Carbon reclamation from biogas plant flue gas for immobilizing lead and neutralizing alkalis in municipal solid waste incineration fly ash

partially attributed to expansion effects on particle size. 
3.3.2. XRD analysis 
This section aimed at exploring the carbonation effect on crystal 
shape of FA via comparing the XRD patterns (
Fig. 7
b). A similar peak 
shape, with a changing crystallinity, was shared by uncarbonated and 
carbonated groups. In the uncarbonated FA, the crystal form was 
composed of the chlorine salt, sulfate and carbonate of light and alkali 
Fig. 5. Pb leaching toxicity based on curing time under three carbonation time.
Fig. 6. Acidic-basic properties of FA samples under different carbonation time. (a) ANC curves for 0 h, 1 h, and 4.5 h carbonated FA; (b) Variation of leachate pH 
with increasing carbonation time. 
Z. Zhu et al.

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Chemical Engineering Journal 435 (2022) 134812
6
metals, in accord with the results of Zhu et al. 
[8]
. The richness of so-
dium chloride (NaCl) and potassium chloride (KCl) might be caused by 
the heavy use of table salt in daily life. The existence of CaCO
3 
peak 
could be attributed to the earlier natural carbonation 
[43]
. Calcium 
sulfate (CaSO
4
) and basic calcium chloride (CaClOH) were respectively 
due to the adsorption of HCl and SO
3 
with calcium hydroxide (Ca(OH)
2
) 
during the carbonation 
[44]
. 
With increasing time from 0.5 h to 4.5 h, the intensities of charac-
teristic peaks attributed to CaCO
3 
continually increased by 24.5%, 
which were similar with previous studies about single CO
2 
treatment 
[45–46]
. But the intensity increase of CaCO
3 
from 3 h to 4.5 h only 
accounted for 3.4%, conforming to the assumptions throughout the 
Sections 3.1 and 3.2
. According to the chemical reactions (R1-R2), the 
disappearance of CaClOH was supposed to occur during the carbon-
ation, but the unexpected intensity increase was found in 0.5 h and 1 h 
groups. This was potentially due to the generation of hydrogen ion via 
steam and acid gas reaction in flue gas, incorporating with dominant 
soluble chlorine salt (accounting for 73.17–95.29% of the total chloride) 
in MSWI-FA to transform into HCl 
[26]
. Thus, CaClOH was produced 
according to reactions (R3-R4). However, calcium chloride was not