The Effect of Concentrated Electron Donors on theSolubility of Trichloroethene

by Eric Hood, David Major, and Greg Driedger

AbstractThe use of enhanced in situ bioremediation to remove chlorinated ethenes in sources areas is an emerging application of

this remediation technology. Contaminant mass removal likely involves a number of different physicochemical processes;however, recent vendor claims suggest that the addition of highly concentrated electron donor solutions to increase the aque-ous solubility of trichloroethene (TCE) is a significant mechanism of contaminant mass removal. To assess the validity ofthese claims, experimental measurements of the solubility of TCE in aqueous solution with eight typical electron donors atconcentrations ranging from 0.5% to 25% are presented. The results of these measurements, in accordance with the theoreti-cal understanding of solubility, indicate that the salts of carboxylic acids, including common electron donors (e.g., sodiumlactate), depress the solubility of TCE in accordance with the theoretical predictions of the Setschenow equation. Three ofthe electron donors tested (ethanol, acetic acid, and lactic acid) increased the solubility of TCE. The largest increase in solu-bility resulted from ethanol addition, with only a fourfold increase at an ethanol concentration of 25%. These results suggestthat salting-out effects are yet another consideration, along with stoichiometric electron donor requirements, adverse geo-chemical impacts, utilization by dechlorinating organisms, and electron donor longevity, for electron donor selection.

IntroductionThe widespread occurrence of chlorinated solvents in

ground water has led to the widespread use of in situ biore-mediation techniques, including the addition of electrondonors to stimulate the reducing conditions favoring anaer-obic reductive dechlorination. Engineering design of thesetechnologies is rapidly evolving, although to date there isconsiderable uncertainty in determining the quantities ofelectron donor required and the impacts of dosing require-ments on field performance.

The use of in situ bioremediation in source areas con-taining dense nonaqueous phase liquids (DNAPLs) is ofparticular interest in light of recent studies, suggesting thatrates of biodegradation are sufficient to accelerate the rateof nonaqueous phase dissolution (Seagren et al. 1993;Cope and Hughes 2001). A number of hypothesized mech-anisms, summarized in Table 1, are proposed to explainthis process. The increase in mass transfer rates resultingfrom contaminant biodegradation (i.e., mechanism I) issupported by laboratory and modeling (Carr et al. 2000;Seagren et al. 2002; Yang and McCarty 2002; Chu et al.2003) studies that demonstrate this process. Morerecently, it is hypothesized that a much wider range of

physicochemical processes (i.e., mechanisms II through V)contribute to enhanced mass transfer by increasing contami-nant solubility, including the addition of concentrated sol-utions of common electron donors such as sodium lactate(Sorenson 2003). This physicochemical process is thesubject of commercialized patent claims (US Patent No.6,783,678).

In this technical note, we examine the scientific basisfor the hypothesized in situ processes enhancing contami-nant solubility and describe the basic theoretical under-standing of the solubility of nonpolar compounds. Usingwell-established and simple protocols, we present experi-mental measurements of the solubility of trichloroethene(TCE) with varying cosolutes and cosolute concentrations.

Hypothesized Mechanisms of Solubility EnhancementEnhanced mass removal (including both DNAPL and

sorbed contaminant mass) resulting from contaminant deg-radation is the subject of numerous studies (e.g., Seagrenet al. 1993; Schnarr et al. 1998; Cope and Hughes 2001),although this process for field-scale applications is lesswell understood.

In contrast, however, there is little literature evidence tovalidate the hypothesis that electron donors or biochemicalprocesses using electron donors increase solubility of TCE(i.e., mechanisms II through V). Sorenson (2003) pre-sents data inversely correlating DNAPL/water interfacial

Copyright ª 2007 The Author(s)Journal compilationª 2007National GroundWater Association.

Ground Water Monitoring & Remediation 27, no. 4/ Fall 2007/pages 93–98 93

tension with sodium lactate concentration; however, solu-bility was not measured. Although the author concludesthat the reduction in interfacial tension resulted in a compa-rable increase in solubility, there is little a priori basis forthis expectation. Lactate’s low molecular weight and ionicstructure are significant dissimilarities with the surfactantsand cosolvents used for remediation purposes.

Hydrophobic partitioning into organic solutes (mecha-nism III) has been demonstrated to result in solubilityenhancements resulting from preferential contaminantpartitioning into some dissolved organic solutes such ashumic substances, humic and fulvic acids (e.g., Hunchak-Kariouk et al. 1997), and cyclodextrins (Bizzigotti et al.1997), all of which have high molecular weights and com-plex molecular structures incorporating highly hydrophobicregions comparable to the hydrophobic interior of a surfac-tant micelle. In comparison, typical electron donors usedfor bioremediation have simple structures in comparisonto these compounds, and there are no experimental datademonstrating a similar partitioning phenomenon for anyof the electron donors in common use.

The formation of fermentation products potentiallyacting as cosolvents and/or mobilizing natural organic car-bon from the aquifer solids into the aqueous phase (mecha-nism IV) is uninvestigated. While many electron donorscan be fermented into highly solvated alcohols underanaerobic conditions, the authors are not aware of field ob-servations of alcohols in ground water at a sufficient con-centration to cause cosolvency (e.g., 10% v/v or higher;Schwarzenbach et al. 1993) and, in our experience, alco-hols such as ethanol rapidly undergo further fermentationin ground water resulting in the formation of organic acids(e.g., acetate).

Finally, there are a number of studies examining bio-surfactant-enhanced solubility effects (mechanism V) (e.g.,Zhang and Miller 1992; Providenti et al. 1995; Noordmanet al. 2000); however, these investigations involve the useof refined biosurfactants that are manufactured ex situ

rather than the in situ production of biosurfactants follow-ing electron donor addition. In situ biosurfactant produc-tion may play an important role in contaminant desorptionand bioavailability (Pennell and Abriola 1998); however, todate the experimental work with biosurfactants emphasizesthe use of biosurfactants manufactured ex situ and is moreproperly considered as a surfactant flushing technologyrather than a key component of in situ bioremediation.

Factors Influencing Contaminant SolubilityAqueous solubility is ‘‘the abundance of the chemical

per unit volume of aqueous phase when the solution is inequilibrium with the pure compound at a specified pres-sure and temperature’’ (Schwarzenbach et al. 1993). Theaqueous solubility of a nonpolar organic contaminantreleased into ground water as a nonaqueous phase liquid,such as PCE or TCE, is limited by the relatively high polar-ity of the solvent (i.e., water). To dissolve the nonaqueousphase compound into aqueous solution, sufficient thermo-dynamic energy must be available to dissociate eachorganic molecule from the organic phase, displace hydro-gen bonds between adjacent water molecules to create acavity for the organic molecule, and then form intra-molecular attractions between each organic molecule andthe surrounding water molecules (Schwarzenbach et al.1993). The magnitude of this thermodynamic barrier to dis-solution depends upon the strength of molecular inter-actions between the organic molecule, surrounding watermolecules, and any cosolutes.

Inorganic ionic cosolutes (e.g., salts) reduce the solu-bility of nonpolar compounds by tying up water moleculesinto a hydration shell around each ion, forming electro-static bonds between the ion and the surrounding watermolecules. Water molecules incorporated into the hydra-tion shell are then unavailable to form hydration shellsaround other solutes. The decrease in the solubility of anonpolar organic compound caused by an inorganic

Table 1Hypothesized Physicochemical Mechanisms Resulting in an Increase in the Solubility of

Hydrophobic Contaminants

Hypothesized Mechanism Description of Physicochemical Process Source

I. Microbial activity atNAPL/water interface

Biodegradation enhances dissolution rate by increasingthe concentration gradient

Seagren et al. (1993)

II. Reduction of interfacial tension Specific electron donors (e.g., sodium lactate) decreaseNAPL/water interfacial tension, which increasesthe solubility of the NAPL constituents

Sorenson (2003)

III. Hydrophobic partitioning Organic contaminants preferentially partition intodissolved organic carbon (e.g., the electron donor)

Payne et al. (2001)

IV. Fermentation product cosolvency Products of electron donor fermentation act as cosolvents,increasing the solubility of hydrophobic contaminants

Payne et al. (2001)

V. Biosurfactant formation Surfactants produced by microorganisms increase solubilityof hydrophobic contaminants

Payne et al. (2001)

Note: NAPL ¼ nonaqueous phase liquid.

E. Hood et al./ Ground Water Monitoring & Remediation 27, no. 4: 93–9894

cosolute, termed ‘‘salting out,’’ is described by theSetschenow equation (Schwarzenbach et al. 1993) asfollows:

logðC0=CÞ 5 Ks�X�

where C0 is the ideal aqueous solubility, C is the solubilityin the specified salt concentration [X], and Ks is an empiri-cal salting constant.

The effects of organic cosolutes are less well under-stood and depend on the cosolute’s relative polarity andthe extent of its intermolecular interactions with thesolvent. Highly nonpolar organic cosolutes such as co-contaminants within a DNAPL depress the solubility ofthe compound of interest in accordance with Raoult’s Law(Pankow and Cherry 1996). In contrast, highly polarorganic cosolutes (e.g., alcohols) at sufficiently high con-centration (generally greater than 10% v/v; Schwarzenbachet al. 1993) increase the aqueous solubility of nonpolarorganic compounds. These cosolutes act a second solvent(i.e., a cosolvent) that can be incorporated into the hydra-tion shells surrounding a nonpolar solute (Schwarzenbachet al. 1993). This process is the basis for cosolvent flushingto rapidly remove DNAPL from the subsurface.

Many electron donors consist of an anionic organicsubstrate (e.g., lactate) with a cationic inorganic cosolute(e.g., Na1, K1). The cation reduces solubility via hydra-tion; however, the effect of the anion likely varies depend-ing upon its molecular structure. For example, a highlypolar anion (i.e., possessing highly electronegative func-tional groups such as an alcohol), which more readilyforms hydrogen bonds with the surrounding water mole-cules, decreases the extent of hydration required to main-tain the cosolute in solution and minimizes any effect onsolubility. Organic anions with widely separated nonpolarand polar moieties can also increase the solubility of a non-polar solute by forming micelles with highly nonpolar in-teriors into which the nonpolar solute can dissolve.However, the net effect on solubility of the interaction ofthese competing effects (i.e., the extent of cation hydrationvs. anion cosolvent or surfactant effects) is not well under-stood for common electron donors.

Experimental ProcedureThe solubility of TCE in aqueous solution with nine

cosolutes was determined using an experimental protocolsimilar to that employed by Broholm and Feenstra (1995).The cosolutes, which include some common electrondonors, and their concentrations are summarized in Table 2.Cosolute concentrations were selected based on the rangeof sodium lactate concentrations previously reported todecrease interfacial tension (Sorenson 2003). Batch experi-ments were performed in 40-mL glass vials sealed withplastic screwcaps containing Teflon�-faced septum. All ex-periments were carried out at the ambient laboratory tem-perature (~20�C). Cosolute solutions were prepared withdeionized water using laboratory-grade ethanol, sodiumchloride, acetic acid, sodium acetate, sodium lactate, potas-sium lactate, lactic acid, and sucrose. Sodium chloride was

selected for comparison purposes since it is a commoncosolute in ground water. Replicate vials were filled witha cosolute solution (triplicate vials for each concentration)and amended with 3.4-mL neat TCE (Fisher Chemicals,Ottawa, Canada) using a gastight syringe (Hamilton Co.,Reno, Nevada). Solution pH was not controlled throughbuffer addition. Identical controls without a cosolute (i.e.,deionized water and neat TCE) were also prepared.

Cosolute solution displaced by TCE addition was al-lowed to overflow and each vial subsequently sealed toavoid forming a headspace. Each vial was agitated for 24 hon a platform shaker (New Brunswick Scientific, Edison,New Jersey) at 200 rotations per minute and then incu-bated at ambient laboratory temperature. Water sampleswere collected from each vial using a 10-lL gastightsyringe. The concentration of TCE in each sample wasdetermined using headspace gas chromatography equippedwith a flame ionization detector (HP 5890 Series II)according to SiREM Standard Operating Procedure #024.

Sucrose is the principal carbohydrate in molasses,which is commonly used as an electron donor. To qualita-tively assess the effect of biological activity on TCE solu-bility, replicate solubility measurements were made usinga fermented sucrose solution to model the production offermentation products and biosurfactants. A concentratedsucrose stock (25%) was prepared under anaerobic condi-tions in a nitrogen-purged glovebox (Coy Laboratory Prod-ucts, Grass Lake, Michigan). The sucrose stock solutionwas amended with 1 mL of an anaerobic seed suspensioncontaining a mixed culture of microorganisms from acontaminated site and incubated at 30�C for 7 d. The pre-sence of volatile fatty acids (acetic and lactic acid) in theincubated solution was subsequently confirmed by ionchromatography. The fermented sucrose solution was filtersterilized using a 0.2-m syringe filter (MilliporeCorporation, Bedford, Massachusetts) and used to preparevials at nominal initial sucrose concentrations of 1%, 11%,and 25%.

Results and DiscussionSummaries of the TCE solubility data collected for

each cosolute are provided in Table 2 and Figure 1. TCEsolubility limits for each experimental condition were com-pared to the TCE solubility in unamended controls usingthe t-statistic calculated for small sample populations (p ¼0.05) to identify statistically significant differences insolubility (Miller and Freund 1985). The average concen-tration of nine unamended controls (1139 mg/L) agreedclosely with the reported solubility of TCE (1100 mg/L;Pankow and Cherry 1996). Significant increases in TCEsolubility were observed with the addition of ethanol, ace-tic acid, and lactic acid; decreases in solubility wereobserved with the addition of all other cosolutes, with theexception of sucrose.

The addition of ethanol at 1 and 10 wt. % had noapparent effect on solubility although the addition of etha-nol at 25 wt. % increased TCE solubility fourfold. In com-parison, Boving and Brusseau (2004) reported a 57-fold

E. Hood et al./ Ground Water Monitoring & Remediation 27, no. 4: 93–98 95

increase in TCE solubility using 50% ethanol. These con-centrations of ethanol are much higher than what couldconceivably be formed in situ by fermentative micro-organisms. The addition of lactic or acetic acid alsoincreased TCE solubility, suggesting that concentratedaqueous solutions of these organic acids may exert a co-solvent effect by decreasing the solution polarity. The pHof these cosolute solutions was not measured; however,additional controls with the pH adjusted to 2 using HCl

confirmed that the decrease in solution pH did not itselfresult in an increase in TCE solubility (data not reported).

The addition of sucrose, a nonionic electron donor, hadno measurable effect on TCE solubility. In contrast, the fer-mented sucrose solution decreased TCE solubility by 16%at the highest initial sucrose concentration (26 wt. %). Theaddition of cosolute salts (i.e., sodium chloride, sodiumlactate, sodium acetate, and potassium lactate) also signi-ficantly decreased TCE solubility. Pooled data for these

y = 0.0697x + 0.0339 R2 = 0.33

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 1 2 3 4 5 6

Concentration (M)

log(

S/S

0)

EthanolAcetic AcidLactic Acid

a)

y = -0.2164x - 0.0054 R2 = 0.88

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.0 0.5 1.0 1.5 2.0 2.5

Concentration (M)

log(

S/S

0)

NaClNa-acetateNa-lactateK-lactateSucroseSucrose (Fermented)

b)

Figure 1. Normalized solubility data for (a) cosolutes increasing TCE solubility (i.e., ethanol, acetic acid, and lactic acid) and (b)cosolutes decreasing TCE solubility (i.e., sodium chloride, Na-acetate, Na-lactate, K-lactate, sucrose, and fermented sucrose).

Table 2Summary of Measured Solubility Limits for TCE with Varying Cosolutes and Cosolute Concentrations

CosoluteCosolute

Concentration (wt %)Mean

Solubility (mg/L)Standard

Deviation (mg/L)NormalizedSolubility1

Significant(p ¼ 0.05)2

Control 0.0 1139 51 —Sodium chloride 0.5 1083 13 0.92 1

4.9 762 20 0.65 1

12 423 28 0.36 1

Ethanol 1.0 1062 206 0.98 �10 1180 73 1.09 �25 4350 428 4.03 1

Sodium acetate 0.8 1095 75 0.93 �7.7 776 33 0.66 1

18 425 37 0.36 1

Sodium lactate 1.0 1045 19 0.90 1

10 685 50 0.59 1

25 398 20 0.34 1

Potassium lactate 1.0 892 57 0.83 1

10 610 20 0.57 1

25 305 15 0.28 1

Acetic acid 11 1960 45 1.67 1

Lactic acid 1.0 1180 22 1.02 1

10 1371 67 1.18 1

25 2165 84 1.86 1

Sucrose 1.0 1156 119 1.00 �10 1101 44 0.95 �25 1099 46 0.95 �

Sucrose (fermented) 1.0 900 153 0.83 �10 1043 94 0.97 �25 911 61 0.84 1

1Reported solubility measurements are normalized to the aqueous solubility of TCE in deionized water.2Statistically significant changes in TCE solubility are denoted by ‘‘1’’; the absence of a significant change in TCE solubility is indicated by ‘‘�.’’

E. Hood et al./ Ground Water Monitoring & Remediation 27, no. 4: 93–9896

cosolutes (Figure 1b) were linearly correlated (r2 ¼ 0.88)and the slope of the line of best fit was used to calculatea Setschenow salting constant of 0.22 L/mol, which agreeswith previously reported salting constants for a wide rangeof organic compounds in sea water cosolute solutions (Xieet al. 1997). The results of this calculation may be used toconstrain the addition of electron donors at concentrationsthat may adversely impact TCE solubility. For example, toavoid decreasing TCE solubility by more than 1%, sodiumlactate concentrations should be less than 2260 mg/L(0.2 wt. %).

In Table 2, the change in normalized solubility is pro-vided on a per electron equivalent basis to comparechanges in TCE solubility for equivalent electron donordosing (typically determined using the stoichiometricdemand exerted by the electron acceptors; AFCEE 2004).For an equivalent electron dose of sodium or potassiumlactate, the decrease in TCE solubility is significantlygreater than the comparable increase in TCE solubility re-sulting from ethanol addition.

ConclusionsTo our knowledge, these are the first data directly

examining the impact of electron donor concentration onTCE solubility. Under ideal experimental conditions, thelimited data presented here demonstrate that six of ninecosolutes (including sodium lactate) either decreased orhad no effect on the aqueous solubility of TCE. Three co-solutes (ethanol, acetic acid, and lactic acid) resulted in anincrease in solubility. In the case of ethanol amendment at25%, the increase in solubility was small in comparison tothe cosolute concentration and, in the judgment of the au-thors, is unlikely to have a significant beneficial impact onthe performance of enhanced in situ bioremediation atmore typical electron donor dosages. Of the other electrondonors tested, potassium lactate resulted in the largestabsolute impact, decreasing the solubility of TCE from1139 to 305 mg/L. For electron donor dosages calculatedusing accepted guidance (e.g., AFCEE 2004), which aretypically less than 0.1%, the impact (positive or negative)of electron donor concentration on the solubility of TCE islikely negligible.

If the results presented in the study are representativeof the solubility impacts at the field scale, the smallchanges in TCE solubility in comparison to the high elec-tron donor concentrations employed suggest that it is diffi-cult to envision circumstances justifying the use of a highelectron donor concentration to enhance TCE solubilityas part of a bioremediation strategy, although the use ofmore concentrated (e.g., 50% to 95%) ethanol solutionswould be appropriate for cosolvent flooding. As part of thedesign of a bioremediation system in a DNAPL sourcearea, salting-out effects are yet another consideration thatshould be balanced against the stoichiometric electrondonor requirements, any adverse geochemical impacts, thesuitability for dechlorinating organisms, longevity. Calcu-lation of the salting-out effects may be easily performedto confirm that adverse impacts to TCE solubility areminimized.

Although high concentrations of lactic acid did resultin solubility increase, the high electron donor concentra-tion required to achieve a significant improvement in con-taminant solubility may be inhibitory to biodegradationand likely to result in adverse water quality impacts. Thepresence of high concentrations of organic acids, eitherdirectly amended or produced by substrate fermentation,can be associated with decreases in pH (e.g., Adamsonet al. 2003) that inhibit reductive dechlorination of cis-1,2-dichloroethene and vinyl chloride to ethene (Zhuangand Pavlostathis 1995). Further, high electron donor con-centrations can result in adverse ground water impacts,including high biochemical oxygen demand, reduced per-meability caused by biofouling, high methane generation,and high dissolved iron and manganese concentrations(Yang and McCarty 2002). If the remedial objective israpid mass removal, then consideration of a cosolventflushing strategy that optimizes the solubility enhancementmay be warranted.

AcknowledgmentsThe authors would like to acknowledge the helpful

input and reviews provided by C. Repta and H. Groenevelt(GeoSyntec) along with those provided by two anonymousreviewers. Funding for this study was provided by Geo-Syntec.

Editor’s Note: The use of brand names in peer-reviewedpapers is for identification purposes only and does not con-stitute endorsement by the authors, their employers, or theNational Ground Water Association.

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Biographical SketchesEric Hood, corresponding author, is a senior scientist with

Geosyntec Consultants, 130 Research Lane, Suite 2, Guelph,Ontario; (519) 822-2230; ehood@geosyntec.com. He holds a Ph.D.in civil engineering from the University of Waterloo.David Major is principal with Geosyntec Consultants, 130

Research Lane, Suite 2, Guelph, Ontario; (519) 822-2230; dmajor@geosyntec.com. He holds a Ph.D in microbiology from the Univer-sity of Waterloo.Greg Driedger is a laboratory technician with SiREM Labs,

130 Reseach Lane, Suite 2, Guelph, Ontario; (519) 822-2265. Heholds a B.Sc. from the University of Waterloo.

E. Hood et al./ Ground Water Monitoring & Remediation 27, no. 4: 93–9898