| Sludge,as a kind of solid waste with both the characteristics of resource recovery and environmental danger,how to properly treat it and make full use of the beneficial components while reducing the environmental risk is an urgent problem to be solved in the future.Sludge pyrolysis technology is the most potential sludge treatment method at present.On the one hand,it can reduce volume,inactivate bacteria,solidify and stabilize heavy metals,and reduce the toxicity.On the other hand,the sludge biochar,with large specific surface area,developed pore structure and rich in plant nutrients,can be used as an improver of soil structure and fertility.In this experiment,biochar was prepared from municipal sludge.The structure and properties of biochar were characterized by means of elemental analysis,SEM,FTIR,XRD,thermogravimetric analysis and DRS.The characteristics of heavy metals and environmental risks in biochar were analyzed by BCR continuous extraction method,TCLP method and potential ecological risk index method,so as to determine the safety and feasibility as a soil improver.Based on the analysis of the effect of biochar on soil structure,moisture and thermal properties,the mechanism of biochar's effect on soil thermal properties and improvement of soil fertility were discussed from the perspective of heat.The main conclusions are as follows:(1)With the increase of pyrolysis temperature,the yield of sludge biochar decreased,the ash content increased,and the pH value of biochar changed from acidic to alkaline.The contents of elements C,H,N and O all decreased with the increase of pyrolysis temperature,the degree of aromatization and stability of biochar increased,hydrophilicity and polarity decreased,and the leaching capacity of inorganic nitrogen decreased.The specific surface area and pore volume of biochar first increase and then decrease with the increase of pyrolysis temperature,and the average pore diameter decreases.High temperature was conducive to the formation of pore structure in depth direction.The surface of sludge biochar has organic functional groups such as-COOH,-OH,C-O-C,C=N,and inorganic components such as sulfate,phosphate,silicate,and carbonate.During the pyrolysis process,the contents of total nitrogen,available nitrogen,available potassium and organic matter decreased with the increase of pyrolysis temperature,and the concentration of total phosphorus and total potassium occurred.(2)The total concentration of heavy metals in sludge and biochar was Zn>Cr>Cu>Ni>Pb>Cd.Heavy metals in biochar were concentrated during pyrolysis,but the maximum total concentration was lower than the national control threshold.The continuous extraction results of BCR showed that high temperature pyrolysis could promote the transformation of heavy metals from bioavailable components to stable components,and the bioavailability of heavy metals was significantly reduced.The results of TCLP toxicity leaching showed that the leaching rate of heavy metals in biochar decreases with the increase of pyrolysis temperature,and pyrolysis can significantly inhibit the leaching capacity of heavy metals in sludge.The results of environmental risk assessment showed that high temperature pyrolysis can significantly reduce the environmental risk level of sludge,indicating that pyrolysis was an effective sludge treatment method for fixing heavy metals,and sludge biochar with high quality and low environmental risk can be prepared by controlling the pyrolysis temperature.(3)The application of sludge and biochars can change the soil structure,increase the proportion of sand and reduce the proportion of silt in the soil,and have little effect on the clay content.When the addition ratio was 10%and 15%,the soil bulk density decreased greatly,the soil capillary porosity and total porosity increased,the soil non-capillary porosity decreased,and the soil pore structure was improved.With the increase of pyrolysis temperature,the degree of sludge carbonization was intensified and the color of biochar was deepened.Biochar prepared at 500?(marked as BC500,the same below)and 700?(marked as BC700,the same below)can significantly reduce the reflection of radiant light from the soil.Biochar prepared at300?(marked as BC300,the same below)had little effect on soil reflectivity.The raw sludge(marked as SS,the same below)increased the reflection of the soil to the radiant light.(4)BC500 and BC700 can significantly increase the soil saturated water content,but SS and BC300 were not significant.When SS,BC300,BC500 and BC700 were applied into the soil in different proportions,the field water holding capacity of the soil increased by2.51%?7.78%,0.80%?2.93%,0.43%?6.76%and 0.09%?13.00%,respectively,SS and BC700had a more significant effect on the field water holding capacity of the soil.Meanwhile,the application of biochar enhanced the soil water retention performance to different degrees,effectively slowed down the evaporation rate of water,and extended the time of soil water retention stability.(5)The addition of biochar can reduce the soil heat capacity,thermal diffusivity and thermal conductivity,which was related to the low thermal capacity,low thermal diffusivity and thermal conductivity of biochar,the decrease of soil bulk density and the increase of porosity.When the bulk density increased from 1.00 g/cm~3 to 1.30 g/cm~3,the average increase range of soil heat capacity,thermal diffusion rate and thermal conductivity was up to 10.39%,13.60%and 25.35%,respectively.The positive effect caused by the decrease of bulk density in porosity offset the negative effect caused by biochar itself and the addition ratio.The thermal bulk density,thermal diffusivity and thermal conductivity of soil under different biochar treatments were positively correlated with the soil water content within a certain range(R~2>0.95).Soil thermal properties were related to pyrolysis temperature,addition ratio,soil organic matter content and soil water content,and biochar addition ratio and soil water content were the main factors affecting soil thermal properties. |
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