Functional starch-based hydrogels: renewable material solutions for wastewater and agriculture industries

Abstract

Nitrogen (N) recovery is of increasing interest across the world, as concern rises about eutrophication, nitrate leaching, nitrous oxide greenhouse gas emissions, and ammonia particulates. The inefficiencies of existing commercial fertilisers in supplying N nutrients to the crop and the absence of effective management strategies to control N losses from the soil have resulted in nitrogen entering surface and ground water streams, leading to environmental degradation of both water and soil. Thus, there is a need for a sustainable solution to address the inadequacies of current nutrient and water management systems. Among emerging technologies, resource management using biopolymers is a novel approach that offers new opportunities towards more circular strategies. Recently, starch has gained considerable attention in this area as an economic alternative to fossil-based feedstocks. In addition to being chemically modifiable, starch is abundant, renewable, and biodegradable. It can be processed into three-dimensional polymeric networks, known as hydrogels. Hydrogels are a novel class of functional biopolymers with a unique set of properties such as high swelling capacity and structural flexibility. Their dynamic response to external stimuli differentiates them from other polymeric materials, and their tailorability makes them excellent candidates for the design of functional devices, applicable in a variety of technological fields. Starch-based hydrogels have recently been identified as having the potential to be successfully employed in a range of unconventional applications such as removal of reactive ions from wastewater and enhancing the efficiency of N-fertilisers. However, the large-scale production of starch hydrogels is currently limited due to the high cost and complexity of their manufacturing methods. Additionally, there are undesirable by-products and toxic chemical residues that can potentially alter their biodegradability and affect the life-cycle assessment of the products.Therefore, this thesis focused on both the design and development of functional starch-based hydrogels as a sustainable solution for efficient water and nutrient management. In order to address the technical limitations of starch hydrogel production, we combined the benefits of continuous manufacturing with the use of green chemistry principles and developed a method based on reactive extrusion (REx) to produce functional starch hydrogels. The key to developing a routine process was maximising the efficiency of starch chemical modification (graft copolymerisation) using REx. Thus, the processing conditions and formulations were optimised based on swelling capacity, by benchmarking our results against base-line materials, which we prepared in solution using the conventional technique. We then employed the optimised REx method for graft copolymerisation of starch with acrylamide monomer, using five different types of non-food grade starches. The results demonstrated the viability of REx as a processing platform to produce starch-copolymers. As a logical next step, we optimised the swelling capacity in all starch-copolymers using an ionic comonomer, 2-acrylamido-2-methylpropane-sulfoacid. This led to a class of pH-responsive hydrogels that behaved similarly to polyelectrolyte gels, with a high uptake capacity for water and ammonium ions, comparable to those of the synthetic sorbents. Microstructural features of the starch substrate controlled the grafting efficiency but had limited influence on monomer conversion. This offered insight into the potential of REx as an effective alternative method for the production of high-performance N-sorbents based on starch hydrogels.Next, we adapted both the synthesis conditions and formulations to improve the swelling capacity and durability of starch hydrogels. We then examined these hydrogels’ potential for use as soil conditioners via evaluation of three sample-specific parameters, viz. graft content, gel stiffness, and swelling capacity. Thus, a simple but effective REx procedure for chemical crosslinking of starch hydrogels was established. These optimised conditions led to the successful production of chemically crosslinked functional starch hydrogels with a robust structure and high swelling capacity in both deionised water and electrolyte solutions, comparable to those of commercial products. In model-scale, incorporation of cationic starch hydrogel (CTS-hydrogel), with 1.8 wt.% crosslinker content, significantly improved the field capacity of sand, demonstrating the potential for these hydrogels to be further developed into effective soil conditioners.Finally, we studied the aerobic biodegradation of starch, CTS-hydrogel, and a commercial polyacrylamide hydrogel under a controlled laboratory environment, by monitoring CO2 evolution in a soil environment, during a 6-month incubation period. The results demonstrated the accelerating effect of starch on the biodegradation rate of the CTS-hydrogel sample. With a moderate biodegradation rate and estimated half-life of ~1 year, CTS-hydrogel was found to be suitable for application as a soil conditioner.Overall, our results demonstrated the feasibility of reactive extrusion as a processing platform for continuous production of functional starch hydrogels as a full-circle sustainable materials solution for wastewater, agricultural and other industries.