Abstract
Release of heavy metals from different industries and industrial waste is a major global threat for as well humans as ecosystems. In this study, immobilization of an industrial filter sludge (FS) with an extremely high content of several heavy metals (24.6 wt% Pb, 21.7 wt% Hg, and 9.00 wt% Se) and sulfate via calcium sulfoaluminate-belite (CSAB) cement was tested. The ratios of 25%, 50% and 75% of CSAB addition were tested, and the target was to achieve immobilization of the hazardous components. The leaching of Pb, Hg, SeO4, SO4, Ni, Cd, Cu, and As was monitored, and the structure of the immobilized materials was examined via X-ray powder diffraction (XRD) and field emission scanning electron microscopy-energy dispersive spectroscopy (FESEM-EDS) analysis. It was observed that Hg, Cu, As, Cd, and Ni were immobilized completely and leaching of Pb was reduced by 69% from the theoretical release. On the other hand, the leaching of SeO4 and SO4 experienced major increase when CSAB was added. XRD indicated significant ettringite formation as the amount of added CSAB increased, and the formation of gypsum as the amount was decreased. FESEM-EDS indicated that the immobilization was largely based on encapsulation into the CSAB binder, but chemical immobilization into the ettringite binder was also observed. It was concluded that the increased release of SO4 and SeO4 might have resulted from an excess amount of sulfates (added gypsum) during hydration.
Highlights
Various industrial processes generate hazardous, heavy metals containing waste materials that need to be treated before their disposal
Leaching of the different components was highly dependent on the solution pH, and this was seen for the filter sludge (FS)
The leaching of lead could be a result of it appearing as anglesite, which is known to have low solubility acidic conditions, but to be soluble at high pH (Keim and Markl, 2015; Rumble, 2019)
Summary
Various industrial processes generate hazardous, heavy metals containing waste materials that need to be treated before their disposal. Several studies have tested the immobilization of hazardous components, for example, heavy metal cations, such as Cr3þ, Pb2þ, Cu2þ, and Zn2þ (Niu et al, 2019, 2018; Wu et al, 2012), and various oxyanions, such as SeO42À, CrO42, AsO43À, and MoO42À (Guo et al, 2017; Kiventera€ et al, 2019; Zhang and Reardon, 2003) into the ettringite structure with encouraging results. The content of hazardous components was relatively low This is the first time a solid material with extremely high content of multiple hazardous components (more than 60%) was stabilized with CSAB cement, and the heavy metal immobilization capacity of the CSAB solidification was studied. In order to get a reference for the level of hazard of the leaching components, the measured leaching rates were compared to the Finnish environmental regulations for landfilled materials (FINLEX ® 331, 2013)
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