(168e) Preparation and Characterization of Poly(vinyl alcohol)/Sodium Alginate/TEMPO-Oxidized Cellulose Nanofiber Hydrogel for Dye Removal

In recent years, water pollutants such as synthetic dyes have become a growing concern due to their threats to the environment and living things. Synthetic dyes are generally used in various industries due to their high solubility in water, easy application and inexpensive properties. However, ingestion of dyes can cause serious health problems. Especially, since cationic dyes (e.g. methylene blue) can interact with negatively charged cell membrane surfaces, they can enter cells easily, leading to long-lasting influences on human organism. Although various methods have been developed for treating dye effluents, adsorption offers various advantages such as the ability to treat a wide variety of dyes, ease of operation, no formation of toxic by-products and secondary wastes. Therefore, it is considered as one of the preferred techniques for dye removal.

Poly(vinyl alcohol) (PVA) hydrogels have been studied for water treatment by many researchers due to their low cost, non-toxic, high chemical stability, and porous characteristics. However, the poor mechanical strength and tendency to agglomerate are the main drawbacks of their applications. Various modification methodologies have been used to enhance their physicochemical characteristics. For instance, crosslinking of PVA hydrogel with sodium alginate (SA) was reported to modify the surface properties of the resulting hydrogel to suppress agglomeration. Cellulose nanofiber (CNF) has also been proved to be an effective mechanical reinforcing agent for enhancing mechanical properties. However, incorporation of both SA and CNF into PVA hydrogel and in detailed examination of synthesis conditions on their properties have not been reported yet.

In this study, the effect of synthesis conditions of the composite hydrogel of poly(vinyl alcohol) (PVA)/sodium alginate (SA)/TEMPO-oxidized cellulose nanofiber (CNF) on their mechanical and dye removal properties was examined in detail. The composite hydrogels were prepared by cross-linking PVA, SA, and CNF with boric acid/borax and calcium chloride by changing weight ratios of SA and CNF with maintaining PVA and (SA + CNF) weight ratio. Especially, we aimed at studying the impacts of weight ratios of SA and CNF on the mechanical strength and dye adsorption capacity of the hydrogels.

Preparation of PVA/SA/CNF hydrogels was carried out as follows: four different 50 mL of polymers solutions containing PVA, SA, and CNF were prepared by changing each component concentration. In this study, while the PVA concentration and the (SA + CNF) concentration were maintained as constant (7.5 and 1.0 wt%, respectively), the weight ratio of SA:CNF was changed from 1.0:0 to 0.7:0.3. By dripping a PVA, SA and CNF mixture solution into 500 mL of cross-linking agent solution containing 5 wt% boric acid/borax and 3 wt% calcium chloride at a dripping rate of 100 mL h^-1, hydrogel beads were obtained. The beads were washed with 400 mL of deionized water for 24 h with stirring and stored in 3 wt% calcium chloride solution. The samples of the obtained hydrogels are referred to as “SA weight ratio”:“CNF weight ratio”. For example, the sample prepared with the weight ratio of SA:CNF of 0.9:0.1 is named as the 9:1 hydrogel.

The mechanical properties of hydrogels were analyzed with a tabletop testing machine (A&D STB-1225S, Japan) using a 50 N load cell. The compressive strength tests were performed with a crosshead speed of 10 mm min^-1. The result shows that the compressive strength of hydrogels (elastic modulus) varies depending on the weight ratio of SA and CNF in the samples in the following order: 7:3 < 8:2 < 9:1 < 10:0. The 10:0 hydrogel shows the largest elastic modulus of 1.0 MPa, and the value was 1.8 times larger than that of the 7:3 hydrogel. Then, the durability of the hydrogels was examined using a Tornade overhead stirring system (As One SM-101, Japan) under very harsh stirring conditions. 100 hydrogel beads were dispersed in 1.0 L of deionized water and stirred very strongly at an agitating speed of 2250 to 3500 rpm for 5 min. Then, the amount of hydrogel beads without any damage (unbroken gel beads) was counted. Based on the result, the breakage ratio of the hydrogel beads was calculated. The result exhibited that the durability of the hydrogels is in the following order: 7:3 ≒ 8:2 < 10:0 < 9:1. When the samples were agitated at high speed of 3000 rpm for 5 min, although all the beads for both of the 8:2 and 7:3 hydrogels were broken, approximately 58% and 88% were remained unbroken for the 10:0 and the 9:1 beads. Finally, dye adsorption kinetics and isotherms of the hydrogels were examined by using methylene blue (MB) as a model artificial dye in batch processes. MB is a cationic dye widely used in textile industry. In this experiment, the hydrogels with high mechanical strength and durability (the 10:0 and 9:1 hydrogels) were selected to perform adsorption test. The experimental data was analyzed by adsorption kinetic model and Langmuir isotherm model. The result showed that the dye adsorption fitted the pseudo-second order kinetics and the Langmuir isotherm model well. Moreover, compared to other reported adsorbents, the maximum adsorption capacity of 243.9 mg/g for the 9:1 hydrogel was 1.4 times higher than that of the highest value reported by other researchers.

In conclusion, the PVA/SA/CNF hydrogels with different weight ratios of SA and CNF while maintaining PVA ratio as constant were prepared. The results of compressive stress tests and durability tests exhibited that the PVA/SA/CNF hydrogels prepared with 7.5 wt% PVA, 0.9 wt% SA and 0.1 wt% CNF (9:1 hydrogel) had the highest durability and second highest young modulus. The 9:1 hydrogel also showed the maximum adsorption capacity for MB of 243.9 mg/g, which is 1.4 times higher than the reported value. These results indicate that in this study the synthesis condition of the 9:1 hydrogel would be appropriate for obtaining MB adsorbent with both of the maximum adsorption capacity and the stability for long term or repeated use.