Dielectric Relaxation, Water Sorption Thermodynamics, and Humidity‐Sensing Mechanisms of Compressed Ulva Biomass Pellets

This study investigates the moisture sorption thermodynamics, dielectric relaxation behavior, and humidity‐sensing performance of compressed Ulva powder pellets prepared from Ulva sp. macroalgal biomass harvested from Abu‐Qir Bay, Alexandria, Egypt. Gravimetric sorption isotherms at 25°C exhibited a sigmoidal profile well‐described by the Guggenheim−Anderson−de Boer (GAB) model, yielding a monolayer moisture capacity ( M 0 ) of 14.94%. Frequency‐dependent dielectric and complex impedance measurements across a relative humidity (RH) range of 11.3%−97.3% demonstrated that moisture sorption leads to a substantial increase in permittivity and a multi‐order‐of‐magnitude reduction in electrical impedance. Fitting the real permittivity spectra to the Debye formulation revealed a systematic increase in static permittivity ( ε s ) and a decrease in relaxation time ( τ ) with rising hydration. Generalized Cole−Cole modeling captured non‐Debye relaxation behavior, supporting the interpretation that the electrical response reflects moisture‐induced dipole reorientation and apparent ionic transport within the polysaccharide matrix. The compressed Ulva powder pellets demonstrated favorable humidity‐sensing characteristics, including a low maximum hysteresis error (~1.63%), complemented by rapid, symmetric transient response (t 90 ) and recovery (t re ) kinetics. These quantitative findings establish a foundation for utilizing sustainable compressed Ulva biomass pellets as functional transducing elements in environmental humidity monitoring.