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Bioremediation for Sustainable Environmental Cleanup

10.4.1.1 Bioleaching

The bioleaching approach by acidophilic bacteria facilitates the solubility of heavy metals from

sediments and contaminated sites. It predominantly affects the concentrations of toxic metals,

including iron and sulfur, in sediments. Thus, groups of bacteria oxidize iron and sulfur compounds

include: Firmicutes (e.g., Alicyclobacillus sp., Sulfobacillus sp.), Nitrospirae (e.g.,Leptospirillum sp.),

Proteobacteria (e.g., Acidithiobacillus sp., Acidiphilium sp., Acidiferrobacter sp., Ferrovum sp.),

Actinobacteria, archaea (Crenarchaeota). These microorganisms are known as bioleaching

microorganisms (Akcil et al. 2015). They can establish an acidic environment by oxidizing minerals,

and therefore dissolve toxic metals ions into an aqueous medium (Akcil et al. 2015).

Chemolitho-autotrophic bacteria Leptospirillum oxidized Fe⁺² under acidic conditions

(pH = 1.5 – 1.8) in a sample adulterated with uranium (U92) (Bertrand et al. 2015), whereas

Thiobacillus is found to be capable of generating energy by oxidation of sulfur and thiosulfate (Xin

et al. 2009). Fungal strain Aspergillus niger is also found to be efficient for leaching the elements

and removing pollutants from sediment (Zeng et al. 2015).

The efficiency of bioleaching depends on various parameters, including abiotic stresses,

characteristics of ions present in sediments, pH, and size of sediments. The biotic factors like the

presence of microbial communities, metabolic pathways used for the process, and adaptability to

minerals also play a significant role. As and Fe are the most leached metals from sediments. With

these, a small percentage of Cu is also leachable. The bioleaching process is not suitable for Hg.

The remediating capabilities of indigenous heterotrophic bacterial isolates of Bacillus have been

observed in the leaching of toxic metals from polluted sites (Štyriaková et al. 2016).

These methods are viable both financially and environmentally. According to different research

studies, bioleaching of toxic metals is more effective than the conventional leaching methods

(Liu et al. 2003, Deng et al. 2013). However, several significant disadvantages restrict the utilization

of this technique for an effective detoxification process. Excessive metal concentrations usually limit

the development or functioning of susceptible microbes (Collinet and Morin 1990). A high load of

solid materials and organic materials in polluted sources also hinders microorganisms, resulting in

decreased bioleaching effectiveness (Cho et al. 2002).

10.4.1.2 Biosurfactant

Biosurfactants are synthesized by various classes of microorganisms, including fungi, and are

known to be a substitute for conventional leaching owing to factors such as low toxicity, high

biodegradability and significant environmental friendliness (Shekhar et al. 2015). Biosurfactants

are the molecules that serve as biological chelating agents for a number of heavy metals. The

biosurfactant-producing microorganisms are used at heavy metal-contaminated locations (Pacwa­

Płociniczak et al. 2011). Biosurfactants are composed of different functional groups, including fatty

acids, glycolipids and phospholipids (Pacwa-Płociniczak et al. 2011). Rhamnolipid is the most

studied biosurfactant for heavy metal elimination (Mulligan et al. 1999).

Z-glycolipid in Burkholderia sp. is capable of being utilized as a biosurfactant to remove a

mixture of toxic metals like Pb, As, Cu, Cd, Zn and As from polluted soil. Due to large acid-soluble

properties and high chelation with biosurfactants, Mn, Zn and Cd have been more efficiently

removed from the soil. The biosurfactant, rhamnolipid, demonstrated efficient heavy metal removal

from polluted river sediments in exchangeable, carbonate bound or iron-manganese oxide–bound

fractions (Chen et al. 2017).

Using biosurfactants to remove pollutants from soil, waste and sediments is an extremely

interesting approach. It may also be used for the pre-treatment of contaminated areas before

proceeding with stabilization/solidification, natural attenuation or electrokinetic processes. This

method has the edge over other bioprocesses due to its ability to operate effectively even at high pH

up to 11, while other bioprocesses (such as bioleaching and biosorption) work effectively only at low

pH levels. Biosurfactants with excellent degradability and biocompatibility provide a considerable

advantage with high environmental acceptability (Mulligan 2005). However, the exorbitantly high