Effect of clay and iron sulphate on volatile and water-extractable organic compounds in bamboo biochars

Improved plant disease resistance, seed germination and plant productivity have recently been associated with mineral-enhanced biochars. This has generated interest in characterizing those biochar organic compounds which may contribute to their favorable properties. This study builds on recent physico-chemical characterization of a series of mineral-enhanced bamboo biochars produced between 350 and 550 °C. Here, these biochars are subjected to aqueous extraction followed by liquid chromatography organic carbon detection (LC-OCD) and thermal desorption gas chromatography mass spectrometry (TD-GC/MS). These techniques provide a structural insight into their more bio-available organic compounds and how they vary with pyrolysis temperature. In comparison to neat biochar, their mineral-enhanced composites produce at least three times the water-extractable organic carbon, nitrogen and VOCs, and this increase is further amplified at pyrolysis higher temperatures. However, the biochar carbon fraction that is mobile is low, with total TD-GC/MS compatible VOCs reporting approximately 0.2% and LC-OCD solubles approximately 1-2%. Prior mineral impregnation of bamboo enhances the release of oxygenated compounds including humics and phenolics from its biochars. This increase in mobile oxygenates occurs at higher pyrolysis temperatures despite these minerals catalyzing lignocellulose condensation and carbonization. This anomaly is explained by oxygenates relatively small contribution and the self inerting batch pyrolysis conditions producing different biochar surface and bulk molecular properties. By describing the impact of mineral amendments on the yield and structure of mobile organic compounds that may be released from biochar, this work contributes to our understanding of biochar efficacy in soils.