ARTICLE 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, Xi0 an, Shaanxi 710062, China ‡ Department of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan0 an University, Yan0 an, Shaanxi 716000, China bS Supporting Information ABSTRACT: The present study aims to investigate the adsorption of anionic dye, alizarin red s (ARS), on activated clay modified by 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 best fit 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 gÀ1 The obtained results indicate that Fe-clay is suitable for adsorption of ARS from aqueous solutions 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 10À15% of the dye produced is lost during the textile dyeing process and finishing 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.5À8 However, most of above treatments suffer from one or another limitation, and they are unsatisfactory in terms of efficiency and economy.5,9 Particularly, anthraquinone dyes like alizarin red S (ARS) used in many fields belong to the group of the most durable dyes, which cannot be completely degraded by general chemical, physical, and biological processes.10 This is attributed to its complex structures of aromatic rings that afford high physicochemical, thermal, and optical stability.11 Therefore, most of the treatments for such dye effluents are largely inadequate By the comparison, adsorption 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 adsorption capacity has been widely employed as an adsorbent for the r 2011 American Chemical Society removal of various contaminants from water.6À8 But the problems 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 alternatives to carbon adsorbent, such as fly ash,4,9 agricultural wastes,15 and wood wastes.16 Especially, natural and modified clay materials have received wide attention due to their low cost, high specific surface areas, and variety of surfaces and structural properties, such as montmorillonite,3,13 kaolinite,17,18 and activated clay.18À20 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.21 Many researchers have explored modified clay materials as adsorbents for organic anions, such as acidification,18,22 surfactant modifying,23,24 and 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 Published: July 07, 2011 9712 dx.doi.org/10.1021/ie200524b | Ind Eng Chem Res 2011, 50, 9712–9717 Industrial & Engineering Chemistry Research 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 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 m2 gÀ1 and 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 the first step, the activated clay was impregnated with 0.05 M Fe(NO3)3 solution (7 mL ferric nitrate solution per g activated clay) After stirring at ambient temperature for h followed by solvent evaporation, the sample was dried at 110 °C for h and then calcined in a muffle furnace at 400 °C for h The BET surface area and average pore size of Fe-clay prepared were determined to be 190.1 m2 gÀ1 and 6.58 nm, respectively Furthermore, the detailed characterization of adsorbent material was described in the Supporting Information, 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 (1,2dihydroxy-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 In this experiment, the synthetic dye solution with various concentrations 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 (100À500 mg LÀ1), adsorbent dosages (2À6 g), and initial pH of solution (3À11) 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 Figure Chemical structure of alizarin red s ARTICLE 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 g with different initial concentration for h The dye solution was drawn out at preset time intervals and immediately filtered through a membrane filter to collect the supernatant The amounts of ARS adsorbed onto the adsorbent (qt, mg gÀ1) were determined as qt ¼ ðc0 À ct ÞV W ð1Þ where c0 and ct are the initial and the residual ARS concentrations in solution at any time t (mg LÀ1), 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 (100À 700 mg LÀ1) at a fixed Fe-clay dosage of g The contents were agitated isothermally for 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 Thereinto, two well-known adsorption equilibrium models, the Langmuir and Freundlich equations, are selected to describe adsorption behavior 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 mathematical kinetic models are employed to investigate the dye removal dynamics to realize the mechanisms and the rate controlling of adsorption, namely the pseudofirst-order, pseudosecond-order, and intraparticle diffusion equations RESULTS AND DISCUSSION 3.1 Preliminary Experiments Various preliminary adsorption 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 Furthermore, 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 molecules to overcome mass transfer resistance between the liquid and the solid phases.3,5 Figure shows the effect of different initial dye concentration on qt (the adsorption capacity of ARS onto Fe-clay) with time The value of qt seems to increase with enhancing the initial dye concentration However, it can be Table Various Models and Equations Utilized in Our Study parameters adsorption kinetic adsorption isotherm theoretical model eqs ref Lagergren pseudofirst-order kinetics k1 logðqe À qt Þ ¼ log qe À 2:303 t pseudosecond-order kinetics intraparticle diffusion ¼ k2 1qe þ qte qt ¼ ki ðt 1=2 Þ 3,5,15 3,15 Langmuir Ce qe t qt ¼ KVm þ Ce Vm log qe ¼ log Kf þ log Ce 1=n Freundlich 9713 7,12 dx.doi.org/10.1021/ie200524b |Ind Eng Chem Res 2011, 50, 9712–9717 Industrial & Engineering Chemistry Research Figure Effects of initial concentration on amount of ARS adsorbed per unit mass (qt) at different contact times (experimental conditions: adsorbent dosage g, temperature 25 °C, natural initial pH without adjustment) 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 concentration gradient, which is defined as the difference between dye molecules in aqueous and on adsorbent Figure 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 LÀ1, where over 90% of dye can be removed within No significant change in adsorption was seen beyond the contact time of 40 min, indicating that the adsorption reached equilibrium in this time with a concentration of 100À300 mg LÀ1 Whereas, for the initial concentration of 400 and 500 mg LÀ1, a relatively slow dye uptake can be observed and adsorption required about 90 to reach nearequilibrium 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 methylene blue.30 3.1.2 Effect of Adsorbent Dosage Figure displays the effect of Fe-clay dosage (2À6 g) on ARS uptake capacity at a fixed initial concentration (400 mg LÀ1) 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 to 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.3 However, the further addition of dosage beyond ARTICLE Figure Effect of adsorbent dosage on ARS removal (experimental condition: initial concentration 400 mg LÀ1, temperature 25 °C, natural initial pH of 4.15) Figure Effect of solution pH on ARS adsorption (experimental condition: adsorbent dosage g, initial concentration 400 mg LÀ1, temperature 25 °C) g cannot enhance the dye removal greatly, indicating the amount of 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 and 11, and the results are shown in Figure Clearly, both the amount adsorbed and the percentage removal of ARS at adsorption equilibrium decreased as the pH of aqueous solution increased from 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 gÀ1 and 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 3.5,31 which may be conveniently combined with the positively charged 9714 dx.doi.org/10.1021/ie200524b |Ind Eng Chem Res 2011, 50, 9712–9717 Industrial & Engineering Chemistry Research ARTICLE Table 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, c0 (mg LÀ1) qe,exp (mg gÀ1) k1 (minÀ1) qe1,cal (mg gÀ1) r12 k2 (g mgÀ1 minÀ1) qe2,cal (mg gÀ1) r22 ki (g mgÀ1 minÀ1/2) ri2 100 6.2 0.008 0.3 0.655 200 12.41 0.007 0.71 0.882 0.272 6.12 0.999 0.028 0.711 0.097 12.22 0.999 0.049 300 18.43 0.013 1.08 0.866 0.862 0.058 18.36 0.999 0.099 400 24.15 0.026 0.738 2.26 0.871 0.036 24.22 0.999 0.202 500 29.87 0.02 0.703 2.62 0.845 0.027 29.93 0.999 0.339 0.676 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.26 This 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 adsorption 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 displays various model parameters, including rate constant (k), the equilibrium adsorption capacity (qe), and the correlation coefficient (r2) Apparently, all the values of the linear regression correlation coefficient reached to 0.999 for the pseudosecondorder 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 gÀ1 minÀ1 as the initial concentration increased from 100 to 500 mg LÀ1, 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 diffusion was the rate-limiting step If the intraparticle diffusion is the rate-limiting step, then plots of qt vs t1/2 would result in a linear relationship and the line passes through the origin.19 As shown in Table 2, the corresponding regression coefficient (ri2) is very low Furthermore, it is clear in Figure that the plots for dye concentration between 300 and 500 mg LÀ1 have the same trend of initial curved portion followed by linear portion and plateau Moreover, all the plots 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 investigated in our study to understand the nature of the interaction between dye molecules and adsorbent Two well-known Figure Intraparticle diffusion kinetic model for ARS adsorption on Fe-clay at different dye concentration models, the Langmuir and Freundlich isotherms were utilized to describe the observed adsorption phenomena of ARS onto Feclay 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 heterogeneous 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 displays the isotherm between dye concentration on the solid and in the liquid phases at equilibrium (qe vs 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.33 Moreover, 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 qe of 32.1 mg gÀ1) It should be noted that these results are characteristic of Langmuir isotherms The above analysis is also supported by the fitting results to Langmuir and Freundlich adsorption isotherm models (Table 3) It can be observed that the Langmuir model is much more satisfactory to fit the experimental data than the Freundlich model, as reflected with the correlation coefficient (r2) This suggests the 9715 dx.doi.org/10.1021/ie200524b |Ind Eng Chem Res 2011, 50, 9712–9717 Industrial & Engineering Chemistry Research ARTICLE 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 (3À11) 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 adsorption capacity reached 32.7 mg gÀ1 on 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 Figure Adsorption isotherms for ARS on Fe-clay at initial dye concentrations between 100 and 700 mg LÀ1, an adsorbent dosage of g, and a temperature of 25 °C Table Estimated Parameters of Langmuir and Freundlich Isotherms for Adsorption of ARS on Fe-clay at an Adsorbent Dosage of g and Temperature of 25 °C 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 K (L mgÀ1) Langmuir parameters 0.306 Vm (mg gÀ1) 32.7 correlation coefficient (r2) 0.999 Freundlich parameters Kf 9.864 1/n 0.275 correlation coefficient (r2) 0.834 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.,7 Wu 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 to fit the adsorption data well in their investigations The reported Vm for adsorption of ARS was found to be 0.064 mg gÀ1 on activated charcoal, 31.8 mg gÀ1 on hybrid gels, and 161.29 mg gÀ1 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 CONCLUSION The results of this study indicate that the activated clay modified by iron oxide can be successfully used for the adsorption of *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, 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