Electron microscopic study on pyrolysis of CCA (chromium, copper and arsenic oxide)-treated wood

The effectiveness of pyrolysis as a possible technique for disposing of CCA (chromium, copper and arsenic oxide)-treated wood was studied. A CCA-treated sample given an extra heat treatment at 450 °C for 10 min was thoroughly investigated in order to establish the details of the reaction in which arsenic is captured in the pyrolysis residue prior to volatilization. Composition and structure of the metal compounds in the pyrolysis residue were examined by transmission electron microscopy (TEM). A large number of particles were found of variable diameter between 10 and 100 nm. The smaller ones were mostly spherical, sometimes faceted. The larger ones were lumpy. CCA compounds and their reaction products like Cr2As4O12 and As2O3 were identified in conventional TEM by selected area electron diffraction. In high resolution, the nanoparticles exhibited lattice fringes as indication of their monocrystalline character, fitting, e.g., the d2 1 0=0.204 nm of Cr. The volatility of arsenic during pyrolysis of CCA-treated wood was measured by XRF (X-ray fluorescence) analysis at temperatures up to 500 °C and at times up to 1 h. Weight change and arsenic content of the pyrolysis residue were measured after dissolution in HNO3. More than 20% of arsenic was already lost at 300 °C, which may have been caused mainly by the volatilization of the unreacted arsenic compound after reduction of As(V) in As2O5 to As(III) in As2O3. Arsenic is probably released as As4O6, which is very difficult to capture and toxic. By an additional heat treatment, this reduction can be prevented and the dry separation of the metals may be allowed.