methanogens, Most frequently, organic compounds are used for that otherwise compete with dechlorinating microor- electron donors. Here, fermentation leads to the produc- ganisms for the electron donor; and (3) the costs for de- tion of acetate and hydrogen, as indicated in the follow- livery of the electron donor per unit of solvent degraded ing transformations, using glucose as an example: are much less when applied to high solvent concentra- Glucose as Electron Donor: tions. The question then arises as to what are the best Fermentationof glucose: donors to use for chlorinated solvent dehalogenation and what are the best strategies for their delivery to C6H12O6 þ2H2O ¼ 4H2 þ2CH3COOHþ2CO2 ð6Þ the DNAPL source area? Dehalogenation : CCl2]CCl2 þ 4H2 ¼ 4HCl þ CH2]CH2 ð7Þ 2. Electron donors Net reaction : Reductive dehalogenation of PCE occurs in a step- ] wise fashion, generally with hydrogen as the preferred C6H12O6 þ CCl2 CCl2 þ 2H2O ¼ 2CH3COOH electron donor although acetic and other organic acids þ4HCl þ 2CO2 þ CH2]CH2 ð8Þ may be used by some dehalogenating microorganisms [6,11,12], converting PCE to TCE to 1,2-cis-dichloro- Organic compounds that have been added to aquifers ethene (cDCE) to vinyl chloride (VC), and finally to to achieve reductive dehalogenation include a variety of ethene. soluble organic compounds (pentanol, lactate, methanol, Reactions when using hydrogen for reductive De- ethanol, molasses, benzoate), materials such as vegeta- halogenation of PCE are: ble oils, precipitated compounds such as calcium oleate, natural organic solids such as compost, and various ] ] CCl2 CCl2 þ H2 ¼ CHCl CCl2 þ HCl ð1Þ commercially available products, such as ‘‘slow hydro- gen release compounds’’ or HRCs. Some of their CHCl]CCl þ H ¼ CHCl]CHCl þ HCl ð2Þ 2 2 relative advantages and disadvantages have been CHCl]CHCl þ H2 ¼ CH2]CHCl þ HCl ð3Þ addressed [14]. A major limitation for DNAPL dissolu- tion as indicated in Eq. (8) is that not only are four ] ] CH2 CHCl þ H2 ¼ CH2 CH2 þ HCl ð4Þ moles of hydrochloric acid produced, but also acetic acid. The latter not only requires additional buffer, but Net : CCl2]CCl2 þ 4H2 ¼ CH2]CH2 þ 4HCl ð5Þ also adds an undesirable organic compound to the Different microorganisms have different abilities aquifer, which can lead to further degradation of water to use PCE, TCE, cDCE, and VC in energy metab- quality through iron, manganese, or sulfate reduction, olism. There are several different bacterial genera and methane formation. capable of using PCE and TCE, but only Dehalococ- There are reports that acetic acid may serve by itself coides has been found to use cDCE and VC in for partial [11] or perhaps complete [6] dehalogenation energy metabolism [3e5,9]. Interestingly, different of PCE and TCE to ethene. This would reduce the con- Dehalococcoides strains are restricted in the chlori- centration of acetic acid, but would lead to the produc- nated species that they can dehalogenate. Thus, in tion of two moles of the weak-acid forming carbon order to obtain complete dehalogenation efficiently, dioxide: a strain that uses VC in energy metabolism is required [3,5]. CCl2]CCl2 þ CH3COOH þ 2H2O ¼ CH2]CH2 Hydrogen itself may be injected into an aquifer, þ 2CO2 þ 4HCl ð9Þ but has several disadvantages. It has low solubility (about 1 mM), which can be a significant limitation Thus, acetic acid utilization for dehalogenation may for DNAPL dehalogenation. It is a hazardous com- have some, but perhaps not a great impact in reducing pound, and as indicated in Eqs. (1)e(5), an end the acid problem with dehalogenation. product of dehalogenation is hydrochloric acid. An ideal compound for DNAPL biodegradation The high hydrochloric acid production can be a would be sufficiently soluble, would not lead to the 278 P.L. McCarty et al. / European Journal of Soil Biology 43 (2007) 276e282 production of acetic acid, and would be self-neutraliz- In addition, the acetic acid that is formed when ing of hydrochloric acid production. A compound that most organics are fermented to form hydrogen also de- meets these characteristics is formate. presses pH: Formate as Electron Donor: CH COOH ¼ Hþ þ CH COOÀ; Â 3ÃÂ Ã 3 Formate disproportionation : þ À À Á H CH3COO À5 ¼ Ka ¼ 1:72 10 at 20 C ð16Þ 4HCOONa þ 4H2O ¼ 4NaHCO3 þ 4H2 ð10Þ ½CH3COOH Reductive dehalogenation of PCE : The pH problem can be especially difficult when ap- plying biological reduction to biologically enhanced CCl ]CCl þ 4H ¼ 4HCl þ CH ]CH ð11Þ 2 2 2 2 2 dense non-aqueous phase (DNAPL) chlorinated solvent Acid neutralization : dissolution. Here, dehalogenation of high concentra- tions of chlorinated solvent can result in production of 4NaHCO3 þ 4HCl ¼ 4NaCl þ 4CO2 þ 4H2O ð12Þ high hydrochloric acid and acetic acid production, necessitating a large buffer capacity to prevent adverse Net reaction : pH drop. Fig. 1 illustrates the change in pH as a function CCl2]CCl2 þ 4HCOONa ¼ CH2]CH2 of the extent of dechlorination with groundwaters start- ing with different bicarbonate alkalinities (mg/L as þ4NaCl þ 4CO2 ð13Þ CaCO3). This is based upon the use of hydrogen alone for reductive dehalogenation. Enhanced dissolution of Formate is enzymatically converted into bicarbonate TCE or PCE DNAPL might lead to the production of and hydrogen (Eq. (10)). The hydrogen is used for re- 10 to 20 mM chloride or more. The figure illustrates ductive dehalogenation (Eq. (11)), and the hydrochloric acid produced is neutralized by the bicarbonate that in order to maintain pH of about 6.5 or above for maximum reduction rates, no more dechlorination (Eq. (12)). The net result is the production of ethene, so- could be achieved than about 3.3 mM with an initial dium chloride, and carbon dioxide gas. Carbon dioxide bicarbonate alkalinity of 400 mg/L, double that to is a highly soluble weak-acid gas, and some bicarbonate 6.6 mM with alkalinity of 800 mg/L, and double that must be present to buffer its impact. again to 13.2 mM with alkalinity of 1600 mg/L. The later of 13.2 mM chloride could result from complete 3. Electron donor, alkalinity, and pH relationships dechlorination of 4.3 mM TCE, which is only about one half of the TCE solubility of 8.4 mM. Ideally for bi- The rates of biological reactions are affected greatly ologically enhanced dissolution of TCE, much greater by pH, with most organisms favoring a near-neutral pH dehalogenation than this would be desirable.