Preparation Of Paulownia Biochar And Its Effect On Phvsiological Indexes Of Wheat Seedlings In Cd Contaminated Soil

With the development of society and industrialization,the gradual accumulation of Cd in agricultural soils due to emissions from industrial and mining enterprises,long-term application of Cd-containing phosphate fertilizers,and irrigation with Cd-containing sewage has led to serious Cd pollution problems in agricultural fields.Traditional physical and chemical remediation techniques are not suitable for remediation of large area low concentration Cd contaminated farmland soils due to the limitations of high cost and small scope of application,therefore,it is of practical value to explore a low cost and simple operation remediation method to ensure the safety of food production.Biomass pyrolysis carbonization technology is an important form of resource utilization of agricultural and forestry wastes,which has received high attention and support from our government.Due to its unique physical and chemical properties,biochar can be used to adsorb heavy metals in soil,reduce the uptake and utilization of heavy metals by plants,and at the same time improve the soil environment and promote plant growth.As a resource-rich fast-growing economic tree species in China,a large amount of residual resources are produced during the production and processing process that are not effectively utilized.Therefore,in this paper,biochar for remediation of heavy metal contaminated soil was prepared by slow pyrolysis technique at 300℃,400℃,500℃,600℃ and 700℃ using common residue resources of paulownia(paulownia bark,peeled branches and paulownia leaves)as raw materials,and modern analytical methods such as thermal analysis,elemental analysis,Fourier transform infrared spectroscopy analysis,specific surface and pore size analysis were investigated.The physical and chemical properties of biochar were investigated by thermal analysis,elemental analysis,Fourier transform infrared spectroscopy,specific surface and pore size analysis,and then the biochar with better performance parameters was selected for further simulated application tests.By comparing with two commercial biochars(corn straw biochar and bamboo biochar)and also conducting pot tests,the effects of different biochar types and application amounts on foliar parameters,photosynthesis parameters,chlorophyll fluorescence parameters,biomass of wheat seedlings in Cd-contaminated soils were analyzed.Chlorophyll fluorescence parameters,biomass,and Cd enrichment and transport in Cd-contaminated soil.The main findings are as follows.(1)Through the preparation and analysis of paulownia biochar,it was found that the pyrolysis process of the three paulownia raw materials showed similar trends and existed in three stages,namely,the water evaporation stage from the initial temperature to 150℃,the rapid pyrolysis stage from 150℃ to 500℃,and the gentle pyrolysis stage above 500℃.The weight loss rate of the three raw materials reached a peak at about 350℃,and the weight loss rate rapidly slowed down to a stable level after 500℃.With the increase of the pyrolysis temperature,the yield of paulownia biochar rose continuously,the surface functional groups decreased gradually,and the aromaticity and stability increased;the adsorption isotherm types of paulownia biochar were all converted from class III H3 type to class Ⅱ H4 type,the specific surface area rose from 1.52～3.35 m2·g-1 to 95.06～512.54 m2·g-1,and the average pore size decreased from 12.29～22.96 nm to 2.40-2.77 nm,and the percentage of micropores increased from 0%to more than 10%;at higher pyrolysis temperatures,the pore structure of vesicular biochar became more complex,the fractal dimension tended to be close to 3,and the pore surface was rougher.(2)The simulated application of biochar showed that reasonable application of biochar promoted the growth of wheat seedling leaves in Cd-contaminated soil,but the promotion effect was weakened by too low or too high application rate.5%biochar application rate showed better promotion effect,and the performance of paulownia biochar was inferior to that of corn straw biochar and bamboo biochar,and corn straw biochar had the best promotion effect on the growth of wheat seedling leaves.The photosynthetic rate of wheat seedlings in Cd-contaminated soils was reduced by the application of biochar,and the reduction was significant at high application rates.photosystem II of wheat seedlings in Cd-contaminated soils was damaged,and the application of biochar had a certain improvement effect,enhancing the maximum light energy conversion efficiency Fv/Fm,actual light energy conversion efficiency Y(Ⅱ)and electron transfer efficiency ETR of photosystem Ⅱ,with the best effect at 5%application rate The best effect was achieved at 5%application rate.Therefore,it was concluded that the photosynthetic capacity of wheat seedlings could be improved by moderate application of biochar from paulownia.(3)Through the simulated application test of biochar,it was found that the application of biochar of paulownia rose the soil pH,and different types of biochar would have different changes on soil pH due to their own pH levels,and the application amount and the degree of effect on soil pH were positively correlated.The application of biochar from promoted the plant height of wheat seedlings in Cdcontaminated soil,had no significant effect on root length,and promoted the root-tocrown ratio.The promotion effect on plant height gradually decreased or even inhibited at high application rate with increasing application rate,but the promotion on root-tocrown ratio was incremental.The application of biochar to Cd-contaminated soil reduced the aboveground Cd content of wheat seedlings and increased the belowground Cd content,did not significantly change the Cd enrichment coefficient of wheat seedlings,and reduced the Cd transport coefficient of wheat seedlings.Therefore,it was concluded that biochar of paulownia has the potential to remediate Cdcontaminated farmland soils.