Desorption of Cl?from Mg-Al layered double hydroxide intercalated with Cl?using CO2 gas and water

Tomohito Kameda,Hiroki Uchida,Shogo Kumagai,Yuko Saito,Keiichi Mizushina,Ichirou Itou,Tianye Han,Toshiaki Yoshioka

1 Graduate School of Environmental Studies,Tohoku University,6-6-07 Aoba,Aramaki,Aoba-ku,Sendai 980-8579,Japan

2 Kurita Water Industries Ltd.,Nakano Central Park East,4-10-1 Nakano,Nakano-ku,Tokyo 164-0001,Japan

Keywords:Mg-Al layered double hydroxide CO2 gas Cl?desorption Semi-dry method

ABSTRACT Mg-Al layered double hydroxide intercalated with (CO3 ·Mg-Al LDH)is effective for treating HCl exhaust gas.HCl reacts with in CO3 ·Mg-Al LDH,resulting in the formation of Cl·Mg-Al LDH.We propose that CO2 can be used for the desorption of Cl?from Cl·Mg-Al LDH to regenerate CO3 ·Mg-Al LDH.Herein,we studied the desorption of Cl?from Cl·Mg-Al LDH by adding water to Cl·Mg-Al LDH and blowing CO2 into it.We also analyzed the effects of temperature and water addition speed on the desorption of Cl?from Cl·Mg-Al LDH.Our results show that the added water adhered to Cl·Mg-Al LDH and that CO2 in the gaseous phase was dissolved in this adhered water,thus generating .Therefore,anion exchange occurred between and Cl?in the Cl·Mg-Al LDH,thus desorbing Cl?.

1.Introduction

There are many studies reported in the literature regarding the removal of diverse components using diverse nanomaterials[1–25].One such material is Mg-Al layered double hydroxide(Mg-Al LDH).It comprises a host layer consisting of an octahedral layer charged positively by replacing a part of Mg2+of Mg(OH)2with Al3+and a guest layer consisting of interlayer water and anions that compensate for the positive charge [26–29].The general formula used to represent Mg-Al LDH ismH2O](An?:n-valent anion,x:Al3+/(Mg2++Al3+),0.20 ≤x ≤0.33).Mg-Al LDH can intercalate various anions in the guest layer and shows an anion exchange capacity [30–37].Currently,HCl exhaust gas generated in waste incineration facilities is mainly processed by the dry method,which produces fly ash containing CaCl2because of the usage of Ca(OH)2[38–41].Fly ash is disposed by landfilling,which increases the load on the landfill and increases the salt concentration in leachates.To solve this problem,we propose here a new method for treating HCl exhaust gas using Mg-Al LDH intercalated with(CO3·Mg-Al LDH)[42–45].

It is known that HCl reacts within the CO3·Mg-Al LDH interlayers and forms Cl·Mg-Al LDH,which is caused by the intercalation of Cl?in the interlayer of Mg-Al LDH,according to Eq.(1)[45].This is classified as chemical adsorption[46].

By shaking Cl·Mg-Al LDH in an Na2CO3solution,anion exchange occurs between and Cl?in the Cl·Mg-Al LDH interlayers,and CO3·Mg-Al LDH is regenerated [45].The CO3·Mg-Al LDH regenerated in this wet method could be reused for the treatment of HCl exhaust gas,but there are some issues for applying it to practical use.Regenerating CO3·Mg-Al LDH by the wet method is a multi-stage process involving shaking,solid–liquid separation,and drying.Therefore,an easy and cost-effective semi-dry method,which means the desorption of Cl?from Cl·Mg-Al LDH using CO2gas and water,was studied,focusing on the practical application of the CO3·Mg-Al LDH regeneration process.In general,CO2reacts with H2O and is present asin H2O.Therefore,CO2gas and water can be effectively used for the regeneration of CO3·Mg-Al LDH,instead of Na2CO3solution in the wet method.In this study,we have examined the desorption of Cl?from Cl·Mg-Al LDH by adding water to Cl·Mg-Al LDH and blowing CO2into it.The effects of temperature and water addition speed on the desorption of Cl?from Cl·Mg-Al LDH were investigated.

Table 1 Elemental composition of Cl·Mg-Al LDH

2.Experimental

Table 1 shows the chemical composition of Cl·Mg-Al LDH[47]used in the experiment.Fig.1 shows a schematic diagram of the experimental apparatus.1.0 g of Cl·Mg-Al LDH was loaded on the fiberglass present in a quartz tube(inner diameter:18 mm),and in addition to room temperature,the tubular electric furnace was set to 100 and 170°C.CO2gas was allowed to flow at 100 ml·min?1for 1.5 h while adding ion-exchanged water at 0,10,and 100 μl·min?1using a syringe-type pump.After the reaction,the substance was dried at 105°C for 12 h,and the water content was calculated.The dried material was dissolved in HNO3,and Cl?in the solution was quantified by ion chromatography.The Cl?desorption rate was calculated from the amount of Cl?present in Cl·Mg-Al LDH before and after desorption.

3.Results and Discussion

Fig.2 shows the effect of temperature on Cl?desorption and water content.The desorption rates at room temperature,100°C,and 170°C were 78%,67%,and 24%,respectively.The desorption rate of Cl?was higher at lower temperatures.The water contents of the material after reaction at room temperature,100°C,and 170°C were 39%,32%,and 1%,respectively.The water content of the substance after reaction was higher at lower temperatures.At room temperature and 100 °C,the water content of Cl·Mg-Al LDH before the reaction was maintained,confirming that the amount of water added was sufficient.Hereinafter,we will consider the desorption of Cl?from Cl·Mg-Al LDH in this reaction system.The added water adhered to Cl·Mg-Al LDH.CO2,which is in the gaseous phase,dissolves in the water adhered to Cl·Mg-Al LDH and generates[Eq.(2)].The anion exchange betweenand Cl?in the Cl·Mg-Al LDH interlayers progresses,and Cl?gets desorbed(Eq.(3)).

In this case,trace amounts of Mg2+are probably dissolved from Mg-Al LDH into water as a counter cation for Cl?.Since the amount of dissolved Mg2+is tiny,the Mg-Al LDH certainly keeps its layered structure.The Cl?desorption rate is extremely low at 170°C because the water could not adhere to Cl·Mg-Al LDH as it evaporated.The Cl?desorption rate at 100°C was slightly lower than that at room temperature because of the evaporation of water.In sum,the above result has clarified the desorption of Cl?from Cl·Mg-Al LDH using CO2gas and water in a semi-dry method.

Fig.1.Experimental apparatus.1—CO2 cylinder;2—N2 cylinder;3—Mass flow controller;4—Syringe-type pump for H2O;5—Electric furnace;6—Quartz tube;7—Fiberglass.

Fig.2.Effect of temperature on Cl?desorption and water content.Time:8 h;Water addition speed:100 μl·min?1.

Considering the cost,it is desirable to reduce the amount of water added for treating the processing solution in the regeneration process by this semi-dry method.Thus,we have examined the effects of water addition speed on Cl?desorption from Cl·Mg-Al LDH by the semi-dry method.Fig.3 shows the effect of water addition speed on Cl?desorption at room temperature.The desorption rates at water addition speeds of 0,10,and 100 μl·min?1were 23%,79%,and 97%,respectively.The respective water contents of the substance after the reaction were 4%,36%,and 39%.The Cl?desorption rate was low at a water addition speed of 0 μl·min?1probably because the CO2in the gaseous phase could not dissolve because the water adhering to Cl·Mg-Al LDH reduces with time due to drying.The Cl?desorption rate increased with the increase in water addition speed.This is probably because the increase in the amount of water adhering to Cl·Mg-Al LDH increases the amount of dissolvedresulting in the progress of anion exchange with Cl?in the Cl·Mg-Al LDH interlayers.Fig.4 shows the effect of water addition speed on Cl?desorption at 100°C.The desorption rates at water addition speeds of 0,10,and 100 μl·min?1were 6%,9%,and 80%,respectively.The respective water contents of the substance after the reaction were 2%,2%,and 33%.The Cl?desorption rate was low at water addition speeds of 0 and 10 μl·min?1.This is probably because the CO2in the gaseous phase was not able to dissolve as the water adhering to Cl·Mg-Al LDH dried because of heating at 100°C.The Cl?desorption rate was high when the water addition speed was 100 μl·min?1.This is probably because with a sufficient amount of water added to Cl·Mg-Al LDH,the amount of water adhering to Cl·Mg-Al LDH increased,which in turn increased the amount of dissolved,resulting in the progress of anion exchange with Cl?in the Cl·Mg-Al LDH interlayers.

Fig.3.Effect of water addition speed on Cl?desorption and water content.Time:24 h;Temperature:room temperature.

Fig.4.Effect of water addition speed on Cl?desorption and water content.Time:24 h;Temperature:100°C.

The above result describes the possibility of regeneration of CO3·Mg-Al LDH using CO2gas and water effectively by the semi-dry method.CO2is contained in incineration flue gas,and occupies about 10%of chimney exhaust gas after the treatment of acidic gases.Therefore,we propose a CO3·Mg-Al LDH regeneration process that makes effective use of this chimney exhaust CO2gas.In this process,a CO3·Mg-Al LDH regeneration facility was installed in the incineration facility.This makes the regeneration of CO3·Mg-Al LDH feasible on-site,without the need to transport the Cl·Mg-Al LDH,which was generated after HCl exhaust gas treatment,outside the incineration facility.

4.Conclusions

Cl?was successfully desorbed from Cl·Mg-Al LDH using CO2gas and water by the semi-dry method.The Cl?desorption rate was higher at lower temperatures.We consider the Cl?desorption from Cl·Mg-Al LDH in this reaction system.The added water adhered to the Cl·Mg-Al LDH.CO2in the gaseous phase was dissolved in the water adhering to the Cl·Mg-Al LDH,generating.This led to progress in the anion exchange reaction betweenand Cl?in the Cl·Mg-Al LDH interlayers,and thus,Cl?was desorbed.The Cl?desorption rate was extremely low at 170°C because the water could not adhere to Cl·Mg-Al LDH as it evaporated.The Cl?desorption rate was low at room temperature and at a water addition speed of 0 μl·min?1.The reason for this was probably that the CO2in the gaseous phase could not dissolve as the water adhering to Cl·Mg-Al LDH reduced with time due to drying.The Cl?desorption rate increased with increasing water addition speed because the increase in the amount of water adhering to Cl·Mg-Al LDH increased the amount of dissolved,resulting in the progress of anion exchange with Cl?in the Cl·Mg-Al LDH interlayers.The Cl?desorption rate was high when the water addition speed was 100 μl·min?1at 100°C.The reason for this it thought to be that with a sufficient amount of water added to Cl·Mg-Al LDH,the amount of water adhering to Cl·Mg-Al LDH increases,which in turn increases the amount of dissolved,resulting in the progress of anion exchange with Cl?in the Cl·Mg-Al LDH interlayers.