Elemental Composition and Heavy Metal Safety of Biochar from Different Tobacco Wastes

To clarify the effect of pyrolysis temperature on elemental composition and heavy metal safety of tobacco biochar,an experiment was conducted to study the effect of two pyrolysis temperatures (450℃ and 600℃) on biochar yield,elemental composition,heavy metal (Cr,Ni,Cu,Zn,As,Cd and Pb) contents and leaching toxicity,using tobacco waste (tobacco stalks,tobacco stems and discarded tobacco leaves) as raw materials.The results showed that as the pyrolysis temperature increased from 450℃ to 600℃,the yield of tobacco waste biochar gradually decreased;the carbon (C) content in biochar increased,and the nitrogen (N) and oxygen (O) contents decreased;The heavy metal contents of Cu,Zn,As,and Pb in tobacco biochar all increased in certain degree compared to those in the raw materials,while the content of Cr in tobacco leaf biochar and Cd content in tobacco stem and leaf biochar under 600℃ pyrolysis temperature were significantly lower than those in the raw materials.The relative enrichment factor (REF) of Cr and As in tobacco stem biochar and Zn,As in tobacco leaf biochar decreased with the increase of pyrolysis temperature.And the corresponding heavy metals in biochar changed from enrichment (REF>1) to volatilization (REF<1).However,Ni and Cu in tobacco leaf biochar changed from volatilization to enrichment.In addition,the toxicity characteristic leaching procedure (TCLP) of heavy metals in tobacco biochar were all lower than those in leaching solution of their raw materials.The contents of Cu,Cd and Pb in leaching solution of each tobacco biochar all decreased with the increase of pyrolysis temperature,but the content of As in them showed the opposite trend.The contents of heavy metals in the leaching solution of tobacco waste biochar were all lower than the limit value of leaching toxicity of solid waste in GB 5085.3-2007,which indicated that the leaching toxicity of heavy metals in tobacco waste biochar was very low.So,the tobacco waster biochar could be applied to farmland safely.