Abstract In this work, the brown alga Dictyota dichotoma was explored as a new reinforcing material for papermaking. Performing the typical chemical tests for cellulosic substrates on D. dichotoma evidenced large amounts of ethanol:benzene extractable substances (7.2%) and ashes, algae-speci ¦ c results. Also, even if lipophilic compounds are removed, brown seaweed are not a primary source of ¦ bers because it contains low proportion of cellulose. However, its elevated content of insoluble carbohydrates (51.4%) suggest there is some potential in association with conventional cellulosic pulps, as ¦ brous elements contribute to sheet forming and other components ¦ ll the spaces in the paper web without noteworthy loss of strength. Extraction was carried out with clean processes: hydrogen peroxide and mixtures (hydrogen peroxide-hydrochloric acid and hydrogen peroxide-sodium perborate), sodium hydroxide and sodium hypochlorite, always aiming for low reagent concentrations, in the range of 1-12%. The results show that sodium hydroxide and sodium hypochlorite were the treatments that lead to paper sheets with better structural and mechanical properties without further bleaching or re ¦ ning, thus highlighting the suitability of these algae for papermaking applications. ntroduction Depending on the meteorological conditions, tides and geography, large amounts of algae can be deposited in coastal areas. These accumulations are managed as municipal waste and often end up in land ¦ lls. Its collection is necessary because they cause environmental and health problems affecting the local tourism (Moral et al., 2019; Rajkumar et al., 2014). Regardless of the dead biomass in beaches, since the beginning of the century there has been a notorious increase in seaweed harvest globally, from 11.4 million wet tons in 2000 to 19 million in 2010, to 28.4 million in 2014 and to 35.8 million in 2019 (FAO, 2018, 2021; Thompson et al., 2019). Cultivation for human food and cosmetics is especially common in China, Indonesia and Rep. Korea but, unless severe extractive processes are carried out, these are not safe applications for beach waste. A frequent suggestion for the non-food valorization of seaweed is biofuel production, aiming to obtain biodiesel from the lipids and bioethanol from the saccharides (El-Sheekh et al., 2019; Patil et al., 2017). Another option, which should not be seen as competing but as complementary, is papermaking. This is a common approach to reuse agricultural waste (Aguado et al., 2018; Jiménez et al., 2008; Nagpal et al., 2020). The lack of lignin in most seaweed can make them even more appealing than ¦ eld residues, as cellulose extraction becomes easier (Moral et al., 2019). However, while green algae have been successfully used as source of cellulosic ¦ bers (Wahlström et al., 2020), the different composition of brown algae present certain challenges and, at the same time, new opportunities. Among all the best-known types of marine algae (red, green and brown), Phaeophyceae (brown algae) are the most commonly found in the debris along the Mediterranean coasts (Deniaud-Bouët et al., 2014) and encompass approximately 2000 species (Thompson et al., 2019). Some studies have shown that the
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