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2738 DUBOLAZOV ET AL.workers 12 investigated the process of complexformation between PAA and UO 2þ 2 , using potentiometryand differential pulse polarographytechniques. It was found that polyelectrolyteforms intrapolymer chelates with actinide ionsand complex formation completes with the compositionof UO 2þ2(carboxylate) 2 . The resulting complexformation constants revealed that PAA hasshown much higher complex ability than that oflow-molecular-weight species such as succinicand propionic acids. This phenomena wereexplained by the assumption that the concentrationof ligands is higher in the domain so thatonce the metal ion is attached to one group onthe polymer chain, the other ligands coordinatemore easily. It was also found that PAA formsmore stable complex with uranyl ion than withcopper ion. 11Many researchers have investigated the complexationbehavior between polymer and metalions using ultrafiltration, ion exchange, ionselective electrode potentiometry, and severalspectroscopic techniques. Luminescence spectroscopyin general and time-resolved fluorescencespectroscopy are very sensitive techniques forthe analysis, speciation, and interactions of actinideand lanthanides. 13 There are however fewpapers in the literature about the complexationbehavior of functional group-containing polymerssuch as PAA with uranyl ions by using luminescencespectroscopy. 14 Both molecular structureand chemical surroundings have an influence onwhether a molecule can produce luminescence.These factors, including chemical structure, temperature,pH, ionic strength, solvent type, concentration,and dissolved oxygen, can affectemission intensity while luminescence occurs. 15The excitation and emission spectra of many fluorophores(fluorescent sensitive molecules) aresensitive to the polarity of their surroundingenvironment. As a consequence, quenching offluorescence is observed if complex affects theelectronic structure of the fluorophore. For somecations, sensing methods were established thatrely on the intrinsic optical properties. Somemetal ions show absorption (ranging from the UVto the near infrared) or luminescence. Examplesare the detection of Cu(II) 16 or the uranyl ion. 14,17Luminescence of hexavalent uranium in solutionand solid state has been known for morethan 150 years. All the electrons in the stronglybonded UO 2þ2structure are paired. Thus, theground electronic level is a singlet. To formhigher-energy electronic states, one of the bondingelectrons is transferred to the 5f nonbondingatomic orbitals of the uranium ion. 18 Thus, thefluorescence spectrum is the result of the transitionsfrom the first excited electronic level to theground singlet and the vibrational levels associatedwith the singlet. Using this fluorescence ofuranyl, the complexation of uranyl ion with someinorganic and organic systems has beenstudied. 13,15 Application of U(VI) luminescenceto speciation of U(VI) however has forwardeddiscrepant results and interpretations. Especiallydependence of fluorescence behavior on pHis discussed in terms of mutually exclusive interpretations.Complexation studies and migrationof radionuclides such as uranium are quiteimportant from environmental pollution andsafety point of view. The advantages of the use ofpolyelectrolytes such as PAA are easy recovery ofthe uranyl ion from aqueous systems.In our previous study 19 the mechanism ofinteraction between PAA and uranyl ions inaqueous solutions and the restrictive role of Coulombicinteractions were confirmed using conductometric,potentiometric, thermal analysis,and FT-IR spectroscopic methods. In this study,we tried to show the effect of some factors suchas pH, salt concentration, and temperature onluminescence properties of UO 2þ2ions complexedwith PAA in aqueous solutions.EXPERIMENTALPAA with weight-average molecular weight (M w )2.3 10 5 was purchased from BDH Chem. Corp.(England) and used without further purification.Uranyl ion stock solutions (c ¼ 0.001 mol/L) wereprepared from hexahydrate of uranyl nitrateUO 2 (NO 3 ) 2 6H 2 O (Merck, Darmstadt, Germany).Inorganic salts (NaNO 3 and KNO 3 ) of analyticalgrade were used as received.The PAA/UO 2þ2solutions were prepared byadding PAA solution (c ¼ 0.002 mol/L on arepeating unit basis) into UO 2þ2stock solutions.In all experiments the [repeating unit of PAA]/[metal] concentration ratio was 2 (stoichiometryof PAA/UO 2þ2complex), keeping the total volumeconstant at 10 mL.To investigate the pH effect on the formationof PAA/UO 2þ2complex, a set of solutions withpH varying from 1.88 to 4.02 were prepared byusing 0.05 M nitric acid or sodium hydroxidesolutions. The pH values of the aqueous mixtureswere determined with a digital pH 211
FACTORS AFFECTING POLYACRYLIC ACID/URANYL ION COMPLEX FORMATION 2739microprocessor pH meter (Hanna Instruments,Germany).The complex solutions with different concentrationsof inorganic salts were prepared by addingcorresponding alkali-metal nitrate to PAAand UO 2þ2solutions separately followed by theirmixing to make up the total solution. Temperatureeffect on the complexation of PAA withuranyl ions was studied by changing the temperatureof complex solution from 25 to 50 8C.UO 2þ2emission spectra were recorded on a PerkinElmerLS 55 luminescence spectrometer (PerkinElmerInstruments, England) under excitationat 310 nm. All measurements were carriedout at ambient temperature within 10 min aftermixing of components for salt concentration andpH investigations. The deconvolution of theemission spectra was performed by resolving thepeaks using a computer program (Peakfit 3.0software).Turbidimetric measurements were carried outwith a Philips model 8715 UV–vis spectrophotometerat 400 nm.RESULTS AND DISCUSSIONThe luminescence properties of uranium speciesin aqueous solutions are known to be stronglyinfluenced by their immediate coordinative environment.20 Earlier 19 we showed the preliminaryresults of very interesting behavior of UO 2þ2emission spectra in the presence of PAA of differentconcentrations: at the concentration ratio[repeating unit of PAA]/[metal] ¼ 2, new hypertransitionshoulder at 483 nm appeared, whichis probably due to complex formation betweenUO 2þ2and PAA.The basic structure of the uranyl nitratehydrates is shown as [UO 2 (NO 3 ) 2 (H 2 O) 2 ]mH 2 O,where m ¼ 0, 1, and 4. 21 Basically, the linearuranyl ion is coordinated to two water moleculesand two bidentate nitrate ions. Other water moleculesin the trihydrate (m ¼ 1) and hexahydrate(m ¼ 4) are linked loosely to the coordinatedwater molecules by means of hydrogen bonding.The removal of the mH 2 O does not affect the fluorescencespectrum significantly. However, it iswell known that the water molecules adjacent tothe uranyl ion affect the fluorescence lifetime.Among the hydrates of uranyl nitrate the hexahydrate,which has the highest number of watermolecules in the unit cell, is expected to lead toan increase in the lifetime.Figure 1. Emission spectra of aqueous UO 2þ2at differentn ¼ PAA on a repeating unit basis /UO 2þ2values:(a) n 2 and (b) (n > 3). k ex ¼ 310 nm.The emission spectra of UO 2þ2aqueous solutionsin the absence and presence of PAA withdifferent concentrations under excitation at310 nm are shown in Figure 1. When n < 2, increasein emission intensity is strongly related toPAA concentration and emission peaks shift tolower wavelengths (Fig. 1a) due to coordinationof UO 2þ2ions by PAA. This can be explained bythe expulsion of some of the water moleculescoordinated to uranyl ions by complex formation.22 It is important to note that the increasein PAA concentration is accompanied by significantchanges in the spectral shape with appearanceof a new shoulder at 483 nm (Fig. 1b) untilstoichiometric ratio reaches the value of n ¼ 2(PAA on a repeating unit basis/UO 2þ2¼ 2:1 Mratio). Formation of shoulders around the mainpeaks of UO 2þ2has also been observed for thecomplexation of uranyl ions with some a-substitutedcarboxylic acids in the work of Moll et al. 23For higher n values however, (PAA on a repeatingunit basis/UO 2þ2>4) decrease in emissionintensity has been observed, which can beexplained by the quenching effect of PAA in
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