Comparison of TCE Degradation in Groundwater vs Degradation in Groundwater in soils
ARS Technologies, Inc. (ARS) has expanded the Zero-Valent Iron (ZVI) reduction technology through the development of an insitu remediation process (Feroxsm) that involves the injection of a highly reactive iron powder directly into the contaminated soils and aquifers . Through this process, dissolved, adsorbed, and free-phase halogenated hydrocarbons can be effectively reduced to target levels.
Through the year 2000, ARS has successfully applied the Feroxsm Technology at seven sites in New Jersey, Alabama and California contaminated with chlorinated VOCs. Prior to the field applications, extensive laboratory treatability studies were performed on groundwater-only samples and groundwater-soil slurry (Dual-Phase) samples collected from the site.
Varying dosages of iron were tested for the each of the two matrices. The primary objective of these treatability studies was to determine the amount of zero-valent iron powder required to reduce TCE to a target concentration within a specific period of time. The treatability studies also evaluated the nature and quantity of the formation of any of the TCE dechlorination by-products, such as the DCE isomers, vinyl chloride and chloride. Changes in pH were also monitored.
Results extracted from a recent treatability studies performed by ARS are presented below:
Groundwater-Only Treatability Study
The groundwater-only treatability study was run in parallel with the groundwater-soil slurry (Dual-Phase) study. The initial concentration of TCE was found to be 30 mg/liter. VC and cis-1,2-DCE concentrations were below the instrument detection limit of 20 ug/liter. The initial pH of groundwater was 7.5.
0.3 g, 0.6g and 1.2 g of iron powder were placed into three sets of micro-batch reactors containing the groundwater being tested. These quantities corresponded to approximately 500:1, 1000:1, and 2000:1 iron-to-TCE mass ratios. One set of micro-batch reactors filled with groundwater without the iron powder served as the control.
Table 1 shows TCE concentrations in groundwater samples on specific days during the study. The rates of TCE reduction in reactors of different iron dosages are presented in Table 2
As expected, the higher iron-to-TCE ratios resulted in more rapid reduction of TCE within the duration of the experiment. TCE reductions ranging from 95.4 to 99.4% were observed within all batch reactors after 30 days of treatment. TCE concentrations were found to remain relatively stable in the control sample.
The formation of daughter products cis-1,2-DCE and VC were monitored throughout the duration of the kinetic studies. VC was not detected in any of the reactors during the 30 days. The results of cis-1,2-DCE analysis are shown in Table 3, corresponding to the different quantities of iron used. The highest concentration of cis-1,2-DCE was detected in the reactor containing the highest quantity of iron.
Changes in pH were monitored throughout the treatability study. The results, presented in Table 4, show a direct correlation between the increases in pH and the dosage of iron used.
Dual-Phase Groundwater-Soil Slurry Treatability Study
Treatability studies were performed on a Dual-Phase system consisting of two clayey soil types and groundwater. Baseline analysis showed the TCE concentration in the groundwater (30 mg/liter) was significantly higher than in the soils (0.12 mg/liter and 2.0 mg/liter). Therefore, it was interpreted that most of the TCE in the reactor was present in the dissolved phase.
The general procedure of the Dual-Phase treatability studies is presented in Figure 1. The results are presented in Tables 5 and 6. The results are presented on a total mass of TCE present in both the groundwater and soil (Table 5). Percent of TCE removal is shown in Table 6.
Rapid reductions in TCE mass were observed within the Dual-Phase micro-batch reactors. As expected, greater initial TCE reductions were observed in batch reactors containing larger iron dosages.
Experimental Procedure for Chlorinated VOC
(CVOC) Degradation in soil with Iron Powder (Fe0)
From these studies, it can be concluded that the rate of TCE reduction in the Dual-Phase system is significantly higher than TCE reduction in the groundwater-only system under the same experimental conditions (compare Tables 2 and 6).
VC and cis-1,2-DCE were not in Dual-Phase reactor systems during the treatability study. This is in contrast to the significant formation of cis-1,2-DCE in the groundwater-only system. One potential explanation of the observation may be that the clayey soils possess compounds which appear to change the reaction pathways whereby controlling the formation of cis-1,2-DCE.
The increase in pH within the Dual-Phase system was significantly lower than that of the groundwater-only system under similar experimental conditions (compare Tables 3 and 7). This effect can play an important role in field applications since a lower pH in the reaction products can minimize precipitate formation and coating of the iron, which in turn may reduce the reactivity of the iron powder.
Results of kinetic studies for the second ground water -soil slurry were identical to these shown in Tables 5-7.
Enhanced Contaminant Reduction with the Aid of Electron Mediators
Published research studies have demonstrated that the presence of certain naturally occuring electron mediators in soils can facilitate the reduction of contaminants by zero-valent iron. The results of these studies indicate that reductive transformation by zero-valent iron is a surface-mediated process. However, the requirement that the substrate of interest (ARS note: e.g. TCE, PCBs, etc.) contact the iron surface for electron transfer to take place can be circumvented by the addition of appropriate water-soluble electron mediators .
The objective of the research, conducted by the USEPA National Exposure Research Laboratory, was to 1) determine whether reduction by zero-valent iron is a surface-mediated process and, if so, 2) whether the transport of the contaminant to the iron surface is the rate-limiting step in the treatment scenarios.
The research team selected an aromatic azo compound, 4-aminoazobenzene (4-AAB), as the test compound or substrate. 4-AAB was chosen because of its susceptibility to reduction by the ZVI. In addition, it contains a reactive amino group that provides the means for attaching the molecule to a nonreactive support matrix in the form of epoxy-activated Sepharose beads to preclude 4-AAB’s transport to the surface of the iron.
To address the first objective above, reduction of the 4-AAB by ZVI was tested under two scenarios. Under one scenario, the 4-AAB was bound to the Sepharose beads and then exposed to ZVI in an aqueous solution. The other scenario entailed exposing unbound or free 4-AAB to the ZVI solution.
Results of the two scenarios were drastically different. There was negligible reduction of the bound 4-AAB over a one-week period, while the free 4-AAB was completely reduced by the iron within two hours. This appeared to indicate that reduction by ZVI is a surface-mediated process. That is, direct contact of the substrate with the iron surface is necessary for electron transfer to occur. This also implied that treatment schemes with ZVI may be limited to water-soluble chemicals.
However, in an attempt to answer the second objective above by investigating an approach to circumvent the need for direct contact of the substrate with the iron surface for reduction to occur, the reseach team discovered that in the presence of electron mediators, the need for direct contact between the pollutant and iron powder may not be necessary. It was demonstrated that the addition of pond sediments to the ZVI-water suspension containing bound 4-AAB initiated the the reduction process . The degradation rate was identical to that observed for the free 4-AAB.
The study went on to conclude that naturally occuring substances in the sediments and soils such as humic materials or soluble silicates are able to serve as electron mediators. Oxidation of iron powder causes the reduction of natural water-soluble products. These compounds are able to transfer electrons from iron powder to contaminants sorbed on a solid support and thereby causing their reduction. As depicted in the figure below, electron mediators act to shuttle electrons from zero-valent iron to the electron acceptor, in this case the bound 4-AAB.
The dissolution of alumosilicate minerals such as kaolinite and montmorillonite have also shown to enhance hexavalent chromium reduction with zero-valent iron, perhaps by providing a proton at the metal surface .
Based upon published results and ARS’s independent research, it has been identified that the presence of clayey soils within a treatment system can result in an increased efficiency of CVOC reduction by iron powder. It is hypothesized that water-soluble electron mediators present in the soil matrix appear to enhance the capture and transfer of electrons produced from the oxidation of the iron powder to the CVOC molecule for its reduction. This finding significantly demonstrate that remediation technologies based on zero-valent iron may be applied to the treatment of hydrophobic contaminants in sediments, soils and sludges.
Our research has shown that the treatment of TCE contaminated groundwater-only batch reactors resulted in formation of cis-1,2-DCE. In contrast, the TCE contaminated batch reactors containing both groundwater and soil (the Dual-Phase reactors) did not show any evidence of the formation of cis-1,2-DCE.
The treatment of TCE contaminated groundwater alone results in an increase in the reaction solution pH from 7.5 up to a range between 9.0 – 9.8, depending upon the iron-to-contaminant ratio used. The pH resulting from the treatment of the dual-phase batch reactors resulted in an increase in pH from 7.0 to a range of 7.6-8.5. It is hypothesized that the smaller pH increases are due to the buffering capacity of the soil material and greater reactivity of the zero-valent iron with the contaminants at the expense of side reactions that lead to pH increases.
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Powell R.M., Puls R.W. et al. Coupled Iron Corrosion and Chromate Reduction: Mechanism for Subsurface Remediation. Environ. Sci. Technol. 29, 1913-1922, 1995.