Sorption of Pd(II) ion by calcium alginate gel beads at different chloride concentrations and pH. A kinetic and equilibrium study

2.1 Chemicals

Alginic acid (by Sigma, lot 051M1864V, molecular weight in the range 70–100 kDa), extracted from Macrocystis pyrifera, with an average content of mannuronic (m) and guluronic (g) acids of 61% and 39%, respectively, was used as sodium salt. Palladium(II) ion solutions were prepared starting from palladium(II) nitrate dihydrate salt (by Sigma–Aldrich, purum). NaNO_3, NaClO₄ and NaCl solutions were prepared by weighing the pure salts (Fluka) after drying at 110 °C for 2 h. Nitric and hydrochloric acids and sodium hydroxide solutions used to adjust the pH of the metal ion solutions and in the ISE-H⁺ potentiometric titrations were prepared by diluting concentrated Fluka ampoules and standardized against sodium carbonate and potassium hydrogen phthalate, respectively. Calcium chloride dihydrate (by Fluka) was used to prepare calcium alginate (Ca-AA) gel beads. Standard solutions of calcium(II) ion were prepared by diluting concentrated Sigma Aldrich ampoules. Standard solutions of palladium(II) ion were prepared from a 1000 μg mL⁻¹ standard solution in 10% HCl (SCP Science). All the solutions were prepared using fresh CO₂-free ultra pure water (ρ ⩾ 18 MΩ cm).

2.2 Preparation and characterization of calcium alginate gel beads

Gel beads were prepared by following the classical procedure used in Reference Cataldo et al. (2013b). The beads were stored in ultrapure water at 4 °C before use.

The physical and mechanical properties of Ca-AA gel beads were investigated by different techniques. The diameter of beads was measured with a digital calliper (MITUTOYO, model 500–181-U) on fifty beads. A helium pycnometer (AccuPyc 1330) was used to measure the density of gel beads at T = 20 °C. The experiment was repeated three times on different gel beads. The water content of gel beads was determined as follows: fifty beads of gel were dried in an oven for three days at 40 °C and the dry weight was measured after a constant weight was reached. The weight of the wet beads was determined by weighting the same number of gel beads after they were placed on a cellulose filter for 30 s. The procedure was repeated three times. The mechanical resistance of gel beads, measured by a Texture Analyser (model TA-XTS2i, Stable Micro Systems, England), represents the mean force (expressed in Newton’s) necessary to generate the 10% compression on a bead placed under a cylinder probe P10 (Batch 2370, Stable Micro Systems, England). The experiments were repeated on 40 beads and the average value was calculated together with the standard deviation.

The morphology of gel beads before and after sorption of Pd(II) ion (at C_Pd2+ = 30 mg L⁻¹, pH = 2, in NaNO₃ medium, at I = 0.01 mol L⁻¹ and T = 25 °C) was investigated by an electron microscope ESEM FEI QUANTA 200F. Before the analysis the gel beads were oven-dried at T = 105 °C for 24 h and their surface was coated with gold in the presence of argon by an Edwards Sputter Coater S150A in order to prevent charging under electronic beam. The electron beam was opportunely set in order to avoid the damage of the samples.

2.3 Experimental equipment and procedures for speciation study

Speciation study of the Pd(II)-AA system in NaCl/NaClO₄ mixed media, at I = 0.1 mol L⁻¹ and T = 25 °C was carried out by using the potentiometric technique. An apparatus consisting of a model 713 Metrohm potentiometer, equipped with a combination glass electrode (Ross type 8102, from Orion), and a Model 765 Metrohm motorized burette was employed. Estimated accuracy was 0.2 mV and 0.003 mL for electromotive force (e.m.f.) and titrant volume readings, respectively. The apparatus was connected to a PC, and automatic titrations were performed using a suitable computer program to control titrant delivery and data acquisition and to check for e.m.f. stability. All titrations were carried out in thermostated cells under magnetic stirring, bubbling purified presaturated N₂ through the solution in order to exclude O₂ and CO₂. In each titration 25 mL of solution containing palladium(II) nitrate 0.4–0.5 mmol L⁻¹, sodium alginate 1–2 mmol L⁻¹ and the right amount of the two salts (NaCl and NaClO₄) of the ionic medium were titrated with NaOH in the pH range 2–5. The titrand solutions were acidified with known amounts of HClO₄ or HCl in order to start the titrations from pH ≈ 2. For each experiment, independent titrations of strong acidic solution with standard base were carried out under the same medium and ionic strength conditions as the system to be investigated, with the aim of determining E⁰ (standard potential). In this way, the pH scale used was the total scale, pH = −log [H⁺], where [H⁺] is the free proton concentration (not activity).

2.4 Experimental equipment and procedures for kinetics and adsorption isotherm

Batch kinetic experiments at 25 ± 1 °C were carried out by putting 25 beads in Erlenmeyer flasks containing each 25 mL of solution of palladium(II) 30 mg L⁻¹ as Pd(NO₃)₂*2H₂O. Kinetic experiments were performed in NaClO₄/NaCl mixed media at I = 0.01 mol L⁻¹ by varying the concentration of chloride in the range 0 ⩽ ${\text{C}}_{{\text{Cl}}^{-}}$ (mmol L⁻¹) ⩽ 10 in the pH range 2–5. The solutions were shaken at 180 rpm using an orbital mixer (model M201-OR, MPM Instruments). Samples were withdrawn at different times in the range 1–360 min and 10 mL of each solution was pipetted and stored in plastic tubes. For each set of kinetic experiments, sixteen samples of 10 ml were collected for the ICP-OES measurements. The variation of H⁺ concentration was checked by a potentiometer (Model 654 Metrohm) equipped with a combined ISE-H⁺ glass electrode (Ross type 8102). For sorption isotherm experiments a different number of Ca-AA gel beads (ranging between 5 and 150) was added to 25 mL of Pd(NO₃)₂ solution at 30 mg L⁻¹. The range of pH as well as the chloride concentration and the temperature of the solutions were similar to those of kinetic experiments. The solutions were shaken at 180 rpm for six hours and then filtered and stored in plastic tubes. The concentration of Ca(II) and Pd(II) ions in the filtrates obtained from both kinetic and isotherm experiments was determined by ICP-OES (Perkin Elmer Model Optima 2100, equipped with an auto sampler model AS-90).

3.1 Calculations in speciation study

The following computer programs (De Stefano et al., 1997) were used for the elaboration of potentiometric data: (i) ESAB2M for the refinement of all of the parameters from the acid–base titration (E°, the ion product constant of water K_w, the liquid junction potential coefficient j_a, and the analytical concentration of reagents), (ii) BSTAC and STACO for the calculation of complex formation constants, (iii) ES4ECI to draw the speciation diagrams. Formation constants of Pd²⁺–AA complex species are given according to the following equilibrium:

3.2 Models for kinetic and thermodynamic studies of palladium(II) sorption and calcium(II) release

The kinetic study of Pd(II) sorption and of Ca(II) release by calcium alginate gel beads was carried out by processing the experimental data by the pseudo second order Eq. (2) (Ho and Ofomajia, 2006):

The thermodynamic approach to the sorption of metal ions by gel beads was made by applying Langmuir and Freundlich isotherm models according to Eqs. (3) and (4), respectively (Gotoh et al., 2004b).

Kinetic and isotherm equations were fitted to experimental data by using the Linear and Non Linear data Analysis home-made computer program (LIANA) (De Stefano et al., 1997).

4.1 Characterization of Ca-AA gel beads

The results of characterization of gel beads are reported in Table 1. They are very close to those reported in Reference Cataldo et al. (2013b) for gel beads of the same biomaterial and this confirms the reproducibility of the method used to produce the gel beads. Among the experimental data reported in the Table, it is worth mentioning the mechanical resistance of the sorbent material. In fact, the hardness of the sorbent material is very important when it is employed in the removal/recovery of metal ions from water solutions. Although the gel beads have a 97.4% of water, they have a good mechanical resistance (0.19 N to generate the 10% compression of the bead).

In Fig. 1 are reported the micrographs obtained from SEM analysis at 100 and 500 magnifications for Ca-AA gel beads after contact for six hours with solutions, at pH = 2, in NaNO₃ medium, I = 0.01 mol L⁻¹ with (Fig. 1b and d) and without Pd(II) ion 30 mg L⁻¹ (Fig. 1a and c). Differences in morphology can be noted after Pd(II) sorption. The surface of gel beads, initially smooth, becomes wrinkled with an increase in creases and pores. The same surface change was noted after Cd(II) and Cu(II) sorption on calcium alginate and calcium alginate/pectate gel beads as reported in a previous paper (Cataldo et al., 2013a) and, as in that case, it can be attributed to the shrinkage of the beads at high concentrations of metal ions different from Ca(II).

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Figure 1

SEM micrographs obtained at 100 and 500 magnifications of (a, c) Ca-AA after contact with a solution at pH = 2, in NaNO₃ medium at I = 0.010 mol L⁻¹ and (b, d) Ca-AA after contact with a solution containing C_Pd2+ = 30 mg L⁻¹ at pH = 2, in NaNO₃ medium at I = 0.010 mol L⁻¹.

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4.2 Speciation study of the palladium(II) – alginate system

To assess the more appropriate solution conditions to be adopted in order to obtain the best efficiency in the palladium(II) sorption by calcium alginate gel beads, a preliminary speciation study was carried out on the Pd-AA system in aqueous solution, at I = 0.1 mol L⁻¹, in mixed NaClO₄/NaCl ionic medium and at different chloride concentrations. As an example, in Fig. S1 is reported the ISE-H⁺ potentiometric curve of a solution containing Pd²⁺ 0.38 mmol L⁻¹, AA 0.57 mmol L⁻¹, in NaCl/NaClO₄ medium, at I = 0.1 mol L⁻¹ and ${\text{C}}_{{\text{Cl}}^{-}}$ = 0.01 mol L⁻¹. Previous investigations on the chemical behaviour of Pd(II) ion in aqueous solution (Gianguzza et al., 2010) show that: (i) Pd(II) undergoes strong hydrolysis starting from very acidic solution condition (pH ≈ 2) and low soluble hydroxide formation occurs over pH ≈ 4.5 in the absence of ligands; (ii) the presence of chloride ions in solution leads to the formation of quite stable PdCl_x^2 − x (x = 1–4) and mixed Pd(OH)Cl₃²⁻ species, (iii) the formation of these complex species allows to keep palladium(II) in solution in a slightly large pH range, up to ∼5, (iv) some solution components, such as nitrate or perchlorate, do not interact with Pd(II) ion. Moreover, quantitative data on the protonation of alginate are reported in ref (Crea et al., 2009).

On the basis of these information, the chemical speciation study, carried out by potentiometric measurements, gave evidence for the formation of a stable Pd-AA species, with formation constant log K₁₁₀ = 9.124 ± 0.008, which achieves the maximum percentage formation at pH ≈ 5 in the presence of chloride ion 0.01 mol L⁻¹ (see speciation diagram reported in Fig. 2). As shown by results reported in Table 2, the formation percentage of Pd-AA species decreases with the decreasing of pH at the same chloride concentration, while it increases with the decreasing of chloride concentration for the same pH value. This different trend is clearly attributable to the gradual protonation of carboxylic groups of alginate with the lowering of pH, to the predominance of hydrolytic species over pH = 2 which lowers the concentration of Pd(II) ion available to interact with alginate ligand and confirms the role of chloride in keeping Pd(II) ion in solution. Although the protonation of carboxylic groups of alginate increases with the decreasing of pH, at pH = 2 a sufficient amount of COOH groups is present in deprotonated form available to react (Cataldo et al., 2013b). On the basis of these results, the Pd(II) sorption process has been set up considering a solution from where Pd(II) must be removed, containing different chloride concentrations (0 ⩽ ${\text{C}}_{{\text{Cl}}^{-}}$ /mmol L⁻¹ ⩽ 10) in a pH range 2–5.

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Figure 2

Distribution of species vs pH in the system Pd²⁺-AA, in NaCl/NaClO₄ ionic medium, at I = 0.1 mol L⁻¹ and at T = 25 °C. Experimental conditions: ${\text{C}}_{{\text{NaClO}}_4}$ = 0.09 mol L⁻¹, C_NaCl = 0.01, C_Pd = 0.28 mmol L⁻¹, C_AA = 0.60 mmol L⁻¹.

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4.3 Kinetic results

Particular attention has been paid to the conditions where the maximum formation percentage of Pd-AA species was achieved, i.e., at pH = 2 in the absence of chloride (PdAA % = 98.75) and at pH 3, 4 and 5 at the lower ${\text{C}}_{{\text{Cl}}^{-}}$ achievable without precipitation of low soluble hydrolytic species of palladium(II) (see the percentages of PdAA reported in Table 2). To verify the hypothesis according to which the ion exchange between the Pd(II) in solution and the Ca(II) present in the calcium alginate sorbent gel is the main sorption mechanism, the kinetic measurements were carried out to check both Pd(II) sorption and Ca(II) release at different contact times.

In Table 3 are reported the results of kinetic experiments at different pH values and for different chloride concentrations for both palladium(II) ion sorption and calcium(II) ion release. In general, the R² values obtained show that the pseudo-second order Eq. (2) fits well the experimental data. No significant trends were found in the rate of Pd(II) sorption and of Ca(II) release by gel beads as function of ${\text{C}}_{{\text{Cl}}^{-}}$ and pH. The kinetic constant (K) values range between 0.21 ⩽ K/g mmol⁻¹ min⁻¹ ⩽ 0.93 and 0.21 ⩽ K/g mmol⁻¹ min⁻¹ ⩽ 0.79 for palladium(II) sorption and calcium(II) release, respectively.

As can be seen, the amount of Pd(II) adsorbed by CaAA at pH = 2 increases with the decreasing of the chloride concentration in solution (q_e = 0.431 and 0.072 mmol g⁻¹ for ${\text{C}}_{{\text{Cl}}^{-}}$ equal to 0 and 10 mmol L⁻¹, respectively). This trend is clearly shown in Fig. 3a where the experimental kinetic data for Pd(II) sorption are reported together with the curve fits obtained by Eq. (2). The chloride content in solution seems to have very low or no influence in the palladium sorption at pH = 3 and 4 in comparison with the effect shown at pH = 2, probably owing to the contemporary formation of hydroxo species (Gianguzza et al., 2010).

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Figure 3

Dependence of q_t (mmol Pd(II) adsorbed/g dry gel beads) (a) or (mmol Ca(II) released/g dry gel beads) (b) on contact time of calcium alginate gel beads with aqueous solutions at different chloride concentrations, at I = 0.01 mol L⁻¹ (NaClO₄/NaCl mixed medium), T = 25 °C and pH = 2. Symbols used for the adsorption of Pd(II) ion at different chloride concentrations (mmol L⁻¹: (0, ♢); (2.5, ∇); (5.0, △); (7.5, ○); (10.0, □). The corresponding solid symbols refer to the release of Ca(II) ions by gel beads. The symbol * refers to the release of Ca(II) of gel beads in solutions that do not contain Pd(II) ion, at the same pH and ionic strength of the corresponding experiments. The lines represent the fits of kinetic equation.

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Therefore, by the first inspection of speciation results it could be affirmed that the best conditions for removal of Pd(II) ion from aqueous solutions are at pH = 2 in the absence of chloride, as shown in Fig. 3a. This hypothesis is also confirmed by the high q_e values obtained from kinetic experiments carried out under the above cited conditions, as reported in Table 3. Nevertheless, these conditions are far from those usually found in the natural waters and wastewaters and can be used only for particular acidic industrial effluents or for the recovery process of palladium(II) ion. Therefore, the adsorption capacity of Pd(II) ion by CaAA has to be checked also under solution conditions more close to natural aquatic systems. Unfortunately, owing to the formation of low soluble hydrolytic species of Pd(II) at higher pH values, the maximum pH value achievable before the precipitation is about 5 in the presence of chloride ions which allow to keep palladium in solution by PdCl_x^2 − x and Pd(OH)Cl₃²⁻ soluble species formation.

Table 3 reports also the results obtained by kinetic measurements carried out at pH 3, 4 and 5 in the presence of variable chloride concentration. As can be seen, it was possible to carry out measurements at pH 4 and 5 only for chloride concentration of 5 and 10 mmol L⁻¹. In general the q_e values obtained for palladium adsorption in the presence of chloride are lesser than the q_e value obtained at pH = 2 in the absence of chloride, as also shown in Fig. S2 of Supplementary data file. From the q_e values obtained in chloride solution at different pH values, the pH = 3 seems to be the best pH value to be adopted for Pd(II) sorption in the presence of chloride. Nevertheless, an acceptable adsorption of palladium can be also noted at pH = 5 in 10 mmol L⁻¹ chloride concentration (the achievable conditions more close to natural aqueous systems), as shown by q_e value: 0.170 mmol g⁻¹ ≈ 18 mg g⁻¹ dried gel beads. This result let us to affirm that a fairly good adsorption of Pd(II) ion by CaAA gel beads can be still obtained by setting-up a pH = 5 in the solution containing an adequate chloride concentration.

The results obtained from kinetic measurements on calcium release by CaAA (see Table 3) can give us some information about the adsorption mechanism. Since we are obliged to operate in an acidic pH range, a consistent part of calcium present in the gel beads is exchanged with protons of solution. This is especially evident for calcium release at pH = 2, where the amount of calcium release in the presence and in absence of palladium(II) ions in solution is nearly the same, confirming that the Ca²⁺–H⁺ exchange is the predominant mechanism. At pH 3, 4 and 5 a more consistent Pd–Ca exchange occurs, as shown by the q_e values for calcium release if compared with the corresponding values obtained in the absence of Pd(II) ion in solution. This trend confirms an ion exchange mechanism between Pd(II) in solution and Ca(II) present in the calcium alginate gel occurs for pH values over 2. Although the great part of Ca(II) released by gel beads at pH = 2 is caused by the exchange with the proton of solution, in our opinion also at this pH the ion exchange can be considered the main mechanism of Pd(II) ion sorption, similar to that experimentally confirmed at higher pH.

4.4 Adsorption isotherms

In order to know the maximum sorption capacity of gel beads towards Pd(II) ion, the sorption isotherm study was made at the same experimental conditions adopted in speciation and kinetic investigations. The data were processed by Langmuir and Freundlich isotherms Eqs. (3) and (4) and the results obtained are reported in Table 4. Both the models closely fit the experimental data (R² ⩾ 0.94) although Langmuir model gave slightly better results. The use of Langmuir isotherm allows to calculate the important parameter q_max that gives quantitatively information on the sorption ability of the sorbent material. As an example, the curve fits of the two equations using the experimental data at pH = 2 and at ${\text{C}}_{{\text{Cl}}^{-}}$ = 0 mol L⁻¹and at pH = 3 and at ${\text{C}}_{{\text{Cl}}^{-}}$ = 0.0075 mol L⁻¹ are reported in Figs. 4 and 5. As can be seen from the data of Table 4, the sorbent material shows higher and comparable, within the experimental errors, sorption ability at pH = 2 and ${\text{C}}_{{\text{Cl}}^{-}}$  = 0 mmol L⁻¹ and at pH = 3 and ${\text{C}}_{{\text{Cl}}^{-}}$  = 2.5 mmol L⁻¹, with q_max values of 119 and 127 mg g⁻¹, respectively. In these experimental conditions a right compromise is achieved between the amount of deprotonated COOH groups of alginate and the amount of palladium(II) present as positively charged species in solution. Owing to the high formation percentages of Pd–Cl species and to the low percentage of deprotonated form of alginate, at pH = 2 and in the presence of chloride in solution, there is a significant decrease of q_max that at ${\text{C}}_{{\text{Cl}}^{-}}$ = 10.0 mmol L⁻¹ is equal to 16 mg g⁻¹. Due to the higher percentage of dissociated COOH groups of alginate, at pH ⩾ 3 the effect of chloride on the sorption capacity of gel beads is less noticeable and causes a more little decrease of the q_max (e.g., the q_max goes from 127 to 74 mg g⁻¹ at pH = 3 and ${\text{C}}_{{\text{Cl}}^{-}}$  = 2.5 and 10.0 mmol L⁻¹, respectively). Even if the formation of chloride and hydroxo species affects the sorption capacity of gel beads, independently of the chloride concentration and of the pH of the solution, at the experimental conditions adopted in this work the q_max values are high enough. This is a confirmation that the calcium alginate gel beads can be considered a good biosorbent material for the removal of Pd(II) from aqueous solution even if its affinity towards the metal ion changes with the changing of experimental conditions (e.g., at pH = 2 the K_L goes from 1.2 to 0.09 L mg⁻¹ for ${\text{C}}_{{\text{Cl}}^{-}}$ = 0 and 10.0 mmol L⁻¹, respectively). The affinity of gel beads towards Pd(II) ions in the pH range considered at ${\text{C}}_{{\text{Cl}}^{-}}$ = 0.01 mol L⁻¹ (the only chloride concentration used for three different pH values) is almost the same (K_L = 0.09, 0.20 and 0.09 L mg⁻¹ for pH = 2, 3 and 4, respectively).

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Figure 4

Sorption isotherms of Langmuir (continuous lines) and Freundlich (dashed lines) for the adsorption of palladium(II) ion (30 ppm) by calcium alginate gel beads in aqueous solution at pH = 2, at I = 0.01 mol L⁻¹, T = 25 °C and ${\text{C}}_{{\text{Cl}}^{-}}$ = 0 mol L⁻¹.

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Figure 5

Sorption isotherms of Langmuir (continuous lines) and Freundlich (dashed lines) for the adsorption of palladium(II) ion (30 ppm) by calcium alginate gel beads in aqueous solution at pH = 3, at I = 0.01 mol L⁻¹, T = 25 °C and ${\text{C}}_{{\text{Cl}}^{-}}$  = 0.0075 mol L⁻¹.

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4.5 Comparison between the Pd(II) sequestration by alginate in aqueous solution and Pd(II) removal by sorption onto alginate in gel phase

The preliminary speciation study on the Pd-AA system was carried out to assess the best experimental conditions to be used and also to define the strength of the interaction between Pd(II) ion and alginate sequestering agent in aqueous solution (log K_PdAA = 9.124 ± 0.008). By assuming that this interaction is quantitatively similar to that of Pd(II) with alginate as sorbent gel material, some correlations can be found between the results obtained for palladium sequestration in aqueous solution and palladium removal by sorption, in the same experimental conditions. To this end, the q_e and q_max values will be compared with pL_0.5 values which give quantitative information on the sequestering capacity of a ligand towards a metal ion in solution. In particular, pL_0.5 represents the concentration of ligand necessary to sequestrate the 50% of a metal ion present in solution, and can be expressed by the following Boltzmann equation:

When the metal ion in solution is present as trace, pL_0.5 is independent of the analytical concentration of the metal ion and varies with the experimental conditions (pH, ionic strength, supporting electrolyte, temperature, etc.).

More details about the calculation and the meaning of the pL_0.5 are reported in References Gianguzza et al. (2012, 2010). In Table 5 are reported the pL_0.5 values of AA towards Pd(II) ion at different pH and different chloride concentrations (the same pH values and chloride concentrations adopted in sorption experiments). As an example, in Fig. 6 are reported the sequestering curves obtained at different concentrations of chloride ion in solution, at pH = 2 and at T = 25 °C. As can be seen, at the same pH the sequestering ability of alginate towards Pd(II) ion increases with the decreasing of chloride concentration.

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Figure 6

Fraction (x) of Pd(II) ion complexed by AA vs. −log C_AA, in mixed NaClO₄/NaCl medium, at I = 0.1 mol L⁻¹ and at T = 25 °C. Experimental conditions: C_Pd2+ = 10⁻¹⁰ mol·L⁻¹ (trace), pH = 2, 0 ⩽ C_NaCl (mmol L⁻¹) ⩽ 10.

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The best sequestering capacity of AA towards Pd(II) ion is obtained for pH = 2 and ${\text{C}}_{{\text{Cl}}^{-}}$ = 0, confirming the results obtained in sorption process at the same experimental conditions. Accordance with sorption results can be also noted in the presence of different chloride concentrations: the effect of chloride on the sequestration of Pd(II) by AA decreases with the increasing of pH. For example, at ${\text{C}}_{{\text{Cl}}^{-}}$  = 0.01 mol L⁻¹ the pL_0.5 is equal to 0.93, 2.57, 3.27 and 3.37 at pH = 2, 3, 4 and 5, respectively.

Moreover, the q_e, q_max and pL_0.5 parameters showed similar trends as function of some variables, such as the pH and the chloride concentration in solution. As an example, in Fig. 7 is reported the linear correlation between the pL_0.5 and the kinetic parameter q_e, both calculated at different pH and chloride concentrations. In Fig. S3 of Supplementary data file is also reported the comparison of the trend of pL_0.5 and of q_max as function of chloride concentration of solution at pH = 2. The good linear correlation (R = 0.97) between q_e and pL_0.5 (Fig. 7) and the similar trend of pL_0.5 and q_max parameters at different chloride concentrations (Fig. S3) indicate that the affinity of alginate towards Pd(II) ion as ligand in solution and as sorbent material is strictly related to each other.

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Figure 7

Linear correlation between pL_0.5 and q_e calculated at different pH and chloride concentrations, at T = 25 °C.

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Therefore, although the gelation could alter the number of available binding sites of alginate (Kalis et al., 2009), it can be affirmed that alginate shows a good and similar sequestration capacity in solution and as sorbent gel material, confirming the assumption made before, and pointing out, once again, the importance of speciation studies for the aquatic ecosystem characterization.

4.6 Literature data comparison

To our knowledge this is the first study on the sorption ability of calcium alginate gel beads towards palladium(II) ion. Some papers are reported in the literature on the binding ability of raw materials (Das, 2010; de Vargas et al., 2004; Mack et al., 2007) and synthetic resins (Hubicki and Leszczynska, 2005; Hubicki et al., 2006, 2008) towards Pd(II) ion. Among the natural polyelectrolytes, only derivative and non derivative crosslinked chitosan gels were used as sorbent material for this metal ion (Ramesh et al., 2008; Ruiz et al., 2000; Zhou et al., 2009). Some q_max literature values are reported in Table 6 together with those we found for alginate gel beads. As can be seen, at the same pH the effect of chloride concentration on the sorption ability of chitosan and of alginate gels is different and this is attributable to the different functional groups of the two polyelectrolytes. The protonated amino groups of chitosan bind preferentially the negatively charged chloride species of palladium [PdCl₃⁻ and PdCl₄²⁻, PdCl₃(OH)²⁻], in the acidic pH range, whilst the carboxylate groups of alginate, partially deprotonated in the pH range investigated, bind the positively charged species of the metal ions [Pd²⁺, PdCl⁺, Pd(OH)⁺]. No data were found on the sorption ability of chitosan in the absence of chloride but it is presumable that in the pH range 2–4 alginate gel is a better sorbent material in comparison with chitosan gel in the absence of chloride in solution. The q_max values reported in Table 6, show that calcium alginate can be considered a valid alternative sorbent material to chitosan gel, especially in solution with low concentrations or without chloride. Moreover, the gel beads used in this work do not contain additional chemical agents, as in chitosan gel material used by the other authors, that cause an increase in cost of their production and also in the following disposal.