An Innovative Plant-Based Hydrogel Module Employing a Bio-Authentic Strategy for Agricultural Waste Reduction

The study focused on developing a sustainable system for creating high-performance hydrogel modules from agricultural waste, such as rice straw, sugarcane bagasse, and maize stover, to achieve better water retention and nutrient delivery. A design concept known as the "Bio-Authentic Strategy" imitates natural biological systems, such as the plant's own vascular system.

1. Core Concept: The "Bio-Authentic" Strategy

Petroleum-based polymers are frequently used in conventional synthetic hydrogels. Two things are given priority in a bio-authentic approach: replicating the three-dimensional structure of plant tissues (xylem/phloem) to enable effective capillary action, and water storage is known as structural mimicry.

Utilising naturally occurring polymers, primarily cellulose, hemicellulose, and lignin, derived from waste materials, ensures the complete biodegradability and chemical compatibility of the final product with the soil ecosystem.

2. Transforming Agricultural Waste

The module examines the "waste to wealth" cycle by processing standard field residues.

• Feedstock: Sugarcane bagasse, wheat straw, fruit peels, dry fruit peels, and other biomass-based materials, which are cellulosic, hemicellulosic, and lignin. 
• Extraction: Cellulose is isolated via alkali treatment or green solvents (ionic liquids).
• Synthesis: Biodegradable agents, such as citric acid, or physical methods, like freeze-thaw cycles, are employed to cross-link the cellulose, thereby yielding a porous and highly absorbent network.

3. Key Benefits in Agriculture

These plant-based modules act as "mini-reservoirs" buried near the root zone, providing several advantages.

4. Current State-of-the-Research (2024–2025)

Recent breakthroughs in this field focus on "smart" hydrogel in reports. These are designed to respond to specific environmental triggers:

• pH-Responsive: releasing stored water only when the soil reaches a certain acidity/alkalinity level.
• Temperature-triggered: expanding or contracting based on soil heat to manage moisture during heatwaves.
• Nano-Reinforcement: Adding nanocellulose (CNCs) to the module to increase its mechanical strength, allowing it to withstand the pressure of heavy soil without collapsing.
• Note on Sustainability: This strategy aligns with the Circular Bioeconomy, as it prevents the open burning of agricultural waste (reducing CO2 and particulate emissions) while simultaneously tackling water scarcity.

From a chemical standpoint, plant-based hydrogels exploit naturally occurring biopolymers—primarily cellulose, hemicellulose, pectin, starch, alginate, and lignin—which possess abundant hydroxyl (–OH), carboxyl (–COOH), and ether (–O–) functional groups. These functionalities enable crosslinking, swelling, in exchange, and biodegradation, making them ideal matrices for waste reduction and reuse strategies.

Chemical Basis of Plant-Based Hydrogel Formation

2.1 Raw Material Chemistry

Agricultural residues (e.g., rice straw, sawdust, sugarcane bagasse, fruit peels) are chemically composed of:

• Cellulose (β-1,4-linked D-glucose units) – primary structural polymer
• Hemicellulose – branched polysaccharides enhancing flexibility
• Lignin (phenolic polymer) – contributes mechanical strength and hydrophobicity
• Pre-treatment methods such as alkaline treatment (NaOH), acid hydrolysis (H₂SO₄), or enzymatic delignification are used to expose reactive functional groups and improve polymer accessibility.

2.2 Hydrogel Network Formation

Hydrogel synthesis involves the creation of a three-dimensional polymeric network capable of retaining large volumes of water. This can be achieved via:

a) Physical Crosslinking 

• Hydrogen bonding
• Ionic interactions (e.g., Ca²⁺ crosslinking of alginate) - Freeze–thaw cycles

b) Chemical Crosslinking

• Esterification (citric acid, tartaric acid)
• Radical polymerisation (initiators such as APS)
• Schiff base reactions (–CHO and –NH₂ groups)
• A bio-authentic strategy emphasises non-toxic, biodegradable crosslinkers, avoiding synthetic agents like glutaraldehyde.

3. Experimental Methodology

3.1 Extraction of Biopolymers

• Mechanical size reduction of agricultural waste
• Chemical pretreatment (alkaline or acid hydrolysis)
• Washing and neutralisation
• Drying and pulverisation
• Polymer isolation (cellulose/pectin/starch extraction)

3.2 Hydrogel Synthesis

• A typical experimental route includes:
• Dissolution of plant-derived polymers in aqueous medium
• Controlled addition of natural cross-linker
• Heating or stirring under inert/ambient conditions
• Gelation and curing
• Washing to remove unreacted components
• The resulting hydrogel module can be moulded into films, beads, blocks, or layered composites, depending on the application.

4. Characterisation Techniques

• To validate chemical structure and performance:
• FTIR – confirmation of functional group interactions and crosslinking
• XRD – crystallinity changes after gel formation
• SEM – porous network morphology
• Swelling studies – water retention and kinetics
• Thermal analysis (TGA/DSC) – thermal stability
• Biodegradability tests – soil burial or enzymatic degradation

5. Agricultural Waste Reduction Mechanism

• The hydrogel module contributes to waste reduction through:
• Upcycling agricultural residues into value-added materials
• Water retention and controlled nutrient release in soil, reducing irrigation demand
• Carrier matrices for compost nutrients or microbial consortia
• Biodegradable soil amendments, eliminating secondary pollution
• As the hydrogel degrades, it releases organic carbon and nutrients, closing the agricultural material loop.

6. Bio-Authentic Strategy and Sustainability

• The bio-authentic approach ensures:
• Renewable feedstocks
• Green chemistry principles
• Non-toxic synthesis pathways
• Complete environmental assimilation after use
• This strategy aligns with circular economy models, where agricultural waste is not discarded but transformed into functional materials supporting sustainable farming systems.

7. Conclusion

The development of an innovative plant-based hydrogel module represents a chemically robust and experimentally viable solution for agricultural waste reduction. By integrating plant biopolymer chemistry with eco-friendly crosslinking and scalable experimental techniques, this approach offers a sustainable, biodegradable, and multifunctional platform with strong potential for agricultural and environmental applications.