| As an important components of the terrestrial system,soil provides the material basis for human survival.Therefore,the quality of the soil ecological environment can directly affect our quality of life.At present,due to the increase of urbanization rate and rapid population growth,human activities directly or indirectly affect the content of heavy metals in the soil,excessively disturb the natural ecosystem,and cause the quality of the soil environment to continue to decline.Therefore,the analysis of distribution law,migration characteristics,ecological and environmental risks of soil heavy metals have certain guiding significance for the improvement of local ecological environment and social and economic development.This research focuses on the soils of different geomorphic units in Yinchuan Plain,taking soil heavy metals as the research object.Collected 17 soil samples of three typical geomorphic units,using X-ray diffraction,X-ray fluorescence spectroscopy,ICP-MS,and total organic carbon tester to analyze soil minerals Composition,major elements,eight types of heavy metals and their five forms,total organic carbon content,and group experiments were used to study the adsorption behavior,a total of 1479 experimental data were obtained,using statistical analysis methods,correlation analysis,environmental risk assessment methods,etc.Methods to conduct research on the distribution and occurrence of pollutants,adsorption behavior,ecological and safety risks.The following results have been achieved:1.Distribution characteristics of soil heavy metals in different geomorphic units(1)The average contents of total heavy metals Cr,Ni,Cu,Zn,Cd,Pb,Hg,As are 81.84mg/kg,39.08 mg/kg,29.69 mg/kg,90.49 mg/kg,0.16 mg/kg,32.70 mg/kg,0.08 mg/kg,15.91 mg/kg.Compared with the background values of soil elements in Ningxia,the levels are higher than 50.28%,79.76%,84.64%,104.77%,60.00%,89.57%,515.38%,74.64%,respectively.(2)The order of heavy metals Cr,Zn and Hg in different geomorphic units is first terrace> second terrace> piedmont alluvial fan,and the content of Cd,Cu and Ni is from large to small to piedmont alluvial fan> second First-level terraces>first-level terraces,the order of Pb and As is piedmont alluvial fan> first-level terraces>second First-level terraces.2.Occurrence forms and influencing factors of soil heavy metals(1)The occurrence forms of Cr,Ni,Cu,Zn and As in the soil of different geomorphic units are mainly in the residue state,which is less harmful to the ecological environment.However,the potential utilization of Cd and Pb is relatively large,the activity and bioavailability are relatively strong,and the ecological risk is relatively strong.(2)In the three geomorphic units,the exchange states and iron binding states of nickel,lead,and copper are in the piedmont alluvial fan> the second terrace> the first terrace;the carbonate and organic binding states of zinc and lead are in the first terrace> second terrace > piedmont alluvial fan;the residual state of Cr,Ni,Pb and Cu is secondary land> first terrace> piedmont alluvial fan.(3)The form distribution of heavy metals is affected by factors such as total amount,p H and TOC.The high salinity in the Yinchuan Plain results in a higher p H of the soil,which is beneficial to reduce the activity of heavy metals in the soil.3.Adsorption and migration characteristics of heavy metals in different soils(1)From the perspective of adsorption kinetics,the adsorption of Cu,Pb,and Zn by soil is more in line with the quasi-secondary model,and the adsorption rate of Pb and Cu is ranked in the order of second terrace> piedmont alluvial fan> first terrace For Zn,the piedmont alluvial fan has the fastest adsorption rate,followed by the second terrace,and the slowest is in the first terrace.(2)From the perspective of isotherm adsorption,the first-level terrace and piedmont alluvial fan are more in line with the Freundlich model,and the second-level terrace is more in line with the Langmuir model.From the adsorption capacity,it can be concluded that the adsorption capacity of Pb and Zn is the second terrace> the first terrace> the piedmont alluvial fan,and the adsorption capacity of Cu is the first terrace> the second terrace> the piedmont alluvial fan.(3)From the perspective of thermodynamics,the adsorption of Cu,Pb,and Zn by the three geomorphological unit soils are all spontaneous endothermic reactions and physical adsorption.From the perspective of the spontaneous degree of adsorption,see the second terrace> first terrace> Alluvial fan in front of the mountain.(4)The migration speed of heavy metals in the soils of the three geomorphological units is piedmont alluvial fan> second terrace > first terrace.The migration capacity of heavy metals in the Yinchuan Plain can be summarized as stronger than in the piedmont flood plain in the central and western regions.In the eastern terraces,it is inferred that the capacity of Yinchuan Plain to hold heavy metals has gradually increased from west to east.4.The evaluation of Heavy Metal Pollution(1)Based on the full amount of heavy metals,the Nemerow pollution index and potential ecological hazard methods are used for evaluation.At the same time,the static environmental capacity of the soil of different heavy metals is evaluated.The heavy metals in Yinchuan Plain are in a state of moderate pollution and slight ecological hazards,of which the main The environmental risks brought by Hg and Cd are higher,and the environmental capacity is lower than other heavy metals.Attention should be paid to the possible risks of Hg and Cd.(2)Pollution evaluation of the effective state of heavy metals based on the bioavailability index method and the risk evaluation index method showed that the risk level of the elements except Cd in the soil is mainly medium risk,and the rest are in a low-risk or non-polluting state,but should pay attention to the ecological risks that Cd may bring.(3)The comparison of the evaluation results of the full amount and the effective state of heavy metals shows that the effective state evaluation combines the effectiveness of heavy metals on organisms,which can modify the single factor index method that only considers the total amount of heavy metals,and can better show the soil in the study area Ecological risks of heavy metals. |
