Synthesis and characterization of anionic/nonionicsurfactant-interceded iron-doped TiO2 to enhancesorbent/photo-catalytic properties

Ajit Sharma, Byeong-Kyu Lee n

Department of Civil and Environmental Engineering, University of Ulsan, Nam-gu, Daehak ro 93, Ulsan 680-749, Republic of Korea

a r t i c l e i n f o

Article history:Received 21 March 2015Accepted 26 April 2015Available online 12 May 2015

Keywords:SDSTriton X-100As(V)TiO2/FePhoto-catalystAdsorbent

a b s t r a c t

We investigated the synthesis, characterization, and application of surfactant-interceded Fenanoparticle-doped TiO2 (TiO2/Fe-S1 and TiO2/Fe-S2) that were used as adsorbents and photo-catalysts for the removal of As(V) ions from aqueous media. Two types of surfactant (anionic (sodiumdodecyl sulfate), S1 and non-ionic (Triton X-100), S2) were used to obtain the separation and mono-dispersion of Fe(III) ions in the reaction solution. The nanocomposites were characterized by Fouriertransform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), UV–vis, scanningelectron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) and elemental mappinganalysis before and after As(V) removal. The Langmuir capacities (qe, mg/g) of the sodium dodecyl sulfate(SDS) and Triton X-100 interceded nanocomposites (TiO2/Fe-S1 and TiO2/Fe-S2, respectively) for arsenicremoval were determined to be 65.79 and 50.76 mg/g, respectively, in aqueous media with As(V) concentration ranges of 0–10 mg/L at pH 6.5.

& 2015 Elsevier Inc. All rights reserved.

1. Introduction

Arsenic removal by adsorption and photo-catalytic processes isadvantageous because of ease of handling, economy and highremoval efficiency at low concentrations [1]. Arsenic removal hasbeen developed extensively using titanium dioxide (TiO2). Arsenicremoval with titanium is mainly dependent on the characteristicsof the adsorbent and the oxidation state of the arsenic ions. TiO2

offers higher sorption capacities and oxidization of As(III) to As(V) under UV irradiation, which provides more hydroxyl radicalsas reactive oxidizing species. Titanium dioxide shows higherremoval of As(V) than As(III) as an adsorbent and does not requirelight irradiation [2–4]. To enhance the arsenic removal efficiency,some bimetal oxide adsorbents have been prepared by researchers[5–9]. TiO2 combined with oxides of cerium, lanthanum andzirconium can significantly enhance the arsenic removal efficiency[2,10,11]. Iron-doped TiO2 has also been reported for arsenicremoval. For example, the adsorption capacity for As(V) of Fe-doped TiO2 was twice as high as the parent TiO2 under similarenvironmental conditions [12]. Nanoparticles of zero valent iron,iron oxide and titanium dioxide used as nano-sorbents areefficient and effective compared with their macro-sized counter-parts because of the high proportion of active sites on their

surface, resulting in increased reactivity and unique catalyticactivity [1,3]. Nanoparticles, such as aluminum, nickel, manganeseand iron are generally used as bi-functional materials with TiO2

[2,13–15].In this study, we focused on size scheming and homogeneous

dispersion of the metal nanoparticles, particularly as dopants, onthe TiO2 nano-texture. One method to obtain the size scheme withhomogeneous dispersion of metal nanoparticles is the use ofsurfactants during the precursor reaction solution. The particlessurface chemistry is important for size scheming to obtain well-defined and uniform dispersions of metal nanoparticles in the hostmatrix. The molecules of surfactant typically react with nanopar-ticles in a disperse medium. The particle surface chemistry ischanged with treatment by surfactant molecules, which arecommonly made as pseudo-aggregates of particle clusters. Thesurfaces of nanoparticles are difficult to modify because they areevenly covered by negatively charged particles on compositematrices. Nanoparticle synthesis by the precipitation method withmolecules of interceded surfactant is an effective process forobtaining uniform modified particle surfaces in a single step andallows controlling the size and surface chemistry [16–18]. Manystudies have reported the use of surfactants, such as sodiumdodecyl sulfate (SDS), cetyl trimethylammonium chloride, cetyltrimethylammonium bromide and Triton X-100, in the synthesis ofnickel oxide, silver oxide and iron oxide nanoparticles [19–23].

Thus, the incorporation of iron nanoparticles into TiO2 mayoffer a relatively inexpensive method that could be effective in

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Journal of Solid State Chemistry

http://dx.doi.org/10.1016/j.jssc.2015.04.0420022-4596/& 2015 Elsevier Inc. All rights reserved.

n Corresponding author. Tel.: þ82 52 259 2864; fax: þ82 52 259 2629.E-mail address: bklee@ulsan.ac.kr (B.-K. Lee).

Journal of Solid State Chemistry 229 (2015) 1–9

removing dissolved arsenic from water. The present work evalu-ates the effectiveness of the surfactant-interceded synthesis ofiron-TiO2 nanocomposite agglomerates with uniform dispersion ofthe dopant for arsenic sorption studies. The doped iron(III) oxideinto TiO2 agglomerate might provide enhanced material proper-ties, such as surface sorption and photo-induced catalysis. Surfac-tants prevent the agglomeration of metal particles in solution.Thus, the aims of this study were (i) to identify the effect of anionic(SDS)/nonionic (Triton X-100) surfactant-interceded synthesis bycharacterizing the TiO2/Fe nanocomposite and (ii) to evaluate theuse of the nanocomposite for arsenic removal by sorption andphoto-induced catalysis from aqueous solution.

2. Materials and methods

2.1. Materials

Commercially available liquid titanium(IV) isopropoxide,hydrated ferric nitrate (purity 499.9%), sodium dodecyl sulfate(SDS) and Triton X-100 were purchased from Daejung Chemicaland Metals Co. Ltd., Korea. The As(V) stock solution was preparedby dissolving disodium hydrogen arsenate (Na2HAsO4 �7H2O),purchased from BDH Chemicals.

2.2. Synthesis of the surfactant-interceded titanium-ironnanocomposite

The novel, iron doped TiO2 (TiO2/Fe) nanocomposites weresynthesized by a modified sol–gel process. Titanium dioxidenanoparticles were doped with hydrated ferric nitrate (Fe(NO3)3 �9H2O) using two different surfactants: (1) SDS (S1), ananionic surfactant or (2) Triton X-100 (S2), a non-ionic surfactant.In a typical synthesis, titanium(IV) isopropoxide (5.58 mL) washydrolyzed using glacial acetic acid (10.74 mL) at 35 1C and thenSDS (S1) or Triton X-100 (S2) (0.1 and 0.4 M) in 50 mL ethanolsolution was added and stirred for 20 min to form a homogeneoussolution. Next, 50 mL of ferric nitrate (0.1 or 0.4 M) in H2O(deionized water) was added into the surfactant-interceded tita-nium-iron solution. The nanocomposites were obtained by thedrop-wise addition of 0.5 M NaBH4 (2 mL) into the solution withstirring at 35 1C. The formation of the surfactant-interceded TiO2/Fe nanocomposite was confirmed visually when the solutionturned slightly black in color. This solution was stirred continu-ously at 40 1C for 12 h to obtain the gel-containing solution as aresult of the complete hydrolysis of the TiO2 and iron nanoparti-cles. Then, the complete gelation was achieved by placing the gel-containing solution into an oven at 85 1C for 10 h. Subsequently,the wet gel was dried overnight at 100 1C under atmosphericpressure to obtain the doped material. The dried material was

calcined at 400 1C for 4 h to remove the surfactant (sodiumdodecyl sulfate or Triton X-100) [22], consequently producingthe desired bimetal-mixed nanocomposite.

The titanium dioxide nanoparticles were synthesize usingabove procedure except no addition of surfactant and hydratedferric nitrate at above identical condition [24].

2.3. As(V) analysis

As(V) standard solution (1000 mg/L) was prepared by dissol-ving disodium hydrogen arsenate (Na2HAsO4 �7H2O) in Milli-Qdeionized water. Arsenic was analyzed by inductively-coupledplasma mass spectrometry (ICP-MS), according to a literaturemethod [25] using an Agilent 7700 series ICP-MS system (Wald-bronn, Germany) equipped with a quartz torch, connector tube,micro-flow nebulizer, Pt skimmer cone and brass skimmer. Theoptimal operation conditions are summarized in Table 1.

2.4. Characterization

Infrared spectra were assessed by Nicolet Nexus 470 Fouriertransform infrared (FTIR) spectroscopy in the region 4000–500 cm�1. The surfaces of the TiO2/Fe nanocomposites were char-acterized by X-ray photoelectron spectroscopy (XPS) using a ThermoScientific K-Alpha XPS spectrometer before and after As(V) adsorption.The nanocomposites were further examined with an Evolution 300spectrophotometer (UV-1700 Shimadzu) to obtain the UV–vis absorp-tion spectra at wavelengths of 300–700 nm. Semi-quantitative ana-lyses and elemental mapping were performed on an energy-dispersiveX-ray (EDX) spectrometer connected to a Hitachi S-4700 scanningelectron microscope (SEM).

2.5. Batch arsenic sorption experiments

Arsenic sorption experiments were carried out under opti-mized conditions using the nanocomposite as an adsorbent in atemperature-controlled incubator shaker set at 120 rpm andmaintained at 30 1C for 1.5 h. The pH (pH analysis by pH ThermoScientific) of the reaction mixture was initially maintained usingeither HCl or NaOH (0.1 M). After the flask was shaken for thedesired time, the suspension was filtered through Whatman(0.45 mm) filter paper and the filtrate, after suitable dilution,was analyzed for the As(V) concentration.

To evaluate the kinetic data, the agitation time was varied from 15to 120min and kinetic studies were performed at 100 mL of 500 mg/LAs(V) solution using a 10 mg nanocomposite dose. To reduce experi-mental error, separate flasks were used for each time interval. Afteradsorption for 1.5 h, the reaction solutions were filtered and thequantity of adsorbed As(V) ions was analyzed by ICP–MS.

Table 1Equipment and operating parameters for As(V).

Operation parameter

aRF power 1600 (W)Carrier gas 0.7 L/minMakeup gas 0.5 L/minDeflect 0 (V)Octopole bias �18 (V)He gas 5.0 mL/minSample uptake time 30 (s)Sample stabilization 30 (s)Integration time per point 1 (s)Integration point 3/mass

a RF power¼radio-frequency (V)¼volt, (s)¼second, (W)¼watt.

Fig. 1. FTIR spectra before and after As(V) removal.

A. Sharma, B.-K. Lee / Journal of Solid State Chemistry 229 (2015) 1–92

For adsorption optimization, various parameters were alteredwhile keeping the others fixed. The study of pH effects for As(V) ions removal, 100 mL of the 500 mg/L As(V) solutions wereadjusted to different pHs, ranging from 4 to 8. The optimum pHwas determined using a 10 mg nanocomposite dose. Nanocompo-site samples of differing iron-to-surfactant molar ratios were usedto study the effects of iron and surfactant on As(V) removal underthe optimal adsorption conditions.

To study the effect of the initial arsenic concentration, differentinitial concentrations ranging from 0 to 10 mg/L were evaluated witha 10 mg nanocomposite. The quantity of As(V) adsorbed per gram ofthe nanocomposite was calculated as the difference between theinitial and the final concentration, using Eq. (1) [9,26].

qe ¼ ðCo�CeÞ � V=W ð1Þwhere qe is the uptake capacity (mg/g) of the nanocomposite, Co andCe are the initial and equilibrium concentrations of As(V) (mg/L), V isthe volume (L) of solution and W is the amount (g) of thenanocomposite used.

2.6. Photo-catalytic activity measurements

Photo-catalytic activity was evaluated by measuring thedecomposition of the aqueous solution of arsenic at a concentra-tion of 1000 mg/L under UV and visible light irradiation. Photo-catalytic processes were carried out in a thermostatic batchreactor equipped with a UV lamp (20 W, black light, λmax¼352 nm,Alim Industry Co., Korea) and a Xe-lamp (Superstar Dulux EL,20 W) that were used as visible light sources. For the photocata-lytic test run, 10 mg of nanocomposite was added into 100 mL ofthe arsenic solution in the quartz tube and the pH was adjusted to6.5 using 0.1 M HCl. The reaction solutions were maintained underconstant air-equilibrated conditions before and during the

Fig. 2. XPS spectra and detailed peak distribution of O1s, Fe2p, and As3d for TiO2 (a), TiO2/Fe (b), TiO2/Fe-S1 (c), TiO2/Fe-S1-As (d), TiO2/Fe-S2 (e), and TiO2/Fe-S2-As (f).

Fig. 3. UV analysis before and after As(V) removal.

A. Sharma, B.-K. Lee / Journal of Solid State Chemistry 229 (2015) 1–9 3

irradiation. The reaction was started by irradiating the reactionsolutions with the light source at room temperature. At given timeintervals, solutions were centrifuged and filtered through What-man (0.45 mm) filter paper to remove the nanocomposite and thefiltrates were analyzed by ICP–MS.

3. Results and discussion

3.1. Properties/characteristics

3.1.1. FTIR analysisFig. 1 shows the FTIR spectra of the TiO2, TiO2/Fe, TiO2/Fe-S1,

TiO2/Fe-S2 nanocomposites before and after arsenic removal. Thestrong band at 1110 cm�1 may be attributable to the hydroxylgroup of the TiO2/Fe oxides [10] and overlap of the stretchingvibration of the hydroxyl groups of the both titanium and ironoxides [27]. Similarly, as addition with SDS (S1) or Triton X-100(S2) surfactant, band at 1110 cm�1 was shifted to 1100 cm�1

(attributed to Ti–O–Fe) with a higher intensity in the TiO2/Fe-S1or TiO2/Fe-S2 samples, likely because the surfactants stimulateagglomeration for better dispersion due to lowering energy of thedoped material surface. The peak at 610 cm�1 which is attributedto the groups of Ti–O and Fe–O in the nanocomposite, was sharpand broader in the surfactant-interceded nanocomposite, com-pared with the TiO2/Fe composite (with no interceded surfactant)

[28]; this may be due to the uniform dispersions of the iron as ahost particles in the TiO2 texture as guest matrices due tointermicellar exchange of surfactants. After As(V) removal, thepeak at 3370 cm�1 shifted to 3355 cm�1 and 3360 cm-1 in thespectra of TiO2/Fe-S1 and TiO2/Fe-S2, respectively. Additionally,the disappearance of the peak at 905 cm�1 and the peak at1110 cm�1 shifted to 1098 cm�1 with low intensity, indicate thatarsenic (V) ions bind with surface hydroxyl groups of the nano-composite and enhance the sorption phenomena [8]. The peak at610 cm�1, attributed to the TiO2/Fe-S1 and TiO2/Fe-S2 groupsdecreased to 590 cm�1 after As(V) sorption, suggesting a changein the metal (M)–O groups after the sorption.

3.1.2. XPS analysisFig. 2 shows the peak positions of oxygen, iron, and arsenic

obtained by XPS. As shown in Fig. 2, the O1s core level could bebest fitted by the surfactant-interceded nanocomposite. The broadpeaks with positions at 529.38, 529.83, 529.86–531.91 and529.80–531.34 eV in the spectra for TiO2, TiO2/Fe, TiO2/Fe-S1 andTiO2/Fe-S2, respectively, could be assigned to titanium/iron oxide,a hydroxyl group bonded to Ti/Fe-OH and adsorbed H2O [29,30].After As(V) removal, the area ratio of metal-bonded hydroxylgroup peak intensity decreased for TiO2/Fe-S1-As and TiO2/Fe-S2-As, indicating a decrease in the quantity of hydroxyl groups onthe sorbent surface during arsenic removal. This also indicated

Fig. 4. SEM, EDX, and elemental analyses of TiO2 (a, e, i), TiO2/Fe (b, f, j), TiO2/Fe-S1 (c, g, k), and TiO2/Fe-S2 (d, h, l), respectively.

A. Sharma, B.-K. Lee / Journal of Solid State Chemistry 229 (2015) 1–94

that the TiO2/Fe-S1 should be more efficient than TiO2/Fe-S2 forarsenic removal. The Fe2p core level of the composite materialsshowed that the peaks at 711.31 eV and 725.58 eV could beallocated to 2p3/2 and 2p1/2 of iron(III), respectively. Thus, theiron element in the composite materials existed mostly in thisoxidation state and the Fe(III) could be combined with the matrixof TiO2 to make Ti–O–Fe bonds after calcination of the iron-dopedTiO2 nanoparticles [25]. In the As3d spectra of the two samples,the two marked peaks were attributed to the arsenic-loadednanocomposite at 36.54 and 45.14 eV for TiO2/Fe-S1-As and37.08 and 45.21 eV for TiO2/Fe-S2-As, which should be ascribedto As(V) [8]. This result indicates that the SDS surfactant-interceded nanocomposite enhanced As(V) removal because ofits high content of iron to TiO2 matrix. In the course of thesurfactant-interceded synthesis, the positively charged iron(III)combined with the negatively charged hydrophilic head of thesodium dodecyl sulfate molecule and formed in the solution forthe iron salt dissolution, preventing the iron(III) ions from accu-mulating with contiguous ions [18].

3.1.3. UV–vis analysisAs shown in Fig. 3, it is known that absorption at wavelengths

below 387t i is caused by the intrinsic band gap absorption oftitanium. It also indicates that the doping of Fe(III) in thenanocomposite contributes to a red shift in the absorption thresh-old and an improvement in the spectral response peak in theultraviolet region, due to the similar radii of Ti4þ (0.68 Å) andFe3þ (0.64 Å). It could also be attributed to the excitation of the 3delectrons of Fe(III) to the TiO2 conduction band (charge transfertransition), subsequently leading to increased band gap energy

[28,31]. Increasing iron concentrations in the surfactant-interceded nanocomposite, with absorbance at 425–675 nm, werealso intensified. The sudden enhancement of absorption in theregion from 500 to 650 nm for surfactant-interceded nanocompo-sites due to the high content of iron is consistent with the resultsof XPS. The band shifts to higher wavelengths occurred in thefollowing order: TiO2oTiO2/FeoTiO2/Fe-S2oTiO2/Fe-S1. Spectralshifting shows the conformation of the arsenic sorption by thenanocomposites, resulting in TiO2/Fe-S1-As and TiO2/Fe-S2-As.

3.1.4. Surface morphology with elemental analysisThe surface morphology and elemental composition of the nano-

composite before and after arsenic removal were examined by SEM/EDX and elemental analyses (Figs. 4 and 5). The shape of the TiO2 wasa nano-sphere while the TiO2/Fe composite was finer and nano-needleshaped (Fig. 4a and b). The TiO2/Fe-S1 sample (Fig. 4c) showed auniform surface with dense growth, compared with the TiO2/Fesample (Fig. 4b). Additionally, the uniformity of the dispersion of theiron nanoparticles with a porous structure in the TiO2/Fe-S1 samplewas superior to that of the TiO2/Fe-S2 samples (Fig. 4d). The TiO2/Fe-S2 samples showed a small iron nanoparticle yield and low uniformity.In the analysis of the EDX spectra (Fig. 4e–h) and the elementalmapping (Fig. 4i, j, k and l) for TiO2, TiO2/Fe, TiO2/Fe-S1 and TiO2/Fe-S2,respectively, the peaks of the Ti and O atoms were attributed to TiO2,while the Fe peak was due to the Fe-doping of the samples prior toSEM analysis. The atomic Fe % (6.8, 14.5 and 17.2, respectively) of thenanocomposites, as determined through EDX and elemental analysis,was in the following order: TiO2/FeoTiO2/Fe-S2oTiO2/Fe-S1.

Fig. 5 (TiO2/Fe-S1-As (a) and TiO2/Fe-S2-As (b)) shows SEMimages after As(V) loading. It is clear from Fig. 4(c and d) that the

Fig. 5. SEM, EDX, and elemental analyses of TiO2/Fe-S1-As (a, c, e), and TiO2/Fe-S2-As (b, d, f), respectively.

A. Sharma, B.-K. Lee / Journal of Solid State Chemistry 229 (2015) 1–9 5

surface morphology of the As(V)-loaded nanocomposite has adifferent surface structure than the parent composite (TiO2/Fe).EDX analysis revealed that Ti, Fe and O were abundant on thesurface and arsenic was detected after the sorption (Fig. 5c and d).To investigate the distribution of As(V) from the surface to thecenter of a particle, the nanocomposite after As(V) adsorption wasacquired for TiO2/Fe-S1-As and TiO2/Fe-S2-As (Fig. 5e and f). It canbe seen that the sorbed As(V) was almost evenly distributed inthe SDS-interceded nanocomposite due to the homogenousdispersion of iron versus the Triton-X100 surfactant-intercedednanocomposite.

3.2. Arsenic adsorption/photo-catalytic experiments

3.2.1. Sorption kineticsFig. 6 indicates the As(V) removal was initially fast; the removal

efficiency reached more than 55.7% for TiO2/Fe-S1 and 51.3% forTiO2/Fe-S2 within 30 min. Then, the adsorption efficiency wasreduced and equilibrium was reached at 1.5 h, with no change in

uptake capacity within 24 h, it may be due to large number of ironnanoparticles with more active sites at the initial stage of sorption.Also, the decreased adsorption rate was due to the surface reactionprocess involved in the adsorption [9]. After 1.5 h of treatment, theequilibrium As(V) concentrations were 4.35 and 13.41 mg/L for theTiO2/Fe-S1 and TiO2/Fe-S2, respectively. The kinetics of As(V) removal is demonstrated in Fig. 6A and B with pseudo-second-order and the second-order rate model following Eqs.(2) and (3), respectively [12].

t=qt ¼ 1=k1q2e þt=qe ð2Þ

1= qe–qt� �¼ 1=qeþk2t ð3Þ

where k1 and k2 are the pseudo-second-order (g/mg/min) and second-order (g/mg/min) rate constants, respectively. The quantity of arsenicions adsorbed (mg/g) at equilibrium and at a time ‘t’ is denoted by qeand qt, respectively. The pseudo-second-order kinetic model showedthe best fit for the kinetics of the As(V) removal by the nanocomposite,indicating that the adsorption process was dominated by the chemicaladsorption phenomenon. The correlation coefficients and rate constantare presented in Table 2, which indicate that the sorption of arsenicremoval was greater in the TiO2/Fe-S1 than the TiO2/Fe-S2 samples ata 500 mg/L As(V).

3.2.2. pH effectFig. 7 shows that the As(V) removal by the four different

sorbents increased with increasing pH before the concentrationof sorbed phase reached its maximum value at pH 6.5. The highestarsenic removal with 500 mg/L initial concentration at pH 6.5 was2.20, 3.50, 4.95 and 4.80 mg/g for TiO2, TiO2/Fe, TiO2/Fe-S1 andTiO2/Fe-S2, respectively (Fig. 7). The uptake of arsenic declined atpH values above 7.0, likely because of more inter-ionic repulsionbetween the arsenic and the oxy-anions or hydroxyl ions. Theremoval mechanism of arsenic ions by nanocomposite is givenreaction (4):

� Ti=Fe�OHþAs2� Ti=FeAsþOH� ð4Þwhere hydroxyl groups bind with titanium and iron ions. In thereaction with arsenic, the surface sites of titanium and ironattached with arsenic ions. Arsenic removal at different pHs indrinking water is dependent primarily on interchange of ligandinteractions between the arsenic ions and the hydroxyl groups[32–33]. However, at pHs above 3.5, As(V) exists in differentanionic forms such as AsO4

3� , HAsO42� and H2AsO4

- in water [3].Thus, the arsenic(V) ions may be removed by the cationic form ofarsenic. The removal of As(V) was greater in the SDS-intercedednanocomposite, versus the Triton X-100 surfactant and othersamples (TiO2 and TiO2/Fe) at pH 6.5.

Fig. 6. Kinetic models for the equilibrium adsorption data: (A) pseudo-secondorder and (B) second-order model plot of As(V) removal at a concentration of500 mg/L using adsorbent dose¼10 mg, pH¼6.5, reaction volume¼100 mL, andtemperature¼30 1C.

Table 2Sorption kinetics of As(V) removal by the surfactant-interceded nanocomposite.

Pseudo second order model Second-order model

(Sample) qe (mg/g) k1 (g/mg/min)

R2 qe (mg/g) k2 (g/mg/min)

R2

TiO2/Fe-S1

6.51 3.85�10�3 0.986 112.35 4.0�10�3 0.759

TiO2/Fe-S2

6.32 3.80�10�3 0.974 322.58 2.0�10�3 0.817

Fig. 7. Effect of pH on the equilibrium concentration of As(V) (initial concentration¼500 mg/L, adsorbent dose¼10 mg, reaction volume¼100 mL, and T¼30 1C).

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3.2.3. Effects of iron-surfactant concentration on As(V) removalTo investigate the effect of iron loading onto the TiO2 in the

presence of surfactant for As(V) removal, experiments were con-ducted with four different concentration conditions of iron (0.1and 0.4 M) and surfactant (0.1 and 0.4 M) for both S1 and S2(Fig. 8). The following combinations were evaluated: TiO2/Fe(0.4)–S1(0.4), TiO2/Fe(0.4)–S1(0.1), TiO2/Fe(0.1)–S1(0.4) and TiO2/Fe(0.1)–S1(0.1), and TiO2/Fe(0.4)–S2(0.4), TiO2/Fe(0.4)–S2(0.1), TiO2/Fe(0.1)–S2(0.4) and TiO2/Fe(0.1)–S2(0.1). The arsenic removal effi-ciencies of TiO2/Fe(0.1) and TiO2/Fe(0.4) were 59% and 70%,respectively (Fig. 8A). This indicates that more binding sites wereavailable for arsenic adsorption in nanoparticles with higher ironcontent. However, iron-doped TiO2 with SDS surfactant showedincreased As(V) removal efficiencies, up to 94.64, 96.94, 98.44 and99.13% for TiO2/Fe(0.4)–S1(0.4), TiO2/Fe(0.4)–S1(0.1), TiO2/Fe(0.1)–S1(0.4) and TiO2/Fe(0.1)–S1(0.1), respectively (Fig. 8B). The surfac-tant played a significant role in increasing the dispersion of ironnanoparticles onto TiO2 and the combination of iron and SDSsurfactant at 0.1 M ratios greatly increased arsenic removal effi-ciency. A similar trend was also observed with the Triton X-100surfactant (Fig. 8C). The arsenic removal efficiencies of the nano-composite with iron and Triton X-100 surfactant were 93.1, 95.64,94.28 and 97.38% for TiO2/Fe(0.4)–S2(0.4), TiO2/Fe(0.4)–S2(0.1),TiO2/Fe(0.1)–S2(0.4) and TiO2/Fe(0.1)–S2(0.1), respectively. Thearsenic removal efficiency for both surfactants can be ranked inthe following order: TiO2/Fe(0.4)oTiO2/Fe(0.1)–S2(0.1)oTiO2/Fe(0.1)–S1(0.1). Adequate loading of the iron nanoparticle canincrease the generation rate of electron pairs, enhancing the

removal of arsenic. At higher loads of iron nanoparticles, thenanocomposite can block some active sites and reduce the effec-tiveness of the active phases [10,18]. Thus, using the optimalcombination of iron and surfactant (TiO2/Fe(0.1)–S1(0.1)) wouldbe most effective in achieving the highest As(V) removal results.

3.2.4. Arsenic sorption isothermFig. 9 shows the equilibrium isotherm data using the Langmuir

and Freundlich isotherm models [32] for the sorption of As(V) atpH 6.5 and 30 1C. The Langmuir isotherm (Fig. 9A) is valid formonolayer sorption and is expressed in the linear form, as shownin Eq. (5):

qe ¼ qmaxbCe=ð1þbCeÞ ð5Þwhere qe is the equilibrium amount of arsenic loaded on thenanocomposite (mg/g), Ce is the equilibrium concentration ofarsenic (mg/L), qmax is the maximum capacity (mg/g) representingthe monolayer sorption capacity, and b is the relative energy(intensity) of sorption, also known as binding constant (L/mg). Theparametric values obtained for the various constants are given inTable 3. It is evident from the data that the maximum removalcapacity of the TiO2, TiO2/Fe, TiO2/Fe-S1 and TiO2/Fe-S2 nanocom-posite toward As(V) were in the order of TiO2oTiO2/FeoTiO2/Fe-S2oTiO2/Fe-S1. The maximum As(V) removal capacity of the SDS-interceded surfactant nanocomposite was about five times greaterthan that of TiO2 and two times greater than that of thecomposites (TiO2/Fe) with no surfactant. High sorption capacityof surfactant-interceded nanocomposite toward As(V) could be

Fig. 8. Effect of the iron-surfactant combination on As(V) removal (initial concentration ¼500 mg/L and pH¼6.5).

A. Sharma, B.-K. Lee / Journal of Solid State Chemistry 229 (2015) 1–9 7

attributed to the formation of strong TiO2/Fe As bond with largeamount of Fe ions.

The Freundlich isotherm (Fig. 9B) describes heterogeneoussurface energies by multilayer adsorption and is expressed inlinear form, as shown in Eq. (6):

In qe ¼ In Kf þ1=n In Ce ð6Þwhere Kf indicates sorption capacity (mg/g) and n is an empiricalparameter related to the intensity of sorption, which varies withthe heterogeneity of the nanocomposite. Higher values of the 1/nrepresent better favorability of adsorption. The values obtained forthe various parameters of these two models are given in Table 3.

3.2.5. Comparative analysis with adsorption versus photo-catalyticactivity

Fig. 10 compares the arsenic removal efficiency of the photo-catalytic (UV and visible phase) and adsorption methods (Darkphase). Arsenic removal efficiency was more prominent with theadsorption method, particularly with the surfactant-intercedednanocomposite, than the photo-catalytic methods. These experi-ments indicated that the photo-catalytic activities of thesurfactant-interceded nanocomposite were quite different fromthose of common photo-catalysts due to the high adsorption ofarsenic onto TiO2/Fe-S1 and TiO2/Fe-S2. The photo-degradation ofarsenic depends on its concentration in bulk solution and on the

catalyst surface [34]. The arsenic removal efficiency of the TiO2/Fenanocomposites for the adsorption method and the two photo-catalytic methods (UV and visible light irradiation) was in thefollowing order: dark phase 4photo-catalyst at visible phase 4atUV region with surfactant-interceded (TiO2/Fe-S1, TiO2/Fe-S2) andTiO2/Fe nanocomposites.

3.2.6. MechanismThe enhanced removal of arsenic(V) ions, via adsorption as a

dark phase or photo-catalysis process, by the TiO2/Fe nanocompo-site with surfactants as adsorbents/photo-catalysts would be dueto the enhanced iron content of the SDS surfactant-intercedednanocomposite. During iron doping by the sol–gel process in thepresence of SDS, the positively charged iron(III) is combined withthe negatively charged hydrophilic head of the sodium dodecylsulfate molecule. This combination prevents the iron(III) ions fromaccumulating with contiguous ions. Consequently, a bulk quantityof iron(III) ions is transferred from solution to the TiO2 along withthe SDS. Then, in the calcination step at 400 1C, SDS decomposesand iron(III) oxide nanoparticles are left behind on the TiO2.Scheme 1 shows the enhancement of arsenic removal withnanocomposite. Triton X-100 surfactant is a neutral molecule thatdoes not attract more iron(III) ions. Thus, the uptake capability ofarsenic(V) ions by Triton-X 100 may not significantly affect thesize reduction of the iron nanoparticles loaded on the nanocom-posite. As discussed above, the size of the iron nanoparticles onthe titaniumwas found to be smaller for SDS than for Triton X-100.In addition, dispersed nanocomposite in an aqueous solution, theadsorption of cations/anions and/or the ionization on surfacegroups generate surface charge; thus, the surfaces of particlesand the dispersed medium are producing electrical potential forfurther removal. This finding indicates that surfactant surfacecharge is main property of the nanocomposite for the dopantdispersion’s stability and for further adsorption or photo-catalysisprocesses.

4. Conclusions

In this study, we investigated a method for preparing an iron-doped TiO2 composite using sodium dodecyl sulfate (SDS) andTriton X-100 for arsenic removal by its action as an adsorbent/photo-catalyst. The surface characterization data and the adsorp-tion results showed that the surfactant-interceded transfer of Fe(III) ions from the doping solution to the titanium and thesubsequent in-situ production of Fe nanoparticles by the calcina-tion of the transferred iron(III)-surfactant onto the TiO2. Inparticular, the anionic surfactant SDS enhanced the transfer ofiron(III) ions to TiO2, resulting in the monodispersity of the Fe

Fig. 9. Equilibrium isotherm data shown as arsenic sorption by TiO2, TiO2/Fe, TiO2/Fe-S1 and TiO2/Fe-S2 at pH¼6.5 and temperature¼30 1C. Plots: (A) Langmuir and(B) Freundlich.

Table 3Isotherm constants and correlation coefficients for the removal of As(V).

Sample Langmuir isotherm Freundlich isotherm

qmax (mg/g) b (mL/g) R2 Kf (mg/g) 1/n R2

TiO2/Fe-S1 65.79 1.767 0.992 1.576 0.461 0.941TiO2/Fe-S2 50.76 1.442 0.992 1.783 0.393 0.913TiO2/Fe 33.34 1.424 0.984 1.827 0.329 0.946TiO2 12.65 0.768 0.994 0.156 0.501 0.959

Fig. 10. Comparative analysis for As(V) removal: adsorption vs. photocatalyticactivities. Experimental conditions: initial As(V) concentration¼1000 mg/L, adsor-bent/catalyst dosage¼10 mg, pH¼6.5, temperature¼30 1C, solar light intensity(UV region and visible phase).

A. Sharma, B.-K. Lee / Journal of Solid State Chemistry 229 (2015) 1–98

nanoparticles on the TiO2. Surfactant-interceded nanocompositeswere effective in removing As(V) present at low concentrations(o10 mg/L) at a pH of 6.5. In the removal of arsenic by surfactant-interceded TiO2 with iron doping, adsorption (in dark phase)removal was much greater than photo-catalytic (UV and visiblephase) removal. This study also demonstrates that the use of asurfactant at the optimum ratio is a useful method for thepreparation of bi-metal mixed nanocomposites as adsorbents/photocatalysts in bulk for the removal of arsenic from aqueoussolution.

Acknowledgments

This work was supported by basic science research programthrough the National Research Foundation of Korea (NRF) fundedby the Ministry of Education (2013R1A1A2065796) and supportedby a grant from the National Research Foundation of Korea (NRF)funded by the Ministry of Science, ICT and Future Planning(2013R1A2A2A03013138).

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Scheme 1. Proposed schematic mechanism of iron doping with surfactant onto TiO2 for As(V) removal.

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