Chromium species adsorption

Table 2 presents the abatement of Cr(Vl) concentration and the rate of all chromium species adsorptions to wit - the adsorption capacity (AC) of different ACF, when fig.l show the growth of Cr(lll) concentration. In all cases the initial pH was 4.7, the final - 6.4. The experiment carried out showed that for ACF-1, ACF-2, ACF-3 pore structure and the value of surface area has no great importance. 

Chromium compounds are widely used in many industries metal finishing and electroplating, leather tanning, pigments manufacturing, photography and catalysts production [1]. The presence of chromium species in wastewater of all of these industries is a problem because of the affect onto die human physiolo. Chromium removal from wastewaters by adsorption onto activated charcoals is an important process in the environmental protection [2]. On the other hand the chromium species adsorption from aqueous solution is one of the processes for chromium catalysts supported on activated c ons production [3],... 

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). 

CHROMIUM (VI) AND (111) SPECIES ADSORPTION FROM AQUEOUS SOLUTIONS BY ACTIVATED CARBON FIBERS... 

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. 

On the other hand in the case of ACF-4 when the total pore volume is around 0.3 cmVg and Sbei=500 - 600 mVg during the initial phase of adsorption (time less then 2 h) the AC decreases twice. At the same time for all non oxidized ACF the final (reached after 48 h) amounts of chromium species adsorbed and Cr (VI) reduced to Cr(lll) are very close. Oxidized sample ACF-2ox, possessing the similar texture as ACF-3, has notably lower adsorption and reduction capacity in respect to non-oxidized sorbents. 

Figure 1. Chromium (lU) concentration variation as a function dtime. Initial pH - 4.7 temperatuie 20 "C. Table 3 Chromium (VI) species adsorption at initial pH - 2,5 as a function of time.

The second step in the development of polymerization activity is an alkylation reaction in which the first chain begins growing on the reduced chromium species. Exactly how this reaction occurs has been unclear. Several possibilities have been snggested over the years, but no clear answer has yet been established (25,31-35). Recent studies suggest that initial adsorption of olefin on Cr(II) sites may cause an oxidation to an alkylated CrdV) site as the active polymerization... 

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. 

G. Micera and A. Dessi, Chromium adsorption by plant roots and formation of long-lived species An ecological hazard /. Inorg. Biochem., 34 (1988) 157-166. 

Nitrogen dioxide, N02, is a fairly small molecule with an unpaired electron and may be expected to be a selective molecule for electron-deficient or Lewis acid sites. Nevertheless, only very little spectroscopic information on the nature of surface species formed on adsorption of N02 is available. Naccache and Ben Taarit (242) have shown by infrared spectroscopy and ESR that N02 forms Cr+N02+ and Ni+N02+ complexes on chromium and nickel zeolites. Thus, N02 behaves as an electron donor and reducing agent in these zeolites. Boehm (243) has studied the adsorption of N02 on anatase and on tj-A1203, which were pretreated at very low temperatures of only 100°-150°C. At 1380 cm-1, a band which is to be attributed to a free nitrate ion, was observed. Boehm (243) has explained the formation of the nitrate ion by the disproportionation by basic OH ions ... 

The inverse relationship between the amount of chromium ions removed and the temperature was found. This fact can be explained in the consideration that the amount of Cr(VI) reduced to Cr(lll) increases with the increase of the temperature and that the adsorption of Cr(lll) ions on ACF-1 is relatively low. So, more the Cr(lll) ions appears in the solution during ACF and Cr(Vl) species contact less the total amount of chromium removed. 

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