Assessment And Remediation Of Heavy Metal Pollution In A Farmland Of Dongdagou,Baiyin City

The situation of heavy metal pollution in farmland soil in China is severe,and there are characteristics such as significant regional differences,long periods and difficulty in a unified treatment.It is urgent to develop targeted,scientific and comprehensive remediation technology to improve the ecological environment of local soil by considering the regional conditions,on the principle of no excessive remediation.This study takes a farmland soil polluted by heavy metal compounds in Dongdagou,which is located in Baiyin,Gansu,as the research object,and the following works have been done:（1）We get the current situation of heavy metal pollution in the soil of the study area through assessment;（2）Four passivators,Hydroxyapatite,Humic acid,Attapulgite and Goethite,have been selected by laboratory soil incubation experiments;（3）Based on the previous experiments,the Hydroxyapatite and Humic acid,combined with Pennisetum giganteum,have been determined as the composite materials for remediating the polluted farmland soil in the study area.According to the changes in the morphological distribution of heavy metals in soil,the physiological response of Pennisetum giganteum,and the diversity of the microbial community,before and after the experiment,the ability of our scheme to remediate the heavy metals in soil and the effect of restoration to the ecological environment of soil have been evaluated.This study aims to explore the technology for removing heavy metal pollution in farmland in the arid area of Northwest China,and contribute to developing the harmonious regional agricultural environment and enhancing food security.The main conclusions are as follows:（1）The surface soil in the range of 0-20 cm is severely polluted,and there is a significant risk of ecological harm.The concentration of Cd(average 17.4 mg·kg^-1)far exceeds the control value outlined in the GB15168-2018.Moreover,the concentration of Cu,Pb and Zn(average 215.6,265.8 and 830.8 mg·kg^-1)also exceeds the filter value in the GB15168-2018.The results of Nemerow index calculation method,Pollution load index method,and Potential ecological hazard index method show that the study area has been severely polluted.The Cd is the main pollution factor that needs to be strictly controlled and treated.（2）Hydroxyapatite,Humic acid,Attapulgite and Goethite can passivate the four heavy metals（Cd,Pb,Cu and Zn）in the study area,among which the effects of Hydroxyapatite and Humic acid are prominent.The usage of Hydroxyapatite and Humic acid significantly reduced the bioavailability and toxic leaching level of Cd,Pb,Cu and Zn.The results of Scanning electron microscope and Fourier transform infrared spectroscopy show that PO₄^3-and Ca²⁺contained in Hydroxyapatite can form precipitation and isomorphic substitution with Pb²⁺and Cd²⁺;The rich carboxyl groups and phenolic hydroxyl groups on the surface of Humic acid could firmly combine with Cu²⁺and Zn²⁺.The addition of Humic acid can increase the organic matter and reduce the pH value in soil,while Hydroxyapatite is the opposite.The combination of the two passivators may complement each other and reduce the disturbance to the environment.（3）In the treatment group of the field experiment,2%Hydroxyapatite combined with 1%Humic acid shows a comprehensive effect on reducing heavy metals’active contents.Under this treatment level,the percentage of active contents of Cd,Pb,Cu and Zn in soil decreases by 18.59%,9.12%,11.83%and 3.33%,respectively.The treatment group could also significantly increase the biomass of Pennisetum giganteum,weaken its oxidative damage and enhance the ability of photosynthesis.In addition,without affecting the pH value and conductivity of soil,the Hydroxyapatite and Humic acid in the treatment group significantly improve soil fertility.The treatment group does not cause secondary pollution and shows a certain degree of environmental friendliness.（4）The application of Hydroxyapatite and Humic acid,combined with Pennisetum giganteum,has been demonstrated as a feasible,reasonable and effective way to remediate the soil polluted by heavy metals in Dongdagou,Baiyin City.Pennisetum giganteum can promote the transfer of heavy metals from soil to plants and realize the removal of heavy metals in soil.Among the four heavy metals in the study area,Pennisetum giganteum shows a strong enrichment ability to transfer Cd from the soil to the underground part and Zn from the underground part to the overground part.The passivation effect of Hydroxyapatite and Humic acid on heavy metals and the enrichment effect of Pennisetum giganteum on heavy metals form a unity of opposites,which can reduce the active contents and concentration of heavy metals in the soil at the same time.（5）Using Hydroxyapatite and Humic acid combined with Pennisetum giganteum,the treatment increases the number of soil microorganisms and enriches the microbial species and community diversity in the study area.The rhizosphere effect of Pennisetum giganteum increases the richness and diversity of bacterial species in soil,which significantly increases the total amount and unique number of OTUs of the bacterial community in soil and improves the composition of species.The application of Hydroxyapatite and Humic acid,combined with Pennisetum giganteum,is beneficial to maintaining the balance and stability of soil ecology and has positive significance for the stability of heavy metals in soil and restoration of the ecological environment.