Effects Of Biochar On Heavy Metals In Microenvironment Of Sludge Composted By Earthworm

Heavy metals are one of the important pollutants in the process of sludge composting,which not only restricts the agricultural use of sludge,but also brings risks to human health.At present,in earthworm-biochar-sludge composting,most of the previous studies have focused on the influence of earthworms on the chemical properties of sludge,or the relationship between the biochar addition ratio and the change of the overall heavy metal concentration of earthworms,but rarely discussed the process and mechanism of heavy metal migration from the matrix to earthworms under the action of biochar.The distribution and accumulation characteristics of heavy metals in earthworm tissues,subcells and vermicompost are less studied.Therefore,in this study,biochar(rice husk charcoal,straw charcoal,bamboo charcoal)was added to earthworm compost to explore the effects of different proportions of biochar(0%,2%,4%,6%,8%,10%)on the physicochemical properties and heavy metals of sludge and vermicompost.Meanwhile,The effects of biochar on heavy metals in earthworm as a whole,tissues(epidermis and intestinal tract)and subcellular components(cell solutes,cell fragments and solid particles)were analyzed.The main conclusions were as follows:(1)The effects of biochar on heavy metals in earthworms after composting:compared with different proportions of the same biochar,the contents of Heavy metals Cr,Cu and Zn in earthworms were increased by three kinds of biochar;Rice husk charcoal and bamboo charcoal could increase the contents of Cr,Cu,Pb and Zn in the epidermis and intestinal tract of earthworm.The effects of rice husk charcoal on heavy metals in earthworm,epidermis and intestine were greater than those of straw charcoal and bamboo charcoal.After biochar addition,the contents of Cr,Cu and Zn in the epidermis of earthworm were higher than those in the intestine.Correlation analysis showed that biochar addition mainly affected the content of heavy metals in earthworms by affecting the p H value,EC value,TP,total Zn and available Zn contents in sludge.The addition of biochar mainly affected the content of heavy metals in earthworm epidermis by affecting the contents of p H,EC,TP,total Cr,Total Cu and available Pb in sludge.The addition of biochar mainly affected the contents of heavy metals in the intestinal tract of earthworms by affecting the p H value,total Cr and available Pb in the sludge.(2)Effects of biochar on heavy metals in subcellular components of earthworm:Compared with different addition ratios of biochar,biochar can increase the contents of Heavy metals Cr,Cu and Zn in solutes,cell fragments and solid particles of earthworm cells.The effects of rice husk charcoal on the increase of heavy metals in solutes,cell fragments and solid particles of earthworm cells were greater than those of straw charcoal and bamboo charcoal.Cu and Zn are mainly distributed in solutes of earthworm cells,while Cr and Pb are mainly distributed in solid particles of earthworm.Correlation analysis showed that biochar mainly affected the heavy metal content in solute of earthworm cells by changing p H value,EC value,total Cr,total Cu,total Pb and available Pb contents in sludge.Biochar affects the heavy metals in earthworm cell fragments mainly by changing the contents of p H value,EC value,total Cu and available Cr in sludge.Biochar mainly changes the content of EC,TN,total Pb,available Cr,available Cu and available Pb in sludge.And then affect the heavy metals in the solid particles of earthworm.(3)Effects of biochar on the properties of vermicompost: compared with different biochar addition ratios,the p H value,TOC and TN contents of vermicompost were increased by rice husk charcoal,straw charcoal and bamboo charcoal,and the contents of EC,TP,TK,Total Cu,Total Pb,total Zn,available Cr,available Cu,available Pb and available Zn were decreased by rice husk charcoal,straw charcoal and bamboo charcoal.Compared with straw charcoal and bamboo charcoal,the contents of Cu and Zn in vermicompost of rice husk charcoal decreased the most.Compared with straw charcoal and bamboo charcoal,rice husk charcoal reduced the content of available Cu and Available Pb in vermicompost.Correlation analysis showed that biochar affected the physicochemical properties of vermicompost mainly by changing p H value and TP content in sludge.Biochar mainly affected the total heavy metals in vermicompost by changing the contents of p H,TP,available Pb and available Zn in sludge and EC,TN,TP and TK in vermicompost.Biochar affects the content of heavy metals in vermicompost mainly by changing the content of EC,Total Cu,Total Pb and available Pb in sludge and the content of EC and TOC in vermicompost.The addition of biochar to vermicompost can change the morphology of heavy metals in vermicompost,mainly converting the exchange state(F1)and organic binding state(F4)of heavy metals in vermicompost to residue state(F5),thus reducing the activity of heavy metals in vermicompost.(4)Effects of biochar on sludge properties in vermicompost: compared with different addition ratios of the same biochar,the three biochar can increase p H value and TOC content of sludge,and decrease EC value,TP content,total Cr,total Cu,total Pb and total Zn content of sludge.Rice husk charcoal and bamboo charcoal reduce the contents of available Cr,available Pb and available Zn of heavy metals in sludge.On the whole,4% or 6% biochar had the greatest influence on the physicochemical properties and heavy metal content of sludge.Under the same addition ratio,compared with different biochar,rice husk char reduced the contents of Cr,Cu,Pb and Zn in sludge more than straw char and bamboo char.Compared with rice husk charcoal and bamboo charcoal,the content of available Cr and Pb in sludge increased more,while the content of available Cu and Zn in bamboo charcoal decreased more than that of rice husk charcoal and straw charcoal.The addition of biochar can change the occurrence state of heavy metals in sludge compost,mainly transforming the exchangeable(F1),ferro-manganese binding(F3)and organic binding(F4)states of heavy metals in sludge into residue state.Biochar can reduce the migration and transformation capacity of heavy metals in sludge.Figure 18 Table 23 Reference 130...