Keywords

1 Introduction

Water pollution is one of the major concerns of the modern world which is linked with rapid urbanization and industrialization even in the remotest and most undeveloped parts of the world and is associated with poor planning for such developmental activities. The demand for clean water supply has increased by many folds over the past few decades, however, urban authorities are unable to keep pace with such high demands. Furthermore, due to rampant pollution of ground and surface water sources even the available clean water reserves are shrinking on a regular basis. These pollutants include heavy metals such as chromium, cobalt and selenium, and organic compounds like chlorinated solvents, nitrates, dyes, arsenic, phenol, halogenated, and nitroaromatic compounds (Sun et al. 2006). The necessity for a reliable and safe water supply is driving developing countries to create new and cost-effective water/wastewater purification and treatment systems (Ombaka et al. 2020; Gil-Díaz et al. 2020). Traditional and advanced treatment methods such as flocculation, Fenton’s oxidation, membrane filtration, adsorption, phytoremediation, bioremediation, photochemical, ion exchange, electrochemical oxidation, electrolytic precipitation, and ozonation have drawbacks such as sludge production and disposal, contaminants transfer from one phase to another and formation of by-products. Therefore, the removal of synthetic chemicals remains an issue of serious concern.

Along with the development in other scientific fields, the synthesis, characterization and application of engineered nanomaterials has also been revolutionized with nanoparticles being synthesized from new materials and with novel shapes and sizes. These novel nanoparticles have found application in all fields imaginable including catalytic reactions, advanced electronic devices, sensors, biomedical fields for both detection and therapeutics, pollutant detection and remediation, etc. (Sun et al. 2007). Among these wide-ranging applications, this chapter is dedicated toward environmental pollutant mitigation using nanoparticles for preventing water pollution and to help with the clean-up of already polluted water sources. For this purpose, the zero-valent nanoparticles are gaining attention due to their environmental remediation capability for various pollutants. They are used as reducing agents and are capable of transforming or degrading a variety of contaminants, which are often found in groundwater and soil. The zero-valent (nZVI)-based technologies act as reductant and are involved in various environmental oxidation–reduction reactions (Deng et al. 2020). This chapter focuses on the recent advances of nZVI and their use in the adsorption/photocatalysis for the treatment of hazardous contaminants. Further reports about immobilization of nZVI onto support and doping of ZVI with other metals are discussed.

2 Types of Nanomaterials in Wastewater Treatment

2.1 Nanoscale Zero-Valent Iron (nZVI)

The zero-valent iron (nZVI) nanoparticles have a typical core–shell configuration. The core is predominantly made up of zero-valent or metallic iron, with the mixed valent [i.e. Fe(II) and Fe(III)] oxide shell forming the metallic iron oxides. As iron is commonly found in the environment as iron(II) and iron(III) oxides, whereas nZVI is a synthesized material, so far, uses of nZVI have mostly centered on its electron-donating capabilities (Mortazavian et al. 2018). nZVI is moderately reactive in water under ambient conditions and can serve as a good electron donor, making it a flexible remediation material. The nZVI serves as a reducing agent, supplying electrons directly to the pollutant for degradation or assisting activities that require electrons for degradation. The direct reactivity of zero-valent iron with groundwater pollutants can provide an abiotic degradation pathway.

The reactivity and mobility of nZVI are greatly influenced by its size, surface, capping substance, oxide layer or support material, and manufacturing method. The technique of synthesis chosen is also determined by the application needs of nZVI (Galdames et al. 2020). Because of its high surface energy and magnetic characteristics, bare nZVI are extremely reactive, estimated to be 10–1000 times more reactive than granular ZVI (Vilardi et al. 2019). nZVI is a moderate reducing reagent with a typical reduction potential of 0.44 V (Khuntia et al. 2019). It is also affordable and non-toxic. nZVI can oxidize organic contaminants in the presence of oxygen dissolved in water. It reacts with O2 to form H2O2 in the first reaction. As a result, produced hydrogen peroxide is reduced to water by ZVI or can react with Fe2+, resulting in hydroxyl radicals.

2.2 Immobilization of ZVI onto Supports

Immobilization of ZVI nanoparticles onto different support systems has been explored in many of the previous studies. This immobilization improves the performance of ZVI nanoparticles as the aggregation and reduction of ZVI could be prevented or at least reduced due to their immobilization. Moreover, immobilization provides stable sites for reaction to take place which is better for the removal of these pollutants. The materials on which ZVI was immobilized were different such as carbon, carbon nanotube, activated carbon, granular carbon, fly ash, resins, polystyrene, silica, graphene oxide and different clay materials (Wu et al. 2020). Depending upon the materials, various methods were used for the synthesis and immobilization of ZVI onto these porous materials. Such porous materials are used for immobilization because they have a larger surface area, and unique structural and property-wise benefits. For example, many of these materials are good adsorbents, and many have good photocatalytic and oxidation–reduction properties which can itself be beneficial for any wastewater treatment. Following are some of the examples of ZVI immobilization in different materials along with the methods used.

Immobilization of ZVI nanoparticles within ion exchange resin was carried out with two polymers (polystyrene resins) composed of the same backbone but different surface functional groups, i.e. N–S and Cl–S (Jiang et al. 2011). For the immobilization of ZVI in the ion exchange resin, beads of two different polymeric resins were prepared and added to the ferric chloride solution. The ferric chloride solution contained hydrochloric acid for N–S beads, and for Cl–S beads; the ferric chloride solution was in ethanol–water (2:3 by volume) mixture. After shaking the beads in respective solutions, the solution was decanted and following washing with alcohol to remove water suspended in sodium borohydride solution to reduce entrapped iron to Fe0. This final step was carried out in an ultrasonic bath, and following the reaction beads were vacuum dried to obtain ZVI-immobilized ion exchange resin beads.

In another study, ZVI nanoparticles were incorporated into electrospun polymer nanofibrous mat for copper removal (Xiao et al. 2011). The mat was prepared using electrospinning of a polymer mixture containing polyacrylic acid and polyvinyl alcohol, and was further strengthened using multiwalled carbon nanotubes (crosslinking at 145 °C). Such prepared mat was further suspended in ferric solution to take up iron which was then reduced to Fe0 with borohydride. Such prepared material showed higher copper removal. A similar enhancement of heavy metal removal property was also reported in a study with ZVI-supported anion exchange (D201) polymer (Liu et al. 2017a). The method used for the synthesis of ZVI-supported ion exchange resin was also similar to previous studies. The ferric iron was first loaded into the resin and then reduced to zero-valent iron with borohydride. In another study, ZVI and zeolite hybrid was prepared by first mixing iron chloride and zeolite together by mechanical agitation (Zhou et al. 2015). In the subsequent step, the adsorbed ferric iron was reduced to Fe0 with sodium borohydride solution. Thus, prepared material was washed with ethanol and stored under vacuum.

Jia and Wang (2013) reported montmorillonite-supported ZVI nanoparticles to prevent its aggregation. This study found that heterogeneous nucleation is better than homogeneous nucleation for the synthesis of ZVI and its reactivity as heterogeneous nucleation produces smaller sized ZVI nanoparticles. The localization ZVI in the inner layers of montmorillonite is better for its stability than on the material’s surface, from where higher loss was reported. Petala et al. (2013) used ZVI supported on mesoporous silica (MCM-41) for hexavalent chromium removal from wastewater. In this method, the ferric chloride solution in ethanol was added to MCM-41, and the solvent was evaporated at 80 ºC. Following this step, the ferric iron bound to MCM-41 is reduced to zero-valent iron using a sodium borohydride solution. After the reaction is complete, the particles were washed with ethanol and vacuum dried before being used.

In a method to synthesize nickel-zero-valent iron immobilized biochar, paper mill sludge was converted to biochar first by pyrolyzing it at a high temperature of 700 ºC (Devi and Saroha, 2015). The ZVI was produced by reducing ferrous sulfate with sodium borohydride. The resulting ZVI was immobilized on the biochar by suspending the reactant in CTMB solution along with ZVI and stirring the solution at 1000 RPM. Further, Ni coating on the material was performed by suspending the ZVI immobilized biochar into a nickel chloride solution and sonicating it to complete the reaction. Vogel et al. (2019) reported ZVI-activated carbon complex synthesis using ferric nitrate as the iron source. In this method, the activated carbon particles were mixed in a ferric solution, dried and then converted using a thermal method by heating at 700 to 850 °C. The resulting dried material contained 20% ZVI nanoparticles and 55% activated carbon.

In a study, a different approach was taken for the immobilization of ZVI onto biochar where chitosan was used to adhere the ZVI particles (Zhou et al. 2014). In this procedure, the chitosan was dissolved in acetic acid in which the ZVI particles were suspended before biochar was added to the mixture. To this solution, NaOH was added under stirring conditions and once the reaction was completed, the excess reactants were washed before the ZVI-immobilized biochar particles could be recovered, dried and applied for experimental investigation.

Liu et al. (2017a, b) prepared ZVI immobilized fly ash-based adsorbent for the removal of lead and chromium. In this study, fly ash, bentonite, coke and iron ore tailings were mixed in a 1:2:2:2 ratio along with dried seaweed (Enteromorpha prolifera) biomass (1% by weight) in a blender for proper mixing. Cylindrical-shaped pellets were formed from this mixture, dried at 105 ºC and sintered at 900 ºC. The resulting materials were used for adsorptive removal of heavy metals from wastewater. In another study, ZVI-supported chitosan was prepared using chitosan-obtained shrimp shell wastes (Ahmadi et al. 2017). This extracted chitosan was dried and added to acetic acid solution to dissolve, to which a ferric chloride solution was added. This mixed solution was reduced by adding borohydride solution dropwise. Once the solution turned black, the reaction was stopped, and the precipitates were washed with ethanol and dried at 100 ºC.

Zhou et al. (2018) used a novel compound, i.e. waste rock wool to immobilize ZVI nanoparticles for the removal of Cr(VI). Waste rock wool was surface functionalized using hydrochloric acid. Then the activated waste rock wool was added to ferrous sulfate solution, and the iron was reduced to zero-valent iron using a borohydride solution. The resulting composite materials were washed with deionized water, alcohol and vacuum dried. Moreover, the results found that this immobilization of ZVI prevented its aggregation and improved its chromium removal performance. The waste rock wool with a higher surface area was an ideal candidate for ZVI impregnation. The addition of zero-valent iron imparted magnetic property to the adsorbent and could be easily separated from the solution.

2.3 Doping of ZVI with Other Metals

Similar to the immobilization strategy, the main reason for doping other metals, particularly transition metals in ZVI, is to prevent their agglomeration. This doping strategy also helps in preventing the ZVI to be oxidized and improving its reactivity. The doping of transition metals to ZVI improves the degradation of organic pollutants through dehalogenation by acting as a hydrogen catalyst or by producing reactive electrons. This way, the reaction rate and kinetics of pollutant degradation improve.

Lai et al. (2014) studied copper doping to zero-valent iron to form bimetallic nanoparticles in which a different Cu amount was doped ranging from 0.05 to 1.81 g per G of Fe. The study reported that the copper doping improved the catalytic activity of ZVI at a concentration < 0.89 g Cu/g Fe. The degradation of p-nitrophenol was very high (98%) at optimum conditions which was much higher than only ZVI-based reaction. In a different study, the incorporation of bismuth into ZVI nanoparticles increased its reactivity (Murtaza et al. 2019). In this method, the ferrous sulfate and bismuth nitrate solution was prepared in a water–ethanol mixture, and reduced using sodium borohydride solution. The governing mechanism for this removal was through the reduction of Cd(II) to Cd0, and the addition of bismuth improved this cadmium removal by 11%.

In order to improve catalytic degradation of hexachlorobenzene, silver doping was done to ZVI at 0.05–0.45% concentration (on weight basis) (Nie et al. 2013). The results showed that a small amount of Ag doping is effective with an optimal value of 0.09%, whereas the addition of more amount of Ag reduced the reactivity of ZVI due to covering the active site on the nanoparticle surface. In another study, nickel doping to produce nickel/iron bimetallic nanoparticles was carried out using the ball milling method (Xu et al. 2012). The results showed bimetallic particles have much higher dechlorination efficiency for 4-Chlorophenol. The nanoparticles were found to be much more stable at pH 6, with very high recyclability without losing any reactivity. In a similar work, Krishnan et al. (2021) examined the efficiency of palladium doping to ZVI nanoparticles immobilized polyurethane support for degradation of azo dye RED ME4BL. The study found the Pd-doped ZVI could effectively degrade (92% removal efficiency) the azo dye and doping improved the process efficiency.

Koutsospyros et al. (2012) performed a comparative study between copper and nickel for preparing bimetallic nanoparticles with iron to be used for the degradation of various explosive compounds (RDX, TNT, HMX, etc.). The results showed that there was no significant difference between the Cu and Ni doping on the kinetics of degradation of these pollutants, however, Fe–Cu particles were highly effective for degradation of all the compounds. Wang et al. (2022) explored copper doping to iron nanoparticles for the reactive removal of hexavalent chromium. These zero-valent Cu and Fe were introduced to montmorillonite support material for preventing their release during the experimental study. The results showed that bimetallic nanoparticles had improved reactivity with a higher amount of chromium removal. He et al. (2016a) demonstrated improved degradation of polychlorinated biphenyls by palladium-doped ZVI nanoparticles. In this study, EDTA was used as a ligand for enhancing the performance of the nanocomposite.

3 Zero-Valent Iron for Wastewater Treatment

3.1 Heavy Metal Removal by ZVI

Heavy metals such as lead, cadmium, zinc, nickel, copper, chromium and arsenic have serious detrimental effects on the environment. The sources of heavy metals in the environment are primarily industrial activity related to mining, coal burning, petroleum refinery, steel plants, etc. (Sinharoy et al. 2020a). The release of these heavy metals untreated into the surface water bodies creates hazardous effects on all the components of the ecosystem. Moreover, due to their recalcitrant and nonbiodegradable nature, they persist in nature for longer duration causing more harm to the environment (Sinharoy and Lens 2020). Hence, various strategies including chemical precipitation, adsorption, membrane separation, filtration, evaporation, reverse osmosis, electro dialysis, electro coagulation, etc. have been explored for removing heavy metals from wastewater (Sinharoy and Pakshirajan 2019; Sinharoy et al. 2020b). However, among these strategies, the use of ZVI has gained a lot of traction recently due to their high degree of heavy metal removal which can often reach complete removal (Table 1). Furthermore, due to the magnetic nature of ZVI, its separation following the treatment is easier (Wu et al. 2020). To improve the performance of ZVI for heavy metal removal, different strategies such as surface modification, or forming conjugate with porous/semiporous materials and clay minerals are also implemented (Zou et al. 2016).

Table 1 Heavy metal removal by adsorption with zero-valent iron (ZVI) and ZVI-based materials
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The mechanism involved in the heavy metal removal using ZVI nanoparticles is adsorption and reduction (Tarekegn et al. 2021). In addition, some studies that have shown oxidation, aggregation, ion exchange, etc. can also be involved in the removal of heavy metals from wastewater by ZVI nanoparticles (Zou et al. 2016; Jacob et al. 2020). Adsorption is a well-known mechanism used for heavy metal removal. ZVI nanoparticles are suited for adsorption due to their high amount of active sites and functional groups on its surface which helps in heavy metal interaction with the nanoparticles (Wu et al. 2020). There are many studies of adsorption-based removal of heavy metals using ZVI nanoparticles. Boparai et al. (2011) studied cadmium (25–450 mg/L) removal using ZVI nanoparticles with an adsorbent dosage of 500 mg/L. The results showed very high cadmium removal with 769.2 mg/g of adsorption capacity. According to the authors, the adsorption mechanism is chemisorption. Statham et al. (2015) reported excellent copper and zinc removal with ZVI nanoparticles with adsorption capacities of 2.2–4.5 and 0.15–0.28 mg/g of ZVI for Cu and Zn, respectively. Xi et al. (2010) studied the mechanism of lead removal using ZVI nanoparticles. The Pb removal obtained in this study was very high (99.9%) at optimum conditions within just 15 min of reaction along with 401.8 mg/g of adsorption capacity at 0.05 g of ZVI dosage. The XPS analysis of the ZVI samples before and after the adsorption process showed that both forms of lead, i.e. Pb0 and Pb(II) are present on the ZVI surface. These findings indicated that the removal mechanism involved rapid adsorption followed by partial reduction of lead to form lead oxide precipitate on the ZVI surface.

Reductive removal of heavy metals using ZVI is also reported in the literature. In this case, ZVI acts as an electron donor and heavy metals as an electron acceptor. But this interaction is dependent upon the properties of ZVI and heavy metals. Moreover, this reduction process can take place either directly by reaction with heavy metals or in a two-step process where first adsorption takes place which is followed by a reduction reaction. This type of reduction process reduces the valence of heavy metals to a more stable form and often mimics their reduction in a natural environment. For instance, Cheng et al. (2021) reported the reductive removal of hexavalent chromium to trivalent chromium using ZVI particles. Liu et al. (2015) reported that among the different mechanisms involved in lead removal using magnesium hydroxide-supported ZVI particles, 47% was due to a reduction reaction which was confirmed to form Pb0 elements following the reduction of Pb(II). In a similar work by Zhou et al. (2015), trivalent antimony was reduced to its elemental form to a very high degree (>80%) by ZVI nanoparticles supported on zeolite.

Liu et al. (2017b) studied copper removal using a ZVI nanoparticle supported on an anion exchange polymer (D201). In this work, the copper removal significantly improved (~50% increase) due to its immobilization onto ion exchange resin. The copper was adsorbed and precipitated onto the polymeric beads, and further reduced to Cu0 by reacting with ZVI present in the beads. Moreover, complete regeneration of the ion change beads containing ZVI could be achieved with Cu removal performance being restored to its original form.

The development of reactors capable of continuous operation is essential for the application of ZVI nanoparticles and ZVI-based materials for heavy metal removal on an industrial scale. There are few studies in the literature which have demonstrated the capability of ZVI nanoparticles under continuous operation. Among the continuous heavy metal removal setup, column experiment is the most common type of study conducted on a lab scale to analyze the heavy metal removal capacity of the adsorbent. Kishimoto et al. (2018) used an experimental setup containing a 10 mm acrylic column with 15 g of ZVI for zinc removal. The study obtained a 74% overall removal for zinc. Further study was conducted on desorbing zinc from ZVI using citric acid with recovery capacity ranging 36–77% for 0.1–10 mM citric acid concentration.

In a different strategy, Madaffari et al. (2017) constructed a column containing a mixture of ZVI and lapillus (a type of unconsolidated volcanic rocks) in different ratios for the removal of nickel. The maximum Ni removal efficiency of 99.7% was obtained at a 50:50 ratio for ZVI and lapillus. However, the next two removal values were not far behind with 99.6% (10:90 ratio) and 99.4% (30:70). The potential of ZVI is greatly highlighted in this work, as even the presence of 10% ZVI in the column filling is able to remove more than 99% of the heavy metal from the influent wastewater.

Large-scale installation with ZVI has also been reported for treating real industrial effluent. For example, a 60 m3 ZVI-based reactor was installed in Jiangxi province (China) for treating highly corrosive and heavy metal-containing wastewater from a copper processing plant (Li et al. 2017). The reactor was fitted with pH and ORP monitoring and controlling strategies. This ZVI-based treatment system was used as a pre-treatment step for removing arsenic (110 mg/L) and heavy metals. Apart from arsenic, other heavy metals present in the wastewater are Cu in large quantities (103 mg/L), and small amounts of Pb (0.35 mg/L), Sb (8.7 mg/L), Ni (0.67 mg/L), Se (4.6 mg/L) and Zn (0.38 mg/L). The performance of the reactor was excellent with more than 99.5% removal of As and other pollutants over a long period of time (>1 year).

In another study, a zero-valent iron-based treatment system was installed in a uranium mill tailings site to remove heavy metals and radioactive compounds (As, Mn, Mo, Se, U, V and Zn) from groundwater located at Durango (Colorado, USA) (Morrison et al. 2002). There were two types of configurations used at this place, one with plates with ZVI powder bounded with aluminosilicate and the other one with granular ZVI mixed with steel wool as a packing material. The treatment unit performed excellently with > 97% removal for As, Se, U, V and Zn. However, Mn and Mo showed slightly less removal with 85% and 67%, respectively. The effluent concentration of these pollutants was far less than what was present in the influent stream which confirmed the suitability of the treated water to be safely released into the environment.

3.2 Photocatalysis for Removal of Organic Pollutants

For the photocatalysis, nZVI nanoparticles, nZVI immobilized with supports and nZVI doping with metal/metalloids are used as catalysts (Table 2). The majority of organic pollutants include synthetic compounds present in wastewater from the textile, printing, leather, paper, food and pharmaceutical industries; these are hazardous even at very low concentrations (Kumar et al. 2015). There are several approaches such as biological oxidation, membrane filtration, adsorption and heterogeneous photocatalysis (Bokare and Choi, 2009) which are used for the removal of organic pollutants from an aquatic medium. Apart from nanosize, ZVI can also be synthesized in microscale sizes. However, the presence of an innate passive layer in microscale ZVI reduces their efficiency compared to nanoscale ZVIs. Moreover, the specific surface area of nanoscale particles is higher than the microscale particles due to the specific surface area being indirectly proportional to the particle size. This property increases the effectiveness of the microscale particles with better treatment efficiency (Shih et al. 2010). To exemplify this improved property of the nanoscale particles, one study reported nanoscale ZVI could be able to degrade 90% of a dye within 24 min of reaction time within which microscale particles could only degrade just 25% under the same reaction condition.

Table 2  Photodegradation efficiencies for specific pollutants achieved with nZVI-based catalysts
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Each hue of dye used in the textile industry has a distinct chemical structure. The dye molecules are great electron acceptors and the nZVI particles are good electron donors. In the aqueous medium, the nZVI particles reduce to Fe2+ and Fe3+ ions, and the hydroxyl, hydrogen ions produced during the reduction process react with dye molecules to cause the chromophore link to break (Mesa-Medina et al. 2021). To decolorize the dye molecules, the nZVI particles must also break the auxochrome link, and the ensuing intermediate organic compounds must be mineralized into CO2, H2O and inorganic ions to achieve complete degradation. It is evident that the color removal process is affected significantly by pH (Liu and Wang 2019). Due to protonation at a low pH value, the iron particles act as an electron donor, hence converting ions into atoms. Resultantly, the chromophoric group is reduced and converted into amines via intermediate formation. As the solution gets more acidic, the H+ concentration increased, thus, iron particles donated more electrons and the color removal process was enhanced.

There are studies which focus on the immobilization of nZVI onto supports, for example, Wang et al. 2016 reported tetracycline degradation using nZVI on polydopamine surface-modified biochar. As biochar is a carbon-rich pyrogenic material, it shows very high adsorption due to its high surface area and has a quite higher rate of degradation due to the presence of higher porosity to achieve enhanced adsorption capacity to organic contaminants (Mandal et al. 2020; Ahmaruzzaman 2021; Ruan et al. 2022). Further, nZVI was immobilized on the organobentonite which could rapidly and completely dechlorinate pentachlorophenol to phenol with an efficiency of 96.2%. Generally, organobentonites are produced by replacing the metal cations in bentonite interlayers with organic cations. The quaternary alkylammonium in the form of (CH3)3NR+ is used most extensively, where R is a long alkyl chain (Li et al. 2011). In addition, nZVI immobilized by Enteromorpha prolifera-based-activated carbons were applied for chloramphenicol treatment (Wu et al. 2018). The modified nZVI exhibited excellent removal capacity for 545.25 mg/g because of the synergetic effects of adsorption and degradation of the nZVI-activated carbon system. Immobilization of nZVI onto graphene matrix can restrain aggregation of iron particles, contributing to its enhanced performance toward ternary nanocomposite of magnetic nZVI/graphene–TiO2 nanowires (Wang et al. 2018). The usage of nZVI/graphene–TiO2 nanowires showed superior activity in the removal of metronidazole (99.3%) compared with TiO2 nanowires (43.0%) and graphene–TiO2 nanowires (67.6%) (Raez et al. 2021).

Heterogeneous photocatalysts are renewable, low-cost, efficient and simple-to-implement advanced oxidation processes. Chemical processes and catalyzers used in advanced oxidation processes generate free radicals. nZVI could produce the required oxygen vacant surface for absorption in the visible spectrum of electromagnetic radiation by the TiO2. The associated excess electrons on nZVI can be transferred to the surface oxygen electron of the TiO2. This may result in the delocalization of the surface electron in TiO2 and cause a significant increment in electron density. The decoloration of 88% observed for orange(II) under simulated solar light was possible in 20 min at pH 2 (He et al. 2016b). Zhang et al. in 2020 for the first time reported a nanocomposite prepared from graphitic carbon nitride and nZVI with superior catalytic activity for methylene blue removal efficiencies of 99.2% in 120 min. Graphitic carbon nitride is a metal-free polymeric photocatalyst that is very promising for utilizing solar energy with excellent structural stability, owing to its suitable bandgap of 2.7 eV. Graphitic carbon nitride doped with nZVI achieved high degradation efficiency of 98.5% for tetracycline due to the formation of a heterogeneous photo-Fenton system (Sheng et al. 2016; Wang et al. 2019). Hua et al. (2014) found that the nZVI and N co-modified TiO2 (nZVI/N–TiO2) nanotube arrays have an improved photocatalytic property under visible light. A similar report was also made by Yu et al. (2013) where the rate of phenol degradation with nZVI/N–TiO2 under visible light was higher than the rate constants obtained using different combinations of titanium oxide compounds, namely N–TiO2, nZVI/TiO2 and TiO2 under the same experimental conditions. The reason behind this observation is attributed to the fact that the electron transfer is much better in nanoscale ZVI spheres than the other compounds (Hsieh et al. 2010).

4 Current Challenges and Future Perspectives

The ZVI-based treatment systems for wastewater treatment have shown promising potential, and due to this many research and review articles have come up in recent times. However, there are a few challenges regarding its commercialization and application on an industrial scale. Firstly, most of the studies reported in the literature are conducted using lab-scale setups and controlled environments. There will be many challenges to replicating these lab-based results on a pilot or industrial scale with real wastewater. As the large variations in physico-chemical parameters of real wastewater along with many organic/inorganic co-pollutants present in real wastewater will surely affect the performance of ZVI, hence, to overcome this challenge more studies should be conducted using real wastewater and efforts should be taken to upscale the lab-based experimental setups.

Another challenge is the release of ZVI particles into the environment along with treated effluent. The ZVI like any other nanoparticle has a detrimental effect on living beings as well as on the overall environment. Hence, their release should be strictly monitored and controlled. The immobilization or entrapment of ZVI on support materials could prevent their release to a certain extent, but it could not be stopped completely. To ensure no ZVI is released from treatment plants, probably an additional setup can be installed which will help in separating ZVI from effluent after treatment. Due to the magnetic nature of ZVI particles, a setup with magnetic properties could be an ideal choice. Although this could increase the installation cost, considering the additional benefits, it will be an intelligent decision.

Agglomeration of ZVI particles is another issue that creates a lot of difficulty in their commercial application. Immobilization, doping with transition metals and coating them with different active compounds can prevent such problems. Rapid oxidation is another problem for ZVI particularly in aerobic environments, which requires special care for their storage and application. These ZVI and ZVI-based compounds work best under an anaerobic environment. Non-target specificity of ZVI during their reaction also possess a challenge in their application as in many cases some other compounds (than the target ones) can be preferentially adsorbed on its surface. Moreover, they have a very narrow pH range for their optimal reactivity. Due to this reason, pH adjustment of the wastewater need to be performed in many cases whose pH lies beyond this optimum range. Furthermore, continuous mixing in ZVI-based system is essential to bring the pollutants in contact with ZVI for their effective removal. This need for mixing can require additional energy consumption, although this could be minimal in comparison to many other conventional treatment systems.

It is also difficult to determine the exact mechanism involved in pollutant removal using ZVI particles, as in many cases a combination of different processes such as adsorption, oxidation–reduction and precipitation are involved in removing a particular pollutant. Furthermore, estimating the role of each of the individual processes in the removal of a typical pollutant is challenging, and requires sophisticated instrumentations. This could be taken up as an objective for future study. Moreover, better understanding of mass transfer aspects and reaction kinetics for ZVI-based pollutant degradation could be helpful for its application in real field scenarios.

5 Conclusion

Nanotechnology is an interdisciplinary field of study involving science, engineering and technology at the nanoscale level. Since its conceptualization in the 1950s, this field of research has come a long way and has found its application potential in almost all fields of science. Application of nanomaterials in wastewater treatment using different approaches and mechanisms has currently developed from a lab scale to a large industrial level. Among the different nanomaterials, ZVI-based systems provide a promising treatment technology for contaminated water. Treatment of both organic and inorganic wastewater has been demonstrated using ZVI particles. Due to their high surface area and high reactivity, they are utilized as an efficient tool for treating various wastewater. The high treatment efficiency achieved using ZVI for various pollutants is much more than obtained using conventional treatment, encouraging its commercial use. The magnetic properties of ZVI could provide a strategy for their separation after treatment. Moreover, immobilization on different support materials, doping with other metals and coating with active compounds can improve the performance of ZVI particles. These modifications also help in overcoming some of the drawbacks of pristine ZVIs. However, for their successful commercial application, more number of experimental studies using a pilot scale should be conducted. Furthermore, a better understanding of their removal mechanism, reaction scheme and environmental implications will help in their universal application.