Zinc arsenate, wood preservative

Substantial arsenic may leach into adjacent soils from wooden structures with CCA or other arsenic-bearing preservatives. Townsend et al. (2003) has shown that 65 surface soil samples from underneath nine CCA-treated wood structures in Florida, USA, contain 1.18-217 mg kg-1 of arsenic when compared with control soils that average only 1.36 mg kg-1 (Table 3.20). In a study involving ammoniacal copper zinc arsenate wood preservatives, arsenic concentrations generally fell to background levels within 150-300 mm away from 19 utility poles in Virginia, New York, and Florida, USA ((Morrell, Keefe and Baileys, 2003) Table 3.20). The environmental effects of arsenic-bearing wood preservatives are further discussed in Chapters 5 and 7. 

Water-borne preservatives are divided into two categories. One group which includes acid copper chromate, chromated zinc chloride, copperized chromated zinc arsenate and fluorchrome-arsenate-phenol is used where the wood is not subjected to excessive leaching. The second group, ammoniacal copper arseuite and three types of chromated copper arsenate which react to become practically water insoluble, are used at about 0.6 lb/ft3 (9.6 kg/m3) when wood is placed in ground contact under severe service conditions. 

Inorganic arsenicals find limited use as pesticides. Examples include arsenious oxide in cattle dips, zinc arsenite as a wood preservative, and calcium and lead arsenates and Paris green (double salt of copper arsenite and copper acetate) as insecticides30. ... 

Metals are major pollutants only at wood preserving plants that treat with water-borne salts of copper, chromium, arsenic, and zinc. 

Estimated end-use distribution of arsenic in the United States was 70% (16,000 tons as elemental arsenic) in industrial chemicals (mainly as wood preservatives), 22% (5200 tons) in agricultural chemicals (mainly as herbicides and desiccants), 4% (900 tons) in glass manufacture, 3% (700 tons) in nonferrous alloys and 1% (300 tons) for other purposes (animal feed additives, pharmaceuticals etc.) in 1989. Estimated end-use of arsenic in Japan was 35% (230 tons) for refining of zinc, 34% (220 tons) for glass manufacture, 15% (100tons) for electronics as ultrapure arsenic metal and 15% (lOOtons) for wood preservatives and agricultural chemicals in 1988. 

Inorganic Salts. A number of the metal salts have fungicidal activity and are used to formulate commercial wood preservatives. The principal metal salts used are compounds of arsenic, chromium, copper, and zinc. In order to provide the desired fungicidal activity, leach resistance, and low corrosivity, combinations of these compounds are used. All of the formulations discussed later are waterborne solutions. Only a brief description of these preservative systems will be presented here more detailed presentations can be found elsewhere (3). 

Lebow ST and Morrell JJ (1995) Interactions of ammoniacal copper zinc arsenate (ACZA) with Douglas fir. Wood and Fiber Science, 27(2) 105-18 Lebow ST and Tippie M (2001) Guide for minimizing the effect of preservative-treated wood on sensitive environments. USDA, Forest Service, Forest Products Laboratory, General Technical Report FPL-GTR-122... 

Chromium copper arsenate as a wood preservative (uses 62% of the chromic acid in the United States) Catalyst for polymerization of ethylene Copper chromite catalyst for hydrogenation Zinc chromate near the zinc anode gives batteries 50-80% more shelf life... 

Historically, almost every chemical or compound known to be toxic has been tried and used to inhibit the action of wood-destroying pests. Among the many chemicals used as wood preservatives are compounds based on creosote, mercury, arsenic, thallium, borate, cyanide, chromium, copper, zinc, nickel, fluorides, and pentachlorophenol. The majority fall into 3 classes ... 

For this reason, heavy metals and metal ions that are used as additives in plastics and rubber (as colorants, stabilisers, plasticisers and so on) should be monitored carefully, and their use as well as the amounts used should be well known and regulated. In addition to their existence in some of the additives used in plastics and rubbers, toxic heavy metals most of which are considered chronic poisons, such as arsenic, lead, mercury, cadmium, nickel, zinc and chromium, are frequently encountered in industrial processing and other manufacturing operations (their main industrial sources include paint, ink, plastic, rubber and plastic film production, leather tanning, wood preserving, battery manufacturing, and so on). 

Both inorganic as well as organo-metallic compounds pi, 19.] have been extensively used in wood preservation. Most commonly products based on arsenic, chromium, copper, tin and zinc have been included. 

Zinc metaarsenite, Zn(As02)2, is used under the commercial name ZMA as an insecticide and wood preservative. It may be detected and identified by the positive responses to the tests for arsenic i and zinc. This material is not soluble in water. The test described in the previous section for the... 

Waterborne Salts. Waterborne salts for wood preservation include zinc chloride, chro-mated zinc chloride, copperized chromated zinc chloride, zinc meta arsenite, chromated zinc arsenate, chromated copper arsenate, ammoniacal copper arsenite, acid copper chromate, and fluor chrome arsenate phenol. These chemicals leave little odor and have little effect on the appearance of the wood, which may also be painted. The zinc chloride salts, at high penetrations, also provide fire retardance.Wood should be reseasoned before use because this type of chemical treatment injects a large amount of water into the wood. 

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). 

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