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2742 DUBOLAZOV ET AL.takes place, but in the latter case there is only adistribution of Me þ metal ions near the polymerchain and no real binding of monovalent metalions. 30 Attraction between PAA and Me þstrongly affects the electric field of polymer surface,which can lead to the rearrangement ofPAA structure. As a general rule the increase insalt concentration of the medium changes theconformation of the polymer chains to a morecompact form. 31 In our case this factor probablyleads to the enhancement of hydrophobicity aswell as stability of PAA/UO 2þ2complex.Recently Iida observed the similar effect ofKCl on activity of Ca 2þ in aqueous PAA solution,using a Ca 2þ ion sensitive electrode. The Ca 2þactivity in the PAA solution at the low degree ofneutralization was increased by the presence ofdilute KCl and decreased by the presence of concentratedKCl. 32The size effect seems to be obvious explanationfor the influence of Me þ nature on thestability of PAA/UO 2þ2polycomplex. Kim and coworkers31 found that in the solutions containingalkali metal ions, aggregation of PAA wasincreased in the order Li þ < Na þ < K þ < Cs þ ,which means that the hydrophobicity of thechain decreases with increasing solvated radiusof the counterion. As it can be seen from Figure4, the addition of KNO 3 has more pronouncedeffect on increasing of I/I 0 value than NaNO 3 aswell as on the critical concentration of complexprecipitation (see Fig. 5). Indeed, the radius ofhydrated Na þ (R H 4Å) is bigger than that ofK þ (R H 3Å). 33 Therefore in our case condensationforce is higher for K þ , which can stabilizeand favor the aggregation of PAA/UO 2þ2polycomplexat lower concentrations than Na þ . Consideringthe difference in the critical concentration ofcomplex aggregation at the onset of phase separation,we conclude that the PAA/UO 2þ2 /KNO 3complex is more hydrophobic and stable thanPAA/UO 2þ2 /NaNO 3.The effect of concentrations of H þ , NaNO 3 ,and KNO 3 on quenching the luminescent intensityof UO 2þ2complexed with PAA has been analyzedby constructing relevant Stern–Volmerplots, namely, I 0 /I versus quencher concentrations,using the following relation:Figure 5. Effect of inorganic salts NaNO 3 (1) andKNO 3 (2) on the turbidity of PAA/UO 2þ2aqueous solutions.k ¼ 400 nm, pH ¼ 3.13, PAA/UO 2þ2¼ 2.I 0 =I ¼ I þ k q hs 0 i½QŠwhere I 0 and I are luminescent intensities in theabsence and presence of quencher, k q quenchingrate constant, hs 0 i average lifetime of the uranylion in water and [Q] concentration of quencher.The data presented in Figures 3a and 4 weretherefore used to prepare Figure 6. The inset inthis figure is for H þ ions. Ascending parts of thecurves correspond to quenching of luminescenceby corresponding salts or acid. By taking the hs 0 ivalue as 5 ls, which is typical average lifetime ofUO 2þ2in aqueous solutions, 14 and using the slopeof ascending parts of the curves of Figure 6quenching rate constant k q was calculated andlisted in Table 1. By analogy to the effect ofquenching, the descending parts of the curveswere evaluated similarly to calculate the rateconstants for enhancement effect, where thecoefficient k q in above given equation is replacedby k e . Both results are listed in Table 1. Thequenching rate constant k q ¼ 1.7 10 8 M 1 s 1determined for H þ is in very good accordanceFigure 6.3a and 4Stern–Volmer plot of the data of Figures
FACTORS AFFECTING POLYACRYLIC ACID/URANYL ION COMPLEX FORMATION 2743Table 1. Quenching and Enhancement Effectsof Some Ions on Luminescence Properties ofPAA-Bonded UO 2þ2Ion k q (M 1 s 1 ) k e (M 1 s 1 )H þ 1.7 10 8 1.1 10 9Na þ 7.6 10 4 1.8 10 7with data reported in the literature 2.8 10 7 ,2.7 10 8 for acrylic acid and methacrylic acidmonomers. 14Stern–Volmer plots obtained in the presenceof Na þ and K þ , and as a function of H þ shownonlinear behavior (Fig. 6). The curves given inthis figure can be considered to be composed oftwo parts; an initial enhancement effect up to acertain salt or H þ concentration, followed by aquenching effect. I 0 /I vs. concentration patternsfor all three quenchers used are quite similar.These type of nonlinear Stern–Volmer plots havebeen also observed with monovalent alkalinesalts (Na þ and K þ ) in another fluorescencequenching study. 34 Such nonlinear behavior hasbeen generally explained to be due to combinedeffects of dynamic and static quenching mechanisms.As it is seen directly from the slopes ofquenching part of the curves corresponding tohigher salt concentrations in Figure 6, and fromthe calculated quenching constants k q given inTable 1, K þ is more effective than Na þ . This canbe due to smaller hydrated ion size of K þ comparedto that of Na þ . A similar effect on quenchingefficiency of ions has been observed in thecase of interactions of monovalent salts (NaCland KCl) with a fluorescent probe. 34are relatively temperature stable in 25–50 8Crange. Rao et al. 35 studied the complexation ofuranium (VI) with malonate using UV–visabsorption and luminescence spectra at differenttemperatures, and provided qualitative informationon the temperature effect. No significantshifts in the positions of the luminescence emissionbands were observed when the temperatureof the uranyl malonate solutions was increasedfrom 25 to 70 8C. However, the overall intensitiesof the bands decreased by about ten times at70 8C, compared to those at 25 8C.CONCLUSIONSThe complexation behavior of UO 2þ2ions withpoly(acrylic acid) (PAA) has been studied byusing fluorescence spectroscopy in the presenceof varying external factors such as pH, salt concentration,and temperature. Following are theconclusions reached from this work:1. UO 2þ2ions form various hydrolyzed speciesdepending on the pH of medium. In acidicsolutions generally UO 2þ2ions are predominant,while the hydrolyzed species predominatewith increasing pH. Fluorescenceemission intensities of PAA/UO 2þ2complexdecreased in strong acidic solutions (pH ¼2.0–3.25) because of the destabilization ofthe complex between uranyl ions and PAAby H þ ions, whereas at pH > 3.25 forma-K þ 1.2 10 6 4.6 10 6 Figure 7. Effect of temperature on the relativeTemperature EffectThe temperature effect on the stability of PAA/UO 2þ2complex is shown in Figure 7. Variation oftemperature from 25 to 50 8C affects I/I 0 value ofcomplex emission band (483 nm) by slightdecreasing of relative emission intensity of complexband (curve 1). It is known that the lifetimeof excited uranyl (VI) ion in aqueous solutionalso decreased with temperature from 1.6 to0.6 ls at 20–50 8C temperature range. 18 Resultsobtained confirmed the absence of enhancementeffect, as well as existing of slight increase ofquenching with increase in temperature (curves2 and 3). Therefore taking into account this factwe concluded that PAA/UO 2þ2complex particlesluminescence intensity of PAA/UO 2þ2(1) and maximumUO 2þ 2 bands in the absence (2) and presence of PAA(3). k ex ¼ 310 nm, pH ¼ 3.13, [UO 2þ2] ¼ 0.001 mol/L,PAA/UO 2þ2¼ 2.
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