Chromate, adsorption rate

Fig. 7 presents partial results of dynamic regime experiments for chromate adsorption and desorption by ODA-clinoptilolite. As shown by breakthrough curves, ODA-clinoptilolite column quantitatively removes chromate species from simulated waste water , apparently more efficiently by lower flow rate. Consequently to similar configuration of chromate and sulfate molecules, such loaded column was more efficient to regenerate with Na2S04 than NaCl solution, as elution curves at the Fig. 7 illustrate. 

Table III. Rate Constants of Adsorption/Desorption of the Phosphate and Chromate...

The role of chemisorption in the mechanism of passivity is borne out by the typical patterns of data, having the same shape as adsorption isotherms, which describe concentration of radioactive chromium on the surface of iron passivated by chromates (11), or by potential changes induced by surface concentration of chromates (12), both as a function of chromate concentration in solution (Figure 1). It is also illustrated by the initially rapid rate, followed by a measurably slow rate, with which metals achieve passivity as followed by potential change with time for iron immersed in chromates or by 18-8 immersed in aerated water (IS) (Figure 2), and by... 

For example, the corrosion rate of steel in 0.01 % Na2Cr207 is <0.0001-in. penetration per year (i.p.y.), but on addition of NaCl to make a 3.5% solution, the rate increases to 0.0017 i.p.y., which is still a low rate. In absence of dichromate, the rate is 0.024 i.p.y. Halide ions catalyze reduction of dichromates probably by introducing imperfections in the adsorbed film (through competitive adsorption), at which areas metal ions not only enter solution but also HjO+ can discharge and hydrogen atoms can adsorb. It is probably the adsorbed hydrogen atoms that reduce adsorbed chromate, or dichromate. The situation is similar to that described by Langmuir... 

The adsorption of an inhibitor onto the metal surface slows the rate of corrosion by blocking part of the surface. The extent of inhibition depends on the equilibrium between the dissolved and adsorbed inhibitor species, expressed by the adsorption isotherm. This mechanism which is particularly important in acids will be discussed in the next section. (Sect. 12.4.2). Certain inhibitors promote the spontaneous passivation of a metal and thus drastically reduce the corrosion rate. Oxidizing species such as chromates fall in this category. Buffer agents that maintain a high pH at the metal surface also favor the passive state. Other inhibitors lead to the formation of surface films by precipitation of mineral salts or of weakly soluble organic complexes. These films reduce the ability of oxygen to reach the surface and, in addition, they may impede the anodic dissolution reaction. 

Like molybdates and other oxyanion analogues of chromates, the inhibitive action of monovanadate anions are attributed to their competitive adsorption on the metal surface, the formation of an adsorbed layer on the oxide film and the formation of a highly insoluble salt with dissolved metal ions, which prevents the penetration of CL ions and consequently decreases the rate of corrosion. ... 

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