Citric Acid‐Mediated Pretreatment of Seaweed Residue: Towards High‐Performance Carboxylated Nanocellulose for Polyvinyl Alcohol Film Reinforcement

ABSTRACT Carboxylic nanocellulose (Car‐SRNC) was successfully isolated from seaweed residue (SR) via aqueous citric acid (CA) pretreatment followed by ultrasonic disintegration, with the objective of enhancing the mechanical performance of polyvinyl alcohol (PVA) composite films. Statistical analysis revealed a strong positive linear correlation between the carboxyl content of Car‐SRNC and both its yield and the mechanical strength of Car‐SRNC/PVA films (Pearson's r ≈ 0.97, R 2 ≈ 0.95), whereas no significant correlation was observed with CNC content. Based upon preliminary single‐factor experiments, a systematic optimization of process parameters was performed using a three‐level five‐factor central composite design integrated with response surface methodology to evaluate the interactive effects of five critical variables: catalyst/SR ratio (0.5–1.5 g/100 g), CA concentration (75%–85%), pretreatment temperature (110°C–130°C), pretreatment time (4–8 h), and liquid–solid ratio (25–35 g/g). The developed models demonstrated high predictive accuracy, as indicated by a coefficient of determination of 0.9517, confirming their reliability for process prediction and optimization. Under the optimized conditions—catalyst/SR ratio of 0.98 g/100 g, CA concentration of 82.8%, pretreatment temperature of 122.5°C, pretreatment time of 7.5 h, and liquid‐to‐solid ratio of 30.8 g/g—the maximum carboxyl content of Car‐SRNC reached 7.20 mmol/g, with a corresponding yield of 64.3%. When incorporated as a reinforcing agent, Car‐SRNC with high carboxyl content significantly enhanced the tensile strength, tensile strain, Young's modulus, and toughness of the composite film by 53.82%, 13.53%, 173%, and 180%, respectively, compared to neat PVA films. This study presents a scalable and sustainable approach for the valorization of SR, a major byproduct of the alginate extraction industry.