Reduction of water activity

Fig. 11.4. Reduction of water activity at the electrode/electrolyte interface in the electrodimerization of acrylonitrile (AN). Water is repelled from the interface by NEt j and this decreases the likelihood of hydrogen evolution competing with the acrylonitrile reduction. (Reprinted from F. Beck, Erdol Kohle Erdges, [Oil, Coal, Natural Gas] Petro-chem. 41 287,1988.)...

Figure 10. The reduction of water activity by high concentration of NaCl. Water activity was measured by the rate of proton dissociation from excited hydroxypyrene trisulfonate (A), excited 2-naphthol-6-suIfonate ( ), or using the published vapor pressure ( ) (Grollman, 1928 Kracek, 1928).

The acid end products of fermentation can inhibit growth and butanediol formation. Large quantities of butanediol, up to 130 g/1, were not strongly inhibitory, while 0.45 g/1 of acetic acid can completely inhibit growth of K. oxytoca. Butanediol inhibition is believed to be due to the reduction of water activity, while acetic acid and not its salt is the inhibitory metabolite (Fond et al. 1985). In Enterobacter aerogenes, ethanol was also shown to be an inhibitory metabolite for growth (Zeng and Deckwer 1991). 

Microbiol Stability. Microbial growth is hindered by reducing water activity and adding preservatives. An overview is available (30). Reduction in water activity is typically obtained by including approximately 50% of a polyalcohol such as sorbitol or glycerol. Furthermore, 20% of a salt like NaCl has a pronounced growth inhibiting effect. 

Mechanistic Approaches. Adequate and appropriate river-quality assessment must provide predictive information on the possible consequences of water and land development. This requires an understanding of the relevant cause and effect relationships and suitable data to develop predictive models for basin management. This understanding may be achieved through qualitative, semi-quantitative or quantitative approaches. When quantitative or semi-quantitative methods are not available the qualitative approach must be applied. Qualitative assessments involve knowledge of how basin activities may affect river quality. This requires the use of various descriptive methods. An example of this kind of assessment is laboratory evaluation of the extent to which increases in plant nutrients, temperature or flow may lead to accelerated eutrophication with consequent reduction of water quality. 

PEMFC)/direct methanol fuel cell (DMFC) cathode limit the available sites for reduction of molecular oxygen. Alternatively, at the anode of a PEMFC or DMFC, the oxidation of water is necessary to produce hydroxyl or oxygen species that participate in oxidation of strongly bound carbon monoxide species. Taylor and co-workers [Taylor et ah, 2007b] have recently reported on a systematic study that examined the potential dependence of water redox reactions over a series of different metal electrode surfaces. For comparison purposes, we will start with a brief discussion of electronic structure studies of water activity with consideration of UHV model systems. 

Murthi VS, Urian RC, Mukeijee S. 2004. Oxygen reduction kinetics in low and medium temperature acid environment Correlation of water activation and surface properties in supported Pt and Pt alloy electrocatalysts. J Phys Chem B 108 11011-11023. 

Reungoat J, Macova M, Escher BI, Carswell S, Mueller IF, Keller J (2010) Removal of micropollutants and reduction of biological activity in a full scale reclamation plant using ozonation and activated carbon filtration. Water Res 44 625-637... 

Redox activity in sediments may be defined by an equilibrium approach, considering that the Eh of an aqueous system is bound in the upper boundary by the oxidation of water and in the lower boundary by reduction of water. The term Eh... 

The Ni(II) complexes of 14-membered tetraaza macrocyclic ligands, cyclam, Lie, L18, and Lie show catalytic activity in H20 or aqueous MeCN. The total mole-for-mole yields of CO and H2 are ca. 1 in most cases. The [Ni(cyclam)]2+ complex is a very effective and selective catalyst for the electrochemical reduction of C02 to CO relative to the reduction of water to H2 in aqueous solution when it is adsorbed onto mercury. The CO/H2 product ratio is >100 for [Ni(cyclam)]Cl2 (79). It is suggested that the greater selectivity for the electroreduction of C02 compared with water is related to the size of the macrocyclic ligand... 

Babcock, G.T. Wickstrom, M. (1992) Oxygen activation and the conservation of energy in cell respiration. Nature 356, 301-309. Advanced discussion of the reduction of water and pumping of protons by cytochrome oxidase. 

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