| Carbon dioxide is the mojor component of greenhouse gases,and the main factor of global warming.The efficient capture and utilization of CO2 has become an urgent problem to solve for the researchers.Using bio-enzymatic to capture CO2 has attract great intention due to its eco-friendly feature and mild reaction conditions.However,there are some practical limitations in the application of CA in the native form,such as low thermal and operational stability,narrow pH range,and difficulties in recovery.Enzyme immobilization technology allows the enzyme to be recycled as a solid catalyst,while maintaining the properties of the enzyme,providing an effective way for the application of the enzyme.At present,immobilized enzymes have been used in many fields such as industrial production,medicine,chemical analysis and environmental engineering.Carbonic anhydrase(CA)is a zinc-based metalloenzyme that accelerate the reversible hydration of CO2,and turn it into HCO3-ion.The greenhouse effect has caused serious danger to the ecological balance.Using immobilized CA for efficient CO2 capture is an important research topic in the field of enzyme engineering and environmental protection.It is very important to study the support materials which can efficiently immobilized CA and to apply to CO2 mineralization or to CO2 absorption by organic amine solution.Reducing the greenhouse gas emission and protecting the human living environment are very important.Firstly,3-aminopropyltriethoxysilane(APTES)was grafted onto the surface of SBA-15 by post-modification method to prepare amino-functional mesoporous materials(AFS).After the corresponding characterization,CA was immobilized on AFS via covalent binding,and the optimum immobilization conditions were studied.The results showed that when under the medium of pH 7.0,which is greater than the isoelectric point of the CA(Ip = 6.4).The electrostatic interaction between the CA molecule and the carrier contributes to the immobilization of CA on AFS.Under the conditions of the AFS dosage of 10 mg/10 mL of CA,immobilization time of 2 h,the capacity of AFS is 63.2 mg/g of AFS,63%of the CA has been immobilized.The enzymatic properties of immobilized CA(CA/AFS)prepared under these conditions were studied.The results showed that CA/AFS had the highest activity at 40?,which increased by 5 ? compared with free CA,and maintained high activity in the range of 40-55 0C.The optimum pH of CA/AFS was 10.0,which was higher than that of free CA(pH = 8.5),and CA/AFS had better pH stability.Remained at 4 ? for 30 days,CA/AFS retained 83%of the initial activity,while free CA only saved 30%activity.After 20 cycles,CA/AFS still has 78%initial activity,which indicated that CA/AFS has great reusability.CA/AFS had better thermal stability than free CA in 5 wt%MDEA solution.It could maintain more than 80%activity at temperatures below 50 ?.When the temperature rises to 90 ?,CA/AFS maintained 11%of the initial activity.While the free CA maintained only 8%activity at 35 ?,and the activity of the free CA was almost undetectable when the temperature exceeded 60 ?.The Km values of free CA and CA/AFS were 2.4 mM and 3.0 mM,respectively.The results indicated that the affinity of CA to the substrate becomes weak after immobilization,since the substrate is difficult to contact with the active site of CA,which is the same as the case of most immobilized enzymes.The kcat/Km values of free CA and CA/AFS were 896.4 M-1·s-1 and 728.2M-1·s-1,respectively,which indicated that the activity of CA decreased after immobilization.Secondly,SBA-15 was modified by y-glycidoxypropyltrimethoxysilane(GPTMS)to prepare epoxy-functionalized mesoporous materials(GFS).After the corresponding characterization,CA was immobilized on GFS by covalent coupling method.The immobilization mechanism of action of CA immobilized on GFS was discussed,and the optimum immobilization conditions were studied.The results show that when the pH of the medium was 6.0,which is smaller than the isoelectric point of CA.The zeta potential of GFS is negative under this condition.CA molecules and GFS had a strong electrostatic interaction that could promote the immobilization of CA.Under the conditions of the GFS dosage of 5 mg/10 mL of CA,the immobilization time of 2 h,the capacity of CA was 72.6 mg/g of GFS,and about 36%of the CA was immobilized.The enzymatic properties of CA/GFS prepared under these conditions were studied.The results showed that the activity of CA/GFS was the highest at 40 ?,which was 5 ? higher than that of free CA and slightly higher than that of CA/AFS in the range of 40-55 ?.The optimum pH of CA/GFS was 10.0,which is higher than that of free CA.Remained at 4 ? for 30 days,and CA/GFS retained 91%of the initial activity.After 20 cycles,CA/GFS still had 87%initial activity,indicating that it had good reusability.In 5 wt%MDEA solution CA/GFS has better thermal stability and could maintain more than 80%activity at temperatures below 50 ?.When the temperature was increased to 90 ?,CA/AFS maintained 13%of its initial activity.The Km values of CA/GFS was 3.1 mM,which indicated that the affinity of CA to the substrate becomes weak after immobilization.The kcat/Km values of CA/GFS were 757.4M-1·s-1,which indicated that the activity of CA decreased after immobilization.Considering the high cost of the two above supports,the commercially available amino functionalized resin(LX-1000EA,EA)was used as the immobilized carrier.The mechanism of CA immobilized on EA is similar to that of AFS.The results of the study on the optimum immobilization conditions show that,under the conditions of the EA dosage of 200 mg/10 mL of CA,the immobilization time of 2 h,the CA capacity was 9.1 mg/g of EA,and about 99%of the CA was immobilized on EA.The enzymatic properties of CA/EA prepared under these conditions were studied.The results showed that the activity of CA/GFS was the highest at 35?,and the activity was higher in the range of 15-55 ? than that of CA,but lower than CA/AFS and CA/GFS.The optimum pH of CA/EA is 9.0,which is higher than that of free CA.After 30 days storage at 4 ?,and CA/EA retained 90%of the initial activity.After 10 cycles,CA/EA still had 83%initial activity.In 5 wt%MDEA solution,the activity of CA/EA was the highest at 35 ?,and 62%of the initial enzyme activity was maintained.The activity of CA/EA decreased with the increase of temperature,and when the temperature increased to 90 0C,CA/EA maintained 8%of the initial activity.The Km value of CA/EA was 8.8 mM,higher than that of CA,CA/AFS,and CA/GFS,which indicated that the affinity of CA/EA to substrate was lower than that of CA/AFS and CA/GFS.Finally,the effects of immobilized CA on CO2 mineralization and CO2 absorption-desorption process of tertiary amine(MDEA)were studied.When the immobilized CA is used for CO2 mineralization,the active site of free CA is more accessible,and its activity is higher than that of immobilized CA.Therefore,the amount of CaCO3 catalyzed by free CA is the largest.The order of the amount of produced CaCO3 was:free CA(247 mg)>CA/GFS(231 mg)>CA/AFS(209 mg)>CA/EA(202 mg).When C02 was absorbed by the MDEA solution,the reaction rate of the four solutions of MDEA,CA ?MDEA,supports + MDEA,and immobilized CA + MDEA was:CA+MDEA>CA/AFS+MDEA>MDEA>AFS+MDEA;CA+MDEA>CA/GFS+MDEA>MDEA>GFS+MDEA;CA+MDEA>CA/EA+MDEA>EA+MDEA>MDEA.It can be seen that the addition of immobilized CA has enhancement effect on the absorption of CO2 by MDEA solution.MDEA solution desorption of CO2 results showed that the addition of immobilized CA and the corresponding carrier had little effect on CO2 desorption.The addition of CA/AFS,CA/GFS,AFS,and GFS can increase the instantaneous desorption of CO2 before the equilibrium state is reached.Due to the inactivation of free CA structure under high temperature conditions,the addition of free CA has no effect on CO2 desorption rate.The support materials were porous material.Their addition could improve the heat transfer and mass transfer process of the desorption process,and then improved CO2 desorption rate.However,the immobilized CA has no effect on the desorption rate. |
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