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

Highlights

• Advances in water treatment by green-synthesized nanocatalysts and nanomaterials, are described.

• Summary of synthetic methods for biogenic nanomaterials.

• Catalytic degradation of various pollutants in water, are presented.

• Mechanistic details are provided for water treatment with green-synthesized nanomaterials.

• Future challenges on water treatment using green-synthesized nanocatalysts and nanomaterials are discussed.

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.

Introduction

Nano-engineered materials, such as nanoadsorbents, nanometals, nanomembranes, and photocatalysts offer promising options for novel water technologies which can be adapted to customer-specific needs. A large majority of them are compatible with existing treatment technologies and can be integrated simply in the existing set-up. There are numerous contaminants in wastewater discharge which have adverse health effects namely pesticides, textile dyes, plasticizers, disinfection by-products, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and emerging pollutants such as perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), endocrine disrupting materials, pharmaceutical and personal care products (Bousselmi et al., 2004; Mozia et al., 2007; Rizzo et al., 2009). Innovative engineered nanomaterials are very encouraging for removal of these hazardous contaminants, as they have high surface areas and remarkable reactivity (Zhang et al., 2019). In this context, the development of greener protocols for the elimination of ionic metal species from water has witnessed profound interest (Iravani, 2011; Shukla and Iravani, 2017; Nadagouda and Varma, 2008; Moulton et al., 2010).

Nanotechnology and nanoscience, an area of research that has a progressed at a very fast pace, present numerous attractive options for water/wastewater treatment. Nowadays, nanostructured materials have garnered attention in the degradation as well as remediation of toxic organic/inorganic pollutants owing to unique physicochemical properties such as their high catalytic activity, high physical/chemical and thermal stability, large specific surface area, significant chemical reactivity, and strong electron transfer ability, among others (Pradhan et al., 2001; Sinha et al., 2013; Xu et al., 2019; Zhang et al., 2014). Indeed, nanomaterials and nanoparticles (NPs) are recently applied to address the environmental issues e.g. water contaminant treatment and/or environmental monitoring/sensing; they are considered as an excellent option, since the reactive nanostructures have potential features that render them more efficient to convert and/or remove hazardous/toxic pollutants into toxic-free substances. In general, nanostructured materials e.g. nanosorbents, nanoparticles (Pd, Au, Ag, Cu, Fe₃O₄, TiO₂, etc.), nanocatalytic membrane systems, are more efficient, require lesser time, environmentally-friendly and constitute low energy approaches but not all these systems are inexpensive or green, and hence are not applied yet to treat the wastewater on large scales. Consequently, there is an essential need to fabricate some green nanomaterials, which must be very effective, having high activity/efficiency, eco-friendly, green and easy to handle. In this respect, green-fabricated nanomaterials can be considered as good candidates for the photocatalysis application in practical water treatment systems, although still more elaborative studies should be performed regarding the application of these nanomaterials.

Organisms specifically fungi and bacteria are capable of surviving and multiplying under stressful conditions due to the presence of higher concentrations of toxic metals (Beveridge et al., 1996; Rouch et al., 1995). It appears that numerous reducing agents in organisms and biochemical trajectories lead to bioreduction of metal ions. In view of the critical function of these agents, there have been more studies pertaining to the role and appliance of genetically engineered and natural organisms in bioreduction of metal ions (Stephen and Macnaughtont, 1999). It has been realized that many organisms reduce various metals, metalloids and radio nuclides such as uranium(VI) (Lovley et al., 1991; Kashefi and Lovley, 2000; Bansal et al., 2004; Mukherjee et al., 2001; Fredrickson et al., 2000; Lloyd and Macaskie, 2000; Lovley and Phillips, 1992; Lovley et al., 1993) and technetium(VII) (Kashefi and Lovley, 2000; Fredrickson et al., 2000; Lloyd and Macaskie, 2000; Philipse and Maas, 2002; Lloyd and Macaskie, 1996) and trace metals including arsenic(V) (Sweeney et al., 2004; Laverman et al., 1995), chromium(VI) (Kashefi and Lovley, 2000; Fredrickson et al., 2000; Zhang et al., 1998, 1996; Wang, 2000; Lovley, 1993), cobalt(III) (Kashefi and Lovley, 2000; Zhang et al., 1996; Sastry et al., 2003; Slawson et al., 1992; Gorby et al., 1998; Caccavo et al., 1994), manganese(IV) (Kashefi and Lovley, 2000; Lovley, 2000), and selenium(VI) (Konishi et al., 2007; Oremland, 1994); majority of them being hazardous environmental contaminants. Therefore, these organisms can be utilized for removing metal and metal oxides contaminants from water and wastewaters (Lee et al., 2004; Grünberg et al., 2001; Lovley, 1995; Lovley and Coates, 1997). As an example, aquatic macrophytes exhibited great potential for eliminating heavy metals (Sood et al., 2012; Gunawardaha et al., 2016; Sarkar and Jana, 1986) which can be harnessed for producing metallic NPs, as well (Gunawardaha et al., 2016; Korbekandi et al., 2014).

The conventional physicochemical strategies for the fabrication of nanomaterials entail the participation of hazardous and volatile materials. This has prompted the researchers to design suitable bioinspired biogenic and greener strategies which are eco-friendly, safer, and cost-effective for the development of novel and efficient nano-scale adsorbents and catalysts which can be harnessed for eliminating and degrading various contaminants in water (Fig. 1, Fig. 2). Indeed, the presence of various phenolic antioxidants in plants and other microorganisms serve as capping and reducing agents for the production of nanomaterials in varied shapes namely, flowers, wires, rods, and tubes.

In this critical review, current trends and future prospects exploiting the application of green-synthesized nanocatalysts and nanomaterials for water and wastewater treatments are discussed. This encompasses advanced nanomaterials and development of novel nanosorbents attained via greener and sustainable processes for removing the contaminants and metal ions from aqueous solutions, including groundwater, drinking water, and wastewater treatment. Recent trends and forthcoming challenges pertaining to green-synthesized nanocatalysts and nanomaterials and their potential applications for treating and purifying wastewater are highlighted. The development of new eco-friendly treatment methods should be perceived as a critical element for the industries producing hazardous, toxic, and chemically-laden wastewater.