Chromate CrO

The Lyondell plant (ARCO Chemical Company) operates one of the first industrial scale chromate recovery systems in the U.S. The chromate recovery system is located in the ethylene plant of the Channelview, Texas, chemical complex. It was designed to treat a 1100 GPM cooling tower blowdown stream containing a 20-25 ppm chromate (CrO, ). A strongbase anion resin is used to selectively remove the chromate. This unit was placed in service in June, 1976. Information presented in this paper is based upon operating experiences. Economic considerations are also included. 

A specific type of electrostatic anion receptor constitutes the compounds capable of chromate (CrO -) or dichromate (Cr2Oj ) recognition. These anions are particularly important because of their toxicity and the presence of these pollutants in the environment (waste water and soils). The corresponding receptors are based on the protonated amines [68,69] (such as 54), protonated forms of similar compounds [70,71] or on the quaternary ammonium salts [72] of type 55. Thus, ammonium salt 54 (in the form of chloride) can transfer chro-... 

The two major forms of chromium(VI) in solution are yellow chromate, CrO r, and orange dichromate, Cr20. The latter predominates in acidic solution, as shown by the following reaction, the equilibrium of which is forced to the left by higher levels of H+ ... 

Chromium concentrations are generally below detection in anaerobic groundwaters that contain ferrous iron. This may reflect reduction of soluble chromate (CrO ") to insoluble chromic ion (Cr(III)) by Fe in a coupled redox reaction that can be written... 

Carbonates (COf ), phosphates (PO ), chromates (CrO ), sulfides (S -) Compounds containing alkali metal ions and the ammonium ion... 

Passivators are inorganic substances possessing oxidative properties whose reaction products with metals form a passive film on the substrate surface, which shifts the corrosion potential of the substrate to the positive side by a few tens of volts. Like a depolarizer, the passivator generates a current on the anodic areas of the substrate of 1 > i density, where i is the critical density of the passivation current. This means that the chemical composition of the passivating film on a metal substrate is the same whether the substrate is passivated by anodic polarization in an acid or is treated with solutions of chromates (CrO ), nitrates (NO ), molybdates (MoO ), tungstates (WO ), ferrates (FeO ) or pertechnates (TcOj). 

While a portion of chromates (CrO ", HCrOj, and Cr207") is reduced by the advection of the EO flow that contains Fe°/Fe ions from the corrosion of ZVI in the barrier, most chromates do directly migrate into the PRB and react with the ZVI. The subsequent reaction products, Cr(III) and Fe(III), will migrate toward the cathode or reside in the soil pore solution or in the barrier to form precipitates of Cr(III)-Fe(III) oxyhydroxides or hydroxides. Figure 23.8 shows the Fe filings... 

In the enviromnent, Cr(VI) is either in the chromate (CrO ) or dichromate (Cr207 ) form. Whether Cr(VI) is predominantly one or the other strongly depends on the pH of the aqueous solution within which it is dissolved. The following ionic equilibrium is established between the two ... 

All sulfites (SO2 ), carbonates (CO3 ), chromates (CrO, and phosphates (P0 ") are insoluble, with the exceptions of those that are made with NH, or the alkali metals. 

The effect of the addition of citric acid on the photocatalytic reduction of hazardous Cr(VI) to less hazardous Cr(III)) with titania catalysts has been studied by means of in situ EPR of chromium species by Meichtry et al. [52] using titania P25 as photocatalyst. Reduction experiments of Cr(VI) solution were performed under near-UV (366 nm) irradiation under acidic conditions (pH 2) with bubbling air. It is found that the addition of citric acid fadhtates Cr(VI) reduction with a stepwise reduction of the chromate CrO/ (V)) via formation of Cr(V) and Cr( IV) and finally Cr(III) spedes observed. In the absence of dtric add, a cycHng between the different valence states of chromium occurs because of reduction and reoxidation processes by OH radicals. The maximum rate (fivefold increase) of Cr(VI) reduction is achieved at an initial citric add/Cr(VI) molar ratio of 1.25. Citric acid is oxidized to its anionic radical by electron abstraction of surface-trapped holes Cit + h+ j, -> Cit . 

Recently, a Raman probe has been designed for a cone penetrometer for studying underground storage tanks at the Department of Energy site in Hanford [143]. These tanks contain a mixture of chemicals and radioactive waste and, with in-tank characterization of such mixtures, significant reduction in personnel exposure, analysis time, and cost is achieved relative to laboratory analysis. RS has been shown to be well suited for detecting many of the chemicals contain within the tanks [43,144,145]. For example, Marston have detected and identified nitrite (NO2), nitrate (N03 ), sulfate (SO4 ), chromate (CrO ),... 

The thermodynamically favoured metal species at a contaminated site depends on the pH and the major element chemistry of the site (Salomons 1995). Generally, low pH conditions favour the mobility of metal cations whereas neutral to high pH conditions favour the solubility of oxyanions such as chromate (CrO "),... 

Chromium is known in all oxidation state between 0 and VI, but is commonly found in oxidation states III and VI. Cr(III) species in acidic solution exist as Cr(H20)g ions and in concentrated alkali have been identified as Cr(OH)g" and Cr(0H)5(H20) . These ions are regular octahedral. Cr(VI) in basic solution above pH 6 exists as tetrahedral yellow chromate, CrO ion. Between pH 2 and 6, HCrOj and the orange-red dichromate, Cr207, are in equilibrium. Under strongly acid conditions, only dichromate ion exists. Addition of alkali to dichromate gives chromate. 

---