The Nutrient Properties And Heavy Metal Ecological Risk Assessment Of Sewage Sludge Biochar By The Co-pyrolysis With Organic Additives

Sewage sludge has been suggested as a raw material for biochar production that could not only conducive to the reuse of solid waste,but also conducive to agricultural production.However,effects of pyrolytic temperature and organic additive addition on the nutrient characteristic and the ecological and environmental risks of heavy metals and their underlying mechanisms are still unclear.In this thesis,a series of sewage sludge biochar（SSB）were produced from the co-pyrolysis of sewage sludge and organic additive with different types（reed straw,brewers’spent grain and sawdust）at 350 and 700°C,respectively,and their basic physicochemical properties,nutrient and heavy metal properties were also analyzed.The main results were as follows:（1）The C content of SSB greatly increased,while the yield,ash content,p H value,electrical conductivity,and elemental ratios of H/C,N/C,and O/C of SSB all decreased,with increasing the addition rate of organic additive.In addition,the ash content,p H value and specific surface area of SSB increased,while the yield,electrical conductivity,C,H,N,O element content and elemental ratios of H/C,N/C,and O/C of SSB decreased,when the pyrolysis temperature rose from 350°C to 700°C.The elevations in the addition rate of organic additive and pyrolytic temperature led to a decrease in electrical conductivity and increases in C content,aromaticity,and stability of SSB,which could be beneficial to the SSB application in agricultural soils.（2）As the pyrolysis temperature increased from 350°C to 700°C,the contents of total phosphorus,total potassium,ferrum,and NO₃^--N of SSB were found to increase.Furthermore,the contents of total phosphorus,total potassium,ferrum,NO₃^--N,and dissolved organic matter of SSB were observed to decrease with increasing the organic additive addition rate.Notably,the contents of available phosphorus,NH₄⁺-N,and other available nutrients also increased with rising organic additive addition rate,when the pyrolytic temperature was at 700°C.Overall,in most cases,the addition of organic additive could promote the nutrient retention of SSB,indicating a greater nutrient recovery potential.（3）The SSBs prepared under a condition of low pyrolytic temperature and low organic additive addition rate were suggested to be more suitable to use as a soil amendment than other SSBs,due to their relatively higher dissolved organic matter contents.However,according to the results of principal component analysis and comprehensive quality score,the pyrolytic temperature was identified as the most important factor affecting the quality score of SSB.Meanwhile,the SSBs prepared under the conditions of high pyrolytic temperature and high organic additive addition rate demonstrated the highest quality scores,and the quality rank of three kinds of organic additive was sawdust,brewers’spent grain and reed straw.（4）The total concentration and the proportions of potentially bioavailable（F3 specie）or non-bioavailable（F4 specie）components increased for each measured heavy metal when the pyrolytic temperature rose from 350°C to 700°C.The total concentration decreased,while the proportions of potentially bioavailable（F3 specie）or non-bioavailable（F4 specie）components increased for each measured heavy metal with rising the organic additive addition rate.In most cases,the ecological risk of heavy metals for the SSBs prepared at700°C with the same raw material were lower than those of 350°C.In addition,the organic additive addition rate had opposite effects on the ecological risk of heavy metals at different pyrolytic temperatures:the negative correlation between ecological risk of heavy metals and addition rate was found at 350°C,while the positive correlation was found at 700°C.The potential ecological risk of heavy metals in all SSBs prepared in this study were extremely low,implying the environmental risk might be very low when SSB is used as a soil amendment.In summary,the co-pyrolysis of sewage sludge and organic additive is an effective method to realize the resource utilization of sewage sludge.The properties of SSB can be significantly improved by controlling the pyrolytic temperature,organic additive type,and addition rate,and eventually enhance the application potential of SSB as a soil amendment.