ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ, 2007, том 81, № 9, с. 1559-1564
^ CHEMICAL KINETICS ^^^^^^^^^^^^^^
AND CATALYSIS
УДК 541.124
KINETICS OF Cu2+ BINDING TO THE POLY(ACRYLIC ACID) HYDROGEL © 2007 A. Kostic*, J. Jovanovic**, B. Adnadjevic**, A. Popovic***
*Faculty of Agriculture, Nemanjina 6, Zemun, Belgrade, Serbia ** Faculty of Physical Chemistry, Studentski Trg 12-16, Belgrade, Serbia *** Faculty of Chemistry, Studentski Trg 12-16, Belgrade, Serbia E-mail: bora@ffh.bg.ac.yu
Abstract - Isothermal kinetics of copper (ion) binding to the poly(acrylic acid) (PAA) hydrogel at 20°, 25°, 35° and 45°C was investigated. Isothermal conversions and kinetics curves of Cu2+ binding to the PAA hydro-gel were determined. It was found that the well known kinetics models of Peppas can not be applied for describing the entire process of Cu2+ binding. The new method for determination the kinetics model of Cu2+ binding process, as well as the activation energy density distribution functions of PAA hydrogel interaction with Cu2+, were established. It was found that dominant influence on the kinetics of the process, at temperatures T > 30°C has Cu2+ diffusion to the active centers (with Ea = 9 kJ/mol), but at T > 30°C and for the degree of bound Cu2+ a = >0.2, the interaction of Cu2+ from the adsorption center with Ea = 26 kJ/mol is dominant.
INTRODUCTION
Polymer materials are widely used type of materials and very popular in present time are hydrogels. Poly-electrolytes hydrogels have been attracted much attention as functional polymers that possess properties of water absorption. In recent time environmental protection has been confronted with increased contamination of waste water by heavy metals ions. They are metabolic poisons and enzyme inhibitors. The polyelectrolyte hydrogels can be applied to the problems of the recovery of precious metal, removal of toxic or radioactive elements from various effluents, and metal preconcen-tration for environmental sample analysis [1]. It has been shown that metal absorption is generally limited by metal diffusion inside the hydrogel and the hydro-gel-water interfacial area [2]. Coordination resins have been used in metal extractions [3] but they demonstrate poor swelling in water, limiting the mobility of ligands. If we considered this fact, hydrophilic polymeric networks (polyacrylamide, polymethacrylic and poly-acrylic acid) will be very advantageous, as they can absorb an amount of water that can be thousand times larger than the mass of the original dry polymer. Fundamental studies on the complexation of metal cations with polyelectrolyte hydrogels have been carried out and used successfully in the recovery of metal ions [4]. Thus, the swelling of a polyelectrolyte network in metal aqueous solution can be applied to the treatment of diluted aqueous effluents in environmental studies [5]. The below cited papers illustrated this application and dealt with the absorption of copper ions with different types of hydrogels.
Recently, H. Ka§göz and co-workers dealt with development and investigation of the sorbents for removal Cu(II) and other metal ions [6-8]. They prepared and used modified polyacrylamide (PAAm) hydrogels for
removal of Cu(II) ion at pH 5.5 (which was considered to be optimal) by the batch equilibrium technique [6, 7]. They found that unmodified PAAm was not effective in removal of Cu(II) ions since amide groups alone cannot form complexes with copper ions. They found that the hydrogels of the modified polymers obtained by transa-midation of PAAm exhibited promising properties as complexing agents for Cu(II) ion removal [6, 7]. The maximum Cu(II) ion removal capacity of 4.07 mmol/g has been obtained with hydrogel based on sulfomethylation reaction products of PAAm [7]. The reported results were based solely on the determined Cu(II) ion removal capacity and there was not any information about the kinetics of the investigated process. Also, the new sor-bent hydrogels based on poly(N-hydroxymethylacryla-mide)modified with introduced amine groups, were developed and used as sorbent for removal of Cu(II) ion (as a model metal ion) from aqueous solutions. The adsorption properties of the hydrogels were investigated at 25°C depending on pH, time and Cu(II) ion concentration. The results obtained from adsorption isotherm were evaluated by using Langmuir and Freundlich adsorption models. It was found that Langmuir isotherm model provided the best fit for the adsorption of Cu(II)ion as well as indigo carmine [8].
The work of I. Katime and E. Rodriguez [5] dealt with the absorption of copper and zinc with poly(acryl-ic acid-co-itaconic acid) hydrogel. The swelling process of xerogels of different compositions in water and salt solutions has been studied at 25° C and influence of pH on the adsorption process was examined. The swelling kinetics, kinetics order and rate constant of the investigated hydrogels have been determined. The swelling processes satisfied to a second-order kinetics. They found that metal absorption increased when pH, salt concentration in external solution and itaconic acid
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content were levelled. They suggested that a complex-ation of -COO- groups with Cu2+ happened forming neutral -COOCuOOC-group.
Some authors suggested that copolymer of a poly-acrylamide and polyacrylic acid should be of use for the recovery of copper from environment [9]. Hydrogels based on acrylic acid are known as specific materials because of their biocompatibility, nontoxicity, hydro-philicity, etc. Due to this properties they have been used in medicine and pharmacy [10, 11], environmental protection, and agriculture. The aim of this paper was to investigate the kinetics of Cu2+ ion binding to the poly (acrylic acid) hydrogel.
EXPERIMENTAL
Materials. Acrylic acid (99.5%) (AA) in glacial form was supplied by Merck A.G., Germany. N,N-me-thylene bisacrylamide (p.a) (MBA) was purchased from Aldrich Chemical Co., Milwaukee, USA. The initiator, 2,2-Azobis-[2-(2-imidazolin-2-il)-propan dihydro-chlorid (VA044), (99.8%) was supplied by Wako Pure Chemicals Industries, Ltd, USA. Copper-sulfate pen-tahydrate, CuSO4 • 5H2O (99.8%) was purchased from Zorka Pharma, Sabac, SCG. All substances were used as received without further purification.
Synthesis. Poly(acrylic acid) hydrogels (PAA) were synthesized by simultaneous radical polymerization and crosslinking using AIPD as an initiator and MBA as a cross-linking agent, in general using experimental procedure previously described [12, 13]. This procedure consists in follows: solution of acrylic acid in the form of 20% (wt) solution was prepared and mixed with solution of MBA (0.1%w). After good stirring of these mixtures to assue the homogenity of reaction mixture and nitrogen bubbling through the mixture for half an hour, initiator solution (0.06 mol % of the monomer) was added and reaction mixture was once again rapidly homogenized with stirring. The prepared solution was placed in Petri dish and stored in a dry oven for 5 hours at 80°C. After the polymerization was finished, the obtained gel-type product was transformed into the Na+ form (60%) by neutralization with a 3% solution of Na2CO3. The resulting hydrogel was cut to approximately equal discs and placed in excess of distilled water. The water was changed 7-times every 5 hours (or 12 hours during the night), in order to remove the unreacted monomers and the sol fraction of polymer. Then the obtained hydrogel was dried in an air oven in temperature regime 80° C for 2 hours, 90°C for 3 hours and 105°C to constant mass. The obtained product was stored in a vacuum exicator before use.
Binding of copper to the PAA hydrogel. Pre-weighed dry hydrogel (xerogel) samples, with average weight of 0.1 g, were immersed in solution of 0.01 M CuSO4 at different temperatures. Aliquots of solution were taken at predetermined time intervals. The amount of copper was determined by atomic absorption spec-
trometry using Varian Spectra AA55 atomic absorption spectrometer. Specific quantity of binded Cu2+ (X) was determined according to (1):
X = Co ~ C- v (1)
m
where C0 is the Cu2+-concentration of the starting copper solution and Ci is the Cu2+ concentration at time t, V is the volume of the Cu2+ solution, and m is the hy-drogel sample mass.
Determination of the degree of bound Cu2+ (a): degree of bound Cu2+ (a) was determined according to eq. (2):
a = X/Xmax, (2)
where Xmax is the maximal quantity of bound Cu2+.
Methods used to evaluate kinetics parameters of Cu2+ ion binding to PAA hydrogel
Peppas' method [14]: To determine the kinetics parameters of Cu2+ ion binding to PAA hydrogel, the results were analyzed applying the linearized form of well-known equation (3):
a = kf, (3)
where n is an exponent indicative for the mechanism of the process, coefficient k is the apparent release rate and t is the interaction time.
Friedman's differential iso-conversional method [15]: the kinetic analysis of experimental data is based on the rate equation:
f = A/(a)exp (-RT), (4)
where T is the temperature, A is the pre-exponential factor, E is the apparent activation energy,f(a) is the kinetics model function of the degree of Cu2+ ion binding and R is the gas constant. The logarithm form of equation (4) leads to:
lnda = lnA + lnf(a) - ^. (5)
For /(a) = const, the plot ln(da/dt) vs. 1/T, obtained from conversional curve should be a straight line whose slope allowed to evaluate the apparent activation energy.
RESULTS AND DISCUSSION
Figure 1 presents kinetic curves of isothermal binding of Cu2+ to the PAA hydrogel at different temperatures.
At the kinetic curves of isothermal binding of Cu2+ to the PAA hydrogel, two characteristic shapes of the changes of the specific quantity of the bound Cu2+ with interaction time, i.e. a linear and non-linear ones can be easily observed at all investigated temperatures. The interaction temperature increase, especially for T > 30°C, leads to an abrupt increase of C
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