Effect Of Modified Fly Ash On Environmental Behavior And Safety Of Soils Contaminated With Pb And Cd

Soil heavy metal pollution is one of the most serious environmental problems in the world.Most of the pollution comes from the release of heavy metal ions during the development of mineral resources and metal smelting,as well as pesticide residues in agricultural production activities,especially,lead（Pb）and cadmium（Cd）in contaminated soil,which are characterized by low mobility,high toxicity,strong durability and difficult to remediate,will not only threaten the quality of the atmosphere,water and crops,but also harm the health of human beings and animals and plants through the food chain.Studies have shown that in-situ immobilization of soil heavy metals is a kind of relatively efficient remediation measures,but there are many differences in the selection of amendments,which will cause various effects..This study aims to synthesize two kinds of low cost and high efficiency amendments with fly ash as raw material on the basis of the previous research of the subject,and apply them to three kinds of soils contaminated by heavy metals.The experiments were carried out by soil column leaching,to explore the stabilization and remediation mechanism of the materials for heavy metals.（1）Preparation and characterization of materials.Fly ash（FA）from coal-fired power plant was used as raw material and Na OH as modifier,a low-temperature roasting and hydrothermal methods were used to modify the fly ash resulting in two new types of adsorption materials-modified fly ash（MFA）and artificial zeolite（ZE）.MFA and ZE were characterized by scanning electron microscopy（SEM）,X-ray diffraction peak（XRD）,Fourier transform infrared spectroscopy（FTIR）and other techniques to explore the modification effect.（2）Design of leaching experiments.The experiment was operated in the form of soil leaching.The tested soils were Guanzhong Lou soil,Hanzhong yellow-brown soil and Hunan paddy soil,respectively.Then,MFA,ZE and natural zeolite（ZO）were mixed with the soil according to the dry weight ratio of 0.5%（wt/wt）,1%（wt/wt）and 2%（wt/wt）,respectively,and the leaching experiment was operated.The experiment was carried out for a total of 6 months,during which the leachate and soil samples were collected and various indicators were tested and analyzed.（3）Analysis of the components of leachate and soils.The effects of amendments on pH,DOC,dissolved Pb and Cd in soil leachate,as well as on organic matter,available Pb and Cd and microbial community diversity in soils were explored by leaching experiments.The results showed that the activity of heavy metals in the three soils decreased significantly after 6 months of leaching experiment,but the effect in Hanzhong soil was most obvious.For Hanzhong soil,after 11 weeks of leaching column operation,the contents of dissolved Cd and Pb in soil leachate decreased below the detection limit due to the stabilizing effect of amendments,and the DOC also decreased and stabilized from160mg L^-1to 30mg L^-1.It was found that among the three amendments,ZE had the best immobilization effect on Cd,and MFA had a better stabilization effect on Pb,the p H of this two treatments increased by 0.9 and 0.45 units,respectively.For the soils of Guanzhong and Hunan,ZE had a significant stabilizing effect on Cd,and the available Cd content decreased by 40.5% and 53.2%,respectively.However,MFA had a stronger stabilizing ability than both ZE and ZO on Pb,and the available Pb content in the soils of Guanzhong and Hunan decreased by 48.3%and 30%,respectively.（4）Analysis of microbial community abundance and its effect on soil environment.After 6 months of operation on the leaching column,it was observed that the microbial communities of the same soil type tended to cluster,and the microbial communities of different soil types were independent of each other.In addition,the application of amendments also has significant influence on the microbial community,among which ZE has the most obvious effect.The richness of bacteria with heavy metal resistance genes decreased,while the richness of other bacteria increased.Proteobacteria and Bacteroidetes were still the dominant phyla,indicating that ZE effectively reduced the toxicity and activity of heavy metals,and it played a positive role in promoting the improvement of soil environmental quality.To sum up,the modified fly ash has a good stabilization effect on heavy metals in both water and soil environment,and is an economic and efficient amendment that helps to improve the quality and safety of soil environment.