# Adsorption of Heavy Metals from Low Concentration Solutions onto Dried _Chlamydomonas reinhardtii_
## 5 Conclusions and Future Developments  
Microalgal adsorption represents a sustainable solution for the removal of heavy metals from polluted water streams. The biomass of dried _C. reinhardtii_ is able to capture Pb, Cr, and Cd from water solution, even when in low concentration, showing a high efficiency removal up to 80-90% (\(w/w\)). The capture process is governed by the negatively charged surface wall of the dried _C. reinhardtii_, which can electrostatically bind the positively charged ions, and weakly (e.g., Van der Waals interactions) bind the neutral species. The exposed electron-rich functional groups of the algae cell wall are involved in the interaction with cations through coordination and electrostatic interactions. pH has a strong influence on the distribution of the species that can actively take part in the adsorption process; however, it apparently does not directly affect the algal functional groups at the surface of the cell wall. _C. reinhardtii_ metal affinity depends on the nature of the ionic species, on steric hinderance, and on ionic radii. The affinity ranking proposed is Pb \(>\) Cd \(>\) Cr for multi-ion solutions. Considering multi-metal solutions, a site competition for Pb and Cd is proposed since they are both bivalent and show a strong difference in charge density and in ionic radius. SEM analysis of the alga before and after metal adsorption showed the disruption, damage, distortion, and collapse of the spherical cell typical of the drying process, while no evidence of an additional effect of the metal capture was found. First experiments show that the HM removal capacity of living algae is even higher than that of dried biomass, probably due to additional bioaccumulation inside the cells. The performed study is particularly useful for wastewater treatments under low metal concentration solutions. In future papers, the investigation should be performed under increased equimolar concentration to better study the sorbent, its affinity, and its selectivity ranking. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app142311057/s1](https://www.mdpi.com/article/10.3390/app142311057/s1)]([https://www.mdpi.com/article/10.3390/app142311057/s1](https://www.mdpi.com/article/10.3390/app142311057/s1)), Figure S1: FTIR in ATR mode low frequency region spectrum of pristine _C. reinhardtii_; Figure S2: EDX analysis of pristine dried _C. reinhardtii_. Au* peak is related to gold plating of samples to make them conductive; Figure S3: FTIR in ATR mode of the microalga before (namely, pristine) and after the adsorption of the HMs at pH 6 full spectra; Figure S4: EDX analysis of the microalga after single metal adsorption at pH 6; Figure S5: EDX analysis of the microalga after multi-metal adsorption at pH 6. Au* peak is related to gold plating of samples to make them conductive; Figure S6: EDX analysis of the living microalga after Cd adsorption. Au* peak is related to gold plating of samples to make them conductive; Figure S7: Hydra-Medusa plots for: (a) Pb(II), (b) Cr(III), and (c) Cd(II) single-ion solution; Figure S8: Hydra-Medusa plots of the multi-ionic solution; Table S1: capture of metal ions using living and dead biomass; Table S2: Metal concentration of industrial wastewater streams (BAT reference documents); Table S3: Kuhl Medium composition for the algae growth; Table S4: Characteristics IR bands and functional groups detected in the samples; Table S5: Dried algae metal adsorption at different contact times at pH 6; Table S6: Nominal results of dried algae metal adsorption at pH 6 in single and in multi-ion solutions; Table S7: nominal results (\(\mathrm{\SIUnitSymbolMicro mol}_{\mathrm{ion}}\)/\(\mathrm{\SIUnitSymbolMicro g}_{\mathrm{algae}}\)) of dried algae metal adsorption at pH 6 in single and in multi-ion solutions after 2 days of contact time. References [10; 35; 36; 37; 38; 39; 41; 42; 46; 47; 51; 52] are cited in the supplementary materials. Conceptualization, G.S. ; methodology, M.G.G., G.S., and C.C. ; software, M.G.G.