It is generally assumed that hexavalent chromium in chromated copper arsenate wood preservative is substantially reduced to trivalent chromium during fixation and that the potential to leach the more toxic CrVI is minimal. However, there are occasional claims some hexavalent chromium remains in wood after fixation and that small amounts of CrVI are available for leaching. One problem is that other metals such as iron can develop the same colour reactions as CrVI with the standard indicator, diphenyl-carbazide which may result in false measurements of CrVI. Ion chromatography can measure CrVI to 0.5 parts per billion and distinguish the reaction from that with other metals. In this study a number of CCA treated wood samples of different wood species and ages and wood cement and wood plastic composites containing CCA treated wood were collected and exposed to laboratory leaching. The leach water was analysed for CrVI and total Cr, Cu and As. For CCA treated wood and wood plastic composites, CrVI in the leachate was near or below the detection limit of the ion chromatograph (<1ppb) compared to total chromium levels of 200 – 2700 ppb. The amount of hexavalent chromium relative to total chromium leached was generally less than 0.1 %. Leachate from wood cement composites showed significant levels of CrVI (up to 50% of the total chromium leached). However, both hexavalent and total chromium levels in the leachate were low (0.2 – 0.4 ppm). These results show that once the CCA is properly fixed in wood, there is no health or environmental risk associated with toxic hexavalent chromium being released from treated wood.

Download document (222 kb)
free for the members of IRG. Available if purchased.

Purchase this document