| At present,the digestates are mostly used as fertilizer,but there is a potential risk of heavy metal pollution.Pyrolysis of digestate can achieve its reduction,recycling utilization and the stabilization of heavy metals.This article takes swine manure digestate as the research object,While studying its pyrolysis characteristics,it focuses on the effect of pyrolysis temperature on the migration and transformation characteristics of heavy metals during pyrolysis.Pyrolysis experiments were carried out at different final temperatures（500℃～900℃）,various physical and chemical characteristics of pyrolysis products were characterized,the energy balance analysis of the pyrolysis process was studied.Using electrical plate digestion methods and BCR sequential extraction procedures,the total amount and speciation of heavy metals（Ni,Cu,Zn,Pb,Cr）in swine manure digestate and its pyrolysis products were analyzed.The acetic acid buffer solution method is used to explore the heavy metal leaching characteristics of digestate and its pyrolysis products,and the contamination and ecological risk of heavy metals are evaluated by three kinds of assessment indices（RAC,PERI and GAI）.The main research conclusions are as follows:Biochar yield is always the highest,and its p H value increases with the increases of pyrolysis temperature.Under medium temperature conditions,biochar will obtain the best surface morphology and pore structure,The energy consumption for drying the swine manure digestate is 5.73MJ/kg dired,d,the energy of biochar and biooil is significantly higher than the energy consumption for drying and pyrolysis of digestate.The order of heavy metal content in swine manure digestate:Zn>Cu>>Cr>Ni>Pb,and Ni,Zn,Pb have high migration and transformation properties.As the pyrolysis temperature increases,the contents of Ni and Cu in each biochar gradually increase,but the contents of Zn,Pb,and Cr increase first and then decrease.Pyrolysis can effectively reduce the acid-exchangeable content of heavy metal acids and increase its residue content,but under high temperature（over 700℃）,the unstable state content of heavy metals have a tendency to increase instead.700℃may be the most appropriate temperature for pyrolysis of digestate,which could balance the enrichment of the total amount of heavy metal and stablization of speciation form.With the increase of pyrolysis temperature,the content of heavy metals in biooil are higher,and the proportion of exchangeable states increases first and then decreases.Ni,Cu,Zn,Pb in 700℃biooil have the highest proportion of exchangeable states.According to the distribution ratio of heavy metals in biochar and biooil,the proportion order of the heavy metals volatilized into biooil:Ni>Cr>Pb>Cu>Zn.At medium and low temperature（not more than 700℃）,the content of each heavy metal biochar are more than 80%,but at higher temperatures,except for Ni,the distribution of heavy metals in biochar decreases significantly.The pyrolysis process can effectively transfer the leaching toxicity of digestate to biooil,thereby reducing the leaching toxicity of heavy metals inbiochar.The percentage of heavy metal leaching concentration in the sample has a certain correlation with the proportion of heavy metal acid-exchangeable state.In the three evaluation systems,the heavy metals Cr in all samples are at no or low risk level.According to the RAC system,although pyrolysis can reduce the RAC value of heavy metals in biochar,the heavy metals in biooil are at high risk levels,especially under high temperature conditions.According to the PERI and GAI index,Cu and Zn in swine manure digestate and pyrolysis products may be at a certain pollution risk level. |
