This study explores the adsorption of methylene blue (MB) dye from aqueous solution using different forms of fenugreek galactomannan and linseed (both individually and in combination) as an adsorbent. Characterization study of adsorbents reveal the involvement of carboxyl, hydroxyl and amine groups in MB adsorption taking place on the amorphous and porous surface of the adsorbent. BET surface area of the adsorbent (F+L) was found to be 394 m2/g. Different parameters including pH, adsorbent dose, adsorbate concentration, adsorption time and temperature were studied. Results of the study shows 86.62, 86.94, 88.04, 88.24, 88.56 and 89.09% of MB dye adsorption with F, L, F + L (1:1), F + L (1:2), F + L (2:1) and F + L (2:2) under optimized conditions. Maximum adsorption take place at pH 12, i.e., highly basic medium. Isothermal study shows the fitness of the experimental data to Langmuir isotherm model. Kinetic modeling shows that both physical and chemical interactions are involved in dye adsorption. Further, thermodynamic study confirms the spontaneity, endothermicity and feasibility of the process. Overall, current research involves the use of low-cost raw material for MB dye removal that aids in waste management and in promoting sustainability.

  • Wastewater treatment is one of the emerging trends in modern society.

  • Use of saccharide materials without any surface modification for MB dye removal promoting waste management and resource preservation.

  • Optimization of various operation parameters for a better understanding of the process.

  • Isothermal, kinetic, and thermodynamic study for better understanding of the adsorption process.

  • Comparative analysis of adsorption capacity with other studied adsorbents.

Environmental issues are drawing researchers’ attention nowadays due to their adverse effects on public health and the environment (Pal et al. 2014). Several kinds of diseases are caused by prolonged exposure to contaminants that are emerging mainly because of anthropogenic activities, including rapid industrial growth (Miyah et al. 2018). These contaminants are responsible for affecting the key mediums of life on Earth such as air, soil, and water (Heibati et al. 2015; Munagapati et al. 2018; Mannan et al. 2022). Among all kinds of pollutants, dyes are the main constituents of waste emerging mainly from the industrial sector (Mittal et al. 2010; Uddin et al. 2017; Mondal & Kar 2018) and are accountable for affecting both the environment and life on Earth due to their carcinogenic and mutagenic nature (Haik et al. 2010; Ai et al. 2011; Anastopoulos & Kyzas 2014). Methylene blue (MB) dye is one of the cationic dyes having the molecular formula of C16H18N3SCl and molecular weight of 319.85 g/mol (Khan et al. 2022). This dye is noxious in nature and is responsible for causing numerous health issues including eye burn, cardiac disorder, fever, headache, and anemia. The non-biodegradable nature of this dye makes its removal from wastewater very difficult, which poses a serious threat to the environment and all life forms on Earth (Ghzal et al. 2023). In view of this, for ensuring the well-being of our ecosystem and sustaining life on this planet, it is crucial to find an appropriate way to treat polluted water (Konicki et al. 2015).

When it comes to treating wastewater, numerous water treatment procedures have been developed and are in use today. These include traditional physicochemical or biological methods for dye removal from wastewater like photochemical degradation, adsorption, reverse osmosis, membrane separation, oxidation, coagulation/flocculation (Gupta 2009; Idrissi et al. 2014; Duta & Visa 2015; Miyah et al. 2017) and biological treatment using some microorganisms (Room & Center 2004). However, each of the aforementioned methods has its own merits and demerits such as the high cost associated with the operation and toxic by-product formation (Zhou et al. 2018) that limit their uses for treating wastewater (Zhou et al. 2018). Among all currently available techniques, adsorption is one of the most effective methods for treating dye-loaded wastewater because of its low cost, remarkable efficiency, and easy operation (Jain et al. 2014; Naushad et al. 2016; Ahmad et al. 2017; Daraei & Mittal 2017; Arora et al. 2020; Kumar et al. 2020; Soni et al. 2020; Arora et al. 2021; Haddad et al. 2021; Patel et al. 2021; Saharan et al. 2021; Mariyam et al. 2021a, 2021b, 2021c; Mittal et al. 2021a, 2021b, 2021c, 2022). In the adsorption treatment, the selection of an appropriate adsorbent for maximum removal of the pollutant plays a crucial role (Azhar-ul-Haq et al. 2022). To date, numerous adsorbents have been studied for treating wastewater such as Cedrus deodara sawdust (Batool et al. 2021), walnut wood (Hajati et al. 2016), coconut husk (Foo & Hameed 2012), loofah sponge (Li et al. 2018), orange peels (Oyekanmi et al. 2019), sugarcane bagasse (Noreen et al. 2020), rice husk (Bhatti et al. 2020), peanut hull (Tahir et al. 2017; Yang et al. 2020), potato peels (Chidi & Kelvin 2018), algae (Kumar et al. 2006), soy waste (Jawad et al. 2020), stone of mango (Shoukat et al. 2017), activated carbons (Pereira et al. 2003), zeolites (Bosso & Enzweiler 2002), silica beads (Krysztafkiewicz et al. 2002), industrial by-products (Garg et al. 2003; Netpradit et al. 2003), agricultural wastes (Robinson et al. 2002a, 2002b), polymeric materials (Lord et al. 2022; Mashkoor et al. 2023), and others (Gupta 2009; Salleh et al. 2011; Miyah et al. 2015, 2016; Miyah et al. 2017).

However, some of these adsorbents are costly and their adsorption efficiency is low in most cases, which can be improved by some physical or chemical treatments, for example, modification or by making composites with other materials (Miyah et al. 2018). Therefore, the use of some non-modified, abundant, low cost, and waste materials as an adsorbent, such as Trigonella foenum-graecum (F) and Linum usitatissimum (L), were considered for efficient MB dye removal, giving an extra economical interest to technical adsorption studies. Linseed (L. usitatissimum L.) crop is commonly cultivated in India. Activated carbon obtained from deoiled cakes finds extensive uses in treating wastewater having numerous metals and dyes in it (Khan & Khan 2016). Fenugreek (T. foenum-graecum) is another valuable crop in India and is commonly known as Methi and is widely available all over the world. Once the leaves of fenugreek are cut for cooking, the residual stems and roots are discarded as waste products that can be employed as adsorbent for treating wastewater (Jain et al. 2020). Upon mixing F and L adsorbents equally (1:1 or 2:2) and in variable (2:1 or 1:2) ratios, adsorptive removal of MB dye can be improved, which is the main focus of the study.

Keeping in view the aforementioned discussion, current research aims to investigate the efficiency of individual F seeds and L as adsorbents for MB dye adsorption, and to investigate their combined effects in four different ratios, i.e., 1:1, 1:2, 2:1, and 2:2, respectively, for MB adsorption, to optimize the adsorption parameters including solution pH, adsorbent dose, dye concentration, contact time, and temperature for maximum dye removal, to apply concentration and contact time data to different isotherm and kinetic models for determining the best fitted model, to explore the adsorption process thermodynamically via the Van't Hoff plot, to investigate the mechanism of MB removal on studied adsorbents and to compare the adsorption capacity of studied adsorbents with already reported adsorbents toward MB dye removal. It is noteworthy to mention here that two different (1:2 and 2:1) and two equal ratios (1:1 and 2:2) of adsorbents were used and the ratio 2:2 is just the double amount of 1:1, but this variation in amount used was done to better understand the effect of doubling the amount of adsorbent. Although many other researches have already been reported in the literature that involve the study of MB adsorption (Mahmoud et al. 2016; Basrur & Ishwara Bhat 2017a, 2017b; Gopalakrishnan et al. 2020; Kuang et al. 2020; Ahmad & Ansari 2021; Işık & Uğraşkan 2021; Sharma et al. 2022; Taweekarn et al. 2022; Zeghioud et al. 2022; Kumari et al. 2023), the current work is novel in a sense that to the best of our knowledge it is the first ever report on studying the individual and combined adsorptive efficiency of F and L in six different forms (i.e., F, L, F + L (1:1), F + L (2:2), F + L (1:2) and F + L (2:1)) for MB dye removal.

Chemicals and instruments used

The analytical-grade chemicals utilized in the current study include sodium hydroxide and nitric acid (for investigating the effect of solution pH on dye adsorption). The dye used here is MB that was purchased from a chemical drug house (CDH). Further, distilled water was used as a solvent throughout the research work. The percentage purity of the dye used was 99.99%. The instruments used were a Fourier transform infrared (FTIR) spectroscope, an X-ray diffractometer (XRD), a scanning electron microscope (SEM), a Brunauer–Emmett–Teller (BET) analyzer, a UV-visible spectrophotometer, a pH meter, a weighing balance, a centrifuge machine, and a hot plate.

Preparation of spike solution of MB dye

A standard MB solution of 1,000 mg/l concentration was prepared by dissolving stoichiometric amount of dye, i.e., 1 g in enough distilled water to make total volume of 1,000 ml. Solutions of required dilute concentrations were then obtained by diluting the stock solution with distilled water.

Preparation of adsorbent

Six different types of adsorbents, i.e., fenugreek, linseed, and combination of fenugreek + linseed in same ratios, i.e., F + L (1:1) and (2:2) and different ratios, i.e., F + L (1:2) and F + L (2:1) were employed throughout the research work. Galactomannan was extracted from fenugreek only (no extraction was done for linseed) and was further purified before its use. For galactomannan extraction, the fenugreek was first soaked in excess of distilled water, which was then grinded with the help of pestle and mortar (to get particle size 355 μm) followed by filtering with the help of muslin cloth. After the extraction of galactomannan, the next step was to isolate and purify the mucilage. For this purpose, filtered galactomannan was first washed thoroughly with methanol and then filtered using Whatman filter paper. The obtained filtrate was then dried in a microwave oven at 60 °C for 60 s till constant weight was obtained. The dried powder was then used both individually and in combination with linseed for MB dye removal. Further, linseed was also washed thoroughly with distilled water (to remove any kind of impurities), grinded with the help of pestle and mortar (to get particle size 355 μm), and dried at 60 °C for 60 s till constant weight was obtained. This powder form was then used both individually and in combination with fenugreek for MB dye removal. Figure 1(a) shows the flow diagram of each step involved for adsorbent preparation and dye adsorption. When the individual polysaccharides were mixed with each other, it resulted in the development of some kind of linkage termed as the 1, 4-glycosidic linkage between the rhamnose monosaccharide of L. usitatissimum seed (L) (present in linseed) (Fedeniuk & Biliaderis 1994) and mannose monomer of galactomannan of T. foenum-graecum (T) (Hannan et al. 2007; Kamble et al. 2013) (Figure 1(b)). Due to this bond formation, a water molecule was released by reaction of hydrogen atom of C1 of one monosaccharide with hydroxyl atom of another monosaccharide attached at C4 position (Tukenmez et al. 2019). This linkage continues throughout the F–L saccharide polymer.
Figure 1

(a) Schematic illustration of galactomannan extraction from fenugreek, isolation, purification of mucilage, and dye adsorption using prepared adsorbents (F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2)), (b) 1, 4-Glycosidic linkage formed between monomeric units of natural saccharide polymers, i.e., Linum usitatissimum seed (L) and Trigonella foenum-graecum (T).

Figure 1

(a) Schematic illustration of galactomannan extraction from fenugreek, isolation, purification of mucilage, and dye adsorption using prepared adsorbents (F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2)), (b) 1, 4-Glycosidic linkage formed between monomeric units of natural saccharide polymers, i.e., Linum usitatissimum seed (L) and Trigonella foenum-graecum (T).

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Adsorption studies

For optimization of experimental conditions for maximum dye removal, batch study was conducted by varying one parameter at a time while keeping others as constant. The physicochemical parameters studied included solution pH (2–12), dose of each adsorbent (0.1–1.2 g adsorbent/0.02L of dye solution), dye concentration (10–120 mg/l), contact time (10–120 min), and temperature (10–50 °C). Totally, 20 ml of dye solution having concentration of 40 mg/l was used for investigating the effect of solution pH and adsorbent dose. For subsequent trials, i.e., contact time and temperature experiment, solution concentration of 80 mg/l was used. Dose of each adsorbent used during all experiments was 1 g of adsorbent/0.02L of dye solution. Each experiment was conducted at 12 pH except for the pH study where the solution pH varies from 2 to 12. A contact time of 40 min was applied to each experiment while studying the effect of solution pH, adsorbent dose, and dye concentration, while for the study of contact time, adsorption time was varied from 10 to 120 min for its optimization where maximum results could be achieved. At the end, temperature study was carried out at the optimized time period of 60 min. All experiments were performed at 313 ± 1 K unless otherwise specified.

For calculating percentage removal of dye and adsorption capacity of each adsorbent toward MB dye removal, Equations (1) and (2), respectively, were used (Tchobanoglous et al. 1991):
formula
(1)
formula
(2)

Herein, and are initial and final absorbance of dye solution, and refer to the initial and equilibrium concentration of the dye, V refers to volume (ml) of the dye solution, and M refers to the mass (g) of adsorbent used. Experimental data from dye concentration and contact time experiment were applied to three different isotherm and kinetic models correspondingly, while a thermodynamic study was carried out on the results of the temperature study.

Adsorbent characterization

For characterization of the prepared adsorbents, different analyses techniques were employed as mentioned previously, including FTIR, XRD, and SEM for determining the functionalities present in adsorbents (Batool et al. 2022; Shah et al. 2024), their crystallinity or amorphous nature (Batool et al. 2022), and morphological analysis of the prepared adsorbents, respectively (Yusuff 2019; Batool et al. 2021).

Characterization results

Characterization results of all six studied adsorbents have been summarized as follows.

Functionalities present in studied adsorbents

For investigating the functional groups present in the studied adsorbents, FTIR analysis was carried out with wavenumber ranging from 4,000 to 500 cm−1 and 32 scans per sample. The results of the FTIR analysis of each individual polymer (L usitatissimum seed oil cake (L) and T. foenum-graecum (F) seed galactomannan) and their combined mixtures are shown in Figure 2 revealing the presence of different organic and inorganic functionalities of the studied adsorbents. The FTIR results of linseed showssharp peaks at 1,843 cm−1 that can mainly be attributed to the stretching vibration of fatty acids present in the polymeric structure, while stretching vibrations of bond due to methylene groups of linseed are confirmed by peaks obtained at 2,869 and 2,764 cm−1, respectively. Furthermore, the peak obtained at 1,182 cm−1 represents the bending vibrations of methylene group of linseed. Stretching vibrations of functionalities were confirmed by peaks obtained at 3,655 cm−1 (Guesmi et al. 2022). For fenugreek polymer, the peak obtained at 3,101 cm−1 represents and stretching vibrations of protein and starch fiber. Further, peaks obtained at 3,023 and 2,996 cm−1 represent asymmetric and symmetric stretching vibrations. Presence of stretching vibration of lipids was clear from the peak value obtained at 1,709 cm−1, while the peaks at 1,428 and 1,188 cm−1 correspond to N–H bending vibrations of primary amide and vibrations, respectively. Furthermore, vibrations of bond appear nearly at 886 cm−1 (El-Bahy 2005). However, the combinations of L and F in four different ratios show some similar functionalities as were present in the individual polysaccharides, i.e., presence of stretching vibration of lipids that was clear from the peak obtained at 1,790 cm−1 for fenugreek. It was observed that bond vibrations of methylene groups that were obtained at 2,764 cm−1 for linseed were diminished after combining fenugreek and linseed conforming to the formation of the F + L combination. P–O–C anti-symmetric stretching vibrations were also confirmed by peaks obtained in the range 1,220–1,100 cm−1. Furthermore, the presence of hydroxyl group was also confirmed by FTIR spectra, but with a little shift in peak value obtained after mixing the individual polysaccharide.
Figure 2

FTIR results of individual and combined Trigonella foenum-graecum (F) and Linum usitatissimum (L) seeds.

Figure 2

FTIR results of individual and combined Trigonella foenum-graecum (F) and Linum usitatissimum (L) seeds.

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X-ray diffraction analysis

XRD analysis was carried out within 2θ range of 5–90°. Figure 3 represents the analysis results of the XRD of combined F and L . Since, a broad peak is obtained nearly at 2θ = 20°, it represents the crystallinity of the studied saccharide material. However, the presence of many other smaller peaks in the spectra confirms the presence of a majority of amorphous regions in the prepared F–L structure (Niknam et al. 2020).
Figure 3

XRD results of combined Trigonella foenum-graecum (F) and Linum usitatissimum (L) seeds.

Figure 3

XRD results of combined Trigonella foenum-graecum (F) and Linum usitatissimum (L) seeds.

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BET surface area analyzer

BET surface area of the studied adsorbent (F + L combination) was observed to be 394 m2/g conforming to the mesoporous surface of the studied adsorbents that follow the type III of hysteresis loop. The results (Figure 4) show that the hysteresis loop of the studied process was found to be between 0.5 and 0.8 with respect to relative pressure. Similar findings are already reported by Ghzal et al. (2023).
Figure 4

N2 adsorption–desorption isotherm.

Figure 4

N2 adsorption–desorption isotherm.

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Morphology of studied adsorbent

For morphological investigation of all studied adsorbents, SEM analysis was carried out, which reveals the porous, heterogeneous, and rough surface of all adsorbents with various kinds of vacant adsorption sites. These empty cavities are mainly responsible for dye adsorption. Once equilibrium in adsorption was achieved, then it resulted in filling the empty sites of the adsorbent surface that ultimately converted the highly heterogeneous adsorbent surface to a somewhat homogeneous surface (Batool et al. 2021; Ghzal et al. 2023). The results of the SEM images for F, L, and F + L adsorbents are shown in Figure 5.
Figure 5

SEM results of the individual ((a) and (b)) and combined (c) Trigonella foenum-graecum (F) and Linum usitatissimum (L) seeds.

Figure 5

SEM results of the individual ((a) and (b)) and combined (c) Trigonella foenum-graecum (F) and Linum usitatissimum (L) seeds.

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Effect of solution pH on MB dye adsorption

Solution pH has a remarkable effect on adsorption of dyes as it affects the charge present on the adsorbent surface. Herein, solution pH varies from 2 to 12, keeping other parameters constant. The results of the study are presented in Table 1 and Figure 6. They reveal that with an increase in solution pH from 2 to 12, there is a continuous increment in percentage removal for all studied adsorbents, i.e., F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2). It is observed that when the F and L  were used individually, their percentage removal efficiency was less as compared with their different combinations in the ratios 1:1, 1;2, 2:1, and 2:2. The maximum percentage removal was obtained with the 2:2 ratio because of the equally combined efficiency of the individual adsorbents. Further it is observed that the basic pH favors adsorption of cationic dye due to deprotonation of the functionalities present on the adsorbent surface. This leads to introducing negative charge on each adsorbent, which attracts and adsorbs positively charged MB dye via electrostatic interactions leading to an increased percentage removal, while conversely, acidic or lower pH of solution makes the surface of the adsorbent positively charged due to the protonation of the adsorbent functional groups. This positively charged adsorbent surface repels positively charged MB dye molecules, which results in a decrease in percentage of removal (Badis et al. 2016; Ahmad et al. 2021; Batool et al. 2021; Azhar-ul-Haq et al. 2022; Bukhari et al. 2022; Ghzal et al. 2023; Urooj et al. 2023).
Table 1

Experimental data showing the effect of solution pH on MB dye adsorption

Parameter% removal
pHFLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
11.42 15.71429 18.35714 19.28571 21.5 22.71429 
21.06 26.66667 28.29787 29.85816 34.68085 34.8227 
36.29 39.23077 41.11888 39.65035 42.18182 44.82517 
46.59 48.88889 50.54861 51.45833 52.47917 55.625 
56.916 55.9589 57.46575 59.65753 58.15068 59.65753 
10 61.30 60.42234 62.6703 61.78474 64.64578 66.62125 
12 66.98 66.30655 68.27144 66.98177 68.87238 71.57326 
Parameter% removal
pHFLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
11.42 15.71429 18.35714 19.28571 21.5 22.71429 
21.06 26.66667 28.29787 29.85816 34.68085 34.8227 
36.29 39.23077 41.11888 39.65035 42.18182 44.82517 
46.59 48.88889 50.54861 51.45833 52.47917 55.625 
56.916 55.9589 57.46575 59.65753 58.15068 59.65753 
10 61.30 60.42234 62.6703 61.78474 64.64578 66.62125 
12 66.98 66.30655 68.27144 66.98177 68.87238 71.57326 
Figure 6

Effects of solution pH on MB dye adsorption.

Figure 6

Effects of solution pH on MB dye adsorption.

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For the better understanding of the pH effect on MB dye adsorption, point of zero charge (pHpzc) was determined according to the methodology adopted by Taqui et al. (2023) and was 6.6 for fenugreek and 5.2 for linseed. Based upon this pHpzc, it is observed that when the solution pH is higher than pHpzc, it makes the surface of the adsorbent negatively charged, which favors the adsorption of the cationic species (MB dye in this study), while on the contrary, when the solution pH is less than pHpzc, it imparts positive charge on the adsorbent surface, which would then favor adsorption of anionic species (Dada et al. 2013). It is obvious from the pH study that all the studied adsorbents show maximum adsorption percentage at higher pH, i.e., 12, where the removal percentage of F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2) were 66.9, 66.3, 68.2, 66.9, 68.8, and 71.6%, respectively. From these observations, a solution pH of 12 was selected as the optimum pH in subsequent experiments for all studied adsorbents.

Effect of adsorbent dose on MB dye adsorption

The adsorbent dose is one of the important parameters that needs to be optimized to achieve maximum adsorption. The results of the adsorbent dose on dye removal are summarized in Table 2 and Figure 7, revealing a direct relationship between adsorbent dosage and removal percentage of dye for all studied adsorbents. The results of the study reveal that with an increase in the combination ratio of F and L from 1:1 to 2:2, the maximum percentage removal of dye takes place as compared with individual F and L adsorbents. This can be attributed to the fact that adsorptive removal efficiency of the individual adsorbents can be improved when combined together in equal ratio. Further, with an increase in adsorbent dose from 0.1 to 1.0 g, there is a continuous increase in percentage removal for all studied adsorbents. Percentage removal of MB dye on F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2) increased from 13.9 to 63.8%, 16.8 to 64.8%, 20 to 64.4%, 23 to 64.9%, 23.8 to 70.8%, and from 24 to 72.6%, respectively, when adsorbent dose was increased from 0.1 to 1.0 g. This can be attributed to the increase in number of available active sites on the adsorbent surface with an increase in adsorbent dose, which leads to greater removal percentage (Javed 2018; Sujata et al. 2019; Haqa et al. 2021; Azhar-ul-Haq et al. 2022; Imran et al. 2022; Javed et al. 2023). However, with further increase in the adsorbent removal, i.e., from 1.0 g to 1.2 g, no significant increase in removal percentage was observed. Hence, in view of the context of conservation of mass and energy, 1.0 g of each adsorbent was elected as the optimum adsorbent dose for subsequent trials.
Table 2

Experimental data showing the effect of adsorbent dose on MB dye adsorption

Parameter% removal
Adsorbent dose (g)FLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
0.1 13.98 16.88 20.00 23.07 23.84 24.04 
0.2 22.63 28.88 29.86 30.69 36.73 41.44 
0.4 39.01 40.88 42.18 42.32 42.88 46.06 
0.6 48.40 49.16 49.93 52.01 53.54 56.31 
0.8 58.81 59.16 59.36 60.20 60.69 61.66 
63.86 64.24 64.46 64.96 70.88 72.62 
1.2 64.92 64.84 66.12 65.91 70.35 72.60 
Parameter% removal
Adsorbent dose (g)FLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
0.1 13.98 16.88 20.00 23.07 23.84 24.04 
0.2 22.63 28.88 29.86 30.69 36.73 41.44 
0.4 39.01 40.88 42.18 42.32 42.88 46.06 
0.6 48.40 49.16 49.93 52.01 53.54 56.31 
0.8 58.81 59.16 59.36 60.20 60.69 61.66 
63.86 64.24 64.46 64.96 70.88 72.62 
1.2 64.92 64.84 66.12 65.91 70.35 72.60 
Figure 7

Effects of adsorbent dose on MB dye adsorption.

Figure 7

Effects of adsorbent dose on MB dye adsorption.

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Effect of dye concentration on MB dye adsorption

For optimization of dye concentration, dye solutions of variable concentrations were prepared. The results of the study (Table 3 and Figure 8) reveal that with an increase in dye concentration, there was a linear increment in dye removal for all studied adsorbents but up to a certain limit after which the percentage of removal started decreasing. The percentage of removal increased from 16.21 to 74.96%, 21.64 to 75.53%, 23.04 to 75.84%, 23.50 to 76.86%, 30.33 to 78.11%, and from 34.60 to 80.86% for F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2), respectively, when the dye dosage was increased from 10 to 120 mg/l. This increase in dye removal might be due to an increased concentration of dye that gets adsorbed on the active sites present on the surface of the adsorbent (Rehman & Afzal 2015; Pyrzynska 2019). In addition, with further increase in dye concentration, no further increment in adsorption was observed. Rather a decrement in removal percentage took place that might have been due to the repulsive forces that exist between the adsorbate molecules or it may also be attributed to the fact that at a fixed adsorbent dose, excess concentration of dye finds no adsorption sites for getting adsorbed on an adsorbent surface, which leads to a decrease in removal efficiency (Rehman & Afzal 2015; Ghzal et al. 2023). Furthermore, it was observed that a higher proportion of individual adsorbents, i.e., 2:2, shows maximum adsorption efficiency at each dye concentration when compared with other studied adsorbents. This is due to the fact that when equal proportions of both F and L adsorbents were mixed, it resulted in higher efficiency due to the combined effect of the individual adsorbents. From the study, it was found that optimum dye dose was 80 mg/l, where the maximum adsorption was obtained for all studied adsorbents.
Table 3

Experimental data showing the effect of dye concentration on MB dye adsorption

Parameter% removal
Concentration (mg/l)FLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
10 16.21 21.64 23.04 23.50 30.33 34.60 
20 31.06 34.50 40.58 44.15 46.71 49.70 
40 62.72 64.19 65.10 65.80 69.79 71.18 
60 63.91 66.60 66.92 68.08 70.10 74.42 
80 74.96 75.53 75.84 76.86 78.11 80.86 
100 73.41 74.88 75.06 75.21 75.06 78.90 
120 70.36 70.29 71.36 71.19 74.82 76.90 
Parameter% removal
Concentration (mg/l)FLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
10 16.21 21.64 23.04 23.50 30.33 34.60 
20 31.06 34.50 40.58 44.15 46.71 49.70 
40 62.72 64.19 65.10 65.80 69.79 71.18 
60 63.91 66.60 66.92 68.08 70.10 74.42 
80 74.96 75.53 75.84 76.86 78.11 80.86 
100 73.41 74.88 75.06 75.21 75.06 78.90 
120 70.36 70.29 71.36 71.19 74.82 76.90 
Figure 8

Effects of dye concentration on MB dye adsorption.

Figure 8

Effects of dye concentration on MB dye adsorption.

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Effect of contact time on MB dye adsorption

Contact time plays a crucial role in attaining equilibrium time for maximum adsorption, and in view of this, time for adsorption of dye molecules varied from 10 to 120 min by keeping other parameters constant. The results of the study (Table 4 and Figure 9) reveal that when contact time increases from 10 to 80 min, there is a continuous increase in percentage removal of dye for all studied adsorbents. This rapid increase in adsorption percentage was mainly attributed to the increased time available for adsorbing dye molecules onto the surface of adsorbents. This increased contact resulted in increasing the percentage removal of dye (Guo et al. 2015; Wekoye et al. 2020). Further, increased proportions of individual adsorbents adsorb dye molecules to a greater extent when compared with individual adsorbents. In general, the adsorption mixture of F and L having a ratio of 2:2 shows maximum efficiency toward MB dye adsorption as compared with other studied adsorbents. It is obvious from the results that maximum adsorption was obtained at 80 min of contact time where F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2) show percentages of removal of 76.70, 78.73, 78.98, 79.24, 79.56, and 80.12%, respectively. However, with further increase in time from 80 to 120 min, a continuous decrement in percentage adsorption was observed for all studied samples, which might be due to the desorption of dye molecules as a result of repulsion between the adsorbed dye molecules. Therefore, contact time of 80 min was selected as the optimum contact time for all the studied adsorbents in subsequent experimental work.
Table 4

Experimental data showing the effect of contact time on MB dye adsorption

Parameter% removal
Contact time (min)FLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
10 38.16 40.44 43.52 48.82 58.76 60.06 
20 60.19 61.66 63.91 68.01 68.20 70.44 
40 72.79 76.10 76.62 78.63 79.80 82.86 
60 84.62 85.12 85.32 85.96 86.28 86.46 
80 76.70 78.73 78.98 79.24 79.56 80.12 
100 72.70 72.94 74.64 74.78 75.53 78.36 
120 70.31 70.63 72.91 73.68 75.38 78.10 
Parameter% removal
Contact time (min)FLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
10 38.16 40.44 43.52 48.82 58.76 60.06 
20 60.19 61.66 63.91 68.01 68.20 70.44 
40 72.79 76.10 76.62 78.63 79.80 82.86 
60 84.62 85.12 85.32 85.96 86.28 86.46 
80 76.70 78.73 78.98 79.24 79.56 80.12 
100 72.70 72.94 74.64 74.78 75.53 78.36 
120 70.31 70.63 72.91 73.68 75.38 78.10 
Figure 9

Effects of contact time on MB dye adsorption.

Figure 9

Effects of contact time on MB dye adsorption.

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Effect of solution temperature on MB dye adsorption

Temperature study has been conducted to evaluate the efficiency of adsorbents for MB dye removal. Table 5 and Figure 10 show the effect of temperature on the removal of the dye using six different proportions of the adsorbents, revealing maximum adsorption at 50 °C for all studied adsorbents. It is clear from the results that when the temperature increased from 10 to 40 °C, percentage of removal of the dye increased linearly. When temperature is increased from 10 to 40 °C, then F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2) exhibit an increase in percentage of MB removal from 40.19 to 86.62%, 41.81 to 86.94%, 43.63 to 88.04%, 48.96 to 82.59%, 53.04 to 86.29%, and 68.24 to 86.49%, respectively. This increase in removal percentage with an increase in temperature was mainly due to an increase in the kinetic energy of adsorbate molecules, which results in increasing the effective number of collisions between adsorbent and adsorbate. The greater the number of effective collisions, the greater will be the percentage of adsorption. Furthermore, a high temperature of the system results in providing sufficient energy for dye molecules to be captured and retained inside the internal structure of the studied adsorbents (Ashizawa et al. 2012; Terdputtakun et al. 2017). However, it was observed that there is not much difference between adsorption percentage obtained at 40 and at 50 °C for all studied adsorbents. Therefore, 40 °C is considered to be an optimum temperature for the maximum MB dye removal. Further, on comparing the efficiency of the studied adsorbents, it is obvious that the adsorbent in which F and L are mixed in the 2:2 ratio shows the maximum adsorption efficiency in comparison with the other studied adsorbents, owing to their combined efficiency.
Table 5

Experimental data showing the effect of solution temperature on MB dye adsorption

Parameter% removal
Temperature (oC)FLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
10 40.19 41.81 43.63 48.96 53.04 68.24 
20 66.60 69.34 70.84 72.48 73.13 76.20 
30 78.96 79.74 81.23 82.59 86.29 86.49 
40 86.62 86.94 88.04 88.24 88.56 89.09 
50 89.03 89.23 89.30 89.44 89.61 89.86 
Parameter% removal
Temperature (oC)FLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
10 40.19 41.81 43.63 48.96 53.04 68.24 
20 66.60 69.34 70.84 72.48 73.13 76.20 
30 78.96 79.74 81.23 82.59 86.29 86.49 
40 86.62 86.94 88.04 88.24 88.56 89.09 
50 89.03 89.23 89.30 89.44 89.61 89.86 
Figure 10

Effect of solution temperature on MB dye adsorption.

Figure 10

Effect of solution temperature on MB dye adsorption.

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Isotherm modeling

For investigating the nature of MB dye adsorption on the studied adsorbents, data from concentration experiments were applied to different isotherm models, namely, Langmuir, Freundlich, and the Dubinin–Radushkevich (D–R) isotherm model, discussed as follows.

Langmuir isotherm model

The Langmuir isotherm model mainly deals with monolayer formation on the surface of the adsorbent having no intermolecular interactions between the adsorbed molecules. This model also assumes the availability of specified number of active sites on the adsorbent surface such that only one dye molecule gets adsorbed on an active site at a time (Ahmad Khan et al. 2021). The following Equation (3) best describes the mathematical expression of the Langmuir model (Ahmad Khan et al. 2021):
formula
(3)
Herein, Qe () represents equilibrium amount of dye that gets adsorbed on the surface of the adsorbent, and b () and Qmax () refer to the maximum adsorption capacity. Ce () in the equation represents equilibrium adsorbate concentration. A unitless factor known as equilibrium factor/separation factor ‘RL’ was used for investigating the feasible and linear nature of the process and is mathematically expressed as Equation (4):
formula
(4)
When RL equals zero, it shows the non-reversible nature of the adsorption process. However, the linear nature of the process is confirmed when RL equals unity. Further, if RL > 1, it reveals the non-feasible nature of the process and the process is feasible in nature if 0 < RL < 1. The results of the study are summarized in Figure 11. From the slope and intercept of the graph (plotted between Ceq on x-axis and Ceq/Cads on y-axis), the values of Qo and b can be calculated, respectively. From the results of the study, it is obvious that the value of regression coefficient, i.e., R2 for all studied adsorbents is greater than other studied models, which reveals the fitness of the experimental data of each adsorbent to the Langmuir isotherm model. The R2 values for the adsorption of MB dye by F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2) were 0.773, 0.790, 0.8406, 0.909, 0.928, and 0.955, respectively. These values of R2 were greater than that of the Freundlich and D–R isotherm models, which reveals the monolayer adsorption of the studied process on each kind of adsorbent. Further, it was observed that the value of RL for each adsorbent was nearly unity, which shows the linear nature of the studied process. Comparable results have already been reported for biosorption of MB dye on coconut dregs (Shukor et al. 2022) and on bamboo-based activated carbon (Hameed et al. 2007), which also follow the Langmuir adsorption isotherm model.
Figure 11

Langmuir isotherm model for adsorption of MB dye on (a) F, (b) L, (c) F + L (1:1), (d) F + L (1:2), (e) F + L (2:1), and (f) F + L (2:2).

Figure 11

Langmuir isotherm model for adsorption of MB dye on (a) F, (b) L, (c) F + L (1:1), (d) F + L (1:2), (e) F + L (2:1), and (f) F + L (2:2).

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Freundlich isotherm model

The Freundlich isotherm model mainly deals with multiple-layer adsorption of dye molecules on the heterogeneous adsorbent surface. The mathematical expression of the Freundlich isotherm model is provided by the following expression in Equation (5) (Ahmad Khan et al. 2021):
formula
(5)
Herein, Ce (mg/l) refers to equilibrium dye concentration, qe (mg/g) refers to the amount of adsorbate that gets adsorbed per gram of adsorbent, and Kf (mg g−1) is Freundlich constant while ‘n’ refers to the Freundlich exponent (Ahmad Khan et al. 2021). The results of the study are summarized in Figure 12 revealing that the value of regression coefficient for all studied adsorbents is less than that obtained from the Langmuir isotherm model, which confirms that the Freundlich adsorption isotherm model does not fit directly to the adsorption data. Values of R2 obtained from the Freundlich isotherm model for F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2) were 0.551, 0.565, 0.599, 0.562, 0.720, and 0.704, respectively, which are less than unity, thus revealing that the current adsorption process does not fit well to the Freundlich isotherm model.
Figure 12

Freundlich isotherm model for adsorption of MB dye on (a) F, (b) L, (c) F + L (1:1), (d) F + L (1:2), (e) F + L (2:1), and (f) F + L (2:2).

Figure 12

Freundlich isotherm model for adsorption of MB dye on (a) F, (b) L, (c) F + L (1:1), (d) F + L (1:2), (e) F + L (2:1), and (f) F + L (2:2).

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D–R isotherm model

The DR isotherm model plays a very important role in determining the nature of the adsorption process, i.e., either physical or chemical via calculating the apparent energy of the adsorption. In other words, it explains the process of adsorption and distribution of Gaussian energy onto a heterogeneous adsorbent surface (Machida et al. 2005; Ahmad Khan et al. 2021). The mathematical expression of this model is given in Equation (6) (Ahmad Khan et al. 2021):
formula
(6)
Here, qe (mg/g) denotes maximum sorption capacity while () is a constant of the D–R isotherm model. Additionally, (kJ mol−1) represents Polanyi potential, which is given as in Equation (7):
formula
(7)
where R denotes the universal gas constant (J/ mol K) and T refers to the absolute temperature (K). The results of the study are summarized in Figure 13, revealing that the value of regression coefficient for all studied adsorbents is less than that obtained from the Langmuir isotherm model, which confirms that the D–R adsorption isotherm model does not fit directly to the adsorption data. The values of R2 obtained from the Freundlich isotherm model for F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2) were 0.546, 0.561, 0.593, 0.559, 0.720, and 0.703, respectively. Hence, it was observed that in addition to the D–R model, some other kinds of models are also required for the complete quantification of the adsorption process. However, the value of energy of sorption for all studied adsorbents confirms the physical nature of the process, i.e., presence of physical interactions between adsorbate molecules and the adsorbent surface.
Figure 13

D–R isotherm model for adsorption of MB dye on (a) F, (b) L, (c) F + L (1:1), (d) F + L (1:2), (e) F + L (2:1), and (f) F + L (2:2).

Figure 13

D–R isotherm model for adsorption of MB dye on (a) F, (b) L, (c) F + L (1:1), (d) F + L (1:2), (e) F + L (2:1), and (f) F + L (2:2).

Close modal

A comparison of parametric values obtained from each model for all studied adsorbents is provided in Table 6. It is clear from the table that each adsorbent shows physical adsorption of MB dye as each adsorption process fits well to the Langmuir isotherm model because the regression coefficient value of each adsorbent is the greatest among other studied isotherm models. Further, the maximum adsorption capacity of the studied adsorbent is the highest, i.e., 31.98 mg/g when both F and L are mixed in the ratio 2:2, showing the effectiveness of the F + L (2:2) composition for MB dye adsorption.

Table 6

Comparison of parameters of different isotherm models for all studied adsorbents

ModelParameterFLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
Langmuir model Qo (mg/g) experimental 1.21 1.26 10.98 16.46 29.42 32.29 
Qo (mg/g) calculated 1.06 1.18 10.66 15.99 29.08 31.98 
b 0.164 0.1552 0.018 0.010 0.004 0.0081 
R2 0.773 0.790 0.8406 0.909 0.928 0.955 
Freundlich model n 0.308 0.313 0.336 0.266 0.188 0.332 
Kf (mg g−11.01 × 107 1.01 × 106 9.90 × 105 1.04 × 106 1.03 × 106 1.00 × 106 
R2 0.551 0.565 0.599 0.562 0.720 0.704 
D–R model β (kJ2 mol−23 × 107 3 × 107 3 × 107 2 × 107 2 × 106 4 × 107 
ε2 (kJ mol−14.08 × 10−4 4.11 × 10−4 4.22 × 10−5 5 × 10−4 1.6 × 10−3 3.5 × 10−4 
R2 0.546 0.561 0.593 0.559 0.720 0.703 
ModelParameterFLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
Langmuir model Qo (mg/g) experimental 1.21 1.26 10.98 16.46 29.42 32.29 
Qo (mg/g) calculated 1.06 1.18 10.66 15.99 29.08 31.98 
b 0.164 0.1552 0.018 0.010 0.004 0.0081 
R2 0.773 0.790 0.8406 0.909 0.928 0.955 
Freundlich model n 0.308 0.313 0.336 0.266 0.188 0.332 
Kf (mg g−11.01 × 107 1.01 × 106 9.90 × 105 1.04 × 106 1.03 × 106 1.00 × 106 
R2 0.551 0.565 0.599 0.562 0.720 0.704 
D–R model β (kJ2 mol−23 × 107 3 × 107 3 × 107 2 × 107 2 × 106 4 × 107 
ε2 (kJ mol−14.08 × 10−4 4.11 × 10−4 4.22 × 10−5 5 × 10−4 1.6 × 10−3 3.5 × 10−4 
R2 0.546 0.561 0.593 0.559 0.720 0.703 

Kinetic modeling

To better understand the adsorption mechanism and kinds of forces involved, different kinetic models, namely, pseudo-first-order, pseudo-second-order, and intra-particle diffusion models, were applied to the experimental data. The results obtained from each model are discussed as follows.

Pseudo-first-order kinetic model

This model relates adsorption rate with number of active sites available on the surface of adsorbents. The mathematical expression of this model is given as in Equation (8):
formula
(8)
wherein the amount of adsorbate adsorbed at time ‘t’ and at equilibrium point is represented by and , respectively. The rate constant for the pseudo-first-order model is given as . Lagergren for the first time gave the linearized form of the pseudo-first-order model (Hayat 2013), which is represented as in Equation (9):
formula
(9)
When a graph is plotted between time t (min) on x-axis and on y-axis, then a straight line is obtained (Yao et al. 2010). From the slope and intercept of this graph, the values of constant k1 (min−1) and Qe (mg/g) are derived, correspondingly. The results of the study for all studied adsorbents are summarized in Figure 14. It is obvious from the results that values of R2 for F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2) were 0.916, 0.951, 0.958, 0.936, 0.911, and 0.982, respectively. Since the value of regression coefficient is close to unity for all the studied adsorbents, this reveals the fitness of the studied model to the pseudo-first-order model conforming to the physical adsorption of MB on each adsorbent, i.e., presence of some physical interactions/forces between each adsorbent and MB dye.
Figure 14

Plot of pseudo-first-order model for (a) F, (b) L, (c) F + L (1:1), (d) F + L (1:2), (e) F + L (2:1), and (f) F + L (2:2).

Figure 14

Plot of pseudo-first-order model for (a) F, (b) L, (c) F + L (1:1), (d) F + L (1:2), (e) F + L (2:1), and (f) F + L (2:2).

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Pseudo-second-order kinetic model

The pseudo-second-order model assumes chemical adsorption as a rate controlling step and can be expressed as in Equation (10) (Chen & Li 2010):
formula
(10)
Herein, k2 ( refers to the pseudo-second-order rate constant, and the amount of adsorbate adsorbed at time ‘t’ and at equilibrium point is represented by and , respectively. In linearized form, this model can be expressed as in Equation (11) (Wang et al. 2010):
formula
(11)
For the pseudo-second-order model, the graph is plotted between time t (x-axis) and (y-axis); from the slope and intercept of this graph, values of K2 and Qeq can be calculated. The results of the study for all studied adsorbents are summarized in Figure 15. It is obvious from results that values of R2 for F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2) were 0.898, 0.919, 0.946, 0.949, 0.988, and 0.992, respectively. By comparing the values of R2 of the pseudo-first-order with the values obtained from the pseudo-second-order model, it is obvious that for F, L, and F + L (1:1), R2 values are higher by the former model, while for the remaining three adsorbents, namely F + L (1:2), F + L (2:1), and F + L (2:2), the R2 values are higher by the pseudo-second-order model. These results reveal the presence of some physical interactions/forces between each adsorbent and MB dye when fenugreek and linseed are used individually or in 1:1 ratio. However, the combination of these adsorbents at higher ratios results in improving the overall performance of these adsorbents due to the development of some chemical interactions between the adsorbent and the MB dye (Shukor et al. 2022). Overall, the results reveal the involvement of both physical and chemical interactions between the studied adsorbents and the MB dye.
Figure 15

Plot of pseudo-second-order model for (a) F, (b) L, (c) F + L (1:1), (d) F + L (1:2), (e) F + L (2:1), and (f) F + L (2:2).

Figure 15

Plot of pseudo-second-order model for (a) F, (b) L, (c) F + L (1:1), (d) F + L (1:2), (e) F + L (2:1), and (f) F + L (2:2).

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Intra-particle diffusion model

Weber and Morris for the first time proposed this model that mainly deals with the diffusion process (Weber & Morris 1963). Mathematically, this model can be expressed as in Equation (12):
formula
(12)
wherein the amount of adsorbate adsorbed at time ‘t’ is presented as , while I denotes the thickness of adsorption layer. Intra-particle diffusion constant is presented as (Weber & Morris 1963). The results of the study for all studied adsorbents are summarized in Figure 16. It is obvious from the results that the value of R2 for F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2) were 0.986, 0.985, 0.981, 0.968, 0.984, and 0.988, respectively. By comparing the values of R2 of the intra-particle diffusion model with the values obtained from the pseudo-first-order and pseudo-second-order models, it is obvious that in addition to some kinds of physical and chemical interactions taking place on the surface of the adsorbent, some diffusion mechanism is also involved in the dye adsorption. Since this model shows good values of R2 for studied adsorbents, it also shows good fit to experimental data conforming to the diffusion of dye molecules into the bulk of the adsorbent. Overall, the results reveal the involvement of both the physical, chemical interactions and the diffusion process for MB dye adsorption on each studied adsorbent.
Figure 16

Plot of pseudo-second-order model for (a) F, (b) L, (c) F + L (1:1), (d) F + L (1:2), (e) F + L (2:1), and (f) F + L (2:2).

Figure 16

Plot of pseudo-second-order model for (a) F, (b) L, (c) F + L (1:1), (d) F + L (1:2), (e) F + L (2:1), and (f) F + L (2:2).

Close modal

Table 7 shows a comparison of all the studied kinetic parameters for each adsorbent. It is observed from the table that for the pseudo-second-order kinetic model, the variation between experimental and calculated for all studied adsorbents is less in comparison with the other studied kinetic models. Also, the R2 value of each adsorbent in case of the pseudo-second-order model is close to unity. All these results show the fitness of experimental data to the pseudo-second-order kinetic model. However, for the pseudo-first-order and intra-particle diffusion models, the results are also good (not too much variation between experimental and calculated and R2 values close to 1) revealing the fitting of data to these models as well. In general, all three kinetic models studied here are good in explaining the mechanism of MB dye adsorption on each studied adsorbent, i.e., F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2).

Table 7

Comparison of parameters calculated from different kinetic models for MB dye adsorption on F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2

ModelParameterFLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
 Experimental  1.071 1.100 1.134 1.153 1.190 1.230 
Pseudo-first-order Calculated  2.13 2.22 3.66 2.68 2.32 3.09 
 0.023 0.024 0.022 0.028 0.036 0.029 
R2 0.916 0.951 0.958 0.936 0.911 0.982 
Pseudo-second-order Calculated  1.92 1.70 1.46 1.43 1.39 1.38 
 0.006 0.008 0.016 0.021 0.034 0.042 
R2 0.898 0.919 0.946 0.949 0.988 0.992 
Intra-particle diffusion Calculated  0.114 0.119 0.106 0.101 0.082 0.076 
 0.196 0.189 0.023 0.080 0.331 0.428 
R2 0.986 0.985 0.981 0.968 0.984 0.988 
ModelParameterFLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
 Experimental  1.071 1.100 1.134 1.153 1.190 1.230 
Pseudo-first-order Calculated  2.13 2.22 3.66 2.68 2.32 3.09 
 0.023 0.024 0.022 0.028 0.036 0.029 
R2 0.916 0.951 0.958 0.936 0.911 0.982 
Pseudo-second-order Calculated  1.92 1.70 1.46 1.43 1.39 1.38 
 0.006 0.008 0.016 0.021 0.034 0.042 
R2 0.898 0.919 0.946 0.949 0.988 0.992 
Intra-particle diffusion Calculated  0.114 0.119 0.106 0.101 0.082 0.076 
 0.196 0.189 0.023 0.080 0.331 0.428 
R2 0.986 0.985 0.981 0.968 0.984 0.988 

Thermodynamic study

Three different parameters, namely, change in Gibbs's free energy (ΔG), enthalpy (ΔH), and entropy (ΔS), were calculated from experimental data for understanding the nature of the adsorption process, i.e., spontaneity or non-spontaneity, endothermic or exothermic nature, and randomness of the adsorption process, respectively. Three thermodynamic parameters are related to each other by the following Equation (13) (Ahmad Khan et al. 2021):
formula
(13)
When it comes to the calculation of enthalpy and entropy change, i.e., ΔH and ΔS, the following Van't Hoff Equation (14) is used:
formula
(14)
In the aforementioned equation, R represents the ideal gas constant while Kc and T correspond to equilibrium constant for sorption process and reaction temperature in K. A constant ‘Kc’ was calculated via Equation (15) (Sahmoune 2019; Ahmad Khan et al. 2021; Batool et al. 2021):
formula
(15)
wherein the amount of MB dye that gets adsorbed on the surface of each adsorbent at equilibrium and residual adsorbate concentration in the solution at equilibrium are represented by Cads (mg L−1) and Ceq (mg L−1), correspondingly. Figure 17 shows the Van't Hoff plot for adsorption of MB dye on each adsorbent being study. From the slope and intercept of each graph, values of ΔH and ΔS can be calculated correspondingly, which are summarized in Table 8. The results of the thermodynamic study of each adsorbent (F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2)) reveal spontaneity of the process as is evident from negative values of change in Gibb's free energy (ΔG). Furthermore, a positive value of enthalpy change (ΔH) confirms the endothermicity of the process, showing that with an increase in temperature MB dye removal increases continuously. It is observed from the study that with an increase in temperature, there was a decrease in entropy, i.e., the randomness of the system starts decreasing revealing that the adsorption process takes place, which results in converting more of the disordered form of the system to the ordered form conforming to the feasibility of the adsorption process (Ahmad Khan et al. 2021).
Figure 17

Van’t Hoff plot for (a) F, (b) L, (c) F + L (1:1), (d) F + L (1:2), (e) F + L (2:1), and (f) F + L (2:2).

Figure 17

Van’t Hoff plot for (a) F, (b) L, (c) F + L (1:1), (d) F + L (1:2), (e) F + L (2:1), and (f) F + L (2:2).

Close modal
Table 8

Calculated thermodynamic parameters at variable temperatures, i.e., T = 10, 20, 30, 40, and 50 °C

ParameterTemperature (K)FLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
ΔG (kJmol−1283 −11,311.9 −11,332.94 −11,158.86 −12,230.06 −12,345.24 −11,364.1 
293 −10,673.5 −10,652.88 −10,691.04 −11,558.86 −11,061.08 −10,677.8 
303 −10,254.8 −10,441.24 −10,247.68 −10,475.92 −10,242.82 −10,084.2 
313 −10,366.3 −10,484.38 −10,055.88 −10,409.08 −9,720.121 −9,675.20 
323 −10,400.6 −10,061.80 −9,861.16 −10,057.30 −9,859.911 −9,297.20 
ΔH (kJmol−1283–323 17.4261 18.97337 20.3510 27.9258 30.395 25.943 
ΔS (Jmol−1K−1283 101.54 107.08 111.34 141.89 151.02 131.83 
293 36.428 36.358 36.488 39.450 37.750 36.442 
303 33.844 34.459 33.820 34.564 33.804 33.281 
313 33.11 33.496 32.126 33.256 31.054 30.911 
323 32.20 31.151 30.529 31.138 30.526 28.783 
ParameterTemperature (K)FLF + L (1:1)F + L (1:2)F + L (2:1)F + L (2:2)
ΔG (kJmol−1283 −11,311.9 −11,332.94 −11,158.86 −12,230.06 −12,345.24 −11,364.1 
293 −10,673.5 −10,652.88 −10,691.04 −11,558.86 −11,061.08 −10,677.8 
303 −10,254.8 −10,441.24 −10,247.68 −10,475.92 −10,242.82 −10,084.2 
313 −10,366.3 −10,484.38 −10,055.88 −10,409.08 −9,720.121 −9,675.20 
323 −10,400.6 −10,061.80 −9,861.16 −10,057.30 −9,859.911 −9,297.20 
ΔH (kJmol−1283–323 17.4261 18.97337 20.3510 27.9258 30.395 25.943 
ΔS (Jmol−1K−1283 101.54 107.08 111.34 141.89 151.02 131.83 
293 36.428 36.358 36.488 39.450 37.750 36.442 
303 33.844 34.459 33.820 34.564 33.804 33.281 
313 33.11 33.496 32.126 33.256 31.054 30.911 
323 32.20 31.151 30.529 31.138 30.526 28.783 

Adsorption capacity of studied adsorbents (F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2)) for MB dye is compared with some already reported adsorbents in the literature as summarized in Table 9. It is clear from the table that some adsorbents show maximum affinity toward MB dye adsorption, while others adsorb MB dye to lesser extents. However, in comparison with the literature, the studied adsorbents possess good adsorption affinity toward MB dye and this can be improved when both fenugreek and linseed are mixed together in equal and higher proportion, i.e., 2:2. Overall, the results show the effectiveness of the studied adsorbents for maximum MB dye removal without any modification.

Table 9

Comparison of adsorption efficiency of studied adsorbents with already reported adsorbents reported in the literature for MB dye removal

AdsorbentQe (mg/g)ReferenceAdsorbentQe (mg/g)Reference
Eggshell 0.80 Tsai et al. (2006)  Banana peel 20.8 Annadurai et al. (2002)  
Eggshell membrane 0.24 Tsai et al. (2006)  Coconut coir 15.59 Sharma & Upadhyay (2009)  
Raw orange tree sawdust 39.68 Azzaz et al. (2017)  Cereal chaff 20.3 Han et al. (2006)  
Orange peel 18.60 Annadurai et al. (2002)  Natural zeolite 19.94 Han et al. (2009)  
Wheat shells 16.56 Bulut & Aydın (2006)  NaOH-treated raw kaolin 16.34 Ghosh & Bhattacharyya (2002)  
Indian rosewood sawdust 11.8–51.4 Garg et al. (2004)  Corn husk 18.06–41.55 Paşka et al. (2014)  
Neem leaf 8.76–19.61 Bhattacharyya & Sharma (2005)  Palm-trees waste 8.4 Singh et al. (2005)  
Fly ash 13.42 Wang et al. (2005)  Data stones 8.8 Singh et al. (2005)  
1.06 Present study F + L (1:2) 15.99 Present study 
1.18 Present study F + L (2:1) 29.08 Present study 
F + L (1:1) 10.66 Present study F + L (2:2) 31.98 Present study 
AdsorbentQe (mg/g)ReferenceAdsorbentQe (mg/g)Reference
Eggshell 0.80 Tsai et al. (2006)  Banana peel 20.8 Annadurai et al. (2002)  
Eggshell membrane 0.24 Tsai et al. (2006)  Coconut coir 15.59 Sharma & Upadhyay (2009)  
Raw orange tree sawdust 39.68 Azzaz et al. (2017)  Cereal chaff 20.3 Han et al. (2006)  
Orange peel 18.60 Annadurai et al. (2002)  Natural zeolite 19.94 Han et al. (2009)  
Wheat shells 16.56 Bulut & Aydın (2006)  NaOH-treated raw kaolin 16.34 Ghosh & Bhattacharyya (2002)  
Indian rosewood sawdust 11.8–51.4 Garg et al. (2004)  Corn husk 18.06–41.55 Paşka et al. (2014)  
Neem leaf 8.76–19.61 Bhattacharyya & Sharma (2005)  Palm-trees waste 8.4 Singh et al. (2005)  
Fly ash 13.42 Wang et al. (2005)  Data stones 8.8 Singh et al. (2005)  
1.06 Present study F + L (1:2) 15.99 Present study 
1.18 Present study F + L (2:1) 29.08 Present study 
F + L (1:1) 10.66 Present study F + L (2:2) 31.98 Present study 

MB dye is a cationic dye and its adsorption is highly favored when the adsorbent surface possesses some kinds of negatively charged functionalities that attract dye molecules and help in adsorption. From the FTIR study it is clear that all the studied adsorbents, namely, F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2), possess several functional groups (already mentioned in sub-section 3.1.1) that become highly negatively charged when basic pH is added to the system. This can be attributed to the fact that at higher basic pH, functional groups of the studied adsorbent's surface become negatively charged owing to deprotonation, which favors cationic dye adsorption. Main functionalities that contribute majorly to MB dye adsorption include groups. These functional groups are responsible for the adsorption of MB dye via electrostatic forces and hydrogen bonding (El-Shafey et al. 2016; Zhang 2018; Batool et al. 2021).

The current research focused on the adsorptive removal of MB dye from aqueous solution using non-modified biosorbents that are easily available, low cost, and meeting the aspects of green chemistry. The biosorbents used in this research include fenugreek galactomannan, linseed, and four combinations of fenugreek and linseed, i.e., 1:1, 1:2, 2:1, and 2:2 in a batch mode. The characterization study reveals the presence of various functionalities in the adsorbents that are amorphous and possess highly rough and heterogeneous surfaces. BET surface area of the adsorbent (F + L combination) was found to be 394 m2/g. The results of the adsorption study reveals the effectiveness of the studied adsorbents as nearly 86.62, 86.94, 88.04, 88.24, 88.56, and 89.09% of MB dye adsorption takes place with F, L, F + L (1:1), F + L (1:2), F + L (2:1), and F + L (2:2) under optimized conditions (pH of 12, adsorbent dose of 1.2 g, dye concentration of 80 mg/l, contact time of 60 min at 40 °C). The isothermal study shows the fitness of the experimental data to the Langmuir isotherm model, while the kinetic modeling shows that both physical and chemical interactions are involved in dye adsorption. Further, spontaneity, endothermicity, and feasibility of the process were also confirmed from the thermodynamic study. Overall, it was observed that the studied adsorbents are effective and promising adsorbents for maximum MB dye removal.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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