INTERACTION OF ALGINATE WITH METAL IONS, CATIONIC SURFACTANTS AND CATIONIC DYES

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1 INTERACTION OF ALGINATE WITH METAL IONS, CATIONIC SURFACTANTS AND CATIONIC DYES SANDU PERETZ Institute of Physical Chemistry I. G. Murgulescu, Department of Colloids, 202 Spl. Independenţei, , Bucharest, ROMANIA, Received May 28, 2004 Systems containing a natural polymer and metal ions in the presence or the absence of oppositely charged surfactants are considered with respect to interactions and phase behavior. Alginate is a natural linear polysaccharide extracted from marine brown algae, a copolymer of β-d-mannuronate and α-l-guluronate residues. Alginate hydrogels are made by ionically crosslinking with calcium or barium ions, in the absence or the presence of two cationic surfactants, dodecyltrimethylammonium bromide and cetyltrimethylammonium bromide. The alginate cross-link complexes can exist either as homogenous clear solutions or precipitates or as beads. The effects of some salts and sodium alginate concentrations on the properties of the gel-like membranes were investigated. The influence of the two surfactants to the domains of steep binding isotherms was also investigated. The capacity of alginate gel beads to remove some cationic dyes from water was studied. Key words: alginate, cross-linked polymer, bead, capsule, cationic surfactant, removal, cationic dye. INTRODUCTION In the cross-linking process the water-soluble polymer such as polysaccharides interact with the metal ions to forms complexes. The formation of complexes between the components of opposite charge can be characterized by steep binding isotherms. It was established that ionic surfactants are bound strongly and cooperatively to polyelectrolytes of opposite charge, synthetic or natural [1, 2]. In steep binding isotherms, the existence of a critical aggregation concentration, [3, 4], often much lower than the critical micellar concentration (cmc) of the surfactant, is a general property of these systems [5 9]. Sodium alginate (Na-Alg), a water soluble salt of alginic acid, is a natural linear polysaccharide from marine brown algae composed of β-d-mannuronic and α-l-guluronic acid residues arranged in a nonregular and blockwise fashion along the chain [10]. Guluronate blocks are responsible for the egg box formation with calcium ions or heavy metals during alginate gelation [11]. The Rom. Journ. Phys., Vol. 49, Nos. 9 10, P , Bucharest, 2004

2 858 Sandu Peretz 2 alginate salts differ from most other polysaccharides exhibiting a sol-gel transition when simply submitted to modification of their ionic environment, e.g., substitution of sodium by divalent cations such as calcium [12, 13]. Alginate has several advantages over other carbohydrates, such as high biocompatibility [14], low toxicity, biodegradability, chelating ability, and the possibility of chemical modification [15]. It is used in many food products, textiles, paper, pharmaceuticals and environment protection as a stabilizer, emulsifier, thickener and gelling agent. Contacting drops of alginate solution with calcium or barium chloride solution, the system can produce homogenous clear solutions, or form precipitates, or gel-like membranes that separate the two aqueous solutions. Dyes with cationic charge such as o-nitrophenol, p-nitrophenol or methylene blue could be released in water, accidentally or as a waste after a technological process. Although many post practices that resulted in substantial releases of these contaminants to the environment have been corrected, treatment technologies for their removal from aqueous wastestreams are still needed [16]. In the past years the use of natural polymers for cleaning the wastewater has received considerable attention. In the present work we studied the ionic interactions between guluronate blocks of sodium alginate and metal ions (Ca 2+ or Ba 2+ ) in the absence or the presence of two cationic surfactants: dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB). The influence of the two surfactants on the domains of steep binding isotherms and on the size and shape of alginate beads was also studied. The process of o-nitrophenol, p- nitrophenol, methylene blue uptake by alginate beads have been investigated through a series of sorption and kinetic experiments under varying amount of calcium alginate (Ca-Alg) and the physical properties of beads. EXPERIMENTAL Sodium alginate from brown algae of low viscosity grade η r = 250 cps, at 25 C (solution 2% w/v), was supplied by SIGMA (France) and it was used without further purification. Calcium chloride anhydrous (CaCl 2 ) 99.9%, barium chloride anhydrous (BaCl 2 ) 99.9%, o-nitrophenol (o-nph), p-nitrophenol (p-nph) 99% (Merck), methylene blue (MB-Fluka) and surfactants: dodecyltrimethylammonium bromide and cetyltrimethylammonium bromide (Merck) were used as received. Bidistilled water was used throughout the study. The concentration of stock aqueous solutions of CTAB and DTAB was M, but in the whole volume of samples the surfactant concentration was M. The stock dyes solutions of o-nph, p-nph was M, and

3 3 Interaction of alginate with metal ions 859 for MB was M. The aqueous solutions of CaCl 2 and BaCl 2 concentrations ranging between 0.01% and 5.0% (w/v) were prepared. The ph of the reaction medium was varied between 4.5 and 7.0. The sodium alginate solution was magnetically stirred at 400 rpm for 5 h on a heated (40 C) plate, and then dropped using a microsyringe into the salt solutions (CaCl 2 or BaCl 2 ). The syringe needle was 0.3 mm in diameter. The alginate solution was dropped approximately 2 cm from the surface of salt solution. The beads form instantaneously when the alginate solution droplets reach the surface of the salt solution. The beads were directly prepared in spectrophotometer cuvettes in order to observe the system, to determine the turbidity and to visualize the shape. The beads evolution was followed through video enhanced microscopy (VEM) using a Logitech video camera for 24 h, then the observations regarding the aspect of the alginate-metal ions complex were used to draw the alginate-salt phase diagrams. Phase diagrams were drawn using samples whose concentrations in sodium alginate increased from each other by base-mol dm 3 and 0.01% (w/v) in the case of salts solutions. To obtain information on the efficiency removal, the dyes concentrations in the water were measured by a spectrophotometer; the turbidity determinations were made using a SPECORD M40 UV-vis spectrophotometer in order to determine the passage from domain C to domains PP or IPA. The average diameter of beads and the thickness of the layer of capsules were performed using an optical microscope (Zeiss) equipped with a micrometrical device. All the measurements were made at 25 C. RESULTS AND DISCUSSION Previous studies concerning the interfacial properties of sodium alginate and alginate-hydrophobically modified derivative in the presence of DTAB showed the formation of electrostatically stabilized dynamic associates, previously named Surfactant-Polyelectrolyte Complexes [17]. From the interaction of sodium alginate (concentration between base-mol dm 3 ) with the oppositely charged surfactants, DTAB and CTAB on a concentration range from M to M we obtained only amorphous precipitates or polymer aggregates. Unlike chitosan, a deacetylated chitin, which forms gel-like membranes in the shape of stable capsules with oppositely charged surfactants [18], for the case of sodium alginate in order to obtain stable structures is necessary to add in the system metal ions. As result of the reactions between sodium alginate and ions from salt solutions, we can observe several behavioral domains, namely: 1. Homogeneous, clear solutions domain C.

4 860 Sandu Peretz 4 2. Precipitates, preceded by the appearance of an advanced turbidity in the PP domain. 3. Irregular polymer aggregate - IPA domain. 4. Formation of gel-like membranes able to separate the two aqueous solutions of polymer and salt in the presence or the absence of surfactants inside domain B (spherical beads) and inside the NSC domain (non spherical capsules). In order to separate the two solutions, the membrane of cross-linked polymer complex formed at the interface must be stable and mechanically resistant. 5. In the IPA domain, at some low concentrations of sodium alginate and CaCl 2 stable ring-shaped formations RS-F appear (see Fig. 1). Fig. 1. Solubility and domain formation of gel-like membranes in the NaAlg-CaCl 2 system: B Beads; NSC Non spherical capsules; IPA Irregular polymer aggregates; PP Precipitates; C Clear solutions; Ring shape formation 1. SR-F 1 ; 2. SR-F 2 ; 3. SR-F 3 ; ( ) no surfactant; ( ) [M] DTAB; ( ) [M] CTAB. In the domain where the gel-like membrane forms, the particles have different evolutions owing to the mechanical resistance, as well as to the osmotic pressure difference at the interface. Particles with ideal size and shape were obtained with a reaction time of at least 15 minutes. Cross-sectional analysis performed by electron microscopy shows that the particles have a relatively rough and uniform surface and a homogeneous internal structure, characterized by a microporosity reflecting the incubation time in the presence of Ca 2+ ions. It must be noted that the amount of calcium ions appear to be a very critical parameter [19]. Other authors clearly show that in some cases the maturation step in the particles preparation process is generally slow, resulting in a steady state equilibrium after about 15 hours,

5 5 Interaction of alginate with metal ions 861 depending to the cation concentration, the ionic strength, and ph [20]. The domain B represents an area where the spherical beads formed have maximum stability. The prepared beads have an average diameter between mm. In order to evaluate the effect of the reaction time on beads morphology, the microspheres were left in the presence of CaCl 2 or BaCl 2 solutions for different periods of time, comprised between 10 minutes and 24 hours. For the range of salt concentration between 0.5 to 5.0% (w/v) we find that in the maturation step, the equilibrium was achieved after 24 hours. In the steep binding isotherms the presence of surfactant molecules ( M) leads to a shifting of area B towards lower salt concentrations. This phenomenon is stronger for DTAB than CTAB on the whole range concentration of sodium alginate solutions (see Figs. 1 and 2). The solubility in the domain B is practically unaffected by the salt concentration up to a critical value, called minimal salt concentration (MSC). The MSC value represents the salt solution concentration at which well defined spherical stable structures (beads) form. It is situated at about 0.5% (w/v) for the samples with surfactant and 0.6% (w/v) for the samples without surfactant. Fig. 2. Solubility and domain formation of gel-like membranes in the NaAlg-BaCl 2 system. At concentration of the salt solutions between % (w/v) complexes are formed in non-spherical, semitransparent capsules (NSC), with an average diameter between mm, which present thin gel capsule wall and striated surface. As a result of the cross-linking process in the maturation step, the thickness of the capsule layer grows from 0.01 mm to 0.05 mm as we can see from the determinations made by VEM and direct optical microscopy. After 24 hours from the preparation, the thickness of the capsule layer remains constant. The presence of DTAB or CTAB in the system leads to a relative

6 862 Sandu Peretz 6 thickening of the capsule layer with 25 50%. Also the final formation time of the gel capsule wall in maturation step decreases at 14 hours. When the surfactant is present in the system, the area occupied by NSC domain has the tendency to displace towards lower salt concentrations at the same time with the increase of the sodium alginate concentration, which is more obvious for DTAB than CTAB. The expansion of NSC domain is bigger for Na-Alg concentrations up to 0.05 base-mol dm 3 and better defined for CaCl 2 than for BaCl 2. We can see that for all the sodium alginate concentration the cross-linking reaction with salt solutions ( % w/v) in the presence or absence of surfactant generates irregular polymer aggregates (IPA domain). The interactions between sodium alginate at concentrations higher than 0.1 base-mol dm 3 with salt solutions of very low concentrations ( % w/v) produce precipitates, in the presence of DTAB; these are more substantial in the presence of CTAB, according to PP domain. For the sodium alginate concentrations until 0.1 base-mol dm 3 and salt solutions (concentrations between % w/v), in absence of surfactants, the system remain clear, homogeneous C domain. From the interaction of sodium alginate (low concentrations until 0.05 base-mol dm 3 ) with CaCl 2 ( % w/v) in the presence of DTAB stable, well defined ring shape-formations appear, which are find together in the RS-F 1,2,3 domains (see Fig. 1). We suppose that hydrocarbon chains of DTAB, shorter than those of CTAB, have an important contribution in the process of formation and the stability of these structures. In order to reach the maximum sorption capacity of calcium alginate gel beads, the sorbing cationic substrates must penetrate through the entire volume of the 2.5 mm diameter of gel beads. Previous studies of copper sorption by Ca-Alg gel beads have suggested that the kinetics of the sorption process is limited by diffusion within the gel matrix [21, 22]. Diffusion limitation is also suggested by the observed correlation between the dissolved sorbate concentration and the square root of time [16]. Calcium ions are located into electronegative cavities, like eggs in an egg-box, from this similitude arises the term egg-box model [23]. The presence of the calcium salt could also have a channelling effect that determined more rapid environment penetration into the inner layers of the matrix [24]. Our studies were conducted to establish the sorption kinetics for o-nph, p-nph, MB by calcium alginate gel beads and their affinity for the cationic dyes. The ionic exchange takes place in the channels of Ca-Alg beads between guluronate blocks charges and the connterions substrate. Up to 89% removal of o-nph was achieved for 0.22 g Ca-Alg, comparatively with only 47% for p-nph and 80% in the case of MB. For all the cationic dyes the efficiency removal is much higher for 0.22 g calcium alginate

7 7 Interaction of alginate with metal ions 863 than that of 0.06 g of alginate beads (see Fig. 3). This shows that the number of functional groups increases with the increase of the amount of alginate beads. Fig. 3. Efficiency of dye removal as function of Ca-Alg amount; ( ) o-nph [M]; ( ) p-nph [M]; ( ) MB [M]. The results (see Fig. 4) show that the removal percent is higher for o-nph and MB than in case of p-nph at the same amount of gel beads (0.22 g). The kinetics of cationic dyes uptake show that the sorption begins with a rapid phase, followed by a relatively slow phase. The rapid phase lasts for 4 to 12 hours, while the slow phase may continue for 12 to 72 hours. Fig. 4. Dye removal kinetics; ( ) o-nph [M]; ( ) p-nph [M]; ( ) MB [M]; Ca-Alg 0.22 [g].

8 864 Sandu Peretz 8 CONCLUSIONS The results presented in this article show the possibility of obtaining gel-like membranes able to separate two aqueous solutions and open the possibility to develop micro-encapsulation processes for wastewater treatment. As a result of cross-linking process between sodium alginate and divalent metal ions gel-like membrane of capsule shape or spherical beads were obtained in the presence or absence of surfactants. Bead formation takes place in a crosslinking process, through the diffusion of calcium or barium ions. The thickening of the gel capsule walls is accelerated by the presence of surfactants. The shifting towards lower salt concentrations of B and NSC domains in the phase diagrams is higher for DTAB than for CTAB. The solubility in the domain B-spherical beads is practically unaffected by the salt concentration up to a critical value, called minimal salt concentration, located around 0.5% (w/v) for the samples with surfactant and 0.6% (w/v) for the samples without surfactant. Calcium alginate gel beads have the capacity to remove efficiently o-nph, p-nph and MB from wastewater. REFERENCES 1. K. Shirahama, The Nature of Polymer-Surfactant Interactions (J. C. T. Kwak Ed.), Polymersurfactants Systems, Dekker, New York Basel Hong Kong, , E. D. Goddard, Polymer-Surfactant Interaction Part II: Polymer and Surfactant of Opposite Charge (E. D. Goddard and K. P. Ananthapadmanabhan Eds.), Interactions of Surfactants with Polymers and Proteins, CRC Press, Boca Raton, , K. Shirahama, N. Ide, The Interaction Between Sodium Alkylsulfates and Poly(ethylene oxide) in 0.1M Sodium Chloride Solutions, J. Colloid Interface Sci., 54, , K. Shirahama, K. Tsujii, T. Takagi, Free-Boundaries Electrophoresis of Sodium Dodecyl Sulfate-Protein-Polyacrylamide Gel Electrophoresis, J. Biochem., 75, , K. Thalberg, B. Lindman, Segregation in Aqueous Systems of Polyelectrolyte and Ionic Surfactant, Colloids Surf., 76, , H. Okuzaki, Y. Osada, Effects of Hydrophobic Interaction of a Surfactant to a Polymer Network, Macromolecules, 27, , H. Okuzaki, Y. Osada, Role and Effect of Cross-Linkange on the Polyelectroyte-Surfactant Interactions, Macromolecules, 28, , J. P. Gong, Y. Osada, Theoretical Analysis of the Cross-Linking Effect on the Polyelectrolyte- Surfactant Interaction, J. Phys. Chem., 99, , R. Y. Lochhead, Water-Soluble Polymers, Cosm. & Toil., 107, , A. Haug, B. Larsen, O. Smidsroed, A Study of the Constitution of Alginic Acid by Partial Acid Hydrolysis, Acta Chem. Scand., 20, , G. T. Grant, E. R. Morris, D. A. Rees, P. J. C. Smith, D. Thom, Biological Interaction Between Polysaccharides and Divalent Cations: the Egg Box Model, FEBS Lett., 32, , A. Page, P. Hubert, L. Choplin, M. C. Houzelle, A. Sinquin, P. Marchal, E. Dellacherie, Rheological Study of Aqueous Solution of Hydrophobically-Associating Derivatives of Propylene Glycol Alginate, Oil & Gas Sci. Technol., 52, , 1997.

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