Oxidation using diluted HNO3 solution is dependent on reactions taking place in the gas phase, resulting in HNO3 regeneration, thus allowing sub stoichiometric reactions (16.7 mmol of C oxidized with 10.4 mmol of HNO3). Reactions for HNO3 regeneration are dependent on temperature gradient along reaction vessel. Thermographic images were obtained of external vessel wall after 2.5, 5 and 7.5 min of heating. Temperature inside reaction vessel was directly measured using a sapphire thermocouple in four different depth (3, 6, 12 and 16 cm). Reactions under inert atmosphere (7.5 bar Ar) were evaluated and normally presented higher temperature gradient than that performed with O2 pressure, since HNO3 regeneration reactions are exothermic. A better understanding of the regeneration of HNO3 and the dependence on the whole reaction vessel temperature in this system was possible combined to residual carbon content and residual acidity measurements to estimate the extent of organic matter oxidation.

A method for digestion of plastics from waste of electrical and electronic equipment (WEEE) was developed using the microwave-assisted wet digestion in single reaction chamber (MAWD-SRC). The determination of As, Cd, Co, Cr, Cu, Ni, Pb, Sb and Zn by inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometry (ICP OES) was carried out after sample digestion. Mercury was determined by flow-injection cold vapor generation coupled to inductively coupled plasma mass spectrometry (FI-CVG-ICP-MS). Results obtained using MAWD-SRC for sample preparation were compared with those obtained using microwave-assisted wet digestion (MAWD) at high pressure. Acid mixtures (HNO3 or HNO3 + HCl) were evaluated and feasibility for further inorganic contaminants determination by ICP-MS and ICP OES was demonstrated. Sample preparation by MAWD-SRC using HNO3 + HCl mixture resulted in better digestion efficiency in comparison to MAWD. In addition, lower limits of quantification were obtained using MAWD-SRC due to the higher sample mass that can be digested (500 mg). The combination of HNO3 and HCl for digestion showed to be crucial for quantitative recovery of some elements, as Cr and Sb. Agreement with certified values was better than 96%.