Pollution Properties Of Heavy Metals In Xuzhou Urban Topsoil And Their Environmental Magnetic Proxies

In recent years, heavy metal pollution in many cities is significantly serious due to the rapid development of industrial activities, transportation as well as urban and suburban construction. In this paper, several properties including enrichment, spatial distribution and sources of heavy metals(i.e., Ti, Cr, Mn, Fe, Ni, Cu, Zn, Mo, Cd and Pb) in Xuzhou urban topsoils were studied according to the four functional zones(i.e., industrial, traffic, residential and public green areas). In accordance with multivariate statistics, environmental magnetism, SEM-EDX and XRD technologies, the magnetic properties of Xuzhou urban topsoil were investigated. Finally, the relationship between the concentration of heavy metals and the values of magnetic parameters was explored. The main conclusions were drawn as follows:1. Among the 10 heavy metals(Ti, Cr, Mn, Fe, Ni, Cu, Zn, Mo, Cd and Pb), the average concentrations of Cd and Zn were 5.8 and 2.24 times of the background values, respectively. The concentrations of Cr, Ni, Cu, Mo and Pb, on averge, were 1.12 to 1.77 of the respective background value. Multivariate statistical results shows that Ti and Mn are mainly from soil parent materials and Cu, Zn, Cd, Pb and Fe are mainly from anthropogenic activities.The block ratios [C0/(C0+C)] of Cu, Fe, Zn, Cd and Pb are all within the range of 0.25-0.75, indicating that Cu, Fe, Zn, Cd and Pb are moderately spatially dependent and their spatial variability was not only affected by soil parent materials, geographical and climatic factors, but also affected by anthropogenic activities. However, the block ratios of Ni, Cr, Ti, Mo and Mn are not large than 0.25, indicating the spatial variability is dominated by dependence variability. Overall, concentrations of Pb, Cd, Cu, Zn and Fe from urban central areas topsoils are significantly higher than those from suburban areas where anthropogenic activities are not so intense.2. The mean value of specific magnetic susceptibility in this present study is 234.6×10-8 m3·kg-1, and is significantly higher than the local background value. This result indicates the presence of enrichment of anthropogenic magnetic minerals in this area. Moreover, significant positive correlation between both SOFT and SIRM indicated that the ferrimagnetic minerals of the MD and PSD grains are the major controller to the magnetic properties of urban topsoils investigated in this present study.In addition, the results from thermomagnetic analysis and hysteresis loop of representative samples showed that the magnetic properties of topsoils from industrial, traffic and residential areas were dominated by magnetite while magnetic characteristics of public green samples were controlled by magnetite and hematite. Spatial distribution of χ, SIRM and SOFT depicts a trend in which the highest value was found in industrial area near Xuzhou Iron-Steel Plant in the northeast of Xuzhou urban area, the sub-high value in topsoil near the road with high traffic intensity, and the low value in residential area and green area. These results indicated that the order of contents of magnetic minerals was industrial area > traffic area > residential area > public green area.3. The magnetic diagnosis model based on the relationships between heavy metals including Zn, Pb, Fe, Cu, Cd and magnetic parameters were established by applying stepwise regression method. When χARM < 64×10-8 m3·kg-1 and SIRM < 278×10-5 A·m2·kg-1, it shows basically no heavy metal pollution in the topsoil within the study area. When 64×10-8 m3·kg-1 ≤ χARM < 147×10-8 m3·kg-1 and 278×10-5 A·m2·kg-1 ≤ SIRM < 1278×10-5 A·m2·kg-1, it shows slight heavy metal pollution. When 147×10-8 m3·kg-1 ≤ χARM < 230×10-8 m3·kg-1, 1278×10-5 A·m2·kg-1 ≤ SIRM < 2278×10-5 A·m2·kg-1, it shows moderate heavy metal pollution. When χARM ≥ 230×10-8 m3·kg-1 and SIRM ≥ 2278×10-5 A·m2·kg-1, it shows strong heavy metal pollution in the topsoil within the study area.4. Results from SEM/EDX of magnetic extracts from fly ashes, cements, street dusts and soil samples showed that all these extracts exhibited typical anthropogenic magnetic particle shape. Also, the presence of some heavy metals in these magnetic extracts was observed. Furthermore, the concentrations of Cu, Zn, Cd and Pb in these magnetic extracts are larger than those of non-magnetic material.