Effects Of Pyrolysis Temperature And Additives On The Heavy Metals Stability And Adsorption Performance Of Cr(Ⅵ) Of Sludge Derived Biochar

Sludge derived biochar has the advantages of low cost,simple manufacture and pore-structure developed,which has a wide range of potential application prospects in the field of water pollution remediation.However,the enrichment of the most of the heavy metal in the sludge derived biochar made it possess a potential ecological concern,which had an impact on and limited the utilization of adsorption.Because of this,researchs into how pyrolysis temperature settings and additives affect the enrichment process and stability of heavy metals in sludge derived biochar is crucial for both theoretical and practical reasons related to the actual utilization of sludge derived biochar.This study focuses on the effects of pyrolysis temperature and additives on the physicochemical properties and the stability of heavy metals as well as their mechanism in sludge derived biochar.The adsorption impact and mechanism of sludge derived biochar applied to the adsorption of Cr（VI）are investigated using the representative contaminant Cr（VI）in water as an example.The research found that:（1）The yields of sludge derived biochar decreased from 64.52%to 49.25%when the pyrolysis temperature climbed from 400°C to 600°C.In addition,the specific surface area increased from 2.04m²·g^-1 to 8.48 m²·g^-1,and the H/C atomic ratio decreased from 1.92 in SS to 0.36 in BC600.The organic matter was more thoroughly decomposed.The degree of aromatization and the pore structure were both improved by the rise in temperature.Comparing with SS,all heavy metals in BC400,BC500 and BC600 were enriched in different degrees.When the pyrolysis temperature was 500℃,the content of all heavy metals in the easy migration state（F1+F2）in BC500 was the minimum,and the risk value of RAC was the lowest.Considering the yields,specific surface area,the cost of pyrolysis treatment,the effect of heavy metal stabilization and the other factors of sludge derived biochar,the optimized pyrolysis temperature in this study was 500℃.（2）The yields of BC-Ca,BC-Fe and BC-RM decreased from 57.43%of BC500 to 45.23%,48.18%and 46.24%,respectively,with the introduction of additives.The specific surface area increased from 8.35 m²·g^-1 of BC500 to 24.18 m²·g^-1,13.20 m²·g^-1 and 18.62 m²·g^-1,respectively.The degree of organic matter cracking in the modified sludge char was further increased compared with that of BC500,and the microstructure was significantly improved.The proportions of Pb,Mn and Cu in（F1+F2）in BC-Ca decreased slightly compared with those in BC500,while the proportions of F4 increased from 38.08%,6.98%and 1.09%in BC500 to51.11%,12.53%and 3.20%,respectively.The amount of each heavy metal in the stable state（F3+F4）of the three modified sludge derived biochars was at its largest,its RAC value was at the lowest point,and its ecological danger was the lowest.The modification impact of Ca O was the best among the three modified sludge derived biochars when taking into account the yields of biochar made from sludge,specific surface area,and heavy metal stability of modified sludge.（3）Pyrolysis temperature and additives have an important influence on the Cr（Ⅵ）adsorption performance of sludge derived biochar.The higher the pyrolysis temperature,the better the adsorption effect of sludge derived biochar.The theoretical saturation adsorption capacity of sludge derived biochar prepared at different temperatures for Cr（Ⅵ）showed the relationship of BC600>BC500>BC400,which was 15.86 mg·g^-1,14.51 mg·g^-1 and 11.73mg·g^-1,respectively.The modification effect of calcium oxide as adsorbent was the best,the theoretical saturation adsorption capacity of modified sludge char for Cr（Ⅵ）was BC-Ca>BC-RM>BC-Fe with the pyrolysis temperature at 500℃,which was 17.49 mg·g^-1,16.55 mg·g^-1and 15.23 mg·g^-1,respectively.The adsorption of Cr（Ⅵ）by all sludge derived biochar was in accordance with Langmuir equation and pseudo-second-order kinetic model.The adsorption modes of unmodified sludge carbons and modified sludge carbons for Cr（Ⅵ）were mainly external liquid film diffusion,surface adsorption and interparticle diffusion,and the adsorption rates are mainly controlled by chemical mechanism.