Environmental Life Cycle Assessment of Permeable Reactive Barriers

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Environmental Life Cycle Assessment of Permeable Reactive Barriers: Effects of Construction Methods, Reactive Materials and Groundwater Constituents Mark S. H. Mak and Irene M. C. Lo* Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China;

bS Supporting Information ABSTRACT: The effects of the construction methods, materials of reactive media and groundwater constituents on the environmental impacts of a permeable reactive barrier (PRB) were evaluated using life cycle assessment (LCA). The PRB is assumed to be installed at a simulated site contaminated by either Cr(VI) alone or Cr(VI) and As(V). Results show that the trench-based construction method can reduce the environmental impacts of the remediation remarkably compared to the caisson-based method due to less construction material consumption by the funnel. Compared to using the zerovalent iron (Fe0) and quartz sand mixture, the use of the Fe0 and iron oxide-coated sand (IOCS) mixture can reduce the environmental impacts. In the presence of natural organic matter (NOM) in groundwater, the environmental impacts generated by the reactive media were significantly increased because of the higher usage of Fe0. The environmental impacts are lower by using the Fe0 and IOCS mixture in the groundwater with NOM, compared with using the Fe0 and quartz sand mixture. Since IOCS can enhance the removal efficiency of Cr(VI) and As(V), the usage of the Fe0 can be reduced, which in turn reduces the impacts induced by the reactive media.

1. INTRODUCTION Green remediation has been proposed by U.S. Environmental Protection Agency (EPA) in recent years for avoiding and reducing the associated environmental impacts during the remediation processes.1 Consumption of energy, water, and material resources as well as waste generation and pollutant emissions can be accompanied by remediation activities. Therefore, assessments have been proposed in order to evaluate and minimize the environmental impacts created by a remediation process. U.S. EPA has suggested several aspects for consideration during the assessment.2 These aspects include pollutant emissions and greenhouse gas emissions, and impacts to water quality and ecosystems.2 To assess the environmental impacts of the remediation activities, life cycle assessment (LCA) can be one of the suitable tools. LCA method has been widely applied for assessing the environmental impacts such as global warming, acidification, carcinogenics (human toxicity), eutrophication, ozone depletion, and smog formation generated from the soil and groundwater remediation sites. LCA has been used for comparing different technologies for soil remediation such as bioremediation, soil washing, and soil excavation.3 7 For groundwater remediation, studies have been conducted for comparing a pump-and-treat (PT) system and a permeable reactive barrier (PRB) system.8,9 Higgins and Olson9 have assessed the environmental impacts from the TCE-contaminated site in Dover Air Force Base in Dover, DE, and found that a PRB system can generate less r 2011 American Chemical Society

environmental impacts than a PT system for long-term operation. The PRB system is a passive treatment technology, which requires a lower amount of energy during the cleanup operation, comparing to the energy-intensive PT system. However, the environmental impacts induced by the construction of the PRB system are higher than that of the PT system as the PRB system involves excavation of soil and emplacement of reactive media.9 Besides, Bayer and Finkel8 have found that changing the use of the funnel construction material from steel to clay can directly reduce the environmental impacts from the overall PRB system. The construction process has been found to be a major factor of the environmental impacts of the PRB system.9 PRBs are commonly installed in a funnel-and-gate configuration,10 which can be generally constructed by two major methods, namely caisson-based method and trench-based method. The PRB in Dover Air Force Base is one of the examples which adopted the caisson-based method,9 whereas the one in Vapokon Site, Denmark is one of the PRBs adopted the trench-based method.11,12 In comparison, the caisson-based method can allow the replacement of reactive media, while the trench-based method cannot. The trench-based method requires the excavation of a larger amount of soil for emplacing all the reactive media at the Received: June 14, 2011 Accepted: October 28, 2011 Revised: October 25, 2011 Published: October 28, 2011 10148

dx.doi.org/10.1021/es202016d | Environ. Sci. Technol. 2011, 45, 10148–10154

Environmental Science & Technology

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Table 1. Conditions of Each Scenario concentration of groundwater contaminants (mg/L)

construction

reactive

scenario

method

media

C1

caisson

Fe0 and quartz sand a

C2 C3 C4 C5 C6

caisson caisson caisson caisson caisson

Cr(VI)

0

a

0

a

0

a

Fe and quartz sand Fe and quartz sand Fe and quartz sand 0b

IOCS and Fe

20 20 20 20 20

0b

IOCS and Fe

20

(mg/L as DOC)

0 0 10

(mg g

1

reactive media) n.a. e

8

1.20

c

n.a. e

1.05

d

0.53 d

0.88

d

0.32 d

1.33

d

0.60 d

1.24

d

0.57 d

c

0

10

reactive media) 1.44 c

8

10

1

0 0

10

(mg g

8

T1 T2

trench trench

Fe and quartz sand Fe0 and quartz sand a

20 20

0 0

0 8

1.44 1.20 c

n.a. e n.a. e

T3

trench

Fe0 and quartz sand a

20

10

0

1.05 d

0.53 d

8

0.88

d

0.32 d

1.33

d

0.60 d

1.24

d

0.57 d

T4 T5 T6

trench trench trench

0

0

Fe and quartz sand 0b

IOCS and Fe

0b

IOCS and Fe

a

As(V)

concentration of NOM removal capacity of Cr(VI) removal capacity of As(V)

a

20 20 20

10 10

0

10

8

Notes: a Assume Fe0 is mixed with quartz sand in 1:1 (w:w). b IOCS and Fe0 are assumed to be mixed in 1:1 (w:w). c The removal capacities were based on the results from Liu and Lo.23 The initial solution pH was 7 and the groundwater flow rate was 400 m/yr. d The removal capacities were based on the results from Mak et al.30 The initial solution pH was 7 and the groundwater flow rate was 100 m/yr. e As(V) is not included in this scenario. initial construction stage.11 As a result, the construction energy may be significantly different with the use of the construction methods. Furthermore, the sizes of the funnels and gates may vary between the construction methods because the regular size of the caisson restricts the size of the gate, whereas the size of the gate using the trench-based method is more flexible.11 This may result in a different consumption of the materials for the funnels and gates, leading to different environmental impacts for the PRBs. Apart from the construction, the material consumption was also found to be another major factor of the environmental impacts of the PRB system.9 Zero-valent iron (Fe0) has been conventionally used as the material of the reactive media of PRBs, which can effectively remove inorganic pollutants such as Cr(VI), As(V) and U(VI),13 15 and organic pollutants such as TCE and DCE.16 Nevertheless, the reactivity of the Fe0 has been shown to be reduced gradually due to the passivation of the iron corrosion products.17 Additionally, some groundwater constituents can affect the reactivity of the Fe0, such as alkalinity, hardness, nitrate and natural organic matter (NOM).18 22 NOM has been reported to significantly inhibit the reactivity of the Fe0 due to the deposition of NOM aggregates on the Fe0 surfaces.23 Some enhancement materials have thus been suggested for improving the performance of the Fe0 PRB systems, in which iron oxide-coated sand (IOCS) has been found to enhance the removal efficiency of Cr(VI) and As(V) by using IOCS with Fe0 together.24 The use of a different material for the reactive media in PRB can alter the environmental impacts of the whole system due to the differences in the production processes and raw materials. On the other hand, the use of a different material for the reactive media can also affect the removal performance of the whole system, leading to a different thickness of the PRBs. Therefore, different environmental impacts in the construction process of the PRBs could be caused. Nevertheless, the previous studies only focused on (i) the comparison of the environmental impacts generated from different remediation technologies such

as the comparison of a PRB system with a PT system,9 and (ii) the effects of changing the funnel material on the environmental impacts generated from PRBs.8 There is a lack of studies addressing the differences in the environmental impacts due to the applications of various construction methods, different materials of reactive media and the presence of groundwater constituents, in which these factors can complicate the investigation of the environmental impacts from the PRBs as each of these factors could have different effects on each of the PRB components. The objective of this study was to assess the associated environmental impacts of a PRB by investigating the twelve scenarios with different construction methods (caisson-based/ trench-based), reactive materials (Fe0 and sand mixture/Fe0 and IOCS mixture), and groundwater constituents (without/with NOM), using the approach of environmental LCA. The PRB system was assumed to be installed in a simulated site for treating 20 000 m3 of groundwater contaminated by either Cr(VI) alone or Cr(VI) and As(V) which are commonly found heavy metal pollutants in groundwater.25,26 The major environmental impacts can be identified and improvements can be suggested for the future design of the PRBs, based on the results of the LCA.

2. MATERIALS AND METHODS 2.1. Simulated Contaminated Site. A PRB was designed for treating a simulated site with a size of a 30 m wide 185 m long 9 m deep groundwater contaminant plume. The contaminant plume was equivalent to a volume of 20 000 m3 groundwater. The conditions for the groundwater constituents and contaminants in different scenarios are shown in Table 1. The contaminants include either Cr(VI) alone or Cr(VI) and As(V). To study the effects of NOM, the groundwater consists of 8 mg/L NOM as dissolved organic carbon (DOC), which is within the common range of the concentration of NOM in groundwater.27 The geological conditions of this simulated site were assumed to be 10149

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similar to the site in Dover Air Force Base.28 The upper depth of the aquitard is 11 m and the aquifer thickness is 9 m.28 The porosity of the aquifer is 0.4. 28 The PRB is constructed in a funnel-and-gate configuration with a width of 41.4 m which is chosen in according to the PRB in Dover Air Force Base. Two types of reactive materials, Fe0 and quartz sand mixture, and Fe0 and IOCS mixture, were used as the reactive media for different scenarios, respectively (Table 1). The Fe0 used in this study was made of gray cast iron, whereas the IOCS is a waste generated from a fluidized and air-aerated bed reactor, which had been used to remove iron ions that are produced in the process of NO 3 reduction by Fe 0 . 29

The amounts of Fe0, quartz sand, and IOCS required in each scenario were determined by using the removal capacity of Cr(VI) alone, or Cr(VI) and As(V) by the Fe0 and quartz sand mixture, or the Fe0 and IOCS mixture, which have been investigated by Liu and Lo, and Mak et al. (Table 1).23,30 The thickness of the reactive media (as shown by “l” in Supporting Information (SI) Figure S1) in the PRB for different scenarios was varied with the removal capacities, while the width and depth of the reactive media were fixed in different scenarios. Two construction methods, caission-based and trench-based, are employed in different scenarios, respectively (Table 1). The PRB using the caisson-based construction method was designed with reference to the PRB installed in the Dover Air Force Base.9,28 A 36.6 m length of funnel and four 2.4 m diameter cylindrical gates is installed (Figure 1a). The funnel is composed of steel sheet pilings sealed together with cementitious grout. The gates are constructed by excavating the soil within a 2.4 m diameter steel caisson, emplacing a 1.2 m wide column of reactive media, and backfilling the pretreatment and post-treatment zones with quartz sand (SI Figure S1a). The gates are removed for replacement of the reactive media after the removal capacity of the reactive media exhausts. Replacement of reactive media is assumed to occur three times. The PRB using trench-based construction method was designed with reference to the PRB installed in Vapokon Site, Denmark.12 A funnel with a length of 31.4 m and a gate with a width of 10 m and a depth of 11 m are installed (Figure 1b). The thickness of the gate of each scenario is varied with the amount of the materials used for the reactive media. Steel sheet pilings are installed for the funnel and gate of the PRB. The steel sheet

Figure 1. PRB design settings based on (a) Caisson method and (b) trench method.

Figure 2. System boundary of a PRB with reference to Higgins and Olson 9. 10150



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Figure 3. Impact categories including (a) global warming, (b) acidification, (c) carcinogenics, (d) eutrophication, (e) ozone depletion, and (f) smog formation generated by different components of the PRBs.

pilings for the funnel are sealed together with cementitious grout. After installing the sheet pilings, the soil inside the gate is excavated and the reactive media are then emplaced. Two sand backfills with a thickness of 1 m are installed on both upgradient and downgradient of the reactive media as pretreatment and post-treatment zones, respectively (SI Figure S1b). The machineries used to install the PRBs and the associated specifications for energy consumption are addressed in SI Table S1, whereas the unit consumption of the materials and energy for the PRB construction are summarized in SI Table S2.The treatment goal was set to follow the drinking water standard established by World Health Organization (WHO). The WHO drinking water standards for Cr and As are 50 and 10 μg/L, respectively.31 2.2. Goal and Scope of LCA. The goal of the LCA was to simulate a PRB system with different scenarios in order to evaluate the environmental impacts due to the effects of the construction methods, the use of different materials of reactive media and the groundwater constituents. The system boundaries consisted of material production, transportation and construction, as illustrated in Figure 2. The functional unit of this study was the successful treatment of 20 000 m3 of the contaminated groundwater to the treatment goal, while the temporal boundary is 30 years.

2.3. Life Cycle Impact Assessment Method. The LCA was conducted using SimaPro 7.1 LCA software,32 and its built-in inventory databases and impact assessment methods.3,9 The major assumptions of this study are addressed in SI Table S3, while the inventories of each scenario are summarized in SI Tables S4 and S5. The impact assessment was conducted with the characterization factors of the Tool for the Reduction and Assessment of Chemical and other environmental Impacts (TRACI) method.33 The environmental impact categories considered in this study include global warming, acidification, carcinogenics, eutrophication, ozone depletion, and smog formation.33 These impact categories are suggested by U.S. EPA for consideration in the environmental impact assessment of remediation technologies,2 and are widely used for the LCA conducted on the environmental impacts generated from remediation technologies.4

3. RESULTS AND DISCUSSION 3.1. Effects of Construction Methods. The environmental impacts of a PRB using the trench-based construction method were noticeably lower than those of using caisson-based construction method (C1 C6 compared with T1 T6; Figure 3). The reduction in the impacts was mainly attributed to the 10151



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Figure 4. Materials and energy consumption analysis of the PRB systems in scenarios (a) C1, (b) C2, (c) C3, (d) C4, (e) C5, and (f) C6. Impact categories include global warming (GW), acidification (Ac), carcinogenics (Ca); eutrophication (Eu), ozone depletion (OD), and smog formation (SF).

reduction of the impacts induced by the funnel component of the PRB (Figure 3). The component of the funnel consists of the material production, transportation, and construction of the funnel. In this component, the impacts generated by the funnel materials were significantly reduced in the scenarios of using the trench-based construction method (as indicated by Material Funnel in Figures 4 and 5). For example, the fraction of global warming impact generated by the funnel materials was reduced from about 42% by using the caisson-based construction method to about 16% by using the trench-based construction method. The reduction of the impacts could be the result of the reduction of the total length of the funnel from 36.6 m with the caissonbased construction method to 31.4 m by using the trench-based construction method. SI Tables S4 and S5 show that the steel and cement for the funnel were significantly reduced by about 14% using the trench-based construction method. Since the production processes of steel and cement are energy-intensive and highly polluted, the decrease in the usage of the steel and cement can greatly reduce the impacts from the production process of the materials. Apart from the usage of materials, the impacts generated by the funnel construction can also be significantly reduced by using the trench-based construction method. The decrease of the usage of the funnel materials can also lead to the reduction of the environmental impacts generated by the transportation. 3.2. Effects of Reactive Materials. Comparing the use of Fe0 and IOCS mixture with the use of Fe0 and quartz sand mixture, the scenarios of using the Fe0 and IOCS mixture had lower environmental impacts in the different categories (C3, C4, T3, and T4 compared with C5, C6, T5, and T6, respectively; Figure 3).

When the groundwater consists of NOM, the effects of using the Fe0 and IOCS mixture on the different impacts were more significant. The impacts generated by the reactive media component decreased slightly in the scenarios without NOM, while those are drastically decreased up to about 43% with NOM. In the reactive media component of the PRB, the material production, transportation, and construction of the reactive media were included, it was clear that the material production of the reactive media was a major part of the total impacts (as indicated by Material Reactive Media in Figures 4 and 5). The reduction in the impacts was mainly due to the reduction of the use of materials. The IOCS is a waste product whereas the quartz sand requires a lower energy consumption for the production process, compared with Fe0. Therefore, the increase in the impacts was mainly due to the increase in the amount of Fe0. Since Fe0 production involves energy-intensive and highly polluted processes, the impacts induced by the production of the reactive media were largely attributed to the Fe0. As shown in SI Tables S4 and S5, the use of Fe0, which was the major factor of the impacts among the three reactive media, was lower in the scenarios of using the Fe0 and IOCS mixture, compared with scenarios of using the Fe0 and quartz sand mixture. In the scenarios using the Fe0 and IOCS mixture and with NOM, the usage of Fe0 was about 43% lower than those using the Fe0 and quartz sand mixture and with NOM. The decrease in the usage of Fe0 could be due to the enhancement effect of using IOCS on the removal capacity of Cr(VI) and As(V). A synergistic effect was found in the Cr(VI) and As(V) removal by the combination of Fe0 and IOCS.30 This resulted in an increase in the removal capacity of Cr(VI) and 10152



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Figure 5. Materials and energy consumption analysis of the PRB systems in scenarios (a) T1, (b) T2, (c) T3, (d) T4, (e) T5, and (f) T6. Impact categories include global warming (GW), acidification (Ac), carcinogenics (Ca); eutrophication (Eu), ozone depletion (OD), and smog formation (SF).

As(V). Therefore, reduced amounts of Fe0 were required. In the presence of NOM, the IOCS can adsorb NOM, reducing the impacts of the NOM on the reactivity of Fe0. This greatly increased the removal of Cr(VI) and As(V) by Fe0 and thus reduced the usage of Fe0. 3.3. Effects of Groundwater Constituents. Comparing the scenarios with and without NOM, the scenarios with NOM generally had higher environmental impacts on each of the categories (C1, C3, C5 compared with C2, C4, C6, and T1, T3, T5 compared with T2, T4, T6; Figure 3). The scenarios with the highest impacts were C4 and T4, respectively, for the caission-based and trench-based construction methods. In C4 and T4 scenarios, the Fe0 and quartz sand mixture was used as the reactive media. The impacts of the different categories in the scenarios with NOM (C4 and T4) were about 10 20% and 40 50%, respectively, higher compared with those without NOM (C3 and T3). Among the three components of the PRB system, the impacts induced by the reactive media were significantly influenced by the effects of NOM, whereas the funnel and gate generate only about the same level in the scenarios with or without NOM. The reactive media component of the PRB consisted of the material production, transportation, and construction of the reactive media, in which the material production of the reactive media generated a significant fraction of the impacts, as shown in Figures 4 and 5 (indicated as Material Reactive Media). As shown in SI Tables S4 and S5, the materials required (Fe0, and quartz sand/IOCS) for the scenarios with NOM were significantly higher than those without NOM (C1, C3, C5 compared with C2, C4, C6, and T1, T3, T5 compared with T2, T4, T6).

The increase in the material requirement for Fe0 was mainly because the NOM can reduce the reactivity of the Fe0.23 Therefore, a larger amount of Fe0 was required to achieve the same treatment goal. As discussed previously, Fe0 was a major factor of the impacts generated from the materials of the reactive media. Therefore, the increase in Fe0 consumption could remarkably increase the impacts.

4. ENGINEERING IMPLICATIONS This study provides a comparison of the environmental impacts of a PRB due to the selection of different construction methods and the materials of reactive media, and the effects of groundwater constituents. The results of this study provide a decision support base for more environmental sustainable PRB design. The results indicate that the construction methods significantly affected the environmental impacts of the PRBs. The trench-based construction method remarkably reduced the environmental impacts by reducing the use of the funnel materials in the PRBs. Therefore, the construction method should be considered when incorporating the green remediation in PRB design. Another factor that should be considered is the effect of groundwater constituents. The presence of NOM in groundwater can cause significant effects on the removal of Cr(VI) and As(V) so that a larger amount of reactive materials such as Fe0 would be required. The increase in the usage of the reactive materials could increase the environmental impacts of the PRBs. The use of NOM adsorbents such as IOCS can help to enhance the removal efficiency of Cr(VI) and As(V). Nevertheless, such enhancement materials should be more environmentally 10153


Environmental Science & Technology friendly reactive media than Fe0. The IOCS used in this study was a waste material, which only has low impacts, compared to Fe0.

’ ASSOCIATED CONTENT

bS

Supporting Information. More detailed information about the design of the PRB, the assumptions and the inventory data are available. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: 852-23587157; fax: 852-23581534; e-mail: [email protected].

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