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Journal of Hazardous Materials 260 (2013) 425– 433

Contents lists available at SciVerse ScienceDirect

Journal of Hazardous Materials

jou rn al hom epage: www.elsev ier .com/ locate / jhazmat

orption properties of a new thermosensitive copolymeric sorbentearing phosphonic acid moieties in multi-component solutionf cationic species

lain Graillota,b, Denis Bouyera,∗, Sophie Mongeb, Jean-Jacques Robinb,ierre Loisona, Catherine Faura,∗

Institut Europeen des Membranes-IEM (UMR 5635 CNRS-ENSCM-UM2) – Equipe Genie des Procedes Membranaires, Universite Montpellier II cc047, Placeugene Bataillon, 34095 Montpellier Cedex 5, FranceInstitut Charles Gerhardt Montpellier (UMR 5253 CNRS-UM2-ENSCM-UM1) – Equipe Ingenierie et Architectures Macromoleculaires, Universiteontpellier II cc1702, Place Eugene Bataillon, 34095 Montpellier Cedex 5, France

i g h l i g h t s

Influence of cation valence on sorp-tion properties on functionalizedcopolymer.Investigation of sorption mecha-nisms depending on the operatingconditions.Sorption selectivity in multi-component systems.Role of Al3+ cation on sorption prop-erties in multi-component system.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

rticle history:eceived 11 March 2013eceived in revised form 14 May 2013ccepted 27 May 2013vailable online xxx

eywords:etal sorption

a b s t r a c t

In this paper, original thermosensitive copolymers bearing phosphonic acid groups, namely the poly(N-n-propylacrylamide-stat-2-(methacryloyloxy)methylphosphonic acid) (P(NnPAAm-stat-hMAPC1)) weresynthesized, and their sorption properties for three divalent cations (Ni2+, Ca2+, Cd2+) and one triva-lent cation (Al3+) have been investigated. The sorption experiments were performed with increasingrelative amount of cationic pollution compared to the amount of sorption sites (Cn+/P ratio) in monoand multi-component solutions to investigate the sorption mechanisms. Cn+/P proved to strongly affectthe sorption capacity and high capacities were obtained for all cations at highest Cn+/P ratios, reaching

n+

hermosensitive copolymershosphonic acid

one mole of Csorbed per phosphonated moiety. For divalent cations, sorption mechanisms were likelyto be described by electrostatic interactions only, whereas for aluminum trivalent cation the sorptionnot only resulted from electrostatic interactions but also from the formation of coordination binding.The selectivity of the phosphonic acid moieties for aluminum cations was demonstrated, highlight-ing the interest of P(NnPAAm-stat-hMAPC1) for their use for the treatment of metallic pollution fromwastewater.

∗ Corresponding authors. Tel.: +33 467143897; fax: +33 467149119.E-mail addresses: denis.bouyer@um2.fr (D. Bouyer),

atherine.Faur@iemm.univ-montp2.fr (C. Faur).

304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2013.05.050

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

In the last decades, a wide spectrum of processes has been

developed to eliminate the metallic pollution from wastewater.The most common processes are based on lime precipitation,electrolytic methods or ion-exchange resins but the evolutionof standards for the metallic pollution in wastewater drove the

4 rdous Materials 260 (2013) 425– 433

stwbtbiptaepwewa(soatsg[itmsdptaTsibmlfAipalesetsscs

avtro[onsc

se

26 A. Graillot et al. / Journal of Haza

cientific community either to improve these usual methods oro find out original solutions to remove metallic pollution fromastewater. Among all of these processes, a special interest has

een given to polymeric sorbents. Their main advantage for thereatment of metallic pollution is that a polymer can be specificallyuilt to remove a targeted metal, i.e. the selectivity of the process

s greatly improved compared to the common processes, such asrecipitation for instance. It has been shown in literature thathe wide range of sorption chemical moieties (alcohol, carboxyliccid, sufonic acid, phosphonic acid, amidoxime, crown ether,tc.) [1] enables to synthesize a broad spectrum of functionalizedolymers, which have therefore specific sorption properties for aide range of elements (transition metals, lanthanides, rare earth

lements, etc.). Within these functional moieties, a special interestas given in this work to phosphonic acid groups since they

llow the sorption of a lot of metallic cations in aqueous solutionAg+, Ni2+, Ca2+, Cu2+, Pb2+, Co2+, Cr3+, etc.) [2–7]. The physicaltate (soluble, insoluble) and properties (thermal, mechanicalr chemical properties) of such functionalized polymers can bedapted to the purification process and also to the separationechnique (decantation, filtration, etc.). In this respect, it is pos-ible to build (i) insoluble polymer sorbents (polymer beads [8],els [9,10], membranes [11,12], polymeric networks [13], fibers14,15], etc.) or (ii) water soluble polymers. On the one hand,nsoluble polymeric sorbents are of great interest for wastewaterreatment since they can be easily removed from the water after

etal sorption. On the other hand, water-soluble polymericorbents ease the sorption step since the sorption groups areirectly in contact with the metallic pollution in water due toolymer hydrophilicity [3]. As they combined both advantages,hermosensitive polymeric sorbents are therefore particularlyttractive. At temperature lower than the Lower Critical Solutionemperature (LCST), the thermosensitive polymeric sorbent isoluble in water; that is, the sorption of the metallic cationss favored in solution. Above the LCST, the polymeric sorbentecomes non-soluble, making the polymer–water separationore favorable. For these reasons, many researchers have high-

ighted the advantages to use thermosensitive polymeric sorbentsor the elimination of metallic pollution in wastewater [3,5,16–18].mong all the thermosensitive polymeric sorbent used, a special

nterest was given in our previous works [5,16] to poly(N-n-ropylacrylamide-stat-2-(methacryloyloxy)methylphosphoniccid) (P(NnPAAm-stat-hMAPC1)) copolymers as they combinedow LCST values and good sorption properties due to the pres-nce of phosphonic acid moieties on the polymer chains. Thesetudies, focusing on the elimination of Ni2+ cations from syntheticffluents, highlighted the link between the sorption capacity andhe temperature, the pH and the moles of Ni vs the moles oforption sites (Ni/P ratio). Additionally, it was shown that the Ni2+

orbed under the LCST were not released above the LCST, once theopolymer–metal complex switched from soluble to non-solubletate in water.

Nevertheless, our previous work only focused on the sorption of specific cation (Ni2+) onto P(NnPAAm-stat-hMAPC1) copolymer inarious conditions of Ni/P ratio but no special attention was giveno the sorption mechanisms. Actually, to date, two different theo-ies are described in the literature to tempt explaining the sorptionf metallic cations onto phosphonic acid groups: (i) ion exchange4,19–21] and (ii) complexation [6,22]. However, the predominancef one mechanism on the other is still unclear and to our knowledgeo detailed work was carried out until now to determine whichorption mechanism is predominant when different experimental

onditions are used.

In this contribution, we demonstrated for the first time that theorption of cationic species on phosphonic acid is due to both ionxchange and/or complexation, notably depending on the metallic

Fig. 1. Chemical structure of P(NnPAAm-stat-hMAPC1) polymeric sorbent and itsarchitecture (©: NnPAAm, �: hMAPC1).

cation and its concentration. This was shown investigating (i) theinfluence of the cation on the sorption properties, (ii) the sorptionmechanisms involved between the phosphonic acid groups anddifferent cations and (iii) the selectivity of the phosphonic acidas a function of cationic species. More concretely, the sorptionproperties of P(NnPAAm-stat-hMAPC1) copolymeric sorbent wereevaluated for divalent cations (Ca2+, Ni2+ and Cd2+) and for atrivalent cation (Al3+) in mono and multi-component solutions andfor various operating conditions. Results allowed to discuss thesorption mechanisms involved between phosphonic acid groupsand cations having different valences and also to conclude on theaffinity of these functional groups.

2. Experimental

2.1. Materials

2.1.1. Thermosensitive polymeric sorbentAll P(NnPAAm-stat-hMAPC1) copolymers used in this study

have been synthesized by free radical polymerization between(dimethoxyphosphoryl)methyl 2-methylacrylate (MAPC1, Spe-cific Polymers, [86242-61-7]-SP41-003) and N-n-propylacrylamide(NnPAAm, Specific Polymers, [25999-13-7]-SP43-0-002) usingAIBN as radical initiator. The copolymerization procedures as wellas the hydrolysis of the phosphonated ester into phosphonic acidgroups of the P(MAPC1) moieties were described in a previouspaper [16]. Chemical structure and architecture of P(NnPAAm-stat-hMAPC1) is reported in Fig. 1.

Copolymers with different NnPAAm/hMAPC1 molar ratios(80/20, 90/10 and 95/05) were used. The amount of sorption groupsper gram of copolymer (mmolP/gpoly) was calculated using 1H NMRspectroscopy. The signals of hMAPC1 at 4.0–4.3 ppm correspond-ing to the hydrogens of the CH2 in � of the phosphorus atom werecompared with those of NnPAAm at 1.2–1.8 ppm attributed to thehydrogen of the methyl group. P(NnPAAm-stat-hMAPC1) 80/20,90/10 and 95/05 contained 1.45 mmol, 0.67 mmol and 0.37 mmol ofphosphonic acid sorption moieties per gram of copolymer, respec-tively.

2.2. Synthetic effluents

The synthetic effluents were prepared by dissolution ofsalts in deionized water. Nickel(II) chloride hexahydrate(NiCl2·6H2O, Fisher Scientific, 97%), calcium(II) chloride tetrahy-drate (CaCl2·4H2O, VWR 99%), cadmium(II) chloride hydrate(CdCl2·H2O, Fisher Scientific, 98%) and aluminum(III) nitratenonahydrate (Al(NO3)3·9H2O, Fisher Scientific, 98%)) salts have

been used as received. The concentrations of metal cations of allsynthetic effluents were checked by Atomic Sorption Spectroscopywith a Perkin Elmer AAnalyst 400, an AutoPrep 50 dilutor and aS10 Auto-sampler. Calibration curves were obtained by automatic

A. Graillot et al. / Journal of Hazardous Materials 260 (2013) 425– 433 427

0 5 10 15 20 250,00,10,20,30,40,50,60,70,80,91,01,11,21,31,41,5

Time (hour s)

Cn+/P=0,50 5 10 15 20 25

0,00,10,20,30,40,50,60,70,80,91,01,11,21,31,41,5

Time (hour s)

Cn+/P=5

a b

m-stat

dconi

3

3

lamstwmimh

3

cSwwdi

33aatf1cd

3sc

Ni Ca

Fig. 2. Sorption kinetics obtained with P(NnPAA

ilution of a standard solution (100 mg L−1). In order to limit theationic species in solution and to avoid bias in the determinationf the sorption properties, the pH of the synthetic effluents wasot adjusted and only depended on the amount of salts introduced

n solution.

. Methods

.1. Sorption experiments

Sorption experiments were carried out using a dialysis tubu-ar membrane as already described in a previous paper [5]. Themount of metallic cations trapped by the copolymer was deter-ined by measuring the concentration of the metal cations in the

ynthetic effluent before and after sorption experiments. The lat-ers were carried out at 20 ◦C during 24 h and the temperatureas controlled by a 6 Liter Fisher Bioblock Scientific Cryother-ostated bath. The sorption properties were only attributed to the

nteractions between the cationic species and the phosphonic acidoieties. Indeed, it was checked that P(NnPAAm) thermosensitive

omopolymers did not have any sorption properties.

.1.1. Sorption kineticsSorption kinetic experiments were performed by measuring the

ation concentrations in solutions at regular time-steps by Atomicorption Spectroscopy. For each sorption kinetic, six experimentsere conducted in parallel in order to avoid bias when samplesere collected. Half of these experiments was launched with 12 helay; thus the sorption kinetics were followed during 24 h without

ntermission.

.1.2. Isothermal sorption experiments

.1.2.1. Mono-component experiments. The ratio between the rel-tive amount of cationic pollution and the amount of phosphoniccid sorption groups (Cn+/P ratio) was obtained by changing eitherhe copolymer concentration in the tubular dialysis membranerom 1.25 g L−1 to 10 g L−1 (corresponding to 0.42 mmolP L−1 to4.5 mmolP L−1 depending on the copolymer used) or the cationoncentration in the effluent. The pH of the effluents was followeduring the sorption experiments.

.1.2.2. Multi-components experiments. For these experiments,ynthetic solutions containing equimolar amount of the targetedations (Ni2+, Ca2+, Cd2+ and Al3+) were prepared.

Cd Al

-hMAPC1) for targeted Cn+/P of 0.5 (a) and 5 (b).

3.1.2.3. Two steps sorption experiments. These experiments wereperformed in order to estimate the sorption properties of a copoly-mer already loaded with cationic species. The first sorption step wasmade as previously described for mono-component experiments.Second sorption step was achieved by taking out the dialysis tubu-lar membrane from the first sorption bath and then immersing itin a second sorption bath containing cationic species in the sameconcentration as the one used in the first sorption step.

4. Results and discussion

4.1. Sorption kinetics

Sorption kinetics experiments were achieved at 20 ◦C for Ni2+,Ca2+, Cd2+ and Al3+ in order (i) to highlight the influence of thecation on the sorption kinetics and (ii) to evaluate the time neededto reach the equilibrium. Sorption experiments were performedwith molar concentrations of metal ions that corresponded to Cn+/Pratios equal to 0.5 (Fig. 2a) and 5 (Fig. 2b). At low Cn+/P ratio (0.5),the amount of cations was in excess compared to the amount ofphosphonic acid sorption, unlike at high Cn+/P ratio (5).

The results obtained at lowest Cn+/P ratio (Fig. 2a) were verysimilar whatever the cation: the sorption equilibrium was reachedafter about 8 h and varied from 0.24 and 0.3 mmolCn+

sorbed/mmolP.

At highest Cn+/P ratio (Fig. 2b), the amount of cations sorbed persorption were comprised between 0.8 and 1.05 mmolCn+

sorbed/mmolP

at equilibrium, which was reached after 13 h in each case. Sorptionexperiments were also conducted during 24 h.

4.2. Influence of Cn+/P ratio on sorption equilibrium

Fig. 2 exhibited that the sorption capacity at equilibriumstrongly depended on Cn+/P ratio between 0.5 and 5. Consequently,this parameter was investigated in more details in this section. Thesorption properties were evaluated for a wide range of Cn+/P ratiosranged between 0 and 22 for three divalent cations (Ni2+, Ca2+, Cd2+)and one trivalent cation (Al3+).

Fig. 3 reports the moles of cation sorbed per mole of sorptiongroups (i) for Cn+/P ratios between 0 and 22 (Fig. 3a) and (ii) with afocus between 0 and 6 (Fig. 3b). Overall, the curves relative to eachcation followed the same trend: the amount of cation sorbed per

sorption site increased from 0 to 1 when increasing Cn+/P. Notethat standard deviations relative to the results obtained at thehighest

∑Cn+/P are high due to analytical factor; hence the abso-

lute values of these last points should be considered carefully. As

428 A. Graillot et al. / Journal of Hazardous Materials 260 (2013) 425– 433

Ni Ca Cd Al

0 2 4 6 8 10 12 14 16 18 20 220,00

0,25

0,50

0,75

1,00

1,25

1,50

1,75

2,00

2,25

2,50

Cn+/P

0 1 2 3 4 5 60,00

0,25

0,50

0,75

1,00

1,25

Cn+/P

Fig. 3. Sorption capacities of P(NnPAAm-stat-hMAPC1) copolymer as a function of Cn+/P ratio: (a) for Cn+/P ratios ranged from 0 to 22; (b) for Cn+/P ratios ranged from 0 to 6.

Table 1Moles of cationic species sorbed and moles of proton released at the sorption equilibrium, at Cn+/P ratios equal to 0.5 and 5.

Targeted Cn+/P = 0.5 Targeted Cn+/P = 5

Ni2+ Ca2+ Cd2+ Al3+ Ni2+ Ca2+ Cd2+ Al3+

mmol n+ /mmolP 0.29 0.24 0.26 0.29 0.86 0.79 0.89 1.05

dcttiwrn

iiomciitaaD

s

Fr

Csorbed

mmolH+released/mmolP 0.65 0.65 0.65

emonstrated in our previous paper for Ni2+ cations [5], differentoordination modes could be considered due to the copolymer sta-istical architecture (Fig. 1): coordination modes involving one andwo phosphonated moieties per Ni cation were considered depend-ng on Ni/P (Fig. 4). At low Ni/P ratio, two phosphonated moieties

ere involved in sorption of one Ni cation whereas at higher Ni/Patio, it was shown that one Ni cation was sorbed on one phospho-ated moiety.

Fig. 3 exhibits that the curves relative to other cations usedn this study (Ca2+, Cd2+, Al3+) followed the same trend, suggest-ng that the same coordination modes could explain their sorptionn the copolymer. For Cn+/P ratios lower than 2.5, metal sorptionainly occurred on neighboring sites and therefore A, A′ and A′′

oordination modes (Fig. 4) are the most relevant. This assumptionmplies that a non-negligible amount of sorption sites were notnvolved in the sorption at low Cn+/P ratios as they were isolated inhe copolymer chains. When increasing Cn+/P, the sorption mech-nisms switched to other coordination modes, which involved an

verage of one phosphonated sorption groups per cation (B, C, D,′ and D′′ coordination modes).

Even though the same coordination modes could explain theorption of these four cations, the sorption capacity at different

ig. 4. Representation of the most probable coordination modes involved in the sorptionatios.

0.90 1.66 1.55 1.80 2.30

Cn+/P ratios was different for Ca2+. The maximum capacitiescorresponding to one metal sorbed per sorption moiety wasreached at Cn+/P between 5 and 7 for Ni2+, Cd2+ and Al3+ whereasit was reached only at Cn+/P = 10 for Ca2+. This difference could bedisplayed by a lower affinity of calcium cations for the phosphonicacid sorption groups compared to the other cations, especially athighest Cn+/P ratios.

4.3. pH during the sorption process

Different sorption mechanisms may explain (i) the coordina-tion modes involved between phosphonic acid and the cations and(ii) the differences observed in the case of Ca2+. These sorptionmechanisms are of two kinds: (i) cation exchange and (ii) com-plexation. Ion exchange processes involve attraction of cations bythe negative charges of the functional groups. Resulting functionalgroups-cations interactions are relatively weak, also reversible andallow ion exchange. To maintain the electronegativity, the neg-

ative charges of the functional groups are always balanced byan equivalent amount of cationic species. Furthermore, complex-ation process results from the formation of coordination bondsbetween one cation and an organic ligand which bears electron

of cations on P(NnPAAm-stat-hMAPC1) copolymer for different conditions of Cn+/P

A. Graillot et al. / Journal of Hazardous Materials 260 (2013) 425– 433 429

F APC1p

d(bdcgIecmsf

4

taTmbertcsmAsfCmttor

ig. 5. Representation of the electrostatic interaction between P(NnPAAm-stat-hMrocess.

onor groups that can be either basic ( NH2, OH, etc.) or acidic COOH, PO3H2, SO3H2, etc.). When two or more coordinationonds are involved in the cation sorption, the functional groups areefined as chelates or chelating agents. One sorption mechanisman prevail on the other, depending on the nature of the functionalroups, but both are most often involved in the sorption process.n order to determine which sorption mechanism prevail in thesexperiments, the pH has been measured during the sorption pro-ess to determine the moles of H+ released per mole of phosphonicoieties and compared to the moles of cations sorbed per sorption

ite. Table 1 summarizes the sorption and release values obtainedor Cn+/P equal to 0.5 and 5 at equilibrium.

.3.1. Divalent cationsFor divalent cations (Ni2+, Ca2+ and Cd2+), Table 1 shows that

he amount of released H+ was nearly two times higher than themount of cations sorbed on the copolymer, whatever Cn+/P ratio.his result suggests that ion exchange is the most probable sorptionechanism since the sorption of divalent cations was regulated

y charge conservation. Also, the divalent cations would be pref-rentially sorbed on adjacent phosphonic moieties at low Cn+/Patio whereas isolated sorption moieties would remain free of sorp-ion [5]. Indeed, as shown in Fig. 5a–c, adjacent sorption moietiesoncentrate the local charge density and therefore promote theorption of divalent cations on the contrary to isolated sorptionoieties, which are characterized by a lower local charge density.t low Cn+/P ratio, the coordination modes involving two sorptionites per cation (A, A′ and A′′ coordination modes, Fig. 4) are there-ore directly explained by the electrostatic interactions. At highn+/P ratio, electrostatic interactions are also consistent with theodification of the coordination modes from one cation sorbed for

wo sorption sites to one cation sorbed per site. Indeed, in a firstime, the sorption would also result on electrostatic interactionsn adjacent moieties as proposed in Fig. 5d. Then, at higher Cn+/Patios, the higher cation concentration gradient would favor the

) copolymer and divalent cations at low and high Cn+/P ratios during the sorption

sorption on sites that remained free (Fig. 5e) and would lead to anaverage of one cation sorbed per site (Fig. 5f), corresponding to B,C, D, D′ and D′′ coordination modes.

Also, the sorption mechanism mainly based on electrostaticinteraction is consistent with the lower affinity of Ca2+ for the phos-phonic acid moieties. The lower calcium electronegativity (� = 1.0)compared to those of nickel (� = 1.9) and cadmium (� = 1.69)implies a lower tendency of Ca to attract electrons than to otherdivalent cations. Hence, the sorption of calcium on isolated phos-phonic acid moieties occurred at higher Cn+/P values in comparisonwith the other divalent cations (Fig. 3).

4.3.2. Trivalent cationsAt low Cn+/P ratio (0.5) and for trivalent Al3+ cations, electro-

static interactions were also involved in the sorption mechanismsas the amount of H+ released in solution was three times higherthan the amount of Al3+ sorbed on the copolymer. But a differenttrend was observed at higher Cn+/P ratio (5): the amount of releasedH+ was only two times higher than the amount of sorbed Al3+. Thisresult was in agreement with the maximum capacity of two H+

released per phosphonic acid site, meaning that at high Cn+/P ratio,all phosphonic moieties were involved in the sorption process. But,in this case, the principle of charge conservation was not checkedas the sorption of one Al3+ (three positive charges) came with therelease of only two H+ (two positive charges). That is, another kindof interactions was probably involved in the sorption mechanism.

One reason could be proposed to explain these results butit was not perfectly convincing: at the beginning of the sorp-tion experiments, the pH was contained between 4.0 and 4.5 andtherefore aluminum was decomposed in various forms, i.e. 10% ofAl(OH)2+, 5% of Al(OH)2

+ and 85% of Al3+. The sorption of divalent2+ +

Al(OH) and monovalent Al(OH)2 species could occur without

charge conservation issues, as illustrated in Fig. 6a. But, at the endof the experiments, the sorption of 1 mmolAl3+

sorbed/mmolP from a

mono-component solution of Al3+ still implied a default of charge

430 A. Graillot et al. / Journal of Hazardous Materials 260 (2013) 425– 433

copoly

cicaamaidstatt(ttp

tb

4

4

epwgrcl

o

Aotwc∑

a

t∑

a

N

Fig. 6. Representation of the interactions between P(NnPAAm-stat-hMAPC1)

onservation, which could not only be explained by electrostaticnteractions. Consequently, the formation of coordination bondsould happen between the phosphonic acid groups and Al3+ cationss illustrated in Fig. 6b. The existence of both electrostatic inter-ctions and coordination bond formation to describe the sorptionechanisms could be explained by the acidity of the phosphonic

cid moieties. Indeed the latter is contained between the acid-ty of sulfonic acid and the one of carboxylic acid and both haveifferent sorption mechanisms [23,24]: the sorption properties ofulfonic acid are mainly due to electrostatic interactions because ofheir strong acidity whereas the sorption properties of carboxyliccid preferentially result from coordination bonds. Additionally,he pH of solutions containing Al3+ was lower (from pH = 4.0–4.5o 2.9–3.0) than the pH of solutions containing divalent cationsfrom pH = 6.5–6.0 to 3.3–3.0), which might also have an impact onhe sorption mechanisms. Lower pH values imply a lower ioniza-ion of the functional groups ( OH rather than O−) and thereforeromote complexation rather than electrostatic interactions.

To check the relevance of this assumption and to investigatehe selective behavior of the copolymers, the sorption competitionetween the divalent and trivalent cations was studied.

.4. Sorption competition and selectivity

.4.1. Multi-component sorption experimentsAdditional experiments were carried out with synthetic efflu-

nts in order to evaluate the competitive sorption on thehosphonic acid moieties. Equimolar mixtures of the four cationsere prepared (25 mol% of each cation in the initial solution). The

lobal cationic concentration was adjusted to reach a global Cn+/Patio ranging from 0.44 to 4.62. As this global Cn+/P ratio takes intoonsideration all the cationic species in solution, it is called

∑Cn+/P

ater in this study.Fig. 7 represents the distribution of the cationic species sorbed

n the copolymer at equilibrium, with increasing∑

Cn+/P ratios.

t the lowest∑

Cn+/P ratio, the distribution of the cations sorbedn the copolymer is almost the same than the one of the ini-ial synthetic solutions. At higher

∑Cn+/P ratios, the distribution

as modified and the aluminum was preferentially sorbed on theopolymer: the mole fraction of sorbed aluminum reached 53% at

Cn+/P of 0.85, 67% at∑

Cn+/P of 1.70, 69% at∑

Cn+/P of 2.75, 81%

t∑

Cn+/P of 3.60 and almost 100% at∑

Cn+/P of 4.62. In the same2+

ime, the proportion of sorbed Ca decreased when increasing the

Cn+/P ratio from 22% at∑

Cn+/P of 0.44 to 4% at∑

Cn+/P of 1.70

nd less than 0.5% when∑

Cn+/P exceeded 3.60. The proportion of

i2+ and Cd2+ decreased with the same order of magnitude when

mer and Al cations at the sorption equilibrium for low and high Cn+/P ratios.

increasing the∑

Cn+/P ratio, but less than for Ca2+: from 22% and

30% for Ni2+ and Cd2+ respectively at∑

Cn+/P = 0.44 until 1% at∑

Cn+/P = 4.62.

These results suggest that, at low∑

Cn+/P, the large numberof sorption sites compared to the global amount of cations didnot allow distinguishing the sorption affinity of all cations on thecopolymer. Increasing the

∑Cn+/P ratio led to the increase of the

Al3+ proportion sorbed on the copolymer. The affinity between thecopolymer and both Ni2+ and Cd2+ was similar and much greaterthan the one of Ca2+. Consequently, the affinity of the phosphonicacid groups for the divalent cations could be classified as follows:Ni2+ ≥ Cd2+ > Ca2+, in agreement with the electro-negativity of thecations (1.91, 1.69 and 1.00, for Ni2+, Cd2+ and Ca2+, respectively).The increasing amount of Al3+ sorbed on the copolymer observedwhen increasing the

∑Cn+/P ratio indicates that phosphonic acid

had greater affinity for the trivalent cation than for divalent ones;that was in agreement with the different interaction mechanismsproposed to describe the sorption of these cations.

Consequently, a special attention was given to the total amountof cations sorbed per site (mmolCn+

sorbed/mmolP) when increasing

∑Cn+/P ratio (Fig. 7). For

∑Cn+/P ranging from 0.44 to 2.75, all

the cationic species were sorbed and the total sorption capac-ity increased from 0.29 to 0.58 mmolCn+

sorbed/mmolP, in agreement

with the results shown in Fig. 3. However, at∑

Cn+/P ≥ 3.60, thetotal sorption capacity decreased to 0.45 mmolCn+

sorbed/mmolP. Such

a decrease could not be attributed to the pH of these solutionssince it remained almost constant for all solutions (from pH = 4.4 at∑

Cn+/P = 0.85 to pH = 4.1 at∑

Cn+/P = 4.6). However, this decreasewas correlated to the high affinity of phosphonic acid groupsfor Al3+ cations. Indeed, the sorption capacity value obtained for∑

Cn+/P = 4.6 was the same than the one corresponding to a mono-

component solution of Al3+ with an Al3+/P ratio equal to 1.15 (Fig. 3),meaning that the multi-component sorption of Al3+ is so favored athigh

∑Cn+/P ratio that the copolymer sorption capacity is similar

to that obtained for mono-component solutions of Al3+.

4.4.2. Sorption of cations on loaded copolymersTo confirm the link between the sorption mechanisms and Cn+/P

ratio and more specifically to better understand the specific inter-actions between the phosphonic acid groups and Al3+, new sorptionexperiments have been carried out. In a first step, sorption exper-

iment was achieved between the copolymer and a first cation(named Cn+

1 ) until equilibrium; then in a second step, the samecopolymer was put into contact with a second cation (named Cn+

2 ).In Fig. 8, the first column corresponds to the amount of cation Cn+

1

A. Graillot et al. / Journal of Hazardous Materials 260 (2013) 425– 433 431

Fig. 7. Repartition of the cationic species sorbed on P(NnPAAm-stat- MAPC1) copolymers starting from equimolar synthetic effluent of the four cations (Ni, Ca, Cd, Al) ford

staiistamw

h

ifferent∑

Cn+/Pratios.

orbed in the first sorption step. The four other columns show (i)he amount of Cn+

2 sorbed in the second sorption step and (ii) themount of Cn+

1 that was still sorbed after the sorption of Cn+2 . The

nitial cationic molar concentration was the same in all cases. Asn Fig. 3, the ordinate represents the total mole number of cationsorbed per mole of phosphonic acid moieties. Fig. 8a–d representshe results obtained at Cn+/P ratio of 0.5 and Fig. 8e–h correspond to

targeted Cn+/P of 5. The total mole number of cations sorbed perole of phosphonic acid moieties reached 0.3–0.4 when Cn+/P = 0.5hereas it increased to 0.8 and almost unity when Cn+/P = 5. When

Fig. 8. Two steps sorption experiments results obta

Cn+/P = 0.5, a significant number of sorption sites remained freefrom cations after the first sorption step, whereas at Cn+/P = 5 almostall sorption sites were saturated at the end of the first sorption step.

At lowest Cn+/P ratio, Fig. 8a–d shows that the sorption of thesecond cation Cn+

2 (second step) did not lead to a significant releaseof the sorbed cation Cn+

1 already sorbed during the first step. Simi-lar results were obtained for all cation couples, suggesting that the

sorption of the second cation Cn+

2 occurred on free sorption sites,i.e. sites that have not been involved during the first sorption step.On the contrary, when the copolymer was previously saturated at

ined at low (a–d) and high (e–h) Cn+/P ratios.

4 rdous

t(chssCCtp(sntct

mcpwcrdscssfkrit

5

chaoc(wao

w(

tcocscalbaetaoasr

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32 A. Graillot et al. / Journal of Haza

he end of the first sorption step, different cases were observedFig. 8e–h). Firstly, for all cations, no additional sorption of the sameation was noticed, which confirmed that all available sorption sitesave been involved in the sorption process at the end of the firsttep. Secondly, when the cation sorbed during the first sorptiontep (Cn+

1 ) was a divalent cation, the sorption of the second cationn+2 occurred on sorption sites already engaged in the sorption ofn+1 which implied a significant release of Cn+

1 . Fig. 8e–h point outhat the amount of cation released depended on the Cn+

1 /Cn+2 cou-

le. Hence, the most important release was observed for Ca2+ (Cn+1 )

between 85% and 100% of Ca2+ released at the end of the secondorption step), whatever Cn+

2 cation. The release of cadmium andickel during the second step was ranged from 70 to 95% and 65o 80%, respectively, depending on Cn+

2 . Furthermore, Fig. 8e and gonfirmed that the affinity of the phosphonic group is higher forhe nickel than for the cadmium.

These results confirmed (i) that ion exchange was the mainechanism involved in the sorption of divalent cations and (ii) the

lassification of the divalent cations based on their affinity withhosphonic acid sorption moieties: Ni2+ ≥ Cd2+ > Ca2+. When Cn+

1as trivalent aluminum cation (Al3+), the sorption of Cn+

2 divalentation in the second step occurred but the amount of aluminumeleased in the solution was relatively low (20–25% of Al3+ releaseepending on Cn+

2 cation). In this case, the amount of Cn+2 cation

orbed in the second step finally represented 15% of the wholeations sorbed on the copolymer, whereas the other 85% corre-ponded to aluminum. This result was in agreement with theorption mechanisms (ion exchange and complexation) proposedor the Al3+ sorption. Indeed, as the electrostatic interactions arenown to be weaker than the coordination bonds, the aluminumeleased in solution probably corresponded to the one sorbed byon exchange whereas Al3+ sorbed by complexation remained onhe copolymer.

. Conclusion

This work focused on the sorption properties of four differentations (Ni2+, Ca2+, Cd2+ and Al3+) on a new P(NnPAAm-stat-MAPC1) thermosensitive polymeric sorbent bearing phosphoniccid moieties and demonstrated that the sorption capacitiesf P(NnPAAm-stat-hMAPC1) depend on the relative amount ofationic pollution compared to the amount of sorption sitesCn+/P ratio). The maximal sorption capacity of the copolymeras strongly influenced by Cn+/P ratio whatever the cation. More,

djusting this ratio allowed reaching a maximal sorption capacityf 1 mmolCn+

sorbed/mmolP, which was significantly higher than that

as observed in the literature for similar di- and trivalent cationsless than 0.2 mmolCn+

sorbed/mmolP) [6,7]. This work also showed that

he sorption phenomena involved between phosphonic acid andationic species depend not only on the cation valence but alson the Cn+/P ratio. Two kinds of interactions should be taken intoonsideration to describe the sorption of cations on P(NnPAAm-tat-hMAPC1): electrostatic and coordination bindings. For divalentations, the sorption mainly resulted from electrostatic interactionsnd the affinity of the phosphonic acid for the divalent cations wasinked to the cations electro-negativities. As a result, the affinityetween the phosphonic acid and the cations could be classifieds follows: Al3+ � Ni2+ ≥ Cd2+ > Ca2+. For aluminum, simultaneouslectrostatic interactions and coordination bonds were involved inhe sorption mechanisms. This assumption is in agreement with thecidic force of the phosphonic acid, which is located between those

f carboxylic acid (which creates coordination bonds with metals)nd sulfonic acid (which mainly interacts with metals by electro-tatic interactions) [23,24]. At low Cn+/P ratios, sorption mainlyesulted from electrostatic interactions whereas, for high Cn+/P

[

Materials 260 (2013) 425– 433

ratios, the formation of coordination bonds prevailed to explainthe metal–polymer interactions.

Acknowledgements

The authors thank the “Agence Nationale de la Recherche” (ANR)for funding this work under the 2009 ECOTECH program (ANRCOPOTERM) and Specific Polymers Society (Montpellier, France) forsupplying both monomers enabling the synthesis of the copolymersstudied in this paper.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013.05.050.

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