Adsorption chromium

As an example of the effect of solution pH on anion adsorption, chromium solutions at various pH levels are contacted with two amine functionalized silica gels. The results show that the extraction efficiency increases as the pH increases up to a maximum value for each extractant and then starts to decrease as shown in Figure 7.16. This behavior could be a result of (1) the protonation of amine groups on the multidonor atom ligand (see Figure 7.22 in Section 7.4), (2) preferential adsorption of multicharged anionic species, and (3) steric hinderance factors between the anionic species and the adjacent protonated amine donor atoms. However, sufficient information is not yet available at the present to analyze this behavior conclusively. 

Chromium(III) hydroxide, like aluminium hydroxide, possesses adsorptive power, and the use of ehromium compounds as mordants is due to this property. 

Ceilings, P. J. and de Jongh, M. A., Grain Boundary Oxidation and the Chromium-depletion Theory of Intercrystalline Corrosion of Austenitic Stainless Steels , Corros. Sc/., 7,413 (1967) Armijo, J. S., Impurity Adsorption and Intergranular Corrosion of Austenitic Stainless Steel in Boiling HNOj-KjCrjO, Solutions , Corros. Sci., 7, 143 (1967)... 

Early studies on oxide films stripped from iron showed the presence of chromium after inhibition in chromate solutionand of crystals of ferric phosphate after inhibition in phosphate solutions. More recently, radio-tracer studies using labelled anions have provided more detailed information on the uptake of anions. These measurements of irreversible uptake have shown that some inhibitive anions, e.g. chromateand phosphate are taken up to a considerable extent on the oxide film. However, other equally effective inhibitive anions, e.g. benzoate" pertechnetate and azelate , are taken up to a comparatively small extent. Anions may be adsorbed on the oxide surface by interactions similar to those described above in connection with adsorption on oxide-free metal surfaces. On the oxide surface there is the additional possibility that the adsorbed anions may undergo a process of ion exchange whereby... 

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

Another possibility is that carbene species are generated via the dissociative adsorption of ethylene onto two adjacent chromium sites [71]. A second ethylene molecule then forms an alkyl chain bridge between the two chromium sites this can subsequently propagate via either the Cossee or the Green-Rooney mechanism. 

Other aquatic weeds such as reed mat, mangrove (leaves), and water lily (Nymphaceae family plants) have been found to be promising biosorbents for chromium removal. The highest Cr(III) adsorption capacity was exhibited by reed mat (7.18 mg/g), whereas for Cr(VI), mangrove leaves showed maximum removal capacity (8.87 mg/g) followed by water lily (8.44 mg/g). It is interesting to mention that Cr(VI) was reduced to Cr(III), with the help of tannin, phenolic compounds, and other functional groups on the biosorbent, and subsequently adsorbed. Unlike the results discussed previously for the use of acidic treatments, in this case, such treatments significantly increased the Cr(VI) removal capacity of the biosorbents, whereas the alkali treatment reduced it.118... 

Gude, S.M. and Das, S.N., Adsorption of chromium(VI) from aqueous solutions by chemically treated water hyacinth Eichhornia crassipes, Indian Journal of Chemical Technology, 15 (1), 12-18, 2008. 

Toxic pollutants found in the mercury cell wastewater stream include mercury and some heavy metals like chromium and others stated in Table 22.8, some of them are corrosion products of reactions between chlorine and the plant materials of construction. Virtually, most of these pollutants are generally removed by sulfide precipitation followed by settling or filtration. Prior to treatment, sodium hydrosulfide is used to precipitate mercury sulfide, which is removed through filtration process in the wastewater stream. The tail gas scrubber water is often recycled as brine make-up water. Reduction, adsorption on activated carbon, ion exchange, and some chemical treatments are some of the processes employed in the treatment of wastewater in this cell. Sodium salts such as sodium bisulfite, sodium hydrosulfite, sodium sulfide, and sodium borohydride are also employed in the treatment of the wastewater in this cell28 (Figure 22.5). 

A method has been developed for differentiating hexavalent from trivalent chromium [33]. The metal is electrodeposited with mercury on pyrolytic graphite-coated tubular furnaces in the temperature range 1000-3000 °C, using a flow-through assembly. Both the hexa- and trivalent forms are deposited as the metal at pH 4.7 and a potential at -1.8 V against the standard calomel electrode, while at pH 4.7, but at -0.3 V, the hexavalent form is selectively reduced to the trivalent form and accumulated by adsorption. This method was applied to the analysis of chromium species in samples of different salinity, in conjunction with atomic absorption spectrophotometry. The limit of detection was 0.05 xg/l chromium and relative standard deviation from replicate measurements of 0.4 xg chromium (VI) was 13%. Matrix interference was largely overcome in this procedure. 

In groundwater, hexavalent chromium tends to be mobile due to the lack of solubility constraints and the low adsorption of CH6 anion species by metal oxides in neutral to alkaline waters (Calder 1988). Above pH 8.5, no CH6 adsorption occurs in groundwater Cr adsorption increases with decreasing pH. Trivalent chromium species tend to be relatively immobile in most groundwaters because of the precipitation of low-solubility Cr 3 compounds above pH 4 and high adsorption of the Cr+3 ion by soil clay below pH 4 (Calder 1988). 

Wehrli, B S. Ibric, and W. Stumm (1990), "Adsorption Kinetics of Vanadyl(IV) and Chromium(III) to Aluminum Oxide Evidence for a Two-step Mechanism , Colloids and Surfaces 51,77-88. 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

The result obtained for Af//°[Cr(CO)6, cr)] is some 50 kJ mol-1 more positive than the recommended value, -980.0 2.0 kJ mol-1 [149], a weighted mean of experimental results determined with several types of calorimeter. The large discrepancy is not due to an ill-assigned thermal decomposition reaction but to a slow adsorption of carbon monoxide by the chromium mirror that covered the vessel wall. This is an exothermic process and lowered the measured Ar//°(9.13). 

To avoid the formation of the metallic mirror and thus the adsorption process, Connor, Skinner, and Virmani used the microcalorimeter to examine the iodination of chromium hexacarbonyl at 514 K ... 

The most stable oxidation states of chromium in the subsurface environment are Cr(III) and Cr(VI), the latter being more toxic and more mobile. The oxidation of Cr(III) in subsurface aqueous solutions is possible in a medium characterized by the presence of Mn(IV) oxides. Eary and Rai (1987), however, state that the extent of Cr(III) oxidation may be limited by the adsorption of anionic Cr(VI) in acidic solutions and the adsorption and precipitation of various forms of Cr(OH). These authors also report a rapid quantitative stoichiometric reduction of aqueous Cr(VI) by aqueous Fe(ll), in a pH range covering the acidity variability in the subsurface even in oxygenated solutions. 

Sterling Jr MC, Bonner JS, Page CA, Ernest ANS, Autenrieth RL (2003) Partitioning of crude oil polycyclic aromatic hydrocarbons in aquatic systems. Environ Sci Technol 37 4429-4434 Stern O (1924) Zur theorie der elecktrolytischen doppelschict. Z Electrochem 30 508-516 Stollenwerk KC, Grove DB (1985) Adsorption and desorption of hexavalent chromium in an alluvial aquifer near Telluride, Colorado. J Environ Qual 14 150-155 Stumm W, Morgan JJ (1996) Aquatic chemistry, 3rd edn. WUey, New York... 

In the 19th century, various carbons were studied for their ability to decolorize solutions and adsorb compounds from gases and vapors. Commercial applications of activated carbon began early in the 20th century. Solutions containing phenols, acetic acid, herbicides, dyes, chlorophenols, cyanide and chromium have been successfully treated by carbon adsorption ( ). 

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