Study On The Immobilization Of Carbonic Anhydrase On Geopolymer Microspheres And Its Catalytic Adsorption Of CO₂

Carbon dioxide(CO₂),especially from anthropogenic activities,is deemed as the one of the major culprits responsible for global warming and corresponding environmental problems.Researchers are continuously committed to settle the matter related to the excessive emission of CO₂but no panacea is available currently.Therefore,the promising approach for CO₂capture and sequestration(CCS)is often cited as one of the most challenging technology for 21^stcentury.CCS is a powerful solution to capture and sequestrate CO₂in the atmosphere or waste gas which can be further converted into high-value industrial chemicals and fuels such as methane,methanol and formic acid.It is broadly recognized as having the potential to play a key role in meeting climate change targets,delivering low carbon heat and power,decarbonizing industry and,more recently,its ability to facilitate the net removal of CO₂from the atmosphere.So far,various techniques have been proposed,including gas phase separation,absorption into a liquid,and adsorption on a solid,as well as hybrid processes,such as adsorption/membrane systems.Particularly,the biological method is regarded as a viable and environmental-benign candidate for tackling the CO₂issues for its low energy consumption and high efficiency.Carbonic anhydrase,commonly abbreviated to CA,is a kind of zinc-containing metalloenzyme which can catalyze the hydration of CO₂into bicarbonate.It is ubiquitous in animals and plants and has at least five distinct categories.Featured like other enzymes,it has the nature of the high catalytic efficiency,excellent specificity and mild reaction conditions.Despite the fact that CA has emerged as a prevailing strategy to tackle the CO₂crisis,the practical applications are confronted with the challenge of the intrinsic fragile nature of enzyme in harsh environment,which makes them vulnerable to detrimental structural changes in industrial conditions,leading to inevitably denaturation,inactivation,poor stability,and laborious recycling.The enzyme immobilization technology overcomes the above problems.The immobilization technology is defined as the enzyme can be reused and recycled while still maintain the catalytic capability when it was confined or bound in a certain area by solid materials.The immobilized enzyme integrates the characteristics of mild reaction conditions,high efficiency and specificity,and makes up for the deficiency of free enzyme owing to its superiority in enhancing stability and overcoming the obstacle of enzyme recycling.The present study entails the immobilization of CA on geopolymer microspheres and the CO₂capture and sequestration performance of immobilized enzyme.The optimum immobilization conditions were found out to be(a)pH value,8.0;(b)temperature,30?;(c)immobilization time,2 h;(d)GMS dose,100 mg;(e)GA concentration,0.2 wt%;(f)cross-linking time,1 h.Compared with free CA,the optimum pH of the immobilized enzyme increased from 7.5 to 8.0 and the optimum temperature increase from 25? to 30?.The thermal stability of immobilized CA was better than its free counterpart.The storage stability of the enzyme at different temperatures was improved,and the relative activity of the enzyme remained 46.1%after five cycles of continuous operation.The Michaelis constant(K_m)and the maximum reaction velocity(V_max)of free CA were determined to be 7.6283 m M and 1.6011 m M·min^-1,while that of the immobilized CA were 21.549 m M and 5.0525 m M·min^-1,respectively.The overall activity(K_cat/K_m)of them were 61.5015 and 12.3593M^-1·s^-1,respectively.The catalytic performance of immobilized CA was investigated by carrying out the CO₂absorption assay.Detailed parameters like temperature,gas flow and enzyme dose were explored.According to the results,under the circumstance of 30?,the absorption of CO₂catalyzed was the most significant.The amount of CO₂absorbed by immobilized CA was 46-folds of the blank experiment.When the gas flow rate was 300 m L/min,the absorption reached the equilibrium.The increasing amount of enzyme dose did accelerate the catalytic speed,but it also made the reaction difficult to control,with the side-effect of inaccuracy and high-cost.The mineralization study was also performed to demonstrate the feasibility of converting CO₂into bicarbonate.The effect of enzyme,temperature and buffer solution on the mineralization of CO₂was investigated.The free CA and immobilized CA shared the similar trend towards the influence on the mineralization of CO₂.In the presence of CA,the catalytic reaction was significant and the precipitation was obvious.Temperature had a dual effect on the catalytic performance of the enzyme.With the increase of temperature,the activity of free enzyme and immobilized enzyme increased,but the solubility of CO₂decreased in the meanwhile.The existence of buffer solution was beneficial to maintain the pH of the reaction system.In the low pH environment,the hydration of CO₂and the ionization of HCO₃^-were inhibited,so it was difficult to produce enough CO₃^2-to react with Ca²⁺to form precipitation.It appeared to us that the immobilization of CA on geopolymer microspheres represented a new strategy for tackling the intrinsic fragile nature of enzyme and was potential in promoting the capture and sequestration of CO₂in a biological route.