Laboratory column study for remediation of MTBE-contaminated groundwater using a biological two-layer permeable barrier

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  • Available at www.sciencedirect.com

    journal homepage: www.elsevier.com/locate/watres

    Laboratory column study for remediation ofMTBE-contaminated groundwater using a biologicaltwo-layer permeable barrier

    She-Jiang Liua, Bin Jianga,b, Guo-Qiang Huanga,b, Xin-Gang Lia,b,

    aSchool of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR ChinabNational Engineering Research Centre for Distillation Technology, Tianjin University, Tianjin 300072, PR China

    a r t i c l e i n f o

    Article history:

    Received 15 February 2006

    Received in revised form

    26 June 2006

    Accepted 16 July 2006

    Available online 7 September 2006

    Keywords:

    MTBE

    Remediation

    Groundwater

    Permeable reactive barrier

    Biodegradation

    A B S T R A C T

    In this study, an in situ biological two-layer permeable reactive barrier system consisting of

    an oxygen-releasing material layer followed by a biodegradation layer was designed to

    evaluate the remediation effectiveness of MTBE-contaminated groundwater. The first layer

    containing calcium peroxide (CaO2) and other inorganic salts is to provide oxygen and

    nutrients for the immobilized microbes in the second layer in order to keep them in aerobic

    condition and maintain their normal metabolism. Furthermore, inorganic salts such as

    potassium dihydrogen phosphate (KH2PO4) and ammonium sulphate ((NH4)2SO4) can also

    decrease the high pH caused by the alkali salt degraded from CaO2. The second layer using

    granular expanded perlite as microbial carrier is able to biodegrade MTBE entering the

    barrier system. Batch experiments were conducted to identify the appropriate components

    of oxygen-releasing materials and the optimum pH value for the biodegradation of MTBE.

    At pH 8.0, the biodegradation efficiency of MTBE is the maximum and approximately48.9%. A laboratory-scale experiment using two continuous upflow stainless-steel columns

    was then performed to evaluate the feasibility of this designed system. The fist column was

    filled with oxygen-releasing materials at certain ratio by weight. The second column was

    filled with expanded perlite granules immobilizing MTBE-degrading microbial consortium.

    Simulated MTBE-contaminated groundwater, in which dissolved oxygen (DO) content was

    0mg/L, was pumped into this system at a flow rate of 500mL/d. Samples from the second

    column were analyzed for MTBE and its major degradation byproduct. Results showed that

    MTBE could be removed, and its metabolic intermediate, tert-butyl alcohol (TBA), could also

    be further degraded in this passive system.

    & 2006 Elsevier Ltd. All rights reserved.

    1. Introduction

    As an alternative to traditional pump-and-treat and dig-and-

    treat methods for the remediation of contaminated ground-

    water, permeable reactive barrier is a relatively new in situ

    technology, and is attracting increased attention (Borden

    et al., 1997; Rasmussen et al., 2002; Wilkin et al., 2003; Carsten

    et al., 2004). The barriers are installed perpendicular to the

    direction of groundwater flow within aquifers. As the ground-

    water passes the barriers under natural hydraulic gradients,

    the contaminants are scavenged or degraded from the water

    by chemical, physical or biological action. The barriers also

    ARTICLE IN PRESS

    0043-1354/$ - see front matter & 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.watres.2006.07.015

    Corresponding author. School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China.Tel.: +86 22 27890628x8019; fax: +86 2227404705.

    E-mail address: lxg@tju.edu.cn (X.-G. Li).

    WAT E R R E S E A R C H 40 ( 2006 ) 3401 3408

  • prevent groundwater contaminants from migrating to un-

    contaminated aquifers, which may be difficult to locate and

    remedy. The main advantage of permeable reactive barrier

    is the lower cost. Once installed, the barriers do not need

    above-ground facilities or energy inputs, and they can take

    advantage of the in situ groundwater flow to bring the

    contaminants in contact with the reactive materials.

    Methyl tert-butyl ether (MTBE) is the most commonly used

    as fuel oxygenate that can enhance the gasolines octane

    rating and improve the combustion efficiency of gasoline. In

    spite of its relatively recent usage, MTBE has become the

    second most common contaminant detected in the urban

    groundwater which gives to the water both an unpleasant

    taste and odour, and poses a significant health threat

    (Squillace et al., 1996; Caprino and Togna, 1998; Kharoune

    et al., 2001). In recent years, many methods for treatment of

    MTBE have been proposed including carbon adsorption (Shih

    et al., 2003), advanced oxidation technologies (AOTs) such as

    UV/H2O2, UV/O3, Fenton reaction, H2O2/O3, TiO2 photocata-

    lysis, sonolysis, and radiolysis. Although MTBE can be

    removed effectively from contaminated water system using

    above methods, most of these technologies are limited in

    factual application for the remediation of the contaminated

    subsurface. Furthermore, the major disadvantage of these

    physical, chemical treatment methods is the potential for

    forming byproducts with higher toxicity than the original

    contaminant (Graham et al., 2004; Zang and Faronood, 2005;

    Bergendahl and Thies, 2004; Safarzadeh-Amiri, 2001; Barreto

    et al., 1995; Kang and Hoffmann, 1998; Hsieh et al., 2004).

    Microcosm studies demonstrated that MTBE, tert-butyl alco-

    hol (TBA) that is currently widely accepted as metabolic

    intermediate or dead-end product of MTBE, may be biode-

    gradable with special bacterial strains or natural isolates

    under aerobic conditions (Fortin and Deshusses, 1999; Deeb

    and Alvarez-Cohen, 2000; Prince, 2000; Fayolle et al., 2001;

    Bradley et al., 2002; Sedran et al., 2002). However, MTBE

    degradation was highly variable under different environmen-

    tal conditions (Bradley et al., 2001). For the evaluation of MTBE

    biodegradation, TBA instead of MTBE got even more concern

    due to its higher toxicity (Schmidt et al., 2004). Besides, the

    microbes using MTBE as sole source of carbon and energy

    under aerobic conditions grow slowly with low yields of

    biomass and are sometimes unstable. As a result, a viable

    bioremediation process for MTBE has not been fully devel-

    oped so far.

    In generally, dissolved oxygen (DO) content is very poor

    within the plum pore and mid-plume areas in the contami-

    nated groundwater, which cannot maintain aerobic biode-

    gradation of some organic contaminants. Several researchers

    have developed an oxygen-releasing compound such as

    calcium peroxide (CaO2) to passively increase DO in the

    subsurface (Cassidy and Irvine, 1999; Arienzo, 2000; Kao et al.,

    2001). CaO2, besides oxygen, releases Ca(OH)2 causing a

    significant rise in pH of solution. Laboratory study showed

    that high pHmight inhibit microbial activity and decrease the

    removal efficiency of contaminants (Kao et al., 2003). Ritter

    and Scarborough (1995) illuminated that pH of environment

    helping for microbial growth should be keep in the range of

    6.58.5. But now, few researchers focus their attention on the

    regulation of pH caused by the oxygen-releasing compound.

    To this disadvantage influence of high pH, it mainly depends

    on microbial adaptive ability and buffer capability of field soil

    to deal. This is inevitable to prolong period and increase cost

    of remediation.

    As microbial carrier, granular expanded perlite with about

    particle size of 23mm was obtained from Tianjin Sanhua

    Corporation Ltd., whose chemical composition (wt%) was as

    follows: SiO2, 72.93; Al2O3, 12.90; TiO2, 0.05; CaO, 0.76; MgO,

    0.16; Fe2O3, 0.53; FeO, 0.18; K2O, 5.3; Na2O, 2.57; MnO, 0.06; H2O,

    4.56. The following reasons make it good candidate for this

    study: (1) the surface of expanded perlite is porous and

    coarse, which helps microbe to adsorb and immobilize; (2) as

    a kind of silicate minerals, expanded perlite does not bring

    any new contaminant into groundwater when it is placed in

    the barriers and (3) expanded perlite is relatively inexpensive.

    Based on the above discussions, we designed a biological

    two-layer permeable reactive barrier system containing

    oxygen-releasing material and biodegradation layers to

    evaluate the remediation effectiveness of MTBE-contami-

    nated groundwater. Oxygen-releasing materials and ex-

    panded perlite granules with immobilized microbes can be

    filled in remediation wells or permeable trenches. The

    schematic diagram of this passive system is shown in Fig. 1.

    The principle of this work was to design a passive

    treatment system to bioremediate groundwater contami-

    nated by MTBE. In this study, batch experiments were

    conducted to identify the components of oxygen-releasing

    materials, which could continuously release oxygen and

    regulate the high pH caused by CaO2. In addition, the

    biodegradation efficiency of MTBE under different pH value

    conditions was also studied. A column experiment was then

    performed to evaluate the feasibility and potential of this

    passive barrier system for biodegradation of MTBE.

    2. Materials and methods

    2.1. Experimental microbes collection, enrichment andacclimation

    The original experimental microbes were collected from the

    soil located 3545cm deep at Dagang Oil Field in Tianjin city,

    China. Under aerobic condition (pumping air at 30mL/min

    into the medium), microbes were enriched and acclimated in

    ARTICLE IN PRESS

    Spill Site

    Clay

    GroundwaterFlow Direction Oxygen

    Oxygen-releasingMaterial Layer

    Water Table

    UncontaminatedGroundwater

    Biodegradation Layer

    Nutrients

    Fig. 1 Schematic diagram of the designed biological barrier

    system.

    WAT E R R E S E A R C H 40 ( 2006 ) 3401 34083402

  • a biochemical culture bottle at room temperature for 2

    months. The components of liquid mineral salts medium

    were as follows (units are in mg/L of water): ammonium

    sulphate ((NH4)2SO4), 1000; potassium dihydrogen phosphate

    (KH2PO4), 500; K2HPO4, 500; MgSO4 7H2O, 30; Fe2(SO4)3, 30;MnSO4 H2O, 40; ZnSO4 H2O, 50; CaCl2, 10. Glucose was usedas sole carbon source in the initial period of 015 days. During

    1530 days, the quantity of glucose decreased gradually from

    2000 to 0mg/L, and the quantity of MTBE increased from 0 to

    2000mg/L. After 30 days, MTBE was used as the sole carbon

    source for the microbial growth. The suspension of accli-

    mated microbe was used for the next experiments.

    2.2. Batch experiments

    2.2.1. Study on the pH controlTechnical CaO2 used in this study was purchased from Tianjin

    Chilong Chemical Engineering Co. Ltd., whose main impurity

    was about 50% of Ca(OH)2 by weight. KH2PO4 and (NH4)2SO4were obtained from Tianjin Chemical Reagent Company,

    whose purities were more than 99%. CaO2 releases oxygen

    upon contact with water according to the following overall

    reaction:

    2CaO2 2H2O! O2 2CaOH2. (1)During the oxygen-releasing process of CaO2, Ca(OH)2

    produced can result in a significant rise in pH of solution

    that may decrease enzymatic activity of microbes. Therefore,

    in this study, the pH was effectively regulated by using

    (NH4)2SO4 and KH2PO4, which are usually used as nutrient

    components in the medium. The appropriate mol ratio of

    nitrogen (N) and phosphorus (P) is very important for

    microbial growth in view of microbial metabolism. According

    to the concentrations of N and P in the above medium, the

    study on the pH control was carried out in an enclosed reactor

    under the condition of fixing mol ratio of N:P at 2.6. Reactive

    mixtures were obtained by taking 90mg of CaO2, adding 90,

    135, 180, 225, 270mg of KH2PO4 and calculated weights of

    (NH4)2SO4 based on that of KH2PO4, respectively. The reaction

    was initiated by adding 200mL of sterile deionized water

    saturated by sparging nitrogen, which made the DO content

    reduce to zero. All above experiments were conducted in a

    reciprocal shaker at constant temperature (20 1C) and rotate

    speed (130 r/min). A PHS-3C pH meter was used to online

    monitor the pH changes.

    2.2.2. Effects of pH on the biodegradation efficiency of MTBEThe biodegradation experiments were performed in five

    600mL enclosed reactors containing 200mL of the mineral

    salts medium described above with the pH value at 6.5, 7.5,

    8.0, 8.5, 9.5, respectively, and MTBE (about 130mg/L) as sole

    source of carbon and energy. Air was sparged into the

    medium before adding MTBE using an air pump for 10min

    in order to increase the DO content. The reactors were then

    enclosed when 20mL of the suspension of acclimated

    microbe was inoculated, respectively. The five reactors, once

    inoculated, were placed in a reciprocal shaker (130 r/min) at

    20 1C. In this study, the control experiments were also

    conducted with abiotic microbes. The liquid concentrations

    of MTBE in the reactors were measured at the beginning and

    end of the experiments. By calculating the degradation

    efficiency of MTBE, the optimum pH value for the biodegrada-

    tion can be determined. In addition, CO2 concentrations in

    the headspace of reactors were also analyzed at the end of

    experiments.

    2.3. Microbial immobilization

    The laboratory column of 100cm length and 5 cm internal

    diameter made of stainless steel was homogeneously packed

    with the expended perlite granules. They have hardly

    chemical adsorptive capacity for MTBE and TBA (data not

    shown). At the same time, the suspension of acclimated

    microbe was injected in this column to submerse the perlite.

    Column feed solution consisting of MTBE (2000mg/L) and

    above mineral salts medium was pumped into the column by

    using a peristaltic pump at a flow rate of 1.5 L/d for

    maintaining the microbial metabolism and permitting the

    development of microbial film on the surface of expended

    perlite granules. In order to ensure this system aerobic

    condition, air was introduced at 30mL/min into the feed

    solution before entering this column. The DO levels in the

    effluent, the concentrations of MTBE in the influent and

    effluent were measured respectively every 23 days. The

    whole process of microbial immobilization lasted 25 days.

    2.4. Column experiment

    A laboratory-scale barrier system was designed using two

    continuous upflow stainless-steel columns. The first oxygen-

    releasing material column (100cm length and 5cm internal

    diameter) was filled with the mixture of oxygen-releasing

    materials prepared by blending CaO2, KH2PO4, (NH4)2SO4,

    sand, trace elements at a ratio of 0.10:0.20:0.26:0.40:0.04 by

    weight. Among the mixture, the ratio of CaO2, (NH4)2SO4,

    KH2PO4 and trace elements which included MgSO4 7H2O,Fe2(SO4)3, MnSO4 H2O, ZnSO4 H2O, CaCl2 was obtained fromthe results of pH control experiment and the components of

    mineral salts medium described above. CaO2 was used as

    oxygen source for the aerobic microbes, (NH4)2SO4 and

    KH2PO4 were used to provide nutrients for in situ nutrient

    supplement and lower the pH caused by CaO2, trace elements

    were used to provide necessary elements for the microbial

    growth, and the sterile quartz sand (about 1mm of grain size)

    was used to increase the permeability of the mixture, which

    may make groundwater flow easy to pass the barriers. The

    column that had immobilized microbes described above was

    used as the biodegradation column, which was equippedwith...

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