In the current study, tannic acid-functionalized iron oxide nanoparticles have been synthesized using a cost-effective co-precipitation method and subsequently characterized using various instrumentation techniques such as Fourier transform infrared spectroscopy, X-ray diffractometer, field emission scanning electron microscopy, and thermal gravimetric analysis. Further, these surface-modified magnetite nanoparticles have been used for the adsorption of toluidine dye from an aqueous solution. The adsorption process was accompanied using batch procedure, and influences of several factors such as adsorbent dose, contact time, pH, temperature, and initial concentration of adsorbate were inspected concurrently. The maximum adsorption capacity of tannic acid-functionalized magnetite nanoparticles was found to be 50.68 mg/g. The adsorption process was observed to follow the Temkin isotherm model, whereas the kinetic study was well described by pseudo-second order. The thermodynamic study revealed the adsorption process to be endothermic and spontaneous in nature with a high degree of freedom between adsorbent and adsorbate. Therefore, the study indicated that the tannic acid-functionalized magnetite nanoparticles have promising adsorption capability and can be used as an excellent adsorbent for the removal of toluidine blue O dye from the aqueous solution.

  • Tannic acid-tailored magnetite nanoparticles were employed as adsorbents for the removal of coloured pollutants from an aqueous solution.

  • Batch adsorption techniques were applied to scrutinize the adsorption behaviour of these magnetite nanoparticles.

  • The adsorption process obeys pseudo-second-order kinetics and the Langmuir model of isotherm.

  • The desorption study suggested the reusability of adsorbents.

Coloured effluents are major water pollutants produced by various industries including the textile industry in large quantities into nearby water sources that result in adverse effects on aquatic life and the quality of the water and thus can lead to various problems due to water toxicity. These organic substances are mainly responsible for the noxiousness and door of water. Due to their toxic effects, dye effluents are very hazardous to humans because they have both a cancer-causing and venomous perspective and could also have an effect on the direct damage of living aquatic entities (Giridhar 2014; Saura & Galindo 2016). Hence, effluents containing dye residues should be treated before they are discharged into the environment (Lairini et al. 2017). The basic dyes that are also termed cationic dyes including methylene blue and toluidine blue O (TBO) dye are a significant group of organic compounds and possess various scientific as well as industrial applications (Ding et al. 2006; Dow et al. 2009; Ghica & Brett 2009). Toluidine O blue dye is a basic thiazine dye, which belongs to the quinone-imine family and is selectively used in various test reactions as well as in staining acidic tissue components. In terms of wastewater purification, it is very difficult to remove cationic dye from wastewater due to its inefficient adsorption of dye effluents, high rate of redevelopment, and low frequency of flow. This issue has been considered as an attractive opportunity for researchers. In this context, various physical and chemical treatment methodology processes have been used, such as adsorption (Ferrero 2010; Islam et al. 2019; Dai et al. 2021), chemical oxidation (Türgay et al. 2011), electrochemical oxidation (Zhao et al. 2010), membrane separation (Wei et al. 2013), and photocatalytic oxidation (Fernandes et al. 2020). Of these methodologies, adsorption treatment is found to be a more efficient process for dye remediation from aqueous solutions due to its high proficiency, easy usage, and accessibility of several adsorbents. An enormous amount of synthetic as well as natural materials such as activated carbon, synthetic silica, activated aluminum oxide (Mangla et al. 2021), Turkish zeolite (Alpat et al. 2008), magnesium oxide/calcium alginate (Cui et al. 2021), carboxymethyl cellulose-based hydrogel (Ma et al. 2021), and orange peel (Lafi et al. 2015a; Ahmed et al. 2020) have been used commercially. However, such type of adsorbents are very expensive to use and show low efficacy of adsorption, less capability to separate their arduous fabrication involves high consumption of energy. To overcome these demerits, uses of new low-priced, maintainable, and advanced adsorbents are required. So, the adsorbents that possess both properties, i.e. easy separation and good adsorption capacity, are recognized as the most effective and proficient adsorbents. Magnetic separation methodology has exponentially attracted the attention of several researchers as a fast and suitable technology for separating materials. Nowadays, magnetic nanoparticles such as magnetite (Fe3O4) nanoparticles have attracted the eyeballs of scientist considerably due to their special characteristics such as superparamagnetism and large surface area-to-volume ratio. Several researchers have focused on the surface modification of these magnetic nanoparticles with a suitable and appropriate material such as chitosan (Aranaz et al. 2019; Singh et al. 2021), poly-acrylic acid (Liao et al. 2017), aminopropyl triethoxy silane (Rajabi et al. 2015), multiwalled carbon nanotube (Hamidi Malayeri et al. 2012), and gum arabic (Kong et al. 2014). The surface modification helps in enhancing the stability as well as the adsorption capacities of magnetic nanoparticles. Yang et al. have synthesized a nanocomposite of magnetic Fe3O4 for the adsorption of methylene blue from aqueous solution (Yang et al. 2008). Rocher et al. developed alginate beads containing magnetic nanoparticles as well as activated carbon for the removal of organic dyes (Rocher et al. 2008). Adok et al. used surfactant-modified alumina for the removal of crystal violet from aquatic environments (Adak et al. 2005). Amended nanoparticles show better results, and therefore, many organic or inorganic substances have been explored to amend magnetic nanoparticles for the removal of contaminants. Tannic acid (TA) is considered humic-like substance, which can be used for the surface modification of magnetic nanoparticles, which in turn improves the properties and the adsorption capacity for the removal of dye. TA is a natural polyphenolic molecule consisting of sugar esters mainly derived from the breakdown of herbs. It has a lot of phenolic, hydroxyl, and carbonyl groups (Bagtash et al. 2016). Coating of TA on the surface of iron oxide nanoparticles enhances the efficiency of nanoparticles for the remediation of cationic dyes from the aqueous solution.

During the last few years, several research methods attracted the eye ball of scientists for the development of novel as well as cost-effective adsorbents. Therefore, the current study emphasizes the cheaper and easier synthesis of magnetic nanoadsorbent, i.e. TA-modified magnetite nanoparticles (TA@Fe3O4), for the removal of TBO dye from the aqueous solution. The physical and chemical characterizations of the synthesized TA@Fe3O4 nanoparticles were accompanied by analytical methods.

Materials

All chemicals were of analytical reagent grade and were used without any further purification. Ferrous sulphate heptahydrate (FeSO4.7H2O), ferric chloride hexahydrate (FeCl3.6H2O), TA, and 25% ammonium hydroxide solution were purchased from SRL (India). TBO dye was used as an adsorbate throughout the experiments. Distilled water was used for the preparation of solutions.

Methods

Techniques used

A digital mechanical stirrer was used for the preparation of unmodified and TA-modified magnetite nanoparticles (TA@Fe3O4) at 2,000 rpm. The Fourier infrared spectrum of unmodified and TA-modified magnetite nanoparticles was recorded using an MB-3000 ABB FTIR spectrophotometer. Thermograms of synthesized unmodified and modified magnetic nanoparticles were obtained using a thermogravimetric (TG) analyzer, Perkin Elmer STA-6000 TG analyzer under specific conditions (5–80 °C/min heating rate and 20–1,000 °C temperature range). The point of zero charge was determined by Zeta-sizer Nano 90plus (Brookhaven Instruments Corporation). The adsorption of initial and final concentrations of dye solutions was measured by a T90 PG Instrument Limited UV-Visible spectrophotometer in the range of 190–900 nm. The size and morphology of both unmodified and TA-modified magnetic nanoparticles were investigated by Hitachi SU-8000 field emission scanning electron microscope. The X-ray diffraction patterns for synthesized magnetic nanoparticles were obtained by a diffractometer working with Cu Kα radiations of 1.540 Å wavelength over a 2θ range of 20–80°.

Method of preparation

The co-precipitation method was adopted as the easiest and most appropriate route way for the preparation of both unmodified and modified magnetic nanoparticles. The ferric and ferrous salts in a molar ratio of 2:1 were dissolved in 100 mL of distilled water. The aforementioned solution was stirred and heated up to 90 °C. After 10–20 min, 10 mL of ammonium hydroxide solution was added to the aforementioned solution rapidly, and the mixture was stirred for another 30 min at 90 °C. A black-coloured solution was obtained (Jangra et al. 2021). The solution was decanted off using a magnet, and black-coloured precipitates of unmodified magnetite nanoparticles (Fe3O4) were collected. To modify the surface of these synthesized magnetic nanoparticles, the solution of TA, coating material (0.5 g/20 mL), was added to the dispersed solution of unmodified magnetic nanoparticles (1.05 g Fe3O4 in 40 mL, sonicated for about 15 min) and stirred continuously at about 1,800–2,000 rpm for approximately 2 h (Atacan et al. 2016). The solution was cooled, and the black precipitates of TA@Fe3O4 nanoparticles were washed several times with water and separated using an external magnetic field.

Preparation of TBO adsorbate solution

TBO dye solution was used to prepare the stock solution (50 mg/L) by dissolving precisely amount (50 mg) of dye in deionized (1 L) and was consequently diluted to the desired concentrations. Therefore, a graph between various concentrations of TBO dye and their absorbance was plotted to obtain the calibration curve at maximum wavelength.

Adsorption behaviour study

The TA@Fe3O4 nanoparticles were used as adsorbents, and TBO dye was used as adsorbate. For this, the stock solution of TBO dye (50 mg/L concentration) was prepared and used for further process. The effect of several experimental parameters such as pH (2.0–8.0), temperature (298–323 K), adsorbent quantity (05–30 mg), and time (0–80 min) was investigated (Eleryan et al. 2023). The amount of known concentration of adsorbate (10 mL) was used, and the preferred pH was maintained with 0.1 M HCl or 0.1 NaOH for further studies. The values of absorbance of dye solutions before and after the treatment were measured by using a UV-visible spectrophotometer at a maximum wavelength of 630 nm using a calibration curve. To investigate the rate mechanism and nature of the adsorption process, many kinetics and isotherm models such as pseudo-first order (Langergren & Svenska 1898), pseudo-second order (Ho & McKay 1999), Langmuir (1918), Freundlich (Freundlich 1907), and Temkin (Temkin & Pyzhev 1940) were examined accordingly.

The adsorption results are represented as the value of adsorption capacity (qe.exp), which can be calculated as follows:
formula
(1)
where the percentage adsorption of the dye or dye removal efficiency can be determined using:
formula
(2)
where Co (mg/L) is the initial concentration of dye, Ce (mg/L) is the equilibrium concentration of dye, V (L) is the volume of the solution, m (g) is the amount of adsorbent, and qe.exp (mg/g) is the adsorption loading capacity.

Desorption study

Desorption experiments were executed to estimate the reusability of TA@Fe3O4 nanoparticles. Thirty milligrams of TBO dye adsorbed TA-modified magnetite (TBO@TA@Fe3O4) nanoparticles was stirred with 30 mL of ethanol for about 100–120 min at room temperature. After which, the magnetite nanoparticles were decanted off using a magnet and the absorbance of the decanted solution was measured at 630 nm. The desorption capacity (qde) was calculated using Equation (3):
formula
(3)
where Cde (mg/L) is the concentration of dye in ethanol, V is the volume of ethanol used, and m is the mass of TBO@TA@Fe3O4 nanoparticles.
The percentage of desorption efficiency (Wde) can be expressed as follows:
formula
(4)

After desorption, the coated nanoparticles were recollected and dried. In addition, the adsorption capacity of recycled nanoparticles was investigated again to examine the reusability.

Characterization studies

Fourier transform infrared spectral studies

Infrared (IR) spectral analysis revealed the presence of functional groups in unmodified and modified magnetic nanoparticles. The spectra of unmodified, TA-modified magnetite nanoparticles and pure TA were obtained for comparative studies. The spectral peaks at low wavenumbers i.e. lower than 700 cm−1 were attributed to the stretching vibrations of Fe-O bonds of iron oxides. The spectra of both unmodified magnetic nanoparticles, Fe3O4, and TA-modified magnetic nanoparticles, TA@Fe3O4, depicted a strong and intense peak near 600 cm−1 and confirmed the presence of Fe-O bond (Figure 1). The IR spectrum of TA@Fe3O4 was observed to be different from that of Fe3O4 between 1,000 and 1,700 cm−1. The broadband in the region 3,000–3,500 cm−1 can be assigned to the –OH stretching of the phenolic group of TA (Silverstein & Bassler 1962; Kim & Kim 2003; Özacar et al. 2008) and also might be attributed to the stretching of –OH bond of unmodified Fe3O4 nanoparticles. Two prominent peaks at 1,704 and 1,080 cm−1 correspond to stretching vibrations of –C = O and –C-O, respectively. Thus, the comparison of both spectra confirmed that TA is effectively coated onto the surface of unmodified magnetic nanoparticles.
Figure 1

FTIR spectra of bare magnetite nanoparticles (Fe3O4), pure tannic acid (TA), and tannic acid-coated magnetite nanoparticles (TA@Fe3O4).

Figure 1

FTIR spectra of bare magnetite nanoparticles (Fe3O4), pure tannic acid (TA), and tannic acid-coated magnetite nanoparticles (TA@Fe3O4).

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X-ray diffraction studies

X-ray diffraction analysis was carried out to study the crystalline nature and the diameter of the synthesized nanoparticles, which showed a pattern having few intense peaks at 30.8°, 36°, 43.7°, 57.6°, and 63.4°. Thus, the position and intensity of these peaks suggested the semi-crystalline nature of surface-modified magnetite nanoparticles (Figure 2). The Debye–Scherrer equation was used for the calculation of the average diameter of synthesized nanoparticles.
formula
where is the wavelength of X-ray (1.540 A°) and β is the width of peak at half maximum θ and is Bragg angle in degree. The calculated diameter or size of synthesized TA@Fe3O4 nanoparticles was found to be approximately 38.5 nm.
Figure 2

X-ray diffraction pattern for tannic acid-coated magnetite nanoparticles (TA@Fe3O4).

Figure 2

X-ray diffraction pattern for tannic acid-coated magnetite nanoparticles (TA@Fe3O4).

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Field emission electron microscopy study

Field emission electron microscopy (FESEM) studies were used to monitor the morphology as well as the size of synthesized magnetic nanoparticles. The FESEM images depicted that the average size of unmodified magnetite (Fe3O4) nanoparticles and TA-modified magnetite (TA@Fe3O4) nanoparticles was 11 and 35 nm, respectively, which are in good agreement with the results obtained by the particle size analyzer (Figure 3). In addition, the synthesized nanoparticles were observed to have a spherical shape. The increased size of TA@Fe3O4 nanoparticles justified that the surface of unmodified magnetite nanoparticles (Fe3O4) was fruitfully modified with TA.
Figure 3

FESEM images and histograms of unmodified magnetite nanoparticles ((a) and (c)) and tannic acid-modified magnetite nanoparticles ((b) and (d)).

Figure 3

FESEM images and histograms of unmodified magnetite nanoparticles ((a) and (c)) and tannic acid-modified magnetite nanoparticles ((b) and (d)).

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TG studies

TG studies of both modified magnetite (Fe3O4) and TA-modified magnetite (TA@Fe3O4) nanoparticles were analysed to examine the presence of coating material, i.e. TA onto the surface of unmodified magnetite nanoparticles (Figure 4). The TG analysis displayed the different stages of weight loss for unmodified (Fe3O4) and surface-modified (TA@Fe3O4) magnetite nanoparticles. The weight loss (∼7–10%) between 10 and 200 °C may be due to the decomposition of loosely bound water molecules. In addition, the significant weight loss of ∼11.6% at the second stage (200–600 °C) may be attributed to the degradation of the organic layer, i.e. TA. Thus, the comparison of thermograms depicted that the total percentage weight loss for Fe3O4 and TA@Fe3O4 nanoparticles was estimated to be 11.76 and 21.6%, respectively.
Figure 4

Thermogravimetric curves of unmodified (Fe3O4) and tannic acid-modified (TA@Fe3O4) magnetite nanoparticles.

Figure 4

Thermogravimetric curves of unmodified (Fe3O4) and tannic acid-modified (TA@Fe3O4) magnetite nanoparticles.

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Zeta potential

The point of zero charge of TA@Fe3O4 nanoparticles refers to the pH of the solution at which their surface charge is equal to zero. The pHZPC of TA@Fe3O4 nanoparticles was determined by the values of zeta potential with a pH of the solution ranging from 2.0 to 8.0. The value of point of zero charge of TA@Fe3O4 nanoparticles was found at pH of ∼2.0 (Figure 5). When a pH of the solution is lower than pHPCZ, the surface charge of the adsorbent is positive due to the protonation of the acidic groups present in the material (Tran et al. 2016). When the pH of the solution is higher than the pHPCZ, ionization or dissociation of the acidic oxygen surface groups occurs.
Figure 5

Zeta potential as a function of pH of the tannic acid-modified magnetite nanoparticles (TA@Fe3O4).

Figure 5

Zeta potential as a function of pH of the tannic acid-modified magnetite nanoparticles (TA@Fe3O4).

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Adsorption analysis

Many adsorption experiments were performed to analyse the effects of variable experimental factors including pH, temperature, adsorbent amount (TA@Fe3O4), adsorption time, as well as initial concentration of adsorbate on the loading capacity and removal efficiency of adsorbent, TA@Fe3O4.

Effects of solution of pH

The dye remediation efficiency of nanoparticles can be influenced by the variation of the surface charge density of surface-modified nanoparticles. The pH effect is a crucial variable for the dye removal process. The variation in pH on the removal of coloured effluents can be scrutinized in the pH range of 2.0–8.0 with a different initial concentration of adsorbate and a fixed amount of adsorbent (20 mg). The pKa value of TBO is 2.4 and 11.6 (Sabnis 2010), which suggests that the rate of degradation of the contaminant will increase in the basic medium. The point of zero charge was also found at a pH of ∼2.0, which suggested that the electrostatic repulsion between sorbent and the adsorbate led to the decrease in the percentage removal efficiency of nanoparticles at low pH. The percentage of dye adsorption efficiency progressively increased with an increase in the pH of the solution ranging from 2.0 to 8.0 (Figure 6). This increasing effect may be attributed to the electrostatic interactions between the adsorbate (which possess the cationic nature) and the adsorbent. At the low value of pH, the functional groups of the adsorbate and adsorbent get protonated and both attain a positive charge and start repelling each other, which results in a decrease in the percentage removal efficiency. As the value of pH increases, the extent of protonation decreases gradually, which in turn enhances interactions between adsorbate and adsorbent molecules and permits the increase in the percentage removal efficiency. The increase in the percentage removal efficiency in the pH range from 7.0 to 8.0 may be accounted on the basis of electrostatic interactions between the negatively charged dye present on the surface of adsorbent (TA@Fe3O4 nanoparticles) and the positively charged TBO dye. Therefore, the study suggested that pH 8.0 is considered the optimum value of pH for better adsorption.
Figure 6

Effect of pH on percentage removal of dye at different concentrations of dye solution.

Figure 6

Effect of pH on percentage removal of dye at different concentrations of dye solution.

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Effect of time

The performance of synthesized adsorbent (TA-coated magnetic nanoparticles), which focused on the elimination of coloured effluents from the aqueous solution, was studied as a function of time using batch adsorption techniques at pH 8.0, a fixed concentration of adsorbate (50 mg/L), and various amounts of adsorbent (05–30 mg) at room temperature. The percentage removal efficiency of modified magnetic nanoparticles showed similar behaviour for different doses of adsorbent, which increases steadily at the initial stage with contact time and attains a state of equilibrium after 35 min (Figure 7). This behaviour of TA-modified magnetic nanoparticles can be explained on account of the fact that initially several numbers of active sites were present on the surface of synthesized TA-modified magnetite nanoparticles for the adsorption of coloured effluents. However, after a time lapse, the number of vacant active sites decreased, and it became a challenging process to fill these sites due to increased repulsive forces between the adsorbate and adsorbent after which no adsorption took place as equilibrium was achieved. Thus, the state of equilibrium should be treated as the optimum state for the better adsorption of TBO dye.
Figure 7

Effect of contact time on percentage removal of dye for tannic acid-coated magnetite nanoparticles for different amounts of adsorbent.

Figure 7

Effect of contact time on percentage removal of dye for tannic acid-coated magnetite nanoparticles for different amounts of adsorbent.

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Effect of temperature

The effect of temperature on the dye elimination efficiency of nanoparticles was examined to investigate the endothermic or exothermic nature of the adsorption process. Several experiments were performed at a temperature range of 298–323 K under specific conditions using a fixed amount of adsorbent. It was noticed that the dye removal efficiency increased with the increase in temperature until the adsorption process achieved the equilibrium point, i.e. 313 K, and further a decrease was observed after 313 K (Figure 8). This decrease in the percentage of dye elimination efficiency may be attributed to the weak forces of interaction between adsorbate and adsorbent molecules (Singh et al. 2014) or may be due to agglomeration of nanoparticles that resulted in a decreased number of active sites for adsorption. Therefore, the temperature of 313 K was considered as the optimum value of temperature.
Figure 8

Effect on temperature on the percentage removal efficiency of tannic acid-modified magnetite nanoparticles.

Figure 8

Effect on temperature on the percentage removal efficiency of tannic acid-modified magnetite nanoparticles.

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Effect of adsorbent (TA@Fe3O4) amount and initial concentration of adsorbate on percentage removal efficiency

The influence of the adsorbent amount on the dye elimination efficiency was examined by varying the amount of adsorbent (5–30 mg) with 10 mL of TBO dye solution of variable concentration (10–50 mg/L) under optimum conditions of pH (8.0), temperature (313 K), and time (35 min). The study revealed that the percentage of dye removal efficiency increased on increasing the amount of adsorbent and decreased on increasing the concentration of adsorbate solution (Figure 9). This increased effect can be elucidated by the fact that an increased amount of adsorbent results in more adsorption sites and a large surface area for adsorption. It was observed that a maximum of 97.23% and a minimum of 41.28% of TBO dye were removed from the aqueous solution of TBO dye (concentration 50 mg/L) for 30 and 5 mg of nanoadsorbent (TA@Fe3O4), respectively.
Figure 9

Effect of adsorbent amount and initial concentration of adsorbate on percentage removal efficiency of tannic acid-modified magnetite nanoparticles.

Figure 9

Effect of adsorbent amount and initial concentration of adsorbate on percentage removal efficiency of tannic acid-modified magnetite nanoparticles.

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Kinetic study

The kinetic study plays a key role in the determination of the rate mechanism of the adsorption process. Numerous adsorption experiments were executed at different time periods to inspect the effect of contact time using a fixed amount of adsorbent (30 mg) and a fixed concentration of dye (50 mg/L). The study led to the conclusion that the loading capacity increases rapidly at an initial stage and then starts to increase gradually till it achieves a saturation stage at 35 min. To inspect the rate of the mechanism of the adsorption process, two models of kinetics including pseudo-first-order model (Equation (5)) and pseudo-second-order model (Equation (6)) were applied simultaneously.
formula
(5)
formula
(6)
where qe (mg/g) and qt (mg/g) are the adsorption loading capacity at equilibrium and at time t (min), respectively, and k1 (min−1) and k2 (g/mg/min) are the rate constants for pseudo-first-order and pseudo-second-order models. The value of k1 can be obtained by plotting the graph between log (qeqt) and time, t, whereas the value of k2 can be calculated from the linear plot of t/qt versus time, t. Moreover, on comparing the kinetic variable and correlation coefficients (R2) of both the pseudo-first-order and pseudo-second-order models, a relative kinetic study was observed to follow the pseudo-second-order model, which demonstrated the chemisorption of dyes onto the surface of nanoparticles (Figure 10).
Figure 10

Pseudo-first-order and pseudo-second-order kinetic models.

Figure 10

Pseudo-first-order and pseudo-second-order kinetic models.

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Adsorption isotherm analysis

The adsorption isotherm analysis is very significant to evaluate the interaction between adsorbate and adsorbent. Several isotherm models such as Langmuir (Equation (7)), Freundlich (Equation (8)), and Temkin (Equation (9)) were applied to examine the nature of the interactions among adsorbent and adsorbate. The linear forms of the isotherms were as follows:
formula
(7)
formula
(8)
formula
(9)
where qm (mg/g) is maximum adsorption capacity, b is the Langmuir constant, qe (mg/g) is the loading capacity at equilibrium, KF is the Freundlich constant, A is the Temkin constant, R is the gas constant, T (K) is the temperature, and Ce (mg/L) is the equilibrium concentration.
These isotherm analyses were performed for 10 mL of adsorbate solution of different concentrations (10–50 mg/L). A fixed amount of modified nanoparticles (5 mg) was added to the adsorbate solution, and the prepared solution was stirred for approximately 30–35 min. After that the nanoparticles were separated using an external magnetic field, and the absorbance was recorded. The measured absorbance values were used for further calculations. The isotherm curves were plotted for Langmuir, Freundlich, and Temkin to analyse the parameters. The calculated data (Table 1) suggested that the adsorption process was found to fit well with the Temkin isotherm model with a high value of coefficient of determination (0.996). Figure 11 presents the Temkin model of isotherm. The Temkin model of isotherm suggested the homogenous and multilayer adsorption of the adsorbate over the surface of the adsorbent. The maximum value of adsorption capacity was 50.68 mg/g, which was calculated from the Langmuir model for the elimination of TBO dye from the aqueous solution using TA-modified magnetite nanoparticles. Moreover, Equation (1) can be used for the calculation of experimental values of adsorption capacities. The experimental values of adsorption capacity for 5 mg of adsorbent are 40.28, 37.52, 32.18, 29.94, and 14.71 mg/g for variable concentrations (29.36, 21.39, 13.91, 7.53, and 2.64 mg/L, respectively).
Table 1

Calculated values of parameters of isotherm models

Isotherm models
Langmuir
Freundlich
Temkin
R2qm (mg/g)KL (L/g)RLR2nKF (mg/g L/mg)R2A (L/g)B (J/mol)
0.913 50.68 0.14 0.59 0.889 2.32 2.7 0.996 0.314 11.03 
Isotherm models
Langmuir
Freundlich
Temkin
R2qm (mg/g)KL (L/g)RLR2nKF (mg/g L/mg)R2A (L/g)B (J/mol)
0.913 50.68 0.14 0.59 0.889 2.32 2.7 0.996 0.314 11.03 

Note: qm is maximum adsorption capacity, KL is Langmuir isotherm constant, RL is equilibrium parameter, KF and n are Freundlich isotherm constants, while A and B are Temkin constants.

Figure 11

Temkin isotherm model. The inset picture of Ce versus qe plot shows the isotherm experimental data attaining the equilibrium plateau.

Figure 11

Temkin isotherm model. The inset picture of Ce versus qe plot shows the isotherm experimental data attaining the equilibrium plateau.

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Adsorption thermodynamic analysis

Adsorption thermodynamic parameters such as Gibbs free energy (ΔG), enthalpy changes (ΔH), and entropy changes (ΔS) are very helpful in the determination of favourability, spontaneity, and feasibility of the process. In the current analysis, the influence of temperature on the adsorption capacity as well as removal efficiency was used to determine the aforementioned parameters using the following equations:
formula
(10)
formula
(11)
formula
(12)
formula
(13)
where R is (8.314 J/mol K) a universal gas constant, T (K) is the temperature, KC is the distribution coefficient, and qe (mg/g) and Ce (mg/L) are the adsorption capacity and concentration, respectively, at equilibrium point. The values of ΔS and ΔH can be obtained from the intercept and slope of the linear graph plotted between log Kc versus 1/T (Figure 12). The positive value of ΔH and negative value of ΔG (Table 2) indicate the endothermic and spontaneous nature of the adsorption process, while the positive value of ΔS demonstrates the high value of degree of freedom among adsorbate and adsorbent during the adsorption process.
Table 2

Thermodynamic parameters for the remediation of dye on tannic acid-coated magnetite nanoparticles

Temperature (K)lnKcΔG (kJ/mol)ΔH (kJ/mol K)ΔS (J/mol K)
298 2.047 −5.072 16.45 72.36 
303 2.152 −5.421 
313 2.467 −6.420 
323 2.528 −6.791 
Temperature (K)lnKcΔG (kJ/mol)ΔH (kJ/mol K)ΔS (J/mol K)
298 2.047 −5.072 16.45 72.36 
303 2.152 −5.421 
313 2.467 −6.420 
323 2.528 −6.791 

Note: ΔG is change in Gibbs free energy, ΔH is enthalpy changes, ΔS is entropy changes, and KC is distribution coefficient.

Figure 12

Graph between ln(Kc) and 1/T for the calculation of thermodynamic parameters for remediation of TBO dye on tannic acid-coated magnetite nanoparticles.

Figure 12

Graph between ln(Kc) and 1/T for the calculation of thermodynamic parameters for remediation of TBO dye on tannic acid-coated magnetite nanoparticles.

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A desorption study was executed to inspect the adsorbent regeneration and recovery. The regeneration capacity of a biosorbent validates its applicability. The reuse of the adsorbent for TBO occurred in two cycles, using ethanol as a desorption solvent. Eighty-seven percent of desorption efficiency was obtained for the adsorbent. In addition, no significant change was observed in the adsorption capacity of recollected TA@Fe3O4 nanoparticles, which revealed that these modified magnetite nanoparticles can be reused for the exclusion of TBO dye from the aqueous solution.

The percentage removal and maximum adsorption capacity of TBO by TA-functionalized magnetite nanoparticles were compared with already reported adsorbents (Table 3). The comparative study revealed that TA-functionalized magnetite nanoparticles are the most potential candidates among them for the removal of TBO from the aqueous solution.

Table 3

Comparison of percentage removal efficiency and maximum adsorption capacity of several adsorbents reported in literature with the present study

Sr. No.Adsorbent usedpHIsothermKinetic order% Removal of TBOAdsorption capacity qm (mg/g)Ref
1. Magnesium oxide – Pseudo-first order ∼90 – Mohammad Salim & Mohammad Salih (2015)  
2. Turkish zeolite 11 Langmuir and Freundlich Pseudo-first order – 33.03 Alpat et al. (2008)  
3. Fly ash – Freundlich Pseudo-second order – Talman & Atun (2006)  
4. Starch sulphate Toth Pseudo-first order – 47.13 Guo et al. (2011)  
5. Gypsum 6.5 Langmuir Pseudo-second order – 28 Rauf et al. (2009)  
6. Multiwalled carbon nanotubes @NiFe2O4 – Langmuir Pseudo-first order 65.8 25 Bahgat et al. (2014)  
7. Pulp fibre 6.5 Langmuir Pseudo-second order – 25 van de Ven et al. (2007)  
8. Orange peel Langmuir Pseudo-second order 72.9 – Lafi et al. (2015b)  
9. TA@Fe3O4 nanoparticles Temkin Pseudo-second order 97.23 40.28 The present study 
Sr. No.Adsorbent usedpHIsothermKinetic order% Removal of TBOAdsorption capacity qm (mg/g)Ref
1. Magnesium oxide – Pseudo-first order ∼90 – Mohammad Salim & Mohammad Salih (2015)  
2. Turkish zeolite 11 Langmuir and Freundlich Pseudo-first order – 33.03 Alpat et al. (2008)  
3. Fly ash – Freundlich Pseudo-second order – Talman & Atun (2006)  
4. Starch sulphate Toth Pseudo-first order – 47.13 Guo et al. (2011)  
5. Gypsum 6.5 Langmuir Pseudo-second order – 28 Rauf et al. (2009)  
6. Multiwalled carbon nanotubes @NiFe2O4 – Langmuir Pseudo-first order 65.8 25 Bahgat et al. (2014)  
7. Pulp fibre 6.5 Langmuir Pseudo-second order – 25 van de Ven et al. (2007)  
8. Orange peel Langmuir Pseudo-second order 72.9 – Lafi et al. (2015b)  
9. TA@Fe3O4 nanoparticles Temkin Pseudo-second order 97.23 40.28 The present study 

The adsorption of TBO dye from an aqueous solution by TA-modified magnetite nanoparticles may be explained well on the basis of different functional groups present on the surface of these nanoadsorbents. TBO may be adsorbed onto the surface of TA-modified magnetite nanoparticles via different kinds of interactions like H-bonding, π-π interactions, and electrostatic interactions (Keshvardoostchokami et al. 2018). The adsorption of imidacloprid on the surface of TA-modified magnetite nanoparticles was found maximum at pH 8.0 as at this pH, hydrogen bonding and electrostatic attraction between them is maximum. From the isotherm studies, it was found that the adsorption of the adsorbate over the surface of the adsorbent was homogenous and multilayer adsorption.

The present study illustrated the synthesis and characterization of unmodified and TA-modified magnetite nanoparticles using several techniques such as Fourier transform infrared (FTIR) spectral study, X-ray diffraction analysis, field emission scanning electron microscopy, and TG study. In addition, numerous experiments were performed to evaluate the adsorption capacity of these nanoparticles for the degradation of TBO dye. Characterization studies suggested the successful coating of TA onto the surface of bare Fe3O4 nanoparticles, and FESEM images confirmed the size, 11 and 35 nm of synthesized bare Fe3O4 and TA@Fe3O4 nanoparticles, respectively. The isotherm and kinetic study depicted that the adsorption process followed the Temkin model of isotherm and pseudo-second-order kinetic with good correlation coefficients, respectively. The thermodynamic study suggested the endothermic and spontaneous nature of the adsorption process. The TA@Fe3O4 nanoparticles show better adsorption properties for the elimination of TBO dye. Thus, the overall study revealed that the TA@Fe3O4 nanoparticles can be employed as an efficient as well as eco-friendly adsorbent for the adsorption of dyes.

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

The authors declare there is no conflict.

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