Regeneration of carbonate-intercalated Mg–Al layered double hydroxides (CO3·Mg–Al LDHs) by CO2-induced desorption of anions (X) from X·Mg–Al LDH (X = Cl, SO4, or NO3): A kinetic study
Graphical abstract
Introduction
The dry method of treating acidic exhaust gases (HCl, SOx, and NOx) produced in waste incineration facilities poses certain problems, e.g., the generation of CaCl2-containing fly ash due to the use of Ca(OH)2 for HCl and SOx capture (Chu and Rochelle, 1989; Dal Pozzo et al., 2018; Karlsson et al., 1981; Kim et al., 2017). The landfilling of this fly ash increases the burden on the final disposal site and affords leachates with high salt concentrations. In the case of NOx removal, even though catalytic denitration results in complete reduction to N2 (Chen et al., 2016, 2018; Gao et al., 2017), the high cost of catalytic denitration facility management poses a problem. Therefore, novel methods of acidic exhaust gas treatment allowing one to circumvent the above issues are highly sought after.
Mg–Al layered double hydroxides (Mg–Al LDHs) are described by the general formula [Mg2+1−xAl3+x(OH)2][An−x/n·mH2O] (An− = n-valent anion, 0.20 ≤ x ≤ 0.33 according to Vegard’s law) (Allmann, 1968; Cavani et al., 1991; Ingram and Taylor, 1967; Mills et al., 2012) and comprise host layers, which are positively charged because of the partial substitution of Mg2+ for Al3+, and negatively charged guest layers, which contain charge-compensating anions and interlayer water. Importantly, Mg–Al LDHs can intercalate various anions into the guest layer, and the ease of this intercalation is positively correlated with anion charge density (Kameda et al., 2003, 2017; Miyata, 1983; Sato et al., 1986), which has been exploited in works on LDH-based wastewater treatment (Hong et al., 2020; Hou et al., 2020; Lyu et al., 2020; Puzyrnaya et al., 2020; Wei et al., 2020; Yazdankish et al., 2020; Zhang et al., 2020; Zhou et al., 2020). Moreover, Mg–Al LDH intercalated with CO32− (CO3·Mg–Al LDH), which is a reusable absorbent with anion exchange ability, has been used to remove HCl, SO2, and NOx (Kameda et al., 2019a, 2019b; Kameda et al., 2020a, 2020b). Specifically, HCl reacts with the CO32− ions present between the LDH layers to form Cl·Mg–Al LDH, while SO2 and NO2 react to form SOx·Mg–Al LDH and NOx·Mg–Al LDH, respectively, and can also be bound via surface adsorption. Agitation of Cl·Mg–Al LDH, SO4·Mg–Al LDH, or NO3·Mg–Al LDH (hereinafter collectively referred to as X·Mg–Al LDH) in aqueous Na2CO3 leads to the regeneration of CO3·Mg–Al LDH via anion exchange, which has been investigated in terms of equilibrium establishment, kinetics, and thermodynamics.
The present study describes a novel method of regenerating CO3·Mg–Al LDH based on the use of CO2, a potent greenhouse gas contributing to climate change. Specifically, CO2 was bubbled through a suspension of X·Mg–Al LDH in deionized (DI) water, and the kinetics of the regeneration reaction was examined as a function of CO2 flow volume and temperature. For comparison, regeneration was performed by bubbling CO2 into a suspension of X·Mg–Al LDH in aqueous NaOH.
Section snippets
Experimental
X·Mg–Al LDHs were synthesized by co-precipitation (Kameda et al., 2016) (Table 1) and regenerated according to Eqs. (1), (2), (3).Mg0.67Al0.33(OH)2(Cl)0.33 + 0.17CO32− → Mg0.67Al0.33(OH)2(CO3)0.17 + 0.33Cl−Mg0.67Al0.33(OH)2(SO4)0.17 + 0.17CO32− → Mg0.67Al0.33(OH)2(CO3)0.17 + 0.17SO42−Mg0.67Al0.33(OH)2(NO3)0.33 + 0.17CO32− → Mg0.67Al0.33(OH)2(CO3)0.17 + 0.33NO3−
For regeneration, a 1000-mL five-neck round-bottom flask filled with DI water (500 mL) was placed in a constant-temperature (10, 30, or
Results and discussion
Fig. 1 shows the effects of CO2 flow rate and time on the pH and efficiency of Cl− desorption from Cl·Mg–Al LDH, revealing that the latter parameter increased with increasing flow rate, equaling 44, 70, and 81% at 60 min for flow rates of 0, 50, and 100 mL min−1, respectively. This behavior was ascribed to the generation of CO32− through the reaction of CO2 and H2O and the subsequent exchange (Eq. (1)) of Cl− in Cl·Mg–Al LDH for CO32− produced by the dissociation of carbonic acid. At a CO2 flow
Conclusions
The bubbling of CO2 through slurries of X·Mg–Al LDH (X = Cl, SO4, or NO3) in DI water resulted in the formation of CO32− anions that were exchanged for X to afford CO3·Mg–Al LDH. All desorption reactions followed the intraparticle diffusion model and featured the diffusion of CO32− in between LDH layers as the rate-determining step. As CO32− anions were produced from the bubbled CO2, the true rate-determining step was identified as the dissolution of CO2. The use of aqueous NaOH instead of DI
Declaration of Competing Interest
The authors report no declarations of interest.
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