Cerium binding activity of different pectin compounds in aqueous solutions

Colloids and Surfaces B: Biointerfaces 77 (2010) 104–110

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Cerium binding activity of different pectin compounds in aqueous solutions Maxim Y. Khotimchenko a,∗ , Elena A. Kolenchenko b , Yuri S. Khotimchenko b , Elena V. Khozhaenko a , Valeri V. Kovalev b a b

Department of Pharmacology, Vladivostok State Medical University, 2 Ostryakova ave., Primorskiy krai, Vladivostok 690990, Russia Laboratory of Pharmacology, Institute of Marine Biology, Far East Branch of Russian Academy of Sciences, Vladivostok 690041, Russia

a r t i c l e

i n f o

Article history: Received 5 August 2009 Received in revised form 5 January 2010 Accepted 22 January 2010 Available online 4 February 2010 Keywords: Cerium Equilibrium study Pectin Degree of esterification Wastewater

a b s t r a c t Cerium binding activity of different water soluble pectin compounds varying according to their degree of esterification and insoluble calcium pectate beads in aqueous solution was studied in a batch sorption system. The cerium uptake by all pectin compounds was highest within the pH range from 4.0 to 6.0. The binding capacities and rates of cerium ions by pectin compounds were evaluated. The Langmuir, Freundlich and BET sorption models were applied to describe the isotherms and constants. Sorption isothermal data could be well interpreted by the Langmuir equation. The results obtained through the study suggest that pectin compounds are favorable sorbents. The largest amount of cerium ions is bound by pectin with the degree of esterification close to zero. Therefore, it can be concluded that low esterified pectins are more effective substances for elimination of cerium ions from aqueous disposals. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Due to the uncontrolled release of heavy metals and radionuclides into surface and sea water, the monitoring of different pollutants in water systems has increased in recent years [1]. Cerium is one of the important fission products; therefore its removal is essential from the point of view of purification of environmental and radioactive waste disposal [2]. Cerium is widely used in various fields of industry, particularly because of its luminescent and magnetic quality as well as its abrasive, coloring, discoloring, and catalytic properties [3]. Also cerium is a product of nuclear degradation of uranium [4] and it is found in nuclear waste underground repositories. Cerium, in particular, its radioactive isotopes, entering human body may cause damages in inner organ, such as lungs, liver, heart and other [3]. It should be taking into account that the rapidly increasing production of engineered nanoparticles containing lanthanides such as cerium has created a demand for particle removal from industrial and communal wastewater streams [5]. Efficient removal is particularly important in view of increasing long-term persistence and evidence for considerable ecotoxicity of specific nanoparticles and other waste products [5,6]. Among the metal ion removal procedures, ion-exchange process is very effective for elimination of various metal ions and can be easily recovered and reused by regeneration operation [7]. Some

∗ Corresponding author. Tel.: +7 4232 280310; fax: +7 4232 310900. E-mail address: [email protected] (M.Y. Khotimchenko). 0927-7765/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2010.01.015

of the best ion-exchange materials are biopolymers, which are industrially attractive because they possess a capability of lowering transition metal ion contents to parts per billion concentrations, they are widely available and environmentally safe [8,9]. Usually biosorbents are used for removal of bivalent ions. For removal of the trivalent ions were proposed ion-exchange resins containing iminodiacetic acid, but their efficacy was significantly lowered by the presence of other metal ions [10]. It was suggested recently that usage of the pectin-rich fruit biomass is effective for removal of the metal ions from water solution [11]. Pectin substances belonging to the group of natural biopolymers are the ionic plant polysaccharides, which main structural features are the linear chains containing more than 100 (1–4)-linked ␣-d-galacturonic acid residues [12]. Their metal binding capacity was proved in the numerous studies under as in vivo as in vitro conditions [13–15]. The main structural characteristic of all pectin substances is the degree of esterification indicating the number of galacturonic acid residues with methanol radical attached. According to the “egg-box” model proposed for the metal binding mechanism of pectins [16], esterified residues are not active whereas negative charges of the free carboxyl groups in pectin molecules form the covalent bonds with the metal ions. Thus, pectin with the low degree of esterification presumably exerts considerably higher metal binding activity [17]. But the model proposed describes binding of bivalent metal ions by pectin substances. It is not known if galacturonic acid can form strong bonds with trivalent ions and wither the binding processes are dependent on the degree of esterification. Therefore, the main aim of this work was to investigate the binding of Ce3+ ions by the pectin compounds and the

M.Y. Khotimchenko et al. / Colloids and Surfaces B: Biointerfaces 77 (2010) 104–110

influence of the degree of esterification on the sorption capacity of pectins. The results of the work may be used as a basis for elaboration of the new materials purposed for removal of cerium and other similar ions from water disposals. There were studied parameters of the cerium binding processes by four pectin compounds with their degree of esterification varying from 1% to 60% and low esterified calcium pectate in a form of water insoluble tiny beads. The influence of experimental conditions such as pH, agitation period, agitation rate and initial concentrations on the parameters of the binding process were investigated. The Langmuir, Freundlich and BET equations were used to fit the equilibrium isotherm. 2. Materials and methods 2.1. Materials High esterified citrus pectin without additives was obtained from Copenhagen Pectin A/S, Lille Skensved, Denmark. The stated degree of esterification of this preparation was 60.0%. The pectin preparation contained no acetyl or amide groups. All other chemicals were of the highest quality available. Distilled water was used throughout. 2.2. Preparation of pectin samples with different degree of esterification Preparation of the pectin samples with the degree of esterification less than 60.0% was executed using the method of alkali de-esterification of high esterified pectin in a water–ethanol solution. Initially the raw high esterified pectin was washed with 50% ethanol solution for purification. In this process 200 g of pectin was suspended in 2 l of the ethanol solution for 30 min, then pectin was separated by filtration, rinsed on filter with 1.5 l of the 50% ethanol solution and 1 l of the 95% ethanol solution, consecutively, and then dried at 70 ◦ C. The ultimate pectin sample was ground and fractionated according to the particle size. In the study a fine disperse fraction sifted through a sieve with a mesh size 74 ␮m. It was made purposefully to minimize the possibility of obtaining the heterogeneous samples regarding degree of esterification due to a different rate of de-esterification process in pectin molecule outside and inside granules caused by different velocity of diffusion of hydrolyzing agent. Process of alkali de-esterification of pectin included initial neutralization of free carboxyl groups of anhydrogalacturonic acid and then, during increase of pH of the media higher than 8.5, proper pectin de-esterification. Amount of alkali required for neutralization of carboxyl groups was calculated as the follows: 1 g of pectin was suspended in 10 ml of 50% ethanol solution and in the presence of Hintone indicator titrated with 1 M NaOH solution in 50% ethanol until the change of the indicator color. According to the titration results the amount of alkali required for pectin neutralization was determined using the following equation: V1 = m × V0

(1)

where V1 is the volume of 1 M NaOH solution necessary for pectin neutralization (ml), m is the weight of a pectin sample (g), V0 is the volume of 1 M NaOH solution used during titration of 1 g of pectin (ml). Amount of alkali necessary for achievement of a set degree of esterification of pectin was calculated using the following equation: V2 =

DEin − DEfin m×A × × 100 176 × 100 100

(2)

where V2 is the volume of 1 M NaOH solution necessary for deesterification of pectin (ml), m is the weight of a pectin sample (g), A is the anhydrogalacturonic acid content in pectin (%), 176 is the

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molar weight of anhydrogalacturonic acid, DEin is the initial degree of esterification of a pectin sample (%), DEfin is the final required degree of esterification of a pectin sample (%). The total volume of alkali necessary for the whole process of pectin de-esterification was found by summarizing of V1 and V2 volumes determined earlier. The proper de-esterification process was carried out as follows. 20 g of pectin powder was suspended in 200 ml of 50% ethanol solution intensively stirring and then gradually 1 M NaOH solution in 50% ethanol was added. For the visual control of the process, the indicator phenolphthalein was added into reactive mixture. The color of phenolphthalein changes at pH 8.2–10.0. The next portion of alkali was added into the reactive volume only after discoloration of indicator. When 95% of calculated amount of alkali was added into reactive volume, the sampling was carried out for determination of the degree of esterification. According to results of this analysis the quantity of alkali was precised. When the preliminary set degree of esterification was achieved the reaction mixture was acidified with 1 M HCl solution in 50% ethanol achieving pH of the media 5–6 under intensive stirring. Obtained pectin preparation was separated from water–ethanol solution with filtration and consecutively rinsed with 300 ml of 50% ethanol solution and 150 ml of 95% ethanol. Rinsed pectin was dried at 70 ◦ C. Total anhydrogalacturonic acid content, non-methylated (deesterified) anhydrogalacturonic acid content, degree of esterification, and intrinsic viscosity were estimated in the pectin samples obtained. For the preparation of calcium pectate beads 100 g of pectin with degree of esterification about 1% was suspended in 500 ml 70% ethanol. Gradually 100 ml of 70% ethanol solution containing 22.6 g CaCl2 ·6H2 O was added. A magnetic stirrer was used to stir the solution. After the process was finished, calcium pectate was separated through a glass filter with a pore size 30–40 ␮m, rinsed with 800 ml 70% ethanol to remove any CaCl2 ·6H2 O, and dried at 60 ◦ C. The beads were then ground by using a laboratory jar mill and sieved to a constant size (<250 ␮m) before use. 2.3. Pectin analysis The galacturonan content of the pectin preparation was determined colorimetrically by the m-hydroxydiphenyl method [18]. The degree of esterification was characterized using titrimetric analysis with 1 M NaOH solution in 50% ethanol in the presence of Hintone indicator [19]. Intrinsic viscosity of low esterified pectin was determined in 0.05 M NaCl/0.005 M Na–oxalate at 25.0 ◦ C and pH 6.0 using an Ubbelohde viscosimeter. The intrinsic viscosity was related empirically to the molecular weight by the Mark–Howink equation [20] presented as  = KM˛ that is generally used for definition of the pectin molecular weight using values for the constants ˛ (0.79) and K (216 × 10−6 ) that are suitable for pectins [21,22]. The calcium content in the calcium pectate sample was assayed by atomic absorption and expressed in mg g−1 of a sample [23]. 2.4. Experimental procedures 0.1 M stock solutions (14.01 g L−1 ) of Ce3+ ions was prepared using analytical-reagent grade Ce2 (SO4 )3 . The stock solution was then diluted to give standard solutions of appropriate concentrations with controlled pH at 6.0 achieved by addition of either 0.1 M HCl or 0.1 M NaOH. Batch sorption experiments were conducted in 20 ml beakers and equilibrated using a magnetic stirrer. Then 1.0 ml aliquots of these standard solutions were placed in 20 ml beakers with 10 ml of solution containing 0.05 g of dry pectin preparation or 10 ml of suspension with 0.05 g of dry calcium pectate added. Then the total volume of the solution prepared was made up to 20 ml by addition of distilled water. Removal of pectin compounds

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from Ce3+ solution was performed by using centrifugal force unit 3000 for 10–20 min and following filtration through a glass filter with a pore size 100–120 ␮m. Concentration of Ce3+ ions in the supernatant obtained was analyzed using an atomic absorption spectrophotometry method. The effect of Ce3+ sorption was studied in a pH range 2.0–6.0. The pH of the initial solution was adjusted to the required pH value using either 0.1 M HCl or 0.1 M NaOH. Pectins were equilibrated at the particular pH for about 120 min at 400 rpm and at initial Ce3+ concentration of 0.6 g L−1 using a bath controlled at 24 ◦ C. Each experiment was triplicated under identical conditions. A negative control experiment with no pectin added was simultaneously carried out to ensure that the cerium removal was caused by the polysaccharide binding activity and not by the beaker or filter. The parameters obtained were subjected to a one-way analysis of variance using a software package SPSS (Statistical Package for Social Sciences) for Windows, version 11.0 with a confidence level of 95% (P < 0.05). The effect of agitation period was also studied to determine the optimum condition for sorption of Ce3+ ions. For batch kinetic studies 0.05 g calcium pectate or 10 ml of solution containing the same amount of dry pectin were equilibrated at optimum condition as mentioned earlier. The sorption system was placed in 20 ml beakers and stirred by a magnetic stirrer. At preset time intervals, the aqueous samples (5 cm3 ) were taken and the concentration of Ce3+ was assessed. Sorption equilibrium studies were conducted at optimum condition using a contact time of 120 min at pH 6.0. pH value 6.0 was considered as most acceptable because all polysaccharides at this point possess highest binding activity and pH control requires minimum amounts of HCl and NaOH. Bath controlled temperature was 24 ◦ C. Isotherm studies were conducted with a constant pectin preparation amount (0.05 g) and varying initial concentration of Ce3+ in the range of 0.05–0.7 g L−1 . Each experiment was at least duplicated under identical conditions. The metal accumulation (q) was determined as follows: qe =

(C0 − Ce )V W

(3)

where C0 is the initial Ce3+ concentration (mg L−1 ), Ce is the final or equilibrium Ce3+ concentration (mg L−1 ), V is the volume of the Ce3+ solution (ml), and W is the weight of the dry samples of pectin, or calcium pectate (g). The amount of the metal ions bound by the pectin compounds was expressed in mg g−1 of the dry sorbent.

Table 1 Physicochemical parameters of pectin preparations used in the study. Parameter

Pectin sample DE = 60% DE = 40% DE = 20% DE = 1% Ca pectate

Degree of esterification, % Total anhydrogalacturonic acid content, % Non-methylated anhydrogalacturonic acid content, % Intrinsic viscosity, ml/g anhydrogalacturonic acid Ca content, mg g−1 Water solubility

60.2 79.4

40.1 70.4

18.8 66.6

1.2 73.7

1.2 73.8

31.6

42.2

54.1

72.8

72.6

915

533

433

408

–

– Yes

– Yes

– Yes

– Yes

38 No

DE—degree of esterification.

But at the very beginning of the de-esterification process a dramatic reduction of intrinsic viscosity was noted. In correspondence with Mark–Houwink equation relating intrinsic viscosity and molecular weight of pectin the difference found in the study conforms to 2.8fold reduction of an average molecular weight. Because molecular weight of initial high esterified pectin was substantially high (more than 200 kDa), the sample with the degree of esterification 1.2% and intrinsic viscosity 408 ml/g may be considered as a high molecular pectin. Physicochemical parameters of the calcium pectate beads prepared from the low esterified pectin were similar to those of that sample, except they were not soluble in water. 3.2. Effect of pH In the study with natural polysaccharides, solution pH is an important parameter because the system is strongly pH dependent due to the properties of both pectin compounds (charge and potential) and the solution composition, i.e. metal ion speciation, which changes with pH [24–26]. The number of active binding sites presented with free carboxyl group residues of the pectin molecule may change with varying pH. Fig. 1 depicts the effect of pH values on the Ce3+ uptake by soluble pectins and calcium pectate. The binding capacities were found to be relatively low at lower pH values and increased with the rise of pH. The optimum pH range of 4.0–6.0 is obtained for all compounds studied except high esterified pectin. The highest Ce3+ binding capacity of this sample was found at pH 6.0 only, whereas at lower pH metal uptake for the high esterified pectin was significantly lower. This can be explained

3. Results and discussion 3.1. Pectin substrate There were obtained four samples of pectins by means of alkali de-esterification method and insoluble calcium pectate beads. The samples were ranged according to more or less gradual reduction of their degree of esterification from 60.2% to 1.2%. Physicochemical parameters of the pectin samples obtained are shown in Table 1. It was found that the pectin samples, which were subjected to de-esterification procedure, are characterized by similar contents of anhydrogalacturonic acid making up from 66.6% to 73.7% of a sample mass. The differences of the anhydrogalacturonic acid content may be explained by partial degradation of uronic acids and removal of side chains during the process of alkali esterification. Also neutral sugars are washed out due to the rinsing with ethanol. Simultaneously with the decrease of the degree of esterification from 60.2% to 1.2% was registered a relative rise of the non-methylated anhydrogalacturonic acid content from 31.6% to 72.8%, i.e. 2.3 times more. Generally intrinsic viscosity of pectin samples reduces with the decrease of the degree of esterification.

Fig. 1. Effects of pH on the Ce3+ uptake by pectin with different degree of esterification and calcium pectate. DE—degree of esterification.

M.Y. Khotimchenko et al. / Colloids and Surfaces B: Biointerfaces 77 (2010) 104–110

Fig. 2. Effect of the agitation period on the Ce3+ uptake pectin with different degree of esterification and calcium pectate. DE—degree of esterification.

by the pKa value of pectins varying from 3.5 to 4.1 for various pectin samples [12]. At the pH values higher than 4.0 the majority of carboxyl groups are in dissociated state, therefore, metal binding processes are easily occurred. Low pH would favor protonation of the carboxyl sites of the pectin molecule resulting in a reversal of the charge leading to a reduced metal binding activity of pectin. In the media with pH values lower than 2.0 all pectin substances are usually precipitated, and this sedimentation results in dramatic decrease of the binding activity of polysaccharides [9]. At pH values higher than 6.0, Ce3+ binding capacity of all compounds studied was close to zero due to the alkaline shift, which obviously results in polysaccharides becoming unstable [24]. Moreover, binding process at these pH values must be disrupted due to the formation of hydro-complexes of Ce3+ ions. According to the literature [27], the chemical species of Ce3+ existing in the solution at pH 6.0 and higher are mostly Ce(OH)3 . Since such species possess a large size and are unable to dissociate, they barely interact with active sites of carboxyl groups in the pectin molecule. Moreover, the Ce(OH)3 molecules are prone to precipitation in the case of high concentration, hence, their interaction with polymer molecules is impossible. Therefore, it was considered unreasonable to measure Ce3+ binding capacity of pectin at pH values higher than 6.0. 3.3. Effect of agitation period Fig. 2 shows the effects of agitation period on the metal uptake by pectins. Period required for equilibrium concentration between pectins and Ce3+ ions to be achieved was found on the base of different Ce3+ uptake values obtained after various periods of interaction between polysaccharide compounds and metal ions being in the batch sorption system. Amount of Ce3+ bound by all water soluble compounds increases within the beginning of agitation period and attains equilibrium in about 60 min. Ce3+ uptake by calcium pectate increases significantly slower and attains equilibrium in 80 min. The main differences in the sorption rate between soluble compounds and insoluble calcium pectate were observed within the first minutes of the agitation period. So, during the first minute of the period soluble pectins have bound 45–53% of their highest uptake under given conditions whereas insoluble calcium pectate beads have bound only 15.7%. In 10 min the difference of the Ce3+ uptake between soluble and insoluble pectin substances was found to be insignificant. Taking to consideration that all pectin were hydrated before taken into the batch, longer period required for calcium pectate to bind Ce3+

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Fig. 3. Equilibrium sorption of Ce3+ from aqueous solution pectin with different degree of esterification and calcium pectate. DE—degree of esterification.

ions depends only on the rate of metal diffusion into pectate beads. 3.4. Effect of agitation rate The influence of agitation rate on the binding of Ce3+ by pectin compounds was studied using the following stirring speeds: 0, 100, 200, 300, 400, and 500 rpm. It was found that agitation rate has no influence on the values of Ce3+ uptake by water soluble pectins and insoluble calcium pectate beads. Even if the batch sorption system was not stirred at all there was found no differences in the Ce3+ uptake values registered. 3.5. Equilibrium studies Removal of Ce3+ ions by pectins with different degree of esterification and calcium pectate as a function of equilibrium metal concentration was studied at pH 6.0. The sorption processes in the batch result in the metal ions being removed from the solution and concentrated on the sorbent sites, until the remaining ions in the solution are in the dynamic equilibrium with the ions bound to the active binding sites. Therefore, there is a defined distribution of the metal ions in the sorption system, which can be expressed by one or more isotherms [28]. Fig. 3 shows the sorption curve indicating the amount of Ce3+ ions bound to the pectin molecules increasing with rise of the equilibrium metal concentration in solution. For description of the interaction between sorbent and the metal ions being bound the sorption isotherm plotting is usually used. The isotherms are characterized by the initial region, which is represented as being concave to the concentration axis. The isotherm reaches a plateau, which can typically be described by the Langmuir, Freundlich or BET isotherms [29], which were used for analysis of the results obtained through this part of experiment. The Langmuir equation is most often used to describe equilibrium sorption isotherm, which is valid for monolayer sorption with a finite number of identical sites and is given by: q=

qmax bCe 1 + bCe

(4)

where qmax is the maximum sorption at monolayer (mg g−1 ), Ce is a final equilibrium concentration of Ce3+ , q is the amount of Ce3+ bound per unit weight of the pectin compound at final equilibrium concentration (mg g−1 ), b is the Langmuir constant related to the affinity of binding sites (ml mg−1 ) and is considered as a measure of the energy of sorption.

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Table 2 Langmuir, Freundlich and BET isotherm constants and correlation coefficients of Ce3+ binding capacity of pectin compounds. Sample

Langmuir b (ml mg

Pectin, DE = 1% Pectin, DE = 20% Pectin, DE = 40% Pectin, DE = 60% Ca pectate

−1

Freindlich )

0.833 0.185 0.140 0.010 0.100

qmax (mg g

−1

)

200.0 200.0 142.8 125.0 200.0

2

R

KF (mg g

1.000 0.998 0.998 0.964 0.998

8.052 7.078 6.494 2.382 7.367

−1

BET )

n

R

qmax (mg g−1 )

B

R2

12.19 8.00 8.33 2.19 9.61

0.854 0.939 0.888 0.840 0.993

125.0 125.0 76.9 8.26 47.60

1.33 1.00 1.44 2.95 1.10

0.123 0.266 0.410 0.739 0.371

2

DE—degree of esterification.

Table 3 RL values based on the Langmuir equation. Ce3+ initial concentration, mg L−1

RL value

20 40 60 80 100

DE = 1%

DE = 20%

DE = 40%

DE = 60%

Calcium pectate

0.056 0.029 0.019 0.014 0.011

0.212 0.118 0.082 0.063 0.051

0.263 0.151 0.106 0.081 0.066

0.827 0.705 0.615 0.545 0.489

0.333 0.200 0.142 0.111 0.090

DE—degree of esterification.

The following linearized plot of the Langmuir equation was used in this study:

RL =

Ce 1 Ce = + qe qmax qmax b

(5)

The widely used empirical Freundlich equation based on sorption on a heterogeneous surface is given by: log qe = log KF +

1 log Ce n

(6)

where KF and n are Freundlich constants indicating sorption capacity (mg g−1 ) and intensity, respectively. KF and n can be determined from linear plot of log qe against log Ce . The BET equation is given by: Ce = (C0 − Ce )qe

 1   B − 1 C  0 Bqmax

able”. The separation factor, RL is defined by:

+

Bqmax

Ce

(7)

where qmax is the maximum uptake at monolayer (mg g−1 ), Ce is the equilibrium concentration of Ce3+ (mg L−1 ), C0 is the saturation concentration of the solute (mg L−1 ), qe is the amount of Ce3+ bound per unit weight of the pectin compounds at equilibrium concentration (mg g−1 ) and B is the BET constant expressive of the energy of interaction with surface. Calculated results of the Langmuir, Freundlich and BET isotherms are given in Table 2. They show that sorption of Ce3+ by pectin compounds were better correlated (R2 > 0.96) with the Langmuir equation as compared to Freundlich and BET equations under the given range of concentration. It can be explained by the presence of finite number of homogenous binding sites on the pectin molecules presented with free active carboxyl groups, which is the basic condition of the Langmuir sorption model [30]. According to calculated Langmuir parameters obtained from the plot C/q vs C (Fig. 4), the highest binding capacity is characteristic of pectins with the low degree of esterification (1% and 20%) and calcium pectate beads. Despite equal value of the qmax of these compounds the sorption coefficient b that is related to the apparent energy of sorption was significantly different. Coefficient b for low esterified pectin was much greater than that of pectin with higher degree of esterification and calcium pectate. The essential features of a Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor, RL that is used to predict if an adsorption system is “favorable” or “unfavor-

1 1 + bC0

(8)

where C0 is the initial Ce3+ concentration (mg ml−1 ) and b is the Langmuir adsorption equilibrium constant (ml mg−1 ). The results of the RL factor calculation (Table 3) showed that based on the effect of separation factor on isotherm shape, the RL values of all pectin compounds studied were in the range of 0 < RL < 1, which indicates that the binding of Ce3+ by these substances is favorable. Thus, de-esterified pectin is a favorable material for binding of the Ce3+ ions. RL values for other pectin compounds were also between 0 and 1, but they were significantly different from those of de-esterified pectin suggesting the lower capacity of these substances to bind Ce3+ . The obvious mechanism of sorption is related to the formation of covalent and hydrogen bonds between the metal ions and non-esterified carboxyl groups and hydrogen atoms located on the pectin molecules and acting as the binding sites [16]. Results obtained show that intensity of binding processes and sorption capacity does not depend on solubility or other physicochemical parameters of the compound studied but closely relates to the number of the free carboxyl groups in its structure [30].

Fig. 4. Langmuir plot for the sorption of Ce3+ by pectin with different degree of esterification and calcium pectate. DE—degree of esterification.

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The qmax parameter of the Langmuir model indicating the number of active binding sites of molecules shows that the lower is the degree of esterification of the pectin molecule the more active sites on the carboxyl groups are taking part in the process of Ce3+ binding. Changes of the b coefficient reflecting affinity of the pectin to metal ions were also strongly corresponding to the degree of esterification with highest values of affinity possessed by pectin with the degree of esterification about 1%. The similar tendency was found after calculation of the Freundlich equation parameters; despite they were considered less significant. This implies formation of the junction zones between free carboxyl groups of pectin and Ce3+ according to the “egg-box” model [16], which is closely related to the degree of esterification of pectin, and was confirmed by the batch studies performed. Combining these data with the value of the rate of sorption found in experiments the main mechanism is obviously chemisorption. Calcium pectate, despite its very low degree of esterification, showed significantly lower cerium binding capacity in comparison with de-esterified pectin. Results obtained through evaluation of experimental data by means of Langmuir equation showed that low esterified calcium pectate contain the same quantity of active binding site as low esterified pectin. But due to the presence of the calcium ions affinity coefficient of this sample was more than 7-fold lower. It was found earlier that affinity of pectin to the Ca2+ ions is relatively high [14], therefore, calcium pectate exerts poor ion-exchange activity regarding Ce3+ ions and does not release calcium ions into the media. The values of the calculated parameters suggest that low esterified pectin possesses higher affinity and sorption capacity regarding the Ce3+ ions in comparison to higher esterified pectin and calcium pectate. In our previous studies we have estimated binding activity of pectins interacting with Zn2+ and Pb2+ ions. We have shown that qmax parameter of the Langmuir model indicating the number of active binding sites of molecules for Zn2+ is 131.6 mg g−1 and for Pb2+ 680.0 mg g−1 [9,31]. Taking to account the difference between atomic weights of these elements we can conclude that trivalent metals are poorer bound by pectin compounds due to smaller amount of junctions zones formed according to the “egg-box” model. It should be noticed that b coefficient showing apparent energy of sorption y of the pectin to the Ce3+ ions is also low if compared to other metals. This may indicate that formation of the junction zones is disrupted in the trivalent ions bound resulting in lower general binding capacity. The results obtained through this study suggest that pectin substances, in particular, the ones with the low degree of esterification possess relatively high cerium binding activity. Such property may be useful for creation of cheap and effective materials purposed for removal Ce3+ ions from various contaminated water sources and reservoirs. These materials can be easily elaborated using a pectin-rich biomass usually disposing after fruit processing. The main advantages of these materials are their availability and low cost because they are a disposal of the food industry. Chemical modifications of pectin substances may markedly increase metal binding activity of natural pectin containing materials purposed for Ce3+ removal from water solution and contribute to higher degree of water purity.

4. Conclusions The chemical structure and physicochemical properties of the pectin substances dramatically influences cerium binding activity of pectin compounds. The main parameter affecting metal uptake exerted by pectins is their degree of esterification. The lower is the degree of esterification, the more Ce3+ ions can be bound to

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the active sites of the pectin molecule. The presence of Ca2+ ions markedly decelerates the binding process even despite the very low degree of esterification. The pH change in the solution has a strong effect of dynamics of the binding process. At pH lower than 4.0 Ce3+ uptake significantly decreases as well as at pH higher than 6.0 caused by the changes of the pectin properties as well as the Ce3+ ion behavior. The optimum pH range for binding of Ce3+ by pectins is 4.0–6.0. The rate of the process provides the equilibrium concentration to be achieved in 60 min for water soluble pectins and in 80 min for insoluble calcium pectate beads. Equilibrium studies showed that pectins are favorable materials for binding and removal of metal ions, particularly Ce3+ , from aqueous solution. On the base of results obtained the most preferable are pectin with lowest degree of esterification being as in watersoluble as in insoluble forms.

Acknowledgement This work was supported by the grant of the Russian Foundation for Basic Research and Far Eastern Brach of the Russian Academy of Sciences (project no. 09-03-98512).

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