Figure. 12.1. Various Approaches for Heavy Metal remediation

Microalgal Bioremediation of Heavy Metals 209

1.Ultrafiltration

2.Coagulation

3.Flocculation

4.Membrane

filtration

5.Ion exchange

Metal Remediation Approaches

Physical

Approaches

Chemical

Approaches

Biological

Approaches

1. Neutralization

2. Solvent extraction

3. Chemical

precipitation

4. Electrochemical

treatment

1.Bioaccumulation

2. Biosorption

3. Microbial

reduction of

oxidation

4. Metabolic

precipitation

5. Metal-

Phytochelatin

Figure 12.1. Various approaches for heavy metal remediation.

By using physical methods, almost all the pollutants can be removed, but these methods have

some limitations. Physical methods based on the distribution of the practical size of pollutants need

further processing and have a comparatively high cost of application. Although, chemical methods

of metal remediation are highly effective in these methods formation of byproducts increases further

downstream processing steps (Mona et al. 2008). The biological methods for metal remediation are

less costly and do not create any secondary pollution (Selvi et al. 2019).

12.1.3 Bioremediation using Microalgae—Merits and Potential

Bioremediation is a technique to exude and modify harmful pollutants (heavy metals) into less

harmful substances and/or eliminate toxic elements from the polluted environment (Eccles 1999).

Microalgae are found to be very potent in the bioremediation of different heavy metals from

wastewaters. Microalgae have several advantages like small size, simple structure, easy handling,

high photosynthetic activity, short life cycle, simple nutrient requirements, high adaptability and

tolerance to different types of stress conditions, which increase their potential for applications in

bioremediation. Therefore, there has been great interest in using microalgae in the phytoremediation

of toxic heavy metals. The cell wall of microalgae shows more binding affinity, the richness of

binding sites and wide surface area, all of which favor effective biosorption of the metals (Cameron

et al. 2018). Moreover, microalgae show good biosorption capacity as living or dead cells, free or

immobilized cells. Besides metal elimination capacity and being eco-friendly, bioremediation of

heavy metals has added significance, such as the development of value-added products.

Chlorella vulgaris and the Chlorella salina (marine alga) have been shown to remove 14 to

100% of heavy metals viz. Fe, Mn, Ni, Zn, Cu, Co and Cr from wastewaters along with other

pollutants such as TDS, pH, COD, BOD, calcium, magnesium, ammonia, nitrate, phosphate, sulfate,

sodium, potassium (El-Sheekh et al. 2016). While several microalgae species have shown very

good metal tolerance, they also show tolerance to certain toxic dyes and have additional merits of

being able to produce biohydrogen (Mona and Kaushik 2015a). Many cyanobacterial species show

excellent co-tolerance to metals and salts (Kiran et al. 2008). The metal-salt co-adapted Lyngbya

and Gloeocapsa strains were found to show better Cr removal capability in the presence of salts

(Kiran et al. 2007a). This indicates that the indigenous strains of microalgae may be more effective

in bioremediation when they have been exposed for a long time to different pollutants.