Advanced Characterization of Biochars Produced from Three Different Organic-based Feedstocks and their Potential Applications

Abstract The present paper discusses the properties of biochars produced from different feedstocks, namely sewage sludge (SS), olive tree pruning (OTP) and walnut shells (WS), after slow pyrolysis at 300–400 °C. The techniques used for their characterization are Χ-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR), Thermogravimetric analysis (TG) and Scanning Electron Microscopy (SEM). In addition, the toxicity of the feedstocks and the produced biochars was assessed with the use of ΕΝ 12457-2 test. Furthermore, the phytotoxicity of substrates, containing test soil, 20 wt% biochar and / or 190 mg/kg Cu was evaluated for three seeds, namely Lepidium sativum (LS) , Sinapis alba (SA) and Sorghum saccharatum (SC) , with the use of Phytotoxkit microbiotest kit (MicroBioTests Inc., Belgium) and the determination of the germination index (GI). The experimental results clearly show that both the feedstock type and the pyrolysis temperature affect the physicochemical properties of the produced biochars. The pyrolysis yield decreases with the increase of pyrolysis temperature and may reach values as low as 10% for OTP biochar. According to the International Biochar Initiative (IBI) sewage sludge biochar is classified as Class III because its C org content varies between 10% and 30%, while OTP and WS biochars belong to Class I as their C org content is well above 60%. The application of EN 12457-2 test indicates that the produced biochars exhibit no toxicity. Finally, the addition of biochar in substrates has either phytostimulant or phytotoxic effect depending on the type of the seed used and the contamination with Cu ions.