The Primary Study On The Stability Of Heavy Metals In Biochar

Biochar is a stable carbon-rich product derived from agricultural and forestry waste biomass and other biomass under the condition of hypoxia and special pyrolysis temperature,which can improve physical and chemical properties of soil,reduce greenhouse gas emissions and improve crop yield.A great deal of researches have shown that the characteristic of large specific surface area,high cation exchange capacity and strong adsorption capacity of biochar can effectively reduce the toxic effects of heavy metals on crop and restore soil contaminated heavy metal.Previous studies have indicated that heavy metals in biomass will be enrichment in biochar during pyrolysis,however,there is relatively little research on the heavy metals in biochar releasing into soil environment and effecting on crop yield and heavy metal enrichment.Thus,this study took biochars,derived from four kinds of straw containing heavy metal at three pyrolysis temperature,as research subjects to investigate the physicochemical properties and enrichment ratio of heavy metals.The TCLP extractant,CaCl2 extractant and DTPA extractant were used to extract heavy metals from biochars,for researching the effects of pyrolysis temperature and raw material on the stability of heavy metals in biochars.Meanwhile,the extracted concentrations of heavy metals of soil mixing biochars before and after cultivation were studied to confirm the change of stability of heavy metal in biochars.The cultivation of Pakchoi in Biochar-Soil system was also studied to determine the effects of biochar on the growth and heavy metal enrichment of Pakchoi.The main results are as follows:(1)The composition and physicochemical properties of biochar are affected by raw materials and pyrolysis temperature.As the pyrolysis temperature increasing,the content of C in biochar increased,the content of O in biochar decreased,the pH value of biochar increased,the specific surface area increased,the pore volume increased and the pore size decreased.The surface micro-structure of biochar was gradually developed,the extent of aromatization of biochar increased and the thermal stability of biochar enhanced,in the process of pyrolysis temperature increasing.(2)The heavy metals could be enriched in biochar at all conditions of pyrolysis temperature and raw materials.The enrichment rate of Cd,Cu and Pb decreased as the pyrolysis temperature increasing.The enrichment rate of Cu in biochar was the highest,followed by Cd enrichment rate and Pb enrichment rate.(3)The mobility of Cd,Cu and Pb in soil environment in biochar derived from different raw materials decreased with the increase of pyrolysis temperature.The CaCl2 exchangeable and DTPA available heavy metals contents have the lowest value in biochars at high pyrolysis temperature,but the proportion of the exchangeable and available heavy metals in total amounts of heavy metals of biochars have the highest value.(4)The heavy metals concentrations extracted by TCLP extractant,CaCl2 extractant and DTPA extractant in soils with 1%and 3%biochar mixed at the end of 10-week cultivation significantly decreased,compared to those concentrations before cultivation.It also showed that the extracted concentrations of heavy metals from biochars would decrease,as the pyrolysis temperature increase during producting biochar.The stability of heavy metals in biochar also improved with the pyrolysis temperature increased.(5)The contents of heavy metals in Pakchoi growing in soil with biochars increased with the increase of content of biochar in soil.The Cu content of Pakchoi in all treatments was no significantly difference,compared to that content of CK.The Cd content of Pakchoi growing in soil with biochar derived at the temperature of 300? and 500? was higher than that in CK,but lower than that in CK growing in 700? biochar-soil system.The Pb content of Pakchoi of all biochar treatments was lower,compared to that of CK.