Quality And Safety Assessment Of Tobacco Waste Biochar And Its Returning Effect

Biochar refers to the solid products of bioorganic materials(biomass)after pyrolysis under anoxic or hypoxic conditions.Biochar has good effects in soil carbon fixation and emission reduction,soil physic-chemical properties improvement,and soil heavy metals availability reduction.However,some studies have shown thatsome potential risks should be noticed after large amounts of biochar applicationin farmland,due to influencing factors such as biochar’s own composition,structural stability,and heavy metal enrichment capacity.In this study,the tobacco biochars were made under three pyrolysis temperatures of 300℃,450℃and 600℃using the wastes of tobacco straw,stalk,and leaf.The morphological characteristics,element composition,polycyclic aromatic hydrocarbons and heavy metals content of different tobacco biochars were investigated.The migration and morphological characteristics of heavy metals in different biochars were evaluated using the Toxic character leaching(TPLC)method and BCR continuous extraction method individually.and the physiological toxicity of different biochar leachates were explored through the experiments of germination rateand enzyme activity of wheat and acute toxicity of earthworms.And finally,the systemic effects of biochar application on soilphysi-chemical properties,soil heavy metal availability,heavy metal accumulation in tobacco,and the yield and quality of tobacco leaf were studied using the long-term field experiment of different dosages of tobacco straw biochar.The main conclusions are as follows:1.The basic physi-chemical properties of tobacco waste biochar were greatly affected by the pyrolysis temperature.The p H,EC and ash content of tobacco waste biochar were increased,while the CEC and production rate were decreased with the pyrolysis temperature.It couldbe seen from the different biochar element composition that the carbon content of tobacco stalk biochar was higher(59.95%～64.19%),and the O/C ratio value was decreased with the pyrolysis temperature.The BET specific surface area and pore volume of biochar were increased with the pyrolysis temperature,and the specific surface area and pore volume of tobacco stalk biochar under 600℃pyrolysis temperature was the largest among all biochars.The Infrared spectroscopy(FTIR)and X-ray diffractometer(XPS)results showed that tobacco waste biochar had abundant functional groups and mineral crystals.And the Bothm titration results of biochars indicated that the content of alkaline groups in the tobacco leaf biochar was relatively the highest of all.2.According to the standards for heavy metal content inbiochar in European Biochar Foundation(EBF)and International Biochar Association(IBI),the heavy metal contents in biochar of tobacco straw,stalk and leaf were all below the limit value,which indicated the safty in use.The available heavy metal content in TPLC leaching solution of biochars were also below the limit value,and the available heavy metal content of biochar was lower than thatof the raw material.Different heavy metals in biochar had different morphological distribution trends.and the speciation distributed of As,Cr,Cu and Pb were mainly oxidizable state and residue state.The results of enrichment index(REF),heavy metal risk index(RAC)and potential ecological risk index(RI)in different biochars showed that tobacco waste biochar were in a safety level,and the stability of heavy metals in biochar were increased with the pyrolysis temperature.3.The effect of biochar leaching solution on the growth of wheat seedlings and the acute toxicity of earthworms showed that under the stress of different tobacco leaching solution,the germination inhibition rates of wheat and the inhibition rate of shoot length and root length were increased with the pyrolysis temperature.Moreover,the antioxidant enzyme activity of wheat seeding was also improved compared to the control.The leaching solution of tobacco stalk had the biggest inhibition rate on the growth of wheat seedlings among three tobacco biochars,and it had also the highest acute mortality of earthworm.while the leaching solution of tobacco leaf biochar under 300℃pyrolysis temperature had the least impact on the acute mortality of earthworm.4.The application of tobacco straw biochar with different amounts to tobacco field could increase soil p H,exchangeable calcium and magnesium and other nutrients.The content of soil available Ni,Pb,and Zn were decreased with the application amount of tobacco straw biochar,while the changes of available As,Cr and Cd were opposite,which shouldbe pay more attention to the risks of these heavy metals.Overall,the content of heavy metals in tobacco leaves were decreased with the amount of biochar application.And the economic benefit,yield and quality of tobacco leaves was the best with tobacco biochar at 5 t/hm~2,which could be used as the reference for the application of tobacco straw biochar in the local tobacco field.