Fast and High Amount of Uranyl Ion Uptake by p(VinylPhosphonic Acid) Microgels Prepared by UV IrradiationTechnique

Nurettin Sahiner

Received: 7 January 2014 /Accepted: 28 April 2014 /Published online: 12 May 2014# Springer International Publishing Switzerland 2014

Abstract Micrometer-size poly(vinyl phosphonic ac-id) (p(VPA)) hydrogel was synthesized by employingUV irradiation of an emulsion containing vinyl phos-phonic acid (VPA) and crosslinker, prepared usinglecithin as surfactant and gasoline as solvent. Thep(VPA) microgels were employed in absorption ofUO2

2+ ions from aqueous environments and havevery high and fast absorption capacity. In about20 min, 670 mg UO2

2+ ions were absorbed per gramof p(VPA) microgel from the prepared UO2

2+ ionsolution, and the absorption capacity increased upto 900 mg at pH 6. Various parameters affectingUO2

2+ absorption characteristics of p(VPA) were in-vestigated. It was found that the Langmuir isothermfitted the absorption characteristics of p(VPA) betterthan the Freundlich isotherm. Moreover, magneticferrite can be prepared within p(VPA) and used as amagnetically responsive p(VPA) microgel compositefor externally controlled absorption of UO2

2+ ionswith little decrease in the absorption capacity of thep(VPA) microgel.

Keywords Hydrogel . p(VPA)microgel . Nanogel .

Uranyl ion absorption . Composite

1 Introduction

Metal ions, such as uranium, are generally toxic, havecarcinogenic effects, and the removal and storage ofuranium including depleted uranium (DU) are trouble-some (Blake et al. 2004; Sahiner et al. 2012). DU is aslightly radioactive and toxic heavy metal used even inkinetic energy weapon systems and/or for penetration oftanks or military armored vehicles. DU can enter thehuman body by respiration or digestion or throughcontact with open wounds. It can lead to many healthproblems due to both its chemical toxic nature andradioactivity (Preetha et al. 2006; Yazzie et al. 2003).There have been many polymeric (Karadag et al. 1995;Pekel et al. 2000, 2001; Sahiner et al. 2000; Sahiner et al.2012; Saraydin et al. 2001) and inorganic materials(Chisholm-Brause et al. 2001; Chisholmbrause et al.1994; Duff and Amrhein 1996; Guerra et al. 2010;Korichi and Bensmaili 2009) used for the removal andadsorption/absorption of uranyl ions from different aquat-ic environments, including ground and surface waters aswell as industrial wastes and contaminated waters(Anirudhan et al. 2012; Guibal et al. 1995; Khedr 2013;Yusan and Erenturk 2011; Zhang et al. 2012). Hydrogelswith different functional groups have been used in theuptake of uranyl ions, offering many advantages due totheir hydrophilic nature, easy modification, and easypreparation techniques (Kundakci et al. 2009; Liu et al.

Water Air Soil Pollut (2014) 225:1982DOI 10.1007/s11270-014-1982-1

N. Sahiner (*)Faculty of Science & Arts, Chemistry Department, CanakkaleOnsekiz Mart University,Terzioglu Campus, 17100 Canakkale, Turkeye-mail: sahiner71@gmail.com

N. SahinerNanoscience and Technology Research and ApplicationCenter (NANORAC), Canakkale Onsekiz Mart University,Terzioglu Campus, 17100 Canakkale, Turkey

2010; Ortaboy et al. 2013; Saraydin et al. 1995). Addi-tionally, hydrogel can be prepared in different dimensions(micrometers to nanometers), shapes (cylinders, disks,spheres and films, etc.), g networks, and so on (Guleret al. 1997; Sahiner 2007; Sahiner et al. 2011a, b; Sahinerand Ilgin 2010a, b). These kinds of materials with tunablephysical and chemical characteristics can produce veryfast responses to external effects or stimuli such as pH,temperature, electric/magnetic field, light, or any analytesand even metal ions, and can even be designed to react tospecific molecules. The use of hydrogels in microm-eter and nanometer dimensions for metal ion ab-sorption, such as for uranium, can offer great ben-efits. Here, for the first time, the use of poly(vinylphosphonic acid) (p(VPA)) microgel prepared by aphotopolymerization technique is reported for ura-nyl ion absorption from aqueous environments.Various parameters affecting the uranyl ion absorp-tion characteristics of p(VPA) were investigated.Additionally, it was demonstrated that the p(VPA)microgels can be made magnetic field responsiveby preparation of magnetic ferrite particles withinp(VPA) hydrogels as a composite. These can beremoved from the absorption media upon comple-tion of tasks under an externally applied magneticfield, providing additional opportunity and/or valuefor the environmental use of these polymeric com-posite particles.

2 Experimental

2.1 Materials

Vinyl phosphonic acid (VPA, 90 %) as monomer,N,N′-methylenebisacrylamide (MBA) as crosslinker,2,2′-azobis(2-methylpionamidine) dihydrochloride(ABMPDH) a s UV i n i t i a t o r , l e c i t h i n a smicroemulsion-forming surfactant, and gasoline as asolvent were used as received from Sigma-Aldrich,Acros, and Fluka chemical companies. Sodium salic-ylate (SS, ReagentPlus, ≥99.5 %, Sigma), sodiumhydroxide (NaOH, 98 %, Sigma-Aldrich), hydro-chloric acid (HCl, 37 %, Riedel-de haen), iron (III)chloride hexahydrate (FeCl3.6H2O, Acros), and iron(II) chloride tetrahydrate (FeCl2.4H2O, Fluka) wereused for magnetic particle preparation. Distilled wa-ter (Millipore Direct-Q3 UV) was used throughoutthe experiments.

2.2 Synthesis of p(VPA) Microgels

Microgel was synthesized from VPA by irradiation of aVPA-containing microemulsion polymerization tech-nique, a modification of previously reported methods(Sahiner and Sagbas 2013). Certain amounts of MBA(0.75 % mol ratio with respect to VPA) dissolved in5 mLVPA monomer and mixed with UV initiator solu-tion were prepared by dissolving 80 mg 2,2′-azobis(2-methylpionamidine) dihydrochloride in 200 μL distilled(DI) water. In 0.5 mL of this solution, microgel precur-sors were dispersed in 15 mL 0.1 M lecithin solutionsprepared from gasoline. Under constant mixing at1,200 rpm, the solutions were irradiated in a photoreactor (LUZCHEM, 420 nm, Canada) for 4 h for si-multaneous polymerization of the crosslinking reaction.After centrifugation at 10,000 rpm, the supernatant so-lution was removed and the obtained microgels werewashed with cyclohexane to remove lecithin. This wasfollowed by rewashing with ethanol-to-water mixture(50:50 by volume) twice to purify the particles of allunreacted monomers, crosslinkers, initiator, and surfac-tant. The prepared microgels were dried at 40 °C in anoven and kept in a closed container for characterizationand absorption studies.

2.3 Synthesis of Magnetic-p(VPA) Microgels

In situ preparation of magnetic ferrites were completedby using previously reported methods (Ozay et al. 2009;Ozay et al. 2010; Sahiner 2006; Sahiner et al. 2011c;Sahiner et al. 2011d). Briefly, 0.43 g FeCl2 and 1.168 gFeCl3 were dissolved in 30 mL DI water. Then, 0.2 gp(VPA) microgel was placed into this solution andmixed under constant stirring at 200 rpm for 2 h. Theiron ion-loaded p(VPA) microgels were separated fromthe iron ion solutions by centrifugation (10,000 rpm)and rewashed with DI water. Finally to generate ferriteparticles within p(VPA) for composite magnetic-p(VPA) microgels, the iron ion-loaded p(VPA)microgels were placed in 0.5 M 30 mL NaOH solutionand mixed constantly at 200 rpm for 1 h, followed bywashing several times and storage under a magneticfield.

2.4 UO22+ Absorption Studies

The batch type uranyl ion absorption studies from aque-ous media were carried out with time using 500 ppm

1982, Page 2 of 8 Water Air Soil Pollut (2014) 225:1982

100 mL UO22+ solution and 50 mg dried p(VPA)

microgels. The effect of pH of the uranyl ion solutionon the p(VPA) absorption capacity was also investigatedby determining the absorption amounts from 500 ppm100 mLUO2

2+ solutions at different pHs: 3, 5, 6, 7, 8, 9,and 11. The contact time for UO2

2+ ions with p(VPA)for all the absorption studies was 2 h throughout thismanuscript, unless otherwise stated.

The effect of the amounts of p(VPA) microgels onuranyl ion absorption was also investigated using 25,50, 75, and 100 mg p(VPA) microgels in absorptionstudies from 500 ppm 100 mL UO2

2+ solutions. Addi-tionally, keeping the p(VPA) microgel amount constantat 50 mg, the uranyl ion concentrations were varied:100, 250, 500, 750, and 1,000 ppm (each 100 mL) todetermine the maximum absorption amount of uranylions. To calculate the amount of uranyl ions, 3 mLUO2

2+ solution was mixed with 0.3 mL 1,000 ppmsodium salicylate for complexing, and the absorbancevalues were measured by UV-vis spectrophotometer

(T80+UV/VIS Spectrometer, PG Ins. Ltd) at 327 nmfrom a previously constructed calibration curve.

3 Results and Discussion

3.1 p(VPA) Microgels and Its Magnetic Composites

The schematic representation of p(VPA) microgels ob-tained by photo polymerization and crosslinking reac-tion is given in Fig. 1a. Although there are some reportson the utilization of phosphonic acid functional groupsin the absorption of uranyl ions (Chi et al. 2013;Dudarko et al. 2008; Ferrah et al. 2011; Migianu-Griffoni et al. 2009; Nash 1993; Sizgek et al. 2009),there is no report on the use of pure p(VPA) acidmicrogels for uranyl ion absorption. Previously, wereported the preparation of p(VPA) for different pur-poses (Sahiner and Sagbas 2013). As can be seen fromthe digital camera images illustrated in Fig. 1b, the

20 µm20 µm

hν+

microemulsion

(a)

(b)

Fig. 1 a Schematic representation of photopolymerization and crosslinking mechanism of p(VPA) microgels. bOptical microscope imagesof p(VPA) microgels

Water Air Soil Pollut (2014) 225:1982 Page 3 of 8, 1982

prepared p(VPA) microgels are of micrometer size. Theuse of micrometer or even nanosized p(VPA) in uranylion absorption may require a more difficult handlingprocedure, such as high-speed centrifugation etc.; there-fore, we also prepared magnetic field-responsivep(VPA) microgels. The metal or magnetic ferrite metalnanoparticles can be readily prepared within hydrogelswith different dimensions (Sahiner 2013a) due to func-tional groups on the hydrogel that readily bind to metalions such as –PO3H2 in the p(VPA) microgels. Themagnetic field responsive mag-p(VPA) microgel com-posites were prepared by loading the microgels withFe(II) and Fe(III) mixture and reduction/precipitationwith NaOH. Figure 2a illustrates the digital cameraimages of mag-p(VPA) microgels and their behaviorunder an externally applied magnetic field. It is obviousthat these materials with magnetic responsiveness canbe directed under an externally applied magnetic field orremoved from the medium upon completing their mis-sion of recovery of uranyl ions. The thermal behavior ofp(VPA) and mag-p(VPA) composites are given inFig. 2b. As can be seen, both particles show similardegradation behavior up to about 300 °C. After that,

p(VPA) has a sharp degradation between 450 and500 °C and keeps degrading slowly up to 950 °C. Themag-p(VPA) composite on the other hand shows similardegradation up to 300 °C, and after that, it slowlydegrades up to 950 °C. The degradation temperaturesfor p(VPA) are 136, 451, and 715 °C, and for mag-p(VPA) are 104, 327, 480, and 780 °C, respectively.The difference between p(VPA) and mag-p (VPA) is57.8wt% suggesting that more than half of the weight ofmag-p(VPA) is ferrite particles (about 58 wt%).

3.2 UO22+ Absorption Studies

To determine how fast p(VPA) microgel can absorblUO2

2+ ions from an aqueous medium, a batch typeabsorption of UO2

2+ ions by 50 mg dried p(VPA)microgels was carried out from 500 ppm 100 mLUO2

2+ ion solutions. The maximum absorbed amountsof metal ion per gram dry hydrogel were calculatedusing the very well known mass balance equation:

q ¼ C0−Ceð ÞV.W ð1Þ

(a)

Temp Cel

800.0600.0400.0200.0

100.0

80.0

60.0

40.0

20.0p(VPA)

Mag-p(VPA)

(b)

(% 57.82 )

TG %

Fig. 2 a Digital camera images of the aqueous suspension ofmag-p(VPA) microgels and mag-p(VPA) microgels under a mag-netic field. b Thermogram of p(VPA), mag-p(VPA) microgels(57.82 %), and uranium-absorbed p(VPA) microgels (62.68 %)

0

200

400

600

800

0 30 60 90 120

qe

(mg

/g)

Time (min)

0

200

400

600

800

1000

3 5 7 9 11 13pH

Ma

x.

ab

sorb

ed a

mo

un

ts(m

g/g

)

(a)

(b)

Fig. 3 a The amount of absorbed UO22+ with time by p(VPA)

microgels and b the maximum absorption of uranyl ions by 50 mgp(VPA) microgel from 500 ppm 100 mL UO2

2+ solutions atdifferent pH values (Conditions: 500 ppm, 100 mL UO2

2+,50 mg p(VPA) microgels)

1982, Page 4 of 8 Water Air Soil Pollut (2014) 225:1982

where q is the amount of UO22+ ions absorbed per unit

mass of p(VPA) in mg/g, C0 and Ce are the initial andequilibriummetal ion concentrations (mg/L),V is volumeof metal ion solution, and W is the weight of the driedhydrogel (in g) used. Figure 3a illustrates that 50 mgp(VPA) microgels can absorb 660 mg UO2

2+ per gramp(VPA) in 20 min. This is very fast and provides a greatadvantage for this system to be used in real applications.These amounts are higher than most of the reportedvalues (Karadag et al. 1995; Liu et al. 2010; Preethaet al. 2006). Figure 3b illustrates the maximum amountsof uranyl ion absorption according to solution pH. Ascan be seen from the figure, the highest value ofsorption is obtained at a pH value of 6, which is inaccordance with the literature values (Pekel et al.2000; Sahiner et al. 1998). When medium pH isbetween 5 and 6, the amount of uranyl ion absorptionis relatively higher, and the absorption amounts herewere 900 mg/g at pH 6. Upon increasing the pHvalues, e.g., between 7 and 11, the amount ofabsorbed uranyl ion significantly decreases due to

the formation of insoluble uranium hydroxides, sucha s UO2 (OH) 2 , UO2 (OH) 3

− , UO2 (OH) 42 − ,

(UO2)3(OH)7−, (UO2)3(OH)8

2−, (UO2)3(OH)82−, and

(UO2)3(OH)104−, as a result of the hydrolysis of ura-

nyl ions (Akperov et al. 2010).To determine the effect of initial UO2

2+ ion solutionconcentration and of the amount of p(VPA) microgel onthe absorption capacity, different initial concentrationsof UO2

2+ ions (100, 250, 500, 750, 1,000 ppm each with100 mL volume) were contacted with 50 mg p(VPA)microgels, and varying amounts of p(VPA) microgels(25, 50, 75, and 100 mg) were contacted with 100 mL500 ppm uranyl ion solutions. The obtained results arepresented in Fig. 4a, b. As can be seen from Fig. 4, themaximum amount of absorbed UO2

2+ increases with theincrease in initial amounts of UO2

2+ solution (a) anddecreases with the increase in amount of p(VPA) from100 mL 500 ppm solutions. This can be explained bythe fact that as the amount of free UO2

2+ insolution increases, they are readily absorbed byp(VPA) hydrogels giving higher absorption capac-ity as in Fig. 4a. As the amount of microgel isincreased for the same amount of free UO2

2+ ionsin solution, there are many sites available to bind

0

200

400

600

800

0 25 50 75 100 125

Amounts of p(VPA) microgel (mg)

Max. ab

sorb

ed a

mou

nts

(mg/g

)

0

200

400

600

800

1000

1200

0 200 400 600 800 1000 1200

Initial concentration, C0 (ppm)

Max. ab

sorb

ed a

mou

nts

(mg/g

)

(a)

(b)

Fig. 4 The maximum absorption of uranyl ions a at differentinitial solution concentrations: 100, 250, 500, 750, and1,000 ppm each with 100 mL volume using 50 mg p(VPA)microgels, and b using different amounts of p(VPA) microgelsfrom 500 ppm 100 mL uranyl ion solutions

y = 0.0009x + 0.0864

R² = 0.9991

0

0.2

0.4

0.6

0.8

0 100 200 300 400 500 600

Ce/q

e

Ce

(a)

y = 0.4484x + 1.7916

R² = 0.941

2

2.5

3

3.5

1 1.5 2 2.5 3

Log q

e

Log Ce

(b)

Fig. 5 The application of a Langmuir and b Freundlich isothermsfor UO2

+2 absorption by p(VPA) microgels

Water Air Soil Pollut (2014) 225:1982 Page 5 of 8, 1982

with in p(VPA); hence, a reduction in the absorp-tion capacity is observed.

3.3 Equilibrium Adsorption Studies

The batch type absorption of UO22+ ions from aqueous

solution by p(VPA) microgels was performed using fivedifferent concentrations of UO2

2+ ion solutions varyingbetween 100 and 1,000 ppm per 100mL. The very well-known Langmuir (Eq. 2) and Freundlich (Eq. 3) equa-tions were applied to determine and define the nature ofthe absorption of the UO2

2+ ions into p(VPA)microgels:

Ce

.qe ¼ Ce

.qm

� �þ 1

.qmKL ð2Þ

where, Ce is the equilibrium concentration of metal ions(mg/L), qe is the amount of metal ions adsorbed pergram absorbent (mg/g), qm is the maximum adsorptionof metal ions (mg/g), and KL is the Langmuir absorptionequilibrium constant (L/mg).

Log qe ¼ logK F þ 1.nlogCe ð3Þ

where, qe (mg/g) is the adsorption capacity at equilibri-um, Ce (mg/L) the equilibrium concentration of metalsalts in solution, and KF and n are physical constantsof the Freundlich adsorption isotherm. KF and n areindicators of adsorption capacity and adsorption in-tensity, respectively. Figure 5a (Ce/qe vs qe) andFig. 5b (log qe vs log Ce) illustrate the applicationof the Langmuir and Freundlich models for UO2

2+

ion absorption by 50 mg p(VPA) microgel for100 mL of 100–1,000 ppm concentration, respec-tively. The results confirm that UO2

2+ ion absorptionby p(VPA) microgel abides by both models but better fitsthe Langmuir isotherm and their corresponding valuesare given in Table 1. The maximum absorption capacity

(qm) was 1,110 mg/g, a value which is higher than thosepreviously reported (Guler et al. 1997; Karadag et al.1995; Kundakci et al. 2009; Pekel et al. 2001; Sahineret al. 2000; Saraydin et al. 1995; Saraydin et al. 2001).This could be attributed to the accessibility of the innerpart of p(VPA) microgels by UO2

2+ ions due to swellingability, leading to open porosity, and ion exchange ca-pability of phosphonic acid groups with high metal ionbinding ability via electrostatic interactions. It is verywell known that functional groups such as–COOH, –NH2OH, –SO3H, and –PO3H2 grant the ability to absorblarge quantities of water and metal ions such as UO2

2+

(Migianu-Griffoni et al. 2009; Nash 1993; Pekel et al.2000; Sahiner 2013b). Assuming 1,110 mg UO2

2+ ioncan be absorbed by 50 mg p(VPA) microgels underoptimum conditions, this implies that the mole ratio ofphosphonic acid functional groups to UO2

2+ ions is 2.25.In other words, for every one UO2

2+ ion, there are more

Table 1 The parameters for the absorption of UO22+ by p(VPA)

microgel from the application of the Langmuir and Freundlichisotherms

Langmuir isotherm constants Freundlich isotherm constants

KL (L/g) qm (mg/g) R2 KF n R2

96 1,110 0.999 61.89 2.23 0.976

Absorption conditions: 100–1,000 ppm with 100 mL volumes,0.50 mg p(VPA) microgel hydrogel, and 30 °C

Temp Cel

800.0600.0400.0200.0

100.0

80.0

60.0

40.0

20.0

p(VPA)

Uranyl ion absorbed p(VPA)

(b)

(a)

Fig. 6 a Suggested binding mechanism for the absorption ofuranyl ions by p(VPA) microgels and a comparison of the thermalstability of uranyl ion-absorbed p(VPA) and bare p(VPA)microgels

1982, Page 6 of 8 Water Air Soil Pollut (2014) 225:1982

than two moles of VPA used. The correct ratio could be2, due to some experimental errors or other reasons, suchas uranyl ions not having 100 % accessibility to func-tional groups or impurities in VPA, crosslinker, andinitiator may also be accountable. Therefore, the poten-tial binding mechanism due to the interaction of uranylions and phosphonic acid in p(VPA) microgels is sug-gested in Fig. 6a as it is known that functional groups,such as phosphonic acid in p(VPA), are the major uranylion absorbing groups. The UO2

2+ absorbed and barep(VPA) microgel thermograms were also comparedand are given in Fig. 6b. As can be seen, the weight lossof UO2

2+ ion-absorbed p(VPA) microgels is about16 wt% and no weight was lost up to 950 °C confirmingvery strong interactions between the UO2

2+ ions andp(VPA) microgels.

The mag-p(VPA) microgels were also used for theabsorption of UO2

2+ ions, and the amount ofabsorbed UO2

2+ ion per gram p(VPA), excludingthe weight of magnetic ferrite, is less than for barep(VPA) microgels, 595±74 and 669±24 mg/g, re-spectively. The reason for the lower UO2

2+ ion ab-sorption by mag-p(VPA) could be due to the entrap-ment or interaction of some of the phosphonic acidfunctional groups with magnetic ferrites.

4 Conclusion

In this investigation it was demonstrated that p(VPA)microgels have very quick uranyl ion absorption ability(about 20 min) with a very high absorption capacity ofabout 900 mg/g at pH 6. Furthermore, magnetic ferriteswere also prepared within p(VPA) microgels as com-posites to potentially allow external targeting of mag-p(VPA)microgels, which opens up new avenues for realusage of these materials in contaminated ground andsurface waters as well as in nuclear power stations.

Acknowledgments The author is grateful to the financial sup-port from the Turkish Ministry of Science, Industry, and Technol-ogy (00533.STZ.2010-1). Support from Selin Sagbas is greatlyappreciated.

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