The fascinating complexity of seagrass bio-fibres: insights from bio-chemo-hydro mechanical analysis for their reuse as soil reinforcement

Beached seagrass bio-fibres are an abundant yet underutilised Mediterranean biowaste with potential for sustainable soil reinforcement. We present the first multiscale bio–chemo–hydro–mechanical characterisation, combining synchrotron nanotomography, FE-SEM, elemental analysis, tensile testing, and water retention measurements. An image-based method applied to Posidonia oceanica links Aegagropile diameter to fibre length, enabling targeted selection to minimise clustering. Air-dried fibres exhibit higher tensile strength, whereas pre-soaked fibres show greater porosity, tortuosity, toughness (~ 2.5×), and water retention (~ 18% more in bundles), due to enhanced capillary storage. Water retention curves reveal two storage domains: intra-fibre adsorption and inter-fibre capillarity. This dual functionality, mechanical enhancement, and improved moisture storage, which mitigates drying effects when interacting with the atmosphere, position Posidonia oceanica fibres as a circular, high-performance alternative to synthetic reinforcements, transforming coastal waste into a valuable geotechnical resource.