Análisis de variables nominales

Chitosan graft copolymerization is increasingly becoming an important reaction due to their potential applications in industrial field. A number of vinyl group and other monomers are used in grafting and represent a powerful technique to produce substantial enhancement in chitosan chemical and physical properties, hence enlarging its range of application. Methyl methacrylic acid has been used in graft polymerization onto chitosan by using various initiator systems, such as potassium persulfate. Lagos and Reyes studied the grafting of poly(methyl methacrylate) onto chitosan backbone by a redox initiator; Fenton's reagent under atmospheric conditions. They found that the grafting percentage and grafting efficiency depend on the concentrations of chitosan, poly(methyl methacrylate), ferrous ammonium sulfate, hydrogen peroxide, time and temperature of reaction (Lagos and Reyes, 1988). El-Tahlawy et al. investigated the grafting reaction of methacrylic acid onto chitosan using 2,2-azobis(2-methylpropionitrile) as an initiator. They reported that the optimum conditions to carry out the reaction are material-to-liquor ratio of 1:30, concentration of 2,2-azobis(2-methylpropionitrile) of 0.125%, and a temperature of 80 °C (El- Tahlawy et al., 2006).

Panic et al. synthesized poly(methacrylic acid) hydrogels with monomer neutralization degree of 0% and 80% and studied their adsorption potential for cationic dye, basic yellow 28 from aqueous solutions. The grafting reaction was conducted using 2,2'-azobis-[2-(2-imidazolin-2-yl)propane]

dihydrochloride as initiator. They found that the adsorption capacities of both modified polymers towards the selected dye are highly sensitive to changes in external conditions; in particular the one affecting the degree o f swelling. Kinetic rate models revealed that the pseudo first order kinetic model is best to fit the data and that adsorption is well described with phase-boundary controlled models. Thermodynamic models showed spontaneous endothermie system. Both hydrogels showed that the chemisorption and physisorption mechanisms were present. The maximum saturated uptake was 102 mg/g and 157 mg/g for poly(methacrylic acid) hydrogels 0% and 80%, respectively. Good removal properties for the dye were reported with higher adsorption capacities, percentage removal and significant acceleration in adsorption were achieved using the monomer up to 80% (Panic et al., 2013).

Xing et al. prepared poly(methacrylic acid) modified chitosan microspheres for adsorbing dyes;

methylene blue and malachite green, from aqueous solutions in batch system. A remarkable increase in the adsorption capacities of the produced microspheres for the two cationic dyes was noted due to the introduction of a large number of carboxyl groups. Langmuir than Freundlich isotherm was better than other models in describing the equilibrium system. Maximum adsorption capacities of 1000 mg/g and 523.6 mg/g were accomplished for methylene blue and malachite

________Kinetic and Removal Mechanisms o f BTEX Compounds from Aqueous Solutions by Chitin, Chitosan and Enhanced Chitosan

green, respectively, according to Langmuir model. Pseudo second order rate model showed better correlation coefficients compared to the other kinetics models, confirming that chemisorption mechanism is controlling the process (Xing et al., 2009). Bayramoglu et al. prepared the poly(methaciylic acid) brush grafted crosslinked chitosan beads using ammonium persulfate as an initiator for grafting reaction. The beads were utilised in ion exchange and adsorption o f lysozyme;

which is a commercial enzyme used in food technology, from aqueous solution. The adsorption capacity reached up to 65.7 mg/g at pH o f 6.0. The Langmuir isotherm equation was best to fit the experimental equilibrium data. Kinetic analysis showed favourable to the second order rate model (Bayramoglu et al., 2007).

In this study, crosslinked chitosan is graft copolymerized with poly(methacrylic acid) using potassium persulfate as initiator; Figure (4.7). This initiator is commonly used in graft copolymerization o f vinyl monomer with the addition o f heat. First radicals generated by this system form from the decomposition o f the initiator into two radicals, or what is known as homolytic bond scissions. These radicals are capable o f abstracting hydrogen atoms from chitosan which create macroradicals. This is followed by the reaction o f these radicals with the vinyl monomer to initiate a grafted chain. Thus, heating the potassium persulfate aqueous solution will result in the decomposition o f this compound into SO4* radicals and other radicals. The chitosan functional groups; amine groups or hydroxyl groups, react with the free radicals and grafted in the backbone o f chitosan.

One o f the reaction mechanisms that have been interestingly proposed for chitosan/persulphate system and consider the direct reaction between the amine group of chitosan with persulphate anions to yield R — NH, OSO3H and S0 4~(Jenkins and Hudson, 2001). However, there were no references reporting such mechanisms where the persulphate anions react directly and in particular with chitosan as proposed. Equations (4.9) to (4.16) demonstrate the grafting reactions that generally proceeds between glutaraldehyde crosslinked chitosan and methacrylic acid (Qudsieh et al., 2004).

S2O#- ^ 2 S 0 ;' (4.9)

s o ; ' + H2O ^ H so; + h c (4.io)

2 H 0 * ^ H 0 0 H (4.11)

HO' + HOOH ^ H2O + HO^ (4.12)

S2 0§ - + HO2 -> H s o ; + s o ; ' + O2 (4.13)

________Kinetic and Removal Mechanisms o f BTEX Compounds from Aqueous Solutions by Chitin, Chitosan and Enhanced Chitosan

o — NH2 + R* ^ o — NH* + RH or

0 — OH + R* —> o — 0* + RH (4.14)

o - NH* + CH2 = C(CH3)C0 0H ^o - NHCH2 - C(CH3)C0 0H or

0 - 0* + CH2 = C(CH3)C0 0H ^ o - OCH2 - C(CH3)C0 0H (4.15)

n C H 2 = C (C H 3 )C 0 0 H

o — NHCH2 — C(CH3)C0 0H --- > Grafted Copolymer or

n C H 2 = C (C H 3 )C 0 0 H

o — OCH2 — C(CH3)C0 0H > Grafted Copolymer (4.16) where o — OH is glutaraldehyde crosslinked chitosan and R* is the free radical species.

The grafting percentage and efficiency is determined by the following equations (Qudsieh et al., 2004);

Wi - Wq (4 17)

P ercentage G ra ftin g = — — x 100 Wq

— Wq (4 18)

P ercentage E ffic ie n c y = — — x 100 Vv2

where

Wq is the weight of chitosan Vfi is the weight of grafted polymer W2 is the weight of the monomer

It was noted that the solubility of the produced graft copolymer is very low in many organic solvents; hence the conformation of graft copolymer cannot be obtained using the nuclear magnetic resonance (NMR) technique. Nevertheless, the indirect methods such as infrared spectroscopy and acid hydrolysis technique can be used in the conformation of grafting reaction (Hebeish et al., 1988, Qudsieh et al., 2004).

Kinetic and Removal Mechanisms o f BTEX Compounds from Aqueous Solutions by Chitin, Chitosan and Enhanced Chitosan

Initiator

OH OH

NHz

Figure (4.7): Graft copolymerization of methacrylic acid with chitosan.