**3. The role of algae in the biological removal of heavy metals**

The use of algae to remove heavy metals from wastewater has been noticed for more than 40 years, and many studies have been carried out to use algae as bio-absorbents, especially for the removal of heavy metals from aquatic environments.

In recent years, the cell structure of algae, modification and genetic change, and effective factors in their biological absorption, such as biomass concentration, primary ion concentration in the environment, size of present ions, temperature, and pH have been studied and investigated and it has been confirmed that the use of algae in The process of biological absorption of toxic and radioactive ions is a cheap, safe and efficient method; Also, valuable elements such as gold and silver can be extracted by them [12, 13].

In terms of mechanism, heavy metals are absorbed by groups such as hydroxyl, phosphoryl, carboxyl, sulfuryl, amino, sulfate, phosphate, and carbohydrate on the surface of algae. The availability of active sites of algae is determined through the FTIR test. Also, the amount of absorption depends on the number of effective groups in algae cells, their availability, the orientation of metal ions, and the chemical state of

**Figure 2.**

*Adsorption of metal ions on the algal cell surface.*


### **Table 1.**

*Specific ligands for the absorption of each ion.*

the active sites. Usually, functional groups (OH-, PO4 2-, RS-, RO, SH-, COO-, NO3-, and RNH2-) create a negative charge on the surface. These bonds are located in the cytoplasm of the cell, especially the vacuole, and improve absorption. This is shown in **Figure 2** [14].

Cytosolic proteins transport ions into the cell. Therefore, the vacuole is an organelle for accumulating metal ions. According to **Table 1**, specific ligands for the adsorption of each ion are listed.

Algal cell walls are the first barrier against absorption, and according to the abundance of cell wall compounds in different algal strains, the capacity to absorb metals will be different. According to research, brown algae is a very good absorbent in this area [12, 15].
