Carbonic Anhydrase Immobilized On Magnetic Poly（GMA-DVB） Microspheres And Its Application In Catalytic CO₂ Hydration

Excess emissions of CO₂ have been shown to be a major contributor to global warming. Currently, amine-based absorption processes are the most available for post-combustion CO₂ in industry, but the main drawbacks of amine-based processes are amine degradation and the parasitic power loss. A novel Integrated Vacuum Carbonate Absorption Process（IVCAP） has been proposed to reduce the energy usage by employing a potassium carbonate aqueous solution as a solvent for CO₂ absorption. However, K2CO3-based system has a much slower CO₂ absorption rate than amine-based system does. Carbonic anhydrase is the most effective known enzyme by far which catalyzes the hydration of CO₂ into bicarbonate and a proton. Studies have shown IVCAP process can significantly increase the CO₂ absorption rate when the carbonic anhydrase is added. The free enzyme has poor thermal and storage stability, while immobilization can improve the stability of the enzyme and make the enzyme easy to be recovered and reused. Moreover, enzyme immobilization provides the possibility for the biocatalyst confinement in continuously operated reactors. With its high efficiency, eco-friendly and other unique advantages, as a new application in the field of carbon capture, carbonic anhydrase attracts researchers’ attention. In the present work, the immobilized carrier materials were prepared and characterized, and the optimal conditions for CA immobilization were determined. In addition, CO₂ hydration and some kinetic parameters of the immobilized CA were also compared with that of the free CA.At first, iron oxide nanoparticles were obtained by solvothermal synthesis, followed by 3-（trimethoxysilyl）propyl methacrylate（MPS） modification to immobilize the active vinyl groups onto the surfaces, and then the monomers were polymerized at the interfaces to form the polymer shells by seeded emulsion radical polymerization with Fe₃O₄ as the seed, glycidyl methacrylate（GMA） as the monomer, and divinyl benzene（DVB） as the cross-linker. The resultant microspheres displayed a high magnetic content（Fe₃O₄, 47 wt%）, well defined core-shell nanostructure, and surface epoxy functionalities, as well as a sustained colloidal stability. The BET surface area of the microspheres was 11.2048 m2 g-1. The total pore volume of the microspheres was 0.1697 cm3 g-1. The microspheres were mono-dispersed in water, and nearly spherical in shape with particle diameter of 200-400 nm. After the polymerization, the microspheres still maintained the spinel structure of Fe₃O₄. The saturation magnetization of the blank microspheres and the microspheres immobilized CA were 48.750 and 48.472 emu g-1, respectively.The microsphere was synthesized for CA immobilization in covalent binding form. The optimal conditions for CA immobilization were determined by an uniform design experiment（U9（96）） tests. The highest enzyme activity recovery was 65.71%. The optimum pH and temperature as well as the thermal stability, storage stability and reusability for the immobilized and free CA were investigated. The results indicated that CA immobilized on the poly（GMA-DVB） microspheres owned a higher thermal stability than that of the free CA. The free CA retained 71.8% of its original activity, whereas the immobilized CA retained 90.9% of its original activity after 30 days storage. After six recycles, the immobilized CA finally maintained 47.6% of its initial activity. Kinetic constants, i.e. Km, Vmax and the catalytic efficiency（Kcat）, of the immobilized and free CA were investigated by using p-NPA as the substrate in the liquid phase.The performance of the immobilized and free CA on CO₂ hydration was carried out in a double stirred tank by using pH=10.5 of K₂CO₃-KHCO₃ buffer as a solvent under 40-60 ° C. In addition, the effect of inhibitors on the enzyme-catalyzed activity, as well as the reusability of immobilized CA was investigated. The results showed that the main factors affecting the CO₂ absorption rate were two reasons: enzymatic action and temperature. With the increase of temperature, the enzymatic action weakened, and the absorption capacity of absorbent decreased at the same time. Compared with free CA, the immobilized CA owned a better toleration to high temperature, alkali environment and inhibitors. After six recycles, the immobilized CA still maintained about 50% of the catalytic activity, rendering them promising biocatalyst for CO₂ capture.