Intimately Coupling Photocatalytic Optical Fibers and Biolm for Rapid and Sustainable Degradation of 4-Chlorophenol

The need for wastewater treatment is progressively rising as the release of copious amounts of industrial wastewater is increasing. Likewise, there is an urgent requirement for renewable energy sources because of the growing energy demand and depletion of fossil fuels. The use of microalgae to convert toxic phenolic wastewater to lipid-enriched biofuel has recently been proposed. Here, we report a new strategy for coupling N-doped TiO 2 -coated photocatalytic optical bers and a microalgal biolm to degrade 4-chlorophenol (4-CP) and produce biomass. In the combined photocatalysis and biodegradation system, the photocatalytic products were directly biodegraded by the heterotroph-enriched (Salinarimonas and Pseudomonas) biolm, promoting biomass production; O 2 produced by the phototrophs (Scenedesmus obliquus) promoted the generation of hydroxyl free radicals using N-doped TiO 2 . Thus, the combined photocatalysis and biodegradation system rapidly and sustainably degraded 4-CP while maintaining the growth of the microalgal biomass. The 4-CP removal, dechlorination, and biolm growth rates reached ~78 µM/h, ~41 µM/h, and 1.8 g/h/m 2 , respectively. Overall, we present a useful synergy between an optical catalyst and a bioreactor that has implications for both wastewater remediation and sustainable microalgal biomass production.


Introduction
4-Chlorophenol (4-CP) is a typical toxic chlorinated organic pollutant in phenol wastewater that causes serious pollution and damage to the environment (Lan et al. 2017). Long-term consumption of water contaminated by 4-CP can cause neurological diseases, such as dizziness, rashes, and itching (Samet et al. 2010 ). Therefore, the rapid and continuous removal of 4-CP from wastewater to protect the ecological environment and public health has become an urgent issue worldwide.
Intimately coupled photocatalysis and biodegradation (ICPB) can effectively degrade phenolic pollutants (Yu et al. 2017;Zhou et al. 2018;Yusoff et al. 2020). This technology uses photocatalysts to degrade refractory compounds, and the partial photocatalytic products are consumed as a carbon source by microorganisms (Rittmann. 2018;Li et al. 2020). Furthermore, it overcomes the shortcomings of incomplete photocatalytic degradation and long-term degradation of bio lms, and thus greatly improves the toxic organic wastewater degradation e ciency. Although the reported ICPB technology has many advantages, the photocatalyst carriers (sponges, porous ceramics, and foams) have poor optical properties, severe light attenuation, and restrictions on the transfer of light energy. Furthermore, bacterial bio lms barely produce oxygen, and even consume it (Al-Amshawee et al. 2020; Carré et al. 2020); thus, an effective coupling between photocatalysis and bio lm technologies is di cult to achieve. This limits the degradation and mineralization of toxic organic wastewater using ICPB technology.
Here, we present a novel ICPB reactor using N-doped TiO 2 coated hollow quartz optical bers (HQOFs) and Scenedesmus obliquus (S. obliquus) as the initial bio lm for the biodegradation of 4-CP (Figs. S1-S2). The photoreactor comprises both upper and lower regions for biodegradation and photocatalysis, respectively. The biodegradation of 4-CP and its photocatalytic products releases O 2 via photosynthesis.
The produced O 2 is transferred to the photocatalytic region to promote photocatalysis for the production of a strong oxidizing group ( • OH). The produced • OH rapidly detoxi es 4-CP for use by the bio lm. The conditions required to obtain rapid and continuous degradation were determined by investigating the effects of the dissolved oxygen (DO) concentration, temperature, and pH on the photocatalytic degradation of 4-CP. For comparison, use of the isolated photosynthetic microalgal bio lm for 4-CP degradation was also studied. Ultimately, the coupled photocatalytic and biodegradation system was used to degrade 4-CP. Furthermore, the changes in the bacterial population in the bio lms were analyzed, and the biomass production and oil accumulation in the bio lms were revealed.

Preparation and operation of the ICPB photoreactor
The ICPB photoreactor (length, width, and height of 140, 59, and 27 mm, respectively) was fabricated using polymethyl methacrylate, as shown in Supplementary Fig. S2. It is separated into the upper region (biodegradation region) and lower region (photocatalytic region) using a nuclear pore membrane. The working volumes of the upper and lower regions were ~ 27 and 57 mL, respectively. The nuclear pore membrane was used to immobilize the S. obliquus cells and transmit the visible light emitted by the photocatalytic optical ber surface, thus facilitating the production of O 2 by S. obliquus photosynthesis, which promotes the generation of photocatalytic products via the degradation of 4-CP in the photocatalytic region. The upper region was composed of a microalgal bio lm and gas phase space, and was used to degrade the photocatalytic products and part of the 4-CP to produce O 2 . The lower region was composed of two rows of 36 photocatalytic optical bers, lled with 4-CP wastewater, and used to rapidly degrade and dechlorinate the 4-CP. The excitation light within the photocatalytic optical bers was obtained from UV-vis LED light sources.
In the isolated photocatalysis test, the circulation of the 100-mL 4-CP (388.9 µM) solution in the photocatalytic region of the ICPB photoreactor was controlled at a ow rate of 1 mL/min via the wastewater inlet and outlet. The initial pH of the 4-CP solution was adjusted to 5.0-10.0, using HNO 3 and NaOH solutions, as required. The solution temperature was adjusted to 25-50 ℃ using a thermostatic water bath (DCW-0530, Shunmatech, China). High-purity oxygen (99.995%) was supplied to the ICPB photoreactor via an external oxygen supplier. The oxygen supply per unit time was controlled within the range of 0 to 8.39 mM using a mass ow meter (Rheonik RHM 007, Germany). The 36 photocatalytic optical bers were then evaluated for their ability to photocatalytically transform 4-CP when illuminated using a UV-vis LED light source with an average irradiance of 20 W at 360-380 nm and 30 W at 400-750 nm.
For the isolated biodegradation test, the ICPB system with bio lm was cultured, as shown in Supplementary Section S1, and was employed to evaluate the degradation of 4-CP. The 36 optical bers without photocatalysis were illuminated with visible light (average irradiance of 50 W at 400-750 nm).
The circulation of the 100-mL 4-CP solution (initial concentration of 388.76 µM) in the photocatalytic region of the ICPB photoreactor was also controlled at a ow rate of 1 mL/min through the wastewater inlet and outlet. The temperature and initial pH of the 4-CP solution were adjusted in the range 25-45°C and 5.0-10.0, respectively.
In the ICPB system test, the ICPB system comprising 36 photocatalytic optical bers and bio lm was used to evaluate the degradation of 4-CP. The setup was the same as that used in the isolated photocatalysis test, except that the bio lm was inoculated on the nuclear pore membrane, and the initial pH, DO concentration, and temperature were controlled at 7.0, 388.9 µM, and 30°C, respectively.

Characterization techniques
The HQOF and PcHQOF characterization, bioinformatics analysis, and liquid-phase analysis are detailed in Supplementary Sections S3-S5.   Figure 1d shows the porous structure of the NT coating. Figure 1e shows that the NT photocatalyst particles had a uniform size with an average of ~ 30 nm.

Results And Discussion
Supplementary Fig. S3a and Fig. 1f characterize the elemental composition of the NT using TEM-EDS plane scans and XRD and XPS analyses. Ti, O, and N are clearly observed in the NT STEM-EDS plane scans. The XRD analysis of the NT sample showed nine main peaks corresponding to solid diamonds (indicated by solid diamonds in Supplementary Fig. S3a) Figure 1f shows two strong peaks at 458.25 eV, and 529.5 eV, corresponding to the characteristic peaks of Ti2p, and O1s, respectively. The N 1s XPS spectrum of NT exhibits a characteristic peak at 399.1 eV, revealing the N linkage in TiO 2 (interstitial N). Together, the results shown in Fig. 1 and Supplementary Fig. S3 demonstrate that NT has been successfully synthesized.
Supplementary Fig. S4a shows the UV-vis absorption spectrum of the NT particles, which reveals a broad absorption edge in the visible region; with the band edge redshifted to 500 nm owing to defect states created by N-doping, enabling sub-bandgap absorption. Supplementary Fig. S4b shows that the surface luminous spectrum of the NT-coated PcHQOFs was lower than that of the HQOFs in the spectral range 200-750 nm, due to absorption by the NT coating. By contrast, the NT-coated HQOFs exhibited high 3.1 Effects of temperature, initial pH, and DO content on photocatalysis Figure S5a and S5b show that the NT-coated HQOFs exhibited the highest photocatalytic degradation of 4-CP at a temperature of 35°C and an initial pH of 7.0. Fig. S5c shows that part of the 4-CP was degraded, as evidenced by the ~ 67% loss of 4-CP, ~ 42% loss of the initial dissolved organic carbon (DOC), and ~ 56% dechlorination, after 10 h. Over the 10-h experiment, the dissolved oxygen (DO) concentration decreased due to the consumption of oxygen for the photocatalytic oxidation of 4-CP (Fig. S5d). The pH also decreased because the 4-CP degradation produced small organic acids. Figure 2a shows that the highest photocatalytic degradation of 4-CP was obtained at 8.93 mM DO in 4 h. Figure 2b shows ~ 100 % removal of 4-CP, ~ 63% loss of DOC, and ~ 71% dechlorination after 14 h. The DOC losses were proportionally less than the loss of 4-CP, indicating that organic residuals remained, which is a desirable feature for the coupling of photocatalysis and biodegradation, providing the residuals are biodegradable. Figure S6 shows that the highest biodegradation of 4-CP occurred at a pH of 7.0 and a temperature of 30°C, which correspond to the most suitable conditions for S. obliquus growth (Breuer et al. 2013). Figure 3a shows that the degree of 4-CP degradation, DOC removal, and dechlorination corresponded tõ 83%, ~ 68%, and ~ 85%, respectively, after 10 h. Figure 3b shows that the DO concentration rapidly increased from 234.68 µM to 265.31 µM over 10 h due to the photosynthetic production of O 2 by S. obliquus; however, the pH was only slightly decreased due to the the bio lm metabolism produced organic acids. The production of O 2 and stabilization of the pH are bene cial for photocatalysis because they can enhance the NT surface oxygen vacancy production of • OH (Wang et al. 2021 Figure 4a shows the rapid removal of 4-CP, which was accompanied by the release of Cl − corresponding to ~ 84% of the Cl in 4-CP. Approximately 99% DOC removal was achieved after 8 h, whereas all 4-CP had been completely removed after 5 h. Overall, the 4-CP removal, dechlorination, and DOC removal rates reached ~ 78, ~41, and ~ 27 µM/h, respectively, which were much higher than those obtained with isolated photocatalysis and biodegradation. Figure 4b shows that whereas the pH continuously declined, the DO initially declined due to photocatalysis consumption, but then increased with photosynthetic activity. The key role of the S. obliquus metabolism is underscored by the increase in biomass over time (Fig. 4c); the growth rate of bio lm in the presence of 4-CP reached 1.8 g/h/m 2 . Furthermore, Fig. S7 shows that the PcHQOFs demonstrated a repeatable transformation of 4-CP and generation of O 2 . The photocatalytic activity of the PcHQOFs was maintained at the same level over eight cycles, as the Ndoped TiO 2 photocatalyst did not detach due to the use of the Triton X-100 cross-linking agent and polyethylene glycol coated on the chemically cleaned HQOFs (Wu et al. 2020). In addition, the photosynthetic production of O 2 and biodegradation of photocatalysis products both increase • OH generation by the PcHQOFs, thus maintaining high photocatalytic activity. Notably, S. obliquus grown in the presence of 4-CP had a normal internal cell structure (Fig. S8a) and the bio lm became enriched in Salinarimonas and Pseudomonas (average relative abundances were ~ 17% and ~ 18%, respectively; Fig.  S8b). Salinarimonas and Pseudomonas are well known for their metabolism of a variety of biorecalcitrant pollutants (Mcleod et al. 2006). In addition, these heterotrophic microorganisms metabolize 4-CP and consume O 2 produced by S. obliquus photosynthesis and produce CO 2 for S. obliquus growth.

ICPB system for degradation of 4-CP
This synergy provides an e cient means to consume photocatalytic products (heterotrophs) and O 2 produced (phototrophs) by the bio lm, promoting the generation of more • OH. The increase in • OH can quickly degrade the high concentration of phenol to achieve a level within the microalgal bio lm adaptation range, thereby avoiding the long-term toxicity of high concentrations of phenol to algal cells, and contributing to the continued growth of the bio lm (Fig. 4c). The synergistic properties of photocatalysis, biodegradation, and photosynthesis enhance the degradation and mineralization of 4-CP and the growth of bio lm biomass.

Conclusion
The ICPB system, comprising NT-coated PcHQOFs, an NpM, and a bio lm that includes S. obliquus and heterotrophic microorganisms, provided rapid and continuous removal of 4-CP. S. obliquus in the bio lm provided O 2 for photocatalysis and for the biodegradation of 4-CP and its photocatalytic products by Salinarimonas and Pseudomonas. Furthermore, the biodegradation of 4-CP and photocatalytic products enhanced the production of • OH and improved its e ciency, which enhanced photocatalysis. The results demonstrated that 100 mL of 4-CP with an initial concentration of 388.9 µM was degraded in 5 h using the ICPB system, with the bio lm maintaining a high activity and growth rate. Thus, the ICPB system with the NT-coated PcHQOFs establishes synergism among photocatalysis, microalgae, and bacteria. This synergism provides an e cient means to rapidly and continually degrade and mineralize 4-CP and convert it to microalgae biomass.

Declarations Disclosures
There are no con icts of interest to declare.