Green-synthesized nanocatalysts and nanomaterials for water treatment: Current challenges and future perspectives

Abstract

Numerous hazardous environmental pollutants in water bodies, both organic and inorganic, have become a critical global issue. As greener and bio-synthesized versions of nanoparticles exhibit significant promise for wastewater treatment, this review discusses trends and future prospects exploiting the sustainable applications of green-synthesized nanocatalysts and nanomaterials for the removal of contaminants and metal ions from aqueous solutions. Recent trends and challenges about these nanocatalysts and nanomaterials and their potential applications in wastewater treatment and water purification are highlighted including toxicity and biosafety issues. This review delineates the pros and cons and critical issues pertaining to the deployment of these nanomaterials endowed with their superior surface area, mechanical properties, significant chemical reactivity, and cost-effectiveness with low energy consumption, for removal of hazardous materials and contaminants from water; comprehensive coverage of these materials for industrial wastewater remediation, and their recovery is underscored by recent advancements in nanofabrication, encompassing intelligent and smart nanomaterials.

Keywords: Biogenic nanomaterials; Green synthesis; Nanocatalysts; Sustainable methods; Water treatment.

Fig. 1.

Biogenic nanomaterials for wastewater treatment: notable advantages and challenges.

Fig. 2.

Categories of biogenic nanomaterials for appliances in wastewater treatment.

Fig. 3.

Parameters for the fabrication of monodispersed and stable NPs. Reproduced with permission from Ref (Singh et al., 2016).

Fig. 4.

(a) Mechanism of extracellular/intracellular synthesis of NPs by microbial enzymes and/or metabolites. (b) Lactobacillus bacterial cell can serve as support and reducing agent for the formation of NPs. Reprinted with permission from Refs (Sengani et al., 2017; Parandhaman et al., 2019; Nair and Pradeep, 2002).

Fig. 5.

Proposed reaction mechanism for the green-synthesized Pd nanomaterials. Redrawn from Ref (Nasrollahzadeh et al., 2020a).

Fig. 6.

Probable components of various plant extracts towards the reduction of metal ions to metal NPs. Redrawn from Ref (Mittal et al., 2013).

Fig. 7.

Gallic acid-assisted (a) Pd(II) reduction and (b) Pd NPs stabilization. Redrawn from Ref (Dauthal and Mukhopadhyay, 2013).

Fig. 8.

Initial step proposed for the reaction of NR dye with hydroxyl radical. Redrawn from Ref (Jaafar et al., 2019).

Fig. 9.

Removal of pollutants by applying nanomaterials. Redrawn from Ref (Eskandarloo et al., 2017).

Fig. 10.

Schematic representation and general mechanism for photocatalytic degradation of dye using green-synthesized NPs. Reproduced with permission from Ref (Shivaji et al., 2020).

Fig. 11.

(a) Calculated values of the band gaps and position of the conduction and valence bands (CB and VB) for MOF-5 in comparison with those of commercial TiO₂. (b) A time conversion plot of the phenol disappearance (y axis represents “mol of phenol decomposed per g per mol”). (c) A possible mechanistic proposal towards the photodegradation of phenol utilizing MOF-5 photocatalyst. Reproduced with permission from Ref (Alvaro et al., 2007).

Fig. 12.

Main pathways proposed for the photodegradation of MO by UTSA-38 under visible or UV/vis light irradiation. Reproduced with permission from Ref (Das et al., 2011).

Fig. 13.

Possible mechanistic pathway for the NaBH₄-assisted reduction of 4-NP by Pd NPs@Zeo. Reproduced with permission from Ref (Nasrollahzadeh et al., 2020b).

Fig. 14.

Biomolecule directed synthesis of a magnetite@chitosan-Au or Pd NPs and (HR)SEM images of well-dispersed spherically-shaped nanocomposite. Reproduced with permission from Ref (Parandhaman et al., 2016).

Fig. 15.

Proposed mechanism for the H₂O₂-assisted H-Bi₂WO₆ photocatalytic degradation of organic pollutants under solar irradiation. Reproduced with permission from Ref (Yi et al., 2018).

Fig. 16.

(a) Schematic diagram illustrating the biomolecule directed synthesis of Al₂O₃/BT/Au NPs, (b,c) (HR)TEM images of the well-dispersed spherically-shaped Au NPs on Al₂O₃ support. Reproduced with permission from Ref (Huang et al., 2011b).

Fig. 17.

SEM images of the produced magnetic iron oxide NPs-Tea. Reproduced with permission from Ref (Lunge et al., 2014).

Fig. 18.

Green-produced iron oxide NPs dispersed onto zeolite by eucalyptus leaf extracts (EL-MNP@zeolite). Reproduced with permission from Ref (Xu et al., 2020).

Fig. 19.

Bimetallic nZVI-Cu and bentonite supported green nZVI-Cu nanocomposite for removing tetracycline (TC). Reproduced with permission from Ref (Gopal et al., 2020).

Fig. 20.

(A) SEM image of GS-NiFe NPs (B) SEM image of GS-NiFe beads. Reproduced with permission from Ref (Ravikumar et al., 2020).

Fig. 21.

(A) PMMs fabrication strategy. (B) PMMs were fabricated by applying MOF as green template for treatment of water; the reported strategy was compared with traditional mixed matrix membranes. Reproduced (Adapted) with permission from Ref (Lee et al., 2014).