Chromium hexavalent, chemistry

The simultaneous extraction of trivalent chromium, hexavalent chromium, and zinc from cooling lower blowdown water was studied in detail by Fuller and Li. They investigated Alamine 336, an oil-solaUe tertiary amine, and Aliquat 336-S, an oil-soluble quaternary amine salt, both manufactured by General Mills, as carriers in various liquid-membrane systems. Alamine was found to be an effective extractant for Cr , Ct, and Zn. However, for Cr and Cr, a pH of 3.0 or less in the external (feed) phase was required while Zn could be extracted only at a pH > 7. Thus, a combined process could not be developed. Aliquat 336-S was more promising. The chemistry of extraction with this carrier involves the following steps ... 

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... 

Figure 4.9. On-line coupling of USAL and detection with (B) and without (A) an intermediate preconcentration-clean-up step for the determination of hexavalent chromium in soil. AS — acid stream, D — debubbler, DPC — diphenylcarbazide, DS — detection system, FC — flow cell, IV — injection valve, L — leachant, LC — leaching chamber, MC — mixing coil, PL — propagating liquid, PP — peristaltic pump, PC — reaction coil, SAEC — strong anion-exchange column, S / — switching valve, UP— ultrasonic probe, 1/1/— waste and 1/1/6 — water bath. (Reproduced with permission of the Royal Society of Chemistry, Ref [10].)...

Second generation Phillips catalysts involve use of titanium compounds that modify the surface chemistry of the support and enables production of polyethylene with higher MI (lower MW) (12). Titanium tetraisopropoxide, also known as tetraisopropyl titanate (TIPT), is the most commonly used modifier for these catalysts. Hexavalent chromium titanate species are probably formed on the surface as shown in Figure 5.3 (13). Catalyst surfaces contain a diversity of active sites and molecular weight distribution of the polymer is broader than that from generation catalysts. 

Chromium wastes are commonly treated in a two-stage batch process. The primary stage is used to reduce the highly toxic hexavalent chromium to the less toxic trivalent chromium. There are several ways to reduce the hexavalent chrome to trivalent chrome including the use of sulfur dioxide, bisulfite, or ferrous sulfate. The trivalent chrome is then removed by hydroxide precipitation. Most processes use caustic soda (NaOH) to precipitate chromium hydroxide. Hydrated lime [Ca(OH)2] may also be used. The chemistry of the reactions is described as follows ... 

Kulkami, P. S., Kalyani, V., Mahajani, V. V. (2007). Removal of hexavalent chromium by membrane-based hybrid processes. Industrial and Engineering Chemistry Research 46 8176-8182. 

Chemistry is used for many aspects of wastewater treatment. Lime or caustic soda can be used to raise the pH, while sulfuric acid lowers it. Cyanide can be destroyed by introducing chlorine, as a gas or in liquid from (with sodium hypochlorite at a pH of 11) [9]. When enough chlorine is added the cyanide is converted to carbon dioxide and nitrogen. Hexavalent chromium can be reduced to trivalent chromium by using SO2 gas or SO2 releasing chemicals [9]. 

The most commonly used types of conversion coatings are phosphates and chromates or a process based upon a combination of these two. More recent developments have been in zirconium- and titanium-based chemistry along with cerium-based processes. The impetus behind the more modem conversion coatings is the replacement of hexavalent chromium, which is used in many conventional processes. Conversion coatings have been applied to most metals the most amenable metals along with a number of applications are given in Tables 1 and 2. 

M. Hirano, M. Murakami, M. Sugano, and H. Sango, Removal of Hexavalent Chromium from Aqueous Sample by Calcium Alumino-zincate, Proceedings of 13th International Congress on the Chemistry of Cement, No.91, 2011. 

In organic chemistry, dilute solutions of hexavalent chromium can be used to oxidize primary or secondary alcohols to the corresponding aldehydes and ketones. Tertiary alcohol groups are unaffected. Because of the oxidation is signaled by a color change from orange to a blue-green, chromic acid is used as a qualitative analytical test for the presence of primary or secondary alcohols. 

Lee, D.G. (1980). The Oxidation of Organic Chemistry by Permanganate Ion and Hexavalent Chromium, Open Court, La Salla, lllionois. 

---