Research On Preparation And Optimization Of Sludge-based Biochar And Its Remediation Of Heavy Metal Contaminated Soil

Recently,with the rapid development of urbanization,the treatment and disposal of sewage sludge have been increasingly concerned with publics.Pyrolysis is a noval option for the treatment and disposal of sewage sludge.Biochar,a solid pyrolysis by-product,is expected to be applied into soils to realize sludge recycling.However,some shortcomings are still existed in the soil application of sludge-based biochars.For example,sludge-based biochars have low contents of carbon and poorly developed pore structures.Potentially toxic elements enriched in the sludge-based biochars may cause certain environmental risks.The properties of dissolved fractions as well as their ecological roles,soil application efficiency and mechanism are still unclear.Therefore,this article regared the sludge-based biochar as the objects,and carried out reaserch from three aspects:preparation,optimization and soil application.Firstly,different pyrolysis temperatures were adopted to investigate their effects on properties of both solid fractions and dissolved fractions in biochars,migration and transformation of potentially toxic elements(PTEs)in sludge,complexation behaviors between dissolved fractions and Cu(II).The performance of sludge-based biochars in biological safety was also evaluated.As pyrolysis temperartures arose to700°C,the contents of C,H,O,N,S in biochars gradually decreased,the intensities of surface functional groups decreased,the specific surface area,pore volumes and pore sized increased to 70.79 m~2/g,0.128 cm~3/g and 7.21 nm.In the dissolved fraction of biochars,the contents of dissolved organic carbon and dissolved nitrogen decreased with the increasing pyrolysis temperatures,and the intensities of hydroxyl,amino C=O and aliphatic C-O-C gradually decreased.With the increase of pyrolysis temperature,the molecular weight distribution becamed much more dispered.One protein-like substance and two humic-like aromatic substances were identified in the dissolved fraction of biochars.With the increasing of temperatures,the contents of Cu,Zn,As,Pb,Cd and Cr in biochar increased by 81.96%,106.27%,35.98%,84.05%,109.33%and 121.94%,respectively.However,the contents of available Cu,Zn,As,Pb,Cd and Cr decreased by 88.60%,20.78%,18.51%,81.41%,54.37%and 44.28%.Higher temperatures reduced the complexation affinities,meanwhile improved the safety performances of biochars in Vibrio fischeri,wheat and soils microorganisms.In addition,sewage sludge and sawdust were co-pyrolyzed to optimize the physicochemical characteristics of bichars and to reduce the potential environmental effects of endogenous heavy metals in sludge.The influence of pyrolysis temperatures and blending ratios on elemental composition,proximate analysis,surface morphology and poro structure,carbon structure,speciation and contents of PTEs was also analyzed.With the increasing of sawdust blending ratios,the contents of C and fixed carbon in co-pyrolysis biochars increased by 11.35-71.56%and 15.11-151.48%,respectively,the contents of ash in co-pyrolysis biochars decreased by 2.12-68.62%.Some functional groups,such as aliphatic–CH,C-O-O-and aromatic C=O,were introduced into surfaces of co-pyrolysis biochars.At 700°C,the surface areas of biochars reached maximum(93.78 m~2/g),when the belend ration of sludge and sawdust was 3:1.The larger surface areas of co-pyrolysis biochar might be attributed to antagonistic effect of carbon retention.A certain proportion of sawdust could effectively reduce the contents of Cu,Zn,Pb,Cd and Cr,and obviously increased the contents of residual Zn,As and Cr.It was confirmed that the immobilization of PTEs was mainly affected by carbon structure,fixed carbon,H/C ratio,O/C ratio,p H and ash.Ultimately,biochars pyrolyzed at differernt conditions were applied into heavy metal polluted soils to screen out the effective and eco-friendly biochars.Physicochemical characteristics of soils,heavy metals passivation efficiency,soil microbial activity and microbial community structure were also investigated.During the soil incubation process,soil p H was improved most significantly by biochars produced at 700°C.Electrical conductivity(EC)and contents of dissolved organic carbon(DOC)in soils were effectively increased by low-temperature biochars.Application of biochars reduced bioavailable Pb and bioavailable Cd by 4.76-55.30%and 4.81-64.54%,respectively.Exchangeable Pb and exchangeable Cd were reduced by 6.65-13.41%and 6.37-15.27%,respectively.However,residual Pb and residual Cd were increased by 3.59-9.97%and 3.32-20.24%in biochar amended soils,respectively.Application of biochars increased soil dehydrogenase activities by 6.29-150.40%,and effectively increased soil microbial biomass.Redundancy analysis showed that p H,residual Pb and residual Cd were the domimant factors that affected soil microbial community structure.The above resulted suggested that the biochar prepared at 700°C with blending ratio of 1:1(S1W1-7)had the highest efficiency for immobilizing heavy metals in soils,and could improve soil microbial activity and soil microbial community structure to the maximum extent.SEM-EDS,XRD and FTIR showed that the high efficiency of S1W1-7 in heavy metals immobilization was ascribed to the formation of organ-mineral complex agglomerates and surface carboxylation.