Thermal behaviour of arsenic trioxide adsorbed on activated carbon

The thermal stability and desorption of arsenic trioxide (As2O3) adsorbed on activated carbon (AC) was investigated as this phenomenon is expected to influence the arsenic release during low temperature pyrolysis of chromated copper arsenate (CCA) wood waste. Firstly, a thermogravimetric (TG) experiment with arsenolite, an allotropic form of As2O3, was performed. The sample starts to sublime at temperatures lower than 200 °C with a sublimation peak temperature of 271 °C. Subsequently, TG experiments with samples of As2O3 adsorbed on AC revealed that only very little (max. 6 ± 3 wt%) As2O3 was volatilised at temperatures below 280 °C, while still 41.6 (±5) wt% of the original arsenic concentration was retained at 440 °C and 28.5 (±3) wt% at 600 °C. The major arsenic volatilisation occurred between 300 °C and 500 °C. The kinetic parameters of desorption, activation energy of desorption (Ed) and pre-exponential factor (A), were determined by fitting an Arrhenius model to the experimental data, resulting in Ed = 69 kJ/mol, A = 1.21 × 104 s−1. It can be concluded that the adsorption of As2O3 on AC can contribute to the thermal stabilisation of As2O3. Consequently, during low temperature pyrolysis of CCA wood arsenic release may be prevented by adsorption of As2O3 on the coal-type product formed during the thermal decomposition of the wood.