Explore The Effect Of Chemical Washing On Soil Aggregate Structure And Heavy Metal Desorption Characteristics

Soil is a kind of natural resource that is difficult to regenerate.It is of great economic value and ecological benefit to restore the original ecological function of soil as much as possible while remediation of heavy metal contaminated soil,so that the recovered soil can be safely and effectively reused.Thus,this thesis took the soil contaminated with multi-heavy metals in a certain site as the research object,aimed to investigate the washing effect of EDTA,citric acid and Fe Cl₃ on heavy metals and the change of the occurrence pattern of heavy metals from soil aggregates which were screened and graded on particle sizes of 0.25-2.00 mm,0.05-0.25 mm and<0.05 mm.Analyze the changes of organic matter,iron and aluminum oxide,carbon source,nitrogen source,mineral composition and micro-morphological characteristics on soil aggregates before and after washing.The aimed to understand the damage mechanism of eluent on the structure of soil aggregates,explore the influencing factors of washing technology on soil quality.It is hoped to provide the scientific basis of chemical washing technology to remediate soil and reuse environmental risk assessment.The main conclusions were as follows:（1）The distribution of Pb and Cd on the aggregates with different particle sizes was very different（P<0.05）.The concentration of Pb on the aggregates with particle sizes<0.05mm was the highest,and the content of Cd gradually rose to the particle size of the aggregates decreased.On the whole,EDTA and Fe Cl₃ had good washing effect,but citric acid washing effect was poor,EDTA and Fe Cl₃ had the highest washing efficiency on the aggregate<0.05 mm.（2）The chemical forms of heavy metals in aggregates with different particle sizes are different,compared with the large particle size aggregates,the effective heavy metal content in the small particle size aggregates is greater.The proportion of effective Pb in small particle size aggregates increased after washing.So it is necessary to pay attention to the change of the potential environmental risk of heavy metals in different particle size aggregates after washing.（3）The chemical eluent will change the morphology of the iron and aluminum oxides on the aggregates,thereby destroying the original agglomeration mechanism of the soil aggregates,and lead to the disintegration of the large-particle water-stable aggregates,destroying the soil aggregates’structure,reducing its stability.But Fe Cl₃ has less damage to the aggregates than EDTA and citric acid.Therefore,considering the extraction efficiency of heavy metal and the impact on the stability of soil structure,Fe Cl₃washing is more conducive to the remediation and reuse of heavy metal soil.（4）The addition of organic matter can promote the formation of aggregates with larger particle size to a certain extent and improve the stability of aggregates,but the increase in organic matter will have a negative effect on the washing effect of citric acid.In addition,chemical washing can change the organic matter,total nitrogen concentration and C/N on aggregates with different particle sizes,thus affecting the activity of microorganisms in soil and indirectly affecting the composition structure of aggregates.（5）The changes of organic matter,C/N,iron/aluminum oxides and soil mineral composition on the soil aggregates after washing should be taken into full account when assessing the environmental risks of soil reuse and exploring techniques to restore the original ecological functions of the soil.