Microalgal Bioremediation of Heavy Metals 211

Table 12.3. Microalgal biosorption of heavy metals.

Name of Microalgae

Metal (s)

Removal efficiency (%)

References

Nostoc linckia

Chromium, Cobalt

58–60

Mona and Kaushik 2015b, Mona

et al. 2011b

Maugeotia genuflexa

Arsenic

24

Ubando et al. 2021

Laurencia obtuse

Chromium, Cobalt,

Cadmium

98.6, 98.2, and 98.0

Hamdy 2000

Nostoc, Gloeocapsa

Chromium

90–95

Sharma et al. 2016

of researchers across the world, leading to a huge amount of literature on the subject. Table 12.3

shows some reports on metal removal by various microalgae species.

Bioaccumulation is a procedure wherein heavy metal is controlled metabolically, producing

energy and altering heavy metal concentration (Arunakumara and Zhang 2008). Bioaccumulation

of metals by living cells depends on both the intra and extracellular processes, and passive gaining

is restricted (Fomina and Gadd 2014). Cladophora herpestica, a green alga that grows abundantly

in the Maruit Lake surface, has shown accumulation of residual nutrients from the atmospheric

and aquatic environment in introduction to heavy metal ions (Dahlia and Hassan 2017). Some

microalgae have also been reported to heavy metals bioaccumulation. Bioaccumulation of heavy

metals (Zn, Fe, Cu, Cd,Als) was shown by Chlorella vulgaris, Phacus curvicauda, Euglena acus

and Oscillatoria bornettia (Abrihire and Kadiri 2011). Amongst these species, Oscillatoria had a

high concentration of metal factor for Zn (0.306), Fe (0.302), Cu (0.091), Cd (0.276), while Phacus

and Euglena had relatively higher concentration factor for Al (0.439).

12.2.3 Factors Influencing Metal Remediation by Microalgae

In each type of environment, many factors present unique influences on metal remediation from

water. The biosorption method is regulated by many operating factors, including pH, temperature,

organic molecules, salinity, primary metal concentration, contact time and co-pollutants. There

have been extensive studies showing the influence of such factors on the metal biosorption process

(García-García et al. 2018).

(a) pH: Solution pH regulates the biosorption process and affects solution chemistry and functional

group activity in the biosorbents and competition with other co-pollutants (Vijayaraghavan and

Yun 2008). The optimum pH for maximum biosorption of the metals shows great variations.

Lyngbya putealis was found to show the highest Cr (VI) biosorption at acidic (pH 3.0). The

isoelectric point for algal

biomass being acidic pH (3.0), there is hydronation of some functional

groups, and the occurrence of hydronium ions near the binding sites lead to greater binding of

Cr (VI) to the algal surface. Cr (VI), which exists as HCrO⁴⁻, Cr2O ²⁻

7 ^{, in solution form at}

optimum sorption pH, tends to bind to the protonated active sites of the biosorbent (Kiran

et al. 2007a). Extracellular polymeric substance (EPA) of Gloeocapsa calcarean and Nostoc

punctiforme remove maximum Cr (VI) at pH 2 (Mona et al. 2008). Cr (VI) and Co (II) are

removed by Nostoc at pH 2 and 3.5, respectively (Mona et al. 2013).

(b) Temperature: Temperature plays a crucial part in the heavy metal removal process from water.

With increasing temperature, the flow of adsorbate diffusion on the biosorbent surface, solubility

of heavy metals, enzymatic activity and metabolism increase, thus generally escalating the

removal process (Igiri et al. 2018). Numerous studies on the impact of temperature on heavy

metals remediation are available, but the response of different microalgae to temperature shows

inconsistent effects on remediation. Kumar et al. (2015) reported varying impacts of temperature

on removal of heavy metals; some indicated a rise in metal remediation with an increase in

temperature, while others showed a decline at high temperature. Aksu (2002) reported that the

dry biomass of Chorella vulgaris shows improved adsorption of Ni at increased temperature.