Green and Sustainable Pathways for Wastewater Purification

CHAPTER 14

Green and Sustainable Pathways for Wastewater Purification Manavi Yadav, Radhika Gupta, Rakesh Kumar Sharma Green Chemistry Network Centre, Department of Chemistry, University of Delhi, Delhi, India

14.1 Introduction With the rapid industrialization and swift growth in human population, the demand on global water resources is expanding, leading to higher costs and stricter regulations. This in turn is pressurizing natural resources and deteriorating environment, thereby, causing serious threat to the sustainability of economic development. Besides, the vast amount of wastewater generated by industries is not only an issue for humans but also a major concern for forthcoming disasters [1, 2]. In the recent years, there is a growing alertness against environmental pollution which is propelling companies and municipalities to reexamine their current methods of water purification and driving them toward a more sustainable future. The biggest atrocious environmental crisis that man is facing today is water pollution [3]. Water affects all aspects of human life, including but not limited to energy, health, food, and economy. The unprecedented increase in pollutants to critical levels has intimidated society with a range of unexplored pollution problems. These contaminants reduce freshwater supplies from both surface and groundwater resources. Moreover, the aquifers are getting less productive and polluted worldwide due to soil erosion, the intrusion of salt water, fertilizers, pesticides, detergents, and heavy metals. In order to protect our natural ecosystems from further damage, it is crucial to eradicate these pollutants from the water bodies in a cost-effective manner. To this end, a number of studies have been performed and adopted, but, none of them is successful in complete removal of pollutants. Due to the ineffectiveness of the conventional techniques, the present scenario requires better and improved methods. This chapter gives a comprehensive overview of green technologies adopted for various pollutants present in surface water.

Advances in Water Purification Techniques. https://doi.org/10.1016/B978-0-12-814790-0.00014-4 Copyright # 2019 Elsevier Inc. All rights reserved.

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356 Chapter 14

14.2 Upsurge in Pollutants: An Alarming Situation Due to high toxicity, high chemical stability, low biodegradability, and bio-recalcitrant nature, pollutants have the ability to bioaccumulate and biomagnify that cause several environmental problems [4]. These contaminants originate from urban run-off, agriculture wastewater, domestic sewage, and industrial effluents including food processing, pulp and paper making (Fig. 14.1). The decomposition of such hazardous waste consumes dissolved oxygen (DO) at a faster rate than its replenishment, thereby depleting oxygen which severely affects the life cycle events and distribution ranges of all forms of biota. Moreover, huge quantities of suspended solids in water bodies curtail sunlight to photosynthetic organisms, rendering them an unsuitable habitat for many invertebrates. Various organic and inorganic pollutants include pesticides, fertilizers, hydrocarbons, phenols, plasticizers, biphenyls, detergents, oils, greases, pharmaceuticals, and heavy metals [5, 6]. Some of them are discussed below.

14.2.1 Persistent Organic Pollutants Persistent Organic Pollutants (POPs) are chemical substances that prevail in the environment resisting bio-degradation, bioaccumulate through the food web, have long-range transport ability and have adverse effects to human health and environment [7]. They can cause reduced reproductive growth, birth defects, behavioral changes and even death. In fact, they are

Fig. 14.1 Various kinds and sources of water pollutants.

Green and Sustainable Pathways for Wastewater Purification 357

Fig. 14.2 Structures of some of the banned/restricted POPs.

suspected carcinogens and endocrine disrupters. Some of the compounds that are either banned or restricted under the Stockholm Convention, a global agreement on POPs, are shown in Fig. 14.2 [8].

14.2.2 Surfactants Surfactants are compounds composed of both hydrophilic and hydrophobic groups because of which they tend to concentrate at the interfaces of aqueous mixtures (Fig. 14.3). Since they are widely employed in industries including textiles, food, fibers, paints, polymers, cosmetics, pharmaceuticals, detergent formulation, oil recovery, pulp paper, etc., there are enough sources through which these surfactants are discharged into water bodies [9]. They are responsible for causing foams in rivers and reduce the water quality. In case of its prolonged exposure to skin, the lipid coating that protects skin gets disrupted. Its presence also creates harmful situation for aquatic flora and fauna as it may interact with oxygen transfer by modifying surface tension. Another hazardous property of surfactants includes its binding ability with other dangerous substances that may damage the environment.

Fig. 14.3 Examples of different types of surfactants.

358 Chapter 14

14.2.3 Dyes Dyes are another major constituent of the wastewater generated from industries including textile, paint and varnishes, ink, plastics, pulp and paper, cosmetics, tannery, etc. More than 10,000 dyes are available commercially today and annual production of dyes is about 700 million kilograms, textile industry being their largest consumer [10, 11]. During the process of coloration a large percentage of the synthetic dye that does not bind to the textile yarn is lost to the waste stream making the effluent highly colored and esthetically unpleasant. High concentrations of these dyes stop the reoxygenation capacity in water bodies and cease sunlight, thereby causing reduction in photosynthesis and DO level. Also, some dyes degrade into compounds that have toxic, mutagenic, and carcinogenic effects on living organisms. Structures of some of the common industrial dyes are shown in Fig. 14.4.

14.2.4 Phenolic Resins Phenolic resins are other major contributors to water pollution due to emissions of phenolic compounds during their synthesis (Fig. 14.5). These resins are used as binding agents in insulation materials, chipboard, paints, and casting sand foundries [12]. Besides, phenolic emission is also found during medicinal preparations including throat lozenges, mouthwashes,

Fig. 14.4 Structures of some common industrial dyes.

Green and Sustainable Pathways for Wastewater Purification 359

Fig. 14.5 Phenol-based compounds responsible for phenolic emissions.

gargles, and antiseptic lotions. Occasionally, phenols form complex compounds with metal ions discharged from industries which are more carcinogenic in nature than the former [13]. Due to the strong corrosive nature of phenols, skin contact cause severe burns, and swelling of the skin. It can also be absorbed through the skin into the body inducing toxic effects to heart, liver, kidney, and nervous system.

14.2.5 Pesticides Pesticides are among one of the major water pollutants (Fig. 14.6). Agricultural practices including large-scale application of pesticides and herbicides in fields and forestry have led to fast growth of agrochemical industries worldwide. However, with the domestic activity of controlling pest, various pesticides and herbicides have entered into the surface and groundwater resources. Phosphate, often used in forms of pesticides or insecticides is water soluble and within a period of a few weeks, phosphate added to the soil converts to less soluble

360 Chapter 14

Fig. 14.6 Structures of some of the major pesticides (values in the circles denote median lethal doses [LD50] in mg kg
1 in mammals).

forms if it has not been taken up by plants and results in run-off [14]. As a consequence, phosphates are responsible for the eutrophication of fresh water resources thereby increasing algal growth and reducing other life forms in water [15].

14.2.6 Heavy Metals Heavy metals are a group of metals and metalloids that have relatively high density and are toxic even at ppb levels [16]. Examples include Pb, As, Hg, Cd, Zn, Ag, Cu, Fe, Cr, Ni, Pd, and Pt. These metals are released into the environment by both natural and anthropogenic sources such as industrial discharge, automobiles exhaust, and mining. Unlike organic pollutants, heavy metals are nonbiodegradable and have tendency to accumulate in living beings. In fact, most of them are known to be potential carcinogens. Various adverse health hazards are known due to long term and continuous exposure to heavy metals. Since they are nondegradable and tend to bioaccumulate, suitable methods need to be established for their efficient removal from the environment. Guideline values for some of the heavy metals are listed in Table 14.1 [17].

14.3 Sustainable Water Treatment and Management: Main Goal Although wastewater treatment is a promising means of conserving and augmenting available water resources, major challenge lies in allocating and effectively managing water with a focus on energy efficiency and sustainability [18]. In order to pursue sustainability goals, industries are taking initiatives for sustainable treatment solutions to reduce operational costs and comply with increasingly stricter regulations. The main goal is to design a sustainable wastewater treatment process that minimizes the volume and toxicity of the final residue since its disposal can incur serious liability. Regardless

Green and Sustainable Pathways for Wastewater Purification 361 Table 14.1 Guideline values of heavy metals (according to World Health Organization) Heavy Metal

Guideline Value (mg L21)

Antimony (Sb) Arsenic (As) Barium (Ba) Cadmium (Cd) Copper (Cu) Chromium (Cr) Lead (Pb) Manganese (Mn) Mercury (Hg) Molybdenum (Mo) Nickel (Ni) Selenium (Se) Uranium (U)

0.02 0.01 0.7 0.003 2 0.05 0.01 0.4 0.006 0.07 0.07 0.01 0.015

of the industry, the selection and evaluation of treatment technologies typically follow a reasonable sequence of steps to achieve this goal. To deal with effluents, the conventional methods have gradually been substituted with a decentralized approach for treating selected wastewater streams in the most efficient and economical way. Besides, additional goals include zero-discharge strategies which will be gradually incorporated to both water and wastewater management approach.

14.4 Recycling: Key Component of Modern Waste Reduction In the present scenario, key point of prevention and minimization of pollution is recycling of waste materials and reusing them as starting materials for other processes. As a consequence of scarcity of pristine water, there is an emerging consensus on recycling and reusing wastewater [19]. This would not only ease the pressure on our water resources but would also reduce the discharge of wastewater to the environment in addition to lowering the cost of freshwater purchase. Utilizing rainwater is an easy and effective way to conserve water supplies and reduce the amount of freshwater use. Other than this, gray water (all nontoilet, domestic wastewater) can be a good water resource during times of drought and water restrictions [20]. However, its reuse can bring health and environmental risks. Therefore, it is better to adopt methods of recycling and reusing low-risk water sources, such as rainwater or storm water; before recycling higher risk source water, such as gray water and sewage.

362 Chapter 14 In comparison to agriculture, only a small fraction of industrial water is actually consumed and most of it is discharged as wastewater. With appropriate management and treatment strategies, industrial water can be employed for variety of purposes including industrial uses such as cooling or material washing or nonindustrial uses such as irrigation or toilet flushing [21]. But before its use, it is important to identify, assess, and manage all the risks appropriately.

14.5 Factors Governing Feasibility of Removal Techniques The selection of the most relevant technique for wastewater treatment depends mainly on its characteristics and other parameters which are mentioned in the Fig. 14.7 [22]. Usually, individual process is not adequate to remove pollutants as each method is associated with some drawbacks. This can be overcome by coupling techniques together. In the next section, some important techniques are discussed.

14.6 Water Remediation Techniques A number of efficient techniques have drawn significant attention for the removal of highly toxic organic compounds from water, and current directives and regulations demand urgency for finding an optimal solution for water treatment so as to minimize hazardous substances

Fig. 14.7 Factors upon which removal techniques depend.

Green and Sustainable Pathways for Wastewater Purification 363 prescribed by the regulations. The choice of suitable process needs appropriate methodologies. Technologies for treating wastewater according to their characteristics are generally divided into three broad categories: biological methods, chemical methods, and physical methods [23]. Table 14.2 gives a brief overview of the same, followed by detailed description of some recent work on widely employed water remediation techniques.

14.6.1 Bioremediation Nowadays, biological processes are gaining tremendous attention in various fields due to less chemical requirement, cost-effectiveness and eco-friendly operating techniques. Bioremediation is one such technology that uses microorganisms, cellular components, and enzymes for the partial transformation of contaminants into harmless or less toxic substances [24]. It is a cost-effective method for heavy metals remediation. Satyapal and group observed that Pseudomonas present in the arsenic (As) contaminated groundwater of middle Gangetic plain, Bihar, India have developed As detoxification mechanism for their survival. Some strains of these bacteria produce arsenite oxidase which are capable of oxidizing toxic As(III) into less harmful As(V) at high rate. The group utilized these strains for As remediation processes [25]. Similarly, Ogugbue and Sawidis isolated a novel bacterial strain Aeromonas hydrophila from textile wastewater treatment plant which was found to be capable of decolorizing triarylmethane dyes within 24 h and with the color removing range of 72%–96% [26]. The decolorization was observed due to the degradation of dyes into smaller fragmented colorless intermediates. The efficiency of decolorization of the bacterium depends on concentration of dye, temperature, pH, and aeration. Rapid degradation, easy handling, and low cost are some of the advantages of the present bioremediation method. Keskin et al. encapsulated Lysinibacillus sp. NOSK bacterial cells in water-based, biocompatible and natural nonpolymeric electrospun cyclodextrin fibers (CD F) [27]. Besides acting as a carrier matrix, it was found that CD F also served as a feeding source for the encapsulated bacteria, improved bacterial viability and protected the bacteria from the external environment. Furthermore, the bacteria/CD F biocomposite was utilized for the removal of heavy metals [Ni(II) and Cr(VI)] and textile dye. Results showed that the removal capability of bacteria/CD F was higher in comparison to the free bacteria. It was due to the fact that CD F acts as an extra carbon source which promoted bacterial growth rate. Fig. 14.8 shows SEM images of CD F encapsulating 1% and 2% (w/w) of bacteria (circles show encapsulated bacteria on CD F). Besides the abovementioned bacterial processes, researchers have introduced the concept of using wastewater as a medium and source of nutrients for algae production which has gained much attention due to its benefits in both environmental and energy-related issues [28]. Conventional wastewater treatment processes use activated sludge for the degradation of

Table 14.2 Technologies for treating wastewater Category

Type

Principle Involved

Pollutants Treated

Merits/Demerits

Biological methods • Use microbial sources coupled with advanced technology. • Include biosorption and biodegradation in aerobic, anaerobic or combined aerobic/ anaerobic treatment processes by bacteria, fungi, plants, yeasts, algae and enzymes.

Membrane Biological Reactor (MBR)

• Includes a semipermeable

Removes micropollutants and organic contaminants

Merits • Compact • High rate of degradation • Possibility of high volumetric load

Chemical methods Used for removing substances by:

• producing insoluble solids and gases

• coagulating colloidal suspension

• converting nonbiodegradable to biological degradable substances

•

Electrocoagulation

membrane barrier system either submerged or in conjunction with an activated sludge process. The membrane is only permeable for water, so the sludge remains inside the biological system, where degradation occurs.

Demerits • Lower removal efficiency, when nutrient concentration is high. • Cost of building and operating is usually higher than those of conventional wastewater treatment.

Removes emulsified oil, It quickly coagulates and Merits hydrocarbons, suspended solids, • Gives clear, colorless and removes the colloidal and and heavy metals. suspended particles, followed by odorless water electro-oxidation of remaining • Removes smallest colloiorganics. dal particle • Avoids use of chemical substances Demerits • Electrodes need to be regularly replaced • Expensive

Table 14.2 Category

Type

• destroying chelating

Advanced oxidation process (AOP)

agents producing simpler substances that can be easily removed

Technologies for treating wastewater—Cont’d Principle Involved

Activated carbon adsorption process

Merits/Demerits

• It involves a set of chemical Converts many organics to CO2, Merit

•

Physical methods • Remove substances by use of naturally occurring forces, such as gravity, electrostatic attraction, van der Waal forces, and physical barriers. • These include sedimentation, flotation, and adsorption, as well as barriers such as membranes, electro dialysis and ion exchange.

Pollutants Treated

treatment procedures to remove organic (and sometimes inorganic) materials within water and wastewater by oxidation. Oxidation procedure includes photochemical degradation processes (UV/ O3and UV/ H2O2), photocatalysis (TiO2/UV, Fenton and photo-Fenton process) and chemical oxidation processes (O3, O3/H2O2, and H2O2/Fe2+).

Activated carbon produced from natural materials can be used as adsorbent for water and wastewater treatment.

water, and to fully oxidized states of other atoms in organic pollutants, including sulfates and nitrates.

• Highly competitive for the removal of organic pollutants that are not treatable by conventional techniques. Demerit

• Costly process, driven by external energy sources such as electric power, ultraviolet radiation or solar light. Removes heavy metals, pesticides, herbicides, carbamate insecticides, chlorobenzene and pchlorobenzene sulfonic acid from wastes.

Merits • Simple design • Low investment in terms of both initial cost and land required Demerits • Removal is pH dependent • Regeneration is difficult and results in adsorbent loss • Requires costly disposal

Coagulation

• It consists of chemical addition via a reaction, forming an insoluble product that serves to remove substances from the wastewater.

Removes suspended and colloidal particles

Merits • Economically feasible • Removes sulfur and vat dyes effectively

(Continued)

Table 14.2 Category

Technologies for treating wastewater—Cont’d

Type

Principle Involved

Pollutants Treated

• Polyvalent metals are com•

Ion exchange

•

•

Demerits • pH dependent • Produces large quantities of sludge • Not good for highly soluble dyes

monly used as coagulating chemicals. Typical coagulants include lime (that can also be used in neutralization), iron containing compounds (such as ferric chloride or ferric sulfate).

• It involves the exchange of ions by attachment to an electrostatically charged ion exchange resin in an electrically neutral solution. It is a reversible chemical reaction wherein positively or negatively charged ions present in the water are replaced by likely charged ions present within the resin. Once exhausted, can be recharged with more replacement ions.

Merits/Demerits

• Removes dissolved inorganic ions effectively

Merits • Possibility to regenerate resin • Relatively inexpensive initial capital investment Demerits • Dye-specific • Large scale dye recovery is expensive • Not capable of removing total dissolved solids (TDSs) levels above approximately 5000 mg L
1

Green and Sustainable Pathways for Wastewater Purification 367

Fig. 14.8 SEM images of CD F encapsulating (A) 1% and (B) 2% of bacteria. Reproduced with permission from San Keskin NO, Celebioglu A, Sarioglu OF, Uyar T, Tekinay T, Encapsulation of living bacteria in electrospun cyclodextrin ultrathin fibers for bioremediation of heavy metals and reactive dye from wastewater. Colloids Surf. B: Biointerfaces 161:169–176, 2018.

organic carbonaceous matter to CO2. Comparatively, algae assimilate organic pollutants into cellular constituents such as carbohydrate and lipid, thereby, reducing pollution in a more eco-friendly manner. Algal species, Chlorella has been widely applied for wastewater treatment by reducing the contents of nitrogen (N), phosphorus (P), and chemical oxygen demand (COD). Thus, it has the potential of replacing activated sludge process in secondary (2 degrees) or tertiary (3 degrees) treatment step. Wang et al. cultivated Chlorella sp. on different points of municipal wastewater treatment plant, that is, wastewater before primary settling, wastewater after primary settling, wastewater after activated sludge tank and centrate. Results showed that algal growth was enhanced in the centrate due to higher levels of N, P, and COD in comparison to the contents at other points of the plant. It was also reported that limited phosphorus content hamper the growth of algae and hence efficient nutrient removal did not take place. Thus, it was recommended that algal system should be employed as the 2 degrees treatment process rather than 3 degrees treatment process [29].

14.6.2 Chemical Precipitation Chemical precipitation is considered to be the most effective method for the removal of heavy metals from wastewater. It is widely employed in industries since it is relatively inexpensive and easy to operate. The traditional chemical precipitation processes include hydroxide precipitation and sulfide precipitation [30]. Out of these two, hydroxide precipitation is more popular due to several advantages over sulfide precipitation, such as, low cost, simplicity, and ease in controlling pH.

368 Chapter 14 The solubilities of the various metal hydroxides are covered within the pH range of 8.0–11.0. Further, the metal hydroxides can be easily removed by flocculation and sedimentation. There are a variety of hydroxides used to precipitate metals from wastewater, based on the low cost and easy handling, lime is considered the best choice of base in industrial settings. Recently, Mirbagheri and Hosseini, removed Cu(II) and Cr(VI) ions from wastewater using Ca(OH)2 and NaOH [31]. In this, Cr(VI) was converted to Cr(III) using ferrous sulfate and maximum precipitation of Cr(III) occurred at pH 8.7 when Ca(OH)2 was added. It was observed that the concentration of chromate reduced from 30 to 0.01 mg L
1. However, hydroxide precipitation also has some limitations since it generates large volumes of relatively low density sludge, which creates dewatering and disposal problems. Also, some metal hydroxides are amphoteric and therefore create problems in selection of ideal pH. In addition, metal hydroxide precipitation is inhibited if complexing agents are present in the wastewater. Sulfide precipitation is another effective process for the treatment of toxic heavy metals ions [32]. It possesses advantages like lower solubilities of the metal sulfide precipitates than hydroxide precipitates. Also, sulfide precipitates are not amphoteric and therefore can achieve a high degree of metal removal over a broad range of pH in comparison to hydroxide precipitation. Moreover, metal sulfide sludges exhibit better thickening and dewatering characteristics than the corresponding metal hydroxide sludges. Lately, new sulfide precipitation process has been reported based on sulfate-reducing bacteria (SRB) that oxidizes simple organic compounds under anaerobic conditions and transform sulfates into hydrogen sulfide which further reacts with divalent metals to form insoluble metal sulfides [33]. But, use of sulfide precipitation has some drawbacks as under acidic medium, it may result in evolution of toxic hydrogen sulfide (H2S) fumes. Hence, it is imperative to perform this process under neutral or basic conditions. Furthermore, metal sulfide precipitation has tendency to form colloidal precipitates that cause separation problems. As an alternative to conventional processes, many companies employed chelating agents to precipitate heavy metals from aqueous systems. Matlock et al. examined the potential of three commonly used commercial heavy metal precipitants, trimercaptotriazine, potassium/sodium thiocarbonate, and sodium dimethyldithiocarbamate [34]. But, due to the lack of necessary binding sites and environmental risks during their use, new chelating agents were explored. Later, Matlock and coworkers developed a new thiol-based compound, 1,3benzenediamidoethanethiol (BDET2
) dianion that can effectively precipitate mercury in the leachate solution and heavy metals from acid mine drainage [35]. Fu and coworkers used dithiocarbamate-type supramolecular heavy metal precipitants, N,N-bis-(dithiocarboxy) piperazine (BDP), and 1,3,5-hexahydrotriazinedithiocarbamate (HTDC) for heavy metal removal in wastewater [36, 37]. It was observed that both BDP and HTDC could efficiently reduce heavy metal ions in wastewater to even lower than 0.5 mg L
1. Another group employed potassium ethyl xanthate to remove copper ions from wastewater over a broad range of copper concentration (50, 100, 500, and 1000 mg L
1) to as low as 3 mg L
1 [38]. Xu et al. synthesized

Green and Sustainable Pathways for Wastewater Purification 369 a new organic heavy metal chelator dipropyldithiophosphate which could reduce the concentration of lead, cadmium, copper, and mercury from 200 mg L
1 at pH 3–6 up to over 99.9% [39].

14.6.3 Advanced Oxidation Processes Advanced oxidation processes (AOPs) are defined as oxidation processes which involve in situ generation of powerful oxidizing agent such as hydroxyl radicals (•OH) in sufficient concentration to effectively decontaminate wastewater [40]. Depending upon the source of generation of hydroxyl radicals, various types of AOPs are there which are described as under:

14.7 Chemical Fenton-related AOPs have been in practice since the 19th century. The well-known Fenton’s reagent involves catalytic decomposition of H2O2 by Fe2+ salts to form complex radical chain mechanism which is capable of treating wastewater, discoloring dye effluents and destroying toxic organic compounds. It is also used in combination with membrane filtration, coagulation, and biological oxidation for more extensive degradation [41].

14.8 Photochemical Photochemical AOPs make use of ultraviolet (UV) irradiation to degrade water pollutants by coupling with powerful oxidants such as ozone (O3), hydrogen peroxide (H2O2) or catalysis with Fe3+ or titania (TiO2). H2O2 can be photolyzed by UV radiations at wavelength (λ) ranging from 200–300 nm, whereas, O3 absorbs UV radiations between 200 and 360 nm [40]. Since the molar absorption coefficient of O3 at λmax 253.7 nm (molar absorption coefficient εmax ¼ 3600 mol L
1 cm
1) is much larger than that of H2O2 (εmax ¼ 18.6 mol L
1 cm
1), O3 photolysis process is much more widely used, particularly in the elimination of toxic persistent organic pollutants (POPs). Photo-Fenton is another means of photochemical AOP which can use several UV regions as light energy source, that is, UVA (315–400 nm), UVB (285–3185 nm) and UVC (<285 nm). Recently, sunlight has been used as renewable and free energy source for the degradation of pharmaceutical waste in solar photo-Fenton process [42]. It was observed that the pharmaceuticals under study were completely removed within 60–120 min of irradiation under acidic conditions (pH 3). Besides the above-mentioned photochemical AOPs, heterogeneous photocatalytic procedures (TiO2/UV) have been widely used for the remediation of wastewater. TiO2 has been accepted as an ideal photocatalyst due to characteristics such as chemical stability, biological inertness, inexpensive and that its band gap is comparable to that of solar photons. Fig. 14.9 represents the general photocatalytic degradation mechanism for water pollutants [43]. It is well established

370 Chapter 14

Fig. 14.9 General photocatalytic mechanism using TiO2.

that TiO2, upon irradiation with light energy (hν) greater than its band gap, creates conduction band electrons (e
) and valence band holes (h+). These electrons react with the adsorbed O2, reducing it to superoxide radical anion O2 
. Simultaneously, the holes react with H2O to form •OH. Both of the so formed oxygen species can react with the dye molecules and thereby cause their degradation. Based on the above mentioned mechanism, Goutam et al. have synthesized TiO2 nanoparticles (NPs) using leaf extract of Jatropha curcas L. plant for the solar photocatalytic degradation of tannery wastewater [44]. For this, filtered secondary tannery wastewater was photocatalytically treated with green synthesized TiO2 NPs by directly exposing to sunlight for 5 h in a parabolic trough reactor. Results showed 82.26% reduction of COD and 76.48% removal of Cr6+.

14.9 Sonochemical Sonochemical-based AOP involve use of ultrasonic irradiation in aqueous medium to either dissociate H2O and O2 molecule homolytically to yield •OH, HO2  , and O• radicals (chemical method) or form cavitation bubbles which grow and collapse and thus create high temperature (2000–5000 K) and pressure (6  104 kPa) conditions (physical method known as sonication) [45]. Under these conditions, sonolysis of water molecules occur, thereby forming highly reactive •OH and H• radicals. Although ultrasonic method is a cleaner way of water remediation, its large scale application is often limited due to the formation of insufficient number of •OH radicals. Therefore, to improve the degradation of organic pollutants in water and to decrease the sonochemical treatment time, ultrasounds are applied in combination with other oxidants such as H2O2 and O2, UV radiation and even with Fenton’s reagent.

Green and Sustainable Pathways for Wastewater Purification 371

14.10 Electrochemical Electrochemical AOP is one of the greenest water remediation techniques because it makes use of electrons, which is a clean reagent. It involves anodic oxidation, where heterogeneous •OH are generated at the surface of anode and electro-Fenton process, where homogeneous •OH are produced in the bulk solution [46]. These processes allow rapid and complete mineralization of organic pollutants and use easy and safe operating equipment with no or few chemical reagents requirement.

14.10.1 Catalytic Degradation Catalytic degradation processes have emerged as one of the important tools in wastewater remediation. Besides providing mild degradation conditions, their catalytic and recyclable nature imparts green credentials to the water treatment approach [47]. Recently, Iqbal and coworkers designed a new multicomponent and recyclable nanocatalyst CDs/Ag@Mg-Al-Ce-LDH for the degradation of organic water pollutants [48]. The catalyst was fabricated by in situ reduction of Ag ions to Ag NPs (catalytic centres) on the surface of Mg-Al layered double hydroxide support doped with Ce (Mg-Al-Ce-LDH) and with the assistance of carbon dots (CDs) which acted as both reducing and stabilizing agent. In the nanocomposite, Mg-Al-Ce-LDH acted as both the support and the cocatalyst. It accelerated the catalytic reaction by facilitating the adsorption of BH
4 on the surface of Ag NPs. The doping of LDH with Ce also enhanced the catalytic activity due to its ability to transfer and migrate electrons. The catalytic activity of the fabricated nanocatalyst was checked in the reduction of 4-nitrophenol and water dyes such as Congo red, rhodamine B, rhodamine 6G, methylene blue, and methyl orange. The CDs/ Ag@Mg-Al-Ce-LDH exhibited excellent catalytic performance in very short reaction time. Besides, the catalyst could be separated from the medium by simple centrifugation and could be reused for several consecutive reaction cycles. Fig. 14.10 shows the representation of degradation process and catalytic function of CDs/Ag@Mg-Al-Ce-LDH.

Fig. 14.10 Representation of degradation of water dyes using CDs/Ag@Mg-Al-Ce-LDH (left) and catalytic function of CDs/Ag@Mg-Al-Ce-LDH (right). Reproduced with permission from Iqbal K, Iqbal A, Kirillov AM, Shan C, Liu W, Tang Y, A new multicomponent CDs/Ag@Mg-Al-Ce-LDH nanocatalyst for highly efficient degradation of organic water pollutants. J. Mater. Chem. A 6:4515–4524, 2018.

372 Chapter 14 In the past few years, zero-valent iron NPs have been investigated as a new tool for dye degradation [49]. Although they provide useful advantages like low cost and easy availability and have the capability to completely degrade contaminants, their catalytic degradation rate can be enhanced by incorporation of a second catalytic metal such as Ni, Zn, Pd, or Pt. Bokare and group fabricated Fe-Ni-based bimetallic NPs for the degradation of azo dyes, specially, Orange G. Results showed that Fe-Ni NPs rapidly degrade the dye molecule and form aniline and naphthol amine as the degradation products [50].

14.10.2 Solid-Phase Extraction Nowadays, a variety of techniques are available for the treatment of wastewater. Among all, solid-phase extraction has displayed most promising and effective results as a wastewater treatment technology. It is a surface phenomenon where species get accumulated onto a solid phase from a bulk phase. It is associated with several fascinating advantages such as convenient operation procedure, low energy requirement, cost-effectiveness, higher efficiency, less time requirement, low consumption of reagents, and reusability of the adsorbents [51–53]. Numerous adsorbents have been widely used for water purification: carbon based, silica gel, clay, and zeolites to name a few. In recent years, sustainable strategies have gained tremendous attention from the researchers worldwide. Some of them are discussed in the following:

14.11 Use of Waste Materials as Alternative Adsorbents High generation costs of adsorbents have shifted the interest of researchers toward the fabrication of low cost and eco-friendly materials [54]. In the past few years, silica, clay, pumice stones, alumina, zeolites, activated carbon, and many other materials have been widely used as low cost adsorbents for the removal of hazardous substances from wastewater. In this regard, researchers have also utilized various types of waste materials for the fabrication of useful adsorbents.

14.11.1 Industrial Waste Gupta and coworkers used bottom-ash and deoiled soya for the fabrication of adsorbents [55]. Bottom ash is a waste material which is obtained from thermal coal power generation plants, whereas, deoiled soya is a by-product of soybean oil extracting mills which has been banned as an edible substance due to its bitter taste and toxicity. Transformation of these waste materials into adsorbents was done by first dipping them in H2O2 solution to oxidize the organic impurities. Moisture content was removed by keeping them in oven at 100°C. Further, the bottom ash was activated by heating 500°C for 15 min. The so formed adsorbents were successfully utilized for the removal of brilliant green dye from water. After doing batch adsorption and optimization studies, bulk removal of the dye was also carried out via column

Green and Sustainable Pathways for Wastewater Purification 373 for both adsorbents. The dye was even recovered from the exhausted columns using sulfuric acid of pH 3 as the eluting agent and the percentage recovery was found to be 76.22% and 88.89% respectively.

14.11.2 Agricultural Waste Lately, agricultural wastes have been converted into low-cost adsorbents and utilized for wastewater remediation. Waste materials such as rice, wheat, tea, coffee, coconut, peanuts, fruit peels, and shells [56] are composed of hemicelluloses, lipids, proteins, lignin, starch, and hydrocarbons and possess a variety of functional groups. Recently, Li et al. utilized peanut shells for the adsorptive removal of four antibiotics from aqueous solutions (sulfamerazine, sulfamethazine, sulfathiazole, and sulfamethoxazole) [57]. The adsorption efficiency was dependent upon adsorbent dosage and antibiotic concentration. Peanut shells can also be reused when methanol was used as an eluting agent.

14.11.3 Municipal Waste Sewage sludges are produced as by-products of wastewater treatment processes. They are largely composed of compounds that cause offensive odor and pathogenic organisms. Conventionally, landfilling has been the major mode of sewage sludge disposal. But, limited landfill space, high cost, and uncompromising environmental standards has forced the researchers to look for other alternative uses of sludge. In this regard, M
endez and coworkers prepared carbon-based adsorbents from sewage sludge using pyrolysis at low temperature and activating them further in the presence of air [58]. The adsorbents so prepared had BrunauerEmmett-Teller (BET) surface area of >100 m2 g
1. They were then utilized for efficient Fe3+ removal which demonstrated that these cheap adsorbents can be used for removing metal impurities from wastewater.

14.12 Use of Magnetic Nanoparticles-Based Adsorbents Till now, numerous studies have been conducted for the development of efficient adsorbents for wastewater treatment. However, removal of suspended adsorbents from wastewater stream still remains a challenge which must be addressed immediately. In this regard, incorporation of magnetism in such adsorbents enables an improvised efficient purification technology [59–61]. Magnetic adsorbents can be formed by incorporating a base adsorbent with magnetic particles which are oxides of Fe, Co, Ni, and Cu. The presence of magnetic core provides easy recovery from the medium by the application of an external magnetic field. Besides, it can combat issues of high capital cost and efficient recyclability of used adsorbents. In this regard, nanotechnology has offered great potential for the treatment of wastewater. In contrast to bulk materials, magnetic NPs possess unique properties such as large

374 Chapter 14 surface-to-volume ratio, enhanced active sites, high stability, and high sorption affinity [60]. However, they have a tendency to agglomerate into large clusters due to strong magnetic interactions which hamper their practical applications. This drawback can be eliminated by coating their surface with suitable protective agents. On this subject, Sharma and coworkers fabricated functionalized silica-coated magnetic NPs for the selective removal of metal ions from differently charged wastewater [62–64]. Various ligands were immobilized to improve the sorption capacity and selectivity toward extraction. Scheme 14.1 and Table 14.3 demonstrate the scheme and the ligands, respectively, by which the nanoadsorbents were fabricated for the selective removal of metal ions. Excellent performance was reported for extraction by the nanoadsorbents in terms of ease of fabrication, economic, and rapid separation (Fig. 14.11), high adsorption capacity, and high specificity. Also, it was observed that the metals can be easily desorbed from the adsorbent under mild conditions and less time.

Scheme 14.1 Schematic illustration for the synthesis of nanoadsorbent.

Green and Sustainable Pathways for Wastewater Purification 375 Table 14.3 Various ligands which were utilized for the synthesis of magnetic nanoadsorbents Name of the Ligand

Structure of the Nanoadsorbent (Ligand-NH2-Si@MNP)

Ion Adsorbed

Ref.

2+

Pb

[63]

1-{4-[(2-Hydroxy-3methoxybenzylidene)amino]phenyl} ethanone (L)

Cd2+

[62]

2,6-Diacetylpyridine monothiosemicarbide (DAPTS)

Pd2+

[64]

Acetoacetanilide (AAA)

AAA-NH2-Si@MNP Selective adsorption of Pb2+ ions

Magnetic separation

Pb2+ ions

Solution containing other ions along with Pb2+

Pb2+ adsorbed AAA-NH2-Si@MNP

Fig. 14.11 Pictorial illustration for the recovery of nanoadsorbent from the solution using simple external magnet.

376 Chapter 14 Different carbon-based materials have also been utilized as potential adsorbents for the removal of various inorganic and organic pollutants. Carbon nanotubes (CNTs) are one of those carbon materials which have gained tremendous attention in the past few years [65]. CNTs consist of graphene or graphitic sheets rolled up in a cylindrical shape and possess conjugated-π structure with a highly hydrophobic surface. They have many unique properties such as large surface area, high porosity, hollow and layered structure, and ability to interact with molecules through hydrophobic and π-π electronic interactions which contribute to superior removal capacities. Considering their advantages, Gupta et al. combined the features of multiwalled CNTs (MWCNTs) and iron oxide to fabricate composites of MWCNT/nano iron oxide for the adsorption of Cr(III). SEM images revealed that clusters of iron oxide are attached on the entangled network of MWCNTs (Fig. 14.12). It was reported that the composite showed better adsorption capability than MWCNTs or activated carbon [66]. Calixarenes are another class of carbon-based materials that have been used in sensing and extraction of contaminants from wastewater [67]. They are well-known for possessing large number of active sites, high surface area and strong host-guest complexation. Besides, they can be selectively functionalized at phenolic OH groups and at the p-position of phenol rings. Recently, Sayin and Yilmaz synthesized magnetic NPs supported calix[4]arene by grafting disubstituted p-tert-butylcalix[4]arene derivative onto [3-(2,3-epoxypropoxy)-propyl]trimethoxysilane-modified Fe3O4 NPs (EPPTMS-MN). The so formed calixarene derivative was utilized for the extraction of As(V), Cr(VI), and U(VI) ions (Fig. 14.13). It was observed that besides providing easy magnetic recovery, the adsorbent showed excellent affinity and was selective for the above ions even in the presence of other competitive foreign ions [68].

Fig. 14.12 SEM images of (A) MWCNTs and (B) MWCNT/nano iron oxide. Reproduced with permission from Gupta V, Agarwal S, Saleh TA, Chromium removal by combining the magnetic properties of iron oxide with adsorption properties of carbon nanotubes. Water Res 45:2207–2212, 2011.

Green and Sustainable Pathways for Wastewater Purification 377

Fig. 14.13 Binding of magnetic calixarenes with As(V), Cr(VI), and U(VI) ions.

Likewise, activated carbons@magnetic NPs composites have been prepared to avoid traditional cumbersome separation procedures such as filtration. In this regard, Ai et al. deposited CoFe2O4 NPs onto the surface of activated carbon and used it as an adsorbent for the removal of malachite green dye from wastewater. It was observed that the process of adsorption was quite rapid. Also, the adsorbent demonstrated good recyclability and reusability [69].

14.12.1 Ion Exchange The most prevalent cation exchangers are strongly acidic resins with sulfonic acid groups (–SO3H) and weakly acidic resins with carboxylic acid groups (–COOH). In both the resins, hydrogen ions serve as exchangeable ions with metal cations. In the process, as the solution of heavy metal passes through the cationic column, metal ions are replaced for the hydrogen ions on the resin. Some other cationic exchangers were tested; one such example includes resin purolite C100 for the removal of Ce4+, Fe3+, and Pb2+ from aqueous systems [70].

378 Chapter 14 It was observed that metal ions adsorption followed the order Ce4+>Fe3+>Pb2+. Likewise, Kang and coworkers tested Amberlite IRN-77 cation-exchange resin for Co2+ and Ni2+ [71]. Another ion exchanger with strong affinity for heavy metal ions had thiocarbamate functional groups which could potentially reduce the ion level from 20 ppm to 1 ppb. Nisso ALM-525 is an example of this type, which is usually employed for selective Hg(II) removal [72]. Lately, some more ion-exchangers based on thioester resins were developed where thiol groups were directly bonded to aromatic ring and were applied for removing Hg(II) present in the industrial wastewaters [73]. In the recent years, researchers have shifted from synthetic resins to natural ones due to their low cost and high abundance. Examples include silicate materials and natural zeolites for the removal of heavy metals. Clinoptilolite is one such material (natural zeolite) that has received considerable attention due to its selectivity for heavy metals [74]. It was reported that the exchange capacity of clinoptilolite-Fe system is so good that it could simultaneously remove copper, zinc, and manganese from drinking water.

14.12.2 Membrane Filtration Membranes provide physical barriers that allow the movement of materials only up to a certain size, shape, or character. In total, there are four crossflow, pressure-driven membrane separation processes currently employed for liquid/liquid and liquid/solid separation [75]: ultrafiltration (UF), reverse osmosis (RO), nanofiltration (NF), and microfiltration (MF) (Fig. 14.14). Membranes are fabricated in a variety of configurations including hollow, fiber, spiral, and tubular shapes and each of these configurations have different degrees of separation capability. The removal of Cu2+ and Ni2+ ions was found successful by the RO process and with Na2EDTA, the rejection efficiency of the two ions increased up to 99.5% [76]. Another group carried out a pilot-scale membrane bioreactor system in combination with RO and observed excellent removal efficiency. But, high power consumption due to the pumping pressures, and the restoration of the membranes will continue to be the major drawbacks of RO process.

14.13 Integrated Approaches In recent years, extensive investigations have been directed toward integrated approaches that couples several techniques together to treat a wide range of hazardous substances in water and wastewater. For this, efficient methods have been optimized and coupled with advanced treatment procedures to treat and reuse contaminated water in an environmentally sustainable and cost-efficient way. Electrocoagulation is one such technique that incorporates the functions and advantages of conventional methods such as flotation, coagulation, and electrochemistry in water and wastewater treatment [77]. Another example include combination of AOP and biological systems for the treatment of a diversity of wastewater [78]. Recently, new

Green and Sustainable Pathways for Wastewater Purification 379

Fig. 14.14 Various membrane filtration methods.

adsorbents, coagulants, membrane filtration, and photocatalysis are frequently considered and widely tested for the treatment of the contaminated water. It appears that a single technique for universal applicability is unrealistic, therefore a combination of different techniques is what is required nowadays to devise a technically and economically viable strategy.

14.14 Conclusions Wastewater reclamation has always been an attractive strategy for conserving indispensable resource for future generations. It will continue to be the focal point of industries as it not only reduces the operational costs, but also cut down the quantity of wastewater discharge and solids sludge generation. However, their use is energy intensive and accompanied with various risks, therefore, requires proper treatment. Over the years, green chemistry has provided a variety of solutions to address the problems of water treatment. They administer the use of sustainable, cost-effective and eco-friendly agents such as microorganisms, enzymes, sunlight, electrons, and discarded solid waste such as industrial, agricultural, and municipal to capture or degrade water pollutants in the most effective manner. Besides, integration of nanotechnology with traditional purification

380 Chapter 14 techniques has also enhanced the greenness and efficiencies of conventional treatment processes. Till now, several methods have been employed for the treatment of wastewater. However, there is no single process that could completely resolve the problem of water purification. Therefore, integrated approaches are required that can possibly bring a paradigm shift of managing water from “treatment” to “profitable utilization.” Moreover, strong collaboration between industries and academia is essential to develop and employ new wastewater treatment innovations as soon as they become available.

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