Materials Science and Engineering C 31 (2011) 1826–1831

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Materials Science and Engineering C

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Characterization and determination of the thermodynamic and kinetic properties ofthe adsorption of molybdenum (VI) onto microcrystalline anthracene modifiedwith 8-hydroxyquinoline

Xu Wang a, Yan Zhang b, Quanmin Li a,⁎a College of Chemistry and Environmental Science, Henan Normal University, Henan Key Laboratory for Environmental Pollution Control, Xinxiang, Henan 453007, PR Chinab College of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang, Henan 453003, PR China

⁎ Corresponding author. Tel./fax: +86 373 3326336.E-mail address: mercury6068@hotmail.com (Q. Li).

0928-4931/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.msec.2011.08.016

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 January 2011Received in revised form 12 June 2011Accepted 30 August 2011Available online 14 September 2011

Keywords:MolybdenumMicrocrystalline anthracene8-hydroxyquinolineAdsorption

A reliable and effective method for the determination of trace molybdenum in effluents is proposed. Molyb-denum (VI) is analyzed by Microwave Plasma Torch Optical Emission Spectrometry (MPT-OES) based on theadsorption collection onto microcrystalline anthracene modified with 8-hydroxyquinoline. The possible reac-tion mechanism was discussed in detail and the optimum conditions for adsorption of Mo (VI) were con-firmed. The experimental data were fitted well with the pseudo-second-order kinetic model and Langmuirmodel at all studied temperatures. The calculated thermodynamic parameters (ΔG , ΔH and ΔS ) showedthat the adsorption of molybdenum onto microcrystalline anthracene was feasible, spontaneous and endo-thermic at 280–320 K. The recovery of this method is in the range of 96.5%–103.3% with preconcentrationfactor of 100 and the limit detection after preconcentration is 0.078 μg L−1. The proposed method has beensuccessfully applied to the determination of trace Mo (VI) in environmental water samples with satisfactoryresults.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Molybdenum as an essential trace element plays a key role for avariety of biological functions, most notably nitrogen fixation [1].Molybdenum deficiency has often been reported, but at large concen-trations, molybdenum may be toxic as it leads to secondary copperdeficiency or a series of problems in health [2,3]. Therefore, it is ofgreat importance and significance for environmental science and lifescience to separate and determine trace amount of Mo (VI) in watersamples.

ETAAS [4], GFAAS [5], ICP-MS [6], ICP-OES [7–11] etc. are availabletechniques for the determination of trace molybdenum in varioussamples. Due to molybdenum (VI) of low-concentration and the com-plicated matrices in environmental samples, it is usually necessary toapply a preconcentration method while separating the analyte fromthe concomitants. Among numerous preconcentration methods, ad-sorption, owing to its simple operation, diversification of solidphase, high enrichment factor, and low consumption of organicsolvents for sample preparation, is receiving increasing attention.Many adsorbents, including novel adsorbents of mesoporous mate-rials or nanometer-scale objects [12,13], such as activated carbon[7], Amberlite XAD [8], silica gel [9], polyurethane foam [10], coir

pith carbon [11], nanometer-sized titanium dioxide [14], maghemitenanoparticles [15], Nanocarbon [16] and microcrystalline naphtha-lene [17] have been employed for the single- or multi-element ex-traction from their initial matrices. However, different kinds ofdrawbacks, for instance, tedious procedures and rigid control of con-ditions involved in the preparation of adsorbent [8–10], pollutioncaused by the nature of adsorbent (Microcrystalline naphthalene,for instance, seriously pollutes environment and possibly does harmto operators in experiments [18] because of its sublimation.), orhard desorption [7], are shown in the previous reports. Furthermore,some of the sorbents have to be packed into columns for separationand enrichment of analytes. Due to the limitation of flow rates orcolumn capability, this operation shows low efficiency [19]. On-lineseparation and preconcentration techniques can ensure the efficiencyof the analysis. However, they suffer from poor reproducibility [20].Since the shortcomings mentioned above, it's significant to find apromising alternative adsorbent for the removal of Mo (VI) fromnatural waters. Table 1 shows different preconcentration methodswith various sorbents developed for Mo (VI) determination indifferent samples.

This paper proposes a novel method using microcrystallineanthracene (MICRO-AN) loaded with 8-hydroxyquinoline (HOx) asan adsorbent for the separation and preconcentration of trace Mo(VI) in environmental water samples. The method is, to be specific,to mix the sample solution with the prepared adsorbent of HOx-MICRO-AN or with certain concentrations of HOx solution and

Table 1Separation and preconcentration procedures for molybdenum determination.

Sorbent/reagent Eluent Adsorption capacity Sample/volume Concentrationefficiency

Detection Range ordetection limit

Reference

Activated carbon/calmagite Concentrated HNO3

at 120 °CSeawater/1000 mL 100 ICP-OES 0.75 μg L−1 [7]

Amberlite XAD/TAN HClO4 Standards/50 mL 3.3 ICP-OES [8]Silica gel/quercetin α-Benzoinoxime 8.54 mg g−1 (0.089 mmol g−1) Extract of plants/5 mL 1 ICP-OES 1 ng L−1 [9]Polyuretane foam/thiocyanate Concentrated HNO3 Seawater/500 mL 50 ICP-OES 1.5 μg L−1 [10]ZnCl2 activated coir pith carbon HCl or NaOH 11.30 mg g−1 (18.90 mg g−1

molybdate)Waters UV–Visible

spectrophotometer[11]

Nanometer-sized titaniumdioxide

0.5 mol L−1

NaOH2.01 mg g−1 Steel samples 100 ICP-OES 0.14 mg L−1 [14]

Maghemite nanoparticles NaOH 33.40 mg g−1 Wastewater/350 mL 17.5 Spectrophotometric 0.78 μg L−1 [15]Microcrystalline anthracene/8-hydroxyquinoline

NaOH or HCl 150.38 mg g−1 Wastewater/1000 mL 100 MPT-OES 0.078 μg L−1 This work

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anthracene solution. In the former case, the adsorbent is capable ofdirectly absorbing Mo (VI) in the solution; the latter, while in the dy-namic formation process of MICRO-AN, HOx and the chelate complexgenerated by HOx and Mo (VI) could be adsorbed onto the surface ofMICRO-AN so as to make Mo (VI) enriched. Advantages of usingMICRO-AN as an adsorbent can be summed up as follows: a) Relativefriendly material to environment. MICRO-AN can hardly be subli-mated into the air and do no harm to operators in experiments.Owing to its hydrophobic property [21], it can easily be regeneratedfrom the solution and used circularly. b) Great adsorption capacity.Under stirring anthracene could highly disperse in the form ofmicrocrystal in the solution, which makes MICRO-AN have highspecific surface area. Compared to many available adsorbents,the adsorption capacity of MICRO-AN toward Mo (VI) is superior(Table 1), which proves its great adsorption capability. c). User-friendly control. Without the rigorous and redundant preparationprocess, MICRO-AN could be prepared immediately after addinganthracene solution into aqueous phase. Moreover, the process ofadsorption in this paper, which is carried out in 1000 mL of watersamples directly instead of using columns, is proved to be time-saving,easy handling and economical. This method has been successfullyapplied to the determination of trace Mo (VI) in various environmentalwaters.

2. Experimental

2.1. Reagents and apparatus

A standard molybdenum solution of 1.0 g L−1 was prepared bydissolving 0.1500 g MoO3 in 10.0 mL of 2.00 mol L−1 ammonia andthen diluting to 1000 mL with doubly distilled water. Working stan-dard solutions were obtained by further dilution of the standard solu-tion. A solution of 1.00×10−3 mol L−1 of salicyl fluorone (SAF) wasprepared by dissolving 0.0336 g salicyl fluorone in 5.00 mL of5.00 mol L−1 hydrochloric acid and diluting to 100mL with ethanol. A1.00×10−2 mol L−1 solution ofHOx in95%ethanol, a 50.0 g L−1 solutionof anthracene in N, N-dimethylformamide and a 5.00×10−3 mol L−1 ofcetyl-trimethyl ammonium bromide (CTMAB) in double distilled waterwere prepared. All the chemicals mentioned above were purchasedfrom Shanghai Reagents Plant, Shanghai, China. All reagents were of ana-lytical reagent grade.

Analyses of all samples were performed using a 1020MPT system(Changchun Jilin University · Little Swan Instrument Company, Chang-chun, China) (Microwave forward power: 80W; Carrier gas (Ar) flowrate: 0.7 L min−1; Support gas (Ar) flow rate: 0.4 L min−1; Sheathinggas (O2) flow rate: 1.0 L min−1; Wavelength: 313.3 nm). A model 722spectrophotometer (Xiamen Analytical Instrument Plant, Xiamen, China)was employed for photometric measurements. All pH measurements

were performed with a pH 3 C digital pH meter (Shanghai Lei Ci DeviceWorks, Shanghai, China).

2.2. Condition experiments

A given amounts of standard molybdenum solution, 1.00×10−2 mol L−1 of HOx and buffer solution were put into a 50 mL beaker.The solution was diluted to 20.0 mL with distilled water. Then 0.30 mLof 50.0 g L−1 (15 mg) anthracene was added drop by drop to it undercontinuous stirring. After stirring for 5 min and standing for a while,1.00 mLof supernatant liquidwas taken to a 10 mL comparison tube. Spe-cific determination of Mo(VI) is carried out according to available litera-ture [22]. The amount of Mo(VI) remained in the solution and theenrichment yield of Mo(VI) (E, %) were calculated.

2.3. Determination of Mo (VI) in environmental samples

Take a 1000 mL water sample, heat, cool and filter to get rid of in-soluble suspended substance. 1.00 mol L−1 HCl was used to adjust pHto 2.00. 8.00 mL of 1.00×10−2 mol L−1 HOx and 4.00 mL of50.0 g L−1 anthracene were added to it in succession under continu-ous stirring. After stirring for 1 h, the mixture was filtered with G4crucible. The residue was washed three times with 0.50 mL of0.50 mol L−1 NaOH each time to desorb Mo (VI) from the surface ofMICRO-AN. After adjusting the pH of eluent and diluting it to thedesired volume of 10.0 mL, the concentration of Mo (VI) was deter-mined by MPT-OES directly. The recovery(R) can be calculatedaccording to the equation as follows: R=(ct−co)/ca, where ct, coand ca represent the total amount determined, the original amountin specific sample and added amount of Mo(VI), respectively.

3. Results and discussion

3.1. Effect of acidity on the E% of Mo(VI)

The effect of acidity on the enrichment yields of Mo (VI) and otherions was investigated in the pH range of 0.00–7.00. The results areshown in Fig. 1. At pH=0.00 ([H+] is 1.00 mol L−1 in the solution.),Mo (VI) cannot be enriched at all. It is probable that HOx has threeforms under different acidity conditions including the protonated(H2Ox+), neutral (HOx) and anionic (Ox−) forms [23]. Under thecondition of high acidity, H2Ox+ formed reduces the chance of HOxandMo (VI) to form the complex of MoO2(Ox)2 [24]. With an increaseof pH value from 1.00 to 2.00, the E% of Mo (VI) is enhanced from70.6% to 99.2% and keeps constant in the pH range of 2.00−5.00.For the reason that accompanied by the decrease of [H+] in the solu-tion, the [HOx]/[H2Ox+] ratio increases. Due to the deprotonation ofHOx, more of MoO2(Ox)2 can be formed leading to the enhancement

Fig. 1. Effect of acidity on the E% ofMo(VI).Metal ions: 50 μg;HOx (1.00×10−2 mol L−1):0.50 mL; anthracene (50.0 g L−1): 0.30 mL; preconcentration time: 5 min; total volume:20.0 mL.

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of the E% of Mo (VI). At pH 6.00, the E% of Mo (VI) decreases from100% at pH 5.00 to 90.3%. At pH 7.00, the E% of Mo (VI) is merely 0.It is probably that various polymerized species of Mo (VI) [25], suchas Mo7O24

6− and MoO42−, are formed with the increasing pH value,

which can not react with HOx to form the neutral complex.At pH 1.00–2.00, Cd(II), Co(II), Mn(II), Ni(II), Pb(II), Zn(II), Al(III),

Fe(III), Cr(III) etc. are not enriched at all, while the E% of Cu(II) in-creases from 50.8% to 70.2%. At pHN2.00, the E% of Cu(II) nearlyattained to 100%. In order to ensure Mo (VI) can be totally enrichedand separated from most of other ions, the enrichment experimentsare carried out at pH 2.00.

In addition, in order to separate Mo(VI) from Cu(II), two types ofeluents including 5.00 mol L−1 of HCl and 0.50 mol L−1 of NaOHwere tested. Under the condition of high acidity (5.00 mol L−1 HCl),both Mo(VI) and Cu(II) could be quantitatively desorbed from thesurface of MICRO-AN (RecoveryN90%); whereas in the alkali medium(0.50 mol L−1 NaOH), Cu(II) formed precipitate as hydroxide, by con-trast, Mo (VI) was desorbed into the solution in the form of Mo7O24

6−

or MoO42− [25]. The aim of selective desorption of Mo (VI) could be

achieved. Therefore, 0.50 mol L−1 of hot NaOH solution was selectedin the procedure of sample analysis.

3.2. Effect of the amount of HOx and anthracene on the E% of Mo(VI)

Keeping the amount of anthracene (15 mg) constant, the effect ofthe amount of HOx on the E% of Mo (VI) was shown in Fig. 2 (curve a).It indicates that the E% of Mo (VI) is 15.1% without HOx. It may be thatMo (VI) in the form of big electrically neutral molecules of H6Mo7O24

Fig. 2. Effect of theamount ofHOx andanthraceneon the E%ofMo(VI) (curve a: anthracene(50.0 g L−1): 0.30 mL; HOx (1.00×10−2 mol L−1): 0–0.50 mL; curve b: HOx (1.00×10−2

mol L−1): 0.50 mL, anthracene (50.0 g L−1): 0–0.50 mL). Mo(VI): 50 μg; pH: 2.00; precon-centration time: 5 min; total volume: 20.0 mL.

and H2MoO4 is adsorbed on the surface of MICRO-AN. An increasefrom 19.4% to 99.2% of the E% of Mo (VI) was observed with an incre-ment of HOx amount from 0.10 to 0.40 mL. The E% of Mo (VI) does notchange with more HOx used. Therefore, 0.50 mL of HOx was chosenin condition experiments.

Under the fixed amount of 0.50 mL of 1.00×10−2 mol L−1 HOx,the effect of the amount of anthracene on the E% of Mo (VI) isshown in Fig. 2 (curve b). The E% of Mo (VI) is 59.7% in the absenceof anthracene, corresponding to 1.01 μg mL−1 of Mo (VI) left in thesolution. It is probable that Mo (VI) can react with HOx to form pre-cipitated MoO2(Ox)2 directly, which is in good agreement with thephenomenon of yellow precipitate observed without anthracene inexperiment. Based on the amounts of Mo (VI) and HOx remained inthe solution, the apparent solubility product constant of precipitatedMoO2(Ox)2 can be calculated as K′sp=5.04×10−13. According tothe formula: αOx(H)=1+β1[H+]+β2[H+]2 (β1

H=1/Ka2=109, β2H=

1/Ka2Ka1=1014.1, βH — cumulative stability constant of proton), theacidic effective coefficient (aOx(H)) of HOx is 1.26×1010 at pH 2.00;thereby, the solubility product constant (Ksp) of precipitated MoO2

(Ox)2 can be obtained as 3.17×10−33. Owing to the small Ksp,MoO2(Ox)2 was directly generated in the solution and the concentra-tion of Mo (VI) decreased from 2.50 to 1.01 μg mL−1.

When the amount of anthracene increases from 0 mL to 0.30 mL,the concentration of Mo (VI) in the solution is decreased from 1.01to 0 μg mL−1 and the E% of Mo (VI) increased to 100%. It is probablethat HOx adsorbed onto MICRO-AN is at higher concentration level.The concentration product of MoO2

2+ left in the solution and HOxadsorbed on MICRO-AN ([MoO2

2+]·[HOx]2) is bigger than the K′sp ofMoO2(Ox)2. Therefore, MoO2(Ox)2 can be generated and retainedon MICRO-AN. This indicates that MICRO-AN loaded with HOx canperfectly adsorb Mo (VI) of low-concentration in the solution. Inorder to economize reagent and ensure Mo (VI) could be completelyenriched, 0.30 mL of anthracene was selected in the subsequentexperiments.

3.3. Adsorption kinetics

At room temperature (T=27 °C), the kinetics of Mo (VI) adsorp-tion onto MICRO-AN was investigated in the range of 0–20.0 min.The results indicate that with an increase of time from 0 to 5 min,the E% of Mo (VI) is enhanced from 97.2% to 100%. The E% of Mo(VI) does not change after 5 min. Pseudo-first-order Lagergren ratemodel (lg (qe−qt)=lg qe−k1t /2.303) and pseudo-second-ordermodel (t/qt=1/k2qe2+ t/qe) [26], were used to determine the pre-dominant kinetic mechanism for Mo (VI) adsorption onto MICRO-AN(Fig. 3), where qe and qt are the amounts of Mo (VI) adsorbed onthe sorbent (mg g−1) at equilibrium and at time t, and k1, k2 arethe rate constants of the first-order adsorption (min−1) and the

Fig. 3. Kinetics investigation of Mo(VI) adsorption (curve a: Pseudo-first-order model;curve b: pseudo-second-order model.).

Fig. 5. Langmuir plots of Mo(VI) adsorption on MICRO-AN of different concentrations.

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second-order adsorption (g mg−1 min−1), respectively. The equa-tions of linear regression of lg (qe−qt) against t, t/qt against t can beobtained as lg (qe−qt)=lg 0.0903−0.3574 t /2.303 (r1=0.9953), t/qt=1/181.4889+ t/3.3369 (r2=1.0000). It is found that the first-order equation of Lagergren does not apply well throughout thewhole range of contact time, and the calculated qe value(0.0903 mg g−1) does not agree with the experimental one(3.3333 mg g−1). The unconformity of the pseudo-first-order kineticmodel may be due to a boundary layer controlling the beginning ofthe adsorption process [27]. The fitting results show at room temper-ature (T=27 °C) the pseudo-second-order model coincide well withthe data obtained from experiments, which is also in agreement withthe theory that the sorption process should obey pseudo-second-order kinetics model at lower initial concentration of solute [26].Therefore, the adsorption of Mo (VI) is best described by thepseudo-second-order equation.

3.4. Adsorption isotherm and adsorption capacity

At different temperatures of 280 K, 300 K and 320 K, the equilibriumadsorption amount of Mo (VI) on MICRO-AN as a function of the equi-librium concentration of Mo (VI) is shown in Fig. 4. The adsorption forMo (VI) increased until the saturation was attained. The Langmuirisotherm (ce/qe=1/bqm+ce/qm) is probably the most widely appliedadsorption isotherm to describe how solutes interact with sorbent[28], where qm is the maximum adsorption at monolayer (mg g−1),ce is the equilibrium concentration of Mo (VI) and b is the Langmuirconstant related to the affinity of binding sites (L mg−1).

The empirical Freundlich equation based on sorption on a hetero-geneous surface is given by: ln qe=ln KF+1/n ln ce, where KF and nare Freundlich constants indicating sorption capacity and intensity,respectively. The Langmuir, Freundlich isotherm constants were de-termined from the plots of ce/qe against ce, ln qe versus ln ce, respec-tively, at 280 K, 300 K and 320 K (Figs. 5 and 6). The isothermconstants and the correlation coefficients are listed in Table 2. Langmuirand Freundlich isotherm models were statistically significant at a95% confidence level. It is found that the adsorption of Mo (VI) onMICRO-AN was correlated better (r2N0.99) with the Langmuir equa-tion than with the Freundlich equation (r2N0.95) under the concen-tration range studied. The increase in bwith increasing temperaturessuggests that the adsorption of MoO2(Ox)2 is an endothermic pro-cess. The essential features of Langmuir isotherm can be expressedin terms of a dimensionless constant separation factor or equilibriumparameter, RL, which is used to predict if an adsorption system is“favorable” or “unfavorable” [29]. The separation factor, RL is definedby: RL=1/(1+bc0). The calculated RL values are in the range of0–1.0000, which indicates that the adsorption of Mo (VI) on MICRO-AN is favorable (Table 3).

Fig. 4. Isotherm of Mo(VI) adsorption on MICRO-AN at different temperatures (280 K,300 K and 320 K; the amount range of Mo(VI)=50–2600 μg).

3.5. Thermodynamic studies

Thermodynamic parameters (ΔGΘ,ΔHΘ,ΔSΘ) were used to describethermodynamic behavior of Mo (VI) adsorption onto MICRO-AN at280 K, 300 K and 320 K. These parameters were calculated from follow-ing equations [30]: lnc=ΔHΘ/RT+D, where c is the equilibriumconcentration of Mo (VI) in solution (mg L−1), and ΔHΘ was as-sumed to be constant for a constant surface coverage; To calculatethe values of the other parameters (ΔGΘ, ΔSΘ), ΔGΘ=−RT ln Kc,ΔSΘ=(ΔHΘ−ΔGΘ) /T, Kc=cs /cm were used respectively, wherecs and cm are the equilibrium concentrations of Mo (VI) on the sorbentand in the solution, Kc is the equilibrium constant. Furthermore, the dif-ference of standard chemical potential (Δμθ), a dimensionless parame-ter, indicating the adsorption direction equals to the change of itspartial molar free energy (ΔGΘ). That is to say, the equation can bealso represented as ln Kc=−(μsθ−μmθ )/RT=−Δμθ/RT=−ΔG /RT[31]. The values of ΔHΘ, ΔSΘ and ΔGΘ were given in Table 3. The de-crease in ΔGΘ value with increase in temperature shows an increasein feasibility of adsorption at higher temperature, and the negativevalue of ΔGΘ, which also demonstrates the μsθ value of Mo (VI) isless than the μmθ one, indicates that the adsorption is spontane-ous with Mo (VI) transporting from the feed to the solid phase.The calculated ΔHΘ, as an indicator of endothermic nature of theadsorption, falls into the heat range of physical sorption (2.1–20.9 kJ mol−1) [32]. Therefore, the adsorption process of Mo (VI)onto MICRO-AN was taken place via physical sorption. The positiveΔSΘ suggests the increased randomness at solid–solution interfaceduring the adsorption process [33]. These figures also corroboratethe previous assumption made on the basis of the analysis of theLangmuir parameter b.

Fig. 6. Freundlich plots of Mo(VI) adsorption on MICRO-AN of different concentrations.

Table 2Langmuir and Freundlich isotherm constants and correlation coefficients at different temperatures.

Temperature/K Langmuir Freundlich

qm(mg g−1) b R2 RL n KF R2

280 K 145.14 16.51 0.9928 0.8392–1.0000 5.18 8.01 0.9931300 K 150.38 39.31 0.9976 0.7193–1.0000 6.56 8.44 0.9693320 K 155.76 137.71 0.9927 0.4032–1.0000 12.76 8.80 0.9566

Table 4The determination results of Mo(VI) in water samples (pH 2.00).

Samples Mo(VI) (afterenrichment)(μg L−1)

AddedMo(VI)(μg L−1)

TotalMo(VI)(μg L−1)

Recovery ofMo(VI) (%)

RSDa (%)(n=5)

Distilled water 0 2 2.04 102.0 1.14 3.96 99.0 0.8

b

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3.6. The reaction mechanism and driving forces for Mo (VI)

Based on the discussion presented above (Section 3.2), it seemsreasonable that the enrichment mechanism of Mo (VI) adsorbed onMICRO-AN is as follows:

1). MICRO-AN, as coprecipitator,makesMo (VI) of high-concentrationin the form of H6Mo7O24 or H2MoO4 adsorbed on the surface of it,when the solution includes MICRO-AN but no HOx.

H6Mo7O24 or H2MoO4 →H6Mo7O24 or H2MoO4

feedð Þ MICRO‐ANphaseð Þ

2). When the solution includes HOx, but no anthracene, MoO22+ of

high-concentration can directly react with HOx to form precipitat-ed MoO2(Ox)2, which leads to the decrease of the concentration ofMo (VI) in the solution.

MoO22þ þ 2HOx→MoO2 Oxð Þ2↓þ 2Hþ

feedð Þ feedð Þ

3).When the solution includesMICRO-AN loadedwith HOx,MoO22+

of low-concentration in the feed displaces H+ of HOx in MICRO-ANphase to form MoO2(Ox)2 by ion-exchange reaction and theexchanged H+ is returned into the feed.

MoO22þ þ 2HOx → MoO2 Oxð Þ2 þ 2Hþ

feedð Þ MICRO‐AN phaseð Þ MICRO‐AN phaseð Þ feedð Þ

Therefore, in the process of MoO2(Ox)2 generated, H+ exchangedby MoO2

2+ was transported from the MICRO-AN phase to the feed.The counter-diffusion of H+ supplied the energy for the transport ofMo (VI) against a concentration gradient, which leads to Mo (VI)being preconcentrated onto MICRO-AN phase. This also coincideswith the previous conclusion gained from thermodynamic investiga-tion, namely, spontaneous nature indicated by negative ΔGΘ and theconclusion of physical sorption obtained by the ΔHΘ value.

3.7. Calibration curve

1.00, 3.00, 5.00, 7.00, 8.00, 9.00 and 10.0 μg of standard Mo (VI)were, respectively, put into a 25 mL volumetric flask. Then 2.50 mLof 0.10 mol L−1 HCl was put into each flask, respectively. The signalintensity was determined directly by MPT-OES. Signal intensity has

Table 3Thermodynamic parameters for the adsorption of Mo(VI) on microcrystallineanthracene.

Temperature/K 280 300 320

Kc 35.76 55.47 74.75ΔGΘ(kJ mol−1) −8.33 −10.02 −11.48ΔHΘ(kJ mol−1) 5.10ΔSΘ (kJ mol−1 K−1) 0.048 0.050 0.052

been plotted as a function of the concentration of Mo (VI). A linear re-gression equation of calibration curve is attained as I=0.442 c+1865.372 (n=5, R2=0.9994), where I=emission intensity, c=concentration (mg L−1). Following the 3σ criteria (n=11) the limitof detection is 7.8 μg L−1. Mo (VI) is enriched from 1000 mL watersample to 10.0 mL, which means that the enrichment factor is 100.The limit of detection after preconcentration is found to be0.078 μg L−1.

4. Applications of the method

4.1. Synthetic sample analysis

The recommended procedurewas applied to determine theMo (VI)content in synthetic water sample (1000 mL), which have a composi-tion similar to certified river water samples SLRS-4 (National ResearchCouncil, Ottawa, Ont., Canada). The average of three determinations ofMo (VI) was found to be 0.20 μg L−1 with R.S.D. value 3.2% (Certifiedvalue for Mo (VI) concentration is: 0.21±0.02 μg L−1). The analyticalresults for the synthetic water sample were in good agreement withthe certified value, which indicates that the proposed method isreliable.

4.2. Environmental sample analysis

To test the accuracy of the proposed method and its applicabilityto the analysis of real samples, some natural water samples havebeen analyzed using the method given in Section 2.3. The recoveriesof the spiked analytes are also determined. The results for the waterof tap, river, lake and rain are given in Table 4.

The results demonstrate that the recovery is in the range of 96.5%–103.3% with relative standard deviation of 0.8–2.3%. It is concludedthat the amount of Mo (VI) in the environmental samples can bedetermined by MPT-OES quantitatively after the separation and pre-concentration using MICRO-AN loaded with HOx.

After the procedure of desorption, MICRO-AN was recovered forsubsequent experiments. It was found that adsorption properties ofMICRO-AN did not change after six cycles of adsorption and desorption.

Tap water 3.66 2 5.70 102.0 1.34 7.79 103.3 2.0

River waterb 6.03 1 7.06 103.0 1.02 7.96 96.5 1.4

Lake waterb 4.56 1 5.58 102.0 2.32 6.50 97.0 1.6

Rain water 2.98 2 4.96 99.0 0.94 6.92 98.5 2.1

a RSD: relative standard deviation.b Tap, River and Lake water samples were collected from our lab, Wei River, Xinxiang,

China and Lake in the people's park, Xinxiang, China, respectively.

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5. Conclusions

In this work, a simple and reliable method was developed for pre-concentration and determination of trace Mo (VI) based on adsorp-tion onto MICRO-AN loaded with HOx. The sorbent showed goodproperties such as fast adsorption equilibrium, easy elution, goodadsorption capacity, favorable reproducing property, etc. Moreover,the preparation of MICRO-AN loaded with HOx was relatively simpleand convenient. However, HOx loaded onMICRO-ANby intermolecularforce may be not as firm as selective reagent modified on thoseadsorbents (e.g. silica gel) by bonding effect. The pseudo-second-order model and the Langmuir isotherm model provided the best fitto the experimental adsorption data for Mo (VI) and the maximum ad-sorption capacity of MICRO-AN towards Mo (VI) was 150.38 mg g−1

(300 K). The thermodynamic parameters (ΔGΘ, ΔHΘ and ΔSΘ) showedthe feasibility, endothermic and spontaneous nature of the adsorptionin the range of 280–320 K. The proposed method was successfullyapplied to the analysis of trace Mo (VI) in environmental samples.Analytical results obtained were very satisfactory.

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