The results in Section 3.1 indicate

The results in Section 3.1 indicate that the spectrum of VL is important for effective photocatalytic disinfection by NS, thus the effect of VL wavelengths on bacterial disinfection was also investigated under VL intensity of 87 mW cm−2 at pH 8 using different colored LEDs (blue, green, yellow, red and white LED). The photocatalytic disinfection efficiencies of NS under blue and yellow LED were higher than that under green LED, and much higher than that under red LED (Fig. 1b). The inactivation process of blue LED was similar to yellow LED, and both of them could totally disinfect 107 cfu mL−1E. coli K-12 within 8 h treatment. Green LED could result in the reduction of a 5-log cell density after 10 h treatment. However, within 10 h red LED irradiation, there was no cell density decreasing, which means the red range of VL has no effect on the photocatalytic disinfection. From Table SM-1, the red colored LED has the longest wavelengths range of approximately 610–650 nm, yellow LED 570–620 nm, green LED 470–570 nm, and blue LED 440–490 nm. Based on the results of Fig. 1b, the most effective range of VL was the wavelength of 440–490 and 570–620 nm for the NS. The longest wavelength of VL from 610–650 nm was not effective for the photocatalytic disinfection by the NS.
Fig. 2 shows the diffuse reflectance UV–Vis absorption spectra of the NS (Li et al., 2009b). We could observe a steep absorption edge at about 420 nm and the adsorption of the NS gradually decreased with the wavelength prolonging to 620 nm. Because the photon energy of blue LED is higher than those of other colored-LEDs and the wavelength of blue LED overlaps with the steep absorption edge of the NS, the blue LED is more effective to disinfect E. coli K-12 than other colored-LEDs. At the same intensity of light, the photon number of yellow LED is more than green LED, which probably leads to the good disinfection efficiency of yellow LED. In addition, there is no adsorption of the NS in the red range of VL, which suggests that the wavelength of 610–650 nm has no effect on the photocatalytic disinfection. This observation is well matched with the result of Fig. 1b that the red wavelength range is not contributed to the disinfection efficiency. At equivalent intensity, the disinfection efficiency under white LED was lower than that under blue and yellow LED ( Fig. 1b), which is probably because of the invalid red wavelength in white LED occupying parts of the intensity of lamp. Thus, selecting an appropriate wavelength range of VL can improve the disinfection efficiency and save energy.

The spectra of white LED and FT both cover with a wavelength of 400–620 nm, which belongs to the absorption region of the NS. And the “continuous spectrum” of white LED occupies almost all blue and yellow range. But the “discrete peak spectrum” of FT occupies a part of green range (550 nm). Combing the results of wavelength effect and the absorption spectra of the NS, the disinfection efficiency under white LED irradiation shall be higher than that under FT irradiation. But the results of spectra effect are opposite, which further confirms the “discrete peak spectrum” supplies power energy to excite the photocatalyst to inactivate bacterial cells.