Mechanisms for abiotic dechlorination of TCE by ferrous minerals

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Environmental Processes

Mechanisms for abiotic dechlorination of TCE by ferrous minerals under oxic and anoxic conditions in natural sediments Charles E. Schaefer, Paul Ho, Erin Berns, and Charles J. Werth Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b04108 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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Environmental Science & Technology

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TITLE: Mechanisms for abiotic dechlorination of TCE by ferrous minerals under oxic and anoxic conditions in natural sediments

AUTHORS:

Charles E. Schaefer1,*, Paul Ho2, Erin Berns3, Charles Werth3

AFFILIATIONS:

CDM Smith, 110 Fieldcrest Avenue, #8, 6th Floor, Edison, NJ 08837 1

2

CDM Smith, 14432 SE Eastgate Way # 100, Bellevue, WA 98007 3

University of Texas at Austin, Civil, Architectural, and Environmental Engineering, 301 E. Dean Keeton St., Stop C1786, Austin, TX 78712

*CORRESPONDING

AUTHOR: Mailing address: CDM Smith, 110 Fieldcrest Avenue, #8, 6 Floor, Edison, NJ 088837. (732)-590-4633. E-mail: [email protected] th

Submitted to Environmental Science & Technology

Key Words: abiotic, TCE, dechlorination, ferrous, hydroxyl

ACS Paragon Plus Environment


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Abstract

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Bench-scale experiments were performed on natural sediments to assess abiotic

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dechlorination of trichloroethene (TCE) under both aerobic and anaerobic conditions. In

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the absence of oxygen (99.5% purity) and gas standards (methane, ethane, ethene, acetylene, propane,

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propylene, methyl acetylene, butane in a nitrogen balance) were purchased from Sigma

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Aldrich (St. Louis, MO). A solution consisting of 5 mM CaCl2 was used in all the batch

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experiments to serve as background electrolyte. Five natural aquifer solids used for this

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study, hereafter referred to as sediments, were collected from US Department of Defense

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facilities.

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All sediments were collected from the saturated zone via direct push drilling, then

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sent overnight to the laboratory where they were stored anaerobically at 3 degrees C.

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Prior to use in the abiotic batch experiments, sediments were sterilized via gamma

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irradiation (2.5 M-rads) to limit microbial activity. Sediments were homogenized by hand

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in a glass bowl within the anaerobic chamber; any large solids (> 5 mm) present in the

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sediments were discarded.

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Batch Abiotic Dechlorination Experiments - Anaerobic Batch experiments to assess and quantify TCE abiotic dechlorination under anaerobic

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conditions were performed based on previously described methods (10). Briefly, all

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preparations were performed in an anaerobic chamber. For each batch system, 5g of

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gamma-irradiated sediment was placed in 40 mL amber glass vials. The 5 mM CaCl2

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electrolyte solution was sparged with nitrogen gas for a minimum of one hour prior to use

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in the anaerobic chamber. A contaminant spiking solution was also prepared using neat

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TCE dissolved into 5 mM CaCl2 that had been sparged with nitrogen. Control bottles (no

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added TCE) were amended with 35 mLs of the TCE-free electrolyte, while TCE spiked

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bottled received 15 mLs (each) of both the TCE-free and TCE-spiked electrolyte; no neat

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TCE was transferred to the 40 mL vials. TCE aqueous concentrations in the TCE-

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amended vials were approximately 3 mM; these elevated TCE concentrations facilitated

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the detection of transformation products. Use of both contaminant-spiked and non

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contaminant-spiked conditions served as a means to distinguish generation of TCE

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dechlorination products due to current abiotic reactions from that of gas generation due to

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any other carbon sources and/or any accumulation of these gases in the sediment samples

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from dechlorination that may have occurred in situ prior to sample collection (10, 11).

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The vials were capped with Mininert® valves (with epoxy seals on the threads) to allow

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for repeated sampling of the approximately 5 mL of headspace in the vials for TCE

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(including potential biotic dechlorination products cis-1,2-dichloroethene (DCE) and

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vinyl chloride (VC)) and reduced gases.

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Vials were prepared in triplicate for each of the five sediment types, for a total of 30 vials (TCE-amended and non TCE-amended controls). In addition, parallel vials were


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prepared (in triplicate for each sediment, and with and without TCE) for headspace

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sampling of O2/CO2 and aqueous monitoring of organic acids (OAs); this amounted to

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another 60 vials. A no-sediment control (with amended TCE) also was prepared in

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triplicate for chlorinated ethene and reduced gas analysis, bringing the total vials

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prepared under anaerobic conditions to 93. All sampling of the anaerobic vials was

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performed under a stream of nitrogen to limit potential introduction of oxygen into the

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vials. Monitoring typically was performed for up to approximately 105 days. At the end

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of the experiment, select vials were sampled to determine both pH and the dissolved iron

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concentration in the supernatant water.

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Batch Abiotic Dechlorination Experiments - Aerobic

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Aerobic batch experiments were performed and monitored similarly to the anaerobic

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experiments (another 93 vials were prepared, including 3 additional no-soil controls),

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except that the aerobic experiments had ultra-high purity oxygen injected into the vial

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headspace. Equal amounts of vial headspace were removed and replaced with oxygen to

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maintain atmospheric pressure. Vial headspace was periodically monitored and

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maintained above 8% O2, which (based on Henry’s Law) resulted in aqueous dissolved

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oxygen concentrations being maintained above 120 µM.

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Analytical Methods and Sediment Characterization All mineral analyses were performed on homogenized soil prior to performing the

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abiotic dechlorination experiments. Sediment mineralogy was determined by MUD

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Geochemical, Inc. (Austin, TX) using x-ray diffraction (XRD) with a Bruker D2 Phaser;


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XRD analysis was performed using duplicate samples. Magnetic susceptibility was

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determined by Microbial Insights, Inc. (Knoxville, TN) using a Barrington MS2B meter

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(17, 18). Ferrous mineral content was determined by McCampbell Analytical, Inc. using

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the 1,10-phenanthroline method (19). Both magnetic susceptibility and ferrous mineral

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content were performed in duplicate. Sieve analysis was performed for each sediment to

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determine the approximate particle size distribution. pH was determined by using an

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Orion 720A+ meter and Orion 9107BN pH probe, and was performed in duplicate for

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each sediment type and condition (i.e., aerobic vs. anaerobic). Total organic carbon was

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analyzed by Fremont Analytical (Seattle, WA) using EPA Method 9060.

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TCE, DCE, VC, and reduced gas (methane, ethane, ethene, propane, acetylene,

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butane, methyl acetylene, and propylene) concentrations were determined via headspace

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analysis using a Shimadzu 2010+ gas chromatograph equipped with a Flame Ionization

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Detector (FID) and an RT-QS-BOND fused silica PLOT column. Aqueous

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concentrations were determined by applying Henry’s Law. Aqueous quantification limits

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for TCE, DCE, and VC were 9.7 x 10-5, 2.4 x 10-4, 3.9 x 10-5 mM, respectively; aqueous

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quantification limits for the reduced gases ranged from 1 x 10-6 to 1 x 10-7 mM. Further

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details of the OA, O2/CO2, and dissolved iron analyses are provided in the Supplemental

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Information.

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Quantification of cumulative hydroxyl radical generation was performed in a parallel

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set of vial experiments using 2 of the sediment types. The methodology (24) used to

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quantify hydroxyl radicals is provided in the Supplemental Information.

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Results and Discussion

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Sediment Properties

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Sediment properties are summarized in Table 1. Sediment texture and mineralogy

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varied among the 5 sediments used in this study. Ferrous mineral content varied nearly 3

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orders of magnitude among the sediments, with ferrous mineral content generally greater

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in the clayey sediments than in the sandy sediments. The ferrous mineral content did not

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appear to be associated with the presence of ferrous-containing minerals identified via

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XRD (i.e., biotite, siderite, illite, and ankerite), suggesting that the ferrous content of

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these minerals varied greatly among the sediments used in this study. It is also possible

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that the 1,10-phenanthroline method used to determine the ferrous iron content did not

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sufficiently quantify the ferrous iron associated with the minerals identified via XRD.

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Chlorinated ethenes

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With the exception of sediment 3, TCE concentrations for all TCE-amended

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conditions remained steady (15%

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were not observed for any other sediment amended with TCE, or for the TCE-amended

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controls prepared without sediment. In addition, no generation of lesser chlorinated

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reductive dechlorination products DCE or VC were observed in any samples.

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Anaerobic Experiments


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Generation of the observed reduced gas transformation products for sediments 1

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through 3 is shown in Figure S1 (Supplemental Information); no generation of reduced

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gases were observed for sediments 4 and 5. Acetylene or ethene were only present in the

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vials spiked with TCE, and not in the vials without amended TCE or in the no-sediment

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controls. Acetylene is not a known biotic reductive dechlorination product of TCE, and

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therefore serves as an indicator of abiotic dechlorination. While ethene can be both a

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biotic and abiotic TCE dechlorination product, the absence of measurable VC and DCE

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accumulation suggests that the ethene is likely an abiotic transformation product (20, 21).

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For sediments 1 through 3, results show that abiotic generation of the TCE dechlorination

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products was occurring throughout the duration of the experiment, as evidenced by the

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increasing trends in acetylene and ethene. However, as noted in the Figure S1 caption, the

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lack of a replicate for the sediment 3 acetylene data at 106 days warrants interpreting this

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final data point with caution. The abiotic generation of reduced gases such as acetylene

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and ethene under anaerobic conditions is consistent with many previous studies using

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either TCE or PCE (e.g, 8-11).

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In addition to the reduced gas generation shown in Figure S1, sediment 1 also

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exhibited generation of glycolic/acetic acid (Figure S2); generation of glycolic/acetic acid

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was only observed in the TCE-amended sediment 1 samples. The generation of this TCE

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oxidation product is further explored in subsequent sections, as the role of trace oxygen

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levels on hydroxyl radical formation is examined. It is noted that the analytical method

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used could not distinguish between glycolic acid and acetic acid. Glycolic or acetic acids

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have been observed as oxidative abiotic chlorinated ethene dechlorination products in

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studies using crushed rock and in electrochemical systems (12, 22). No sustained


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generation of OAs was observed for the other sediments. However, quantification limits

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for the OAs were much greater than for those of the reduced gases - approximately 5 x

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10-3 as a molar fraction of the TCE present (using the y-axis units on Figure S1) - so low-

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level OA generation at a magnitude similar to that exhibited by acetylene could have

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occurred. Similarly, no generation of CO2 was observed in any experiment, but CO2

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levels of 2 x 10-3 to 52 x 10-3 (depending on the background CO2 levels) as a molar

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fraction of the TCE present would have been required for detection. Regardless, for all

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sediments, the fraction of TCE transformed for the duration of the experiment was a very

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small (

Mechanisms for abiotic dechlorination of TCE by ferrous minerals

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