Microbial Adhesion And Transport

The presence of appropriate microorganisms (depending on electron-acceptor and nutrient availability) at the actual site of contamination (sometimes at a micro-scale) has long been recognized as a key factor in determining if biodegradation/biotransformation will occur, as well as in influencing the rate of biodegradation. Microorganisms can be present in the subsurface in suspension (in the pore fluid), as microcolonies or as a biofilm. Bacterial adhesion to porous media often influences the nature and extent of colonization of a subsurface medium. The biofilm development model (Characklis and Wilderer, 1988), summarized below, can provide some understanding into bacterial transport and adhesion in subsurface environments. The "first three steps" in biofilm development are: (1) Surface conditioning, (2) Transport of microoganisms to the conditioned surface and, (3) Sorption of microorganisms to the surface. In biofilm literature, it is recognized that the nature of the bacterial cell surface is a key parameter in determining its adhesion to any media and eventually the biofilm architecture (Reynolds et al, 1989; Characklis and Wilderer, 1988; van Loosdrecht et al, 1987). The sorption of microorganisms is due to reversible sorption (governed by charge/electrostatic interactions) and irreversible sorption (due to production of extracellular exoploymers and formation of matrix). The dependency of reversible sorption of bacteria on charge/electrostatic interactions indicates that an applied electric field may play a significant role in bacterial adhesion and transport in subsurface environments. Further, the role of bacterial surface (eg., lipid content and surface charge) itself could influence the electrophoretic mobility of bacteria in porous media. Data available in literature indicate that the presence of heavy metals alter the electrokinetic properties of bacteria, although the context of these researchers was not hazardous waste decontamination (Collins and Stotzky, 1992). The imposed dc electric field is also expected to affect the electrokinetic properties, adhesion and transport of microorganisms. It is not clear what will be the extent of this effect; however, the authors noted that microorganisms tend to stick and attach themselves to the electrodes in experiments conducted using diluted sludge samples under dc fields. The type or activity of microorganisms attached to the electrodes was not evaluated. Electrode polarity did not seem to have an effect on adhesion as microorganisms were attached not only to the anode but also to the cathode. This might indicate that the adhesion to electrode surface is not necessarily due to the electric attraction between the negatively charge microrganisms and the positively charged anode. Further evaluation is needed to verify if this adhesion is due to reversible sorption, irreversible sorption, or due to electrode charge.

Electric fields will not only impact microorganisms adhesion but will also affect their transport in porous media. As microbes are generally negatively charged, dc fields will cause their transport towards the anode. DeFlaun and Condee, (1997) demonstrated electrokinetic transport of a pure bacterial culture in bench-scale soil samples. Generally, the rate of transport is related to the effective electrophoretic mobility in soils and is affected by soil physical parameters such as porosity, pore size distribution and tortousity. The results of DeFlaun and Condee (1997) demonstrate the potential of using microorganisms electrophoresis for the purpose of bioaugmentation to enhance in situ bioremeidation. It is also necessary to note that microorganisms transport in porous media under electric field might not be strictly governed by electrophoresis alone, since microbes, as living entities, may be subject to other influences or "attractors" and also tend to form colonies and attach to the soil particle surface.

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