Reactive orange 5 removal from aqueous solution using hydroxyl

ammonium ionic liquids/layered double hydroxides intercalation composites

Qingqing Zhou(School of Chemistry and Chemical Engineering, Wuhan Textile University)

Abstract: A series of hydroxyl ammonium ionic liquids/layered double

hydroxides intercalation composites (ILs/LDHs) were synthesized and

adopted to study the adsorption process of anionic dye reactive

orange 5 from aqueous solutions. The ILs/LDHs and LDHs were

characterized by infrared spectroscopy (IR), X-ray diffractometry

(XRD), thermogravimetric analysis (TG), total organic carbon (TOC)

analyzer and BET surface area measurement. The successful

intercalation of the anion of ILs (2-hydroxyethylammonium acetate)

into the interlayer space of LDHs was confirmed. The effects of

contact time, temperature, adsorbent dosage and solution pH on the

adsorption experiments were investigated. The experimental results

showed that the maximum adsorption capability of ILs/LDHs reached

up to 300.9 mg/g, which was obviously higher than that of LDHs. The

adsorption isotherms were well described by Freundlich model in the

presence of the LDHs and ILs/LDHs. The adsorption kinetics followed

the pseudo-second order kinetic model. The negative value of ΔG0 and

the positive value of ΔH0 indicated spontaneous and endothermic

nature of reactive orange 5 adsorption. For ILs/LDHs(b), desorption

percentages were 57.91%, 46.67%, and 37.34% in each cycle,

respectively. This innovative approach, using ILs/LDHs, was more

efficient and could be envisaged as a promising process for reducing

the pollution of the textiles manufacturing.

Keywords：2-Hydroxyethylammonium acetate (ILs); Intercalation;

Layered double hydroxides; Adsorption; Reactive orange 5

0 IntroductionTextile industry is one of the fast growing industries and

significantly contributes to the economic growth in china. However,

the textile dyeing industry consumes large quantities of water and

produces large volumes of wastewater. The major pollutant of textile

wastewater comes from the dyes which are not dyed or washed down

after dyeing[1]. From the point of view of production practice, ten to

twenty percent of dyes are poured into the dyeing-printing

wastewater during the dyeing and printing process[2]. Unfortunately,

on account of the complex and stable chemical structure, most of

these dyes are toxic and poor biodegradation, and dyeing wastewater

discharge is able to reduce aquatic diversity by blocking the passage

of sunlight through the water and affect human health.

At present, many methods, such as coagulation or flocculation,

chemical oxidation, biological treatment, membrane filtration,

photodegradation, adsorption and etc[3–8], have been studied in order

to remove the dyes from the textile wastewater. Among the above

treatment techniques, the adsorption method is regarded as one of

the most economical and effective way to deal with the dyed

wastewater[9,10]. For example, activated carbon has been used as the

adsorbent in some industrial water treatment devices to remove the

dyes from the wastewater. In recent years, a lot of attention has been

focused on preparing the adsorption material with low-cost and high

adsorption capacity, like clay minerals, polymers, nano materials,

silica and so forth[11–14].

Layered double hydroxides (LDHs), known as the anionic clay,

have become an important kind of adsorbents for treating the

wastewater owing to its efficient adsorption and reusability. In brief,

hydrotalcites are referred to as layered double hydroxide with the

general formula [M1−xⅡMxⅢ(OH)2]x+[An－]x/n·mH2O, where MⅡ and MⅢ stand

for a divalent and a trivalent cation[15,16], respectively, An － is the

interlayer anion, such as CO32－, Cl－, NO3－ and etc, which is located in

the interlayer and the lamellar surface. Due to their anionic exchange

capacity, LDHs are suitable for sorption of anionic species[17].

Moreover, the adsorption capacity of LDH is largely influenced by the

anion.

Nowadays, LDHs have been modified in various forms in order

to improve their adsorption capacity[18–20], such as calcination and the

organic modification. Zaghouane-Boudiaf et al.[21] investigated the

adsorption of methyl orange on calcined MgNiAl LDHs and their

precursor and found a much higher adsorption on calcined MgNiAl

LDHs than their precursor without heat treatment. Miranda et al. [22]

used dodecylsulfate (DS) and dodecylbenzenesulfonate (DSB) anionic

surfactants to modify hydrotalcite-iron oxide magnetic so as to

improve the adsorption efficiency of the LDHs, and the removal

percentages of methylene blue (MB) dye had increased by 81% and

73%, respectively. Liu et al.[23] even used LDHs-bacteria aggregates to

enhance the decolourisation of methylene blue. In a word, utilization

of LDHs and derivations could bring great economical and

environmental benefits to dyeing wastewater industries.

In recent years, room temperature ionic liquids (ILs) have

become the research emphasis in consequence of their unique

properties, such as extremely low vapor pressures, fine stability and

outstanding solubility. Accordingly, ILs can certainly be used as dye

extractant to achromatize aqueous solution[24,25]. However, such kind

of green material is difficult to have a role in the dyes wastewater

treatment because of high price, low liquidity and unsatisfactory mass

transfer efficiency. Facing these problems, immobilization of ILs on

solid supports may improve their applicability in industrial processes.

Gao et al.[26] had synthesized a functional ionic liquid cross-linked

polymer to adsorb anionic azo dyes, the experimental results showed

that the performance of it is superior to other adsorbents and

adsorption capacity could reach 547–925 mg/g for different anionic

azo dyes. Ghaedi et al.[27] and Absalan et al.[28] had immobilized ILs on

inorganic materials to improve the adsorption properties of these

materials.

As mentioned above, both LDHs and immobilized ILs are

promising class of sorbents. However, to the best of our knowledge,

the combination of LDHs and ILs has not been reported in literature.

Therefore, the objectives of our work are to prepare a series of

hydroxyl ammonium ionic liquids/layered double hydroxides

composites (ILs/LDHs), which used as adsorbent for the removal of

anionic dye reactive orange 5 from aqueous solution. The effects of

various factors such as solution pH, and adsorbent dosage on

adsorption have been investigated. Kinetic and isotherm models have

also been discussed. The experimental results show that the

adsorption capacity and performance of LDHs have improved

significantly after inserting the ILs in their interlamination. This study

can provide a reference for the dyes wastewater treatment using

inexpensive and easily-obtained the intercalation composites.

1 Experimental

1.1 Materials

Ethanolamine, 2-(2-aminoethoxy) ethanol, triethanolamine, formic

acid, ethanoic acid, lactic acid, ethyl alcohol, aluminum nitrate

nonahydrate, magnesium nitrate hexahydrate, sodium hydroxide and

sodium carbonate were all AR grade and purchased from Sinopharm

Chemical Reagent Co., Ltd. reactive orange 5 was purchased from

Jiangsu Shenxin Dyestuff Chemicals Co., Ltd. All reagents were used

without further purification.

1.2 Sample preparation

Hydroxyl ammonium ionic liquids were synthesized according to

the procedures described in literature[29].

The co-precipitation method was adopted for the preparation of

ILs/LDHs intercalation composites. The reaction was conducted in a

500 mL four neck round bottom flask with a magnetic stirrer, two

dropping funnel and a reflux condenser. 20 mL ILs and 35 mL water

were mixed up uniformly and put into the flask. The mixture was

heated to 90℃ in a thermostat water bath. A solution was prepared

by mixing Mg(NO3)2·6H2O and Al(NO3)3·9H2O (Mg2+/Al3+ molar ratio of 2

and Mg2+ concentration is 0.6 M) in 100 mL of deionized water. This

solution and the aqueous solution of 0.8 M NaOH were simultaneously

added dropwise into a flask under vigorous stirring. Meanwhile, the pH

value of the mixed suspension liquid should be controlled at about 10

during the dropping process. Afterwards, the samples should be

acutely stirred for 3 h at 90℃ and then aged at 100℃ for 24 h. The

precipitate was separated by suction filtration, washed with deionized

water and dried in an oven at 80℃ for 12 h.

1.3 Characterization of the prepared materials

X-ray diffraction (XRD) patterns of the samples were obtained by

Bruker D8 Advance, using filtered CuKα radiation (λ = 0.154 nm). 2θ

angle of the diffractometer was stepped from 3° to 80° at a scan rate

of 10°/min. FT-IR spectra were recorded on a Tensor 27 IR

spectrometer (Bruker, Germany) using KBr disc technique. Thermal

decomposition of LDHs and ILs/LDHs were evaluated by

thermogravimetric analysis (TG) carried out on Diamond TG/DTA

instrument under nitrogen atmosphere at 10℃/min from room

temperature up to 700℃. The proportion of ILs(b) containing in

ILs/LDHs(b) composite was analyzed by total organic carbon (TOC)

analyzer (VarioTOC, Elernentar, Germany) at 950℃. The pore

structures of the LDH and ILs/LDHs were analyzed by N2 adsorption–

desorption at 77 K on an Automatic specific surface and porosity

analysis physical adsorption instrument (ASAP 2020-M, Micromeritics,

USA).

1.4 Adsorption

1.4.1 Effect of initial pH

This effect was studied on suspensions of adsorption material

in 180 mg·L－1 of reactive orange 5 dye solutions (solid/solution ratio =

0.5 g·L － 1). The initial pH (the values varied from 4 to 11) of dye

solutions was adjusted with 0.1 M Na2CO3 and 0.1 M ethanoic acid

solutions (used to adjust pH in dyeing and printing industry). The

suspensions were stirred at 25℃ during equilibrium times and then

centrifuged. The dye equilibrium concentration in the supernatants

was measured by visible spectrophotometer on METASH (V-5600) UV–

vis spectrophotometer at 478 nm.

1.4.2 Effect of LDHs and ILs/LDHs dose

The effect was studied on suspension of LDHs or ILs/LDHs

(solid/solution ratio varied from 0.1 to 0.9 g·L－1) in 60 or 180 mg·L－1 of

reactive orange 5 dye solution at natural pH of the dye (6.48). The

suspensions were stirred during equilibrium times and then

centrifuged. The dye equilibrium concentration in the supernatants

was determined as above.

1.4.3 Kinetic study

Kinetic studies were conducted to find out the equilibrium time

and the kinetic models of reactive orange 5 sorption by LDHs and

ILs/LDHs. In the experiment, the solid (LDHs and ILs/LDHs) /solution

ratio was 0.5 g·L－1, the initial concentration of reactive orange 5 was

180 mg·L－1. Suspensions were stirred for different time interval (5 min

to 8 h) at 25℃ and then centrifuged.

1.4.4 Sorption isotherms

The sorption isotherms were established using LDHs and

ILs/LDHs suspension in reactive orange 5 dye solutions (solid/solution

ratio = 0.5 g·L － 1) at different concentrations (80–250 mg·L － 1). The

suspensions were stirred in a thermostatic reciprocating shaker bath

at 25℃ during equilibrium times and then centrifuged. The dye

equilibrium concentration in the supernatants was determined as

above.

1.4.5 Adsorption thermodynamics

In order to evaluate the effect of temperature on the

adsorption process, the experiments were studied on suspensions of

LDHs and ILs/LDHs (solid/solution ratio = 0.5 g·L － 1) in 50 and 200

mg·L － 1 of Reactive orange 5 dye solution, respectively. The

suspensions were stirred in thermostatic reciprocating shaker bath at

30, 45 and 60℃ during equilibrium time and then centrifuged. The

dye equilibrium concentration in the supernatants was determined as

above.

The adsorption capacity qe (mg of reactive orange 5 per g of

sorbent) was calculated using the following equation:

(1)

where C0 is the initial dye concentration (mg·L － 1) in the

solution; Ce is the dye concentration (mg·L－1) at equilibrium; V is the

initial volume (L) of the dye solution and m is the mass of LDHs or

ILs/LDHs (g).

1.4.6 Desorption study

The process of desorption of ILs/LDHs for the adsorption of

reactive orange 5 was studied for consecutive three cycles. In each

cycle, 0.5 g·L－1 of the adsorbents was added in the 180 mg·L－1 of the

reactive orange 5 solution (pH = 6.0, 25℃) and shaken in the water

bath for 2.5 h. Then, the adsorbent/adsorbed was separated from the

dye solution and added to 15 mL of solution (pH = 11.0, 35℃) and

shaken for 4 h. The adsorbents were separated by centrifugal and the

amount of adsorbed dye was determined through the same method

used in the adsorption experiments. After each cycle of desorption,

the adsorbents were washed with deionized water several times until

the residual water coloration was not significant. The amount of

desorbed dye was calculated from the concentration of desorbed dye

in the liquid phase.

1.4.7 Statistical analysis

All experiments were conducted in triplicate under identical

conditions and statistically analyzed by F test. When Fc > 10Fα(n, n－p

－1), α = 0.005, the results were statistically highly significant.

2 Results and discussion

2.1 Characterizations of ILs/LDHs and LDHs

The XRD patterns of ILs/LDHs and LDHs are shown in Fig. 1. The

XRD patterns of LDHs show sharp and symmetric peaks which give

clear indication the sample is well crystallized, the peaks

corresponding to (003), (006), (009), (015), (018), (110) and (113)

planes are characteristic of hydrotalcite with a layered structure [30].

The peak at about 11° is assigned to the (003) reflections and can be

calculate the basal spacing between the layers. For convenience, the

LDHs inserted with different ILs are designated as ILs/LDHs(a),

ILs/LDHs(b) and ILs/LDHs(c), and so on. The results are shown in

Table1 and Fig. 1. As shown in Fig. 1, the intercalation of ILs

containing formate into the interlayer of the LDHs causes the

disappearance of (006) plane. And using ILs containing lactate as

guest molecules, the broadened peaks for intercalation composites

are mainly owing to its less ordered layer stacking. The crystal

structure of ILs/LDHs(b) is the best among the synthesized

intercalation composites, and the adsorption capability is in

agreement with the crystallinity of intercalation composites, which

maybe due to larger specific surface area. So ILs/LDHs(b) will be

chosen for the further detailed adsorption study.

Table 1 Synthesized ILs intercalated into the interlayer of the LDHs, and the maximum

adsorption amount of reactive orange 5 onto each ILs/LDHs.

Synthesized ILs composites Intercalated qe

2-hydroxyethylammonium formate (ILs(a)) ILs/LDHs(a) -- 165

2-hydroxyethylammonium acetate (ILs(b)) ILs/LDHs(b) √ 320

2-hydroxyethylammonium lactate (ILs(c)) ILs/LDHs(c) -- 54

tri-(2-hydroxyethyl)ammonium formate

(ILs(d))

ILs/LDHs(d) -- 156

tri-(2-hydroxyethyl)ammonium acetate (ILs(e)) ILs/LDHs(e) -- 105

tri-(2-hydroxyethyl)ammonium lactate (ILs(f)) ILs/LDHs(f) √ 280

2-(2-hydroxyethoxy)ammonium formate

(ILs(g))

ILs/LDHs(g) -- 128

2-(2-hydroxyethoxy)ammonium acetate

(ILs(h))

ILs/LDHs(h) √ 233

2-(2-hydroxyethoxy)ammonium lactate (ILs(i)) ILs/LDHs(i) -- 60

Note: -- the anion of ILs did not intercalate into the interlayer of the LDHs or

intercalation composites have less ordered layer stacking. qe (mg•L－1) is the

equilibrium adsorbing capacity of the sorbents, solid/solution ratio = 0.5 g•L－1, 25℃,

dye solution concentration = 180 mg•L－1, natural pH (6.48) of dye solution.

Fig. 1 XRD patterns of the synthesized ILs/LDHs and pure LDHs samples.

The position of diffraction peaks (003), (006) and (009) of

ILs/LDHs(b) moved to small angle integrally corresponding to the

increase of interlayer spacing (Fig. 1). The d003 of pure LDHs, which

corresponds to an adjacent distance of hydroxide layers, is

determined to be 0.7644 nm. The interlayer spacing of ILs/LDHs(b) is

enlarged to 1.229 nm. It can be concluded that the acetate radical of

ILs has successfully intercalated into the gallery of LDHs. And, cation

of ILs is spread outside of laminates of LDHs, which will increase the

adsorption by electrostatic repulsion force between the –SO3− group of

dye molecule and cation of ILs.

From the results of total organic carbon (TOC) analyzer, the

proportion of ILs(b) containing in ILs/LDHs(b) composite was 6.9128

wt%.

To further verify the above conclusion, the prepared samples are

characterized by FTIR. Fig. 2 presents the FTIR spectra of LDHs and

ILs/LDHs(b) intercalation compounds. For LDHs, the broad peak is

found at 3488 cm － 1 ( － OH stretching vibration), caused by the

interlayer water molecules and hydroxyl groups in the brucite-like

layers. The weak band around 1642 cm－1 region(δ-HOH) is due to the

H2O from the interlay water. The 1381 cm－1 peak corresponds to the

carbonate group[31]. Following the ionic exchange of carbonate by

ILs(b), several characteristic bands are observed at 1559 cm －1, 1410

cm － 1 and 1168 cm － 1. The weak absorption peak at 1168 cm － 1 is

assigned to CN stretching vibration. The corresponding bands at 1559

cm－ 1 and 1410 cm－1 are ascribable to antisymmetric and symmetric

vibrations of － COO － groups. In addition, characteristic absorption

peak of the carbonate group can still be observed, which partially

overlaps with －COO－ groups[32]. These data indicate that the acetate

radical of ILs has successfully intercalated into the gallery of LDHs and

replace part of the carbonate group.

Fig. 2 FI-IR patterns of the synthesized ILs/LDHs(b) and pure LDHs samples.

The TG thermograms of ILs/LDHs(b) intercalation compounds

are shown in Fig. 3, with ILs(b) and neat LDHs as the controls. The

sharp weight loss of ILs(b) started from 110℃, and it lost all the

weight at 190℃. LDHs exhibit two-step degradation at 50–210℃ and

250–610℃. A first weight loss of 10.1% is observed from about 50 to

210℃ and is caused by the elimination of adsorbed and interlayer

water[33]. A second weight loss of approximately 37.1% in the

temperature range of 250–610℃, corresponding to the decomposition

of carbonate anion in the brucite-like layers and the deeper

decomposition of brucite layer anions OH － [34]. The ILs/LDHs(b)

intercalation compounds mainly exhibited two weight losses at about

90–321 and 321–520℃. A first step of weight loss is owing to the

degradation of ILs(b) and the elimination of interlayer water.

Compared with pure ILs(b), the degradation of intercalated ILs(b) in

the LDHs gallery is obviously delayed, because of the protection from

the inorganic layers and electrostatic bonding with the layers[35]. The

second step of degradation can be attributed to the decomposition of

residual carbonate anion. Because most part of the carbonate group is

replaced by ILs(b), decomposition temperature of ILs/LDHs(b) is lower

than pure LDHs.

Fig. 3. TG thermograms of ILs(b), LDHs and ILs/LDHs(b)

The N2 adsorption–desorption isotherms for LDHs and

ILs/LDHs(b) are revealed in Fig. 4. LDHs and ILs/LDHs(b) show the

isotherm of type IV according to IUPAC classification, characteristic of

mesoporous solids. H3-type hysteresis loop, is observed for the LDHs

sample, which is characteristic solids with slit-shaped irregular

pores[15]. The isotherm of ILs/LDHs(b) is of type IV with a broad H2-type

hysteresis loop, which also indicated that mesoporous in ILs/LDHs(b)

can be classified as regular and large[36]. The specific surface area of

LDHs and ILs/LDHs(b), as derived from the adsorption data using a

BET equation, are 81.6 and 96.7 m2/g, respectively. It has been found

that many organic compounds have a strong affinity to the surface of

clay minerals. Clearly, a large specific surface area can be an

important factor in the adsorption of colored species by the sorbents.

Fig. 4 N2 adsorption-desorption curves of the synthesized ILs/LDHs(b) and pure LDHs samples.

2.2 Study of reactive orange 5 removal with LDHs and ILs/LDHs(b)

2.2.1 Effect of initial pH

The solution pH is one of the most important parameters in the

adsorption process, which can affect the chemical properties of both

the dye molecule and the adsorbent. The influence of pH values on

the adsorption of reactive orange 5 on the two adsorbents is

investigated and illustrated in Fig. 5. It is obviously that the sorption

capacity of the two adsorbents decreased gradually with the

increasing of solution pH. Reactive orange 5 contains negatively

charged sulfonate group. The adsorption capacity is related to the pH

values of the solution, which may be due to the electrostatic

attraction negatively charged dye molecule and adsorbents. At the

lower solution pH, a large amount of H+ causes hydroxyl groups

became protonated (－OH+2) in the surface of the adsorbents, which

promoted the electrostatic attraction between the surface of the

adsorbents and the － SO3－ group of reactive orange 5. The uptake

decreased with the increase of OH － in solutions, indicating that high

pH is not in favor of the adsorption of reactive orange 5. At the higher

solution pH, the surfaces of adsorbents had negative charges, which

cause the electrostatic repulsion between the negatively charged

surface sites and －SO3－ group of dyes. Meanwhile, the competition

between OH－ excess in the solution and －SO3－ of the dyes was fierce

with the increase of pH.

Fig. 5 Effect of initial pH on reactive orange 5 adsorption on LDHs and bILs/LDHs.

2.2.2 Effect of LDHs and ILs/LDHs(b) dose

The effect of the dosage of LDHs and ILs/LDHs(b) on the

removal of reactive orange 5 is examined by varying dosages from

0.1 to 1.0 g ·L－1. Fig. 6 shows that the percentage removal increased

sharply from 14.6% to 73.0% and 17.6% to 98.6% with increasing

adsorbent dosage from 0.1 to 0.9 g·L － 1 for LDHs and ILs/LDHs(b),

respectively. The equilibriums removal percentage are both obtained

at absorbent dose 1.0 g·L － 1 for LDHs and ILs/LDHs(b). It is also

observed from Fig. 6 that the adsorption capacity decreased with

increase in adsorbent dosage. This can be explained that the low

adsorbent dosage causes the dispersion of sorbent grains in aqueous

solution, all types of sites of the adsorbent surface are entirely

exposed which would facilitate saturated quickly, and a large number

of sites are accessible to the dye molecules. Furthermore, because

the probability of collision between solid particles and particle

aggregations increased, the higher particle concentrations cause an

increase in diffusion path length and a decrease in the total surface

area[21], and overdosing adsorbent dosage will result in high cost.

Furthermore, confirming as expected, the adsorbed amount on

ILs/LDHs(b) is much higher than on LDHs, which can be explained by

the larger specific surface area of ILs/LDHs(b) which could lead the

adsorption more available.

Fig. 6 Effect of adsorbent dose on reactive orange 5 adsorption on LDHs and ILs/LDHs(b).

2.3 Adsorption kinetics

Kinetic modeling of sorption process provides a prediction of

sorption rates and allows determination of suitable rate expression

characteristic for possible sorption mechanisms. In this study, three

kinetic models are used for the analysis of the adsorption kinetic

process: the pseudo-first order model, the pseudo-second order model

and the Weber and Morris intra-particle diffusion model[37].

The pseudo-first order model is expressed by the following

equation:

The pseudo-first order model is expressed by the following

equation: (2)

The pseudo-second order kinetic model can be expressed as:

(3)

The mathematical expression of the intra-particle diffusion model is:

(4)

where qe and qt (mg·L－1) are the adsorption amount of reactive

orange 5 at equilibrium and at the time t, respectively. K1(min－1)

represents the first-order rate constant, K2 (g(mg·min) －1) is the

second-order adsorption rate constant, and Ki (mg·g－1·min－0.5) is the

intra-particle diffusion rate constant, C is the adsorption constant.

These statistical parameters (R2) and the non-linear regression

coefficients were obtained and shown in Table 2. The results are

plotted in Fig. 7.

Table 2 Kinetic parameters for reactive orange 5 adsorption by LDH and ILs/LDHs(b).Models Parameters LDHs bILs/LDHs

qe (exp) (mg·g－1) 56.82 253.01

Pseudo-first order qe (cal) (mg·g－1) 52.88 238.12

K1(min－1) 0.0169 0.1122

R2 0.9782 0.9182

Fc 54.53 66.83

10F0.005 38.8 38.8

Pseudo-second order qe (cal) (mg·g－1) 61.56 249.47

K2(g (mg·min)－1) 3.24×10－4 8.21×10－4

R2 0.9905 0.9897

Fc 311.58 273.51

10F0.005 38.8 38.8

Intra-particle diffusion model

Ki(g (mg·min0.5)-1) 2.43 4.35

C(mg·g-1) 10.16 175.14

R2 0.9142 0.7524

Fc 18.14 15.65

10F0.005 38.8 38.8

Fig. 7 Adsorption kinetics of reactive orange 5 on LDHs and ILs/LDHs(b) fitted with pseudo-first order, pseudo-second order and intra-particle diffusion models.

The adsorption of reactive orange 5 on the LDHs and

ILs/LDHs(b) might be described by intra-particle diffusion model,

which is a process involving migration of dye into layer of LDHs

and ILs/LDHs(b). The values calculated from the intra-particle

diffusion model obtained values of statistical parameters R2=0.91

and R2=0.75 for LDHs and ILs/LDHs(b), respectively, which

suggests that the intra-particle diffusion model does not

appropriately describe the adsorption processes. It means that

intra-particle diffusion has not played the key role in this the

adsorption process.

The values calculated from the pseudo-first order kinetic

model obtained values of statistical parameters R2 = 0.978 and R2

= 0.918 for LDHs and ILs/LDHs(b), respectively. And, relatively

great differences between experimental (qe(exp)) and calculated

(qe(cal)) are observed, especially with ILs/LDHs(b) as adsorbent. It

can be concluded that the pseudo-first order kinetic model does

not appropriately describe the adsorption processes.

The plots of the non-linearized form of the pseudo-second

order model for the adsorption are showed in Fig. 7. The R2 are

much greater in this case, confirming a very good agreement with

experimental data. It is also obviously, for the pseudo-second

order kinetic model, the values of the Fc for both materials are

much higher than others. In addition, the results of qe which

calculated from the pseudo-second order rate model are

consistent with the experimental values. Therefore, we consider

that the pseudo-second order kinetic model is the most suitable

model in describing the adsorption kinetic of reactive orange 5 on

LDHs and ILs/LDHs(b).

2.4 Sorption isotherm

In order to research the characteristics of the adsorption

isotherms, the Langmuir, the Sips and Freundlich isotherm models are

studied to analyze the equilibrium adsorption data.

The non-linearized form of Langmuir isotherm model are

expressed as the following equation:

(5)

where KL(L·mg － 1) is the Langmuir adsorption constant which is

related to the energy of adsorption, Ce (mg·L － 1) is the equilibrium

concentration of the dye, qe and qmax (mg·g－1) are the equilibrium and

maximum adsorption capacity.

The non-linearized form of Freundlich isotherm model can be

described as the following equation:

(6)

where KF is the Freundlich constant related to adsorption

capacity, n is related to the intensity of adsorption.

The Sips isotherm model is obtained by introducing a power

law expression of the Freundlich isotherm into the Langmuir isotherm.

The non-linearized form of Sips isotherm model can be given as

follows:

(7)

where Ks is the Sips isotherm constant representing the energy of adsorption, and m is the empirical constant.

Sips isotherm equation is characterized by the heterogeneity

factor, m. and it can be employed to describe the heterogenous

system. qmax is the maximum adsorption capacity. When m=1, Sips

isotherm equation reduces to the Langmuir equation and it implies a

homogeneous adsorption process.

The adsorption isotherms of reactive orange 5 onto LDHs and

ILs/LDHs(b) are shown in Fig. 8, and the fitted parameters for the

three models are given in Table 3. As can be seen, the values of R2

and Fc for the Freundlich model were much higher than other models,

indicating that the equilibrium data for the adsorption of reactive

orange 5 onto both sorbents can be well described by the Freundlich

isotherm model. This is indicative of the heterogeneity of the

adsorption sites on the LDHs and ILs/LDHs(b). The value of n (>1)

which was calculated from the Freundlich equations indicated a

favorable adsorption process[38,39]. The high KF (20.12,250.73) values

indicate that both adsorbents, LDHs and ILs/LDHs(b), have high

adsorption capacity and affinity by dye molecules.

Fig. 8. Adsorption isotherm of reactive orange 5 on LDHs and ILs/LDHs(b) fitted with Langmuir, Freundlich and Sips models.

Table 3 Isotherm constants for reactive orange 5 adsorption by LDH and ILs/LDHs(b).Models Parameters LDHs bILs/LDHs

Langmuir isotherm model Qmax (mg·g-1) 53.83 300.91

KL (L·mg-1) 0.14 16.16

R2 0.9595 0.9548

Fc 57.97 12.01

10F0.005 70.1 56.6

Freundlich isotherm model KF (mg1-1/n·L1/n·g-1) 20.12 250.73

1/n 0.2060 0.0554

R2 0.9922 0.9955

Fc 278.17 223.57

10F0.005 70.1 56.6

Sips isotherm model Qmax (mg·g-1) 53.09 315.94

Ks((L·mg-1)m) 0.1975 3.6544

m 0.6827 0.5988

R2 0.8304 0.9177

Fc 9.13 12.93

10F0.005 83.8 62.3

A comparison of the dyes removal performance between the

ILs/LDHs(b) in this study and other sorbents reported in literature was

given in Table 4. It is found that the ILs/LDHs(b) in this work has a

relatively higher adsorption capacity, which makes it to be used as

potential efficient adsorbent for the anionic dyes removal from

aqueous solutions.Table 4 A comparison between the performance of the ILs/LDHs composites in this

study and other sorbents reported in literature.Material Adsorbate q (mg·g-1) Contact time

(min)Reference

GO/CS/ETCH Reactive congo red 121.48 600 [8]

Mg-Fe-CO3-LDH Reactive congo red 104.6 30 [18]

Mg/Fe-CLDH Orang G 128.6 540 [42]

Mg/Fe-CLDH Acid brown 14 369 250 [15]

Mg/Al-CLDH Orange acid 10 303 300 [44]

RR-Fe4O3/Mg/Al-LDH Reactive red (RR) 101 30 [45]

ILs/LDH(b) Reactive orange 5 320 150 This study

2.5 Adsorption thermodynamics

The study of the temperature effect on reactive orange 5

adsorption on LDHs and ILs/LDHs(b) enabled us to determine the

thermodynamic parameters (Gibbs free energy ΔG0, enthalpy ΔH0 and

entropy ΔS0), which can be calculated by the following equations[40]:

(8)

(9)

where R is the ideal gas constant, T is the temperature (K), Kd

is the distribution coefficient. The plot of lnKd against 1000/T gives a

straight line for both of sorbents, the slope and the intercept

correspond to ΔHo/R and ΔSo/R, respectively. Values of ΔGo at different

temperatures can be calculated by the following equation: (10)

The results of the thermodynamic parameters are summarized

in Table 5. Generally, the change in adsorption enthalpy for

physisorption is in the range of −20 to 40 kJ/mol, but chemisorption is

between −400 and 80 kJ/mol[43]. The positive ΔH0 values (16.36 and

38.23 kJ/mol) reveal that the adsorptions on LDHs and ILs/LDHs(b) are

endothermic and physical in nature. The positive values of ΔS0, which

indicate an increase of randomness at the interface

adsorbent/adsorbate during adsorption process, suggest that the

removal of dye by LDHs gives a less ordered system than by

ILs/LDHs(b). Furthermore, the negative values of ΔG0 indicate that the

adsorptions onto LDHs and ILs/LDHs(b) is spontaneous.Table 5 Thermodynamic parameters for the adsorption of reactive orange 5 on

LDHs and ILs/LDHs(b).

Sorbents So

(Jmol-1K-1)Ho

(kJmol-1) Go (kJmol-1)

303.15K 318.15K 333.15KLDHs 151.34 38.23 -7.65 -9.92 -12.19

ILs/LDHs(b) 66.87 16.36 -3.96 -4.91 -5.92

2.6 Adsorption mechanisms

To elucidate adsorption mechanisms between reactive orange 5

and ILs/LDHs(b), FT-IR analyses are conducted. The FT-IR spectra (Fig.

9) of ILs/LDHs(b) before and after adsorption of dyes are recorded and

compared each other in the range of 400–2000 cm － 1. As it can be

seen from the spectra of after adsorption, there are new adsorption

peaks, which are due to the functional groups of dyes. the peaks at

1190 cm － 1 and 1047 cm － 1 are mainly attributed to symmetric

vibrations and antisymmetric of sulfate S ＝ O bond, the peak at 782

cm － 1 was due to the stretching vibration of S － O, and the peak at

1139 cm－1 corresponds to stretching vibration of －C－O from phenol.

Through the FT-IR of ILs/LDHs(b) before and after adsorption of dyes,

it is clear that the dye is successfully adsorbed into ILs/LDHs(b).

Moreover, it can be seen that a weakening in the relative intensity of

1559 cm－ 1 (corresponding to the －COO stretching from the interlay

anions) suggesting that part of species of dye replace the interlayer

anions. In other words, the ion exchange is occurred between

interlayer anions and dye species.

Fig.9. FI-IR patterns of the synthesized ILs/LDHs(b) before and after the adsorption of reactive orange 5.

The adsorption behavior is affected by the functional groups of

the adsorbent and adsorbate. Considering the groups of ILs/LDHs(b)

(C＝O,－CN, －OH) and reactive orange 5 ( －SO3－ ,C－O,N－H), the

adsorption of reactive orange 5 onto ILs/LDHs(b) may occur through

electrostatic attraction (O － H+/ － SO3－ ) under acidic conditions and

hydrogen bond (C＝O/N－H, O－H/C－O) interactions. As mentioned,

the adsorption capacity significantly decreased with increasing pH

value, the results indicate that the electrostatic interaction is the main

responsible. In addition, van der Waals interactions may also occur.

The process of dyes adsorption onto ILs/LDHs(b) is the combination of

these factors.

2.7 Desorption analysis

Desorption experiments of reactive orange 5 were performed to

evaluate the recyclable availability. The experimental results show

that the desorption percentages were 57.91%, 46.67%, and 37.34% in

each cycle, respectively (in Fig.10) As it was described before, an

increment in the solution pH favors reducing the interaction with dye

and sorbents, and the behavior of desorption sustains the results

obtained in the previous adsorption at different pH experiment, and it

also could be concluded that the principal interaction force that lead

the adsorption onto ILs/LDHs(b) is the ionic interaction between －SO3

－ of dye and ILs/LDHs(b)[44].

Fig.10. Performance of ILs/LDHs(b) by three cycles of adsorption/desorption reactive orange 5.

3 Conclusions

In this study, 2-hydroxyethylammonium acetate is successfully

intercalated into the hydrotalcite (LDHs) by co-precipitation method,

which is proven by the marked increase in interlayer spacing of host

structure and presentation of the － COO － group characteristic

vibration peaks in the FT-IR spectrum of the intercalation composites

(ILs/LDHs(b)). The BET surface of ILs/LDHs(b) is larger than that of

LDHs. LDHs and ILs/LDHs(b) are applied to remove anionic dye

reactive orange 5 from aqueous solutions. The experimental factors

such as adsorbent dosage and pH to affect the adsorption were

measured. The pseudo-second order model accurately described the

reactive orange 5 adsorption kinetic for LDHs and ILs/LDHs(b). The

adsorption isotherm data are in agreement with the Freundlich model.

Thermodynamic data indicated reactive orange 5 adsorptions were

spontaneous and endothermic nature. The maximum adsorption

capacity of reactive orange 5 onto ILs/LDHs(b) was 300.9 mg/g, which

was higher than that of pure LDHs(53.9 mg/g). The process of dyes

adsorption onto ILs/LDHs(b) is the combination of ion exchange,

electrostatic attraction, hydrogen bond interactions and van der

Waals interactions. The desorption percentages were 57.91%,

46.67%, and 37.34% in each cycle, respectively. On the basis of

above results, ILs/LDHs(b) could be used as potential adsorbent for

reactive orange 5 removal from aqueous solutions.

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项目来源：2015 年湖北省自然科学基金面上项目（B2015304） 作者简介：周青青(1990.07-)，女，主要研究方向：染整清洁生产工艺。