Influence of leaching medium and drying time between successive leaching periods on the emission of chromium, copper, and boron from treated wood

Abstract

Laboratory studies were conducted to determine leaching of Cu, Cr, and B from pine sapwood treated with Cr-Cu-B (CCB) preservative in different leaching media (ultrapure water [UPW], rainwater [RW], and soil water [SW]). Losses of metals from CCB-treated wood, expressed in percentage of initial content, were always highest for B (11.2-63.5%) and lowest for Cr (0.02-0.36%) in any of the leaching media assayed. Losses of B and Cr from treated wood were not affected by the leaching medium; however, losses of Cu were 1.5 to 2.5 times higher in SW than in UPW and RW. The influence of drying time between successive leaching periods on release of metals also was determined. Treated wood was submitted to wetting-drying cycles of 24 h/d, 18 h/d, and 6 h/d in contact with the leaching medium over a total contact time of 276 h. For an equivalent contact period of CCB-treated wood with either UPW or RW, leaching of B was from 1.1 to 1.9 times higher for continual wetting than under intermittent wetting conditions. Leaching of Cu was not affected by the wetting-drying periods when wood was in contact with RW, but it was 1.2 to 1.5 times higher for continual wetting with UPW. In general, leaching of Cr was not affected by the wetting-drying cycles when in contact with UPW or RW. However, when CCB-treated wood was in contact with SW under discontinuous wetting of 6 h/d, Cu and Cr emissions were approximately 1.2 and 2.5 times higher, respectively, than with continuous wetting. This study shows that the type of leaching medium and the drying time between successive wetting periods must be considered when evaluating metal losses from CCB-treated wood during service life.