Fate of Heavy Metals in Waste to Energy (wte) Processes

This study was done to increase the understanding of how heavy metals in the aqueous phase are removed at low initial concentrations in different pH and Eh values. The reaction that has been studied is mainly hydroxide precipitation and adsorption in a condensate treatment. In the study, data from one of Vattenfalls waste incinerators was analysed and the results from the data were then compared to previous studies. To increase the understanding, modelling of the heavy metals behaviour in the given concentrations was then made with Medusa and PHREEQC. The heavy metals that were analysed were Sb, As, Pb, Zn, Cr, and Cd. The low initial concentration that vary between 36.1-23600 μg/l complicates the removal process because it corresponds in a low driving force and the results are hard to compare to other studies whose initial concentrations vary between 10-100 mg/l. From the modelling and the measurement data it can be seen that Pb, Zn, Cr, and Cd was removed by hydroxide precipitation at pH 10. According to the speciation calculations, the dominant species at this pH are Pb(OH)2 , Cd(OH)2, Zn(OH)2 and Cr(OH)3. For arsenic a clear conclusion could not be drawn from the modelling and the measurement data because of low precision. Due to the limited thermodynamic parameters of antimony in comparison with other heavy metals in the database of Medusa and PHREEQC, the modelling of antimony behaviour in condensate treatment has relatively larger uncertainty. The modelling results show that the main species in acidic solutions for antimony is Sb(OH)3 and in basic solutions Sb(OH)-6. Further investigation for antimony in needed for a clear conclusions to be drawn.