Interactions between laponite and microbial biofilms in porous media: implications for colloid transport and biofilm stability

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  • Water Research 38 (2004) 361

    ndt

    wHans-Curt Flemming

    Institute for Interface Biotechnology, University of Duisburg-Essen, Geibelstrasse 41, 47057 Duisburg, Germany

    of colloids. Since these events are likely to occur in subsurface environments, our results suggest that colloidbiolm

    interactions will have implications for colloidbound contaminant transport and the remobilisation of pathogens.

    size range, i.e. approximately between 109 and 106m, mund et al., 1996; Roy and Dzomak, 1997). Under

    ARTICLE IN PRESSconditions which stimulate the retention and aggrega-

    tion of colloids, e.g. high ionic strength, the colloids

    form an integral part of the subsurface structural matrix.

    The importance of colloid-facilitated transport is em-

    phasised by the fact that changes in environmental

    *Corresponding author. Tel.: +49-0-203-379-3827; fax:

    +49-0-203-379-1941.

    E-mail address: strathmann@uni-duisburg.de

    (M. Strathmann).

    0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.wr 2004 Elsevier Ltd. All rights reserved.

    Keywords: Biolms; Clay; Colloid; Contaminant transport; Laponite; Porous media; Subsurface

    1. Introduction

    The movement of particulates through the subsurface

    has been studied and modelled in detail, and extensive

    reviews are available (Ryan and Elimelech, 1996).

    Movement of particulates which fall in the colloidal

    have proven implications for contaminant transport

    (McCarthy and Zachara, 1989), either when such

    colloids represent contaminants (e.g. heavy metals, some

    organics, and microbial pathogens) (Kersting et al.,

    1999), or when relatively innocuous colloids such as clay

    minerals enhance the transport of pollutants (Groli-Received 14 November 2003; received in revised form 20 April 2004; accepted 12 May 2004

    Abstract

    Quartz sand columns and sand-lled microscope ow cells were used to investigate the transport characteristics of the

    clay colloid laponite, and a biolm-forming bacterium, Pseudomonas aeruginosa SG81. Separate experiments were

    performed with each particle to determine their individual transport characteristics in clean sand columns. In a second

    set of experiments, bacterial biolms were formed prior to introduction of the clay colloids. In the independent

    transport experiments, bacteria and laponite each conformed to known physicochemical principles. A sodium chloride

    concentration of 7 102 M caused complete retention of the laponite within the sand columns. P. aeruginosa SG81was generally less inuenced by ionic strength effects; it showed relatively low mobility at all ionic strengths tested and

    some (albeit reduced) mobility when introduced to the columns in 1M NaCl, the highest concentration tested, but

    nevertheless showed reproducible trends. Under conditions favourable to laponite retention and biolm stability

    (7 102 MNaCl), laponite suspensions were able to remobilise a portion of the attached bacterial biomass. At lowionic strength, the prole of laponite elution was also altered in the presence of a P. aeruginosa biolm. These

    observations suggest that while a reduction in ionic strength has a dominant inuence on the mobilisation of biological

    and inorganic colloids, the presence of laponite and biomass can have a distinct inuence on the mobility of both typesInteractions between laponite amedia: implications for colloid

    C. Felipe Leon-Morales, Andreatres.2004.05.00943626

    microbial biolms in porousransport and biolm stability

    P. Leis, Martin Strathmann*,

  • the model assumes a Fickian type of movement (rate of

    ARTICLE IN PRESSC.F. Leon-Morales et al. / Water Research 38 (2004) 36143626 3615conditions (e.g. sudden reductions in ionic strength) can

    cause remobilisation in a process known as colloid

    generation. Colloids generated in this way can be an

    important mechanism for the transport of strongly

    sorbing pollutants (Grolimund et al., 1996), and other

    colloids.

    Little is known about the in situ mobility of

    microorganisms under extreme conditions such as

    pronounced changes in ionic strength resulting from

    rainfall events or de-icing of streets with salt, although

    laboratory simulations would suggest that these condi-

    tions will have an impact on the mobility of biocolloids

    (Deshpande and Shonnard, 1999). Microbial deposition

    in the subsurface raises an important aspect of microbial

    transport through porous materials: the ability of

    microorganisms to colonise a porous medium, produce

    extracellular polymeric substances (EPS) and grow as

    biolms (Cunningham et al., 1991). Biolm formation

    and release can be considered analogous to the process

    of colloid generation or release described earlier. As a

    consequence, these microbial formations are expected to

    inuence colloid transport in several different ways.

    These inuences can be direct as is the case of particle

    retention within biolm compartments (Okabe et al.,

    1997) or indirect as is the case for changes in

    hydrodynamic conditions caused by extensive biolm

    growth. Changes in hydrodynamic conditions within the

    porous media caused by thick biolm formation have

    been recognised (Sharp et al., 1999). Thick, conuent

    biolms are common in engineered systems or in

    nutrient rich environments (i.e. heavily contaminated

    environments) whereas thin and patchy biolms are

    commonly found in pristine subsurface environments.

    The transport of microorganisms through porous

    media has received considerable attention (Camesano

    et al., 1999; Deshpande and Shonnard, 1999; Jewett

    et al., 1999; Smets et al., 1999), although the focus of

    these studies has been varied. These include studies on

    preventing bacterial migration through aquifer systems

    (e.g. ltration of pathogens) (El-Masry et al., 1995;

    Tufenkji et al., 2002) and studies centred on enhancing

    microbial transport to a contaminated site for bioreme-

    diation (Li and Logan, 1999). Sand columns are

    commonly used to obtain colloid transport parameters,

    such as collision efciencies. As will be expanded upon

    in the theoretical background section, deposition para-

    meters in sand columns rely on the quantication of

    inuent/efuent concentrations. The quantication of

    retained colloids or biocolloids typically involves

    destructive methods such as extrusion of the porous

    medium and slicing to determine cell numbers by

    labelling methods like microbe and radiolabel kinesis

    (MARK) (Gross et al., 1995; Li and Logan, 1999). The

    confocal microscopy approach reported in the present

    study takes advantage of the ability to acquire informa-

    tion on the retention of uorescently-labelled bacteriasolute spread grows linearly with time). As will be seen

    later, these conditions are met in our experimental

    setting.

    Column parameters such as pore volumes, tracer

    dispersivities and average travel times can be obtained

    from experimental tracer breakthrough curves (BTC).

    One can either t experimental data to an analytical

    solution of the transport equation (1) using a standard

    nonlinear least-squares procedure or they can be

    obtained by moment analysis of the BTC (Dyson,

    1990). In sand columns with high Peclet numbers both

    procedures yield similar results (Grolimund et al., 1998).

    For either method it is necessary to obtain a normal-

    isation constant, n0; which represents the total amountusing online, non-destructive technology which can be

    independent of inuent/efuent bacterial concentrations

    and which at the present state of development can serve

    as a semi-quantitative tool for the determination of

    colloidal particle deposition. The main aim of the

    present report is to highlight the differences between

    the transport of the synthetic clay mineral laponite, and

    a microorganism, Pseudomonas aeruginosa SG81, within

    sand-packed laboratory columns. Both particles were

    studied separately, and the transport of laponite and its

    effects on biolm stability were studied subsequent to

    the establishment of microbial biolms within the

    porous medium.

    2. Theoretical background

    According to classical groundwater hydrology, the

    transport of solutes through porous materials can be

    described by accounting for advection (or mechanical

    movement) and hydrodynamic dispersion (as a sum of

    molecular diffusion and mechanical dispersion). The

    equation describing these relationships is known as the

    advectiondispersion equation (ADE). In the case of the

    transport of colloidal particles, several researchers

    (Kretzschmar et al., 1997; Grolimund et al., 1998,

    2001) have commonly used a modied version of the

    ADE that includes a term accounting for particle

    deposition (ltration). The concentration of suspended

    particles at a determined column depth and time, cx; tcan then be written as

    @C

    @t

    @2C

    D@x2 v

    @C

    @x kc; 1

    where v is interstitial colloid particle velocity, D is the

    hydrodynamic dispersion coefcient and k is the particle

    deposition rate coefcient. For this equation to be valid,

    the column should be initially free of colloidal particles

    (clean bed), colloidal release should be minimal com-

    pared with deposition, and colloid concentration should

    be small (no ripening or blocking effects). Additionally,

  • where kdf is the