SYNTHESIS AND APPLICATION OF NATURAL POLYMER BASED HYDROGELS

Abstract

The depletion of fossil-based resources and increasing environmental concerns have intensified the search for sustainable, biodegradable, and eco-friendly materials. Natural polymer-based hydrogels derived from humic acid (HA), lignin, and lignite have emerged as promising candidates due to their multifunctional properties, including their ability to enhance soil moisture retention, control agrochemical release, adsorb pollutants, and contribute to energy storage applications. This research focuses on the synthesis, characterization, and application of these bio-based hydrogels, systematically investigating their structural, thermal, rheological, and morphological properties to optimize their performance for various applications. The study begins with a comprehensive literature review exploring the significance of humic acid, lignin, and lignite in hydrogel synthesis and their applications in agriculture, and environmental remediation and energy storage. Key research gaps were identified, highlighting the need for more extensive studies on the multifunctionality of these natural polymers in hydrogel systems. The first experimental phase involved the synthesis and application of a lignosulfonate-grafted sodium acrylate hydrogel (LS-g- SAH) for controlled urea release. This hydrogel demonstrated 60% release of urea from LS-g-SAH in 24 h, significantly reducing nutrient leaching and improving soil fertility. The soil's water-holding capacity remarkably raised from 21.27 to 77.3 g using synthesized hydrogel. Also, the water evaporation rate reduced from 99 to 76.69% of the total added water. Enhanced water retention further contributed to increase in plant growth efficiency, making it a potential alternative to conventional fertilizers. vi Following this, a humic acid-grafted sodium acrylate hydrogel (HA-g-SH) hydrogel was developed for controlled pesticide release, taking dinotefuran as model pesticide. The hydrogel enabled sustained pesticide release over 49 h 78.45%, improving bioavailability and increasing pest control efficiency of dinotefuran. This approach minimized environmental toxicity by reducing pesticide leaching in soil and water. To further enhance the stability of pesticides, a lignite- sodium acrylate hydrogel (Lt-g- SAH) was synthesized for UV protection of dinotefuran. The study revealed that after 30 days of UV irradiation, Lt-g-SAH group exhibited remarkable stability, with only a marginal decrease of 1.58% in Dinotefuran release, resulting in a final concentration of 41.73 mg. In stark contrast, the Ctrl group experienced a substantial reduction of 28.11% in Dinotefuran release, with the final concentration diminishing to 25.42 mg. This marked disparity in pesticide release profiles can be attributed to the differential susceptibility of the formulations to UV-induced degradation over time. Based on the controlled-release capabilities, HA-g-SAH, LS-g-SAH, and Lt-g-SAH hydrogels were explored for comparative delivery of thiamethoxam. These hydrogels facilitated a controlled pesticide release for 49 h, reducing pesticide’s application frequency, and minimizing environmental contamination. The release pattern followed the Fickian release mechanism as stated by the Korsmeyer-Peppas model and the Weibull model. The swelling index revealed a distinct order, with HA-g-SAH exhibiting the highest absorbency, followed by LS-g-SAH, Lt-g-SAH, and the control group. Moreover, the synthesized hydrogels demonstrated a significant impact on reducing soil water evaporation rates from 99 to 72.85% (HA-g-SAH), 74.79% (LS-g- SAH), 78.23% (Lt-g-SAH), and 85.23% (Ctrl) of the total added water on the 54th day. vii Beyond their agricultural applications, these hydrogels were investigated for their potential in textile wastewater treatment. The comparative study demonstrated that HA- g-SAH, LS-g-SAH, and Lt-g-SAH hydrogels exhibited high adsorption capacities for methylene blue (MB), a common industrial dye, Under optimized conditions, HA-g- SAH achieved a maximum removal efficiency of 93.7 ±1.1% at 323.15 K, significantly outperforming LS-g-SAH (92.4 ±1.3%), Lt-g-SAH (82.4 ±1.4%), and the control (Ctrl) hydrogel (67.9 ±0.9%). The presence of functional groups in humic acid and lignin enhanced their binding affinity, making these hydrogels promising candidates for large- scale wastewater treatment applications. This thesis provides a comprehensive analysis of natural polymer-based hydrogels and their potential to address critical challenges in agriculture, and environmental management. The findings suggest that humic acid, lignin and lignite based hydrogels can significantly reduce environmental pollution, enhance soil fertility, improve plant growth, and remove contaminants from water. Further research should focus on optimizing the scalability, durability, and biodegradability of these hydrogels to maximize their practical benefits and commercial viability.