Recovering Heavy Metal Ions from Complex ... - ACS Publications

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Recovering Heavy Metal Ions from Complex Solutions Using Polyethylenimine Derivatives Encapsulated in Alginate Matrix Caroline Bertagnolli,* Andrey Grishin, Thierry Vincent, and Eric Guibal Centre des Matériaux des Mines d’Alès (C2MA), Ecole des mines d’Alès−6, Avenue de Clavières, F-30319 Alès cedex, France S Supporting Information *

ABSTRACT: A new sorbent was manufactured by immobilization of polyethylenimine (PEI) derivatives (glutaraldehyde-crosslinked PEI, and carbon disulfide-grafted PEI) into an alginate matrix. These composite materials take advantage of the sorption properties of both PEI and alginate, and the encapsulating properties of the biopolymer. The sorption properties of these composite materials were tested for the recovery of Cd(II), Cu(II), and Zn(II) ions from aqueous solutions. Kinetic, equilibrium, and desorption behaviors were successively investigated. Special attention was paid to competitive sorption and to the effect of inorganic ions on sorption efficiency. Sorbent morphology was observed using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray microanalysis system (EDX). SEM-EDX results show a homogeneous distribution of heavy metals in beads after metal sorption. Alginate was found to have a better sorption capacity for all heavy metals studied in simple systems. Conversely, calcium and sodium salts strongly reduce the sorption of heavy metals. In complex systems, the presence of other reactive groups (amine groups from PEI) contributes to improved sorption performance.

1. INTRODUCTION Heavy metals pollution is one of the most important problems of this century because of its significant threat to human health and environment. Both intrinsic acute toxicity and cumulative effect in the food chain contribute to its hazardous effects. Chemical precipitation, coagulation−flocculation, membrane filtration, and other methods for removing heavy metals from wastewater have been extensively studied.1−4 These processes frequently face economic and technical constraints that limit their use and efficiency to respect the environment in accordance with international regulations. Among them, sorption processes may efficiently remove heavy metals when the initial concentrations are less than 100 mg L−1 and the discharge concentrations are less than 2 mg L−1. Natural polysaccharides, such as alginate or chitosan, offer considerable sorbent potential because of their availability, biodegradability, and the substantial number of reactive hydroxyl, carboxyl, and/ or amino groups. The sorption capacity of these biopolymers was reported by several authors.5−8 The biopolymers have quite good sorption properties for metal ions, but the behavior of these sorbents in complex systems (high ionic strength, multimetal solutions) was not extensively studied. There is still a need for further studies and development of alternative sorbents that are capable of recovering metal ions (hazardous metals or valuable metals) from complex solutions. Real industrial wastewater is generally a very complicated system. Various heavy metal ions coexist along with all kinds of inorganic ions. Therefore, inorganic ions may compete for sorption sites or influence the speciation of metal ions. This, in turn, strongly reduces or inhibits the removal of heavy metals. Polyethylenimine (PEI), which is known for its metalchelation properties due to the presence of a large number of amine groups per molecule, is often used to modify sorbent surface to increase the sorption capacity of given sorbent.9−11 Sulfur-containing reactive groups are well-known for their high © XXXX American Chemical Society

affinity for metal ions, and generally grafting these reactive groups allows using these sorbents in a wider range of pH. The reaction of PEI with carbon disulfide is supposed to form dithiocarbamate moieties with strong reactivity for metal ions.12 These facts appeared to be interesting for improving metal binding: higher density of sorption sites and strong reactivity for metal cations. Thus, PEI derivatives (obtained by glutaraldehyde cross-linking or by CS2 grafting) were synthesized as powder. Large-size particles generally have poor efficiency due to their resistance to intraparticle diffusion therefore sorption may be limited to sorption sites at the surface of the material. Reducing the size of sorbent particles improves sorbent efficiency but risks complications when recovering spent sorbent (due to size effect) at the end of the process. This problem may be overcome using an encapsulation process for the immobilization of micrometer-size particles in a suitable matrix. In this case, the challenge lies in the management of both mass-transfer properties (limitations due to encapsulating matrix) and reactive microparticles confinement. Thus, a good compromise seems to associate the small PEI-derivatives particles with an encapsulating biopolymer. Alginate is both an excellent encapsulating agent and an efficient sorbent for metal ions due to the presence of carboxylic groups (associated with guluronic and mannuronic acids, the monomers that constitute alginate).13,14 These reactive groups may interact with metal cations via chelation, but they also contribute to the biopolymer gelling (divalent cations with carboxylic groups from vicinal chains). Alginate was used for encapsulating PEI-derivatives microparticles, and these composite sorbents have been tested for Cd(II), Cu(II), Received: December 8, 2015 Revised: January 22, 2016 Accepted: January 29, 2016

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DOI: 10.1021/acs.iecr.5b04683 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

For the kinetic studies, 0.25 g of alginate, PEI/alginate, or PEI-CS2/alginate beads were shaken with 0.5 L of metal solution (C0, 0.3 mmol L−1; pH0 4; 150 rpm; 20 ± 1 °C). Samples were taken from the solution at known time intervals over a total period of 1620 min (i.e., 27 h). Uptake kinetics are generally controlled by a combination of mechanisms such as resistance to bulk diffusion (in the core of the solution), resistance to film diffusion, resistance to intraparticle diffusion, and the proper reaction rate.17 A sufficient agitation allows, in most cases, reducing the contribution of bulk diffusion and film diffusion in the control of sorption kinetics. In the case of poorly porous materials the resistance to intraparticle diffusion may play a predominant role in the control of kinetic profiles. Though a complete approach combining the different mechanisms would be closer to the reality of the process, the numerical solving requires sophisticated and complex mathematical tools17 and a simplification in the description of sorbent particles (spherical, homogeneous, etc.) that fails sometimes to approach the reality of the complex structure of sorbent materials. For these reasons we used a simplified approach that disconnects resistance to intraparticle diffusion to other mechanisms, and the intraparticle diffusion coefficient (De, effective diffusivity, m2 min−1) was determined using the Crank equation, eq 1a, assuming the solid to be initially free of metal, and the kinetics to be only controlled by intraparticle diffusion resistance:18

and Zn(II) recovery from simple and complex solutions (synthetic solutions with different metal cations, and in the presence of salts that increase ionic strength).

2. MATERIALS AND METHODS 2.1. Sorbent. Two sorbents were developed using a branched polyethylenimine (low molecular weight of 600− 800, water free, Aldrich, Switzerland). The first material was obtained by cross-linking between the amine groups of different chains of PEI and glutaraldehyde (50 wt % solution in water, Aldrich, Switzerland). The second material was obtained by an intermediate reaction step between amine groups of PEI and carbon disulfide (dithiocarbamate formation). Subsequently, cross-linking reaction by glutaraldehyde was conducted. Six grams of PEI were added to 400 mL of demineralized water under gentle agitation. Then, 6 mL of glutaraldehyde (50% w/w) were added dropwise, and the mixture was maintained under agitation for 24 h. An alternative material was produced by mixing PEI with 1 mL of carbon disulfide (RPE, Carlo Erba Reagents, France) for 1 h, before adding glutaraldehyde (6 mL of 50% w/w glutaraldehyde solution). After filtration the product was extensively washed before being dried at room temperature. The powder was immobilized in an alginate matrix (Protanal LF 240 D, 30−35% guluronic, FMC BioPolymer, USA), because it was impossible to use the fine particles in column systems (head loss and clogging effect). Fifty grams of sodium alginate solution (4%, w/v) were mixed with 2 g of PEI derivative powder and 48 g of distilled water. The mixture of alginate and powder was distributed dropwise into a CaCl2 (20 g L−1) stirred solution. The gel beads were kept in the CaCl2 solution for 24 h, for complete gelation. Finally, calcium alginate beads were rinsed with demineralized water and freezedried (−52 °C, 0.1 mbar). Beads containing only alginate were also produced to evaluate the alginate contribution on the heavy metal sorption. An environmental scanning electron microscope, Quanta FEG 200, equipped with an OXFORD Inca 350 energy dispersive X-ray microanalysis system was used to investigate the morphology and elemental composition of the sorbents, before and after metal sorption, in order to evaluate the distribution of composite material and detect if all reactive groups are accessible to the target metal. 2.2. Batch Sorption Experiments. Heavy metal solutions were prepared from the nitrate salt of copper (Cu(NO3)2· 3H2O), sulfate salt of zinc (ZnSO4·7H2O) and chloride salt of cadmium (CdCl2·H2O). Solution pH was adjusted using either 0.1 M NaOH or HCl, as required. Several studies have shown the increase of sorption capacity with the pH, while the precipitation of copper, zinc, or cadmium metal ions occurs around (or higher) pH 6,15,16 thus pH0 4 was chosen to study the sorption of these heavy metals in this paper to prevent the interference of metal precipitation in all experimental conditions. In addition, the sorption experiments generally involve a more or less important pH increase due to protonation/deprotonation mechanisms. It is thus important to have an appropriate selection of the initial pH to avoid metal precipitation after this pH increase. Initial and equilibrium metal concentrations were subsequently determined using inductively coupled plasma atomic emission spectrometry (ICP-AES, Jobin-Yvon Activa-M). The final pH was systematically determined in order to evaluate the potential precipitation phenomena associated with pH variation.

∞

q(t ) =1− qeq

∑

⎛ −Deq 2t ⎞ 6α(α + 1) exp⎜ 2 n ⎟ ⎝ r ⎠

n=1

9 + 9α + qn 2α 2

(1a)

q(t) and qeq are the concentrations of the metal in the resin at time t and equilibrium, respectively, r is the radius of the particle (2 mm), and qn is the nonzero roots of the equation:

tan qn = with mqeq VCo

=

3qn 3 + αqn 2

(1b)

1 1+α

(1c)

m (g) is the mass of sorbent added to the solution. The Mathematica software was used for the determination of the intraparticle diffusion coefficient, De, and for the simulation of experimental data. For sorption isotherms experiments, 25 mg of beads were maintained in contact under agitation with 25 mL of metal ions solution at varying concentrations (in the range 0.6−8 mmol L−1), pH0 4, 20 ± 1 °C. The Langmuir equation, eq 2, was applied to analyze the experimental data. q=

qmbCeq 1 + bCeq

(2)

where qm is the maximum sorption capacity of the sorbent (mmol g−1); b is the constant related to the energy of sorption (L mmol−1), and Ceq (mmol L−1) is the equilibrium metal concentration. The competitive effects were investigated comparing the sorption properties from multicomponent solutions (binary and ternary mixtures) under the same experimental conditions used for the study of sorption isotherms (initial concentration B


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Figure 1. SEM-EDX micrographs of alginate beads with incorporation of PEI derivatives (freeze-dried materials).

for each metal: C0, 2 mmol L−1). The separation factor (α21), eq 3, is calculated for defining the possible preference of the sorbent for a given metal in a mixture of metal ions:19 α21 =

potassium sulfate salts were performed. Testing the sorption in the presence of alkaline metals is also important to evaluate the stability of the sorbent (more specifically the stability of the encapsulating matrix). These alkaline metal ions may be exchanged with calcium ions that were used for the crosslinking of the alginate matrix (which, in turn, may induce a partial leakage of the biopolymer and the dispersion of encapsulated material). The experiments followed the same experimental procedure for batch tests, but the working solutions were prepared by dilution of metal stock solutions (1 g L−1) with a salt solution. The initial metal concentration was fixed at 2 mmol L−1 (pH0 4, sorbent dosage 1 g L−1), and

qeq1Ceq2 qeq2Ceq1

(3)

If α21 > 1, ion 1 is preferred, alternatively, ion 2 is preferred. The effect of Na(I), K(I), and Ca(II) ions on sorption was also studied. Industrial effluents, particularly from ceramics, surface treatment and mine industries, are generally complex and may contain alkaline and alkaline earth metals. Heavy metal sorption in the presence of calcium chloride salt and sodium or C


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Industrial & Engineering Chemistry Research increasing concentrations of salts were tested. Flasks were shaken for 48 h at 200 rpm, 20 ± 1 °C. Desorption of metal ions was carried out by using HCl 0.1 M and acidified water (pH 2) sorbent dosage, 2.5 g L−1 of eluent, 20 ± 1 °C. Desorption percentage (D%) was calculated by eq 4: D% =

mmol of heavy metal desorbed × 100 mmole of heavy metal sorbed

(4)

3. RESULTS AND DISCUSSION 3.1. SEM and SEM-EDX Analyses. Figure 1 shows the SEM images of composite materials (i.e., polyethylenimine compounds encapsulated with alginate matrix). Beads have a spherical form with a diameter of approximately 2 mm. Alginate composites reveal well-defined biopolymer internal walls (foils): these results are consistent with previous observations reported by Shu et al. (2014)20 for freeze-dried alginate beads. The hybrid materials are characterized by an opened structure (alveolar structure) with agglomerates of granular PEI derivative. PEI/alginate has a more homogeneous distribution. PEI powder occupies more space in the alveolar structure of the composite than PEI-CS2. In addition, both PEI and PEI-CS2 derivatives (immobilized in alginate matrix) contain fewer pores compared to the alginate bead. In terms of compactness the sorbents can be classified according to PEI/alginate > PEI-CS2/ alginate > alginate. EDX spectra show the presence of C, O, Ca, Na, and Cl common elements in calcium alginate beads. The presence of S element in the alginate matrix identified on the EDX spectrum (Figure 1a) may probably be associated with fucoidan residues (present in the brown seaweed from which alginate was extracted) and the Al is an impurity associated with alginate extraction step. Obviously the composite obtained by alginate encapsulation of PEI-CS2 derivative shows a large peak for S element. Though the EDX analysis is not quantitative, the differences in the intensities of the relevant peaks are large enough to justify this interpretation. Nitrogen detection is usually difficult by EDX technique (being superimposed with the large peaks of C and O elements): it is thus difficult at this stage to chemically identify the presence of additional amine groups associated with the incorporation of the PEI derivative. Alginate and PEI derivatives alginate encapsulated were analyzed by SEM-EDX after sorption. Figure S1 (Supporting Information) shows that metal distribution is homogeneous through the entire mass of the bead (no concentration gradient); this means that all reactive groups remain accessible providing a sufficient time is given for metal ions to migrate to the center of the particle. Copper and cadmium demonstrated similar behavior. Complementary characterization is currently being performed to evaluate acid−base properties of sorbents (titration and interpretation of binding mechanisms). 3.2. Uptake Kinetics. The kinetic profiles (Figure 2) show that Zn(II), Cu(II), and Cd(II) sorption takes place in two phases: (a) a first fast step, lasting for about 2 h, that corresponds to the sorption of 80%, 60%, and 70% of the heavy metal by alginate, PEI/alginate, and PEI-CS2/alginate, respectively, followed by (b) a slow sorption phase that corresponds to the time required to reach equilibrium. The equilibrium time strongly depends on the type of sorbent: while 7 h were sufficient for alginate material, 24 h of contact were

Figure 2. Uptake kinetics for the sorption of Cu(II), Cd(II), and Zn(II) using alginate and PEI derivatives alginate encapsulated (C0, 0.3 mmol L−1; SD, 0.5 g L−1; pH0 4; agitation speed, 150 rpm; 20 ± 1 °C).

necessary for reaching the equilibrium in the case of PEI derivatives that were alginate encapsulated. This is consistent with the differences in porous structures of the materials (see SEM images, Figure 1). SEM analyses showed that substantial differences can be identified in the porous structure of the different sorbents: alginate beads have more “opened” structure than composite materials. In addition, the sorption sites are limited to external reactive groups or those present on the foils of the internal structure, while for composite materials the reactive groups that are present in the core of the PEI derivatives agglomerates are less accessible and require larger contact time for equilibrium and saturation. It is noteworthy that, in most cases, the kinetics profiles are not significantly affected by the type of metal. Figure 3 shows the modeling of uptake kinetics with the Crank equation (resistance to intraparticle diffusion coefficient, eq 1a. The model roughly fits experimental curves: the worst fit was obtained in the case of copper binding with alginate beads; the model fails to correctly fit the curvature of the kinetics (underestimation of sorption performance). The intraparticle diffusion coefficient (De) ranges between 2 × 10−11 and 3.6 × 10−10 m2 min−1 (Table 1). De for copper is generally lower than that for cadmium and even more so to that for zinc. The D


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Figure 4. Sorption efficiency (%) as a function of initial metal concentration for Zn(II), Cu(II), and Cd(II) using alginate and composite materials (SD, 1 g L−1; pH0 4; 20 ± 1 °C).

Figure 3. Modeling of uptake kinetics with the Crank equation (resistance to intraparticle diffusion) (see experimental details in Figure 2).

sorption efficiency: as expected increasing the concentration of the metal reduces the sorption efficiency; however, at low metal concentrations (i.e., 0.6 mmol L−1) the sorbents allow removing more than 80% of Cu(II), and about 70% and 60% for Cd(II) and Zn(II), respectively. Subsequently, these sorbents are efficient for the treatment of dilute solutions. The experiments performed at high metal concentrations (i.e., 6−8 mmol L−1) are more interesting for approaching the maximum sorption capacity at saturation of the sorbent. These data have been used for plotting sorption isotherms (see Figure S2, Supporting Information). The experimental data were analyzed using the Langmuir equation (eq 2). Table 2 reports the values of the Langmuir parameters for the different systems. Regardless of the type of

Table 1. Modeling of Uptake Kinetics for the Sorption of Zn(II), Cd(II), and Cu(II) Using Alginate, PEI/Alginate, and PEI-CS2/Alginate Beads (Crank Equation) sorbent alginate

PEI/alginate

PEI-CS2/alginate

a

metal

De × 10−11 (m2 min−1)

MSDa

Cd Cu Zn Cd Cu Zn Cd Cu Zn

17.8 1.98 36.2 12.3 2.87 10.2 8.40 4.19 15.6

0.018 0.164 0.12 0.019 0.029 0.048 0.013 0.040 0.031

MSD: mean square deviation.

Table 2. Langmuir Isotherm Constants for the Sorption of Zn(II), Cd(II), and Cu(II)

comparison between the different sorbents does not show a significant trend: this means that the incorporation of solid microparticles hardly changes the diffusion properties of composite materials. De values are 3 orders of magnitude lower than the free diffusivity of zinc, cadmium, and copper in water (4.218 × 10−8, 4.314 × 10−8, 4.284 × 10−8 m2 min−1, respectively).21 This fact is evidence for the contribution of diffusion restrictions in the control of sorption kinetics. 3.3. Sorption Isotherms. Batch tests for Zn(II), Cu(II), and Cd(II) in single-metal solutions were performed to estimate the maximum sorption capacity of the sorbents. Figure 4 shows the impact of initial concentration on the

sorbent alginate

PEI/alginate

PEI-CS2/ alginate

E

metal

qm (mmol g−1)

b (L mmol−1)

R2

qm × b (L g−1)

Cd Cu Zn Cd Cu Zn Cd Cu Zn

1.48 1.67 1.00 1.32 1.55 0.99 0.97 1.13 0.64

2.22 6.28 2.12 3.83 14.84 5.53 3.13 21.59 4.13

0.95 0.97 0.97 0.89 0.94 0.91 0.96 0.95 0.92

3.30 10.51 2.13 5.06 17.15 5.46 3.04 24.5 2.64


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Industrial & Engineering Chemistry Research sorbent, the maximum sorption capacities of the materials can be ranked according the series: Cu(II) > Cd(II) > Zn(II). Haug et al. (1961)22 investigated the interactions of alginate with a series of metal cations. They determined and compared their ion exchange properties (between metal cations in solution and protons bound to the biopolymer) and established the following ranking for the affinity of alginate with these metal cations: Pb(II) > Cu(II) > Cd(II) > Ba(II) > Sr(II) > Ca(II) > Co(II) > Ni(II) > Mn(II) > Mg(II). Differences in metal uptake are due to the chemical structure of each metal and the intrinsic properties of the sorbent (i.e., type of reactive groups). More specifically, these properties are generally controlled by the HSAB theory23 that sets that hard acids (low electronegativity and small size) have a preference for hard bases (high electronegativity and low polarizability), and vice versa. Papageorgiou et al. (2006)24 studied heavy metals sorption by calcium alginate beads from Laminaria digitata. The authors observed the following sorption sequence: Pb(II) > Cu(II) > Cd(II) and they correlated the differences in conformation of polyguluronic and polymannuronic blocks to their differences in affinity for selected cations. Metals with low binding strength (i.e., Cd(II)) are bound by polymannuronic blocks, while metals with higher binding strength (i.e., Pb(II)) are adsorbed by all available carboxyl groups (polyguluronic and polymannuronic). Agulhon et al. (2012)25 investigated the influence of the cation and the alginate structure (guluronic, G, and mannuronic, M, contents) on the ionotropic gelation of alginate: they observed that Cu(II) equally interacted with G and M units whereas other gels obtained by Co(II) or Zn(II) ionotropic gelation were more sensitive to the distribution of G and M units. Table 2 compares both maximum sorption capacities (i.e., qm) and affinity coefficients at low residual metal concentration (b). These two parameters do not systematically follow the same variation trend. For example in the case of PEI/alginate the maximum sorption and the affinity coefficient for Cd(II) were 1.32 mmol Cd g−1 and 3.83 L mmol−1, respectively; whereas for Zn(II) the sorption capacity decreased to 0.99 mmol Zn g−1 while the affinity coefficient increased to 5.53 L mmol−1. These changes in the order of parameter values can be associated with the effect of increasing occupancy of sorption sites. In addition the parameter qm × b (L g−1) corresponds to the initial slope of the sorption isotherm. The values are of the same order for Cd(II) and Zn(II) (in the range 2.1−5.5 L g−1) and much lower than the levels reached for Cu(II) (in the range 10.5−24.5 L g−1). This is another evidence of the stronger affinity of the sorbents for Cu(II) compared to Cd(II) and Zn(II). The analysis of isotherm data reveals that alginate beads had the greatest qm and the lowest b, regardless of the metal. This means that in pure synthetic solutions, the incorporation of PEI derivative into the alginate matrix does not improve sorption performance and does not show specific interest. However, the sorption properties of PEI derivatives have been tested under more complex conditions that could approach the composition of real effluents. PEI is expected to improve the sorption property and selectivity over alkaline and alkaline earth metal ions and to decrease the impact of anions. 3.4. Influence of the Composition of the Solution on Sorption Properties. The sorption of zinc, copper, and cadmium was tested in the presence of chloride salts of calcium or sodium and sulfate salts of sodium or potassium. Figure 5 summarizes the sorption efficiency for metal cation recovery

Figure 5. Sorption of Zn(II), Cd(II), and Cu(II) in the presence of different concentrations of calcium. (C0, 2 mmol metal L−1; SD, 1 g L−1; pH0 4; 20 ± 1 °C). Standard deviation ≈ 2%.

using PEI derivatives encapsulated in alginate beads in the presence of calcium salt. Calcium ions compete for sorption sites from alginate, thus the sorption efficiency decreases in the presence of calcium for all alginate sorbents. PEI/alginate composite presents a reduction of its sorption capacity but remains the most efficient among the different sorbents even in the presence of a large excess of salt. The increase in calcium chloride concentration slightly increases the sorption of zinc and copper by PEI/alginate (concentration >0.5 mol L−1). Calcium in solution may contribute to stabilize the structure of the bead (egg-box structure of alginate) and to enable metal binding: functional groups held on PEI replace carboxylic groups of alginate for metal binding. The stability of the sorbent and the efficiency of sorption thus require an optimization in terms of calcium content. The sorption behavior in the presence of monovalent ions was also observed (cation concentration, 0.5 mol L−1). Alginate shows a stronger ion exchange affinity for sodium than potassium. Sodium cations are thus more effective in competing with calcium ions.26 As a consequence, the presence of these monovalent cations in solution involves ion exchange with Ca(II) and contributes to destabilize the structure of the beads. PEI derivative beads after sorption test in the presence of sodium or potassium ions were disintegrated. After Zn(II) and Cd(II) sorption by PEI/alginate composite, for example, only the powder was observed in solution. The degree of inhibition F


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Industrial & Engineering Chemistry Research on sorption by the inorganic ions followed the sequence: Na(I) ≈ K(I) > Ca(II). Liu et al., (2013)27 studied the influence of inorganic ions on Pb(II), Cd(II), Cu(II), and Cr(III) sorption onto titanate nanotubes. The authors observed a decreasing trend in their sorption capacities with increasing concentrations of Ca(II). The inorganic ion competed for sorption sites, decreased the activity of heavy metal ions, and promoted aggregation of sorbent particles. Because the ion-exchange properties of Na(I) with Ca(II) bound to alginate biopolymer, the presence of sodium cation strongly impacts the stability of sorbent capsules (in addition to its potential competitor effect). Different concentrations of calcium (0, 1, 5 and 10%) were added to the solutions containing sodium to determine the minimum amount of Ca(II) (compared to Na(I)) necessary to prevent the degradation of the beads. When an amount of Ca(II) (compared to Na(I)) greater than 5% was added to the solution the beads were not degraded. The mechanical stability of the sorbents is confirmed by the photographs presented in Figure S3 (Supporting Information): the morphological and physical integrity of the beads at the end of the experimental sorption step is maintained. Calcium ions help to stabilize the “egg-box” structure of the alginate matrix. The ion prevents the disruption of the capsule and the release of microparticles of PEI derivatives. Zinc sorption was tested under different sodium concentrations, respecting the calcium percentage for beads stability. Figure 6 confirms a strong reduction of sorption capacity for alginate in the presence of sodium and calcium ions, whereas for PEI sorbents the inhibiting effect was less marked.

Figure 7. Single and multicomponent sorption of Zn(II), Cd(II), and Cu(II) (C0, 2 mmol each metal L−1 for multicomponent sorption and equimolar concentration for single sorption; SD, 1 g L−1; pH0 4; 20 ± 1 °C): (a) ternary and (b) binary systems. Standard deviation ≈ 3%.

sorbent Cu(II) was more efficiently bound compared to Zn(II) and Cd(II): this is consistent with the results obtained in monocomponent sorption systems (in terms of both sorption capacity and affinity coefficients). In binary solutions (Figure 7b), the presence of copper (which remains preferentially sorbed) strongly reduces the capacity of the sorbents to bind either Zn(II) or Cd(II). The efficiency of copper in binary systems was seven times greater than that of zinc by PEI derivatives and 4.5 times that by alginate. Cu(II)/Cd(II) binary systems present a copper removal two and five times greater than cadmium by alginate and PEI derivatives, respectively. On the other hand, in the binary Cd(II)/Zn(II) systems the sorption efficiency for Cd(II) was greater than that for Zn(II) (Table S1, see Supporting Information). This analysis indicates that the sorbents (independently of their type) have a decreasing affinity for the metals according to Cu(II) > Cd(II) > Zn(II). Overall, the sorption capacity of alginate sorbents for Cd(II) and Zn(II) decreases more in ternary systems (competing effect with two other metals) as compared to binary systems. Mohan and Singh (2002)28 studied the sorption of the same heavy metals using activated carbon derived from bagasse. For binary systems, they observed that the presence of other metal ions compete with Cd(II) and Zn(II) ions, reducing their sorption capacity, while Cu(II) was less affected by the

Figure 6. Sorption of Zn(II) in the presence of increasing concentration of sodium and respecting the 10% proportion of calcium to avoid bead degradation (C0, 2 mmol metal L−1; SD, 1 g L−1; pH0 4; 20 ± 1 °C).

3.5. Competitive Sorption in Multicomponent Solutions. Sorption in multicomponent systems involves complex solute−surface interactions.28 The sorption properties (sorption capacities) for Cd(II), Cu(II), and Zn(II) of the different materials have been compared for mono-, bi-, and tricomponent solutions at pH 4 (Figure 7). For ternary solutions (Figure 7a, Table S1) the sorption efficiency for Cu(II), Zn(II), and Cd(II), respectively, were 44.0%, 9.9%, and 20.6% for alginate, 62.1%, 6.8%, and 12.3% for PEI/alginate, and 43.0%, 8.0%, and 9.0% for PEI-CS2/alginate composites. Regardless of the G


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Industrial & Engineering Chemistry Research presence of other ions. Pang et al. (2011)29 studied the competitive sorption of the same heavy metals by PEI-grafted magnetic porous sorbent, but the preferential sorption order observed was Cu(II) > Zn(II) > Cd(II). They correlated the maximum sorption capacity and initial sorption rate with properties of the heavy metals, which may explain the sorption order. In this study, the sorption affinity order was matched with Pauling’s electronegativity order, which are Cu (1.90), Cd (1.69) and Zn (1.65)30 and reverse order of hydrated radius, Cu (4.19 Å), Cd (4.26 Å) and Zn (4.30 Å).31 The data indicate that higher electronegativity and smaller hydrated radius ensure a more favorable sorption through inner sphere surface complexation or sorption reactions. The sorption capacity of cadmium is slightly better than zinc, and the same trend is shown by their respective electronegativities. Allen and Brown, (1995)32 and Yu et al., (2014)33 obtained the same affinity sequence and properties correlation studying copper, cadmium, and zinc sorption on lignite and modified waste phoenix tree’s leaf, respectively. They proposed that more electronegative metal ions will be more strongly attracted to the surface. Gao et al., (2006)16 studied the copper, zinc, and cadmium sorption by PEI/SiO2 particles. The adsorbing ability followed the order Cu(II) > Cd(II) > Zn(II). According to the authors, Cu(II) presents a greater second ionization energy than those of Cd(II) and Zn(II), thus Cu(II) accepts the electron pair of N atoms of ligand more easily than the other ions. To highlight the preference of the sorbents for given metal ions the separation factors (α21) have been calculated for the different systems. Obviously, a separation factor greater than 1 indicates a preference of the sorbent for metal 1. Table 3 cleary

contributes to this effort of solute concentration. In addition, the desorption of the metal opens the route for sorbent recycling that also contributes to make the process more competitive (especially for low-cost metals and complex or expensive sorbents). Metal desorption is frequently operated using either acidic solutions (or alkaline solutions for some specific metal ions such as oxo-metalates, i.e., molybdate or vanadate or complexing agents).34−38 The main drawback at using complexing solutions is associated with the relative difficulty to valorize or precipitate complexed metals and, as far as possible, changing the pH (using either acid solutions or alkaline solutions, depending on the metal) is preferred. Preliminary tests have shown that acidic solutions are quite efficient for heavy metal desorption, and the study of Zn(II), Cu(II), and Cd(II) desorption focused on the use of 0.1 M HCl solutions and acidified water (controlled at pH 2 with HCl; i.e., ∼0.01 M). A reference test was also performed using only demineralized water. Figure 8 shows that demineralized water has a negligible effect on metal desorption (less than 7%). For Zn(II) and Cd(II), the acidity of the eluent has a limited impact on the desorption efficiency, which exceeded 90% for Zn(II) and 80% for Cd(II): the type of sorbent hardly affected the desorption efficiency (in the case of Cd(II) and alginate beads the 0.1 M HCl solution showed significantly better desorption). In the case of Cu(II) the desorption efficiencies are roughly comparable for the different sorbents. It is noteworthy that a substantial decrease in desorption efficiency (from 97 to 100% to 69−78%) is observed when the pH of the solution is increased from 1 to 2. As already reported above (affinities and sorption capacities), the sorbents have a stronger interaction with copper compared with zinc and cadmium; this may explain that a more concentrated HCl solution is necessary for efficient desorption of copper. Another important issue in the design of the desorption process is the ability to recycle the sorbent after metal release. The recycling of the sorbent is a critical criterion. The reuse of the sorbent can be limited by several reasons such as progressive poisoning of the sorption sites, the progressive saturation of sorption sites (when desorption is not complete) or the degradation of the sorbent. Complementary experiments are currently under investigation for evaluating the recycling of the sorbent over 3 or 4 sorption/desorption cycles (not shown). Preliminary results point out the importance of an extensive washing/reconditioning of the sorbent between desorption and sorption cycles to maintain more specifically appropriate pH value for optimum sorption. Insufficient rinsing/washing may lead to low pH values, which, in turn, contribute to decrease metal sorption performance. In addition, the presence of a small concentration of calcium ions may be important for efficient reuse of the sorbent; indeed, calcium ions contribute to ionotropic gelation of the sorbent and prevents partial degradation and sorbent loss during the cycles.

Table 3. Separation Factor binary systems metal 1 + 2

α2

1

Zn + Cd Zn + Cu Cu + Cd

0.35 0.12 3.14


0.55 0.05 10.48


0.29 0.08 7.05

preferred Alginate Cd Cu Cu PEI/Alginate Cd Cu Cu PEI-CS2/Alginate Cd Cu Cu

ternary systems* α21

preferred

0.43 0.14 3.02

Cd Cu Cu

0.52 0.04 11.66

Cd Cu Cu

0.87 0.12 7.57

Cd Cu Cu

*

Calculated between each two heavy metals from multicomponent solution.

confirms the preference order for Cu(II) over both Zn(II) and Cd(II) when α1,2 > 1. These results are in agreement with predictions of the eq 3. In the binary system Zn(II)/Cd(II) whatever the sorbent, a significant preference was observed for Cd(II). The separation factors for ternary systems were obtained by a combination between each two heavy metals. The factors values present the same magnitude and preference order than the separation factors for binary systems. 3.6. Metal Desorption. The desorption of metals from loaded sorbent is important for the recovery of valuable metals or for concentrating hazardous metals before their confinement. All the sorption processes are entitled to contribute to the concentration of a specific target and, as such, the desorption

4. CONCLUSIONS Composite materials have been prepared by immobilization of PEI derivatives (i.e., PEI cross-linked with glutaraldehyde and PEI grafted with CS2 through glutaraldehyde linkage) in alginate capsules. SEM and SEM-EDX analyses have been performed in order to characterize the structure and distribution of PEI derivative microparticles in the composite material; in addition, the SEM-EDX analysis confirmed that the metal ions after saturation of the sorbent are homogeneously distributed in the capsules: all reactive groups appear to be H


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compromise should be found between sorption properties and sorbent stability. The three sorbents have a marked preference for Cu(II) over both Zn(II) and Cd(II) when used for metal recovery from multicomponent solutions, according to the sequence:Cu(II) ≫ Cd(II) > Zn(II). Metal desorption exceeds 80−90% depending on the metal and the sorbent, and in most cases a 0.1 M HCl solution is sufficient for achieving good levels of metal recovery.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04683. Zn(II) mapping of alginate and PEI derivative beads after sorption (Figure S1); heavy metals sorption isotherms (Figure S2); stability of sorbents (Figure S3) and sorption percentages in multicomponent systems (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is part of EU research project (BIOMETALDEMO). The research leading to these results has received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) managed by REA-Research Executive Agency (http://ec.europa.eu/research/rea) under Grant No. 619101. The authors thank Jean-Marie Taulemesse for SEM and SEM-EDX analysis.

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REFERENCES

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Figure 8. Desorption test for Zn(II), Cd(II), and Cu(II) recovery from metal-loaded sorbents. (Sorption: C0, 2 mmol metal L−1; SD, 1 g L−1; pH0 4; 20 ± 1 °C. Desorption: 10 mL of eluent, 24 h).

accessible. These materials have been successfully tested for the sorption of heavy metal cations (i.e., Cu(II), Zn(II), and Cd(II)). In pure synthetic solutions the incorporation of PEI derivatives does not improve sorption properties. The incorporation of PEI derivatives contributes to a decrease in mass transfer properties: uptake kinetics are slightly less favorable; though the determination of the diffusion coefficients did not show substantial differences. This means that the effect of the incorporation of sorbent microparticles in the composite alginate beads was much less marked than expected. In more complex solutions (i.e., containing chloride or sulfate anions, or in the presence of alkaline and alkaline-earth metal ions) the hybrid materials show higher sorption capacities. The presence of high concentrations of Na(I) leads to Ca(II) ion exchange and particle degradation of alginate capsules: adding calcium in the solution (at a concentration close to 5−10% of Na(I) concentration) contributes to stabilize the sorbents; a I


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