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Bioremediation for Sustainable Environmental Cleanup
The temperature has the highest effect on metal remediation, but it has less impact than the
impact of pH (Lau et al. 1999). Therefore, it is important to find the optimum temperature for
each microalgal system before designing the biosorption operation for different metals.
(c) Organic matter: Organic matter concentration influences the biosorption method, and is
dependent on the type of metal and microalgal species used in bioremediation. For example,
biosorption of copper decreases from 3.2–2.3 mg L–1 and arsenic from 2.2–0.0 mg L–1 in
Chlorella, whereas in the case of Scendesmus almeriensis, removal declines from 2.1 to
1.6 mg L–1 for copper and 2.3 to 1.7 mg L–1 for arsenic in the existence of organic matter
(Saavedra et al. 2019). Since most wastewaters have some organic load, and organic matter
tends to chelate the metals-forming complexes, therefore it is paramount to comprehend the
effect of organic matter on the overall metal bioremediation protocol.
(d) Carbon dioxide: Microalgae need carbon dioxide for their photosynthesis as raw material, and
carbon dioxide is emitted by several industries. Microalgae can be grown by using this waste
gas and converting the same into useful biomass, which can then be used as a biosorbent to
clean metal contaminated waste streams. Oedogonium sp. shows rapid uptake of a few heavy
metals (particularly Cd, Al, Ni and Zn), and its biomass productivity gets increased when CO2
concentration is higher than normal conditions and escalates with productivity (Roberts et al.
2013).
(e) Nutrients: Algal growth is largely affected by the availability of nutrients. There are some
dominant species likes Chlamydomonas, Spirogyra, Euglena and Dinoflagellates that use
carbon and other nutrients from the wastewater for their maturation and photosynthetic activity,
increasing the efficiency of metal remediation microorganisms (Sayara et al. 2021).
Many other factors like initial metal concentration, types of metals such as tertiary and
quaternary multi-metal systems and contact time are also important factors for metal removal (Kiran
and Kaushik 2012).
12.2.4 Role of Exopolymers
Biofilms are formed on the surface where the algae get attached due to the secretion of exopolymer
substances that are essentially composed of proteins, lipids, nucleic acids, polysaccharides,
lipopolysaccharides and glycoproteins (Zeraatkar et al. 2016). The structure of exopolymers varies
depending upon the type of microorganism, age of biofilms and on environmental conditions.
Exopolysaccharides are surface-active bio-agents originating from algae, fungi, bacteria,
cyanobacteria and help in sequestering toxic metals from an aqueous solution (Sharma et al. 2009,
Mona and Kaushik 2015b). Chelating agents produced by exopolysaccharides for the elimination of
positively charged metal ions from water (Potnis et al. 2021).
The yield of exopolysaccharides by algae increases when disclosed to a higher concentration
of metal ions, which seems to play a crucial part in metal biosorption (Sharma et al. 2009). The
bioremediation of heavy metals by exopolysaccharides of the algae is gaining importance for future
use in bioremediation programs of wastewaters containing these metals.
12.2.5 Bioremediation Mechanisms
The remediation of pollutants (heavy metals) in algae occurs in two ways; first is fast passive
biosorption that takes place where the pollutants assimilate on the biomass surface within a lesser
period, and this step is not based on the metabolism of cells, and the second step is slow active
sorption of pollutants (heavy metals) into the cytoplasm of algal biomass, and this step is completely
based on metabolism of biomass (Monteiro et al. 2012).
Various analytical techniques have been used by researchers, which mainly include atomic
“Scanning Electron Microscope (SEM) or Transmission Electron Microscope (TEM) coupled
with energy-dispersive X-ray spectroscopy, X-ray spectroscopy, Infrared spectroscopy or Fourier-