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Alginate, Deep Eutectic Solvents, Precipitation

Abstract

Brown seaweed is an abundant marine resource mainly exploded for its content of an industrial-relevant hydrocolloid: Alginate. This phycocolloid represents a relevant biomaterial with a broad set of current and potential applications. It is used as a thickener and as a gelling agent in the food industry; or within the medical sector, as a coating material for improving drug delivery and for the production of dental imprints. The alginate extraction
comprises several steps with different basic or acid chemical additions. Additionally, other chemicals such as paraformaldehyde could be added to remove pigments and for seaweed preservation to avoid the costly drying
process (Saji et al., 2022). Together with the harsh chemicals, the usage of large amounts of water for better handling of the viscous alginate solution, makes the process unsustainable.

One important green technology recently explored for the valorisation of bio-compounds more sustainably, is the utilization of Deep Eutectic Solvents (DES). These types of solvents are defined as a mixture of two or more
components: Brønsted-Lewis bases and acids. They possess high melting points individually but when mixed the melting point of the mixture decreases dramatically. Normally composed of a Hydrogen-bond acceptor (HBA) and
a Hydrogen-bond donor (HBD). Hydrogen bonds between the compounds determine their good solvent properties (Naik et al., 2022). Deep eutectic solvents have drawn attention since they can be prepared from natural and
biodegradable compounds, the so-called Natural Deep Eutectic Solvents (NaDES). Eutectic mixtures could be obtained from mixing ammonium quaternary salts such as choline chloride with amides, polyols, sugars, organic
acids, amino acids, etc. This makes their manufacture potentially more sustainable and inexpensive. Additionally, as many combinations could be achieved and with different proportions of each compound, a DES can be tuned to
provide the optimal conditions for the target (s) compound extraction. Several investigations have been conducted to extract compounds such as lipids, pigments, and polyphenolic compounds with potential bioactive applications
with relative success (Kaoui et al., 2023). However, the extraction of macromolecules represents a bigger challenge due to the complex interactions that could come up with the size and the chemical nature of the molecule.
Before the extraction, a reliable method for alginate quantification needs to be developed. The polymer content is normally determined, based on the analysis of its uronic acids which are released after an acid hydrolysis process.
Colorimetric methods exist, however, unexpected reactions occur with some DES compounds. Besides, care must be taken to maintain the structure integrity of the target molecule after the extraction for further applications, and
the analytical procedure should reflect this. Therefore, the separation of alginate from DES for analytical purposes should be accomplished. Alginate in water solutions could be chemically separated via precipitation with acid,
ethanol or calcium-rich solutions and the pellet used for further analysis. This research work aimed to evaluate different chemical methods to separate the alginate from DES solutions for quantification purposes and identify
potential interactions between the solvents and the biopolymer.

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Figure 1. Approach considered for the isolation and quantification of alginate in DES.

DES containing sodium alginate were subjected to precipitation with sulfuric acid 0.2 M (pH 1.6), ethanol-water mixture (80 % v/v) and calcium chloride (1 % w/v CaCl2·2H2O) (Figure 1). Alginate in precipitates was quantified
and used to evaluate the performance of each separation technique. The highest recovery yields (51.2 ± 1.3 %) were obtained using the ethanol-water mixture followed by calcium chloride (45.7 ± 1.2 %), except for polyols
(e.g. sorbitol). The lowest recovery yields were obtained with acid, with a particularly low recovery yield when urea was used as HBD (9.6 ± 1.3 %) (Reynaga-Navarro, 2024). This research work provides insights into the
feasibility of chemical techniques for alginate separation from DES and it will contribute to developing a sustainable seaweed biorefinery platform using this emerging technology.

Acknowledgement

This research work was funded by the Dutch Research https://app.dimensions.ai/details/grant/grant.12924032. Grant number: 19479. Council (NWO)

References

  • Kaoui, S., Chebli, B., Basaid, K., & Mir, Y. (2023). Deep eutectic solvents as sustainable extraction media for plants and food samples : A review. Sustainable Chemistry and Pharmacy, 31(September 2022), 100937. https://doi.org/10.1016/j.scp.2022.100937
  • Naik, P. K., Kumar, N., Paul, N., & Banerjee, T. (2022). Deep Eutectic Solvents in Liquid–Liquid Extraction. In Deep Eutectic Solvents in Liquid–Liquid Extraction. https://doi.org/10.1201/9781003231158
  • Reynaga-Navarro, W. (2024). Isolation and quantification of alginate in choline chloride-based deep eutectic solvents. International Journal of Biological Macromolecules, 262(January). https://doi.org/10.1016/.ijbiomac.2024.130103
  • Saji, S., Hebden, A., Goswami, P., & Du, C. (2022). A Brief Review on the Development of Alginate Extraction Process and Its Sustainability. Sustainability, 14(9), 5181. https://doi.org/10.3390/su14095181