Abstract
Nitric acid (HNO3)-treated carbon fiber (CF) rich in hydrophilic groups was applied as a cell-immobilized carrier for xylitol fermentation. Using scanning electron microscopy, we characterized the morphology of the HNO3-treated CF. Additionally, we evaluated the immobilized efficiency (IE) of Candida tropicalis and xylitol fermentation yield by investigating the surface properties of nitric acid treated CF, specifically, the acidic group content, zero charge point, degree of moisture and contact angle. We found that adhesion is the major mechanism for cell immobilization and that it is greatly affected by the hydrophilic–hydrophilic surface properties. In our experiments, we found 3 hto be the optimal time for treating CF with nitric acid, resulting in an improved IE of Candida tropicalis of 0.98 g∙g−1 and the highest xylitol yield and volumetric productivity (70.13% and 1.22 g∙L−1∙h−1, respectively). The HNO3-treated CF represents a promising method for preparing biocompatible biocarriers for multi-batch fermentation.
Highlights
Utilization of immobilized cells eliminates time-consuming and cost-intensive steps involved in the isolation and purification of intracellular enzymes [1]
The grooves on the carbon fiber (CF) carrier were formed by removing the amorphous carbon during the nitric acid treatments, which may help to increase the roughness and binding sites on material surface [18]
Despite the similar surface morphology of CF-Nh3 and CF-Nh6,more cells adhered to CF-Nh3 than CF-Nh6 carrier, after the batches’ fermentation immobilized with C. tropicalis (Figure 1e,f), which suggests that the capacity of cell immobilization could be improved after the treatment by nitric acid and the amount of treated time has an optimum value
Summary
Utilization of immobilized cells eliminates time-consuming and cost-intensive steps involved in the isolation and purification of intracellular enzymes [1]. The carrier material should be biodegradable, available in large quantities at low cost, and exhibit appropriate physical properties for the sufficient adherence of specific microorganisms [7]. Different materials such as porous glass, Ca-alginate, chitosan and sodium alginate have been investigated. The biocompatibility of carbon fiber (CF) was shown to possess a high capacity for bacterial adhesion and has been adopted as a biofilm support for wastewater treatment [12,13,14,15]. Zhu et al [23] show that the hydrophilicity and biocompatibility of carbon materials is enhanced by increasing the content of surface oxygenic functional groups. The surface morphology, the content of acidic groups, point of zero charge (pHpzc ), degree of moisture (DM), and contact angle (CA) of the HNO3 -treated CF was evaluated to study the effects on immobilized efficiency (IE), xylitol yield and volumetric productivity in thebatchesfermentation of xylitol by Candida tropicalis (C. tropicalis)
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