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Bioremediation for Sustainable Environmental Cleanup

16.4.1 Nano-photocatalyst

Photocatalysis is a photochemical reaction based on electron/hole pair redox interactions when

exposed to light (Danish et al. 2021). Photocatalysis has the potential to degrade contaminants

that are difficult to decompose. The photocatalytic process is regarded as an effective remediation

method for converting harmful chemicals into environmentally friendly products. In the presence

of abundant solar radiation, photocatalysts accelerate chemical reactions. In general, photocatalysis

is a redox process in which holes are created in the Valence Band (VB) of a photocatalyst and

electrons are generated in the Conduction Band (CB), resulting in the emergence of highly

energetic and reactive species such as hydroxyl (OH^–) and superoxide radicals (O^2–). Due to the

production of extremely energetic radicals that function as potent oxidizing agents, photocatalysis

serves as a fruitful technique for the removal of hazardous chemical compounds (Ajmal et al.

2014). Nanomaterial photocatalysis for environmental cleanup has been the subject of significant

research for the past decade. To cleanse soil, purify the air and detoxify wastewater, a wide

range of nanostructured photocatalysts have been synthesized (viz., oxides and sulfide of metals,

composite oxides, carbon derivatives, graphene-based photocatalysts, dendrimers and polymeric

nanocomposites (Danish et al. 2021). In comparison to conventional photocatalysts, graphene-based

photocatalysts have been found to enhance the activity due to their large surface area, nanosize

and greater electronic motions. Oxide-based nanomaterials, such as Fe3O4, TiO2, ZnO and their

composites, are also excellent catalysts for the removal of heavy metals. Several researches

have reported the use of these oxides in environmental remediation, notably in photocatalytic

degradation of organic contaminants (Czech et al. 2020, Sabzehei et al. 2020, Masudy-Panah

et al. 2019). By reducing the distance between photon absorption sites and limiting electron-hole

(e– -h+) recombination, these nanoparticles with a large surface area and porosity have greater

photocatalytic activity. Transition metal oxides and their composites have strong photocatalytic

activity for organic pollutant photodegradation. TiO2 has demonstrated outstanding photocatalytic

destruction of contaminants due to its high resistance against photochemical, exceptional surface

qualities, microstructural features and large surface area; nanosized metal oxides are employed

in adsorption processes (Hitam et al. 2018, Rahman et al. 2011). The surface energy of metal

absorbents is improved by decreasing their size and generating more active sites on their surface for

the adsorption of pollutant molecules (Gusain et al. 2019). Immobilizing ZnO NPs on polymer

substrates is another agreeable technique in the production of modified-ZnO photocatalysts.

According to research, ZnO/polymer nanocomposite achieves the requisite photocatalytic activity

(Shirdel and Behnajady 2020). Hydrothermal synthesis and homogeneous precipitation are the

most appropriate procedures to create ZnO photocatalytic materials. A large number of studies have

demonstrated CuO’s remarkable performance in the photocatalytic breakdown of contaminants.

Chen et al. (2020) reported a 97% photodegradation of crystal violet dye utilizing monoclinic

crystalline CuO nanoparticles when exposed to visible light. Carbon nanotubes (CNTs) are also

used as innovative materials for photocatalyst due to their higher quantum efficiency and excellent

chemical stability. Gupta (2017) successfully developed ultrathin photocatalyst (SWCNTs-TiO2) for

wastewater treatment.

16.4.2 Nanoadsorbents

Heavy metal discharge from industrial, municipal, agricultural and household wastewater has

become a significant ecological concern. Over the last few decades, a new class of nano-adsorbents

has been developed to combat this rising menace. Adsorption is commonly used for heavy metal

removal due to its low cost, efficiency and simplicity. They have acquired recognition as a result

of their particular characteristics, and have demonstrated outstanding promise in the treatment

of wastewater and industrial effluents for reuse in a wide range of applications for the long-term

sustainability of the environment. Adsorption has long been known as a phenomenon in water

treatment. It is a common experience in the gaseous phase, but is used effectively in the treatment