Magnetic characterization and iron oxide transformations in Technosols developed from thermal power station ash

Fly and bottom ash originating in thermal power stations (TPSs) are industrial wastes containing magnetic minerals, primarily Fe-bearing phases, which are formed during coal combustion. Ash deposited on disposal sites becomes the parent material for technogenic soils (Technosols), in which the transformations of Fe-bearing magnetic minerals accompany the soil-forming processes. This study presents the application of magnetic methods together with mineralogical techniques for the determination of the magnetic properties and mineral composition of (1) fresh (unweathered) fly and bottom ash resulting from bituminous coal and lignite combustion, and (2) two Technosols developed on dry ash disposal site and former ash settling pond. Magnetite with a mean coercivity (B₁/₂) of 36–47 mT and hematite (B₁/₂ of 200–900 mT) were found to be the major magnetic minerals of fresh fly and bottom ash. The size of magnetic grains of ash ranged from 1 to 20 µm, which corresponds to the typical pseudo-single-domain grains. The heterogeneity of the parent material (i.e. TPS ash) along the soil profiles significantly affected the magnetic parameters and composition of magnetic minerals. In the soil developed from bituminous coal ash, the magnetic susceptibility ranged from 771 to 2538 × 10⁻⁸ m³ kg⁻¹, whereas in the soil developed from lignite ash it was 622–7657 × 10⁻⁸ m³ kg⁻¹. The most abundant Fe oxides in Technosols were (i) magnetite with (ii) a few percent of hematite admixture (both minerals mainly inherited from fresh ash and partly neoformed), and (iii) low-coercivity maghemite (B₁/₂ ~ 15–20 mT) which most likely formed by the surface oxidation of fine magnetite grains inherited from the parent material. Magnetic methods were found to be a valuable research technique in the case of TPS ash-derived Technosols, for the identification of Fe-bearing magnetic minerals that are hardly recognizable by mineralogical methods, and for tracking the transformations of magnetic minerals resulting from soil-forming processes.