Greener citrate-assisted extraction of sodium alginate: process optimization and the mechanical performance of alginate-based films

Abstract

Alginate, a biopolymer sourced from brown seaweed, is of growing interest for bio-based materials, highlighting the need for sustainable extraction methods tailored to deliver physicochemical properties suitable for material fabrication. This work investigates a low-hazard sodium citrate chelation process that replaces multi-step extraction with a single mild-pH operation for cation removal and alginate isolation. The effects of temperature, time, and citrate concentration on the yield and physicochemical properties were evaluated using the response surface methodology. Optimum conditions (49.5 °C, 1 h, and 0.125 M citrate) resulted in a 21.0 ± 0.6% yield and a high molecular weight (508 ± 18 kDa), a ninefold increase compared to that with non-optimized extraction. The method was reproducible at a 20-fold scale and verified by Fourier-transform infrared spectroscopy analysis. Films derived from the optimized alginate exhibited enhanced flexibility (tensile strain = 11 ± 2%) and lower stiffness (Young's modulus = 2091 ± 236 MPa) relative to those from lower molecular weight commercial alginate. Fourier-transform infrared spectroscopy, differential scanning calorimetry, and scanning electron microscopy showed that the optimized films maintained comparable microstructural density and glass transition temperature to commercial films, with subtle differences in ionic coordination and thermal stability. In terms of environmental assessment, the optimized extraction reduced the total energy use by 62% and global warming potential by 17%, compared to non-optimized extraction. This work advances a scalable, lower impact process consistent with green chemistry principles for producing alginate suitable for bio-based plastics.