Characteristics Of Tobacco Straw Biochar And Its Role And Mechanism In Remediating The Cadmium-Contaminated Soil

Tobacco as one of the extremely important cash crops in some regions of China inevitably leads to a large volume of straw wastes.It is therefore urgent to develop innovative measures for the resourceful use of tobacco straw in these regions.In this study,tobacco straw was sufferred from slow pyrolysis under series temperatures of 300°C,400°C,500°C,600°C and700°C for 3 h.The solid residues as tobacco straw biochar were collected and labelled as T300,T400,T500,T600 and T700,respectively.The biochar properties were first studied by various techniques.And then their Cd²⁺adsorption behaviours were investigated by batch tests.Finally,the biochar prepared at 500℃was added to a Cd-contaminated farmland soil in series of amounts such as 0,5,15 and 30 g/kg,denoted as B0,B5,B15 and B30respectively.The soils were grown firstly with C.coronarium and then C.endivia in pots,in order to study their effect on reducing Cd uptake by the vegetables.The aims were to provide with fundamental data in resource utilization of tobacco straw.The main findings were as following:（1）The biochars prepared with tobacco straw under different temperatures were significantly different in both chemical properties and Cd²⁺adsorption behaviours.The acidity of the biochar increased from p H9.05 to p H 11.54 when the temperature increased from 300°C to 700°C.The contents of H,O and N and the atomic ratios of H/C,O/C and（O+N）/C all decreased with the increase of the pyrolysis temperature,showing a higher degree of aromatization for the high temperature biochar.These biochars were predominantly mesoporous.The low temperature biochars tended to have relatively large specific surface area and abundant in functional groups,while the high temperature biochar had well-developed porous structure.（2）Both the quasi-secondary kinetic equation and the intraparticle diffusion equation were able to well fit the adsorption process of Cd²⁺by the prepared straw biochars at different temperatures,which might indicate that the adsorption was heterogeneous chemisorption.The high temperature biochar such as T700 had a strong adsorption capacity for Cd²⁺and a hihg removal rate of Cd²⁺by 99.01%.The Langmuir-Freundlich equation was able to well simulate the Cd²⁺uptake behaviour of the biochar.The maximum adsorption capacity（Qm）ranged from 54.40 to 106.16 mg/g,highest in T700,which was also much higher than those of the biochar from either rice straw,wheat or other crop straws.Our results showed that all the biochars prepared with tobacco straw under different temperatures had a strong adsorption capacity for Cd²⁺and thereafter might perform good value in remediating environmental Cd²⁺pollution.（3）amending tobacco straw biochars significantly increased soil p H,organic matter content,cation exchange capacity（CEC）,available phosphorus and potassium content（P≤0.01）.The forms of Cd were largely changed fowllowing the amendment of tobacco straw biochars.In particular,the acid-soluble Cd content was eventually reduced from 1.43mg/kg to 1.15 mg/kg.Accordingly,the percentage of the acid-soluble Cd,in the total decreased from 71.91%to 52.94%.The Cd content of the edible part（above ground）of both C.coronarium and bitter wheat cabbage was reduced by 53.47%～91.69%and45.50%～80.71%respectively compared to the control and therefore reached the national limits（Cd≤0.2）.（4）The amendment of tobacco straw biochars induced lower Cd bioconcentration factors（BCF）in either above-or below-grownd biomass for both C.endivia and C.coronarium.The Cd transport factor（TF）of C.endivia significantly increased by biochar addition,while little change in C.coronarium.Our results showed that adding appropriate rate of tobacco straw biochar such as 5-15g/kg could not only improve the soil quality,promote the growth of vegetables and increase the yield,but also reduce the Cd availability and inhibit the uptake and accumulation of Cd in plant tissues.The Cd-sensitive crops like C.coronarium showed better resposne to the biochar amendment.