A Study On Immobilization of Carbonic Anhydrase On TiO2 Nano-Particles and TiO2 Functionalized Membranes

Carbon capture and storage (CCS) from sources such as flue gases from coal-fired power station is a vital option to reduce the impact of CO₂ on climate change. To reduce the environmental impact of such processes, the use of gas-liquid membrane contactor with benign solvents such as water has received considerable attention for CO₂ capture but the rate of uptake for CO₂ remains a challenge. One approach is to utilize carbonic anhydrase (CA), the fastest known CO₂ hydration enzymatic catalyst, to accelerate the bicarbonate formation. While the low stability of CA in free form limits its potential applicability, enzyme immobilization on nano-structured materials offers a great opportunity to improve their stability. In this work, a systematic study was carried out to investigate the performance of CA immobilized on both TiO₂ nanoparticles and TiO₂ functionalized membranes.

Modification and functionalization of TiO₂ particle surface could drastically improve the immobilization performance over simple adsorption.  Chemically modified TiO₂ particles exhibited substantially higher specific activity (2,072 U/mg CA, assayed by p-NPA) and better activity recovery (92 %) in comparison with the un-modified TiO₂ particles (specific activity: 210 U/mg CA, activity recovery: 9 %). In terms of reusability, chemically modified TiO₂ particles preserved up to 90 % of CA activity after 20 assay cycles, while the CA immobilized on un-modified TiO₂ particles lost more than 90 % of its initial activity after 20 cycles. When comparing the stability of the immobilized CA with free CA, the CA immobilized on chemically modified TiO₂ particles preserved 80 % of its initial activity after 20 days of storage at 25 ˚C, while the free CA only maintained 40 % during the same period.

In terms of the TiO₂ functionalized membranes, the results illustrated that the membrane architecture especially the distribution of the TiO₂ particles had strong influence on the bio-catalytic performance. The TiO₂ functionalized membrane with majority of the particles located on the membrane surface (e.g. TiO₂ sol-gel membrane prepared via low temperature hydrothermal process) is preferred compared to the membrane with most of the TiO₂ particles distributed throughout the membrane matrix (e.g. TiO₂ blended membrane), as the former significantly increased the available covalent bonding sites readily exposed to enzyme. Such a strong bonding assisted the preservation of the original conformation of CA and mitigated potential enzyme detachment or leaching. As a result, 8 times higher specific activity and activity recovery were observed on the TiO₂ sol-gel membrane than TiO₂ blended membrane.

The promising results from this work demonstrated the potential of utilizing these bio-catalytic TiO₂ nanoparticles and TiO₂ mediated membranes in a gas-liquid membrane contactor for a robust and high efficiency CO₂ capture solution from post-combustion.