# Biosorption of Heavy Metals by the Bacterial Exopolysaccharideaccharide FucoPol
## 3 Results and Discussion
### Evaluation of the Metal-Binding Ability of FucoPol
The biosorptive capacity towards different metals can be speculated through the covalent index, calculated as \(\mathrm{X^{2}_{m}}r\), in which the electronegativity (X\({}_{m}\)) and ionic radius (\(r\)) of the metals are considered. Pb\({}^{2+}\) has the higher covalent index (6.41), followed by Co\({}^{2+}\) (2.65), Cu\({}^{2+}\) (2.64) and finally Zn\({}^{2+}\) (2.04) [39], which was the preferential sequence of specific metal uptake by FucoPol (Pb >> Co > Cu > Zn). Furthermore, the FucoPol-Pb attraction might have been weaker than with other ions (higher repulsion due to the higher electronic density). This would lead to a less compact FucoPol structure in the presence of the metal. Hence, the same amount of Pb\({}^{2+}\) occupied less surface area and less negatively charged functional groups than when the other metals were used [21]. This allowed that more Pb\({}^{2+}\) could bind to FucoPol's molecule within the equilibrium period. Several studies with other bacterial EPS reported a similar trend, wherein there was a better metal biosorption performance towards Pb\({}^{2+}\) removal compared to other metals (Cu\({}^{2+}\), Zn\({}^{2+}\), Co\({}^{2+}\), Cd\({}^{2+}\), Ni\({}^{2+}\), Fe\({}^{2+}\)). Examples include the anionic EPS synthesized by _P. stutzeri, P. jamiae_ and _B. firms_, in which the order of metal uptake was Pb > Co > Cu, Pb >> Cu > Zn >> Co and Pb > Cu > Zn, respectively [20; 21; 32]. However, the interaction between EPS and metals highly depends on the structure, composition and surface area of the biosorbent, as well as on the attractive forces and the conformation established between the EPS and the metal [4; 20], thus different results might be achieved depending on the characteristics of the EPS. In fact, contrasting results were reported for the EPS produced by _M. organophilum_ and _P. aeruginosa: M. organophilum_ EPS removed preferentially Cu\({}^{2+}\) over Pb\({}^{2+}\) (21% and 18%, respectively)[35], while _P. aeruginosa_ EPS removed 87.4% of Cu\({}^{2+}\), followed by Zn\({}^{2+}\) (80.6%) and Pb\({}^{2+}\) (79.7%) [36]. Both EPS had less negative charged groups (e.g., uronic acids, pyruvate and acetate) in their composition than FucoPol and the EPSs that showed preferential removal of Pb\({}^{2+}\) (Table 1). Despite the increase in metal concentration inside the dialysis tubing observed in all tests, no insoluble species were formed, which indicated that FucoPol interacted with the metals in solution without chemically converting them into less soluble forms. As so, a biosorption mechanism, or a combination of several mechanisms, must be involved the FucoPol-metal interaction. Possible interaction mechanisms include microprecipitation, ion exchange, coordination, complexation, adsorption, chelation, and electrostatic interactions [4; 17]. Based on these results, it was clear that FucoPol has a good performance for Pb\({}^{2+}\) sequestration. Lead poisoning causes serious health problems in the central nervous system, the kidneys, the gastrointestinal tract and the reproductive system [5]. Moreover, [PERSON] et al. [2] reported that up to 50% of Pb\({}^{2+}\) adsorbed in adults was due to the ingestion of contaminated drinking water. Therefore, the subsequent experiments focused on the different factors that influenced the adsorption of Pb\({}^{2+}\) by FucoPol, envisaging the optimization of its use as biosorbent.