A closed-loop process to transform mixed plant biomass waste into cellulose acetate bioplastic as innovative growing substrates in plant cultivation

Biomass waste is an abundant resource. The current biomass valorisation is focused on purified or more homogeneous streams of waste, such as spent grains from breweries. Mixed plant biomass waste is usually not valorised due to the high cost of separation, collection and transportation. Moreover, the challenge extends to space exploration, where the management of mixed plant biomass waste through storage or incineration is not sustainable for long-duration missions. Additionally, Rockwool, widely used in hydroponics both terrestrially and in space, presents health hazards and environmental disposal issues. There's a critical need to valorise mixed plant biomass waste and to develop safer, more eco-friendly substrates for plant growth. This study presents a circular approach to convert mixed plant biomass waste into bioplastics for plant growth substrates, with the process completed by recycling plant residues back into bioplastics. Mixed garden waste was treated with alkaline hydrogen peroxide (AHP) to enhance cellulose content by 75% while decreasing hemicellulose and lignin contents. The effectiveness of AHP in extracting cellulose was verified using FTIR and SEM analyses. Cellulose was transformed into cellulose acetate (CA) using a green acetylation process, achieving a degree of substitution of 2.5. Triethyl citrate (TEC) plasticized the CA, which was then 3D printed into porous structures for growing garden peas. Post-harvest, the plant debris was reconverted into CA, showcasing a sustainable cycle of plant biomass waste valorisation. This research has proved the concept of converting mixed plant biomass waste into growth substrates for hydroponic systems, offering advantages to the space, horticulture, waste management, and bioplastics sectors.