# Biosorption of Heavy Metals by the Bacterial Exopolysaccharideaccharide FucoPol
## 1 Introduction
Conventional physical and chemical methods have been implemented to remove metals from aqueous solutions, namely, ion exchange, reverse osmosis, chemical precipitation, chemical extraction, and electrochemical treatment [4]. However, they have some drawbacks, such as the difficulty for large scale implementation, the generation of undesirable by-products, high costs, and inefficiency to treat effluents contaminated with low metal concentrations [4,16,17]. Consequently, environmental biotechnology is focused on developing novel, attractive and economic alternatives to remove heavy metals from contaminated wastewaters. One of the most promising methods is the biosorption of heavy metals, which utilizes living or dead microorganisms, or products from their metabolism, such as polymers [4,18]. Microbial cells are capable of binding and accumulating metal ions present in water, reducing their bioavailability. This biosorption phenomenon occurs due to the presence of biopolymers, rich in acidic functional groups, in the cellular structures that can bind metallic cations, independently from the cellular metabolism [4]. Biosorption of metal ions by polymers may involve different physical and chemical processes. Physical sorption occurs due to electrostatic and Van der Walls interactions, or due to the imbalance of metal concentration between the surface of the biosorbent and the solution [4,19]. Chemical mechanisms responsible for biosorption are complexation, chelation, coordination, microprecipitation, and ion exchange [4,17]. Numerous extracellular polysaccharides (EPS) secreted by different microorganisms were reported to be effective in metal sequestration. For instance, the EPS produced by _Paenibacillus jamiae_[20], _Bacillus firmus_[21], _Bacillus licheniformis_ KX657843 [18], _Herbospirillium_ sp. [22] and _Paenibacillus peoriae_ TS7 [23] were capable of removing several heavy metals (e.g., Zn\({}^{2+}\), Pb\({}^{2+}\), Ni\({}^{2+}\), Cu\({}^{2+}\), Cd\({}^{2+}\), Co\({}^{2+}\), Hg\({}^{2+}\)) from aqueous systems. These reports suggest that the use of EPS is a promising solution for heavy metal sequestration and recuperation from water systems. The ability of EPS to adsorb metal cations is due to the presence of negatively charged functional groups (carboxyl, phosphoryl and hydroxyl groups) in these macromolecules' structures, which can establish electrostatic interactions and bind the positively charged metal ion [4; 24]. Moreover, the use of microbial polysaccharides as biosorbents is advantageous since they are eco-friendly and non-toxic, and their production is easy and cost-effective [25]. Compared to the biosorption using living bacterial cells, metal biosorption with EPS has a reduced complexity and no metabolic interferences [4]. Moreover, living cells have nutritional requirements and are affected by abiotic stress factors [24]. FucoPol is an anionic EPS synthesized by the bacterium _Enterobacter_ A47 [26]. This high molecular weight EPS is composed of fucose, galactose, glucose and glucuronic acid sugar residues (2.0:1.9:0.9:0.5 molar ratio), and the acyl groups acetyl, pyruvateyl and succininyl, that represent 12.3 wt% of FucoPol's dry mass. FucoPol possesses a\(\rightarrow\)4)-\(\alpha\)-L-Fucp-(1\(\rightarrow\)4)-\(\alpha\)-L-Fucp-(1\(\rightarrow\)3)-\(\beta\)-D-Glcp(1\(\rightarrow\)trimer backbone. The branches, present at position 3 of the first fucose, are composed of an \(\alpha\)-D-4,6-pyruvateyl-Galp-(1\(\rightarrow\)4)-\(\beta\)-DGlcAp-(1\(\rightarrow\)3)-\(\alpha\)-D-Galp(1\(\rightarrow\)trimer, with two pyruvate caps in the terminal galactose, specifically at position C-4 and C-6 [27; 28]. FucoPol was employed for the preparation of a bioactive silver nanocomposite [27] and as a coating agent for iron oxide magnetic nanoparticles (MNP). The use of these FucoPol-MNP was evaluated for human antibody purification [29] and as cell labeling nanoprobes for Magnetic Resonance Imaging [30]. The potential of using FucoPol as biosorbent for the removal of heavy metals from water streams was never explored.