Introduction
Saponites are widespread clay minerals found in the weathering zone of magnesium-bearing rocks. Known deposits of saponite are located in Denmark (the Faroe Islands), Japan (Wakamatsu mine), the United States (Montana), and other countries (Treiman et al. Reference Treiman, Morris, Agresti, Graff, Achilles, Rampe, Bristow, Ming, Blake, Vaniman, Bish, Chipera, Morrison and Downs2014; Sokol et al. Reference Sokol, Sprynskyy, Ganzyuk, Raks and Buszewski2019). The largest deposits in Europe are located in Ukraine, in the Khmelnytskyi region (Ulashaniv, Varvariv, and Tashkiv deposits), with reserves of >30 million tons (Vogels et al. Reference Vogels, Kloprogge and Geus2005).
Saponites are clay minerals ((M + x–y ·nH2O)(Mg3–y R 3+ y )(Si4–x Al x )O10(OH)2, where R indicates trivalent cation substitutions in the octahedral site, y is the number of trivalent substitutions, M is the exchangeable cation in the interlayer (univalent example given here), x represents the number of substitutions for Si by Al in the tetrahedral sites, and n is a variable). Due to a particular spatial structure, saponites, which are magnesium smectites, exhibit significant adsorption and ion-exchange properties (Hover et al. Reference Hover, Walter, Peacor and Martini1999; Shao and Pinnavaia Reference Shao and Pinnavaia2010; Tarasevich et al. Reference Tarasevich, Poljakov, Ivanova and Trifonova2011; Polyakov and Tarasevich Reference Polyakov and Tarasevich2012; Vogels et al. Reference Vogels, Kloprogge and Geus2005).
The use of saponite as a sorption material has been developed widely in many fields of the chemical, agrochemical, food, and medical industries (Choy et al. Reference Choy, Choi, Oh and Park2007; Szabo´ et al. Reference Szabo', Mitea, Leeman, Premachandra, Johnston, Szekeres, De'ka'ny and Schoonheydt2008; Polyakov and Tarasevich Reference Polyakov and Tarasevich2012; Chanturiya et al. Reference Chanturiya, Minenko, Suvorova, Pletneva and Makarov2017; Petra et al. Reference Petra, Billik, Melichová and Komadel2017; Tangaraj et al. Reference Tangaraj, Janot, Jaber, Bechelany and Balme2017; Kumaresan et al. Reference Kumaresan, Radheshyam, Bhavesh and Hari2019; Nityashree et al. Reference Nityashree, Price, Pastor-Perez, Manoharal, Garcia, Maroto-Valer and Reina2020). The low cost and the wide variety of saponite clay minerals with diverse structural characteristics and sorption properties make them convenient for the formation of high-efficiency low-cost sorbents which can be used in water-purification processes, for example. Acid-activated saponite is used in water-purification processes (Petra et al. Reference Petra, Billik, Melichová and Komadel2017; Tangaraj et al. Reference Tangaraj, Janot, Jaber, Bechelany and Balme2017; Zhoua et al. Reference Zhoua, Zhoua, Wua, Petitd, Jianga, Xiaa, Lib and Yua2019) to remove heavy metal and ammonium ions and is characterized by a large specific surface area, which exceeds significantly the specific surface area of saponite-rich raw material (Rodriguez et al. Reference Rodriguez, Suarez, Lopez and Bañares1994). Additional modification of saponite by TiO2 and Ag demonstrated photocatalytic and antibacterial properties for water-purification processes (Fatimaha et al. Reference Fatimaha, Nurillahia, Sahronia and Muraza2019; Sprynskyya et al. Reference Sprynskyya, Sokolb, Rafińskaa, Brzozowskaa, Railean-Plugarua, Pomastowskic and Buszewskia2019).
Saponite-rich raw material is used successfully as a complex mineral admixture for farm-animal feeds, in the detoxification of anthropogenic and radiation-contaminated soils, as an active sorbent for the extraction of radionuclides and heavy-metal salts from human and animal organisms, plants and nutrition products, and as a preserving agent in root-crop storage, etc. (Murray Reference Murray1999; Samara et al. Reference Samara, Matsi, Zdragas and Barbayiannis2019). The presence of metal ions such as lithium, chromium, molybdenum, germanium, gold, and platinum, in micro-quantities in saponite clay, metals which are vital for human biochemical processes, means that saponite-based medicines offer promise in medical contexts (López-Galindo et al. Reference López-Galindo, Viseras and Cerezo2007; Carretero and Pozo Reference Carretero and Pozo2009; Shao and Pinnavaia Reference Shao and Pinnavaia2010; Donauerová et al. Reference Donauerová, Bujdák, Smolinská and Bujdáková2015; Vogels et al. Reference Vogels, Kloprogge and Geus2005).
Saponite clay from the Tashkiv deposit (Ukraine) was modified with magnetite nanoparticles (the amounts of Fe3O4 in the nanocomposites were 2, 4, 7, and 10 wt.%) (Makarchuk et al. Reference Makarchuk, Dontsova and Krymets2017a, Reference Makarchuk, Dontsova, Perekos, Skoblik and Svystunovc). The modification by magnetite resulted in an increase in the specific surface area and in the sorption activity of these composites to various forms of dye. In the present study, the next step was implemented, i.e. the study of the effect of additives such as nanosized graphene-like molybdenum and tungsten sulfides on the surface and textural characteristics of composites based on native saponite and MMS. The layered chalcogenides of transition metal elements (Mo, W) are known to have significant sorption, photocatalytic, and semiconductor properties with a small energy gap width (∆Е (MoS2) = 1.84 еV, ∆Е (WS2) = 1.97 еV) (Hu et al. Reference Hu, Hu, Xu and Pan2010; Borisenko et al. Reference Borisenko, Krivosheeva and Shaposhnikov2016; Qiao et al. Reference Qiao, Hu, Tian, Hou and Li2016). Such properties of layer graphene-like chalcogenides of Mo and W and the large specific surface area of nanosized MoS2 particles (Dontsova et al. Reference Dontsova, Kulikov and Astrelin2017) make them interesting in terms of the formation of active sorbent surfaces on clay materials.
Previous results and the unique properties of the graphene-like chalcogenides of Mo and W indicate that an investigation of the effects of modification in texture of saponite-rich raw material and magnetite-containing saponite should be interesting for their future use as sorbents and/or catalysts. This investigation involves the study of the solid phase and acid-base characteristics in both aqueous and air environments. The objective of the present study was to identify the effect of modification of native and MMS with graphene-like Mo and W sulfides on the surface (acid-base) and textural characteristics of the composites with the purpose of creating sorption and/or catalytic materials based on them.
Materials and Methods
Saponite
The saponite clay used in the present study is from the Tashkiv deposit (Khmelnytsky region, Ukraine), and is a trioctahedral smectite; for the present study the <63 μm particle-size fraction was used. In previous research (Makarchuk et al. Reference Makarchuk, Dontsova and Krymets2017a, Reference Makarchuk, Dontsova and Perekosb), the chemical composition of the saponite was given as (wt.%): Mg (5.34–7.18); Al (4.90–5.76); Si (20.68–22.67); S (0.00–0.18); Ti (2.22–2.98); V (0.15–0.20); Mn (0.87–1.21); Ca (24.57–26.86); Fe (35.94–37.35); Ni (0.05–0.12); Cu (0.11–0.14); Zn (0.09–0.18); Sr (0.05–0.1). Other mineral phases present in the raw material include quartz and calcite.
Reagents
The reagents used to define the acid-base and sorption properties were obtained from: Carlo Erba Reagents (Chaussée du Vexin, France) for fuchsine, bromophenol blue, methyl red, bromothymol blue, phenol red, thymol blue, indigo carmine, methylene blue, and congo red; Mainchem Co., Ltd. (Xiamen, China) for o-nitro aniline; and Merck & Co. (Kenilworth, New Jersey, USA) for μ-dinitrobenzene.
Magnetic fluid and Magnetic Sorbents
Synthesis of magnetic fluid and magnetic sorption of nanomaterials, with a magnetite particle size of up to 10 nm was implemented following the procedure described in detail by Mykhailenko et al. (Reference Mykhailenko, Makarchuk, Dontsova, Gorobets and Astrelin2015) and Makarchuk et al. (Reference Makarchuk, Dontsova and Perekos2017b).
Dichalcogenides
Mo and W chalcogenides (MoS2 and WS2) (Vasilyeva et al. Reference Vasilyeva, Asanov and Kulikov2015) were used as modifiers of native saponite and MMS. Graphene-like 2H MS2 nanoparticles (M = Mo, W) were obtained by chemical-vapor deposition (CVD) at low temperature (650–670 K) in self-oscillating temperature modes (Reference Vasilyeva, Asanov and Kulikov2015).
Modification by Dichalcogenides
Modification of saponite and MMS was conducted in the following order: water paste-like slurries of saponite clay and MMS were prepared using an ultrasonic (40 kHz) bath (UZM-004-1, Medpromplylad Ltd., Ukraine). After the formation of a suspension of saponite or MMS, modifying metal chalcogenides (Mo, W) were added and mixed for 30 min, then dried for 120 min in a drying chamber at 80°C. Six samples were obtained (Table 1).
Table 1. Compositions of six saponite-based, modified sorbents (wt.%)
Characterization of the Samples Obtained
Specific Surface Area of Saponite Composite Materials
The structural adsorption properties of the composite materials were analyzed using a Quantachrome Autosorb Nova 2200e (Boynton Beach, Florida, USA) instrument. The specific surface area and pore-size distribution were calculated using the Brunauer-Emmett-Teller (BET) equation and the Barrett-Joyner-Halenda (BJH) method, respectively (Brunauer et al. Reference Brunauer, Emmett and Teller1938; Villarroel-Rocha et al. Reference Villarroel-Rocha, Barrera and Sapag2014) (following Method BS 4359-1:1996 (ISO 9277:1995)). The weight of the sample was 0.3 g, the temperature of adsorption of the adsorbate gas (nitrogen) was 77 K, the pressure in the inert gas (helium) supply line was 70 kPa, and the degassing temperature was 400°C.
SЕМ
Morphological analysis of the composite samples was conducted using a scanning electron microscope (SEM-106I, Selmi Ltd., Ukraine). Precise chemical analysis of the surface was conducted in each case. The SEM-106I has a built-in microanalysis system with a database of recorded standards for all pure elements.
Acid-Base Properties of Sorbent Surface
The pH of the suspensions of all composite samples was determined using a pH-meter (Portlab 102 pH-Meter, Russia). A 1% suspension of each sample was prepared in twice-distilled water and the change in the pH of the resulting suspension was recorded at regular intervals (t, s). From the data obtained, a pH = f(t) dependence diagram was constructed (Fig. 4).
The effect of the distribution of the sorbent surface centers on the acid-base characteristics was studied by means of the Hammett method in aqueous solutions (Hammett and Deyrup Reference Hammett and Deyrup1932). For the study, the following dyes (Hammett indicators) were selected: o-nitro aniline (pK = −0.29), fuchsine (pK = 2.1), bromophenol blue (pK = 3.9), methyl red (pK = 5.25), bromothymol blue (pK = 6.8), phenol red (pK = 7.6), thymol blue (pK = 8.8), indigo carmine (pK = 12.8), and μ-dinitrobenzene (pK = 16.8). The sample weight was 0.02 g. The number of centers (q, μg/L g) at a given acid strength was calculated by the formula:
where C i and V i are the concentration and volume (μg L–1 and L, respectively) of the indicator; m 1 and m 2 are the mass of sorbent sample while measuring D 1 and D 2 (g); D 0 is the optical density of initial solutions of indicators; D 1 is the optical density of the solutions after the establishment of adsorption equilibrium and decantation of the solid phase; D 2 is the optical density, which takes into account the effect of changes in the pH of the medium upon contact of the solution with the sorbent; D 0, D 1, and D 2 were measured simultaneously in parallel experiments. Application of the Hammett method in aqueous solutions was described in detail by Dontsova et al. (Reference Dontsova, Yanushevskaya, Nahirniak, Makarchuk, Ivanets, Roshchina, Kutuzova and Kulikov2018).
Vacuum Thermogravimetric Method for Determining the Acid-base Characteristics of a Solid Substance.
Acid and base properties of the samples were investigated by the Quasi-Equilibrium Thermo Desorption (QE-TD) method. Stepwise thermodesorption of ammonia experiments were conducted at 50–400°C under vacuum (constant pressure of 0.133 Pa was maintained by vacuum pump) using a thermogravimetric apparatus with a quartz spring balance of the McBain type (original handmade equipment). The quasi-equilibrium mode of conducting a thermal desorption experiment suggests that thermal desorption of the probe substance proceeds through the achievement of a sequential series of equilibrium states. The probe substance desorbed during the gradual temperature increase after adsorption onto the test sample. With each temperature-increase increment, the sample was maintained until adsorption equilibrium with a gaseous (CO2 or NH3) substance was reached, as measured by the mass “sample + adsorbed substance” (Puziy et al. Reference Puziy, Poddubnaya, Kochkin, Vlasenko and Tsyba2010; Vlasenko et al. Reference Vlasenko, Kochkin, Telbiz, Shvets and Strizhak2019a, Reference Vlasenko, Kyriienko, Valihura, Yanushevska, Soloviev and Strizhakb). Composite surfaces were degassed to a constant mass by heating the sample in vacuum at 400°C for 15 min (Rodriguez et al. Reference Rodriguez, Suarez, Lopez and Bañares1994). The number and strength of acid and base centers were evaluated by using ammonia (NH3) and carbon dioxide (CO2), respectively, as probes. The temperature was increased stepwise (5°C/min between steps, step size 50°C) with holding at constant temperature until constant weight was achieved at each step for 10–15 min. As a result, the experimental conditions corresponded to the state of adsorption equilibrium of the probe gas at each step.
The amount of probe gas adsorbed per 1 g of solid (∆m) at each temperature was calculated as:
where m sample was the mass of the sample before probe adsorption; m (sample+probe) was the mass of a sample with the adsorbed probe gas at a given temperature; M probe is the molecular weight of the probe gas (NH3 or CO2).
The differential curves of NH3/CO2 sorption were obtained as the differentiatial of the Δm vs T plot using the Origin program.
Sorption properties of sorbents.
To study the sorption capacity of the composite samples (Saponite, Sap+MoS2, Sap+WS2, MМS, MМS+MoS2, MМS+WS2), a series of experiments was carried out in which the adsorption of selected dyes (methylene blue (MB) and сongo red (CR)) from their aqueous solutions was measured. To do this, 0.01 g of sorbent was added to the 50 mL solution of the corresponding dye with various concentrations (100 mg/L, 400 mg/L, 600 mg/L, or 1000 mg/L) and mixed in a shaker for 120 min before the establishment of adsorption equilibrium, then the suspension was centrifuged and the separated filtrate was analyzed by spectrophotometry for the presence of residual dye. For the spectrophotometric studies, a 721 UV/Vis Spectrophotometer (Shanghai Metash Instruments Co. Ltd, Shanghai, China) was used. The concentrations of standards used to prepare the calibration curve were: 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 mg/L. The maximum absorption wavelength for MB was 663 nm and for CR, 490 nm.
The adsorption capacity, q e (mg g–1), of the sorbents was calculated using Eq. 3:
where С 0 and С e are the initial and residual concentrations of dye (mg g–1), respectively .
RESULTS AND DISCUSSION
Specific Surface Area and Porosity of Saponite Composite Materials
The adsorption-desorption of nitrogen isotherms for all the samples tested (Figs. 1 and 2) revealed that all of the isotherms are type II, and are characteristic of mesoporous materials, and can be described adequately by the BET model. The BET method of surface evaluation of the samples indicated an increase in the specific surface area of magnetite-modified composites compared to the saponite-rich raw material. When saponite was modified with magnetic fluid, the specific surface area of MMS increased from 35 to 53 m2/g (Figs 1a and 2a). According to the pore distribution curves, both samples exhibited a pronounced mesoporous structure (average pore size of the order of 4 nm), which was due to the porous saponite structure. The pore-size distribution of the MМS sample (Fig. 2a) revealed a maximum of 15 to 18 nm, suggesting that Fe3 O4 nanoparticles form a secondary mesoporous structure on the saponite surface, i.e. the MМS sample is characterized by bimodality (Dontsova et al. Reference Dontsova, Yanushevskaya, Nahirniak, Makarchuk, Ivanets, Roshchina, Kutuzova and Kulikov2018).
Fig. 1 Nitrogen adsorption-desorption isotherms for a saponite, b Sap+MoS2, c Sap+WS2
Fig. 2 Nitrogen adsorption-desorption isotherms for a MMS, b MMS+MoS2, c MMS+WS2
Modification of saponite-rich raw material by nanosized graphene-like particles of Mo and W chalcogenides led to the opposite effect compared with magnetite. After the application of MoS2 and WS2 (Fig. 1b, c) as modifiers, the specific surface area of the saponite decreased from 35 to 24 and 19 m2 g–1, respectively, while the average mesopore size remained at 3.96 nm for Sap+MoS 2 and 3.85 nm for Sap+WS2. The cause of this phenomenon may be the spontaneous conglomeration of chalcogenide nanoparticles due to high natural hydrophobicity in the aquatic environment in the process of sample preparation and blocking of the microporous areas of saponite, resulting in a decrease in specific surface area.
In contrast, when modified by MMS-based chalcogenides (Fig. 2b, c), the specific surface area of the composite increased slightly from 53 m2 g–1 for MMS to 57 and 58 m2 g–1 for MMS+MoS2 and MMS+WS2, respectively. This increase in the specific surface area can be explained by the fact that the hydrophobic nanostructured conglomerates of Mo and W sulfides are sorbed onto the secondary mesoporous structure formed by magnetite, which results in the disappearance of the bimodality of the MMS sample. The prevailing pore diameter for these samples was also at the level of 4 nm. In this case, therefore, the incorporation of nanoparticles of chalcogenides of Mo and W into the magnetite-modified saponite contributed to the increase in specific surface area of the composite.
Analyses by SEM (Fig. 3) demonstrated that the morphologies of the saponite and MМS samples are similar, characterized by the existence of separate, distinct particle shapes. The samples of Sap+MoS2, Sap+WS2, MМS+MoS2, and MМS+WS2 modified by nanosized graphene-like particles of Mo and W chalcogenides differed slightly but were also similar. Compared to the saponite and MMS samples, their surface appeared to be covered with a dark layer, indicating deposition of MoS2 and WS2 particles on the surface of the saponite and MMS.
Fig. 3 SEM images of a saponite, b Sap+MoS2, c Sap+WS2, d MMS, e MMS+MoS2, f MMS+WS2. Scale bars: 50 μm
As a result of the spot chemical analysis of the Sap+MoS2, Sap+WS2, MMS+MoS2, and MMS+WS2 surfaces, in addition to the saponite-specific base elements (Si, Al, Fe, Ca, Mg), Mo, W, and S were also detected, which confirmed the deposition of chalcogenides of Mo and W on the surface of the saponite and MMS.
Acid-base Characteristics of the Surface of the Composites
According to modern concepts (Gregg and Sing Reference Gregg and Sing1982), the active surface of adsorbents is bifunctional, being a combination of acid and base centers of both the Brönsted and Lewis types. The determination of the nature and concentration of active centers enables prediction, therefore, of the sorption and catalytic abilities of the surface. A fairly accurate method for assessing the acid-base surface properties is the Hammett method, which is implemented in an aqueous medium. A limitation of the method is the possible transformation of the aprotic Lewis centers into Brönsted proton centers in the aqueous medium (Tanabe et al. Reference Tanabe, Misono, Hattori and Ono1990). This method is, therefore, relevant primarily for characterizing adsorbents and catalysts for processes occurring with the participation of a hydrated solid surface; in particular, water-purification processes. The methods implemented in a gas medium or in a vacuum, when the surface of the material does not undergo changes, are more universal. These methods include quasi-equilibrium thermal desorption. The information obtained with the application of these methods is free from the influence of a solvent and may be useful for predicting the adsorption and catalytic characteristics of materials for a wider range of applications. For this reason, in the current work, study of the active centers of modified composite adsorption was conducted in both aqueous and gas media.
Evaluation of the resulting acidity of the surface in the aqueous medium, performed by measuring the pH of the respective composite suspensions, demonstrated that all the samples had a base characteristic; but, after modification by chalcogenides of Mo and W, a slight increase in the acidity of the modified samples was observed (Fig. 4a,b) indicating a weak hydroxyl-acceptor property of the sample surface (Dai et al. Reference Dai, Yan, Liang and Xu2017).
Fig. 4 Time dependence of the total acidity of composites in aqueous suspension
This fact correlates well with studies of the distribution of acid-base centers by the Hammett method (Fig. 5a,b). For example, for the MMS sample, the total number of Brönsted acid centers was 1.71 μg g–1 in contrast with 0.63 μg g–1 for saponite. For Sap+Mo 2, Sap+WS2, MMS+МоS2, and MMS+WS2 these values were 6.43, 2.16, 0.7, and 2.68 μg g–1, respectively. The numbers of Brönsted base centers evaluated by the Hammett method for the saponite, MMS, Sap+MoS2, Sap+WS2, MMS+МоS2, and MMS+WS2 samples were 2.1, 0.63, 2.51, 1.46, 1.75, and 1.11 μg g–1, respectively. These results demonstrated that the number of Brönsted base centers is greatest for saponite and Sap+MoS2. Modification of saponite material, therefore, led to an increase in the acidic properties of the sorbents. An exception to this was the small but noticeable increase in the basicity of the MMS+MoS2 sample surface in an aqueous medium, which could probably be explained by the synergistic effect that occurs when both magnetite and molybdenum sulfide are applied.
Fig. 5 Distribution of acid and base centers on the surface of composite materials determined by the Hammett method
The work described above demonstrates that additives (magnetite, MoS2, and WS2) may enter in ways that cause changes to the textural characteristics and acid-base states of surfaces. The most significant change, namely the increase in the number of active centers, in the surface properties of modified samples of saponite was achieved by using magnetite, and, as already mentioned, by magnetite and nanostructured graphene-like molybdenum sulfide (i.e. their contemporaneous application). More active centers and a larger specific surface area resulted in better sorption properties with respect to pollutants of various natures, as presented by Makarchuk et al. (Reference Makarchuk, Dontsova and Krymets2017a, Reference Makarchuk, Dontsova and Perekosb), for the example of magnetite-containing saponite.
Vacuum thermogravimetric studies of the acid-base characteristics of sample surfaces (Figs. 6 and 7) indicated a somewhat different character. According to this technique, acid and base centers were absent from the native saponite. When native saponite was modified with graphene-like WS2, centers did not appear, but when modified with MoS2 a small amount (0.28 mmol NH3 g–1) of the powerful acid centers (Т desorption = 475°C) did appear (Table 2).
Fig. 6 Differential curves of ammonia desorption for the samples based on saponite
Fig. 7 Differential curves of a ammonia and b carbon dioxide desorption for the samples based on MMS
Table 2. Acid-base characteristics of samples
*Limit strength of acid sites
**Limit strength of base sites
C a(w), C b(w): number of weak acid and base centers, respectively; C a(m), C b(m): number of medium acid and base centers, respectively; C a(s), C b(s): number of strong acid and base centers, respectively; and ƩC a, ƩC b: total number of acid and base centers, respectively.
MMS has some active centers, as shown by the presence of a small number of weak acidic centers (0.08 mmol NH3 g–1, T desorption = 225°C) on its surfaces. A significant increase in the number and activity of acid centers was observed only after the addition of graphene-like molybdenum sulfide (MMS+MoS2), characterized by the adsorption of 1.25 mmol NH3 g–1 at 250°C.
The modification of sorbents based on MМS with graphene-like molybdenum sulfide (MMS+MoS2) generates simultaneously a much smaller number of base centers compared to the number of acid centers, but a much greater force (0.48 mmol СО2 g–1, T desorption = 350°C) (Fig. 7b). On the other hand, modification of MМS by tungsten sulfide led to the appearance of base centers of average force but only insignificant numbers of them (0.14 mmol СО2 g–1, T desorption = 300°C). The largest number of acid and base centers occurred, therefore, in the MMS+MoS2 composite, i.e. saponite modified by nanosized magnetite (7%) and molybdenum sulfide (1%).
The data obtained on the distribution of acid-base centers by vacuum thermogravimetry correlated with the data obtained by the Hammett method, namely, the samples based on saponite-rich raw material were characterized by a smaller total number of acid-base centers than the samples based on MMS. The existing differences in the acidic and basic characteristics of the samples obtained by the different methods (QE-TD method and Hammett method) are explained by the fact that the QE-TD method determines primarily the aprotic Lewis centers, and the Hammett method determines the protonated and hydroxylated Brönsted centers.
These experiments led to the conclusion that the most interesting sorbent was that with magnetite and modified with graphene molybdenum sulfide (MMS+MoS2), which, compared to other samples, possessed the largest specific surface area (58 m2 g–1) and the greatest number of acid-base centers, as defined by the Hammett method (6.43 μg g–1 of acid centers and 2.51 μg g–1 of base centers) and by thermogravimetry (1.25 mmol NH3 g–1, T desorption = 250°C; 0.48 mmol CO2 g–1, T desorption = 350°C). This may be related to the synergistic effect between the secondary magnetite structure and the metal chalcogenides.
Analysis of the data above leads to the further conclusion that modification is a powerful tool for changing the surface structure of materials and changing their acid-base characteristics, which may be used to create new sorption and catalytic materials.
Acid-base properties are known to affect sorption and catalytic properties significantly (Tangaraj et al. Reference Tangaraj, Janot, Jaber, Bechelany and Balme2017; Nityashree et al. Reference Nityashree, Price, Pastor-Perez, Manoharal, Garcia, Maroto-Valer and Reina2020). In the current paper, testing of the sorption properties of modified and unmodified samples of native and magnetite-containing saponite was motivated by a need to create less expensive saponite-based sorption materials. The sorption capacity of modified and unmodified clay minerals with respect to dyes of various natures relies only on their total acidity (Table 3). Sorbents having base characteristics are more active towards cationic dyes, as evidenced by the increased sorption capacity of twice-modified saponite (MMS+WS2; 185 μg MB g–1 of sorbent) compared to unmodified saponite (103 μg MB g–1 of sorbent). A similar situation was observed for the anionic dye, where the sorption capacity of twice-modified MMS+MoS2 (75 μg CR g–1 of sorbent) was greater than for unmodified saponite (39 μg CR g–1 of sorbent). An increase in the number of acid or base centers (in this case, the Brönsted centers) after modification led, therefore, to an increase in the sorption activity of both cationic and anionic dyes, which depended also on the specific surface area; the sorption capacity decreased as the specific surface area decreased, and vice versa.
Table 3. Sorption capacity of modified and unmodified clay minerals
Conclusions
This study of the modification of saponite and MMS by graphene-like MoS2 and WS2 revealed the influence of modification on the texture characteristics of composites: the modification of saponite-rich raw material led to a decrease in the specific surface area (from 35 m2 g–1 to 19 m2 g–1); of MMS, it led to a slight increase (from 53 m2 g–1 to 58 m2 g–1). The research demonstrated that the acid-base surface state of saponite-rich raw material and MMS changed significantly after their modification by graphene-like MoS2 and WS2; the more promising of these is MoS2. The sorption capacity of the sorbents in relation to cationic and anionic dyes was correlated with the total acidity of the samples. All samples had at least three times greater sorption capacity for cationic than for anionic dyes, which is explained by the general basicity of composite samples. The results showed that targeted modification of relatively inexpensive clay minerals to produce highly efficient sorbents and catalysts with improved surface and texture characteristics is a promising and relevant area in water purification and catalysis. Further studies into catalytic processes are needed, however.
ACKNOWLEDGMENTS
The authors thank Svitlana Nahirniak for her support in conducting this research.
Compliance with Ethical Statements
Conflict of Interest
The authors declare that they have no conflict of interest.
