Chemically mediated membrane processes

In chemically mediated membrane processes the throughput or productivity (the rate a compound passes through the membrane) is related to the chemical potential driving force as follows ... 

Figure 2.6 Mass transfer in a chemically mediated membrane process. The chemical potential gradient from the bulk feed to the bulk permeate streams is the driving force for mass transfer (shown as yellow line). Three main resistances to mass transfer are shown—fluid boundary layers on the feed and permeate sides of the membrane and diffusion through the membrane.

The subject of carrier-mediated or facilitated gas transport in liquid film membranes has been an active field of research the past 15-20 years. It refers to the enhancement of gas transport attributable to reversible reaction of physically dissolved gas with non-volatile, diffusible solutes. The underlying physics are analogous to those governing gas absorption with chemical reaction. However, the two processes differ insofar as the membrane process has been primarily operated in the steady-state, and without the occurrence of irreversible reactions. 

From Eq. (1.42), it can be inferred that the substrate breaks down into carbon dioxide and proton along with electricity as a by-product. The concept of MFC was demonstrated by Potter in 1910, who used platinum electrodes as well as living cultures of Escherichia coli and Saccharomyces. The anodophile species of the microbes can transfer the electrons directly to the anode. Otherwise, the electron mediators are required in the cell for enhanced power output and increased efficiency. The direct electron transfer to the anode is hindered by a majority of microbial species due to the presence of non-conductive Upopolysaccharides, peptididoglycans and lipid membrane in their outer layers. Hence, mediators are used which capture electrons from the membrane and are reduced. Furthermore, these mediators will again become oxidised once they move across the membrane and release the electrons to the anode. Hence, the electron transfer process keeps the anode replenished which maintains the sustainability of cell. Anthraquinone-2, 6-disulphonate, 2-methylene blue, thionine, 2-hydroxy-l, 4-naphthoquinone, Fe (III) EDTA, Meldola s blue and neutral red are some of the common chemical mediators which enhance electricity production. MFC is known as the... 

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