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Materials Chemistry and Physics 143 (2014) 1469e1480

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

The bonding of heavy metals on nitric acid-etched coal fly ashesfunctionalized with 2-mercaptoethanol or thioglycolic acid

M.I. Muñoz a, A.J. Aller a,*, D. Littlejohn b

aDepartment of Applied Chemistry and Physics, Area of Analytical Chemistry, Faculty of Biological and Environmental Sciences, Campus de Vegazana, s/n,University of León, E-24071 León, SpainbWestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, UK

h i g h l i g h t s

* Corresponding author. Tel.: þ34 987291536; fax:E-mail address: aj.aller@unileon.es (A.J. Aller).

0254-0584/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2013.12.002

g r a p h i c a l a b s t r a c t

� Coal fly ashes were organically-functionalized.

� Organically-functionalized coal flyashes were spectrometricallycharacterized.

� Organically-functionalized coal flyashes can be used as an effectivesolid sorbent for metal(oid)s.

� This retention system provides a verypromising future in designingeffluent treatment systems.

a r t i c l e i n f o

Article history:Received 1 June 2012Received in revised form2 December 2013Accepted 8 December 2013

Keywords:OxidesEnergy dispersive analysis of X-raysRaman spectroscopy and scatteringFourier transform infrared spectroscopySurface properties

a b s t r a c t

Coal fly ash is a waste by-product of the coal fire industry, which generates many environmentalproblems. Alternative uses of this material would provide efficient solutions for this by-product. In thiswork, nitric acid-etched coal fly ash labelled with 2-mercaptoethanol or thioglycolic acid was assessedfor retention of Al(III), As(III), Cu(II), Cd(II), Fe(III), Mn(II), Hg(II), Ni(II), Pb(II) and Zn(II) ions. The bondingcharacteristics between the organic compounds with the solid support, as well as with the metal ions,were evaluated using various surface analytical techniques. Visualization of the organically-functionalized coal fly ash particle was possible using scanning electron microscopy (SEM), while theelemental composition of the functionalized material, before and after retention of the metal ions, wasobtained by energy dispersive (ED)-X ray spectrometry (XRS) and electrothermal atomic absorptionspectrometry (ETAAS). Fourier transform infrared (FT-IR) spectrometry and Raman spectrometry wereused to obtain information about the functional groups. It was found that some metal(oid) ions (As, Ni,Pb, Zn) were coordinated through the mercaptan group, while other metal(oid)s (Al, Cd, Cu, Fe, Hg, Mn)were apparently bonded to oxygen atoms. A low-cost and effective solid phase retention system forextraction of heavy metals from aqueous solutions was thus developed.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

The removal of heavy metals from dilute aqueous samples hasattracted considerable interest in the emergent field of

þ34 987291883.

All rights reserved.

environmental clean-up technologies. It has been established thatthe use of solid supports, with chelating and coordinating agentsembedded within the pores or bonded to their surfaces, representsa reasonably cost effective and efficient means for the retention ofheavy metals and many other applications [1]. Supported coordi-nating agents offer several advantages for metal ion removal fromaqueous solutions, allowing selection of the ligands to be used forgiven types of metal ions. Thus, chalcophile elements can be

M.I. Muñoz et al. / Materials Chemistry and Physics 143 (2014) 1469e14801470

particularly retained by ligands containing sulphur and lithophileselements by ligands supporting e.g. oxygen functional groups.Functionalized solid supports successfully used include: poly-styrene impregnated with b-diphenylglyoxime to retain palladium[2], ion-exchange resins for chelating agents easily derivatized orionized for cadmium, cobalt, copper, lead, manganese, nickel, andzinc [3], silica gel functionalized with long-chain hydrocarbon li-gands to remove several trace elements including cadmium, cobalt,copper, nickel, lead and zinc, among others [4], n-dodecylbenzoy-lacetone for copper [5], and oleoylacetone or n-dodecylsalicyldox-imine for nickel and cadmium [6]. Various straight chainbifunctional compounds, including aminoalcohols and mercap-toalcohols, which can formmonolayer films onmany surfaces, havealso been tested for some applications [1,7]. In addition, differentwaste by-products, such as coal fly ashes, have been used to preparezeolitic materials to remove heavy metals [8,9], although little isknown about organically-functionalized coal fly ash and themechanisms involved in the chemical functionalization and metalion retention processes [10].

Coal fly ash is a mixture of oxides, mainly silica and alumina.This amorphous material possesses a framework consisting oftetrahedral units of SiO4 connected by siloxane bridges SieOeSi,together with a similar structure for alumina, where the formationof chemical bonds of SieOeAl is supposedly the reason for theamorphicity of coal fly ash [11]. The framework topology of coal flyash is very similar to that shown by zeolites (hydrated, porouscrystalline aluminosilicate), which also contain an assemblage ofSiO4 and AlO4 tetrahedral units joined together by sharing of oxy-gen atoms. The alumina/silica surfaces are usually covered withdifferent types of very reactive hydroxyl groups when in contactwith an aqueous environment [12]. The silanol groups of the silicasurfaces show weak acidic properties (pKa z 9) [13], while pKa

values for alumina are usually lower, although it depends on thealumina surface considered [14]. The acidebase properties of thesesurface hydroxyl groups are the cause of the adsorption capabilitiesof the alumina/silica surfaces. However, the retention capability onthe surface can be improved using organically-functionalized solidsupports, with the organic ligand bonded to the solid surface pri-marily involved in the retention process [10]. Slightly acidicalumina/silica surfaces facilitate reactions with molecules con-taining oxygenated functional groups [13], where normally thehydrated alumina surfaces show higher reactivity toward carbox-ylic acids than the hydrated silica surfaces [13]. Surface chemi-sorption of organic molecules usually results from an esterification-type reaction, noted strongest at a pH close to the pKa of the organicmolecule [15].

In this work, organically-functionalized coal fly ash was inten-ded to be used as a solid support for the retention of several met-al(oid) ions, Al(III), As(III), Cu(II), Cd(II), Fe(III), Mn(II), Hg(II), Ni(II),Pb(II) and Zn(II), where elucidation of the bond type involved in thefunctionalization and retention processes was the main goal of thestudy. The present work considered condensation as a way ofattaching a mercaptan on to the coal fly ash particle surface. Themercaptans investigated were 2-mercaptoethanol (HSeCH2eCH2e

OH) and thioglycolic acid (HSeCH2eCOOH), which show similarchemical characteristics, with OH and SH groups at either end ofthe molecule. It was expected that these small organic moleculeswould be suitable owing to their polar nature and compact struc-ture [10]. They are also quite strong hydrogen bond acceptors andcapable of coordinating with many metal ions, through their donorsites. The hydroxyl functionality was though to be used to bind theorganic molecules to the solid support, exposing the mercaptogroup above the surface to bind to themetal ion. Nitric acid-etchingand functionalization of the nitric acid-etched coal fly ash, as wellas the retention of heavy metals, are essentially surface chemical

processes and were studied in this work using various comple-mentary surface analytical techniques. Thus, scanning electronmicroscopy/energy dispersive-X ray spectrometry (SEM/ED-XRS)allowed the morphology and the general elemental composition ofthe coal fly ash particles to be evaluated. Raman spectrometrytogether with Fourier transform infrared (FT-IR) spectrometry wereused to investigate functional group and additional vibrationalfeatures associated with the functionalization and retention pro-cesses. In addition, electrothermal atomic absorption spectrometry(ETAAS) was used to determine the amount of metal(oid) retainedby the functionalized coal fly ash.

2. Experimental section

2.1. Instrumentation

2.1.1. SEM/ED-XRSElectron microprobe analysis was performed in a JEOL scanning

electron microscope (Model JSM-6100), equipped with an energy-dispersive X-ray detection system (LINK), operated under recom-mended conditions (15 kV acceleration voltage and 5 nA probecurrent).

2.1.2. ETAASA Thermo Jarrel Ash atomic-absorption spectrophotometer (SH

11) equipped with an electrothermal graphite atomizer (ModelCTF-188) was used. The atomiser was operated with standard un-coated rectangular graphite tubes and standard pyrolytic graphitecoated graphite platforms. Argon (99.995% purity) served as thepurge gas.

2.1.3. Raman spectrometryRaman spectrawere obtained using two spectrometers. The first

was a Renishaw Ramanscope spectrometer fitted with a CCD de-tector and an Olympus optical microscope model BH2-UMA,objective X50 and automatic focal positioning. The Raman spec-trometer was calibrated using a multi-wavelength source. Ramanspectrometry measurements were performed at room temperatureusing the 514.5 nm line of an argon-ion laser as the excitationsource; the power level was set nominally at 100%, but it had to bereduced on several occasions due to saturation of the detector. Theexperimental conditions were 10 s accumulation time, 1 minacquisition time, and a �20 objective; spectra were scanned from200 to 4000 cm�1 with a spectral resolution of 4 cm�1 and theresults were processed using GRAMS20 software.

The other instrument was a BWTEK portable Raman spec-trometer, i-Raman model, fitted with a refrigerated CCD detector.Raman spectra were recorded in the back scattering geometry andat room temperature. Raman spectrometry measurements wereperformed using the CleanLaze 785 nm laser (>300 mW) as theexcitation source; the power level was set nominally at 100%, but ithad to be reduced on several occasions due to saturation of thedetector. The experimental conditions were 10 s accumulation timeand 1 min acquisition time; spectra were scanned from 200 to3100 cm�1 with resolution 4 cm�1. The results were processedusing Grams AI software.

2.1.4. FT-IRInfrared spectra in the 400e5000 cm�1 regionwere recorded on

a Perkin Elmer System 2000 Fourier transform spectrometer(Norwalk, CT, USA) equipped with an air-cooled deuterium trygli-cine sulphate (DTGS) detector. The attenuated total reflection (ATR)accessory utilized was a Perkin Elmer in-compartment HATR ACCY-FLAT (2000), with flat top-plate fitted with a 25-reflection, 45�, 50-mm ZnSe crystal. Reproducible contact between the crystal and the

Table 1Removal of metal(oid) ions (0.5 mg l�1) using thioglycolic acid- and 2-mercaptoethanol-functionalized coal fly ash (CFA) (150 mg) at pH 7.0 for a 20 minretention time.

Ion(M)

Concentration,mM

Thioglycolic acid 2-Mercaptoethanol

Extraction,%

Metal(oid)retained,mmolesM g�1 CFA

Extraction,%

Metal(oid)retained,mmolesM g�1 CFA

Al(III) 18.5 98.0 � 5.0 1.21EXP(-3) 97.0 � 4.0 1.20EXP(-3)As(III) 6.7 98.0 � 2.0 4.36EXP(-4) 96.0 � 3.0 4.27EXP(-4)Cd(II) 4.5 96.8 � 0.5 2.88EXP(-4) 97.0 � 1.0 2.88EXP(-4)Cu(II) 8.0 92.0 � 1.0 4.83EXP(-4) 94.0 � 5.0 4.94EXP(-4)Fe(III) 9.0 99.0 � 6.0 5.89EXP(-4) 97.0 � 4.0 5.70EXP(-4)Hg(II) 2.5 91.0 � 2.0 1.52EXP(-4) 93.0 � 3.0 1.55EXP(-4)Mn(II) 9.0 93.0 � 3.0 5.64EXP(-4) 94.0 � 2.0 5.70EXP(-4)Ni(II) 8.5 95.0 � 1.0 5.39EXP(-4) 96.0 � 2.0 5.45EXP(-4)Pb(II) 2.5 99.2 � 0.3 1.60EXP(-4) 97.0 � 3.0 1.56EXP(-4)Zn(II) 7.5 99.0 � 3.0 5.05EXP(-4) 96.0 � 4.0 4.90EXP(-4)

M.I. Muñoz et al. / Materials Chemistry and Physics 143 (2014) 1469e1480 1471

sample was ensured by use of a variable pressure clamp assembly(2000/GX). Prior to each analysis, a ZnSe background was scannedat 2 cm�1 resolution for each spectrum; 400 scans were coadded. Inan effort to minimize problems from baseline shifts, the spectrawere baseline-corrected and normalized using the maximume

minimum normalization in the OPUS software.A pH-meter (Crison model Digit 505) was used to measure the

acidity of the aqueous phase, when necessary. A Mettler AE 240semi-micro analytical balance (sensitivity � 0.01 mg) was used forweighing the chemicals and samples.

2.2. Chemicals and solid supports

The stock solutions of each metal(oid) contained 1000 mg l�1,which resulted in the following molar concentrations for alumi-nium(III) (w37 M), arsenic(III) (w13 M), cadmium(II) (w9 M),copper(II) (w16 M), iron(III) (w18 M), lead(II) (w5 M), man-ganese(II) (w18 M), mercury(II) (w5 M), nickel(II) (w17 M) andzinc(II) (w15 M), were prepared from nitrate salts in distilled,deionized water. Nitric acid (d ¼ 1.42, 69.8% w/w) and 1 M sodiumhydroxide solutions were employed to adjust pH. In order to cleanall containers and glassware, a 3 M nitric acid solutionwas used forat least 24 h and rinsed three times with distilled, deionized waterbefore use. Distilled, deionized water with a specific resistivity of18 MU cm, from a Millipore water purifier system, was used for thepreparation of the sample and standard solutions. The workingsolutions were prepared immediately prior to their use by serial

Fig. 1. ED-X ray spectra of the untreated coal fly ash (a) and nitric acid-etched coal flyash (b).

dilutions of the stock solutions. Thioglycolic acid, 2-mercaptoethanol and the other chemicals used in this study wereof analytical reagent grade and were purchased from E. Merck(Darmstadt, Germany).

2.3. Procedures and functionalization of the solid sorbent

The coal fly ash used in this work was collected from a factorylocated near to León, Spain. The untreated coal fly ash was treatedwith 1M nitric acid at 293 K for 18 h to oxidize the adhering organicmatter and to remove other soluble species [10]. The resultingmaterial (Fig. 1) was then washed with deionized water, dried at373 K, powdered, ground, and sieved to the desired particle size(below 400 mesh, <37 mm). The density and porosity were1.01 g cm�1 and 0.36 fraction, respectively. The surface area of thesorbent was 45.0 m2 g�1 measured by BET. The average composi-tion (n ¼ 3) of the resulting coal fly ash was derived from ED-XRS(Table S1). The material can be stored in a vacuum desiccator formonths before use. Functionalization of the coal fly ash was carriedout by treating it with 1 M solutions of thioglycolic acid or 2-mercaptoethanol at pH 7.0 and 293 K for 18 h with agitation on amagnetic stirrer hot plate [10]. The functionalized material wasseparated by filtration, washed with deionized water and dried atroom temperature. This material can be stored before use for abouttwo months without loss of its retention capacity. As a generalstatement, untreated material shows a compact mass, while nitricacid-etched and functionalized coal fly ash shows a finer powderedmaterial.

Table 2pH variation during retention of individual metal ions (0.5 mg l�1) with thioglycolicacid-functionalized coal fly ash under neutral conditions. (Similar changes in pHwere noted for the mercapto material).

Metal ion Initial pH Final pH

None 6.7 6.6Al(III) 6.8 5.6As(III) 6.7 4.9Cd(II) 6.6 4.8Cu(II) 6.8 5.1Fe(III) 6.8 5.0Hg(II) 6.6 4.5Mn(II) 6.7 4.8Ni(II) 6.8 4.9Pb(II) 6.7 5.2Zn(II) 6.8 4.7

M.I. Muñoz et al. / Materials Chemistry and Physics 143 (2014) 1469e14801472

2.4. Retention procedure and measurements

Batch experiments were carried out using a series of Erlenmeyerflasks covered with Teflon sheets to prevent any contamination.Aqueous samples (pH 5e7) (10 ml) containing different amounts ofthe heavy metal ions studied were treated for 20 min with thefunctionalized-coal fly ash (15 mg ml�1) [10]. The solid sorbentcontaining the retained metal ion was then separated by centrifu-gation. The isolated pellet was dried before analysis using SEM/ED-XRS, Raman spectrometry and FT-IR spectrometry. The amount ofmetal retained was determined by ETAAS following treatment ofthe isolated pellet with 5 M nitric acid solution. Three separate150 mg samples of coal fly ash were used for each experiment andthe results averaged; this helped compensate for any heterogeneityin the composition and surface features of the powder. The accu-racy of the analytical data obtained in the quantitative analyses wasassessed through recoveries of known amounts of metal ions addedto three separate samples of the solutions. Blanks were preparedwith distilled deionized water.

3. Results and discussion

3.1. Preliminary studies

Table 1 gives details of the retention efficiency of theorganically-functionalized coal fly ash labelled with 2-mercaptoethanol or thioglycolic acid when 10 ml of 0.5 mg l�1

concentrations of individual metal ions at pH 7 were treated with150 mg for 20 min. The retention efficiencies ranged from 91 to 99%for thioglycolic functionalized material and 90e97% for 2-mercaptoethanol functionalized material. The retention effi-ciencies for nitric acid-etched material always ranged from 50 to70%. The retention efficiency for each metal ion solution did notvary much with pH in the range 5e7. However, as Table 2 indicatesfor thioglycolic functionalized coal fly ash, the pH of the solution

Fig. 2. ED-X ray spectra of the coal fly ash functionalized using 0.01 M 2-mercaptoethanol (and 1 mg l�1 Pb (B, D).

after treatment with the powder decreased to different extents fordifferent metal ions (from pH 6.8 to pHs in the range 5.6 (Al(III)) to4.5 (Hg(II))). The reasons for the variable decrease in pH are relatedto differences in the mechanism of the retention process. Althoughthe main goal of the study was investigation of the mechanisms offunctionalization and metal ion retention, and not optimization ofthe retention efficiency, it is clear that for the concentration of themetal ions evaluated good retention characteristics were demon-strated for both functionalized materials.

3.2. Study of the functionalization/retention process e SEM/ED-XRS

To make a preliminary evaluation of the mechanism involved inthe functionalization and retention processes, samples of the un-treated, nitric acid-etched, functionalized coal fly ash and thefunctionalized coal fly ash after retention of the metal(oid) ionswere analyzed by SEM/ED-XRS. Fig. 1 shows ED-XRS spectra of theuntreated coal fly ash and nitric acid-etched coal fly ash. The effectof nitric acid was to clean and homogenize the coal fly ash surface,as a result of an oxidation process. The most noticeable character-istic of the spectrum of nitric acid-etched coal fly ash was theelimination of the sulphur peak, while the calcium peak goes downin intensity and the K and Fe peaks go up in the intensity (Fig. 1). Incontrast, the ED-XRS spectra of the coal fly ash functionalized with2-mercaptoethanol featured the sulphur peak (Fig. 2) confirmingthat the organic molecule was bonded to the particle surface;similar observations were made for coal fly ash functionalized withthioglycolic acid. All the ED-XRS spectra contained the expectedpeaks of the main matrix elements in coal fly ash (e.g. Al, Si, Fe, Ca,K), but when the functionalized material was used to extract metalions, additional or increased peaks appeared in the ED-XRS spectra,as demonstrated for Pb (Fig. 2), and other elements (Fig. 3).

The SEM element distribution maps provided informationabout the spatial distribution of each element on the coal fly ashsurface. However, the retention of As and Pb was clearly stated by

ME) solutions (A, B) and 1 M ME solutions (C, D) and used to retain 0.2 mg l�1 Pb (A, C)

M.I. Muñoz et al. / Materials Chemistry and Physics 143 (2014) 1469e1480 1473

SEM/ED-XRS studies because the X-ray As/Al and X-ray Pb/Al peakratios grown with the amplification used for visualization. The X-ray As peak, as well as the X-ray Hg peak, grown as the X-ray S peakincreased. On the other hand, Fe does not seem to be bound tosulphur, because when the X-ray Fe peak grown, the X-ray S peakdecreased. It also seems that metals like Al and Mn were bound tooxygen. Nonetheless, information related to some elements, suchas Al and Fe, should be considered doubtful because these elementsare main constituents of the coal fly ash matrix. It was also notpossible to follow any general conclusion about other elements,such as Ni, Cu and Zn, from the SEM/ED-XRS studies.

3.3. Study of the functionalization/retention processes e Ramanspectrometry

Study of the sorbent material by Raman spectrometry wasaffected by the presence of a fluorescence background in the

Fig. 3. ED-X ray spectra of the coal fly ash functionalized with 2-mercaptoethanol (ME

spectra. However, shifts in frequencies and variations in the in-tensity of some bands were noted, depending on the treatmentfollowed. Fig. 4 shows Raman spectra of the untreated coal fly ashand the nitric acid-etched coal fly ash (Fig. 4A), as well as of thematerial functionalized with 2-mercaptoethanol (Fig. 4B) and thi-oglycolic acid (Fig. 4C, D). A Raman peak at 1120e1124 cm�1,assigned to SiO2 asymmetric stretching vibrations [16], wasobserved with different intensities in all Raman spectra. The Ramanspectra of the untreated coal fly ash also showed a weak band at510 cm�1 (Fig. 4A and Table 3) assigned to a SeS stretch, indicatingthe presence of small amounts of disulfide probably due to airoxidation of adjacent thiol groups on the surface of the untreatedcoal fly ash [17]. For the nitric acid-etched coal fly ash, the peak at510 cm�1 disappeared, but two new Raman peaks noted at 576 and667 cm�1 were assigned to magnetite, Fe3O4 [18], clearly distin-guishable from other Fe-oxides of a spinel (or inverse-spinel)structure. Spectral changes also occurred on the coal fly ash

) and with thioglycolic acid (TA) used to retain As (A), Hg (B), Mn (C) and Cu (D).

Fig. 4. Raman spectra of the untreated (A), nitric acid-etched (A) and functionalized (B, C, D) coal fly ash (CFA).

Table 4Assignment of Raman peaks in spectra of the metal-retained functionalized coal flyash.

M.I. Muñoz et al. / Materials Chemistry and Physics 143 (2014) 1469e14801474

surface owing to the bonding of the two ligands (Tables 3 and 4).Thus, the mercaptan SeH stretch vibrations (Fig. 4D) appeared asweak bands at 2162 cm�1 [19] and at 2576 cm�1 [17]. Other bandsat w2550 cm�1 and 944 cm�1 (Table 4) were also assigned to SHstretching vibrations [20]. There are other peaks at 394, 615, 736,821 and 968 cm�1 using 2-mercaptoethanol (Fig. 4B) and at 615 and736 cm�1, 1324 and 1412 cm�1 using thioglycolic acid (Fig. 4C). Thepeak at 394 cm�1 (Fig. 4B) was assigned to a n2 SO4

2� bending mode

Table 3Main peaks in Raman spectra of various forms of coal fly ash (See text and Table 4 forassignments).

Metalretained

Raman peaks, cm�1

Untreated-coalfly ash

Nitricacid-etchedcoal fly ash

Coal flyash-MEa

Coal fly ash-TAb

None 510, 1124 576, 667 394, 736, 821,968

615, 736, 1324,1412

Al(III) 679, 932, 1257 808As(III) 555, 620, 1224 549, 562, 574,

1240, 1424Cd(II) 1364 808, 1006,

1303, 1380Fe(III) 620, 954Hg(II) 1070 1053Mn(II) 926, 1364 690Ni(II) 1196Pb(II) 610, 738 768, 892Zn(II) 1186 1233

a ME: 2-mercaptoethanol.b TA: thioglycolic acid.

[21], while the peak at 968 cm�1 was due to the Si-Onbr stretching[22]. The Raman peak occurred at 821 cm�1 (Fig. 4B) was related tothe bending mode of the OeC]O group (out of plane OeH vibra-tion of carbonyl group) [23], although this peak could also be due to

Coal fly ash Peaks, cm�1 Assignment Reference

Functionalized 2553e2544 SeH [20]944 SeH [20]650 SOx [37]460 SeOeS [38]

As(III) 426 AseS [40]375 AseS [40]

Cd(II) 410 CdeO [43]Cu(II) 502 CueO [48]

348 CueO [48]334 CueO [48]

Fe(III) 652 FeeO [44]516 FeeOeSi [45]498 FeeO [46]

Hg(II) 585 HgeO [49]570 HgeO [49]

Mn(II) 654 MneO [47]Ni(II) 370 NieS [41]

350 NieS [41]Pb(II) 460 SeOeS [38]

450 PbeS [39]210 PbeS [39]170 SeH [20]

Zn(II) 647 ZneS [42]616 ZneS [42]

M.I. Muñoz et al. / Materials Chemistry and Physics 143 (2014) 1469e1480 1475

the CeSeC asymmetric stretching vibrations [24]. The band at1324 cm�1 (Fig. 4C) was ascribed to the CH2 wagging mode [19],while the strong peak at 1412 cm�1 was caused by the HCH in-plane deformation [25]. However, the Raman peak at 1412 cm�1

(Fig. 4C) can also be due to ns(CO2) vibrations [26]. According to thesurface selection rules by Moskovits and others [27], the vibrationsignals of the retained organic molecules, which have a componentperpendicular to the surface, would be enhanced. The most infor-mative signals about the molecular conformation are the CeSstretching modes that appeared in the region 600e750 cm�l [28].The Raman bands at 615 and 736 cm�1 were due to the CeSstretching vibrations, particularly related to gauche and trans con-formations, respectively (Fig. 4BeC). However, the trans confor-mation (736 cm�1) usually dominated the spectra, which meansthat the organic molecules were strongly bonded, although incertain disorder.

When the metals were retained by the functionalized coal flyash, several changes occurred in the Raman spectra (see Figs. 5e7and Tables 3and 4). First, the bands assigned to the SeH vibrationdisappeared. This observation would indicate that metals werechemically bonded to the SeH bonds with little or no free SeHbonds present. Once themetal was bonded to the sulphur atom, theu-hydroxyalkanemercaptans would interact through van derWaalsforces and would be ordered in different conformations. Because ofelectrodonation from the sulphur to the metal, the CeS bondweakened, resulting in a red shift of the stretching frequency. Theextent of ordering appeared to be somewhat different dependingon the ligand and metal retained by the functionalized coal fly ash.The CeS bonds have a significant component perpendicular to thesurface, which means that the organic molecules would be orderedon the surface mainly in trans conformations. In the presence of Pb,

Fig. 5. Raman spectra of the metal-retained thiog

the main Raman peaks appeared at 768 and 892 cm�1 with thio-glycolic acid (Fig. 5) and at 610 cm�1 and 738 cm�1 with 2-mercaptoethanol (Fig. 6). The band observed at 892 cm�1 wasdue to ns of the OeSieO from (Si2O7)6� units [29], while the peak at768 cm�1 can be related to the ring-plane bending modes of COO�

[30]. In the low Raman shifts region (Fig. 7A), the peaks at 210 and450 cm�1 confirmed the presence of the PbeS bond (Table 4).

For the functionalized coal fly ash treated with arsenite, an AseSstretch was present at w375 and 426 cm�1 (Fig. 7B), indicating theexistence of an AseS bond [31]. Additionally, the decrease in in-tensity of the SeH stretch with arsenite retention, gives evidencefor the binding of the arsenite directly to the mercaptan group. Theinability of some mercaptan ligands to bind with arsenic would berelated to the inaccessibility of some of these mercaptan groups.However, the Raman spectra also provided information to supportthat some metals were bonded to oxygen instead of sulphur. Thevery small Raman peaks at 549, 562 and 574 cm�1 for thioglycolicacid (Fig. 5B) and at 555 cm�1 for 2-mercaptoethanol (Fig. 6A) wereassigned to AseOeAs symmetric stretching vibrations [32] andsymmetric breathing vibration of the As4O6 tetrahedron [32].Further, the As Raman peak at 620 cm�1 for 2-mercaptoethanol(Fig. 6A) was related to AseO asymmetric stretching vibrations[33].

When Cu was retained on the functionalized coal fly ash, someRaman spectra showed strong bands in the ranges 300e350 and500e550 cm�1 (Fig. 7C and Table 4) due to copper oxide [34]. As thestrong copper oxide band overlaps the weak trans CeS band, it isdifficult to identify this band. The very small peak at 615 cm�1

(Fig. 5C) from the Raman spectra of the iron-retained functional-ized coal fly ash, usually assigned to the CeS stretching [28,35], canshow some contribution from the FeeO stretching vibrations [36].

lycolic acid-functionalized coal fly ash (CFA).

Fig. 6. Raman spectra of the metal-retained 2-mercaptoethanol-functionalized coal fly ash (CFA).

M.I. Muñoz et al. / Materials Chemistry and Physics 143 (2014) 1469e14801476

The Raman spectra of the nickel-retained functionalized coal fly ashprovided a peak at 1196 cm�1 (Fig. 5C), widely assigned to rt(CH2)vibrations [26]. Two other peaks at 350 and 370 cm�1 (Fig. 7B) wereassigned to NieS vibrations. Several other Raman peaks were alsofound (Table 4) [20,37e49] from the low Raman shifts region(Fig. 7), confirming the bonding of Pb, As, Ni and Zn to sulphur [39e42], while the other elements, Cd, Fe, Mn, Cu and Hg, seem to belargely bonded to oxygen [42e49].

3.4. Study of the functionalization/retention process e FT-IRspectrometry

The results obtained by FT-IR spectrometry confirm those foundby Raman spectrometry and consequently the IR spectra will onlybe briefly discussed. The region 3800e2000 cm�1 showed somecharacteristic OH vibrations and particularly CeH stretching vi-brations of the CH2 group when the organic molecules were joinedto the coal fly ash (Figs. 8 and 9A). At this region, the nas(CH2) andns(CH2) narrow bands at 2923 cm�1 and 2850�cme1, respectively,were easily identified [50]. In addition, a very weak n(SeH) vibra-tion was also noted at 2580 cm�1 (Fig. 9A) when Mn was retained[25], particularly from the thioglycolic acid-functionalized coal flyash. Broad bands at 3374 cm�1 (Fig. 8) and 3390 cm�1 (Fig. 9A), dueto stretching vibrations of monomeric hydrogen bonds, corre-sponded to the loosely held water molecules [51].

In the IR region 2000e600 cm�1, the peak at 1740 cm�1,together with the small narrow peak at 1719 cm�1, only appearedwhen Mn and Al were retained (Fig. 9B), and are due to eCOO�

symmetric stretching vibrations [52]. In addition, the relativelystrong IR band at 1540 cm�1 was caused by the eCOO� asymmetric

stretching [52,53], suggesting certain interaction of the carboxylicacid groups with the coal fly ash surface (Fig. 8), thus confirmingchemisorption to the matrix components through an esterification-type reaction, and with some metal ions (Al, Cd, Mn) (Fig. 9B). Theband at 1650 cm�1, noted in several IR spectra (Figs. 8 and 9B), wasdue to the water bending mode [1,52], whose intensity decreasedslightly with the metal-retained functionalized coal fly ash [54].The IR peak at 1598 cm�1, which only appeared when As, Cu, Mnand Ni were retained (Fig. 9B), could originate from water mole-cules weakly hydrogen bonded in the second sphere of hydration[55], but was overlapped by the peak at 1560 cm�1, ascribed to theasymmetric stretching of the carboxylate ion [52]. The IR peaks at1465, 1424, 1400 and 1329 cm�1 (Figs. 8 and 9B) were indicative ofthe CH2 bend vibrations overlapping with CH2 scissoring de-formations [52,56]. The peak at 1329 cm�1 disappeared (Figs. 8 and9) after the retention of the metal ions.

The IR peak at 1048 cm�1 (Figs. 8 and 9C) was due to theasymmetric stretching vibration in SieO(Si) and SieO(Al) structuralunits [57e59], which confirm the amorphous character of thismaterial. This peak increased substantially after bonding of themetal ions (Figs. 8 and 9). The very small peak at 877 cm�1 (Fig. 9C)was ascribed to the non-bonding oxygen in the SieOH bonds/silicanetwork [60] with two additional diagnostic peaks of quartz at 780and 798 cm�1 [61] (Fig. 9C), respectively assigned to the symmetricstretch, ns(SieOeSi), and deformed OeSieO ring vibrations [62].The last peak has also been reported as characteristic of amorphoussilica [50,60,63]. However, the peak at 798 cm�1 can also be due tothe CeSH deformation band [64], indicating some free mercaptangroups. The peaks appeared at 679 and 692 cm�1 (Fig. 9C) origi-nated from deformed 4- or 6-membered ring vibrations of SiO4 or

Fig. 7. Low Raman shifts spectra of the 2-mercaptoethanol-functionalized (A) andmetal-retained 2-mercaptoethanol-functionalized coal fly ash (CFA) (A, B, C).

Fig. 8. FT-IR spectra of the untreated and 2-mercaptoethanol-functionalized coal flyash (CFA).

M.I. Muñoz et al. / Materials Chemistry and Physics 143 (2014) 1469e1480 1477

AlO4 tetrahedra, although the peak at 679 cm�1 has also beenassociated with FeeO out-of-plane vibrations [50]. In addition, thepeak at 645 cm�1 (Fig. 9C) was related to SieOeSi bending [65].

The low wavenumber region provided specific additional in-formation about the metal bonds (Fig. 10) [66e73], suggesting thatPb was partially bonded to sulphate (Fig. 10A and Table 5) [66],although there were other IR spectra peaks ascribed to the PbeSbond [74] (Table 5). Similarly, several other ions, such as As and Niwere also retained as sulphide [68,71] (Fig. 10B and Table 5), whileCd, Fe, Mn and Cu were seen particularly bonded to oxygen[69,70,72,73] (Fig. 10C and Table 5). No characteristic IR peaks werefound related to the retention of the other metal ions considered inthis work.

3.5. Discussion

The functionalization process can be considered analogous tothe combination of two alcohol moieties to produce an ether-likeproduct for 2-mercaptoethanol and an ester-like product for thio-glycolic acid, where n-alkanemercaptans supposedly would form amonolayer on the surface of the coal fly ash. The reaction betweenthe mercaptan (HO-R-SH) and the coal fly ash support (hAl/SiOH)can be schematically summarized as follows,

hAl/SiOH þ HO-R-SH 4 hAl/SiO-R-SH þ H2O

with the following probable mechanism [75],

hAl=Si�OþH2þHO

���� �R�SH4hAl=Si�O

���� �HþH2O

þ�R�SH

hAl=Si� O���� � HþH2O

þ� R � SH4hAl=Si� HO

þ� R � SH

þH2O4hAl=SiO� R � SHþ H3Oþ

where, HS-R-OH refers to HS-CH2-CH2-OH or HS-CH2-CO-OH andhAl/SiO-R-SH represents the functionalized coal fly ash. Thefunctionalization process was found favourable under neutralconditions (pH 7.0), probably because the metal(oid)s studied cangenerate abundant hydroxylated species at high pHs, while theorganic molecules would be deprotonated. In contrast, low pHvalues can produce changes in the chemical structure of the organicmolecules, particularly as a result of a protonation process throughtheir strong donor heteroatoms. Considering the smaller bondstrengths of Al and Si (main matrix constituents of the coal fly ash)when joined to sulphur compared to oxygen (Table 6), thefollowing alternative functionalization reaction,

hAl/SiOH þ HS-R-OH 4 hAl/SiS-R-OH þ H2O

is much less plausible.The bonding of the metal (M2þ) ions by the functionalized coal

fly ash can be seem through a selective coordinative binding of themetals to the sulphur atoms,

2hAl/SiO-R-SH þ M2þ 4 (hAl/SiO-R-S)2M þ 2Hþ

where, (hAl/SiO-R-S)2M represents the metal-retained function-alized coal fly ash. The validity of the proposed reaction pathway

Fig. 9. Three shortened FT-IR spectra of the metal-retained thioglycolic acid-functionalized coal fly ash (CFA).

Fig. 10. Three shortened low wavenumber region FT-IR spectra of the 2-mercaptoethanol-functinalized (A) and metal-retained 2-mercaptoethanol-function-alized coal fly ash (CFA) (A, B, C).

M.I. Muñoz et al. / Materials Chemistry and Physics 143 (2014) 1469e14801478

can be justified by the pH variation found (Table 2) during theretention of the metal(oid)s by the functionalized solid sorbentmaterial. Some elements in solution, particularly arsenic, are pre-sent as anions (i.e., AsO3H¼, AsO3H2

�) at neutral pHs, probablysupporting a different retention mechanism, where a condensationreaction could also be involved, although this behaviour also de-pends on the redox properties of the environment (Fig. S1). Results

from Table 2 indicate that the amount of protons lost as a conse-quence of the coordination between the mercaptan moiety and themetal(oid) ion is almost quantitative to the amount of the metal(-oid) ion at the onset of the experiment. The weakly acidicmercaptan moiety (pKa ¼ 9.72 for 2-mercaptoethanol andpKa ¼ 10.61 for thioglycolic acid) [76] would facilitate the bond tothe metal(oid) ions. Participation of protons from the hydroxyl

Table 5Characteristic IR peaks of the metal-retained functionalized coal fly ash.

Coal fly ash IR peaks, cm�1 Assignment Reference

Functionalized 1149 SO42� [66]

710 CeS [67]620 SeS [67]

As(III) 360 AseS [69]343 AseS [69]

Cd(II) 550 CdeO [70]Cu(II) 606 CueO [74]

503 CueO [74]437 CueO [74]

Fe(III) 631 FeeO [70]572 FeeO [70]

Hg(II) Unidentified peaksMn(II) 566 MneO [73]Ni(II) 395 NieS [71]

315 NieS [72]Pb(II) 835 PbeS [74]

668 PbSO4 [68]623 PbSO4 [67]592 PbSO4 [67]

Zn(II) Unidentified peaks

M.I. Muñoz et al. / Materials Chemistry and Physics 143 (2014) 1469e1480 1479

group for the case of 2-mercaptoethanol, and from the carboxylicgroup for the thioglycolic acid, in the retention of the metal(oid)ions through the interchange proton/metal(oid) ion would not bepossible because the hydroxyl group shows no acidic propertiesand the carboxylic group (pKa ¼ 3.6) would be already deproto-nated at the neutral pHs used in the retention process.

It is known that Al, Fe and Mn ions, with an inert gas electronicstructure and showing high charge/radii ratios at neutral pHs,exhibit a tendency to be joined to oxygen atoms, which agrees withthe experimental results from this work for these elements. It isworth bearing in mind that at neutral pHs, Al, Fe and Mn can bepartially precipitated as hydroxides (Mn(II) is easily oxidized toMnO2), which would be removed together with the solid sorbentduring the centrifugation stage. On the other hand, Al, Fe and Mnoxides are main constituents of the coal fly ash matrix whichcomplicates interpretation of data when trying to assess theretention of aqueous ions of these elements by the functionalizedcoal fly ash. Other elements, such as Zn, Pb, Ni and As, whose ionsdo not adopt an inert gas electronic structure, tend to be joined tosulphur, which also partially agrees with the results found in thiswork. However, differences in the bond strengths for some metalswhen joined to sulphur and oxygen are not so important (Table 6).In addition, Hg, Cu and Cd, should be theoretically included in thisgroup, but the experimental results do not support this proposition,probably due to the low solubility product of the correspondingoxides/hydroxides. In conclusion, part of these findings can betheoretically explained by the differences between the bond

Table 6Bond strengths for the metal(oid) ions (M) considered in this work when joined tosulphur (S) and oxygen (O) and the corresponding solubility products of the hy-droxides and sulphides [76].

Ion MeS, kJ mol�1 MeO, kJ mol�1 Kps(M-OH) Kps(M-S)

Al(III) 373.6 � 7.9 511 � 3 10�33.5 e

As(III) 379.5 � 6.3 481 � 8 10�15.0 10�5.3

Cd(II) 208.4 � 20.9 235.6 � 83.7 10�14.4 10�27.4

Cu(II) 276 269.0 � 20.9 10�20.4 10�35.2

Fe(III) 322 390.4 � 17.2 10�38.8 10�85.0

Hg(II) 217.1 � 22.2 220.9 � 33.1 10�25.5 10�52.4

Mn(II) 301 � 17 402.9 � 41.8 10�12.7 10�13.5

Ni(II) 344.3 382.0 � 16.7 10�15.2 10�25.7

Pb(II) 346.0 � 1.7 382.0 � 12.6 10�15.2 10�27.9

Zn(II) 205(13) 284.1 10�15.5 10�23.8

Si(IV) 623 799.6 � 13.4 e e

strengths of the metal(oid) ions when joined to sulphur and oxy-gen, as well as the corresponding solubility products of hydroxidesand sulphides (Table 6) [76]. However, the real retention processwould be much more complex because the standard enthalpychange during such a process is really the contribution from theintrinsic enthalpy change, related to the fixation of metal ions onthe retention sites, and the hydration standard enthalpy of themetal ion, related to the interaction strength between the solubi-lized ion and its hydration sphere. The intrinsic standard enthalpychange is always exothermic, while the hydration standardenthalpy change is endothermic. Those ions showing high charge/size ratio and consequently relatively high hydration enthalpiesusually show exothermic retention behaviour, but this behaviourcan change depending on the conditions used. In addition to thestandard enthalpy, other parameters such as entropy changes, thecoordination number or spatial configuration (steric effect), pHchanges in the solid microenvironment, metal(oid) concentration,amongst others can also show simultaneously strong effects on theformation and stability of bond. This means that any comparisonand/or evaluation using standard thermodynamic data are onlytentative approximations and conditional constants should beconsidered, particularly because a mixture of several mechanismscould be involved. In other words, each particular process needs tobe theoretically solved for each individual situation. Simultaneousremoval of different ions with this functionalized solid sorbentwould follow similar patterns for each of the studied ions withsome competition between them, but quantitative informationabout such process is really out of the aim of this work.

4. Conclusion

Functionalization of the nitric acid-etched coal fly ash with 2-mercaptoethanol or thioglycolic acid was spectrometrically char-acterized, and the mechanism of retention of metal ions fromaqueous solution investigated The use of the bifunctional sub-strates containing a hydroxyl moiety was shown to be an effectiveapproach to the chemical modification of the coal fly ash solidsupport. It was also indicated that the mercaptan group wasparticularly available for coordination to some metal(oid) ions (As,Ni, Pb, Zn). Nonetheless, a partial oxidation of the mercaptan groupseems to be possible, as some of the results also suggest that Pbcould be partially retained as sulphate. However, other metal(oid)s(Al, Cd, Cu, Fe, Hg, Mn) were apparently bonded to oxygen atoms,probably from the coal fly ash framework and less probably fromthe organic molecule. Some of these elements are main oxideconstituents of the coal fly ashmatrix whichmade interpretation ofthe spectral data more complex regarding metal retention anisms.Use of these coal fly ash-supported bifunctional compounds canafford a relatively inexpensive method of effective removal ofselected heavy metal(oid) ions and could make a considerablecontribution to environmental clean-up technologies. The resultsreported also show that measurements by SEM/ED-XRS com-plemented those of Raman and IR spectrometry to provide a betterunderstanding of the binding of metal(oid) ions to the organically-functionalized coal fly ash surface. In general, results from this workprovide important data contributing to knowledge about func-tionalization of the coal fly ash and retention of metal(oid)s.

Acknowledgements

The financial support by a grant (Ref.: LE050A06) from theConsejería de Educación (Junta de Castilla y León) is gratefullyacknowledge. One of the authors (MIM) is thankful to the SpanishMinisterio de Educación y Ciencia for support under the FPUProgram.

M.I. Muñoz et al. / Materials Chemistry and Physics 143 (2014) 1469e14801480

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.matchemphys.2013.12.002.

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