Chromium preservation

Wood Preservation. The use of chromium compounds ia wood preservation is largely because of the excellent results achieved by chromated copper arsenate (CCA), available ia three modifications under a variety of trade names. The treated wood (qv) is free from bleeding, has an attractive ohve-green color, and is paintable. CCA is widely used, especially ia treating utility poles, bull ding lumber, and wood foundations. About 62% of all the chromic acid produced ia the United States is consumed by the wood preservation industry (77,167) (see Building materials, survey). 

Chromium-containing wood preservatives and their chemical compositions are Hsted ia Table 13 (199). Chromium compounds have a triple function ia wood preservation (200). Most importantiy, after impregnation of the wood the Cr(VI) compounds used ia the formulations react with the wood extractives and the other preservative salts to produce relatively insoluble complexes from which preservative leaches only very slowly. This mechanism has been studied in the laboratory (201—206) and the field (207). Finally, although most of the chromium is reduced to chromium (ITT), there is probably some slight contribution of the chromium (VT) to the preservative value (208). 

Zinc in contact with wood Zinc is not generally affected by contact with seasoned wood, but oak and, more particularly, western red cedar can prove corrosive, and waters from these timbers should not drain onto zinc surfaces. Exudations from knots in unseasoned soft woods can also affect zinc while the timber is drying out. Care should be exercised when using zinc or galvanised steel in contact with preservative or fire-retardant-treated timber. Solvent-based preservatives are normally not corrosive to zinc but water-based preservatives, such as salt formulated copper-chrome-arsenic (CCA), can accelerate the rate of corrosion of zinc under moist conditions. Such preservatives are formulated from copper sulphate and sodium dichromate and when the copper chromium and arsenic are absorbed into the timber sodium sulphate remains free and under moist conditions provides an electrolyte for corrosion of the zinc. Flame retardants are frequently based on halogens which are hygroscopic and can be aggressive to zinc (see also Section 18.10). 

The broad spectrum of the raw goods occurring in the leather and fur industry [95] necessitates various wet treatment processes in which surfactants and emulsifiers play a big role, e.g., in the regeneration of raw goods, which are preserved with salt, or by drying short-chain sulfosuccinates. To achieve hydro-phobizing effects, sulfosuccinate as emulsifiers are fixed on the surface by salts of aluminum or chromium. 

Bardgett, R.D., T.W. Speir, DJ. Ross, G.W. Yeates, and H.A. Kettles. 1994. Impact of pasture contamination by copper, chromium, and arsenic timber preservative on soil microbial properties and nematodes. Biol. Fertil. Soils 18 71-79. 

Although CCA is an exceedingly effective preservative in service, attention has increasingly been focused on the fate of CCA when the treated timber products are disposed of. This has led to concerns especially regarding the ultimate release of arsenic and chromium into the biosphere. 

Cox, T.R.G. and Richardson, B.A. (1978). Chromium in wood preservation health and environmental aspects. International Research Group on Wood Preservation, Doc. No. IRG/WP 3120. 

Kazi, E.K.M. and Cooper, P.A. (2002). Rapid extraction oxidation process to recover and reuse copper chromium and arsenic from industrial wood preservative sludge. Waste Management, 22(3), 293-301. 

The chemical or physical form of trace metals in water is often of interest. The form in which a specific element is present will often influence is toxic effects. For instance the chemical state of chromium affects its toxicity i.e., Cr+6 is more carcino genic than Cr+3, Kopp (48) has described the various forms in which metals may he present. The categories include dissolved metals, suspended metals, total metals, extractable metals and organometallics. In addition, Kopp describes sample preparation requirements for each category. Gihhs (20) has also studied metal species in river water. It should be obvious that the desired analytical result has to he considered beforehand. For example, if dissolved metal concentrations were desired and normal acid preservation performed, suspended metals could possibly be solubilized to a large extent. Both Hamilton (25) and Robertson (81) have shown vast differences between acidified and non-acidified samples. Many other publications have dealt with this subject (16, 37, 80, 30). 

Imagine that a friend has asked you to review the title and abstract of a paper that is being written with other researchers on chromated copper arsenate (CCA), a compound used to preserve wood. The research team examined the chemical structure of arsenic (As) and chromium (Cr) in CCA to determine if the oxidation state of As and Cr changed over time due to weathering. 

At the Coast Wood Preserving, Inc., Superfund site (Ukiah, California), the technology was used to remove metal contamination to comply with both state and federal cleanup standards [50 parts per billion (ppb) arsenic, 50 ppb chromium, and 1 ppm copper]. The estimated total cost for the source control component of the remedy was 1,000,000, and the estimated total operational and maintenance costs was estimated to be 19,500 for a 20-year period (D16888B, p. 3, Report Documentation p. 2). 

The Lewis ENVIRO-CLEAN process removes and recovers metals such as chromium, copper, nickel, mercury, lead, zinc, iron, and cadmium and has effectively demonstrated that it can treat a matrix of multiple metals in a single stream with positive results. The process treats wastes from wood preserving, metal finishing, mining, surface and groundwaters. The two-step process uses granular-activated carbon and electrolytic metal recovery to yield a salable metallic by-product. 

The rate constants for hydrogen abstraction from Rh H, O H, and C-H bonds by chromyl ions and Craq002+ are summarized in Table VI. Also listed in the table are selected relative rate constants for hydrogen abstraction by tert-butoxyl and tert-butylperoxyl radicals, expressed as .buo/AbuOO- The difference between the two sets of data is striking in that alkoxyl radicals are 105-107 times more reactive than alkylperoxyl radicals, but in the chromium series the ratio kcrolkcrOO is only about 102 for all the reactions studied. This ratio is preserved over about 103-fold change in absolute rate constants within each series. 

High-carbon austenitic structures can be preserved at ambient temperatures if the iron is alloyed with sufficient nickel or manganese, since these metals form solid solutions with 7-Fe but not with a-Fe. If over 11% chromium is also present, we have a typical austenitic stainless steel. Such steels are corrosion resistant, nonmagnetic, and of satisfactory hardness, but, because the a-Fe 7-Fe transition is no longer possible, they cannot be hardened further by heat treatment. Figure 5.9 summarizes these observations. 

Chromium copper arsenate is a commonly used preservative for softwood. To a lesser extent, ammoniacal copper arsenate is also used. 

Chromium triphenyl iodide, (CeHg CrlXgHg.O.CaHg, is obtained by the interaction of potassium iodide or hydriodic acid and an aqueous solution of the base in the presence of chloroform. The aqueous layer is discarded, the chloroform solution dried, the bulk of the solvent removed at 40° to 50° C. in vacuo and the residue added dropwise to eight or ten volumes of dry ether. The iodide separates as an oil which gradually solidifies. The salt is hygroscopic, can be preserved for some time over concentrated sulphuric acid in a vacuum, but is unstable in air. It dissolves more readily in hot than cold water, and i also soluble... 

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