Heavy Metals in Pyrolysis of Contaminated Wastes: Phase Distribution and Leaching Behaviour

Abstract

Pyrolysis is a recognized alternative for the sustainable management of contaminated organic waste, as it yields energy-rich gas, oil, and a carbon-rich biochar product. Low-volatility compounds, however, such as heavy metals (HMs; As, Cd, Cu, Cr, Ni, Pb, and Zn) typically accumulate in biochars, limiting their application potential, especially for soil improvement. The distribution of HMs in pyrolysis products is influenced by treatment temperature and the properties of both the HMs and the feedstock. There is a significant knowledge gap in our understanding of the mass balances of HMs in full-scale industrial pyrolysis systems. Therefore, the fate of HMs during full-scale relevant pyrolysis (500–800 °C) of seven contaminated feedstocks and a clean wood feedstock were investigated for the first time. Most of the HMs accumulated in the biochar (fixation rates (FR) >70%), but As, Cd, Pb, and Zn partly partitioned into the flue gas at temperatures ≥ 600 °C, as demonstrated by FRs of <30% for some of the feedstocks. Emission factors (EFs, mg per tonne biochar produced) for particle-bound HMs (<0.45 µm) were 0.04–7.7 for As, 0.002–0.41 for Cd, 0.01–208 for Pb, and 0.09–342 for Zn. Only minor fractions of the HMs were found in the condensate (0–11.5%). To investigate the mobility of HMs accumulated in the biochars, a novel leaching test for sustained pH drop (at pH 4, 5.5 and 7) was developed. It was revealed that increasing pyrolysis temperature led to stronger incorporation of HMs in the sludge-based biochar matrix: after pyrolysis at 800 °C, at pH 4, <1% of total Cr, Cu, Ni, and Pb and < 10% of total As and Zn contents in the biochars were leached. Most interestingly, the high HM mobility observed in wood-based biochars compared to sewage-sludge-based biochars indicates the need to develop specific environmental-management thresholds for soil application of sewage-sludge biochars. Accordingly, more research is needed to better understand what governs the mobility of HMs in sewage-sludge biochars to provide a sound basis for future policy-making.