Factors affecting the complexation of polyacrylic ... - ResearchGate

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