Microalgal Bioremediation of Heavy Metals 215

binds to the algal cell in a polynuclear form; after binding of aluminum ions, these ions prevent the

other metal ions from binding on the binding surface of the biomass (Bottero et al. 1980).

12.2.6 Metal Removal in Continuous Packed Bed-Reactors

For the industrial application of microalgal bioremediation of metals, the operation should be

carried out in a continuous mode, where packed bed reactors are found useful. For the dynamic and

static mode studies, Lyngbya putealis HH-15 cyanobacteria extracted from a metal-polluted surface

was used as a biosorbent of Cr (VI) from aqueous solutions (Kiran and Kaushik 2008). Through

regularly flow column experiments, the data effect of an initial concentration (5–20 mgL^–1), flow

rate (1–3 mL min^–1) and bed height (5–10 cm) on breakthrough time and adsorption capacity of

the immobilized alga was developed into Bohart-Adams model (Singh et al. 2012). The chromium

elimination efficiency and regeneration capacity of this biosorbent recommend its application use

in industrial activities, and the data generated strongly suggested potential for more increase in

the adsorption process. There is another discovery for metal sorption of a column packed with

Spirogyra granules, which could be successfully used up in many cycles of sorption and removing

Cu (II) and Pb (II), respectively (Singh et al. 2012). The excellent metal bonding ability of algae

has been proved by the assessment of maximum sorption capacity based on isotherm studies by

various researchers. Continuous flow studies in packed bed columns appear highly effective and

economically suitable than the metal absorption by batch operation. Thomas mass transfer model,

Adam-Bohart advection–dispersion-reaction equation and bed-depth-service-time model have been

established for understanding the breakthrough curve. Some fresh approaches, such as artificial

neural networking, may prove still more useful for elucidating breakthrough curves and metal

sorption in multi-metal systems (Kumar et al. 2016).

12.2.7 Pretreatment and Immobilization Approaches for Improved

Bioremediation

Microalgal biosorption of metals can be improved by adopting physical and chemical pretreatments

that provide extra binding sites on the cell surface by changing the cell surface structure. Physical

pretreatment (heating, boiling, freezing, crushing and drying) enhanced metal ion biosorption

(Ahluwalia et al. 2007). These kinds of pretreatments increase the cell wall surface area and

favor improved biosorption of the metals (Uzunoglu et al. 2014, Kiran et al. 2016). General algal

pretreatments include treating with calcium chloride, formaldehyde, glutaldehyde, NaOH and HCl.

The impact of temperature and acid treatment on the absorption of tetravalent chromium [Cr (IV)]

by the Chlamydomuns reinhardtii was observed and noted that biosorption capacity is higher than

the untreated biomass (Zeraatkar et al. 2016). Biofilm generation on natural or artificial packing,

engagements of biomass inside matrices, adsorption and binding of cells to a surface are considered

as the prime immobilization technique used in the metal elimination process (Nasirpour et al. 2017).

Saccharomyces cerevisiae immobilized polyacrylamide was used to bioaccumulate Cu²⁺, Cd²⁺,

Co²⁺ (Duncan et al. 1997). Sequestration of Cr and Co by exopolysaccharides (EPS) of freshwater

microalga has been demonstrated with high biosorption capacities 14.3mg Cr g^–1 EPS and

17.9 mg Co g^–1 EPS (Mona and Kaushik 2015b).

12.3 Challenges and Future Opportunities

Bioremediation of heavy metals, particularly the biosorption technique, has been one of the

most detailed studied subjects from the past six decades, with more than 13,000 research papers

in reviewed journals. A vital understanding of the complex biosorption mechanism has made it

possible to quantify the process based on equilibrium and kinetics studies and optimize the process

by manipulating various operational parameters using modeling approaches based on RSM. Though

testing of the process in pilot projects and at the industrial-scale is still in the initial stage, the