CO2 conditioning

Equilibrium distributions of aqueous and solid-phase uranium at various pH and CO2 conditions were calculated by the computer code MD TEQA2 [7]. The version 3.11 of MD TEQA2 contains 63 kinds of complexation reactions of U with ligands and the stability constants of each reaction. The U species, considered in these complexation reactions are U02 and U. The ligands such as hydroxide, chloride, carbonate, fluoride, sul te, phosphate, and silicate are included. In this study, 20 kinds of complexation reactions of U were added to the code to increase the reliability of the model calculations. The stability constants in the code were also updated. New complexation reactions and stability constants were referred to by the studies of Grenthe and Bond [8,9]. In this model calculation, complexation reactions of U02 with hydroxide and carbonate ions were considered. The species of and ligands such as chloride, sul te and phosphate were not included considering our experimental conditions. 

Equilibrium distributions of uranium species by the model calculation are shown in Figures 1 and 2. Figure 1 is the calculated result of U speciation under 0% CO2 condition. Uranium almost exists as U02 at pHs 5.5 or below. Uranium also exists as a form of UO2OH and (U02)3(0H)s between pH 5 and 6.5. Uranium is precipitated as a form of 0 -U02<0H)2(s) between pH 6 and 9. The distribution percentage of the solid-phase is over 80%. Uranium exists as U02(0H)3 at pHs 9 or above. The dominant solid-phase is uranyl hydroxide because of the CO2 free condition. The distribution of U species under air condition is shown in Figure 2. Uranium exists as U02 at pHs 5.5 or below and as U020H and (U02)3(0H)s between pH 5 and 6.5. Uranium is precipitated as species of 0 -U02(0H)2(s) between 6 and 7.5. The maximum percentage of the solid-phase is 54%. 

This solid-phase disappears at pH 7.5 and U species including carbonate is formed above pH 7. The dominant species in the pH range of 7-9 and above 9 are (U02)2C03(0H)3" and 1102(003)3, respectively. The aqueous phases of uranyl ion, uranyl hydroxyl carbonate and uranyl carbonate are formed as the pHs of solution increase. The solid-phase is uranyl hydroxide around pH 7. It is found the equilibrium model calculations that the dominant species at pHs 5.5 or below is uranyl ion although the CO2 conditions were varied. Uranium is precipitated as a hydroxide form of 3 H02(0H)2(s) at a neutral pH. The aqueous phase of uranium hydroxide, hydroxyl carbonate and carbonate are dominant species at a high pH. These species have anionic charge. 

Figure 1. Calculated distribution of uranium species in the aqueous and solid-phase of IxlO M solution equilibrated under 0% CO2 condition.

The adsorbed amounts of U as a function of pH under different CO2 conditions are also shown in Figure 3. Under nearly 0% CO2 condition, less than 40% of initial U was adsorbed below pH S. The adsorbed amount of U increased rapidly as the pH increased. The adsorbed percentage reached more than 95% of initial U above pH 7 and then the amount was constant in the pH range from 7 to 9.5. But the adsorbed amount began to decrease above pH 9.5. When the CO2 condition is air atmosphere, 28% of initial U was adsorbed at pH 4.4. The adsorbed amount increased sharply in the pH range of 4-7. The adsorbed percentage decreased slowly at pH 7 or above. 

In C. littorale, the inhibition of photosynthetic oxygen evolution and carbon uptake, and the growth of air-grown cells subjected to H-CO2 conditions can be explained by the activity change in PS II. The increase in PS I activity found in the adaptation period suggests that ATP produced by cyclic electron flow around PS I should be used to cope with H-COj stress and for the recovery of PS II activity. 

Another characteristic of DMC that is important is hydrolysis. Dimethylcarbonate can react with water to produce methanol and CO2. Conditions that favor this reaction are elevated temperature, excess free water, and alkali metal carbonates. Under normal circumstances, in gasoline without any free water, the reaction does not proceed at all. With a small amount of free water and adequate time and temperature, some of the DMC will enter aqueous phase and hydrolyze. In a completely aqueous system with an alkali carbonate catalyst present and elevated temperature, DMC hydrolyzes slowly. For example, DMC in water held at 70°C (158°F) for Iweek with ample K2CO3 present was hydrolyzed to methanol and CO2 with a conversion of 50%. 

Villalobos, M. and Leckie, J.O., Carbonate adsorption on goethite under closed and open CO2 conditions, Geochim. Cosmochim. Acta, 64, 3787, 2000. 

The catalysts (MPcS chitosan) were obtained by impregnation of chitosan aerogel beads with an aqueous solution of sulfonated metal phthalocyanine. After impregnation, the solids were dried again under supercritical CO2 conditions. The textural properties are maintained and surface areas were greater than 140 m g (Table 4). 

Fig. 3.11. Species distribution diagram for covellite under open (atmospheric CO2) conditions (Acar and Soma-sundaran, 1990).

Figure 6. The o-Ps lifetime and intensity before (time<0) and after (time>0) CO2 conditioning of the polyimides and polyethylene. In the conditioning the sample polymer was immersed in 50 atm CO2 gas overnight and then the gas was evacuated. In the polyimides both o-Ps lifetime and intensity rise by the conditioning and show gradual change, while for polyethylene essentially no change is induced.

Figure 7. Comparison of the hole size distribution before (O) and after ( ) CO2 conditioning of polyimides and polyethylene. The experiments are the same as in Fig.6, but the data are analyzed to extract the distribution using the CONTIN program. (Reproduced with permission from ref 21, copyright The Chemical Society of Japan)...

In terms of CO2 as a reaction medium, a novel one-step process involving supercritical CO2 and enzymahc hydrolysis of cellulose has been shown to produce a 100% glucose yield [47]. However, to maintain the high pressure and temperature (160 bar and 50 °C) means the technology may have limited viability for industrial production, but it is an ideal technology for specialty products and possibly for other applications. For example, butyl butyrate can be synthesized via enzymatic esterification and transesterification using a lipase, Novozym 435, under supercritical CO2 conditions. Butyl butyrate is a component of pineapple flavor used by... 

In Eig. 14.2, the results of the electron beam experiments performed at ambient-pressure and high-pressure CO2 conditions are presented. At ambient pressure, Eig. 14.2 a clearly shows the additional peak of the newly formed polymer of Mw 79800 Da produced by the radicals in the aqueous phase induced by the... 

Figure 19.11 Oxidation profiles of samples (a) 02,(b) CO,(c) CO2. Conditions 10 Kmin, 5%02/He no water added.

Photosynthesis was measuredwith a Clark-type 0 electrode at a nonsaturating PAR of 55 pmol m s and at 38 C. Prior to the measurements, 20 pi of 1 M NaHCO solution was added to certify saturating CO2 conditions. 

The contents of the substrate RuBP-binding sites were 79.3 13.2 and 90.5 7.1 (n = 3) nmol/mg Chi in the spinach and radish leaves, respectively, used in this study. Figure 3 shows the progress of the direct and activated carboxylase reactions of RuBisCO "extracted" at liq. N2 -temperature from leaves in steady photosynthesis at various CO2 concentrations. The direct reactions of the "extracts" of the leaves at 1000 and 3000 ppm CO2 showed large inflections a few min after the start of the reaction, and were similar to those of the activated "extracts" and of Panel A in Fig. 2, indicating that RuBisCO was functioning as the form of ECMRs under such CO2 conditions. The RuBP content of the same leaf powder was 200 to 265 nmol/mg Chi. RuBisCO... 

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