Chromated Copper Arsenate-Treated Wood Research Articles

Kinetics of the low-temperature pyrolysis of chromated copper arsenate-treated wood

In this paper, a kinetic evaluation of non-isothermal thermogravimetric (TG) data is carried out by the method of least squares for both untreated and chromated copper arsenate (CCA)-treated wood. The type of wood used is Pinus sylvestris and the slow-heating decomposition in an inert (nitrogen) atmosphere (low-temperature pyrolysis) is studied. In the case of untreated wood, best fit parameters are determined for the three independent parallel reactions scheme, proposed by several other researchers. For CCA treated wood a comparative study is performed, in which five different kinetic models are considered. These are based on the assumption of: (1) a single reaction; (2) independent parallel reactions; (3) competitive reactions; (4) a chain of successive reactions and (5) a combination of two independent parallel reactions and one subsequent reaction. The choice between which of these models offer the best fit, is based partly on chemical considerations and partly on the quality of the fit. Altogether, the DTG curve of CCA treated wood is well described by assuming two independent parallel reactions at low temperature, which may be catalyzed by the metals, followed by the successive decomposition of the residue. A comparison is made with the kinetic parameters derived for untreated wood, resulting in the following observations and conclusions drawn therefrom: hemicellulose decomposition is catalyzed by the metals (lower activation energy, pre-exponential factor and peak temperature and higher contribution to volatiles production); cellulose decomposition starts once the major part of chromium(III) arsenate is decomposed and volatilised (higher activation energy) and may then even be catalyzed by the metals (higher pre-exponential factor) and lignin decomposes very slowly at higher temperatures together with intermediate products from hemicellulose and cellulose decomposition (low pre-exponential factor and activation energy). During this slow charring process the metals act no more as catalysts. This hypothesis is confirmed by a TG analysis, performed on a chromium(III) arsenate precipitate, which is the major arsenic compound present in CCA treated wood, and by a study concerning the microdistribution of copper, chromium and arsenic in the pyrolysis residue of CCA treated wood.

Accumulation of metals in consumers associated with chromated copper arsenate-treated wood panels

Metals leached from chromated copper arsenate (CCA)-treated wood, taken up by epibiota and trophically transferred to their motile consumers or taken up directly by the consumers, were measured in caged organisms exposed to treated and untreated wood panels for 3 months. Epibiota on treated panels had more copper and arsenic than epibiota on untreated panels, and amphipods living on the former had elevated copper. However, metal concentrations in grass shrimp ( Palaemonetes pugio) and two teleost fish, the naked goby ( Gobiosoma bosci) and mummichog ( Fundulus heteroclitus), were unaffected by CCA-treated wood exposure. In addition, there was no evidence of biomagnification in the consumers other than the amphipods. Thus, trophic transfer was not demonstrated to the consumers, suggesting that the treated wood was not presenting a hazard to higher trophic levels. Fish may have more efficient mechanisms for regulating metal levels in their tissues. ©

Evaluating the Environmental Risks Associated with the Use of Chromated Copper Arsenate-Treated Wood Products in Aquatic Environments

Chromated copper arsenate (CCA)-treated wood has been used worldwide since 1933. In properly fixed wood, the chromium, copper, and arsenic are tightly bound to the wood fibers. However, it has long been known that small amounts of these metals do leach from properly treated wood products. These metals, particularly the cupric ion (Cu2+), are known to be highly toxic to aquatic organisms at concentrations as low as 6 μg Cu l−1. The United States Environmental Protection Agency has established a marine water quality standard of 2.9 μg Cu l−1 total copper. Washington State has established a marine water quality standard of 2.5 μg Cu l−1 and a marine sediment standard of 390 mg Cu g−1 (dry sediment). Comparison of these standards with the results of bioassays suggests that these levels are sufficiently low to protect even the most sensitive organisms. Metal losses are an inverse function of time and decline to background levels within 60–90 d of treated wood immersion. Increasing use of CCA-treated wood to construct residential bulkheads in estuarine environments on the east coast of the United States has raised concerns regarding the near-term environmental risks associated with these uses. This paper reports the development of a spreadsheet-based computer model to predict water column and sediment concentrations of copper leached from bulkheads and piling. Model output is compared with data in Weis et al. (1991, 1993) and Weis and Weis (1992). The model predicts environmental levels of copper that are highly dependent on input parameters but below regulatory levels for most projects constructed in reasonably well-circulated bodies of water. The model does demonstrate the possibility of exceeding regulatory standards for up to 3 wk when bulkheads are installed in very poorly flushed aquatic environments or where the surface area of the CCA-treated wood structure is a significant proportion of the water body surface area.