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Materials Chemistry and Physics 129 (2011) 168– 175

Contents lists available at ScienceDirect

Materials Chemistry and Physics

jo u rn al hom epage : www.elsev ier .com/ locate /matchemphys

ynthesis of functionalized silica gel with poly(diethylenetriamine bis(methylenehosphonic acid)) and its adsorption properties of transition metal ions

ing Yin ∗, Yuan Tian, Zengdi Wang, Rongjun Qu ∗, Xiguang Liu, Qiang Xu, Qinghua Tangchool of Chemistry and Materials Science, Ludong University, Yantai 264025, PR China

r t i c l e i n f o

rticle history:eceived 27 August 2010eceived in revised form 25 March 2011ccepted 27 March 2011

a b s t r a c t

A novel functionalized silica gel with poly(diethylenetriamine bis(methylene phosphonic acid))(SH–DETA–PDBMPA) was synthesized in order to improve the adsorption for transition metal ions fromaqueous solutions. The material was characterized using infrared spectra (IR), X-ray diffractometry (XRD),scanning electron microscope (SEM), Brunauer–Emmett–Teller (BET) surface area measurement, porous

eywords:unctionalized silica geloly(diethylenetriamine bis(methylenehosphonic acid))ransition metal ions

analysis, thermogravimetric analysis (TG) and 29Si Magic Angle Spinning Nuclear Magnetic Resonance(29Si MAS NMR). The application of this material in adsorption of transition metal ions revealed thatSH–DETA–PDBMPA had excellent adsorption amount and high selectivity for Au(III), which provided anovel material for adsorbing this specious metal ions from aqueous solutions.

© 2011 Elsevier B.V. All rights reserved.

dsorption

. Introduction

In recent years, increasing attention has been paid to silica gelue to its excellent thermal and mechanical stability, unique largeurface area, well-modified surface properties. Generally, it is dif-cult for organic functional groups to bond to silica gel directlyecause of the relative inertness of the original surface of silica gel.owever, bonding of organic functional groups to silica gel sur-

ace can be achieved after surface activation and modification. Asn amorphous inorganic polymer, silica gel is composed of internaliloxane groups (Si–O–Si) with a large number of silanol groups

Si–OH) distributed on the surface. The kind of functionalizedaterials based on silica gel has received a great deal of atten-

ion recently because of their excellent performance in the fieldf chromatography, adsorption, ion exchange, solid phase extrac-ion, metal ion preconcentration, catalysis and so on [1–4]. The

ost common method of silica gel surface modification invol-es the reaction of surface hydroxyl groups with silane couplinggents which act as precursors for further immobilization of orga-ic groups, chemical modification of the skeleton of silica gel viahe covalent coupling of an organic moiety is a promising approacho obtain such kind of silica gel-based materials [5], and it has beenound that the behavior of these solids used as adsorbent are mainly

ependent on the presence of active donor atoms such as O, S and

of the incorporated organic moieties [6–8].

∗ Corresponding author. Tel.: +86 535 6696162/6699201; fax: +86 535 6697667.E-mail addresses: yinping426@163.com (P. Yin), rongjunqu@sohu.com (R. Qu).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.03.067

On the other side, the contamination of heavy metals has recen-tly increased in the environment, especially in the industrial areas.Heavy metals are not biodegradable and thermodegradable, andcan be harmful to human, plants and animals. Therefore, the remo-val and recovery of heavy metal ions from industrial wastewaterhave been a significant concern in most industrial branches for eco-nomic and environmental factors. In the recent years, attention wasfocused on the methods for recovery and reuse of metals ratherthan disposal. Many treatment processes such as chemical pre-cipitation, electrodialysis, adsorption, are currently used. Amongthese methods, adsorption is highly effective and economical, andis a promising and widely applied method [9,10]. Consequently,effective adsorbents with strong affinities and high loading amountfor heavy metal ions were subsequently prepared by functiona-lizing the surface of various substrates, such as activated carbon[11], clay [12], zeolite [13], resin [14] and so on. In the relativeresearch work, people found phosphonic acid groups had the abilityof cation exchange, and the existence of oxygen atoms in P–O andP O groups makes the group coordinate with a variety of transitionmetal ions. If the organophosphonic acid groups are grafted on thesolid matrix such as silica gel, this kind of chemical modificationcan overcome its shortcomings of being toxic and soluble in water,and be used in adsorption of metal ions from aqueous solutions.

The aim of this work is to prepare functionalized silica withorganophosphonic acid groups and to get more information on itsadsorption properties of transition metal ions from aqueous solu-

tions. The introduction of the organic phosphonic acid groups ontosilica gel can make the material form the stable chelating com-pounds with transition metal ions. In the present work, we exploredthe synthesis of silica gel supported with poly(diethylenetriamine

P. Yin et al. / Materials Chemistry and Physics 129 (2011) 168– 175 169

OH

sil ica-g el

SiH3COOCH3

OCH3

Cl H2NHN NH2

HO

OSi

HN

OCH3

OCH3

NH

NH2

SH-DET A

SH-DETAHCl

H

HO

OHPH

HO

O

SH

OSi

HN

OCH3

OCH3

NH

N

POH

OH

POH

OHO

e pro

bddavHwgewcatMva

2

2

YsttijCca

pCaxap

2

(t

Scheme 1. Synthesis of th

is(methylene phosphonic acid)) with both N donor atoms and Oonor atoms, which could make the material have excellent coor-ination properties with transition metal ions and obtain a noveldsorbent with high loading of metal ions and sorption selecti-ity for some particular metal ions. After its adsorption of Au(III),g(II),Cu(II), Pb(II), Co(II), Zn(II), Ni(II), Cr(III) and Cd(II) metal ionsas studied, the results displayed that SH–DETA–PDBMPA had

ood adsorption amount for Au(III), Hg(II) and Cu(II) metal ions,specially for Au(III) ion. It is well known that precious metals areidely used in the fields of industry and medicine due to their spe-

ific physical and chemical properties. Just because of the valuend scarcity of precious metals such as gold, it is necessary to treathe waste aqueous solutions and try to recover them economically.

oreover, the adsorption mechanism and the adsorption selecti-ity of this functionalized silica gel adsorbent for Au(III) ions fromqueous solutions were investigated.

. Experimental

.1. Chemicals and materials

Silica gel (SH) of chromatographic grade (120–200 mesh) was purchased fromantai Chemical Institute, China. The silica gel was activated with nitric acid aqueousolution (HNO3:H2O = 1:1) at refluxing temperature of 112 ◦C for 3 h, and then it wasreated with hydrochloric acid aqueous solution (HCl:H2O = 1:1) at room tempera-ure for 6 h, Subsequently, the silica gel was washed with distilled water and driedn muffle at 160 ◦C for 12 h. In our study, organic solvent toluene was redistilledust before use. The silylant agent 3-chloropropyltrimethoxysilane (CPTS) (Jianghanhemicals Factory, Jinzhou, China), diethylenetriamine (DETA) (Shanghai Chemi-al Factory of China) and the other reagents such as sodium hydroxide etc. werenalytical grade and used without any further purification.

Stock solutions of containing various metal ions at a certain concentration wererepared by dissolving their relative metal salts (Sinopharm Chemical Reagento., Ltd., China) in distilled water. The pH value of solution containing Au(III) wasdjusted with hydrochloric acid aqueous solution (1 mol L−1) and sodium hydro-ide aqueous (1 mol L−1), while those of solution containing other metal ions weredjusted with ammonium acetate/nitric acid solutions. Distilled water was used torepare all the solutions.

.2. Instruments

Infrared spectra (FTIR) of samples were obtained on a Nicolet MAGNA-IR 550series II) spectrophotometer, USA. Test conditions were as follows: scanning 32imes, KBr pellets in the 4000–400 cm−1 region with a resolution of 4 cm−1, and

SH-DETA-PDBMPA O

duct SH–DETA–PDBMPA.

the data were treated with Thermo Nicolet Corporation OMNIC32 software. Themorphology of the compounds was examined on JEOL JSF5600LV scanning electronmicroscope, JEOL Co., Japan. The EDXAS was performed on a NORAN LEVER-2 EDXanalytical instrument. Before observation, the sample was placed on a specimen stubcovered with a conductive adhesive tab and provided with a sputtered 15-nm pla-tinum coating. Porous structure parameters were characterized using an automaticphysisorption analyzer ASAP 2020 (Micromeritics Instruments Corporation, USA)by BET and BJH methods through N2 adsorption at 77 K. The N analysis of productswas analyzed by elemental analyzer Vario EL III. Thermogravimetric analysis (TG)was recorded on a Netzsch STA 409, Test conditions: type of crucible, DTA/TG cru-cible Al2O3; nitrogen atmosphere, flow rate 30 mL min−1; heating rate: 10 K min−1.The solid-state 29Si Magic Angle Spinning Nuclear Magnetic Resonance (29Si MASNMR) measurements were performed on the Varian Infinityplus-400 spectrometerequipped with a 5 mm MAS probe. Atomic absorption analysis of transition metalions was performed with a flame atomic absorption spectrophotometer (GBC-932A,made in Australia).

2.3. The synthesis of silica gel chemically modified adsorbent SH–DETA–PDBMPA

The synthesis of intermediate silica gel functionalized with polyamino-terminated polymers (SH → SH-DETA) were carried out as reported in our previousworks [15], the synthesis route of SH–DETA–PDBMPA was shown in Scheme 1, andthe detailed procedures were as follows:

Under nitrogen atmosphere, a mixture of 47.0 mL of diethylenetriamine and10.0 mL of CPTS were stirred at 80 ◦C in 100 mL of ethanol solution for 12 h, andthen the product was distilled until there is no ethanol in it, and then 8.0 g of acti-vated silica gel was added with 120 mL toluene as solvent. The mixture was stirredat 110 ◦C for 12 h, and then the solid was filtered off and transferred to a Soxhletextraction apparatus for reflux-extraction in ethanol for 24 h. The solid product wasdried in vacuum at 50 ◦C over 48 h, and it was referred to as SH-DETA. Furthermore,8.0 g of SH-DETA were added to 70 mL ethanol at room temperature for 12 h, thenadded 2.7 g of paraformaldehyde (CAS number: 68476-52-8), 7.3 g of phosphorousacid and 2.5 mL of hydrochloric acid. After refluxing at 90 ◦C for 12 h, the product(SH–DETA–PDBMPA) was filtered off, then washed thoroughly with distilled watertill acid-free and finally dried under vacuum over 48 h at 50 ◦C.

2.4. Adsorption experiments for transition metal ions

Saturation adsorption experiment was employed to determine the adsorptionamounts of SH–DETA–PDBMPA for different kinds of metal ions, and they werecarried out with shaking 0.02 g of adsorbents with 10 mL of metal ion solution(2 mmol L−1). The mixture was equilibrated for 24 h on a thermostat-cum-shaking

assembly at 25 ◦C.

The adsorption amount was calculated according to the Eq. (1):

q = (Co − Ce)VW

(1)

170 P. Yin et al. / Materials Chemistry and Physics 129 (2011) 168– 175

F

wlv

2

0nmt

3

3

fssdavistfiSbanwycv3stdwtraNsobtstob

ig. 1. FT-IR spectra of silica gel SH(1), SH-DETA(2) and SH–DETA–PDBMPA (3).

here q is the adsorption amount (mmol g−1); Co and Ce are the initial and equi-ibrium concentrations of metal ions(mmol ml−1) in solution, respectively; V is theolume of the solution (mL); W is the weight of SH–DETA–PDBMPA (g).

.5. Competitive adsorption

In order to investigate the adsorption selectivity of the adsorbent for Au(III),.02 g of adsorbents were added into 10 mL solutions (binary system which contai-ing equal initial concentrations (2.5 mmol L−1) of Au(III) ion and other coexistingetal ions) and the mixture were shaken for 12 h, and the initial pH was adjusted

o 2.0 at 25 ◦C.

. Results and discussion

.1. FT-IR analysis

In this synthesis process, silanol groups on the silica gel sur-ace were the mainly initiator sites, which readily react with theilane coupling agent 3-chloropropyltrimethoxysilane to introduceurface functional groups. Therefore, pre-treatment process of aci-ification by nitric acid and hydrochloric acid and the subsequentctivation process in muffle at high temperature before use areery necessary. The infrared analysis is a very useful technology indentifying the immobilization process by comparing the precur-or and modified surfaces [16], and it was determined to confirmhat the structure characteristic of the obtained chemically modi-ed silica gels. The FT-IR spectra of the original Silica gel SH, andH-DETA and SH–DETA–PDBMPA were shown in Fig. 1. A largeroad between 3200 and 3500 cm−1 of the silica gel spectrum wasttributed to the presence of the O–H stretching frequency of silia-ol groups and also to the remaining adsorbed water. Comparedith the spectrum of silica gel, the spectrum of SH-DETA displa-

ed that two new bands appeared at 2933 and 2851 cm−1, whichorresponded to the typical asymmetric and symmetric stretchingibration of –CH2–, due to the presence of the carbon chain of-chloropropyltrimethoxysilane and polyamines attached to theilica gel. Moreover, new band appeared at 1391 cm−1 was assignedo the bending vibration of N–H transferred to lower frequenciesue to the stretching vibration of remaining Si–O of the silica gelhich strongly absorbed at 1633 cm−1 [6]. Furthermore, we found

hat the band around 968 cm−1 of the free silanol groups disappea-ed because of the reaction with the alkoxide groups of the silylantgent functionalized with polyamines, and stretching vibration of–H were overlapped in the range of 3725–3052 cm−1. These facts

uggested that organic polyamines were successfully introducednto the surface of silica gel. As to that of SH–DETA–PDBMPA, theand at 1391 cm−1 ascribed to the –NH2 and –NH– bending vibra-ion and the strong peak centered at 3432 cm−1 contributed to the

tretching vibration of N–H were both weakened, indicating thathe organophosphonic acid groups were successfully introducednto the amino-terminated chelating resin silica gel. However, theonds P O at 1 175 cm−1 and the characteristic sorption peak of

Fig. 2. XRD patterns of silica gel SH(1), SH-DETA(2) and SH–DETA–PDBMPA (3).

P–OH around 930 cm−1 were both overlapped in the broad bandbetween 1341 and 870 cm−1.

3.2. XRD analysis

The XRD patterns of silica gel SH, SH-DETA andSH–DETA–PDBMPA were displayed in Fig. 2, and it represen-ted that all the diffraction peaks of silica gel SH and its derivativesappeared at 23◦, which was the amorphous diffraction peaks ofsilica gel. This result showed that there was no an essential changeoccurred in topological structure of silica gel before and afterthe functionalized reactions, which implied that silica gel wasstable enough to experience the chemical modification reactions.Moreover, it was noted that the polarity of grafted organic moietieshad an evident effect on the relative intensity of the diffractionpeaks. The introduction of amine groups and organophosphonicacid groups led to the slight decrease of intensity of diffractionpeak. This may be explained by the formation of hydrogen bondbetween basic amines/acidic phosphonic acids and unreactedsurface silanols. The decrease in the intensity of the XRD diffrac-tion peak in SH–DETA–PDBMPA provided further evidence thatgrafting mainly occurred inside the mesopore channels, since theattachment of organic functional groups in the mesopore channelstended to reduce the scattering power of the mesoporous silicatewall. It also suggested that the structural order of the synthesizedmaterial was maintained after the functionalization form the XRDpattern of the functionalized silica gel [17,18]. On the other side,no novel diffraction peak appeared after reactions meant that thehighly branched polymers on the surface of silica gel existed in aform of non-crystalline state.

3.3. SEM analysis

Morphologies of the silica gel, the intermediate and the productobtained from the relative synthesis process were characterized bySEM shown in Fig. 3, and SEM was performed on the bare silicaand chemically modified particles in order to detect differences intheir surfaces. Apparently, the surface of bare silica gel was smoothand became rough after the modification reactions. Moreover, weobserved that no clog between particles occurred during the prepa-ration process, and the particles maintained regular lumpy shape.It could be seen that the particle appearance and size of thesethree samples were similar, demonstrating that the particles ofsilica gel had good mechanical stability and they had not been des-troyed during the whole reaction. Scanning electron micrographsof SH, SH-DETA and SH–DETA–PDBMPA in Fig. 3 were obtained

at 100×, 750×and 5000× magnification. Low magnification SEMof SH–DETA–PDBMPA (Fig. 3(a)) was displayed to clarify the un-agglomeration of the silica gel particles after treatment to supportthe claiming of regular distribution of the functional group on the

P. Yin et al. / Materials Chemistry and Physics 129 (2011) 168– 175 171

-DETA

wgobb

3

Sbtwicttp

Fig. 3. SEM images silica gel SH(1-a, 1-b), SH

hole surface. Moreover, it was evident that the loaded functionalroups were distributed on the whole surface that made the surfacef the intermediate SH-DETA and the product SH–DETA–PDBMPAecome rough and may block part of the pore region, which woulde verified by the following porous structure analysis.

.4. Porous structure analysis

The nitrogen adsorption–desorption isotherms for silica gel SH,H-DETA and SH–DETA–PDBMPA were shown in Fig. 4, and It coulde seen that silica gel and its derivatives were type IV accordingo the IUPAC classification and each had a hysteresis loop thatas representative of mesopores. The volume adsorbed steeply

ncreases at a medium relative pressure (p/p0) indicating capillary

ondensation of nitrogen within the uniform mesoporous struc-ure, and the two lines are approximately parallel representing thathe pores of silica have uniform radius and are open. These openores are very favorable for the bond reaction conducted on the sur-

(2-a, 2-b) and SH–DETA–PDBMPA (3-a, 3-b).

face of silica gel, and the inflection position shifted toward lowerrelative pressures and the volume of nitrogen adsorbed decreasedwith functionalization. The BJH desorption pore size distributionsof silica gel and its derivatives were displayed in Fig. 5. As illus-trated in Fig. 5, the pores between 5 and 15 nm were dominantfor all products. With the proceeding of the reaction, the amountof the pores between 5 and 15 nm became smaller gradually andthe pore size distribution moved to the smaller pore size. However,those relative values of SH–DETA–PDBMPA became greater thatthose of SH-DETA. The porous structure parameters of the sam-ples derived from the basis of the nitrogen adsorption data weresummarized in Table 1. As seen in Table 1, the values of BET sur-face area, BJH desorption average pore radius and BJH desorptioncumulative volume of pores for silica gel decreased with the func-

tionalization reactions. This fact could be interpreted as follows:the molecules of reagent first diffused into the interior of pores ofsilica gel, and then reacted with the active sites. With the forma-tion of grafted pendant organic chains and crosslinking products,

172 P. Yin et al. / Materials Chemistry and Physics 129 (2011) 168– 175

Fig. 4. Nitrogen adsorption–desorption isotherms of

FS

tdtItS

TP

interval, an first mass loss 4.0% was attributed to physisorbed watermolecules released and a second mass loss 2.2% form 170 ◦C to800 ◦C was attributed to the condensation of silanol groups bondedto the surface. Similar to silica gel, the functionalized silica gels also

ig. 5. BJH desorption pore size distributions of silica gel SH (1), SH-DETA (2) andH–DETA–PDBMPA (3).

he pores size of silica gel became smaller and even some poresisappeared In addition, the presence of pendant organic chains onhe surface partially blocked the adsorption of nitrogen molecules.

t should be noted that BET surface area, BJH desorption cumula-ive volume of pores and BJH desorption average pore radius ofH–DETA–PDBMPA were 199.15 m2 g−1, 0.70 cm3 g−1, and 45.25 A,

able 1arameters of porous structure of silica gel SH, SH-DETA and SH–DETA–PDBMPA.

Adsorbents BET surfacearea (m2 g−1)

BJH desorptioncumulative volume ofpores (cm3 g−1)a

BJH desorptionaverage poreradius (Å)

Silica gel 265.12 0.85 47.95SH-DETA 150.78 0.56 42.68SH–DETA–

PDBMPA199.15 0.70 45.25

a The total volume of pores between 8.5 A and 1500 A radius.

silica gel SH, SH-DETA and SH–DETA–PDBMPA.

respectively, which were greater than those of SH-DETA, whichcould be interpreted by the formation of hydrogen bond betweenphosphonic acid group with unreacted surface silanol as well asthe partial hydrolysis of the sample with sodium hydroxide utili-zed to remove superfluous hydrochloric acid during the chemicalmodification process.

3.5. Thermal analysis

The thermogravimetric curves reflect the thermal stability of thematerials and the thermal stabilities of silica gel and its derivativeshave been determined by thermogravimetric analysis at the rangeof 25–800 ◦C, and the results were shown in Fig. 6. As could be seenin curve 1, for silica gel in the room temperature (25 ◦C) to 170 ◦C

Fig. 6. Thermogravemetric curves of silica gel SH (1), SH-DETA (2) andSH–DETA–PDBMPA (3).

P. Yin et al. / Materials Chemistry and Physics 129 (2011) 168– 175 173

paaclaodgb

3

aMSi−acmwwIato

3

Amywpa00aCpccoi

2 3weakening of peaks at 1626 cm−1 attributed to the characteristicpeak of –NHCO and –NH2 can be considered as the evidence of thecoordination of–NHCO and–NH2 with Au(III). Moreover, The static

Table 2The regeneration properties of SH–DETA–PDBMPA.

Regenerationtimes

SH-DETA-PDBMPA

1 q (mmol g−1)Desorption rate (%)

1.4199.52

Fig. 7. 29Si MAS-NMR spectra of SH and SH–DETA–PDBMPA.

resented two distinct stages of mass loss. The first mass loss wasssigned to adsorbed water and the other mass loss from which wasttributed to the organic ligands anchored on the surface and theondensation of remaining silanol groups to produce siloxane. Theosses of SH-DETA and SH–DETA–PDBMPA below 255 ◦C are 8.0%nd 4.0%, respectively, and the total losses at the temperature rangef 255–800 ◦C are 14.8% and 10.0%, respectively. Therefore, theseata indicated that the resulting product SH–DETA–PDBMPA hadood thermal stability and it should be applied at the temperatureelow 255 ◦C.

.6. 29Si MAS-NMR spectra

In order to investigate the changes of surrounding of siliconfter the functionalization, the 29Si Magic Angle Spinning Nuclearagnetic Resonance (29Si MAS NMR) measurements of SH and

H–DETA–PDBMPA were carried out, and the spectra were shownn Fig. 7. Unmodified silica (SH) showed main peaks at −112 and105(sh) ppm and these could be assigned to ((SiO)4Si) silica sitesnd ((SiO)3SiOH) silanol sites, respectively. ((SiO)4Si) sites werelearly the dominant peak in both spectra because they were theost abundant sites. The spectrum of the functionalized silica sho-ed a marked decrease in the intensity of the ((SiO)3SiOH) sites,hich verified the anchoring of the functional groups to Si–OH [18].

n addition, one new peak appeared at −62(sh) ppm, which could bessigned to ((SiO)3Si–R) sites. These results further indicated thathe organophosphonic acid groups were successfully introducednto the silica gel solid matrix.

.7. Adsorption properties

The saturation adsorption amounts of SH–DETA–PDBMPA foru(III), Hg(II),Cu(II), Pb(II), Co(II), Zn(II), Ni(II), Cr(III) and Cd(II)etal ions were shown in Fig. 8. The research result displa-

ed the static adsorption amounts for Au(III), Hg(II) and Cu(II)ere 1.173 mmol g−1, 0.456 mmol g−1, and 0.298 mmol g−1, res-ectively. However, those for Pb(II), Co(II), Zn(II), Ni(II), Cr(II)nd Cd(II) metal ions were 0.189 mmol g−1, 0.0760 mmol g−1,.0659 mmol g−1, 0.0606 mmol g−1, 0.189 mmol g−1 and.0760 mmol g−1, respectively. Obviously, the as-synthesizeddsorbent had good adsorption amount for Au(III), Hg(II) andu(II) metal ions, especially for Au(III) ion,. Through the aminohosphonic acid groups, SH–DETA–PDBMPA can form the stable

helating compounds with many transition metal ions, espe-ially with Au(III). As we compared the adsorption amountf different types of adsorbents using for Au(III) adsorptionn the literature, It was clear that the adsorption amount of

Fig. 8. The saturation adsorption amounts of SH–DETA–PDBMPA for transitionmetal ions: initial solution 10 ml 2 mmol l−1; pH = 5.0 (except for pHAu = 2.0); 25 ◦C.

SH–DETA–PDBMPA was relatively higher than those of severalother adsorbents such as thiol cotton fiber, Alfalfa biomass,vinylbenzylchloride–acrylonitryle–divinylbenzene copolymersmodified with tris(2-aminoethyl)amine, and l-lysine modifiedcrosslinked chitosan resin [19–22]. According to the theory of hardand soft acids and bases (HSAB) defined by Pearson, metal ionswill have a preference for coordinating with ligands that havemore or less same electronegative donor atoms. Chelating agentswith N and O donor atoms are highly efficient for the selectivesorption of precious metal ions. Therefore, in the following part,the adsorption of the adsorbent SH–DETA–PDBMPA for Au(III) wasinvestigated particularly.

To test the reusability of the adsorbent SH–DETA–PDBMPA,the Au(III) ion loaded SH–DETA–PDBMPA sample were treatedwith 0.1 mol L−1 hydrochloric acid and different concentrationsof thiourea, respectively, for 12 h to remove the Au(III) ions andthen neutralized and followed with a second round of metal ionadsorption testing. The results for Au(III) ion adsorption using theregenerated adsorbents are summarized in Table 2. Only a littledecrease of the adsorption efficiency was seen in the second use,and the samples retain their Au(III) uptake amounts of 91.12% aftertwice use cycles. Then the Au(III) uptake amounts decreased gra-dually in the next successive uses. Therefore, the high adsorptionamount and good reproducibility make this hybrid material had asignificant potential for removing Au(III) from aqueous solutionsusing adsorption method.

The FT-IR spectra of SH–DETA–PDBMPA before (2) and after (1)adsorption for Au(III) were shown in Fig. 9. The peak at 1388 cm−1

in the spectrum of SH–DETA–PDBMPA–Au is a characteristic peakfor the stretching vibration of NO in the residual NO − ions. The

2 q (mmol g−1)Desorption rate (%)

1.2095.98

3 q (mmol g−1)Desorption rate (%)

1.0191.12

174 P. Yin et al. / Materials Chemistry and Physics 129 (2011) 168– 175

Table 3The adsorption selectivity of SH–DETA–PDBMPA for Au(III)(Au(III)concentration: 2.5 mmol L−1; concentration of coexisting metal ions: 2.5 mmol L−1; pH = 2.0; T = 25 ◦C).

Adsorbent System Metal ions Adsorptionamount(mmol g−1)

Selectivecoefficient

Au(III)–Hg(II) Au(III)Hg(II)

1.460.35

˛Au(III)/Hg(II) = 4.2

Au(III)–Cu(II) Au(III)Cu(II)

1.380.11

˛Au(III)/Cu(II) = 12.4

Au(III)–Cd(II) Au(III)Cd(II)

1.320.04

˛Au(III)/Cd(II) = 33

Au(III)–Pb(II) Au(III) 1.29 ˛Au(III)/Pb(II) = ∞SH–DETA–PDBMPA Au(III)–Ni(II) Pb(II)

Au(III)Ni(II)

0.001.290.00

˛Au(III)/Ni(II) = ∞

Au(III)–Zn(II) Au(III)Zn(II)

1.340.00

˛Au(III)/Zn(II) = ∞

Au(III)–Cr(II) Au(III)Cr(II)

1.400.00

˛Au(III)/Cr(II) = ∞

Au(III)–Co(II) Au(III)Co(II)

Fig. 9. FT-IR spectra of SH–DETA–PDBMPA before (2) and after (1) adsorption forAu(III).

awof

rsgmA(TiaT

o

otSimasc

[10] A. Stafiej, K. Pyrzynska, Sep. Purif. Technol. 58 (2007) 49.[11] K.C. kang, S.S. Kim, J.W. Choi, S.H. Kwon, J. Ind. Eng. Chem. 14 (2008) 131.[12] E. Eren, J. Hazard. Mater. 159 (2008) 235.

dsorption amounts of SH-D-PDBMPA for Au3+ was 1.17 mmol g−1,hich was close to the ligands content (0.62 mmol g−1). Thus, most

f the functional groups of resin SH–DETA–PDBMPA seemed toorm 1:2 complex with Au3+.

The most important properties of a chelating adsorbent mate-ial that influence its application are sorption amount in addition toorption selectivity which is basically an attribute of the functionalroup of the adsorbent. So, the competitive adsorption experi-ents by SH–DETA–PDBMPA were carried out for Au (III)–Hg(II),u (III)–Cu(II), Au (III)–Cd(II), Au (III)–Pb(II), Au (III)–Ni (II), Au

III)–Zn (II), Au (III)–Cr (III), and Au(III)–Co(II) binary systems.he initial Au (III) concentration as well as other transition metalons such as Zn (II), Ni (II), Pb (II) and Cu (II) was 2.5 mmol L−1,nd the obtained results at 25 ◦C and pH 2.0 were presented inable 3.

The selective coefficients were the ratio of adsorption amountsf metal ions in binary mixture:

The selective coefficient t = q′/q′′, where q′ is adsorption amountf Au(III) ion in binary mixture and q′′ is adsorption amount ofhe other metal ion in binary mixture. The results displayed thatH–DETA–PDBMPA had excellent adsorption for Au (III) in binaryons systems, thus, this novel functionalized silica gel chemically

odified by poly(diethylenetriamine bis(methylene phosphoniccid)) SH–DETA–PDBMPA had high adsorption amount and goodelectivity for Au (III), which can be applied for removing this pre-

ious metal element from aqueous solutions.

[[

1.280.00

˛Au(III)/Co(II) = ∞

4. Conclusions

Chemical modification of silica gel had been successfu-lly carried out, and a novel silica gel functionalized withpoly(diethylenetriamine bis(methylene phosphonic acid))SH–DETA–PDBMPA was synthesized and characterized in thispaper. Both the X-ray diffraction and pore structure analysisindicated that the silica gel matrix was so stable that the porousstructure still remained. The thermogravimetry analysis showedthat the resulting adsorbent satisfied adsorption experimentsbecause the organic functional groups were not decomposed lessthan 255 ◦C. Moreover, its adsorption results of Au(III), Hg(II),Cu(II),Pb(II), Co(II), Zn(II), Ni(II), Cr(III) and Cd(II) metal ions displayedthat SH–DETA–PDBMPA had good adsorption capability for Au(III),Hg(II) and Cu(II) metal ions, especially for Au(III) ion. The highadsorption amount and good selectivity make this functionalizedsilica gel have a significant potential for uptaking Au(III) fromaqueous solutions using adsorption method.

Acknowledgements

We greatly appreciate the support provided by the Natio-nal Natural Science Foundation of China (51073075), the NatureScience Foundation of Shandong Province (2009ZRB01463), theBasic Project of Educational Bureau of Shandong Province of China(J07YA16), and the Foundation of Innovation Team Building ofLudong University (08-CXB001).

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