Regulation Mechanism Of Adsorption-Migration Of Cu And Zn In Farmland Soil During Snowmelt By Biochar Application

Seasonal freeze-thaw cycle is the most important natural phenomenon in cold region ecosystem,which significantly affects the soil physicochemical properties and the migration and transformation process of heavy metals.During the soil freeze-thaw cycle,biochar applied to the soil as a soil remediation agent will undoubtedly experience frequent freeze-thaw cycles,resulting in changes in biochar properties,which will have an important impact on soil environmental remediation.At the same time,heavy metals attached to the snow in the cold region will cover the surface of the soil during the snowfall process.In the snowmelt period,heavy metals will infiltrate into the soil with the snowmelt water,changing the vertical profile distribution of copper and zinc in the soil.Therefore,under freeze-thaw conditions,this research of the adsorption-migration regulation mechanism of biochar on Cu and Zn in farmland soil during snow thawing period is of great scientific significance to the restoration of farmland soil ecological environment.In black soil in northeast China as the research object,through the indoor test to explore the mechanism and the rule of the outdoor test analysis the way of combining in indoor experiment,set up a 0 to 30 times（1-10 cycles）freeze-thaw cycles under the condition of biochar samples,analyzed the natural conditions and the freeze-thaw cycle under the condition of biochar pH,SA,PD、PV and the change of functional groups,explores the freeze-thaw biochar physical and chemical properties and its adsorption performance relationship and significant level of heavy metals,combined with the adsorption equation of heavy metals Langmuir and Freundlich,the adsorption mechanism of biochar for heavy metals Cu and Zn was analyzed.In outdoor experiment,set up three different biochar addition amount,analyzes the bare land（S1）,covering biochar 3kg/m²（B3）,6 kg/m²（B6/S2）,9 kg/m²（B9）under the condition of 0-100 cm soil moisture content,dry density,porosity and spatial change of pH value differences,and established the physicochemical properties of soil and soil heavy metal content in the fitting relationship function,at the same time,further analyzes the bare land（S1）,covering biochar 6 kg/m²（B6）conditions in the early stage of the falling snow,and snow melt period of soil Cu and Zn content of spatial variations.The enrichment factor method and geoaccumulation index method were used to measure the pollution degree of heavy metals copper and zinc.The regulation mechanism of biochar on the migration and transformation of heavy metals Cu and Zn in the soil during snow thawing period was studied by combining the T_j and C_ij.The main research results are as follows:1.Effect of freeze-thaw cycles on physicochemical properties of biochar.Freezing and thawing cycles can significantly alter the chemical properties and surface structure of biochar.Specifically,after 10 cycles（30 times）,the pH value of freeze-thaw biochar decreases from 8.80 to7.99,the SA increases from 6.28 m²/g to 20.26 m²/g,and the PV and PD decrease from 0.009mL/g and 1.692 nm to 0.003 mL/g and 1.423 nm,respectively.Compared with CK group,the maximum adsorption capacity Q_m of Cu and Zn adsorbed by 10-cycle biochar increased by 72.00%and 44.55%,respectively.Meanwhile,the number of oxygen-containing functional groups-OH,-COOH and-C=O increased significantly.2.Statistical analysis of physicochemical properties and adsorp tion properties of freeze-thaw biochar.Correlation analysis showed that the concentration of copper adsorbed by freeze-thaw biochar was negatively correlated with biochar PV,PD and pH,and positively correlated with SA.The concentration of heavy metal zinc adsorbed by freeze-thaw biochar was negatively correlated with biochar PV,PD and pH,and positively correlated with SA.The significant level analysis showed that when the PV,PD or pH of freeze-thaw biochar falling,the adsorbed Cu content augmented significantly.When the SA of biochar augmented,the adsorption of heavy metal copper content was significantly enhanced.When the PV,PD or pH of freeze-thaw biochar decreased,the adsorbed heavy metal zinc content augmented significantly.When the SA of biochar augmented,the adsorbed Zn content of biochar augmented significantly.So the freeze-thaw cycle of biological carbon absorption ability of the physical and chemical properties and performance not only as to a condition or a single index correlation,is characterized by multiple factors and a few index the interaction between common have a major impact on it.3.Effects of different amounts of biochar on soil physicochemical properties.Under B3,B6and B9 treatments,the soil dry bulk density of each soil layer was decreased compared with that of S1,which was S1>B3>B9>B6;Soil moisture content,porosity and pH value of each soil layer in B3,B6 and B9 were all increased compared with those in S1,and the effect was shown as B6>B9>B3>S1.Therefore,when biochar supplemental level was 6 kg/m²,it had the greatest contribution to effectively improving soil physicochemical properties,and had a more significant effect on surface soil moisture content compared with other soil layers.4.Correlation analysis of soil physicochemical properties and heavy metal content in soil.Correlation analysis showed that the heavy metal copper in soil was positively correlated with soil moisture content and dry bulk density,but negatively correlated wi th soil pH value.The zinc in soil is positively correlated with soil pH and porosity,and negatively correlated with soil moisture content.The enrichment degree of Cu in vertical soil profile i s not related to soil porosity,and the zinc is not related to soil dry bulk density.The enrichment degree of cooper and zinc in soil is not completely consistent with the correlation between the physical and chemical properties of soil,which is closely related to the adsorption capacity of Cu and Zn by soil,soil type,climate conditions and sample handling methods.Significant level analysis showed that when soil moisture and dry bulk density decreased or pH increased,the content of heavy metal Cu in soil decreased,but the decrease was not significant.When soil pH and porosity increased or water content decreased,the content of heavy metal Zn in soil increased,but the increase was not significant.5.Regulation characteristics of biochar on heavy metal migration in soil during snowmelt period.In the snowmelt period,the concentration of copper and zinc in the 0-10 cm soil layer under S1 treatment improved by 21.84 mg/kg and 41.99 mg/kg compared with that before snowfall,and the contents of cooper and zinc in the 0-10 cm soil layer under S2（B6）treatment augmented by 11.11 mg/kg and 13.15 mg/kg compared with that before snowfall.After S2（B6）treatment,the augmented contents of Cu and Zn in 0-10 cm soil layer decreased by 10.73 mg/kg and 28.84 mg/kg,respectively,compared with S1 treatment.In the snowmelt pe riod,compared with S1,the leaching rates of Cu and Zn in the surface soil,the ground accumulation index and the enrichment factors all showed a decreasing trend after the addition of biochar.In the snowmelt period,the leaching rates of Cu and Zn in the 0-10 cm soil layer under S2（B6）treatment decreased by 0.22 and 0.24,the ground accumulation index decreased by 0.52 and 0.23,and the enrichment factor decreased by1.20 and 0.09,respectively,compared with S1 treatment.At the same time,atmospheric PM_2.5has a strong ability to enrich chemical substances.In conclusion,this study believes that only reasonable application of biochar can have a positive effect on improving the seasonal freeze-thaw soil and water environment.Moreover,the application of biochar in the early snowfall period can intercept the heavy metals in the snow during the snowfall period,which effectively inhibits the infiltration of heavy metals carried by the snowmelt aquatic stream into the soil during the snowmelt period,and has a good effect on the improvement of the soil physical and chemical properties.Considering the comprehensive effect of seasonal freezing-thawing on the snowfall,biochar and soil mixture,it is suggested that the application amount of biochar should be controlled at about 6 kg/m²,which is beneficial for the biochar to maintain the best adsorption state,better fixation of heavy metals and the soil to maintain the best structure condition.This study provides a reliable basis for understanding the application of biochar in seasonal permafrost regions,and has important guiding significance for improving farmland soil and water environment in cold and arid regions and promoting sustainable development of regional agricultural ecology.