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Chapter 13
Iron Nanoparticles for In Situ Groundwater Remediation of Chlorinated Organic Solvents in Taiwan Yu-Ting Wei1, Shian-Chee Wu1, Chih-Ming Chou2, De-Huang Huang3 and Hsing-Lung Lien2 1
Graduate Institute of Environmental Engineering, National Taiwan University, Taipei, Taiwan, ROC 2 Department of Civil and Environmental Engineering, National University of Kaohsiung, Kaohsiung, Taiwan, ROC 3 Chinese Petroleum Corporation, Kaohsiung, Taiwan, ROC
A 200-m2 pilot-scale field study successfully demonstrated that palladized nanoscale zero-valent iron (NZVI) is capable of remediating groundwater contaminated with a variety of chlorinated organic solvents including vinyl chloride (VC), dichloroethanes and dichloroethylenes in southern Taiwan. Major contaminant is VC that has a concentration ranging from 10 to 5000 μg/L. The concentration distribution is depthdependent at the site where contaminant concentrations increased with depth. A total iron mass of about 20 kg on-site synthesized NZVI (Pd 0.05 wt%) suspended in 8,500 L water was injected via gravity into the sandy aquifer. Thirteen multilevel monitoring wells allowing to collect samples from three different depths (6, 12, 18 m) were installed. For a monitoring period of 3 months, a spatial and temporal decrease in VC concentrations was observed. The degradation efficiency was greater than 90% at both upper and middle layers but was about 20-85% at the bottom layer. Oxidation-reduction potential (ORP) measurements indicated a homogeneous reducing condition (ORP -450 ~ -280 mV) was achieved in the testing field. Analysis of total iron concentrations found iron was mainly trapped at the upper layer. NZVI-enhanced biodegradation was observed.
© 2009 American Chemical Society
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Introduction The use of nanoscale zero-valent iron (NZVI) for the remediation of groundwater impacted by a variety of contaminants including chlorinated hydrocarbons and heavy metals has received much research attention over the past decade (1). The NZVI technology has been demonstrated to be suitable for in-situ treatment of contaminant “hot-spots” given its high reactivity and flexible deployment in the field (2-6). Taiwan has long been known as an industrial island. Major economic activities rely on advanced electronic devices and petroleum manufactures. In this study, a groundwater contaminated site was selected from a vinyl chloride monomer (VCM) manufacturing plant. In this paper, we present the first field test for groundwater remediation by NZVI technology in Taiwan. The study focuses on a pilot-scale field demonstration of injecting the palladium-catalyzed and surfactant-dispersed NZVI to control the contaminated plume. The VCM manufacturing plant is located in southern Taiwan. High concentrations of VC (4562 μg/L), 1,1-dichloroethylene (430 μg/L), cis-1,2dichloroethylene (1151 μg/L) and trichloroethylene (682 μg/L) in groundwater were detected from the monitoring well nearby the plant (Figure 1). The NZVI pilot test was conducted in a small area (10 meters by 20 meters) south of the VCM plant in downstream groundwater direction. The unconfined aquifer, composed of medium to coarse sand and few silt, lies approximately 4 to 18 meters below ground surface (m bgs).
Materials and Methods Test Area Design Three injection wells and thirteen nested multi-level monitoring wells were installed on a 200-m2 pilot. In the downstream direction of each injection well, four additional multi-level monitoring wells were installed. The positions of the four nested monitoring wells were approximately one, two, three, and five meters from the injection well. The injection wells were all eighteen-meter deep with fifteen-meter screens. In addition, every nested monitoring well included three separate wells which were approximately six, twelve and eighteen-meter deep with three-meter screens (Figure 2). There was one nested monitoring well located upstream for the purpose of background monitoring. Initially, about 1,000 liters of on-site synthesized NZVI were injected into well IW-3 by gravity. Another 7,500 liters of NZVI suspension was injected into IW-1 via gravity after ten days. The total iron mass was about 20 kilograms companioned with 100 g of palladium catalyst (0.05% of total iron mass).
In Environmental Applications of Nanoscale and Microscale Reactive Metal Particles; Geiger, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
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Figure 2. Injection and monitoring locations within the test area.
In Environmental Applications of Nanoscale and Microscale Reactive Metal Particles; Geiger, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
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Production of On-Site Synthesized NZVI A semi-continuous reactor system was used to produce NZVI on site (Figure 3). The reactor system is designed to load on a trailer for convenient mobility. The on-site synthesized NZVI was prepared by slowly adding ferrous sulfate solution into sodium borohydride (>98.5%, Beckman Coulter, Inc.) solution containing nonionic surfactant (Taiwan NJC Corp., industrial-grade) at the concentration of 5,000 mg/L in a 1,000-liter tank. After the reaction was completed, palladium acetate was mixed with the NZVI suspension. The NZVI mixture was then pumped into a storage tank for the injection later. Because of the high reactivity, dry iron nanoparticles tend to explode in contact with air. Nevertheless, the on-site synthesized NZVI preserved in aqueous solutions can be safely handled without the danger of explosion. TEM analysis indicated that the on-site synthesized NZVI has the particle size in the range of 80-120 nm with a specific surface area of 29.3 m2/g.
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Figure 3. A semi-continuous reactor system for on-site synthesis of NZVI. Analytic Methods Volatile organic compounds were measured by GC/MS (Angilent 6890/5973 with a DB-624 capillary column) using a purge and trap sampling equipment (OI Analytical. Model 4560). Methane, ethane and ethene were measured from the headspace of serum vials containing water samples after equilibration. The headspace was analyzed for the target gases by GC/FID (HP 5890 with GS-GASPRO column). Dissolved oxygen (DO), oxidation-reduction potential (ORP) and pH were measured by a portable equipment (YSI 650 MDS-6600 V2-4 Sonde, YSI Inc.). Total iron was measured by atomic absorption spectrophotometer (Perkin Elmer Aanalyst 800) after acid digestion.
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Results and Discussion
Injectability of NZVI is evaluated by the total iron concentration in the aquifer. The background concentration of total iron in the testing site was about 10 mg/L. As shown in Figure 4a, the total iron concentration measured at the upper layer increased significantly at the whole testing site after NZVI was injected. The iron concentration was in a range of 40-370 mg/L. In general, the iron concentration decreased with increasing distance downstream from the injection well. Figure 4b shows the iron concentration at the testing site after NZVI was injected for 60 days. A dramatic decrease in iron concentrations with time was observed. This suggests that iron either was consumed through the corrosion or transported through the groundwater flow. In terms of the depth, it was found that the iron concentration decreased in the order: upper layer > middle layer > lower layer. It is believed that with the gravity injection, because of the aquifer heterogeneity, much of the NZVI first seeped through channels in the unsaturated zone, causing NZVI to accumulate more in the upper layer as compared to those in the lower layer.
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Figure 4b Iron concentration at the testing site after NZVI was injected for 60 days. Mobility As it has been observed in field tests of the NZVI technology, the pH and Eh profiles at given monitoring locations over time can be used as a convenient indicator for the NZVI reactivity and to track the migration path of the nanoparticles (2-4). In this study, our data of the iron concentration shown in Figure 4a suggests that the NZVI is mobile. Furthermore, as illustrated in Figure 5, the ORP decreased from about -100 to -400 mV at the central area of the testing site during the 30 days. The Eh profiles shown in Figure 5 suggest the NZVI gradually migrated downstream. This is consistent with the previous studies indicating the ORP can serve as a convenient indicator for the mobility of NZVI (2-4). Overall, the data from this study suggest that NZVI is an effective means of achieving highly reducing conditions in the subsurface environment.
In Environmental Applications of Nanoscale and Microscale Reactive Metal Particles; Geiger, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
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(b) Figure 5. ORP changes at the upper layer within 30 days. Effectiveness The concentrations of VC monitored in various times for the upper, middle and lower layers are totally summarized in Figure 6a. It is clear that the VC concentration steadily decreased as the test date progressed, with few
In Environmental Applications of Nanoscale and Microscale Reactive Metal Particles; Geiger, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
240 exceptions. Furthermore, the decrease in VC concentrations corresponded to a decrease of ORP (Figure 6b). This is in agreement with the previous studies suggesting that ORP can act as an indicator for the NZVI reactivity (4). The average removal efficiency determined at most of the monitored wells was 5099%. The lowest removal efficiency was about 20%, which was found at the bottom layer of the monitoring well #5M1. 1.0
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In Environmental Applications of Nanoscale and Microscale Reactive Metal Particles; Geiger, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
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Microbial Interaction Figure 7 shows the concentrations of hydrocarbons and total organic carbon (TOC) measured along the downstream direction in the bottom layer. An unexpected high concentration of ethylene was determined in the testing site. The TOC concentration is used to reflect the influence of added biodegrable surfactants. An unexpected high concentration of ethene was determined in the testing site. Currently, the cause is still unclear. Nevertheless, methane was observed at 25 days after the injection.The gradual increase of methane concentration suggests methanogenesis took place at the testing site. The methanogenic conditions require the ORP value lower than -240 mV, which can be established in this specific testing site. A small amount of ethane was also found. The formation of ethane is likely due to the reduction of VC to ethane by NZVI, and the other being the presence of 1,1-dichloroethane that would reduce to chloroethane and ultimately to ethane. A gradual increase in TOC concentration followed by a subsequent leveling off provides further evidence to support enhanced biodegradation occurring in the testing site because the added biodegrable surfactant may serve as the carbon source to stimulate microbial growth (7-8).
Conclusion This paper presents a successful pilot-scale field study for applying on-site synthesized nanoscale zero-valent iron to remediate groundwater contaminated with chlorinated organic compounds. A total amount of 20 kg palladized NZVI was injected into the groundwater via gravity at a 10 m × 20 m testing site. The VC degradation efficiency determined at most of the monitoring wells was 5099%. High concentrations (up to 20 mg/L) of methane and ethylene were detected. Though the cause of which is still unclear at the current stage, it is likely that enhanced bioremediation was involved at the testing site because of its strongly reducing conditions. An increase in VC degradation efficiency corresponded to a decrease of ORP values, which is in agreement with the previous studies suggesting that ORP can serve as a proper indicator for the NZVI reactivity.
In Environmental Applications of Nanoscale and Microscale Reactive Metal Particles; Geiger, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
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Fig. 7 TOC and molar of methane, ethane and ethylene found in the bottom layers.
Acknowledgements The authors would like to thank the National Science Council (NSC), Taiwan, R.O.C. for the financial support under Grant no. NSC 95-2221-E-002162-MY2 and NSC95-2221-E-390-014-MY2. We also like to thank Chinese Petroleum Corporation for its on-site assistance.
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