Microalgal Bioremediation of Heavy Metals 217

adsorption capacity (15–18 mg/g) for Cr (VI) and Cd metals, and the efficiency is higher when the

spent biomass is obtained at a relatively earlier stage (Mona et al. 2013). Such studies suggest that

careful planning can lead to the highly successful integration of the algae-based bioremediation­

cum-bioenergy production.

These studies explore the application of waste algal biomass from biofuel producing systems for

metal bioremediation and use microalgal biomass produced in wastewater for biofuel production.

12.4.2 Using Microalgae for Bioremediation Adopting Biorefinery

Approach

Mass cultivation of algae can be done in a cost-effective way by growing them in wastewaters

that generally contain carbonaceous matter, nitrates and phosphates nutrients required for algal

growth. The cost of cultivating 1 kg microalgae using synthetic fertilizers in the culture medium or

freshwater is very high. While growing in such wastewaters, microalgae utilize all the inorganic and

organic nutrients (TP, TN, NH4

+, NO3–, TOC). These nutrients are removed from the wastewater,

and COD and BOD are also decreased. When algal production is carried out by raceway ponds,

the cost decreases of biofuel production from wastewater treatment increases with added benefits

of less impacts of environment (Park et al. 2011). Chlorella sp. extracted from wastewater showed

high lipid production efficiency in a bioreactor under heterotrophic and photoautotrophic conditions

(Viswanath and Bux 2012). Under heterotrophic conditions, it produced 3.6-fold more biomass

than photoautotrophic growth conditions and enhanced lipid production by 4.4-fold. Microalgal

heterotrophic growth is an efficient way for producing high biomass and biochemicals of algae,

which could lower the cost of producing microalgal biomass. Since various microalgal species also

have an innate ability to produce certain peptides having functional groups such as amine, sulfate,

phosphate, carboxyl and hydroxyl, that can be attached to heavy metals; these biomolecules take

part in conclusive to the metal ions compactly (Gong et al. 2005). The dead microalgal cells also

show biosorption function for bioremediation of heavy ccc metals. As demonstrated by Kaushik

et al. (2011), even the spent biomass of algae from bioreactors shows very good metal biosorption

capability; thus, there are enormous opportunities that need to be explored and investigated

systematically for adopting a biorefinery approach (Figure 12.3).

There are several methods to produce different types of fuels from algal biomass using wastewater

as a biorefinery approach (Craggs et al. 2011), while several value-added biomaterials, including

pigments and carbohydrates, can also be obtained from the microalgae along with remediation of

the wastewater (Markou and Nerantzis 2013, Preeti et al. 2015).

Biodiesel production from microalgae occurs via transesterification, i.e., the conversion of lipids

into fatty acid methyl esters. The biodiesel formation from microalgae completed the sequential

steps: biomass production, lipid extraction from biomass, lipids transesterification and biodiesel

purification.

Bioethanol production from algal biomass or algal meals takes place through saccharification

and fermentation. The carbohydrates in algal biomass, such as starch, cellulose, glucose and

hemicellulose, get converted to bioethanol through fermentation. Porphyridium cruentum,

Spirogyra sp., Nannochloropsis oculate and Chlorella sp. are some of the most used microalgae for

carbohydrate synthesis (Markou and Nerantzis 2013).

Biogas and biomethane are created by fermented bacteria during the controlled, anaerobic

breakdown of microalgal biomass (Li et al. 2019). Methane fermentation involves a series of

biochemical reactions carried out by specific microbes, including hydrolysis, acidogenesis and

methanogenesis (Jankowska et al. 2017). Biobutanol can be made from green waste left behind

after extracting microalgae oil. It has a higher energy density than biomethanol or bioethanol and

a chemical structure like gasoline. It can also be used as a solvent in industries, in addition to

being a biofuel (Yeong et al. 2018). Microalgal strains which have high starch and sugar content,

such as Chlorella vulgaris, Tetraselmis subcordiformis and Scenedesmus obliquus, are favored for

biobutanol synthesis.