Figure 1a–b show the surface morphology of uncoated HQOF and NT-coated PcHQOF, respectively. Figure 1b–c reveal that the dense and uniform NT photocatalyst was successfully coated on PcHQOF. 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.
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) at 2θ = 25.26, 38.01, 48.09, 53.91, 55.07, 62.7, 68.8, 70.23, and 75.09, which correspond to the characteristic diffraction peaks of anatase TiO2. 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 TiO2 (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 visible luminous intensity in the spectral range of 750–950 nm, which was caused by the increase in the refractive index of the NT coating. The good light absorption by the NT coating and the surface luminosity of NT-coated PcHQOFs can enhance the photocatalytic activity of the PcHQOFs (Wu et al. 2020) and provide light energy for photosynthesis in the biofilm.
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.
3.2 Effects of temperature and initial pH on biofilm degradation of 4-CP
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 to ~ 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 O2 by S. obliquus; however, the pH was only slightly decreased due to the the biofilm metabolism produced organic acids. The production of O2 and stabilization of the pH are beneficial for photocatalysis because they can enhance the NT surface oxygen vacancy production of •OH (Wang et al. 2021).
3.3 ICPB system for degradation of 4-CP
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 biofilm in the presence of 4-CP reached 1.8 g/h/m2. Furthermore, Fig. S7 shows that the PcHQOFs demonstrated a repeatable transformation of 4-CP and generation of O2. The photocatalytic activity of the PcHQOFs was maintained at the same level over eight cycles, as the N-doped TiO2 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 O2 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 biofilm 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 bio-recalcitrant pollutants (Mcleod et al. 2006). In addition, these heterotrophic microorganisms metabolize 4-CP and consume O2 produced by S. obliquus photosynthesis and produce CO2 for S. obliquus growth. This synergy provides an efficient means to consume photocatalytic products (heterotrophs) and O2 produced (phototrophs) by the biofilm, 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 biofilm adaptation range, thereby avoiding the long-term toxicity of high concentrations of phenol to algal cells, and contributing to the continued growth of the biofilm (Fig. 4c). The synergistic properties of photocatalysis, biodegradation, and photosynthesis enhance the degradation and mineralization of 4-CP and the growth of biofilm biomass.