(660f) Characterization of Chitosan Hydrogel with Improved Acid Stability Switched on By Carbon Dioxide

Along with the development of chemical technology comes the obvious drawback of industrial processes: wastewater pollution. There have been many examples of water contamination around the world, which all seem to deliver the same message: it is now urgently necessary to develop more advanced, effective and sustainable technologies for wastewater treatment.

Amongst the wastewater treatment methods currently available, adsorption has been the focus of many studies for its effectiveness and relatively straightforward operation. There are various types of adsorbents, such as zeolite, modified-alumina, activated-clay, activated-carbon to bentonite, fly ash, pine leaves and chitosan [1]. As the world is switching its focus to “sustainable technology”, adsorbents should be cheap, environmentally-friendly and widely abundant. One candidate that can meet those requirements is chitosan hydrogel. Chitosan, a derivation from the N-deacetylation of chitin which can be extracted from the shells of crustacean animals, is one of the most abundant natural bio-polymer [2]. In many studies, this material has been reported to possess excellent adsorption capacities towards many synthetic dyes and heavy metals. The adsorption capacity of chitosan hydrogel for anionic dye pollutants is particularly high at low pH due to the protonation of amino groups on chitosan molecules. In this process, NH₂ groups are protonated, which would allow chitosan hydrogel to strongly and quickly adsorb negatively-charged ions on anionic dye molecules.

In spite of the promising results, there are two existing problems with chitosan adsorbent materials. First of all, at acidic conditions, chitosan hydrogel is protonated and thus can be easily dissolved. This is a conflict for many researchers, as using chitosan at low pH conditions for anionic pollutants would require making the material stable in acids. To achieve this, chitosan is usually cross-linked using glutaraldehyde (GLA), epichlorohydrin (ECH) or ethylene glycol diglycidyl ether (EDGE). However, although these cross-linkers can increase chitosan’s acid stability, they have been regularly avoided due to their high toxicity and the negative impact on chitosan’s adsorption capacity [3]. The second problem is that, even if chitosan is successfully modified to survive in acids, the use of conventional acidic compounds to lower the pH (usually HCl or H₂SO₄) would lead to further wastewater treatment such as using basic compounds, for example NaOH, to neutralise the mixture afterwards. Furthermore, HCl or H₂SO₄ are known to corrode process equipment, which may result in malfunctioning and extra maintenance costs.

To tackle these problems, we have devised a new technique, CO₂-switchable system. CO₂-switchable system is a concept originally proposed by Jessop’s group where carbon dioxide can be deployed as a trigger for modifications of various solvents’ properties, such as degree of viscosity, polarity or hydrophilicity [4] [5]. In our case, we decided to utilise carbon dioxideto decrease the solution’s pH, which is a much “greener” method compared with using inorganic acids such as HCl or H₂SO₄. With our results up to now, it can be shown that in CO₂-switchable system, chitosan undergoes protonation to become a cationic polymer, which can significantly speed up the adsorption mechanism between chitosan hydrogel and anionic dyes. Another interesting result that was obtained is the enhancement of chitosan hydrogel’s stability in acid when the temperature of CO₂-switchable system was increased to 45°C or higher. At this condition, the material became mechanically stable, retained its shape and weight and most importantly, it did not dissolve throughout the whole adsorption process. Nevertheless, to achieve a detailed comprehension of the reaction between chitosan and carbon dioxide, further characterization was urgently needed.

Based on that motivation, in this study the effects of carbon dioxide on chitosan hydrogel at temperatures higher than 45°C were investigated. To understand the changes inside chitosan’s structure, FT-IR spectra were studied. Dynamic light scattering method was also employed to investigate the zeta-potential of chitosan hydrogel after making contact with CO₂ at different temperatures. Last but not least, the stability of chitosan hydrogel in acid after pre-treatment with CO₂ was tested with acetic acid aqueous solution

II. Experiment

Chitosan flakes were dissolved in CH₃COOH solution to form a hydrogel mixture. This hydrogel mixture was left under stirring for 5 hours before being collected into a syringe. From this syringe, chitosan hydrogel was dropped into NaOH solution for coagulation. The coagulated beads were left overnight before being washed extensively with DI water until a neutral pH was reached.

After that, chitosan hydrogel beads were put in an aqueous solution. The solution was heated in 65°C with continuous CO₂ bubbling for 3 h. After that, the hydrogel beads were taken out and oven-dried at 50°C.

Chitosan hydrogel after pre-treatment by CO₂ and oven-drying was analyzed by Fourier-Transform Infrared Spectroscopy (FT-IR). Zeta-potential was measured by Dynamic Light Scattering (DLS) method.

For analysis of acid stability, these hydrogel beads were put in an aqueous solution of acetic acid. The weight of chitosan hydrogel was measured and recorded at different time intervals.

III. Results and discussion

The FT-IR spectra revealed noticeable changes to represent the formation of a carbamate cross-link, which is produced from the reaction between a primary amine and CO₂ as reported by literatures [6,7].

DLS was employed as a useful tool for inspecting the role of CO₂ on turning chitosan hydrogel into a cationic polymer. The result of the treated chitosan hydrogel shows positive zeta potentials. It is important to be reminded again that at 65°C, chitosan hydrogel was found to possess a higher acid stability than at 25°C. This, coupled with DLS results, further confirms the assumption from FT-IR spectra that a carbamate cross-link has been formed.

The results from the acid stability experiment show the changes in chitosan hydrogel’s weight for both chitosan hydrogel treated by CO₂ switchable system and chitosan hydrogel that is not pre-treated. For the treated chitosan, after 3 hours the weight of chitosan was largely unchanged and in fact, slightly increased due to swelling.

IV. References

1 M. T. Yagub, T. K. Sen, S. Afroze and H. M. Ang, Adv. Colloid Interface Sci., 2014, 209, 172–184.

2 M. Li, S. Cheng and H. Yan, Green Chem., 2007, 9, 894.

3 R. Huang, Q. Liu, J. Huo and B. Yang, Arab. J. Chem., 2013.

4 P. P. G. Jessop, D. J. D. Heldebrant, X. Li, C. A. C. Eckert and C. C. L. Liotta, Nature, 2005, 436, 1102.

5 P. G. Jessop, S. M. Mercer and D. J. Heldebrant, Energy Environ. Sci., 2012, 5, 7240.

6 D. Nagai, A. Suzuki, Y. Maki, H. Takeno, Chem. Commun., 2011, 47, 8856.

7 V. Stastny and D. M. Rudkevich, J. Am. Chem. Soc., 2007, 129, 1018–1019.