Chalcogenides-Based Nanomaterials for Contaminant Removal in Wastewater Treatment

Abstract

The pollution has been increasing day by day which highly affects the environment. The longer we wait to take action to save the environment, the harder it will be. Increasing organic and inorganic waste production has made widespread pollution and water contamination due to rapid growth in population. It is believed that contaminated water poses a significant danger to water security. Precipitation, adsorption, electrochemical, photocatalysis, and membrane filtration are just some of the methods for purifying the water supply. One of the most efficient methods for eliminating dissolved metal ions from wastewater is photocatalysis. High efficiency, cost-effectiveness, avoiding residual pollutants, and direct application of solar energy are only a few of the benefits of the photocatalytic approach compared to other methods. Due to their lower band gaps, charge carrier mobility, and visible-light absorption, nanomaterials based on chalcogenides are widely employed as photocatalysts. A more significant number of active sites per unit surface area and a longer distance over which charge carriers could diffuse are two novel qualities that emerged due to the quantum size effect, caused by the reduction in the size of chalcogenides. In this chapter, we will dive deep into the novel application of nanomaterials based on chalcogenides for contaminant removal in wastewater treatment. Water contamination, its treatment, and the other environmental toxins are explored in detail. These chalcogenide-based compounds are used as catalysts to purify water from industrial wastes and remove environmental toxins.

1. Introduction

The current economic status depends on industrialization, making it one of the most sought-after requirements in all areas. The rate at which pollutants are discharged into the environment, and thus the severity of pollution from all sources, rises in tandem with the rate of industrialization [1]. Humans, animals, and other species dependent on water for survival are all negatively impacted by water pollution because of the widespread use of dangerous chemicals dumped into water sources. It is estimated that 2.2 million people worldwide die yearly from drinking contaminated water in the developed and developing countries. Organic and inorganic contaminants can be found in wastewater [2]. Emerging contaminants include pharmaceuticals, endocrine disruptors, personal care products, other refractory organic pollutants, and more common pollutants like dyes and heavy metals. As such, water pollution is a multifaceted environmental issue that calls for a comprehensive approach.

Chlorination is an inefficient water treatment process for removing inorganic contaminants, organic molecules, heavy metals, and more complex compounds. When substances are introduced into water and subsequently released into rivers, it poses a threat to the entire water purification system. Common issues are typically caused by an imbalance between the chemical oxygen demand and the biological oxygen demand of the aquatic organism [3]. Heavy metal ions (HMIs), azo dyes, and other organic contaminants are the most common types of potentially toxic elements (PTEs) in wastewater [4, 5, 6]. Waterborne disease epidemics affects the lives of millions of people every year, primarily in developing nations where clean water is scarce [7].

Humans are particularly susceptible to the health dangers caused by HMI, including Pb, Hg, and Cd, even at relatively modest exposure levels. Fatigue, thirst, depression, headaches, dementia, gene mutation, Wilson disease, kidney or liver damage, neurological dysfunctions, cancer, sleeplessness, diarrhea, and mortality are only some of the outcomes [8]. Human health and economic development depend on effective methods for cleaning polluted water supplies for consumption and other uses. Toxins in wastewater have been effectively eliminated using various methods including electrochemical processes, absorption, ultraviolet radiation, chemical oxidation, and different, complex filtration techniques [9]. Microorganisms’ contribution to wastewater treatment, alongside more conventional approaches, are significant. Various enzymatically driven organisms, such as metal oxidation/reduction and even precipitation via enzymatic ligand synthesis, are involved in treating wastewater by microbes [10]. It is generally accepted that nanomaterials aid in developing cutting-edge methods for treating wastewater. Nanotechnology research leads to the development of nanomaterials, which are the results of nanoscale engineering. The term “nanomaterial” refers to a broad category of substances. Material properties are process- and structure-dependent. Nanostructured materials are distinct from bulk materials in ways beyond size alone [11]. These chemical reactivity, energy absorption, and biological mobility differ from those of bulk materials. These materials have recently entered the realm of cutting-edge healthcare. From the agents to targeted gene transfer, they have found several uses in medical imaging [12]. These buildings, however, raise several ecological and societal concerns. Materials like sulfides, selenites, and tellurites are some of the examples of nanostructured metal chalcogenides (MCs) because they include at least one metal cation and a chalcogen.

Materials and nanoparticles based on metal chalcogenides are widely used in energy conversion and storage devices like fuel cells, solar cells, light-emitting diodes (LEDs), ion batteries, supercapacitors, thermoelectric devices, semiconductor diode lasers, photovoltaic cells, optoelectronics, sensors, and bioelectronic components due to the remarkable physical-chemical properties of the compounds formed during the combination [13]. Hydrogen release from water splitting, wastewater treatment, and selective oxidation are just a few applications of chalcogenides as photocatalysts [14]. Nanotechnology has resulted in a proliferation of approaches for cleaning up wastewater. Current technologies that make use of nanoscale chalcogenides are discussed in this chapter. Some examples include using chalcogenide nanomaterials as catalysts for the disinfection of wastewater bacterial communities and the breakdown of organic pollutants in polluted water treatment.

2. Basics of chalcogenides and properties

Chalcogenides are a class of compounds with a wide range of structural complexity and exciting chemical and physical properties. Chalcogenides has been ideal for visible-light harvesting and related applications because the band gaps between their energies are lower than those of most oxides. Among chalcogenides, metal sulfides have drawn the most significant number of studies because of their low band gap energy, better light harvesting, and diverse applications [15]. It is possible to classify chalcogenides into three distinct categories based on the presence of either alkali metals or alkaline-earth metals, transition metals, or chemicals from the primary group. Therefore, this chapter of the book explores the various chalcogenide classification systems. Photocatalytic activity of metal sulfides, selenides, and tellurides, as well as general chalcogenide properties, are also covered [16]. Some of the characteristics that set chalcogenides apart are listed below.

• All chalcogens react strongly with alkali earth metals.

• The ionic forms of all chalcogens can be found in metallic ores.

• The less dense chalcogens vital to all life forms are not harmful.

• Tellurium, selenium, and polonium are examples of heavier chalcogens that are poisonous and potentially dangerous.

• The atomic sizes of chalcogens exhibit significant variation. Nevertheless, these elements possess a common characteristic: they each possess six valence electrons.

• A higher atomic weight results in a greater density, higher melting and boiling temperatures, and a larger nuclear radius.

• Nonmetals including oxygen, sulfur, and selenium and semimetals like tellurium and polonium conduct electricity.

When a chalcogenide bonds to a transition metal, two different structures can result: dichalcogenides and tetrahedral structures. This system will form if chalcogen is bonded to specific transition metals. When chalcogenide elements connect with Mn, Co, Fe, Cu, Ni, and Zn, a tetrahedral metal-chalcogenide structure is created, but a dichalcogenide structure is generated when Ti, Cr, V, Zr, Mo, Nb, Tc, Ta, Hf, W, and Re are present [17]. Due to their unique properties, chalcogenides are an effective solid-state electrolyte for batteries. Because of the wide variety of metal halides and chalcogenides, researchers have split them into three distinct classes based on their physical and chemical characteristics: the primary category, the phase transition group, and the group of rare earth metal chalcogenide halides [18]. The properties of several chalcogenides are shown in Table 1. The classification of chalcogenides is shown in Figure 1.

Chalcogenide type	Chalcogenide property
Chalcogenides of alkali metals and alkaline-earth elements	Compounds that are salt-like and typically colorless and soluble in water.
Chalcogenides of transition metals	Displaying a covalent, nonionic nature, tetrahedral bonding, the crystal has a hexagonal symmetry.
Chalcogenides with a nanostructure	Facilitate the segregation of electrons and phonons with their exceptional qualities; materials have a cube-like (or rock crystal-like) form.
Glasses made from chalcogenide elements	Act like semiconductors and display the characteristics of amorphous semiconductors with band gap energies between 1 and 3 eV. So-called “lone pair” semiconductors, they have features in between those of organic polymers and oxide glasses. Unlike organic polymers, the structure of atomic bonds is relatively stable. They exhibit more pliability than oxide glasses.

Table 1.

Properties of chalcogenides [19].

Figure 1.

Classification of chalcogenides.

2.1 Chalcogenide classification based on the number of constituents

Binary, ternary, and quaternary systems, the presence or absence of metals, the presence or absence of chalcogen ions, and so on are all used to categorize chalcogenides. In contrast to quaternary chalcogenides, extensive research has been conducted on binary and ternary chalcogenide molecules. Chalcogen derivatives exist for every element in the periodic table, except the noble gases. Their stoichiometries, such as silicon disulfide (SiS₂), boron sulfide (B₂S₃), and antimony trisulfide (Sb₂S₃), typically adhere to the common valence trends [20]. Directional covalent bonding, rather than close packing, determines the architecture of many primary group materials.

2.1.1 Binary chalcogenides

Metal cations and a chalcogen anion make the ions in binary chalcogenides. One of the most researched chalcogenides is cadmium sulfide (CdS). It is relatively active when exposed to visible light and has a band gap energy of 2.3 eV. Its unusual optical and electrical properties, which vary in size and structure, make it useful in various applications. Due to their wide range of possible uses, cadmium chalcogenides are the most essential materials, particularly for photoluminescence devices and solar cells. It was determined by Bajpai et al. that cadmium thiolate powder, when heated, nucleates to CdS particles of the desired size [21].

2.1.2 Ternary chalcogenides

Adding a third group III, IV, or V element to a binary chalcogenide produces a ternary chalcogenide. Adding a group III or IV metal to a binary chalcogenide, such as copper sulfide (CuS), results in the formation of a ternary chalcogenide. Group VI of the periodic table contains the elements sulfur and selenium. They have many properties and can be found in hydrocarbon compounds in various mixtures [22]. In addition, sulfur and selenium are frequently employed together or separately in ternary and quaternary chalcogenide compounds. There has been three ions in ternary chalcogenides; the chalcogen anion is one of them. The ability to precisely control electronic characteristics by adjusting the Cd:Zn atomic ratio makes ternary chalcogenides like ZnCdX (where X = S, Se) particularly significant. The band gap energy of zinc cadmium sulfide (ZnCdS), for instance, can range from about 2.4–3.7 eV. ZnCdS has many applications, including solar cells with heterojunctions, cathodoluminescence at low voltages, UV diodes, etc. Liu et al. reported the thermochemical synthesis of one-dimensional (1D) nanostructures of ternary chalcogenide from ZnS and CdS precursors. As-prepared 1D nanoscale ZnCdS nanotubes have shown promise in emerging nanoelectronics and photonics applications [20].

2.1.3 Quaternary chalcogenides

In addition, the quaternary chalcogenide formed by substituting half the indium in copper indium sulfide (CuInS₂) with zinc and another half with tin is a derivative compound from chalcopyrite, or ternary chalcogenide, consisting of the I2-II-IV-VI4 quaternary semiconductor compound. The kesterite or stannite structure of CuZnSnS₄ (CZTS (copper, zinc, tin, and sulfide))/Se₄ results from this substitution. The only difference between the two structures is their arrangement Cu and Zn atoms. Because of its excellent thermodynamic stability, CZTS material typically occurs in the kesterite phase rather than the stannite form [23]. A chalcogen anion is one of the four ions present in quaternary chalcogenides. Quaternary chalcogenides are a large class of materials that have several potential applications. These include serving as absorbers in solar cells, photocatalysts for solar water splitting, etc. Unlike in binary or ternary chalcogenide, many elements can be accommodated in quaternary materials, allowing for relatively complex electrical and structural features. It has been reported that the visible-light absorption of a series of sulfides quaternary chalcogenides, including AgInZn₇S₉, is superior to that of their oxide counterparts. Therefore, when exposed to visible light, these sulfides exhibit high photocatalytic activity for the generation of H₂ [24].

2.2 Metal-based chalcogenide classification

Different varieties of chalcogenides can be distinguished by the amount and types of metals they contain.

2.2.1 Chalcogenides of alkali metals and alkaline-earth metals

Common examples of mono-chalcogenides include alkali metals and alkaline-earth metals. Hydrolysis of mono-chalcogenide sulfides of alkali and alkaline-earth metals results in the formation of derivatives containing disulfide anions. They are chemical substances that resemble salts but lack color and generally soluble in water. The antifluorite structure and the sodium chloride motif are frequently observed during the crystallization of alkali metal chalcogenides. KLiX (X = S, Se, Te) is an example of an alkali metal chalcogenide, while MX (M = Ca, Sr., Ba, and X = S, Se, Te) is an example of an alkaline-earth metal chalcogenide [24, 25].

2.2.2 Chalcogenides of transition metals

Many different stoichiometries and structural forms of transition metal chalcogenides (TMCs) have been discovered. Commonly studied transition metal chalcogenides include zinc (ZnS, ZnSe, and ZnTe), cadmium (CdS, CdSe, and CdTe), molybdenum (MoS2), and mercury (HgCdTe). TMCs, with their zinc-blend or wurtzite structure, exhibit strongly covalent, nonionic activity. TMCs can be broken down, even further by subcategories [21, 25].

2.2.3 Metal-containing chalcogenides

In metal-rich chalcogenides, transition metals exhibit II or higher oxidation states. These compounds usually include necessary metal-to-metal bonding and can be distinguished by their structural and physical characteristics. Electrocatalytic hydrogen generation is aided by metal-rich chalcogenide, which Siegmund et al. reported comprises a significant components. Nb₂₁S₈ is a sulfide containing many metals [26].

2.3 Classification of chalcogenides according to the number of chalcogen ions

This categorization is based on the relative abundance of chalcogen ions.

2.3.1 Mono-chalcogenides

In the MX structure of mono-chalcogenides, M represents a transition metal and X represents a chalcogenide anion like S_2-, Se_2-, or Te_2-. The surfaces of these compounds are highly durable and have a low coefficient of friction. This is due to their lamellar structure that consists of several thin, stiff sheets formed by strongly bound atoms. Gallium sulfide (GaS) or gallium selenium (GaSe) crystals have been solved, and their designs show two layers of Ga atoms sandwiched between S and Se ions [25].

2.3.2 Dichalcogenides

For instance, MoS₂, TiSe₂, MoS₂, and WSe₂ are all examples of metal dichalcogenides, which have the formula MX2, where M = transition metal and X = a chalcogen anion. They crystallize in two-dimensional (2D) structures with a stoichiometry of exactly 1:2. The 2D design consists of X, M, and X sheets held apart by van der Waals forces. The sheets have metal ions that are sixfold coordinated, making a pyramidal shape that is either octahedral or body-centered [16, 25].

2.3.3 Trichalcogenides

Ti, V, Cr, and Mn are only a few early metals found in trichalcogenide groups. M4+ (C2 2-) (C2 -) is a standard notation for these substances, where C = S or Te. One well-known example is niobium triselenide (NbSe₃). The acid treatment of tetra-thiomolybdate results in the formation of niobium triselenide [25].

3. Wastewater treatment techniques

Due to the high degree of population growth, the environment quickly becomes polluted and unusable, resulting in severe drought and limited drinking water availability. Municipal and industrial effluents and biological activities represent a severe threat to the quality of water supply, rendering it unfit for human use. For instance, phenols in wastewater are potentially dangerous as they can build up in food chains and become poisonous over time, wreaking havoc on people’s health. Heavy metals, suspended particles, pathogens, organic materials, and others are some of the other wastewater components. Conventional procedures, such as physical particle separation, chemical oxygen demand assessment, and biological oxygen demand calculation, which can be used to remove these contaminants and render the water fit for human consumption or other purposes. Large quantities of phosphorus, nitrogen, and other ionic compounds are discharged into the environment and freshwater sources, reducing dissolved oxygen levels in the treated water. Because of the heavy chemical load used to eradicate harmful species before dumping, the water will be carcinogenic, and the local flora and fauna will be decimated. Researchers are developing scalable approaches for fast wastewater treatment, considering current technologies in terms of expense and investment in capital. With its ability to synthesize materials quickly and efficiently and its high effectiveness in eliminating contaminants, nanotechnology is seen as a potential boon to the wastewater treatment and remediation process [27]. Nanoparticles with a diameter of less than 100 nm have a larger surface area than their volume, giving them many active sites and boosting their efficiency. Nanotubes, quantum dots, nanowires, nanocolloids, and nanofilms are only a few of the nanomaterials reported in research. These nanomaterials are crucial for developing nanoparticles with specific properties [28]. Other nanomaterials with physical, chemical, and electrical properties useful for water purification have also been developed. Another option is soft nanomaterials, such as the surfactants, typically employed to regulate coagulation and flocculation [29]. Nanotechnology has allowed us to see the several stages of treatment more clearly, including those involving catalytic oxidation, adsorption and separation, and disinfection [19]. Various treatment methods for wastewater are shown in Figure 2.

Figure 2.

Wastewater treatment methods.

3.1 Photocatalysis

Groundwater, municipal sewage treatment plant, and surface water have all been contaminated due to the widespread use of PTEs and the problems involved with their eradication from wastewater. PTEs with poor biodegradability are particularly common in wastewater from various industries, including the textile, agricultural, and pharmaceutical sectors [30, 31]. Conventional pollutant removal techniques have been hampered by the difficulty of decomposing and mineralizing these complexes. Solar energy to destroy organic pollutants is known as photocatalysis, an eco-friendly process. Photocatalysis is one of the advanced oxidation processes (AOPs) that have attracted much attention in recent years, especially the discovery of photoelectrochemical water-splitting reactions using semiconductors. Wastewater treatment plants have used this eco-friendly, energy-saving, sustainable technology to eliminate contaminants of high concentration, high complexity, and low biodegradability [32]. Photocatalysts have been demonstrated to be effective in degrading nonbiodegradable organic contaminants in water, according to findings from the literature [33]. Metal oxides like zinc oxide and titanium dioxide (ZnO and TiO₂) have found extensive use as semiconductors in photocatalysis. Reactive oxygen species (ROS), such as the highly oxidizing superoxide radical anion (O₂) and hydroxyl radicals (OH), are produced when the photo-induced electrons and holes react with water (H₂O), the hydroxyl group (OH), and oxygen (O₂). ROS is the driving force behind the deterioration of polar organic compounds (POCs) in wastewater [3].

3.2 Adsorption

As a surface phenomenon, adsorption involves the accumulation of the adsorbate on the absorbent’s surface. Adsorption occurs when a solution containing an adsorbable solute comes into contact with a highly porous surface structure, where part of the solute molecules are deposited due to liquid-solid intermolecular forces of attraction [34]. The atoms that make up a typical bulk material provide each other with the necessary bonding interactions, whether covalent, metallic, or ionic. Adsorbents tend to attract adsorbates as their surfaces are not covered by all their atoms [35, 36, 37]. Adsorption can be roughly classified as chemisorption (the bonding of adsorbate due to electrostatic attraction), with the precise bonding dynamics depending on the properties of the involved species [38]. Physisorption (attaching the adsorbates to the surface via weak van der Waals forces) and covalent bonding are other possibilities.

3.3 Antimicrobial

Treatment of medications, cosmetics, and colors that remain in the environment requires the employment of nanomaterials with potent antibacterial capabilities [39]. The method by which nanomaterials exert their antibacterial action is poorly understood. However, limited research suggests that this process typically begins with producing reactive oxygen species (ROS), which interact with proteins in the microbial cell wall, denaturing the wall. By breaching the cell wall, ROS can disrupt the microbe’s respiratory chain, killing the cell [3]. Several applications involving transition metal chalcogenide nanoparticles contains genetically modified microbes [19], as shown in Figure 3.

Figure 3.

Genetically engineered microbes in transition metal chalcogenide synthesis for various environmental applications.

3.4 Peroxydisulfate/peroxymonosulfate activation

Homogeneous catalysis has been widely used to degrade organic contaminants, but the difficulty of isolating the catalyst has hindered it. Because of their high reaction selectivity, extended lifespan, and broad pH operating range, sulfate radical-based procedures have received much attention [40]. Peroxydisulfate (PDS) and peroxymonosulfate (PMS) can be efficiently activated with transition metals like Cu+; Mn²⁺; Co²⁺ through electron transfer, light irradiation, and heat to create sulfate radical [41]. Sulfate radicals can be activated in a general way by the metal ions Fe⁰, Fe²⁺, and Fe³⁺ [42].

3.5 Fenton-like process

One method that shows promise and safe for the environment is the heterogeneous Fenton oxidation process used in the waste water treatment. It is a highly desirable technique because of its efficiency, low cost, and simple operating conditions [43, 44]. Heterogeneous Fenton/Fenton-like processes generate hydrogen peroxide (H₂O₂) locally by activating molecular oxygen, unlike the more cumbersome and potentially dangerous H₂O₂ storage required by the traditional Fenton process. Chemical, electrochemical, or photocatalytic processes could all be used to activate oxygen. Catalytic decomposition of locally produced H₂O₂ can result in the formation of •OH using a wide variety of iron-based and iron-free materials [45, 46, 47]. Wastewater treatment using the Fenton-like process has used several catalysts based on transition metal oxides (TMOs) and TMCs [46].

4. Chalcogenide nanomaterials

Chalcogen comes from the Greek and Latin words for copper (khalkos) and “born” (genes), respectively. The chalcogenide compounds comprise group VI elements like arsenic, and the elements play a vital role in biosynthetic processes. These chalcogenides are part of the oxygen family of metals, including sulfides, selenides, and tellurides. A chalcogenide chemical compound, amorphous or crystalline, that contains a minimum of one chalcogen anion, such as Se, S, or Te, and a minimum one other electropositive element linked by covalent bonds. Metal oxides, sulfides, selenides, and tellurides based on group VI elements (Zn, Cd, Pb, Cu, Ag, etc.) contain semiconducting chalcogenide binary compounds. However, metal chalcogenides would corrode when exposed to radiation without any electron donors. It employs appropriate band engineering to prevent the rapid recombination of h+/e couples and retrograde reactions. Particle surface modification is essential for enhancing photocatalytic activity by blocking electron-hole recombination. Metal chalcogenide materials can be made better in a few ways. Dopants might establish traps between the photocatalyst’s valence band (VB) and conduction band (CB), which could absorb light and prevent recombination by trapping electrons or protons. Dopants can potentially improve charge separation, which is crucial for the photocatalytic reaction [48].

(a) Metal ion: due to their excellent electrical and optical properties, these nanomaterials have many optoelectronic applications in photovoltaics, display devices, thermoelectric generators and refrigerators, lithium-ion batteries, fuel cells, and supercapacitors. (b) Ions of a nonmetal reduce the band gap energy because their impurity states are located near the VB and push the VB edge upward. They exhibit the ability to couple with a wide range of metals and chalcogenides [49]. The polymer sensitizes the surface. The recombination of h+/e− couples can be prevented by using surface imperfections as adsorption sites, where a charge transfer to the adsorbed species occurs. Dye sensitization broadens the photocatalyst’s range of excitation energies into the visible area, allowing it to harness solar radiation more effectively. Combining two semiconductors with the correct CB and VB of the photocatalyst can improve the collection of photogenerated electrons and holes on the various semiconductor surfaces and boost the redox reactions of the electrons and holes [50].

Metal chalcogenides are used in various applications, including solar cells, optoelectronic sensors, particle detection and tracking by fluorescent labeling, and even cancer diagnosis [51]. Superconductors, fuel cells, photovoltaics, photocatalysts, and energy storage are exciting uses for chalcogenide nanostructures [52]. Chalcogenide materials are amenable to a wide range of synthesis techniques, including hydrothermal/solvothermal, microwave-assisted, sonochemical, electrochemical, and vacuum synthesis. In addition, chalcogenide materials can be combined with graphene and other forms of advanced carbon [22]. NiS, a sulfide-based chalcogenide, has garnered much attention due to its potential application as a rechargeable lithium battery and its abundance of phases [53]. The utilization of chalcogenide-based materials and other nanomaterials are employed for the removal of various pollutants from the environment, including dyes, chlorinated organic compounds, organophosphorus chemicals, volatile organic compounds, and halogenated herbicides. Environmental pollution and energy constraint are two of the major environmental problems caused by the expansion of industrial and technical sectors. For this reason, it is critical to combat the negative effects on the environment by creating novel nanocomposites that can mediate such effects. Catalysts based on transition metal chalcogenides have being studied for potential uses in various fields because of their stability, optoelectronic behavior, and indirect band gaps, which allow them to absorb visible light, which is abundant in solar radiation [54].

Metal chalcogenide nanostructures are being considered for use in various sensors, light-emitting diodes, Li-ion batteries, solar cells, supercapacitors, thermoelectric devices, fuel cells, and storage devices. In recent years, metal chalcogenides have become popular as potential components in solar absorber devices. The well-known quantum size effect causes metal chalcogenides’ physical and chemical properties to emerge at the nanoscale, where they had previously been hidden from view. In addition, the specific surface area of nanostructured metal chalcogenides is much higher than that of their bulk counterparts. It is helpful for energy devices since it facilitates a more rapid reaction or interaction between them and the interacting medium. Nanostructured metal chalcogenides, in contrast to their bulk counterparts, can have a significantly higher specific surface area, which is beneficial for energy devices since it allows for a faster reaction/interaction among the components and their surrounding medium [50]. Developing new nanostructured materials of varying sizes and morphologies has benefited the many uses of metal chalcogenides. Nanostructured materials have several benefits over their bulk equivalents when used as a catalyst or electrode material in electrochemical storage systems, including (1) Higher specific surface areas and more active sites, (2) increased durability after repeated uses, (3) enhanced electrical conductivity, and (4) decreased electron transport route lengths.

4.1 Chalcogenides that are metal-based

Cadmium sulfide has gained much interest because of its remarkable chemical, physical, and optical features; nevertheless, it has a high recombination rate. As a result, Aboud et al.’s spray pyrolysis technique for synthesizing Cu-doped CdS to lower the recombination rate was successfully implemented [55].

4.2 Chalcogenides containing transition metals

Cocatalysts made of noble metals like Pt have been found to boost the photocatalytic efficiency of chalcogenide materials, particularly CdS [56]. Rapid one-pot solvothermal synthesis technique yields excellent Pt/CdS photocatalyst, as described by Fu et al. The photocatalytic activity was reported to increase when exposed to visible light significantly [57]. Solvothermal processing was used to make the materials [56, 57].

4.3 Chalcogenide composites

MoS₂/CdS is one of the many chalcogenide composites that have been produced and found to be effective as photocatalysts. A simple solvothermal approach was used in the MoS₂/CdS nanocomposite synthesis by He et al. It has been observed that the photocatalytic activity of the synthesized MoS₂/CdS is five times that of pure CdS. The nanocomposite of CdS and graphene is yet another instance. According to Khan et al., the substance was synthesized utilizing a simple and one-step precipitation technique. CdS-graphene nanocomposites increased photocatalytic activity under visible-light irradiation [58].

5. Synthesis of chalcogenide nanomaterials

Several techniques, such as bottom-up synthesis by vapor-phase deposition, solvothermal method, and top-down strategy via micromechanical cleavage, have been developed to manufacture 2D metal chalcogenides. The advent of nanotechnology opened up a wide variety of new doors for several fields, including public health, medicine, environmental protection, and early detection and diagnosis. The commercialization of chalcogenide nanostructures exposes society to a range of risks that are difficult to predict without extensive research. Using chalcogenide, nanomaterials have benefits and drawbacks, just like any other nanomaterial. Table 2 highlights the advantages and disadvantages of chalcogenide nanoparticles selectively. Therefore, it is essential to regularly choose suitable microorganisms and monitor and adjust variables, such as pH, temperature/incubation temperature, metal ion concentration, and biomass content. During the development phase, microbes oxidize organic compounds using chalcogen oxyanions, which are reduced to sulfur oxyanions, selenium oxyanions, and tellurium oxyanions, respectively [51].

Benefits	Drawbacks
Low-temperature, high-pressure synthesis	Inadequate dimension formation
Easy alteration	The shape complicates the process of achieving the intended shape
Reduced use of energy	Size and shape control is challenging and frequently unreliable
Uniformity	Microbe generation is slower than chemical synthesis; thus, it takes more time
Electron donors are made from sustainable resources.	Expensive
Highly transparent crystals and a high rate of production	A laborious process is being followed

Table 2.

Nanomaterials that have been synthesized: their benefits and drawbacks.

6. Application in wastewater treatment

Environmental, medicinal, materials science, and technological fields have all shown interest in chalcogenide nanoparticles over the past two decades. The chalcogenides are widely regarded as a great material class with numerous promising applications in science and technology. Chalcogenide particles exhibit tunable quantum efficiencies, long photostability, narrow emission spectra, and continuous absorption. Nanocomposites of metal chalcogenides and graphene, as well as core-shell materials composed of metal chalcogenides, have demonstrated a wide range of uses beyond composites. Hydrogen purification heterojunctions involving various chalcogenide semiconductor materials (e.g., CdS, CdSe, Bi2S3, Cu2S, Ag2S, and CuI) and noble metals (Au and Ag) have been the subject of the most research. For different purposes, scientists looked into chalcogenide-based nanoparticles made by reducing selenite using Veillonella atypica. Selenium nanospheres can be produced by the anaerobic bacterium Veillonella atypica by reducing selenium oxyanions. Bacteria can further decrease selenium nanospheres to reactive selenide. This reactive selenide can be precipitated with a metal cation to create nanoscale chalcogenide precipitates, like zinc selenide, which potentially have optical and semiconducting properties. When reducing selenite, the entire cell population relied on hydrogen as an electron donor; adding a redox mediator, such as anthraquinone disulfonic acid, accelerated the reduction rate [19].

Degradation of pollutants, photoreduction of pigments and dyes, and photocatalysts are just some of the many current applications of photocatalysis. Other applications include photocatalysis, solar cells, photochemical reactions applications, optoelectronic materials for LEDs, etc. [19]. The uses of chalcogenide nanoparticles range widely and include the following: Biochemical infrared sensing with selenide glass fibers, ZnSe as an infrared (IR) window IR sensor, solar panels comprised solar cells made of polymer, and sensors that use electrochemistry. Typically, the practical applicability of binary metal chalcogenides is limited due to their reduced charge separation efficiency and susceptibility to photocorrosion. Chalcogenide-based nanomaterials find applications in a wide variety of activities, such, uses in medicine, imaging using X-rays, decontamination by microorganisms, metabolic profiling: a tool for studying organisms of all kinds, disposal of heavy metals, electrocatalysis, treatment of wastewater, managing infectious disease-causing organisms, and organic matter breakdown. The applications of chalcogenide-based nanomaterials in various fields are shown in Figure 4.

Figure 4.

Applications of chalcogenide-based nanomaterials in various fields.

7. Challenges and prospects of chalcogenides

For the future sustainable and precise manufacture and usage of chalcogenide-based nanomaterials for practical applications, the following difficulties need to be addressed from a photographic perspective:

• For chalcogenide-based nanomaterials, size, shape, and monodispersity are crucial criteria. To achieve the desired optical properties of chalcogenide composites, it is necessary to systematically investigate and confirm methods for effective control of the particle size distribution, morphology, and monodispersity by varying the synthetic parameters and solvents used.

• By designing heterojunction metal chalcogenides that are multirich electrons and the separation of charge carriers (holes and electrons), one can achieve the desired optical properties of chalcogenide composites.

8. Conclusion

Sulfide, selenide, and telluride-based chalcogenides are typical examples of chalcogenides. Many technologies use chalcogenide materials or their derivatives, such as those based on binary, ternary, and quaternary chalcogenides. Hydrothermal, one-pot, solvothermal, sonochemical, microwave-assisted, and electrochemical processes are few of the ways chalcogenide compounds can be synthesized. Raw material composition, synthesis strategies, and post-synthesis processing all have influenced the morphology of chalcogenide materials. An intriguing platform for light-harvesting applications are using chalcogenides and chalcogenide-based nanomaterials with smart hybridization. These novel materials show great promise for photocatalytic water-splitting reactions to generate hydrogen gas (H₂), degrading organic and inorganic contaminants, reducing carbon dioxide (CO₂) to create renewable fuels, and other solar-to-energy conversion processes.

Due to their potential usefulness in various technological contexts, metal chalcogenides have recently gained attention as a significant material class. The immobilization of nanomaterials on reactor surfaces necessitates paying close attention to trapping the nanoparticles onto specific membrane surfaces to prevent them from escaping and entering the drinking water due to the risks they bring to human health and the environment. To isolate and keep the nanoparticles in suspension, new research approaches should be developed to bind them onto the reactor/membrane surfaces firmly. During the environmental rehabilitation process, these chalcogenide-based compounds were used as catalysts for removing various contaminants and converting industrial effluents into drinkable water. Remediation-related chalcogenide-based nanoparticles are produced by Veillonella atypica through selenite reduction. Severe environmental problems, such as pollution and a lack of renewable energy, have resulted directly from progress in the industrial and other sectors. To counteract these unfavorable environmental effects, novel nanocomposites must be developed and used in ecological restoration. Transition metal chalcogenides were utilized even in highly specialized environmental remediation applications, such as dealing with radionuclides found in legacy wastes from nuclear weapons development or wastes generated during nuclear power generation or nuclear fuel reprocessing.

The following suggestions, are essential for promoting the use of TMOs, TMCs, and their composites in water and wastewater treatment operations. It is not scientifically sound to utilize any poisonous material to remove another harmful material; hence, more research is needed to analyze the toxicity of the TMOs and TMCs and their composites. For commercial uses of TMOs and TMCs, it is necessary to produce them on a large scale, but this cannot be done without conducting a scientific investigation to determine the best synthesis methods. Surface modifications to TMOs and TMCs can lessen charge carrier recombination, but additional research into the recombination mechanism is necessary before photocatalysis can be optimized for broader uses.