Removal of safranin-O dye from aqueous solution using modified red mud: kinetics and equilibrium studies

Abstract

The object of this study was to evaluate the efficiency of safranin-O dye removal from aqueous solutions with application of red mud waste as an adsorbent. For this propose, the surface of the red mud was modified by the addition of sodium dodecyl sulphate as an anionic surfactant. Different instrumental techniques were used to understand the physiochemical properties and the surface properties of the modified red mud for the adsorption of safranin-O dye. Batch adsorption experiments were carried out to understand the optimum conditions with respect to solution pH, contact time and initial dye concentration. Kinetics data were best fitted with a pseudo-second-order model. The adsorption isotherms data were a good fit to the Langmuir model with a maximum monolayer adsorption capacity of 89.4 mg g⁻¹ at 308 K. Desorption experiments were carried out by testing several solvents such as water, sulphuric acid, hydrochloric acid and acetic acid and the maximum (93.2%) desorption was found using hydrochloric acid.

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1. Introduction

Organic dyes are widely used in different industries to colour their products but they are one of the major groups of water pollutants.¹ The wastewater discharged from industries like textile, leather, cosmetics, paper, printing, plastic, pharmaceutical and food without treatment, affects the photosynthetic aquatic life due to the lack of light perception.^2,3 Dyes are responsible for genetic mutations in human beings and can easily accumulate in living tissues.⁴ In addition, many dyes and their by-products are harmful to human and animal life.^5,6 In view of the toxic properties of dyes, immediate attention is required to develop suitable methods for the removal of dyes from wastewater. Various processes are available in the literature to remove dyes from wastewater^7–14 but due to different limitations, the processes are not very efficient. Researchers are striving hard to develop suitable methods and processes to remove dyes from water. Recently, many low-cost and effective adsorbents^15–21 have been used for the removal of dyes from water.

Red mud is a caustic by-product of the alumina refinery and an estimated of 120 million tonnes of red mud are being generated annually worldwide.²² Red mud slurry is highly alkaline having a pH > 12. Both by its chemical composition and the amount generated every year, red mud is a serious pollutant in the environment. It cannot be disposed of easily because of its high alkalinity and hence the slurry is stored in a large pond or reservoir, but that process of storing is also not free from problems like quantity seepage into the underground water and alkaline airborne dust. There are many problems for the management of red mud slurry. Therefore, it is necessary to develop an effective utilization of bins of red mud. In addition, red mud has a high potential for water treatment as it is an efficient adsorbent due to its chemical stability, mechanical stability and structural properties.

There is growing interest in research on both natural untreated and modified red mud minerals, including attempts to improve the characteristics of the materials regarding the removal of hazardous dye from industrial wastewaters. To enhance the dye adsorption properties of red mud waste, different techniques of modification have been applied: acid activation,^23–25 heat and chemical treatment,²⁶ and chemical modification using inorganic based materials.²⁷ To the best of our knowledge, the investigation of sodium dodecyl sulphate (SDS) surfactant modified red mud as an adsorbent for the removal of safranin-O dye has not been reported before. The surface modification using SDS surfactant represents an environmentally friendly and inexpensive method in comparison with other methods. A list of low-cost adsorbents for the adsorption of safranin-O from aqueous solutions is given in Table 1.

Table 1 Comparison of adsorption capacities of safranin-O dyes onto different adsorbents

Adsorbent(s)	pH	Adsorption capacity (mg L^−1)
Calcined mussel shells^28	Above 9.2	154.34
Pineapple peels^29	6–8	21.7
Calcined bones^17	6.2	135.32
NaOH treated rice husk^30	8	37.97
Alkali-treated rice husk^31	8	9.77
SDS/RM (present study)	4	89.4

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In this study, red mud was chemically treated and the surface was modified by sodium dodecyl sulphate anionic surfactant for the removal of safranin-O from aqueous solution. The red mud surface was modified with SDS surfactant to prevent the agglomeration of a large ratio of surface area and volume which reduces the surface energy. Sodium dodecyl sulphate (SDS) was selected for the surface modification of red mud, because of its extensive use for its admicellar sorption properties as well as its high biodegradability. The effectiveness of the proposed method for dye removal and the factors influencing the adsorption process were investigated in detail. The kinetics and equilibrium data of the adsorption process were studied to understand the adsorption characteristics. Finally, the safranin-O removal mechanism of SDS/RM was discussed.

2. Experimental section

2.1. Materials

All the reagents used in this study were analytical grade obtained from Merck, Mumbai (India) and Rankem, New Delhi (India). The cationic safranin-O dye and the anionic surfactant used in this study were obtained from Sigma-Aldrich, Steinheim (Germany). The physiochemical properties of safranin-O are shown in Table 2.

Table 2 The physiochemical properties of the safranin-O dye

Molecular structure
3D model structure
Molecular formula	C20H19N4Cl
Molecular weight (g mol^−1)	350.85
IUPAC name	3,7-Diamino-2,8-dimethyl-5-phenylphenazinium chloride
λmax (nm)	520
Generic name	C.I. basic red 2

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A 1000 ppm stock solution was prepared by dissolving 1 g safranin-O in double distilled water (1 L). The required standard solutions were prepared from the stock solution by dilution and their concentrations were confirmed using the calibration curve. Regeneration of the adsorbent was carried out using water, sulphuric acid (0.1 M), hydrochloric acid (0.1 M) and acetic acid (0.1 M).

2.2. Adsorbent preparation and characterization

The bauxite residue (red mud) used in this study was obtained from Vedanta alumina Ltd, Lanjigarh, Odisha. 10 g of red mud and was placed in a 500 mL conical flask. Then 30 mL of H₂SO₄ (16 mol L⁻¹) was slowly added and the reaction mixture was stirred for 2 h at 80 °C. After that, 100 mL of deionized water was added into the flask to extract soluble compounds from the resultant mixture. The resulting solution was then centrifuged at 3000 rpm for 10 min to separate the solid product and washed three times with distilled water and dried at 100 °C in an oven for 24 h.

2.3. Modification of the adsorbent surface using sodium dodecyl sulphate (SDS) surfactant

For the modification of the surface of the red mud, 0.1 g of the above prepared chemically treated red mud was first dispersed in 50 mL of distilled water for 2 h at room temperature using a magnetic stirrer to swell and reach homogeneity. The pH of the solution was maintained at pH 4 using 0.1 M hydrochloric acid. Then 1.0 mL of 5% (m/v) sodium dodecyl sulphate (SDS) surfactant was added and the mixture was stirred for 1 h. The SDS surfactant was added to the solution of red mud at concentrations lower than its critical micelle concentration (CMC = 2.3 g L⁻¹) to modify the surface of the red mud. The mixture was separated by centrifugation and dried in an air oven at 30 °C. The sodium dodecyl sulphate modified red mud (SDS/RM) was kept in an airtight container until used.

2.4. Characterization techniques

The surface area and particle size of the samples were measured using N₂ adsorption isotherms at −196 °C by applying the Brunauer, Emmett and Teller (BET) method (Quantachrome AUTOSORB-1, USA) and using a particle size analyser (Nano-ZS 90, MALVERN, UK) respectively. XRD studies of the samples were recorded using a Philips X’Pert X-ray diffractometer for the characterization of the material before and after adsorption. FT-IR analysis of the samples were performed using a PerkinElmer RX-I spectrometer in the range 500–4000 cm⁻¹. A JEOL, JSM-6480LV analyser recorded the SEM images of SDS/RM before and after adsorption of safranin-O. The pH of all solutions were checked by using a PHAN Lab India pH analyser. The concentrations of safranin-O in water were determined from the calibration curve at λ_max = 520 nm using a UV-visible spectrophotometer (PerkinElmer).

2.5. Batch adsorption experiment

Batch adsorption experiments were performed by adding 0.25 g of SDS/RM to 100 mL of a known concentration of dye solution in a 250 mL stoppered conical flask and shaking for 1 h at 180 rpm in a rotary shaker until the equilibrium was reached. The pH of the solutions were adjusted by adding HCl (0.1 M) or NaOH (0.1 M) as required. Ionic strength of the aqueous solutions were adjusted with NaCl. To get the optimum condition, the experiments were performed by varying the initial dye concentration in the range of 10 mg L⁻¹ to 50 mg L⁻¹, pH 2 to 12 and time 10 to 90 min. The concentration of residual safranin-O was determined spectrophotometrically at λ_max = 520 nm. The percentage removal (eqn (1)) and uptake capacity (q_e) (eqn (2)) were calculated using the following equations:where C₀ and C_e (mg L⁻¹) are the initial and final concentration of dye, respectively. V = volume of the solution (L) and M = mass of the adsorbent (g).

2.6. Adsorption kinetics models

The study of adsorption kinetics is important as it controls the process efficiency.³² The pseudo-first-order, pseudo second-order, Elovich model, Bangham’s equation and intraparticle diffusion models were used to establish which model shows the best fit with experimental data.

The pseudo first-order kinetic model is represented by the Lagergren rate equation:³³

where q_e (mg g⁻¹) and q_t (mg g⁻¹) are the amounts of the adsorbate adsorbed on the adsorbent at equilibrium and time t (min), respectively. The rate constant k₁ (min⁻¹) and q_e were calculated from the linear plot of log(q_e − q_t) versus t.

The linear form of the pseudo second-order kinetics model for the boundary condition q_t = 0 at t = 0 and q_t = q_t at t = t, is expressed as:

where k₂ (g mg⁻¹ min⁻¹) is the equilibrium rate constant. The values of k₂ and q_e were calculated from the linear plot of t/q_t versus t.

The Elovich model is:³⁴

where α (mg g⁻¹ min⁻¹) is the adsorption rate, and β is related to the extent of surface coverage. The Elovich constants α and β were obtained from the slope and intercept of the linear plot of q_t versus ln t, respectively.

Bangham’s equation is:³⁵

where, C₀ is the initial concentration of the adsorbate (mg L⁻¹), V is the volume of the solution (mL), m is the weight of the adsorbent used per liter of solution (g L⁻¹), q (mg g⁻¹) is the amount of the adsorbate retained at time t, α (<1) and k₀ (g) are constants. log log[C₀/(C₀ − q × m)] values are plotted against log t in Bangham’s plot.

When the adsorbent is porous, pore diffusion is also expected in addition to surface adsorption. To confirm whether safranin-O adsorption on SDS/RM is dominated by intraparticle diffusion, the data are analyzed using an intraparticle diffusion model (Weber Morris).

The intraparticle diffusion model (Weber Morris) is described as follows:³⁶

q_e = k_ipt^0.5 + C (7)

where q_t (mg g⁻¹) is the uptake capacity of safranin-O dye at time t. k_ip (mg g⁻¹ min⁻¹) is the intraparticle diffusion rate constant and C (mg g⁻¹) indicates the thickness of the boundary layer obtained from the slope and intercept of the plot q_t versus t^0.5 respectively at different initial safranin-O concentrations.

2.7. Mass transfer study

The double nature of the intraparticle diffusion plot confirms both film and pore diffusion, hence in order to predict the actual slow step involved, the kinetics data are further analyzed using the model given by Boyd et al.:³⁷or,where m is an integer and F is the fraction of solute adsorbed at time t and is obtained by the expression:where q_t (mg g⁻¹) represents the amount of dye adsorbed at time t (min) and q_∞ is the amount of safranin-O adsorbed at infinite time (mg g⁻¹). B is the time constant (min⁻¹), and r is the radius of the adsorbent particle, which is assumed to be spherical.

For every calculated value of F, the corresponding values of Bt are computed from the following equation:

Bt = −0.4977 − ln(1 − F) (12)

The plot of calculated Bt values versus time t is used to identify whether the adsorption mechanism is via film diffusion or a particle controlled mechanism. If the linear straight line plot passes through the origin, it indicates that the adsorption studies are governed by a particle diffusion mechanism.³⁸ The calculated B values were used to evaluate the effective diffusion coefficient, D_i (m² s⁻¹) using eqn (11). The linear plot of ln D_i versus 1/T allow the use of the Arrhenius equation for the determination of D₀, E_a and ΔS^# as per the following formulas:

where D₀ is the pre-exponential constant, E_a is the energy of activation, ΔS^# is the entropy and D is the distance between the active site of the adsorbent and is taken as 5 Å. k_B, h and R are the Boltzmann, Planck’s and gas constant, respectively.

The mass transfer studies are according to the model (eqn (15)) suggested by McKay and co-workers,³⁹ which is:

where C₀ (mg L⁻¹) and C_t (mg L⁻¹) are the concentration of the safranin-O dye at initial time and time t, respectively. K (L g⁻¹) is constant. k_f (cm s⁻¹) is the mass transfer coefficient. m (g L⁻¹) and S_s (cm⁻¹) respectively denote the mass of the adsorbent and the outer surface area of the adsorbent particles.

The value of S_s is calculated by the following equation:

where d_p (cm) is the particle diameter, ρ_p (g L⁻¹) is the density of SDS/RM and ε_p is the porosity of the adsorbent. The k_f value is determined from the slope and intercept of the plot ln[(C_t/C₀) − 1/(1 + mK)] versus t, which proves the sanctity of the above model.

A further model developed by the Waber–Mathews equation is used⁴⁰ to determine k_f by the following equation:

where C₀ (mg L⁻¹) is the initial concentration of adsorbate, C_t (mg L⁻¹) is the concentration of the adsorbate at time t and S_s is the specific particle surface area of the adsorbent. From the Weber model k_f values were calculated from the slope of the straight line obtained from the plot of C_t/C₀ versus t.

2.8. Adsorption isotherm models

In this study, a number of isotherm models namely, Langmuir, Freundlich, Dubinin–Radushkevich, Temkin, Elovich and Harkin–Jura were used to fit to the experimental data for the adsorption of safranin-O at different temperatures, where q_e refers to the amount of safranin-O adsorbed (mg g⁻¹) at equilibrium, and C_e is the equilibrium concentration of safranin-O (mg L⁻¹) for every isotherm.

The Langmuir isotherm assumes that the uptake of metal ions occurs on a homogeneous surface by monolayer adsorption without any interaction between the adsorbed ions and is given by the equation:

where the constant q_m and b were calculated from the plot of C_e/q_e versus C_e. The values of b decreased with an increase in temperature, which accounts for the exothermic nature of the adsorption process. A dimensionless parameter R_L is calculated by the following equation:

According to the Langmuir theory, R_L values indicate the isotherm type. When R_L > 1, the isotherm is unfavourable, when R_L = 1 the isotherm is linear, when 0 < R_L < 1 the isotherm is favourable, and it is irreversible when R_L = 0.

The Freundlich isotherm expresses multilayer adsorption on a heterogeneous surface, accompanied by the interaction between adsorbed molecules:

The Freundlich constants K_f and 1/n indicate the adsorption capacity and the adsorption intensity respectively and are calculated from the plot of log q_e versus log C_e.

The Dubinin–Radushkevich (D–R) isotherm model is:

ln q_e = ln q_m − Kε² (21)

where ε is the Polanyi potential. The values of constants q_m and K were calculated from the plot of ln q_e versus ε².

The Temkin isotherm model is:

q_e = B₁ ln K_T + B₁ ln C_e (23)

The Temkin constant K_T and B₁ (RT/b) were determined from the plot of q_e versus ln C_e.

The Elovich model is:

where K_E is the Elovich equilibrium constant. The slope and the intercept of the plot of ln (q_e/C_e) versus q_e give the value of the isotherm parameter.

The Harkin–Jura⁴¹ adsorption isotherm model is:

where A and B are constants and calculated from the linear plot of 1/q_e² versus log C_e.

3. Results and discussion

3.1. Physicochemical characterization of the SDS/RM adsorbent

The BET-N₂ surface area of the red mud and SDS/RM were found to be 33.5 and 67.10 m² g⁻¹, respectively. The surface area of the SDS/RM increased significantly, which is helpful for the adsorption of safranin-O dye from aqueous solution. Fig. 1 shows the XRD patterns of SDS/RM and safranin loaded SDS/RM. From the XRD data, the following important mineral phases were identified: hematite (α-Fe₂O₃), goethite (α-FeO(OH)), gibbsite (γ-Al(OH)₃), rutile/anatase (TiO₂) and quartz (SiO₂). From the XRD figure, some new peaks appear and some parent peaks disappear after the adsorption of safranin-O dye, also some peaks of SDS/RM before adsorption are shifted from their original position after adsorption which suggested that the safranin-O dye was loaded onto the SDS/RM adsorbent surface.

Fig. 1 XRD of SDS/RM and safranin loaded SDS/RM showing TiO₂, FeO(OH), ♥ SiO₂, ◆ Fe₂O₃, ● Al(OH)₃.

The SEM micrographs of SDS/RM and safranin loaded SDS/RM are shown in Fig. 2. From the SEM images, significant differences are observed between the SDS/RM and safranin loaded SDS/RM. This may be because after chemical treatment the surface of the SDS/RM becomes coarser. These properties should facilitate the adsorption of safranin-O to the surface of SDS/RM, implying high adsorption and rate.

Fig. 2 Scanning electron micrograph image of (a) SDS/RM and (b) safranin loaded SDS/RM.

FT-IR spectra of the SDS/RM, safranin-O and safranin-O loaded SDS/RM are presented in Fig. 3. A broad adsorption at 3338 cm⁻¹ in safranin-O and safranin-O loaded SDS/RM shows the presence of the –OH and –NH₂ groups, respectively. The peak at 3183 cm⁻¹ is due to the N–H group. The presence of the main peaks of safranin-O on safranin-O loaded SDS/RM in the IR spectra indicates that the safranin-O is unchanged on the surface of the adsorbent.

Fig. 3 FT-IR spectra of SDS/RM, safranin-O and safranin-O loaded SDS/RM.

3.2. Effect of pH

The pH is one of the most significant environmental factors influencing the chemistry of safranin-O adsorption. The effect of pH on the adsorption of safranin-O onto SDS/RM was studied at a pH range of 2.0–12.0 with an initial dye solution of 50 mg L⁻¹. The obtained results are shown in Fig. 4. It can be found that the adsorption decreased with an increase in pH. In the pH range from 2 to 4 the uptake capacity increases smoothly, then decreases sharply with an increase in pH values from 4.0 to 12.0.

Fig. 4 Effect of the initial pH on the adsorption of safranin by SDS/RM (initial dye concentration = 50 mg L⁻¹, weight of the adsorbent = 0.25 g, contact time = 45 min, and temperature = 303 K).

The variation in uptake capacity of the safranin-O dye with pH depends on the surface charge of the adsorbent. At acidic pH the surfactant molecule adsorbed on the surface of the red mud by hydrophobic bonding and created surface aggregates (hemimicelles, mixed hemimicelles or admicelles) on the hydrophilic adsorbent surface. Thus the surface charge of the adsorbent (SDS/RM) becomes negative due to the –SO₃⁻ anion of the surfactant. In this condition, the cationic dye can be adsorbed onto the surface of the SDS/RM adsorbent via electrostatic interactions. On the other hand, at alkaline pH, the surface of red mud is negatively charged and the surfactant is negative too. So, the density of –SO₃⁻ groups on the outer surface of the adsorbent decreases and consequently the adsorption capacity decreases.³⁰ The maximum adsorption capacity of safranin-O takes place at around acidic pH 4, thus pH 4 was selected for all further adsorption experiments. The mechanism of the adsorption capacity at different pH is illustrated in Fig. 5.

Fig. 5 Proposed mechanism for the safranin-O adsorption onto SDS/RM.

3.3. Effect of contact time

Fig. 6a shows the effect of contact time on the adsorption of safranin-O onto SDS/RM with different initial concentrations (20–50 mg L⁻¹). The adsorption increased initially and attained equilibrium at 45 min. The equilibrium adsorption of safranin-O was found to rise from 4.83 to 8.69 mg g⁻¹ by changing the initial concentration from 20 to 50 mg L⁻¹ at 45 min. It is clear that, at a higher initial concentration, the adsorption of safranin-O increases, which can be attributed to an enhance driving force¹⁷ or may be due to the availability of the uncovered surface area and the active sites of SDS/RM.

Fig. 6 (a) Effects of contact time for safranin adsorption onto SDS/RM at different concentrations, (b) pseudo-first order, (c) pseudo-second order, (d) Elovich, (e) Bangham’s, and (f) the intraparticle diffusion model for the adsorption of safranin onto SDS/RM.

3.4. Adsorption kinetics

The different kinetics models are shown in Fig. 6, and the values of the kinetics parameters were obtained from the plot represented in Table 3. From Table 3 it is clear that the pseudo second order model was best fitted with the correlation coefficient R² > 0.97, signifying the dominating role of chemisorption, and meaning that the overall rate of the safranin-O adsorption process seems to be controlled by the chemical process via electrostatic attraction.²⁰ Satisfactory linear curves were not obtained for Bangham’s equation and the intraparticle diffusion model which indicates that the diffusion of the adsorbate into the pores of the adsorbent was not the only rate controlling parameter.

Table 3 Kinetics parameters for the adsorption of safranin-O onto SDS/RM at different initial concentrations

Kinetics model	Parameters	Initial concentration (mg L^−1)
		20	30	40	50
Pseudo-first-order	qe (mg g^−1)	8.8614	10.5291	12.1916	13.9123
	k1	0.0071	0.0068	0.0065	0.0062
	R^2	0.5522	0.5419	0.5330	0.5771
Pseudo-second-order	qe (mg g^−1)	6.9836	8.0358	9.0197	10.0594
	k2	0.0107	0.0099	0.0095	0.0088
	R^2	0.9739	0.9775	0.9798	0.9866
Elovich model	α (mg g^−1 min^−1)	1.0057	1.2926	1.6201	1.9901
	β (g mg^−1)	0.6334	0.5651	0.5156	0.4761
	R^2	0.8080	0.7977	0.7891	0.8287
Bangham’s	k0 (g)	0.5685	0.6410	0.7048	0.7724
	α	0.1529	0.1372	0.1248	0.1118
	R^2	0.7747	0.7648	0.7569	0.7949
Intraparticle diffusion	kip	0.4661	0.5214	0.5706	0.6232
	C	2.0358	2.5302	3.0402	3.5218
	R^2	0.6954	0.6841	0.6744	0.7205

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3.5. Mass transfer study

Different plots of mass transfer studies are shown in Fig. 7 and the values of D_i, D₀, E_a and ΔS^# are given in Table 4. A perusal of Table 4 indicates that the diffusion coefficient decreases with an increase in temperature because with increasing temperature the adsorption of the SDS surfactant onto the red mud surface decreases. The negative values of ΔS^# obtained by the system reveal that the internal structure of the red mud does not go through any significant internal change during the adsorption of safranin-O dye. The mass transfer coefficient (k_f) values indicate that the transition took off significantly and conspicuously from a bulk liquid phase to a solid phase to trap safranin-O onto SDS/RM.³⁸ The k_f values of the McKay et al. equation and the Waber–Mathews equation are found to be 3.49 × 10⁻⁴, 4.61 × 10⁻⁴ and 2.13 × 10⁻⁴, 3.11 × 10⁻⁴ at temperatures of 308 and 328 K respectively. It could be understood from the k_f value that SDS/RM adsorbed safranin-O faster at a lower temperature for both the McKay et al. equation and the Waber–Mathews equation.

Fig. 7 (a) Boyd plot, (b) ln D_i vs. 1/T plot, (c) McKay plot and (d) Waber–Mathews plot for the adsorption of safranin onto SDS/RM.

Table 4 Values of D_i, D₀, E_a, and ΔS^# for safranin-O at different temperatures

Dye	Di × 10^−7 (m^2 S^−1)			D0 × 10^−8 (m^2 S^−1)	−Ea (kJ mol^−1)	−ΔS^# (J K^−1 mol^−1)
	308 K	318 K	328 K
Safranin-O	3.95	3.74	3.55	6.4	4.81	73.64

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3.6. Effect of the initial safranin-O concentration

The effect of the initial safranin-O concentration on the adsorption is studied in the range of 10–50 mg L⁻¹ at the temperatures of 308, 318 and 328 K by keeping the other parameters (pH 4, adsorbent dose 0.25 g, time 45 min) constant. The effect of safranin-O concentration on adsorption is shown in Fig. 8a. The uptake capacity of SDS/RM increased initially and reached a constant after a certain concentration. This may be due to the lack of an active site for the adsorption of a high initial concentration of safranin-O dye.

Fig. 8 (a) Effect of the initial concentration for safranin adsorption on SDS/RM at different temperatures, (b) Langmuir, (c) dimensionless parameter, (d) Freundlich, (e) Dubinin–Radushkevich, (f) Temkin, (g) Elovich, and the (h) Harkin–Jura isotherm model for the adsorption of safranin on SDS/RM.

3.7. Adsorption isotherms

The isotherms are shown graphically in Fig. 8 and the isotherm parameters are listed in Table 5. From Table 5, the value of R_L is in between 0 and 1, which confirmed the favourable uptake of safranin-O. The calculated R_L values at different initial safranin-O concentrations and different temperatures are shown in Fig. 8c. From the Freundlich isotherm, the values of 1/n less than 1 represent a favourable adsorption.⁴¹ With the increase in temperature, the Freundlich constant decrease shows the adsorption process is favourable at low temperature and exothermic in nature.³⁰ The smaller value of the Temkin constant (B₁) suggested that adsorption of safranin-O on SDS/RM is favourable. The comparison of coefficients indicates that the Langmuir isotherm fit more precisely (R² = 0.99) than the Freundlich isotherm (R² = 0.92), Dubinin–Radushkevich isotherm (R² = 0.94), Temkin isotherm (R² = 0.93), Elovich isotherm (R² = 0.93) and Harkin–Jura isotherm (R² = 0.90). The basic assumption of the Langmuir adsorption isotherm is based on the monolayer coverage of the adsorbate on the surface of adsorbent, which is an indication of the fact that the adsorption of safranin-O onto SDS/RM generates a monolayer formation.³⁰

Table 5 Isotherm parameters for the adsorption of safranin-O onto SDS/RM at different temperatures

Isotherm model	Parameters	Temperature (K)
		308	318	328
Langmuir	qm (mg g^−1)	89.471	84.193	79.423
	b (L mg^−1)	1.0265	0.3859	0.2585
	RL	0.0891	0.2057	0.2789
	R^2	0.9999	0.9980	0.9973
Freundlich	Kf	58.9604	37.7345	20.0975
	1/n	0.1237	0.1422	0.1948
	R^2	0.9258	0.9491	0.8916
Dubinin–Radushkevich	qm (mg g^−1)	80.927	74.890	69.254
	β (mol^2 K J^−2)	0.1207	0.7795	1.7856
	E	2.0352	0.8008	0.5291
	R^2	0.9448	0.9928	0.9943
Temkin	KT (L mg^−1)	417.9285	106.8732	13.6380
	B1	0.9423	0.9682	1.1813
	R^2	0.9380	0.9604	0.9086
Elovich	qm (mg g^−1)	17.397	27.571	37.230
	KE	37.6249	2.2383	0.5532
	R^2	0.9313	0.7680	0.7017
Harkin–Jura	A	100.0207	69.5651	40.0643
	B	2.6810	2.5776	2.2395
	R^2	0.9009	0.9245	0.8578

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3.8. Desorption studies

Desorption experiments were carried out by a batch method using different solvents such as water, sulphuric acid (0.1 M), hydrochloric acid (0.1 M) and acetic acid (0.1 M). The comparison of desorption efficiency is shown in Fig. 9. The desorption by distilled water is only 6% indicating that physical adsorption is not the dominant factor while acetic acid confirmed thin desorption efficiency (64%) showing the presence of a chemical-complexation type dye–adsorbent interaction route. On the other hand, HCl and H₂SO₄ show maximum desorption efficiencies of 93 and 71% respectively, because, the anionic surface, i.e. sodium dodecyl sulphate present in the red mud helps to absorb H⁺ of acids but not from the dye molecules. However, the crest desorption by hydrochloric acid approved the chemisorption of dyes onto SDS/RM.

Fig. 9 Desorption of safranin dye using various desorbing agents.

4. Conclusions

Red mud modified with sodium dodecyl sulphate (SDS) is an excellent adsorbent for the treatment of aqueous solutions of safranin-O dye. The solution pH was a critical parameter for the removal of safranin-O dye. Generally, adsorption capacity of safranin-O by modified red mud decreases with an increase in the solution’s initial pH. The maximum adsorption of safranin-O occurs at pH 4 with an equilibrium time of 45 minutes. The pseudo second order kinetics model was found to be best fitted to the experimental data. Equilibrium data were found to follow the Langmuir isotherm model with a maximum adsorption capacity of 89.471 mg g⁻¹. The adsorption of safranin-O onto SDS/RM was determined to be faster at lower temperature, and the process was regulated by mass transfer. The obtained results represent a fundamental study of the adsorption of safranin-O dye from aqueous solutions by SDS modified red mud. The study shows that the investigated red mud, as an economical and efficient adsorbent, has potential for application in the treatment of dye-contaminated wastewaters. Further investigations will be directed toward applications to real samples and the enhancement of the adsorption properties of the studied red mud.

Acknowledgements

The authors are thankful to Vedanta Pvt. Ltd, Lanjigarh, India, for funding the research project. The authors are also grateful to Prof S. K. Sarangi, Director National Institute of Technology, Rourkela for providing the research facilities.

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