| In recent years,the environmental pollution caused by the rapid growth of coal chemical industry has become a major restriction on the development of modern coal chemical industry in China.Above all kinds of pollutions,the heavy metal pollution in soil is one of the urgent problems need to be solved,since it is harmful to the ecological environment and can be very toxic to human health.Therefore,studies on the prevention technologies of soil heavy metal pollution and the protection methods for human health are highly significant for the development of modern coal chemical industry,economic development and national energy strategy in China.Based on the accumulation and the spatial distribution characteristics of the heavy metals in soil in the coal chemical plant area,the source determination of the heavy metals and the human health risk assessment are necessary methods to protect the environment and human health from heavy metals pollution.In this research,a chemical plant located in the northwest of China is the study object.the current accumulation and spatial distribution characteristics of the heavy metals including Pd,Cd,Cr,Hg and As in the soil near the plant were analyzed.Based on the leaching experiments in the lab and the simulation model,the accumulation and spatial distribution characteristics of Pd,Cd,Cr,Hg and As in this area in the next 10,15 and 20years were predicted.The source of the heavy metals including Pd,Cd,Cr,Hg and As were analyzed based on the distribution of the heavy metals in soil and the production units in the plant.The risk levels of Pd,Cd,Cr,Hg and As to human health were also assessed and a spatial risk level distribution map of those heavy metals was obtained using the geographic information system and the geostatistics method.The human health risk warning was also performed according to the content prediction of the above mentioned heavy metals.The main conclusions of this study are as follows:1)The accumulation and spatial distribution characteristics of heavy metals in soil near the coal chemical plan.?1?The study on the accumulation characteristics of the heavy metals shows that the average concentrations of Pd,Cd,Cr,Hg and As are 17.087,0.553,93.432,0.092,12.840mg·kg-1 respectively.The concentrations of all 5 heavy metals listed above have meet the secondary environmental quality standard of inorganic pollution for industry land.However,the average concentrations of Cd,Cr and As are 4.94,1.56 and 1.08 times higher than the reference value in Ningxia Province,which indicates that the coal chemical production process has significant influence on the accumulation of those heavy metals in soil.?2?The study on the spatial distribution of the heavy metals reveals the areas where the highest contents of different heavy metals in soil are observed.The highest Pd content is obtained in the soil near the coal entry corridor and Pd distributes along the transportation route.The highest Hg content is obtained in the soil near the power unit and the south area of it.The Hg content is reducing from the power unit to the surroundings.The highest Cd content is found in the soil near the coal entry corridor and the doorway area for people.Cd distributes along the transportation route to the inside area of the plant.The highest Cr content is observed in the soil near the northeast of the gasification unit and the power unit,and it reduces to the surroundings.The highest As content exists in the area between the right side of the power unit and the coal entry corridor.It distributes to the surroundings with a reducing trend.2)The prediction of the accumulation and spatial distribution characteristics of heavy metals in soil near the coal chemical plant area.?1?The relationship between time and the contents of heavy metals in soil was studied.The contents of Pd in 2021,2026 and 2031 in the same plant area will be 1.75,2.01 and 2.22 times higher than the value in 2011 respectively.The contents of Cd will be 10.85,13.40 and 15.08 times higher.The contents of Cr???will be 2.20,2.20 and2.20 times higher.Hg's contents will be 29.71,35.57 and 45.00 times higher and the contents of As will be 2.69,3.26 and 3.70 times higher.?2?The study of the relationship between time and the spatial distribution of heavy metals in soil near the coal chemical plant shows clear changing trends for different heavy metals in soil.The enrichment area of Pd is always located at the both sides of coal entry corridor.After 2026,the areas near the main road in the plant and the surroundings of chimneys in the power unit will have a high level of Pd accumulation.The enrichment area of Hg is situated at the power unit and the south area of it.The pollution range will extend to its surroundings over time.After 2021,the increasing speed of Hg in the main area will be 0.050 mg·kg-1 per 5 years.Meanwhile,in the surrounding areas,the speed will be decreased as the increasing distance to the main area.The main pollution area of Cd is found at the main road in the plant,the downwind area of the fuel gas outlet and the coal entry corridor.The content of Cr???has the highest value in soil near the gasification unit and the power unit.The content reduces from the slag yards in the two units to their surroundings in a ladder-like manner.In 2021,the concentration of Cr???will exceed the standard value in the whole plant range and the highest value will be184.32%of the standard.In 2021,the high As content area is located near the fuel gas outlet in the power unit.In 2026,a As pollution area with a water drop shape will appear near the fuel gas outlet and the southeast area of it.In 2031,the concentration difference of As between the pollution area and its surroundings will be further expanded.3)The study on the sources of the heavy metals in soil near the coal chemical plant?1?The analysis of the soil sample collected from different locations in the coal chemical plant reveals a significant enrichment for Se,Ag and Cd,a moderate enrichment for Cr,As and U and a slightly enriched state for Ni and Pb.No enrichment is found for Hg.In addition,Cr,Cd,Pb and Ni have the same enrichment level,while As,Hg,Se,Ag and U shows two different enrichment levels in the sample collected from different locations.?2?The correlation analysis between different heavy metals was performed.The correlation levels between different heavy metals are indicated as their correlation coefficients,which are 0.624 for Pd and Cd,0.632 for Hg and As,0.842 for Hg and Se,0.689 for As and Se,0.881 for Cr and Ni,0.761 for Cr and Ag and 0.795 for Ag and Ni.?3?The cluster analysis of heavy metals suggests 3 groups for 8 heavy metals in soil based on their correlations.The first group includes Cr,Ni and Ag;the second group includes Se,Hg and As;and Cd and Pd are in the third group.?4?The main factor analysis and the spatial distribution analysis of factor scores reveals three pollution sources,which are transportation pollution source as factor 1,slag yard pollution source as factor 2 and fuel gas pollution source as factor 3.Factor 1 is located near the doorway for people,the transportation route in the plant and the coal entry corridor.Factor 1 mainly affects the distribution of Pd and Cd in soil near the plant.Factor 2 which mainly affects the distribution of Cr,Hg and Ni in soil exists near the gasification unit,the power unit and the methanol synthesis unit.Factor 3 which affects the distribution of Hg,As and Se is found in the downwind area of chimneys in the power unit.4)The human health risk assessment of heavy metals in soil near the coal chemical plant.?1?The average carcinogenic risks values of Cd,As and Cr???are 1.75E-06,1.05E-05 and 2.08E-05 respectively.The carcinogenic risk exposure pathways of Cd and As are oral ingestion and inhalation,and the risk exposure pathway of Cr???is inhalation.?2?The average non-carcinogenic risks values of Cd,As,Cr???,Pb and Hg are1.95E-02,0.54,8.38E-02,5.94E-04 and 5.91E-03,which are all lower than the safe value for non-carcinogenic risk.Inhalation is the main pathway of non-carcinogenic risk exposure for Cd,and As.Oral ingestion is the main risk exposure for As,Pb and Hg.Skin contact is the main risk exposure for Cr???.?3?According to the risk distribution map of heavy metals,the area with the highest Cd risk is found near the coal entry corridor and the doorway area for people.The risk of As covers the whole plant and the As pollution level is much higher than the standard.The center area of the As risk exists at the southeast of the power unit.It covers the most area of the power unit and a small part of the gasification unit.The risk of Cr???distributes in the whole plant and is also higher than the standard.The center area of the pollution is situated at the gasification unit and the power unit.The non-carcinogenic risk of Pb distributes clearly along with the transportation route.The highest non-carcinogenic risk area of Hg is located at the power unit and the area between the power unit and coal entry corridor.?4?Heavy metal health risk warning indicates:Human harm caused by As includes carcinogenic risks and non-carcinogenic risk,whose high risk is around the chimney downwind and expanding to the factory over time;Human harm caused by Cd and Cr???is mainly carcinogenic risks,the high risk of Cd is around where people flow into and out of the corridor,which expanding twords the field along the traffic line;Cr???highly carcinogenic regions are located in the power unit and the gasification unit which are virtually unchanged over time;The non-carcinogenic risk of Pb and Hg is low. |
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