Table 2 Chemical characterization of Dictyota dichotoma and comparison with other raw materials. HWS: hot water solubility. EBE: ethanol-benzene extractives. WICH: water-insoluble carbohydrate content. ARCH: alkali-resistant carbohydrate content. KLAS: acid-insoluble compounds Genus Dictyota Ulva Rhizoclonium Cladophora HWS (%) 20.2 ± 0.6 33.4 34.6 - EBE (%) 7.2 ± 1.1 3.8 9.43 - ASH (%) 15.8 ± 3.1 19.8 15.9 2.48 WICH (%) 51.4 ± 4.5 47.8 44.1 21.4 ARCH (%) 30.6 ± 1.1 40.7 - 17.1 KLAS (%) 16.1 ± 0.5 7.9 3.8 4.64 Source This work Moral et al. 2019 Chao et al. 1999 Mukherjee and Keshri 2019 A ¦ fth of the seaweed mass, including inorganic salts and some carbohydrates, was found to be soluble in hot water (20.2 ± 0.6 %). Seemingly, inorganic salts accounted for most of that, as the ash content in D. dichotoma was as high as 15.8 ± 3.1 %. Sand, deposits or encrusted carbonates greatly contribute to the mineral fraction. In spite of silt being commonly removed during ash determination, in practice it represents part of the chemical composition of the harvest (Sculpthorpe, 1967). Rupérez and Saura- Calixto (2011) found ashes in some Spanish seaweeds to be very abundant and variable (21-39.8%) in all the species studied. The percentage of water-insoluble carbohydrates (51.4 ± 4.5%) is well below any cellulosic or lignocellulosic raw material, in which this fraction encompasses α -cellulose and hemicelluloses accounting for 60-80% (Jiménez et al., 2008). However, it was unexpected that 30.6 ± 1.1% resisted alkaline extractions, which makes way for an easy solubilization of proteins and lipids while keeping enough material to be used as papermaking additive. Finally, the acid-insoluble content (16.1 ± 0.5%), while lower than the aromatic-rich lignocellulosic sources (26.2%) (Jiménez et al., 2008), looks surprisingly high when compared to other algae (Chao et al., 1999; Moral et al., 2019; Mukherjee and Keshri, 2019). Taking into account the absence of structural lignin, this value can be due to certain lignin- like compounds, aromatics, alkyl derivatives and some salts. Carbohydrate extractions and effects on paper properties As can be seen in Table 3, the yield obtained after oxidation with hydrogen peroxide was lower when algae were exposed to more severe conditions. It should be remembered that, although not repeated here, a higher concentration was accompanied by a higher temperature and a longer time (Table 1). Interestingly enough, severe conditions increased breaking length, stretch, burst and tear indexes and brightness. The sheet having better characteristics was the one formed with a lower proportion of alga (25 % seaweed and 75% pine) and subjected to treatment with hydrogen peroxide and hydrochloric acid at the highest concentration (6%). Hydrogen peroxide is a powerful oxidizer that, at least without
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