Selective removal of heavy metal ions using sol–gel immobilized and SPE-coated thiacrown ethers

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  Selective removal of heavy metal ions using sol–gel immobilized and SPE-coated thiacrown ethers
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  Analytica Chimica Acta 555 (2006) 146–156 Selective removal of heavy metal ions using sol–gel immobilized andSPE-coated thiacrown ethers Bahruddin Saad ∗ , Ching Ching Chong, Abdussalam Salhin Mohamad Ali, Md Fazlul Bari,Ismail Ab Rahman, Norita Mohamad, Muhammad Idiris Saleh School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia Received 10 May 2005; received in revised form 22 August 2005; accepted 23 August 2005Available online 29 September 2005 Abstract Sorbent materials based on three thiacrown ethers, 1,4,7,10-tetrathiacyclododecane (12S4), 1,4,7,10,13-pentathiacyclopentadecane (15S5) and1,4,7,10.13,16-hexathiacyclooctadecane(18S6)werepreparedeitherbyimmobilizingtheligandsintosol–gel(SG)matrixorcoatingoncommercialsolidphaseextraction(SPE)column.SGsorbentswerecharacterizedbyFT-IR,energydispersiveX-raymicroanalysis(EDX)andthermogravimetricanalysis/derivative thermogravimetric analysis (TGA/DTG). A marked thermal stability of the ligands when immobilized in sol–gel matrix wasnoted. The competitive sorption characteristics of a mixture of eleven metal ions (Mg 2+ , Zn 2+ , Cd 2+ , Co 2+ , Mn 2+ , Ca 2+ , Cu 2+ , Ni 2+ , Ag + , V 4+ ,Hg 2+ ) using: (i) batch method with ligands trapped in SG matrices, and (ii) off-line SPE column containing coated ligands were studied usingICP-MS. The extraction of metals were optimized for key parameters such as pH, contact time/flow rate, particle size (for SG sorbents) and ligandconcentration.Undertheoptimizedconditions,alltheimmobilizedthiacrownethersexhibitedhighestselectivitytowardAg + ,withlesserresponsestoHg 2+ whiletheextractionofothermetalionswerenegligible.AmongtheSGsorbents,18S6-SGofferthehighestcapacityandthebestselectivityover Hg 2+ . However, for practical applications such as for selective isolation and preconcentration of Ag + , the SPE type especially based on 18S6is preferred as analysis time and recoveries are favorable. The sorbents can be repeatedly used three times as there was no significant deteriorationin the metal uptake (%  E  >90%) or interference from other metal ions. The optimized procedures were successfully applied for the separation andpreconcentration of traces Ag + in different water samples.© 2005 Elsevier B.V. All rights reserved. Keywords:  Thiacrown ethers; Sol–gel; Solid phase extraction; Metal ions 1. Introduction Thiacrownetherisoneoftheclassesofcrownethersinwhichthe donor oxygen atoms on the macrocyclic ring are partially ortotally replaced by sulfur atoms [1,2]. Thiacrown ether is classi-fiedassoftLewisbaseanditiswellknowntointeractselectivelywithsoftmetalionssuchasAg + ,Hg 2+ ,Cu + andPd 2+ [3,4].The1980s and early 1990s have witnessed intensified research onthe fundamental processes that govern the complexation of thi-acrown ethers with metal ions [5–10]. Profound interest in thesesystems stem from the inherent stability of the metal–ligandcomplex which may serve as useful model for studies onelectron-transfer processes between enzymes and metals. ∗ Corresponding author. Tel.: +60 4 6577888; fax: +60 4 6574854.  E-mail address:  bahrud@usm.my (B. Saad). These ligands may also have important utility as sequesteringagents for the treatment of heavy-metal ion poisoning. Ana-lytical applications involving extraction-spectrohotometry[11,12] and ion-sensing have also been reported[13,14].The earlier studies involving conventional liquid–liquidextraction(LLE)usingthiacrowns[15–18],aresubjectedtosev-eral limitations such as being time consuming, labor intensive,use of gross amounts of organic solvents and difficult separa-tion of phases caused by formation of emulsions [19–21]. Thesteady increase in the use of solid phase extraction (SPE) whichcan overcome the disadvantages of the LLE, has aroused muchinterest.SPEoffersseveraldistinctadvantagesnamely,repeateduse,obviatetheuseoforganicsolventsandamenabletoautoma-tion [20,21]. This is achieved by the immobilization (physically or chemically) of functional groups and ligands onto varioussupportssuchassilicagel[22,23],amberliteXAD[24,25],acti- 0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.aca.2005.08.070   B. Saad et al. / Analytica Chimica Acta 555 (2006) 146–156   147Fig. 1. Chemical structures of thiacrown ethers studied. vated carbon [26], metal alkoxide glass [27,28], executed as column or disk  [29,30].Several reports on the immobilization of thiacrown ethersand its derivatives have been published. Bruening et al. [22],have chemically bonded 1,4-dithia-19-crown-6 and 1,4,7,10-tetrathia-18-crown-6 onto silica gel, which was subsequentlypacked into column for the preconcentration of Pd 2+ , Au 3+ ,Hg 2+ and Ag + . Moyer et al. [31] have physically immo-bilized 1,4,8,11-tetrathiacyclotetradecane (14S4) on cationexchange beads for the batch extraction of Cu 2+ , while Bagheriand Shamsipur have physically immobilized the 1,4,7,10,13-pentathiacyclopentadecane (15S5) and hexathia-18-crown-6-tetraone onto C 18  membrane disk for the preconcentration of Au 3+ and Hg 2+ [29,30], respectively.Porous glass-like materials produced by sol–gel from metalalkoxides at low temperatures in which complexing ligands areimmobilized to improve the specificity of the sorbent have beendescribed [32,33]. During extraction, small analytes can readily diffuse into the pores of the sol–gel matrix and interact with thetrappedligand.Thesol–gelimmobilizedcrownether,1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-bis(malonate),fortheselective separation of Sr 2+ was first demonstrated by Yost et al.In order to exploit the selective complexation phenomenonfor practical low-cost applications such as in the elimina-tion of matrix interference, preconcentration prior to ana-lytical determinations and the possible removal of tracesof toxic heavy metals from wastewaters, three cyclic thi-acrownethers,namely,1,4,7,10-tetrathiacyclododecane(12S4),1,4,7,10,13-pentathiacyclopentadecane (15S5), 1,4,7,10,13,16-hexathiacyclooctadecane (18S6) (Fig. 1) that were physicallyimmobilizedintotwodifferenttypesofsupportsnamelysol–gelmatrix (sorbents herein referred to as 12S4-SG, 15S5-SG and18S6-SG)orcoatedontothesurfaceofcommercialSPEcolumn(sorbents referred to as 12S4-SPE, 15S5-SPE and 18S6-SPE)are reported. The resulting materials were used for the selectiveremoval of traces of heavy metal ions using two approaches, i.e.(i) batch-wise for the case of sol–gel immobilized ligands, and(ii) column for the case of SPE sorbents. Key factors that con-tributetotheextractionsuchasequilibriumtime,pH,ligandandmetalionconcentration,foreignmetalionsandreusabilityofthesolid support were investigated. A key difference of the presentworkisthecompetitivesorptioncharacteristicswhereamixturecontainingelevenmetalionswereinvestigatedwhilemostofthereportedworkstudiedonemetalatatime.Thepresentapproachis considered more realistic as in real samples, many ion speciesare present. Thermal stability of the sol–gel materials are alsostudied. 2. Experimental 2.1. Apparatus A Perkin-Elmer Elan 6100 inductively coupled plasma-mass spectrometry (ICP-MS) with version 2.0 software wasused. The instrument conditions and general method parame-ters are listed in Table 1. FT-IR spectrum (4000.0–400.0cm − 1 )in KBr were recorded using Perkin-Elmer 2000 FT-IR system.LEO Supra 50vp field emission scanning electron microscope(SEM) equipped with Oxford INCA 400 energy dispersive X-ray microanalysis system (EDX) was used to study the sur-face morphology and semi-quantitative analysis of the sol–gel.The sample were sputtered with a thin layer of gold usingPolaron(Fisons)SC515sputtercoater.Thermogravimetricanal-ysis (TGA) was performed with Perkin-Elmer thermogravimet-ric analyzer (TGA 7) at heating rate of 10 ◦ Cmin − 1 undernitrogen atmosphere. An Orion ion analyzer (model WA940)was used for pH measurements. A mechanical shaker (StuartScientific, UK) was used for extraction and de-ionized waterwas produce from Millipore Milli-Q plus. 2.2. Chemicals and reagents Two hundred and fifty milliliters stock solution (100ppm)containingmixturesofmetals(sulfatesofZn 2+ ,Cd 2+ ,V 4+ ,Ni 2+ ,Cu 2+ , Mn 2+ ; nitrates of Mg 2+ , Co 2+ , Ag + and chloride of Hg 2+ )were prepared by dissolving appropriate amounts of metal saltsin 2% (w/v) HNO 3  solution. A 1M sodium acetate (Riedel-de Hean) solution was prepared by dissolving 34.02g of thehydrated salt in 250mL de-ionized water.Thiacrown ethers, 12S4, 15S5 and 18S6 (Aldrich) wereused without further purification. Tetraethoxysilane (TEOS)(Fluka) was used as sol–gel precursor and Florisil 3mL SPEcolumn was purchased from International Sorbent Technology(IST), UK. Other solvents including ethanol (EtOH) (Systerm),tetrahydofuran (THF) (Fisher Scientific), 1,2-dichloroethane Table 1ICP-MS instrumental operating conditions and data acquisition parametersCondition/parameters ResultICP parametersrf power (W) 1000Coolant argon flow rate (Lmin − 1 ) 15Auxiliary argon flow rate (Lmin − 1 ) 1Nebulizer argon flow rate (Lmin − 1 ) 97Operating frequency (MHz) 40Sample introduction system Cross flow nebulizerSample cone Nickel with a 1.1mm orificeSkimmer cone Nickel with a 0.9mm orificeScanning mode Peak hoppingPressure (quadrupole analyzer) ICP( × 10 − 5 Torr)4.18Number of replicate 2  148  B. Saad et al. / Analytica Chimica Acta 555 (2006) 146–156  (Merck), HCl (Fisher Scientific) and HNO 3  (Merck) were usedas received.Realwatersamplesforanalysis,namely,tapwater,rainwater,river water (Waterfall River) and lake water (Youth Park), allfrom Penang, Malaysia were collected and filtered to removesuspended particles before use. 2.3. Preparation of sol–gel immobilized thiacrown ethers The sol solution was prepared by stirring a mixture of TEOS(3.28mL),EtOH(4.56mL),andHCl(0.36mL,4M)for15min.Thiacrownether(12S4,15S5and18S6,respectively)whichhasbeen dissolved in tetrahydrofuran (THF) was then added sepa-rately to the sol solution and stirred vigorously for 45min. Theresulting clear and homogeneous solution was aged in an oven(60 ◦ C) for 2 days. During the drying stage, shrinkage of thegel occurred causing it to crack. The gel was then soaked in de-ionized water for 1 day to condition it. The gel was next dried(60 ◦ C) for 1 day and ground into small pieces (1–5mm diame-ter) using mortar and pestle. Blank sorbent was prepared usingthe same procedure, except that no thiacrown ether was added. 2.4. Preparation of column The column method was performed using 3mL column(internal diameter, 8.3mm) containing 100mg of FL silica. Thecolumn was washed sequentially with 10mL of methanol, 5mLof HNO 3  (0.1M) and 10mL water to remove contaminants.After drying the column by passing air through, 2mL of thi-acrown ether ligand (0.02M) in 1,2-dichloroethane was loadedonto the column, and was drawn slowly through the column byapplying a negative pressure at the other end of the cartridge.Finally, the column loaded with thiacrown ether was dried inan oven (60 ◦ C) to evaporate the solvent before it was ready forextraction. 2.5. Extraction and preconcentration of metal ions The batch method was conducted with sol–gel immobilizedthiacrownether(SG)sorbentwhiletheoff-linepreconcentrationwas carried out with thiacrown ether loaded SPE column. 2.5.1. Batch method  Sol–gel sorbent (0.5g) was placed in a glass vial along withmixture of metal ions solution (1ppm, 5mL). The mixture wasshaken mechanically at room temperature (25 ◦ C) for 30min.Aftertheequilibriumtime,themixturewasfilteredandtheunex-tracted metal ions in the filtrate were determined by ICP-MS.Once the loading is completed, the sorbent was regenerated byshaking it with 10mL of HNO 3  (1M). After that, the sorbentwasrinsedseveraltimeswithde-ionizedwateranddried(60 ◦ C)before the next extraction cycle was conducted. 2.5.2. Column method  The thiacrown ether-loaded column (Section 2.4) was first wetted with 5mL of water, then the sample solution containingAg + (2.5ppm)waspassedthroughthecolumn.Aftertheextrac-tion, Ag + was stripped from the column with 10mL of HNO 3 (0.5M) and determined colorimetrically using eosin as reagent[34]. 3. Results and discussion 3.1. The effect of solvent on the sol–gel immobilized thiacrown ether  Due to the poor solubility of thiacrown ethers in polarsolvents such as water and ethanol, a few solvents, namely,chloroform, acetonitrile, 1,2-dichloroethane and tetrahydrofu-ran (THF) have been tested instead. When chloroform, ace-tonitrile or 1,2-dichloroethane were used, inhomogeneous solsolution was observed, with white ligand spots clearly seen.Leaching of thiacrown ether ligands from the sol occurred read-ily. However, when dissolved in THF, transparent and homoge-nous gel was formed. Therefore, THF was used to dissolve thethiacrown ether during the preparation of the gel. 3.2. FT-IR analysis sThe FT-IR spectrum of free thiacrown ether ligands(12S4, 15S5 and 18S6) show characteristic absorptionband at 620cm − 1 due to C S stretching, while the S CH 2 stretching bands appear at 1433, 1419 and 1384cm − 1 for12S4, 15S5 and 18S6, respectively [35] (FT-IR spectra notshown). Both the blank and thiacrown ether sol–gels producesimilar spectra, therefore FT-IR fail to provide conclusiveevidence on the presence of thiacrown ether in gel matrix.This can be attributed to the too small amount of the ligandin the network, as was also experienced by other researchers[36]. 3.3. EDX analysis EDX results of the free thiacrown ether ligand, blank gel andsol–gelimmobilizedthiacrownetheraresummarizedinTable2.For the blank gel, the main elements are Si (43.2%) and O(35.8%). This result supports the FT-IR spectrum of the blank sol–gel, in which the Si–O–Si is the backbone of this material.Analysis on 12S4-SG, 15S5-SG and 18S6-SG show that sulfur,ranging from 0.7 to 1.6% was detected, indicating the presenceof thiacrown ether in the sol–gel matrix. Table 2EDX results of free thiacrown ether and sorbent materialsElement MaterialThiacrown etherligandBlank-SG 12S4-SG 15S5-SG 18S6-SGCarbon 40.0 21.0 5.5 5.1 4.8Oxygen – 35.8 44.5 55.6 48.0Silicon – 43.2 48.6 39.2 46.5Sulfur 53.3 – 1.6 0.7 0.8   B. Saad et al. / Analytica Chimica Acta 555 (2006) 146–156   149Fig. 2. Thermogravimetric and derivative thermogravimetric analysis (dotted line) of the free ligands: (A) 12S4, (B) 15S5, (C) 18S6, and (D) blank gel. Heatingrate, 10 ◦ Cmin − 1 ; atmosphere, nitrogen.  150  B. Saad et al. / Analytica Chimica Acta 555 (2006) 146–156  Fig. 3. Derivative thermogravimetric analysis of (A) blank-SG, (B) 12S4-SG, (C) 15S5-SG, and (D) 18S6-SG. Heating rate, 10 ◦ Cmin − 1 ; atmosphere, nitrogen. 3.4. Thermogravimetric analysis The thermal stability of materials were investigated usingthermogravimetric analysis (TGA) over 30–700 ◦ C in nitrogenatmosphere at a constant heating rate of 10 ◦ Cmin − 1 . The threeligands decompose in a single step (Fig. 2) and essentially lost almostalloftheirmasses(>99%)at280,330and340 ◦ C,respec-tively. It is interesting to note on the gradual increase in thedecompositiontemperaturewithincreasingsizeofthethiacrownether. 3.4.1. Blank sol–gel The thermogram of the blank gel shows three distinct stepsof mass loss (Fig. 2D). The first loss which takes place below 200 ◦ C is associated with the physical desorption of water andthe evaporation of ethanol. The preferentially adsorbed of wateron the surface of silanol sites have been recognized [37,38]. A very broad peak (236–635 ◦ C) occurs in the second step, prob-ably due to the evaporation of trapped water molecules in thesilica, which requires significant thermal energy to release it[39]. Another possibility is due to the combustion of organiccomponentssuchascarbon,hydrogenandoxygen[40].Asharp peak at 660 ◦ C is not accompanied by major mass loss (1.5%)as compared to the first and second decomposition steps whichcontribute to the 13.4% of mass loss. It can be inferred that thegel produced is thermally stable since 84.8% of the total massis still retained when heated to 700 ◦ C. 3.4.2. Sol–gel immobilized thiacrown ethers The blank sol–gel and all the sorbent materials exhibitedsimilar mass loss profiles below 200 ◦ C (Fig. 3). The 12S4- SG, 15S5-SG and 18S6-SG lost approximately 10.4, 8.9, and9.7mass% below 200 ◦ C, that is associated with the loss of thephysically absorped water, as is the case for the blank gel. How-ever, there is no significant mass losses between 280 and 340 ◦ Cas was observed in the free thiacrown ethers (Fig. 3B–D), indi- catingtheenormousenhancementonthethermalstabilityoftheligands when immobilized in the sol–gel matrix. The immobi-lization of thiacrown ethers in sol–gel matrix do not seem toaffect the stability of the material as the final masses of thesethree sol–gels when heated to 700 ◦ C are similar to those of theblank sol–gel. 3.5. Optimization for the extraction of metal ions withsol–gel immobilized thiacrown ethers (batch method) When the blank gel was used, metal ions ranging from 0 to20% were removed (Fig. 4). 3.5.1. Effect of pH  The extent of metal ions extracted was examined by shak-ing the metal ions (3h) over pH 2–9. The profiles of extractionare shown in Fig. 5. In almost all conditions, Ag + is the mostextracted, followed by Hg 2+ . This can be correlated with thestability of the complexes where log stability constants of 12.67in methanol between the free 18S6 ligand and Ag + was found[10], while log stability constants of macrocyclic tetrathiaether Fig.4. Percentageextractionofmetalionswithblankgelconductedusingbatchmethod. Mixture containing 1ppm of each metal ion was used. Shaking time,3h; pH, 5;  n =3.
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