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and Japan, respectively. Hardwood bleached kra pulp (HBKP), sodium hypochlorite (NaOCl), sulfuric acid (H 2 SO 4 ), sodium carbonate (Na 2 CO 3 ), calcium chloride (CaCl 2 ), and ethanol were purchased from Fuji lm Wako Pure Chemical (Osaka, Japan). Deionized water (DW) used for the entire experiment was ob- tained from a WR600G ultraviolet water puri cation system (Yamato Scienti c, Tokyo, Japan). 2.2 Bleaching treatment of the brown seaweeds 10 g of dried seaweed was bleached with a NaOCl solution containing 3% active chlorine obtained by diluting 5% commercial active chlorine (Fuji lm Wako Pure Chemical Corporation, Japan) for 10 min to eliminate the pigments. The bleached seaweeds were then rinsed until the pH was almost neutral. 2.3 Extraction process with sodium alginate The bleached seaweed and unbleached seaweed were rst immersed in 200 mL of 0.05 M H 2 SO 4 at 40 °C for 1 h, sieved, washed until the pH was almost neutral, and centrifuged, so that they can be separated easily from the washing water. Subsequently, the acid-treated seaweed was reacted with 200 mL of 2% Na 2 CO 3 solution at 70 °C for 2 h. Then, ltration was carried out through a muslin cloth to remove agglomerates and centrifuged. Finally, the crude sodium alginate solution was obtained. The samples processed via the bleaching process are referred to as Bleached Sargassum (BS) and Bleached Kelp (BK), while the samples not subjected to the bleaching process are referred to as Unbleached Sargassum (US) and Unbleached Kelp (UK). 2.4 Puri cation and precipitation process of sodium alginate Twodi ff erentpuri cation processes were used, and the calcium and acid routes were compared with the crude sodium alginate solution obtained in Fig. 1. The precipitation process followed the method described by Gomez et al. 21 with some modi cations. 2.4.1. Acid route. In this route, the crude sodium alginate solution was mixed with 200 mL of 0.5 M H 2 SO 4 using a magnetic stirrer at room temperature (RT) for 1 h (pH 2). Next,
Fig. 2 Mechanisms involved in the production of alginate fi bres by a chemical reaction between sodium alginate and calcium chloride with SEM images of calcium alginate fi bres surfaces with its spinning process.
the alginic acid gel was washed and separated via centrifuga- tion. The alginic acid gel was added to 200 mL of 0.5 M Na 2 CO 3 and mixed using a magnetic stirrer at RT for 1 h to convert it to sodium alginate. The solution was then precipitated by the addition of ethanol (1 : 1 volume ratio), centrifuged, and dried at 40 °C. 2.4.2 Calcium route. The crude sodium alginate solution was precipitated by the addition of 200 mL of 0.5 M CaCl 2 solution. Calcium alginate was washed with DW and ltered. Then, 200 mL of 0.5 M H 2 SO 4 was added and stirred at RT for 2 h. The alginic acid was washed and separated by centrifuga- tion and 200 mL of 0.5 M Na 2 CO 3 was added and mixed using a magnetic stirrer at RT for 2 h to convert it to sodium alginate by ion exchange. The solution was precipitated by addition of ethanol (1 : 1 volume ratio), centrifuged, and dried at 40 °C. The samples produced through the acid route process are referred to as the Bleached Sargassum Acid Route (BSAR), Bleached Kelp Acid Route (BKAR), Unbleached Sargassum Acid Route (USAR) and Unbleached Kelp Acid Route (UKAR). While the samples that undergo the calcium route process are referred to as the Bleached Sargassum Calcium Route (BSCR), Bleached Kelp Calcium Route (BKCR), Unbleached Sargassum Calcium Route (USCR), and Unbleached Kelp Calcium Route (UKCR). 2.5 Calcium alginate bre production through the wet spinning process A 3% sodium alginate solution was syringed with a 28-gauge needle and then extruded into a 5% CaCl 2 solution bath. The extruded solution became brous, and it was dried at RT. The reproduced calcium alginate bres were cut into short bres approx. 2 mm long. Fig. 2 shows the chemical reaction between sodium alginate and the calcium chloride solution. In this process, three types of calcium alginate bres were prepared, namely, a commercial alginate bres (CA), kelp calcium algi- nate bres (KA), and sargassum calcium alginate bres (SA). CA
Fig. 1 Schematic diagram for the extraction of sodium alginates.
RSC Sustainability , 2025, 3 , 599 – 610 | 601
© 2025 The Author(s). Published by the Royal Society of Chemistry
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