origin.
In recent years, scientific communities have paid attention to the economic aspects of biological processes and from here the concept of a “circular economy” has been introduced for biosurfactant production and its application. The circular economy strategy is focused on waste prevention or where it is generated it can be utilized in bioeconomical ways. In a circular economy, biorefining, which is the sustainable dispensation of waste biomass into a spectrum of marketable products and energy, play a crucial role. The biosurfactant synthesis and its bioeconomical utilization help in upliftment of a deteriorating environment condition, especially by heavy metals. Therefore, the aim of the present chapter is to provide information about biosurfactant production and their potential application in a bioeconomic way as environment‐friendly products in metal remediation.
4.2 Concept of Surfactant and Biosurfactant for Heavy Metal Remediation
The wide, extensive, and significant role of surfactants has been introduced in various sectors, such as environmental pollution mitigation, the petroleum industry, the detergent industry and food production facilities [23]. Due to the amphiphilic nature of surfactants, the surface tension in an oil–water interface decreased and hence water‐immiscible substance solubility increased. The surfactant environmental impacts and their easy disposal after utilization is of principal concern due to sundry and widespread utilization in the environment. This demanded screening of an environmentally friendly alternative to synthetic surfactants with equal efficiency. These classes of novel surfactants, having a biological origin, stated as “biosurfactant,” shows significant variation in terms of their chemical composition, structure, and mode of action. In present environmental remediation techniques, biosurfactants are gaining more attention due to their biodegradability and eco‐friendly attributes [24].
When focusing on biosurfactants of microbial origins, the extremophiles have gained more attention in the last few years as they can also show efficient remediation capacity in extreme punitive conditions. Their chemical structure involves hydrophilic polar moiety as oligo‐ or monosaccharide and proteins as well as polysaccharides or peptides and the hydrophobic moiety has unsaturated or saturated fatty alcohols or hydroxylated fatty acids [25]. The balance between the hydrophilic and lipophilic ends of biosurfactants grounds the hydrophilic and hydrophobic ends to be determined in substances that are surface active. The amphiphilic behavior of biosurfactants enables them to enhance the hydrophobic substance surface area as well as to give them modification ability in the microbial cell surface property.
The real breakthrough in the biosurfactant production and application research comes only after the knowledge of genetically engineered microorganisms because they have the capability of giving high yields. The detailed knowledge of genetics of the microorganisms plays a crucial role in this esteem. Previous studies on the molecular genetics and biochemistry of several biosurfactants revealed the operons, the enzymes, and the metabolic pathways for their extracellular production. One of the examples is surfactin – a cyclic lipopeptide biosurfactant. Its production occurred as a result of catalysis of a large multienzyme peptide synthase complex, i.e. surfactin synthase via a non‐ribosomal biosynthesis pathway. Similar enzyme complexes are responsible for the synthesis of other lipopeptides such as iturin, lichenysin, and arthrofactin. A very high level of compositional similarity has been shown by various lipopeptide synthesizing non‐ribosomal peptide synthetases (NRPSs). Psuedomonas species required plasmid‐encoded‐rhl A, B, R, and I genes of an rhl quorum‐sensing system for production of glycolipid biosurfactants. The molecular genetics of biosynthesis of alasan and emulsan synthesized by Acinetobacter species, along with some other biosurfactants of fungal origin including mannosylerythritol lipids (MEL) and hydrophobins, have also been researched by scientists.
Microbes involved in biosurfactant production show their excellent efficiency in metal 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.