Effect Of Magnetic Wheat Straw Biochar On Cadmium Bioavailability In Soil-crop System

Crop straw is an important agricultural resource.Biochar is an efficient amendment for soil pollution.Using straw to produce biochar can not only effectively utilize resources,but also protect the soil environment.To further improve the efficiency of biochar immobilization of cadmium(Cd),the biochar was magnetically modified.To explore the adsorption and immobilization of magnetic biochar on Cd in alkaline soil and the effect of magnetic biochar on the growth,Cd absorption and migration of wheat and maize,the following studies were conducted.In this study,potassium ferrate(K2FeO4)was employed as a modifier and wheat straw was used as a raw material to prepare hierarchical porous magnetic biochars by one-step impregnation-pyrolysis method,and the Cd(II)adsorption test,soil indoor culture test,wheat seedling indoor pot experiment and wheat-corn rotation pot experiment were carried out.The effects of magnetic biochar on Cd bioavailability in the soil-crop system were investigated by measuring and analyzing the soil pH,electrical conductivity(EC),available Cd amount and the content of Cd in different forms,as well as the Cd content,migration coefficient,and enzyme activity in various parts of the wheat and maize plant.The results are as follows:(1)Three types of hierarchically porous magnetic biochars(HMB600?HMB700and HMB800)were prepared by pyrolyzing wheat straw and K2 FeO4 under a N2 atmosphere at 600,700 and 800 ?,respectively,which could be used as amendments for Cd in water and soil.Batch sorption experiments showed that Cd(II)sorption onto HMBs were well-described by a pseudo-second-order model and Sips(FreundlichLangmuir)model,indicating that HMBs removed Cd(II)mainly through chemical adsorption.The adsorption behavior of HMB700 to Cd(II)could be explained by surface complexation(–OCd,–COOCd),precipitation(Cd(OH)2 and Cd CO3),physical adsorption(rich pore structure)and ion exchange(K+,Ca2+,Mg2+).Furthermore,adding HMB600 ? HMB700 and HMB800(1 wt%)(incubation 60 days)could significantly increase soil pH and EC,and significantly reduce the content of available Cd in soil.HMB700 fabricated at 700 ? had the best remediation performance for Cd in water and soil,which was mainly due to its largest specific surface area and micropore volume.(2)Application of 700 ? biochar(BC)and hierarchically porous magnetic biochar(HMB)could increase soil pH,EC,reduce soil available Cd content,and reduce Cd content in wheat parts,but the effect of adding HMB was stronger than BC.Wheat biomass,photosynthesis gradually increased,and Cd-induced oxidative stress weakened in wheat roots and leaves by enhancing antioxidant enzyme activity with the increase of BC dosage.Although supplementing the soil with 2.0% HMB can minimize the soil available Cd and the Cd accumulation in the wheat tissue,the high pH and EC values of the soil inhibit the growth and physiology of wheat.Among all the treatments,application of 1.0% HMB to the soil corresponds to wheat plants with the largest biomass,the strongest photosynthetic performance and the lowest oxidative stress damage.(3)With the increase of the application rate of BC and HMB,the soil pH and EC gradually increased,the bioavailable Cd content of the soil gradually decreased,and the biological potential and difficult-to-use Cd content gradually increased.Among all the treatments,the activities of catalase,invertase,urease and alkaline phosphatase in Cd-contaminated soil treated with 1.0% HMB were the highest,while those treated with2.0% HMB were the lowest.With the increase of BC and HMB application rates,the Cd content in each part of wheat in Cd contaminated soil gradually decreased.The Cd content of wheat treated with the same amount of HMB was the lowest,but with the exception of 2.0% HMB treatment,the cumulative amount of Cd in wheat leaves increased with the increase of BC and HMB application.Application of HMB can significantly increase the migration of Cd from wheat stems to leaves and stems to rachis.(4)Both BC and HMB showed certain aftereffects in Cd contaminated soil,which could significantly increase soil pH and EC,reduce soil available Cd content,and promote the conversion of highly active Cd to stable Cd in the soil.The aftereffects of BC and HMB could reduce the Cd content in maize roots,stems,leaves and grains,and gradually decrease with the increase of the application amount.The aftereffects of HMB could significantly reduce the Cd accumulation and migration coefficient of each part of maize.