Surface Modified Magnetic Nanoparticles as an Efficient Material for Wastewater Remediation: A Review

 Richa Tembekar¹, Kallol K. Ghosh^1,*, Angel Minj¹, Abhishek Katendra¹  

¹School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur-492010, Chhattisgarh, India

 

ABSTRACT

Novel methods for management of environmental quality is directly proportional to the growth of the society. This paper reviews the advanced developments in the synthesis methods, surface modifications and applications of magnetic nanoparticles (MNPs) in environmental applications such as wastewater treatment and others. Surface modifications of magnetic nanoparticles with various inorganic, organic and biomolecules such as silicon dioxide, surfactants, metals etc. enhances the properties of the nanoparticles. The present review access the use of MNPs in removing the organic and inorganic contaminants present in the water bodies. Such methods are cost-friendly, eco-friendly, sustainable and easy to access as compared to other methods or techniques. Various organic and inorganic contaminants such as dyes, heavy toxic metals, pesticides, insecticides, pharmaceuticals etc. can be adsorbed or degraded by MNPs with high removal efficiency upto >95% and recycling upto 5 cycles with a minimum time span of 15 to 25 min. The novelty of the work on surface modified magnetic nanoparticles for wastewater remediation lies in several aspects such as enhanced adsorption affinities for particular contaminants, diverse functionalizations that enables targeting wide range of contaminants, synergic effects can enhance overall remediation and magnetic recovery provides access of easy separation and minimizing waste. The need for removal of such contaminants are necessary to reduce the harmful effects on plants, human as well as to aquatic beings.

Keywords: Magnetic nanoparticles, inorganic contaminants, organic contaminants, dyes, surface modifications, toxic effects, wastewater, removal mechanism.

1. INTRODUCTION

Nanoscale magnetic materials recently gained much more interest due to their potential functionalities (Abdullaera et al., 2012). Magnetic nanoparticles (MNPs) are nanoparticles (NPs) in the range of 1-100 nm with exclusive properties, such as high surface area to volume ratio and magnetic properties (Alagiri et al., 2011). These exclusive properties make MNPs particularly strong, versatile and reactive compared to their counterparts. MNPs including iron, cobalt and nickel NPs along with their respective oxides are one of the most promising materials that can be studied using external magnetic fields (Kroll et al., 1996). MNPs based materials such as pure magnetic metals (Fe, Co), alloys (nickel alloys) and ferrites (nickel ferrite, cobalt ferrite) with high saturation magnetization are usually preferred (Mahmoudi et al., 2009). Nowadays, researchers are focusing on synthesis of MNPs with various physical and chemical modifications. MNPs exhibit excellent physical and chemical properties like low toxicity, biocompatibility, large surface area, magnetism, high reactivity, high magnetic susceptibility etc. which makes them different from other nanoparticles (Ahmad et al., 2019, Zamay et al., 2020). Some of the well-known MNPs are hematite (α-Fe₂O₃), maghemite (γ- Fe₂O₃), magnetite (Fe₃O₄), wustite (FeO), and cobalt ferrite (CoFe₃O₄) (Choudhury et al., 2013, Sun et al., 2004). MNPs exhibit the most fascinating properties such as superparamagnetism, recyclability, reproducibility and the presence of non-equivalent ions in the crystal structure (Amiri et al., 2013). Among the various MNPs iron oxides became the most popular MNPs due to their high colloidal stability and biocompatibility compared to others. Magnetic properties of MNPs depend on the magnetic susceptibility, which can be defined by the ratio of induced magnetization (I) and the applied magnetic field (H) (Rodriguez et al., 2018). The magnetic susceptibilities of these materials depend on the temperature, external applied magnetic field and atomic structures (Zuluaga et al., 2007). MNPs show high performance in terms of chemical stability, superparamagnetic behaviour and sensitivity in comparison to other nanomaterials, which enables the different applications such as biosensing, catalysis, diagnosis and purification (Zhang et al., 2019; Kreissel et al., 2021; Berger et al., 2001).

The engineered MNPs with precision have been universally explored for the different novel applications in different fields (Gill et al., 2007). The wide range of applications of MNPs is due to their unique challenging and modular magnetic properties, MNPs have many interdisciplinary applications such as data storage, optical filters, tissue-specific targeting etc (Reziq et al., 2006). As a demand of time a new trend is developing for using MNPs for food analysis, wastewater treatment, catalysis etc. due to their high selectivity, sensitivity, adsorption and photocatalytic properties (Wierucka et al., 2014, Cao et al., 2012, You et al., 2021, Huang et al., 2015). In this respect MNPs are of noticeable importance and have been applied widely in the main field of research. The characteristic properties of MNPs such as high saturation magnetization, chemical stability and capacity to work at molecular and cellular levels enables them for huge varieties of applications in biological field (Gu et al., 2006, Cui et al., 2011; Santos 2014; Wang et al., 2015; Bai et al., 2014; Li et al., 2020; Cheng et al., 2009; Hasanzadeh et al., 2015; Wang et al., 2011). A wide variety of MNPs are applied in biological applications such as cell and protein separation, biosensor, tissue engineering and magnetic resonance imaging etc. (Marcus et al., 2018; Cole et al., 2011; Cole AJ, David et al., 2011; Xue et al., 2001; Kroll et al., 1996). The properties of MNPs varies with the different morphology, constituents, formulation parameters and design (Albornoz et al., 2006).

The MNPs can be synthesized by various methods such as sonochemical, co-precipitation, hydrothermal, electrodeposition, green method etc. (Low et al., 2018, Othman et al., 2018, Mdlovu et al., 2020, Yang et al., 2016). There are various advancements for controlling the physicochemical properties of MNPs. The regardless efforts are put to design, develop and improve stable and compatible MNPs (Zhang et al., 2021). MNPs stability is the crucial requirement as the pure metal forms are very sensitive in nature. Due to the instability and extremely reactive nature towards oxidizing agents, water and humid air (Jadhav et al., 2016). Thus, stabilization of MNPs is of prime importance and this can be achieved by surface functionalization (Wei et al., 2012; Shafaei et al., 2019). Surface functionalization must be done in a controlled manner with conjugated molecules that changes the structure, morphology and surface of the MNPs. Surface modification or fabrication of MNPs is done to enhance the stability, compatibility and uniformity of MNPs (As shown in Fig. 1.). This can be achieved with different surface stabilizing groups such as organic (carbohydrates, carbon coating etc.), inorganic (metal oxides, graphene etc.), biomolecules (amino acids, vitamins etc.) and different surface-active groups (Prasad et al., 2016; Yuvakkumar et al., 2014; Rajiv et al., 2017; Behestkhoo et al., 2018). The surface modification or fabrication of MNPs improves both colloidal and physical stability of the particles, increasing water dispersibility and provide conjugation sites (Jin et al., 2014). The MNPs based composites and surface modification enhances the properties of MNPs. The structures of composite, nanoparticles made of MNPs and organic molecules such as ligands as well as nature of interaction and associated molecular structure enhances the physicochemical properties of MNPs (Arvand et al., 2017). The synthesis of MNPs composites can be achieved with the help of monomers or polymers gives the structural stability to the MNPs by generating polymeric shelf. Likewise, protection of MNPs from oxidizing and colloidal stability of MNPs can be achieved by surface coating by using polymeric stabilizers or surfactants such as dextran, polyvinyl alcohol (PVA) etc. Similarly, lipids like liposomes etc. can be used for surface functionalization which can be considered as promising candidates for tumor hyperthermia, magnetic cell separation and various biological applications (Lin et al., 2006).                              

 

Fig. 1 The scheme of MNPs design workflow

2. SYNTHESIS OF MAGNETIC NANOPARTICLES

There are various physical, chemical and biological known methods for the synthesis of MNPs and its composites. Physical method involves ball milling, vapour deposition pattern, gas phase deposition, electron beam lithography, electrical explosion of wires (Carvaeho  et al., 2013; Granata et al., 2013; Hyeon 2003). Whereas chemical method involves co-precipitation, sonochemical, hydrothermal, microemulsion, pyrolysis, sol-gel method etc. (Mohammed et al., 2017, Perez et al., 1997, Xu et al., 2007, Andrade et al., 2009, Hong et al., 2007). There are various biological methods also, for the synthesis of MNPs in which plants and microorganisms plays a crucial role (Hasanpour et al., 2007).

2.1. PHYSICAL METHODS

2.1.1. Ball Milling

One of the known physical methods is ball milling method in which large particles are crushed into ultrafine small particles without the involvement of chemical reactions (Zhang et al., 2020). For the synthesis of different MNPs, different ball milling powders are used. For iron oxide nanoparticles, wet iron powder is added, for cobalt oxide NPs, cobalt powder is added to a container along with several heavy balls and mechanical energy is applied on it with the help of high-speed rotating ball (Kurlyandskaya et al., 2014). This method is considered as one of the best methods for large scale production of MNPs with high purity.

2.1.2. Electrical Explosion of Wires

The electrical explosion of wires is the rarest method for the synthesis of MNPs. In this method intense current is provided to make the metal wire gets evaporated (Bao et al., 2016). The NPs produced by this method are very pure and spherical in shape. It is considered to be as green method as this method does not produces any toxic by-products.

2.1.3. Electron Beam Lithography

Electron beam lithography use electron beams with 10-100 keV energy per electron. In this method electron beam is used to convert the larger target atom into nano-sized materials (Wang et al., 2019). This method is one of the low costs, easily accessible and less time-consuming method to produce stable MNPs like iron oxides (Fe₃O₄, Fe₂O₃), cobalt oxide (Co₃O₄) NPs (Swihart, 2003).

2.1.4. Vapour Deposition Pattern

 MNPs can be synthesized by the formation of a continuous film and filling holes in the available templates. The deposition of particles occurs through vapour deposition techniques like evaporation, laser ablation, electrodeposition and sputtering etc. (Franzel et al., 2012; Hasanzadeh et al., 2016; Wagener et al., 1999). This method is advantageous as it can be employed for large scale production with high quality of MNPs. Chen and co-workers (Chen et al., 2002) synthesized the platinum cobalt MNPs by chemical vapour deposition method. Platinum and cobalt NPs were homogenously distributed on the carbon support, followed by thermal treatment.

 

2.2. CHEMICAL METHODS

2.2.1. Co-Precipitation Method

Co-precipitation is easy and most convenient method for the synthesis of MNPs. In this method a base like NaOH or NH₄OH is added to a solution containing mixture of metal precursors with different oxidation states for the synthesis of MNPs (Katepetch et al., 2011). Wulandari et al. (Wulandari et al., 2017) used ex situ co-precipitation method to produce chitosan coated MNPs. Ferric chloride and ferrous sulphate were used as metal precursors and ammonium hydroxide as base. Surface modification by chitosan was employed to prevent oxidation of magnetite to maghemite.

2.2.2. Hydrothermal Method

The hydrothermal method is a known method for the synthesis of MNPs. In this method the solution is enclosed in a sealed reactor under high pressure, high temperature and other reaction conditions (Ozel et al., 2015). Moreover, this method is advantageous over others as it produces MNPs with desired shape, size and crystallinity. Wu et al. (Wu et al., 2014) synthesized magnetic Fe₃O₄@C NPs by a simple hydrothermal method. Fe₃O₄ was prepared by ferrous chloride and ferric chloride and its functionalization was done by glucose as a source of carbon. The synthesized material was further studied for the adsorption of dyes.

2.2.3. Sol Gel Method

The sol- gel method includes mixing of compounds containing chemically active ingredients in a liquid phase environment (Moradnia et al., 2021). A reaction such as hydrolysis or polycondensation forms a stable transparent sol system in a solution. Sols get converted into a transparent gel and further dried or heated to prepare the NPs. Shankar et al. (Shankar et al., 2023) synthesized MNPs by sol gel by using ferric nitrate and ethylene glycol as precursors with varying the temperature. The method is environment friendly, inexpensive and produced size controlled MNPs.

2.2.4. Microemulsion Method

In microemulsion method, precipitation of MNPs from microemulsion solution takes place. In this process agglomeration, growth and nucleation occurs. In order to form the microemulsion, it is important to adjust the ratio of surfactant, oil phase and solvent phase (Salabat et al., 2018). Tonghan et al. (Tonghan et al., 2019) used a multiphase segmented flow reaction system for the synthesis of MNPs by microemulsion method.

2.2.5. Sonochemical Method

Sonochemical method is one of the best methods for the synthesis of MNPs. It is advantageous over other methods as it has very low requirements and synthesis takes place under ultrasonic sound waves (Abba et al., 2012) Garcia et al. (Garcia et al., 2020) used ultrasonic chemical reactions to rapidly synthesize MNPs with uniform particle size. The method is rapid and less time consuming. The group used FeSO₄ as precursor salt and NaOH as reducing agent in aqueous medium. The concentration of NaOH was varied throughout the synthesis to optimize the size of formed MNPs.

2.2.6. Electrodeposition Method

The electrodeposition method is the process by which a precursor is deposited onto a substrate to form a nanostructure. The reaction is usually carried out with dissolved metal ions like Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺ ions as a precursor (Tartaj et al., 2001). The preparation of MNPs has a broad application prospects. Wang et al. (Wang et al., 2012) synthesized a Ni/Fe₃O₄ composite and coated it with carbon fibers by electrodeposition method. The developed composites i.e. carbon fibers coated with Ni/Fe₃O₄ NPs composites coating exhibits higher thermal stability and saturation magnetization.

TOP DOWN AND BOTTOM UP APPROACHES OF MAGNETIC NANOPARTICLES SYNTHESIS

(A) TOP DOWN

Top down method generally refers to the approach where larger materials are converted into nanoscale materials via several synthesis methods (as shown in Fig.2)

Ø  Ball milling method

Ø  Lithography method

Ø  Etching method

(B) BOTTOM UP

In contrast, bottom up approach is a process in which nanosized materials are formed by many tiny atoms or molecules via several synthesis methods:

Ø  Sol-gel method

Ø  Self assembly method

Ø  Chemical vapour deposition method

 

Fig.2 Schematic representation of Top down and Bottom up synthesis methods

 

2.3 BIOLOGICAL METHODS

To overcome the cost, time and environmental hazards related issues, it’s an urge to ecofriendly method. The biological method is gaining much more attention as chemical requirements are very less in this method and hence does not produce harmful effects (Table 1).  Microbes and plants both offers pathways for the synthesis of MNPs (Joshi et al., 2018). Parts of the plants like barks, tissues, roots are used for the synthesis of different MNPs. This method is much more beneficial over other methods as they give better reproducibility, scalability, higher yield and controlled size of the NPs. Majidi et al. (Majidi et al., 2016) synthesized the MNPs by using various parts of plants including leaves, barks, tissues etc. The extract of plants contains reducing and stabilizing agents such as polyphenols, catechin, flavonoids, ascorbic acid, citric acid, dehydrogenases and reductases for the synthesis of MNPs.

     Table 1. Microorganisms doped for the synthesis of other metal nanoparticles.

 

2.4. COMPARISION BETWEEN PHYSICAL, CHEMICAL AND BIOLOGICAL SYNTHESIS METHODS

A comparative study was done among all the three methods - physical, chemical and biological/green methods for the synthesis of MNPs (Fig. 3.). According to the study it was found that the chemical methods for the synthesis of NPs are more suitable over physical methods, as physical method requires more manpower, high energy consumptions (Kim et al., 2013). Chemical methods also have some of the adverse effects as they are toxic, costly and sometimes causes threats to the environment as well as hazardous to human beings as well. Nowadays green or biological synthesis provides an edge over physical and chemical methods. They are more advantageous as they are nontoxic, easily available and raw materials (Table 2) (microorganism, plants) are cost friendly and eco-friendly as well. Ali et al. (Ali et al., 2017) reported biological methods are advantageous over other methods as they are obtained from natural sources like plants and microorganisms.

Fig. 3  Different methods for synthesis of MNPs  Table 2. Advantages and disadvantages of synthesis methods of nanoparticles.

SYNTHESIS METHODS	ADVANTAGES	DISADVANTAGES
PHYSICAL METHOD	ü  Less use of toxic chemicals ü  Uniform size and shape ü  Large production	ü  High energy consumption ü  Large amount of waste ü  Expensive
CHEMICAL METHOD	ü  Easy synthesis ü  Fast reaction ü  Controlled size	ü  Hazardous ü  Low purity ü  Unstable
BIOLOGICAL METHOD	ü  Biocompatible ü  Simple and facile ü  Non-toxic	ü  Poor control over size ü  Time consuming ü  Low yield

 

3. SURFACE MODIFICATIONS

The surface modification of different materials of MNPs can be done by surface fabrication or functionalization, which enhances the properties of MNPs as well as its area of applications. Different, surface coating protects the synthesized MNPs from aggregation, its further oxidation and increases its stability. Surface modifications of MNPs can be done by materials such as surfactants, silicon dioxides, polymers, metals, metal oxides/sulphides, carbon coating, biomolecules etc. (Yang et al., 2019; Tiefenauer et al., 1993; Wang et al., 2008). Some of the surface modified MNPs such as nano chitosan coated MNPs, starch coated MNPs.

3.1 Silicon Dioxide

Due to the widespread application of MNPs in the various fields, coating silica on its surface has become a research hotspot (Vojoudi et al., 2017). Non-toxic silica is a very ideal surface functional coating for MNPs, as it forms an inert external shielding layer to protect the nanoparticles. Jouyandeh et al. (Jouyandeh et al., 2018) first synthesized magnetite nanoparticles, then modified them with silicon dioxide and chitosan to finally prepare Fe₃O₄/SiO₂/chitosan nanocomposites. Therefore, various functional group modifications can be performed on the surface of MNPs for applications such as catalysis, adsorption and magnetic separation.

3.2. Surfactants

To prevent the agglomeration of MNPs, surface modifications of MNPs can be done with the help of surfactants like cetyltrimethyl ammonium bromide (CTAB), sodium dodecyl sulphate (SDS), cetyl pyridinium chloride (CPC) (Duan et al., 2020). Surfactant-functionalized MNPs can be easily divided into oil-soluble and water-soluble (Guo et al., 2020).  Heidari et al. (Heidari et al., 2016) reported a method for synthesized MNPs and its surface modification with CTAB. Maleki and co-workers (Maleki et al., 2023) synthesized SDS micelles coated Fe₃O₄/SiO₂ MNPs and studied its adsorption on crystal violet.

3.3. Metals

Another simple way to prevent MNPs from oxidation is to modify their surface with metals like gold, manganese, silver (Bradley et al., 2013). The protection of the metal layer expands the application ranges in interdisciplinary areas and its application in the field of biomedicine, environment and catalysts gradually increases. The effect of different metals on their surface brings a diversification in their properties. Wang et al. (Wang et al., 2016) successfully synthesized Au coated MNPs with different functions and hence is used for bacterial detection.

3.4. Carbon Coating

The surface modification of MNPs can also be achieved by carbon coating with the help of carbon-based materials like carbon fibers, graphene etc. Among them carbon fiber has excellent electrical properties, high strength, and low density. In particular, iron oxide NPs composite coatings have gained more and more scientific and industrial interest due to their electrical conductivity and high permeability. Qiao and co-workers (Qiao et al., 2020) used a simple and controllable method to synthesize Fe₃O₄@carbon composite microspheres. The deposition of Fe₃O₄ nanoparticle composite coating on the surface of carbon fiber fields increases its application in the field of degradation, microwave absorption, batteries, drug loading etc.

 

3.5. Metallic Oxide/Sulfides

Surface modification of MNPs can also be done by metal oxides like SnO₂,MnO₂,Al₂O₃, TiO₂, MgO etc. (Zhang et al., 2016, Banerjee et al., 2018, Wang et al., 2018) or metal sulfides like ZnS,Co₃S₄, Ni₃S₂ etc. (Li et al., 2017, Du et al., 2017, Bujnakova et al., 2017) can also act as protective shells for MNPs. Rasouli et al. (Rasouli et al., 2023) developed a ternary nanocomposite made of titaniumdioxide (TiO₂) and ferric oxide (Fe₂O₃), which can act as a photocatalysts. MNPs can also be protected by metal oxide surfaces or metal sulfides which have widespread applications in various fields. Sun et al. (Sun et al., 2018) synthesized Fe₃O₄/ZnO nanocomposites by coating zinc oxide magnetic core (Fe₃O₄) with ammonium hydroxide as the basic medium.

3.6. Biomolecules

Surface modification of MNPs with biomolecules enhances its properties as well its applications (Maltas et al., 2011) (Fig. 4.). Biomolecules such as proteins, polypeptides, antibodies, amino acids, vitamins can bind with MNPs through a certain functional group (Soshnikova et al., 2013).  Biocompatibility of the MNPs can be widely applied in biological and environmental applications. Chen and co-workers (Chen et al., 2019) synthesized L-cysteine modified magnetic mesoporous silica microsphere for endogenous recognition of glycopeptides.

 Fig. 4 Interactions of functionalized MNPs with targeting agents

4. PROPERTIES OF MAGNETIC NANOPARTICLES

MNPs exhibits various physical and chemical properties like catalytic property, magnetic property, adsorption property, optical property and others which are discussed below.

As (Fig. 5). (Mohammed et al., 2017) depicts the schematic representation of adsorption and reduction of MNPs. M⁺ stands for the metal ion and here iron in the metal which is represented in the form of Fe₃O₄ (magnetite) and ϒ-Fe₂O₃ (maghemite). The overall cycle explains two processes, firstly the transitions of M⁺ to M⁰ during the process of reduction where iron gets reduced from +1 to 0 oxidation state and secondly M⁺ to M⁺ during adsorption where no change in oxidation state takes place. In Fe⁰ condition a layer of iron oxide gets formed.

Fig. 5 Schematic representation of adsorption and reduction potential of MNPs  4.1. Plasmonic Properties:

 Optical properties of NPs arise due to the localized surface plasmon resonance (LSPR) that is associated with the oscillations of electrons, in the presence of electromagnetic radiation (Meenach et al., 2010).  It is based on the size, shape and surface of the MNPs which can be applied as optical detectors, laser sensors, solar cell, photocatalysts etc. Villegas et al. studied the photocatalysis of nitrobenzene. At 15 ppm concentration, its absorbance peak was obtained at 254 nm.

4.2. Magnetic Properties: 

Metals like iron (Fe), cobalt (Co), nickel (Ni) usually show magnetic properties. Evaluating the magnetic properties of nanopaticles is very crucial for determining their efficiency, recyclability, reusability, separation and recovery. Permanent magnetization is not seen when the paramagnetic materials are withdrawn from the magnetic fields in contrast ferromagnetic materials shows persistent magnetic behaviour when removed from magnetic fields. Their properties depend on the surface area, volume and capping agents (Chorny et al., 2007). However, some of the NPs also shows magnetic properties when they are capped with appropriate molecules, the charge localized at the particle surface gives rise to ferromagnetic like behavior. Such property leads MNPs to act as sensing devices. Khan et al. (Khan et al., 2021) reported that the magnetic properties also helps in separating the catalyst by centrifugation, filtering, or extraction processes for the treatment of wastewater. MNPs acts as a catalyst that efficiently remove various organic and inorganic contaminants from wastewater (Mcbain et al., 2007).

4.3. Electrical Properties:

Electrical properties of MNPs deal with the electrical conductivity of the NPs, nanomaterials or nanocomposites. Metals generally possess high electrical stability and excellent electrical conductivity. Due to their high conducting capacities, they can be utilized in developing electronic sensors conducting electronic devices etc. (Liu et al., 2011).

 4.4. Catalytic Properties:

MNPs play a significant role in catalysis due to their increased surface to volume ratios (Luo et al., 2008). Hence this property of MNPs helps in minimizing the harmful effects on both environment and mankind. The MNPs was synthesized by green synthesis method and the degradation of bromophenol blue was influenced by the addition of H₂O₂ under UV light.

 5. APPLICATIONS OF MAGNETIC NANOPARTICLES                                         

 MNPs have a wide range of notable applications in the environmental, industrial, agricultural, biomedical, fabrics, minerals, magnetic fluids, chemical industries and other fields such as catalysis, wastewater treatment etc. (Cui et al., 2016, Pepping 1999, Pandya et al., 2016, Kheilkordi et al., 2022, Mourdikoudis et al., 2018, Jiaqi et al., 2019) These applications highlights  the importance and versatile behavior of MNPs in the world of technology and innovations.

5.1. Application of MNPs in Wastewater Treatment

MNPs are gradually gaining attention as promising materials for environmental applications. The unique physiochemical properties of MNPs due to large surface area, ease of synthesis and inherent superparamagnetic properties which leads to their widespread applications (Weteskog et al., 2017). Like silica based nanoadsorbent removal of pharmaceutical substances etc. Wastewater has adversely affected human as well as plants and animals’ life. The treatment of wastewater is one of the serious issues that cannot be avoided (Sivakami et al., 2020). There are some pre-established methods like water treatment plants, but they are very expensive and requires manpower. MNPs have very good adsorption capacity, as well as their property enhances due to surface functionalization (Rana et al., Bui et al., 2018). As MNPs possesses large surface area for adsorption, they can be used for the removal of dyes during wastewater treatment. MNPs are widely used in the detection and removal of toxic metals like Cr, Cd etc. (Cui et al., 2014, Harkness et al., 2010).

Fig 6 Stepwise process of wastewater treatment by MNPs.

5.1.1 ORGANIC CONTAMINANTS

The cyclic process of wastewater treatment is discussed briefly in (Fig.6). (Huang et al., 2013) Surface functionalized magnetic nanoparticles are dispersed into the wastewater containing several types of contaminants. Due to its excellent superparamagnetic properties, the surface functionalized MNPs adsorbs the contaminants on the surface of MNPs and hence gets separated. On applying the magnetic field the MNPs gets detached from the contaminants and are ready to be used for another cycles for wastewater treatment.

MNPs is efficiently used for the removal of organic pollutants present in the water bodies. Organic wastes which enters the water bodies due to waste disposals from households and hospitals, agricultural activities, combustion processes, industrial activities (textile, cosmetics, pharmaceuticals, paint, leather and food industries) that leads to contamination of water (Mollarasouli et al., 2021, Li et al., 2018, Jung et al., 2011, Chen et al., 2017). Pollutants such as dyes, oil, pesticides, fertilizers and other phenolic compounds are common which leads to environmental pollution especially water contamination (Fig. 7.) . Organic dyes are highly toxic, carcinogenic in nature as well as creates many harmful effects on human beings and threat to aquatic lives (Table 3.). Huang et al. (Huang et al., 2015) synthesized iron nanoparticles (Fe-NPs) for the degradation of malachite green (MG) and hence studied the conditions impacting on its reactivity. Finally degradation study showed that 90.56% of MG was removed using Fe-NPs. Aydin et al. (Aydin et al., 2021) synthesized simple and cost effective method for removal of psychiatric drugs from wastewater. The group studies the adsorption capacity of magnetite red mud nanoparticles (RM-NPs) for effective removal of carbamazepine from wastewater treatment plant effluents. Nawara et al. (Nawara et al., 2012) studied the adsorption of doxorubicin drug on the citrate stabilized magnetic nanoparticles. They synthesized the citric-acid-stabilized magnetic nanoparticles with very good magnetization behaviour. They reported a novel method utilizing a ternary system for the determination of interactions between drug and citric-acid-stabilized nanoparticles. Weng and co-workers synthesized iron-based nanoparticles from green tea extract. As green tea contains polyphenols and catechins which acts as reducing agent in the synthesis. The synthesized GT-MNPs was later studied for the degradation of malachite green (MG). The pH, initial concentration of MG, the dosage of GT-MNPs and the reaction temperature was also investigated. It emerged that 96% of MG was removed with a 50 mg/L at 298 K (Weng et al., 2013). Kinetics studies showed that the removal of MG fitted well to the pseudo first-order mode. Islam and his group (S. Islam et al., 2020) studied the antimicrobial activity of citric acid functionalized iron oxide nanoparticles and their superparamagnetic effects on it. They synthesized the iron oxide nanoparticles by sol gel method. These magnetic nanoparticles are functionalized with different concentrations of citric acid such as 0.1 M, 0.2 M, 0.3 M, 0.4 M and 0.5 M. 0.3 M concentration showed super paramagnetic behaviour. Iron oxide functionalized with citric acid concentration of 0.3 M resulted in high saturation magnetization of 85emu/g with hydrodynamic diameter size almost equal to 25 nm. Thus it was confirmed that 0.3 M concentration of citric acid functionalized nanoparticles  appeared to be beneficial for antimicrobial activity. Atta et al. (Atta et al., 2020) synthesized poly (ionic liquid) functionalized silver and magnetite nanoparticles. Magnetite nanoparticles were prepared with protic poly(ionic liquid) based on a quarternized diethylethanolamine cation combined with 2-acrylamido-2-methylpropane sulphonate-co-vinylpyrrolidone (QAMPSA/VP) as a capping and reducing agent. The kinetics of the catalytic reduction of MB with QAMPSA/VP-Ag NPs was investigated using UV–vis spectroscopy. The intensity of the MB band at 662 nm disappeared completely after 12 minutes and it was also noticed that the colour of MB  changed from blue to colourless. (Table 4 and 5).

Fig 7. Classification of Dyes.

Fig. 7 Classification of organic dyes

Table 3. Toxic dyes and their adverse effects in human body.

DYES	CLASS	TOXIC EFFECTS IN HUMAN BODY
Alizarin Red S	Anthraquinone	Mutagenic, carcinogenic, causes oxidative damage
Azocarmine B	Quinone-imine	Allergic reactions, vomiting, carcinogenic, poisonous
Bromophenol Blue	Triaryl methane	Respiratory tract infection, skin irritation, carcinogenic
Congo Red	Diazo	Genotoxic, teratogenic, mutagenic, carcinogenic
Crystal violet	Triaryl methane	Chromosomal damage, respiratory and renal failure, digestive tract disorders
Eosin B	Fluorone	Nausea, chest pain, skin irritation, dizziness, rapid heart rate
Eriochrome Black T	Azo	Cytotoxicity, skin irritation, respiratory issues
Erythrosine	Azo	Allergies, neurotoxicity, DNA damage behavior, carcinogenic
Fluorescein	Fluorone	Hypotension, skin inflammation, renal failure, pulmonary edema, nerve palsy
Indigo	Indigoid	Allergic reactions, vomiting, intestinal problems
Malachite Green	Triaryl methane	Liver damage, spleen damage, tumors in lungs and ovary
Methylene Blue	Thiazin	Respiratory disorders, overactive reflexes Central nervous system failure, dermatological issues
Methyl Orange	Azo	Vomiting, gastrointestinal irritation, respiratory tract infection
Phenolphthalein	Phthalein	Abdominal pain, allergic skin rash, dizziness, digestive tract or respiratory tract irritation
Rhodamine B	Rhodamine	Liver dysfunction, pre-mature birth, kidney  damage, cardiovascular diseases
Rhodamine 6G	Rhodamine	Skin irritation, blindness, respiratory tract infection, carcinogenic
Safranin	Azo	Liver infection, kidney damage, respiratory tract irritation
Thionin	Thiazin	Itching, skin rashes, fast heart rate
Vat Green 1	Anthraquinone	Skin irritation, eyes irritation, carcinogenic, allergic reactions

 

 

Table 4. Surface modified MNPs and their applications in organic dye removal. S.No.	Name of MNPs	Contaminants removed	Removal mechanism	Removal efficiency	Contact Time	Amount	Reusability/ Recyclability	References
1.	Starch coated MNPs	Rhodamine B dye	Photocatalytic degradation	97%	30 min	10 mg/ 3 ml	Upto three cycles for 92% removal	Sharma et al., 2019
2.	Copper doped ZrO2 MNPs	Methyl orange dye	Photocatalytic degradation	98%	100 min	-	Up to four cycles for 90% removal	Reddy et al., 2020
3.	Epoxy-Triazinetrione- Functionalized MNPs	Malachite green	Adsorption	95%	15 min	-	Up to six cycles for 61% removal	Nejad et al., 2020
4.	Ionic liquid coated MNPs	Rhodamine B	Adsorption	91%	14 min	-	Upto three cycles for 55% removal	Chen et al., 2016
5.	Combination of Fe2O3 and Fe3O4	Bromophenol blue dye	Photocatalytic degradation	98%	60 min	5 mg/L	Up to three cycles for 95%  removal	Fatimah et al., 2020