Development of Cellulose-Based Superabsorbent Hydrogels from Coconut Fiber

Abstract

The upsurge in the global population leads to a rise in agricultural production to meet the growing food demands which in turn results in increased waste generation. Moreover, the rise in water demand for farming purposes necessitates the need to utilize the little we get effectively and prevent water loss. Thus, the use of superabsorbent hydrogels is a possible solution to water conservation. In this study, cellulose-based hydrogels were synthesized from coconut fiber. This was achieved through extraction of cellulose using soda process, then modified to carboxymethyl cellulose (CMC) by esterification, followed by crosslinking with commercial hydroxyethyl cellulose (HEC) using citric acid in a non-polar solvent. The bulk density, tapped density, and swelling capacity of cellulose, CMC and the CMC-HEC were then evaluated. The functional groups, the thermal properties, and degree of crystallinity of cellulose, carboxymethyl cellulose and the hydrogels were evaluated using a Fourier Transform infrared spectrophotometer (FTIR), Thermal Gravimetric Analyzer (TGA), Differential scanning calorimeter (DSC), and an X-ray diffractometer (XRD). The water-holding capacity of the synthesized hydrogel was evaluated by growing tomatoes in hydrogel-modified soils for 54 days at different water regimes. The tomatoes grown in soils without the hydrogels were used as a control group. The statistical analysis showed that the cellulose yield, bulk density, and swelling capacity was 42.1 ±1.5%, 0.08 ±0.0 g/cm3 and 8.5 ±0.14 g of water per gram of cellulose respectively. Upon introduction of the carboxymethyl groups, the degree of substitution and the swelling capacity of the resulting carboxymethyl cellulose was found to be 1.82 ±0.12 and 11.3 ±0.28 g of water per gram of CMC respectively. This swelling capacity was further enhanced by chemical crosslinking of CMC with commercial HEC using citric acid as a crosslinking agent. An optimum swelling capacity of 30 g of water per gram of hydrogel was obtained when the hydrogel was prepared using 2.75% of citric acid as the crosslinking agent. Infrared spectra of the hydrogels displayed a carbonyl functional group frequency at 1652 cm-1, which was an indication that the crosslinking reaction between CMC and HEC had occurred. Changes in the degree of crystallinity and thermal stability observed from TGA/DSC thermograms, and X-ray diffractograms were a result of reactions that occurred when cellulose was converted to CMC and the hydrogels. Plants grown in soils containing the hydrogel had a higher plant height, leaf length, average number of leaves, moisture content, and higher root density as compared to the controls at the different water regimes. In conclusion, the cellulose-based superabsorbent polymer obtained from coconut fiber improved the water retention of soil mixed with the hydrogels and thereby resulted in improved plant growth and is recommended.