Modified xanthan gum (XG-AM-TTE) was employed as an adsorbent to study the adsorption behavior, thermodynamics and kinetics of crystal violet (CV) from an aqueous solution. Fourier transform infrared spectroscopy analysis indicates that the functional groups present in the adsorbent, such as carboxyl, ester and hydroxyl groups, are included on the external surface of the material, and these groups are potential active sites for interaction with CV. According to X-ray diffraction results, the structure of XG-AM-TTE after CV adsorption became more disordered, and the microstructure change is an indication of effective adsorption of CV to the surface, with CV becoming remarkably dispersed in the adsorbent according to the scanning electron microscopy observations. The adsorption kinetics and adsorption equilibrium were best described by the pseudo-second-order model and Freundlich isotherms, respectively. The thermodynamic parameters, as the Gibbs-free energy (ΔG), enthalpy (ΔH) and entropy (ΔS), indicated that the adsorption is a spontaneous, endothermic and entropy increase process. The maximum adsorption capacity of XG-AM-TTE was 183 ± 12 mg/g, suggesting that XG-AM-TTE is an efficient adsorbent.

Crystal violet (CV) is always used as a mutagenic and bacteriostatic agent in medical solutions and as an antimicrobial agent to prevent fungal growth in poultry feed. Despite its many uses, CV has been reported as a recalcitrant dye molecule that persists in the environment for a long period of time and poses the threat of toxic effects (Mani & Bharagava 2016). Many works have been conducted on the removal of CV, including adsorption (Hayasi & Karimi 2017), catalytic ozonation (Wang et al. 2016), and biodegradation (Kwasniewska 1985). Among these, adsorption has been extensively regarded as an efficient and economically sustainable technology for the removal of CV effluents. Adsorption has received global attention due to its simplicity and flexibility of design, high selectivity and efficiency, low operating cost and high-quality-treated effluents. Thus far, the published research has focused on inorganic adsorbents, such as amino silica (Zhou 2014), magnetic nanocomposite (Singh et al. 2011) and Nb2O5/SiO2 (Umpierres et al. 2017).

Xanthan gum (XG) is a water-soluble hetero-polysaccharide with many hydrophilic groups in the main chains (Gils Ray & Sahoo 2009), always used as dispersing agent and stabilizer of emulsions and suspensions (Garcíaochoa et al. 2000). Its property can be customized and extended by hybridization with synthetic polymers such as acrylamide (AM) (Cheng et al. 2017), polyacrylamide (PAM) (Ghorai et al. 2014), poly(2-acrylamido-2-methyl-1-propane sulfonic acid)/montmorillonite (P(AMPS)/MMT) (Aflaki et al. 2016), polyacrylic acid (PAA) (Li et al. 2013), and ethylene diamine tetraacetic acid (EDTA) (Qiu et al. 2016). Among these, AM is one of the most important grafting materials. Maia et al. (2012) studied the reaction conditions of grafting of AM on XG. Cheng et al. (2017) synthesized XG-graft-AM and used as a Cu (II) ion adsorbent. Trimethylolpropane triglycidyl ether (TTE) is a cross-linking agent. Sodium polyacrylate has been grafted on guar gum and the polymers were then surface-crosslinked using TTE to improve its swollen property (Xiong et al. 2015). A novel hydrogel base on AM and TTE grafted XG (XG-AM-TTE) demonstrated a conspicuous adsorbing effect for CV in our previous investigation (Zheng et al. 2019). Deeply understanding its absorption activity is of practical importance since it affects subsequent processing operations and the application in pollution control.

The kinetic, isotherm, and thermodynamic behaviors of absorption behavior of some absorbents have been reported, such as bottom ash (Mittal et al. 2010), XG-g-AM (Cheng et al. 2017) and XG-g-P (AMPS)/MMT (Aflaki et al. 2016). Several models have been developed to represent the characteristics of absorption behaviors (Cheng et al. 2009; Li et al. 2013). The pseudo-second-order model was concluded to best predict the kinetics of removal of methylene blue and methyl violet dyes from aqueous solution using a nanocomposite of hydrolyzed polyacrylamide grafted XG and incorporated nanosilica (Ghorai et al. 2014). The Gibbs-free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) were used to discuss the thermodynamic behavior of removal of Cu (II) from wastewater by modified XG with ethylene diamine (EDA) (Qiu et al. 2016). Different absorbents for different absorbates show various absorption behaviors. The isotherm behavior of XG-PAA for methylene blue is Langmuir (Li et al. 2013), the isotherm behavior of XG-g-AM for Cr (III) is Freundlich (Cheng et al. 2009), and the isotherm behavior of XG-g-AM for Cu (II) is Langmiur and Freundlich (Cheng et al. 2017). So far, the kinetic, isotherm, and thermodynamic behaviors of XG-AM-TTE for CV have not been reported.

The main objective of this study was to evaluate the absorption behavior of removing CV using XG-AM-TTE. Adsorbent experiments were carried out to investigate the effects of the adsorbent XG-AM-TTE dose, initial pH, contact time, temperature and initial CV concentration.

Materials

Xanthan gum (XG, USP, PubChem CID: 7107) was purchased from Aladdin Industrial Corporation. Its CAS number is 11138-66-2, its molecular formula is C35H49O29, and it is moisture sensitive. Acrylamide (AM, AR, 99.0%, PubChem CID: 6579; C3H5NO) was purchased with a molar mass of 71.08 g/mol. Trimethylolpropane triglycidyl ether (TTE, Tech, PubChem CID: 103015) was purchased from Sigma-Aldrich. All other chemical reagents were of analytical grade.

Synthesis of modified xanthan gum hydrogel

The modified XG (XG-AM-TTE) was synthesized in a four-necked flask under a pure nitrogen gas environment and equipped with a mechanical stirrer, a thermometer, and a reflux condenser. An aliquot of 1 g XG was dissolved in 150 mL distilled water under constant stirring for 60 min at room temperature. Then, AM (15 g) and 260 μL TTE were added sequentially into the XG solution, and the resulting mixture was stirred and saturated with pure nitrogen to remove the dissolved oxygen. The temperature was held at 70 °C, and 0.3 g of potassium persulfate (KPS) was added and reacted for 4 h to initiate the graft copolymer. The precipitate was filtered, washed thoroughly with an ethanol/water mixture (v: v = 4:1) three times under high-speed stirring, and soaked in the ethanol/water mixture (v: v = 4:1) for 24 h. The XG-AM-TTE was collected by filtration and freeze drying.

Characterization of XG-AM-TTE

Fourier transform infrared spectroscopy (FT-IR) analysis

FT-IR spectra of XG-AM-TTE before and after CV absorption were recorded in the solid state using KBr pellets with an AVATAR 360 FT-IR spectrometer in the range from 400 to 4,000 cm−1 at 2 cm−1 resolution.

X-ray diffraction (XRD) analysis

The structures of XG-AM-TTE before and after CV absorption were investigated using an Empyrean XRD instrument (Rigaku, Miniflex 600) with a Cu Kα1 radiation source. The X-ray generator was operated at 40 kV and 15 mA. The reflection angle 2θ was monitored from 5° to 60° at a scanning speed of 10°/min and a step size of 0.02 (λ = 1.5406 nm).

Microstructure analysis

The microstructures of XG-AM-TTE before and after CV absorption were examined by a scanning electron microscope (SEM) (JSM-6380LV, JEOL), operating with secondary electrons under low vacuum at 15.0 kV. The samples were coated with Au prior to the SEM examination.

Batch adsorption experiments

A certain amount of XG-AM-TTE and 100 mL of CV solution was sealed in a 250 mL conical flask with a cap to minimize evaporation. Adsorption was allowed to proceed at 25 °C at 170 rpm in a thermostatic shaker for a predetermined time. The pH of the solution was adjusted to the desired values using 0.1 M HCl or 0.1 M NaOH with stirring before the addition of XG-AM-TTE and then shaken in a constant-temperature water bath. CV concentrations in the bulk solutions were analyzed using an xMark (BioRad) instrument at 590 nm.

The adsorption capacity (Qe, mg/g) and the removal rate (ŋ, %) were calculated using Equations (1) and (2), respectively.
formula
(1)
formula
(2)

where Ci and Ce are the initial and final CV concentrations (mg/L), respectively, Vi and Ve are the initial and final volume of CV (L), and m is the weight in grams of XG-AM-TTE used for the adsorption measurements (g).

Characteristics of XG-AM-TTE

The FT-IR spectra of XG-AM-TTE before and after CV adsorption are shown in Figure 1. XG-AM-TTE shows a broad peak at 3,449 cm−1 due to the stretching vibrations of O-H and N-H. The peaks at 1,655 cm−1 and 1,611 cm−1 are attributed to amide-I (the C = O stretching vibrations) and amide-II (the N-H bending vibrations). The peak at 1,655 cm−1 is due to the characteristic absorption of CONH2, the peak at 1,420 cm−1 is due to stretching vibrations of C-N, and the peak at 1,117 cm−1 is due to the wagging vibration of NH2. Examination of the FT-IR spectrum obtained after CV adsorption shows that the frequencies and intensities of some peaks have shifted or substantially decreased compared to their preadsorption values. This can be ascribed to the occurrence of a complex process between the ionizable functional groups and CV. The chemical structure of CV is shown in Figure S1 (available with the online version of this paper). The FT-IR analysis results indicate that the functional groups present in the adsorbent, such as the carboxyl, ester and hydroxyl groups, are included in the external surface of the material, and these groups can be the potential active sites for interaction with CV.

Figure 1

FT-IR spectra from 4,000 to 400 cm−1 of XG-AM-TTE before (black line, a) and after (red line, b) CV adsorption. The full color version of this figure is available in the online version of this paper, at http://dx.doi.org/10.2166/wst.2019.038.

Figure 1

FT-IR spectra from 4,000 to 400 cm−1 of XG-AM-TTE before (black line, a) and after (red line, b) CV adsorption. The full color version of this figure is available in the online version of this paper, at http://dx.doi.org/10.2166/wst.2019.038.

Close modal

The XRD measurements probed the crystal behavior of the sample. The XRD patterns of XG-AM-TTE before and after CV absorption are displayed in Figure 2. As seen, there is no specific peak in the XRD spectrum of XG-AM-TTE before and after CV adsorption, implying that the XG-AM-TTE before and after CV adsorption is almost amorphous and revealing that there are covalent bonds inside the XG-AM-TTE. The peak becomes broad after the introduction of CV, so it is possible that the CV destroyed the covalent bonding in the material, making its structure more disordered.

Figure 2

X-ray diffraction pattern of XG-AM-TTE before (red line, a) and after (black line, b) CV adsorption. The X-ray generator was operated at 40 kV and 15 mA. The XRD chromatograms were recorded over an angular range of 5° to 60° (2θ) at a scanning speed of 10°/min and a step size of 0.02 (λ = 1.5406 nm). The full color version of this figure is available in the online version of this paper, at http://dx.doi.org/10.2166/wst.2019.038.

Figure 2

X-ray diffraction pattern of XG-AM-TTE before (red line, a) and after (black line, b) CV adsorption. The X-ray generator was operated at 40 kV and 15 mA. The XRD chromatograms were recorded over an angular range of 5° to 60° (2θ) at a scanning speed of 10°/min and a step size of 0.02 (λ = 1.5406 nm). The full color version of this figure is available in the online version of this paper, at http://dx.doi.org/10.2166/wst.2019.038.

Close modal

The SEM images of the surface structure of the XG-AM-TTE before and after CV adsorption are shown in Figure 3. It is clear that, prior to adsorption, the external surface of the sample is rough and irregular and contains pores of different sizes and shapes, causing the CV to become trapped and adsorbed into these different parts of the adsorbent. After adsorption, welling and drying are carried out; a significant change is observed in the structure of the XG-AM-TTE, which can be taken as an indication of effective adsorption of CV to the surface and makes the CV become remarkably dispersed in the adsorbent.

Figure 3

SEM micrograph of XG-AM-TTE before ((a) ×3000) and after ((b) ×500) CV adsorption. SEM analyses were performed using a JSM-6380LV (JEOL) operated with an accelerating voltage of 15.0 kV. The samples were coated with Au prior to the experiment.

Figure 3

SEM micrograph of XG-AM-TTE before ((a) ×3000) and after ((b) ×500) CV adsorption. SEM analyses were performed using a JSM-6380LV (JEOL) operated with an accelerating voltage of 15.0 kV. The samples were coated with Au prior to the experiment.

Close modal

Effect of adsorbent dose

The adsorbent dose is one of the parameters that strongly affect the adsorption capacity in an aqueous solution (Zafar et al. 2008). The efficiency of mass dosage of the adsorbent was studied by agitating the different masses (0.05, 0.15, 0.25, 0.35, 0.5, 0.75, 1 and 2 g/L) of XG-AM-TTE and 100 mL of 30 mg/L CV using a shaker at room temperature. The experiments were conducted at an optimized pH of 7.0. The results of the experiments with varying adsorbent doses are presented in Figure 4. Increasing the XG-AM-TTE dose resulted in an increase in the CV removal. More specifically, the increase rate of this parameter was high for higher XG-AM-TTE doses due to the greater availability of active sites on the surface of the materials and low for lower XG-AM-TTE doses due to the progressive saturation of these active sites (Saha et al. 2010). However, the amount of CV adsorbed per XG-AM-TTE mass unit decreased considerably as the XG-AM-TTE dose increased. This can be attributed to the adsorption sites remaining unsaturated during the adsorption reaction (Chowdhury & Saha 2010). These results were in agreement with other similar studies on CV adsorption (Saeed et al. 2010). In addition, it could also be noticed that for the majority of the tested materials, the steady state was reached for an adsorbent dose between 0.25 and 0.75 g/L. Therefore, the optimum adsorbent dose of 0.75 g/L was selected for use in all subsequent experiments.

Figure 4

Effect of mass dosage of adsorbent on the adsorption capacity (continuous line) and the removal (dotted line) of CV by XG-AM-TTE at room temperature, with a contact time of 60 min, a 30 mg/L dye concentration and pH 7.0. The error bars represent the standard error.

Figure 4

Effect of mass dosage of adsorbent on the adsorption capacity (continuous line) and the removal (dotted line) of CV by XG-AM-TTE at room temperature, with a contact time of 60 min, a 30 mg/L dye concentration and pH 7.0. The error bars represent the standard error.

Close modal

Effect of initial pH on adsorption of CV

The pH of an aqueous solution plays an important role in controlling the parameters in the dye adsorption process because it affects the surface charge of the sorbent material and the degree of ionization of the dye molecule (Ramakrishnan & Nagarajan 2009). pH has been related to changes in the structural stability and color intensity of the dye molecule and charged chromophore (Monash & Pugazhenthi 2009). Figure 5 shows the effects of an initial pH in the 2.0–6.0 range on the equilibrium adsorption capacity of CV on XG-AM-TTE. The results show that the Qe of CV on the XG-AM-TTE, as well as ŋ, increases as the initial solution pH increases in the 2.0–7.0 range. The maximum Qe and ŋ of CV were obtained at pH 7.0. This can be explained as being due to the availability of dye-binding sites on the XG-AM-TTE at these pH values. The poor Qe and ŋ of the XG-AM-TTE in the low-pH region is attributed to the large number of hydrogen ions in acidic conditions, which gives rise to high mobility and competition with CV for the active sites on the XG-AM-TTE, leading to reduced attraction between the XG-AM-TTE and greater Qe and ŋ. The Qe and ŋ values tend to be stable with an increase in pH in the range of 7.0–10.0. Therefore, all subsequent studies were carried out at pH 7.0 as the optimum pH.

Figure 5

Effect of pH on the adsorption capacity (continuous line) and the removal (dotted line) of CV by XG-AM-TTE at room temperature, with a contact time of 60 min, and a 30 mg/L dye concentration. The error bars represent the standard error.

Figure 5

Effect of pH on the adsorption capacity (continuous line) and the removal (dotted line) of CV by XG-AM-TTE at room temperature, with a contact time of 60 min, and a 30 mg/L dye concentration. The error bars represent the standard error.

Close modal

Effect of contact time

Contact time is an important parameter for the assessment of the prospects for the practical application of the adsorption process (Jethave et al. 2017). Figure 6 shows the Qe and ŋ of CV in the contact time range of 0–90 min with an initial CV concentration of 30 mg/L and an XG-AM-TTE concentration of 0.75 g/L at pH 7.0. By increasing the contact time, increases in both the amount of CV adsorbed per XG-AM-TTE mass unit and the CV removal were obtained. The ŋ of CV was increased by the contact time so that approximately 58% of CV removal occurred in the first 10 min. In most cases, the rapid initial adsorption during the first few minutes of contact was followed by a slower adsorption process until an equilibrium state was reached. This was due to the existence of abundant vacant active sites, on which adsorption continued with progressive saturation of these active sites with longer times. In addition, the required time to reach equilibrium was approximately 30–90 min because an increase in the contact time to 90 min did not have any significant effects. A short equilibrium time indicates that CV has a high tendency for rapid migration and diffusion to the XG-AM-TTE surface. Therefore, the rest of the experiments were performed using a 30 min contact time.

Figure 6

Effect of contact time on the adsorption capacity (continuous line) and the removal (dotted line) of CV by XG-AM-TTE at room temperature and pH 7.0, with an adsorbent dosage of 75 mg in 100 mL and a 30 mg/L dye concentration. The error bars represent the standard error.

Figure 6

Effect of contact time on the adsorption capacity (continuous line) and the removal (dotted line) of CV by XG-AM-TTE at room temperature and pH 7.0, with an adsorbent dosage of 75 mg in 100 mL and a 30 mg/L dye concentration. The error bars represent the standard error.

Close modal

Effect of initial CV concentration

The mechanism of CV adsorption from an aqueous solution is particularly dependent on the initial CV concentration in the solution. Figure 7 shows the Qe and ŋ of CV in the initial CV concentration range of 10–50 mg/L, with 0.75 g/L XG-AM-TTE at pH 7.0. Increasing the initial CV concentration caused an increase in the amount of CV adsorbed per XG-AM-TTE mass unit. This was due to the increase in the concentration gradient, which is the driving force for mass transfer.

Figure 7

Effect of the initial CV concentration (10, 15, 35, 45, 50 mg/L) on the adsorption capacity (continuous line) and the removal (dotted line) of CV by XG-AM-TTE at room temperature and pH 7.0, with an adsorbent dosage of 75 mg in 100 mL. The error bars represent the standard error.

Figure 7

Effect of the initial CV concentration (10, 15, 35, 45, 50 mg/L) on the adsorption capacity (continuous line) and the removal (dotted line) of CV by XG-AM-TTE at room temperature and pH 7.0, with an adsorbent dosage of 75 mg in 100 mL. The error bars represent the standard error.

Close modal

Effect of contact temperature

Figure 8 presents the effect of contact temperature on CV adsorption and removal. The adsorption and removal of CV were achieved at 0.75 g/L XG-AM-TTE, an initial CV concentration of 30 mg/L and pH 7.0. By increasing the contact temperature, an increase in both the amount of CV adsorbed per XG-AM-TTE mass unit and the CV removal was obtained. This may be attributed to the lower solvent viscosity and higher average kinetic energy of ions under high-temperature conditions. This fact suggests that the adsorption of CV is endothermic in nature. Therefore, to reduce the operational costs, the experiments were performed at 303 K.

Figure 8

Effect of contact temperature on the amount of CV adsorbed per adsorbent mass unit (continuous line) and on the CV removal percentage (dotted line). The error bars represent the standard error.

Figure 8

Effect of contact temperature on the amount of CV adsorbed per adsorbent mass unit (continuous line) and on the CV removal percentage (dotted line). The error bars represent the standard error.

Close modal

Adsorption kinetics

To study the kinetics of CV adsorption, three kinetic models were applied (Pellera et al. 2012): the pseudo-first-order model (Equation (3)), the pseudo-second-order model (Equation (4)) and the intraparticle diffusion model (Equation (5)).
formula
(3)
formula
(4)
formula
(5)
where k1 is the adsorption rate constant (min−1), t is the time (min), Qe and Qt are the amounts of CV adsorbed per adsorbent mass unit at equilibrium and at time t (mg/g), respectively, and Qe;exp and Qe;calc are the amounts of CV adsorbed per adsorbent mass unit at equilibrium (mg/g), obtained from the experiments with varying contact time (experimental) and from the fitting to the pseudo-first-order model (calculated), respectively. k2 is the adsorption rate constant (g/mol/min), and h = k2Qe2 represents the initial adsorption rate for the pseudo-second-order model. Finally, kid is the adsorption rate constant (mol/g/min1/2), and C is the intraparticle diffusion constant.

The results shown in Table 1 indicate that the pseudo-second-order model provides the best description of the CV adsorption kinetics for XG-AM-TTE. This is demonstrated by its correlation coefficient value of 0.9981, as well as by the Qe value of 25.84 mg/g, which was very close to the experimental value of Qe;exp of 25.77 mg/g. This implies that the adsorption process is probably a chemical process (Wu et al. 2009), and it could be rate limiting in the sorption step involving the ion exchange of the sorbent and sorbate. In the intraparticle diffusion model, the linear plots passing through the origin indicate that single particle diffusion is the rate-limiting step (Lorenc-Grabowska & Gryglewicz 2005). However, in this study, the linear plots do not pass through the origin, which may be due to the difference in the rate of mass transfer in the process of adsorption and also shows that the intraparticle diffusion model is not the only rate-limiting step. Particle diffusion and boundary layer diffusion interactions affect the adsorption of CV on XG-AM-TTE as well. In the description of the adsorption kinetics by the pseudo-second-order and the intraparticle diffusion models, the rate-limiting step is chemical sorption (chemisorption) between the XG-AM-TTE and the CV. For the pseudo-first-order model and the intraparticle diffusion model, although in some cases the correlation coefficient values were quite high (R2 > 0.9), the value of Qe;calc (14.39 mg/g) was much lower than the experimental value of Qe;exp (25.77 mg/g). The applicability of the pseudo-second-order model to the materials examined in this study is in agreement with the result obtained by Mittal (Mittal et al. 2010).

Table 1

Kinetics parameters for CV adsorption onto XG-AM-TTE

Qe, exp mg/gPseudo-first-order model
Pseudo-second-order model
Intraparticle diffusion model
k1 min−1Qe,calc mg/gR2k2 g/mg/minQe mg/gh = k2Qe2R2k3 mg/g/min1/2CR2
25.77 0.084 14.39 0.9368 0.034 25.84 22.94 0.9981 0.839 19.64 0.9208 
Qe, exp mg/gPseudo-first-order model
Pseudo-second-order model
Intraparticle diffusion model
k1 min−1Qe,calc mg/gR2k2 g/mg/minQe mg/gh = k2Qe2R2k3 mg/g/min1/2CR2
25.77 0.084 14.39 0.9368 0.034 25.84 22.94 0.9981 0.839 19.64 0.9208 

Equilibrium isotherms

The equilibrium adsorption isotherm provides some of the most important data for the description of the adsorption behavior of an adsorption system. The adsorption process is normally described by the Henry (Equation (6)), Langmuir (Equation (7)) and Freundlich isotherms (Equation (8)) (Chen et al. 2016).
formula
(6)
formula
(7)
formula
(8)
where Kd is the linear adsorption coefficient (L/g), Qe is the amount of CV adsorbed per XG-AM-TTE mass unit (mg/g) at equilibrium, Ce is the equilibrium CV concentration in the solution (mg/L), Q is the maximum dye uptake (mg/g), b is the adsorption energy (L/mg), Kf is the distribution coefficient (mg/g(L/mg)1/n) indicating the adsorption capacity (L/g), n is the heterogeneity factor, indicating the degree of nonlinearity, and 1/n is a dimensionless parameter indicating adsorption intensity.

The results obtained from the application of the three isotherms are shown in Table 2. It can be seen that the Freundlich isotherm provided the best description of the adsorption equilibrium, with high correlation coefficient values (R2 > 0.99). This suggests monolayer coverage of the surface of XG-AM-TTE by CV, indicative that the adsorption was surface adsorption. This also shows the inhomogeneity on the surface of XG-AM-TTE, the reversibility of the adsorption and the coexistence of various interactions, as suggested by FT-IR, XRD and SEM.

Table 2

Adsorption equilibrium parameters

Henry
Langmuir
Freundlich
K (L/g)R2Q (mg/g)b (L/mg)R2K (mg/g/(L/mg)1/n)1/nR2
1.0022 0.9626 −7.7882 −0.0351 0.8878 0.0296 2.2067 0.9918 
Henry
Langmuir
Freundlich
K (L/g)R2Q (mg/g)b (L/mg)R2K (mg/g/(L/mg)1/n)1/nR2
1.0022 0.9626 −7.7882 −0.0351 0.8878 0.0296 2.2067 0.9918 

Thermodynamic parameters

The mechanism of adsorption may be determined through thermodynamic quantities such as the change in ΔS, ΔG, and ΔH. The thermodynamic equilibrium constant K can be calculated using Equation (9) and the increase in K with an increase in temperature indicates the endothermic nature of the process. The ΔS, ΔG and ΔH values were calculated using the van 't Hoff equation, given by Equations (10) and (11) (Chen et al. 2016).
formula
(9)
formula
(10)
formula
(11)
where Kd is the thermodynamic equilibrium constant (L/mol), Qe is the amount of CV adsorbed per adsorbent mass unit (mg/g) at equilibrium, Ce is the equilibrium CV concentration in the solution (mg/L), R is the universal gas constant (8.314 J/mol−1/K), T is the temperature (K), ΔS is the entropy (J/mol/K), ΔH is the change in the enthalpy of adsorption (J/mol), and ΔG is the Gibbs-free energy (J/mol).

Using linear regression analysis, the values of ΔH and ΔS are determined from the intercept and slope of the plot of ΔG vs. T according to Equation (10). The calculated thermodynamic parameters are listed in Table 3. The thermodynamic constants at an initial CV concentration of 50 mg/L were calculated. The positive value of ΔH is indicative of the endothermic nature of the adsorption interaction, which is consistent with the adsorption increasing with increasing temperature. The positive value of ΔS shows the affinity of CV and the increasing randomness at the solid-solution interface during the adsorption process, which is consistent with the XRD results. The decrease in the negative value of ΔG with temperature shows that the adsorption process becomes more favorable at higher temperatures, and the negative value of ΔG over all of the examined temperature range also indicates the feasibility of the adsorption process and the spontaneous nature of CV adsorption. Adsorption of CV appears to be spontaneous and endothermic. ΔG is less than −80 kJ/mol, indicate that, the adsorption was chemical adsorption.

Table 3

Thermodynamic parameters for the adsorption of CV

ΔHΔSΔG (J/mol)
(J/mol)(J/mol/K)313 K323 K
24,489.72 79.06 −256.16 −1,046.77 
ΔHΔSΔG (J/mol)
(J/mol)(J/mol/K)313 K323 K
24,489.72 79.06 −256.16 −1,046.77 

Adsorption capacity

The adsorption capacities and adsorption behavior of various adsorbents included modified XG were compared in Table 4. For the same absorbate, XG-AM-TTE showed higher adsorption capacity on CV than inorganic adsorbent, such as Nb2O5/SiO2 (116 mg/g) (Umpierres et al. 2017), magnetic nanocomposite (112 mg/g) (Singh et al. 2011), and amino silica (40 mg/g) (Zhou 2014). While, it showed lower adsorption capacity on CV than bottom ash (887 mg/g) (Mittal et al. 2010), which was inferred to be activated carbon. The other modified XG has also been reported and been used to adsorb dyes and heavy metals (Table 4). XG-g-AM showed the highest absorption capacity on heavy metals (Cheng et al. 2009). XG-PAM showed higher absorption capacity on methyl violet (Ghorai et al. 2014). In this work, the modified XG, XG-AM-TTE, showed absorption capacity on CV.

Table 4

Comparison of the adsorption capacities of various adsorbents

AdsorbentsAbsorbateQe (mg/g)KineticIsothermThermodynamicReference
Mino silica Crystal violet 40 – – – Zhou (2014)  
Magnetic nanocomposite Crystal violet 112 – – – Singh et al. (2011)  
Nb2O5/SiO2 Crystal violet 116 – – – Umpierres et al. (2017)  
Bottom ash Crystal violet 887 Pseudo-second order Langmuir Spontaneous
endothermic
entropy increase 
Mittal et al. (2010)  
XG-g-AM Cu(II) – Pseudo-second order Langmiur, Freundlich Spontaneous
endothermic
entropy increase 
Cheng et al. (2017)  
XG-g-P(AMPS)/MMT Cu(II) 29 Pseudo-second order Langmuir Spontaneous
endothermic
entropy increase 
Aflaki et al. (2016)  
XG-PAA Methylene blue 30 – Langmuir – Li et al. (2013)  
XG-EDA Cu(II) 46 Pseudo-second order Langmuir – Qiu et al. (2016)  
XG-g-AM Cr(III) 205 Pseudo-second order Freundlich – Cheng et al. (2009)  
XG-PAM Methyl violet 378 Pseudo-second order Langmuir – Ghorai et al. (2014)  
XG-AM-TTE Crystal violet 183 ± 12 Pseudo-second order Freundlich Spontaneous
endothermic entropy increase 
This work 
AdsorbentsAbsorbateQe (mg/g)KineticIsothermThermodynamicReference
Mino silica Crystal violet 40 – – – Zhou (2014)  
Magnetic nanocomposite Crystal violet 112 – – – Singh et al. (2011)  
Nb2O5/SiO2 Crystal violet 116 – – – Umpierres et al. (2017)  
Bottom ash Crystal violet 887 Pseudo-second order Langmuir Spontaneous
endothermic
entropy increase 
Mittal et al. (2010)  
XG-g-AM Cu(II) – Pseudo-second order Langmiur, Freundlich Spontaneous
endothermic
entropy increase 
Cheng et al. (2017)  
XG-g-P(AMPS)/MMT Cu(II) 29 Pseudo-second order Langmuir Spontaneous
endothermic
entropy increase 
Aflaki et al. (2016)  
XG-PAA Methylene blue 30 – Langmuir – Li et al. (2013)  
XG-EDA Cu(II) 46 Pseudo-second order Langmuir – Qiu et al. (2016)  
XG-g-AM Cr(III) 205 Pseudo-second order Freundlich – Cheng et al. (2009)  
XG-PAM Methyl violet 378 Pseudo-second order Langmuir – Ghorai et al. (2014)  
XG-AM-TTE Crystal violet 183 ± 12 Pseudo-second order Freundlich Spontaneous
endothermic entropy increase 
This work 

AM is acrylamide, AMPS is 2-acrylamido-2-methyl-1-propane sulfonic acid, MMT and OMMT are montmorillonite, EDA is ethylenediamine, PAM is polyacrylamide, PAA is polyacrylic acid, TTE is trimethylolpropane triglycidyl ether, Qe is the absorption capacity.

The adsorption behavior of XG-AM-TTE for CV was investigated. Various impact factors, such as adsorbent dose, pH, contact time, temperature and initial CV concentration, were optimized. The experimental results can fit perfectly with the pseudo-second-order model and the Freundlich isotherm model. A negative value of ΔG and a positive value of ΔH confirm the spontaneous and endothermic nature of the adsorption process. The adsorption process is mainly controlled by both surface and chemical reactivity and obeys the pseudo-second-order model for the entire adsorption period. Overall, XG-AM-TTE is an effective adsorbent for the removal of CV from aqueous solutions.

This work was supported by International Science and Technology Cooperation and Exchange Program of Fujian Agriculture and Forestry University (KXGH17001), Fujian Provincial Foreign Cooperation Project (2018I0003), Fujian Provincial Science and Technology Program of Regional Development Project (2018N3001) and National Natural Science Foundation of China (31628016).

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Supplementary data