End-of-life waste criteria: synthesis and utilization of Mn–Zn ferrite nanoparticles as a superparamagnetic photocatalyst for synergistic wastewater remediation

Abstract

Dyes are toxic compounds that are widely included in industrial discharge. Their efficient and economical remediation can be explored through Fenton’s oxidation. Mn_0.6Zn_0.4Fe₂O₄ nanoparticles are prepared via a simple and efficient co-precipitation technique and its chemical composition is confirmed through X-ray diffraction and its morphology via high-resolution transmission electron microscope. The prepared superparamagnetic photocatalyst based on Mn_0.6Zn_0.4Fe₂O₄ is applied for almost complete Synozol Blue dye removal (98% dye removal and 87% COD removal) and the results confirmed that the process is an efficient sustainable technique for the easily magnetically recoverable catalyst. Central composite design analysis was chosen to optimize the parametric conditions of the magnetized Fenton’s variables through 13-level of a quadratic model. The optimized system variables were attained at 39 and 404 mg L⁻¹ for catalyst and H₂O₂, respectively, at pH 3.0 with model correlation coefficients over than 98%. Recover and reuse are a viable option for ‘close the loop’ waste after final treatment as an ‘end-of-life’ waste potential and high removal efficiency is attained up to the 8^th cycle of catalyst reuse. Kinetics of Synozol Blue oxidation fitted through the first-order kinetic model. Finally, the thermodynamic parameters values concluded that the process is non-spontaneous and endothermic in nature.

Introduction

Water is a finite resource that becomes in deficiency due to the increasingly modern activities exist in our societies (Ercin and Hoekstra 2014; Ashour et al. 2014). Estimates by Boretti and Rosa (2019) exhibiting the projected utilization of water up to 2050, based on number of drivers of change, i.e. population/economic/technological growth and utilization pattern, indicate that the global water consumption will be 20–30% greater in 2050 than at present, driven primarily by demand for water. Industry is the big sector everywhere around the world that consumes massive quantities of water since the overall water used in industry accounts about 20% of the total (Zhao et al. 2009; Thabet et al. 2021a, b). Textile industry is gaining a great concern since it consumes vast amounts of water and their discharge is one of the main environmental impacts as the effluent containing dyes, colouring compounds and organic materials (Tayeb et al., 2019; Thabet et al. 2020; Ghaneian et al. 2008). Such heterogeneous discharge causes deterioration in the water bodies. Thus, the treatment of such effluents before its final disposal is imperative and gaining a concern for both industry and academia to meet the environmental regulations. However, colour removal from such discharge remains the major environmental concern since only 47% of dyestuffs used are biodegradable and the rest of them remain in the environment (Bia et al. 2009).

Up to now, various treatment schemes are introduced for dye removal including Biological (Tian and Yu 2020), chemical (Torrades and García-Montano 2014), and physical (Ashour and Tony 2020) techniques. However, chemical treatments are the most efficient techniques, that is, adsorption (Ashour and Tony 2020a, b; Bayantong et al. 2021); catalytic oxidation (Tony and Lin 2021) and visible light-induced catalysis (Rezgui et al. 2021). Recently, Li et al. (2021a, b) applied nanoscale zero-valent iron for eliminating organic pollutants. Lui et al. (2021) and Fang and his co-workers (2021) introduced metal–organic framework as a water stable catalyst for removing organics, heavy metals and pharmaceuticals. More recently, Zou et al. (2022) introduced the aluminosilicate clay mineral-based composites as a photocatalysts for eliminating pollutants from aqueous stream.

Notably, advanced oxidation processes (AOPs) as chemical oxidation technology are gaining a considerable scientists’ attention for reclaiming dye contaminating water since they are green choice, simple in operation and reliable for practical applications (Wang et al. 2021). The emerging field of AOPs is based on either heterogeneous or homogeneous mixture consisting of a strong oxidizing agent and transition metal. Although Fenton’s reagent, which combines iron with H₂O₂, gained a vital role among the AOPs matrix for treating textile effluents; its practical applications are still limited. This is due to the significant challenge of the secondary waste collected after treatment (Rezgui et al. 2021). Hence, the determination of the appropriate catalyst is important to attain a considerable efficiency as well as minimizing the final waste. To date, most of the literature cited is based on applying iron-based salts as a source of Fenton’ reagent.

Under the researchers' best policy scenario, achieving ‘end-of-waste’ status would be cleaner, inexpensive and superior solution equipped to fight the environmental damage (Li et al. 2011; Tony 2020a, b). In this regard, ‘end-of-waste’ photocatalysts by recovering catalyst materials can support the recycling of waste after wastewater treatment for beneficial use to avoid secondary waste materials without damaging human health and environment. Thus, the particularity of the current study is focused on the application of the superparamagnetic Mn–Zn ferrite as an alternative of classical iron source to be oxidized via H₂O₂ to treat textile dyed contaminated wastewater as an easy to recover novel photocatalyst.

Mn–Zn ferrites materials are technologically valuable since they posses high magnetic permeability (Rath et al. 1999), low core losses (Xuan et al. 2007) and they are thermally and chemically stable in aqueous systems (Garcia-Munoz et al. 2020). In adding up, the low band gap of Mn–Zn ferrite materials in comparison with anatase TiO₂ are attaining the advantage of being capable to absorb a part of visible light besides the UV light. That property allows them to display the energy positions of both valence and conduction bands (Yang et al. 2014; Garcia-Muñoz et al. 2020). Due to their elite properties, those materials are applied extensively in numerous fields such as electronic applications (Rath et al. 1999). Such characteristics are alarming them as suitable candidates for wastewater oxidation applications either by the generation of highly reactive hydroxyl (·OH) radicals, the reduction of protons and/or dioxygen into the superoxide radical. However, according to the authors’ knowledge, there is a lack in literature for applying Mn–Zn ferrites as a source of superparamagnetic Fenton’s photocatalyst. As previously stated in the literature (Vinosha and Das 2018), the magnetic catalysts posses the advantage of recoverable and recyclable materials for successive use.

Herein, the current investigation highlighted the application of the high magnetic behaviour, Mn_0.6Zn_0.4Fe₂O₄ nanoparticles as a photocatalyst for dye polluted wastewater oxidation as an easy recoverable catalyst. In the light of the dependence of magnetic properties with Mn and Zn ions concentrations variations, the composition Mn_0.6Zn_0.4Fe₂O₄ was selected at which the maximum magnetization is given. As well known, as the magnetization value of the magnetic nanoparticles is higher as the collecting process of them is easier. Nanoparticle was synthesized by an economically reliable co-precipitation technique and their catalytic activity for successive use is studied to confirm their novelty as a superparamagnetic photocatalyst for sustainability use. This in turn diverts waste after wastewater remediation from disposal, keeping it in the economy as a resource, which can reduce the environmental impacts arising from waste management.

Experimental section

Synthesis of Mn–Zn–Fe₂O₄ nanostructures

Nanosized Mn_0.6Zn_0.4Fe₂O₄ particles have been synthesized via the common co-precipitation synthesis routes as a fast simple technique using mild temperature range (Vinosha and Das 2018). The required metal reagents masses are attained according to the following reaction relation (Xuan et al. 2007):

$$\left( {1 - x} \right){\text{Mn}}^{2 + } + x{\text{Zn}}^{2 + } + 2{\text{Fe}}^{3 + } + 8{\text{OH}}^{ - } \to {\text{Mn}}_{(1 - x)} {\text{Zn}}_{x} ({\text{Fe}}_{2} {\text{O}}_{4} ) + 4{\text{H}}_{2} {\text{O}}$$

(1)

where x is representing the molar percent (0 < x < 1). The precursors used during the current co-precipitation reaction are Fe₂(SO₄)₃, ZnSO₄ and MnSO₄ and all of them are used as received from the supplier (Sigma-Aldrich) without any further refinement. Thus, following the molar contents of ferrite material ${\text{Mn}}_{1 - x} {\text{Zn}}_{x} {\text{Fe}}_{2} {\text{O}}_{4}$, the stoichiometric compositions of precursors are calculated and the essential fractions were added then the solution was magnetically stirred to achieve homogeneity. Aqueous NaOH was used to adjust the pH value. Afterwards, the precipitated solution was continuously stirred under heating (80 °C) to obtain a thick precipitate. Thereafter, the as-synthesized nanoparticles were subjected for successive washing using distilled water. As the result, Mn_0.6Zn_0.4Fe₂O₄ nanoparticles were obtained and characterized.

Characterization of the prepared nanosized Mn_0.6Zn_0.4Fe₂O₄

The composition, microstructure and sizes of the synthesized Mn–Zn ferrites are analysed. The phase structure of the prepared samples characterized by X-ray powder diffractometry XRPhillips X’pert (MPD3040) diffractometer with Cu-Kα radiation at room temperature. Furthermore, the particle size and morphology of the synthesized Mn_0.6Zn_0.4Fe₂O₄ nanoparticles investigated using HR-TEM (type Tecnai G20, FEI). Additionally, hysteresis loop was traced at 300 K using Lake Shore Cryotronics Model 7410 Vibrating Sample Magnetometer (VSM).

Synozol blue wastewater and photocatalytic oxidation

Synozol Blue CA (Reactive Blue 19) is used as a reactive dye source for preparing the synthetic textile wastewater solution, which supplied by DyStar Ltd., German, and used without further purification. Firstly, a 1000-ppm aqueous stock dye solution was prepared and the solution was further diluted as required. The as-synthesized Mn_0.6Zn_0.4Fe₂O₄ was used as the catalyst source of Fenton reaction and hydrogen peroxide (30% w/v) was used to initiate the Fenton’s reagent reaction. The pH value of the synthetic wastewater solution was adjusted to the desired values, if needed, by using diluted sulphuric acid or sodium hydroxide. All chemicals were supplied by Sigma-Aldrich and used as received without further treatment.

100-mL dye-containing solution was added into a glass container; then, the Fenton reagent at specific concentrations was added after pH adjustment in order to demonstrate the effect of heterogeneous Fenton reaction. Thereafter, the solution was subjected to a magnetic stirring to ensure mixing and dispersion before exposed to a UV illumination of 253.7 nm (15 W-UV lamp, 230 V/50 Hz) by immersing the sleeved UV lamp inside a glass container containing the solution. Aliquot samples afterwards were withdrawn periodically at different time intervals for analysis. The graphical presentation of the experimental procedures is illustrated in Fig. 1.

Fig. 1

Schematic illustration of experimental procedure

Analysis

A UV–vis spectrophotometer, Unico UV-2100 model, USA, was used to investigate the residual Synozol Blue dye concentration corresponding to the maximum wavelength of 590 nm at set time intervals. Additionally, chemical oxygen demand (COD) of the treated samples was analysed using Lovibond Checkit direct COD Photometer, Germany. Prior to the samples subjected for measurements, the nanoparticles were separated using a micro-filter. The samples’ pH value was adjusted, if needed, using a digital pH meter model, AD1030, Adwa instrument, Hungary.

Central composite design model

Statistical tool based on response surface methodology, RSM, is applied for optimizing the experimental variables. Central composite experimental design (CCD) is chosen as a statistical technique. Such one is based on a multivariate nonlinear model for optimizing the response surface. That response influencing the various variables as well as categorizing the correlation between the controllable variables and the attained responses. In the current study, the influence of the most affecting parameters in the Fenton reaction, i.e. catalyst dose and hydrogen peroxide concentration was chosen to investigate their influencing on colour ($\xi$ ₁) and COD ($\xi$ ₂) removals’ efficiency of Synozol Blue dye from aqueous matrix. Four levels were chosen for each of the two variables: catalyst (τ₁) and H₂O₂ (τ₂) as tabulated in Table 1 in their original and coded values.

Table 1 Experimental range and levels of the independent factors of central composite design for Synozol Blue oxidation*

The full factorial CCD experimental design for fitting the second-order polynomial model that requires only a minimum number of experiments is given in Table 2. Generally, the optimization technique including main steps, firstly: conducting the statistically designed experiments (Table 2), followed by the estimation of the coefficients in a mathematical model, and finally predicting the response and checking the adequacy of the model. An empirical model was developed to correlate the response to the dye removal process that is based on second-order quadratic model as given by Eq. (2) to interact the interaction variables.

$$\xi_{i} = \beta_{o} + \sum \beta_{i} {\Gamma }_{i} + \sum \beta_{ii} {\Gamma }_{i}^{2} + \sum \beta_{ij} {\Gamma }_{i} {\Gamma }_{j}$$

(2)

where ${\varvec{\xi}}$ _i is the colour and COD removal responses; β_o, β_i, β_ii and β_ij are the model coefficient of the linear effect and double interactions; ${\varvec{\varGamma}}_{{\varvec{i}}}$ and ${\varvec{\varGamma}}_{{\varvec{i}}}^{2}$ are the independent variables.

Table 2 Experimental central composite design matrix of Synozol dye oxidation

Results and discussions

Structural, magnetic and morphological characterization

X-ray diffraction pattern (XRD) of the prepared Mn_0.6Zn_0.4Fe₂O₄ nanoparticles ferrites is shown in Fig. 2. The obtained XRD reveals that the powder is crystallized in a single cubic spinel phase with definite crystalline planes reported on each peak in the figure.

Fig. 2

XRD of the synthesized Mn_0.6Zn_0.4Fe₂O₄ nanopowder with M_H loop as an inset

Transmission electron micrographs, TEM, were performed for the prepared Mn_0.6Zn_0.4Fe₂O₄ ferrites nanoparticles and they are illustrated in Fig. 3 with their corresponding histogram. The micrographs reveal that the obtained particles are almost spherical in shape with nano-size ranges from 3 to 47 nm with most abundance particle size (MAPS) about 15.12 nm. It is also obvious that small number of agglomerated particles is existed together with the separated ones in the obtained powder. The agglomeration can be referred to Van der Walls or electrostatic forces between particles (Eltabey et al. 2017). As a direct proves for the obtained powder has superparamagnetic behaviour, the hysteresis loop (M-H loop) at room temperature was measured and it is illustrated as inset in Fig. 1. The obtained loop is closed one with no coercivity that is considered to be a typical superparamagnetic behaviour.

Fig. 3

TEM micrograph of the synthesized Mn_0.6Zn_0.4Fe₂O₄ nanopowder with particle size histogram

Effect of different treatment systems on reaction time

Catalytic oxidation of Synozol Blue CA dye for its removal from aqueous solution has been investigated. In this light, the effects of reaction time on Mn_0.6Zn_0.4Fe₂O₄/H₂O₂; Mn_0.6Zn_0.4Fe₂O₄/H₂O₂/UV and H₂O₂/UV oxidation systems were examined to investigate an experimental condition for further research. The efficacy of those techniques was tested in the terms of color and COD removals. Thus, firstly, the as-prepared Mn_0.6Zn_0.4Fe₂O₄ was added to the wastewater at either its natural pH value (6.8) or pH 3.0 and thereafter the reaction was initiated by adding 400 mg L⁻¹ of H₂O₂ for oxidizing Synozol Blue dye using Dark Fenton test. The results displayed in Fig. 4a and b demonstrate that both colour and COD removal are higher when the pH value of wastewater is adjusted at 3.0. The dye removal efficiency reached to only 18% and 1% for colour and COD removals, respectively, using the wastewater without pH control in comparison to 44 and 27% for colour and COD removals, respectively, when adjusting the wastewater’s pH at 3.0. Therefore, in the other set of experiments, wastewater was adjusted at 3.0 to conduct Fenton’s reaction. Thus, photo-Fenton’s reaction, Mn_0.6Zn_0.4Fe₂O₄/H₂O₂/UV, using 40 and 400 mg L⁻¹ applied under UV illumination and the removal efficiency reached to 76 and 58% for colour and COD removals, respectively. Comparing the combined Mn_0.6Zn_0.4Fe₂O₄/H₂O₂ system with the solo hydrogen peroxide system, under the UV illumination, the removal reached to 31 and 7% for colour and COD removals, respectively.

Fig. 4

Effect of different treatment systems on Synozol Blue removal (a) Colour removal; (b) COD removal

It is clear from comparing the results in Fig. 4a and b that the UV illumination is an effective tool for solo H₂O₂ oxidation or photo-Fenton reaction as the removal is much higher in comparison to the dark reaction test. It is noteworthy to mention that the photo-Fenton reaction is the most effective reaction for dye oxidation. Also, 15 min of reaction time is enough to attain such removals for the Fenton reaction time, t_R and after that time a plateau is attained in all Fenton combinations. But, with prolonging of oxidation time, the Synozol Blue dye removal slowed down and its removal efficiency became stabilized. However, 60 min time is needed for the solo hydrogen peroxide system to complete the reaction.

This rapid oxidation rate in the initial stage for all the Fenton systems and H₂O₂ oxidation is associated with the ˙OH radicals as the reaction intermediates. Such radicals are the main responsible of the dye oxidation. As the time exceeds, ˙OH radicals produced are declined. Besides, a reduction in the H₂O₂ amount is attained with the formation of other radicals that presents in the reaction medium and inhibits the oxidation rate. Furthermore, Fenton reaction is utilizing H₂O₂ reagent for activating the catalyst. However, such reagent, H₂O₂, is consumed after the initial oxidation period. Afterwards, more radicals are produced and the reaction becomes more complicated. The radicals generated in such case are so-called hydroperoxyl radicals, HO₂, which declines the overall oxidation rate with the time increase. Thus, those radicals inhibit the oxidation of Synozol dye rather than oxidizing them (He and Lei 2004).

In the solo H₂O₂ system, as the time proceeds, the peroxide is consumed and therefore, the dye oxidation rate is reduced. According to the previous investigations (Najjar et al. 2001; He and Lei 2004; Tony 2021a, b, c, d), numerous studies are dealing with the phenomenon of the oxidation rate that is higher at the initial reaction time and declines while the reaction proceeds. This is related to the ˙OH radicals’ production that is too high at the first reaction stage. Although the same rapid initial dye oxidation tendency occurs in the initial stage of all the Fenton’s systems, the removal is limited to only 18% for dye removal using Dark Fenton test. This is related to the presence of the UV illumination that initiating more ˙OH to produce and thus the oxidation rat is enhanced (Maroudas et al. 2021).

Effect of contaminant load

Fenton oxidation set was made at various initial dye concentrations ranged from 15 to 100 ppm in synthetic dye medium and the treatment data are displayed in Fig. 5a and b for colour and COD removals, respectively. From a quick scan of previous studies (Shanmugam et al. 2019; Tony and Lin 2020a; Abdollahzadeh et al. 2020), it was observed that oxidation rate is so far highly dependent on the initial pollutants load. Hence, in this regard, the effect of dye load is investigated at pH value 3.0 with 40 and 400 mg L⁻¹ of catalyst and H₂O₂ concentrations, respectively. According to the experimental results in Fig. 5, the dye oxidation rate was found to be dependent on the initial dye concentration. A maximum of 96% dye oxidation could be achieved within 15 min Fenton’s oxidation when 15 ppm of dye load is treated. However, the higher dye load is significant in making the process feasible for practical applications. Thus, higher dye loads are also investigated; however, the oxidation rate is declined with increasing the dye load. At a glance of Fig. 5a and b, the rate declined from 96 and 78% to 75–34% and 60–17% for colour and COD removals, respectively, for the gradual increase in the dye loads from 15 to 100 ppm.

Fig. 5

The effect of Synozol Blue load on the Mn_0.6Zn_0.4Fe₂O₄/ H₂O₂/UV oxidation (Catalyst 40 mg L¹, H₂O₂ 400 mg L⁻¹, pH 3.0)

The rate and efficacy of Synozol Blue dye oxidation decrease with increasing the initial dye load. This could be attributed to the increased concentration of the toxic dye in the reaction medium leads to a decrease in the ˙OH radicals, while the Fenton’s reagent concentrations kept constant. Such explanation is in a good agreement with that earlier stated as the oxidation reaction is highly dependent on those radicals (Zafar et al. 2020; Tony 2021a, b, c, d). In addition, Shanmugam and his co-workers (2019) had been previously reported the same trend in treating acid blue 113 dye using Fenton’s reagent.

Fenton’s multiple parameters

Catalyst, H ₂ O ₂ and pH value effect

Decomposition of H₂O₂ in the presence of a catalyst, Mn_0.6Zn_0.4Fe₂O_4, for generating hydroxyl radicals is essential for Fenton’s reaction. However, it is crucial in keeping either H₂O₂ or catalyst concentrations minimal since the higher dosage declines the reaction efficiency. Thus, to examine the influence of Mn–Zn ferrites catalyst on the Fenton’s oxidation of Synozol Blue CA dye, experiments were undertaken to determine the influence of catalyst dose on reaction kinetics.

Figure 6a and b illustrates the effective dye oxidation process by varying the concentration over the range of 20–80 mg L⁻¹. It is obvious that the oxidation efficiency increases from 51 to 72% when the superparamagnetic Mn_0.6Zn_0.4Fe₂O₄ catalyst concentration increases from 10 to 20 mg L⁻¹. Additionally, further catalyst increase to 40 mg L⁻¹ results in higher removal efficiency reaches to 96%. With a further increase in catalyst more than to 40 mg L⁻¹, a marginal decrease in the removal rate to 87% is attained. The same trend is observed for the COD reduction efficiency.

Fig. 6

Effect of operating parameters on Mn_0.6Zn_0.4Fe₂O₄/H₂O₂/UV system: Effect of catalyst (H₂O₂ 400 mg L⁻¹, pH 3.0) on (a) colour and (b) COD removals; Effect of H₂O₂ (Catalyst 40 mg L¹, pH 3.0) on (a) colour and (b) COD removals; (c) Effect of pH (Catalyst 40 mg L¹, H₂O₂ 400 mg L⁻¹) on (a) colour and (b) COD removals

Mn, Zn and iron ions are critical for creating the photoactive hydroxo-complexes, which absorb the photons in the UV illumination and then generate the (·OH) radicals. The metal ions are formed and react with H₂O₂ to form further ·OH radicals and metal ions again. Synozol dye contains aromatic structures, ·OH attack the aromatic rings in the dye molecules, then get hydroxylated and build up a hydroycyclohexadienyl radical as a result from the ·OH attack on the aromatic ring. The overall reaction is strongly oxidizing the dye molecules (Chen and Pignatello 1997).

Figure 6c and d displays the effects of the increase in H₂O₂ concentration on Synozol Blue oxidation rate while all other parameters are kept constant (catalyst 40 mg L⁻¹ and pH 3.0). The results demonstrate that there is an increase in the dye removal rate (96 and 73% for colour and COD, respectively) with the peroxide increase. Moreover, the results showed that 400 mg L⁻¹ is recorded the optimal peroxide dose. Such investigation correlates with previous investigation by the author (Tony 2020a). At excess H₂O₂ dosage more than the optimal dose, H₂O₂ itself would act as hydroxyl radical scavenger than a producer. Hence, the result is a terminal effect on the dye removal (71% for 800 mg L⁻¹ H₂O₂ addition). This observation was previously remarked by Rezgui et al. (2021).

According to the previous studies (Shanmugam et al. 2019; Rezgui et al. 2021), Fenton reaction is so sensitive to the pH value of the aqueous media. Figure 6e and f displays that the decrease in the pH value is corresponding to a high colour and COD removal efficiency. This fact illustrates that the Fenton reaction is more active in the acidic range, while in alkaline conditions; hydrogen peroxide oxidizes itself into H₂O and O₂ that renders the oxidation reaction (Shanmugam et al. 2019). Therefore, the pH of the wastewater was kept in the acidic range by manual addition of sulphuric acid for oxidizing via Fenton reagent.

Oxidation mechanism

Under the UV illumination, Mn_0.6Zn_0.4Fe₂O₄ was irradiated, and an electron (e)/hole (h) pair was generated on its surface according to Eq. (3). This photo-induced hole could react with water (H₂O) or may be hydroxyl ion (OH) to produce hydroxyl radicals (˙OH) (as seen in Eq. (4 and 5). Also, the photo-induced electron could be captured via the hydrogen peroxide reagent and then ˙OH radicals are produced. Such reaction may limit the recombination of holes and electrons and hence improving the catalytic activity of the catalyst. Additionally, H₂O₂ will react with Fe³⁺ and Fe²⁺ to activate the Fenton reaction that generating more hydroxyl radicals (Eq. 6, 7). Thus, hydroxyl radicals are produced through different pathways, and an enhancement in the rate of oxidation was observed (Ramirez et al 2007; Sharma et al. 2015).

$${\text{Mn}}_{0.6} {\text{Zn}}_{0.4} {\text{Fe}}_{2} {\text{O}}_{4} + hv \to {\text{Mn}}_{0.6} {\text{Zn}}_{0.4} {\text{Fe}}_{2} {\text{O}}_{4} \left( {e_{{{\text{CB}}}}^{ - } + h_{{{\text{VB}}}}^{ + } } \right)$$

(3)

$$h_{{{\text{VB}}}}^{ + } + {\text{H}}_{2} {\text{O}} \to {\text{H}}^{ + } + {\text{OH}}^{.}$$

(4)

$$h_{VB}^{ + } + {\text{H}}^{ - } \to {\text{OH}}^{ \cdot }$$

(5)

$${\text{Fe}}^{3 + } + {\text{H}}_{2} {\text{O}}_{2} \to {\text{Fe}}^{2 + } + {\text{HOO}}^{ \cdot } + {\text{H}}^{ + }$$

(6)

$${\text{Fe}}^{2 + } + {\text{H}}_{2} {\text{O}}_{2} \to {\text{Fe}}^{3 + } + {\text{OH}}^{ - } + {\text{OH}}^{ \cdot }$$

(7)

H₂O₂ promotes the reaction medium with ˙OH radicals after initiation with UV illumination according to Eq. (8). However, higher doses of such reagent more than the optimal concentration inhibit the oxidation rate since the peroxide competing the pollutants for reacting with the ˙OH radicals (Eq. 9) (Neamtu et al. 2002). Also, recombination reaction of ^•OH radicals and likewise ^•OH radical is further scavenging the overall radicals produced (Eq. 10, 11) (Tamimi et al. 2008). Such generated ˙OH radicals attacking the Synozol dye molecules that consists of aromatic structures to get them hydroxylated and build up a hydroycyclohexadienyl radical as seen in Eq. (12).

$${\text{H}}_{2} {\text{O}}_{2} + hv \to 2^{ \cdot } {\text{OH}}$$

(8)

$${\text{H}}_{2} {\text{O}}_{2} + {\text{HO}}^{ \cdot } \to {\text{HO}}_{2}^{ \cdot } + {\text{H}}_{2} {\text{O}}$$

(9)

$${\text{HO}}_{2}^{ \cdot } + {\text{OH}}^{ \cdot } { } \to {\text{H}}_{2} {\text{O}} + {\text{O}}_{2}$$

(10)

$${\text{OH}}^{ \cdot } { } + {\text{OH}}^{ \cdot } { } \to {\text{H}}_{2} {\text{O}}_{2}$$

(11)

(12)

The ability of hybrid combined metal nanoparticles to absorb light as a photocatalyst affecting the pollutants oxidation. Combining Mn with ZnO in ferrite molecule explains the high performance of such photocatalyst in the visible light-driven band. The photoelectrons transferred in the catalyst surface and thus^.O₂ radicals is formed via the reaction with the solvated oxygen. In the other route, the left H⁺ in the valence band may additionally react with H₂O to form hydroxyl radical, which then could together with^.O₂ may oxidize the organics. Therefore, via such mechanism, the electron–hole pairs are more effective and their lifetime is prolonged (Thakur et al. 2020; Abdel Maksoud et al. 2020; Li et al. 2021a, b; Fang et al. 2021). Moreover, the active ·OH radicals generated through the reaction attack the aromatic structure contained on the dye molecules to hydroxylate it by forming intermediate radicals to finally mineralize the dye molecules (Chen and Pignatello 1997; Thabet et al. 2021a).

Comparison of the Synozol dye treatment technologies using the superparamagnetic modified Fenton type reaction (Mn_0.6Zn_0.4Fe₂O₄/ H₂O₂/UV) from the current study with those from previous studies, which are stated in the literature, is tabulated in Table 3. It could be concluded that Mn_0.6Zn_0.4Fe₂O₄/H₂O₂/UV oxidation attained an efficient superior treatment for Synozol dye removal (96%) in comparison to other used systems. Although Fenton treatment is a dual treatment system from a combination of a catalyst and a H₂O₂ compared to other treatment systems reported in Table 3, such systems exhibited some drawbacks. Such drawbacks including the long reaction time, the need of biomass and quantities of chemical agents are not recorded in the current Fenton system. Thus, the end products from such treatments in comparison to Fenton system are the formation of toxic end products. Also, the other treatments’ cost is expensive compared to the Fenton’s cost. Besides, the use of the physicochemical treatments, i.e. adsorption treatment, the result is the formation of secondary effluents, which may need further treatments since they could not mineralize the pollutants. Hence, such groups of disadvantages are not related to the current Fenton system besides Fenton’s reagent is an environmentally friendly technique compared to that listed in Table 3. Therefore, this is recommending such technology for reactive dye removals.

Table 3 Comparison of different treatment methodologies for Synozol dye contaminated wastewater matrix*

Experimental design and optimization

The effect of the two independent critical parameters, i.e. catalyst and H₂O₂ concentrations on the Fenton oxidation of Synozol dye was simulated using central composite experimental design (Table 2). The quadratic polynomial model equation form validates the associated response functions colour removal ($\% \xi_{1} )$ and COD removal $(\% \xi_{2}$) as follows (SAS 1990):

$$\xi_{1} \left( \% \right) = 95.40 - 4.22\Gamma_{1} + 1.34\Gamma_{2} - 15.51 \Gamma_{1}^{2} + 2.00\Gamma_{1} \Gamma_{2} - 13.26\Gamma_{2}^{2}$$

(13)

$$\xi_{2} \left( \% \right) = 89.60 - 3.92\Gamma_{1} + 1.114\Gamma_{2} - 14.61 \Gamma_{1}^{2} + 1.75\Gamma_{1} \Gamma_{2} - 13.11\Gamma_{2}^{2}$$

(14)

Fisher’s statistical test (F-test) for analysis of variance (ANOVA analysis) (Table 4) was performed to investigate the statistical significance and the suitability of the proposed quadratic model. Generally, the model is recognized to be the best fit with a low standard deviation, a small probability value (Pr < 0.005) and a high regression coefficient (r²) (SAS, 1990). As seen from Table 5, r² for the oxidation reaction is high for the two models with a low probability (Pr) value of 0.0001. Additionally, a good correlation between the predicted and experimental values of the quadratic model was attained for the two models as given in Fig. 7a and b.

Table 4 ANOVA for the regression model and the respective models terms*

Table 5 Photocatalytic kinetic models and statistical analysis of Synozol Blue oxidation via Mn_0.6Zn_0.4Fe₂O₄/H₂O₂/UV*

Fig. 7

Central composite design (CCD) optimization of Mn_0.6Zn_0.4Fe₂O₄/H₂O₂/UV system: (a) Original and estimated ζ₁ response plot; (b) Original and estimated ζ₂ response plot; (c) 3-D surface and contour plot of ζ₁ model; (d) 3-D surface and contour plot of ζ₂ model; (e) Overlay contour plots for the optimal region of two responses (ζ_1, ζ₂); (f) Comparison of Synozol dye oxidation at manually optimized conditions and via CCD values

The experimental parameters effects on the examined responses are presented graphically in Fig. 7 (c, d) that illustrates the response of each experimental variable and the major interactions between those variables. Examination of the 3-D surface graphs and contour plots in Fig. 7c and d illustrates that the colour and COD removals increase with increasing the concentrations of both Mn_0.6Zn_0.4Fe₂O₄ and H₂O₂. However, the curvature of the 3-D surface shown in Fig. 7c and d reveals that there is an interaction effect between catalyst and H₂O₂ doses. Such interaction supports the formation of hydroxyl radical intermediates that positively oxidizing the Synozol Blue molecules and mineralizing them. However, a further increase in the reagent doses results in a reduction in the dye removal rate. Thus, the optimal ratio of the catalyst/H₂O₂ is needed to maximize the hydroxyl radical production yield.

Also, a graphical optimization displayed in Fig. 7e locates that the area (lighter portion zone) of the optimal condition of Fenton reaction (Mn_0.6Zn_0.4Fe₂O₄ based system) for Synozol Blue CA dye oxidation. It can be seen that the optimal feasible response values are in the factors space. All the independent variables and their effective responses (colour and COD removals) with the particular high and low limits of the experimental region are significant and fit the standards.

To verify the validity of the proposed models, the statistically optimized values of the operating parameters attained from Mathematica software (V 5.2), for both catalyst and H₂O₂ were 39 and 404 mg L⁻¹, respectively, and additional three replicates of experiments were performed. The attained predicted removal values of colour and COD (which are presented in Fig. 7f and the inset) are compared with the manually optimized conditions. After 15-min of reaction time, the measured percentage of colour and COD removals attained as 98 and 87%, respectively, that is close to the predicted value (96 and 89%), using CCD. These results verify that the RSM based on CCD is adequate approach for optimizing the operating parameters influencing dye oxidation via Fenton’s reaction based on Mn_0.6Zn_0.4Fe₂O₄ nanoparticles as a superparamagnetic catalyst.

Continuous-flow performance

To further investigate the feasibility of treating Synozol Blue dye wastewater on a large scale, a continuous-flow laboratory-scale experiment was examined as an indication for practical application (Ashour and Tony 2017; Wang et al. 2021). A stock solution of Synozol Blue dye wastewater was pumped at various volumetric flow rates at a steady rate to a container under the UV reactor that is the same as in the batch experiment and stirred with a magnetic stirrer.

The essential parameter examined in this series of experiments was the effect of the hydraulic retention time, HRT, in the UV illumination reactor on the degree of oxidation of the Synozol Blue in wastewater as illustrated by the following relationship: ${\varvec{HRT}} = \frac{{\varvec{V}}}{{\varvec{Q}}}$, where V is the volume of the wastewater in litre and Q is the volumetric flow rate in L min⁻¹. The results displayed in Fig. 8 illustrate the rate of the dye oxidation for the continuous process operating at various HRTs. The data showed that, following the initial period of each experiment, there was a transient phase during which the dye oxidation rate evaluated by the colour removal rates is increased, followed by a steady state removal rate. This is in accordance with the might reasonably be estimated, the high HRT attain an increase in stabilization of the effluent and vice versa. Such results are in agreement with the previously reported in the literature (Zou et al. 2020) that treated pharmaceuticals contaminated wastewater with such Fenton’s reagent.

Fig. 8

Synozol Blue dye oxidation with the reaction time for the continuous Mn_0.6Zn_0.4Fe₂O₄/H₂O₂/UV system (Catalyst 40 mg L¹, H₂O₂ 400 mg L⁻¹, pH 3.0)

Temperature effect on kinetics and thermodynamics

Temperature is an energetic influencing factor on the oxidation reactions since it affects the reaction rates. To examine its influence on the reaction, experiments over a temperature range of 26 -60ºC were conducted. The data in Fig. 9a and b revealed that a decrease in reactive dye removal efficiency from 96 to 67% and from 78 to 49% for colour and COD removals, respectively, through the temperature increase in the studied range. This may be attributed to, at high temperatures; H₂O₂ decomposes into O₂ and H₂O rather than the formation of the highly reactive hydroxyl radicals. Thus, H₂O₂ reagent role becomes a free radical scavenger rather than a producer, which reduces the oxidation reaction. Previous work reported similar reaction trend in treating organic loaded wastewater via Fenton’ s reaction (Guede et al. 2003).

Fig. 9

Effect of temperature on Synozol Blue removal (a) Colour removal; (b) COD removal

Temperature's influence on Synozol Blue oxidation also was evaluated by kinetic and thermodynamic parameters. Kinetic parameters were obtained by applying the linearized form of zero-, first- and second-order kinetic models, to give estimation for the reactor design and system control that highlighting the treatment cost for both capital and operating expenses (Al Momani et al. 2008; Tony 2021a). Table 5 displays the estimated zero-, first- and second-order kinetic models. Examination of the regression coefficient (r²) values shows that the highest values are corresponding to the first-order kinetic model. Thus, this verified the first-order is the best fit of the experimental results of Synozol oxidation through the modified Fenton’s type reaction. As tabulated in Table 5, the first-order model rate constants (k_F) are decreased from 0.2901 to 0.0746 L mg⁻¹ min⁻¹ as the temperature increasing from 26 to 60 °C. Also, the corresponding half-reaction time (t_0.5) are decreased with increasing the temperature. This examination is in agreement with the previously recommended in the literature (Pintor et al. 2011; Ioannou and Fatta-Kassinos 2013) as they suggested the maximal Fenton’s reaction yield is attained within the temperature range of 17–38 °C.

To further explore the temperature effect on the oxidation process of Synozol Blue, the thermodynamic activation parameters were obtained by the Arrhenius equation that is based on the first-order kinetic model ($\ln k_{{\text{F}}} = { }\ln A - \frac{{E_{{\text{a}}} }}{RT}$), where A is the pre-exponential factor constant; Ea is the energy of activation (kJ mol⁻¹); R is the gas constant (8.314 J mol⁻¹ K⁻¹) and T is temperature (K). The linear plot of ln k_F versus 1/T gives a relationship whose slope is corresponding to (− Ea/R) that could be used to calculate Ea (Fig. 10). The thermodynamic activation parameters of the oxidation are evaluated by the Eyring equation ($k_{{\text{F}}} = \frac{{k_{{\text{B}}} T}}{h}{\text{e}}^{{\left( { - \frac{{\Delta G^{^\circ } }}{RT}} \right)}} )$, where k_B and h are Boltzmann and Planck’s constants, respectively. Thus, the enthalpy (∆Hº) and the entropy (∆Sº) of activation could be calculated from the relation of $\Delta H^{^\circ } = E_{a} - RT$ and $\Delta S^{^\circ } = \left( {\Delta H^{^\circ } - \Delta G^{^\circ } } \right)/T$, respectively (Ahmadi et al. 2016). The results attained from such relations are displayed in Table 6 indicated that the non-spontaneously of the oxidation process since ∆Gº > 0 and the degree of non-spontaneity increases as the temperature increases. With the positive ∆Hº values, the endothermic oxidation reaction nature is verified. Also, the negative ∆Sº values confirm the non-spontaneous nature of the oxidation process which exhibited a decrease in the degree of freedom of the dye molecules and maintained a high radicals species, ·OH, yield. Previous work by Argun and Karatas (2011) reported the endothermic Fenton oxidation process. Additionally, Pourali et al. (2020) highlighted the non-spontaneous behavior for treating dye-polluted wastewater using ZnO catalyst. Also, the results display that the dye oxidation via modified Fenton system typically proceeds at 34.53 kJ mol⁻¹. This is considered as a low energy barrier. Such result is in a agreement with the results reported by Ahmadi et al. (2016) (45.84 kJmol-1) for oxidizing Reactive Yellow 84 dye by potassium peroxydisulfate and Sun et al. (2007) (53.96 kJ mol⁻¹) for p-nitroaniline oxidation with Fenton’s reagent.

Fig. 10

Plot of ln k_F versus 1000/T for the Synozol Blue oxidation reaction (solid lines represent least-squares fitting)

Table 6 Thermodynamics parameters of Synozol Blue dye oxidation via Mn_0.6Zn_0.4Fe₂O₄/H₂O₂/UV

Recyclability

The superparamagnetic Mn_0.6Zn_0.4Fe₂O₄ catalyst stability and recyclability is considered a superior importance for its long-term applicability in the field of wastewater treatment. Heterogeneous Mn_0.6Zn_0.4Fe₂O₄ based catalysts are an excellent example as a long-term catalyst reusability example since they possess an excellent magnetic characteristic. Thus, their ability for separation could be easily applicable from the reaction medium after Fenton’s reaction through an external magnet. After the catalyst separation, it is subjected for successive washing with distilled water then followed by oven drying (105 °C) (Tony 2021b; Tony and Lin 2020b). Then, the recovered and regenerated catalyst is used and the mechanism is repeated. The results displayed in Fig. 11 demonstrated that a well catalytic activity is achieved during the range of experimental study till the eighth cycle of catalyst reuse, which confirms the high stability of the synthesized Mn_0.6Zn_0.4Fe₂O₄ photocatalyst. Thus, the study verified their promising sustainability for the use in treating industrial wastewater effluents. The typical oxidation percentage of the Synozol Blue dye via superparamagnetic Mn_0.6Zn_0.4Fe₂O₄ based Fenton system for 8 successive cycles are presented in Fig. 11. It is worth to mention that due to the superparamagnetic catalyst features’, releasing the metal ions in the aqueous solution is difficult to occur which confirms the catalyst sustainability.

Fig. 11

Reusability of Mn_0.6Zn_0.4Fe₂O₄ for successive cycles use under similar conditions

Conclusion

The current study attempted to explore the applicability of Fenton process using the superparamagnetic Mn_0.6Zn_0.4Fe₂O₄ nanopowder for oxidizing textile dying wastewater containing Synozol Blue dye. The experimental results of the study revealed that Fenton process by magnetic catalyst could be an appropriate treatment alternative for Synozol Blue dye removal, providing high dye and COD removal efficiencies with minimum chemicals consumption. The colour and COD oxidation as target efficiencies were found to be a function of the initial pH, H₂O₂ and catalyst doses. Response surface methodology (RSM) based on central composite design (CCD) has been successfully employed for process optimization and the optimum operating values were 39 and 404 mg L⁻¹ of Mn_0.6Zn_0.4Fe₂O₄ and H₂O₂, respectively, at 15 min of reaction at acidic pH conditions. The pseudo-first-order kinetic model is accepted to be well fitted with the experimental data. Overall, the results obtained for Synozol Blue dye removal using the Mn_0.6Zn_0.4Fe₂O₄ based Fenton reaction verify the significant role of the recyclable magnetic nanoparticles in oxidizing dye molecules from aqueous effluents. Such experimental data, derived from the synesthetic textile dying wastewater, add to a knowledge base for treating real textile wastewater matrix treatment applications.