Methylene blue dye: Toxicity and potential elimination technology from wastewater

1. Introduction

The production of synthetic dyes is on the increase due to their high demand, most especially in the textile and clothing industries. These chemicals (dyes) are massively produced annually in thousands of tons worldwide [1]. Specifically, the annual industrial production of dye compounds is ∼7 × 105 tons [2]. A few examples of synthetic dyes are aniline blue, alcian blue, basic fuchsin, methylene blue, crystal violet, toluidine blue, and congo red. The fall-out associated with the huge production and usage of synthetic dyes lies in the fact that they end up being in the environment post-dyeing and finishing processes [1,3,4]. Synthetic dyes remain in the physical environment since most of them are difficult to biodegrade [5] and are not usually eliminated during the conventional water treatment processes and, as such, persist in the environment due to their high stability to temperature, light, water, and other substances including soap and detergents [6]. Consequently, the so-called ‘treated water’ apparently becomes a threat to biotic components in the environment [7].

Methylene blue (3,7-bis(dimethylamino) phenothiazine chloride tetra methylthionine chloride) is one of the synthetic dyes that is applied in large amount as colorant for papers, in wool, silk, and cotton [8]. In addition, food, cosmetics and pharmaceuticals industries are not left behind in consuming a large quantity of MB dye for their productions [9]. Although MB has been proven to possess some medicinal effects, but occurs when it is safely used as clinically instructed/prescribed [10], unlike intake via contaminated water. For example, it can be used to treat vasoplegia after transplant operation [11], malaria treatment dosage: 36–72 mg/kg over 3 days) [12] and heparin neutralization [13]. However, the release of partially or untreated MB dye-loaded wastewater from any of the aforementioned industries could cause a lot of health risks. For instance, in humans, MB dye can induce various ailments such as cyanosis, tissue necrosis, Heinz body formation, vomiting, jaundice, shock, enhanced heart beat rate, amongst others [14]. Also, with respect to plants, the presence of MB has become a major challenge, such as growth inhibition, reduction of pigment, and protein content of microalgae Chlorella vulgaris and Spirulina platensis [15]. Thus, the negative effects associated with MB dye-loaded wastewater warrants the need for effective removal prior to industrial discharge.

Furthermore, the pollution of water bodies by untreated MB dye effluents discharged from industries has been recently associated with the shortage of clean water in the society [[16], [17], [18]]. This is common in developing countries where a high volume of wastewater is released into the physical environments without effective and efficient management [19]. Since the menace posed by MB dye in the physical environment, scientists have been on the search for ways to remediate the environment with respect to the elimination of MB dye. Different treatment methods, including biological methods (use of enzymes and microorganisms), chemical methods (use of advanced oxidation processes), and physicochemical methods (mostly adsorption), have been widely applied to eliminate dye from the environment [5,20]. The adsorption methods include, but are not limited to, the use of rice husk, cow dung, and biochar [21,22], oil palm wastes-derived activated carbons [23], activated banana peel waste [17], Palmyra (Palm) shell [24], zeolite [25], activated carbon [26], and carbon nanotube incorporated eucalyptus derived activated carbon-based [27,28], which have been investigated and applied with respect to the removal of dyes from the environment. Among these methods of environmental remediation of organic or inorganic dyes, the use of biochar has been reported to be highly efficient, cost-effective, and environmentally safe [21,29].

This synthesis attempts to comprehensively discussed the various review and research publications on various available and applicable removal methods for MB dye. The section two of this review describes the toxicities and various health related negative impacts of MB dye. In section three, the traditional removal strategies are comprehensively discussed, challenges highlighted and methods compared with subsection of physical removal strategies, chemical removal strategies and biological removal strategies. In section four of this synthesis, the influence of important operating parameters for adsorption technology in the removal of MB dye are critically discussed and summarized.

Some interesting reviews related to this synthesis have been published, but with few gaps that are filled in this review. For instance, Khan et al. [30], reviewed the properties, uses, toxicity and photodegradation of methylene blue dye. However, the elimination strategies by membrane technology and adsorption technology were not discussed. Similarly, photocatalytic degradation of several dyes was evaluated or reviewed by Waghchaure et al. [31], without comparison of this technique with other conventional removal techniques for those dyes. These gaps are filled in this review while we conclude by highlighting the various areas for future research and point out the research gaps that need to be filled to achieve cheaper, safer, and faster treatment strategy for MB dye-containing (waste)water.

2. Methylene blue dye

Methylene blue dye is a heterocyclic aromatic chemical compound with a planar structure [32].It has a molceular weight and chemical formula of 319.85 g/mol and C16H18N3Sl (Fig. 1), respectively [33]. MB dye is a prevalent blue, cationic, and thiazine type of dye that has been widely applied in the textile industry as a fiber coloring agent [21,22], and also in the field of medicine as staining agents, and for prophylactic and therapeutic purposes [21,34,35]. Table 1 presents the extensive details of the other physicochemical properties of MB dye are presented.

Table 1. Some physico-chemical properties of MB.

Entry	Parameters	Values/names
1.	Maximum wavelength of absorption ( $λ_{\max})$	664 nm
2.	Another name	Swiss blue
3.	Ionization	Basic
4.	Degree of solubility	3.55%
5.	Color index name	Basic blue 9
6.	Color index number	52015
7.	Aqueous pH	Strongly acidic between pH 2.0–3.5

2.1. Applications of methylene blue dye

The applications of MB is vast. Medically, among the earliest synthesized antimalarial drugs was methylene blue, which was tested at the end of the nineteenth century [36]. It was also used in combination with amodiaquine and reported to be effective for falciparum malaria, especially in African children and adults [36,37]. Methylene blue has also found its wide use in medicine where it is indicated for the treatment of children and adult patients diagnosed with methemoglobinemia (a blood disorder in which an abnormal level of methemoglobin is produced) [38]. Other clinical applications of methylene blue include improvement of hypotension related to different clinical states, therapeutic use against hypoxia, and hyperdynamic circulation in cirrhosis of the liver, among others [39].

Furthermore, MB has also been reported to be applied for the treatment of recalcitrant hypotension related ailment in septic endocarditis by adding it to cardiopulmonary bypass (CPB) priming solution at a concentration of 2 mg/kg and infusing it into the patient at 0.25–2 mg/kg/h. This is applied intraoperatively [40]. In post-operation treatments, MB is used to cure severe vasoplegia after transplant operation in patients. Previous reports/survey have revealed that the use of MB, compared to placebo, has extensively reduced the rate of mortality in vasoplegic (hypotensive) patients [41]. These medical applications further strengthen the vast application of MB in drug formulation and also as drug for various ailments.

Industrially, MB is applied primarily in the clothing and textile industries for dyeing various fabrics [22]. It is also used for dyeing papers and leathers [42,43]. In the food industry, MB has been surprisingly used as an indirect food additive [38]. Methylene blue dye is also used in aquaculture for the treatment of various ailments in fishes [42,43] and as a sensitizer in the photo-oxidation of compounds (organic) in medicine, microbiology, and diagnostics [24,44]. Based on the mentioned uses of MB, it is, therefore, unarguably a relevant dye. However, MB has been reported to be harmful to human health at a certain concentration due to its strong toxicity [5] and recalcitrant nature [45], making it a potential threat to the ecosystem and human health [45,46].

2.2. Toxicity of methylene blue dye

As relevant as MB is in diverse sectors, when not properly managed in an environmentally friendly manner, the presence of MB could pose serious threat to the environment and health of humans [47,48]. This chemical compound has been reportedly teratogenic and embryotoxic. The toxic effects were confirmed in an exposure study of MB to angelfish and rat, respectively [38]. Manufacturing industries, such as textiles, paints, pharmaceutical, cosmetics etc. that utilize dyes including MB in their production processes may release a large amount of such products into the environment as waste. From survey, about 67% of dyestuff market and/or consumption is accounted for by textile industries [49] while ∼ 120 cubic meter of industrial wastewater is discharged per tons of fiber manufactured [50].

Moreover, sequel to the indiscriminate channeling of industrial wastewater into natural water sources, which has been observed to be a prominent disposal pathway; groundwater and/or surface water, the aquatic fauna, beneficial microbes and human lives/health has been threatened . This is because at high concentration, the toxicity of dyes, such as MB, has been found to be substantial [51]. MB is carcinogenic and non-biodegradable owing to characteristic stability of aromatic ring in the molecular structure of MB, as depicted in Fig. 1 [52]. The health risks associated with the contact of MB range from gastrointestinal complications, respiratory disorder, central nervous system, cardiovascular issues, genitourinary complications and to dermatological effects [53], amongst others.

Additionally, MB is commonly regarded as an inert dye, but has recently been shown to act as a potent reversible monoamine oxidase (MAO) inhibitor [53,54]. This property makes MB to find several applications in the field of medicine. As previously mentioned, it has been previously proven and/or used to ameliorate a clinical condition known as hypotension [250] and to improve hypoxia and hyper-dynamic circulation in cirrhosis of liver and severe hepatopulmonary syndrome . However, if infused intravenously at a dosage higher than the recommended concentration, MAO inhibitors may precipitate serious serotonin toxicity [54]. Serotonin toxicity, also called serotonin syndrome, is a deadly disorder that is linked to elevated serotonergic activity in the brain and spinal cord [55]. Also, the contact of MB with the skin, as a result of using improperly or untreated MB-loaded water, may result in skin redness and itching, and also in skin necrosis; a condition that cause the death of most or all of the cells in organ or tissue [56]. In the area of human reproduction, the persistent contact with concentrated solutions of MB has been reported to have negative impact in the ability of sperm to move more efficiently [57].

Aesthetically, the characteristic coloration possessed by MB can render streams or rivers unappealing [58]. In addition, MB generally has high molar absorption coefficient (∼8.4 × 104 L mol−1 cm−1 at 664 nm), and this can result in an attenuation of sunlight transmittance, and thereby thwarts the illumination reaching such stream/river [59]. Therefore, presence of MB can negatively affect photosynthesis process [60] chemical oxygen demand (COD), biological oxygen demand (COD) and oxygen requirement levels, thereby affecting the entire water ecosystem [59]. All these associated MB toxicities and negative health and environmental impacts makes it imperative to assess or evaluate the various available technologies for remediating MB-loaded wastewater by synthesizing scattered but useful and available information from previous research studies. Some other examples of illness that could result from contact with MB are graphically represented in Fig. 2.

3. Removal strategies for methylene blue

Methylene blue has strong affinity for water at normal temperature condition [61] and also generally known to be difficult to biodegrade and remove from wastewater using simple conventional treatment methods [62,63]. The removal of MB from effluent wastes is environmentally important in order to prevent the toxic effects it poses to human health and the environment [64]. The use of MB in medicine can be controlled vis-a-vis controlling the dosage to be administered. Therefore, the removal strategies focus mainly on wastewater and effluents emanating from industries. Many technologies have been proposed and developed for the removal of MB from its waste/effluent constituents before they are released into the environment. Dutta et al. [65], highlighted current successful removal technologies to be broadly classified into physical, chemical or biological method. This implies that the three mentioned methods are conventional and have been well researched by scientists and/or environmentalists.

Furthermore, the chemical/electrocoagulation, oxidation, photocatalyzed degradation, biodegradation, biocatalytic degradation and adsorption processesare among the several sub-separation methods that have been proposed for the removal of MB from effluent and sewage [66]. Other techniques are phytoremediation, vacuum membrane distillation, liquid-liquid extraction, ultrafiltration, nanofiltration, and microwave treatment [67,68]. However, studies have shown that most of these conventional methods are characterized with some drawbacks like being expensive, costly electricity demand, and large amount harmful wastes generation, etc [69]. These demerits and other comparisons shall be discussed and drawn out in this synthesized MB removal methods.

3.1. Physical removal methods

There are several physical methods that are available for the remediation of dyes from their effluent discharges [65]. Amongst the most successfully used are filtration processes (membrane, nanofiltration, ultra/microfiltration), reverse osmosis, ion exchange, irradiation, electrolysis, coagulation-flocculation, and adsorption techniques [67]. For example, because of the effectiveness and efficiency in removing dyes, the coagulation–flocculation technique has been scantily studied. This is corroborated by a search on Scopus database using keywords ‘flocculation of methylene blue’ and ‘coagulation of methylene blue’ showed that there were only 110 and 384 documents (published articles) from 1948 to 2022 as at September 26, 2022. This indicates that there is dearth of extensive research in this type of removal technology (Scopus, 2022).

Briefly, for coagulation process, salts such as that of iron and aluminum are used to coalesce the dyes laden effluent, sometimes lime is blended with this for effectiveness. Properties such as the pH and the nature of the wastes are important factors to be considered in order to know the amount of coagulation salts to apply to the effluent. One of the shortcomings of this method is the increase in amount of salt content and substantial volumes of slurry, that may make up for about 5% of the treated effluent water [70]. The coagulation technique has some limitations in its application for industries that utilize huge volumes of detergents. This is because, it has been found to cause a reduction in water surface tension. However, it has been established that effluent treatment can be improved when activated sludge are mixed with coagulation process [71].

3.1.1. Adsorption technology

Adsorption technology for the removal of dyes, such as MB, employed the use of solid sorbents. The technique has been widely and effectively used to remove MB from wastewater. There are many adsorbents that have been researched and applied successfully to reduce dye concentrations from aqueous solutions. There are many adsorbents that have been researched and applied successfully to reduce dye concentrations from aqueous solutions. Economically, the use of commercial granulated activated charcoal and powdered activated charcoal are expensive and they have several regeneration or disposal problems [72] even though adsorption technology is itself economical and has potential to treat commercial wastewater than any other method [73]. This has steered many researchers to study the effectiveness of several low-cost adsorbents extensively. A few examples of low-cost adsorbents are pomegranate peel biochar [74], ava bean peel waste [47,48], cashew nut shells [75], chitosan lignin membrane [76], Granular waterworks sludge-biochar composites [77], activated carbon from KOH-activated dragon fruit peels [78], African almond [79], and pomegranate peels [80]. Mechanistically, different type of interactions between cellulosic, hemi-cellulosic biomass sourced adsorbents materials have been found. For instance, hydrogen bonding, π-π interaction and electrostatic interactions between MB and adsorbent surface have been highlighted as possible uptake mechanism from the FTIR data of adsorbent surface characteristic study by Giraldo et al., [81]. Table 2 shows the various biomass related and/or sourced adsorbents and the corresponding adsorption capacityfor MB.

Table 2. Adsorption capacity of biomass related adsorbents for MB.

Entry	Adsorbents	Operating conditions	Maximum adsorption capacity (mg/g)	References
1.	Acacia wood	Interaction time: 180 min; Temperature: 30 °C; pH: 7	210.21	[82]
2.	Date stone (sulphuric acid treated)	Interaction time: 60 min; Temperature: 50 °C; pH: 5.57	515.6	[83]
3.	Bagasse (tartaric acid treated)	Interaction time: 35 min; Temperature: 30 °C; pH: 9	59.88	[84]
4.	Coconut leaves	Interaction time: 90 min; Temperature: 50 °C; pH: 8	87.72	[85]
5.	Rice straw	Interaction time: 120 min; Temperature: 35 °C; pH: 6	32.60	[86]
6.	Corn husk	Interaction time: 80 min; Temperature: 25 °C; pH: 4	462.96	[87]
7.	Ficus carica bast	Interaction time: 80 min; Temperature: 30 °C; pH: 7.8	47.62	[88]
8.	Neem bark (formaldehyde treated)	Interaction time: 30 min; Temperature: 25 °C; pH: 4	90	[89]
9.	Wheat straw (SDS modified)	Interaction time: 120 min; Temperature: 30 °C; pH: 8	126.6	[90]
10.	Potato peel (acid treated)	Interaction time: 30 min; Temperature: 25 °C; pH: 12	40	[89]
11.	Onion membrane	Interaction time: 60 min; Temperature: 20 °C; pH: 7.1	1923	[91]
12.	Corn cob	Interaction time: min; Temperature: 30 °C; pH: 7.6	417.1	[92]
13.	Garlic pee	Interaction time: 210 min; Temperature: 30 °C; pH: 6	82.64	[93]
14.	Cashew nut	Interaction time: 60 min; Temperature: 30 °C; pH: 10	250	[94]
15.	Neem bark (acid Treated)	Interaction time: 30 min; Temperature: 25 °C; pH: 2	1000	[89]
16.	Wheat straw	Interaction time: 120 min; Temperature: 30 °C; pH: 8	55	[90]
17.	Pea shell	Interaction time: 180 min; Temperature: 25 °C; pH: 5.86	246.91	[95]
18.	Walnut shell	Interaction time: 120 min; Temperature: 25 °C; pH: 6	51.55	[96]
19.	Coconut coir dust	Interaction time: 20 min; Temperature: 30 °C; pH: 6	29.50	[97]
20.	Onion skin (cold plasma treated)	Interaction time: 150 min; Temperature: 30 °C; pH: 10	250	[94]
21.	Mango leaf powder	Interaction time: 120 min; Temperature: 25 °C; pH: 7	156	[98]
22.	Corn husk	Interaction time: 15 min; Temperature: ; pH: 6.2	30.33	[99]
23.	Walnut shell	Interaction time: min; Temperature: 30 °C; pH: 8	33.63	[100]
24.	Rice husk	Interaction time: min; Temperature: 25 °C; pH: 7	25.46	[101]
25.	Sugarcane bagasse	Interaction time: 30 min; Temperature: ; pH: 5	84.74	[102]

Furthermore, studies have shown that interests to manufacture inexpensive and yet efficient adsorbents from readily available materials in the environment have received great attention. Materials such as the ones derived from aluminosilicate minerals (clay) and silica containing compounds are being understudied (Grini, 2006). The popular ones include activated charcoal, aluminosilicate minerals, peels of fruits (orange, banana); shells of grains (wheat), Silicon dioxide, metal–organic frameworks (MOFs) and a host of others [103,104].

Amongst these common adsorbents (e.g. clay, activated carbon from biomass and MOFs), plant-based materials have received great consideration as non-conventional approach. A few more list include skin of citrus fruits [105]; banana bark [106]; fiber sludge, rice crust [107]; betonite clay [108]; dust from nimtree leaves [109]; powdered activated sludge [72]; perlite [108]; powder from bamboo trees, and various agricultural crops [110] [111]; and manure slurry [112]. These have found beneficial applications in the use as adsorbents. Straw, leaf powder, perlite, duckweed and sewage sludges have all been used in the investigation of methylene blue dye removal from its waste [108,[110], [111], [112]]. A review by Rafatullah et al. provided an exhaustive list of these mentioned adsorbents, highlighting their adsorption capacity and limitations for clays, zeolites, agricultural solid wastes, industrial solid wastes and so on [113].

Furthermore, there are currently several ongoing studies that are being carried out to determine the excellent and most effective adsorbents. Environmental friendliness, low-cost, high sorption rate, efficiency as well as availability are the focal points being considered (Grini, 2006). Properties such as equilibrium isotherms, kinetics mechanism of sorption rate and operating conditions such as pH, temperature, contact time, and concentrations of dyes have been greatly researched [112], whereby the adsorption equilibrium isotherm models have been utilized for effective elucidation of the behaviour of adsorbents [114]. The operating conditions such as pH, temperature, contact time, and concentrations of MB are discussed in-depth in section four (4) of this article. Adsorption equilibrium isotherm defines the relationship that exists between the concentration of the adsorbate and the adsorption capacity. This is useful in deciding an effective and efficient adsorbent for a particular procedure [115]. The most popular among these for pollutant removal are Langmuir, Freundlich, and Dubinin-Radushkevich models.

In a study by Vadivelan and Kumar [115], to determine rice husk's adsorption efficiency, these models postulated by Langmuir and Freundlich were adopted. The observed parameters of pH, the temperature and concentration were found to fit well with these models. Wang et al. [116], investigated the impact some physiochemical parameters have on walnut shell in the removal process of methylene blue dye. The parameters considered were the quantity of the adsorbent, the agitation time, the pH of the aliquot, the grain size and the concentration of sodium chloride. They discovered that under the standard operating conditions set for the parameters under investigation, the wall nut shell is a very effective adsorbent to be considered and practicable in the removal process of methylene blue dye. The adsorption isotherm models to achieve a good fit for the kinetic data proposed by Wang et al. [116], were Langmuir, Dubinin-Radushkevich, and Freundlich.

Coffee residues, which are inexpensive adsorbent materials, have been also suggested for removing methylene blue dye from aqueous solution by Nitayaphat et al., [117]. The physicochemical parameters of interest for their study are the pH of the solution, the amount of adsorbent to be used, and the time taken for adsorption process. Under set conditions of the parameters under investigation, the following were observed; it takes a short period of time (∼18 min) for the adsorption of methylene blue dye to attain equilibrium, if about 1 g of adsorbent is used, the starting pH of 11 is needed to achieve 100% maximum removal of methylene blue dye. Also, as more quantity of adsorbent is applied, it will take lesser time to attain equilibrium. The Langmuir model has been described to be the good adsorption isotherm fit for coffee residues.

Clay materials used as adsorbents have also been found to show similar adsorption isothermal consistency with that of agricultural wastes [112]. The experimental results using clay materials have demonstrated similar patterns with aforementioned models. Elass et al. [118], employed a natural clay called ghassoul as adsorbent. They investigated the impact some selected parameters have on adsorption capability on methylene blue dye. Amongst the factors considered for their study are contact time, the starting concentrations of the dye under investigation, temperature and the pH of the aliquot. Their findings suggested that the adsorption isotherms are well fitted using the models proposed by Langmuir model. In addition, a pseudo-second order rate equation can be used to demonstrate how effective ghassoul is in the removal process of methylene blue dye when the parameters are well optimized. This result has shown that, ghassoul is a very effective and inexpensive adsorbent that can be considered if available [118].

3.1.2. Membrane technology

Membrane technology has been reported to be an ecofriendly and sustainable practice [119] and its use has been reported to be effective for removing dyes such as the methylene blue dye from wastewater [120]. Although, membrane separation has been applauded to be a better application in the removal process of dyes; the use of this technique has been described to be expensive. Thus, scholars are continually investigating methods that are more reliable and effective especially in terms of using renewable membrane materials.

Therefore, studies have focused in researching hybrid nanomaterials that combines properties of inorganic and carbonaceous substances in order to achieve inexpensive materials that are practicable and equally efficient [121]. Techniques such as reverse osmosis and ultrafiltration have been indicated to be among the most effective membrane procedures to achieve optimal removal of numerous harmful dyes [70]. Remarkably, recent studies have shown that the use of hybrid materials comprising adsorbents and membrane filters have produced efficient desirable result when deployed to remove methylene blue dye [119]. The combination of adsorptive and membrane make it easier for the system to prevent infiltration of the contaminants by applying physical and chemical processes simultaneously. As a result, great volumes of dye contaminants are effectively removed using this separation technique [121].

Furthermore, polymer membranes that come with micro- and nano-structures have also been a subject of interest. However, to use polymer materials, some important features and properties need to be considered. This is because, depending on the technique adopted, the polymer material will experience some transformation. The features include the adsorption capability of the material, its surface area etc. [122]. The membrane material used is primarily made up of polymeric sheet, solvent diluent, non-solvent liquid and some additives/extracts [123]. Substances such as the polyvinylidene fluoride or any of its copolymers are excellent polymeric materials that can be used in the membrane synthesis [27,28]. The solvent generally used as diluent is N-methyl pyrolidone because water does not mix with most organic solvents and it is therefore a suitable non-solvent of choice. The importance of additives in the effectiveness of membrane cannot be overstressed as they determine the physical (permeability and mechanical strength) and chemical features of the polymers to be formed [124].

However, despite its applicability, the membrane technology suffers a major limitation – fouling of membrane. The membrane needs to be regularly replaced once it becomes fouled or overburden [121]. Therefore, to be considered as a good alternative technology for methylene blue dye removal, the financial implications and sustainability of the type of membrane to be used have to be incorporated in any decision making.

3.2. Chemical removal methods (photochemical and non-photochemical methods)

3.2.1. Non-photochemical methods

3.2.1.1. Ozonation

The highly reactive ozone, $O_{3},$ has been used for the treatment of dye-polluted water. It is advantageous because it improves both color and taste of the water. However, ozone is not stable and it has low solubility which limits its application [125,126]. To improve the application of ozone for the decontamination of MB in water, other materials are often used alongside ozone. For example, titanium oxide and activated carbon are few of the materials that have been used for this purpose. It was observed that the efficiency of MB removal improved when these materials were used along with ozone [125]. Hydrogen peroxide ( $H_{2} O_{2}$ ) was also used along with ozone to form peroxone which degrade MB [127]. The efficiency of peroxone is enhanced by passing current through it to form electro-peroxone system [128]. Previous studies revealed when compared to lack of electric field application, the ozone concentration/solubility in water is improved by 43.1% to when there is presence of an electric field [129].

Meanwhile, other investigations are being carried out for improving the efficiency of MB removal from different systems. For instance, to improve on the application of ozones for the decolourization of MB in the non-steady industrial wastewater, Jibouri et al. developed a model for the application of ozone in a continuous system [130]. Babar et al. investigated the effectiveness of ozonation catalyzed by Fe-loaded biochar as catalyst and the use of ozonation approach alone for decontamination of MB [131]. It was found that at neutral pH, ∼76% of MB is degraded during single ozonation process in 1 h. However, ozonation catalyzed by Fe-loaded biochar showed 95% MB elimination from aqueous solution. The effectiveness of Fe-loaded biochar was diminished by 52% in the presence of hydroxyl radical (.OH) scavenger, revealing that hydroxyl radical is the main oxidant during heterogeneous catalytic ozonation process for MB removal. Zhang et al. evaluated the influence of solution pH on ozonation and carbon-assisted ozonation processes for MB elimination [132]. The authors found that there was an intensification in MB as observed from the hydroxyl radical reaction route for MB mineralization as pH increased to 11, corroborating the fact that .OH radical is responsible for MB decontamination with [.OH] of 5.7 × 10−7 M at pH 11 against 0.64 × 10−7 M at pH 3. It can be inferred that pH affect MB removal and that the use of ozonation is limited in terms of low water solubility, which required the use of electric field to enhance its concentration for effectual MB mineralization.

3.2.1.2. Fenton system

In Fenton system, the utilization of the reaction of hydrogen peroxide and divalent iron to generate highly-reactive hydroxyl radicals and trivalent iron is employed, as against the use of ozone in ozonation process. The divalent iron ion is regenerated by the reaction of the excess hydrogen peroxide with the trivalent iron ion in the product of the reaction (Melgoza et al., 2009). In Fenton-like reaction, catalytic decomposition of hydrogen peroxide leads to the generation of hydroxyl radicals as shown in equations (1), (2), (3):(1) ${F e}^{2 +} + H_{2} O_{2} \to {F e}^{3 +} + {O H}^{-} + {O H}^{.}$ (2) $H_{2} O_{2} + {O H}^{.} \to {H O}_{2}^{.} + H_{2} O$ (3) ${F e}^{2 +} + {O H}^{.} \to {F e}^{3 +} + {O H}^{-}$

There was over 98% methylene blue dye removal within 1 h of Fenton-like reaction in acidic medium. This removal efficiency was achieved when the ratio of 1:1.15:14.1 was maintained for dye:Fe2+:H2O2 [133]. Other substances have been used to replace hydrogen peroxide in Fenton-like process for degrading methylene blue dye. For example, Ferrocene was used to catalyze the Fenton process. There was total methylene blue degradation when ordinary 1.12% of the Ferrocene was introduced into the system [134]. Aqueous ammonia has also been used in the Fenton process for the degradation of 40 mg L−1 of MB within 40 min (Zhou et al., 2014). This implies that addition Fenton reaction can be enhanced via the addition of some reagents (ferrocene and/or ammonia as ‘catalysts’).

3.2.2. Photochemical methods

3.2.2.1. UV light assisted advanced oxidation process

With the assistance of UV light, highly reactive radicals such as hydroxyl radicals and sulphates are generated which in-turn attack the methylene blue dyes leading to the formation of non-toxic or less toxic products [135]. Ordinary UV light is limited in the acidic and natural media for the removal of methylene blue dyes [136]. Hence, UV light has been used to assist other methods/reagents.

The use of hydrogen peroxide with UV light has been reported for the degradation of methylene blue. This method was reported to decolourize over 90% of the methylene blue within 1 h in a report. Though, the time for the decolourization varies with the initial concentration of methylene blue dyes [137]. Also, increase in the concentration of hydrogen peroxide affects the efficiency of this method which may be related to radical scavenging ability of hydrogen peroxide [138]. Due to this, other UV-assisted methods have been investigated. Plasma technology is one of the methods that has been assisted with UV light for the degradation of methylene blue dye. In an investigation [139], the relevance of UV in the operation of plasma technology for the degradation of methylene blue was carried out. It was revealed that the set-up without UV light experienced reduced methylene dye degradation compared to the set-up with UV light which showed that the presence of UV light in plasma technology is important for proper degradation of methylene blue dye.

The use of UV for the activation of persulfate which then led to the degradation of more than 64% of methylene blue dye is another method that has been reported [140,141]. Wen et al., used the combination of persulfate, ultraviolet and vacuum-ultraviolet radiation for degrading methylene blue dyes in a fluidicreactor. The result revealed that the degradation of methylene blue dyes was significantly higher compared to what was obtained without including vacuum-ultraviolet radiation. The reason was because more reactive species are generated in the presence of vacuum-ultraviolet [135]. UV light is also used with other light-sensitive materials to activate persulfate for the degradation of methylene blue. Examples of such materials are the doped graphene oxide[142,143] and lanthanium ferrite [144].

3.2.2.2. Catalyst assisted advanced oxidation process

The mechanism of photocatalytic degradation of organic pollutants has been widely reported in the literatures [[145], [146], [147], [148], [149], [150]]. The summary from these literatures is summarized in Fig. 3. Briefly, solar light is absorbed by the photocatalysts to generate pairs of electrons and holes when the light possesses enough photon energy which is more than or equal to the band gap energy of the photocatalyst. The generated electrons-holes pairs migrate to the surface of the photocatalyst to undergo a redox reaction. However, a lot of these holes and electron pairs recombine to dissipate energies in the form of light and heat [151]. The unrecombined electrons are excited to the conduction band of the photocatalyst while the holes in the valence band of the photocatalyst attacks the methylene blue dye and some of the hole also react with water to generate hydroxyl radicals which in-turn strongly oxidize the methylene blue dye. The excited electrons on the other hand further attack the oxygen to form superoxide anion radicals which also degrade the methylene blue dye in the presence of photocatalyst and light into carbon dioxide, water and other less toxic or non-toxic substances. The efficiency of the photocatalysisdepends on the light intensity and the type of photocatalyst used.

Oxides of metals has proven to be a good photocatalyst for the degradation of methylene blue dyes. In an investigation, 89.11% of methylene blue was degraded by using zirconium oxide nanoparticles as photocatalyst under solar irradiation [246]. Doped metal oxide has also been used as photocatalyst for degrading methylene blue and it was observed that the doped metal oxide (Sn-doped MgO) performed better than the un-doped metal oxide. In fact, the performance was related to the amount of metal dopant that was present in the photocatalyst [152]. Heterojunction systems of two metallic oxides have also been found to be effective as the photocatalysts for the degradation of MB dye. Yadav et al. successfully degraded over 85% of MB in contaminated water by using a system containing nickel oxide-cobalt oxide [153]. The heterojunction chalgogenides are often deposited on carbon-based materials to enhance their separation after usage. For instance, CuBi2O4/AgBiO3 heterojunction was deposited on the carbon nanotubes and there was 97.94% degradation within 1 h of visible light irradiation. Also, the nanocomposites remained stable even after 10 cycles of reusability test [154]. Among these carbon-based materials, graphitic carbon nitride is unique because it is also a photocatalyst with bandgap energy of 2.7 eV [155,156]. This implies that it does not only act as support for the other photocatalyst, but it collectively enhances the rate of photocatalytic degradation of MB dye. This was demonstrated in the 2.6 folds increase in photocatalytic efficiency when α-Fe2O3/g-C3N4 was used as photocatalyst compared to when neat α-Fe2O3 as photocatalyst for the degradation of MB under UV light irradiation [157]. Summarily, doped materials as photocatalysts often enhance the photodegradation performance, by several folds, as revealed in Sn-doped MgO [152], α-Fe2O3/g-C3N4 [157], CuBi2O4/AgBiO3 [154], TiO2/g-C3N4 [158], and Ag/WO3 [159], by attributing this effect to reduction in the band gap of the doped photocatalysts, for instance from 2.6 eV to 2.1 eV for Ag/WO3 [159]. Other few examples of photocatalysts that have been used for the degradation of MB dye, together with their photodegradation performances are shown in Table 3.

Table 3. Photocatalytic degradation of MB.