Metal Oxide Nanoparticles’ Green Synthesis by Plants: Prospects in Phyto- and Bioremediation and Photocatalytic Degradation of Organic Pollutants

Abstract

Over the past few decades, the production of metal oxide nanoparticles (MONPs) has developed into an exciting and sophisticated research area. Green metal oxide nanoparticles have played an extremely imperative role in various fields, including biomedical, environmental, energy, agricultural applications, catalytic, bioactive, antibacterial, poisonous, and biocompatible. To achieve sustainability and adopt environmentally friendly practices, the production of MONPs is now increasingly focused on exploring green chemistry and alternative pathways. When made using green synthesis techniques, the metal oxide nanoparticles are especially important because they do not require external stabilizers, capping agents, dangerous chemicals, or harsh operating conditions (high pressure and temperature). Plant-mediated synthesis of different MONPs using either whole cells or extracts has several advantages, including rapid synthesis (compared with other biogenic processes (using fungi and bacteria)), being more stable than other types, being available in nature, and being non-toxic. This review provides a comprehensive overview of the green synthesis of MONPs using plant parts, factors affecting the synthesis, and the characterization of synthesized NPs. Additionally, it highlights the potential of these environmentally friendly nanoparticles that are widely used to treat environmental pollutants, including the removal of heavy metals, antibacterials, and the degradation of organic pollutants.

1. Introduction

For the development of green technology, science and technology have advanced most quickly [1,2,3,4]. The word “nanomaterial” (NM) refers to materials that have a size between 1 and 100 nm, at least in one dimension, and distinct properties in terms of porosity, form, size, and other functions [5,6]. It has gained lots of attention mainly owing to its electrical, magnetic, chemical, distinctive optical, and mechanical properties that set it apart from its bulk counterparts. The dimensions and shape of a nanoparticle play a key role in determining its essential characteristics and properties, including its optical, electrical, catalytic, bioactive, antibacterial, poisonous, and biocompatible traits [7,8,9].

Metal oxide nanoparticles are primarily synthesized using physical and chemical methods. However, these techniques involve the use of highly reactive and toxic reducing agents like sodium borohydride and hydrazine hydrate, which have harmful effects on the environment, plants, and animals. Furthermore, these methods require sophisticated equipment, complex procedures, and strict experimental conditions, posing significant challenges. Consequently, physical methods result in the production of heterogeneous nanoparticles with high energy consumption, while chemical methods rely on synthetic agents for capping, reducing, and stabilizing and generate non-eco-friendly byproducts, although they yield the desired homogeneous metallic nanoparticles. Additionally, the nanoparticles produced through physical and chemical processes cannot be used for medical purposes due to their adverse effects on human health [3].

Green synthesis has become a more popular term to describe these biological techniques. The term “green synthesis” refers to methodologies, approaches, and processes that synthesize nanoparticles with the least amount of energy consumption while avoiding harmful byproducts through regulation, control, remediation, and cleanup. Waste prevention and minimization, a decrease in byproducts and pollution, the use of greener (or nontoxic) solvents and auxiliaries, and renewable feedstock are the guiding principles for green synthesis [10]. Numerous methods of green synthesis of nanoparticles include mild reaction conditions, mild reducing agents, microwaves, microbes, solar energy, and ultrasound methods [11]

Many biological sources, including yeast, fungi, bacteria, actinomycetes, microalgae, seaweeds, waste aquaculture, plant extracts, disposed-of food products, and horticultural food materials [12], have already been used as substances to produce steady and well-functionalized nanomaterials [13,14,15] (Figure 1). Plant parts, such as leaves, roots, etc., are considered an environmentally friendly and dependable method for producing metal oxide nanoparticles, and synthesizing them is a popular choice for eco-conscious synthesis [16,17]. Also, biological methods have gained much interest lately due to their simple nature, ease of usage, and ecological safety. Alkaloids, phenolic acids, terpenoids, polyphenols, and polysaccharides are a few phytochemicals found in the extracts of plants that function as bioresistant and capping agents in the biogenic production of metal oxide nanoparticles [18,19].

Plant-based synthesis of green MONPs has gained popularity as a phrase to designate these biological methods for the advancement of green technology. Recently, scientists have put a lot of effort into developing chemical processes that are reliable, simple, and environmentally friendly. Sundrarajan et al. [20] produced titanium dioxide nanomaterials (TiO₂NPs) from Morinda citrifolia leaf extract. SEM, XRD, FTIR, and EDX approaches were applied to characterize the produced nanomaterial. Interestingly, 10 nm-sized average crystallites of tetragonal rutile phase TiO₂ were obtained. TiO₂NPs have antimicrobial properties with more efficiency.

Using Annona squamosa leaf extract as a bioreductant, a noble ZnO-NP synthesis has demonstrated intriguing antibiotic and anticancer activities with a focused action by securing the natural microbiota [21]. In another study, Madan, et al. [22] conducted research into the eco-friendly production of multifunctional zinc oxide nanoparticles with diverse morphologies in the hexagonal wurtzite structure. To achieve this, they employed a low-temperature solution combustion method by utilizing the extract of Azadirachta indica (Neem) leaf as a model, used as an antibacterial and photocatalytic for organic pollutants.

The study by Momeni et al. [23] employed a regenerative and nontoxic leaf extract of Euphorbia prolifera as a stabilizing agent and a mild reducing agent. This plant was used to produce Cu/ZnONPs with a size range of 5–17 nm. When used to break down dyes such as Congo red and methylene blue in their aqueous solutions while being exposed to NaBH₄, the produced composite nanomaterial demonstrated outstanding catalytic activity. Using Aloe barbadensis leaf extracts with lowering and capping potential produced ZnO-NP as an efficient, cost-effective, and biogenic fabrication technique [24].

Heavy metals, organic dyes, organic wastewater, and other organic pollutants have been found in various harmful forms in the environment in recent years [25,26,27,28]; they are difficult to eradicate using old technologies, which necessitates the development of new advanced technologies [29,30].

Due to their minimal environmental risks, plant extracts are the primary focus of this study’s recent breakthroughs in the biosynthesis of MONPs. MONPs are nanoscale particles that come from anthropogenic and natural processes and contain both artificially created and naturally occurring particles [31]. As a result, they are better suited for usage in several applications, for example, lithium-ion batteries, sensors, catalysis, and environmental ones. Green MONPs are also widely employed as adsorbents, photocatalysts, and antimicrobials to remove various contaminants [32]. As a result, this work aims to encourage green chemistry through the quick, inexpensive, and ecologically friendly green synthesis of metal oxide nanoparticles by employing plant extracts; a relatively new technique that results in reliable green chemistry and highlights the use of such synthetically created ecologically friendly metal oxide nanoparticles, particularly in wastewater treatment.

2. Green Synthesis Definition

The adoption of green fabrication as an alternative to traditional chemical methods of producing MONPs is gaining momentum as an environmentally friendly option. Green synthesis includes technologies, strategies, and procedures aimed at reducing energy consumption and eliminating harmful by-products through regulation, control, processing, and cleaning. Utilizing ecologically friendly or non-toxic solvents and additives, as well as renewable raw materials, is the fundamental tenet of green synthesis, together with waste reduction and minimization [10]. Because reduction and capping agents are needed through the synthesis methods, metallic nanoparticles can be made more effectively utilizing green synthesis approaches. Aside from cyanobacteria and plant parts (fruit, root, flower, and leaves), natural sources for the ecologically friendly synthesis of nanoparticles include actinomycetes, bacteria, yeast, fungal organisms, and cyanobacteria [33]. As a reducing medium to create nanoparticles and as a capping medium to stabilize those nanoparticles, the whole cell of an algae, a microbe of a plant, or an extract can be employed [34]. It is necessary to produce chemicals and molecules based on “green synthesis” principles, which call for environmentally friendly procedures. Green chemistry adheres to the 12 green chemistry principles that were previously discussed, which is why it has been given that name [35].

3. Metal Oxide Nanoparticles

Nanomaterials are elements having at least one dimension between 1 and 100 nm, with distinct properties in terms of pores, dimensions, form, and other elements [36]. The reaction between a metal cation and oxygen gas results in the creation of metal oxides. Metal oxides are a fascinating group of inorganic substances that have been widely investigated and studied due to their diverse forms and properties. Transition metal oxide nanoparticles are particularly significant because of their attractive electrical, optical, and magnetic properties. These inorganic materials, known as metal oxides, have piqued the interest of researchers, attributed to their diverse forms and properties. Transition metal oxide nanoparticles, in particular, possess attractive electrical, optical, and magnetic properties, which make them well-suited for an extensive range of uses, including environmental applications, catalysis, lithium-ion batteries, and sensors. As a result, there is a growing need to explore their unique features and properties in various applications [31]. Metal oxides have bonds that can be virtually ionic, extremely covalent, or even metallic, and synthesis is more complicated than pure metals [37]. Similarly, MONPs are used in a variety of sectors, including magnetic storage media, catalytic processes, sensors, electronics, and solar energy conversion, thanks to their varied characteristics. Among various metal oxide types, TiO₂, Fe₃O₄, CuO, and ZnO are the most widely researched for the synthesis of MONPs using plant-based methods [38].

4. Factors Affecting the Synthesis of Inorganic Nanoparticles

pH, exposure duration, reaction time, temperature, and stirring rate are a few variables that affect the synthesis of NPs. The number of bioactive compounds in the plant extract is another consideration. The synthesis of nanoparticles as well as their shape, properties, size, and uses are all influenced by these variables [39].

4.1. pH

The form and size of the MONPs produced may change in the reaction’s pH. The buffer strength (also known as pH), which controls NPs’ shapes and sizes, must remain constant throughout the NPs’ generation procedure. Larger particles are created by comparing lower acidic pH values to higher acidic pH values [13]. In contrast, a high pH environment favors the growth of spherical NPs, and synthesis is excellent for the synthesis of small-sized MONPs. The surface plasmon resonance (SPR) peak develops wide and deflects towards a longer wavelength region, which results in a wide range of NPs (usually triangular and cylindrical shaped), compared with synthesizing small-sized NPs when the pH is low [40].

4.2. Temperature

Temperature increase has shown catalytic behavior by boosting the reaction rate and efficiency of nanoparticle synthesis while using biological entities to formulate nanoparticles. Several chemical processes, such as electrochemical, solvothermal, or templating procedures, are significantly influenced by the reaction’s temperature. In general, temperatures of close to 100 °C are required for the synthesis of NPs utilizing green techniques. While chemical synthesis requires mild temperatures, physical reactions require temperatures higher than 350 °C [41]. The average size of the group shrinks as their conversion rate rises. Researchers have postulated that a crucial enzyme included in the synthesis of nanoparticles may have high temperatures [13].

4.3. Concentration of Extracts

Constituents of plant extracts serve as both reductants and stabilizers of generated nanoparticles. The number of bioreducing agents in a plant extract influences the characteristics of nanoparticles by regulating the particle’s size and shape. High plant extract content accelerates nanoparticle synthesis by increasing nanoparticle nucleation. As the concentration of the reducing agent in the plant extract increases, the nucleation rate increases. However, smaller nanoparticles are produced by rapid synthesis rates at high extract concentrations [42].

4.4. Reaction Times

The time taken for a reaction plays a vital role in the synthesis of nanoparticles. In fact, conducting the same experiment while varying the reaction time can lead to variations in the size of the particles produced. The period during which metal salts are exposed to a plant extract is referred to as the “contact time” and is used as a measure of the reaction duration. The concentration of the plant extract also has a noteworthy influence on both the reaction and contact time. Plant extracts containing higher concentrations of reductive groups tend to facilitate faster nanoparticle formation [42].

4.5. Stirring Speed

The stirring speed plays a crucial role in regulating the rate and size of nanoparticle synthesis. Bhattacharya et al. [43] revealed that, when the stirring speed was increased to 900 rpm, the size of CuO NPs decreased from 42.54 nm at 500 rpm to 3.6 nm. Moreover, the smaller size may be accredited to the bigger nanoparticles being broken up into smaller ones at a faster stirring speed. Increased nanoparticle synthesis rate brought on by uniform distribution of metal salts and bioreductant species of the plant extract is another benefit of faster stirring. Homogeneous distribution speeds up the reduction of metal salts to nanoparticles by enhancing the interaction among species of bioreductant and metal salts [11].

4.6. Choice of Species

The cost-effectiveness of NP biosynthesis depends on the physical–chemical factors listed above, as well as (a) choosing the right organism or species based on crucial intrinsic traits, including metabolic pathways, growth rate, and enzyme activity; (b) the amount of its inoculum; and (c) the choice of biocatalysts, which are vital to speed up the rate of reaction (i.e., reduction) (although this should be taken into account) [3]. Enzymes, raw cells, or whole cells can all be employed as biocatalysts (e.g., NADPH, FAD). Because coenzymes are expensive, they can be recycled along the pathway, and they also have a large amount of effectiveness [44].

5. Various Techniques for Synthesizing MONPs

MONPs can be created using either a “top-down” process or a “bottom-up” strategy. A top-down strategy is used to manufacture MONPs by breaking up bulk materials into tiny particles and reducing their thickness [45]. Also, bottom-up synthesis (referred to as “self-assembly”) is a method of producing nanoparticles from smaller units, including atoms, molecules, and smaller connecting unit particles. In contrast, the top-down technique involves breaking down larger structures using manufacturing methods, such as cutting, grinding, and shaping, to achieve the desired nanoscale dimensions [46]. Contrarily, the bottom-up approach uses chemical interactions to create nanoscale structures [47]. For this procedure, physical and chemical techniques can be utilized, including mechanical methods (such as crushing and grinding) [45], sputtering [48], chemical etching [48], thermal evaporation [49], pulsed laser ablation [13], and photoreduction methods [50]. The top-down technique has a key disadvantage in that the surface structure is not perfect [51].

In order to mitigate toxicity and enhance the effectiveness of chemically synthesized nanoparticles (NPs), scientists have utilized plant metabolites as adjuvants. This approach involves utilizing biological extracts derived from microorganisms or plants as reducing agents to convert metal ions into nanoparticles. This reduction process can occur either outside the cell (extracellularly) or within the cell (intracellularly). By employing plant metabolites in this manner, researchers aim to improve the safety and efficiency of the synthesized nanoparticles. The extract components operate as capping agents and assist in producing homogenous stable MONPs by inhibiting aggregation [11]. This synthetic method is used to create MONPs that are biocompatible, scalable, non-toxic, repeatable, and helpful in medicine on a big scale. Furthermore, it is anticipated that metabolites consumed during the reduction process will adhere to the surface of NPs and boost activity [48]. The manufacture of NPs using biological entities is simple (one-pot synthesis), economical, quick, and environmentally friendly [52]. Nanoparticles have a basic advantage over biological ones in that their size and form can be controlled, and they can be produced in greater numbers utilizing chemical and physical processes [53]. The plant parts that follow will describe how to synthesis MONPs utilizing a variety of biological materials found in nature.

6. Biosynthesis of MONPs by Plants

Due to its environmentally friendly nature, using plant systems for the synthesis of MONPs has been hailed as a “green” method and a dependable technique [54]. To synthesize MONPs through biogenic means, various phytochemicals found in plant stems, leaves, and flowers, including polyphenols, alkaloids, tannins, terpenoids, phenolic acids, and polysaccharides, serve as both reductants and capping agents [55]. Many areas of plant parts can collect heavy metals. As a result, methods for the synthesis of nanoparticles using plants have gained popularity, as they are rapid, efficient, practical, and economical. They are a fantastic substitute for traditional preparation methods (Figure 2). In a “one-pot” synthesis approach, a variety of plants can be used for the reduction and maintenance of metallic nanoparticles. Proteins, polysaccharides, and coenzymes are examples of biomolecules that plants can use to transform metal ions into nanoparticles. The synthesis of nanoparticles is based on several factors, including the type of plant extract used, the quantity of the extract and metal salt, temperature, pH level, and time of contact. Plant nanoparticle synthesis is cost-effective and environmentally friendly and it is safe for humans. In another study, Sharma et al. [56] utilized the Neem, Azadirachta indica, leaf extract as a reducing agent in the synthesis of nanoscale tetragonal hausmannite crystals of Mn₃O₄. This study created a very effective chemical sensor for identifying the presence of 2-butanone using the generated nanomaterial as a working electrode.

By reducing the thermal decomposition temperature of ammonium perchlorate to 175 °C in a single decomposition phase, the sensor was able to function effectively. The study by Al-Ruqeishi et al. [57] used polyphenols contained in Omani mango (disambiguation) tree leaves to create a green synthesis of Fe₂O₃-NP nanorods. The average diameter and length of the Fe₂O₃-NPs generated were 15 nm and 3 nm, respectively. The study by Kumar et al. [58] described the production of zinc oxide nanoparticles using grapefruit extract. ZnONPs had a size range of 12–72 nm (TEM) and functioned well as an antioxidant and photocatalyst in the methylene blue degradation process (>56% efficiency for photocatalysis). The information that is currently available on the size, characterization, and applications of some MONPs that are produced using different plant extracts in a green manner is reported in

Table 1. The synthesis of MONPs by different plants.

Species	Plant Portion	MONPs	Dimensions (nm)	Form	Refs.
Cynodon dactylon	Leaf	TiO2	13–34	Hexagonal	[59]
Azadirachta indica	Leaf	Mn3O4	18.2	Spherical	[60]
	Leaf	ZnO	9.6–25.5	Hexagonal wurtzite	[61]
Moringa oleifera	Leaf	ZnO	12.27–30.51	-	[62]
	Peel	CeO2	45	Spherical	[63]
Kalopanax pictus	Leaf	MnO2	19.2	Spherical	[64]
Artocarpus hetero-	Leaf	ZnO	15–25	Hexagonal wurtzite	[65]
Allium sativum	Bulb	ZnO	14–70	Hexagonal wurtzite	[66]
Petroselinum crispum	Leaf	ZnO	14–70	Hexagonal wurtzite	[66]
Artocarpus gomezi- anus	Fruits	ZnO	11.53	Spherical, hexagonal wurtzite	[67]
Asparagus racemosus	Root	Fe2O4	30–40	Spherical	[68]
Chlorophytum comosum	Leaf	Fe3O4	100	-	[69]
Punica granatum	Seed	Fe3O4	25–55	Semi-spherical	[70]
Avicennia marine	Flower	Fe3O4	30–100	Honeycomb	[71]
Cornelian cherry	Fruit	Fe3O4	29–37	Spherical	[72]
Borassusflabellifer	Seed	Fe3O4	10–40	Hexagonal	[73]
Laurus nobilis	Leaf	Fe3O4	8.03 ± 8.99	Spherical and hexagonal	[74]
Punica granatum L.	Leaf	Fe3O4	21–23 nm	-	[75]
Annona squamosal	Peel	TiO2	23 ± 2	Spherical	[76]
Plectranthus amboinicus	Leaf	ZnO	50–180	Rod shape	[77]
Plantain peel	Peel	Fe3O4	<50	Spherical	[78]
Saraca indica	Flower	SnO2	2–4	Circular	[79]
Phyllanthus niruri	Leaf	ZnO	25.61	Hexagonal wurtzite	[80]
Carica papaya	Leaf	CuO	140	Rod shape	[81]
Aspalathus linearis	Leaf	SnO2	8.23	-	[82]
Cassia fistula	Leaf	ZnO	5–15	Hexagonal wurtzite	[83]
Hibiscus Sabdariffa	Flower	CdO	113	Cuboid shape	[84]
Abutilon indicum	Leaf	CuO	16.78	Hexagonal wurtzite	[85]
Leucas aspera	Leaf	CeO2	4–13	Uniform microspheres	[86]
Brassica oleracea	Leaf	SnO2	3.62–6.34	Quasi-spherical	[87]
Amorphophallus	Root	ZnO	19.6	Rice	[88]

.

7. Mechanism of Nanoparticle Synthesis by Plant Extract

There are two methods commonly employed for the synthesis of metallic nanoparticles: intracellular and extracellular.

(a)

The extracellular technique, which utilizes plant extract, is often preferred for nanoparticle synthesis due to its simplicity in handling, cost-effectiveness, and the absence of an additional step required to disrupt cells in order to recover internal nanoparticles for subsequent processing. In the extracellular method, metal salts are converted into metal nanoparticles using a suspension of plant extract. Plant extracts are abundant in various bioactive compounds, which serve as both reducing agents and capping agents during nanoparticle production. These compounds play a crucial role in reducing metal ions and stabilizing the resulting nanoparticles [89]. Metal nanoparticles are formed through the reduction of metal salts using bioactive substances released by plant cells in an extract medium. The process involves two stages: nucleation and crystal growth. During the nucleation stage, precursor metal salts (M⁺) are transformed into metal nanoparticles (M⁰) through a reaction facilitated by a component present in the plant extract. This reaction initiates the formation of small clusters of metal atoms that serve as the building blocks for the nanoparticles. In the crystal growth stage, the metal nanoparticles (M⁰) undergo complexation with other metal salts (M⁺), resulting in the formation of M⁰-M+ complexes. This complexation process enables the nanoparticles to grow in size. Additionally, the nanoparticles undergo adsorption, where they bind together to form nanocrystals, often in the form of metal nanoparticle dimers. Overall, the combination of nucleation and crystal growth processes using bioactive substances from plant extract allows for the production of metal nanoparticles [90].

(b)

In intracellular synthesis, metal salts are reduced within the cells through the assistance of various enzymes such as nitrate reductase and NADH- and NADPH-dependent enzymes. These enzymes are involved in essential metabolic processes like photosynthesis, nitrogen fixation, and respiration. Additionally, plant pigments such as chlorophyll also contribute to the intracellular reduction process [90]. Both living and dry biomasses of plants are capable of performing intracellular synthesis of nanoparticles. This process occurs as a defense mechanism of plant cells against stress conditions induced by metals. When exposed to metal ions, the metabolic machinery of plant cells is redirected to mitigate the toxic effects of metals. As a result, the cells utilize their inherent enzymatic systems and pigments to reduce metal salts and produce nanoparticles intracellularly [91]. The process of nanoparticle formation inside cells involves the binding of positively charged metal ions to the surface of the cell wall or to negatively charged protein or enzyme groups within the cytoplasm. Once the metal ions are trapped, they undergo reduction to form small nuclei, which serve as the starting point for nanoparticle synthesis. This process can lead to the production of nanoparticles with various morphologies. In the case of algal cells, there is an intracellular enzyme present that facilitates the internal conversion of metal salts into nanoparticles. This enzyme plays a crucial role in the reduction process, enabling the transformation of metal ions into nanoparticles within the cellular environment [91].

8. Characterization of MONP Plant Synthesis

Characterization refers to the process of describing a nanoparticle’s properties after its synthesis, including its size, capping agents, shape, crystallinity, surface plasmon resonance, associated functional groups, polydispersity, and surface charge. These physical and chemical properties are essential in designing nanomaterials for specific practical applications.

Table 2. Techniques for MONPs’ characterization.

No.	Studied Properties (Information)	Characterization Techniques
1	Elemental–chemical composition	NMR, SEM-EDX, ICP-MS, XRD, XPS,
2	Shape	TEM, AFM
3	Chemical state–oxidation state	XPS
4	Size	TEM, XRD, DLS, SEM, ICP-MS, UV-Vis
5	Crystal structure	XRD
6	Size distribution	DLS, ICP-MS, SEM
7	Growth kinetics	NMR, TEM
8	Surface composition, arrangement, density, ligand binding	NMR, FTIR, TGA, XPS
9	Surface area	NMR, BET, Zeta potential
10	Agglomeration state	TEM, UV-Vis, Zeta potential, SEM, DLS
11	3D visualization	AFM, SEM
12	Dispersion of MONPs in matrices	AFM, TEM, SEM
13	Detection of NPs	TEM, SEM
14	Optical properties	UV-Vis, PL

displays the different methods used to characterize MONPs for these properties, and the following descriptions outline the techniques used to determine each feature [92].

9. Adsorption Method for the Elimination of Various Pollutants

Adsorption methods are applied most often to remediate contaminated liquids. Adsorption methods are used for removing organic and inorganic pollutants from aqueous solutions, with the aggregation of metal ions onto adsorbent pores and surfaces creating a layer that includes ions as other impurities [93].

For a number of reasons, adsorption is preferred over traditional metal removal methods. Adsorbents are inexpensive and synthesis of them is economical. They also have the following benefits: (1) no sludge is produced, (2) they are regenerative, i.e., the adsorbent can be reused after the pollutant ions have been removed, (3) they use less energy and chemicals, (4) there may be metal recovery, and (5) their performance is competitive, i.e., their achievements are comparable (Figure 3) [9]. Parameters that influence the sorption efficiency include the adsorbate–adsorbent interaction, adsorbent surface area, temperature, contact time, amount of adsorbent, initial pollutant concentration, particle size, pH, etc. [11,94].

9.1. Application of Green MONPs Using Plant Extracts in Water Treatment

9.1.1. Removal of Heavy Metals

Due to the simplicity of modifying their surface functionality and their high surface area-to-volume ratio, MONPs have enhanced adsorption capacity, efficiency, and reusability. In addition, several researchers have investigated the use of MONPs for heavy metal adsorption [95]. According to Srivastava et al. [96], green ZnONPs show a good removal efficacy for Cd(II). Within an hour of contact time, 92% of Cd(II) was removed when the adsorbent dosage was 200 mg L⁻¹. Therefore, ZnONPs might be used to successfully remove cadmium metal from effluent wastewater.

Additionally, Ehrampoush et al. [97] investigated the fabrication of spherical Fe₂O₃-NPs with an average size of 50 nm by tangerine peel extract in order to eliminate Cd(II) from wastewater. At a pH of 4 and an adsorbent dosage of 0.4 g 100 mL⁻¹ during a 90 min contact period, the peel extract of Fe₂O₃-NPs effectively removed cadmium ions by about 90%. Fazlzadeh et al. [98] studied the synthesis of ZnONPs by Peganum harmala seed extract. The nanoparticle offered by the authors was for the removal of Cr(VI) from effluent water, and it displayed 97.9% elimination efficiency at 1000 mg L⁻¹ initial concentration of Cr(VI), pH 2 in 30 min, and 2 g L⁻¹ of the MONPs.

Table 3. Removal of different pollutants using biosynthesized MONPs.

Plant Extract	Green MONPs	Pollutant Elimination	Initial Conc. (mg L−1)	Contact Time (min)	pH	Amount NPs (g L−1)	Removal Efficiency (%)	Sorption Capacity (mg g−1)	Ref.
Peganum harmala	ZnO-NPs	Cr(VI)	1000	30	2	2	97.6	74.7	[98]
Aloe vera	α-Fe2O3-NPs	As(V)	2–30	-	6–8	0.5	-	38.4	[99]
Casein	ZnO-NPs	Cd(II)	500	30	7	2	85.6	156.7	[100]
Tangerine peel	Fe2O3-NPs	Cd(II)	15	90	4	0.4	88.7	-	[97]
Zingiber zerumbet	ZnO-NPs	Pb(II)	25	60	5	0.1	93	19.6	[101]
Ananas comosus	Fe3O4-NPs	Cd(II)	60	10	6	0.1	96.1	49.1	[102]
Emblicao fficinallis	ZnO-NPs	As(III)	0.002	60	5	2.5	96.7		[103]
Calotropis procera	CuO-NPs	Cr(VI)	5	120	2	18.8	99.9		[104]
Jatropha curcas	TiO2-NPs	Cr(VI)	-	-	8.2	-	76.5	-	[105]
Psidium guajava	CuO-NPs	Nile blue					93		[106]
Ziziphus spina-christi	CuO NPs	CV dye		7.5 min			95		[107]
Microbac-terium sp	NiO	Ni(II)		120	7	2.172	95		[108]
Mint leaf extract	CuO	Pb(II) Ni(II) Cd(II)	20	60 min	6	0.5		88.8 54.9 15.6	[109]
Punica granatum leaf extract	CuO	SO dye	-	180 min	6.1	1		189.5	[110]

presents the removal of heavy metals and dye pollutants using biosynthesized MONPs.

The study by Somu and Paul [100] investigated the synthesis of ZnONPs on a case-by-case basis to simultaneously purify wastewater of heavy metals, dyes, and harmful bacteria. The ZnONP, at pH 8.0, displayed the highest adsorption efficiency of 95.35% for Pb(II), 85.63% for Cd(II), and 71.23% for Co(II). The dye degradation was 93.72% for Congo red at pH 6.0 and 93.58% for MB at pH 7. Adsorption loss for loaded ZnONPs after five cycles was only 10–12% for both colors and metals, indicating that the ZnONPs were reusable when the adsorption–desorption methods were repeated five times. The green synthesis of cupric oxide nanoparticles (CuONPs) arbitrated by Calotropis procera latex was used for removing Cr(VI) from aqueous solutions. For the experiment, initial Cr(VI) concentrations and pH were 5 and 2 mg L⁻¹, respectively [104].

While Srivastava et al. [96] revealed that ZnO NPs had a high removal efficiency for Cd(II) using 20 mg L⁻¹ of adsorbent dose, the removal efficiency was ~55%; upon increasing the dose to 200 mg L⁻¹, a 92% removal of Cd(II) within 1 h of contact time occurred. Thus, the ZnO nanoparticle could be successfully used for the removal of Cd from effluents. Debnath et al. [111] prepared ZnO nanoparticles by a green synthesis route from Hibiscus rosa-sinensis leaf extract for the removal of Congo red (CR) dye from an aqueous solution. The maximum adsorption capacity was 9.615 mg/g, with a high percentage of removal efficiency of 95.55%. Through a green approach, Nayak et al. [112] synthesized ZnONPs from Ocimum sanctum (Tulsi leaf) leaf extract as an adsorbent for the removal of Congo red (CR) dye. The removal capacity of ZnO-T was higher (97%) than commercially available ZnO (78%), and the maximum adsorption capacity of ZnO-CT was 74.07 mg/g. Furthermore, Mittal et al. [113] synthesized ZnO nanoparticles through an in situ process by using gum Arabic grafted polyacrylamide (GA-Cl-PAM). The GA-Cl-AAM-ZnO composite was used as an effective adsorbent to take up malachite green (MG) dye from an aqueous solution. The maximum adsorption capacity was 766.52 mg/g, with 99% high adsorption efficiency.

9.1.2. Fluoride Removal

One of the most severe health issues in the world is the fluoride pollution of drinking water. Excessive intake of fluoride leads to the depletion of calcium from the tooth matrix, leading to the formation of cavities and the eventual development of dental fluorosis. With prolonged exposure, skeletal fluorosis may also occur. Fluoride adsorption is greatly favored by metal oxide nanoparticles’ increased surface area [114]. Given their strong adsorption capacity, constrained water solubility, non-toxic composition, and favorable desorption potential, MONPs are regarded as promising materials.

Many different NP types, including Fe₃O₄@Al(OH)₃ and Ce-Ti@Fe₃O₄, have been utilized to remove fluoride from aqueous solutions because of their large surface area, self-assembly, strong reactivity, specificity, and recyclable nature. The former has a high adsorption rate, whereas the latter has a maximum adsorption capacity (q_max = 91.04 mg g⁻¹) at pH 7. Also, the author mentioned that, after five cycles of usage, there was no discernible loss in adsorption capacity. Composite Fe₃O₄@Al(OH)₃ nanoparticles combine the advantages of magnetic separation and Al₂O₃, because aluminum oxide has a large affinity for fluoride [115]. Furthermore, the study by Chai et al. [116] developed separated Fe₃O₄/Al₂O₃ nanoparticles with sulfate doping for the removal of fluoride from drinking water. Nearly 90% of the adsorption could be accomplished in about 20 min, and only 10–15% of additional elimination happened in 8 h. By using the Langmuir model, the nano-adsorbent had a fluoride absorption capacity of 70.4 mg g⁻¹ at pH 7.0.

The study displayed by Silveira et al. [117] discovered that fluoride ion adsorption was beneficial, with maximum adsorption at pH 7. They created FeONPs by employing Moringa oleifera leaf extract as a model for the removal of fluoride ions from wastewater. With an adsorption potential of 1.40 mg g⁻¹ and an adsorption kinetics test, equilibrium was reached in 40 min. FeONPs can be reused three times according to the regeneration process.

In another study by Kumari and Khan [118], Simmondsia chinensis (jojoba) defatted meal was used to synthesize Fe₃O₄ for defluoridation applications in drinking water. However, in the pH scope of 5.0–9.0, the F⁻ removal percentage decreased. While F⁻ adsorption decreased at alkaline pH levels because of competition among F⁻ ions and hydroxyl ions for active surface sites, F⁻ removal decreased at acidic pH levels owing to the formation of hydrofluoric acid (HF). At an 80 min contact time, it was found that 93% of the fluoride had been removed. The Fe₃O₄ NPs’ polyurethane foam (PUF) exhibited a greater water defluoridation capacity of 34.48 mg g⁻¹.

9.2. Photocatalytic Activity of MONPs

The term “photocatalysis” derives from combining “photo” and “catalysis,” where “photo” refers to light and “catalysis” refers to modifying a chemical reaction using a catalyst [119]. Photocatalysts are catalysts that become active when exposed to light radiation and can convert pollutants in the air or water to less toxic compounds, such as water and CO₂. When exposed to visible sunlight, photocatalysts can degrade organic molecules similarly to semiconductors. A hole is created in the valence band because of excitably moving an electron from the valence band to the conduction band [120,121,122].

In the presence of oxygen and water, the holes and electrons created during this process can generate anionic superoxide and hydroxyl radicals, which are capable of decomposing organic molecules [123]. To be effective, a photocatalyst must have a bandgap energy that is less than 3 eV so that it can absorb light in the visible spectrum. Either homogeneous or heterogeneous photocatalytic development is possible, based on the physical characteristics of the catalyst and the interacting species [124].

9.2.1. Advantages of Nano-Photocatalysts

The reduction or elimination of harmful organic compounds is mostly aided by nano-photocatalysts. The main benefit of nano-photocatalysts is related to the quantum size effect, which widens the energy bandgap (Eg) and reduces the size of the nanoparticles. Therefore, there are many benefits to using the photodegradation approach, including generally full deterioration, cheap cost, and being reusable. Despite all these developments, toxicity and catalyst recovery remain issues for nano-photocatalysts. The main advantages and disadvantages of the several physicochemical treatments discussed in this work are reported in

Table 4. The main advantages and disadvantages of nano-photocatalysts [125].

Advantages	Disadvantages
Lower toxicity	Long duration time
Excellent photoactive properties
Availability
High chemical stability	Limited applications
Low cost
Eco-friendliness

.

9.2.2. Factors Affecting the Photocatalytic Degradation

(a) 

Amount of Catalyst

In general, as a catalyst dosage is increased, there is an increase in the quantity of photomineralization of water pollutants. This is because the photocatalyst has more active sites exposed to light, which absorb more photons and generate more ^•OH radicals and positive holes when exposed to light. The rate of pollutant breakdown accelerates as a result of these positive holes and hydroxyl radicals taking part in the photocatalytic activity. The majority of the solar light is passed directly from the solution when utilizing a lower dose catalyst, and just a small fraction is utilized by the catalyst to function [126]. A larger catalyst dosage, after a certain threshold, lowers the rate of pollutant photodegradation. As a result, fewer photoactive species are active, which lowers the percentage of deterioration. Another explanation could be that there are fewer active surface sites available due to the aggregation of nanoparticles at high concentrations in the solution [126].

(b) 

Amount of Pollutant

The quantity of organic pollutants adsorbed onto the surface of the photocatalyst is one of the key parameters affecting the degradation percentage in a photocatalyst reaction. The quantity of contamination in the bulk aqueous media has no bearing on this. [127]. The synthesis and interaction of OH radicals with the contaminants affects how quickly the pollutants degrade. The photocatalyst’s capacity for adsorption is affected by the initial concentration of pollutants in the medium, which normally rises as the pollutant concentration increases. Photodegradation efficiency is enhanced when there is a higher initial concentration of pollutants, as it is easier for more contaminant molecules to be stimulated by light irradiation [128].

(c) 

Influence of pH

The pH of the aqueous solution affects a variety of processes, including surface charge, agglomeration, ionization, valence band potential of the photocatalyst, and contaminant adsorption. The three main factors that contribute to photodegradation are: (a) oxidation of pollutants owing to the high oxidation potential of positive holes, (b) attack by hydroxyl radicals (^•OH), and (c) reduction of contaminants by conduction electrons. As a result, determining the influence of pH on the effectiveness of catalysts for photodegradation is a difficult task [129].

(d) 

Temperature, Time, and Morphology

Temperature, time, and shape all have a massive effect on photocatalytic performance. With the use of environmentally friendly ZnONPs produced in the temperature range of 25 to 40 °C, Hassan et al. [130] showed how temperature might affect the rate of anthracene degradation. According to the degradation efficacy values, a minor rise in reaction rate is caused by a temperature increase. Raising the temperature may have little detrimental effect on photocatalytic degradation, but it may also induce the molecules to collide more frequently.

9.2.3. General Reaction Mechanism of Photocatalysis

Numerous groups have considered the fundamental mechanics of photocatalytic pollution degradation. In the study by Theerthagiri et al. [131], they stated that the primary goal of a photocatalyst in breaking down pollutants is to expedite the oxidation and reduction methods when light is present. The process of photocatalytic degradation of pollutants includes three general stages: (1) the separation of electron and hole under light irradiation, (2) the dispersion of charge carriers on the photocatalyst’s surface, and (3) the light-driven catalytic oxidation and reduction that occur on the catalyst’s active sites. Equation (1) through (5) identify the significant steps in the photocatalytic degradation pathway. Firstly, the photocatalyst generates holes (h+) and electrons (e⁻) in the valence band (VB) and conduction band (CB), respectively, once exposed to light. These photogenerated charges transfer to the photocatalyst’s surface to initiate the required oxidation and reduction reactions. The interactions of h+ and e⁻ with H₂O and O₂ create the highly reactive radical species O₂ and ^•OH, respectively, as depicted in step (2). As shown in step (3), the breakdown of these pollutants occurs as a result of the interaction of these radical species with organic molecules.

$$\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}+\mathrm{h}\mathrm{v}\to {\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}+{\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}$$

(1)

$${\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}+{\mathrm{O}}_2\to {\mathrm{O}}_2^{-}$$

(2)

$${\mathrm{O}}_2^{-}+{\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}+2{\mathrm{H}}^{-}\to {}^{•}\mathrm{O}\mathrm{H}+{\mathrm{O}\mathrm{H}}^{-}$$

(3)

$${\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+{}^{•}\mathrm{O}\mathrm{H}$$

(4)

$$\mathrm{D}\mathrm{y}\mathrm{e}+{}^{•}\mathrm{O}\mathrm{H}\to \mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s} ({\mathrm{C}\mathrm{O}}_2,{\mathrm{H}}_2\mathrm{O},\mathrm{e}\mathrm{t}\mathrm{c}.)$$

(5)

9.2.4. Application of Green MONPs’ Organic Pollutant Treatment by Photocatalytic Activity

According to Jayaseelan et al. [132], excellent photodegradation effectiveness (>80%, 0.24 gL⁻¹ of ZnONPs, 1 h) was achieved by ZnO nanoparticles produced from Jackfruit (Artocarpus heterophyllus) leaves for the removal of Rose Bengal dye emitted by the textile industry’s activities in the water. Cassia fistula leaves were utilized to produce ZnO nanoparticles, which were then used to photodegrade methylene blue dye in an aqueous solution.

In contrast to the same nanoparticle, Ref. [133] manufactured ZnONPs by using C. citriodora leaf extract as a bioreductant for the removal of MB dye. Under visible light irradiation for 90 min, the bioinspired ZnONPs (20 mg) showed an MB degradation efficiency of 83.45%, while the conventionally synthesized ZnONPs only had a degradation efficiency of 59.47%. Consistent with the study by Fulekar et al. [134], TiO₂ nanoparticles for the removal of methyl orange dye under UV irradiation were created utilizing sorghum root extract. The results displayed that 99.7% of the methyl orange dye solution was degraded by TiO₂ nanoparticles. Concerning Prasad [34], who synthesized ZnO NP from lemon juice, under UV light irradiation, 100 mg nanogranules of ZnO was evaluated to cure three different dye solutions: Congo red, CR (67.16 mg L⁻¹), MB (31.98% mg L⁻¹), and Rhodamine, Rh-B (47.90 mg L⁻¹). Photocatalytic degradation efficiency was obtained for 91.17% of the MB solution after 70 min, 90% for the CR solution after 35 min, and 98% for the Rh-B solution after 50 min.

According to Muthukumar and Matheswaran (2015), the green synthesis of FeO NPs was mediated by an extract from the leaves of Amaranthus spinosus. When employing as-synthesized FeO-NPs under solar radiation, the photocatalytic color removal effectiveness for MO (75%) was greater than that of MB (69%) [135]. More recently, Rather and Sundarapandian [136] synthesized rod-shaped magnetic iron oxide nanostructures utilizing Wedelia urticifolia leaf extract in an environmentally friendly manner. These nanostructures had a strong ability to breakdown methylene blue dye. Another study by Prasad et al. [137] used an aqueous extract of Pisum sativum peels to determine an environmentally friendly approach for the fabrication of Fe₃O₄ nanoparticles. The outcomes verified that Fe₃O₄ NPs were ferromagnetic. UV-visible spectroscopy was used to examine the catalytic capabilities of Fe₃O₄ NPs for the breakdown of methyl orange (MO) dye in an aqueous solution. The biosynthesis of iron oxide nanoparticles utilizing peel extracts from Citrus paradisi was recently introduced by Kumar et al. [138] when 1, 1-diphenyl-2-picrylhydrazyl was present; the prepared iron nanoparticles showed antioxidant activity and eliminated the dyes MR (96.65%, 50 mg L⁻¹), MB (80.76%, 10 mg L⁻¹), and MO (89.64%, 10 mg/L).

ZnO NPs were synthesized using a green method (Corymbia citriodora leaf extract), and their photocatalytic activity was demonstrated by Yuhong et al. [133] by degrading MB under UV light, and the photocatalytic activity of hydrothermal and green-synthesized ZnO NPs was examined. Because of their smaller size, the ZnO NPs made using the green synthetic method had higher photocatalytic activity (83.45%) after 90 min of irradiation than ZnO NPs made using the hydrothermal method (59.47%). CuO NPs were synthesized by Nethravathi et al. [139] utilizing leaf extract from Tinospora cordifolia. When exposed to UV and sunlight, the produced particles in the size range effectively degraded MB, exhibiting a structure akin to a sponge. The findings showed that, when the catalytic load was raised, the effectiveness of MB photodegradation rose. At pH 2, the photodegradation efficiency was 91.23% and, at pH 4, the dye degradation climbed to 96.93%.

Amaranthus spinosus leaf extract was used in the green production of FeO NPs, as reported by Muthukumar and Matheswaran [135]. Under solar radiation, the percentage of color reduction achieved with leaf-mediated FeO NPs was 75 ± 2% for MO and 69 ± 2% for MB. The green synthesis of TiO₂ NPs employing the Rhizoma coptidis root was reported by Sreekanth et al. The photocatalytic activity of MB and MG dyes in both dark and UV light settings was assessed using the produced NPs. After 30 min in the dark, about 39% (MB) and 4% (MG) had been absorbed. The TiO₂ NPs showed good photocatalytic activity under UV irradiation; after 60 min, 71% of the MB and 78% of the MG dyes were removed [140]. In addition, NiO NPs were synthesized by Ezhilarasi et al. [141] utilizing leaf extract from Aegle marmelos. According to this study, 4-chlorophenol mineralization was less than 1% at 90 min, but it swiftly rose to 87% at 180 min, and total mineralization was seen at 210 min. Yadav et al. [142] used watermelon juice and a straightforward solution combustion approach to synthesize CeO₂ NPs. Under UV radiation, about 98% of the MB dye degradation was seen after 180 min, whereas 93% of the dye degradation occurred in the presence of sunshine.

Other studies on the photocatalytic activity of metal oxide nanoparticles for the elimination of dye pollutants are recorded in

Table 5. Photocatalytic activities of metal and MONPs.

Plants	Photocatalyst	Types of Dyes	Degradation (%)	Refs.
Banana crust	CuONPs	Congo Red (CR)	88	[143]
Amaranthus spinosus, leaf	FeONPs	Methyl Orange (MO)	75	[144]
	FeONPs	Methylene Blue (MB)	69
Cynometra ramiflora	FeONPs	MB	High	[145]
Asparagus racemosus	FeONPs	MO	Good	[68]
Hibiscus sabdariffa, flower	FeONPs	Congo Red	96.1%	[146]
Ruellia tuberose, leaf	FeONPs	Crystal Violet	80%	[147]
Peltophorum Pterocarpum, leaf	Fe3O4	Rhodamine	High	[148]
Sugar cane, juice	SnO2NPs	Rose Bengal	99.3	[149]
Sugar cane, juice	SnO2NPs	MB	96.8
Rhizoma Coptidis	TiO2NPs	MB	71	[140]
Parthenium hysterophorus	TiO2NPs	MB	92.5	[150]
	TiO2NPs	MO	81.5
	TiO2NPs	Crystal Violet (CV)	79.7
	TiO2NPs	Alizarin Red	77.3
Starch	TiO2NPs	MB	100	196
Aspalathus linearis	NiONPs	MB	Good	[151]
Eucalyptus globulus Eucalyptus globulus	ZnONPs	MB	98.3	[152]
	ZnONPs	MO	96.66
Lagerstroemia speciosa	ZnONPs	MO	93.5	[153]
Cassia fistula, leaf	ZnONPs	MB	98.71	[83]
Artocarpus hetero phyllus, leaf	ZnONPs	Rose Bengal	85	[65]
Lemon juice	ZnONPs	MB	91.17	[154]
	ZnONPs	CR	90
	ZnONPs	Rhodamine B	98
Citrus aurantifolia (lemon), Citrus paradisi	ZnONPs	MB	97	[155]
Punica granatum, peel	ZnO	Antibacterial property	Good	[156]
Eucalyptus globulus		MB	98.3	[157]
Moringa oleifera, peel	CeO2	CV	97.5	[63]
Cauliflower	SnO2	MB	91.89	[87]
Cauliflower waste, potato peel	CuO	MB	87.37–96	[158]
Vitis rotundifolia, fruit	CoO	Acid blue	High	[159]

.

9.2.5. Water Disinfection from Microbes

Numerous engineering and natural nanomaterials have also been found to possess potent antibacterial capabilities [160]. ZnONPs may be utilized to eliminate total coliform bacteria from municipal wastewater effluent, according to a study conducted by Mostafaii et al. [161]. The effectiveness of nanoparticles was 96% at 0.3 gL⁻¹ in 20–90 min, 75–98.66% at 0.5 gL⁻¹ in 20–90 min, and 65% at 0.7 gL⁻¹ in 20 min, reaching 99% in 90 min. Finally, 100% of the total coliform (TC) bacteria was completely eliminated from municipal sewages in 90 min at a concentration of 1.1 g L⁻¹ of ZnO.

Aeromonas hydrophila was employed to synthesize new ZnO nanoparticles, which were then evaluated against pathogenic microbes. E. coli, A. hydrophila, S. aureus, A. flavus, S. pyogenes, E. faecalis, A. niger, P. aeruginosa, and C. albicans were the objectives of the good diffusion tests. It has been reported that using Bauhinia tomentosa leaf extract as the bioreducing agent produced ZnONPs with an average size of 22–29 nm. The bioinspired NP’s antibacterial efficacy was estimated against P. aeruginosa, S. aureus, B. subtilis, and E. coli. ZnONPs were discovered to have a larger bactericidal effect on Gram-negative bacteria than Gram-positive bacteria owing to the structural differences between the two types of bacteria [162]. Bacterial growth was significantly halted after bactericidal doses of ZnONPs were administered [163]. The utilization of an aqueous leaf extract of Origanum vulgare to create CuONPs by the environmentally conscious generation led to a reduction in the cyanobacterium Microcystis aeruginosa population. This bacterium is commonly found in eutrophic waterways and produces secondary metabolites that are harmful to both humans and animals. CuONPs suppressed Microcystis aeruginosa colony growth by about 89.7% at a load of 50 mg L⁻¹ CuONPs and by 62.6% at a low dose of 25 mg L⁻¹ [164].

10. Conclusions and Recommendations

The innovative features of nanoparticles produced by their synthesis processes lead to new synthesis directions in a variety of areas. Chemical, physical, and biological mechanisms can be used to create metal oxide nanoparticles. The biosynthetic approach provides a number of advantages over conventional chemical-based techniques. It has been confirmed that several biological sources, including bacteria, fungi, and plants, are capable of producing nanoparticles. The green approach to the manufacture of MONPs provides many advantages over conventional approaches, including simplicity in scaling up, affordability, and ease of synthesis. With this context in mind, this review highlights the many types of synthesized MONPs and nanocomposites, as well as their essential mechanisms in environmental pollution remediation. An innovative, inexpensive, and non-toxic technique for producing metal oxide nanoparticles on a wide scale could be offered by plant-mediated synthesis, given the expected rise in demand for nanoparticles. Heavy metals and some organic contaminants have recently been found in the environment in a variety of harmful forms that are challenging to eliminate using conventional methods, necessitating the urgent need for new material-removal solutions. Owing to their high nanoscale size effect properties, which make them excellent candidates to provide a solution for environmental remediation, metal oxide nanomaterials have recently garnered much attention. In addition to them, inexpensive green synthetic MONPs made from various plant and algal extracts may also be utilized, with massive benefits and results in the remediation field. Despite promising results, it has also been noted that the use of algal metal oxide nanoparticles for the recovery of hydrocarbon pollutants has been comparatively underexplored. Plant-mediated synthesis offers a promising solution for the large-scale synthesis of metallic nanoparticles at a low cost and without toxicity, which will be increasingly important as nanoparticle demand grows in the future. The recommendation from the current study can be pointed out as follows: Additional research is required to investigate the fate and transport of metal oxide nanoparticles as well as their toxicity when introduced into the marine environment. More research will also be needed to ascertain the effectiveness of green MONPs in large-scale remediation by various plant species (fixed-bed or fluidized-bed studies), in addition to their usage for industrial wastewater treatment.