# Photocatalytic Inactivation as a Method of Elimination of _E. coli_ from Drinking Water
## 3 Inactivation Process by Photocatalysts
### Metal Free Photocatalysts for the Inactivation of _E. coli_  
One of the metal-free materials that has been used for inactivating _E. coli_ is three-dimensional reduced graphene oxide and graphene oxide under visible light irradiation. The inactivation rate was boosted by five times when the peroxisulphate was introduced as an activator [78]. The inactivation was carried out through non-radical and radical reaction pathways. Holes, hydroxyl radicals, hydroperoxyl radicals and peroxyl radicals were found to be the active radicals against the bacteria as revealed by the scavenger investigations carried out by the non-toxic scavengers. These non-toxic scavengers were selected to prevent damages to bacteria cells so as to obtain accurate quenching effects [78]. Graphene oxide quantum dots were also effective against _E. coli_, but when it was functionalized with nanorods and nanoflakes of zinc oxide, it was observed that these functionalized photocatalysts displayed better antibacterial efficiency than when ordinary graphene oxide quantum dots was utilized as photocatalysts [79]. Another metal-free photocatalyst that was used to inactivate _E. coli_ is graphitic carbon nitride, and it was found to be effective against _E. coli_. However, the rate of inactivation was found to be better when graphitic carbon nitride was composited with molybdenum sulphide and bismuth oxide [80]. The graphitic carbon nitride was coupled with copper (I) oxide and used as photocatalyst under visible light to inactivate _E. coli_. Investigations by electron spin resonance and reactive species trapping revealed that the inactivation was effected by holes, hydroxyl radicals and peroxyl radicals [81]. [PERSON] et al. [82] used graphitic carbon nitride to inactivate _E. coli_ k-12, and this was carried out in the presence of visible light irradiation. When the photocatalytic inactivation was carried out for 4 h, there was complete inactivation of the bacteria as confirmed using a bacteria regrowth test, which showed that there was no bacteria count after four days of incubation. It was also observed that, when graphitic carbon nitride was used in the dark, there was no effect on the bacteria. This showed that the inactivation was affected due to the presence of light. Graphitic carbon nitride nanosheet has also been used as a photocatalyst to eliminate 100% of _E. coli_ within 240 min in the presence of visible light. The performance of this graphitic carbon nitride nanosheet was better than that of bulk graphitic carbon nitride because the bulk graphitic carbon nitride could only eliminate 77.1% of the bacteria under the same conditions [23]. In a similar research, 5 log and 3 log of the _E. coli_ were killed when nanosheet and bulk graphitic carbon nitride were respectively used as a photocatalyst in the presence of visible light radiation. When a single layer of the graphitic carbon nitride was used, 2 \(\times\) 107 cfu mL\({}^{-1}\) was inactivated under the same conditions [19]. Another metal-free photocatalyst that has been used as an effective photocatalyst against _E. coli_ is graphene quantum dot. Electrochemical techniques have been used to produce graphene quantum dots which killed _E. coli_ by damaging the cell membrane in the presence of light. The killing of the bacteria was confirmed by the use of the plate count method, where the amount of propidium iodide that was consumed was used for the estimation. When the experiment was repeated under the same conditions in the absence of light, no inactivation took place. In addition, the use of ordinary light for the inactivation was not effective against the bacteria strains [83].