Study Of The Effect And Mechanism Of Biochar On The Solidification And Stabilization Of Lead And Cadmium In Soil

Soil is always the foundation of human settlements,but the rapidly developing economy and society have also brought heavy metal pollution to the soil,which has negative effects on people’s life and production safety.At present,many methods could be used to deal with heavy metal pollution in the soil.Solidification and stabilization,as one of the methods,has the characteristics of rapidly effective and low-cost which has been widely utilized in soil amendment.To realize the solidification and stabilization,cheap-effective stabilizers are one of the necessary factors.Sludge and straw,as common industrial and agricultural wastes,have the characteristics of large total amount and rich C content which could be prepared into biochar providing a reasonable solution to these wastes.As a new type of material prepared by high-temperature pyrolysis under anaerobic environment,biochar usually has the characteristics of easily-derived raw materials,strong stability,developed pore structure and excellent adsorption capacity which has become a hotspot of soil remediation researches.In this paper,industrial sludge,municipal domestic sludge,landfill sludge and rice straw were used to prepare biochars by pyrolysis in an anaerobic environment.Meanwhile,these raw materials were thoroughly mixed with a solution rich in iron ions and then pyrolyzed to prepare magnetic biochar at 500℃.Utilize elemental analyzer,scanning electron microscope and Fourier infrared spectrometer and other means to characterize the obtained biochars.Subsequently,all biochar were applied to the lead and cadmium complex contaminated paddy soil with different dosages,and the available content and form distribution of lead and cadmium were determined through soil incubation experiments,combined with the analysis of soil microbial communities and three-dimensional fluorescence analysis,aiming to explore the effect and mechanism of biochar on the solidification and stabilization of lead and cadmium in soil.The main conclusions are as follows:（1）In the experiment,textile and dyeing sludge,municipal domestic sludge,landfill sludge and rice straw were selected for pyrolysis at 300°C and 500°C to prepare biochar（labeled as TDSBC300、SSBC300、TJSBC300、RSBC300、TDSBC500、SSBC500、TJSBC500、RSBC500）.The characterization results showed that ash content,p H and specific surface area of the biochar erived from the same raw material increased with the pyrolysis temperature increasing.In addition,textile and dyeing sludge,municipal domestic sludge and rice straw were mixed with Fe solution for magnetic modification and then pyrolyzed at the temperature of 500℃aiming to prepare magnetic biochar labeled as MTDSBC500、MSSBC500、MRSBC500.After characterization,it was found that the magnetic biochar which has a rougher surface and was loaded with more particulate matters is slightly different from that of the original biochar.It could be confirmed by scanning electron microscope-X-ray microanalysis（SEM-EDS）that the magnetically modified biochar had been successfully loaded with iron oxides.（2）The original biochars were applied to the lead and cadmium complex contaminated paddy soil at the dosage of 1%/3%/5%（w/w）,and then a 60-day soil incubation experiment was conducted.The diethylenetriaminepentaacetic acid（DTPA）extraction method and the BCR stepwise extraction were used to analyze the available content and different form contents of heavy metals,respectively.The results were shown as follows:（1）Adding biochar into the soil could increase soil pH at the beginning of the experiment.The pH of the soil with 5%RSBC500 increased the most among them（The soil pH on the 15th day was 8.75,which was 15.7%higher than the initial polluted soil pH）.However,at the end of the incubation experiment,the pH finally increased slightly compared with the Blank.In addition,the soil TOC content also had a remarkable increase at the beginning of the experiment（On the 30th day,the TOC content of the soil was 43.66 mg/kg,which was increased by 14.05%compared to the initial contaminated soil）,then slowly decreased and had a flat trend at the end of the incubation experiment.Three-dimensional fluorescence analysis was carried out on the soil sample extracts after 60 days of incubation.As a result,the soil extracts mainly contained short-wavelength humus-like substances,long-wavelength humus-like substances,tyrosine-like and visible-light fulvic acids.Studies have shown that the fluorescence intensity of protein-like proteins in rivers that are severely polluted by industry become relatively high.Furthermore,after 60 days of incubation,the content of tyrosine-like in the soil extracts with the application of biochar decreased,indicating the reduction of heavy metal pollution.（2）The application of biochar could reduce the available content of Cd and Pb.Among them,the reduction effect of sludge-based biochar on the available state of lead and cadmium is better than that of straw biochar.Then,the effect of 500℃pyrolysis biochar is better than that of 300℃pyrolysis biochar and 5%dosage is the optimal dosage.（3）The application of biochar could lower the relatively less-stable form content（acid-soluble and reducible state）of lead and cadmium in the soil,increase the more stable form content（oxidizable state and residue state）.In the incubation experiment,the acid-soluble state of Cd in the soil with 5%SSBC500 decreased by 49.70%（from 25.13 mg/kg to 12.64 mg/kg）,and the residue state increased by nearly 2 times（from 12.98 mg/kg to 25.19 mg/kg）;Pb acid-soluble state decreased by 73.07%（from 27.37 mg/kg to 7.37 mg/kg）,residue state increased by 93.21%（from 27.67 mg/kg to 53.46 mg/kg）.（4）The 16s r RNA analysis of the soil after 60 days of incubation experiment reflected the fact that the application of biochar increased the microbial diversity.As a result,the relative abundances of Chloroflexi and Actinobacteriota increased notably while Proteobacteria decreased at the phylum level.Studies have shown that Sphingomonas can remove lead from seawater incubation media which has great potential in environmental protection;Pseudarthrobacter can accumulate Cd²⁺through intracellular and extracellular chelation.The more this kind of bacteria,the less the Cd pollution.In this experiment,at the genus level,the relative abundances of Pseudarthrobacter and Sphingomonas both have increased,while the relative abundances of Massilia and Flavisolibacter decreased notably,reflecting that the application of biochar could alleviate the pollution.（3）The magnetic biochars were applied to the lead-cadmium complex contaminated paddy soil at a dosage of 5%（w/w）,and then conducted a 60-day soil incubation experiment.Conclusions were as follows:（1）Owing to the high pH,the magnetic biochar raised more soil pH at the beginning of the experiment when comparing with the original biochar group.The pH of the soil added with5%MRSBC500 increased the most（the soil pH was 9.48 on the 30th day）.However,at the end of the experiment,the pH also gradually decreased and then leveled off.Besides,the TOC content of the soil applied with magnetic biochar also had a greater increase than the original group.Take the soil applied with 5%MRSBC500 as an example,the total organic carbon of the soil increased the most which was up to 28.07 mg/kg on the 7th day.Furthermore,same to that of the original biochar group,the organic matter in the soil extracts of magnetic biochar groups were also divided into short-wavelength humus-like substances,long-wavelength humus-like substances,tyrosine-like and visible-light fulvic acids.However,the difference is that the content of humus-like substances was higher than that of the original soil.The reason may be that the magnetic biochars contained more inert carbon sources resulting in high contents of two kinds of humus.In addition,the change of tyrosine-like content is not much obvious,implying that the mitigation effect of magnetic biochars may not be as good as the original biochars.（2）Similar to the original biochars,the application of magnetic biochars could remarkable reduce the available content of Cd and Pb.For example,the available state of Cd decreased by 57.51%（from 53.68 mg/kg to 22.81 mg/kg）,and the available state of Pb decreased by 55.63%（from 86.53 mg/kg to 38.39 mg/kg）compared with the Blank group.（3）The application of magnetic biochars could lower the acid-soluble content（acid-soluble and reducible state）of lead and cadmium in the soil,increase the stable residue content（oxidizable state and residue state）.Among them,the acid-soluble state of Cd in the soil with 5%MTDSBC500 was reduced by 36.85%（from 27.60 mg/kg to 17.43 mg/kg）,and the residual state increased by nearly 3 times（from 7.60 mg/kg to 21.05 mg/kg）;The acid-soluble state of Pb decreased（from 27.73 mg/kg to 11.43 mg/kg）,and the residue content increased（from 18.97 mg/kg to 44.59 mg/kg）.（4）It was found that the application of magnetic biochars increased the microbial diversity of the soil.Specifically,the relative abundances of Chloroflexi and Actinobacteriota had a great increase while the relative abundance of Firmicutes reduced at the phylum level.In addition,at the genus level,the relative abundance of Alkaliphilus and Symbiobacterium had increased while the relative abundance of Bacillus has been remarkable reduced.For the fact that the relative abundances of these two kinds of bacteria are negatively correlated with soil heavy metal pollution,the increase in their abundances also reflected that the application of magnetic biochars could effectively alleviate heavy metal pollution in the soil.