# Adsorption of Heavy Metals from Low Concentration Solutions onto Dried _Chlamydomonas reinhardtii_
## 3 Results
### Biosorption on Dried Algae  
#### 3.2.1 Single-Ion Solutions  
The adsorption process of Pb(II), Cr(III), Cd(II) to dried _C. reinhardtii_ was studied by contacting the dried microalga with single-ion solution at a fixed initial concentration, namely 0.5 mg/L for Pb(II), 0.2 mg/L for Cr(III), and 0.2 mg/L for Cd(II). Experiments were performed at two different contact times, 2 and 4 days, and two operating pH, i.e., 6 and 7. The remaining concentration of these metals after the contact time was recorded by ICP-OES. Results of ICP-OES on the supernatant solution after centrifugation are plotted in Figure 5a,b, and summarised in terms of absolute values in Table S5.  
then it was monitored throughout the experiment. Maximum pH variation during the contact time was \(\pm\)0.5; hence, no further pH correction was required. It is evident that by operating at pH 6, a larger amount of Pb and Cr ions were captured by the microalgae, while no or negligible pH effect occurred in the case of Cd solution (Figure 5b). Accordingly, pH 6 was adopted for further experiments.  
After the adsorption process, the algae were dried and further characterized. FTIR spectrum of _C. reinhardtii_ after metal adsorption at pH 6 is plotted in Figure 6; here, the spectrum of the pristine alga is reported for comparison (full spectra of the microcalga before and after adsorption at pH 6 can be found in Figure S3). The biomass structure is not heavily affected by metal adsorption, given that the positions of the main FTIR bands of the exhausted _C. reinhardtii_ are close to those detected in the pristine microcalga (Figure S3). However, small differences common to all the spectra of the used biomass, regardless the adsorbed metal ion, can be noticed. First, the main and broad band, centred at 3291 cm\({}^{-1}\), is reduced in intensity, the small band at 3010 cm\({}^{-1}\), associated to the =C-H stretching mode disappears. Also, a slight modification of the complex band at 2955-2855 cm\({}^{-1}\) is evident. In Figure 6, the shoulder around 1720 cm\({}^{-1}\) almost disappears, and the band at 1244 cm\({}^{-1}\) is strongly reduced in intensity, as well as the band at 881 cm\({}^{-1}\). Some variations are also detectable in the region of CO/CC stretching around 1030-1000 cm\({}^{-1}\), characterising the carbohydrate component, whose bands are more defined after adsorption.  
SEM analysis of _C. reinhardtii_ after single-metal adsorption of HMs is reported in Figure 7a,b. By comparing the images of the algae after HMs adsorption with those of the pristine one, no remarkable differences are evident, even at large magnification (Figure 7b). As already reported for the pristine alga, only the typical spherical structures of the _C. reinhardtii_ cells arise, as well as the effect of the freeze-drying process effects. Indeed, signs of pitting and damage of the cell as well as distortion and collapse are clear, together with a residual number of undisrupted cells. This suggests no or limited effects of the metal on the pristine _C. reinhardtii_ morphology.  
In EDX analysis of Cd(II) and Cr(III) treated algae, the adsorbed ions are not visible (Figure S4), only Pb(II) is barely evident. Moreover, in EDX analysis, a small amount of metals, such as Mg(II), K(I), Ca(II), and small amount of P, already present in the pristine microcalga were detected.  
Figure 6: FTIR-ATR spectra of the alga before (pristine alga) and after the adsorption of the HMs at pH 6 low frequency region.