Effect Of Aging On The Adsorption And Stabilization Of Heavy Metal Cadmium By Biochar

With the acceleration of industrialization and urbanization in China,environmental problems such as excessive heavy metals in agricultural soils are receiving more and more attention from all walks of life in China.Biochar is a carbon-rich solid obtained by high-temperature pyrolysis of biomass raw materials under oxygen-limited conditions.Its large specific surface area,loose and porous structure,and rich oxygen-containing functional groups make biochar have good adsorption and fixation effects on heavy metal ions,so biochar has good prospects for remediation of heavy metal-contaminated soils.Most of the current research on biochar focuses on the short-term remediation of newly prepared biochar for heavy metal contaminated soil,but after entering the soil,biochar will be gradually aged by various natural earth forces over time,and the aging process will have a non-negligible effect on the physicochemical properties and adsorption performance of biochar.In this paper,two typical agricultural and forestry biomass raw materials,rice and corn straw,were prepared into biochar at 600 ℃.The newly prepared biochar was subjected to artificially accelerated freeze-thaw cycles,alternating wet and dry and UV aging in the laboratory to investigate the effects of aging on the physicochemical properties of different types of biochar,as well as the adsorption performance and bioeffectiveness of the heavy metal Cd,in order to provide a theoretical basis for the long-term remediation of mild to moderate Cd-contaminated soil.The main results of the study are as follows:(1)Three aging methods,namely freeze-thaw cycling,alternating wet and dry aging,and UV aging,changed the physicochemical properties of biochar.After aging,the p H of both rice and corn straw biochar decreased,the surface structure was broken,the specific surface area increased,the average pore size decreased,the C content decreased,the O content increased,and the O/C,H/C,and(O+N)/C increased to different degrees.The aging effect increased the number of oxygen-containing functional groups,reduced the aromatization and enhanced the polarity of biochar.(2)The three aging effects enhanced the adsorption capacity of rice and corn straw biochar on Cd.The adsorption kinetics of rice and corn straw biochar on Cd before and after aging were consistent with the quasi-secondary kinetic model,and the fitted results of isothermal adsorption were consistent with the Langmuir isothermal adsorption model.The adsorption mechanism of biochar on heavy metal Cd was a single-molecule layer adsorption process with multiple modes co-existing,and the first 1 h was mainly a physical adsorption process,and after 1 h,the adsorption was gradually dominated by chemisorption,and the adsorption equilibrium was gradually achieved at 24 h.(3)The addition of biochar changed the basic physicochemical properties of the soil.Compared with the application of freshly prepared rice and corn straw biochar,the aging effect reduced the p H of the soil,increased the organic matter content in the light to moderate Cd-contaminated soil,reduced the content of active state Cd in the soil,increased the biomass of Brassica napus,reduced the content of Cd in the stems and roots of Brassica napus,inhibited the transfer of heavy metal Cd from the roots to the stem and leaf parts of Brassica napus,and reduced the phytoeffectiveness of heavy metal Cd.(4)Comparing the biochar made from two biomass feedstocks,rice and corn straw,the rice straw biochar was more capable of adsorbing heavy metal Cd and reducing the bioeffectiveness of heavy metal Cd than the corn straw biochar.Comparing the three different aging methods,the UV aging method was more effective in increasing the heavy metal adsorption performance and reducing the bioeffectiveness of heavy metals.