Page 96 - EER-1-1

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

solution efficiently reacts with the alkaline components in 2 wt%. The inhibitory effect diminishes when (NH ) CO
4 2
3
IBA, facilitating the accelerated carbonation process. concentration exceeds 4%, aligning with the observed
trend in carbonation efficiency (Figure 1B). In contrast,
3.3. Leaching behavior of native IBA carbonation demonstrates lower efficacy in reducing
Cd, Cr, Cu, Ni, Pb, and Zn are selected as metals of concern the leaching of Cr, achieving only a 45% IR even with a
due to their high leaching rate, potential hazards, and risks higher (NH ) CO concentration of 10 wt%. The leaching
3
4 2
to human health and the environment if they leach into situation worsens when the (NH ) CO concentration
3
4 2
the surrounding soil and water. The heavy metal leaching is 2%. Cr in incineration residues is likely to be present
results for native IBA of different sizes are presented in mainly as Cr(VI) due to their oxidized and high pH
Table 3, revealing notably higher leaching rates for IBA characteristics. Its mobility is mainly controlled by pH
with particle sizes below 2 mm across most heavy metals. and redox reactions. 50,51 When the degree of carbonation
This is because the dissolution or leaching of elements increases at a higher concentration of (NH ) CO , the IR of
3
4 2
typically increases with surface area and porosity. The Cr increases too, but less significantly compared to other
leaching rate of Cu and Cr in IBA <0.6 mm is particularly heavy metals. IBA-AC10 shows the highest IR of 46% for
higher, probably due to their much higher content and Cr. Pb and Zn are known to undergo carbonation to form
surface area, as shown in Tables 1 and 2, respectively. water-insoluble PbCO and ZnCO , which may explain
51
3

3
It has been reported that at a pH above 12, Pb, Zn, the low leaching rate of native IBA samples, which have
and Cu are the primary heavy metals leaching from IBA, been stored for several months before carrying out this
due to their amphoteric property. In our case, despite all study. Both carbonation and pH decrease help immobilize
49
52
leachates registering a pH above 12 (Table 3), the leaching Ni. As for Cu, apart from the carbonation resulting in
rate of Zn and Pb is notably low, particularly for Pb. This lower pH and formation of metal carbonate, complexation
can be attributed to the fact that our samples were stored with organic material and sorption Al-Fe-oxides may
30
for a few months after collection, and aging or natural also contribute to its leaching inhibition. Indeed, it
weathering during this period effectively diminished is challenging to identify the causes of the changes in
the leaching of Pb and Zn, which agrees with the result leaching behavior for all heavy metals after accelerated
obtained by Arickx et al. It was observed that leaching carbonation, especially differently sourced IBAs have
30
of Zn and Pb for IBA of 0.1 – 2 mm decreased within different composition and the influencing factors interact
st
the 1 week to below the limit value. It is noteworthy that with each other.
for fresh IBA, the concentration of Pb in the leachate was Noteworthy is the extremely high concentration of
exceedingly high, reaching ppm levels (the result is not copper and zinc in the carbonating solution, particularly
presented here). for copper, reaching 125 ppm (Figure 3B). This is attributed
2+
3.4. Leaching test of carbonated IBA to the generation of Cu(NH ) and Zn(NH ) 2+ on NH
3
3 4
3 4
release during carbonation. Notably, the former exhibits a
In Figure 3A, the impact of various concentrations of higher formation constant of 2.1 × 10 , surpassing the value
13
(NH ) CO on the inhibition rate (IR) for different heavy of 2.9 × 10 for Zn(NH ) . In contrast, the concentration
9
2+
3
4 2
metals is illustrated. The IR is defined as IR = (L − of other heavy metals in the carbonating solution remains
3 4
bc
L )/L , where L and L represent the leaching rate of relatively low due to their lower propensity to form
ac
bc
ac
bc
heavy metals before and after carbonation, respectively. complexes with NH .
Figure 3A illustrates the notable inhibitory effect of 3
carbonation on metal leaching, with an impressive 75% pH values of the leachates were measured with pH
IR for Cu even at a low (NH ) CO concentration of meter and the results are demonstrated in Figure 4.
4 2 3
Carbonation neutralizes the alkaline components in
Table 3. Heavy metal leaching results (in µg/kg of IBA, leading to a decrease in pH. As shown in Figure 4,
metal/IBA) for native IBA with different particle size and pH the pH of the leachate drops to below 11 from 12.25 for
of leachates native IBA. A notable decrease of 1.36 in pH is observed
with 2 wt% (NH ) CO , and a further decrease of 0.44
IBA particle size Cd Cr Cu Ni Pb Zn pH 4 2 3
<0.6 mm 8.3 631.6 2267 304.2 2.1 74.8 12.12 occurs with 4 wt% (NH ) CO . Subsequently, the decline
3
4 2
in pH becomes gradual. This pattern aligns with the
0.6−1.12 mm 4.8 362.0 1436 347.7 0.9 311.1 12.39 results of carbonation, where efficiency increases most
1.12−2 mm 4.3 365.7 1502 374.6 0.5 395.2 12.37 significantly when (NH ) CO concentration is below 4
4 2
3
2−2.8 mm 3.3 315.2 1105 284.0 0.2 232.6 12.27 wt%. Correspondingly, the inhibitory effect on leaching is
Abbreviation: IBA: Incineration bottom ash. most potent at lower (NH ) CO concentrations.
3
4 2
Volume 1 Issue 1 (2024) 6 doi: 10.36922/eer.4609