# Adsorption of Heavy Metals from Low Concentration Solutions onto Dried _Chlamydomonas reinhardtii_
## 3 Results
### Biosorption on Dried Algae
#### 3.2.2 Multi-Ion Solutions  
Wastewater streams are complex solutions containing different pollutants. To better study microalgae behaviour in polluted water streams, and their ability to remove HMs, multi-ion systems maintaining the previous concentrations were studied.  
In Figure 8, metal capture by dried cells in multi-ion solution at pH 6 is reported and compared with the corresponding single-ion ones, while quantitative results are reported in Table S6. In multi-ion solution, the removal of Pb(II) and Cr(III) increased, respectively, from 80%/_w_/_w_) to 91% (_w_/_w_) and from 65% (_w_/_w_) to 85% (_w_/_w_), while the capture of Cd ions decreased from 76% (_w_/_w_) down to 59% (_w_/_w_), suggesting a possible competition effect.  
Results of metal capture in \(\mu\)mol\({}_{\text{ion}}\)/g\({}_{\text{microalgae}}\) in both single- and multi-ion solutions are plotted in Figure 9 and reported in Table S7. The overall absolute capture behaviour of the microalga follows the same trend in both systems: Cr(III) > Pb(II) > Cd(II). However, the multi-metal system pictures a total increase in captured moles; in particular, Pb(II) adsorption increases by 0.8 \(\mu\)moles, and Cr(III) by 2.5 \(\mu\)moles, while Cd capture decreases by 1 \(\mu\)mole. The decrease of Cd(II) adsorption (1 \(\mu\)mol\({}_{\text{tot}}\)/g\({}_{\text{microalgae}}\)) does not account for the total increase of Pb(II) and Cr(III) (3.3 \(\mu\)mol\({}_{\text{tot}}\)/g\({}_{\text{microalgae}}\)).  
Figure 8: Comparison of metal capture in multi- and single-ions solutions.  
Figure 7: SEM pictures of the microalga before and after metal adsorption in single-ion solutions at pH 6 at different magnifications: (a) 2.50 KX and (b) 5.00 KX (5.21 KX for Cr).  
Also, the effects of metal-treated algae upon adsorption experiments in multi-ion solutions were fully characterized. In Figure 10, FT-IR spectra of the pristine alga before and after metal adsorption in multi-ion systems are plotted.  
No marked differences were found when spectra of the treated algae were compared with those of the pristine one (Figure 10). A broadening of the main band, centred at 3291 cm\({}^{-1}\), occurred, as well as of the complex band at 2955-2855 cm\({}^{-1}\). Broadening is also observed for the shoulder around 1720 cm\({}^{-1}\), while the band at 1244 cm\({}^{-1}\) is reduced in intensity. However, the most remarkable difference between mono- and multi-ion systems regards the region around 900 cm\({}^{-1}\), where the out-of-plan bending of the OH groups is still visible. This band completely disappeared upon the adsorption of metal ions in all single-ion solutions, while it is still weakly visible in the plot of multi-ion solution (Figure 10). This result suggests that, upon contacting the bio-sorbent with the multi-ion system, a different interaction of the metals with OH functional groups are active. Thus, a possible different adsorbate-sorbent interaction is possible, related to the modification  
Figure 10: FTIR-ATR spectra of the alga before and after metal adsorption in multi-ion systems.  
Figure 9: Metal capture in \(\upmu\)mol\({}_{\text{ion}}\)/g\({}_{\text{microlalgae}}\) in both single- and multi-ion solutions.  
of the binding sites and/or different ionic species to be adsorbed. However, this point, considering the system complexity, deserves further studies to be confirmed.  
In Figure 11, SEM micrographs of the alga before and after the metal capture in multi-ionic solution are reported at two different magnifications and compared with those of the pristine alga.  
Also, in this case, only the effect of cell drying, such as pitting, cell damage, distortion and collapse, is observable [55], with no evidence of the algae-metal interaction. Similarly, EDX analysis (Figure S5), did not show any evidence of Pb(II), Cr(III), and Cd(II) adsorption, and only Mg(II), K(I), and Ca(II), and P originally present in the pristine algae, were detected.