Evidence is accumulating as to poor distribution and fixation of CCA in tropical hardwoods, and there is therefore a necessity to investigate the permanency of CCA in tropical hardwoods. The relationship between soil pH and leaching of Cu, Cr and As from CCA pressure-treated sapwood of Kenyan-grown Eucalyptus saligna was tested under laboratory conditions. Small sapwood samples were pressure-treated with 6% CCA-C (oxide type) to a retention of 25.4 Kg/m3, and leached with mild agitation in soil-extract water representative of soil pH 3.5, 4.5, 5.0, 7.5, 8.5, and in tap water (pH 6.8), for 3, 6, 9, 12, 15 and 18 days under controlled laboratory conditions. The results revealed that leaching of Cu, Cr and As was generally high under acidic conditions, low under alkaline situations and lowest at neutral pH. After leaching for 18 days at pH 3.5, average extraction of Cu, Cr and As was respectively 60.8 ppm, 51.3 ppm and 53.8 ppm. Leaching at pH 6.8 extracted low average amounts of Cu, Cr and As, being 0.15 ppm, 0.25 ppm and 0.42 ppm respectively. At pH 8.5 losses were Cu: 1.9 ppm, Cr: 2.4 ppm and As: 1.4 ppm. The patterns of leaching of the elements were irregular in relation to pH and the order established after 18 days of leaching was Cu>As>Cr at pH 3.5, As>Cr>Cu at pH 6.8 and Cr>Cu>As at pH 8.5. Apart from the influence of pH alone, the chemical, physical and microbiological properties of the soil-extract water used may also have influenced leaching. The results indicate that CCA-treated E. saligna used in ground contact, especially in acidic soils, may lose significant amounts of Cu, Cr and As to the environment. The environmental risks that the large number of CCA-treated hardwood poles and posts used in tropical soils represent raise the question as to whether CCA should remain the preferred wood preservative for tropical hardwoods, especially the eucalypts.

Download document (438 kb)
free for the members of IRG. Available if purchased.

Purchase this document