Magnetic nanomaterials assisted nanobiocatalysis to abate groundwater pollution

Abstract

Magnetic nanoparticles are of great interest for research as they have a wide range of applications in biotechnology, environmental science, and biomedicine. Magnetic nanoparticles are ideal for magnetic separation, improving catalysis's speed and reusability by immobilizing enzymes. Nanobiocatalysis allows the removal of persistent pollutants in a viable, cost-effective and eco-friendly manner, transforming several hazardous compounds in water into less toxic derivatives. Iron oxide and graphene oxide are the preferred materials used to confer nanomaterials their magnetic properties for this purpose as they pair well with enzymes due to their biocompatibility and functional properties. This review describes the most common synthesis methods for magnetic nanoparticles and their performance of nanobiocatalysis for the degradation of pollutants in water.

• •Magnetic nanomaterials have been synthesized for their application in nanobiocatalysis and treating groundwater.
• •The most used method for magnetic nanoparticle preparation is the co-precipitation technique.
• •Peroxidase and oxidase enzymes have great potential in the remotion of multiple contaminants from groundwater.

Graphical abstract

Specifications table

Subject area:	Environmental Science
More specific subject area:	Ground water
Name of your Method:	Literature review
Name and reference of original method:	N.A.
Resource availability:	Scopus, Web of Science

Introduction

Water pollution is an environmental problem of worldwide concern that has been exacerbated by rapid industrialization, expanding economies, and growing populations. Moreover, most of the wastewater derived from human activities is continuously released into the aquatic environment with partial or without proper processing [1]. Therefore, concerning concentrations of an ample range of contaminants have been detected in groundwater, surface water, as well as the effluents of wastewater treatment plants. The United Nations have vocalized the importance of solving this environmental issue in the Sustainable Development Goals [1]. Thus, urgent action is required from a prevention, detection, and remediation perspective.

There are two interesting approaches for remediation, whole-cell and isolated-enzyme biocatalysis (Fig. 1), which have been employed for the removal of pollutants in an efficient, safe, and cost-effective manner [2,3]. However, their potential is still limited due to their many disadvantages including side reactions, enzyme deactivation, cross-reactivity, and reduced recyclability, among others [4]. Enzyme immobilization onto appropriate supports has been presented as a suitable solution to overcome those barriers [4,5]. Typically, enzymes can be immobilized through adsorption, crosslinking, covalent, and entrapped methods [6]. In this manner, different materials have been used as supports for the fabrication of biocatalytic systems including metal-organic frameworks, polymers, and silica materials [7].

Fig. 1.

Schematic illustration of various methods for the design and development of biocatalytic processes. Reprinted from [4] with permission from Elsevier. License Number: 5,381,661,443,297.

The development of nanostructured materials and the synergistic innovative interaction between biocatalysis and nanotechnology led to the fabrication of effective adsorbents and nanobiocatalysts, in which the nanomaterials play a fundamental role [8]. In this manner, nanobiocatalysts have been designed and employed for a variety of applications such as biofuel production, extraction of bioactive ingredients, and degradation of pollutants [4,9]. Their potential for the degradation of complex pollutants relies on their advantageous features like specificity, durability, efficiency, and enhanced performance. Moreover, magnetic nanomaterials provide additional benefits such as a facile magnetic recovery, which enables good reusability [10]

In this review, the most recent studies on magnetic nanobiocatalysis for water remediation are discussed in terms of their synthesis methods as well as contaminant removal performance. Finally, a comparative discussion of the advantages and limitations of the use of different nanostructures is presented.

Method details

The most common methods of treating dye-contaminated water are coagulation, adsorption, and oxidation. In more recent years, advanced oxidation processes have been adopted such as UV radiation, UV and ozone treatment, membranes, flocculation as well as biological treatments [11]. At present the costs associated with using these methods are high. Adsorption on materials such as bentonite, sawdust, chitosan, zeolite and clay are often the preferred methods due to their simplicity, low cost and ease of performance [11], [12], [13]. However, adsorption requires further treatment before disposal, which is why there is a need for new removal methods that degrade dyes into less harmful compounds [14,15]. One proposed strategy is the use of biocatalysts such as enzymes that are already found in nature and have been proven to successfully remove a variety of contaminants. Enzyme based treatments have the advantage of being ecofriendly, fast, economic, and respond to a wide range of recalcitrant dyes as well as form byproducts that are less toxic [2,3,16]. Enzymes from the oxidase family such as peroxidase, laccase, glucose oxidase and phenol oxidase have demonstrated great potential for the removal of multiple contaminants such as dyes. Laccase (EC 1.10.3.2) is an enzyme widely found in fungi, insects, and plants. Its structure contains copper, and this enzyme has been used to degrade phenolic as well as nonphenolic dyes [17]. Nevertheless, using the enzyme in its free form exposes it to inhibitors and prevents its recyclability [11]. As a result, many studies have explored different conditions of immobilization. Peroxidases are glycoproteins with a heme group that oxidize organic and inorganic compounds such as phenols, cresols, and synthetic dyes. They are abundant in nature and are already used in many industries such as medicinal, industrial, and biotechnological sectors. They have the advantage of being resistant to extreme conditions and have a wide substrate specificity [16].

Iron oxide can be used to create magnetic nanoparticles (MNPs) that facilitate their separation from a reaction mixture, allowing their use in multiple cycles to increase contaminant removal efficiency. Iron oxide can be paired with Manganese peroxidase (MnP) to degrade dyes. The resulting immobilized enzyme possesses a high catalytic activity, stability, reusability [16]. In recent years carbon nanomaterials have gained interest due to their high surface area and biocompatibility. Graphene oxide (GO) has excellent properties such as biocompatibility, water solubility, and functionalization amenability due to its sheet the oxygen containing functional groups in its surface. GO is a material composed of carbon atoms that confer it the advantage of not altering the biomolecules it is attached to [19,20]. In recent years, graphene oxide-based materials have been extensively used in drug delivery, biocatalysts, and bio-sensing. When paired with an enzyme, GO aids catalysis due to its increased surface and conductivity. The use of enzyme functionalized GO allows for simultaneous dye sorption and degradation [20]. Additionally, when functionalized with iron oxide, GO can also possess magnetic properties, allowing for easy separation when using an external magnet [21]. The co-precipitation method is the most common mixed oxide preparation as well as for magnetic nanoparticles as it is straightforward, cost-effective, fast, and scalable [22,23].

Synthesis of carbonaceous materials

GO can be synthesized by dissolving graphite powder in a 2:1 mixture of HSO₄ and HNO₃, adding KClO₃ and leaving it to react for 16 h at room temperature. The resulting GO must be washed with distilled water until the pH reaches 6.5 [18].

The most used method for magnetic nanoparticle preparation is the co-precipitation technique [11,21,24]. In order to create iron oxide magnetic nanoparticles, 40 g of Fe (SO₄)₂·7H₂O (53 mmol) are solvated in 280 mL of deionized water and stirred at 90 °C. Subsequently, 3.24 g of KNO₃ (16 mmol) and 22.4 6 g of KOH (200 mmol) dissolved in 120 mL of deionized water are added to the previous solution and left to react at 90 °C for 1 h. The resulting nanoparticles can be removed by magnetic separation, washed with deionized water, and then dried at 60 °C overnight in a vacuum drying oven [16]. The co-precipitation technique is similar when using GO. Its method consists of dissolving a FeCl₃·6H₂O and FeCl₂·4H₂O in a molar ratio of 2:1 in degassed H₂O, and then adding this mixture to 40 mL of (5 mg/ml) GO under vigorous stirring and in a nitrogen environment. After this, 30 mL of 25% tetramethylammonium hydroxide solution are added dropwise at a constant temperature between 80 and 85 °C and left to react for 45–60 min (for higher temperatures, less time is needed). The magnetic nanoparticles can be separated using a magnetic field. Finally, the precipitate is rinsed three times with Milli-Q water [21]. In the case that the magnetic nanoparticles will be stored for future use, they can be dried in a vacuum oven or in an oven at 45 °C for 12 h [11,20,24]. Enzyme immobilization is often undergone through physical binding by dissolving magnetic nanoparticles in the presence of the enzyme and left to interact at 4 °C or through covalent binding though diamines or (3-Aminopropyl) triethoxysilane (APTES) [16,18,20].

Some other nanomaterials have been explored such as magnetic metal-organic frameworks (MMOF), magnetic carbon nanotubes and magnetic nanosheets as well as magnetic nanofibers. MMOFs can be created through the modification of magnetic nanoparticles by adding citric acid in a dropwise manner (500 mg MNPs in 50 mL DI and 2.5 mL citric acid 2.0 M) [11]. The reaction occurs at 90 °C under nitrogen. Then, the modified MNPs are added to 30 mL of a 50% ethanol-water solution with HCl (0.05 mmol) and Zn (NO₃)₂·6H2O (0.5 mmol). After 15 min, 10 mL of a solution of 50% ethanol with 2–3 mM methylimidazole and 100 mg of PVP (MW 4000) and 100 mg of laccase. Finally, the MMOF are washed with (0.1 M, pH 5.0) acetate buffer [11]. One study synthesized carbon nanotubes (CNT) through the thermal chemical vapor deposition (CVD) method. They made these CNTs magnetic by dissolving with triethylene glycol (TREG) in water and adding 20 mg of Fe(acac)₃. The solution is then moved to Teflon lined containers and incubated in an oven at 278 °C for 8 h. The resulting magnetic carbon nanotubes (mCNT) are collected with a magnet, washed three times with ethanol and then dried in a vacuum oven at 50 °C. The density of iron oxide nanoparticles on the nanotubes can be adjusted through the ratio of Fe(acac)₃ and CNT [25]. Magnetic nanosheets can be created using carbodiimide chemistry by binding using the carboxyl groups of NTA after binding it to MGO [21]. Poly (methyl methacrylate)(PMMA) magnetite nanofibers can be synthesized using electrospinning by dissolving PMMA in HFP to create a 12% (w/v) solution and then adding 200 mg of magnetite nanoparticles. After stirring for 3 h at room temperature, the material is electrospun using a 75  mm/sec moving head, a voltage of 20 kV, a feed rate of 1.0 mL h⁻¹ and a distance between needle tip and collector of 100 mm. The resulting fibers are then dried for 48 h at 27 °C in a vacuum dryer [24].

As shown in Table 1. Multiple studies immobilized laccase such as in a study where the authors synthesized nanosheets with Cu²+ and Ni²+ to immobilize CotA laccase through metal affinity adsorption. They presented a high adsorption capacity of their material and evaluated the decolorization efficiency before and after immobilization [21]. Another report used laccase and peroxidase mimicking MMOF. The kinetics of the MMOF with and without enzymes were studied using methylene blue and crystal violet in batch and continuous reactors [11]. A study covalently immobilized Trametes versicolor laccase (TvL) on functionalized graphene oxide through covalent linking. They compared the kinetic parameters of free and immobilized enzymes on textile industry effluents [18]. A paper detailed the immobilization of MnP on iron oxide nanoparticles for the removal of various dyes. The authors report the enzyme kinetic parameters as well as the reusability of the construct and the stability of the immobilized enzyme under different temperatures and pHs. The resulting construct was optimized for maximal removal efficiency [16]. Out of the studies discussed in the table, one used a novel enzyme from sheep rumen metagenome called PersiManXyn1. The enzyme was immobilized on magnetic GO nanosheets. This construct was very stable and efficient in removing dye in the presence of NaBH₄ [20].

Table 1.

Types of nano biocatalysts, with their nanoscale component and synthesis method.

Nanomaterial	Nanostructure	Immobilized enzyme	Pollutant	Synthesis method	Efficiency (%) and Time of degradation	Removal conditions	Refs.
Magnetic graphene oxide, and NTA-NH2a	Nanosheets	Laccase	Congo Red	Co-precipitation method	100%, after 5 hb	60 °C and pH 8	[14]
MMOFsd	Nanoparticles	Laccase	Textile dyes	Co-precipitation method	95%, after 30 minc	55 °C and pH 3.5 to 4.5	[9]
Graphene oxide	Nanoassembly	Laccase	Textile dyes	Co-precipitation method	71.25 to 88.65%, less 60 minc	65 °C and pH 6	[11]
PMMA/Fe3O4f	Nanofibers	Laccase	Tetracycline	Co-precipitation method	100 to 94%, in 30 minc	25 °C and pH 5	[15]
Fe3O4f nanoparticles, and magnetic carbon nanotube	Nanocomposite	Horseradish peroxidase - laccase	Malachite green and acid orange 7 (AO7)	Chemical vapor deposition	85% for acid orange 7 and ∼90% for malachite green, after 20 minc	25 °C and pH 7.4	[16]
Graphene oxide, Fe3O4f, Au g, and citric acid	Nanoparticles	Horseradish peroxidase - laccase	4- Chlorophenols	One-step strategy	98%, after 180 minc	25 °C	[12]
Fe3O4f	Nanoparticles	Manganese peroxidase	Direct red 31 and Acid black 234	Co-precipitation method	92% for acid black 234 and 100% for direct red 31, after 24 hb	35 °C and pH 4.5	[10]
Magnetic graphene oxide	Nanocarrier	PersiManXyn1 (mannase and xylanase)	Methylene blue	Co-precipitation method	∼100% in 2.5 minc	50 °C and pH 8	[13]

^aNTA-NH2: N, N-Bis(carboxymethyl)-l-lysine hydrate.

^bh: hours.

^cmin: minutes.

^dMMOFs: metal organic frameworks

^ePMMA: poly (methyl methacrylate).

^fFe₃O₄: iron oxide.

^gAu: gold.

There were a variety of nano components employed such as the novel electrospun PMMA material with magnetic nanoparticles. PMMA has the advantage of being porous, stable, magnetic and can be functionalized. The authors that used PMMA also studied the degradation of tetracycline as well as the reusability of the system reported [24]. Another study created a reusable nanocatalyst for the removal of acid orange (AO7) and malachite green (MG). Their nanoflowers with iron oxide nanoparticles and mCNT were used to immobilize laccase and horseradish peroxidase. The use of magnetic carbon nanotubes facilitated their removal for subsequent reuse as well as increased dye removal, possibly by physical adsorption [25]. Another study synthesized a flower-like construct of graphene oxide with citric acid and gold or iron oxide pistils to remove 4-chlorophenols when hydrogen peroxide is in proximity. Their synthesis strategy used polyols to allow for a simple, environmentally friendly one step creation of their structures. The flower-like constructs were both magnetic and hydrophilic. Finally, the coupling time and enzyme concentration's impact on immobilization efficiency was studied as well as the reaction time and number of cycle's effect on 4-chlorophenols removal [19].

Magnetic nanobiocatalysts for pollutants removal

In recent times, nanobiocatalysts have been developed for their application in contaminated water owing to easier separation, higher reusability, and higher removal efficiency [26]. For instance, a paper mentioned the preparation a magnetic nano-support consisting of NH₂-modified silica-coated Fe₃O₄ nanoparticles which were used to immobilize horseradish peroxidase (HRP) (Fig. 2). The authors achieved a high enzymatic activity by optimizing the immobilization conditions. Additionally, the material had a higher resistance to temperature and pH variations in comparison to the free enzyme. The nanobiocatalyst was employed for the removal of 2,4-dichlorophenol, with a maximum removal of around 80% within 180 min of reaction. The nanobiocatalyst was stable after reuse, retaining around 85% of original activity after four cycles [27].

Fig. 2.

Magnetic nanomaterials as supports in nanobiocatalytic approaches.

Similarly, graphene oxide/Fe₃O₄ nanoparticles were used to immobilize HRP for the removal of phenol and 2,4-dichlorophenol (Fig. 2). Moreover, polyethylene glycol was added to prevent the inactivation of HRP. The synthesized nanobiocatalyst was able to remove 2,4-dichlorophenol almost completely, whereas around 70% of phenol was degraded. Interestingly, when both pollutants were mixed, the phenol removal increased to 94% since 2,4-dichlorophenol facilitated the removal of phenol. Satisfactory results were obtained by the nanobiocatalyst in terms of thermal stability and removal efficiency; However, only 40% of the initial enzymatic activity was retained after four cycles (Table 2). Therefore, further research is needed to improve reusability for practical application in large-scale systems [28].

Table 2.

Performance of different magnetic nanobiocatalysts for the degradation of pollutants in water samples.

Nanomaterial	Nanostructure	Immobilized enzyme	Pollutant	Synthesis method	Efficiency (%) and Time of degradation	Removal conditions	Refs.
NH2−Modified Magnetic Fe3O4a/SiO2b	Nanoparticles	Horseradish peroxidase	2,4-dichlorophenol	Co-precipitation	∼80%, after 180 min d	30 °C and pH 6.4	[17]
Graphene oxide/Fe3O4a	Nanoparticles	Horseradish peroxidase	2,4-dichlorophenol and Phenol	Co-precipitation	∼100%, for 2,4-dichlorophenol and ∼70%, after 180 min d	25 °C and pH 7.4	[18]
ZnO c nanowires/macroporous SiO2b composite	Nanowires	Horseradish peroxidase	Acid Blue 113 and Acid black 10 BX	Hydrothermal technology	95.4% for Acid blue 113 and 90.3% for acid black 10 BX, after 35 min d	30 °C and pH 7 for acid blue 113, and pH 3 for acid black 10 BX	[19]
Amino-functionalized ionic T liquid-modified magnetic chitosan nanoparticles	Nanoparticles	Laccase	2,4-dichlorophenol, Bisphenol A, Indole and Antracene	Co-precipitation	100% for 2,4-dichlorophenol, after 240 min d; 100% for Bisphenol A, after 72 min d; 70.5% for Indole, after 72 min d; and 93.3% for Antracene, after 72 min d	35 °C and pH 3.5	[20]
Chitosan-functionalized super magnetic halloysite nanotubes	Nanoparticles	Laccase	Direct Red 80	Co-precipitation	87%	30 °C and pH 4.2	[21]
Amino-functionalized magnetic MMOFs f	MMFOs	Laccase	2,4- dichlorophenol		87%, after 12 h e	25 °C	[22]

^aFe₃O₄: iron oxide.

^bSiO₂: Silicon dioxide.

^cZnO: Zinc oxide.

^dmin: minutes.

^eh: hours.

^fMMOFs: metal organic frameworks.

HRP has been immobilized on a wide diversity of nano-supports. In this regard, Sun et al. immobilized HRP in ZnO nanowires/microporous SiO₂ composites for the degradation of dyes (Fig. 2). The nanomaterial selection and synthesis provided a large pore size in the microporous silica increasing diffusion, positive charges and surface area of ZnO nanowires. In this manner, the inorganic support played a fundamental role in accelerating the reaction by attracting the pollutants and HRP together. As a result, the material removed 90.3% of Acid black 10 BX and 95.4% of Acid blue 113 after 35 min of reaction [29].

Similarly, laccases have been immobilized onto magnetic nano-supports for pollutant removal. In a study a nanobiocatalyst formed by laccase, ABTS, and magnetic chitosan nanoparticles was prepared for the removal of 2,4-dichlorophenol [30]. Even at high concentrations (50 mg/L), the pollutant was completely removed by the nanobiocatalyst, which also exhibited satisfactory reusability. The formed nanobiocatalyst exhibited a high enzyme loading, enhanced enzymatic activity, higher resistance to temperature and pH variations, and enhanced storage and thermal stability in comparison to the free enzyme. In addition, the pollutant removal capacity was extended to several pollutants including bisphenol A, anthracene, and indole; all of which showed good degradability by the nanobiocatalyst. The authors concluded that ABTS promoted the degradation of pollutants by accelerating the electron transfer between the laccase and the substrate [30]. Another laccase-based nanobiocatalyst was recently employed for the degradation of dyes where the enzyme was immobilized onto chitosan-functionalized supermagnetic halloysite nanotubes to obtain a nanobiocatalyst with excellent reusability capacity and outstanding pH and thermal stability. The nano-support was able to immobilize a high enzyme load, which achieved a Direct Red 80 degradation efficiency of 87% [31].

Metal-Organic Framework (MOF) has also been applied for the preparation of nanobiocatalysts [32]. As a representative example, amino-functionalized magnetic MOF was used to immobilize laccase exhibiting high immobilization capacity, high recovery, and excellent resistance to temperature and pH variations. The good stability was demonstrated by the 89% enzyme activity retained after storage for 28 days. 2,4-dichlorophenol was successfully removed with an efficiency of 87% after reaction; the result was explained by the excellent degradation capacity of laccase and the outstanding adsorption capacity of MOF [33]. One of the most important aspects to determine the success of a water treatment process is its reproducibility. To improve the reproducibility of magnetic absorbents and promote their regeneration Dominguez et al. designed and automatic detection system for the determination of organophosphate insecticides in real water samples base on Sequential injection analysis (SIA) technology with a retention mechanism of magnetic nanospheres (300 nm, Ademtech Kit) [34]. Thus, the implementation of process with nanomagnetic materials could be improved with techniques that generate continuous and reproducible processes.

Conclusions

There is a wide range of sources that allow pollutants enter the environment. As a result, groundwater quality decrease by compounds from agriculture, animal waste, untreated wastewater or landfill leachate. Techniques combined with magnetic nanomaterials have effectively replaced traditional process for the determination and depletion of pollutant in water due to their low consumption of solvents, rapid clean-up, selectivity, simple automation, and high throughput. The use of magnetic nanoparticles in biocatalytic approaches has a positive impact on remediation of water due to their properties of separation through external magnet. The immobilization of enzymes on this kind of support enhances their performance, efficiency, stability and allow their reusability in the degradation of emergent contaminants in groundwater. Graphene and Iron oxide-based materials are favorable for enzyme immobilization and keep higher rate of efficiency after several cycles compared with other materials. Degradation effects on dyes, heavy metals have been awarded to functional groups of graphene. In future near the synthesis of carbonaceous materials needs optimizations to face the challenges for widespread application and the removal of multiple contaminants under real conditions and deal with obstacles for its commercialization and utility on large scale for industrial application. Additional work to developing smarter and advanced materials with less toxicity and environmental impact is required to apply the systems towards green technologies.

CRediT authorship contribution statement

Lizeth Parra-Arroyo: Writing – original draft. Reyna Berenice González-González: Writing – original draft, Data curation, Software. Rocio A. Chavez-Santoscoy: Writing – review & editing, Investigation. Elda A. Flores-Contreras: Writing – review & editing. Roberto Parra-Saldívar: Writing – review & editing. Elda M. Melchor Martínez: Supervision, Writing – review & editing, Methodology. Hafiz M.N. Iqbal: Conceptualization, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data Availability

• No data was used for the research described in the article.