Evaluation Of The Aging Process Of The Sewage Sludge Biochar And Its Potential Ecological Risk Of Heavy Metal Release In The Soil Environment

With the rapid development of China’s economy and society,the sludge production is increasing year by year,and a large amount of surplus sludge becomes a new environmental problem.Currently,pyrolytic carbonization is an effective means of sludge treatment,but heavy metals are enriched in sludge biochar after pyrolysis,which becomes an important factor limiting the resource utilization of sludge biochar.In addition,the aging and degradation process of sludge biochar in soil environment is long,and the changes of soil properties and microbial community structure during the aging process need to be studied in depth,and there is also a lack of systematic research on the migration and release pattern of heavy metals from sludge biochar in soil,which limits the resourceful and safe utilization of sludge biochar.Based on this,sludge biochar prepared at three temperatures was used in this paper to study the changes of its properties,heavy metal morphology and ecological risk during aging,and to explore the effects of sludge char on organic carbon,enzyme activity,heavy metals and microbial community structure in soil,so as to provide a theoretical basis for realizing the resource utilization of sludge biochar.The following conclusions were drawn:（1）In the H₂O₂ simulated aging test,it was found that H₂O₂ oxidation changed the specific surface area and surface morphology of the sludge biochar,and aromatic organic matter in the sludge biochar,but there was no significant change in the inorganic mineral fraction in the sludge biochar.Cr,Ni,Pb,and Zn were more stable in the sludge biochar at 600℃and 800℃.After graded extraction by BCR（proposed by the European Committee for Standard Test Analysis）,it was found that the proportion of heavy metal oxidation state and residue state in sludge biochar increased with the increase of pyrolysis temperature compared with the original sludge,and combined with the RAC（risk assessment index method）analysis,pyrolysis substantially reduced the ecological risk of the original sludge,and H₂O₂ oxidation had little effect on the RAC index at 600℃and 800℃.The results of the comprehensive study can be obtained that the temperature control at 600℃has the best effect of heavy metal passivation and the lowest ecological risk.（2）In a study on the effect of different soil water holding conditions on the aging of sludge biochar in soil environment,it was found that the soil organic carbon content increased 3.3 fold-6.3 fold after the addition of sludge and sludge biochar compared with the original soil,and the soil peroxidase activity increased by 3%-6%,and 60%soil water holding capacity was more favorable for the stabilization of organic carbon and peroxidase.The stabilization of organic carbon and catalase was more favorable at 60%soil water holding capacity.After sequential extraction by BCR,it was found that the bioeffective states of Cr,Ni,Pb and Zn were higher at 100%water holding capacity,but the heavy metal forms did not change much with time.Combined with the RAC analysis,the risk factor of sludge biochar at 400℃at 100%water holding capacity was elevated,and the risk of heavy metals in all other dirty peats decreased to no risk or low risk.The RAC values of heavy metals were all higher than those of peat after the addition of sludge.In conclusion,60%water holding capacity was more favorable for the stability of heavy metals in sludge biochar in soil,and the sludge biochar prepared at 600℃and 800℃had better heavy metal passivation effect.（3）The results of the pot experiment showed that the addition of sludge or sludge biochar increased the soil organic carbon content and was in a stable state,and the soil peroxidase activity increased by about 4%.Compared with the original soil,the addition of sludge biochar reduced the migration rate of Cr and Pb to Bermuda plants by 60%-80%for Cr and 10%-45%for Pb,but increased the migration rate of Ni by16%-40%and Zn by 6%-58%.（4）The addition of sludge or sludge biochar increased the richness and diversity of soil bacterial communities,and over time,the soil supplemented with sludge biochar was more conducive to the stabilization of microbial bacterial community richness and diversity.Analysis of the soil bacterial community structure showed that the Proteobacteria,Actinobacteria,Chloroflexi,Acidobacteria,Planctomycetes,Firmicutes,Bacteroidetes,Armatimonadetes,and Gemmatimonadetes became the dominant phyla in the soil.There were correlations between the phylum changes and organic carbon,peroxidase activity and heavy metals.In summary,sludge biochar has higher heavy metal stability compared to sludge,reduces the ecological risk of heavy metals in soil,and reduces the migration of Cr and Pb from soil to plants.In addition,the addition of sludge biochar increased soil organic carbon content,enhanced soil peroxidase activity,and contributed to the richness and diversity of the soil bacterial community.Thus,sludge pyrolysis carbonization is an important way to solve sludge treatment,and sludge biochar resource utilization has a broad development prospect.