| The output of municipal sludge is increasing day by day,and the traditional treatment methods（such as landfill,incineration,agricultural utilization,etc.）are not sufficient for complete treatment,which may results in toxic gas generation,high cost and environmental safety risks.Pyrolysis technology overcomes many shortcomings,and can convert sludge into methane,tar,biochar.Therefore,sludge pyrolysis treatment has been attracted by several researchers.Due to the complex composition of sludge,the specific process and mechanism of sludge pyrolysis need to be clarified,and the relevant studies on the impact of the pyrolysis process on the environment are relatively scarce.In this paper,the effects of sludge pyrolysis temperature on the structural components of biochar were studied,and the biological toxicity was characterized by the bright luminescent bacteria method.In addition,necessary basic information for the application of sludge pyrolysis technology were also discussed.In the present work,the dehydrated sludge from Tangxunhu sewage treatment plant was selected as the target treatment plant.The sludge biochar was prepared by anoxic pyrolysis of dry sludge in a tubular furnace at 200°C,300°C,400°C,500°C and 600°C.By means of X-ray diffractometer,thermogravimetric analyzer and infrared spectrometer,the physical and chemical parameters of sludge composition,structural characteristics and functional groups before and after pyrolysis were analyzed.The leaching test of sludge and sludge biochar was carried out by sulfuric acid and nitric acid method and acetic acid buffer solution method.The influence of pyrolysis temperature on the organic components of the leaching solution was studied by means of total organic carbon analyzer,UV-vis spectrum,three-dimensional fluorescence spectrum and gel exclusion chromatography.The bio-toxicity was measured and analyzed by the method of luminescent bacteria.Finally,sludge biochar was modified and its adsorption of Pb（II）was investigated.The main conclusions of the experiment are as follows:Results:（1）with the increase of pyrolysis temperature,biochar yield,conductivity,total organic carbon and molecular weight were gradually decreased.（2）the metal content in DSS（dry sludge）was:Al>Fe>Mn>Ni>Li>Cr,while the heavy metals such as,Pb,Cd,Co and As,were found absent.Thermogravimetric analysis showed that the pyrolysis was mainly divided into three stages:primarily,water evaporation stage（0～180°C）,secondly,thermal decomposition stage（190～500°C）of thermally decomposable organic matter,and finally,thermal decomposition stage of inorganic salt（≥500°C）.（3）The leachate compounds are mainly composed of protein-like substances,including benzene rings,conjugated systems,aldehyde groups and ketone groups.Bio-toxicity of sludge biochar at different pyrolysis temperatures:（1）bio-toxicity of heavy metals:Pb>Co>Cd>As>Cr>Ni.The EC50（half effect concentration）of Pb,Cr,Ni,Cd,Co and As were 0.77,7.16,72.82,4.06,3.64 and 4.53 mg/L,respectively.However,the content of heavy metals in the sludge was relatively low,which is far less than the EC50value,so the biological toxicity was not caused by heavy metals in the experiment.（2）bio-toxicity decreases with the increase of pyrolysis temperature.Particularly,when the pyrolysis temperature was≥400°C,the bio-toxicity of the biochar leaching solution gradually became stable and close to non-toxic.The change curve of bio-toxicity of biochar was basically the same as that of the leaching solution of TOC with the pyrolysis temperature,so the bio-toxicity of the leaching solution was mainly caused by some toxic organic compounds（such as halogenated hydrocarbons）.Results:（1）HCl modification can inhibit the adsorption of Pb（II）by biochar,while KOH modification can promote the adsorption of Pb（II）by biochar.（2）optimal adsorption conditions:when p H value and biochar dosage are 5.0 and 2.0 g/L,respectively,the time required for unmodified biochar and KOH modified biochar to reach adsorption equilibrium is 12 h and 2 h,respectively.The adsorption capacity of unmodified biochar decreases with the increase of pyrolysis temperature.Whereas,the adsorption capacity of KOH modified biochar increases with increase in pyrolysis temperature.Under the conditions of p H:5.0,temperature:25°C,rotation speed:160 r/min,and adsorption time:24h,the adsorption capacity of SSB600-KOH（biochar produced by pyrolysis at 600°C modified by KOH）was the largest,and has the maximum adsorption capacity of 135.32 mg/g.（3）on varying the adsorption temperatures from 25°C to 45°C at 10°C intervals.It was observed that the maximum adsorption capacity of KOH modified SSB600 for Pb（II）was 135.32 mg/g,161.03 mg/g and 207.47 mg/g,respectively.The adsorption reaction of KOH modified SSB600 to Pb（II）was endothermic. |
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