Effect of K2CO3 doping on CO2 sorption performance of silicate lithiumbased sorbent prepared from citric acid treated sediment

Junya Wang,Kai Chen,Yi Wang,Jiuming Lei,Abdullah Alsubaie,Ping Ning,Shikun Wen,*,Taiping Zhang,,*,Abdulraheem S.A.Almalki,A.Alhadhrami,Zhiping Lin,Hassan Algadi,Zhanhu Guo

1 Faculty of Environmental Science and Engineering,Kunming University of Science and Technology,Kunming 650500,China

2 Ecological Environment Monitoring Station,Linxiang Branch,Lincang Municipal Bureau of Ecology and Environment,Lincang 677000,China

3 Department of Physics,College of Khurma,Taif University,P.O.Box 11099,Taif 21944,Saudi Arabia

4 Department of Chemistry,Faculty of Science,Taif University,P.O.Box 11099,Taif 21944,Saudi Arabia

5 Integrated Composites Laboratory (ICL),Department of Chemical and Bimolecular Engineering,University of Tennessee,Knoxville,TN 37996,United States

6 Department of Electrical Engineering,Faculty of Engineering,Najran University,P.O.Box 1988,Najran 11001,Saudi Arabia

7 College of Materials Science and Engineering,Taizhou University,Taizhou 318000,China

Keywords:CO2 sorbents Li4SiO4 Sediment Citric acid Doped with K2CO3

ABSTRACT In this paper,a low-cost and environmental-friendly leaching agent citric acid(C6H8O7)was used to treat the sediment of Dianchi Lake(SDL)to synthesize lithium silicate(Li4SiO4)based CO2 sorbent.The results were compared with that treated with strong acid.Moreover,the effects of preparation conditions,sorption conditions and desorption conditions on the CO2 sorption performance of prepared Li4SiO4 were systematically studied.Under optimal conditions,the Li4SiO4 sorbent was successfully synthesized and its CO2 sorption capacity reached 31.37%(mass),which is much higher than that synthesized from SDL treated with strong acid.It is speculated that the presence of some elements after C6H8O7 treatment may promote the sorption of synthetic Li4SiO4 to CO2.In addition,after doping with K2CO3,the CO2 uptake increases from the original 12.02% and 22.12% to 23.96% and 32.41% (mass) under the 20% and 50%CO2 partial pressure,respectively.More importantly,after doping K2CO3,the synthesized Li4SiO4 has a high cyclic stability under the low CO2 partial pressure.

1.Introduction

With the advancing awareness of healthcare [1-6],stringent requirements of advanced materials [7-15] and energy units[16-28],and increasing consumption of biomass usages [29-32],a large amount of CO2and pollutants are released [33].The role of carbon dioxide (CO2) sorbent in sorption-enhanced steam methane (methane: an important renewable green energy)reforming (SESMR) have been extensively studied [34-38].In SESMR reaction,the CO2can be removed from the reaction thereby providing additional benefits of increased productivity of CH4to H2(another important renewable green energy) [39-44].Therefore,suitable CO2sorbent is very crucial.Lithium silicate (Li4SiO4),as a kind of CO2solid sorbent,has been widely accepted due to its excellent sorption property at high temperatures,especially using in SESMR reaction[34,39,45].According to the reaction(1),Li4SiO4can theoretically sorption 36.7 % of CO2[46,47].

A double-shell mechanism theory is usually used to describe the CO2sorption process [48-50].According to the above mechanism,improving the chemical sorption rate and the diffusion process is an important factor to enhance the CO2uptake of Li4SiO4[51,52].Up to now,researchers have focused on improving the CO2sorption properties of Li4SiO4,mainly through (i) structural modification [53];(ii) eutectic doping (alkali carbonates) [54,55];(iii)defects doping[56,57](Al,Fe,Y,Ca,Cr and Ti);(iv)substitution of silicon and lithium sources [58,59];and (v) modification by organic acids [60,61].As reported,the sorption of CO2by Li4SiO4is affected by its own microstructures,including particle size,surface properties [58].Moreover,it has been found that loading of alkali salts (such as K2CO3) in Li4SiO4and other sorbents can improve its CO2capture property by forming a molten eutectic layer [34,56,58,62-64].Furthermore,Gaueret al.demonstrated from the design method of solid-state ionic conductor that Li4SiO4doped with heterogeneous elements can improve the ion mobility and ultimately improve the CO2sorption performance [65].

In addition,different silicon sources will probably give a yield of Li4SiO4sorbents with different microstructures,thus affecting their CO2sorption performance [46,66].In order to reduce costs,there are some silicon-containing wastes (e.g.biomass ash,fly ash and diatomiteetc.) are used to prepare Li4SiO4[67-70].In our previous study,Li4SiO4sorbents was synthesized from the sediment of Dianchi Lake (SDL) and showed good CO2sorption performance,especially superior cyclic stability [67].However,to remove the impurities elements,treatment with strong acids leaching is normally required [71].The use of strong acids not only increases the cost of treatment,but also causes greater harm to the environment.Therefore,our goal is to find a low-cost and environmental-friendly leaching agent to treat the wastes to synthesize Li4SiO4.Citric acid(C6H8O7) is produced in large quantities from natural sources and has widely used in food,fuel and beverage industries with a reasonable price [25,72-74].Therefore,in this paper,citric acid was used as the leaching agent to treat the SDL to fabricate Li4SiO4.The properties between different acids treated SDL derived Li4SiO4sorbents were comparatively studied.The sorption conditions and desorption conditions under different CO2partial pressures(paCO2)were comprehensively investigated.Finally,the relationship between the loading ratio of K2CO3and the sorption performance of Li4SiO4sorbents was systematically discussed.The results were compared with that treated with strong acid in our previous study.

2.Experimental

2.1.Sorbents preparation

The SDL was first washed by deionized water and then dried at 105 °C in the oven for further use.The C6H8O7(Tianjin Fengchuan Chemical Reagent Technology Co.,ltd.,China.AR) solvents with different concentrations(0.1,0.25 and 0.5 mol·L-1)were prepared.The dried SDL (10 g) with 200 ml C6H8O7solvents was stirred at 100 °C for 2 h.Following,the mixture was centrifugally separated and followed by repeated washing with water until neutral.The separated solid sample was then dried at 105 °C and denoted as C-SiO2.Li4SiO4was fabricated by solid-state reaction as reported in our previous study [67].

2.2.Characterization and CO2 sorption methods

The characterization methods in this study are the same as our previous study [45,67].Briefly,the X-ray fluorescence (XRF,Axios mAX,PANalytical,Netherlands) spectrometer was used to analyze the chemical composition of samples.XRD-7000 X-ray diffractometer (Shimadzu,Japan) was used to test the structures of the samples.The morphology of the samples was observed by Nova SEM 50 microscope (FEI,USA).

A thermogravimetric analyzer (TGA,DTG-60 h,Shimadzu,Japan) was used to detect the CO2sorption capacity and cycling stability.The CO2sorption and cycling test procedures are the same as our previous study [67].

3.Results and Discussion

3.1.CO2 sorption

It has been shown in our previous study that the main composition of the SDL is SiO2(42.09%),Al2O3(20.10%),Fe2O3(13.05%)and the XRD result has proved that the main phase of SiO2is crystalline silicon [67].In this study,in order to explore the effect of Li4SiO4fabricated from SDL treated with different concentrations of C6H8O7on the CO2sorption performance,the SDL was treated with 0.1,0.25 and 0.5 mol·L-1C6H8O7,and then mixed with(LiNO3,AR) powders with the molar ratio (MR) of Li:Si=5.After calcination at 750 °C for 6 h,the final samples were obtained.To explore the effect of Li4SiO4fabricated from SDL treated with different concentrations of C6H8O7on the CO2sorption performance,Fig.1 shows the CO2uptake of the synthesized sorbents (sorption at 650 ℃for 120 min under the 100%(vol)CO2).When the concentration of C6H8O7was 0.1,0.25 and 0.5 mol·L-1,the CO2uptake of the synthesized Li4SiO4-based sorbent was 25.93%,26.84% and 23.20%(mass),respectively.The experimental results found that the concentration of C6H8O7would affect the CO2sorption property of the synthetic Li4SiO4sample.Therefore,0.25 mol·L-1of C6H8O7was selected to treat SDL for further study.

Fig.2 shows the SEM images of the synthesized Li4SiO4-x(0.1,0.25,0.5 mol·L-1) sorbents.It shows that the morphology of the Li4SiO4derived from C6H8O7treated SDL was very different from that of Li4SiO4produced by SDL treated with HCl/HNO3in our previous study[67].The Li4SiO4derived from C6H8O7treated SDL presented a layered structure of small particles,which may improve the CO2diffusion on the surface of Li4SiO4and promote the sorption property.The results also showed that the concentration of C6H8O7has no obvious influence on the morphology of the synthesized Li4SiO4-based sorbent.

3.1.2.The influence of synthesis and sorption conditions

To calculate the MR of Li to Si accurately,the chemical composition analysis of SDL treated by 0.25 mol·L-1of C6H8O7is tested by XRF as shown in Table 1.It showed that the content of SiO2is increased to 63.70% from the original 42.09%,and the content of Al2O3,Fe2O3,CaO are 18.70%,8.09% and 0.62%,respectively.It means that there are still some Al2O3and Fe2O3and a small amount of Ti,K,Mg and other elements in the sample,which have not been removed by C6H8O7.Gaueret al.[65] have proved that doping hetero elements on Li4SiO4can improve the ion mobility and ultimately improve the sorption performance of CO2.Therefore,it is speculated that the presence of these elements after C6H8O7treatment may promote the CO2sorption of the synthesized Li4SiO4.According to the results of Table 1,the effect of MR of Li to Si on the CO2uptake for the synthetic Li4SiO4was also explored.The MR of Li:Si was tested with 4:1,5:1,6:1,7:1,and 8:1.All the synthesized Li4SiO4samples here were evaluated at 650 °C for 120 min under the 100% (vol) CO2.As shown in Fig.3,when the MR of Li:Si=4,the CO2sorption capacity is 21.02%.As the ratio of Li:Si increases from 5 to 7,the CO2uptake increases from 26.84% to 31.37%.When the ratio of Li:Si further increases to 8,the CO2uptake decreases to 27.16%.

Fig.1.The CO2 sorption property of the synthesized Li4SiO4-x (x=0.1,0.25,0.5 mol·L-1).

Fig.2.SEM images of the synthesized Li4SiO4-x (x=0.1,0.25,0.5 mol·L-1).

Table 1XRF analyses of the SDL treated by 0.25 mol·L-1 C6H8O7.

Fig.4 shows the XRD patterns of Li4SiO4fabricated by different MRs of Li:Si.In all the samples,the main characteristic peaks of Li4-SiO4all can be observed clearly (see Fig.4),which means that Li4-SiO4can be fabricated with different MRs of Li:Si.In addition,the characteristic peaks of Li5AlO4and LiOH also appear.When the MR of Li:Si=7,the characteristic peaks of Li4SiO4(2θ=16.713°,22.206°,22.606°,34.195°),Li5AlO4(2θ=23.643°,34.398°),and LiOH (2θ=20.475°,32.583°,35.750°) appeared.The theoretical MR of Li:Si in the synthesis of Li4SiO4is 4;however,the presence of Al3+may consume part of the Li+.Therefore,the MR of Li: Si was increased.In combination with its CO2sorption effect,MR of Li:Si=7 was selected for the subsequent study.

Fig.3.CO2 sorption capacity of the synthesized Li4SiO4 (MR of Li:Si=4,5,6,7,8).

Moreover,the pretreatment and sorption conditions of synthesized Li4SiO4were optimized in this study.The experimental results are in Fig.5.When the pretreatment temperature is 650-750°C and the pretreatment time is 5-6 h,the CO2sorption capacity of the synthesized Li4SiO4is not very different.When the pretreatment temperature is 750 °C,the pretreatment time is 6 h,and the sorption temperature is 650 °C,the CO2uptake reaches the maximum value of 31.37% (mass),which is larger than that of the Li4SiO4derived from SDL treated with HCl/HNO3in our previous study [67].It is possible that the presence of other elements in the synthesis of Li4SiO4improves the ionic mobility,and ultimately improves its CO2sorption performance [65].

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3.1.3.Sorbent cyclability

Cyclic usability of Li4SiO4was tested by carrying out 22 cycles of CO2sorption/desorption using typical pressure swing adsorption(PSA) and temperature swing adsorption (TSA) procedures.All the sorbents were desorbed in pure N2.Fig.6(a)shows the PSA procedure with the sorption at 650°C for 30 min in 100%CO2and desorption at 650 °C for 10 min in pure N2.In the 1st cycle,the CO2uptake reaches 18.61%.However,with the number of cycles increases to 2,the CO2uptake decreases sharply to 7.63%.As the number of cycles increases from 3 to 22,the CO2sorption also shows a trend of increasing.However,after 22 sorption-desorption cycles,the CO2uptake is only 12.37%.

Fig.4.XRD patterns of the synthesized Li4SiO4 (MR of Li:Si=4,5,6,7,8).

Fig.5.Influence of(a)pretreated temperature,(b)pretreated time,and(c)sorption temperature on the CO2 capture performance of the synthesized Li4SiO4.

Moreover,in the TSA procedure,the sorption temperature was 650 °C,the desorption temperature was 700 °C,the sorption time was 30 min,and the desorption time is 10 min.Fig.6(b)shows that in the 1st cycle,the CO2uptake reaches 18.34%,whereas,with the cycle number increasing to 2,the CO2uptake suddenly decreases to 8.69%.As the cycle number increases from 3 to 22,the CO2uptake gradually decreases.After the 22th sorption-desorption cycles,the CO2uptake is reduced to 5.28%.

Compared with our previous study,the Li4SiO4derived from SDL treated with HCl/HNO3has a better cycling performance,and it can be desorbed completely within 5 min in the PSA procedure[67].However,in this study,after the 1st cycle,the CO2uptake of Li4SiO4sorbent derived from SDL treated with C6H8O7decreases sharply.In order to investigate the CO2desorption rate of the Li4-SiO4sorbent,one CO2sorption/desorption cycle of Li4SiO4sorbent is chosen for study as shown in Fig.7.In the PSA procedure,Fig.7(a) shows that after 10 min of desorption,the CO2desorption rate was only 28.53%.While in the TSA procedure,when desorption temperature increases to 700 °C,the desorption rate is only 36.48% after 10 min of desorption (see Fig.7(b)).It means that the CO2cannot be desorbed at 700 °C in 10 min.

To improve the desorption rate of the synthesized Li4SiO4,we tried to increase the desorption temperature.Fig.7(c) shows the cycling curves of Li4SiO4sorption at 650°C for 30 min and desorption at 650°C for 120 min.The CO2uptake is 17.95%after sorption for 30 min,and the desorption rate is 53.15% after desorption for 120 min.When the desorption temperature is increased to 700 °C,it is found that the CO2can be desorbed completely after 100 min (see Fig.7(d)).With the desorption temperature further increasing to 750 °C,it is clear that the CO2can be desorbed completely within 60 min(see Fig.7(e)).The results show that increasing the desorption temperature can shorten the desorption time.However,a high desorption temperature will lead to the sintering of Li4SiO4,thus reducing its CO2sorption capacity.

3.2.Loading with K2CO3

It has been reported that the CO2sorption performance of the Li4SiO4is related to the CO2concentration.Under different CO2partial pressures,the uptake of synthetic Li4SiO4increases with the increase of CO2concentration[75].However,in the actual flue gas emission process,the CO2concentration is relatively low.Researchers found that the loading K2CO3can improve the CO2sorption performance of Li4SiO4under low CO2concentrations[76].Therefore,in this study,we explored the CO2sorption properties of Li4SiO4with loading K2CO3under different CO2partial pressures and systematically investigated the relationship between the loading ratio of K2CO3and the sorption performance of Li4SiO4.

3.2.1.CO2 capture property of K2CO3 promoted Li4SiO4 under 20%(vol)paCO2

The sediment was treated with 0.25 mol·L-1of C6H8O7to synthesize Li4SiO4with LiNO3according to the MR of Li:Si=7,and the synthesized Li4SiO4was doped with K2CO3according to the mass percentage of 0%-30%.Fig.8(a) shows the influence of the doping amount of K2CO3on the CO2sorption property of Li4SiO4(sorption at 650 ℃under the 20% (vol) paCO2).Without doping K2CO3,it is found the CO2uptake of Li4SiO4is only 12.02% (mass).When the Li4SiO4is doped with 5% (mass) K2CO3,the CO2uptake rapidly increases to 20.59% (mass).As the doping amount of K2CO3continues to increase to 10%,the CO2uptake of Li4SiO4continues to increase to 23.96%.To further increase the doping amount of K2CO3to 15%-30%,the CO2uptake of Li4SiO4does not change significantly.Therefore,under 20%(vol)paCO2,the optimal loading ratio of K2CO3to Li4SiO4is 10% (mass).Fig.8(b) shows the SEM image of Li4SiO4doped with 10%(mass)K2CO3.It can be seen from the picture that the surface of the Li4SiO4particles is rough and covered with small pores (such as the red circle) on the surface of the particles,which may promote the diffusion of CO2.It has been proved that the optimal sorption temperature of Li4SiO4will decrease with the doping of K2CO3at low paCO2[4].To determine the optimal sorption temperature under the 20% (vol) paCO2,the 10% (mass) K2CO3doped Li4SiO4was tested at 550,600 and 650°C,respectively.The results are shown in Fig.8(c).The highest CO2uptake of 10%(mass)K2CO3doped Li4SiO4happened at 600°C under 20% (vol) paCO2,whereas it does not show a fast sorption rate within the first 10 min.

Fig.6.CO2 cycling performance of the synthesized Li4SiO4 in (a) PSA procedures,and (b) TSA procedures.

Fig.7.One CO2 sorption/desorption cycle of the synthesized Li4SiO4 in (a) PSA procedure,and (b) TSA procedure.Two CO2 sorption/desorption cycles of the synthesized Li4SiO4 (c) desorption at 650 °C for 120 min,(d) desorption at 700 °C for 120 min,and (e) desorption at 750 °C for 120 min.

Fig.8.(a)The influence of the K2CO3 loading amount on the CO2 sorption performance of the synthesized Li4SiO4;(b)the SEM image of the synthesized Li4SiO4 after doping with 10% (mass) K2CO3,(c) the influence of sorption temperature on the CO2 sorption performance of the synthesized Li4SiO4 after doping with 10% (mass) K2CO3.

The cyclic performance of 10% (mass)K2CO3doped Li4SiO4was also tested through two processes of PSA procedure and TSA procedure.Fig.9(a) shows the cyclic results of Li4SiO4under 20% (vol)paCO2(sorption at 600 °C for 30 min and desorption at 600 °C for 10 min).After the first cycle,the CO2uptake reaches 18.80%.After the second cycle,the CO2uptake is reduced to 15.94%.Moreover,the CO2uptake shows a slow decrease trend during the remaining cycles.After 22 sorption/desorption cycles,the CO2uptake of Li4SiO4is reduced to 14.41%.Fig.9(c) shows one cycle of PSA procedure.After 5 min,the desorption equilibrium is reached,but the desorption rate is only 78.70%,the incomplete desorption may be due to the low desorption temperature.Fig.9(b)shows the cyclic results of Li4SiO4under 20%(vol)paCO2(sorption at 600°C for 30 min and desorption at 700°C for 10 min).After the 1st cycle,the CO2uptake reaches 16.74%.The CO2uptake increases gradually with increasing the cycle number.After 22 cycles,the CO2uptake of Li4SiO4is 19.08%.Fig.9(d) shows one cycle of TSA procedure.When the desorption temperature increases to 700 °C,it is clear that the Li4SiO4could be desorbed completely within 15 min.The experimental results show that the Li4SiO4loaded with 10% (mass) K2CO3has a relatively good cycling performance in the TSA process under the condition of 20% (vol) paCO2.

Fig.10(a)shows the SEM analysis of Li4SiO4sorbent doped with 10%(mass)K2CO3after 22 cycles in the PSA process.After 22 cycles,the surface of Li4SiO4particles is smooth and dispersed.There are large pores (red circles) inside the particles,which may be due to the doping of K2CO3enhancing the dispersion between Li4SiO4particles,and the existence of large pores promoting the diffusion of CO2.Fig.10(b) shows the SEM analysis of Li4SiO4doped with 10%(mass)K2CO3after 22 cycles in the TSA process.The sorption temperature was 600°C,and the desorption temperature was 700°C.It can be seen that the surface of the particles is rough,and there are still pores between the agglomerated particles,which may promote the diffusion of CO2,so as to improve the sorption capacity of CO2.

3.2.2.CO2 capture property of K2CO3 promoted Li4SiO4 under 50%(vol)paCO2

The influence of the loading ratio of K2CO3on the CO2sorption property of Li4SiO4under the 50%(vol)paCO2was also investigated(Fig.11(a)).The CO2uptake of Li4SiO4can reach 22.12% without doping K2CO3at 600 °C.It is proved that the higher the CO2concentration is,the better the CO2sorption performance of Li4SiO4is.Then,when the Li4SiO4was doped with 5% (mass) K2CO3,the CO2uptake rapidly increases to 24.96%.As the doping amount of K2CO3continues increasing to 10% (mass),the CO2uptake of Li4-SiO4continues to increase to 26.17%.With increasing the doping amount of K2CO3to 15% (mass),the CO2uptake of Li4SiO4was 25.46%.It is found that the low K2CO3loading had no significant effect on the CO2sorption capacity of Li4SiO4under the 50% (vol)paCO2.To further increase the amount of K2CO3to 20%-30%(mass),the CO2uptake increases obviously.The maximum CO2uptake of Li4SiO4was 32.41%with 30%(mass)K2CO3loading.However,when the doping amount of K2CO3further increase to 35%(mass),the CO2sorption capacity of Li4SiO4shows a decreasing trend.Therefore,under 50% (vol) paCO2,the optimal loading ratio of K2CO3is 30%(mass).Fig.11(b)shows the SEM analysis of Li4SiO4doped with 30%(mass)K2CO3.It can be seen from the picture that the particles are irregular cubes with lots of space stacked on the top of each other.Similarly,the influence of sorption temperature of the 30% (mass) K2CO3doped Li4SiO4under the 50% (vol) paCO2was studied,and the results are shown in Fig.11(c).It is found that 600 °C is also the optimal sorption temperature for 30% (mass)K2CO3doped Li4SiO4under the 50%(vol)paCO2,and it shows a fast sorption rate within the 10 min.

Fig.9.Cyclic performance test of the synthesized Li4SiO4 after doping with K2CO3 in(a)PSA procedures,and(b)TSA procedures,One CO2 sorption/desorption cycle of(c)PSA procedure,and (d) TSA procedure.

Fig.10.SEM images of the synthesized Li4SiO4 doped with K2CO3 after 22 sorption-desorption cycles of (a) PSA procedure,and (b) TSA procedure.

Moreover,the cyclic performance of Li4SiO4doped with 30%K2CO3was also tested through PSA procedure and TSA procedure under the 50% (vol) paCO2.In the PSA procedure,the sorption and desorption temperature are both 600 °C,while,in the TSA procedure,the sorption temperature is 600 °C,and the desorption temperature is 700 °C.The results in Fig.12(a) and (b) show the same trend with the results in Section 3.4.2.Fig.12(c) shows one CO2sorption/desorption cycle of PSA procedure.After 5 min,the desorption equilibrium is reached,and the desorption rate is 80%.Fig.12(d) shows one CO2sorption/desorption cycle of TSA procedure.When the desorption temperature is increased to 700 °C,it is clear that the Li4SiO4sorbent can be desorbed completely within 10 min,and the desorption rate is 88.75%.The experimental results show that the Li4SiO4doped with 30%(mass)K2CO3has a relatively good cycling performance in the TSA process under the condition of 50%(vol)paCO2.Fig.13(a)shows the SEM analysis of Li4SiO4sorbent doped with 30% (mass) K2CO3after 22 cycles in the PSA process.After 22 cycles,the surface of Li4SiO4particles is smooth and the channels exist between the aggregated particles.Fig.13(b)shows SEM analysis of Li4SiO4sorbent doped with 30% (mass)K2CO3after 22 cycles in the TSA process.It can be seen that the surface of particles is relatively rough,which results in a stable sorption capacity.

Fig.11.(a)The influence of the doping amount of K2CO3 on CO2 sorption performance of the synthesized Li4SiO4;(b)the SEM image of the synthesized Li4SiO4 after doping with 30% (mass) K2CO3,(c) the influence of sorption temperature on CO2 sorption performance of the synthesized Li4SiO4 after doping with K2CO3.

4.Conclusions

In this study,a low-cost and environmental-friendly leaching agent citric acid was used to treat the SDL to fabricate Li4SiO4-based CO2sorbents.It shows that the concentration of citric acid,MR of Li:Si,pretreated conditions,sorption temperature and CO2partial pressure all affect the CO2sorption property of the synthetic Li4SiO4.When the concentration of C6H8O7is 0.25 mol·L-1,MR of Li:Si is 7,the pretreated temperature is 750 °C,and the pretreated time is 6 h,the CO2uptake of the synthesized Li4SiO4can reach 31.37%(mass),which is higher than that of Li4SiO4produced from SDL treated with HCl/HNO3.At 20% and 50% (vol) paCO2,the CO2uptake of the synthesized Li4SiO4increases from the original 12.02% and 22.12% to 23.96% and 32.41%,respectively with the optimized K2CO3doping.Even more noteworthy is that the cycling stability of Li4SiO4is significantly enhanced after doping with optimized K2CO3under the low paCO2.Due to its remarkable CO2sorption property and superior cycling stability,the Li4SiO4derived from C6H8O7treated SDL is expected to be a promising CO2sorbent.In addition,citric acid has the potential to become a promising leaching agent in the future.

Data availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are very grateful to the financial support from National Natural Science Foundation of China (21868015,51802135),and the Applied Basic Research Programs of Yunnan Province (140520210057),and Taif University Researchers Supporting Project number (TURSP-2020/163),Taif University,Taif,Saudi Arabia.

Fig.12.Cyclic performance test of the synthesized Li4SiO4 after doping with K2CO3 in(a)PSA procedures,and(b)TSA procedures,one CO2 sorption/desorption cycle of(c)PSA procedure,(d) TSA procedure.

Fig.13.SEM images of the synthesized Li4SiO4 doped with K2CO3 after 22 sorption-desorption cycles of (a) PSA procedure,(b) TSA procedure.