Response Surface Optimization of a Citric Acid Mechanochemical Alginate Extraction from Laminaria digitata for Fiber Production

Alginate fibers are a promising alternative to conventional synthetic fibers; however, the upstream process of extracting alginate remains energy and time intensive. This study aimed to optimize a mechanochemical alginate extraction method using a Box–Behnken design (BBD) and response surface methodology (RSM) and demonstrate its application through the wet spinning of alginate fibers and characterization of their properties. Acid extraction time, citric acid to algae loading ratio, and blade speed were evaluated against six responses: crude alginate yield, ash, phenolic content, protein content, intrinsic viscosity, and M/G ratio. Five responses show statistically significant models and were further investigated using Derringer’s desirability function identified the optimal conditions as 10 min acid extraction time, 4% loading ratio, and 1100 rpm blade speed. The optimized extraction design produced an 18.77% crude alginate yield, 9.01 dL g−1 intrinsic viscosity, 18.75% ash content, total phenolic content 10.07 mg GAE g−1, and M/G ratio of 1.99. The optimized alginate was successfully wet spun into continuous fibers. SEM images showed ribbed surface morphology and fiber diameter ranged from 182 to 269 µm. The tensile strength for the optimized alginate fibers averaged at 8.17 cN/tex and had a fineness of 472.6 dtex. Overall, the results show that a 10 min citric acid blade-based pretreatment can produce calcium alginate fibers from brown macroalgae.