11.2 Microalgae for Wastewater Treatment

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the cell wall and the insoluble fraction gets accumulated intracellularly. Different

species of microalgae such as C. vulgaris, Scenedesmus sp., Chlorococcum sp.,

Lyngbya spiralis, Tolypothrix tenuis, Stigonema sp., Phormidium molle, Aphanothece

halophytica, and Chroococcus paris can remove heavy metals like Hg(II), Cd(II),

and Pb(II) from water bodies.

Leather tanning, electroplating, and tincture wood preservative industries dis-

charge a considerable amount of Cr(III) into the water bodies. Dictyosphaerium

chlorelloides can sequestrate a high amount of Cr(III) into its cell wall, chloroplast,

vacuoles, and cytoplasm through biosorption, accumulate, and use it to produce

sugars and fats. Cr(III) can be oxidized to Cr(VI) by MnO2 and bacteria. Cr(VI)

when compared to Cr(III) is highly mutagenic, carcinogenic, and is 500–1000 times

toxic to a living cell. It is reported that C. vulgaris can convert Cr(VI) into its less toxic

trivalent form [23], whereas Spirulina platensis has 60.92% Cr(VI) removal efficiency

when cultivated in a mixture of artificial medium and wastewater [24]. Scenedesmus

quadricauda species was shortlisted from 11 microalgal species to treat synthetic

wastewater, reducing 30 mg/l of nickel (Ni) and zinc (Zn) to 0.4 mg/l within

90 minutes which is attributed to its higher surface area compared to the other

microalgae [25] Arsenic (As) is a toxic water pollutant and its high concentration

can lead to skin and respiratory diseases, neurological, cardiovascular, gastrointesti-

nal, and urinary disorders, increasing the risk of high blood pressure and diabetes.

The inorganic forms As(III) and As(V) are more toxic compared to its organic form.

A study has shown that Ostreococcus tauri, a marine microalga converts inorganic

As into its organic form, integrating into biogeochemical cycles and catalyzing As

volatilization through the metabolic mechanism of biomethylation [26].

11.2.5

Xenobiotic Compounds

Various industries producing high concentrations of synthetic toxic chemicals

should be treated before releasing into water bodies which is expensive and

complex. Microalgae are efficient in treating these xenophobic compounds by

dispersion, chemical transformation, and bioaccumulation. PAHs are highly toxic

with carcinogenic properties, and therefore, their presence and levels need to be

checked. Several species of microalgae are reported to assimilate and transform into

a less toxic compound. C. vulgaris, Spirulina platydiscus, S. quadricauda, and Sele-

nastrum capricornutum were tested for their ability to remove fluoranthene, pyrene,

and their mixture. S. capricornutum showed the highest removal efficiency ranging

from 88% to 98% for different concentrations [3]. Monoaromatic hydrocarbons

are highly mutagenic and carcinogenic xenobiotic compounds from production,

transportation, and storage of oil and its products, which can be sequestrated using

microalgae. Chlorophenols are another class of xenobiotics with carcinogenic

properties mostly used in the production of pesticides and wood preservatives. A

study showed that Tetraselmis marina metabolized 2,4-dichlorophenol, whereas C.

vulgaris and C. pyrenoidosa showed 100% p-chlorophenol removal efficiency [27]

and Chlorella fusca showed 90% Bisphenol A removal efficiency [28].