Optimization of microbial adsorption (Biofilm Creation) on activated carbon by surface modification of substrate.
Date
2014-02-25
Authors
Sabbagh, Lee-At
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Abstract
Conversion of biomass to useful bio-products is a lengthy and often inefficient process. Research
has looked at the conversion of cellulose to ethanol by means of termite bacterial consortium in
fluidized bed bioreactor, where the bacteria are attached as biofilm to an activated carbon or
cellulose support. High conversion rates were achieved in this reactor and the process was fairly
robust and flexible. However establishing the biofilm on the activated carbon was a lengthy process
in terms of the time it took the bacteria to attach to the various substrates (Activated Carbon and
Cellulose) and form flocculants. The formation of such flocculants substantially increases the
reaction surface and hence should optimize the production of ethanol.
Many physical, chemical, and biological interactions facilitate the attachment of bacteria to surfaces.
It was in this study that the electrostatic attraction was investigated. The understanding of the
physical modifications of surface charge was chosen to be investigated in order to understand the
ideal conditions to propagate and increase biofilm creation. Bacteria carry a negative surface charge
and hence for increased attraction, the surface charge of the substrate should be modified to be
positive. This research, performed as batch processes, has shown that with the correct surface
charge modifications of the substrate the electrostatic attraction forces between the surface and the
bacteria are maximized. As a result of the strong electrostatic attraction forces between the two
surfaces the bacteria adsorbs and attaches to the substrate quicker and creates a biofilm on the
surface.
Prior to the attachment investigation it was important to attempt to understand the bacteria
consortium within the termite gut.. The “worker” termites collect the food and feed off the soil
whilst building their mound.. The bacteria found within the termite are in line with cellulose
degradation, which can be manipulated for biofuels production.
This study aimed to investigate a series of procedures of charge manipulation to the surfaces of both
the substrate and bacteria in order to see the influence of electrostatic interaction on biofilm
creation. It was seen that when only the activated carbons surface charge was modified to have a
positive net charge the attachment of bacteria was most prominent. In addition it was proven that
after being charged to a pH of 5 (4.28 x 104 Bacteria/mm2 Activated Carbon), 6 (3.90 x 104 Optimization of Microbial Adsorption (Biofilm Creation) on Activated Carbon by Surface Modification of Substrate 2013
Bacteria/mm2 Activated Carbon) and 7 (1.58 x 105 Bacteria/mm2 Activated Carbon) were achieved. The greatest attachment was seen when the activated carbon was charged to a pH of 7. This can be explained as the optimal positive pH the activated carbon should be charged to if its pHPZC is 9.7. If the pH is dropped lower than 7 although the surface becomes more positive, the surface actually approaches the surface charge of bacteria and the electrostatic interaction dissipates.
In an effort to use the optimal pH modification to create a biofilm on the surface of the activated carbon time experiments were performed. After 13 days biofilms on the surface of the activated carbon surfaces were seen. The bacteria attach to surface of the activated carbon and produce Extracellular Polymers Substance (EPS) fluid which then causes other bacteria to attach to them resulting in the formation of Biofilms.