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Metal ions recovery with alginic acid coupled to ultrafiltration membrane
European Polymer Journal 45 (2009) 573–581
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Metal ions recovery with alginic acid coupled to ultrafiltration membrane Antonio Maureira, Bernabé L. Rivas * Polymer Department, Faculty of Chemistry, University of Concepción, Casilla 160-C, Concepción, Chile
a r t i c l e
i n f o
Article history: Received 3 October 2008 Accepted 14 November 2008 Available online 21 November 2008
Keywords: Alginic acid Functional polymer Water-soluble polymer complexes Metal ions Membranes
a b s t r a c t Alginic acid (AA) is a natural polysaccharide derived from brown algae. Naturally AA is present in cellular wall forming insoluble complexes with ions as calcium, magnesium, and sodium. This polymer is composed of uronic acids as D-manuronic acid and L-guloronic acid (units differing in C5 configuration) which are disposed in blocks or alternating on principal chain due its spatial configuration. In its structure only hydroxy and carboxylic acid are present, with a pKa alginic acid = 3.45. At pH = 4.3 this polymer is completely soluble in water. Metal ion retention was evaluated using liquid-phase polymer-based retention (LPR) technique elution method, and metal ions studied were Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+ at different pH and filtration factor. A high efficiency for all metal ions at all pH was reveled with a maximum at pH = 4.5 of 100% of majority of metal ions. To evaluate the maximum retention capacity (MRC) of AA, LPR technique concentration method was used. Metal ion/polymer ratio from 48 to 325 mg/g for Zn2+ and Ag+ were studied, respectively. Homopolymer and polymer–metal ion complexes were characterized using FT-IR, Far-IR spectroscopy, dynamic light scattering (DLS), and thermogravimetric analysis. FT-IR revealed relevant shifts between AA and PMC, which involve carboxylic acid, hydroxy, and ether groups. DLS shows non-pH-dependent sizes of alginic acid–silver complexes. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Pollution of waterbodies by heavy metal due the irrational disposal of industrial and domestic water threatens all kind of inhabiting organism. Therefore, if these waters return to waterways must be diminished of it heavy metal charge, because these pollutants cause several damage to aquatic life, and can be finally used in agriculture or human uses carry out all contamination associated problems. As a result of operations on the discharge of the industrial wastewater containing heavy metals capable to enter the natural watercourses imposed by authorities responsible for control of the environmental pollution, many companies have already developed more or less cheap but efficient methods to reduce the metal content of the commercial effluences. Among the most popular appeared these, which are based on the chemical precipitation, * Corresponding author. Tel.: +56 41 2203373; fax: +56 41 2245974. E-mail address: [email protected] (B.L. Rivas). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.11.021
electrolytic methods, adsorption onto activated carbon, membrane process, ion exchange using different chelating resins and fibers. Biosorption (name of the phenomenon of removing heavy metals in water by microorganisms or their metabolic products by various mechanisms), of heavy and/or radioactive metals can be considered as an alternative technology competitive with respect to conventional technologies applied in the wastewater treatment. To recover the aforementioned metals from aqueous solutions it uses cheap adsorption materials, mainly of the natural origin [1,2]. Different studies have used whole algae to metal ion sorption basically as a pretreatment, where dead cells have a better adsorption capability than live cells. Alginic acid (AA) is a natural polysaccharide straightchain, hydrophilic, colloidal derived from brown algae. Naturally AA is present in cellular wall forming insoluble complexes with ions as calcium, magnesium, and sodium. These polymers are composed of uronic acids as D-manuronic acid and L-guloronic acid (units differing in C5 configuration), more specifically it is an polyuronic acid typically
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arranged in three combinations of disaccharide subunits: a-L-(1-4) guluronic–guluronic (GG), a-L-(1-4) mannuronic–guluronic (MG), b-D-(1-4) mannuronic–mannuronic (MM), which are disposed in blocks or alternating on principal chain due its spatial configuration, and its proportion in polymer chain depends on type of algae source. The presence of carboxylic acid groups in both monomeric units makes possible it interaction with different low molecular weight species, and in this sense has been studied its complexation ability point out ions with nutritional value or heavy metal ion present in aqueous media for water purification, then several authors have proposed interaction models between AA and specific metal ion [3–5] and others have studied the removal of different metal ions with natural and modified (cross-linked, oxidized, magnetically-modified) AA with high efficiency [6–12]. Recent advances of AA point out to synthesis of fibers to manufacture dressing materials for medical applications [13–14]. Commonly physicochemical methods as precipitation, solvent extraction, adsorption, ion exchange, etc., are the most effective methods for removal of these pollutants from aquatic or aqueous systems. The efficient and selective separation of metal ions can be achieved by using water-soluble polymers, WSPs, in combination with membrane filtration [15–17]. This technique termed liquid-phase polymer-based retention (LPR) is based on the separation of metal ions bound to WSP with chelating groups, which is the term used to refer to polychelatogens from non-complexed metal ions [18–19]. This technique has found application in the recovery of metal ions from diluted solutions on both an analytical and technical scale. The general principle of this technique is to add WSP binding reagents to a multi-component solution so that these agents will form macromolecular compounds only with the target ions. Thus, the size of the metal ion must be increased significantly whereas the size of the non-target species would remain unchanged. If such a solution were then passed through a ultrafiltration membrane, the target metal ion would be separated from the non-target species. In the LPR process, the membrane represents a barrier that retains all ions bound to the polymer reagent, allowing permeation of all unbound ions. The separation process will be successful if the polymer employed meet the following requirements: high affinity towards the target metal ion, inactivity towards the non-target metal ion, high molecular mass, possibility of regeneration, chemical and
mechanical stability, low toxicity, and low cost. Considering these requirements, basic, neutral, and acid hydrophilic polymers have been designed and investigated respect to the analytical determination of metal ions [20–22]. These WSP’s can be synthesized by different routes such as radical, cationic, and spontaneous polymerization, but the most usual synthetic procedures are addition polymerization, especially radical polymerization, and by functionalizing polymer backbone through polymer-analogous reactions. The polychelatogens may be homo- and co-polymers, and can contain one or more ligand or coordinating groups. These groups are placed at the backbone or at the side chain, directly or through a spacer group [23–24]. The aim of this paper was focused on study of heavy metal ion recovery using a natural polymer coupled to ultrafiltration membranes. Determination of better condition for metal ion retention in aqueous media for AA, maximum retention capacity (MRC) determination for all ions study, and chemical and physical characterization to propose a interaction model between this natural polymer and mono- and di-valent metal ions Scheme 1. 2. Materials Alginic acid from brown algae was obtained from Sigma, FW = 176.1 g/mol. All metal ions were purchased from Merck: Co(NO3)2 6H20, 99%, p.a.; Ni(NO3)2 6H20, 99%, p.a.; Cu(NO3)2 3H20, 99%, p.a.; Zn(NO3)2 6H20, extra pure; AgNO3, 99.8%, p.a.; Cd(NO3)2 4H20, 99%, p.a.; Pb(NO3)2, 99%, p.a. Sodium hydroxide (NaOH, Merck), nitric acid 70% (HNO3, Caledon) were used to adjust pH. 2.1. Study of physical properties of alginic acid Molecular weight was determined in a Perkin-Elmer chromatographer Series 200, using a PLaquagelOH column at 20 °C, dissolving 50 mg of AA in 5 ml of bidistilled water at pH 5.0. In order to estimate the particle size of natural polymer and polymer–metal complexes in aqueous media, this parameter was evaluated in a Nanoparticle Size Analyzer (Brookhaven 90Plus) at different pH from AA and its water-soluble complexes with Cu2+, Ag+, and Pb2+ obtained by mixing 1.0 102 mmol/l of AA and 2.5 104 mmol of metal ion in a WTW inoLab pH level 1 pH meter. Thermogravimetric analysis were measured in a Q50 Thermal Analysis (TA Instruments) with a heating rate of 20 °C/min and a flux rate of 60 ml/min of N2 (g).
Scheme 1. General structure of alginic acid with metal ions.
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2.2. Study of metal ion retention by LPR Technique (washing method) To ensure a high level of ligand sites, the co-polymer repeat unit: metal ion ratio (in mol) was 40: 1. Then, 20.0 ml of a solution containing 1.0 102 mmol/l of a water-soluble homopolymer (0.1160 g of fraction >100 kDa) and 2.5 104 M of metal ions (5 mmol of each metal ion or 5, 10, and 15 meq for mono- and di-valent metal ions, respectively) are placed into the solution cell provided with a ultrafiltration membrane with a molar mass cut off, MMCO, of 10 kDa (Millipore, Amicon). Metal ions studied at pH 3, 5, and 7 were Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+. The pH was adjusted with dilute HNO3 and NaOH. A washing solution (water at pH = 4.0, 4.5, and 5.0, depending on the metal ion) was passed under constant pressure (3.5 bar of N2) from the reservoir through the cell solution (2–4 drops by sec). As the in- and out flux are rapidly equaled, the initial volume (20.0 ml) is kept constant during the experiment. Ten fractions of 20 ml were collected. Each fraction was collected in graduated tubes, and the corresponding metal ion concentration was determined in a Unicam Solaar M5 series Atomic Absorption Spectrometer (AAS) Scheme 2. The binding and elution processes may be formulated as a chemical reaction, where reversible reaction in combination with an irreversible transfer of metal ions across the membrane is responsible for metal ion retention. Retention, R(Z), is defined for any species as the fraction per unit of the species under study remaining in the cell during filtration. The metal ion (M) remaining in the cell during filtration consists of the sum of the metal ion bound to the polymer chain and the metal ion free in the solution. These values are a function of F, i.e. the extent of the filtration run constant during filtration; retention may be formulated as follows:
RðZÞ ¼
575
cfree ðZÞ þ cbound ðZÞ cinit
where cfree is the concentration of M free in the solution, cbound is the concentration of M bound to the polymer, and cinit is the initial metal concentration. Z is the valence of the metal ion considered. Retention can be plotted versus the filtration factor, and a retention profile is obtained.The filtration factor (Z) is defined as the volume ratio of the filtrate (V f ) versus volume in the cell (V c ). V c is kept constant at 20 ml. Z is also a qualitative measurement of the strength of the interaction between the ligand group and the metal ion.
Z¼
Vf Vc
2.3. Determination of maximum retention capacity (MRC) by LPR Technique (concentration method) To obtain polymer–metal complexes, the liquid-phase polymer-based retention (LPR) technique by concentration method was used. This method consists in passing a metal ion solution, known concentration, through a solution of water-soluble polymer (20 ml), keeping a constant volume. To develop the enrichment (or concentration) method only water-soluble polymer are placed into the ultrafiltration cell and the metal ion solution are placed in the reservoir. When metal ions pass through ultrafiltration cell the macromolecules uptake the metal ions until saturation and the metal ion non-retained are collected in 5 and 10 ml assay tubes and quantified by AAS. As those polymer–metal ion complexes will be used to determine its activity as antibacterial, an elution with 100 ml of twice-distilled water was made after each one MRC experiment to eliminate all the metal ions not bounded to polymer in order to only
Scheme 2. LPR arrangement (1) filtration cell with polymeric and metal ion solution, (2) membrane filtrate, (3) magnetic stirrer, (4) pressure trap, (5) selector, (6) reservoir with water.
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observe the polymer–metal complex’s effect. The same polymer fraction, >100 kDa, and membrane, 10 kDa, were employed to this study. A blank experience with metal ions and without polymer is needed to determine the effect of ultrafiltration membrane in metal ion retention. The amount of metal ion bound to the water-soluble polymer was calculated by determination of the difference between the concentration curve slope and the blank curve.The MRC are calculated by the relationship:
MRC ¼
MV Pm
where, MRC is expressed as milligrams of metal ion retained per gram of polymer (or expressed in mole), M is the metal ion concentration in mg/l, V is the filtrate volume through the membrane free of metal ion in l), and Pm is the mass of polymer in g. 3. Results and discussion Alginic acid (AA) is a natural polymer derived from brown algae. In its structure are present carboxylic acid, ester, and hydroxy functional groups as chelating sources,
chemically these groups could be classified according to Pearson´s principle as hard bases. This polymer as acid and its salts with divalent ions gelificate in water, meanwhile its salts from monovalent ions are soluble in water upper pKa. pKa of AA has been analyzed in several studies and the value ranged between 3.5 and 5.0. AA analyzed was dissolved in bidistilled water generating an opaque solution of pH = 3.5, upper pH = 4.3. This polysaccharide is completely soluble in aqueous media (transparent solution), it means that carboxylic acid groups must be as carboxylate form to produce an adequate solubilization. The presence of carboxylic and hydroxy groups offers stable media to stabilize cations in its structure. It is well known, naturally AA form insoluble complexes with several cations as calcium, magnesium, and sodium and several study indicate the gel formation or precipitation with different di-valent metal ions where metal ions will be act as crosslinker reagent between different polymer chains [3–5]. To evaluate the retention of a mixture of different metal ions 0.0704 g (2 mmol) of AA were dissolved in 20 ml of bidestilled water and 0.08 mmol of metal ion were added to AA solution previously prepared, pH was adjusted by adding NaOH until obtain a solution where no gelification
Fig. 1. Retention (%) at Z = 10 for alginic acid for different metal ions at pH = 4.0, 4.5, and 5.0.
Fig. 2. Maximum retention capacity (MRC) curves for Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+.
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bility of metal ions [pH 5.0 for Cu2+ and Pb2+, and pH 7.0 for Ag+, Co2+, Ni2+, Zn2+ and Cd2+]. For Cu2+, Cd2+ and Zn2+, and slightly for Ag+, Ni2+ and Co2+ the complexes form gels at low metal ion concentration in ultrafiltration cell, only Pb2+ stays soluble and allows determinate its MRC (see Fig. 2). Table 1 shows the MRC determined and concentration of polymer: metal ion. Assuming a quantitative retention of different metal ions, the enrichment factor (E) was determined according to the following relationship:
processes is observed. For the most common carboxylic polymer the higher interaction occurs upper pH > 5 (above pKa), specially for those cations that no precipitate as M(OH)x, as Ag+, Co2+, Ni2+, Cd2+ and Zn2+, and how AA has a higher solubility in water and in hydroxy media, these facts will suppose that AA has a similar behavior. Due to poor solubility of AA lower pH 4.0, the speed run is very low (lower than that 1 drop by min) and it application in these conditions means that this technique is inconvenient. These experiments were developed from pH 4.0. Fig. 1 shows results for elution experiences, this figure shows a high interaction, close to 100% for Cu2+ at pH 4, and approx. 100% retention for different metal ions with AA from pH 4.5 (when polymer is completely soluble). Then a slightly pH dependence and high stability of polymer metal ion complexes. This high polymer: metal ion ratio allow obtain water soluble PMC, without precipitation in ultrafiltration cell. Studies at higher pH do not give additional information therefore, its development was not necessary. Maximum retention capacity (MRC) for mono- and divalent metal ions was evaluated at pH of maximum solu-
E¼
ðP CÞ M
where, P is polymer concentration, (g/l), C is maximum capacity of the polymer, (mg/g), and M is initial concentration of the metal salt, (mg/l). Based on previous studies well detailed, about characterization of AA, which indicate the interaction of carboxylic acid, hydroxy, and ester groups with different metal ions [25–26], the FT-IR spectra for AA and PMC synthesized were analyzed. Fig. 3 shows significant differences between these species as the reduction intensity of m(COO)sym of AA at
Table 1 Maximum retention capacity (MRC), pH study, and TGA values of AA for Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+. Sample
AA AA–Ag+ AA–Co2+ AA–Ni2+ AA–Cu2+ AA–Zn2+ AA–Cd2+ AA–Pb2+
MRC
TGA
pH
mg/g
meq/g
E
TD50
dm/dTmax (%/°C)
Residual mass (%)
– 7 7 7 5 7 7 5
– 61.67 1.66 2.08 2,50 79.1 60.4 49.86
– 0.57 0.02* 0.02* 0.02* 0.61* 0.27* 0.12
– 2.06 – – – – – 1.12
284 465 321 326 295 284 401 465
185 183 237 240 244 177 271 191
29 48 24 20 32 28 26 47
TDT50: Temperature at which 50% of weight is loss. dm/dTmax (%/°C): Temperature for maximum weight variation. * Relationship for polymer: metal ion precipitation.
Fig. 3. FT-IR of alginic acid (AA) and its complexes with Ag+, Co2+, Cu2+, Zn2+, Cd2+, and Pb2+.
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Fig. 4. Far FT-IR of alginic acid (AA) and its complexes with Ag+, Co2+, Cu2+, Zn2+, Cd2+, and Pb2+.
Fig. 5. TGA of AA, and AA-complex of Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+.
1730 cm1, an increase intensity for m(COO)sym (1620 cm1) and m(COO)as (1460 cm1) in PMC, dCOH at 1250 cm1 are not present in PMC, mCO and d(CCO) at
1100 and 1030 cm1, respectively, reduced its intensity in PMC. The existence of both sugar moiety anomers can be evidenced by analysis of the spectral range of 820 cm1
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Fig. 6. Weight derivate (dm/dTmax) v/s Temperature (°C) for AA, and its metal ion complexes with Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+.
Scheme 3. (a) Size variation for a polyacid with an increase of pH, (b) interchain interaction for polymer–metal ion complexes, (c) intrachain interaction for polymer–metal ion complexes.
(dCH b-anomer) to 870 cm1 (dCH a-anomer), which are present in AA and PMC. Meanwhile, far FT-IR spectra show slightly differences between AA and PMC, at 89 cm1 it is possible observe the Ag0 signal due to reduction of Ag+ retained for AA (see Fig. 4). All spectra were recorded on a Magna Nicolet 550 spectrophotometer.
3.1. Thermogravimetric analysis Thermogravimetric analysis (see Figs. 5 and 6) shows differences between AA and metal ion complexes weight loss. AA shows four decomposition steps; first step for dehydratation, and from second the decomposition
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Fig. 7. Particle size of alginic acid and its polymer–metal ion complexes changing pH, with a constant polymer : metal ion ratio of 40:1.
corresponding to biopolymer (higher value at 185 °C). This value keeps practically constant for those metal ion complexes, which have complete CMR curves, especially for Pb2+ (191 °C) and Ag+ (183 °C), which moreover shows the higher TDT50 Pb2+ at difference of other complexes shows a decomposition pattern similar to AA, this means that Pb2+ do not affect significantly the decomposition of carboxylic groups to carbon dioxide. Complexes with Co2+, Ni2+, Cu2+, Cd2+, and Zn2+ form gels at low concentration of metal ions and shows the higher shift on decomposition temperatures (dm/dTmax), corroborating the higher inter-action between these metal ions and AA. The residual products of metal ion complexes (possibly oxidized metals or carbonates) show expected values, which are comparable with the values of MRC (see Table 1).
Results are shown in Fig. 7, it can be observed the expected behavior for AA, where highest sizes were found at pH 4.0, and lower size is achieved between pH 5.0 and 6.0. Polymer–metal ion complexes show a similar behavior to AA, only at pH 4.5 the inter-chain interaction can explain clearly the increase of diameter in these polymer– metal ion complexes (Scheme 3b). Alginic– and alginate– silver complexes show a behavior non-pH-dependent with sizes between 3 times (pH 6.0) up to 10 times (pH 4.0) lower than AA. These results could be explained by a intrachain polymer–metal ion interaction (Scheme 3c), but these results were not expected due to its general characteristics as atomic radii (1.44 Å), radii of copper (1.28 Å) and lead (1.75 Å), and it electronic configuration is similar to copper, throughout other experiences must be developed in the near future to explain this result.
3.2. Study of macromolecular dimensions in aqueous media 4. Conclusions Polyelectrolytes show a macromolecular arrangement in aqueous media dependent of pH, ionic strength, and/or counterion. For AA with a known pKa (3.5–5.0), this behavior can be observed clearly by analyzing it size changing pH and counterion. For all polyeletrolytes its maximum size in aqueous media is achieved for completely protonated and deprotonated state and repulsions between functional groups on polymer chain are maximum, and the lower size is achieved when the polyelectrolyte is partially deprotonated and the attracting forces (as hydrogen bound) make to polymer folding, but always the polymer maintain its solubility. For polymer–metal ion complexes the size will depend if this is product of inter- or intrachain interaction, for inter-chain interaction this size is increased, meanwhile the intra-chain interaction drives to a polymer folding (see Scheme 3a). Solutions, in the same condition of the study of polymer–metal ions interaction (1.0 102 M of AA, and 2.5 104 M of metal ion) were analyzed by dynamic light scattering, as only Ag+- and Pb2+-complexes which were completely soluble in MRC experiences. Cu2+ was studied to compare with those insoluble complexes. All data reported were pooled mean and standard deviation of four repeated experiments.
Alginic acid is a natural polymer derived from brown algae composed for uronic acids and is insoluble in water in its protonated form (is present as gel), this condition makes unadequate the LPR technique to remove metal ions with AA. Alginate, non-protonated form of AA, is completely soluble in water from pH 4.3. LPR technique, elution method
Scheme 4. Interaction mechanism proposed for alginate–metal ion complexes.
A. Maureira, B.L. Rivas / European Polymer Journal 45 (2009) 573–581
allows the complete recovery (100%) of mono-, and di-valent metal ions at pH = 5.0. MRC indicate that only Pb2+ complex is completely soluble in water solution, others form gels at different concentrations. Thermogravimetric analysis showed similar behavior between alginic acid and metal ion complexes, with differences at TDT50 higher for Pb2+ and Ag+, and similar for Pb2+ and Ag; AA forms gels with metal ion as Co2+, Ni2+, Cu2+, Zn2+, and Cd2+, complexes which showed a higher decomposition temperature than that soluble polymers and PMC. FT-IR spectroscopy allowed establish the functional groups of AA involved on polymer metal ion generation, and these are carboxylic acid, ester, and hydroxy groups, see Scheme 4.
[10]
[11]
[12]
[13]
[14]
[15]
Acknowledgments [16]
Authors thank to Fondecyt No 1070542. A.M. thanks to CIPA the post-doctoral Grant. [17]
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Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Metal ions recovery with alginic acid coupled to ultrafiltration membrane Antonio Maureira, Bernabé L. Rivas * Polymer Department, Faculty of Chemistry, University of Concepción, Casilla 160-C, Concepción, Chile
a r t i c l e
i n f o
Article history: Received 3 October 2008 Accepted 14 November 2008 Available online 21 November 2008
Keywords: Alginic acid Functional polymer Water-soluble polymer complexes Metal ions Membranes
a b s t r a c t Alginic acid (AA) is a natural polysaccharide derived from brown algae. Naturally AA is present in cellular wall forming insoluble complexes with ions as calcium, magnesium, and sodium. This polymer is composed of uronic acids as D-manuronic acid and L-guloronic acid (units differing in C5 configuration) which are disposed in blocks or alternating on principal chain due its spatial configuration. In its structure only hydroxy and carboxylic acid are present, with a pKa alginic acid = 3.45. At pH = 4.3 this polymer is completely soluble in water. Metal ion retention was evaluated using liquid-phase polymer-based retention (LPR) technique elution method, and metal ions studied were Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+ at different pH and filtration factor. A high efficiency for all metal ions at all pH was reveled with a maximum at pH = 4.5 of 100% of majority of metal ions. To evaluate the maximum retention capacity (MRC) of AA, LPR technique concentration method was used. Metal ion/polymer ratio from 48 to 325 mg/g for Zn2+ and Ag+ were studied, respectively. Homopolymer and polymer–metal ion complexes were characterized using FT-IR, Far-IR spectroscopy, dynamic light scattering (DLS), and thermogravimetric analysis. FT-IR revealed relevant shifts between AA and PMC, which involve carboxylic acid, hydroxy, and ether groups. DLS shows non-pH-dependent sizes of alginic acid–silver complexes. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Pollution of waterbodies by heavy metal due the irrational disposal of industrial and domestic water threatens all kind of inhabiting organism. Therefore, if these waters return to waterways must be diminished of it heavy metal charge, because these pollutants cause several damage to aquatic life, and can be finally used in agriculture or human uses carry out all contamination associated problems. As a result of operations on the discharge of the industrial wastewater containing heavy metals capable to enter the natural watercourses imposed by authorities responsible for control of the environmental pollution, many companies have already developed more or less cheap but efficient methods to reduce the metal content of the commercial effluences. Among the most popular appeared these, which are based on the chemical precipitation, * Corresponding author. Tel.: +56 41 2203373; fax: +56 41 2245974. E-mail address: [email protected] (B.L. Rivas). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.11.021
electrolytic methods, adsorption onto activated carbon, membrane process, ion exchange using different chelating resins and fibers. Biosorption (name of the phenomenon of removing heavy metals in water by microorganisms or their metabolic products by various mechanisms), of heavy and/or radioactive metals can be considered as an alternative technology competitive with respect to conventional technologies applied in the wastewater treatment. To recover the aforementioned metals from aqueous solutions it uses cheap adsorption materials, mainly of the natural origin [1,2]. Different studies have used whole algae to metal ion sorption basically as a pretreatment, where dead cells have a better adsorption capability than live cells. Alginic acid (AA) is a natural polysaccharide straightchain, hydrophilic, colloidal derived from brown algae. Naturally AA is present in cellular wall forming insoluble complexes with ions as calcium, magnesium, and sodium. These polymers are composed of uronic acids as D-manuronic acid and L-guloronic acid (units differing in C5 configuration), more specifically it is an polyuronic acid typically
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arranged in three combinations of disaccharide subunits: a-L-(1-4) guluronic–guluronic (GG), a-L-(1-4) mannuronic–guluronic (MG), b-D-(1-4) mannuronic–mannuronic (MM), which are disposed in blocks or alternating on principal chain due its spatial configuration, and its proportion in polymer chain depends on type of algae source. The presence of carboxylic acid groups in both monomeric units makes possible it interaction with different low molecular weight species, and in this sense has been studied its complexation ability point out ions with nutritional value or heavy metal ion present in aqueous media for water purification, then several authors have proposed interaction models between AA and specific metal ion [3–5] and others have studied the removal of different metal ions with natural and modified (cross-linked, oxidized, magnetically-modified) AA with high efficiency [6–12]. Recent advances of AA point out to synthesis of fibers to manufacture dressing materials for medical applications [13–14]. Commonly physicochemical methods as precipitation, solvent extraction, adsorption, ion exchange, etc., are the most effective methods for removal of these pollutants from aquatic or aqueous systems. The efficient and selective separation of metal ions can be achieved by using water-soluble polymers, WSPs, in combination with membrane filtration [15–17]. This technique termed liquid-phase polymer-based retention (LPR) is based on the separation of metal ions bound to WSP with chelating groups, which is the term used to refer to polychelatogens from non-complexed metal ions [18–19]. This technique has found application in the recovery of metal ions from diluted solutions on both an analytical and technical scale. The general principle of this technique is to add WSP binding reagents to a multi-component solution so that these agents will form macromolecular compounds only with the target ions. Thus, the size of the metal ion must be increased significantly whereas the size of the non-target species would remain unchanged. If such a solution were then passed through a ultrafiltration membrane, the target metal ion would be separated from the non-target species. In the LPR process, the membrane represents a barrier that retains all ions bound to the polymer reagent, allowing permeation of all unbound ions. The separation process will be successful if the polymer employed meet the following requirements: high affinity towards the target metal ion, inactivity towards the non-target metal ion, high molecular mass, possibility of regeneration, chemical and
mechanical stability, low toxicity, and low cost. Considering these requirements, basic, neutral, and acid hydrophilic polymers have been designed and investigated respect to the analytical determination of metal ions [20–22]. These WSP’s can be synthesized by different routes such as radical, cationic, and spontaneous polymerization, but the most usual synthetic procedures are addition polymerization, especially radical polymerization, and by functionalizing polymer backbone through polymer-analogous reactions. The polychelatogens may be homo- and co-polymers, and can contain one or more ligand or coordinating groups. These groups are placed at the backbone or at the side chain, directly or through a spacer group [23–24]. The aim of this paper was focused on study of heavy metal ion recovery using a natural polymer coupled to ultrafiltration membranes. Determination of better condition for metal ion retention in aqueous media for AA, maximum retention capacity (MRC) determination for all ions study, and chemical and physical characterization to propose a interaction model between this natural polymer and mono- and di-valent metal ions Scheme 1. 2. Materials Alginic acid from brown algae was obtained from Sigma, FW = 176.1 g/mol. All metal ions were purchased from Merck: Co(NO3)2 6H20, 99%, p.a.; Ni(NO3)2 6H20, 99%, p.a.; Cu(NO3)2 3H20, 99%, p.a.; Zn(NO3)2 6H20, extra pure; AgNO3, 99.8%, p.a.; Cd(NO3)2 4H20, 99%, p.a.; Pb(NO3)2, 99%, p.a. Sodium hydroxide (NaOH, Merck), nitric acid 70% (HNO3, Caledon) were used to adjust pH. 2.1. Study of physical properties of alginic acid Molecular weight was determined in a Perkin-Elmer chromatographer Series 200, using a PLaquagelOH column at 20 °C, dissolving 50 mg of AA in 5 ml of bidistilled water at pH 5.0. In order to estimate the particle size of natural polymer and polymer–metal complexes in aqueous media, this parameter was evaluated in a Nanoparticle Size Analyzer (Brookhaven 90Plus) at different pH from AA and its water-soluble complexes with Cu2+, Ag+, and Pb2+ obtained by mixing 1.0 102 mmol/l of AA and 2.5 104 mmol of metal ion in a WTW inoLab pH level 1 pH meter. Thermogravimetric analysis were measured in a Q50 Thermal Analysis (TA Instruments) with a heating rate of 20 °C/min and a flux rate of 60 ml/min of N2 (g).
Scheme 1. General structure of alginic acid with metal ions.
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2.2. Study of metal ion retention by LPR Technique (washing method) To ensure a high level of ligand sites, the co-polymer repeat unit: metal ion ratio (in mol) was 40: 1. Then, 20.0 ml of a solution containing 1.0 102 mmol/l of a water-soluble homopolymer (0.1160 g of fraction >100 kDa) and 2.5 104 M of metal ions (5 mmol of each metal ion or 5, 10, and 15 meq for mono- and di-valent metal ions, respectively) are placed into the solution cell provided with a ultrafiltration membrane with a molar mass cut off, MMCO, of 10 kDa (Millipore, Amicon). Metal ions studied at pH 3, 5, and 7 were Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+. The pH was adjusted with dilute HNO3 and NaOH. A washing solution (water at pH = 4.0, 4.5, and 5.0, depending on the metal ion) was passed under constant pressure (3.5 bar of N2) from the reservoir through the cell solution (2–4 drops by sec). As the in- and out flux are rapidly equaled, the initial volume (20.0 ml) is kept constant during the experiment. Ten fractions of 20 ml were collected. Each fraction was collected in graduated tubes, and the corresponding metal ion concentration was determined in a Unicam Solaar M5 series Atomic Absorption Spectrometer (AAS) Scheme 2. The binding and elution processes may be formulated as a chemical reaction, where reversible reaction in combination with an irreversible transfer of metal ions across the membrane is responsible for metal ion retention. Retention, R(Z), is defined for any species as the fraction per unit of the species under study remaining in the cell during filtration. The metal ion (M) remaining in the cell during filtration consists of the sum of the metal ion bound to the polymer chain and the metal ion free in the solution. These values are a function of F, i.e. the extent of the filtration run constant during filtration; retention may be formulated as follows:
RðZÞ ¼
575
cfree ðZÞ þ cbound ðZÞ cinit
where cfree is the concentration of M free in the solution, cbound is the concentration of M bound to the polymer, and cinit is the initial metal concentration. Z is the valence of the metal ion considered. Retention can be plotted versus the filtration factor, and a retention profile is obtained.The filtration factor (Z) is defined as the volume ratio of the filtrate (V f ) versus volume in the cell (V c ). V c is kept constant at 20 ml. Z is also a qualitative measurement of the strength of the interaction between the ligand group and the metal ion.
Z¼
Vf Vc
2.3. Determination of maximum retention capacity (MRC) by LPR Technique (concentration method) To obtain polymer–metal complexes, the liquid-phase polymer-based retention (LPR) technique by concentration method was used. This method consists in passing a metal ion solution, known concentration, through a solution of water-soluble polymer (20 ml), keeping a constant volume. To develop the enrichment (or concentration) method only water-soluble polymer are placed into the ultrafiltration cell and the metal ion solution are placed in the reservoir. When metal ions pass through ultrafiltration cell the macromolecules uptake the metal ions until saturation and the metal ion non-retained are collected in 5 and 10 ml assay tubes and quantified by AAS. As those polymer–metal ion complexes will be used to determine its activity as antibacterial, an elution with 100 ml of twice-distilled water was made after each one MRC experiment to eliminate all the metal ions not bounded to polymer in order to only
Scheme 2. LPR arrangement (1) filtration cell with polymeric and metal ion solution, (2) membrane filtrate, (3) magnetic stirrer, (4) pressure trap, (5) selector, (6) reservoir with water.
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observe the polymer–metal complex’s effect. The same polymer fraction, >100 kDa, and membrane, 10 kDa, were employed to this study. A blank experience with metal ions and without polymer is needed to determine the effect of ultrafiltration membrane in metal ion retention. The amount of metal ion bound to the water-soluble polymer was calculated by determination of the difference between the concentration curve slope and the blank curve.The MRC are calculated by the relationship:
MRC ¼
MV Pm
where, MRC is expressed as milligrams of metal ion retained per gram of polymer (or expressed in mole), M is the metal ion concentration in mg/l, V is the filtrate volume through the membrane free of metal ion in l), and Pm is the mass of polymer in g. 3. Results and discussion Alginic acid (AA) is a natural polymer derived from brown algae. In its structure are present carboxylic acid, ester, and hydroxy functional groups as chelating sources,
chemically these groups could be classified according to Pearson´s principle as hard bases. This polymer as acid and its salts with divalent ions gelificate in water, meanwhile its salts from monovalent ions are soluble in water upper pKa. pKa of AA has been analyzed in several studies and the value ranged between 3.5 and 5.0. AA analyzed was dissolved in bidistilled water generating an opaque solution of pH = 3.5, upper pH = 4.3. This polysaccharide is completely soluble in aqueous media (transparent solution), it means that carboxylic acid groups must be as carboxylate form to produce an adequate solubilization. The presence of carboxylic and hydroxy groups offers stable media to stabilize cations in its structure. It is well known, naturally AA form insoluble complexes with several cations as calcium, magnesium, and sodium and several study indicate the gel formation or precipitation with different di-valent metal ions where metal ions will be act as crosslinker reagent between different polymer chains [3–5]. To evaluate the retention of a mixture of different metal ions 0.0704 g (2 mmol) of AA were dissolved in 20 ml of bidestilled water and 0.08 mmol of metal ion were added to AA solution previously prepared, pH was adjusted by adding NaOH until obtain a solution where no gelification
Fig. 1. Retention (%) at Z = 10 for alginic acid for different metal ions at pH = 4.0, 4.5, and 5.0.
Fig. 2. Maximum retention capacity (MRC) curves for Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+.
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bility of metal ions [pH 5.0 for Cu2+ and Pb2+, and pH 7.0 for Ag+, Co2+, Ni2+, Zn2+ and Cd2+]. For Cu2+, Cd2+ and Zn2+, and slightly for Ag+, Ni2+ and Co2+ the complexes form gels at low metal ion concentration in ultrafiltration cell, only Pb2+ stays soluble and allows determinate its MRC (see Fig. 2). Table 1 shows the MRC determined and concentration of polymer: metal ion. Assuming a quantitative retention of different metal ions, the enrichment factor (E) was determined according to the following relationship:
processes is observed. For the most common carboxylic polymer the higher interaction occurs upper pH > 5 (above pKa), specially for those cations that no precipitate as M(OH)x, as Ag+, Co2+, Ni2+, Cd2+ and Zn2+, and how AA has a higher solubility in water and in hydroxy media, these facts will suppose that AA has a similar behavior. Due to poor solubility of AA lower pH 4.0, the speed run is very low (lower than that 1 drop by min) and it application in these conditions means that this technique is inconvenient. These experiments were developed from pH 4.0. Fig. 1 shows results for elution experiences, this figure shows a high interaction, close to 100% for Cu2+ at pH 4, and approx. 100% retention for different metal ions with AA from pH 4.5 (when polymer is completely soluble). Then a slightly pH dependence and high stability of polymer metal ion complexes. This high polymer: metal ion ratio allow obtain water soluble PMC, without precipitation in ultrafiltration cell. Studies at higher pH do not give additional information therefore, its development was not necessary. Maximum retention capacity (MRC) for mono- and divalent metal ions was evaluated at pH of maximum solu-
E¼
ðP CÞ M
where, P is polymer concentration, (g/l), C is maximum capacity of the polymer, (mg/g), and M is initial concentration of the metal salt, (mg/l). Based on previous studies well detailed, about characterization of AA, which indicate the interaction of carboxylic acid, hydroxy, and ester groups with different metal ions [25–26], the FT-IR spectra for AA and PMC synthesized were analyzed. Fig. 3 shows significant differences between these species as the reduction intensity of m(COO)sym of AA at
Table 1 Maximum retention capacity (MRC), pH study, and TGA values of AA for Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+. Sample
AA AA–Ag+ AA–Co2+ AA–Ni2+ AA–Cu2+ AA–Zn2+ AA–Cd2+ AA–Pb2+
MRC
TGA
pH
mg/g
meq/g
E
TD50
dm/dTmax (%/°C)
Residual mass (%)
– 7 7 7 5 7 7 5
– 61.67 1.66 2.08 2,50 79.1 60.4 49.86
– 0.57 0.02* 0.02* 0.02* 0.61* 0.27* 0.12
– 2.06 – – – – – 1.12
284 465 321 326 295 284 401 465
185 183 237 240 244 177 271 191
29 48 24 20 32 28 26 47
TDT50: Temperature at which 50% of weight is loss. dm/dTmax (%/°C): Temperature for maximum weight variation. * Relationship for polymer: metal ion precipitation.
Fig. 3. FT-IR of alginic acid (AA) and its complexes with Ag+, Co2+, Cu2+, Zn2+, Cd2+, and Pb2+.
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Fig. 4. Far FT-IR of alginic acid (AA) and its complexes with Ag+, Co2+, Cu2+, Zn2+, Cd2+, and Pb2+.
Fig. 5. TGA of AA, and AA-complex of Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+.
1730 cm1, an increase intensity for m(COO)sym (1620 cm1) and m(COO)as (1460 cm1) in PMC, dCOH at 1250 cm1 are not present in PMC, mCO and d(CCO) at
1100 and 1030 cm1, respectively, reduced its intensity in PMC. The existence of both sugar moiety anomers can be evidenced by analysis of the spectral range of 820 cm1
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Fig. 6. Weight derivate (dm/dTmax) v/s Temperature (°C) for AA, and its metal ion complexes with Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+.
Scheme 3. (a) Size variation for a polyacid with an increase of pH, (b) interchain interaction for polymer–metal ion complexes, (c) intrachain interaction for polymer–metal ion complexes.
(dCH b-anomer) to 870 cm1 (dCH a-anomer), which are present in AA and PMC. Meanwhile, far FT-IR spectra show slightly differences between AA and PMC, at 89 cm1 it is possible observe the Ag0 signal due to reduction of Ag+ retained for AA (see Fig. 4). All spectra were recorded on a Magna Nicolet 550 spectrophotometer.
3.1. Thermogravimetric analysis Thermogravimetric analysis (see Figs. 5 and 6) shows differences between AA and metal ion complexes weight loss. AA shows four decomposition steps; first step for dehydratation, and from second the decomposition
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Fig. 7. Particle size of alginic acid and its polymer–metal ion complexes changing pH, with a constant polymer : metal ion ratio of 40:1.
corresponding to biopolymer (higher value at 185 °C). This value keeps practically constant for those metal ion complexes, which have complete CMR curves, especially for Pb2+ (191 °C) and Ag+ (183 °C), which moreover shows the higher TDT50 Pb2+ at difference of other complexes shows a decomposition pattern similar to AA, this means that Pb2+ do not affect significantly the decomposition of carboxylic groups to carbon dioxide. Complexes with Co2+, Ni2+, Cu2+, Cd2+, and Zn2+ form gels at low concentration of metal ions and shows the higher shift on decomposition temperatures (dm/dTmax), corroborating the higher inter-action between these metal ions and AA. The residual products of metal ion complexes (possibly oxidized metals or carbonates) show expected values, which are comparable with the values of MRC (see Table 1).
Results are shown in Fig. 7, it can be observed the expected behavior for AA, where highest sizes were found at pH 4.0, and lower size is achieved between pH 5.0 and 6.0. Polymer–metal ion complexes show a similar behavior to AA, only at pH 4.5 the inter-chain interaction can explain clearly the increase of diameter in these polymer– metal ion complexes (Scheme 3b). Alginic– and alginate– silver complexes show a behavior non-pH-dependent with sizes between 3 times (pH 6.0) up to 10 times (pH 4.0) lower than AA. These results could be explained by a intrachain polymer–metal ion interaction (Scheme 3c), but these results were not expected due to its general characteristics as atomic radii (1.44 Å), radii of copper (1.28 Å) and lead (1.75 Å), and it electronic configuration is similar to copper, throughout other experiences must be developed in the near future to explain this result.
3.2. Study of macromolecular dimensions in aqueous media 4. Conclusions Polyelectrolytes show a macromolecular arrangement in aqueous media dependent of pH, ionic strength, and/or counterion. For AA with a known pKa (3.5–5.0), this behavior can be observed clearly by analyzing it size changing pH and counterion. For all polyeletrolytes its maximum size in aqueous media is achieved for completely protonated and deprotonated state and repulsions between functional groups on polymer chain are maximum, and the lower size is achieved when the polyelectrolyte is partially deprotonated and the attracting forces (as hydrogen bound) make to polymer folding, but always the polymer maintain its solubility. For polymer–metal ion complexes the size will depend if this is product of inter- or intrachain interaction, for inter-chain interaction this size is increased, meanwhile the intra-chain interaction drives to a polymer folding (see Scheme 3a). Solutions, in the same condition of the study of polymer–metal ions interaction (1.0 102 M of AA, and 2.5 104 M of metal ion) were analyzed by dynamic light scattering, as only Ag+- and Pb2+-complexes which were completely soluble in MRC experiences. Cu2+ was studied to compare with those insoluble complexes. All data reported were pooled mean and standard deviation of four repeated experiments.
Alginic acid is a natural polymer derived from brown algae composed for uronic acids and is insoluble in water in its protonated form (is present as gel), this condition makes unadequate the LPR technique to remove metal ions with AA. Alginate, non-protonated form of AA, is completely soluble in water from pH 4.3. LPR technique, elution method
Scheme 4. Interaction mechanism proposed for alginate–metal ion complexes.
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allows the complete recovery (100%) of mono-, and di-valent metal ions at pH = 5.0. MRC indicate that only Pb2+ complex is completely soluble in water solution, others form gels at different concentrations. Thermogravimetric analysis showed similar behavior between alginic acid and metal ion complexes, with differences at TDT50 higher for Pb2+ and Ag+, and similar for Pb2+ and Ag; AA forms gels with metal ion as Co2+, Ni2+, Cu2+, Zn2+, and Cd2+, complexes which showed a higher decomposition temperature than that soluble polymers and PMC. FT-IR spectroscopy allowed establish the functional groups of AA involved on polymer metal ion generation, and these are carboxylic acid, ester, and hydroxy groups, see Scheme 4.
[10]
[11]
[12]
[13]
[14]
[15]
Acknowledgments [16]
Authors thank to Fondecyt No 1070542. A.M. thanks to CIPA the post-doctoral Grant. [17]
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