AbstractThe performance of CCA and ACA treated wood in soil contact has been investigated with particular reference to the effect of high concentrations of nitrogen on the decay process, fixation and leachability of preservative elements and the role of lignin in the binding of nitrogen and preservative elements to wood.
Concentrations of surface nutrients, including nitrogen, present in CCA treated small blocks of both the hardwood lime (Tllia vulgaris, Hayne) and the softwood pine (Pinus sylvestris, L) exposed to soil, increased the toxic thresholds of CCA and accelerated nitrogen transfer to the wood. Wood blocks treated with low concentrations of CCA showed significant losses of preservative elements and the presence of surface nutrients increased the percentage losses observed.
The effect of air-drying, after two weeks of curing at high relative humidity, in minimising the leachability of CCA from treated wood blocks of lime and pine was investigated. Percentage losses of both copper and chromium, during aqueous leaching, were very similar from undried and air-dried blocks.
The leachability of preservative elements from CCA treated lime and pine blocks leached in distilled water, aqueous soil extract or a bacterial suspension in an aqueous soil extract were compared. Neither soluble components of soil, nor large numbers of soil bacteria caused significant solubilisation of CCA.
ACA treated lime, pine and spruce (Picea sitchensis, Carr) blocks contained elevated levels of nitrogen and ammonium nitrogen, relative to untreated blocks, both before and after aqueous leaching and nitrogen contents of blocks increased with increasing ACA treating concentration. During aqueous leaching, approximately 20% of the copper was lost from blocks at all ACA treating concentrations.
Toxic thresholds of the fungicide copper in ACA treated lime, pine and spruce blocks were determined in a largescale soil burial experiment. The toxic thresholds for ACA treated lime and pine blocks were higher than those for similar blocks treated with CGA, previously exposed to soil. Considerable nitrogen inputs from soil to ACA treated wood blocks occurred during the decay process. Losses of copper from unleached ACA treated blocks exposed to soil exceeded those observed during aqueous leaching.
The lignin nitrogen contents of untreated and ACA treated pine blocks increased during soil burial. This accumulation of nitrogen on lignin was associated with an increase in wood nitrogen content and with decay.
Aqueous leaching of CCA treated normal, holocellulose and periodate lignin blocks of lime and pine showed that the majority of each preservative element was resistant to leaching from both the lignin and polysaccharide fractions of wood. Percentage losses of preservative elements were higher from holocellulose than from lignin.
The implications of the findings of these studies in relation to the performance of CCA and ACA treated timber in service in soil contact are discussed.
|Date of Award||Mar 1987|