Magnetic properties of different pollution receptors around steel plants and their environmental significance

Our aim is to effectively monitor environmental pollution around steel plants using magnetic properties of different material: fly ashes on the surfaces of tree bark, grass and garden, soil, and tree ring cores from four different trees. Samples were collected front the vicinity of two steel plants, located at Xinglong County (Hebei Province) and Loudi County (Hunan Province), respectively. Environmental pollution on different time-scales could be assessed by using grass as short-term receptor tree bark and soil as short-to medium-term receptors and tree ring cores as medium- to long-term receptor. Magnetic measurements included temperature-dependence of magnetic susceptibility and remanence, hystersis loops, First-Order Reversal Curves (FORCs) and low-temperature experiments, and microscopic SEM, EDX and XRD analyses. The results indicate that the fly ashes and soil samples are predominated by magnetite, hematite and pure iron. Magnetite grains are spherical and grain size in different receptors ranges from 8 to 50 mu m whilst the pure iron grains generally have an irregular shape. Low-temperature measurements show that magnetite grains exist in the tree ring cores of willow, whereas the tree rings cores of polar, maple and camphor are most likely dominated by para-or supermagnetic minerals whereas ferromagnetic minerals are scarce or absent. This suggests that only the willow tree ring core could serve as a magnetic indicator for long-term pollution induced by the heavy industry activity. We conclude that rock magnetic and microscopic investigations could be an effective method for determining magnetic minerals and grain sizes of ferromagnets induced by heavy industry activity; they provide a rapid and useful technique for monitoring environmental pollution.