Arsenic release during pyrolysis of CCA treated wood waste: current state of knowledge

摘要

Low-temperature pyrolysis is evaluated as a possible technique for the disposal of chromated copper arsenate (CCA) treated wood waste. Theoretical and experimental studies are performed in order to gain more insight in the mechanism of arsenic release. In this paper, the most important observations and results of modelling are brought together in order to present the current state of knowledge. Lab-scale pyrolysis experiments show that pyrolysis at 300℃ for 20 min gives rise to inconclusive release for the three metals. However, arsenic releases are substantial for all experiments using a temperature of 320℃. A speciation study shows the presence of trivalent arsenic in the pyrolysis residue, indicating that arsenic, present in pentavalent state in the CCA solution and in the CCA treated wood, is partly reduced to the trivalent state during pyrolysis. Arsenic release can be modelled by a simple first order single-reaction kinetic scheme for the temperature range from 350 to 450℃. Based on the order of magnitude of the kinetic constants, the reaction is identified as a reduction reaction. Thermogravimetric (TG) analysis of hydrated chromium arsenate (the major arsenic compound in CCA treated wood) reveals that thermal decomposition probably results in the formation of solid Cr_2O_3 and gaseous H_2O, O_2 and As_4O_6, indicating again that As(Ⅴ) is reduced to As(Ⅲ) and arsenic is released in trivalent form. TG studies of the pure arsenic oxides (As_2O_5 and As_2O_3) show that at temperatures lower than 500℃ the reduction reaction As_2O_5→As_2O_3+O_2 does not take place. At higher temperatures decomposition is observed. As_2O_3, however, is already released at temperatures as low as 200℃. This release is driven by temperature dependent vapour pressures. It can be concluded that the wood, char and pyrolysis vapours form a reducing environment, thereby influencing the thermal behaviour of the arsenic oxides. An other explanation for the difference in thermal behaviour could be that part of the arsenic in the CCA treated wood is already present in the trivalent form.