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

1. Introduction 
Due to the growing output of municipal solid waste (MSW), the by- 
product, namely MSW incineration fly ash (MSWI-FA), has increased 
explosively in the last five years 
[1–2]
. Statistics has predicted that the 
annual production of MSWI-FA in China would exceed 10 million tons 
by the end of 2020 
[3]
. The dominant disposal method of MSWI-FA is 
still landfill treatment, but a range of hazardous properties should be 
noted, of which the excessive heavy metal concentration and strong 
basicity in leachate are the worst 
[4–5]
. Previous study of our group 
reported that lead (Pb) in MSWI-FA was measured as the only heavy 
metal element, whose leaching toxicity was about eleven fold of the 
limiting value stipulated in the latest regulations of hazardous waste 
landfill in China 
[6–7]
. It would be attributed to the presence of 
exchangeable fraction (8%–9%) 
[8]
. If the prestabilization is lacking, 
the highly toxic Pb probably endangered the soil, crops, and even human 
beings 
[9–10]
. On the other hand, raw FA sample is rich in alkaline 
oxides and hydroxides, whose mass ratio account for more than half of 
FA, leaving a strong alkalinity of its leaching solution (pH > 12) 
[11]
. 
This property might impose negative effects on the landfill soil, resulting 
in the deterioration of its physical characteristics and following inhibi-
tion of crop growth 
[12–13]
. On the basis of the landfill standard, a 
limited pH range (6–9) of effluent must be obeyed. Hence, pursuing 
pretreatment method for simultaneously stabilizing Pb and neutralizing 
alkalis is required. 
The current Pb stabilization methods in China include washing, 
separation and extraction, melting in high temperature, and solidifica-
tion/stabilization 
[14–17]
. Chelating agent (CA) stabilization, equipped 
with the advantages of strong reliability and availability, is wildly 
recognized as the leading technology. Previous studies found that 
inorganic agents (e.g., sulfides, phosphates, and molysite) are unable to 
stabilize Pb that up to standard until supplying huge dosage of more 
than 13%, meanwhile, this Pb speciation would be easily destructed 
under acidic and parlkaline conditions 
[7,14]
. To handle the above 
limitations, stabilization using organic CA including thiourea de-
rivatives, chitosan, dithiocarbamates was put forward as an alternative 
method 
[7–8,18–19]
. These chelates could achieve standardized leach-
ing of Pb with a less dosage and reinforced stability in extreme 
* Corresponding author at: The State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji Uni-
versity, 1239 Siping Road, Shanghai 200092, China. 
E-mail addresses: 
[e-mail] 
(Z. Zhu), 
[e-mail] 
(T. Zhou).
Contents lists available at 
ScienceDirect 
Chemical Engineering Journal 
journal homepage: 
www.elsevier.com/locate/cej 
https://doi.org/10.1016/j.cej.2022.134812 
Received 5 November 2021; Received in revised form 24 December 2021; Accepted 17 January 2022

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Chemical Engineering Journal 435 (2022) 134812
2
environments, but the high cost and potential biotoxicity remains a 
burning question. 
To prevent the soil alkalization, chemical amelioration, phytor-
emediation, colloidal flocculation, and water washing were used as main 
treatment methods 
[20–21]
. Calcium sulfate serve as the primary inor-
ganic conditioner, providing Ca in replacing exchangeable Na mean-
while reducing soil bulk density and promoting saturated hydraulic 
conductivity 
[22]
. However, a long-term supplement of inorganic 
conditioner would result in the disruption of soil fertility 
[23]
. Applying 
organic amendment would effectively handle the soil alkalization as 
well as increase the capacity of soil organisms 
[21]
. Nevertheless, such 
amelioration could not simultaneously chelate Pb up to standard, 
causing unpleasant cost and operation complexity. 
Accelerated carbonation is defined as the reaction between alkaline 
metal oxides and CO
2 
in high concentration, which forms relatively 
stable carbonates. This promising process had already been investigated 
for immobilizing heavy metals in MSWI-FA 
[24]
, and current research 
mainly focuses on the optimization of reacting factors, including gas 
component and concentration, liquid to solid ratio (L/S), solution pH, 
etc 
[25–27]
. Results indicated that carbonation immobilized Pb, Cu, Zn, 
and As in MSWI-FA, but exerted an adverse influence on Cd and Sb 
[26]
. 
The introduction of SO
2 
into reaction gas weakened the immobilization 
effect on MSWI-FA, and the CO
2 
content was a negligible factor with 
increasing content from 12% to 100% 
[25]
. 0.25 and 9.5–10.5 were 
investigated as the optimum L/S ratio and solution pH towards MSWI- 
FA carbonation 
[26,28]
. 
Currently, some blanks about MSWI-FA carbonation should be 
noticed and handled: (i) All previous studies only considered carbon 
dioxide (CO
2
) in different volume fractions as the reactant gas, or uti-
lized the mixture of CO
2 
and sulfur dioxide to simulate the typical 
components of incineration flus gas 
[25,29–30]
. But there is still a blank 
for effect assessment with an actual flue gas; (ii) Previous work revealed 
that the Pb leaching, treated by single carbonation, hardly satisfied the 
latest regulation, making the incorporation with other pre-stabilization 
methods vital 
[31]
. Moreover, the cost analysis of incorporated stabili-
zation was essential for further application. However, the involved 
research is limited; (iii) Previous research confirmed the carbonation 
could reduce the alkaline capacity while sequestering CO
2 
in MSWI-FA 
[24]
. However, the relationship between carbonation time and leachate 
pH or CO
2 
capture capacity is lacked. Hence, it’s quite necessary and 
promising to investigate whether the combination between carbonation 
with actual flue gas and agent treatment could concurrently realize the 
Pb immobilization, alkali neutralization, carbon emission reduction, 
and economic benefit improvement. 
In this study, the investigates were as follows: i) to determine the Pb 
leaching behavior under different carbonation time while varying the 
agent type, leaching pH and curing durations; ii) to obtain the rela-
tionship between carbonation time and acid-base neutralization; iii) to 
summarize the carbonation mechanisms through the characterization of 
raw and carbonated FA; iv) to calculate the maximum values of 
carbonation conversion and cost reduction rate compared with the 
uncarbonated.