*4.6.1 Zerovalent metal nanomaterials*

Zerovalent metal is a significant wastewater treatment nanomaterial which is highly reactive because of small size and high surface area. Recently, several zerovalent metal nanoparticles for example silver, zinc, iron, aluminum and nickel received attention of researchers for contaminant removal. Silver nanoparticles have potential antimicrobial properties and are generally used as disinfectant to eliminate a large amount of microorganisms, like viruses and bacteria, as well as fungi [39]. It is extremely reactive, cost effective, environment friendly and has multiple pathways for wastewater treatment. Iron nanomaterial can proficiently remove contaminants such as cadmium nitrates, colorant and antibiotics from wastewater by adsorption, redox reaction, and co-precipitation technique. Li et al. [40] reported two-step technique to form zero-valent metal nanomaterials covered with silica and polydopamine (nZVI/SiO2/PDA) for use as sorbent which shows high capacity, selectivity and reusability up to 10 cycles.

## *4.6.2 Metal-oxide nanomaterials*

Metal-oxide nanomaterials like ferric oxides, manganese oxides, aluminum oxides and titanium oxides have been effectively utilized in removing noxious waste such as arsenic, uranium, phosphate, and organics. Titanium oxide nanomaterial is a capable photocatalyst having band gap of 3.2 eV with high photostability, low price and outstanding photocatalytic behavior. TiO2 nanomaterials are suitable for degradation of pollutants like organic chlorine, polycyclic aromatic compounds, pigments, phenols, pesticides, and heavy metals [41].

**Figure 5.** *Various groups of nanomaterials.*

Zinc oxide (ZnO) nanomaterial is competent material for purifying wastewater having a strong oxidizing capacity, wide wavelength and admirable photocatalytic properties. ZnO nanomaterial is environment friendly and captures more light as compared to other metal oxides possessing semiconducting properties. Iron oxide nanoparticles have versatility and are available as potent sorbent material-removing heavy metals from wastewater [42].

### *4.6.3 Carbon-based nanomaterials*

Carbon nanomaterials comprise distinctive structural and electronic properties duet to which they perform complex applications particularly in adsorption [43]. They have high adsorption capacity for removal of various pollutants, high surface area and aromatic selectivity. These nanomaterials are categorized as carbon beads, nonporous carbon, carbon nanotubes (CNTs) and carbon fibers. CNTs have welldefined cylindrical structures, stronger physicochemical interactions, porosity, large surface area, adaptable hydrophobic side and high adsorption capacity for dichlorobenzene, ethylbenzene, dyes, Pb2+, Zn2+, Cd2+ and Cu2+ [44].

Another class is graphene-based nanomaterial which is a single carbon atom layer having honeycomb like structure [45]. Graphene oxide is a graphene layer consisting of hydroxyl, epoxy, carboxyl, and carbonyl groups and is identified for eradicating heavy metals such as lead, zinc, copper, cadmium, mercury and arsenic. Graphene hybrid with nanoparticles of manganese ferrite can be exploited to proficiently remove Pb(II), As(III), and As(V) from contaminated water. Rajabi et al. [46] compared the adsorption efficiency of MWCNTs and functionalized CNTs by varying experimental conditions including pH, times, and temperatures. From results it was clear that f-CNTs possess a higher removal capacity than pristine CNTs. The maximum removal capacity (166.7mg g−1) of methylene blue (MB) with functionalized multi-walled carbon nanotubes (f-MWCNTs) was higher as compared to MWCNTs, which was 100mg g−1.
