## 4 ZnO-Based Nanomaterials for Antimicrobial Application in Water Treatment
### Research Methods for Antimicrobial Activities of ZnO Nanostructures
#### 4.1.1 Disk-Diffusion Method  
The disk-diffusion method is a simple and efficient antimicrobial test, also known as the Kirby-Bauer antibiotic test (KB test) [133]. Mueller-Hinton agar and Brucella blood agar are commonly used as media for this method [133]. During the disk-diffusion method, the  
\begin{table}
\begin{tabular}{c c c c} \hline
**Water Type** & **Sample Type** & **Microphial Pollution Target** & **Types of Adsorbents** & **References** \\ \hline \multirow{4}{*}{sDrinking water} & \multirow{4}{*}{Domestic drinking water} & \multirow{4}{*}{Bacteria; virus; protozoan;} & Activated carbon; & [110] \\  & & & Carbon-graphene; & [111] \\  & & & Nano adsorbent; & [112] \\  & & Microbial contaminants. & Carbon-graphene. & [113] \\ \cline{2-4}  & \multirow{4}{*}{Drinking water source} & \multirow{4}{*}{Drinking water source} & Biological contaminants; & Nano adsorbent; & [114] \\  & & Microbial contaminants; & Iron Oxide Nanoparticles; & [115] \\  & & _E. coli._ & & Mixed matrix. & [116] \\ \hline \multirow{4}{*}{Wastewater} & \multirow{4}{*}{Lake wastewater} & \multirow{4}{*}{Pathogenic microorganisms.} & Zinc oxide; Iron oxide; Silver oxide nanoparticles. & [117, 118] \\  & & & Pathogenic microorganisms; & Acidated carbon; & [119] \\  & & & V. fischeri, B. subtilis, _E. coli._ & Carbon nanotube; & [121] \\  & & & Copper oxide; Zinc oxide; Silver oxide; & [122, 123] \\  & & & Platinum oxide. & [122, 123] \\ \cline{2-4}  & \multirow{4}{*}{Industrial wastewater} & \multirow{4}{*}{Pathogenic microorganisms;} & Graphene oxide nanosheets; & [124] \\  & & Virus. & Silver nanoparticles; & [125] \\ \hline \multirow{4}{*}{Laboratory water} & \multirow{4}{*}{Laboratory simulated water} & \multirow{4}{*}{Laboratory water} & \multirow{4}{*}{_Enterbakter, Cinewkter, Hights_} & Biochar stabilized; Iron oxide; Copper oxide nanoparticles; & [126] \\  & & & Fe\({}_{0}\)Co\({}_{2}\)Co\({}_{2}\)SiO\({}_{3}\); NiO; & \\ \cline{1-1}  & & & PAN/boehmite nanofibers. & [127] \\ \cline{1-1}  & & & Electro spurn; Nanofibers; & [128] \\ \cline{1-1}  & & & Silver @ Eggshell; & [83, 129] \\ \cline{1-1}  & & & Nanocomposte. & \\ \cline{1-1} \cline{2-4}  & \multirow{4}{*}{Laboratory pure water} & \multirow{4}{*}{_S. aureus, E. coli, C. albicans_; & Ag nano-embedded pebbles; & [85] \\ \cline{1-1}  & & & Nano cellulose; & [130, 131] \\ \cline{1-1}  & & _E. coli._ & Nanofibers; Granular activated carbon; & [130, 131] \\ \cline{1-1} \hline \end{tabular}
\end{table}
Table 2: Various kinds of adsorbents for microbial removal in different water resources.  
pH of the medium is usually controlled at around 7.2. The bacterial suspension containing a specific concentration was spread on the above agar medium and the experimental environment was required to be absolutely dry [22; 24; 93; 94; 97; 134; 135; 136]. Under sterile conditions, a certain number of ZnO-based nanomaterials were soaked on the filter paper disk using the selected solvent according to the specific requirements of the experiment. Let the disk dry and carefully mount it on the medium in a Petri dish. Solvent-soaked disks were used as controls to ensure the accuracy of the test. Next, the dishes were incubated at 37 \({}^{\circ}\)C for 24 h, providing the right conditions for bacterial growth. Subsequently, due to the bactericidal activity of the ZnO-based nanomaterials, no bacterial growth was observed around the disks at specific distances. The minimum concentration at which ZnO-based exhibited antibacterial activity was called the minimum inhibitory concentration (MIC) and the area around the disk with no bacterial growth observed was called the zone of inhibition. The lower the MIC value, the larger diameter of the inhibition zone and the higher antibacterial activity. Based on this, the antibacterial properties of ZnO-based nanomaterials were judged [137; 138].