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Explora: Environment
and Resource Accelerated carbonation for heavy metal inhibition in incineration bottom ash

alternatives to address the impending space limitations to choose ammonium carbonate as a carbonating agent:
and environmental concerns. (i) Ammonia solution demonstrated higher efficiency
IBA, the non-combustible portion of MSW, comprises in low CO capture scenarios and its usage incurred
2
80 – 90% of total incineration residues. The primary lower energy penalties compared to conventional
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38,39
components in IBA are metals and minerals. Metal scraps monoethanolamine (ii) in response to the distinctive
3
can be recycled, while the recovery of minerals in IBA CO capture requirements of power plants in Singapore,
2
can help extend the lifespan of landfill and slow down natural gas with a low concentration (3 – 5%) of CO in
2
40
the natural mineral resource depletion. The potential the flue gas is utilized; (iii) the resulting CO -absorbed
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applications of IBA in civil engineering have been solution contains ammonium carbonate and ammonium
extensively explored, with minerals from IBA utilized in bicarbonate as major components (Equations I and II),
ceramics and construction materials. 4-15 However, heavy which can be directly employed for carbonation; and (iv) on
metals such as Cr, Cu, Ni, Cd, Zn, and Pb, commonly reaction between alkaline minerals in IBA with ammonium
found in IBA, pose a hindrance to its broader and extended carbonate or ammonium bicarbonate, calcium carbonate
utilization. Studies have shown that heavy metals from IBA is formed (Equations III and IV), helping immobilize
can mobilize and pose environmental risks. 16-21 In view of heavy metals; in the meantime, ammonia is released and
sustainable waste management, treatment of IBA is both the solution is recycled. This proposed approach avoids
important and necessary to make it meet the civil-technical the need for regeneration of CO by desorption, which is
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and environmental requirements. The acceptance criteria an energy-intensive process. As carbonate ions are readily
for utilizing recycled bottom ash from municipal solid available in this case, the carbonation rate is expected to
waste incinerator (MSWI) in road construction usually speed up greatly. A circular process is therefore developed
depends on its leaching potential. in this way.
Various techniques have been explored to mitigate CO + 2NH 3(aq) + H O  (NH ) CO 3(aq) (I)
2
4 2
2(g)
(l)
heavy metal leaching from IBA, including washing, 22-24 CO + NH 3(aq) + H O  (NH )HCO 3(aq) (II)
4
2
(l)
2(g)
chemical extraction, 25,26 thermal treatment, 27,28 natural Ca(OH) + (NH ) CO  CaCO + 2NH
aging, and accelerated carbonation. 30-37 Among them, 2(s) 4 2 3(aq) 3(s) 3(aq)
29
natural weathering stands out as the most popular and cost- + 2H O (l) (III)
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effective treatment method for IBA. However, carbonation, a Ca(OH) + (NH )HCO 3(aq)  CaCO 3(s) + 2NH 3(aq)
2(s)
4
crucial aging process involving the absorption of CO from + 2H O (IV)
2
atmosphere by alkaline material in IBA, resulting in a pH 2 (l)
decrease and calcite precipitation, is inherently slow, taking This investigation focuses on the impact of ammonium
months to years to complete, depending on the degree of carbonate concentration on carbonation efficiency by fixing
exposure to atmosphere of the samples. This gradual process the volume of the solution and the impact of carbonation
becomes impractical, particularly in land-scarce regions like on the inhibition of heavy metals leaching from IBA. This
Singapore. In contrast, accelerated carbonations by contacting is the first attempt to mitigate heavy metal leaching from
IBA with CO -enriched gas, coupled to the introduction of an IBA through carbonation with the solution method.
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appropriate amount of moisture (to promote the formation of 2. Materials and methods
carbonate ions), have demonstrated its potential advantages.
Accelerated carbonation can improve carbonation kinetics 2.1. IBA sampling and characterization
and conversion considerably. Besides, when flue gas is used IBA samples were sourced from a local MSW incineration
as a CO -enriched source, mineralization of IBA is indeed a plant and subjected to drying at 100°C until achieving a
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process of CO capture and storage (as calcium carbonate has constant weight. Due to the highly variation in size of the
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high geochemical stability). samples, IBA was sorted into a few fractions by sieving
Accelerated carbonation with CO depends on many (0.6, 1.12, 2.0, and 2.8 mm). Subsequently, small portions
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factors, including IBA composition, CO concentration and of samples of varying sizes were ground into powder below
2
pressure, reaction temperature, water-to-solid ratio, reaction 200 µm for X-ray fluorescence (XRF) and X-ray diffraction
time, and so on. Liquid/solid ratio plays an important (XRD) analyses. IBA fraction with particle size exceeding
role in carbonation kinetics, because it strongly affects the 2.8 mm was excluded from the analysis.
dissolution and diffusion of both CO and metal ions, as well The chemical composition of the IBA was analyzed
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as the mixing conditions in slurry-phase systems. with XRF (Bruker S8 TIGER Series, Bruker, USA). XRD
In this work, we used ammonium carbonate as the analysis was performed on Bruker D8 advance A25/
accelerated carbonating agent. There are a few reasons XRK900 diffractometer (Bruker, Germany) to ascertain
Volume 1 Issue 1 (2024) 2 doi: 10.36922/eer.4609