Introduction
Organic dyes represent a large group of pollutants in wastewaters of textile, paper, plastic, food, and other industries. With the production of 70,000 tons of ~10,000 types of dyes and pigments worldwide annually, about 1–15% of dyes are estimated to be lost as wastes during dyeing processes (Elmoubarki et al., Reference Elmoubarki, Mahjoubi, Tounsadi, Moustadraf, Abdennouri, Zouhri, Albani and Barka2015). Organic dyes, especially cationic dyes, are used widely due to their ease of application and durability, but the side-effects are significant. Cationic dyes are recognized as carcinogenic and mutagenic, and have a strong staining effect on the entire ecosystem when disposed as waste into the environment, even at low concentrations (Tkaczyk et al., Reference Tkaczyk, Mitrowska and Posyniak2020). Therefore, reducing the concentration of dyes in wastewaters before discharge into the environment is important and the development of appropriate purification processes is essential (Tripathi et al., Reference Tripathi, Singh, Pathak, Kasaudhan, Mishra, Bala, Garg, Singh, Singh, Singh, Shukla and Pathak2023). Methylene blue (MB) is a cationic dye that has a very wide application and is becoming one of the most common colored pollutants in waters. Among several chemical and physical methods of wastewater treatment, adsorption is an effective and efficient method for removing dyes, using various clay-based adsorbents such as bentonite (Jawad et al., Reference Jawad, Saber, Abdulhameed, Farhan, ALOthman and Wilson2023), Moroccan clay (Souhassou et al., Reference Souhassou, Khallouk, El Khalfaouy, El Gaidoumi, Nahali, Fahoul, Tanji and Kherbeche2023), montmorillonite (Mundkur et al., Reference Mundkur, Khan, Khamis, Ibrahim and Nancarrow2022), and others. In general, raw kaolin-based clays (Koteja and Matusik, Reference Koteja and Matusik2015) are not suitable adsorbents due to the limited specific surface area and low layer charge, which limit the extent of ion-exchange processes on their surfaces. Nevertheless, studies have shown that kaolin-based clay minerals can be modified to become better adsorbers by intercalating or grafting organic compounds into the interlayer space, leading to hybrid organic-mineral materials with suitable properties for adsorption and several other applications (Koteja and Matusik, Reference Koteja and Matusik2015; Mukhopadhyay et al., Reference Mukhopadhyay, Sarkar, Palansooriya, Dar, Bolan, Parikh, Sonne and Ok2021). Other modifications have been reported to increase the adsorption capacity of kaolin-based clays, such as pillaring (Mnasri-Ghnimi and Frini-Srasra, Reference Mnasri-Ghnimi and Frini-Srasra2019; Silva et al., Reference Silva, Kalmakhanova, Massalimova, Sgorlon, Diaz de Tuesta and Gomes2019; Reimbaeva et al., Reference Reimbaeva, Massalimova and Kalmakhanova2020; Zhang et al., Reference Zhang, Wang, Zhao, Bai, Wen, Kang, Song, Song and Komarneni2021), acid activation (Silva et al., Reference Silva, Kalmakhanova, Massalimova, Sgorlon, Diaz de Tuesta and Gomes2019; Zhang et al., Reference Zhang, Wang, Zhao, Bai, Wen, Kang, Song, Song and Komarneni2021), and magnetic activation (Ahmadi et al., Reference Ahmadi, Foroutan, Esmaeili and Tamjidi2020; Esvandi et al., Reference Esvandi, Foroutan, Peighambardoust, Akbari and Ramavandi2020; Baimuratova et al., Reference Baimuratova, Kalmakhanova, Shynazbekova, Kybyraeva, Díaz de Tuesta and Gomes2022; Baimuratova et al., Reference Baimuratova, Ermekov, Silva, Gomes and Kalmakhanova2024); but, to the best of the present authors’ knowledge, organic grafting remains largely unexplored for adsorption processes, especially for MB adsorption (see Fig. S1a in the Supplementary material). Clays with embedded organic compounds are expected to provide a better adsorption capacity for dyes (Haleem et al., Reference Haleem, Anum, Chen and Nazar2023). In particular, amines, such as triethanolamine (TEAO), act as Lewis bases (Zakharova et al., Reference Zakharova, Masoumifard, Hu, Han, Kleitz and Fontaine2018) and, thus, increase the number of active centers for the adsorption of cationic dyes (Yang et al., Reference Yang, Wang, Chen, Xie and Li2019), such as MB, over a wide pH range.
Clays belonging to the kaolin group (1:1 layers) characteristically have a smaller cation exchange capacity and surface area than many 2:1 layered minerals, such as smectite (Qi et al., Reference Qi, Yu, Shah, Shah and You2023), and thus are only good for adsorption purposes if their layers are opened by means of intercalation. Only a few molecules can be intercalated into the 1:1 layer (Lagaly et al., Reference Lagaly, Ogawa, Dékány, Bergaya and Lagaly2013). As a result, they are not as widely modified by organic grafting as 2:1 layered clays (such as bentonites and montmorillonite; see Fig. S1b in the Supplementary material). Furthermore, few studies exist in the literature with kaolinite clays used for the adsorption of dyes with organoclays (Ferreira et al., Reference Ferreira, Ciuffi, Nassar, Vicente, Trujillano, Rives and Faria2017). To the best of the present authors’ knowledge, the application of kaolinite clays modified with TEAO in the adsorption of MB, reported here, has not been tested previously. Because direct intercalation of bulky compounds (such as TEAO) into the interlayer space of kaolinite is not feasible (Letaief and Detellier, Reference Letaief and Detellier2011), due to strong interactions between the tetrahedral and the octahedral sheets of the clay, the smaller dipolar dimethyl sulfoxide (DMSO) was pre-intercalated (yielding Shymkent-DMSO) and used as the precursor for further intercalation of TEAO. In this process, TEAO is inserted into the interlayer spaces of kaolinite upon displacement of DMSO, yielding stable kaolinite organo-grafted clays resulting from the formation of a covalent bond between the hydroxyl groups of TEAO and the aluminol groups of the internal surfaces of kaolinite. Both raw and modified clays were tested in the adsorption of MB, and adsorption conditions were evaluated using the Shymkent-TEAO sample. The amine center introduced in the sample allowed fast MB adsorption over a wide pH range, supporting the hypothesis that was tested regarding the development of highly adsorptive kaolinite organo-grafted clays with TEAO.
Materials and methods
Materials and reactants
In the present study, a natural kaolinite clay was obtained from a deposit found at Shymkent, Kazakhstan, and is referred here as Shymkent clay. Dimethyl sulfoxide (DMSO, C2H6OS, 99.5%) was supplied by Stanlab PURE, dioxane (C4H4O2, 99.5%) was supplied by DiAKiT, isopropanol (C3H8O, 99.7%) was supplied by LaborPharm (Astana, Kazakhstan), triethanolamine (TEAO, C6H15NO3, 95%) was supplied by Tomas.kz. Methylene blue (98%) was supplied by Merck. All reactants were used without modification. Distilled water (resistivity of 18.2 Mohm-cm) was used throughout.
Synthesis of organo-modified clay materials
For the manufacture of materials with adsorption properties, the natural clay was first ground to powder in a mill and passed through a sieve with 0.063 mm maximum particle size. To modify the natural clay, 6 g of the clay was initially suspended in 6 L of water and then added to a mixture containing 30 mL of DMSO and 2.5 mL of water. The suspension was kept under magnetic stirring at a temperature of 80°C for 5 days, and then the mixture was left at room temperature for 2.5 days. The resulting material was extracted after two series of centrifugation using, first, dioxane (2×25 mL), and then isopropanol (2×25 mL). The product was dried at 50°C, leading to the modified clay Shymkent-DMSO. Shymkent-DMSO clay was further grafted with TEAO, generating a modified Shymkent-TEAO sample. One gram of Shymkent-DMSO was added to 6 g of TEOA and stirred at 180°C for 2 h, and then washed with isopropanol (3×50 mL). The resulting material was dried at 50°C for 24 h, leading to Shymkent-TEAO.
Characterization of the materials
Thermogravimetric analyses were performed using a Derivatograph device (MOM, Budapest, Hungary). The method used is based on recording changes in the thermochemical and physical parameters of a substance that are caused when that substance is heated. The thermochemical state of the sample is described by the T (temperature), DTA (differential thermal analysis), TG (thermogravimetric analysis), and DTG (differential thermogravimetric analysis) curves, the last of these being a derivative of the TG function. The materials did not undergo any specific treatment for the TGA analysis, and the analysis of the materials was carried out in air atmosphere, in the temperature range from 20 to 1000°C at a heating rate of 10°C min–1.
The textural properties of the materials were determined by analysis of N2 adsorption–desorption isotherms at –196°C, obtained using a Quantachrome NOVAtouch XL4 adsorption analyzer following procedures reported elsewhere (Diaz de Tuesta et al., Reference Diaz de Tuesta, Silva, Faria and Gomes2018; Karimi et al., Reference Karimi, Diaz de Tuesta, Gonçalves, Gomes, Rodrigues and Silva2020; Diaz de Tuesta et al., Reference Diaz de Tuesta, Saviotti, Roman, Pantuzza, Sartori, Shinibekova, Kalmakhanova, Massalimova, Pietrobelli, Lenzi and Gomes2021). All calculations were carried out using the software Quantachrome TouchWin TM v1.21. Prior to analysis, the materials were degassed at 120°C for 16 h. The surface areas (S BET and S Langmuir) were determined using the methods of BET and Langmuir with P/P 0 in the range 0.05–0.35. The external surface area (S ext) and micropore volume (V mic) were obtained by the t-plot method (thickness was calculated using the ASTM standard D-6556-1). The micropore surface area, S mic, was determined by the subtraction of S EXT and S BET, and the pore width (W mic) was obtained by approximation (W mic = 4–V mic/S mic). The equations used for determining S Langmuir and S BET are described in the Supplementary material (see also Lowell et al., Reference Lowell, Shields, Thomas and Thommes2004; Bardestani et al., Reference Bardestani, Patience and Kaliaguine2019; Diaz de Tuesta et al., Reference Diaz De Tuesta, Roman, Marques, Silva, Silva, Bosco, Shinibekova, Aknur, Kalmakhanova, Massalimova, Arrobas, Silva and Gomes2022). The total pore volume (V total) was determined at P/P 0=0.98. Studies of the morphology of the samples were carried out using a scanning electron microscope (SEM) JEOL JSM-6490LV with INCA Energy Microanalysis and HKL basic structural analysis systems. The multi-purpose SEM (useful magnification of 300,000×) combined the possibilities of working in both standard and low-vacuum modes, allowing the direct examination of samples in powder form without spraying with a conductive layer.
The pH of the point of zero charge (pHpzc) was determined using pH drift tests, adapting the procedure described by Silva et al. (Reference Silva, Kalmakhanova, Massalimova, Sgorlon, Diaz de Tuesta and Gomes2019). In short, five solutions of NaCl (0.01 M) were prepared as electrolyte with a variable initial pH (in the range of 1–12, using HCl and NaOH 0.1 M solutions). Clay sample (0.05 g) in powder form was combined with 20 mL of each NaCl solution. The equilibrium pH of each suspension was measured after 48 h under stirring (320 rpm) at room temperature. The pHpzc value was determined by crossing the curve ‘final pH vs initial pH’ with the straight line ‘final pH = initial pH’.
The X-ray diffraction (XRD) patterns were acquired using an automated diffractometer DRON-3 instrument with SiKα radiation and a β filter. X-ray phase analysis on a semi-quantitative basis was performed using XRD patterns of powder samples. The materials did not undergo any specific treatment before the analysis. The quantitative ratios of the crystal phases were determined. The interpretation of patterns was carried out using data from the ICDD card file: PDF2 powder diffraction database (Powder Diffraction File) and patterns for minerals free of impurities. The DRON-3 diffractometer tube settings were 35 kV and 20 mA. Fourier-transform infrared (FT-IR) spectra were acquired using a Perkin Elmer UATR 2 FT-IR spectrophotometer at a resolution of 1 cm−1 and scan range of 4000 to 450 cm−1. Prior to analysis, the materials were dried carefully at 105°C overnight. The materials (~1 mg) were previously mixed with KBr (~100 mg), crushed and pressed into a pellet using a manual hydraulic press (GS15011, Specac, UK) applying 10 tons of pressure.
Adsorption process and analytical methods
The MB adsorption tests were carried out by adding 0.125 g of adsorbent (natural and modified clays) to 50 mL of a 50 mg L−1 MB solution with a total contact time of 8 h at a temperature of 50°C. Sampling was undertaken at pre-determined intervals (0, 15, 30, 60, 120, 240, 360, or 480 min). The effects of various pH values of MB solution were tested (pH 1, 3, 6, 9, and 12). The adsorption tests also considered variations in the concentration of adsorbent (2.5, 0.5, and 0.25 g L−1) and variations in the concentration of MB solution (50, 100, and 500 mg L−1). The adsorption of MB was monitored at 664 nm using a UV-Vis spectrophotometer.
Results and Discussion
Thermal analysis (DTA and TGA)
As a result of the dynamic heating of these samples, the DTA, DTG, and TG curves revealed various types of reactions in the system (see Fig. S2 in the Supplementary material). Among them were the processes associated with the release of H2O and hydroxyls into the atmosphere during the decomposition of clay minerals and reactions with CO2 emissions because of the combustion of organic matter, as well as during the destruction of calcite. During the thermal decomposition of its mineral components, two pronounced effects were recorded, associated with the decomposition of gibbsite (Al(OH)3) (in the range 20–200°C) and with the dissociation of calcium carbonate (in the range 400–725°C) (Fig. S2a).
Several thermal effects, mainly endothermic reactions, were observed in the heating curves of modified Shymkent-TEAO clay samples (Fig. S2b). The reactions that caused these manifestations were accompanied by the evolution of molecular water (Δm1 (1.7%), see Table 1 for definitions), hydroxyls (Δm2 (1.4%), Δm5 (1.0%)), carbon dioxide from the decomposition of calcite, as well as a result of joint losses (Δm3 (2.5%) = OH (2.2%) + CO2 (0.3%)) at the destruction of kaolinite and combustion of S, respectively, as revealed by the reported semi-quantitative analysis (Table 1).
According to the thermogravimetric curve, in the first case, gibbsite was dehydrated (in two stages) with a weight loss equal to Δm1 – 0.3% = 1.7%, which corresponded to the presence of 8.1% of gibbsite in the Shymkent modified clay under dynamic heating from 20 to 1000°C, leaving a series of endothermic manifestations on the DTA curve at 190, 260, and ~500°C and one exothermic inflection point in the 900°C region. These endothermic reactions also took shape on the DTG curve in the form of clearly defined inflection points in the same temperature regions. Such processes were caused by thermal dehydration of minerals that were part of the test sample. At these temperatures, each such discharge from the water system left a corresponding step of weight loss on the thermogravimetric (TG) curve (Table 1). According to the atlas of thermal curves of minerals and rocks (Lorentz et al., Reference Lorentz, Shanahan, Stetsko and Zayed2018), the endothermic mass loss centered at 260°C was caused by the decomposition of gibbsite. The dehydration of this mineral, due to defects in its structure, proceeded in two stages. The total weight loss in this case reached the value of Δm1 + Δm2 = 8.58%, which corresponded to 13.1% of gibbsite in the sample. Along with this mineral, kaolinite was present in the sample; this was determined using the above-mentioned characteristics, viz. the endothermic effect at 500°C and the exothermic peak at 900°C (Khan et al., Reference Khan, Khan, Azizli, Sufian, Man, Siyal, Muhammad and Faiz ur Rehman2017). The effect found in the 500°C region was caused by dehydroxylation of the kaolinite structure.
Table 1. Thermogravimetric data for the natural clay and of Shymkent-TEAO clay, in the temperature range 20–1000°C
When heated, kaolinite left on the DTA curve a weak peak in the range of 300–500°C and an equally weak feature in the region of 900°C. Thermal dissociation of this mineral occurred together with the combustion of organic matter. At the same time, kaolinite registered a weight loss from the modified Shymkent-TEAO clay of 2.2% (OH↑), and of organic matter of 0.3% (CO2↑). Thus, the amount of this clay mineral corresponded to 8.3%, and the amount of organic matter in the sample, in accordance with the emissions from the CO2 system, was 0.3% of the mass of the modified Shymkent-TEAO clay. Calcite showed itself most clearly during heating, leaving clearly defined events on the DTA and DTG curves in the region of 700°C due to carbon dioxide emissions into the atmosphere. At the same time, the weight loss of the modified Shymkent-TEAO clay was 8.25%, which corresponded to 18.8% of calcium carbonate in its composition (Yuan et al., Reference Yuan, Southon, Liu, Green, Hook, Antill and Kepert2008).
N2 adsorption–desorption isotherms at 196°C
The N2 adsorption–desorption isotherms of the adsorbents (Shymkent natural and modified clays) (Fig. 1) and the corresponding textural properties (Table 2) revealed isotherm patterns identified as type IV (Brunauer et al., Reference Brunauer, Deming, Deming and Teller1940) with a type H3 hysteresis loop (Gregg and Sing, Reference Gregg and Sing1982), in accordance with the IUPAC classification. Type IV showed that the material had a mesoporous character, and the type H3 hysteresis loop showed the characteristics of a layered material with gaps similar to the pores formed by a plate layer with a space between arrays of plates (Sing et al., Reference Sing, Everett, Haul, Moscou, Pierotti, Rouquerol and Siemieniewska1985; Auerbach et al., Reference Auerbach, Carrado and Dutta2004). This also implied that DMSO and TEAO could maintain the area between the clay layers to remain supported to create a wider gallery.
Figure 1. N2 adsorption-desorption isotherms at –196°C of the (a) Shymkent natural clay, (b) Shymkent-DMSO, and (c) Shymkent-TEAO.
Table 2. Textural properties of the materials
The surface area decreased upon modification, which may indicate that DMSO and TEAO block the clay pores, especially the micropores (S mic), with TEAO completely blocking access to the microporosity of the natural clay. Similar behavior was reported previously, with a S BET decrease from 25 to 4 m2 g–1 for montmorillonite-derived materials (Godarziani et al., Reference Godarziani, Ramezanipour Penchah and Ghaemi2022), and from 97 to 37–66 m2 g–1 (Elkhalifah et al., Reference Elkhalifah, Bustam, Azmi and Murugesan2014a; Elkhalifah et al., Reference Elkhalifah, Bustam, Mohd Shariff, Ullah, Shimekit and Riaz2014b) for bentonite-derived materials when modified with TEAO. The micropore area has also been reported to decrease upon intercalation of TEAO (Godarziani et al., Reference Godarziani, Ramezanipour Penchah and Ghaemi2022).
Scanning electron microscopy
SEM images showed different morphologies for the Shymkent natural clay and the modified Shymkent-DMSO and Shymkent-TEAO clays (Fig. 2). The SEM images showed that the surface morphology of the Shymkent natural clay was typical of kaolinite (Cheng et al., Reference Cheng, Liu, Zhang, Yang and Frost2010; Guerra et al., Reference Guerra, Airoldi and Sousa2008; Sun et al., Reference Sun, Li, Li, Yu, Zhang and Fei2011; Caglar, Reference Caglar2012; Machado et al., Reference Machado, Groszewicz, Castro, Wypych and Nakagaki2012). The Shymkent clays modified with DMSO and TEOA retained this aspect, but the size of the kaolinite particles was smaller due to intercalation of TEAO molecules (Guerra et al., Reference Guerra, Airoldi and Sousa2008; Kausar and Bhatti, Reference Kausar and Bhatti2013). The modified clays had more spongy particles because of changes to the surface charge of the particle and the time spent preparing it (Caglar, Reference Caglar2012; Machado et al., Reference Machado, Groszewicz, Castro, Wypych and Nakagaki2012).
Figure 2. SEM images of: (a) Shymkent natural clay, (b) Shymkent-DMSO, and (c) Shymkent-TEAO.
Table 3. Results of semi-quantitative X-ray phase analysis of crystal phases
The pH of point of zero charge (pHpzc)
Procedures used to modify the corresponding pHpzc value (Fig. 3) showed that the starting material (Shymkent natural clay) had a slightly basic character (pHpzc=7.45). Upon modification with TEAO, the pHpzc decreased and became slightly acidic (pHpzc=6.25). Accordingly, the surface at pH<pHpzc was positively charged. The value of pHpzc depended directly on the amount of DMSO and TEАO introduced into the clay materials (Sing et al., Reference Sing, Everett, Haul, Moscou, Pierotti, Rouquerol and Siemieniewska1985).
Figure 3. Determination of the pHpzc values of the Shymkent natural clay and Shymkent-TEAO.
X-ray diffraction analysis
To determine the quantitative ratio of the crystalline phases of alumina in the natural clay from Shymkent, the samples were subjected to analysis by XRD (Table 3; Fig. 4). The results of the analysis established that the Shymkent clay consisted of layered silicates, primarily kaolinite Al2(Si2O5)(OH)4, quartz, and calcite, with a small amount of chlorite (Mg,Fe)5Al(Si3Al)O10(OH)8 and admixtures of muscovite KAl2(AlSi3O10)(OH)2, albite Na(AlSi3O8), Ca(SO4)(H2O)2, KAlSi3O8, and tremolite (Ca,Na,Fe)2Mg5Si8O22(OH)2. Other impurities may be present in small concentrations but were not identified unambiguously due to low-intensity reflections and/or poor crystallization (Serikbayeva et al., Reference Serikbayeva, Kalmakhanova, Massalimova, Zharlykapova and Bazarbaev2021).
Figure 4. XRD pattern of the Shymkent natural clay.
Fourier-transform infrared spectroscopy
The FT-IR spectra of the clays (Fig. 5) revealed characteristic vibrations for tetrahedral Si-O-Si in the region 950–1250 cm−1 and octahedral Al-Al-OH at 913 cm−1, Al-Fe-OH at 865 cm−1, and for Mg-Mg-OH at 650 cm−1 (Ahmed et al., Reference Ahmed, Chaker, Belarbi, Abbas, Chotard, Abassi, Van Nhien, El Hadri and Bresson2018). Furthermore, bands due to hydroxyls and water molecules were observed at ~3200–3400 and 1435 cm−1, respectively. The interactions between organic molecules and silicates were observed in the region 1200–2000 cm–1 (Junior et al., Reference Junior, da Silva, Saltarelli, Crispim, Nassar, Trujillano, Rives, Vicente, Gil, Korili, de Faria and Ciuffi2020). The typical bands of amine groups were observed at 1630 cm−1, confirming the grafting. The intensities of the bands in the high-wavenumber region decreased as a result of the condensation of silanes with Mg-OH or Al-OH groups from the fibrous structures. In addition, new bands at 3479 and 3410 cm−1 were ascribed to amino groups. The absorption band at 2930 cm−1 corresponded to the C–H stretching vibration of CH2 groups (Ahmed et al., Reference Ahmed, Chaker, Belarbi, Abbas, Chotard, Abassi, Van Nhien, El Hadri and Bresson2018).
Figure 5. FT-IR spectra of the Shymkent natural clay, and of versions modified with DMSO and TEAO.
The FT-IR spectra of the natural clays (Fig. 5) contained characteristic peaks for kaolinite and showed evidence of intercalation and grafting of DMSO and TEAO. The kaolinite band at 850 cm–1, attributed to the vibration of hydroxyl groups of the inner surface during smooth bending was sensitive to intercalation and grafting, the band shifting to 726 cm–1. In addition, intercalation of TEAO led to a significant decrease in the intensity of the hydroxyl band of the inner surface at 3600 cm–1 and to the appearance of a new peak at 3044 cm–1 (Fig. 5; DMSO and TEAO). The peak is formed as a result of an H bond between TEAO and hydroxyl groups of the clay inner surface at 1798 cm–1. The stretching peak of Shymkent-TEAO at 1792 cm–1 changed to 1549 cm–1 for the Shymkent-DMSO, indicating that the DMSO molecule forms an H bond with hydroxyls through carbonyl oxygen. In addition, the stretching peak of Shymkent-TEAO at 1310 cm–1 was observed at 1385 cm–1 for Shymkent-DMSO. In addition, the N–H deformation peaks of Shymkent-TEAO were shifted from 1549 cm–1 to 1377 cm–1, and the peaks of C–H stretching of Shymkent-TEAO are shifted from 2968, 2937, and 2798 cm–1, to 2968, 2937, and 2576 cm–1, respectively.
Adsorption of methylene blue
Effect of modification
Methylene blue was adsorbed by the Shymkent natural clay, Shymkent-DMSO, and Shymkent-TEAO (Fig. 6), using a dosage of 50 mg MB and 2.5 g of adsorbent. Similar results were found for each adsorbent in which nearly complete removal of MB was achieved after only 15 min. Very little additional adsorption occurred after even 480 min. Studies were then carried out to determine the optimum MB to adsorbent ratio.
Figure 6. Effect of clay modification on adsorption of methylene blue. Conditions: [MB]0=50 mg L–1, C adsorbent=2.5 g L–1, T=50°C, pH0=3.0.
Effect of adsorbent dosage
The adsorbent dosage was varied from 0.25 to 0.5 g per 50 mg of MB considering both the natural and TEAO-modified Shymkent clay (Fig. 7). When using the Shymkent natural clay at an adsorbent dosage of 2.5 g per 50 mg of MB, the adsorption, as previously reported (Fig. 6), was very quick: within the first 15 min of contact time, all MB had been adsorbed. By reducing the dosage to 0.5 g per 50 mg of MB, this behavior changed drastically: within the first 45 min of contact time, scarcely 35% of the MB was removed, whereas at an adsorbent dosage of 0.25 g per 50 mg of MB the removal rate was ~20%. When using Shymkent-TEAO at the dosage levels of 0.5 and 0.25 g per 50 mg of MB, a sharp improvement compared with the natural clay was observed. After 480 min of contact time, both dosages led to removals of >86% of the original concentration of MB (which is equivalent to an adsorbed quantity of Q=159 mg g–1, calculated according to Saeed et al., Reference Saeed, Munir, Nafees, Shah, Ullah and Waseem2020). The greater degree of adsorption by the modified clay was in the range of 1.10–4.80×, depending on the adsorbent concentration and contact time. Similar observations have been reported previously, with increases in the adsorption capacity of TEAO-modified kaolinites in the range of 1.25–2.5× greater than the raw clay (Koteja and Matusik, Reference Koteja and Matusik2015) and 5–20× greater for halloysite-modified clay (Matusik and Wścisło, Reference Matusik and Wścisło2014) for adsorption of various heavy metals. Adsorption is a process that is not only governed by surface area, but the surface chemistry plays a significant role in adsorption process (Ruiz-Rosas et al., Reference Ruiz-Rosas, García-Mateos, Gutiérrez, Rodríguez-Mirasol and Cordero2019; Kalmakhanova et al., Reference Kalmakhanova, Diaz de Tuesta, Malakar, Gomes and Snow2023). In the case of the present study, given that the surface area was small even before organic grafting, surface area was not the key parameter driving adsorption. TEAO is a molecule containing nitrogenated moieties, which can behave as a Lewis base (Matusik and Wścisło, Reference Matusik and Wścisło2014) and allow interaction with MB. The increase in adsorbent also resulted in increased adsorption. For natural clay, this increase was in the range of 1.10–1.40× for an adsorbent concentration of 0.5 g per 50 mg of MB compared with 0.25 g per 50 mg of MB, depending on contact time. For Shymkent-TEAO this difference was in the range of 1.15–1.30×. Thus, for this range of low adsorbent doses, the effect of modifying the clay is much more pronounced than the adsorbent concentration, allowing for a more sustainable and economic implementation of the process. The adsorption of a range of dyes by organoclays, as described in the literature, is noted in Table S1 (Suchithra et al., Reference Suchithra, Vazhayal, Mohamed and Ananthakumar2012; Fan et al., Reference Fan, Zhou, Jiang, Huang and Lang2014; Ferreira et al., Reference Ferreira, Ciuffi, Nassar, Vicente, Trujillano, Rives and Faria2017; Ali et al., Reference Ali, Ali, Ullah, Ali, Duclaux, Reinert, Lévêque, Farooq, Bilal and Ahmad2020; Rahmani et al., Reference Rahmani, Zeynizadeh and Karami2020; Saeed et al., Reference Saeed, Munir, Nafees, Shah, Ullah and Waseem2020; Teixeira et al., Reference Teixeira, Lima, Benetti, Naushad, Thue, Mello, Reis, Rabiee, Franco and Seliem2023). As observed, the results reported here compare well with previous reports in the literature, surpassing them in some cases. Note that commercially or industrially available clays that often are used may be purer than the natural clays used in the present study.
Figure 7. Effect of adsorbent concentration on methylene blue adsorption by Shymkent and Shymkent-TEAO. Conditions: [MB]0=50 mg L–1; C adsorbent=0.25 and 0.5 g L–1; T=50°C; pH0=3.0.
Effect of pH
The effect of the solution pH may play an important role in the adsorption process, as this parameter changes the chemical composition of the dye solution and the charge of the functional groups of the adsorbents (Nadeem et al., Reference Nadeem, Manzoor, Iqbal and Nisar2016). The effect of pH was studied (Fig. 8). Usually, at a low pH, the removal percentage of an anionic dye from the solution would increase due to the electrostatic attraction between the positive surface charge of the adsorbent and the anionic dye. On the other hand, an electrostatic attraction between a negatively charged adsorbent and a positively charged dye molecule would occur if the solution had a high pH (basic), which would lead to a decrease in the percentage of anionic dye removal (Rashid et al., Reference Rashid, Bhatti, Iqbal and Noreen2016; Tahir et al., Reference Tahir, Bhatti and Iqbal2016a; Naeem et al., Reference Naeem, Bhatti, Sadaf and Iqbal2017; Shoukat et al., Reference Shoukat, Bhatti, Iqbal and Noreen2017). At high pH, the adsorption capacity and removal of cationic dyes should increase, as the positive charges in the dye should ensure their attraction by the anionic adsorbent; an electrostatic attraction would thus occur between the positive charges of the dye and the negative surface of the adsorbent (Sharma Reference Sharma, Borah, Das and Das2015; Tahir et al., Reference Tahir, Bhatti, Iqbal and Noreen2016b; Tahir et al., Reference Tahir, Iqbal, Abbas, Tahir, Nazir, Iqbal, Kanwal, Hassan and Younas2017). However, a decrease was observed in the ability of Shymkent-TEAO clay to adsorb MB with an increase in the pH of the initial suspension (Fig. 8). The adsorption of the cationic molecule of MB should be greater at higher pH, but, as seen in Fig. 8, there is no significant effect of pH. The maximum MB removal was observed at pH=3.0, where the modified clay removed up to 98% of MB. Given the pHpzc of Shymkent-TEAO (pHpzc=6.25) (Fig. 3), an excess of cations (H+) at the surface of the adsorbents was expected. However, as previously discussed, the nitrogenated moiety from the TEAO molecule behaves as a Lewis base, being negatively charged (Matusik and Wścisło, Reference Matusik and Wścisło2014); thus, it allowed sufficient interaction between the adsorbent surface and the MB. The Shymkent-TEAO demonstrated flexibility, allowing its use over a wide range of pH values.
Figure 8. Effect of pH on adsorption of methylene blue by Shymkent-TEAO. Conditions: [MB]0=50 mg L–1; C adsorbent=2.5 g L–1; T=50°C; pH0=1.0, 3.0, 6.0, 9.0, and 12.0.
Effect of methylene blue concentration
As expected, the adsorption efficiency of MB on Shymkent-TEAO modified clay at various initial MB concentrations (Fig. 9) increased as the MB concentration decreased. The main active center for the Shymkent-TEAO sample is the nitrogen moiety from the TEAO molecule. Adsorption is governed by the free energy of adsorption of the surface, so the extent of adsorption depends on both the magnitude of the free energy and the amount of surface area available. As the clay developed here has a relatively small surface area (9 m2 g–1), the adsorption capacity is limited. Thus, the adsorbents prepared here would be most effective when applied in industrial wastewater streams in which the MB concentration is not too large, e.g. 10–50 mg L–1 (Yaseen and Scholz, Reference Yaseen and Scholz2019). This range of MB contamination is found widely, thus making Shymkent-TEAO suitable for MB clean-up, with the added advantage of enabling lower doses of adsorbent to be used than those for unmodified clay.
Figure 9. Effect of the initial concentration of MB on amount adsorbed by the Shymkent-TEAO clay. Conditions: C adsorbent=2.5 g L–1; [MB]0=50, 100, and 500 mg L–1; T=50°C; pH0=3.0.
Conclusions
Clays (natural and modified) are inexpensive adsorbents that have been used successfully over recent decades in the adsorption of dyes from wastewater. The incorporation of a tertiary amine, TEAO, in the natural clay allowed a significant increase in the adsorption capacity of the clay, even if the textural properties were not improved. The reason ascribed to this phenomenon was the Lewis base character of the nitrogenated moiety. The presence of such an active center allowed a greater interaction of the adsorbent with MB. Due to its improved characteristics, Shymkent-TEAO was able to ensure the effective removal of MВ from aqueous solutions. It was also demonstrated that the solution pH was not the key factor in the adsorption, specifically due to the presence of the Lewis base active center. This feature opens future prospects for the development of economically viable adsorption-based technologies that can reduce the spread of MВ in urban water cycles. However, further studies should include an assessment of environmental toxicity in real application scenarios in which several micro-pollutants are present in trace concentrations, and not just one pollutant in a concentration exceeding that observed in urban water cycles.
Supplementary material
The supplementary material for this article can be found at http://doi.org/10.1017/cmn.2024.34.
Data availability statement
Necessary data has been provided along with this manuscript.
Acknowledgements
None
Author contribution
Aizhan M. Serikbayeva: Conceptualization, Investigation, Formal analysis, Methodology, Visualization, Writing – original draft. Fernanda F. Roman: Investigation, Writing – Review and Editing. Helder T. Gomes: Funding acquisition, Project administration, Writing – Review & editing, Supervision. Marzhan S. Kalmakhanova: Writing – Review & Editing, Funding acquisition, Project administration, Supervision.
Financial support
This research was funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (grant no. AP13067715) and by Base Funding of CIMO (UIDB/00690/2020) through FEDER under Program PT2020.
Competing interest
The authors declare none.
