Kinetic study of dye adsorption on chitosan and graphene oxide-chitosan composite materials in aqueous media

Nowadays one of the major cause of water pollution arises from dye pollution. Adsorption is very common and efficient method among many physical, biological and chemical techniques. In present work, chitosan films were used as an adsorbent to remove pyranine dye from its aqueous solution. Adsorption process progresses with time. Adsorption rate is determined by depending on the arrival rate and the percentage of molecules of the adsorbent surface. Adsorption kinetic models explain that adsorption is following what kind of a mechanism and also affinity of dye to the adsorbent surface. Pseudo first order, pseudo second order, Elovich kinetic models and intraparticle diffusion model are some of the most well-known kinetic models. This thesis discusses dye adsorption kinetics of chitosan and graphene oxide-chitosan films. Chitin is a homopolymer of N-acetyl-D-glucosamine units. Natural polysaccharide chitin exists mostly skeleton of crustaceans and it is a structural material of these organisms. Chitosan is biopolymer that composed of D-glucosamine and N-acetyl-D-glucosamine monomers linked with beta-1,4 glycosidic bonds. Chitosan can be produced with partial deacetylation of chitin in basic medium. Due to its biocompatibility and biodegradability, chitosan was chosen in present study. Pyranine is fluorescence dye and it can dissolve in water. It has a pKa of 7.3 and pyranine ionizes pH of between 6 and 10 with the help of its hydroxyl group. Dye emits a strong fluorescence at 510 nm. Free pyranine molecules were excited at 340 nm wavelength. Subsequent to this, their emission spectrums at 510 nm were monitored with a spectrofluorometer. During all adsorption experiments, emission spectrums of free pyranine molecules in solution were saved per minute. Fluorescence intensities of pyranine in solution was decreased because of dye adsorption to chitosan films. Fluorescence excitation light always passed through the solution, never contacted with films. Equilibrium time was determined before all adsorption studies as approximately 375 minutes, after time optimization experiment at time drive mode of the spectrofluorometer with observing pyranine adsorption to sodium sulfate crosslinked chitosan films for 24 hours. As a first step of adsorption experiments, type of ionic crosslinking reagent for chitosan was determined. Since adsorption experiments would be performed in the aqueous medium, chitosan films were crosslinked in sodium sulfate, trisodium citrate and sodium tripolyphosphate solutions to ensure mechanical and long-term strength of the films. Chitosan biopolymer can be crosslinked with physical and chemical crosslinking methods. Because some of chemical crosslinkers such as glutaraldehyde and epichlorohydrin have toxic characteristics, ionic gelation technique was preferred in this study to obtain harmless adsorbent. Chitosan has reactive amine and hydroxyl groups on its polymeric chains. If it is dissolved in acidic medium, free amino sides will be easily protonated. Acetic, formic, lactic organic acid solutions are used very commonly to solve chitosan. Chitosan solutions were prepared by solving 1% chitosan in 1% (v/v) acetic acid with continuously stirring for 6 hours at room temperature. Volume of 30 mL of chitosan solution was poured into plastic petri dish of 9 cm diameter and waited until it becomes a film in a laboratory refrigerator. Chitosan films were crosslinked by immersing them into 1% (w/v) sodium sulfate, trisodium citrate and sodium tripolyphosphate aqueous solutions. All chitosan films were washed with deionized water after crosslinking. Crosslinking procedure was applied for one hour to three type of crosslinker. In aqueous medium, pyranine dye adsorption studies were performed to differently ionically crosslinked chitosan films. Films were dipped into dye solutions and adsorption was followed by fluorescence spectroscopy. The removal efficiency of chitosan films was compared between sodium sulfate, trisodium citrate and sodium tripolyphosphate crosslinked chitosan films. It was realized that chitosan films, which were crosslinked in 1% (w/v) sodium sulfate solutions, have removed pyranine from its solution more than the other two types crosslinked chitosan films within adsorption time. Adsorption kinetic models were applied to experimental results and kinetic parameters were calculated. Considering both removal efficiency results and kinetic constants of sodium sulfate crosslinked film, sodium sulfate was chosen to chitosan film as a crosslinker. In the second step of this study, the ionic cross linking of chitosan films was made with varying proportions of sodium sulfate in crosslinking solutions. Thereby changing the crosslinker concentrations in solutions was investigated with regard to how it will affect the kinetics of adsorption pyranine by chitosan sodium sulfate crosslinked films. Crosslinking of chitosan films was done by immersing them into 50 mL of 1, 5, 10, 15 and 20% (w/v) sodium sulfate crosslinking solutions at room temperature for one hour. Films were washed with same portions of deionized water and dried at laboratory room temperature. Then these sodium sulfate ionically crosslinked chitosan films were subjected to pyranine adsorption studies. Adsorption was performed by fluorescence spectroscopy as measuring emission intensity of pyranine at 510 nm wavelength for every minute of adsorption time. It was seen that dye removal efficiency of sodium sulfate crosslinked films dropped as the amount of sodium sulfate increased from 1% to 20%. Sodium sulfate crosslinked chitosan films pyranine adsorption experimental datas were applied to first order kinetic model, Elovich kinetic model and also intraparticle diffusion model. Exchange between the coefficients were similar in all three models; but considering the regression values, comparisons of kinetic parameters of chitosan films were made on the best compatible first order kinetic model. 1% (w/v) sulfate crosslinked film was showed the most rapid behavior of adsorption among the others. In the last step of the study, chitosan films were prepared from the composite material graphene oxide and crosslinking of films were made with selected crosslinker 1% sodium sulfate in aqueous solution. Graphene oxide (GO) which is oxidized form of two dimensional graphene is very popular material. Studies in recent years showed that it can change electrical, mechanical, thermal characteristics of many biopolymeric systems. Monolayer sp2 nanocarbon structure graphene has large surface area, mechanical flexibility and many of original qualities such as thermal and chemical stability. Graphite occurs as a result of van der Waals interactions between the graphene layers. Graphite form of carbon consisting of clustered graphene sheets is a natural and cheap source for generating graphene and graphene oxide. For difficulties arise in the processing and usage of graphite in synthesis, it is not preferred as an additive in nanocomposites. Despite that graphene oxide and reduced graphene oxide offers good alternatives to the graphite. When preparing graphene oxide-chitosan hydrogel films, first chitosan solutions were doped with between 40-120 µg/mL different amounts of graphene oxide and then chitosan composite films were crosslinked in 1% sodium sulfate solution. After crosslinking, films were washed with same amounts of deionized water and dried at laboratory room temperature as a follow-up procedure. Pyranine dye adsorption experiments were done to composite films in aqueous solution as applying dye experimental conditions of other chitosan films. Experimental results were used for modelling of adsorption kinetics. Adsorption rate of chitosan films has increased up to addition of 60 µg/mL graphene oxide in chitosan films. In conclusion, sodium sulfate crosslinked chitosan and graphene oxide-chitosan films prepared in this study are inexpensive and easily obtainable materials. Chitosan and graphene oxide-chitosan films are suggested to use in adsorption studies of pyranine similar structure other toxic dyes as an effective and efficient adsorbents in aqueous media.