environmental life cycle assessment of permeable reactive barriers: effects of construction methods,...

Published: October 28, 2011

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

ARTICLE

pubs.acs.org/est

Environmental Life Cycle Assessment of Permeable Reactive Barriers:Effects of ConstructionMethods, Reactive Materials and GroundwaterConstituentsMark 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

1. INTRODUCTION

Green remediation has been proposed by U.S. EnvironmentalProtection Agency (EPA) in recent years for avoiding andreducing the associated environmental impacts during the re-mediation processes.1 Consumption of energy, water, and ma-terial resources as well as waste generation and pollutantemissions can be accompanied by remediation activities. There-fore, assessments have been proposed in order to evaluate andminimize the environmental impacts created by a remediationprocess. U.S. EPA has suggested several aspects for considerationduring the assessment.2 These aspects include pollutant emis-sions and greenhouse gas emissions, and impacts to water qualityand ecosystems.2 To assess the environmental impacts of theremediation activities, life cycle assessment (LCA) can be one ofthe suitable tools.

LCA method has been widely applied for assessing theenvironmental impacts such as global warming, acidification,carcinogenics (human toxicity), eutrophication, ozone depletion,and smog formation generated from the soil and groundwaterremediation sites. LCA has been used for comparing differenttechnologies for soil remediation such as bioremediation, soilwashing, 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 impactsfrom the TCE-contaminated site in Dover Air Force Base inDover, DE, and found that a PRB system can generate less

environmental impacts than a PT system for long-term opera-tion. The PRB system is a passive treatment technology, whichrequires a lower amount of energy during the cleanup operation,comparing to the energy-intensive PT system. However, theenvironmental impacts induced by the construction of the PRBsystem are higher than that of the PT system as the PRB systeminvolves excavation of soil and emplacement of reactive media.9

Besides, Bayer and Finkel8 have found that changing the use ofthe funnel construction material from steel to clay can directlyreduce the environmental impacts from the overall PRB system.

The construction process has been found to be a major factorof the environmental impacts of the PRB system.9 PRBs arecommonly installed in a funnel-and-gate configuration,10 whichcan be generally constructed by two major methods, namelycaisson-based method and trench-based method. The PRB inDover Air Force Base is one of the examples which adopted thecaisson-based method,9 whereas the one in Vapokon Site, Den-mark is one of the PRBs adopted the trench-based method.11,12

In comparison, the caisson-based method can allow the re-placement of reactive media, while the trench-based methodcannot. The trench-based method requires the excavation of alarger amount of soil for emplacing all the reactive media at the

Received: June 14, 2011Accepted: October 28, 2011Revised: October 25, 2011

ABSTRACT: The effects of the construction methods, materials of reactive mediaand groundwater constituents on the environmental impacts of a permeablereactive barrier (PRB) were evaluated using life cycle assessment (LCA). ThePRB 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 constructionmethod can reduce the environmental impacts of the remediation remarkablycompared to the caisson-based method due to less construction material con-sumption by the funnel. Compared to using the zerovalent iron (Fe0) and quartzsand mixture, the use of the Fe0 and iron oxide-coated sand (IOCS) mixture canreduce the environmental impacts. In the presence of natural organic matter(NOM) in groundwater, the environmental impacts generated by the reactivemedia were significantly increased because of the higher usage of Fe0. Theenvironmental impacts are lower by using the Fe0 and IOCS mixture in thegroundwater with NOM, compared with using the Fe0 and quartz sand mixture. Since IOCS can enhance the removal efficiency ofCr(VI) and As(V), the usage of the Fe0 can be reduced, which in turn reduces the impacts induced by the reactive media.

10149 dx.doi.org/10.1021/es202016d |Environ. Sci. Technol. 2011, 45, 10148–10154

Environmental Science & Technology ARTICLE

initial construction stage.11 As a result, the construction energymay be significantly different with the use of the constructionmethods. Furthermore, the sizes of the funnels and gates mayvary between the construction methods because the regular sizeof the caisson restricts the size of the gate, whereas the size of thegate using the trench-based method is more flexible.11 This mayresult in a different consumption of the materials for the funnelsand gates, leading to different environmental impacts forthe PRBs.

Apart from the construction, the material consumption wasalso found to be another major factor of the environmentalimpacts of the PRB system.9 Zero-valent iron (Fe0) has beenconventionally used as thematerial of the reactivemedia of PRBs,which can effectively remove inorganic pollutants such as Cr-(VI), As(V) andU(VI),13�15 and organic pollutants such as TCEand DCE.16 Nevertheless, the reactivity of the Fe0 has beenshown to be reduced gradually due to the passivation of theiron corrosion products.17 Additionally, some groundwater con-stituents can affect the reactivity of the Fe0, such as alkalinity,hardness, nitrate and natural organic matter (NOM).18�22 NOMhas 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 forimproving the performance of the Fe0 PRB systems, in whichiron oxide-coated sand (IOCS) has been found to enhance theremoval efficiency of Cr(VI) and As(V) by using IOCS with Fe0

together.24 The use of a different material for the reactive mediain PRB can alter the environmental impacts of the whole systemdue to the differences in the production processes and rawmaterials. On the other hand, the use of a different material forthe reactive media can also affect the removal performance of thewhole system, leading to a different thickness of the PRBs.Therefore, different environmental impacts in the constructionprocess of the PRBs could be caused. Nevertheless, the previousstudies only focused on (i) the comparison of the environmentalimpacts 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 environmentalimpacts generated from PRBs.8 There is a lack of studiesaddressing the differences in the environmental impacts due tothe applications of various construction methods, differentmaterials of reactive media and the presence of groundwaterconstituents, in which these factors can complicate the investiga-tion of the environmental impacts from the PRBs as each ofthese factors could have different effects on each of the PRBcomponents.

The objective of this study was to assess the associatedenvironmental impacts of a PRB by investigating the twelvescenarios with different construction methods (caisson-based/trench-based), reactive materials (Fe0 and sand mixture/Fe0 andIOCS mixture), and groundwater constituents (without/withNOM), using the approach of environmental LCA. The PRBsystem was assumed to be installed in a simulated site for treating20 000 m3 of groundwater contaminated by either Cr(VI) aloneor Cr(VI) and As(V) which are commonly found heavy metalpollutants in groundwater.25,26 The major environmental im-pacts can be identified and improvements can be suggested forthe 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 fortreating a simulated site with a size of a 30 m wide�185 m long�9 m deep groundwater contaminant plume. The contaminantplume was equivalent to a volume of 20 000 m3 groundwater.The conditions for the groundwater constituents and contami-nants in different scenarios are shown in Table 1. The contami-nants include either Cr(VI) alone or Cr(VI) and As(V). To studythe effects of NOM, the groundwater consists of 8 mg/L NOMas dissolved organic carbon (DOC), which is within the commonrange of the concentration of NOM in groundwater.27 Thegeological conditions of this simulated site were assumed to be

Table 1. Conditions of Each Scenario

scenario

construction

method

reactive

media

concentration of groundwater

contaminants (mg/L) concentration of NOM

(mg/L as DOC)

removal capacity of Cr(VI)

(mg g�1 reactive media)

removal capacity of As(V)

(mg g�1 reactive media)Cr(VI) As(V)

C1 caisson Fe0 and quartz sand a 20 0 0 1.44 c n.a. e

C2 caisson Fe0 and quartz sand a 20 0 8 1.20 c n.a. e

C3 caisson Fe0 and quartz sand a 20 10 0 1.05 d 0.53 d

C4 caisson Fe0 and quartz sand a 20 10 8 0.88 d 0.32 d

C5 caisson IOCS and Fe0 b 20 10 0 1.33 d 0.60 d

C6 caisson IOCS and Fe0 b 20 10 8 1.24 d 0.57 d

T1 trench Fe0 and quartz sand a 20 0 0 1.44 c n.a. e

T2 trench Fe0 and quartz sand a 20 0 8 1.20 c n.a. e

T3 trench Fe0 and quartz sand a 20 10 0 1.05 d 0.53 d

T4 trench Fe0 and quartz sand a 20 10 8 0.88 d 0.32 d

T5 trench IOCS and Fe0 b 20 10 0 1.33 d 0.60 d

T6 trench IOCS and Fe0 b 20 10 8 1.24 d 0.57 d

Notes: aAssume 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). cThe removalcapacities 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.dThe removal capacities were based on the results from Mak et al.30 The initial solution pH was 7 and the groundwater flow rate was100 m/yr. eAs(V) is not included in this scenario.



similar to the site in Dover Air Force Base.28 The upper depth ofthe aquitard is 11 m and the aquifer thickness is 9 m.28 Theporosity of the aquifer is 0.4. 28

The PRB is constructed in a funnel-and-gate configurationwith a width of 41.4 mwhich is chosen in according to the PRB inDover Air Force Base. Two types of reactive materials, Fe0 andquartz sand mixture, and Fe0 and IOCSmixture, were used as thereactivemedia for different scenarios, respectively (Table 1). TheFe0 used in this study was made of gray cast iron, whereas theIOCS is a waste generated from a fluidized and air-aerated bedreactor, which had been used to remove iron ions that areproduced in the process of NO3

� reduction by Fe0.29

The amounts of Fe0, quartz sand, and IOCS required in eachscenario were determined by using the removal capacity ofCr(VI) alone, or Cr(VI) and As(V) by the Fe0 and quartz sandmixture, or the Fe0 and IOCS mixture, which have beeninvestigated by Liu and Lo, and Mak et al. (Table 1).23,30 Thethickness of the reactive media (as shown by “l” in SupportingInformation (SI) Figure S1) in the PRB for different scenarioswas varied with the removal capacities, while the width and depthof the reactive media were fixed in different scenarios.Two construction methods, caission-based and trench-based,

are employed in different scenarios, respectively (Table 1). ThePRB using the caisson-based construction method was designedwith reference to the PRB installed in the Dover Air ForceBase.9,28 A 36.6 m length of funnel and four 2.4 m diametercylindrical gates is installed (Figure 1a). The funnel is composedof steel sheet pilings sealed together with cementitious grout.The gates are constructed by excavating the soil within a 2.4 mdiameter steel caisson, emplacing a 1.2 mwide column of reactivemedia, and backfilling the pretreatment and post-treatmentzones with quartz sand (SI Figure S1a). The gates are removedfor replacement of the reactive media after the removal capacityof the reactive media exhausts. Replacement of reactive media isassumed to occur three times.The PRB using trench-based construction method was de-

signed with reference to the PRB installed in Vapokon Site,Denmark.12 A funnel with a length of 31.4 m and a gate with awidth of 10 m and a depth of 11 m are installed (Figure 1b). Thethickness of the gate of each scenario is varied with the amount ofthe materials used for the reactive media. Steel sheet pilings areinstalled 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.



pilings for the funnel are sealed together with cementitious grout.After installing the sheet pilings, the soil inside the gate isexcavated and the reactive media are then emplaced. Two sandbackfills with a thickness of 1 m are installed on both upgradientand downgradient of the reactive media as pretreatment andpost-treatment zones, respectively (SI Figure S1b).The machineries used to install the PRBs and the associated

specifications for energy consumption are addressed in SI TableS1, whereas the unit consumption of the materials and energy forthe PRB construction are summarized in SI Table S2.Thetreatment goal was set to follow the drinking water standardestablished by World Health Organization (WHO). The WHOdrinking 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 tosimulate a PRB system with different scenarios in order toevaluate the environmental impacts due to the effects of theconstruction methods, the use of different materials of reactivemedia and the groundwater constituents. The system boundariesconsisted of material production, transportation and construc-tion, as illustrated in Figure 2. The functional unit of this studywas the successful treatment of 20 000 m3 of the contaminatedgroundwater to the treatment goal, while the temporal boundaryis 30 years.

2.3. Life Cycle Impact Assessment Method. The LCA wasconducted using SimaPro 7.1 LCA software,32 and its built-ininventory databases and impact assessment methods.3,9 Themajor assumptions of this study are addressed in SI Table S3,while the inventories of each scenario are summarized in SITables S4 and S5. The impact assessment was conducted withthe characterization factors of the Tool for the Reduction andAssessment of Chemical and other environmental Impacts(TRACI) method.33 The environmental impact categories con-sidered in this study include global warming, acidification,carcinogenics, eutrophication, ozone depletion, and smogformation.33 These impact categories are suggested by U.S. EPAfor consideration in the environmental impact assessment ofremediation technologies,2 and are widely used for the LCAconducted on the environmental impacts generated from reme-diation technologies.4

3. RESULTS AND DISCUSSION

3.1. Effects of Construction Methods. The environmentalimpacts of a PRB using the trench-based construction methodwere noticeably lower than those of using caisson-based con-struction method (C1�C6 compared with T1�T6; Figure 3).The reduction in the impacts was mainly attributed to the

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



reduction of the impacts induced by the funnel component of thePRB (Figure 3). The component of the funnel consists of thematerial production, transportation, and construction of thefunnel. In this component, the impacts generated by the funnelmaterials were significantly reduced in the scenarios of using thetrench-based construction method (as indicated by Material �Funnel in Figures 4 and 5). For example, the fraction of globalwarming impact generated by the funnel materials was reducedfrom about 42% by using the caisson-based construction methodto about 16% by using the trench-based construction method.The reduction of the impacts could be the result of the reductionof the total length of the funnel from 36.6 m with the caisson-based construction method to 31.4 m by using the trench-basedconstructionmethod. SI Tables S4 and S5 show that the steel andcement for the funnel were significantly reduced by about 14%using the trench-based construction method. Since the produc-tion processes of steel and cement are energy-intensive andhighly polluted, the decrease in the usage of the steel and cementcan greatly reduce the impacts from the production process ofthe materials. Apart from the usage of materials, the impactsgenerated by the funnel construction can also be significantlyreduced by using the trench-based construction method. Thedecrease of the usage of the funnel materials can also lead to thereduction of the environmental impacts generated by thetransportation.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 lowerenvironmental 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 theFe0 and IOCS mixture on the different impacts were moresignificant. The impacts generated by the reactive media com-ponent decreased slightly in the scenarios without NOM, whilethose are drastically decreased up to about 43% with NOM. Inthe reactive media component of the PRB, the material produc-tion, transportation, and construction of the reactive media wereincluded, it was clear that the material production of the reactivemedia was a major part of the total impacts (as indicated byMaterial�Reactive Media in Figures 4 and 5). The reduction inthe impacts was mainly due to the reduction of the use ofmaterials. The IOCS is a waste product whereas the quartz sandrequires a lower energy consumption for the production process,compared with Fe0. Therefore, the increase in the impacts wasmainly due to the increase in the amount of Fe0. Since Fe0

production involves energy-intensive and highly polluted pro-cesses, the impacts induced by the production of the reactivemedia were largely attributed to the Fe0. As shown in SI TablesS4 and S5, the use of Fe0, which was the major factor of theimpacts among the three reactive media, was lower in thescenarios of using the Fe0 and IOCS mixture, compared withscenarios of using the Fe0 and quartz sand mixture. In thescenarios using the Fe0 and IOCS mixture and with NOM, theusage of Fe0 was about 43% lower than those using the Fe0 andquartz 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 ofCr(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 Thisresulted in an increase in the removal capacity of Cr(VI) and

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. Impactcategories 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 thepresence of NOM, the IOCS can adsorb NOM, reducing theimpacts of the NOM on the reactivity of Fe0. This greatlyincreased the removal of Cr(VI) and As(V) by Fe0 and thusreduced the usage of Fe0.3.3. Effects of Groundwater Constituents. Comparing the

scenarios with and without NOM, the scenarios with NOMgenerally had higher environmental impacts on each of thecategories (C1, C3, C5 compared with C2, C4, C6, and T1,T3, T5 compared with T2, T4, T6; Figure 3). The scenarios withthe highest impacts were C4 and T4, respectively, for thecaission-based and trench-based construction methods. In C4andT4 scenarios, the Fe0 and quartz sandmixture was used as thereactive media. The impacts of the different categories in thescenarios with NOM (C4 and T4) were about 10�20% and40�50%, respectively, higher compared with those withoutNOM (C3 and T3). Among the three components of the PRBsystem, the impacts induced by the reactive media were sig-nificantly influenced by the effects of NOM, whereas the funneland gate generate only about the same level in the scenarios withor without NOM. The reactive media component of the PRBconsisted of the material production, transportation, and con-struction of the reactive media, in which the material productionof the reactive media generated a significant fraction of theimpacts, 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 weresignificantly higher than those without NOM (C1, C3, C5 com-paredwithC2,C4,C6, andT1, T3, T5 comparedwithT2, T4, T6).

The increase in the material requirement for Fe0 was mainlybecause the NOM can reduce the reactivity of the Fe0.23 There-fore, a larger amount of Fe0 was required to achieve the sametreatment goal. As discussed previously, Fe0 was a major factor ofthe impacts generated from the materials of the reactive media.Therefore, the increase in Fe0 consumption could remarkablyincrease the impacts.

4. ENGINEERING IMPLICATIONS

This study provides a comparison of the environmentalimpacts of a PRB due to the selection of different constructionmethods and the materials of reactive media, and the effects ofgroundwater constituents. The results of this study provide adecision support base for more environmental sustainable PRBdesign. The results indicate that the construction methodssignificantly affected the environmental impacts of the PRBs.The trench-based construction method remarkably reduced theenvironmental impacts by reducing the use of the funnelmaterials in the PRBs. Therefore, the construction methodshould be considered when incorporating the green remediationin PRB design. Another factor that should be considered is theeffect of groundwater constituents. The presence of NOM ingroundwater can cause significant effects on the removal ofCr(VI) and As(V) so that a larger amount of reactive materialssuch as Fe0 would be required. The increase in the usage of thereactive materials could increase the environmental impacts ofthe PRBs. The use of NOM adsorbents such as IOCS can help toenhance the removal efficiency of Cr(VI) and As(V). Nevertheless,such enhancement materials should be more environmentally

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. Impactcategories include global warming (GW), acidification (Ac), carcinogenics (Ca); eutrophication (Eu), ozone depletion (OD), and smog formation (SF).



friendly reactivemedia than Fe0. The IOCS used in this study wasa waste material, which only has low impacts, compared to Fe0.

’ASSOCIATED CONTENT

bS Supporting Information. More detailed informationabout the design of the PRB, the assumptions and the inventorydata are available. This material is available free of charge via theInternet at http://pubs.acs.org.

’AUTHOR INFORMATION

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

’ACKNOWLEDGMENT

We thank the Research Grants Council of The Hong Kong SARGovernment for providing financial support under the GeneralResearch Fund (account No. 617309) for this research study.

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