Recovery of alkali in Gracilaria agar production by electrodialysis and membrane concentration

Industrial Gracilaria agar extraction consumes a large amount of alkali. The previous strategy was to directly reuse the alkali to extract agar. It eventually led to the accumulation of chemical oxygen demand (COD) in the alkali solution, resulting in the failure of alkali functions. The large amount of COD and alkali from alkali-treated Gracilaria results in resource waste and environmental damage. For this reason, electrodialysis and membrane concentration were used to recover the low-concentration alkali in alkali-treated Gracilaria, and the physicochemical properties of Gracilaria agar extracted by recovered alkali from different methods were evaluated. Results shown that the COD removal rate and the alkali recovery rate of the recovered alkaline solution by electrodialysis reached 69.97 % and 45.31 %, respectively. Furthermore, the optimal conditions for lowconcentration alkali membrane concentration were pressure of 0.50 MPa, flow rate of 40 L/h, and pore size of 1000 Da. Under these conditions, the highest COD removal rate and the alkali recovery rate of recovered alkaline solution by membrane concentration reached 68.46 % and 11.20 %, respectively. The gel strength of agar extracted by electrodialysis and from a low-alkaline solution after five times of membrane concentration increased by 18.2 % and 15.7 %, respectively, compared with those extracted by the traditional method. The Xray fluorescence spectra indicated that elemental K and Ca from the recovered alkali could improve the gel strength of agar. Electron microscopy analysis and Fourier transform infrared spectroscopy revealed no significant differences in the gel skeleton and functional group structure between the recovered alkali-extracted agar and traditional method-extracted agar. After electrodialysis and membrane concentration, the cumulative alkali recovery rate of alkali-treated Gracilaria could reach 37.14 %. This study provided new ideas for the clean production of agar and other marine polysaccharides.