Environmental relevance of solid by-products from Municipal Solid Waste Incineration assessed by combining magnetic and mineralogical analysis

Bottom (BA) and Fly Ashes (FA) from Municipal Solid Waste Incineration (MSWI) represent huge amounts of solid by-products that still pose environmental and health problems. Some routes for MSWI ashes management/reuse have been proposed (e.g., inertization, landfilling, reuse as filler, geopolymers), but not all of BA and FA hazardous components are fully understood. Precise identification of minerals from BA and FA is challenging due to several factors: high number of phases, MSWI combustion temperature, and variable chemical composition of feedstock materials. In addition, (trans)formation of magnetic iron oxides, which have been correlated with heavy metals pollution and the presence of toxic ultrafine superparamagnetic (SP) grains in a range of materials, occurs during incineration and quenching. We have undertaken the study of BA and FA samples from Italian MSWI plants by combining magnetic and mineralogical analysis for probing mineralogy and the extent of SP grains. The BA and FA samples are characterized by narrow hysteresis curves and low coercivity (Bc, 7.2 – 14.1 mT), suggesting significant reversible component of magnetization. Also, the analysis of thermomagnetic properties shows that both BA and FA gain magnetization during cooling. The Low temperature remanent curves by Magnetic Properties Measurement System (MPMS) show magnetite-like shapes for most of samples, but the Vervey transition in FA samples is not clear probably due to the presence of oxidized/impure magnetite or unblocking of SP grains. Measurements of AC susceptibility by MPMS might support the fact of a significant contribution of SP grains in FA (FA show larger frequency dependence than BA). We performed XRD analysis on FA and BA samples, including different magnetic extracts of BA in order to shed some light on iron oxides phases. The main mineralogical phases found in BA are quartz, calcite-vaterite, melilite group minerals and plagioclase; FA contain Ca-aluminosilicates and more sulphates and chlorides with respect to the BA. Iron oxides such as wurstite, hematite, and the magnetic spinel-type iron oxides are noted both in BA and FA. The XRD pattern on BA magnetic extracts confirms that magnetite or impure spinel-type iron oxides (e.g., Mg-magnetite, Ti-magnetite, and maghemite) are in charge of their strong magnetic response. However, on the basis of previously obtained chemical analysis, the presence of impure magnetite containing Cr, Zn, Mn, and Cu cannot be ruled out neither in BA nor in FA. Rietveld refinement to assess the extent of minor metals substitution is ongoing. These preliminary observations emphasise the metastable nature of MSWI ashes and might lead to a better assessment of the environmental impact related to iron oxides.