Effects Of Biochar Application On The Loss Characteristics Of Cd From An Acidic Red Soil Under Simulated Rainfall Conditions

In recent years,with the rapid development of our country’s industry,agriculture and urbanization,a large number of pollutants have been discharged into the soil environment,and the accumulation and excess of heavy metals in soil have become increasingly prominent.When the polluted soil is invaded by the rainfall process,it will pose a huge threat to the surrounding environment and human beings.As an efficient passivator,biochar has received high attention in soil heavy metal remediation.Relevant studies have shown that biochar has a significant effect on improving soil structure and nutrients,but these studies have ignored the impact of biochar on the actual environment,which will change the runoff,sediment volume,and Cd migration characteristics under rainfall conditions.In this study,acidic red soil was used as the research object.Through adsorption and desorption experiments,soil hydrophobicity experiments and simulated rainfall experiments,combined with the results of previous studies,the adsorption capacity of biochar to soil aggregates for Cd,soil aggregate stability,Cd adsorption capacity,soil aggregate stability,Cd Availability,runoff and sediment loss characteristics,and effects of migration.Among them,considering the effect of biochar on the adsorption capacity of soil aggregates,the adsorption characteristics of aggregates with different particle sizes on Cd were studied,and the effects of biochar addition and p H on adsorption performance were investigated.The runoff,sediment,and Change characteristics of Cd migration,and explore whether biochar addition has a greater impact on Cd migration.This study designed a total of three aspects of the research content,and reached the following conclusions:1.Biochar addition can significantly increase the content of organic matter,CEC and free iron oxide in red soil;with the increase of biochar addition ratio,the adsorption amount of Cd in soil increases significantly,and the unit ratio of Cd addition per unit of biochar addition increases.The order of adsorption amount is:1%biochar>3%biochar>7%biochar>5%biochar,too high biochar dosage will reduce the utilization efficiency of biochar;Langmuir and Freundlich models are both good The isothermal adsorption characteristics of Cd in red soil aggregates before and after biochar addition（0.958≤R~2≤0.974）were described.Biochar addition had different effects on increasing the Cd adsorption capacity of aggregates with different particle sizes.In general,the smaller the soil particle size,the better the Cd adsorption.The adsorption capacity is stronger,but the relative increase of Cd adsorption in aggregates>1 mm is the largest（42.9%）;the addition of biochar can reduce the desorption of Cd in different aggregates,and the large particle size（>1 mm）The desorption amount of Cd in the agglomerates decreased most obviously;at the same time,the addition of biochar could still effectively increase the Cd adsorption amount of the agglomerates at higher p H.All showed the highest Cd adsorption capacity.In general,biochar had a good fixation effect on Cd in the acid red soil aggregates developed in granite.2.Soil hydrophobicity affects soil structure,aggregate stability,and soil water evaporation.Soil hydrophobicity experiment was conducted to study the effect of biochar addition in different proportions on soil hydrophobicity by aging test.The results showed that the hydrophobicity of soil increased significantly with the increase of biochar addition,and the soil was slightly hydrophobicity when the biochar addition proportion reached 7%（water drop penetration time>5 s）;The more biochar is added,the greater the increase of hydrophobicity in the aging process.3.In the simulated rainfall experiment,the runoff yield decreased with the increase of biochar addition under the two rainfall intensities,while the sediment produced by rainfall erosion generally decreased under the rainfall intensity of 120 mm h^-1.At 60 mm h^-1rainfall intensities,biochar addition tended to increase the loss of medium-sized（1-0.05 mm）aggregates in runoff sediments,while at 120 mm h^-1rainfall intensity decreased the loss of such aggregates.In most cases,biochar addition can significantly reduce the Cd concentration in the runoff and reduce the total amount of Cd loss（sediment+runoff）.Excessive levels（7%）of biochar additions should be avoided in soil remediation practices,as excessive additions may exacerbate soil erosion and lead to more Cd loss.