Preparation of Eggshell–Sodium Alginate/Polyvinyl Alcohol Composite Hydrogel Carrier-Embedded Rhizobacteria and Their Storage Properties

Rhizobial inoculants play a critical role in sustainable agriculture by enhancing biological nitrogen fixation; however, maintaining rhizobial viability during storage remains a formidable challenge. In this paper, six embedded rhizobial formulations, designated E-SA/PVA R1 through E-SA/PVA R6, were prepared via an embedding method using sodium alginate (SA), polyvinyl alcohol (PVA), and eggshell powder as primary materials with varying component ratios. Structural characterization confirmed the suitability of the composite hydrogel carriers for rhizobial encapsulation: Fourier-transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA) revealed effective crosslinking between SA and PVA, yielding a stable matrix. Scanning electron microscopy (SEM) confirmed the successful encapsulation of abundant rhizobia within the carriers. Mercury intrusion porosimetry (MIP) was employed to characterize the pore architecture of the composite carriers, revealing the relationships among porosity, average pore size, and permeability. Mass transfer evaluation using three probe molecules of differing molecular weights yielded diffusion permeability coefficients (DPC) that quantitatively elucidated the size-dependent transport behavior within the carrier matrices. All embedded formulations displayed favorable biodegradability in soil environments. Notably, the embedded agents maintained viable cell counts above 8 log CFU/g across all formulations even after 60 days under various pH and temperature conditions, satisfying the microbial fertilizer standard in China, establishing them as promising candidates for practical rhizobial inoculant applications.