MXene-based hybrid composites as photocatalyst for the mitigation of pharmaceuticals

Highlights

• Sources of pharmaceuticals and their toxicological consequences are discussed.

• Photocatalytic removal of pharmaceuticals by MXene-based composites is given.

• Ongoing limitations and future research directions are recommended.

• Structural attributes, fabrication, and catalytic properties of MXenes-based composites.

Abstract

Environmental contamination is a burning issue and has gained global attention in the present era. Pharmaceuticals are emerging contaminants affecting the natural environment worldwide owing to their extensive consumption particularly in developing countries where self-medication is a common practice. These pharmaceuticals or their degraded active metabolites enter water bodies via different channels and are continuous threat to the whole ecological system. There is a dire need to find efficient approaches for their removal from all environmental matrices. Photocatalysis is one of the most effective and simple approach, however, finding a suitable photocatalyst is a challenging task. Recently, MXenes (two-dimensional transition metal carbides/nitrides), a relatively new material has attracted increasing interest as photocatalysts due to their exceptional properties, such as large surface area, appreciable safety, huge interlayer spacing, thermal conductivity, and environmental flexibility. This review describes the recent advancements of MXene-based composites and their photocatalytic potential for the elimination of pharmaceuticals. Furthermore, present limitations and future research requirements are recommended to attain more benefits of MXene-based composites for the purification of wastewater.

Introduction

Pharmaceuticals are biologically active compounds consisting of several thousands of synthetic chemical substances utilized as therapeutic agents for human and veterinary (Arnold et al., 2014; Fonseca et al., 2021). Although these modern pharmaceuticals have revolutionized the health sector around the globe and are still being explored to treat various diseases, but they have emerged as one of the hazardous class of pollutants. According to the regulatory and legislative initiatives, pharmaceuticals are one of the emerging pollutant of serious concern that cause serious threats to living organisms and their ecosystem (European Commission, 2019/840). The most commonly identified pharmaceutical pollutants include anti-inflammatories, antibiotics, antidepressants, anticonvulsants, steroids, beta-blockers and lipid regulators (Aguilar-Pérez et al., 2020).

The pharmaceuticals as well as their degraded products become part of water bodies after usage via different channels and are the cause of serious threat to clean environment for living organisms (Kümmerer, 2008; aus der Beek et al., 2016). The whole ecosystem is disturbed upon consumption of such water by the human and other animals (Monteiro and Boxall, 2010). Recently, it has been reported that low concentrations of pharmaceutical drugs are detectable in surface water, municipal wastewater, groundwater and sometimes in drinking water. However, long term exposure even to low concentrations of pharmaceuticals is a serious threat (Sayadi et al., 2010; Rasheed et al., 2020). The presence of pharmaceuticals in waste water can cause chronic diseases, tumors, allergic reactions, endocrine disorders and other health problems. For the prevention of deteriorating effects of such pharmaceuticals, it is necessary to remove these pharmaceuticals by environmentally friendly approach and that is why it is a top focus of the present era (Yaqoob et al., 2020).

There are different water treatment technologies, which have been reported and are of great practical significance to remove diverse pollutants from wastewater (Ahmaruzzaman and Gupta, 2011; Zeng et al., 2018; Wu et al., 2020e). Some notable among these are adsorption (Ahmaruzzaman and Gupta, 2011), advanced oxidation (Wu et al., 2020e), biological methods (Kim et al., 2011), membrane separation (Zeng et al., 2018), and photocatalytic degradation (Saravanan et al., 2016) etc. It has been established in the literature that using conventional wastewater treatments, pollutants are only partially eliminated (Renew and Huang, 2004; Kim et al., 2007). Therefore, it is the ultimate need of the day to focus on the development of water treatment methods and materials and both should supplement each other (Dong et al., 2021). Two-dimensional materials have found much attraction after the success of graphene (Singh et al., 2011; Kuila et al., 2012; Tang and Zhou, 2013). The two-dimensional (2D) nanosize materials have thin structures with lateral sizes ranging from tens nanometers to a few micrometres (Li and Wu, 2019; Schultz et al., 2019). The interesting properties and importance of 2D materials in environmental remediation applications are by their reduced size and large surface area (Dávila et al., 2014; Li et al., 2014; Liu et al., 2014). Several composites of graphene have been reported till now for such purposes. The pool of 2D nanomaterials was quite expanded since 2011 by the discovery of two-dimensional transition metal carbides/nitrides referred to as MXenes (Naguib et al., 2012). MXene and its composites have gained an increasing interest towards water treatment due to their remarkable properties. Several characteristics of MXenes, such as high catalytic ability, chemical stability, sorption capacities, adjustable chemical properties, hydrophilicity and large conductivity have been known for their effective use in water treatment and environmental remediation (Naguib et al., 2014; Feng et al., 2020). In the field of photocatalysis, MXenes have become popular as they can act as a host material to accommodate one or more catalytic substance on their surface to provide enhanced photocatalytic efficiency. Such linkage between MXene and catalytic substance reduces the recombination of photogenerated electrons and holes. The large surface area of MXene layers is also advantageous in photocatalysis as it contributes to provide more photocatalytic sites and hence improve the photodegradation of pollutants (Ihsanullah, 2020a; Ihsanullah and Ali, 2020; Hong et al., 2020).

Some reviews on MXene and MXene based materials have been reported that elaborate their structure, properties, synthetic pathways, and role in environmental remediation (Lei et al., 2015; Jun et al., 2019; Ihsanullah, 2020b; Im et al., 2020; Zhan et al., 2020; Khatami and Iravani, 2021; Lu et al., 2021; Tunesi et al., 2021; Zhang et al., 2021). In spite of this, a compact data is still lacking regarding their role for the photodegradation of pharmaceuticals. In the present review, we discuss the latest progress of MXenes and their photocatalytic behavior towards pharmaceuticals exclusively. This will aid the scientific community in developing MXenes to target pharmaceutical's removal by photocatalysis, an efficient and most focused method for water treatment. An outlook of future research directions to explore this new and exciting material field is also given at the end.