Pollution degree assessment using magnetic methods
Iron is one of the most abundant elements in the Earth’s crust and forms, in combination with oxygen and sulphur, magnetic minerals which are omnipresent in our environment. They result from various natural geo(bio)chemical processes, but also from manmade activities such as, iron works, industrial combustion processes, traffic, and waste discharge. Magnetic minerals have excellent adsorbent properties and do attract heavy metal ions. Generally, particles resulting from combustion processes contain between 2 and 20 weight % of magnetic iron oxides. The highest values are found in fly ashes resulting from fossil fuel combustion, in particular coal combustion. Magnetite and related spinels as well as haematite are the principal magnetic minerals found in industrial fly ashes often occurring as spherules. Heavy metals can be incorporated into the magnetic mineral structure or adsorbed to the surface of the magnetic particles. They are emitted during the com bustions processes in power plants, foundries, incinerators, cement factories and traffic and both follow the same sedimentation path. Hence, magnetic methods can be used for the characterisation and assessment of pollution and some magnetic parameters like magnetic susceptibility are proxies for the heavy metal content. However, some conventional geochemical measurements are required for calibration.
Magnetic susceptibility (MS) is a measure of the ability of a sample to acquire a magnetisation in a magnetic field and is expressed as the ratio of the magnetisation M induced in the sample over the inducing magnetic field H. This magnetic parameter can be used as alternative, non destructive, cost-efficient proxy method to map polluted soils. Figure 1 displays magnetic susceptibility maps of the soil surface from two allotment gardens. The left map shows considerably lower MS values compared to the right. Geochemical control measurements confirm indeed that the lead concentration is lower in the left garden.
Fig.1
Magnetic characterisation of rocks, soils, minerals and materials
Formation processes and environmental conditions control the nature, quantity, magnetic grain size and properties of magnetic mineral populations. Hence, structure sensitive magnetic property measurements (e.g. magnetic susceptibility, coercive force, induced and remanent magnetisation) yield information about their origin and ambient formation conditions. Applying magnetic property analyses one can find out for instance if the magnetic minerals present in a soil originate from industrial processes or possibly from natural soil formation. Magnetic minerals resulting from the latter process are in general much smaller and differ from industrial emitted particles. Due to their small grain size the remanent magnetisation decays within short time, while this is not the case for larger particles. Soil magnetic properties can also be used to study soil erosion, soil mass movement and ancient soil occupation. In general, the field of applications of rock magnetic properties of materials is very large. They are needed for the interpretation of magnetic prospections, they are applied for the determination of the magnetic mineral content in ores, diagraphie of cores, stratigraphic purposes but also for characterisation of magnetic particles used in molecular biology, medicine, magnetic separation techniques, water and waste treatment.
Measurements of magnetic susceptibility in function of temperature can shed light on the magnetic mineral type present in a sample. Magnetic minerals have distinct high and low temperature transitions where the susceptibility changes sharply and which are characteristic for the magnetic mineral type. Magnetite for instance has an order-disorder transition, called Curie-temperature, of 580°C and shows further a crystallographic transition, called Verwey transition, at -153 °C that are both intrinsic properties of this spinel (see Figure 2). Haematite, in contrast, has a Néel-temperature of 675° C and Morin-transition at about -13°C (Figure 3).
Fig.2 (a) High and low temperature variation of MS in a magnetically enriched ancient soil sample of the Loess Plateau in the P.R. China at Huangling, showing the Verwey transition and Curie temperature of magnetite, indicating that magnetite was present before heating the sample. (b) Low temperature variation of MS of a natural magnetite sample in phyllades at Beaurieux (Belgium) showing a sharp Verwey transition.
Fig. 3 Low temperature MS of an artificial haematite sample with a very sharp Morin transition at -12.5 °C.
Fig. 4 For magnetic remanence measurements CPG uses a three-axis 2G cryogenic rock magnetometer with an access of 76mm, equipped with DC squids and a pulse cryogenic refrigerator, which is mounted on top.
Fig. 5 Low field magnetic susceptibility measurements in the field in function of depth a) Measuring unit (red box) and plastic tube that contains the detection coil that moves up and downwards. b) Examples of different MS profiles displaying maximum values at depths less than 15cm.
Fig. 6 Hysteresis parameters are obtained on loose samples in a J-coercivity meter. The sample is mounted inside the rotating Plexiglas disk (arrow). While rotating, the magnetic field is incremented each time before the sample passes in the air gap of the electromagnet. Induced and remanent magnetisation are measured in two different induction coils at each field increment.
