Factors affecting the complexation of polyacrylic ... - ResearchGate

2740 DUBOLAZOV ET AL.Figure 2. Convoluted emission spectrum of PAA/UO 2þ2complex in aqueous solution. k ex ¼ 310 nm,PAA/UO 2þ2¼ 2, pH ¼ 3.13. Experimental spectrum(—) obtained from UO 2þ2(---) and PAA/UO 2þ2(—)peaks.excess of UO 2þ2complex stoichiometry with nondissociatedgroups of polyelectrolyte.As the emission spectrum shown in Figure 2gives the envelope of individual peaks within thespectral range deconvolution software has beenused to determine peak positions and the exactintensities of PAA/UO 2þ2spectrum. According tothe changes in intensity of uranyl PAA complexband under external stimuli factors we attemptedto use the I/I 0 ratio, where I and I 0 are theluminescence intensity of PAA/UO 2þ2complexband in aqueous solution with and withoutexternally affecting factors such as, H þ or OH ,salt concentration (Na þ and K þ ), and temperature.Relative luminescence intensity of polycomplexband can serve as a qualitative evaluationcriterion of enhancement or weakening of interactionsbetween polymer and metal ions: increaseof I/I 0 value can be interpreted to be asdue to a more compact structure as well as morestable. On the other hand, if I/I 0 value is lowerthan 1 it means that fluorescence properties ofUO 2þ2species are diminished under the quenchingeffect. Contribution of both enhancement(destabilization) and quenching effect to thePAA/UO 2þ2emission spectra under external stimulieffects are separately considered later.pH EffectThe results of the previous studies showed thatchelation of metal ions by polymeric ligandsdepend on the equilibrium pH of medium. Sebastianand coworkers 24 investigated the PAA complexationwith various metal ions at different pHvalues (3.45–6.0). The optimum pH for maximummetal-ion uptake by this polymer was found tobe as follows: 5.4 for Fe(III), 5.4 for Co(II), 6.2for Ni(II), 5.4 for Cr(III), and 6.4 for Zn(II). Thelimitation of pH interval was attributed to themetal hydrolysis and precipitation at higher pH.However, Bodzek et al. 25 investigated the possibilityof removing Cu, Ni, and Zn ions frommodel and real galvanic wastewater at 2–10 pHregion, using PAA as flocculant. In all casesthere was an increase in the permeate flux concomitantwith increasing of pH. This phenomenonwas explained by formation of metal–PAAcomplexes more easily at higher pH.Most of the studies about complexation ofUO 2þ2ions with functional group containing polymersshowed that ion exchange has been consideredas the main mechanism responsible forUO 2þ2ion separation. 26 The ion exchange mechanismfor UO 2þ2ion binding to the polymer is complicatedby the fact that the UO 2þ2ion is hydrolyzedin aqueous solutions within the pH rangeof the studied adsorption system and differenttype of hydrolyzed UO 2þ2ions are formed. At pH4.0, more UO 2 (OH) þ ions exist in the solution. 27Application of fluorescence spectroscopy hasbeen hampered by inconsistent interpretation offluorescence lifetimes observed in solutions withincreasing pH. While it has been well knownthat the UO 2þ2species has a fluorescence lifetimeof about 1 ls in acid solution (pH < 2), a secondlifetime contribution had been reported toappear on increasing the pH value to basic solutions.This second lifetime of about 2.5 ls hasbeen interpreted because of different fundamentalprocesses, for example, intersystem crossingor various hydrolyzed uranyl species formation.18 At pH 4, UO 2þ2is the predominant species(96%), UO 2 (OH) þ is less than 4%; at pH 5, UO 2þ2is around 69%, UO 2 (OH) þ is less than26%, (UO 2 ) 2 (OH) 2þ2is less than 5%; at pH > 5.5,the fractions of UO 2 (OH) þ (35%) and (UO 2 ) 2 (OH) 2þ2are decreased and the fraction of (UO 2 ) 2 (OH) 3þis increased with increasing pH. 28For studying the pH effect on luminescenceproperties of PAA/UO 2þ2complex we investigated1.88–4.02 pH range to avoid hydrolyzed forms ofUO 2þ2. The pH dependence of the relative luminescenceintensity of the complex emission bandis shown in Figure 3a. As it can be seen from figure,I/I 0 values are lower than 1 and reachingthis value only at pH 3.25, which probablymeans that the presence of excess of H þ or OHions is unfavorable for complexation between