Chromium (VI)
Table 2.2 Biosorption of heavy metals by different algae.
| Algal biomass(biosorbent) | Metal ions(biosorbate) | Functional groups | Reference |
|---|---|---|---|
| Palmaria palmate | Chromium | Amino, carboxyl, sulfur groups | (Murphy et al., 2008) |
| Ulva lactuca | Cadmium | Amino, hydroxyl, carboxyl groups | (Lupea et al., 2012) |
| Spirulina platensis | Copper | Amino, carboxylic groups | (Çelekli et al., 2010) |
| Oedogonium hatei | Nickel | Amino, amide, hydroxyl and carboxyl groups | (Gupta et al., 2010) |
| Maugeotia genuflexa | Arsenic | Carboxyl, hydroxyl, amide groups | (Sarı et al., 2011) |
| Cladophorasp | Lead | Carboxyl, amino, amide, hydroxyl groups | (Lee and Chang, 2011) |
| Laminaria japonica | Zinc | Carboxyl, hydroxyl groups | (Davis et al., 2003) |
| Spirogyra hyalina | Cobalt | Amino, hydroxyl groups | (Viraraghavan and Srinivasan, 2011) |
| Sargassum sp. | Mercury | Amino, sulfur groups | (Subhashini et al., 2011) |
The cell wall components differ between algal species. Brown algae have three primary components in their cell walls: cellulose that provides structural support, alginic acid (mixture of polymers with their respective salts), and sulfated polysaccharide with large carboxylic groups implicated in metal biosorption. Red algae have garnered attention for biosorption due to the presence of sulfated polysaccharides formed from galactans (high concentrations of hydroxyl and carboxyl groups). Green algae have polysaccharide‐bound cellulose with a large amount of protein that includes amino, sulfate, hydroxyl, and carboxyl functional groups (Flouty and Estephane, 2012).
The algal cell wall consists of polysaccharides that include chemical groups (sulfate, amino, hydroxyl, phosphate, imidazole, mannan) known to function as binding sites for metal (Romera et al., 2007). Electrostatic interaction attracts cations due to the negative charge of the cell wall. In addition to the charge, which plays an essential role in biosorption, hydrophobicity, species and structure, ionic strength of the metal, and chemical makeup of the ionic solution of metals are also important factors to consider when choosing algae for biosorption. (Adewuyi, 2020).
The mechanism of metal absorption by algae is similar to bacteria: metal ions are bound to the outer membrane, followed by internalization. Algal biosorption involves one of two mechanisms:
Ion exchange process: ions such as calcium, magnesium, sodium, and potassium are replaced by metal ions on the algal surface.
Functional groups and metal ions combine to form complexes.
Ionic strength and covalent interactions are hypothesized in the case of the metal uptake process. Carboxyl and sulfate groups form ionic bonds, but amino and carboxyl groups require covalent interaction between the metallic ligand and the chemical group. Phytochelatins are formed within the algal body in response to metal
