Decreased with chromium concentration increase

It is clear from the data in Table 4.6 that the corrosion rates increase with increase in chromium in sulfuric acid solution. The presence of 10% Ni in the Fe-Cr alloy results in decreasing corrosion rate with chromium concentration. The corrosion rates of Fe-Cr alloys in ferric sulfate decrease with increasing concentration of chromium in the alloy. These observations are supported by the data on corrosion potentials of stainless steels in boiling acids and chlorides measured against a saturated calomel electrode. 

Two ions of the transition metal take part in this reaction. However, in the case of supported one-component catalysts the formation of the active bond seems to occur on the interaction of the monomer with isolated ions of the transition metal. That may be illustrated by the data showing that the activity of chromium oxide catalysts decreases linearly with decreasing chromium content (or even increases per chromium ion) to the rather low (0.01%) chromium concentrations on the catalyst surface (62, 69). In... 

In a very interesting and innovative study recently, the ultrasound-assisted microbial reduction of chromium [22], Mathur et al. reported the reduction of hexavalent chromium using Bacillus sp, isolated from tannery effluent contaminated site. The optimum reduction was found at pH 7 and 37°C. The percent reduction increased with an increase in biomass concentration and decreased with an increase in the initial concentration of hexavalent chromium. 

The kinetics of chromium (III) oxide deposition from solution during the leaching of skeletal copper has been studied, and a linear rate was found that is not affected by chromate concentration but decreases with increasing hydroxide concentration [55], The total amount deposited was greater as the chromate concentration increased. 

Additive or more-than-additive toxicity of free cyanide to aquatic fauna has been reported in combination with ammonia (Smith et al. 1979 Leduc et al. 1982 Alabaster et al. 1983 Leduc 1984) or arsenic (Leduc 1984). However, conflicting reports on the toxicity of mixtures of HCN with zinc or chromium (Towill et al. 1978 Smith et al. 1979 Leduc et al. 1982 Leduc 1984) require clarification. Formation of the nickelocyanide complex markedly reduces the toxicity of both cyanide and nickel at high concentrations in alkaline pH. At lower concentrations and acidic pH, solutions increase in toxicity by more than 1000-fold, owing to dissociation of the metallo-cyanide complex to form hydrogen cyanide (Towill et al. 1978). Mixtures of cyanide and ammonia may interfere with seaward migration of Atlantic salmon smolts under conditions of low dissolved oxygen (Alabaster et al. 1983). The 96-h toxicity of mixtures of sodium cyanide and nickel sulfate to fathead minnows is influenced by water alkalinity and pH. Toxicity decreased with increasing alkalinity and pH from 0.42 mg CN/L at 5 mg CaCOj/L and pH 6.5, to 1.4 mg CN/L at 70 mg CaCOj/L and pH 7.5 to 730 mg CN/L at 192 mg CaCOj/L and pH 8.0 (Doudoroff 1956). 

Gebhard and Killmann measured the thickness of the adsorbed layer and the adsorbance for poly(methyl methacrylate) adsorbed onto a high-vaccum vapored chromium plate and a platinum plate from theta solvents 1-chlorobutane and acetonitrile. Both quantities increased with rising molecular weight, but the average concentration of adsorbed poly(methyl methacrylate) decreased with molecular weight. The thickness of the adsorbed layer was not proportional to the square root of M. 

Autopsy studies in the United States indicate that chromium concentrations in the body are highest in kidney, liver, lung, aorta, heart, pancreas, and spleen at birth and tend to decrease with age. The levels in liver and kidney declined after the second decade of life. The aorta, heart, and spleen levels declined rapidly between the first 45 days of life and 10 years, with low levels persisting throughout life. The level in the lung declined early, but increased again from mid life to old age (Schroeder et al. 1962). 

It is interesting to note the effect of chromium content on reaction rate at high pressures (,—500 p.s.i.g.). Experiments (5) were carried out with normal air-activated catalysts (Figure 4). Catalysts were used with chromium contents ranging from 0.7 to 0.0005 wt. % of the total catalyst. Results of one-hour ethylene polymerization tests at 132°C. and 450 p.s.i.g. with these catalysts, activated at 500°C., are given. As the concentration of chromium was decreased, catalyst charge was increased to compensate for poisoning of catalyst sites by trace impurities and to keep total rate of production about constant. 

Molecular weight also changes with the concentration of chromium on the support (5). For example, the number average molecular weight increases about 40% as chromium concentration decreases from 0.75 to 0.001%. This increase is in line with the supposition put forth above in connection with increasing activity per atom of chromium with decreasing chromium content. 

Prepared iroin rayon several activated caiixm fibers (ACF) with difibient pore structure were used to remove Ci(VI) and/or Cr(lll) species fiom solutions. The a oiption experiments were carried out to determine the influence of ACF/solution contact time, pH, temperature, initial Cr(VI) and Ciflll) concentration on the efficiency of chromium ions removal by ACF. It was found that for ACF with total pore volume more than 0.4cm Vg the porous texture has no great importance for the amount of chromium retained. For all non-oxidized ACF the amounts of Cr species adsorbed and Cr(VI) reduced to Ciflll) after 48 h of ACF/solution contact are very close. At the beginning phase of ACF/solution contact the latent period of CtfVI) to Cr(III) reduction was observed. Oxidized ACF has lower adsorption capacity to Cr(VI) species and higher to Ctflll) ions in respect to non-oxidized ACF. The increase of initial CifVI) concentration increases the chromium species removal but the increase of pH and temperature decreases it. 

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