Doctor of Philosophy
Chemical and Biochemical Engineering
Superabsorbent polymers (SAPs) have attracted tremendous attention, with researchers noting that their high water absorption capacity (AC) is valuable for various applications, especially in agricultural contexts. Two types of materials can be used to produce SAPs: fossil-based (which are harmful to the environment) and bio-based (which are significantly more environmentally friendly, given their biodegradability and minimal toxic side effects). Although bio-based SAPs (Bio-SAPs) are preferable due to their environmental merits, their preparation tends to be time consuming and labour intensive, and their AC is still far below expectations. To address these problems, a novel, eco-friendly, cellulose-based superabsorbent polymer (Cellulo-SAP) was developed in this study through facile preparation via free radical synthesis using esterified pure cellulose. First, pure cellulose was esterified with maleic anhydride to evaluate the effects of the catalyst, solvent, and cellulose–maleic anhydride molar ratio. Second, an approach for graft copolymerization of esterified pure cellulose with acrylic acid was developed; this involved free radical synthesis using polyethylene glycol diacrylate as the crosslinker, resulting in the production of Cellulo-SAP. Third, the absorbency, thermal/pH stability, reusability, and biodegradability of Cellulo-SAP were evaluated. This new polymer demonstrated reusability as a water reservoir, in addition to high thermal and pH stability. More importantly, Cellulo-SAP achieved an AC of 475 g/g and exhibited superior biodegradability compared to a commercial, fossil-based SAP (Sigma-Aldrich sodium polyacrylate). These results suggest that Cellulo-SAP can be used in agriculture as an effective alternative to fossil-based SAPs. Finally, crude cellulose was obtained via organosolv fractionation with a mixture of acetic acid, formic acid, and water added to the biomass from cornstalk residues. The crude cellulose consisted of 13.87% lignin, 27.58% hemicellulose, and 57.46% a-cellulose, and it had a 0.41 degree of esterification after fractionation. This crude cellulose was used as a raw material to obtain a Bio-SAP (CrudeCellulo-SAP) via the methodology developed for graft copolymerization and crosslinking of Cellulo-SAP. CrudeCellulo-SAP demonstrated an AC of 369 g/g.
Summary for Lay Audience
Materials with a high absorption capacity (AC) have many applications, which makes them very useful. A common application of such materials is in diaper granules, which absorb large amounts of liquid and expand greatly. Materials of this sort—which absorb more than 300 times their weight—are called “superabsorbent polymers” (SAPs). SAPs also have applications in industries such as construction (e.g., concrete additive), medical care (e.g., drug releaser), and agriculture (e.g., water releaser for plants).
The first step in the agricultural applications of SAPs is hydrating them; a hydrated SAP is known as a “hydrogel.” The second step is mixing the hydrogel with the soil, where it functions as a water reservoir for a plant’s roots. Finally, the water contained in the hydrogel is released; the roots absorb the water, enhancing the plant’s growth and optimizing water irrigation. Since the hydrogel is durable, it can absorb and desorb water several times. After the SAP/hydrogel has been used, it remains in the soil. In the case of fossil-based SAPs—the only SAPs available on the market—this is problematic because they biodegrade very slowly and leave harmful residues in the soil.
SAPs have been benefitting agriculturalists for more than 30 years, and they are increasingly in demand in this industry. SAPs are imperative, but it is also vital to use alternative, natural sources to produce them and make them biodegradable. To this end, the present dissertation explains the development of a methodology to obtain bio-based SAPs, or “Bio-SAPs” (i.e., SAPs that contain natural materials in their composition), as alternatives to fossil-based SAPs. The Bio-SAPs formulated in this study were cellulose-based (Cellulo-SAP) and crude cellulose-based (CrudeCellulo-SAP). Evaluations of Cellulo-SAP and CrudeCellulo-SAP showed high AC in both, highlighting their viability as alternatives to fossil-based SAPs.
Arredondo Ramirez, Rosa Maria, "Production of cellulose-based superabsorbent polymers for soil water retention" (2022). Electronic Thesis and Dissertation Repository. 9093.
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