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.