# Biosorption Potential of Sargassum for Removal of Aqueous Dye Solutions
## 1 Introduction  
Sargassum is a broad genus or family of brown seaweed with over 300 species [1]. Three species of Sargassum have recently been causing environmental and economic issues [2; 3; 4]. _S. muticum_, which is native to the northwest Pacific region [5], appeared on the shores of Europe in the 1970s, displacing native species and having negative environmental impacts and is classified in many European countries as an invasives species [6; 7; 8]. Pelagic Sargassum, particularly _Sargassum fufuians_ and _S. natus_, is a tremendous ecological resource floating in the open ocean [9; 10; 11]. Small quantities arriving on the beach give environmental benefits, such as dune stabilisation [3; 4; 9; 11; 12]. Nevertheless, beaches across the Caribbean and the Gulf of Mexico have experienced massive inundations of pelagic Sargassum since 2011, negatively impacting the environment, human health and the local economies [3; 13; 14; 15; 16; 17; 18; 19; 20; 21]. Removing and disposing of Sargassum is costly [3; 9; 11; 22; 23] and applications that generate revenue are being researched [3; 4; 22].  
The living and dead cells of seaweeds can effectively remove heavy metals and other pollutants from wastewater [24; 25; 26; 27]. Both _S. muticum_ and pelagic Sargassum were suggested as biosorbents [2; 29; 26; 28; 29]. Synthetic dyes are found in the wastewater streams of many industries and can be highly resistant to conventional biological wastewater treatments, leaving behind highly coloured and toxic effluents [30; 31]. Methylene blue (MB) is a cationic dye that is commonly used in the textile industry. Ingestion may cause respiratory issues, nausea, jaundice and skin irritation [32]. Some initial studies have shown that both _S. muticum_ and pelagic Sargassum can effectively remove synthetic dyes, such as Methylene Blue [28; 33]. However, these studies used freshwater washed and pretreated Sargassum with H\({}_{2}\)O\({}_{2}\), CaCl\({}_{2}\), HCl and formaldehyde to improve the absorption capacity via chemical modification, including protonation and chemical crosslinking [28; 33]. Enhanced biosorption capacity is attributed to the protonation of cell wall componentsand improved stability by the Ca\({}^{2+}\) ions of alginate molecules via the formation of the characteristic alginate arrangement, known as the egg-box structure.  
Azo dyes, which are synthetic colours containing an azo group (-N=N-), are the most used dyes (60-70%) in industrial applications, including printing, textiles, tanning, packaging and, to a rapidly reducing extent, in the food industry [34; 35; 36]. Congo Red (CR) is an anionic dye that is often used in experiments as a typical example of one of the hundreds of potential azo dyes [34; 35; 36]. Brilliant Blue R (BB), which is a disulfonated triphenylmethane dye, is an anionic dye that is widely used in biotechnology. It was effectively removed using a fungal biosorbent (_A. tubingensis_), with the highest removal efficiency at lower pH values (pH 2) where the nitrogen-containing functional groups of the fungus are positively charged, allowing for electrostatic interaction with the OH\({}^{-}\) groups of the dye [37].  
This study examined the biosorption of three dyes (MB, CR and BB) using two types of Sargassum with a minimum of pretreatment (_S. muticum_ freshwater rinsed and freeze-dried and mixed pelagic Sargassum drained and freeze-dried). This research also studied the effect of CaCl\({}_{2}\) pretreatment of Sargassum on the biosorption of MB.