# Adsorption of Heavy Metals from Low Concentration Solutions onto Dried _Chlamydomonas reinhardtii_
## 4 Discussion
At pH 7, the presence of Pb\({}^{2+}\), PbOH\({}^{+}\), Cd\({}^{2+}\), and Cr\({}_{2}\)O\({}_{3}\) is confirmed, while all the other species are negligible. The constant and high capture of lead can be explained by the interaction of charges of opposite sign; the simple positive ion species Pb\({}^{2+}\) easily interacts with the negative surface of the bio-sorbent (ZP = \(-\)18.8 mV). On the contrary, the decrease of chromium adsorption, observed for increasing pH, can be related to the progressive formation of neutral species, i.e., Cr\({}_{2}\)O\({}_{3}\), which are able to interact with the sorbent in a weaker way (e.g., Van Der Waals interaction) than the charged ones, i.e., CrOH\({}^{2+}\), present at pH 6. It is not surprising that a lower Cr adsorption is detected at pH 7, where the progressive increase of neutral speciesformation occurs at the expense of the charged Cr(OH)\({}_{2}\)+ and CrOH\({}^{2+}\). Moreover, Cr(III) adsorption at pH 7 could be affected by a higher steric hindrance of the Cr species, despite charged one, thus preventing higher metal capture. This is different to the case of Cd(II), where pH effect on the adsorption is not explained by charge interaction. Indeed, Cd ions are always present as Cd\({}^{2+}\), and the formation of low amount of neutral species, as reported by [61], is not able to fully explain the observed marked adsorption decrease from pH 6 to pH 7 (from 76% (\(w/w\)) down to 65% (\(w/w\))). It can be hypothesized that charged species at different pH are characterized by different water molecules' coordination spheres which imply a larger steric hindrance at pH 7, hence a decreased availability of the adsorption sites. As a matter of fact, more sterically encumbered species cover larger surface portions and/or suffer for repulsions due to overlapping of the coordination sphere. The nature and the extent of the coordination sphere of the different ions can be explained by considering ionic radii and charge density. Indeed, Pb(II) ions are reported to be weakly hydrated because of their low charge density. Pb(II) compounds normally crystallize without water coordination and thereby are unable to form a complete hydration shell [62]. On the contrary, complexes with high coordination number are reported for Cd ions, such as Cd\({}_{2}\)+(H\({}_{2}\)O)\({}_{6}\)(H\({}_{2}\)O) [63]. Accordingly, the resulting steric hindrance can be accounted to explain the difference in metal capture, mainly at higher pH. Furthermore, considering this algal biomass, a correlation has been proposed between the ionic radii of the metal cations and the affinity of the biosorbent; the larger the ionic radius the stronger the affinity [37]. Considering the ionic radii ranking, Pb\({}^{2+}\) (1.32 A) \(>\) Cd\({}^{2+}\) (1.03 A) \(>\) Cr\({}^{3+}\) (0.65 A) [64], a preferential biosorption of Pb ions, followed by Cd and Cr, may occur. According to the performed characterization studies, an electrostatic interaction between the metals and some microalgal functional groups, i.e., amino, hydroxyl, acyl-glycerol, and phosphate groups, is suggested by FT-IR analysis. Indeed, these are electron-rich groups that are able to interact with the positively charged cations consistently with the negative ZP measured, and the process conditions do not affect their availability to the adsorption phenomena. The EDX results show a decrease of the peak-intensity of the light metals (i.e., Mg(II), K(I), and Ca(II)) naturally present in the pristine alga upon Pb, Cr, and Cd solution, suggesting an ion-exchange mechanism between those ions already present on the cell surface and those of interest. The absence of Cr(III) peak EDX spectra can be related to the very low metal amount used and to the overlapping of detection regions [65]. Considering the evidence of (1) experimental data, the (2) nature of the species in solution, the (3) effect of steric hindrance, and (4) ionic radii, the affinity ranking Pb \(>\) Cd \(>\) Cr is considered highly plausible in case of the use of _C. reinhardtii_ as biosorbent in our conditions.