Mechanisms for co-stabilization of multiple heavy metals in thermally treated solid waste systems

Abstract

Hazardous heavy metals in the solid wastes pose a great threat to environment and ecosystems. Ceramic sintering technique is a promising method for achieving the excellent stabilization of heavy metals-laden solid wastes together producing useful ceramic products. However, more complicated sintering backgrounds as well as the multiple heavy metals co-existing situations in the practical solid wastes may enhance the difficulties for the stabilization of heavy metals. This study aims to explore the mechanisms for heavy metals stabilization accompanied with their stabilization efficiencies in the sintered products from the practical or simulated solid wastes sintering systems. Firstly, by using α-Fe2O3 and γ-Al2O3 as ceramic precursors, a low temperature sintering system was developed to stabilize hazardous nickel (Ni) in the nickel-laden electroplating sludge (Ni sludge) via phase transformation into stable crystal structures. A quantitative analysis of the components in the sintered products was performed by using the Rietveld refinement. The generated NiFe2O4 and NiAl2O4 spinel, as the predominant Ni-hosting phases, could go against strong acid attacks, leading to the excellent Ni stabilization effect (leached Ni ratio below 0.06%) for both α-Fe2O3 and γ-Al2O3 precursor series at attainable temperature condition. This study proposed a promising and quantitative method for controllable Ni stabilization of the hazardous industrial sludge via developing spinel structures in the sintered products, which may provide a feasible strategy for the treatment and beneficial utilization of heavy metal-laden solid wastes.

And then, a mixture of NiO+Cr2O3 was selected to simulate the multiple heavy metals containing solid waste. After sintering with α-Fe2O3 or Fe3O4 precursor, the hazardous Ni and Cr could be successfully incorporated into the NiCr2-yFeyO4 spinel solid solutions. The incorporation mechanisms of Ni and Cr as well as the structural properties of the as-formed NiCr2-yFeyO4 spinel solid solutions were deeply explored with the assistance of several characterization techniques including X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). This study proposes an innovate strategy for simultaneous stabilization of Ni and Cr in multi-heavy metals containing industrial wastes.

To explore the effect of CaO/SiO2 on the phase transformation in the sintering systems of heavy metals contaminated solid wastes, two sintering systems including the CdO-NiO--Al2O3-SiO2-CaO sintering matrix and the CdO-PbO--Al2O3-SiO2-CaO sintering matrix were developed to explore the effect CaO/SiO2 mole ratios on the phase transformation and stabilization effect of heavy metals in the simulated solid wastes sintering systems with the co-existence of multiple heavy metals. Particularly, it was firstly revealed that the oriented regulation of Cd/Ni- and Cd/Pb-containing phases in the sintered products could be achieved by adjusting the mole ratio of CaO/SiO2 during the sintering process. Besides, Ca was proved to play a critical role on the determination of the leaching behaviors for heavy metals. The studies provide a pathway for achieving the targeted control of heavy metals-containing phases and their stabilization efficiencies, which sheds light on future studies about heavy metals co-stabilization in practical CaO/SiO2-containing solid wastes.