Abstract
Air pollution control (APC) residues, which are known to be the byproducts of incineration treatment, exhibit a high leaching potential of toxic metals. Calcium silicate hydrate (C-S–H), which is a major hydration product of hardened cement and immobilizes toxic metal, can be formed by the reaction of Ca with pozzolanic Si in a highly alkaline environment. Toxic metals might be immobilized by the addition of pozzolanic material to APC residues (instead of using cement), which is a Ca source and provides an alkaline condition. In this study, diatomite, which mainly comprises amorphous silica (SiO2·nH2O), was investigated as a pozzolanic material for Pb immobilization in APC residues obtained from a municipal solid waste incinerator. APC residues were cured with and without the addition of diatomite at different temperatures. When diatomite was added to APC residues, pozzolanic phases such as C-S–H gel were formed via the consumption of Ca(OH)2 and CaClOH. Compared to APC residues cured without diatomite, the leaching of Pb decreased by 99% for APC residues cured for 14 days with 10% diatomite at 70 °C. The results of sequential chemical extraction showed that water-soluble Pb in APC residues was reduced from 10.3% to nearly zero by the pozzolanic reaction. Consequently, the leaching amount of Pb dropped below 0.3 mg/L (Japanese criteria for landfill disposal). Overall, these experiments provide promising results regarding the possibility of using diatomite for pretreating APC residues.
Highlights
Municipal solid waste incineration (MSWI) has been initiated for hygienic control (Gohlke and Martin 2007)
It is unclear whether other cement hydrates except Calcium silicate hydrate (C-S–H) gel contribute to metal immobilization in actual systems (Geysen et al 2004), the mineralogical characteristics of Air pollution control (APC) residues might have the potential for metal immobilization without using cement
This study aims to investigate the feasibility of using diatomite on metal immobilization in APC residues instead of using cement
Summary
Municipal solid waste incineration (MSWI) has been initiated for hygienic control (Gohlke and Martin 2007). Thermal treatments reduce leaching and the volume of residues, thereby producing a material that is suitable for reuse (Zacco et al 2014) Among these treatments, cement-based S/S is commonly used for metal immobilization in APC residues worldwide (Polettini et al 2001; Benassi et al 2016). Owing to the high Ca content and alkaline pH, the addition of amorphous silica to APC residues may induce C-S–H gel formation via pozzolanic reactions for metal immobilization. Because the main components of APC residues are similar to those in cement (Liu et al 2018), cement hydrates can be formed in APC residues without cement addition (Montagnaro et al 2003) It is unclear whether other cement hydrates except C-S–H gel contribute to metal immobilization in actual systems (Geysen et al 2004), the mineralogical characteristics of APC residues might have the potential for metal immobilization without using cement. The mineralogical characteristics of APC residues and the chemical form of Pb which changed following diatomite addition were investigated
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