*3.6. Photocatalytic Disinfection Efficiency in Real Water Systems*

As the results show (Figure 9), Ag@ZnO exhibits a better disinfection profile as compared to pure semiconductors in case of all the real water samples. The results correspond well with our previous results [11]. The superiority of the Ag@ZnO as compared to the traditional photocatalysts can be attributed to many causes. It is a well-known and established fact that the photocatalyst that is being used here has a core shell nanocomposite structure. The structure itself has many advantages over its traditional counterparts. It is a matter of general observation that the metal ions in the composite structure are protected by the shell in the composite structure. This has many advantages: firstly it solves the problem of leaching out of the silver metal ion. Silver is itself a very poisonous metal ion and detrimental and harmful to various organisms [4,5,11]. At the same time, the target pathogens *E. coli* and *S. aureus* are unable to survive and escape its effects. The core shell morphology also increases the surface area of the photocatalyst. As the surface area increases, so does the effectivity of the photocatalyst. Both the traditional photocatalysts used here, namely TiO2 and ZnO lack in this property. The lack of a proper nanocomposite structure in the cases of TiO2 and ZnO can also explain the lesser efficiency that these photocatalysts show in the photocatalytic degradation of real water samples.

**Figure 9.** Effect of different photocatalysts on the relative reduction in the (**a**) *E. coli* and (**b**) *S. aureus* cell count (*N/N0*) in real water samples after 120 min of solar irradiation at a catalyst loading of 2 mg/L and 3 mg/L catalyst concentration, respectively. In each case the initial bacteria concentration = <sup>5</sup> <sup>×</sup> <sup>10</sup><sup>6</sup> CFU/mL, Temperature = 35 <sup>±</sup> <sup>2</sup> ◦C.

Various studies have already shown that at various concentrations both zinc and silver are detrimental to the growth of microorganisms [35]. The photocatalyst that we have used contains both these elements, so as a result, a better result can always be expected than that from the traditional ones, namely the likes of ZnO and TiO2. The combined effect of toxicity of these two potent antimicrobial agents, combined with the lesser amount of leaching due to the unique structure is indeed a deciding factor in increasing the efficiency of the photocatalyst against the traditional players [11].

However, the issue of safety can be raised, regarding the compatibility of silver and zinc in various water streams and water bodies, as both of these metals are known to be toxic to organisms [36,37]. To attend these sensitive issues, we did an MP-AES assay, and it was observed that the concentration of zinc and silver was below the detectable levels. Thus it addresses most of the toxicity-related issues.
