Technologies for Removal of Emerging Contaminants from Wastewater

Abstract

Emerging contaminants (ECs) include both natural and man-made compounds that have recently been found to be present in wastewater and have a harmful effect on human health and aquatic environment. Several ECs such as pharmaceuticals, antibacterial, hormones, synthetic dyes, flame retardants are directly or indirectly discharged from hospitals, agricultural, industrial and other sources to the environment. Strategies have been developed to overcome the challenges faced by contaminated water treatment technologists. Advanced treatment technologies such as physical, chemical, and biological methods have been studied for ECs removal as well as for reduction of effluents levels in discharged water. Techniques such as membrane filtration, adsorption, coagulation-flocculation, solvent extraction, ion exchange, photodegradation, catalytic oxidation, electrochemical oxidation, ozonation and precipitation, etc., have been investigated. Based on past research, these techniques significantly remove one or more pollutants but are insufficient to remove most of the toxic contaminants efficiently from wastewater. Nanomaterial incorporated technologies may be a proficient approach for removing different contaminants from wastewater. These technologies are costly because of high-energy consumption during the treatment of wastewater for reuse on large scale. Consequently, comprehensive research for the improvement of wastewater treatment techniques is required to obtain complete and enhanced EC removal by wastewater treatment plants.

1. Introduction

Wastewater is the water having surplus substances that may be dissolved or suspended solid particles or organic and inorganic substances or other impurities that critically influence its quality and make it unsuitable for use [1]. Wastewater composition varies and is highly dependent on major sources of generation as industries, commercial and residential areas, agricultural sources, etc. [2]. In developing countries, the risk of consumption of contaminated water and its sanitation problem is increasing day by day.

Water covers about 70% of the earth’s shells and is essential for all living organisms to survive and also for various manufacturing industries. About 3% of the total water on earth is fresh water of 0.01% is available for human use. The discharge of untreated contaminants from various industries directly to groundwater hinders the favorable use of water in normal operations of the ecosystem and causes water scarcity. Water deficiency is considered one of the most significant alarms for humanity and sustainable development [3]. According to the UNO report, about 1.2 billion people are affected by severe water scarcity due to the increasing world population and in future, 1.8 billion citizens are predicted to be affected by water insufficiency. Beyond water scarcity, water pollution also poses a greater threat to human health and aquatic life as well as the environment. Several new compounds recently detected in drinking, ground, and surface water have a major effect on water parameters. Water is a universal solvent and water quality is affected due to contamination by toxic substances dissolved in it which causes water pollution [4]. Water requirement is increasing due to adaptation in atmosphere, industrialization, increase in population, and obliteration of the surroundings [5]. The occurrence of organic and inorganic pollutants in wastewater is a major challenge to recycle water sources. To determine small amounts of unknown pollutants in the evaluation of emerging contaminants, the latest modern treatment techniques are still limited [6].

Currently, various analytical methods have been developed for different kinds of emerging pollutants. The separation of these toxic pollutants from water becomes important before the discharge of industrial wastewater into the aquatic environment. For this purpose, the development of proficient techniques has been a major area of environmental research. In general, traditional clean-up methods are classified as biological, physical, and chemical. Biological treatment is of low cost and simple, but not effective for synthetic dyes as they are resistant to aerobic biodegradation. Chemical treatments produce toxic by-products and are low efficient, while physical treatment is usually effective. For treating these organic pollutants present in water, several techniques such as membrane filtration, coagulation-flocculation, solvent extraction, ion exchange, catalytic oxidation, electrochemical oxidation, precipitation, etc. have been tested. However, these techniques are less effective, very expensive, and do not eliminate the contaminants from polluted water which makes this issue more challenging for the researchers. Besides these techniques, adsorption and photocatalytic degradation are considered the most potential approaches to removing wastewater contaminants [7].

The current chapter focuses on classification, potential sources, occurrence, prevention, control, and elimination of emerging contaminants. The major objective of this section is to study available technologies currently used for the removal of ECs from wastewater. This chapter also focuses on selecting the best available technology for removing emerging contaminants from wastewater. A schematic representation of treatment technologies, their principal advantages, performance efficiency, and limitations are discussed in the present study. Furthermore, future research opportunities are examined to provide more suitable and strategic recommendations for ECs removal from the aquatic environment.

2. Emerging contaminants

Emerging contaminants (ECs), termed contaminants of emerging concern, emerging pollutants (EPs), micro-pollutants, or trace organic compounds (TrOCs) are derived from different natural as well as anthropogenic sources that extensively influence water quality [8]. They are termed as emerging not because they are new but due to enhancement in the level of concern. These contaminants are generally in small concentrations, ranging from nano-gram per liter (ng L⁻¹) to micrograms per liter (μg L⁻¹) in the atmosphere. United States Environmental Protection Agency (USEPA) describes ECs as new chemical compounds that have the potential to cause harmful effects on individual health and the surroundings [9]. It is essential to treat and recycle wastewater to an acceptable standard to fulfill water demands.

2.1 Classification of ECs

ECs are classified into organic, inorganic micro-pollutants like pesticides, personal care products (PCPs), pharmaceuticals, synthetic organic dyes, polycyclic aromatic hydrocarbons (PAHs), heavy metals ions, plasticizers, per-fluorinated compounds, flame retardants, surfactants, etc. (Figure 1) generated by human activities such as domestic, health care units, agricultural and industrial pathways [10]. These compounds are a source of concern due to their physical and chemical properties because they are widely distributed in the environment which is harmful to humans and wildlife. These pollutants are difficult to detect and have varied activities and miscellaneous sources of production. Their presence in small concentrations causes chronic toxicity, endocrine disruption, and the expansion of pathogen resistance [11].

Figure 1.

Classification of emerging contaminants.

2.1.1 Pesticides

Pesticides, a class of organic contaminants, based on their physical and chemical properties are categorized as fungicides, herbicides, bactericides, and insecticides which are used in the agricultural sector to control dangerous insects, weeds, and microorganisms, etc. Based on their application sites, pesticides are frequently detected in groundwater causing toxicity and may bio-accumulate in humans and plants, or sediments depending on solubility, reactivity, and characteristics of soil and environment. Among the pesticide contaminants, dichlorodiphenyltrichloroethane (DDT) and hexachlorocyclohexane are commonly used pesticides (about 67%) as compared to other compounds such as phorate, chlorpyriphos, Atrazine, methyl parathione, Bentazone, Diazinon, Cyanazine, Simazine, phosphamidone, Terbuthylazine, Alachlor and Dimethoate [12].

2.1.2 Pharmaceutical industry

Pharmaceuticals are major emerging organic contaminants occurring in small amounts in water resources worldwide [13]. Pharmaceuticals are extensively used on daily basis in human healthcare as well as veterinary medicine such as nutrition, investigative aids, therapy and preventive medicine. Many pharmaceutical products such as drugs (both prescribed and non-prescribed), hormones and antibiotics are extensively detected in the aquatic environment, surface and groundwater and have adverse effects on humans, poultry, livestock and fish farming, etc. Generally, livestock is given medications to reduce diseases and infections. Researchers have examined more than 3000 chemicals used in therapeutic products but only small proportion (ng L⁻¹ doses) has been studied in the field, which possibly will lead to negative effects on human and wildlife. To enhance animal farming, organic fertilizer such as manure and purines as medicines are used which indirectly affect the atmosphere and can reach living organisms through food stuff. Commonly reported pharmaceuticals in wastewater are antibiotics, diclofenac, antacids, clofibric acid, steroids, antidepressants, ciprofloxacin, propranolol, beta blockers, analgesics, salicylic acid, fluoxetine, antipyretics, anti-inflammatory drugs, nitroglycerin, tranquilizers, lipid-lowering drugs and stimulants [14].

Natural or synthetic hormones are also essential ecological contaminants, because of their estrogenic and androgenic impacts on wildlife. Organic and inorganic hormones consist of 17α-estradiol, 17β-estradiol, estrone, equiline, equilenin, estriol, mestranol and norethindrone which can enter atmosphere through farming, and are not completely eliminated from wastewater and harm aquatic life and humans.

2.1.3 Personal care products (PCPs)

Personal care products (PCPs) are household chemicals commonly used for health, odor, beauty, or cleaning. These chemicals are used in personal care products like ornamental cosmetics, soaps, hair and skin care products, lotions, fragrances and sunscreens. PCPs are used in large quantities throughout the world due to which the release of these pollutants in the environment is increasing day by day [15]. Mostly these substances are bioactive and bioaccumulative and harm the environment and humans [16]. The most probable emerging contaminants in PCPs are antiseptics, perfumes pollutants like galaxolide, pest repellants, preservatives diethyl phthalate ultraviolet (UV) filters and Triclosan (TCS) and triclocarban as disinfectant pollutant. Parabens are antimicrobial preservatives used in cosmetic items, pharmaceuticals, and some food stuffs such as benzyl, butyl, ethyl, isobutyl, isopropyl, methyl, and propyl hydroxybenzoates. Polycyclic musks are used in numerous products such as clean-up products, shampoos, hair care and washing products and cosmetic products. Their use on the outside of human skin increases its discharge in environment without any metabolic changes. Among all these products, cosmetics are frequently used, thus its occurrence in air at low quantity may be a source of damage to human beings, wildlife and environment.

2.1.4 Surfactants

Surfactants are synthetic organic compounds used all over the world in making of household products such as emulsifiers, detergents, paints, and pesticides, in addition to personal care products and are harmful to aquatic species [17]. They are classified as cationic, anionic and zwitterionic surfactants. Frequently used surfactants such as fatty alcohol ethoxylates, linear alkyl benzene sulfonates, lignin sulfonates, and alkyl phenol ethoxylates are produced on a large scale. Furthermore, octylphenol and nonyl-phenol ethoxylates, are highly toxic even at low concentrations and must be substituted in all their uses.

2.1.5 Food additives

Numerous artificial sweeteners such as acesulfame, saccharin and sucralose which are extensively used in foodstuff, pharmaceuticals and hygiene products find their way to domestic wastewater via human excretion. These moderately metabolized sweeteners which pollute the environment are usually hard to remove. Though, latest calculated ratio for predicted environmental concentration (PEC) and predicted no effect concentration (PNEC) of compound sucralose for marine system is below 1 indicating limited threat to aquatic system (plants, algae and fish) [18].

2.1.6 Flame retardants (FRs)

Among all flame retardant compounds, organophosphate ester flame retardants (OPEFRs) class of phosphorus-containing flame retardants (FRs) and halogenated FRs such as polybrominated diphenyl ethers (PBDEs)) are known FR groups that decrease the flammability of industrial and consumer products. Organophosphate flame retardants (OPFRs) are used in furnishings, textiles, construction materials, electronics and as plasticizers in floor polishes and coatings. The discharge of OPFRs from wastewater treatment plants (WWTPs) into the surface water polluted marine environment causes toxicity. PBDEs are hydrophobic in nature and are mostly used as FRs in the manufacturing of carpets, computers, polyurethane foams, electronic cables, etc. [19].

2.2 Potential sources of emerging pollutants

Emerging contaminants sources are the same as those of traditionally known contaminants and they are released to environment by agricultural, domestic, mining and industrial activities and hospitals. These sources are categorized as: point sources and non-point sources [20]. Contaminants from point sources are discharged from a particular site in high concentration and enter the ecological system in a spatially distinctive way. Examples are discharges from industrial activities, mineral extraction and sewage treatment plants. While non-point sources also termed as diffuse sources release pollutants from indistinguishable disperse sources usually over large areas in low quantity. Examples are runoff of bio-solids or fertilizer applied to soils and rain overflow in urban or industrial areas (Figure 2) [21].

Figure 2.

Sources and their pathways of emerging contaminants.

Water resources contamination by ECs from Wastewater is taking place all over the world particularly in those areas where wastewater treatment is not properly organized. Frequent use of drugs and personal care products lead to discharge of low quantity of different by-products. For example, triclosan, bisphenol-A and phthalates are significant industrial compounds integrated into several commercial household products. Their existence in water and environment affects their physical and chemical properties. PPCPs and other ECs metabolites are complex and hydrophobic in nature when released in water and settle at water surface. Thousands of these ECs and their metabolites have been discovered in the marine environment and are more noxious and harmful. Basically, wastewater treatment plants are not specifically designed for the effective removal of emerging contaminants [22].

2.3 Toxicological effects of emerging contaminants

The adverse effects of ECs on living bodies have been widely reported which confirm that even small amount of ECs pose negative effects such as chronic toxicity and endocrine disruption in humans and animals. The major route of human contact with endocrine-disrupting chemicals (EDCs) is taking of foods and drinks connected to contaminated soil, water and microorganisms leading to bio magnification and bioaccumulation in human body (Figure 3). Currently, researchers are focusing on ECs present in surface waters for many reasons: firstly, surface waters commonly contain high quantity and a diverse range of contaminants particularly when surface water is directly associated with industrial discharges and secondly it is easily monitored as compared to groundwater [23].

Figure 3.

Harmful effects on human health.

3. Traditional wastewater treatment methods

Conventional techniques for the treatment of wastewater consist of physical, chemical and biological techniques for the removal of soluble and insoluble pollutants. Benefits and challenges of wastewater treatment technologies are given in Table 1. Biological treatment is of low cost and simple, but not effective for synthetic pollutants such as dyes as they are resistant to aerobic bio-degradation. Chemical treatments produce toxic by-products and are less efficient, while physical treatment is usually effective. Different phases included in wastewater treatment preliminary, primary, secondary and tertiary [24].

Treatment	Basic methodologies	Benefits		Challenges
Screening	Coarse Screening: removes solid materials with a size below 6 mm.	Minimizes interruption and blockage of the treatment technologies		Not effective in the removal of the ECs
	Fine Screening: removal of contaminants in between 0.001 and 6 mm	Efficiency regulated by altering the fineness of the screen openings.		The screen must be cleaned due to the blockage in the small openings
		Highly recommended to regulate the temperature of the process
		Less expensive and less complex process
Adsorption	Process of removing soluble substances by solid substrates of very specific surface area	Capable of very specific removal of the ECs		Accumulation of cyanotoxin in the adsorbent
		Accurate and efficient removal		Difficult to remove the unknown type of contaminants, since adsorbents are highly specific
		Can assist other treatment processes
		Less complex and less expensive, adopted easy		Regulation of selectivity of the membranous system is difficult
				Reverse osmosis requires external energy;
Biosorption	Immobilization of the microbes on absorbents	Efficient treatment		Absorbents need to be cleaned at a certain interval of time
		Specific removal of certain ECs

Table 1.

Benefits and challenges of wastewater treatment technologies.

3.1 Preliminary treatment

Preliminary treatment helps in the removal of suspended materials like dead animals, papers, oils, grease, etc., from wastewater. Different components such as screening, accumulation and floatation tanks and skimming reservoir are used in preliminary treatment. The accumulation tank is used for the elimination of sand and grit while oils and greases are removed by floatation units and skimming tanks.

Figure 4.

Various treatment methods used for wastewater.

3.2 Primary treatment

In primary treatment, organic and inorganic components are removed by floatation and sedimentation processes. Throughout this treatment, untreated nitrogen, unrefined phosphorus, and heavy metals related with suspended impurities are drained off. This method reduces biochemical oxygen demand (BOD) ranges by 5–40%, 50–70% of entire floating particles and oil and grease up to 65% from wastewater. In various developed countries, primary treatment is required for the reuse of wastewater irrigation, i.e., for crops not used by humans.

3.3 Secondary treatment

The secondary or biological treatment is used to eliminate organic effluent that escapes from primary treatment. This method modifies organic matter and transforms it into stabilized form by oxidation or nitrification. This treatment method for sewage is divided into two groups known as filtration and activated sludge methods. Different filters such as contact beds, irregular sand and trickling filters are used in this treatment [25].

3.4 Tertiary treatment

Tertiary treatment is employed for the removal of specific effluents which cannot be completely removed by secondary method. During this process, around 99% of all contaminants are eliminated. This process removes inorganic substances such as nitrogen and phosphorous and recovers wastewater quality which can be reused for irrigation and drinking and have no harmful effect when discharged to the environment [25].

4. Available technologies for wastewater treatment

Generally, conventional wastewater treatment plants are not constructed to eliminate emerging contaminants. The occurrence of ECs in the environment affects public health, marine life and produces resistant bacteria, neurotoxin effects, endocrine interruption, and tumors. To eliminate these organic pollutants from water, several techniques such as membrane filtration, coagulation-flocculation, solvent extraction, ion exchange, catalytic oxidation, electrochemical oxidation, and precipitation, etc. have been tested (Figure 4). However, these techniques are less effective, very expensive and do not eliminate the contaminants completely from polluted water which makes this issue more challenging for the researchers. Besides these techniques, adsorption and photocatalytic degradation are considered potential approaches to remove wastewater contaminants [26].

4.1 Membrane filtration

Membrane technology is a physical method implemented to eliminate emerging contaminants from aquatic system. Membranes are formed from substances having filtering properties such as specific surface charge, pore size and hydrophobicity to remove suspended contaminants. Membrane filtration is categorized as ultra-filtration (UF), nano-filtration (NF), microfiltration (MF), forward osmosis (FO) and reverse osmosis (RO). Major membrane processes including forward osmosis, membrane refinement and electro-dialysis of the membrane have the ability to reduce emerging contaminants upto greater than 99% but still have not been executed on large scale [27].

The ultrafiltration technique works at low pressure for the removal of colloidal, suspended or dissolved pollutants depending on the membrane and pollutant type. UF has pore size in range of 0.001–0.1 μm which is larger than dissolved hydrated metals ions, thus easily pass through it. Polymer enhanced ultrafiltration (PEUF) and Micellar enhanced ultrafiltration (MEUF) processes were studied to enhance the removal efficiency of metal ions such as copper, zinc, chromate, arsenate, cadmium, nickel, serinium, and organics like phenol, o-cresol, etc.

Microfiltration has pore size ranges from 0.1 to 10 μm and is commonly operated at atmospheric pressure but cannot effectively remove contaminants of size greater than 1 μm. Reverse osmosis and forward osmosis depend on the osmotic pressure gradients and use semi-permeable membrane to efficiently remove dissolved particles up to 1 nm from water. Nanofiltration membrane possess small pore size ranges from 1 to 10 nm and have high competency for removal of ECs based on type of membrane and contaminant. NF can be used for removal of pharmaceuticals and natural hormones such as anti-inflammatory drugs, sulfonamide and fluoroquinolone antibiotics, testosterone, estradiol, and progesterone [28].

4.2 Coagulation-flocculation

Coagulation-flocculation process is effective for the elimination of larger colloidal or suspended particles of disperse dyes colored wastewater. Coagulation is a procedure in which dye solution systems are dispersed to form flocs and agglomerates while in flocculation aggregated flocs are joined to form larger agglomerates which settle down due to gravity [29]. Coagulation/flocculation is economically feasible and simply operated and commonly used in textile industries to purify wastewater. In this method, coagulants like lime (Ca(OH)₂), ferric sulfate (Fe₂(SO₄)₃∙7H₂O), aluminum sulfate (Al₂(SO₄)₃∙18H₂O), and ferric chloride (FeCl₃∙7H₂O), combine with the pollutants and remove them by electrostatic interactions or sorption. Use of aluminum sulfate (Al₂(SO₄)₃ for removal of pharmaceuticals such as betaxolol, chlordiazepoxide, bromazepam, warfarin and hydrochlorothiazide by coagulation-flocculation has been reported. This technique diminishes suspended matter, soluble dyes, colloidal particles and coloring agents from wastewater [30].

4.3 Solvent extraction

Solvent extraction is widely used technique for the elimination of organic and inorganic pollutants discharged into wastewater from various industries. It is based on three major operations. First is the extraction/transferring of solute particles to solvent from water. Secondly, the separation of solute from solvent and the third stage is the solvent recovery stage. Solvent extraction is mostly operated for exclusion of phenols, creosols and other phenolic acids from contaminated water containing low quantity of solute arising from petroleum processing plant, coke-oven plants in the steel and plastics manufacturing [31].

4.4 Adsorption

Adsorption is one of the most efficient techniques used for treating wastewater due to its simple design, high competence and ease of operation, capital cost, easy recovery, adaptability and technical feasibility without producing harmful by-products. This technique is not new but is recognized throughout the world because of removal capacity and regeneration of adsorbents. This technique has been broadly applied for both organic inorganic toxins from household and industrial wastewater [32]. Various research efforts have been devoted to discover low-cost adsorbents having large surface area and excellent binding capacity to enhance their adsorption efficiency. Different types of adsorbents, e.g., peat, bamboo dust, chitosan, silica gel, activated carbon,, fly ash, zeolites, metal organic frameworks nano-adsorbents for example carbon nanotubes and graphene have been applied for elimination of emerging contaminants [33, 34]. Activated carbon is widely used as traditional adsorbent because of highly porous surface area, convenient pore composition and thermo stability for removal of dyes and pharmaceutical products, e.g., 17β-estradiol, 17α-ethynylestradiol, bisphenol A, and fluoroquinolonic Caffeine from wastewater [35].

4.5 Advanced oxidation process

Advanced oxidation processes (AOPs) have been introduced as proficient technology in wastewater treatment. AOPs are based on the generation of hydroxyl (OH) or sulfate radicals for oxidation of ECs while sometimes ozone and UV irradiation are used for enhanced removal efficiency. AOPs methods efficiently remove biologically injurious or non-degradable compounds such as pesticides, aromatics, petroleum essentials and volatile organic compounds (VOCs) rather than transferring these to another phase. AOPs are applicable for the removal of many organic contaminants at the same time without producing any hazardous substance in water, as OH˙ is reduced to form H₂O as byproduct. AOPs include ozonation (O₃), hydrogen peroxide (H₂O₂), electrochemical oxidation, Fenton process, UV light and photocatalytic process [36].

4.5.1 Non-photochemical processes

Ozonation:Ozone is an extremely efficient oxidizing agent and has the potential for elimination of organic and inorganic compounds from industrial effluents. It is a complex oxidation method. Pre-oxidation processes give significant development in biological degradation while post-oxidation process improves effluent quality. The limitation of ozonation is low solubility, stability and short half-life. O₃/H₂O₂ and catalytic ozonation have been investigated for generation of hydroxyl radical which efficiently removed organic pollutants such as antibiotics, antiphlogistics, beta blockers, lipid regulators and their metabolites, natural estrogen estrone, antiepileptic drug carbamazepine and musk fragrances in wastewaters effluents.

Electrochemical method:Electrochemical procedure is generally applied for removal of toxic contaminants from textile effluents by direct or indirect oxidation. This procedure is commonly applied for elimination of ECs like dye by using either mercury electrode, graphite rod, boron doped diamond electrode, platinum foil or titanium/platinum as anode while SS304 is used as cathode in textile sewage treatment. This process is cost effective as minute amount of chemical is required and stability is easily attained by manipulating the electric current.

Fenton process:Reaction between ferrous iron and hydrogen peroxide is termed as Fenton’s reaction. Fenton method is used for removal of organic pollutants like phenols, reactive dyes and pesticides. Fenton process is of low cost as no energy is required for activating H₂O₂, environmental friendly, easy to control and efficient for elimination of organic pollutants.

4.5.2 Photolytic chemical process

4.5.2.1 Homogeneous photolytic chemical process

Ultraviolet lamp (UV): In this process oxidizing agent like H₂O₂ is initiated by UV process to produce OH˙ and can degrade micropollutants efficiently which can be affected by various parameters such as pH, structure of dye, composition of effluent and intensity of UV radiation. Generally, UV process occurs at standard wavelength of 254 nm at low pressure. A pilot plant with UV/H₂O₂produced hydroxyl radical to treat effluent, achieved 98% removal of mecoprop and diclofenac. O₃/H₂O₂/UV processes were examined in treatment of textile effluent to achieve complete degradation [36].

Photo-fenton process:In this process, formation of hydroxyl radical is improved by UV light in the presence of Fe and competently degrades wastewater effluents. Fenton process and photo-fenton process are similar but in the later process mineralization is much better. Removal of numerous ECs like pharmaceuticals, beta blockers and pesticides excluding triclosan by photo-fenton process is enhanced significantly (95–100%).

4.5.2.2 Heterogeneous photolytic chemical process

Mostly semiconductor consists of two energy bands, high conduction band and low energy valence band and these two are separated by band gap. In heterogeneous processes, semiconductor sensitized photolytic chemical oxidation produces OH radical. Adsorption of photon having energy (≥band gap energy of catalyst) is needed for photocatalytic reaction to occur. ZnO, strontium titanium trioxide and TiO₂ have been utilized extensively as photocatalysts for commercial application. Photo-catalysis is commonly used for dyestuff degradation from textile wastewater. Photocatalytic process enhances efficiency in the presence of H₂O₂ up to 100% for numerous pollutants such as bisphenol A, pesticides, pharmaceuticals [37].

4.6 Application of nanotechnology for ECs removal

Nanomaterials are generally defined as the materials having at least one dimension smaller than 100 nm. Nanomaterials have higher density and larger surface area resulting in increasing adsorption efficiency, surface reactivity, and resolution mobility. Current investigation in the exploitation of nanomaterials has facilitated the application of nanotechnology in wastewater treatment via adsorption, AOPs and filtration. Nanomaterials have been reported to effectively eliminate emerging contaminants from wastewater. A variety of nanomaterials have been reported for wastewater treatment (Figure 5) such as zerovalent metal nanoparticles, metal-oxide nanoparticles, carbon nanomaterials and nanocomposites [38].

Figure 5.

Various groups of nanomaterials.

4.6.1 Zerovalent metal nanomaterials

Zerovalent metal is a significant wastewater treatment nanomaterial which is highly reactive because of small size and high surface area. Recently, several zerovalent metal nanoparticles for example silver, zinc, iron, aluminum and nickel received attention of researchers for contaminant removal. Silver nanoparticles have potential antimicrobial properties and are generally used as disinfectant to eliminate a large amount of microorganisms, like viruses and bacteria, as well as fungi [39]. It is extremely reactive, cost effective, environment friendly and has multiple pathways for wastewater treatment. Iron nanomaterial can proficiently remove contaminants such as cadmium nitrates, colorant and antibiotics from wastewater by adsorption, redox reaction, and co-precipitation technique. Li et al. [40] reported two-step technique to form zero-valent metal nanomaterials covered with silica and polydopamine (nZVI/SiO₂/PDA) for use as sorbent which shows high capacity, selectivity and reusability up to 10 cycles.

4.6.2 Metal-oxide nanomaterials

Metal-oxide nanomaterials like ferric oxides, manganese oxides, aluminum oxides and titanium oxides have been effectively utilized in removing noxious waste such as arsenic, uranium, phosphate, and organics. Titanium oxide nanomaterial is a capable photocatalyst having band gap of 3.2 eV with high photostability, low price and outstanding photocatalytic behavior. TiO₂ nanomaterials are suitable for degradation of pollutants like organic chlorine, polycyclic aromatic compounds, pigments, phenols, pesticides, and heavy metals [41].

Zinc oxide (ZnO) nanomaterial is competent material for purifying wastewater having a strong oxidizing capacity, wide wavelength and admirable photocatalytic properties. ZnO nanomaterial is environment friendly and captures more light as compared to other metal oxides possessing semiconducting properties. Iron oxide nanoparticles have versatility and are available as potent sorbent material-removing heavy metals from wastewater [42].

4.6.3 Carbon-based nanomaterials

Carbon nanomaterials comprise distinctive structural and electronic properties duet to which they perform complex applications particularly in adsorption [43]. They have high adsorption capacity for removal of various pollutants, high surface area and aromatic selectivity. These nanomaterials are categorized as carbon beads, nonporous carbon, carbon nanotubes (CNTs) and carbon fibers. CNTs have well-defined cylindrical structures, stronger physicochemical interactions, porosity, large surface area, adaptable hydrophobic side and high adsorption capacity for dichlorobenzene, ethylbenzene, dyes,Pb²⁺, Zn²⁺, Cd²⁺ and Cu²⁺ [44].

Another class is graphene-based nanomaterial which is a single carbon atom layer having honeycomb like structure [45]. Graphene oxide is a graphene layer consisting of hydroxyl, epoxy, carboxyl, and carbonyl groups and is identified for eradicating heavy metals such as lead, zinc, copper, cadmium, mercury and arsenic. Graphene hybrid with nanoparticles of manganese ferrite can be exploited to proficiently remove Pb(II), As(III), and As(V) from contaminated water. Rajabi et al. [46] compared the adsorption efficiency of MWCNTs and functionalized CNTs by varying experimental conditions including pH, times, and temperatures. From results it was clear that f-CNTs possess a higher removal capacity than pristine CNTs. The maximum removal capacity (166.7 mg g⁻¹) of methylene blue (MB) with functionalized multi-walled carbon nanotubes (f-MWCNTs) was higher as compared to MWCNTs, which was 100 mg g⁻¹.

5. Future challenges

Management of ECs in water sources is extremely challenging for humans. World population is increasing considerably every year, which leads to increase of freshwater demand for domestic use and also generate wastewater causing water deficiency. Besides, technological advancement, demand of water from cultivation, urban areas and industries, are main causes of water scarcity, resulting in adverse effects on environment. That is why, a highly efficient and low cost waste water management methods and society alertness is necessary.

Currently wastewater treatment is a difficult challenge as it has considerable effect on bio-physical environment and living organisms and depends on socioeconomic circumstances. Discovery of a general technique for complete elimination of all pollutants from wastewaters is complicated. A number of biological, physical, and chemical technologies for wastewater management have been studied to eliminate emerging pollutants but unable to identify best method to overcome challenges in operational obscurity, ecological impact, efficiency, feasibility, probability, and cost-efficiency. For enhanced removal, two or more techniques are merged to reach favorable water quality at low cost.

To overcome these challenges, some proposed potential directions in future are required as follows:

• To incorporate new concepts such as nano-technology and genetic engineering for production of environment friendly and non-hazardous techniques for synthesis of nanoparticles for pollution degradation.

• Effective assessment of treatment to select most suitable treatment technique depending on numerous parameters like water quality, environmental compatibility, consistency, elasticity, working and effective costs technique.

• Apply green technologies on an industrialized scale like membrane filtration nanotechnology, and microbial fuel cells as competent and cost maintenance solution.

• The exploration of cross treatment systems, e.g., combination of photo Vs electro-Fenton, UV photolysis, ozonation and biological treatment technologies is required for the development of the appropriate model.

6. Conclusion

Emerging contaminants are man-made toxic compounds discharged into wastewater. This chapter includes sources of emerging contaminants, their toxicity and treatment techniques. Pharmaceuticals, personal care products and fertilizers are the main sources of ECs. Their presence, even in small concentrations, cause toxic impacts on human health as well as marine organisms. They cannot be successfully eliminated by conventional wastewater treatment methods. Various treatment methods like membrane technology, coagulation-flocculation, solvent extraction, adsorption, advanced oxidation processes and nanotechnology have been discussed. These techniques have their advantages and limitations. Hybrid systems have been found more effective for EC elimination than individual techniques however they have issues regarding time, energy and cost. To overcome these limitations nanotechnology is a promising approach. Thus, comprehensive research on waste water treatment technologies which are technically and economically feasible is required to attain complete and efficient removal of ECs from contaminated water.

Acknowledgments

The authors would like to convey their gratitude to National Centre of Excellence in Physical Chemistry, University of Peshawar for providing us necessary support.