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Article

Efficient Methylene Blue Degradation by Activation of Peroxymonosulfate over Co(II) and/or Fe(II) Impregnated Montmorillonites

by
Niurka Barrios-Bermúdez
1,2,
Arisbel Cerpa-Naranjo
3 and
María Luisa Rojas-Cervantes
1,*
1
Departamento de Química Inorgánica y Química Técnica, Facultad de Ciencias, Universidad Nacional de Edcuación a Distancia, UNED, 28232 Madrid, Spain
2
Departamento de Ciencias, Escuela de Ingeniería, Arquitectura y Diseño, Universidad Europea de Madrid, 28670 Madrid, Spain
3
Departamento de Ingenierías, Escuela de Ingeniería, Arquitectura y Diseño, Universidad Europea de Madrid, 28670 Madrid, Spain
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(8), 479; https://doi.org/10.3390/catal14080479 (registering DOI)
Submission received: 5 July 2024 / Revised: 22 July 2024 / Accepted: 25 July 2024 / Published: 27 July 2024
(This article belongs to the Section Catalytic Materials)
Browse Figures
Versions Notes

Abstract

:
Two commercial montmorillonites, namely montmorillonite K10 (MK10) and montmorillonite pillared with aluminum (MPil) were impregnated with cobalt(II) and/or iron(II) acetates by incipient wetness impregnation and used to activate peroxymonosulfate (PMS) for the degradation of methylene blue (MB) dye in water. Various characterization techniques, including ICP-MS, XRD, SEM and TEM with EDX, and N2 physisorption, confirmed the successful impregnation process. The removal of the dye resulted from a combined effect of adsorption and PMS activation through Co3+/Co2+ redox couples. The MK10 series exhibited a higher degree of dye adsorption compared to the MPil series, leading to enhanced dye decomposition and superior catalytic performance in the former. The influence of catalyst mass, dye concentration, and initial pH was investigated. SO4•− radicals were found as the dominant reactive oxygen species. Co2+-impregnated montmorillonites showed better performance than their Fe2+-impregnated counterparts, with MK10-Co achieving complete MB removal in just 20 min. High degradation values of MB were achieved using lower PMS/MB ratios and amount of catalyst than others reported in the literature, showing the efficiency of cobalt-impregnated montmorillonites. Moreover, the catalysts maintained excellent catalytic activity after three reaction cycles.

Graphical Abstract

1. Introduction

Over the past two decades, advanced oxidation processes have found widespread application in the removal of stubborn organic compounds from water [1]. Within this category, processes utilizing peroxymonosulfate have garnered special attention [2,3], primarily owing to the elevated redox potential and extended shelf-life of sulfate radicals in comparison to OH radicals [4]. Furthermore, peroxymonosulfate shows a higher selectivity for unsaturated bonds and aromatic constituents. Potassium peroxymonosulfate (2KHSO5·KHSO4·K2SO4), known by the commercial names of Oxone (from DuPont) and Caroat (from Evonik), stands out as an asymmetric oxidant with the capability to generate both sulfate radicals (SO4•−) and hydroxyl radicals HO• [5], together with peroxysulfate radicals (SO5•−), that are less efficient to attack the organic compounds because of their weaker oxidizing ability. In order to enhance the efficacy in generating reactive oxygen species, activation of peroxymonosulfate can be achieved through various means, such as transition metal ions [6], metal oxides [7], ultraviolet radiation [8], heat, [9], ultrasound [10], and so on.
Iron and cobalt ions have demonstrated efficiency as activators for peroxymonosulfate [11,12]. Nonetheless, the effectiveness of this method is impeded by challenges such as metal leaching, leading to significant secondary pollution, and the agglomeration of metal oxide particles, which hinders the efficiency of contaminant degradation. These substantial drawbacks can be successfully addressed by employing heterogeneous catalysts, such as iron or cobalt systems supported on oxides, carbons, perovskites, and other materials [3,13,14,15].
Montmorillonites, being abundant and cost-effective natural aluminosilicates, possess distinctive attributes such as high expansion capacity, significant specific surface area, and elevated cation exchange capacity, coupled with their economic viability. These qualities render them excellent candidates for deployment as promising adsorbents [16] or support for heterogeneous catalysts in environmental remediation efforts [17,18,19,20]. Among the methods available for supporting metals on montmorillonites, incipient wetness impregnation stands out as a simple and cost-effective approach, offering reproducible metal loading, although it is limited by the solubility of the metal precursor [21].
Synthetic dyes, characterized by being colored organic compounds, often contaminate and colorize effluent water discharged from industries including textile, leather, food processing, paper, and dye manufacturing [22]. The risks derived from them are diverse, depending on their chemical composition, and the problems associated with the environment have been revised by different authors [23,24]. Due to their intricate aromatic structures, these dyes prove highly stable and challenging to eliminate through conventional wastewater treatments, and therefore, the development of new and more efficient systems has become a critically important objective. Among these methods, advanced oxidation processes, such as Fenton-like reactions or those using the activation of peroxymonosulfate, which are based on the generation of different oxidizing radicals, have been demonstrated to be very effective [25,26] in the degradation of dyes.
Methylene blue, [7-(Dimethyl-amino)phenothiazin-3-ylidene]-dimethylazanium chloride, is a cationic organic dye with different applications in the textile industry, to dye paper and also in medicine for diagnostic and therapeutic purposes; its presence in wastewater is toxic to aquatic organisms. Montmorillonites have been effectively applied to eliminate this dye by adsorption [16,27] or by reduction with NaBH4 [28]. The removal of methylene bluethrough the activation of peroxymonosulfate by different heterogeneous catalysts has been widely reported. Among others, materials based on perovskites [29,30], MOFs [31,32,33], nanoparticles [34,35,36], or carbon materials [37,38,39], have been investigated. However, to our knowledge, there are no articles studying the use of montmorillonites in this system.
In this work, we present the utilization of several catalysts based on two types of montmorillonites, a montmorillonite MK10 and a pillared (Al) montmorillonite, MPil, as activators of peroxymonosulfate for the degradation of methylene blue. These montmorillonites have been impregnated with iron(II) and/or cobalt(II) acetates to synergize the favorable surface properties of montmorillonites with the high efficiency of both Fe3+/Fe2+ and Co3+/Co2+ pairs in decomposing peroxymonosulfate. Various parameters, including catalyst amount, methylene blue concentration, and reaction pH, have been systematically investigated. Furthermore, the stability and recyclability of the catalysts have also been studied in this work.

2. Results and Discussion

2.1. Characterization of the Samples

Figure 1 illustrates the diffractograms of the samples, with clear indications of the main planes corresponding to the diffraction peaks of montmorillonites.
In the MK10 sample and its associated impregnated samples, distinct quartz and mica impurities are evident. The MPil diffractogram reveals the presence of a diffraction peak attributed to the 001 reflection (basal reflection), resulting from the pillaring of the sheets. Upon impregnation of montmorillonites with the respective metal salts, a reduction in crystallinity is observed. However, no additional peaks associated with phases of iron and/or cobalt compounds have been detected, indicating a high dispersion of the metallic oxides or hydroxides formed, leading to particles with a crystallite size less than 4 Å. It is plausible that the percentage of metal employed in the impregnation may not be adequate to ensure the formation of crystals with a size detectable by XRD. This aligns with findings from Milovanovic et al. [40], who noted an absence of Co3O4 crystals in the diffractograms of montmorillonites impregnated with cobalt nitrate for loads less than 10%.
The metal content values, determined by ICP-MS, are presented in Table 1. Given that the pristine montmorillonites inherently contain iron, the iron incorporated post-impregnation (denoted as Fei in Table 1) was computed as the subtraction between the content of the impregnated samples and that of the corresponding support, MK10 or MPil. It is worth mentioning that the overall metal concentrations are slightly below the theoretical values, although for MK10Co and MPilCo, they are very close to the expected levels.
All isotherms of the MK10 and MPil series belong to the IV type according to the IUPAC and are reversible at lower equilibrium pressures with the H3 type of hysteresis loop for P/P0 > 0.4 (Figure 2), indicating of multilayer nitrogen adsorption and capillary condensation within montmorillonite mesopores [41,42], typical features of layered samples [43]. These findings are expected for materials consisting of aggregated planar particles and slit-shaped channels type of pores [44]. In the case of the MPil series, the isotherms exhibited a significant increase in the amount of adsorbed nitrogen at pressures corresponding to micropores, which is proof of successful pillaring [45] in the commercial sample used as support.
Samples based on MK10 predominantly exhibited mesoporosity, as evident from the comparison of Vmes and Vp, along with the minimal or absent contribution of Smic to SBET values (see Table 2). The mesoporosity in these samples is primarily associated with interparticle spaces. In contrast, samples from the MPil series contain a higher proportion of micropores as a result of the pillaring process in the pristine sample, with micropores significantly contributing to the BET surface (compare Smic and SBET values). Upon impregnation of montmorillonite supports with acetates, a decline in SBET and Vp occurs due to the blockage of both micropores and mesopores by the metallic phases. In the case of impregnated pillared samples, the decrease in mesopore volume (39–43%) relative to MPil is notably higher than the reduction in micropore volume (ranging between 10 and 29%). This suggests that for this series, metallic phases were primarily incorporated into mesopores, although micropores were also filled, as deduced from the reduction in the surface area of micropores (Smic) after impregnation. Additionally, there is a shift towards a higher average mesopore diameter (Table 2, Figure S1B). Conversely, the decrease in mesopore volume is less pronounced in the impregnated samples of the MK10 series (compared to MK10), especially when compared to the MPil series. The BJH distribution curves of the MK10 series remain unaltered (Table 2, Figure S1A). In summary, the samples from the MPil series contain a lower mesopore volume and larger mesopores compared to the corresponding samples from the MK10 series.
The metal dispersion and the morphology of samples were studied by TEM and SEM. The TEM images of samples of the MK10 series are shown in Figure 3. Montmorillonite sheets with smooth and well-defined edges can be observed in Figure 3A,B. The presence of iron in pristine MK10 was detected in the EDX spectrum (Figure 3B), affording a value of 2.15 wt%, somewhat higher than that found by ICP-MS (1.4%, Table 1). The incorporation of the compounds of iron and/or cobalt into the matrix of montmorillonites was also corroborated by analyzing the corresponding EDX spectra of TEM images of the catalysts (Figure 3D–F). For MK10-Fe, the value of Fe (7.34%) is similar to the total determined by ICP-MS (7.70%, Table 1). For MK10Fe-Co and MK10-Co, the values for Fe and Co are higher than those reported in Table 1. Considering that the ICP-MS technique determines the metal content in the bulk of the sample and analysis by TEM-EDX is more focused on the surface, these results indicate an irregular dispersion of the metals and an enrichment for these specific zones studied by TEM.
The TEM images of the samples of the pillared series (Figure S2) and their EDX spectra also confirm the lamellar structure and the incorporation of the iron and cobalt species into the montmorillonite matrix. According to the metal content determined by EDX spectra, an enrichment in metals of the surface with respect to that measured in the bulk phase is observed, similar to what occurred for the MK10 series.
Figure 4 displays SEM images and their corresponding EDX spectra for selected samples. The pristine montmorillonite MK10 exhibits a smooth and well-defined flake structure (Figure 4A), with Fe/Si and O/Si atomic ratios of 0.02 and 4.03, respectively. These ratios increase to 0.17 and 5.64 for the MK10Fe-Co sample (EDX spectrum in Figure 4B) and 0.12 and 20 for MK10Fe, indicating the presence of iron (hydro)oxides. The EDX spectra in Figure 4D,E further confirm the incorporation of metallic phases into the montmorillonite matrix in the MPil series. The presence of cobalt is detected in the spectrum of MPilCo (Figure 4E), with an associated increase in the O/Si ratio from 10.62 for the MPil sample to 15.19 for MPilCo, which indicates the existence of cobalt (hydro)oxides.
As a resume, the analysis performed by SEM and TEM demonstrates the incorporation of the iron and cobalt species into the matrix of both pristine montmorillonites, MK10 and MPil.

2.2. Degradation Performance of MB by Montmorillonites/PMS

The ability of both series of montmorillonites to facilitate the decomposition of PMS and generate active radicals for the oxidation of methylene blue (MB) was explored. Previous studies [3,46,47] have established that both couples of species, Fe3+/Fe2+ and Co3+/Co2+, are able to activate the PMS as follows (where S represents the surface of the catalyst, in this case, the montmorillonite):
S-Fe3+ + HSO5− → S-Fe2+ + SO5•− + H+
S-Fe2+ + HSO5− → S-Fe3+ + SO4•− + OH−
S-Fe2+ + HSO5− → S-Fe3+ + SO42− + •OH
S-Fe2+ + SO4•− → S-Fe3+ + SO42−
S-Co3+ + HSO5− → S-Co2+ + SO5•− + H+
S-Co2+ + HSO5− → S-Co3+ + SO4•− + OH−
S-Co2+ + HSO5− → S-Co3+ + SO42− + •OH
S-Co2+ + SO4•− → S-Co3+ + SO42−
SO4•− + OH− → SO42− + •OH
SO4•− (•OH, SO5•−) + organics → by-products + CO2 + H2O
Hence, three distinct reactive radicals, namely sulfate (SO4•−), hydroxyl radicals (•OH), and peroxy-sulfate (SO5•−), can be produced through the activation of PMS by iron or cobalt. However, it is important to note that the peroxy-sulfate radical is less effective in attacking organic compounds because of its weaker oxidizing ability (E(SO5•−/SO42−) = 1.1 V) compared to that of SO4•− (2.5–3.1 V) and •OH (2.8 V) [2].
Moreover, based on their redox potential values (E(Co3+/Co2+) = 1.81 V; E(Fe3+/Fe2+) = 0.77 V), Co3+ ions can be reduced to Co2+ by the action of Fe3+ ions:
S-Co3+ + S-Fe2+ → S-Co2+ + S-Fe3+
The MB removal curves for various catalytic systems, working in the conditions outlined in Section 3.3, are illustrated in Figure 5. Notice that PMS alone achieved only a 7% degradation of MB within 120 min, and no further degrading was produced for longer reaction times, indicating its limited efficacy in removing this organic pollutant. As observed in Figure 5A, the catalytic activity of the MK10 series follows the order: MK10 < MK10Fe < MK10Fe-Co < MK10Co. The MK10 sample exhibits a low MB degradation at the initial stages, with 27% of the pollutant removed after 45 min. The incorporation of Fe3+/Fe2+ couples by impregnation of MK10 with iron(II) acetate results in a moderate increment of the MB removal at the initial stage of reaction, as compared to the pristine one, MK10, the final MB degradation value (70%) being, however, similar for both catalysts. Conversely, upon impregnation of MK10 with cobalt acetate, a substantial enhancement in MB removal is evident. Remarkably, MK10Fe-Co and MK10Co achieve removal values of 68% and 100%, respectively, within just 20 min of reaction time, and the first catalyst removes 92% of MB after 45 min. These results show the efficacy of the Co3+/Co2+ pairs present in the form of oxides or hydroxides (as deduced by SEM-EDX analysis) in enhancing MB decomposition through reactions (5)–(7). It is noteworthy that MK10Co removes 63% of MB within the first five minutes of the reaction.
The degradation kinetics of MB were adjusted to pseudo-first-order reaction kinetics, according to Equation (12):
ln (Ct/C0) = −kobs·t
where kobs is the pseudo-first-order apparent rate constant, and the constants were calculated from the slopes of the straight lines by plotting ln (Ct/C0) as a function of removal time (t) (Figures S3–S5). As deduced from the values of the constants (Table 3), the order in the kinetic decomposition of MB was MK10Co > MK10Fe-Co > MPilCo > MPilFe-Co > MK10Fe > MK10 > MPilFe > MPil.
The analysis of UV-vis spectra was conducted to demonstrate the effectiveness of the catalysts in MB degradation. Figure 6 illustrates the UV–vis spectral changes in MB collected at different time intervals after the addition of MK10Fe-Co and PMS. The intensity of the absorption peak at 664 nm diminishes significantly with reaction time, indicative of MB removal. Additionally, a new peak at approximately 683 nm is observed, suggesting the formation of degradation products. This peak initially manifests as a shoulder in the curve at 45 min, and its intensity increases with reaction time. These alterations in the UV-vis spectra indicate the successful degradation of MB.
The same order of catalytic activity is observed for the system based on MPil (Figure 5B) compared to the MK10 series. Consequently, the impregnated samples exhibit higher activity for MB removal than MPil, and once again, cobalt-containing catalysts perform much more effectively than those containing only iron. While the values of MB removal with the MPil series are notably lower than those obtained with the MK10 series, some catalysts in the MPil series still demonstrate remarkable activity. For instance, MPilCo removes 82% of the pollutant in just 90 min.
A comparison of Figure 5A,B reveals that the MK10-based samples are more efficient in degrading MB than those impregnated over MPil, with the plateau generally reached at 120 min for the first series. Furthermore, the values of the reaction constants (Table 3) for the MK10 series samples are of the order of 2.5–3.6 times higher than their MPil series counterparts. ICP-MS analysis (Table 1) indicates that, in general, samples of the MPil series contain a slightly higher amount of impregnated metal than their counterparts in the MK10 series. Therefore, a higher number of Co3+/Co2+ and Fe3+/Fe2+ couples would be available as active sites to decompose PMS in samples based on MPil, suggesting a potentially higher removal of MB for the MPil series. However, the obtained results are not in agreement with those expected according to the available active sites, and additional factors, such as the adsorption capacity of the samples for MB, may play a role in the observed discrepancies.

2.2.1. Adsorption of MB on Montmorillonites

To understand the differences in catalytic behavior and study the possible contribution of adsorption to dye removal, adsorption experiments of MB on the catalyst, in the absence of PMS, were conducted for both series. The results are depicted in Figure 7.
The MK10 series samples adsorb between 15% and 38% of the pollutant after 1 h, with the order of adsorption matching the observed for the reaction of MB with PMS (i.e., MK10 < MK10Fe < MK10Fe-Co < MK10Co). Notably, the order of adsorption differs from the order for the SBET of samples (Table 2), suggesting that factors other than textural properties play a role in the adsorption process. One such factor could be the existence of a negative charge on the surface of layers of montmorillonite, onto which the molecules of cationic dye are likely adsorbed by electrostatic interaction.
In comparison to the MK10 series, the adsorption of the dye on catalysts based on MPil occurs to a lesser extent. In some cases, after the initial adsorption of dye molecules, desorption takes place, leading to final values of only 5–10% of adsorbed pollutants (Figure 7B). These results align with findings from other authors [29], who reported negligible adsorption of tartrazine over pillared montmorillonite impregnated with cobalt. Therefore, the difference in catalytic behavior for MB removal between the samples of the MK10 and MPil series could be attributed to the higher adsorption of MB on the samples of the first series, contributing to an enhanced elimination of the dye.

2.2.2. Influence of Catalyst Dosage and Dye Concentration

The catalyst dosage plays a crucial role in the treatment of organic wastewater. Increasing the catalyst amount is expected to enhance the number of redox couples serving as active centers, thereby leading to a greater removal of contaminants. Considering this, some experiments with the double catalyst mass, i.e., 30 mg, equivalent to a concentration of 250 mg/L, were carried out. The results of these experiments are presented in Figure 8.
In the MK10 series, specifically with the MK10 and MK10Fe samples, doubling the catalyst amount results in the elimination of an additional 15% and 20% of the dye within the initial 10 min of the reaction, as evident when comparing Figure 5A and Figure 8A. This effect is likely attributed to an increased contribution from dye adsorption. Subsequently, the rate of reactions decelerates, leading to final dye removals that are only 12% and 15% higher for MK10 and MK10Fe samples, respectively.
The enhancement in catalytic activity is particularly pronounced in the case of the MK10Fe-Co sample, which achieves complete dye elimination after 20 min, in contrast to the 68% of dye removed with half the catalyst amount (Figure 5A). For this catalyst, the higher concentration of active cobalt centers likely contributes to an elevated decomposition of PMS through reactions (5)–(7). The MK10Co sample was not tested with 30 mg of catalyst, as complete dye elimination was accomplished using 15 mg within just 20 min.
In the pillared series, a notable increase of 15–25% in the dye removal is observed between 15 and 25 min for all catalysts, with the ones containing cobalt displaying the most significant enhancement. Consequently, with MPilCo and MPilFe-Co, the complete removal of the dye is achieved after 60 and 180 min, respectively, and their apparent constants are increased in a factor of 1.3–1.9 with respect to those obtained with 15 mg of catalyst (Table 3).
To investigate the impact of dye concentration, experiments were conducted with 0.052 mM (16.7 mg/L) and 0.156 mM (50 mg/L). This study focused on the three most active catalysts, i.e., MK10Co, MK10Fe-Co, and MPilCo, utilizing 15 mg of each catalyst and keeping the PMS/MB ratio constant at 3. The results are presented in Figure 9.
As the concentration of dye decreases, there is a corresponding increment in the final percentage of dye removed, as expected. Additionally, the kobs values also show an increase (Table 3). When the concentration is reduced from 0.078 to 0.052 mM, the most significant change occurs for the MPilCo sample, which achieves complete dye removal after 45 min, the kobs being increased by a factor of 2.2 (Table 3). In the case of MK10Co, the reaction proceeds rapidly with the standard concentration of 0.078 mM, decomposing all the dye within 20 min. Reducing the dye amount resulted in a 5-min earlier completion, with all MB removed after 15 min. It is also worth noting that, at the highest dye concentration (0.156 mM), the reaction stabilizes after 60 min, and the catalysts are no longer capable of degrading additional dye molecules. This behavior is consistent across all three catalysts, demonstrating the ability to degrade approximately 60% of the dye, irrespective of the catalyst used.
As a summary, Table 4 presents a comparative analysis of reaction conditions and outcomes achieved with our catalysts in contrast to findings reported by other authors. Certain articles indicate dye removal values that surpass those attained with montmorillonites under similar reaction times. However, it is important to highlight that the quantities of PMS employed in those studies are significantly higher than those utilized in the current work, along with elevated catalyst/MB ratios in some cases. Consequently, the efficiency of cobalt-impregnated montmorillonites as catalysts in the PMS/MB system is remarkable, as evidenced by the achievement of 100% dye removal within 15 to 45 min, utilizing a PMS/MB ratio of only 3.

2.2.3. Dominant Reactive Species

Quenching experiments were applied to probe the primary reactive oxygen species (ROS) involved in the degradation of MB degradation within the Montmorillonites/PMS system. Methanol (MeOH) is well-known for its high reactivity towards both •OH and SO4•− radicals (k·OH/MeOH = 9.7 × 108 M−1s−1; kSO4−/MeOH = 1.6–7.7 × 107 M−1s−1, while tert-butanol (TBA) is an effective •OH quencher (k·OH/TBA = 3.8–7.6 × 108 M−1s−1), with significantly lower reactivity towards SO4•− radicals (kSO4−/TBA = 4–9.1 × 105M−1s−1) [48,49,50]. Consequently, a series of experiments using two different concentrations for both quenchers were conducted with one of the most active catalysts, namely MK10Fe-Co (Figure 10). Specifically, quenching agent/PMS molar ratios of 3 and 6 were utilized, corresponding to 0.702 and 1.404 mM of quenching agent, respectively.
The degradation efficiency of MB shows a decline from 97 to 70% when 0.702 mM of methanol is added into the MK10Fe-Co/PMS system. Doubling the methanol concentration leads to a further decrease in degradation to 54%. This result suggests that at least one of •OH and SO4•− radicals was generated in the system. However, the degradation efficiency of MB only decreases from 97 to 90% when 0.702 mM of TBA is added. Therefore, the quenching effect of TBA is significantly less than that of methanol, implying that •OH is not the predominant reactive species for MB degradation and indicating a higher involvement of SO4•− radicals in the oxidation of MB in the MK10Fe-Co/PMS system.

2.2.4. Influence of pH of Reaction

Another studied factor was the pH of the reaction. The pH of a 0.078 mM MB solution, initially at 5.8 (natural pH), was adjusted to either pH 3 or pH 11 by adding 0.1 M HCl or NaOH, respectively. Catalytic activity results for the MK10FeCo and MPilFe-Co samples are presented in Figure 11. For both catalysts, at pH 3 and pH 11, significantly poorer dye removal results are observed compared to the natural pH of the solution, resulting in reduction values between 18% and 33% in the final activity. The explanation for this reduction is detailed below.
Sulfate radicals (SO4•−) were identified as the primary reactive radicals at the natural pH of the dissolution. At pH 3, competition between protons and cationic dye molecules for adsorption on the negatively charged sites of the montmorillonite surface may occur. This competition can lead to reduced dye adsorption and, consequently, limited access to the active sites of the catalyst. Moreover, under strongly acidic conditions, it has been reported that H+ can undergo adsorption onto the surface of HSO5− and establish hydrogen bonds with O-O bonds, this interaction inhibiting the decomposition of HSO5− into SO4•− and •OH by impeding bond breakage [51]. Additionally, the elevated concentration of proton at pH 3 might predominantly consume a significant portion of SO5•, causing a shift in the equilibrium of reactions (1) and (5) towards the left. Consequently, all these factors would contribute to a decrease in the removal of dye at pH 3, as compared to natural pH, as observed in Figure 11.
Within the pH range of 3–9, OH− undergoes moderate oxidation into •OH by SO4•− (as described by reaction (9)) [52]. As pH increased, this reaction was favored, as OH− with bare extra electrons became more efficient at transferring electrons to PMS, generating hydroxyl radicals (•OH) [53]. Beyond a solution pH of 10, the formation of hydroxyl radicals became predominant, making it the dominant reactive radical. Consequently, as their oxidant power is less than that of SO4•−, the dye removal efficiency decreases. Additionally, PMS exists primarily in the mono-anionic form (HSO5−) at pH less than 9.4, transforming to the less oxidizing di-anionic form (SO52−) through deprotonation at higher pH [54]. This implies a smaller number of HSO5− species capable of intervening in reactions (1)–(3) and (5)–(7) when the reaction is carried out at pH 11, and a reduction in the removal of MB is observed (Figure 11).
Hence, optimal results are obtained when operating at the natural pH of the solution. This is significant as it obviates the requirement to introduce acids or bases for the reaction, offering a more straightforward process and reducing the need for additional chemical adjustments.

2.2.5. Leaching of Metals and Recyclability of Catalysts

Recyclability experiments were conducted using three catalysts, specifically MK10Co, MK10Fe-Co, and MPilCo, which were identified as the most active. The outcomes are depicted in Figure 12. Previously, the determination of iron and cobalt leaching was carried out with the purpose of studying the possible contribution of homogeneous reaction to the activation of PMS. The amounts of metals leached into the aqueous solution after the reaction finished, expressed in mg/L, and their corresponding percentages, are listed in Table 5. It can be observed that cobalt is leached to a significantly greater extent than iron, and consequently, a contribution of the homogeneous reaction of Co3+/Co2+ ions to the activation of PMS cannot be discarded.
Notice that a decrease in the activity of around 8–9% at the initial times in the case of MK10FeCo and MPilCo and about 15% for MK10Co occurred from the first to the second cycle, probably because of the leaching of metals into the solution. The reduction in activity after three runs was between 5.7% for MPilCo and 10% for MK10FeCo. Despite a slight decline in activity between the first and the third cycle, MK10Co demonstrated remarkable performance by achieving complete dye elimination. In the second cycle, elimination occurred at 45 min, and in the third cycle, it took 60 min. Despite the leaching of metals and the decrease in the activity, the amount of remaining active sites in the catalysts was still sufficient to carry out the reaction in consecutive cycles, making them promising candidates for the studied process.

3. Materials and Methods

3.1. Reactants and Materials

The commercial pristine montmorillonites, montmorillonite K10, and montmorillonite pillared (Al) were provided by Sigma-Aldrich) (Merck Life Science S.L.U, Madrid, Spain), and they are designed as MK10 and MPil, respectively. Iron acetate (Fe(CH3COO)2) (95%) and cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O) were purchased from Sigma-Aldrich (Merck Life Science S.L.U, Madrid, Spain). Sodium hydroxide (NaOH) and chlorohydric acid (HCl) were obtained from Panreac. Peroxymonosulfate (KHSO5·0.5KHSO4·0.5K2SO4) was provided by Sigma-Aldrich (Merck Life Science S.L.U, Madrid, Spain)). Methanol (MeOH) and tert-butanol (TBA) from Honeywell Fluka (Fisher Scientifica S.L, Madrid, Spain) were used for the quenching experiments. All chemicals were of analytical grade. The deionized water used in the experiments was produced by the Milli-Q purification system.

3.2. Preparation of the Fe-Co Impregnated Montmorillonites

The commercial montmorillonites, MK10 and MPil, were impregnated with iron and/or cobalt acetates by the incipient wetness impregnation method [55]. A solution containing the corresponding mix of acetates in concentrations suitable for achieving a metal loading of 7 wt% (with respect to montmorillonite) was added dropwise to 1.5 g of montmorillonite. The resulting solid was air-dried at room temperature for 24 h and further dried at 60 °C for 16 h before undergoing pyrolysis under a nitrogen flow at 400 °C for 180 min. Six catalysts were prepared, three for each series (MK10 and MPil), and designated as follows: M’Fe and M’Co for the samples impregnated with iron acetate or cobalt acetate (7 wt% of metal), respectively, and M´Fe-Co for samples impregnated with both acetates (3.5 wt% of each metal), where M’ = MK10, MPil.

3.3. Characterization of Samples

The crystalline structure of the samples was analyzed using X-ray diffraction, employing an X’Pert Pro Panalytical (Malvern Panalytical, B.V., San Sebastián de los Reyes, Madrid, Spain) diffractometer with CuKa radiation (1.5406 Å). The instrument was operated at 40 kV and 40 mA, scanning in the 2θ range of 5–80 °C. The textural properties were determined through nitrogen adsorption–desorption isotherms at −196 °C, utilizing Micromeritics ASAP 2010 equipment (Micromeritics, Méringac, France). Prior to analysis, the samples underwent outgassing at 150 °C for 8 h until a vacuum set point of 200 µm Hg was reached. The surface area and micropore surface were assessed using the BET method and t-plot method, respectively, while mesoporosity characteristics were obtained via the BJH method. The morphology of the samples was examined using Scanning Electron Microscopy (SEM) on a JEOL JSM 6335F microscope (JEOL, Austin, TX, USA) operating at 200 kV, equipped with a device for energy-dispersive X-ray spectroscopy (EDX) measurements. Metal dispersion was monitored through High-resolution Transmission Electron Microscopy (HRTEM) using an Oxford Instrument, model X-Max (Oxford Instruments Nanoanalysis &Asylum Research, High Wycombe, UK) of 80 mm2, with a resolution between 127 eV and 5.9 KeV.
The content of iron and cobalt in the solid samples and the amount of metal leached after the reaction procedure (evaluated by measuring the concentration of metal in the final solution after filtration through 0.45 mm Durapore-membrane syringe filters) were determined by inductively coupled plasma mass spectrometry (ICP-MS) on a Nexion 300D Perkin-Elmer instrument (PerkinElmer INC, Waltham, MA, USA).

3.4. Catalytic Activity

To conduct peroxymonosulfate (PMS, hereafter) mediated experiments for the decomposition of methylene blue (MB, hereafter), a 0.078 mM MB solution (120 mL) was introduced to a batch reactor, along with 15 mg of catalyst. Under these conditions, MB and catalyst concentrations were adjusted to 25 mg/L and 125 mg/L, respectively, resulting in a [catalyst]/[MB] ratio of 5. Additionally, 8.7 mg of PMS was added to achieve a concentration of 0.234 mM, thereby establishing a [PMS]/[MB] ratio of 3. Different aliquots were extracted at selected times and filtered through 0.45 mm Durapore membrane syringe filters, and the MB concentration was determined by absorption at 664 nm using a Cary-1-UV-VIS (Varian Analytical instruments, Madrid, Spain) spectrophotometer. To ensure the replicability of the experiments, all the absorbance measurements were carried out three times. Previously, a calibration curve was obtained that showed a good linear relationship between the absorbance at 664 nm and the concentration (Figure S6). The percentage of decomposed MB was calculated using the formula:
Cdec = (Ct/C0)·100
where C0 represents the initial MB concentration, and Ct is the concentration at each selected time, t. Prior to the catalytic tests, MB degradation by PMS was conducted under identical conditions, excluding the catalyst.
For the adsorption experiments, the reactions were initiated by introducing 15 mg of catalyst to 120 mL of MB solution (0.078 mM) under stirring at 700 rpm and 25 °C, without the addition of PMS. The adsorption rate of MB in the experiment was calculated using the formula:
Percentage adsorption = [(C0 − Ct)/C0] · 100
Here, C0 represents the initial concentration of MB, and Ct is the concentration of MB in the solution at each selected time, both determined by UV-Vis as indicated above.
Several experiments were conducted to assess the recyclability of catalysts. Following each reaction, the catalysts were separated by filtration through 0.45 mm Durapore membrane syringe filters, washed with Milli-Q water, and dried at 110 °C in a vacuum oven for 7 h to prepare for the subsequent cycle. To account for the catalyst loss occurring between successive cycles, the quantities of PMS and MB were adjusted proportionally based on the catalyst amount.
Radical quenching experiments were additionally performed by adding two different quenchers, methanol or tert-butanol. The experiments were carried out under the same conditions as that for the standards, as explained above, i.e., mixing 15 mg of catalysts with 120 mL of 0.078 mM MB solution. To this solution, the appropriate amount of the quencher, methanol, or tert-butanol, was added. Specifically, quenching agent/PMS molar ratios of 3 and 6 were utilized, corresponding to 0.702 and 1.404 mM of quenching agent, respectively. Finally, 8.7 mg of PMS was added to each solution, and the reaction time started.

4. Conclusions

Two series of catalysts, derived from montmorillonites MK10 and MPil, were synthesized via incipient wetness impregnation with cobalt(II) and/or iron(II) acetates and subsequently evaluated for their efficacy in activating peroxymonosulfate (PMS) for the degradation of methylene blue. The catalysts of MK10 and MPil series impregnated with cobalt(II), especially those based on the MK10 series, demonstrated notable activity. Remarkably, MK10Co achieved complete dye removal within just 20 min at a [catalyst]/[MB] ratio of 5, while MK10Fe-Co exhibited a 92% elimination at 45 min, increasing to 100% in 20 min when the amount of catalyst was doubled. The final MB removal values for MPilCo and MPilFe-Co catalysts, 80 and 65%, respectively, were lower than those of their MK10 series counterparts. However, doubling the catalyst amount led to complete dye elimination after 60 and 180 min, respectively.
The efficiency of cobalt-impregnated montmorillonites as catalysts in the PMS/MB system was remarkable when compared with other catalysts reported in the literature because nearly 100% of MB was removed in the present work under mild conditions, using lower PMS/MB ratios than others reported and a low catalyst dose.
The reaction seems to occur because of the combined effect of MB adsorption and PMS activation by Co3+/Co2+ pairs, generating SO4•− radicals as the primary species responsible for dye oxidation. The reaction proceeded better at the natural pH of the solution, minimizing the need for additional chemical adjustments. As a result of the leaching of cobalt, a decrease in MB removal of around 6–10% occurred from the first to the third cycle for two of the tested catalysts. However, the activity remained high, with MK10Co particularly standing out for achieving complete dye elimination even after three cycles. These findings are crucial for assessing the practicality and sustainability of using these catalysts in industrial or research applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14080479/s1, Figure S1. Adsorption BJH curves for the montmorillonites. (A) MK10 series; (B) MPil series; Figure S2. TEM images and wt% of the elements determined from EDX spectra for MPil series. A and B: MPil; C and D: MPilFe; E and F: MPilFe-Co; G and H: MPilCo; Figure S3. Decomposition kinetics of MB (C0 = 25 mg/L) at 25 °C on montmorillonites. (A): MK10 series; (B): MPil series. C0 of PMS: 0.234 mM. Ccat: 125 mg/L; Figure S4. Decomposition kinetics of MB (C0 = 25 mg/L) at 25 °C on montmorillonites. (A): MK10 series; (B): MPil series. C0 of PMS: 0.234 mM. Ccat: 250 mg/L.; Figure S5. Decomposition kinetics and kobs for different MB concentration at 25 °C on montmorillonites. C0 of PMS: 0.234 mM. Ccat: 125 mg/L; Figure S6. Calibration line for Methylene Blue.

Author Contributions

Conceptualization, M.L.R.-C.; methodology, M.L.R.-C. and N.B.-B.; validation, N.B.-B.; formal analysis, N.B.-B.; investigation, N.B.-B., A.C.-N. and M.L.R.-C.; resources, M.L.R.-C. and A.C.-N.; data curation, N.B.-B.; writing-original draft preparation, M.L.R.-C.; writing—review and editing, M.L.R.-C.; supervision, M.L.R.-C. and A.C.-N.; project administration, M.L.R.-C. and A.C.-N.; funding acquisition, M.L.R.-C. and A.C.-N. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Spanish Ministry of Science, Innovation and Universities (PID2021-126579OB-C32) and by the European University of Madrid project (2022/UEM17).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. X-ray diffraction patterns of montmorillonites. (A) MK10 series; (B) MPil series. q: quartz; m: mica.
Figure 1. X-ray diffraction patterns of montmorillonites. (A) MK10 series; (B) MPil series. q: quartz; m: mica.
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Figure 2. N2 adsorption–desorption isotherms of montmorillonites. (A): MK10 series; (B): MPil series.
Figure 2. N2 adsorption–desorption isotherms of montmorillonites. (A): MK10 series; (B): MPil series.
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Figure 3. TEM images and wt% of the elements determined from EDX spectra for MK10 series. (A,B): MK10; (C,D): MK10-Fe; (E): MK10Fe-Co and (F): MK10Co.
Figure 3. TEM images and wt% of the elements determined from EDX spectra for MK10 series. (A,B): MK10; (C,D): MK10-Fe; (E): MK10Fe-Co and (F): MK10Co.
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Figure 4. SEM images (left) and composition determined by EDX spectra (right) from the entire observed area for selected samples. (A): MK10; (B): MK10Fe-Co; (C): MK10Co; (D): MPil and (E): MPilCo.
Figure 4. SEM images (left) and composition determined by EDX spectra (right) from the entire observed area for selected samples. (A): MK10; (B): MK10Fe-Co; (C): MK10Co; (D): MPil and (E): MPilCo.
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Figure 5. Decomposition kinetics of MB (C0 = 25 mg/L) at 25 °C on montmorillonites. (A): MK10 series; (B): MPil series. C0 of PMS: 0.234 mM. Ccat: 125 mg/L.
Figure 5. Decomposition kinetics of MB (C0 = 25 mg/L) at 25 °C on montmorillonites. (A): MK10 series; (B): MPil series. C0 of PMS: 0.234 mM. Ccat: 125 mg/L.
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Figure 6. UV-vis spectra of MB (C0 = 25 mg/L) at 25 °C for catalytic tests with MK10Fe-Co. Ccat: 125 mg/L.
Figure 6. UV-vis spectra of MB (C0 = 25 mg/L) at 25 °C for catalytic tests with MK10Fe-Co. Ccat: 125 mg/L.
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Figure 7. Adsorption kinetics of MB (C0 = 25 mg/L) at 25 °C on montmorillonites. (A): MK10 series; (B): MPil series.
Figure 7. Adsorption kinetics of MB (C0 = 25 mg/L) at 25 °C on montmorillonites. (A): MK10 series; (B): MPil series.
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Figure 8. Decomposition kinetics of MB (C0 = 25 mg/L) at 25 °C on montmorillonites. (A): MK10 series; (B) MPil series. C0 of PMS: 0.234 mM. Ccat: 250 mg/L.
Figure 8. Decomposition kinetics of MB (C0 = 25 mg/L) at 25 °C on montmorillonites. (A): MK10 series; (B) MPil series. C0 of PMS: 0.234 mM. Ccat: 250 mg/L.
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Figure 9. Effect of the MB concentration on the decomposition kinetics of MB at 25 °C on montmorillonites. C0 of PMS: 0.234 mM. Ccat: 125 mg/L. (A): MK10Fe-Co; (B): MPilCo; (C): MK10Co.
Figure 9. Effect of the MB concentration on the decomposition kinetics of MB at 25 °C on montmorillonites. C0 of PMS: 0.234 mM. Ccat: 125 mg/L. (A): MK10Fe-Co; (B): MPilCo; (C): MK10Co.
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Figure 10. Effect of quenching in the degradation of MB (C0 = 25 mg/L) at 25 °C over MK10Fe-Co (Ccat: 125 mg/L).
Figure 10. Effect of quenching in the degradation of MB (C0 = 25 mg/L) at 25 °C over MK10Fe-Co (Ccat: 125 mg/L).
Catalysts 14 00479 g010
Catalysts 14 00479 g011
Figure 11. Effect of pH of reaction in the degradation of MB (C0 = 25 mg/L) at 25 °C. (A): MK10Fe-Co; (B): MPilFe-Co.
Figure 11. Effect of pH of reaction in the degradation of MB (C0 = 25 mg/L) at 25 °C. (A): MK10Fe-Co; (B): MPilFe-Co.
Catalysts 14 00479 g011
Catalysts 14 00479 g012
Figure 12. Recyclability of catalysts for the degradation of MB (C0 = 25 mg/L) at 25 °C. Ccat: 125 mg/L. (A): MK10Fe-Co; (B): MPilCo; (C): MK10Co.
Figure 12. Recyclability of catalysts for the degradation of MB (C0 = 25 mg/L) at 25 °C. Ccat: 125 mg/L. (A): MK10Fe-Co; (B): MPilCo; (C): MK10Co.
Catalysts 14 00479 g012
Table 1. Content of metal (wt%) of samples determined by ICP-MS *.
Table 1. Content of metal (wt%) of samples determined by ICP-MS *.
CatalystFe (wt% ± sd)Fei (wt% ± sd)Co (wt% ± sd)(Fe + Co)i (wt% ± sd)
MK101.54 ± 0.04---
MK10Fe7.70 ± 0.106.16 ± 0.10 (7.0)-6.16 ± 0.10 (7.0)
MK10Fe-Co4.32 ± 0.022.78 ± 0.04 (3.5)3.40 ± 0.10 (3.5)6.18 ± 0.10 (7.0)
MK10Co1.45 ± 0.02-6.90 ± 0.14 (7.0)6.90 ± 0.14 (7.0)
MPil0.77 ± 0.01---
MPilFe6.74 ± 0.135.97 ± 0.13 (7.0)-5.97 ± 0.13 (7.0)
MPilFe-Co3.83 ± 0.113.06 ± 0.11 (3.5)3.45 ± 0.07 (3.5)6.51 ± 0.11 (7.0)
MPilCo0.75 ± 0.01-6.97 ± 0.02 (7.0)6.97 ± 0.02 (7.0)
* Between Brackets: values corresponding to the theoretical ones.
Table 2. Textural properties of samples of MK10 and MPil series.
Table 2. Textural properties of samples of MK10 and MPil series.
CatalystSBET (m2/g)Smic (m2/g)Vp (cm3/g)Vmes (cm3/g)Vmic (cm3/g)dmeso (nm)
MK10231.714.20.3600.3400.0066.5
MK10Fe200.1-0.3180.300-6.2
MK10Fe-Co191.3-0.3150.296-6.5
MK10Co161.8-0.2890.271-6.6
MPil175.3119.10.1860.1750.05212.3
MPilFe143.880.30.1560.1020.0479.4
MPilFe-Co141.985.00.1700.1070.03712.1
MPilCo164.299.70.1680.1000.04410.3
SBET = specific surface area; Smic = micropore surface area determined by t-plot; Vp = pore volume at single point at P/P0 = 0.967; Vmes = mesopore volume by BJH between 2 and 50 nm; Vmic = micropore volume determined by t-plot; dmes = average mesopore diameter (4V/A) by BJH.
Table 3. Values of apparent constants for pseudo-first-order kinetics, kobs (min−1), for the decomposition of methylene blue.
Table 3. Values of apparent constants for pseudo-first-order kinetics, kobs (min−1), for the decomposition of methylene blue.
CatalystCcat: 125 mg/L
CMB: 0.078 mM		Ccat: 250 mg/L
CMB: 0.078 mM		Ccat: 125 mg/L
CMB: 0.052 mM		Ccat: 125 mg/L
CMB: 0.156 mM
MK100.006320.01359--
MK10Fe0.010210.03631--
MK10Fe-Co0.053490.146520.094220.03417
MK10Co0.17178-0.396700.03423
MPil0.002320.00820--
MPilFe0.004200.00764--
MPilFe-Co0.016190.03095--
MPilCo0.048090.065880.107130.02824
Table 4. Reaction conditions and MB removal values.
Table 4. Reaction conditions and MB removal values.
ReferenceCatalystCatalyst/MB Ratio (mg/L/mg/L)PMS/MB Molar RatioRemoval Efficiency
[29]Ag-La0.8Ca0.2Fe0.94O3−δ perovskite10012.590% in 40 min
[30]Mixed conducting perovskite 538.3100% in 15 min
[31]CoFe2O4-MOF2.514.595% in 60 min
[32]Cu@Co-MOF320100% in 30 min
[33]NbCo-MOF1015100% in 30 min
[34]Mn3O4 nanoparticles2016086% in 20 min
[35]MnCo2O4.5 nanoparticles126100% in 25 min
[36]Co nanoparticles210.5100% in 10 min
[37]Activated carbon composite10104100% in 60 min
[38]CuFe2O4@GO101393% in 30 min
[39]Reduced Graphene oxide50100100% in 120 min
This workMK10Co53100% in 20 min
This workMK10Fe-Co5392% in 45 min
This workMK10Co7.53100% in 15 min
This workMK10Fe-Co7.53100% in 45 min
This workMPilCo7.53100% in 45 min
This workMK10Fe-Co103100% in 20 min
This workMPilCo103100% in 60 min
This workMPilFe-Co10385% in 60 min
Table 5. Leaching of metals after 5 h of reaction determined by inductively coupled plasma mass spectrometry (ICP-MS).
Table 5. Leaching of metals after 5 h of reaction determined by inductively coupled plasma mass spectrometry (ICP-MS).
CatalystIron (mg/L)Iron * (%)Cobalt (mg/L)Cobalt * (%)
MK10Fe-Co0.00931.740.57313.48
MPilCo0.00230.241.15613.27
MK10Co0.00320.21.15417.83
* Percentage of metal leached off with respect to the initial content in the montmorillonites samples.
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Barrios-Bermúdez, N.; Cerpa-Naranjo, A.; Rojas-Cervantes, M.L. Efficient Methylene Blue Degradation by Activation of Peroxymonosulfate over Co(II) and/or Fe(II) Impregnated Montmorillonites. Catalysts 2024, 14, 479. https://doi.org/10.3390/catal14080479

AMA Style

Barrios-Bermúdez N, Cerpa-Naranjo A, Rojas-Cervantes ML. Efficient Methylene Blue Degradation by Activation of Peroxymonosulfate over Co(II) and/or Fe(II) Impregnated Montmorillonites. Catalysts. 2024; 14(8):479. https://doi.org/10.3390/catal14080479

Chicago/Turabian Style

Barrios-Bermúdez, Niurka, Arisbel Cerpa-Naranjo, and María Luisa Rojas-Cervantes. 2024. "Efficient Methylene Blue Degradation by Activation of Peroxymonosulfate over Co(II) and/or Fe(II) Impregnated Montmorillonites" Catalysts 14, no. 8: 479. https://doi.org/10.3390/catal14080479

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