The effects of pH on leaching of copper-chrome-arsenate (CCA) from pressure-treated Kenyan-grown Eucalyptus saligna and Acacia mearnsii: Initial findings.

IRG/WP 02-30298

R Venkatasamy

The effects of pH on leaching of CCA from pressure-treated Kenyan-grown Eucalyptus saligna and Acacia mearnsii were tested under laboratory conditions. Small samples of the two species (100mm x 30mm x 40mm) were smooth sawn from 8-year old trees to represent equal amounts of both sapwood and heartwood, air-dried to 12% moisture content, end-sealed, and pressure-treated at a commercial treatment plant (6% CCA-C oxide type). They were then conditioned and air-dried to 12% moisture content and average retentions for each species calculated, (18.2Kg/M3 for eucalyptus and 17.3Kg/M3 for acacia). Batches of samples of each species were then separately leached for 3, 6, 9, 12, 15, 18, and 21 days in tap water, with the pH adjusted to 3.0, 6.0, and 8.0. After each leaching regime samples were removed, oven-dried to 12% moisture content, and the amount of CCA leached out calculated on a weight loss basis. The results revealed that, after 21 days, leaching of CCA from the two species was highest at pH 3.0 (9.8Kg/M3 or 53.8% for E. saligna; 10.2Kg/M3 or 58.4% for A. mearnsii). Lower amounts of CCA were leached at pH 8.0 after 21 days, 5.5Kg/M3 or 30.2% for E. saligna and 6.0Kg/M3 or 34.1% for A. mearnsii. Leaching after 21 days was lowest at pH 6.0, 4.4Kg/M3 or 24.2% for E.saligna, and 3.0Kg/M3 or 17.3% for A. mearnsii. For each of the pH level tested, both species appeared to lose more CCA during the firsts 12 days of leaching, amounts being then lower up to 21 days. The results indicated that CCA-treated poles and posts of E. saligna and A. mearnsii are likely to suffer from excessive preservative depletion, especially when used in acidic soils, leading to short service lives. Further investigations are necessary to establish whether depletion of CCA from these two species may be due to high lignin and tannin contents, or inappropriate treatment techniques.

Keywords: Leaching, pH, CCA, Acacia mearnsii, Eucalyptus saligna, service life

Conference: 02-05-12/17 Cardiff, Wales, UK

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