ion bioremediation from both aqueous and terrestrial environments. Some of the key characteristics of these biological compounds that play a crucial and target‐specific role during the remediation process are their higher surface activity with high tolerance to various environmental factors. The biosurfactants also can withstand from mean to extreme conditions, such as ionic strength, temperature, acidity or basicity of medium salt concentrations, biodegradable nature, demulsifying–emulsifying ability, anti‐inflammatory potential and antimicrobial activity.
The scientists and researchers have identified and characterized different types of biosurfactants produced from various biological sources [26–28]. The main criteria behind biosurfactant classification are their chemical structure, source of origin, antimicrobial activity, efficiency of pollutant removal from the environment, and surface tension reduction ability [29]. A variety of materials have been used by the microbial community as carbon and energy sources for their production. Microorganisms release tensio‐active substances as biosurfactants in the medium during degradation of hydrocarbons [30].
4.3 Mechanisms of Biosurfactant–Metal Interactions
Two main pathways have been identified for the desorption of metal ions from contaminated land using biosurfactants [31]. In the first pathway, there is a complex formation between the free, non‐ionic form of metal and biosurfactant molecules. In this interaction, using the principle of Le Chatelier, the solution phase activity of the metal ions is reduced and thus its desorption from the medium increases. In the second pathway, it is proposed that there is an accumulation of biosurfactants at the solid–solution interface and absorption of metal ions occurs as the interfacial tension reduces between the two.
According to Rufino et al. [32], ion exchange, precipitation dissolution, counter‐ion association, and electrostatic interaction are some of the chief mechanisms that govern metal–biosurfactant binding in the contaminated environment. Studies reveal that the complex formation capability of the biosurfactant with the metal ions is the chief cause for their usefulness in metal ion remediation. Precisely, ionic bonds formed in between metal ions and anionic biosurfactants lead to a generation of stronger stabilizing forces as nonionic complexes form, which, of course, are stronger as compared to metal and soil interaction. Because of the neutral charge of the complex with a subsequent amalgamation of the metal into micelles, the complex form of metal–biosurfactants desorb from the soil matrix and move into the soil solution. A detailed study of the proposed mechanism reveals that either an outer‐sphere surface complex formation occurs in between negatively charged surfaces and metals due to strong electrostatic attraction or an inner‐sphere surface complex formation is established between the metal ions and biosurfactant molecules due to chemical bonding in which hydroxide groups serve as ligands. The mechanism of metal binding through both pathways is smoothed in the presence of water molecules and easy protonation and deprotonation of oxide functional groups. The mechanism of metal–biosurfactant interaction is represented in Figure 4.1.
Figure 4.1 Biosurfactant mediated heavy metal remediation.
4.4 Substrates Used for Biosurfactant Production
Microorganisms are identified as the most important source for production of biosurfactants. Willumsen and Karlson [33] in their study found that many of the biosurfactant‐producing microorganisms are hydrocarbon degraders. The proficiency of microbial biosurfactant in the bioremediation as well as in the enhanced oil recovery have been researched extensively [34]. The verities of substrate used for common biosurfactant production is represented in Figure 4.2.
Figure 4.2 Classification of biosurfactants and the respective producing microorganisms.
4.4.1 Biosurfactants of Bacterial Origin
In the growth medium, the hydrocarbons are emulsified by ionic surfactants excreted by some of the bacteria and yeast. Pseudomonas sp. that produce rhamnolipids (RLs) and Torulopsis sp. that are mainly involved in the production of sophorolipids are some examples of these groups of biosurfactants [35, 36].
Some bacterial species have the ability to alter their cell membrane structure by producing some nonionic or lipopolysaccharide biosurfactants. Examples of some nonionic trehalose corynomycolates producing bacterial strains are: Rhodococcus erythropolis, Arthrobacter sp., and various Mycobacterium sp. [37]. Acinetobacter sp. produce lipopolysaccharides, such as emulsan, and Bacillus subtilis produces extensive quantities of lipoproteins, such as surfactin and subtilisin [38, 39]. Table 4.1 depicts biosurfactants produced by various strains of bacteria.
Table 4.1 Biosurfactants derived from bacteria.
| Bacteria | Biosurfactant |
|---|---|
| Serratia marcescens | Serrawettin |
| Rhodotorula glutinis, Rhodotorula graminis | Polyol lipids |
| Rhodococcus erythropolis, Corynebacterium sp. Mycobacterium sp., Arhtrobacter sp., Nocardia erythropolis | Trehalose lipids |
| Pseudomonas sp., Thiobacillus thiooxidans, Agrobacterium sp. | Ornithine lipids |
| Pseudomonas fluorescens, Leuconostoc mesenteriods | Viscosin |
| Pseudomonas aeruginosa, Pseudomonas chlororaphis, Serratia rubidea | Rhamnolipids |
| Pseudomonas fluorescens, Debaryomyces polmorphus | Carbohydrate‐lipid |
| Pseudomonas aeruginosa | Protein PA |
| Lactobacillus fermentum | Diglycosyl diglycerides |
4.4.2 Biosurfactanats of Fungal Origin
Only a few species of fungi are identified for the production of biosurfactants in comparison to bacterial species. Some of the typical fungal strains explored for the production of biosurfactants, as investigated by researchers are, Candida bombicola [40], Candida ishiwadae [41], Candida lipolytica [42], Candida batistae [43], Aspergillus ustus [44], and Trichosporon ashii [45]. The best part of these fungal strains is that they have produced biosurfactants using low‐cost raw materials as their growth substrate. Glycolipids and sophorolipids are one of the most important class of biosurfactants produced by these fungal strains. The various biofactants produced by fungi are shown in Table
