Research On Subsequent Reduction Of Lead-Cadmium Enriched Plants And Preparation Of Biogas Residue Biochar

Phytoremediation technology is a low-cost and environmentally friendly remediation technology.The subsequent treatment and disposal process of plants is immature.How to deal with heavy metals after absorption by plants is the bottleneck of the development of phytoremediation technology.Creating a newer treatment technology to reduce the impact of secondary pollution caused by phytoremediation is one of the hot spots in the research of heavy metal pollution control and remediation in farmland.In this study,three different families and genera of plants as experimental materials,Lolium perenne L.（Gramineae）,Solanum nigrum L.（Solanaceae）,Galinsoga parviflora（Asteraceae）,and anaerobic fermentation experiments were carried out on these three different lead（Pb）and cadmium（Cd）enriched plants using Bacillus subtilis to promote fermentation.The purpose of this study is to explore the adsorption characteristics of Pb and Cd by biochar produced by pyrolysis of Pb and Cd enriched plants,so as to provide theoretical basis and technical support for the subsequent treatment of phytoremediation of heavy metals.The main results of this study are as follows:（1）Anaerobic fermentation can effectively reduce the amount of heavy metal contaminated biomass.After the fermentation test,the amount of Lolium perenne L.biomass decreased by 73.89%on average,the amount of Solanum nigrum L.biomass decreased by 57.41%on average,and the amount of Galinsoga parviflora biomass decreased by 71.30%on average.Among them,the number of plants with fewer stems,such as Lolium perenne L.,decreased most significantly,while the number of plants with more stems,such as Solanum nigrum L.,decreased relatively less.Although biogas fermentation reduced a large amount of biomass,leaving only a small amount of biogas residue,most heavy metals remained in the biogas residue and a small amount entered the biogas slurry.Compared with the biomass before fermentation,the Cd content of Lolium perenne L.biogas residue increased by 84.09%on average and the Pb content increased by81.21%on average,while the Cd content of Solanum nigrum L.biogas residue increased by 90.39%on average and the Pb content increased by 69.01%on average;Compared with the biomass before fermentation,the content of Cd and Pb in the biogas residue of Galinsoga parviflora increased by 82.74%and 68.80%respectively.After 30 days of fermentation of Lolium perenne L.,the concentration of Cd in biogas slurry is 0.07 mg·L^-1,and the concentration of Pb is 1.2 mg·L^-1;After 30 days of fermentation of Solanum nigrum L.,the concentration of Cd in biogas slurry was 0.08 mg·L^-1,and the concentration of Pb was 0.2 mg·L^-1;After 30 days of fermentation of Galinsoga parviflora,the concentration of Cd in the biogas slurry was 0.10 mg·L^-1,and the concentration of Pb was0.06 mg·L^-1.According to GB/T 40750-2021 in China,the biogas slurry reached the standard of returning to the field.（2）Heavy metals in biogas residue have a tendency to accumulate during the pyrolysis process.After pyrolysis at 350℃,the Cd concentration of Lolium perenne L.biogas residue biochar increased by 53.14%,and the Pb concentration increased by 22.39%;The Cd concentration and Pb concentration of Solanum nigrum L.biogas residue biochar increased by 42.62%and 19.25%respectively;The concentration of Cd and Pb in the biomass of Galinsoga parviflora increased by 35.82%and 19.40%respectively.However,with the increase of pyrolysis temperature,the concentration of heavy metals in biochar began to decrease.After pyrolysis at 750℃,the concentration of Cd and Pb in biochar from Lolium perenne L.biogas residue decreased by 69.94%and 83.17%respectively;Cd concentration and Pb concentration of biochar from Solanum nigrum L.biogas residue decreased by 78.69%and 63.07%respectively;The Cd concentration and Pb concentration of biomass charcoal from Galinsoga parviflora biogas residue decreased by 45.06%and36.67%respectively.Moreover,pyrolysis can effectively convert unstable heavy metals into relatively stable forms,and after pyrolysis,most of the heavy metals in biochar exist in a stable residual state.After pyrolysis of three kinds of plant biogas residues,the residual content of Cd is more than 80%,and the residual content of Pb is more than 70%.（3）From the adsorption test of biochar to aqueous solution Pb²⁺and Cd²⁺,it can be concluded that the pollution-type biochar made from the pyrolysis of three kinds of biogas residues can effectively adsorb Pb²⁺and Cd²⁺in aqueous solution,among which the biochar obtained from the pyrolysis of Lolium perenne L.biogas biogas residue at 750℃,the biochar obtained from the pyrolysis of Solanum nigrum L.biogas residue at 750℃,and the biochar obtained from the pyrolysis of Galinsoga parviflora biogas residue at 550℃are the biochar obtained from the pyrolysis of three different plant biogas residues at different temperatures to the group with the best adsorption effect of Pb²⁺and Cd²⁺.The adsorption of Pb²⁺and Cd²⁺in aqueous solution by these three biochars was more in line with the Langmuir isothermal adsorption model.The theoretical maximum adsorption capacity of Pb²⁺by these three biochars was 114.30 mg·g^-1,129.93 mg·g^-1and 137.11mg·g^-1,the theoretical maximum adsorption capacity of Cd²⁺by these three biochars was76.23 mg·g^-1,74.08 mg·g^-1and 77.09 mg·g^-1,respectively.It shows that these three biochars have the potential to become good adsorption materials.（4）According to the characterization results,the three preferred biochars have a loose and porous surface structure,and have significant adsorption for Pb²⁺and Cd²⁺.The adsorption process is mainly chemical adsorption.Heavy metal ions may have chelation with the biochar surface,electrostatic interaction,and generate metal carbonates.In addition,the two heavy metals also combine with each other to form composite metal oxides.