Effective adsorption of anionic dye, alizarin red s, from aqueous

The adsorption process reached equilibrium within 90 min, and the removal efficiency increased with the enhancement of initial dye concentration, adsorbent dosage, and contact time, but de

Published: July 07, 2011

r 2011 American Chemical Society 9712 | Ind Eng Chem Res 2011, 50, 9712–9717

pubs.acs.org/IECR

Effective Adsorption of Anionic Dye, Alizarin Red S, from Aqueous Solutions on Activated Clay Modified by Iron Oxide

Feng Fu,†,‡ Ziwei Gao,*,† Lingxiang Gao,† and Dongsheng Li‡

†Key Lab of Applied Surface and Colloid Chemistry of the Ministry of Education, School of Chemistry and Materials Science, Shaanxi Normal University, Xi0an, Shaanxi 710062, China

‡Department of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan0an University, Yan0an, Shaanxi 716000, China

bS Supporting Information

iron oxide (Fe-clay) in a batch reactor The adsorption process reached equilibrium within 90 min, and the removal efficiency increased with the enhancement of initial dye concentration, adsorbent dosage, and contact time, but decreased with the enhancement of solution pH The adsorption kinetics was investigated according to three theoretical models, but the bestfit was achieved by the pseudosecond-order kinetics model The adsorption isotherms could be well-defined with the Langmuir isotherm model instead of the Freundlich isotherm model, and the calculated maximum adsorption capacity was found to be 32.7 mg g1 The obtained results indicate that Fe-clay is suitable for adsorption of ARS from aqueous solutions

1 INTRODUCTION

Over the past few decades, a large amount of wastes containing

dyes and pigments have been discharged into the receiving

aquatic environment due to the rapid development of the

modern textile industry.1 It is reported that approximately

1015% of the dye produced is lost during the textile dyeing

process andfinishing operations every year.2,3

The dye-bearing wastewater is not only aesthetically displeasing but also affecting

light penetrating into the stream and resulting in the destruction

of the aquatic ecosystem.4 Moreover, some dyes and their

degradation products are also toxic and even have carcinogenic

and mutagenic effects to aquatic biota and humans.5

To address the above severe issue, there is an urgent need to remove dyes

from textile effluents prior to their discharge into receiving water

Until today, various physicochemical and biological treatment

technologies have been developed to remove dyes from aqueous

solutions such as coagulation, precipitation,filtration, oxidation,

activated sludge processes, and adsorption.58However, most of

above treatments suffer from one or another limitation, and they

are unsatisfactory in terms of efficiency and economy.5,9

Parti-cularly, anthraquinone dyes like alizarin red S (ARS) used in

manyfields belong to the group of the most durable dyes, which

cannot be completely degraded by general chemical, physical,

and biological processes.10This is attributed to its complex

struc-tures of aromatic rings that afford high physicochemical, thermal,

and optical stability.11Therefore, most of the treatments for such

dye effluents are largely inadequate By the comparison,

adsorp-tion is superior to other techniques, which provides an attractive

alternative for the removal of dyes from aqueous solution,7,9,12

especially the removal of dyes that are chemically and biologically

stable

For years, activated carbon (AC) due to its excellent

adsorp-tion capacity has been widely employed as an adsorbent for the

removal of various contaminants from water.68 But the pro-blems with AC in terms of cost and regeneration make it impractical for treatment of industrial wastewater with high volumes.13,14 Recently, much attention has been focused on developing other low-cost and commercially available alterna-tives to carbon adsorbent, such asfly ash,4,9

agricultural wastes,15 and wood wastes.16Especially, natural and modified clay materi-als have received wide attention due to their low cost, high specific surface areas, and variety of surfaces and structural properties, such as montmorillonite,3,13kaolinite,17,18and acti-vated clay.1820 Unfortunately, the clay adsorbents display relatively low adsorption capacity on anionic pollutants owing

to the existence of a permanent net negative charge on the surface.21Many researchers have explored modified clay materi-als as adsorbents for organic anions, such as acidification,18,22

surfactant modifying,23,24and metal cations modifying.21,25

In this paper, we report our investigation in utilization of activated clay modified by iron oxides (Fe-clay) as an adsorbent for removal of anionic dye ARS from aqueous solutions Previous studies have revealed that iron oxides exhibit strong affinity toward numerous anions such as alizarin,26 phosphate,27 arsenate,28 and selenite,29 which can be due to the surface hydroxyl group’s intervention during dissociative chemisorption

of the adsorbate.26 Hence, the Fe-clay was prepared in our experiments by a simple wet impregnation method, which was expected to achieve better removal efficiency on organic anions when compared with traditional clay adsorbents The main objective of this work was to study the adsorption performance

Received: March 16, 2011 Accepted: July 7, 2011 Revised: May 30, 2011

of ARS on the Fe-clay in batch jar tests, including the factors of

the initial dye concentration, contact time, adsorbent dosage, and

initial pH of the solution The adsorption equilibrium isotherms

and kinetics were also evaluated

2 MATERIALS AND METHODS

2.1 Clay Adsorbent.The activated clay (Ac) was obtained

from Chemical Reagent of Da-Tang Co (Xi’an, China), the

chemical composition of which was mainly 2% MgO, 11% Al2O3,

and 85% SiO2 The BET surface area and average pore size

were determined to be 179.5 m2g1and 6.64 nm, respectively,

from N2 adsorption isotherms in an ASAP 2020 apparatus

(Micromeritics, USA)

Herein, the adsorbent of Fe-clay was prepared by a wet

impregnation method In thefirst step, the activated clay was

impregnated with 0.05 M Fe(NO3)3solution (7 mL ferric nitrate

solution per 1 g activated clay) After stirring at ambient

temperature for 3 h followed by solvent evaporation, the sample

was dried at 110°C for 6 h and then calcined in a muffle furnace

at 400°C for 3 h The BET surface area and average pore size of

Fe-clay prepared were determined to be 190.1 m2 g1 and

6.58 nm, respectively Furthermore, the detailed characterization

of adsorbent material was described in the Supporting

Informa-tion, which included X-ray diffraction (XRD), Fourier transform

infrared (FT-IR), scanning electron microscopy (SEM), and

adsorption isotherm analysis

2.2 Adsorbate.An anthraquinone dye of alizarin red S

(1,2-dihydroxy-9,10-anthraquinonesulfonic acid sodium salt) was of

analytical grade obtained from the Institute of Xinchun Reagent

in Tianjin The chemical structure of ARS is shown in Figure 1 In

this experiment, the synthetic dye solution with various

concen-trations was prepared dissolving ARS in distilled water, and the

pH of aqueous solution was adjusted to the desired value by

addition of NaOH (0.1 M) or H2SO4(0.1 M)

2.3 Adsorption Studies.Various batch adsorption tests were

carried out in 250 mL of dye solution with Fe-clay adsorbent at a

constant temperature of 25°C and agitation of 180 rpm (unless

otherwise stated in this paper) Preliminary experiments were

carried out to investigate the effects of initial ARS concentration

(100500 mg L1), adsorbent dosages (26 g), and initial pH

of solution (311) In each experiment, the procedure of test

was performed under the condition where one parameter was

changed at a time while the other parameters were fixed The

ARS content in solution before and after adsorption was

measured by UV/visible spectrophotometer (UV-7504, China)

at 423 nm

Kinetic experiments were determined by agitating the dye solution at fixed Fe-clay dosage of 4 g with different initial concentration for 3 h The dye solution was drawn out at preset time intervals and immediatelyfiltered through a membrane filter

to collect the supernatant The amounts of ARS adsorbed onto the adsorbent (qt, mg g1) were determined as

qt ¼ðc0 ctÞV

where c0and ctare the initial and the residual ARS concentrations

in solution at any time t (mg L1), respectively, V is the volume

of ARS solution (250 mL), and W is the weight of adsorbent used (g)

Adsorption isotherms were determined using a set of 250 mL

of ARS solutions with different initial concentrations (100

700 mg L1) at a fixed Fe-clay dosage of 4 g The contents were agitated isothermally for 7 h, which have been shown by preliminary experiments to be more than sufficient time for reaching adsorption equilibrium

The computation models used for the analysis of adsorption kinetic and isotherm data are given in Table 1 Thereinto, two well-known adsorption equilibrium models, the Langmuir and Freundlich equations, are selected to describe adsorption beha-vior of liquid/solid phase By applying these models, some very important information during the adsorption process can be determined, including adsorption capacity and the interaction between adsorbate and adsorbent Furthermore, three mathe-matical kinetic models are employed to investigate the dye removal dynamics to realize the mechanisms and the rate controlling of adsorption, namely the pseudofirst-order, pseudo-second-order, and intraparticle diffusion equations

3 RESULTS AND DISCUSSION 3.1 Preliminary Experiments.Various preliminary adsorp-tion experiments were carried out to investigate the effects of initial concentration, Fe-clay adsorbent dosage, and initial pH of solution on the removal efficiency of anionic dye, ARS Further-more, the ARS adsorption on the commercial activated clay was also carried out, but no satisfactory results can be obtained under the same experimental conditions Thus, only the experimental data about Fe-clay performance is reported in this paper 3.1.1 Effect of Initial Dye Concentration It is well-known that the initial dye concentration plays as an important role in the adsorption process, which can impel strongly the solute mol-ecules to overcome mass transfer resistance between the liquid and the solid phases.3,5Figure 2 shows the effect of different initial dye concentration on qt(the adsorption capacity of ARS onto Fe-clay) with time The value of qtseems to increase with enhancing the initial dye concentration However, it can be Figure 1 Chemical structure of alizarin red s

Table 1 Various Models and Equations Utilized in Our Study

q t ¼ 1

2 q e2þ t

q e ¼ 1

KV m þ C e

9714 9712–9717

calculated that almost all the dye is removed from the water by

Fe-clay after a certain time at the experimental conditions

Therefore, the change of qt is mainly related to the large

adsorption of the Fe-clay other than the effect of the

concentra-tion gradient, which is defined as the difference between dye

molecules in aqueous and on adsorbent

Figure 2 also shows that the initial dye concentration has a

marked effect on the contact time necessary to reach adsorption

equilibrium It can be found that a rapid uptake occurred for the

initial concentration below 400 mg L1, where over 90% of dye

can be removed within 5 min No significant change in

adsorp-tion was seen beyond the contact time of 40 min, indicating that

the adsorption reached equilibrium in this time with a

concen-tration of 100300 mg L1 Whereas, for the initial

concentra-tion of 400 and 500 mg L1, a relatively slow dye uptake can be

observed and adsorption required about 90 min to reach

near-equilibrium conditions Besides, an asymptotic trend for dye

adsorption can be seen in all the cases At low concentration, the

ratio of dye molecules to the number of available adsorption sites

on adsorbent is small and consequently the adsorption process

may mainly occur on the exterior surface of Fe-clay The rate of

adsorption is fast in this stage, resulting in short equilibrium time

required in the low concentration With an increase in the

amount of dye molecules, the situation changes and lots of dye

molecules are probably adsorbed by the interior surface of

adsorbent by pore diffusion after the adsorption of the exterior

surface reaches saturation Similar discussion has been reported

by Hameed et al for studying adsorption processes for

methy-lene blue.30

3.1.2 Effect of Adsorbent Dosage Figure 3 displays the effect

of Fe-clay dosage (26 g) on ARS uptake capacity at a fixed

initial concentration (400 mg L1) As expected, an increase in

adsorbent dosage leads to an increase in the percentage removal

of ARS Initially, a rapid enhancement of dye removal efficiency

can be observed with the increase of the Fe-clay dosage from 2 to

4 g For a fixed initial solute concentration, the increase of the

adsorbent amount can provide greater adsorption surface area

and the available adsorption sites and thus enhances the extent of

ARS removal.3However, the further addition of dosage beyond

4 g cannot enhance the dye removal greatly, indicating the amount of 4 g is the optimum adsorbent dose in our experimental conditions

3.1.3 Effect of pH The pH of the dye solution plays as an important factor in the adsorption process, which can alter the surface properties of the adsorbent as well as the degree of ionization of the dye In this study, the influence of pH on adsorption capacity is investigated for the initial pH solution between 3 and 11, and the results are shown in Figure 4 Clearly, both the amount adsorbed and the percentage removal of ARS at adsorption equilibrium decreased as the pH of aqueous solution increased from 3 to 11 However, unlike other clay adsorbents, the decline of above indexes was not significant with the increasing pH of Fe-clay, which reduced from 24.6 to 23.3 mg

g1and 98.4% to 93.1%, respectively It is logical to assume that the adsorption of the anionic dye, ARS can be promoted under the acidic conditions, where the positive charge density on the surface of adsorbent is great Moreover, the monovalent ARS molecules are dominant at the pH of solution below 3.5,31which may be conveniently combined with the positively charged

Figure 2 Effects of initial concentration on amount of ARS adsorbed

per unit mass (qt) at different contact times (experimental conditions:

adsorbent dosage 4 g, temperature 25°C, natural initial pH without

adjustment)

Figure 3 Effect of adsorbent dosage on ARS removal (experimental condition: initial concentration 400 mg L1, temperature 25°C, natural initial pH of 4.15)

Figure 4 Effect of solution pH on ARS adsorption (experimental condition: adsorbent dosage 4 g, initial concentration 400 mg L1, temperature 25°C)

adsorption sites With the increase of pH value, the negative

charge increases on the surface of adsorbent and the ARS

molecules mainly exist in the form of multivalent anions,31

leading to the reduction of adsorption efficiency On the other

hand, the adsorption capacity of ARS on Fe-clay does not reduce

remarkably at basic conditions due to the presence of iron species

that hold lots of adsorption sites for anionic dyes.26This indicates

Fe-clay adsorbent can be utilized on removal of anionic dye, ARS

over a broad pH range

3.2 Adsorption Kinetics.In order to investigate the

adsorp-tion mechanism of ARS on Fe-clay, the dynamic data were

analyzed by pseudofirst-order and pseudosecond-order kinetics

models with the intraparticle diffusion model Table 2 displays

various model parameters, including rate constant (k), the

equilibrium adsorption capacity (qe), and the correlation

coeffi-cient (r2) Apparently, all the values of the linear regression

correlation coefficient reached to 0.999 for the

pseudosecond-order kinetic model, which were closer to unity than those for the

other models Moreover, the calculated equilibrium adsorption

amount (qe,cal) was also much closer to the experimental values

(qe,exp) in the pseudosecond-order kinetic model, while the qe,cal

obtained in the pseudofirst-order kinetic model did not agree

with the experimental ones The rate constant (k2) decreased

from 0.272 to 0.027 mg g1min1as the initial concentration

increased from 100 to 500 mg L1, indicating that the adsorption

is dependent on the initial concentration Thus, it is reasonable to

infer that the ARS adsorption on Fe-clay can be represented

effectively by the pseudosecond-order kinetic model, and the

process may be chemisorption controlled.5,32

For a porous material, the diffusion of solute molecules into

the pores cannot be neglected, so the adsorption dynamical data

were further analyzed to determine whether intraparticle

diffu-sion was the rate-limiting step If the intraparticle diffudiffu-sion is the

rate-limiting step, then plots of qtvs t1/2would result in a linear

relationship and the line passes through the origin.19As shown in

Table 2, the corresponding regression coefficient (ri2) is very low

Furthermore, it is clear in Figure 5 that the plots for dye

concentration between 300 and 500 mg L1 have the same

trend of initial curved portion followed by linear portion and

plateau Moreover, all the plots do not have a zero intercept,

indicating that ARS removal is mainly a surface process under

experimental conditions On the basis of above results, it can be

concluded that the pseudosecond-order adsorption mechanism

is predominant in the dye adsorption process and the overall rate

of adsorption appears to be controlled by chemisorption

3.3 Adsorption Isotherms.Adsorption isotherms were

in-vestigated in our study to understand the nature of the

interac-tion between dye molecules and adsorbent Two well-known

models, the Langmuir and Freundlich isotherms were utilized to describe the observed adsorption phenomena of ARS onto Fe-clay The former is applicable under the following assumption: (i) the solid has a uniform surface, (ii) there is no interaction between adsorbed molecules, and (iii) the adsorption process takes place in a single layer The latter is an empirical model used

to explain the observed phenomena for the nonideal hetero-geneous adsorption system, where the adsorbed dye on the adsorbent will increase as long as there is an increase in the dye concentration in the solution

Figure 6 displays the isotherm between dye concentration on the solid and in the liquid phases at equilibrium (qevs Ce) The shape of the isothermal curve looks L-type, indicating that no strong competition exists between the solute and the solvent molecules to occupy the vacant site available, and the adsorbate is not vertically oriented on the surface of adsorbent.33Moreover, the adsorption amount at equilibrium increases dramatically at the low solution concentration, indicating a high affinity between the dye molecule and the Fe-clay surface;7 then the adsorbed amount reaches a plateau at the high equilibrium concentration, reflecting the saturated adsorption (maximum qeof 32.1 mg g1) It should be noted that these results are characteristic of Langmuir isotherms

The above analysis is also supported by thefitting results to Langmuir and Freundlich adsorption isotherm models (Table 3)

It can be observed that the Langmuir model is much more satisfactory tofit the experimental data than the Freundlich model,

as reflected with the correlation coefficient (r2

) This suggests the

Table 2 Kinetic Parameters for the Removal of ARS on Fe-clay at Different Initial Concentrations

pseudofirst-order kinetic model pseudosecond-order kinetic model intraparticle diffusion initial concentration,

c 0 (mg L1) q e,exp (mg g1) k 1 (min1) q e1,cal (mg g1) r 1 k 2 (g mg1min1) q e2,cal (mg g1) r 2 k i (g mg1min1/2) r i2

Figure 5 Intraparticle diffusion kinetic model for ARS adsorption on Fe-clay at different dye concentration

9716 9712–9717

homogeneous feature presented on the surface of Fe-clay and

demonstrates the formation of monolayer coverage of dye

molecule on the surface of adsorbent Moreover, the monolayer

capacity (Vm) calculated from the Langmuir equation is also

coherent with the maximum adsorption amount measured in

the tests Thus, the adsorption is almost completed when the

surface of Fe-clay is covered with a monolayer of ARS; and the

chemisorption dominates in this process since the Langmuir

model assumes that the adsorption is chemical combination It is

rarely reported about the adsorption behaviors of ARS on various

adsorbents as far as we know Nonetheless, similar adsorption

isotherms for ARS have been reported by Iqbal et al.,7Wu et al.,34

and Ghaedi et al.,35 where activated charcoal, hybrid gels, or

multiwalled carbon nanotubes were used as adsorbents The

Langmuir isotherm was also found tofit the adsorption data well

in their investigations The reported Vmfor adsorption of ARS

was found to be 0.064 mg g1on activated charcoal, 31.8 mg g1

on hybrid gels, and 161.29 mg g1 on multiwalled carbon

nanotubes, respectively In comparison with these adsorbents,

Fe-clay could be employed as a low-cost adsorbent for the

removal of ARS

4 CONCLUSION

The results of this study indicate that the activated clay modified

by iron oxide can be successfully used for the adsorption of

anionic dye ARS from aqueous solutions The uptake of ARS was very fast initially, and the adsorption capacity scarcely decreased significantly over a broad pH range (311) The kinetics of adsorption can be well described by the pseudosecond-order model, indicating that both dye concentration and adsorbent dosage play the important role in the adsorption process According to Langmuir isotherm model, the monolayer adsorp-tion capacity reached 32.7 mg g1on Fe-clay The linearity of the Langmuir isotherm plots indicated the chemical nature of the interactions between the adsorbate and the adsorption sites In one word, Fe-clay can be used in the removal of anionic dye rather than other clay materials

’ ASSOCIATED CONTENT

bS Supporting Information Supplementary data associated with this article can be found, in the online version Figure S1 shows the XRD analysis on adsorbent material, Figure S2 displays the FT-IR results of adsorbent material, and Figure S3 shows the SEM results of adsorbent material Figure S4 shows the adsorption isotherm of adsorbent material This material is available free of charge via the Internet at http://pubs.acs.org

’ AUTHOR INFORMATION Corresponding Author

*E-mail: ziweigao@gmail.com Fax: +86 29 85303733 Tel: +86

29 85303733 8001

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21041004, 20771071), the Program for New Century Excellent Talents in University of China (NCET-07-0528), and the Fundamental Research Funds for the Central Universities (2010ZYGX023, GK200902001)

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