Freeze-thaw Biochar Enhances In-situ DEHP Remediation And Microbial Response Mechanism In Black Soil

Di（2-ethyl）hexyl phthalate（DEHP）is often used as a plasticizer and is widely used in the production process of industrial and agricultural products（such as agricultural film）.Due to the complex chemical structure of DEHP,DEHP freed into the soil is easy to accumulate,which will cause long-term pollution of the soil environment.DEHP can destroy the function of hypothalamus-pituitary gland-thyroid,leading to human endocrine dysfunction,which poses a great risk to human health.Therefore,the development of efficient DEPH remediation technology is a practical problem that needs to be solved urgently.In recent years,the research of biochar remediation of organic contaminated soil has received extensive attention and has good prospects.This study uses straw as a material to prepare biochar and subject it to a freeze-thaw cycle to analyze the effects of biochar（BC）and freeze-thaw biochar（FBC）on the adsorption behavior and hysteresis of DEHP in black soil;By constructing a model and studying the effects of BC and FBC on the physical and chemical properties of black soil,soil microbial community structure and functional genes reveal the ecological mechanism of biochar-enhanced black soil DEHP in situ remediation.The main results of this research paper are as follows:（1）The characterization results of biochar showed that:compared with BC,the O/C ratio,specific surface area,functional group,and polarity of FBC have increased.FBC contains more oxygen-containing functional groups,and the cation exchange capacity（CEC）is positively correlated with the number of oxygen-containing functional groups on its surface.The effect of BC and FBC on the adsorption behavior of DEHP showed that the adsorption of 100 mg·L^-1DEHP by the two kinds of biochar reached equilibrium within 12 hours.The adsorption capacity and adsorption rate of FBC to DEHP are higher than BC,which may be due to the higher porosity of FBC surface and more abundant adsorption sites.The adsorption kinetics fitted with the pseudo-second order kinetic model.The adsorption and desorption experiments showed that the desorption of DEHP by biochar is not a completely reversible adsorption process.The adsorption isotherm conforms to the Freundlich model.The adsorption behavior of DEHP by FBC can reduce the migration rate of DEHP in the soil,which is beneficial to reduce the ecological risk of DEHP in the soil.（2）FBC can effectively improve the physical and chemical properties of DEHP-contaminated soil.The content of total nitrogen,ammonium nitrogen,nitrate nitrogen,organic matter,and available phosphorus in the soil increased with the addition of BC and FBC,compared with the initial level.When the addition amount is 2.0%,the content of ammonium nitrogen and nitrate nitrogen in the soil added with FBC and BC reaches the peak.Compared with BC,the ammonium nitrogen and nitrate nitrogen in the soil added with FBC increased by 9.07%and 8.77%,respectively.Combined with the results of catalase kinetics,soil catalase activity,soil p H and the number of bacteria in the soil increase with the increase of FBC concentration.It showed that adding FBC can create a better living environment for microorganisms,thereby promoting the degradation of DEHP by indigenous microorganisms,increasing the biotransformation rate of DEHP,and providing a theoretical basis for in-situ restoration in the soil.（3）The influence of biochar on the microbial community structure in DEHP-contaminated soil was further analyzed by pot experiment and high-throughput sequencing.The DEHP stress reduced the diversity of soil bacterial communities.Compared with the DEHP stress group,the diversity of bacterial communities in the soil with BC and FBC increased,respectively.According to the analysis of the abundance of nitrogen cycle functional genes in the soil,the application of FBC can increase nitrification to a certain extent and inhibit the progress of denitrification.After FBC is applied,it can increase the content of ammonium nitrogen and nitrate nitrogen,and can significantly increase the abundance of nitrifying bacteria and promote the strengthening of nitrification.In addition,there is a certain relationship between nitrogen cycle microbial species variables and environmental variables.Under DEHP stress,the effect of FBC on nitrogen cycle-related functional genes is predicted,and the results show that FBC can significantly promote the abundance of nitrogen cycle functional bacteria.