O2 Fluxes

Thus the key experimental observation Equation (7.11), is satisfied in presence of spillover. When an external overpotential AUWR is applied, with a concomitant current, I, and O2 flux I/2F, although UWR is not fixed anymore by the Nemst equation but by the extremally applied potential, still the work function Ow will be modified and Equations (7.11) and (7.12), will remain valid as long as ion spillover is fast relative to the electrochemical charge transfer rate I/2F.21 This is the usual case in solid state electrochemistry (Figs. 7.3b, 7.3d) as experimentally observed (Figs. 5.35, 5.23, 7.4, 7.6-7.9). 

The calculations in Section 6.2 indicate that the root system as a whole can sustain considerable rates of O2 loss to the rhizosphere without compromising their internal O2 requirements. The standard O2 flux in the calculations in Section 6.2 was 0.5 nmol dm (root) s for the parts releasing O2. For rice roots grown in soil, Begg etal. (1994) obtained values of 0.1-1.2nmol dm (root) s from rates of Fe + oxidation and Fe(III) accumulation near planar layers of rice roots in anaerobic soil, and Kirk and Bajita (1995) obtained 0.1-0.2 nmol dm (root) s with the same experimental system but a soil with a smaller ferrous iron content. These values probably underestimate the total O2 release because they did not allow for O2 consumed by soil microbes. Revsbech et al. (1999) obtained values of 1-3 nmol dm (root) s from measurements of O2 gradients made with a microelectrode near rice roots in the soil used by Kirk and Bajita (1995). These values are in the middle of the range described above. 

Although most product-removal applications feature H2 removal, some decomposition reactions in which O2 is removed are of environmental interest. These use dense mixed-oxide materials (see Section 2.3) to remove O2 only. Early work involved NO and CO2 decompositions using zirco-nias. ° Itoh et al. have recently reconsidered the CO2 decomposition reaction to try to run it with reasonable conversion at lower temperatures using a PBMR. Dixon et al. considered the impact of new, high O2 flux perovskite materials on both CO2 decomposition and NO decomposition. It appears that further advances in materials will be needed before these applications become viable. 

Tsai et a/. also approached the problem of increasing O2 flux and stability. Their approach was to balance the substitutions on the A and B sites of the ABO3 perovskite. Stability is strongly influenced by a stable BO3 skeletal sublattice, and the choice of Fe with a mild Co substitution gives stability with reasonable oxygen anion conductivity. Then different amounts and types of aliovalent cations (Ca, Sr ", Ba " ) were partially substituted for La in a LaFeo.8Coo.203 s perovskite framework to attain higher electron conductivity. 

Song JM, Zhan BQ, Li PC (1997b) Dissolved oxygen distribution and O2 fluxes across the sea-air interface in the East China Sea waters. Chin J Oceanol Limnol 14(4) 228-232... 

The water-air O2 flux F, g/(m d)) in the Changjiang River Estuary was calculated as follows ... 

Fig. 4.60 (Song et al., 1996) shows the daily variation of O2 flux across the sea-air interface at the continuous stations 410 and 111. It can be seen that oxygen released to air was the main process in one day and night, and that the amount released in daytime was more than that at night. This was because photosynthesis produced more oxygen in daytime, and the high temperature of seawater also increased the release of oxygen. This feature was very obvious in Leg 9410. At station 410, the oxygen in air dissolved into seawater from 18 00 to 06 00, but it was O2 from seawater to air from 06 00 to 18 00. The oxygen released into air was —8.63 L/(m d) in Leg 9404. This reflected the fact that the seawater was supersaturated and the photosynthesis was very...

Fig. 4.61. Variation of O2 flux strength afong transects A (a) and B (b) (Gao and Song, 2008) (With permission from Efsevier s Copyright Cfearance Center)...

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