Extraction of heavy metals from CCA treated wood by supercritical fluids
Sustainable process-product development & green chemistry
SCF as Solvent Substitutes (T1-8P)
Keywords: SCF, extraction, metals, Cyanex
For more than fifty (50) years, a water borne salt formulation commonly known as chromated copper arsenate (CCA) has been used as wood preservative to protect the wood from degradation. However, upon impregnation, the element components fix substantially to the wood matrix, remain there and could later leach to the environment and when disposed off could potentially causing severe health problems. As part of a study to device a cost effective and environmentally sustainable way of extracting the CCA from wood matrix, the chemical species in the wood were characterized and the potency of supercritical carbon dioxide using Cyanex 301 and 302 as modifiers, and sodium tetrahydridoborate (NaBH4) as reductive agent were tested. The results obtained indicated that different complex formation involving compounds and combinations of Cu, Cr and As were formed in the wood, while especially high extraction efficiencies of the CCA were obtained using Cyanex 301 and 302 in supercritical carbon dioxide. Leaching tests with NaBH4 solution on the other hand gave lower extraction efficiency of As although thermodynamic calculations clearly predicted the feasibility of reducing all the elements and thereby converting As into gas. At the same time NaBH4 only could leach very small amounts of Cu and no Cr. These results may be due to the higher solubility of Cu (I) compounds as compared to Cu(II) and the conversion of Cr into the non soluble oxide Cr(III)oxide. Cyanex 301 and 302 could be employed to extract CCA from CCA treated wood but the possibility of regenerating carbon dioxide and the chelating agent need to be assessed.
Presented Monday 17, 13:30 to 15:00, in session SCF as Solvent Substitutes (T1-8P).
