Removal of Methyl Orange from Aqueous Solution by Adsorption onto a Hydrogel Composite

Abstract

Poly(acrylic acid-co-2-acrylamide-2-methyl-1-propanesulfonic acid)/kaolin hydrogel composite was synthesised using acrylic acid (AA), 2-acrylamide-2-methyl-1-propanesulfonic acid (AMPS) and kaolin (KL) as main materials. The composite was characterised and used to remove methyl orange (MO) from aqueous solutions. Effect of adsorption conditions, including initial concentration of MO, contact time, pH values and ionic strength, on the adsorption capacities was studied. Maximal adsorption capacity was 506 mg/g as the initial concentration of MO was 1000 mg/L. It showed that the adsorption process was spontaneous, and the isotherms and kinetics were in good agreement with the Freundlich isotherm model and pseudo-second-order equation, respectively.

1. Introduction

The dyeing and printing industry is of great importance in the development of national economy. However, large amounts of dye wastewater with high levels of chroma (500∼5000000) and toxicity are produced with the production and usage of dyes. Various methods have been tried to remove dyes from wastewater, including physical-chemical methods, biological methods, adsorption and electrochemical methods, etc. ¹ Among them, adsorption seems to be superior to others in terms of its low investment, ease of operation, and renewable capability. ² I'nal et al ³ have synthesised novel blend hydrogel beads to remove anionic textile dyes such as reactive red-120 (RR), cibacron brilliant red 3B-A (CBR) and remazol brilliant blue R (RBB) through adsorption. The maximum adsorption capacities are 116.8, 73.3 and 55.3 mg/g for RR, CBR and RBB, respectively. The adsorption is well described by pseudo-second-order kinetics and Langmuir isotherm. Chen et al ⁴ have studied the adsorption behaviour of both cationic and anionic dyes using a synthetic polymer. The highest adsorption capacities for victoria blue, crystal violet, neutral red and methylene blue are 5.821, 6.825, 6.135 and 5.381 mmol/g, respectively. Adsorption kinetics and isotherms analysis suggest that all the adsorption processes are exothermic and spontaneous.

Methyl orange (MO) is a kind of the p-aminoazobenzene (p-AAB) dyes. It is widely used in textile dyeing industry and chemical experiments as an acid-base indicator. However, the aqueous solution of MO is poisonous and irritating. Thus, removal of MO from aqueous solution is of great significance.

Kaolin (KL), whose chemical formula is Al₄[Si₄O₁₀](OH)₈, is one of the clay minerals with hexagonal flaky structures. Due to the asymmetry between the silicon oxygen tetrahedron and aluminium oxygen octahedrons, the interlayer adhesion of kaolin is extremely strong. Besides, owing to the numerous hydroxyl groups on its surface, kaolin is highly hydrophilic. As a result, kaolin is considered to be an alternative mineral resource with good stability and swelling abilities. ⁵ However, kaolin itself is impractical as an adsorbent due to its poor mechanical properties. By being incorporated into polymers or copolymers, the mechanical behaviour, thermal stability as well as the adsorption performance of the product can be improved greatly. Thus, great attention has been paid to the preparation and application of kaolin/polymer composites. ⁶ Chen et al ⁷ have prepared a novel urea-formaldehyde resin/ reactive kaolin composite by in situ polymerisation. It showed that the thermal stability was much better than that of pure urea-formaldehyde resin. Zhu et al ⁸ used kaolin/sodium alginate-grafted poly(acrylic acid-co-2-acrylamide-2-methyl -1-propanesulfonic acid) hydrogel composite as an adsorbent to remove lead, cadmium and zinc ions in aqueous solutions. Maximal adsorption capacities for Pb²⁺, Cd²⁺ and Zn²⁺ were 834.7, 69.9 and 139.8 mg/g.

In this paper, poly(acrylic acid-co-2-acrylamide-2-methyl-1-propanesulfonic acid)/kaolin (P(AA-co-AMPS)/KL) hydrogel composite was synthesised and characterised. The composite was used to remove MO from aqueous solutions by means of adsorption. Effect of adsorption conditions, including initial concentration of MO, contact time, pH values and ionic strength, on the adsorption capacities was studied in detail. The adsorption isotherms and kinetics were also discussed.

2. Experimental

2.1 Materials

Acrylic acid (AA, chemically pure, Sinopharm Chemical Reagent Co., Ltd, China), 2-acrylamide-2-methyl-1-propanesulfonic acid (AMPS, analytical reagent, Shandong Lianmeng Chemical Group Co., Ltd, China), Kaolin (KL, chemically pure, Shantou Xilong Chemical Co., Ltd, Guangdong, China). Ammonium persulfate (APS, analytical reagent, Tianjin Guangcheng Chemical Reagent Co., Ltd, Tianjin, China), N, N'-methylenebisacrylamide (MBA, analytical reagent, Chengdu kelong Chemical Reagent Factory, Sichuan, China), Methyl orange (MO, chemically pure, Shanghai Reagent Factory, China). The rest of the reagents were all analytical reagents.

2.2 Characterisation

IR spectra were recorded on a Nicolet 380 Fourier Transform Infrared Spectrometer using KBr pellets. XRD analysis was performed on an Ultima IV X-ray diffractometer (Rigaku Corporation). Morphology photos were obtained on a KYKY-2800B scanning electron microscope instrument with an accelerating voltage of 20 kV. TGA was carried out on a TGA/SDTA851^e thermogravimetric analyser using nitrogen purge at a heating rate of 10°C/min. Absorbance of the samples was obtained from a WFG-7200 spectrophotometer (Shanghai Instruments Co., Ltd, China) at a wavelength of 560 nm using deionised water as a reference.

2.3 Synthesis of P(AA-co-AMPS)/KL Hydrogel Composite

7.2 g of AA was added in a beaker with 20 mL distilled water. NaOH solution (25 wt%, 12.8 g) was dropped into the beaker to neutralise AA to a degree of 80%. AMPS (1.44 g, 0.007 mol) was added to the neutralised AA solution. After stirring for 10 min the mixture was poured into a four-necked flask, which was equipped with a stirrer, a condenser, a nitrogen line and a thermometer. 1.57 g KL powder was dispersed into the mixture. After stirring for 20 min in a water bath at 40°C, MBA solution (0.005 g/mL, 2 mL) was added dropwise under nitrogen atmosphere. The mixture was slowly heated to 75°C and APS solution (0.1 g/mL, 2 mL) was dropped into it. The above mixture was allowed to react for 3 h at 75°C. The resulting gel was cut into small pieces and dried in an oven till constant weight.

2.4 Adsorption Experiments

A certain amount of the dry P(AA-co-AMPS)/KL hydrogel composite was immersed in a series of MO solution with different initial concentrations at room temperature. After adsorption for a certain period of time, the swollen samples were filtered and the filtrates were obtained for further determination of absorbency. Concentrations of MO in the filtrate were determined from the standard curve of MO. The adsorption capacity of MO on P(AA-co-AMPS)/KL hydrogel composite was calculated on the basis of Eq. (1): ⁹

$$q=\frac{c_0{V}_0-c_t{V}_t}{m}$$

(1)

where c₀ and c _t are the initial and equilibrium concentration of MO (mg/mL), respectively. V₀ and V _t are the volumes of MO solution before and after adsorption (L), respectively. m is the weight of the dry adsorbent particles(g), and q is the adsorption capacity (mg/g).

3. Results and Discussion

3.1 IR Spectra

IR spectra of kaolin, P(AA-co-AMPS) copolymer and P(AA-co-AMPS)/KL hydrogel composite are presented in Figure 1 . In Figure 1(A) , the peaks at 3695 and 3621 cm⁻¹ correspond to the stretching vibration of hydroxyl groups. Those at 1032 and 914 cm⁻¹ are attributed to the stretching vibration Si–O bond. The peaks at 794 and 694 cm⁻¹ are the stretching and bending vibrations of Si–O–Al, respectively. The bending vibration peak of Si–O–Si is at 471 cm⁻¹. In Figure 1(B) , the strong and broad absorption peak at 3438 cm⁻¹ corresponds to the stretching vibration of carbonyl groups. The peak at 1643 cm⁻¹ is the stretching vibration of C=O, and those at 1565 and 1409 cm⁻¹ are the anti-symmetric and symmetry stretching vibrations of RCOO^- in PAA, respectively. Those at 1116 and 1046 cm⁻¹ are ascribed to the anti-symmetric and symmetrical stretching vibration of S=O, respectively. The characteristic peak of -CH₂-produced by the copolymerisation of AA and AMPS appears at 2927 cm⁻¹, indicating the production of P(AA-co-AMPS) copolymer. In Figure 1(C) , it can be seen the peaks of such groups as -OH (strong and broad absorption peak at 3438 cm⁻¹), -CH₂- (2940 cm⁻¹), C=O (the strong peak at 1643 cm⁻¹), RCOO^-(1565 and 1409 cm⁻¹, ascribed to the anti-symmetric and symmetry stretching vibration of RCOO^-, respectively) and Si–O–Si in kaolin (471 cm⁻¹). However, the characteristic peak of C=C is absent, which proves the formation of P(AA-co-AMPS)/KL hydrogel composite.

Figure 1

IR spectra of kaolin (A), P(AA-co-AMPS) copolymer (B) and P(AA-co-AMPS)/KL hydrogel composite (C)

3.2 XRD Patterns

XRD patterns for KL (A) and P(AA-co-AMPS)/KL hydrogel composite (B) are shown in Figure 2 . It can be seen from Figure 3 (A) that KL exhibits two strong characteristic peaks at 2θ=12.34° and 2θ=24.95°. However, in the case of P(AA-co-AMPS)/KL hydrogel composite ( Figure 3 (B) ), despite the existence of characteristic peaks of KL, the strength of the peaks is reduced greatly, which indicates that KL has been dispersed in the poly(AA-co-AMPS) copolymer evenly during the process of intercalation copolymerisation. ¹⁰

Figure 2

XRD patterns for KL (A) and P(AA-co-AMPS)/ KL (B)

Figure 3

SEM images for P(AA-co-AMPS)/KL hydrogel composite (A) and P(AA-co-AMPS) copolymer (B)

3.3 SEM Images

The morphologies of poly(AA-co-AMPS) copolymer and P(AA-co-AMPS)/KL hydrogel composite are displayed in Figure 3 . It can be seen that the surface P(AA-co-AMPS) copolymer is compact and smooth, while that of P(AA-co-AMPS)/KL is coarse and poriferous. As a result, it is more favourable for the composite to adsorb dye molecules.

3.4 Thermogravimetric Analysis

TGA curves of P(AA-co-AMPS)/KL hydrogel composite (A) and P(AA-co-AMPS) copolymer (B) are shown in Figure 4 . It can be seen that both samples have three degradation stages. The first stage starts at 180°C, attributed to the loss of free moistures. The second stage is in the range of 230–590°C, mainly due to the degradation of the polymeric main chains. In this stage the decomposition temperature of P(AA-co-AMPS) copolymer is 315°C (weight loss 14.8%), while that of P(AA-co-AMPS)/KL is 348°C (weight loss 13.1%). The third stage is over 600°C, mainly due to the removal of SO₂ from the side chains of the polymer. Thus, it can be concluded that the thermal stability of P(AA-co-AMPS)/KL is better than that of the P(AA-co-AMPS) copolymer.

Figure 4

TGA curves of P(AA-co-AMPS)/KL hydrogel composite (A) and P(AA-co-AMPS) copolymer (B)

3.5 Effect of Initial Concentration of MO on Adsorption

A series of methyl orange solutions with same volume (50 mL) but different initial concentrations are added into the corresponding 100 mL beakers. After the addition of 50 mg dried hydrogel composite to each beaker, respectively, the composites are allowed to adsorb MO for 24 h. The adsorbing capacities are calculated. Effects of initial concentrations of MO on the adsorption capacity are shown in Figure 5 .

Figure 5

Effect of initial concentration of MO on the adsorption capacity (composite 50 mg, initial pH 7, contact time 24 h, temperature 15°C.)

It shows that the adsorption capacity is improved with the increase of the initial concentrations of MO. Maximal adsorption capacity is 506 mg/g as the initial concentration of MO reaches 1800 mg/L. Despite the interactions such as ion-exchange, electrostatic attraction and hydrogen bonds between the hydrogel composite and dye molecules,^11,12 the good swelling performances of the hydrogel composite play an important role in the adsorption of dyes. It is known that dye molecules will distribute in the solution phase, hydrogel phase and the aqueous phase which is formed by the swelling of the composite. Thus, as the initial concentration of MO increases, the concentration of MO in both the hydrogel phase and the aqueous phase increases, and the adsorption capacities go up accordingly.

3.6 Effect of Contact Time on Adsorption

The effect of contact time on the adsorption capacity is shown in Figure 6 . It can be seen that with the extension of contact time the adsorption capacities increase sharply during the first 7 h, and then become almost constant. Maximum adsorption capacities are 180 and 327 mg/g after 24 h as the initial concentration of MO is 500 and 1000 mg/L, respectively. At the beginning of the adsorption process the volume together with the internal surface area of the hydrogel composite continue expanding, which makes it easier for MO molecules to squeeze into the inside of the hydrogel composite. ¹³ As the adsorption reaches a certain degree it is more and more difficult for MO molecules to go inside due to the increase of the electrostatic repulsion and steric hindrance.

Figure 6

Effect of contact time on adsorption capacity (composite 50 mg, initial pH 7, temperature 15°C)

3.7 Effect of pH Values on Adsorption

The initial pH value of the solution plays an important role during adsorption because it influences the surface charges of both adsorbent and dye molecules. A series of HCl and NaOH solutions whose pH values range from 0.1 to 13.8 are used to adjust pH values, regardless of the volume changes of the solution. he effect of pH values on adsorption is shown in Figure 7 . It can be seen that adsorption is efficient over a wide range of pH values. With the increase of pH, the adsorption capacity increases first (pH=2-7), and then decreases slowly (pH=7-11). However, further increase of pH (pH>11) leads to a sharp decrease of the adsorption capacity. It is known that in an acidic medium methyl orange exists in its cationic form (see Scheme 1), the number of negatively charged groups such as -COO^-, -SO₃^- and -OH^- on P(AA-co-AMPS)/KL hydrogel composite are rather limited. With the increase of pH value, the number of negatively charged groups on P(AA-co-AMPS)/KL rises accordingly. However, the positive charges on MO become less and less when MO changes into its basic form gradually. Therefore, the electrostatic attraction between MO and P(AA-co-AMPS)/KL is poor either in extremely acidic or basic media. Proper pH value for adsorption of MO onto P(AA-co-AMPS)/KL hydrogel composite is neutral. ¹⁴

Figure 7

Effect of initial pH on adsorption capacity (composite 50 mg, contact time 24 h, temperature 15°C)

Scheme 1.

The molecular structure of methyl orange

3.8 Effect of Ionic Strength on Adsorption

The effect of Na⁺ and Zn²⁺ concentration on adsorption is investigated by adding different amount of sodium chloride or zinc nitrate into a series of MO solutions, and the result is shown in Figure 8 .

Figure 8

Effect of ionic strength on the adsorption capacity (composite 50 mg, initial concentration of MO 1000 mg/L, contact time 24 h, 15°C)

It is clear that both metal ions cause inhibition of adsorption. The effect of Zn²⁺ is particularly strong. On one hand, high concentration of metal ions will significantly reduce the solubility of MO, which brings about a decrease of adsorption capacities. ¹⁵ On the other hand, unlike Zn²⁺, certain amount of Na⁺ in the solution causes little impact to the swelling properties of the hydrogel composite for sodium polyacrylate is one of the main components of P(AA-co-AMPS)/KL. Thus, it is much more difficult for MO molecules to enter the inner part of the composite in the solution of Zn²⁺ than Na⁺.

3.9 Adsorption Isotherms

Adsorption isotherms are conducive to the study of the interact forces between the adsorbate and the surface of adsorbent. Several equilibrium models are used to study the adsorption behaviour, which are the Langmuir, Freundlich and Redlich-Peterson equations, as shown in Eqs. (2), (3) and (4), respectively.

Langmuir isotherm equation:

$$\frac{C_e}{q_e}=\frac{1}{q_m{K}_l}+\frac{C_e}{q_m}$$

(2)

where C_e is the equilibrium dye concentration (mg/L), q_e is the equilibrium adsorption capacity (mg/g), q_m is the maximum adsorption capacity (mg/g), and K _l is the Langmuir adsorption coefficient (L/mg).

In the Langmuir adsorption theory, the surface of the adsorbent is homogeneous, where each adsorption site has the same adsorptivity. The process is monolayer adsorption, and there is no interaction between adsorbed molecules. ¹⁶

Freundlich isotherm equation:

$$q_e=K_f{C}_e{}^{1/n}$$

(3)

where K_f and n are the Freundlich constants which represent the adsorption capacity and adsorption intensity, respectively. n is the constant which reflects the character of adsorption. If 0.1<1/n<0.5, it is easy to adsorb, otherwise, if 1/n>2, the adsorption is difficult.

The Freundlich isotherm is an empirical equation which can be applied to both monolayer and heterogeneous adsorption, especially under the condition that the concentration of the adsorbate is low. ¹⁷

Redlich-Peterson (R-P) isotherm equation:

$$q_e=K{C}_e/(1+\alpha C_e^{\beta })$$

(4)

where K and α are the Redlich-Peterson constants, and β ranges from 0 to 1.

As the combination of both Langmuir and Freundlich systems, R-P model is always used to describe the adsorption on heterogeneous surfaces. ¹⁸

The data of Figure 5 are fitted according to Eqs. (2), (3) and (4), respectively, and the results are given in Table 1 .

Table 1

The calculated constants and correlation coefficients for adsorption of MO onto P(AA-co-AMPS)/KL hydrogel composite

Model	Constant		R2
Langmuir	qm (mg/g)	990.0990	0.7088
	Kl (L/mg)	4.18×10−4
Freundlich	n (L/mg)	0.7058	0.9736
	Kf (L/mg)1/n	2.1392
Redlich-Peterson	K (L/mg)	1609.5906	0.9584
	(L/mg)	1049.0429
	β	0.2461

It is concluded that the adsorption process is in accordance with the Freundlich model from the values of R ² .

3.10 Thermodynamic Parameter

The equilibrium constant obtained from the Freundlich equation are used to determine the change of the standard Gibbs free energy (ΔG^θ) during adsorption according to Eq. (5):

$$\Delta G=\Delta G^{\theta }+RTInQ$$

(5)

where ΔG and ΔG^qθ are the changes of Gibbs free energy in the standard and non-standard conditions, respectively, and Q is the reaction quotient. As a fact that ΔG is zero, meanwhile Q is equal to the standard equilibrium constant at equilibrium, Eq. (5) is derived as follows:

$$\Delta G^{\theta }=-RTIn{K}^{\theta }$$

(6)

where K^θ is the standard equilibrium constant. By substitution of the following values, K^θ=(K_f)^1/n, R=8.314 (J/mol/K) and T=288.15 (K) into Eq. (6), the value of ΔG^θ is calculated as −2.581 (kJ/mol). Since ΔG^θ is negative, it follows that the adsorption of MO onto P(AA-co-AMPS)/KL hydrogel composite is spontaneous at 15°C.

3.11 Adsorption Kinetics

Further study shows the adsorption kinetics during adsorption. Two models are adopted, which are the pseudo-first-order equation and pseudo-second-order equation,^19,20 as shown in Eqs. (7) and (8), respectively.

Pseudo-first-order equation:

$$\log (q_e-q_t)=\log q_e-k_{1}t/2.303$$

(7)

Pseudo-second-order equation:

$$t/q_t=1/(k_2{q}_e^2)+t/q_e$$

(8)

where q_e is the equilibrium adsorption capacity (mg/g), q_t is the amount of adsorption dye (mg/g) at time t (min), k₁ is the rate constant of pseudo-first-order (min⁻¹) and k₂ is the rate constant of pseudo-second-order (g/mg/min).

The data of Figure 6 (1000 mg/mL) are fitted according to Eqs. (7) and (8), respectively, and the results are given in Table 2 .

Table 2

The calculated kinetic parameters and correlation coefficients for adsorption of MO onto P(AA-co-AMPS)/KL hydrogel composite

Model	Constant		R2
pseudo first-order	k1 (L/min)	0.0017	0.8916
	qe(cal, mg/g)	13.348
pseudo second-order	k2 (mg/gmin)	9.85×10−8	0.9889
	qe(cal, mg/g)	456.621

It is obvious that the adsorption of MO onto P(AA-co-AMPS)/KL conforms to the pseudo-second-order model, which indicates that the rate-determining step may be a chemical process by sharing or exchanging electrons between the adsorbent and adsorbate. ²¹

4. Conclusions

In this study, P(AA-co-AMPS)/KL hydrogel composite was synthesised and applied for the removal of MO from aqueous solutions. It was shown that neutral media were more effective to adsorb MO than acidic and basic media. Metal ions such as Zn²⁺ and Na⁺ restrained the composite from adsorbing MO. Maximal adsorption capacity was 506 mg/g under the conditions that the dry composite was 50 mg, initial concentration of MO was 1000 mg/L, contact time was 24 h and the temperature is 15°C. The adsorption was spontaneous and in good agreement with the Freundlich isotherm equation. A kinetic study showed that the adsorption conformed to the pseudo-second-order model.