In Situ Dechlorination in Soil and Groundwater Using Stabilized Zero

Downloaded by PURDUE UNIVERSITY on May 30, 2013 | http://pubs.acs.org Publication Date (Web): March 18, 2013 | doi: 10.1021/bk-2013-1123.ch006

Chapter 6

In Situ Dechlorination in Soil and Groundwater Using Stabilized Zero-Valent Iron Nanoparticles: Some Field Experience on Effectiveness and Limitations Man Zhang and Dongye Zhao* Environmental Engineering Program, Department of Civil Engineering, 238 Harbert Engineering Center, Auburn University, Auburn, Alabama 36849, United States *Phone: (334) 844 6277. Fax: (334) 844 6290. E-mail: [email protected]

To facilitate in situ degradation of chlorinated solvents in the subsurface, Auburn university developed and patented (US7,887,880 B2) a new class of stabilized ZVI nanoparticles using carboxymethyl cellulose (CMC) as a stabilizer. Laboratory experimental results revealed some unique attributes of CMC-stabilized ZVI nanoparticles: 1) they are deliverable in soil; 2) they offer much greater dechlorination reactivity than non-stabilized counterparts; and 3) the particles can effectively degrade soil-sorbed contaminants such as trichloroethylene (TCE). These novel features allow the nanoparticles to be delivered into contaminated source zones and facilitate in situ dechlorination in soil and groundwater. This chapter reports preliminary results from two field tests of the in situ remediation technology. These field tests confirmed the soil deliverability and reactivity of the nanoparticles for in situ degradation of chlorinated solvents. In a sandy aquifer at Alabama, at an injection pressure of 100 times greater reactivity than commercially available iron powders. Reduction of chlorinated compounds by Fe-Pd bimetallic particles is a hydrodechlorination process, where Fe acts as the electron source and Pd (typically at 0.1 wt% of Fe) as a catalyst. The presence of trace amounts of Pd greatly facilitates generation and sorption of the reactive atomic hydrogen, which is critical in the dechlorination reaction. In addition, the galvanic couples formed between iron and paaladium are critical for the generation of the reactive atomic hydrogen. Under ambient conditions, TCE is reduced by ZVI nanoparticles to environmentally innocuous compounds, such as acetylene, ethylene and ethane. Some chlorinated intermediates (cis-1,2,-dichloroethylene (DCE), 1,1-DCE, vinyl chloride (VC)) may be produced but usually account for less than 1% of the TCE degraded (3). Compared to conventional granular iron, which has been commonly applied in permeable reactive barriers (PRBs), truly nanoscale iron particles offer the advantage that the nanoparticles can be injected into the contaminated source zones or other down-gradient contaminant plumes, thereby facilitating in situ degradation of chlorinated solvents (4, 5). Compared to conventional remediation technologies such as PRBs and pump-and-treat, such a proactive in situ approach by means of the nanoparticles is expected to greatly shorten the remediation timeframe and reduce the overall remediation cost. In addition, the in situ remediation technology can often reach contaminant zones that are located deep in the ground or underneath built infrastructures, which are not accessible by conventional technologies. 80 In Novel Solutions to Water Pollution; Ahuja, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


ZVI nanoparticles tend to aggregate into micro- or milli-meter scale particles due to strong magnetic and electrostatic interactions. The aggregated particles are no longer transportable through the porous media, even through sand (6, 7), and thus, cannot be delivered or dispersed in the target subsurface contaminated zones. In addition, particle agglomeration also reduces the specific surface area of iron particles, thereby reducing the available active surface sites and the chemical reactivity. To prevent nanoparticle aggregation and facilitate particles deliverability, various particle stabilization techniques have been developed. For examples, nanoparticles are stabilized using emulsified oil (8), carrier supports (9, 10), polymers (11, 12) or surfactants (13, 14). To facilitate soil deliverability of ZVI nanoparticles, Auburn University researchers developed a new class of highly dispersible palladized iron (Fe-Pd) nanoparticles using sodium carboxymethyl cellulose (CMC) as a stabilizer (7). Unlike traditional methods, the CMC stabilizer was added during the particle synthesis, which allows better control of nanoparticle nucleation and growth by manipulating the type and concentration of the stabilizer. In addition to its macromolecular structure, the adsorption of negatively charged CMC molecules (pKa = 4.3) onto the ZVI nanoparticles results in an electrical double layer on the surface of the nanoparticles. Consequently, the nanoparticles are stabilized through the concurrent coulombic repulsion between the CMC-capped particles and steric hindrance due to the coated CMC macromolecules. Figure 1 shows the TEM image of CMC-stabiblized Fe-Pd nanoparticles with an average particle diameter of 4.3±1.8 nm.

Figure 1. TEM image of freshly prepared CMC-stabilized Fe-Pd nanoparticles. Fe = 0.1 g/L; Pd/Fe = 0.1 wt%; CMC (sodium form)= 0.2 % (7). Reproduced with permission from Refererence (7). Copyright 2007, American Chemical Society. Laboratory studies have revealed that CMC-stabilized ZVI nanoparticles offer much improved deliverability through various porous media ranging from sand to field soil (15). Based on column breakthrough curves and filtration modeling, the particle travel distance in soil was found strongly dependent on the groundwater flow rate (or injection pressure). For example, CMC-stabilized ZVI nanoparticles could travel only 16 cm at a groundwater pore velocity of 0.1 m/day in a sandy soil. However, the travel distance can be extended to 146 m when the pore water velocity is elevated to 61 m/day under the otherwise identical 81 In Novel Solutions to Water Pollution; Ahuja, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


conditions (16). Therefore, the effective area of the ZVI nanoparticles can be controlled by regulating the injection pressure. In addition, it is noteworthy that once delivered, the nanoparticles will remained in a very confined domain under typical groundwater conditions. The stabilized Fe-Pd nanoparticles also exhibited superior dechlorination reactivity. Compared to the non-stabilized counterparts, the stabilized nanoparticles exhibited 17 times higher reactivity for reductive dechlorination of TCE. With 0.2 g/L of CMC-stabilized Fe-Pd nanoparticles (Pd = 0.1 wt.% of Fe), 50 mg/L of TCE was completely degraded to chloride in less than one hour without detection of byproducts such as DCE and VC (7). While numerous laboratory studies have been reported, field data have been rather limited on the deliverability and dechlorination effectiveness of CMCstabilized Fe-Pd nanoparticles. In addition to the complex hydrogeochemical conditions in the field, the spatial and temporal variation of contamination (14) and soil heterogeneity (17) are likely to affect the effectiveness of the dechlorination. In addition, soil sorption and dissolved organic matter also affect the efficacy. This study summarizes results from two field demonstration experiments that were designed to investigate the deliverability and dechlorination reactivity of the CMC-stabilized Fe-Pd nanoparticles under field conditions (18, 19). In addition, this study reports laboratory investigation on the effectiveness of CMC-stabilized iron nanoparticles for soil-sorbed TCE, and on the effect of dissolved organic matter on the reactivity of the nanoparticles (20).

Field Test at a California Site A series of single-well, push-pull tests were conducted at an aerospace facility near San Francisco Bay in California. The goal was to test the soil deliverability of CMC-stabilized ZVI nanoparticles into the subsurface and the dechlorination reactivity towards chlorinated contaminants such as TCE and Tetrachloroethylene (PCE). The push-pull test was carried out by first injecting a suspension of CMC-stabilized ZVI nanoparticle into an injection well; and then, an extraction was performed from the same well and recovery of the injected nanoparticles was determined. A conservative tracer (KBr) was mixed in the nanoparticle suspension to provide insight into the hydrodynamic characteristics of the fluid in the groundwater aquifer. Details on this field experiments have been reported by Bennett et al. (18). The test area mainly consists of silts and clays, interlined with coarse-grained channel deposits. The coarse-grained sediments were the primary permeable and water-bearing zones in the subsurface. Three water-bearing zones were detected at depths from 3.8 ~4.8 m, 6.4 ~6.5 m and 9.2 ~9.8 m below ground surface (bgs), where the uppermost water-bearing zones was confined. Soil samples collected from 4.1 m bgs and 9.5 m bgs were characterized as poorly degraded sands and the corresponding hydraulic conductivity were estimated to be approximately 105 and 28 m/day, respectively. The test site was primarily contaminated by a dissolved TCE/PCE plume originating from the upgradient of the groundwater flow. Some typical biological 82 In Novel Solutions to Water Pollution; Ahuja, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


dechlorination byproducts such as cis-DCE and vinyl chloride were detected in the groundwater, suggesting active biological dechlorination is operative at the site. Soil samples collected from the depth of 4.7 m and 9.5 m contained TCE at 0.29 and 2.1 mg/kg, respectively; however, there was no chlorinated ethenes detected in the soil samples from 2.3 m and 6.4 m bgs (detection limit = 0.25 mg/kg). A significant amount of chlorinated ethenes were detected from 9.3 m to 10 m bgs. A multi-level injection well I-1 was constructed for both nanoparticle injection and groundwater extraction. The well was screened at three depth intervals, which corresponded to the three water-bearing zones in the subsurface. Three push-pull tests were performed with CMC-stabilized iron nanoparticles at three different screened intervals. Approximately 117 L of 1.0 g/L (CMC = 0.8 wt%), 110 L of 0.2g/L (CMC = 0.4 wt%) of ZVI nanoparticles and 110 L of 0.35 g/L Fe-Pd bimetallic nanoparticles (CMC = 0.8 wt%, Pd = 0.1 wt.% of Fe) were prepared and injected in three separate injection events on April 25, 2006, April 26, 2006 and May 1, 2006, respectively. The CMC-stabilized ZVI nanoparticles were synthesized on site before each injection through the aqueous phase reduction of ferrous iron by sodium borohydride in the presence of CMC. The freshly-prepared nanoparticles were then injected into the water-bearing stratum of the aquifer along with a conservative tracer (bromide) and then extracted with the groundwater from the same well. The groundwater samples were collected during the extraction stage to assess the nanoparticle recovery and dechlorination reactivity. The mass recovery of the injected iron was compared with that of the injected tracer to evaluate the transportability of the nanoparticles within the aquifer. Figure 2 shows the concentration histories of total iron and bromide in the groundwater collected from I-1 during the extraction ("pull") stage during the third push-pull test (18). The area under curves were used to estimate the total mass of iron/bromide recovered. There was no lag time between the injection and extraction test, i.e. the nanoparticles/groundwater mixture drawn towards I-1 immediately after injection. Mass balance calculations indicated that approximately 31% of total iron (113 g) was recovered during the extraction compared to 76% for bromide, suggesting that the stabilized nanoparticles are partially transportable in the aquifer. A lower recovery of iron nanoparticles was observed when there was a residence time between the end of injection and the beginning of extraction, as was the case in the first and second push-pull tests. For example, in the first push-pull test, when the extraction was initiated about 13 h after the end of injection, only 2.6 % of the total iron was recovered against 61% of bromide recovery. The prolonged residence time of the nanoparticles in the aquifer may facilitate the adsorption of iron in the soil phase (20) and render the nanoparticles less mobile over time. Upon injection of three batches of ZVI nanoparticles, the ORP (oxidationreduction potential) of groundwater rapidly decreased from the baseline value of -173 ~-234 mV to -400 ~-500 mV (18), indicating formation of a strong reducing environment at the site. The decrease of ORP is attributed to the strong reducing power of Fe-Pd nanoparticles and consumption of pre-existing oxidants such as dissolved oxygen. 83 In Novel Solutions to Water Pollution; Ahuja, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 2. Corrected iron concentration and bromide concentration versus volume extracted (18). Reproduced with permission from Refererence (18). Copyright 2010, Elsevier.

Concentrations of dissolved chlorinated contaminants in groundwater were monitored during the extraction phase of the push-pull tests. In a control test when tap water, instead of ZVI nanoparticles, was pumped to the underneath aquifer during the injection phase, the total chlorinated contaminant concentration at the end of extraction phase (Vext/Vinj >1.5) was approximately 35 % lower compared to the baseline condition (18), although more than 90% of the injection bromide was successfully recovered. The lower rebound concentration of chlorinated contaminants suggested that a direct comparison of the pre- and post-injection chlorinated contaminant concentrations could not provide reliable evidence for effective degradation of chlorinated contaminants in this study. Consequently, the accumulation of daughter products associated with reductive dechlorination using ZVI nanoparticles, such as ethane and ethene, was used as an indicator for in-situ abiotic degradation. Ethane, which is the primary product of abiotic dechlorination using Fe-Pd nanoparticles (21), was reported under the detection limit (5 µg/L) in all of baseline samples (18). As shown in Figure 3, immediately after the injection of the stabilized Fe-Pd nanoparticles in the third push-pull test, ethane increased significantly from being non-detected to 65 µg/L in less than 2 h, suggesting the occurrenc of an effective and complete degradation of the chlorinated contaminants in the vicinity of injection well I-1. The high concentration of ethane was observed in the early extraction stage (Vext/Vinj

In Situ Dechlorination in Soil and Groundwater Using Stabilized Zero

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