Slow pyrolysis of metal(loid)-rich biomass from phytoextraction : characterisation of biomass, biochar and bio-oil

Plants have successfully been used for phytoextraction of metal contaminated soils, however the use of these plants for energy production has been a subject of debates due to the potential conversion of the metals in the plants into airborne respirable particles. The aim of this study was to investigate the deportment of metal(loid)s during pyrolysis of a biomass cultivated in a highly contaminated soil in order to engineer best practice environmental approach for utilization of this biomass. A heavy metal(loid) contaminated mangrove (Avicennia marina var. australasica) biomass was used as a feedstock in this study. The biomass was subjected to slow pyrolysis under the heating rate of 60 degrees C/min and different pyrolysis temperatures. Inductively coupled plasma mass spectrometry, thermogravimetric analysis, Fourier-transform infrared spectroscopy, X-ray fluorescence spectroscopy and gas chromatography-mass spectrometry were introduced to characterise the biomass, biochar and bio-oil samples. Results showed that biochar yield decreased from 57.4 % to 35.3 % with the increase in pyrolysis temperature from 300 to 700 degrees C. Heavy metal(loid)s (chromium, manganese, iron, copper, zinc, arsenic and lead) were mainly bound in the biochar produced at 300 degrees C, while the recovery decreased substantially with the increase of pyrolysis temperature. Phenols, carboxylic acids and alcohols were the dominant compounds in all bio-oil samples. This study suggested further requirements of biochar quality and environmental risk assessment to provide a safe and value-added way of phytoextraction residual applications.