| Pyrolysis of crop residues for soil amendment has been recommend as one of the ten key methods for biomass resources recycling.It is reported that pyrolysis technology could minimize the environment risks for waste biomass treatment.In China,heavy metal pollution in agricultural soils has been becoming serious,although very serious policies and financial support to control soil contamination and to remediate polluted lands.However,there have not yet been financially viable technologies so far to ensure the contaminated lands remediated.Therefore,crop production may still continue to take place in the un-remediated lands,leaving high health risk of grains and straws from the fields.On the other side,the volatiles given off during the pyrolysis or the biochar can contain certain level of soluble organic compounds being potentially beneficial to plant growth or useful for leaf and root activities.Use of these organic compounds as foliar fertilizer has been reported to increase seedling resistance to environmental stress and improve plant yield.It would be of significant merit if extractable compounds from heavy metal contaminated crop residues derived biochar can be used as an effective plant growth promoter,thereby increasing the economic and environmental sustainability or biochar production form contaminated biomasses.The aims of this study is try to understand the influence pyrolysis temperature on potential toxic elements(PTEs)content,fractions and environmental risk of heavy polluted and unpolluted wheat straw biochars.Meanwhile,the soluble organic and inorganic elements content in biochar surface was analyzed and used for foliar application for Chinese cabbage growth in a pot experiment to test the agronomic performance using a pot experiment.Subsequently,the extracted biochar was amended to polluted soils to test its effect on soil heavy metal immobilization.Finally,we want to find the solution for safely recycling and value added utilization of metal contaminated and clean crop straw.The results of this study as follows:1.Influence of pyrolysis temperature on biochar properties and PTEs contentIncreasing with pyrolysis temperature,the fixed organic carbon in wheat straw biochar was increased and volatiles content and cation exchange capacity was decreased.Compared to wheat straw,its biochars were rich in C,N,P and K.The pH,and content of C,N,O,P,K in biochar was increase with pyrolysis temperature.The mains pools of heavy metals content in biochar were organic and residues fractions.The concentrations of Cd?Pb?Cu?Zn?Ni were much higher in biochars pyrolyzed at 450? and decreasing with pyrolyzed temperature to 550?.The content(12.77?40.65 mg kg-1)of PAHs in biochar was decreased with increased pyrolysis temperature in this study which was mainly composed with low rings and toxcity monomers.Therefore,the toxicty of PAHs of biochar in this study was low and below the standards the proposed by International Biochar Initiative.2.Influence of pyrolysis temperature on soluble fractions content in biochar surfaceThe concentrations of organic carbon(OC),N,P,Ca,Mg in biochar were significantly decreased with increasing pyrolysis temperature.On the opposite,the pH value and K content was increased with higher pyrolysis temperature.Compare to hot water extracts,the KOH extracts contents more OC,N,P but with less Ca,Mg and Mn.The concentration of Pb in hot water extract from both polluted and unpolluted biochar contained was below detect limit and only trace Cd(0.73-2.82 ?g L-1)was found in hot water extract of polluted biochar.For unpolluted biochar KOH extract,none Pb was detected but contained very low concentration of Cd(0.10-0.59?g L-1).The concentrations of Cd and Pb in KOH extract from polluted biochar were in the range of 0.40-2.65?g L-1,0.15-0.32 mg L-1,respectively.The concentrations of ?16PAHs in both hot water and KOH extracts was in the ragne of 0.40-2.65?g L-1 with main organic compound of phenanthrene and minute quantities of fluoranthene.In summary,the PTEs content in biochar extracts was far below the state standard of foliar liquid fertilizer(GB/T 17419-1998).3.The effect of biochar extract on plant growth and qualityBiochar extracts was diluted to 50 times and its pH was adjusted to 7.0 with H3PO4 before application to Chinese cabbage.The results of two season's pot experiment proved that the extracts of low temperature(350?)pyrolyzed biochar showed most effective on plant yield and quality.Meanwhile,these positive effects was decreased with pyrolysis temperature.In addition,the results confirmed that the temperature biochar extracts could increase expression of BcNR and BcGS,therefor,decrease plant NO3-content and promote the protein synthesis.Furthermore,application of biochar liquids also significantly decreased Chinese cabbage Cd and Pb accumulation and the decrease rate was ranged from 49.2%to 11.0%,and from 12.7%to 30.5%.4.The effect of residue biochar on soil heavy metal bioavailabilityThe residue biochar was collected and amended to Cd and Pb polluted soil after extracting by hot water and KOH,respectively.After 12 months incubation,the CaC12 extractable heavy metals was detected.The results indicated that the available Cd,Pb and Zn was significantly decreased with amendment of bulk biochar and residue biochar,being no difference between pyrolyzing temperatures of a single biochar.Compare to the control,the decrease rate of CaC12 extractable Cd,Pb and Zn was 37.5-78.7%,39.1-89.6%and 44.3-91.1%.In contrast,soil extractable Cu was more or less increased by 37.1-121.0%under bulk biochar treatment but significantly decreased by 33.7-57.2%under residue biochar treatment,again without a significant difference between pyrolyzing temperatures.In summary,the soluble fraction of organic and inorganic compounds was significantly influenced by pyrolysis temperature.Our result proved that these compounds play a very important role on plant growth.Thus,wheat biomass could be treated with pyrolysis at 350? for production of biochar,from which the liquid extract separated for foliar use for vegetable production while the residue biochar for soil amendment to immobilize metals in contaminated soils.This would provide an option to add value in biochar industry as a way to recycle waste biomass in agriculture. |
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