Physico-chemical and antibacterial properties of crosslinked hydrogel from regenerated cellulose and different types of polysaccharides for slow-release fertilizer

Abstract

Regenerated cellulose (RC) hydrogel is an alternative for planting medium to reduce water usage and nutrient leaching as a method of sustainable agriculture. In this study, a facile simple-one-pot method was employed to produce RC hydrogel as slow-release fertilizer (SRF) medium. However, RC hydrogel alone is lacking some properties such as exposure to pathogens attack that causes degradation of the hydrogel as well as less flexible in water re-uptake process in an acidic and alkaline environment. Besides, very limited studies are reported on using sodium hydroxide/urea (NaOH/urea) solvent uses to produce RC can dissolve other materials other than cellulose. Also, SRF ability is highly depending on the formulation of the crosslinked hydrogel matrix. The aims of this study were, (i) to study the effect of the addition of other polysaccharides in the same NaOH/urea solution on the properties of RC hydrogel, (ii) to evaluate the effect of different crosslinkers on the properties of RC hydrogel, (iii) to investigate the effect of antibacterial polysaccharides addition on the properties of RC hydrogel in the aspect of antibacterial and pH-responsive abilities and (iv) to compare the effect of different crosslinkers toward the loading and slow-release of fertilizer. RC hydrogel was fabricated alongside different polysaccharides chitosan (CS) (0.75, 1.00, 1.25 and 1.50 wt%) and alginic acid (AA) (0.75, 1.00, 1.25 and 1.50 wt%) as antibacterial agents as well as carrageenan (CG) (2.00 wt%). There were two types of RC hydrogel formulations were fabricated, (i) RC, CG and CS (RCCGCS) and (ii) RC, CG and AA (RCCGAA). Three different crosslinkers were used to crosslink the hydrogel formulations – epichlorohydrin (ECH), N,N-dimethylbisacrylamide (MBA) and citric acid (CA). All hydrogels were fabricated by using NaOH/urea solution. The morphological, chemical and physical properties of all hydrogels were determined. CA crosslinked hydrogels showed de-swelled swelling capacity, thus, MBA- and ECH crosslinked RCCGCS and RCCGAA cryogels were tested for SRF ability was tested with potassium nitrate (KNO3) (3, 5 and 10 wt%). NaOH/urea cellulose dissolution system was able to dissolve CS and AA homogenously with all crosslinkers tested. Also, the addition of CS and AA enabled the hydrogels to have pH-responsive with the highest water re-uptake at pH 7. For antibacterial activity, CS and AA successfully reduced the growth of Gram-negative and Gram-positive bacteria. Also, the addition of CS and AA increased the swelling percentage of ECH-crosslinked hydrogels on RCCGCS (CS 1.25 wt%) and RCCGAA (AA 1.25 wt%) that recorded 133.96 ± 3.87% and 141.63 ± 0.11%, respectively. For MBA-crosslinked hydrogel, MBA-RC hydrogel showed the highest swelling capacity at 313.31 ± 11.1% due to the hydrophilic and amorphous nature of MBA has interfered with the crystallinity of cellulose. For the SRF study, ECH-crosslinked hydrogels showed good gel fraction and the ECH-crosslinked RCCGCS was able to hold a high percentage of KNO3 (10 wt%) with only 20.45 ± 0.03% released after 24 h. The EDX mapping of potassium and nitrogen also confirmed the high entrapment of KNO3 in ECH-crosslinked RCCGCS. This study had shown the flexibility of RC to be blended with other polysaccharides in the same dissolution system. Also, high water retention by hydrogel and able to release fertilizer slowly were economical plantation mediums for sustainable agriculture.