Structural and Rheological Properties of Chitosan‐Biochar Composites Prepared With Waste Biomass‐Derived Biochar

ABSTRACT

Chitosan‐biochar composites, which combine chitosan's cationic biopolymer matrix with the porous carbon framework of waste‐derived biochar, are widely reported for adsorption and enzyme immobilization. However, their rheological behavior and constitutive modeling, which are critical for processability in drilling fluids and injectable formulations, remain largely uninvestigated. In this work, biochar was produced from vine pruning residues by ZnCl ₂ activation (1:3 biomass:ZnCl ₂ mass ratio) and pyrolysis at 600°C, and then dispersed in a 2% (w/v) chitosan matrix at 0.25%, 0.50%, 0.75%, and 1.00% (w/v) loadings. SEM showed that chitosan partially coated the biochar surface without visibly obstructing its macro and mesoscale pore openings. Nitrogen adsorption–desorption at 77 K yielded a BET surface area of 202 m ² /g and a total pore volume of 0.113 cm ³ /g, with a 45% micropore contribution. TGA revealed a broadened and shifted DTG maximum in the composite relative to pure chitosan (peak near 300°C), and XRD showed that the characteristic chitosan reflections at 2 θ ≈10° and 20° were largely replaced by a broad turbostratic carbon halo centered at 2 θ ≈22°–24°, indicating substantial disruption of polymer crystallinity. FTIR showed slight broadening and a shift in the O–H/N–H stretching region, consistent with interfacial hydrogen bonding. Steady‐state flow curves were recorded from 0 to 45,000 s ⁻¹ over 24°C–33°C. All formulations exhibited shear‐thinning behavior, and biochar addition progressively raised shear stress at comparable shear rates. The Carreau–Yasuda, Cross, and Bingham plastic models were fitted to the flow curves under physical parameter bounds. The Bingham plastic model gave the most reliable description, with an apparent yield stress in the range 2050–3030 Pa and a plastic viscosity between 0.017 and 0.025 Pa·s across all samples and temperatures. The Carreau–Yasuda and Cross fits gave well‐constrained shear‐thinning indices ( n  = 0.02–0.19; m  = 0.81–0.98), consistent with a pronounced pseudoplastic response. At 1.00% loading, increased data scatter and diminished model compliance signaled incipient particle aggregation and marked the practical upper limit of effective reinforcement. This study establishes, for the first time, a quantitative constitutive framework linking biochar loading to macroscopic flow response in chitosan matrices, and defines a 0.50%–0.75% loading window in which flow properties are tunable, predictable, and reproducible.