Effects of β-Cyclodextrin Introduced by Different Methods on the Immobilized Phenol-Degrading Bacteria in Photocrosslinked Spherical Hydrogels

Abstract

In this study, we developed two types of lattice-type β-cyclodextrin (β-CyD)-containing spherical hydrogels to immobilize phenol (PhOH)-degrading bacteria. One type, ENTG-mix-βCyD/HDI, consists of mixed-type spherical hydrogels containing β-CyD ring-bearing polymer microparticles embedded within the gel matrix. The other type, ENTG-co-PSβCyD, consists of copolymerized spherical hydrogels in which β-CyD-substituted monomers are copolymerized and crosslinked. The former features an aggregated distribution of β-CyD rings, whereas the latter exhibits a uniform distribution. Continuous PhOH degradation experiments revealed that both of the β-CyD-containing spherical hydrogel catalysts exhibited catalytic activity exceeding that of the ENTG spherical catalyst without β-CyD. Immobilized bacteria were distributed both on the surface and within the structure of the copolymerized carrier, whereas in the mixed carrier, many bacteria were dispersed throughout. Analysis of the PhOH-degrading flora revealed that Pseudomonas putida formed a niche in the copolymerized hydrogels, whereas Sphingomonas sp. formed a niche in the mixed hydrogels. Batch experiments using p-xylene instead of PhOH demonstrated that the degradation rates of the copolymerized and mixed gels were 2.4 times and 1.6 times greater than that of the ENTG gel, respectively. The copolymerized gel exhibited a faster p-xylene degradation rate due to the reactivity of P. putida. Continuous phenol (PhOH) decomposition experiments were carried out with PhOH-degrading bacteria immobilized in a copolymerized spherical hydrogel (ENTG-co-PSβCyD) and a mixed spherical hydrogel (ENTG-mix-βCyD/HDI). Similar to the results obtained from the batch PhOH degradation experiments, both β-CyD-containing spherical hydrogels, which feature cylindrical hydrophobic intramolecular spaces, exhibited higher activity than the ENTG spherical hydrogels lacking β-CyD. The bacterial cells were extensively distributed on the surface and inside the copolymerized carrier and throughout the entire mixed carrier, while only a small amount of bacteria were found on the surface of the ENTG carrier. Furthermore, the PhOH-degrading bacterial flora (microflora) in the copolymerized and mixed spherical gel matrices were identified. Pseudomonas putida formed a niche in the copolymerized spherical hydrogel, and Sphingomonas sp. formed a niche in the mixed hydrogel. In the continuous PhOH degradation experiment, the performance of both hydrogels was almost identical because the ability of both strains to degrade PhOH is similar. However, in the batch removal experiment using p-xylene as the substrate instead of PhOH, the rates of substrate removal by the copolymerized gel and mixed gel 2.4 times and 1.6 times greater than that of the ENTG gel, respectively. This occurred because the bacterial species in the mixed gel was Sphingomonas sp. instead of P. putida, and the high substrate removal by the copolymerized gel was a result of the high reactivity of P. putida.