Article pubs.acs.org/est
Enhanced Reductive Dechlorination of Tetrachloroethene Dense Nonaqueous Phase Liquid with EVO and Mg(OH)2 Kirsten M. Hiortdahl† and Robert C. Borden*,†,‡ †
Department of Civil, Construction and Environmental Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States ‡ Solutions-IES, Inc., 1101 Nowell Road, Raleigh, North Carolina 27607, United States S Supporting Information *
ABSTRACT: In situ treatment of dense nonaqueous phase liquids (DNAPL) by enhanced reductive dechlorination (ERD) can be limited by contaminant toxicity, low pH, and challenges in effectively delivering electron donor. Flushing emulsified vegetable oil (EVO), colloidal Mg(OH)2 buffer, and a bioaugmentation culture (BC) through a zone containing neat tetrachloroethene (PCE) was effective in reducing contaminant toxicity, limiting pH declines, and accelerating bioenhanced dissolution of the DNAPL. In the effluent of porous media columns with little fine material, PCE concentrations reached a maximum of 40−50 times PCE aqueous solubility in water, demonstrating NAPL PCE was distributed throughout the 1.5 m column length. In a column treated with only EVO+BC, reductive dechlorination was limited. However, a single injection of EVO+Mg(OH)2+BC was effective in reducing PCE to below detection for over 400 days with a large increase in Cl− and dichloroethene (DCE), accelerating bioenhanced DNAPL dissolution. Dechlorination rates gradually increased over time with the rate of total ethene (TE) release from the Mg(OH)2+EVO+BC column reaching 5−6 times the TE release rate from the EVO+BC column. The accelerated dechlorination was likely due to both Mg(OH)2 addition which limited pH declines from HCl, volatile fatty acids (VFAs), and inorganic carbon (IC) production, and formation of a mixed PCE-vegetable oil NAPL which provided a readily accessible electron donor, resulting in rapid PCE degradation with reduced PCE toxicity.
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DNAPL.8 However, contaminant degradation rates can be limited by contaminant toxicity, low pH, and challenges in effectively delivering electron donor to the NAPL. Some dechlorinating bacteria can survive at PCE concentrations near the aqueous solubility of ∼1 mM.5,9 However, other investigators have reported toxic inhibition by PCE10−12 and daughter products.13−15 The upper limit for PCE dechlorination appears to vary from 0.4 to 0.8 mM2,11,16 with higher concentrations tolerated by mixed cultures that can form biofilms and aggregates.11,14 In the laboratory, toxic inhibition can be controlled by mixing the PCE with an inert hydrocarbon to reduce the aqueous concentration of the chloroethenes.1,4,6,7 However, addition of 29.4 g of emulsified vegetable oil (EVO) emulsion to a column containing ∼75 g of TCE DNAPL was not effective in controlling TCE toxicity over the first 240 days of operation.17 Eventually DCE production did increase near the column inlet as TCE was depleted. pH can decline during ERD due to release of hydrochloric acid (HCl), volatile fatty acids (VFAs), and carbonic acid (H2CO3) contributing to reduced dechlorination efficiency.6,18 The optimum pH for dechlorination is near neutral.19,20 Dechlorination of PCE and TCE to DCE can occur at pH
INTRODUCTION Enhanced Reductive Dechlorination (ERD) has been successfully applied for in situ treatment of dissolved chlorinated solvents. However, in situ treatment of dense nonaqueous phase liquid (DNAPL) is more challenging due to contaminant toxicity, low pH, and challenges in effectively delivering electron donor. Under ideal conditions, tetrachloroethene (PCE) and trichloroethene (TCE) may be reduced all the way to nontoxic end-products (ethene and ethane). However in the presence of DNAPL, substantial amounts of dichloroethene (DCE) and vinyl chloride (VC) are often produced.1−3 Conversion of PCE and TCE to DCE can still accelerate source zone remediation by reducing contaminant concentrations in the aqueous phase near the NAPL-water interface, increasing the dissolution rate.4 DCE and VC produced by dechlorination can partition back into the DNAPL. However, since DCE and VC are more soluble than the parent compounds, the effective solubility of the resulting multicomponent DNAPL can be considerably higher than the original DNAPL, accelerating cleanup.1 DCE is over thirty times as soluble as PCE, so large increases in the dissolution rate could be possible, if reductive dechlorination can be stimulated. However, in several labscale studies that used 1-D column experiments and 2-D flow cells, observed dissolution rates only increased by a factor of 3 to 5 times.3,5−7 For efficient DNAPL removal, the contaminant degradation rate must be high relative to contact time with the © 2013 American Chemical Society
Received: Revised: Accepted: Published: 624
September 23, 2013 December 10, 2013 December 11, 2013 December 11, 2013 dx.doi.org/10.1021/es4042379 | Environ. Sci. Technol. 2014, 48, 624−631
Environmental Science & Technology
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down to 5.5, while reduction of DCE and VC is more sensitive, with little or no ethene production observed below a pH of 6.21,22 In the laboratory, near neutral pH can be maintained using high strength buffers.6,12 In columns treated with EVO, up to 71.4 mM (6.0 g/L) of sodium bicarbonate was required for dechlorination of ∼5 mM TCE.17 When using soluble electron donors, ERD treatment efficiency can be limited by poor delivery of the donor to the NAPL interface. In a study using 2-D flow cells, bioenhanced dissolution was limited by poor contact between the soluble electron donor and the residual PCE DNAPL due to fermentation to methane before the substrate reached the DNAPL and/or flow bypassing around the contaminant source due to clogging with biomass or gas bubbles.3 In a pilot test of DNAPL bioremediation in a sheet-pile test cell at Dover AFB, substrate addition with bioaugmentation did increase conversion of PCE to ethene.23 However, there was no statistically significant impact on DNAPL removal. The authors report that “clogging changed the flow paths within the test cell and resulted in the occlusion of some PCE from receiving electron donor”. These experimental observations are supported by modeling studies that predict development of a high biomass zone with reduced permeability that slows DNAPL dissolution.24 These problems with substrate delivery to the DNAPL interface were reduced by mixing pure PCE with olive oil, providing the dechlorinating bacteria with both electron acceptor and electron donor at the same location.2 The success of this approach lead to research on application of electron donors that can be delivered in a dissolved form and partition into the NAPL.25 In this work, we evaluate the impact of adding emulsified vegetable oil (EVO), colloidal Mg(OH)2, and bioaugmentation culture (BC) on bioenhanced DNAPL dissolution. Flushing EVO through a zone containing residual DNAPL generates small droplets of mixed NAPL containing both PCE and vegetable oil which should provide both electron donor and electron acceptor to reduce substrate delivery limitations for ERD and reduce the equilibrium concentration of PCE in water, reducing contaminant toxicity.1,4,6,7 In this work, Mg(OH)2 is used a slow-release pH buffer. Mg(OH)2 slowly dissolves over time due to its low aqueous solubility (pK°sp = 10.4),26 releasing OH− as acidity is produced during ERD, preventing large increases or decreases in pH. Colloidal Mg(OH)2 can be transported through porous media, allowing the pH buffer to be placed in close proximity to the DNAPL where acidity is produced and pH declines are expected to be greatest. Use of EVO in combination with Mg(OH) 2 overcomes many of the problems encountered in previous attempts to bioremediate DNAPL3,5−7,17,23 while providing nontoxic, slow-release electron donor2,27,28 and slow-release pH buffer, reducing requirements for frequent substrate and buffer addition. A single short-term injection of EVO, colloidal Mg(OH)2, and BC was effective in stimulating ERD of a PCE DNAPL. The injection process was effective in redistributing a portion of the PCE DNAPL and distributing oil and Mg(OH) 2 throughout the column. The combination of EVO and Mg(OH)2 addition accelerated PCE conversion to DCE while limiting pH declines, reducing PCE to near the analytical detection limit for over 400 days, and accelerating bioenhanced DNAPL dissolution.
Article
EXPERIMENTAL SECTION
Two sets of column experiments were conducted. Short-term transport experiments were conducted to evaluate the transport and retention of EVO, Mg(OH)2, and PCE in porous media with low-silt clay content. Long-term bioremediation experiments examined the effect of EVO, Mg(OH)2, and BC addition on biologically enhanced dissolution of residual PCE in porous media with higher silt-clay content. Both sets of experiments were conducted in 2.6 cm diameter × 1.5 m long clear PVC columns. PVC use allowed the columns to be frozen at the conclusion of the experiments and sectioned to measure the spatial distribution of Mg(OH)2. For both short- and long-term experiments, the columns were saturated with groundwater, then 1.6 g of pure PCE DNAPL dyed with Oil-O red was injected into the center of the porous media at 15 cm from the inlet and allowed to drain downward toward the inlet. The columns were then flushed with the diluted EVO or diluted EVO+Mg(OH)2 followed by BC maintenance media (MM) to displace EVO and Mg(OH)2 that had not been retained on porous media surfaces (see photo of column during flushing in SI Figure S1). The EVO used in this study was EOS 598 B42 (provided by EOS Remediation, LLC, Raleigh, NC), a commercially available EVO containing 60% vegetable oil, 4% soluble substrate, 2% yeast extract, 10% emulsifiers/additives, and 24% water by weight. The EVO+Mg(OH)2 formulation used in this work was modified from the original EOS formulation to minimize retention of the positively charged Mg(OH)2 particles (zeta potential of Mg(OH)2 = +25 mV at pH = 10) by the negatively charged surfaces of the porous media.26 The final formulation referred to as MH5 contained 50% vegetable oil, 4% glycerol, 7% emulsifiers, 34% water, and 5% colloidal Mg(OH)2 (D50 < 1 μm) by weight. The MM consisted of 12 mM sodium lactate, 14 mM NaHCO3, 1 g/L yeast extract, phosphate buffer, mineral salts, trace metals, and resazurin redox indicator.21 Previous work has shown that EVO is strongly retained by aquifer material, and oil droplets will not be effectively transported until most of the attachment sites on the soil particles are occupied by oil droplets.29,30 The amounts of EVO and MH5 used in this work were selected to provide a large excess of biodegradable organic carbon for reductive dechlorination and to saturate the porous media surfaces, allowing some oil to be discharged in the column effluent. The short-term transport experiments were designed to evaluate the transport and retention of EVO, Mg(OH)2, and PCE in fine porous media with very little silt or clay (D50 = 434 μm, 0.2% finer than 200 sieve). The experimental procedure involved injecting 20 mL of EVO or MH5 diluted with 80 mL of MM (100 mL of 20% dilution) followed by 400 mL of MM (1.6 pore volumes or PV) MM. After resting overnight with no flow, 500 mL of MM (2 PV) were flushed through the column to displace EVO and MH5 that had not been retained by the porous media. Throughout the injection cycle, the injection flow rate was 10 mL/min which resulted in an effective transport velocity of 6 cm/min, similar to conditions that might occur near an injection well during EVO injection. The long-term bioremediation experiments examined the effect of EVO, Mg(OH)2, and BC addition on biologically enhanced dissolution of residual PCE in fine porous media. EVO and MH5 injection procedures were similar to the transport experiments. However, the first 100 mL (0.4 PV) of fluid injected on the second day was replaced with 100 mL of 625
dx.doi.org/10.1021/es4042379 | Environ. Sci. Technol. 2014, 48, 624−631
Environmental Science & Technology
Article
Table 1. PCE, Mg, and Volatile Solids (VS) Initially Present or Injected, Discharged, and Retained by the Porous Media during Initial Amendment Injection Phase of the Transport and Bioremediation Experiments with the EVO and MH5 Amendmentsb transport PCE (mmol)
Mg (mmol)
VS (g)
VS/PCE (g/g) a
bioremediation
fraction
EVO
MH5
EVO
MH5
initial discharged retained injected discharged retained injecteda discharged retained discharged retained
9.65 5.44 (56%) 4.21 (44%) 15.4 13.9 (90%) 1.5 (10%) 15 2.2
9.65 6.35 (66%) 3.3 (34%) 16.45 5.47 (33%) 10.98 (67%) 12.8 8.9 (70%) 3.8 (30%) 8 7.0
9.65 1.71 (18%) 7.94 (82%) 15.4 12.6 (82%) 2.7 (26%) 45 2.1
9.65 0.86 (9%) 8.79 (91%) 16.45 9.22 (56%) 7.23 (44%) 12.8 9.5 (75%) 3.2 (25%) 67 2.2
VS injected includes both MM (7−8% of total) and EVO or MH5 (92−93%). bFraction of amount added shown in parentheses.
DCE, and all results are presented for total DCE. Anions (lactate, acetate, butyrate, propionate, chloride, nitrate, nitrite, and sulfate) were analyzed by ion chromatography with a Dionex ICS 2500 using a hydroxide mobile phase (column: Dionex IonPac AS11-HC). TOC and IC were analyzed with a Shimadzu TOC-5000 and autosampler. Volatile solids (VS) are determined by weight loss on ignition for 1 h at 550 °C. At the completion of each experiment, the columns were frozen, cut into 15 cm sections, homogenized, and analyzed for alkalinity by acidifying dried soil samples to pH < 2 with 2 N HCl, equilibrating for one week, and then back-titrating to pH 4.5 with 0.1 or 1.0 N NaOH. The porous media could not be analyzed for chlorinated ethenes due to interferences in extraction of porous media treated with EVO.
SDC-9 BC (Shaw Environmental, Inc., Princeton, NJ) at an OD550 = 121 to enhance biological activity. The BC was a consortium of methanogenic, sulfate-reducing, and dehalogenating bacteria including strains of Dehalococcodies (DHC) known to degrade PCE and TCE to ethene.21 Following injection of the emulsion and BC, groundwater was pumped batch-wise through the columns at 50 mL (0.2 PV) per week resulting in an average groundwater flow velocity of 15 m/yr. A parallel control column was flushed with an equivalent amount of groundwater during the amendment addition phase and then operated the same as the treated columns to evaluate PCE flushing with groundwater only. The porous media used in the long term bioremediation experiments had a higher silt-clay content to increase retention of EVO in the columns,29,31 providing more oil to reduce aqueous PCE concentrations. The porous media came from two different batches, resulting in slightly different characteristics (MH5: D50 = 542 μm, 12% finer than 200 sieve, EVO and control: D50 = 585 μm, 3.7% finer than 200 sieve). The porous media was not autoclaved prior to packing to simulate naturally occurring conditions. Transport and biodegradation of the oil, Mg(OH)2, and PCE was evaluated by monitoring the column effluent for organic carbon (OC), inorganic carbon (IC), Mg, pH, PCE, TCE, dichloroethene isomers, VC, ethene, ethane, methane, chloride (Cl−), and VFAs (acetate, butyrate, propionate, and lactate). 50 mL of column effluent was collected once per week in a wetted glass syringe by quickly pumping 50 mL of groundwater into the column influent with a syringe pump. Gases discharged in the column effluent were also collected in the glass syringe allowing us to monitor the volume of gas discharged. Liquid and gas samples were discharged from the glass syringe directly into autosampler vials for analysis. Use of this procedure allowed rapid sample collection with low potential for volatilization. Methane, ethene, ethane, and VC were analyzed with a with a Teledyne Tekmar 7000 headspace autosampler and Agilent Technologies 7890A GC with a flame ionization detector (FID) and J&W Scientific GS-Pro column. PCE, TCE, and DCE isomers (1,1-dichloroethene, trans-1,2-dichloroethene, and cis-1,2-dichloroethene) were measured with a Teledyne Tekmar AQUAtek 70 vial autosampler, Teledyne Tekmar Stratum purge and trap system with concentrator, and a Shimazu GC-2014 with FID (column: J&W Scientific DBVRX). Over 99% of the DCE isomers produced were 1,2-cis-
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RESULTS AND DISCUSSION Amendment Injection Phase. Table 1 presents a summary of the amount of PCE, Mg, and VS added to each column, discharged in the effluent, and retained by the porous media. PCE concentrations in the EVO and MH5 column effluent during the amendment injection phase are shown in Figure 1 (short-term transport) and Figure S2 (long-term
Figure 1. Effluent PCE concentrations in MH5 and EVO transport columns showing enhanced PCE transport following emulsion injection (0 pore volumes is start of emulsion injection). Emulsion was visually observed in column effluent from 0.8 to 2 pore volumes. 626
dx.doi.org/10.1021/es4042379 | Environ. Sci. Technol. 2014, 48, 624−631
Environmental Science & Technology
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increased alkalinity throughout the length of the transport column with somewhat higher levels in the first half of the column (Figure 2a). These results demonstrate that colloidal Mg(OH)2 can be transported significant distances in porous media and may be effective for in situ pH adjustment and/or buffering.
bioremediation). VS were effectively transported through all columns, with between 70 and 90% of injected VS discharged in the column effluent, indicating a large excess of EVO was added. EVO retention was higher in the column with more silt +clay, consistent with previous work.29 However, there was little difference in VS retention in columns treated with MH5. Flushing droplets of oil present in both the EVO and MH5 through DNAPL zone enhanced the transport of PCE through the 1.5 m long column. In the transport columns packed with porous media with little silt or clay, effluent PCE concentrations reached a maximum of 40−50 times the PCE aqueous solubility in water of 1 mM, with over half of the NAPL PCE transported through the column and discharged in the effluent. However in the bioremediation columns packed with porous media with a higher silt/clay content, effluent PCE concentrations were lower, reaching a maximum of 7−17 times the PCE aqueous solubility with a much smaller amounts discharged in the column effluent. In all columns, the maximum PCE concentrations occurred at roughly 1 PV after emulsion injection when emulsion was visually observed in the effluent. TCE and DCE were below detection throughout the amendment injection phase. PCE concentrations in the control column were below 0.01 mM indicating minimal flushing of PCE by groundwater, consistent with the low aqueous solubility of PCE (data not shown). The enhanced transport of PCE observed in the column experiments is hypothesized to occur through a process similar to surfactant-enhanced solubilization.32 As the ∼1 μm oil droplets are transported through the DNAPL zone, dissolved PCE partitions into the oil droplet (oil:water partition coefficient = 1238, ref 33), reducing the aqueous phase concentration and increasing dissolution of the DNAPL globules. However the rapid movement of oil droplets through in the DNAPL zone limited PCE dissolution, allowing a substantial portion of the DNAPL to remain near the column inlet, which is consistent with visual observation of the dyed DNAPL (Figure S1). Substantially more PCE was discharged from the short-term transport columns than the long-term bioremediation columns. This difference is likely due to differences in the spatial distribution of the DNAPL near the column inlet. In the low silt/clay content transport columns, the DNAPL was redistributed throughout the first 15 cm of the column resulting in a ∼2.5 min contact time (groundwater velocity = 6 cm/min). However in the higher silt/clay content bioremediation columns, visual observations showed the injected DNAPL was retained in a 3−4 cm long zone near the injection point, reducing the oil droplet-DNAPL contact time to
