Surfactants used in environmental engineering
Biosurfactants are surface-active metabolites secreted by microorganisms during their metabolic processes when they are cultured under certain conditions. Compared with chemically synthesized surfactants, biosurfactants have many unique properties, such as structural diversity,
biodegradability, wide range of biological activities, and mildness to the environment [1]. Due to the influence of raw materials, price and product performance, chemical synthetic surfactants often seriously pollute the environment and jeopardize human health in the process of production and use. Therefore, with the enhancement of human environmental protection and health consciousness, in the past two decades, the research on biosurfactants is increasing and developing rapidly, foreign countries have applied for patents on a variety of biosurfactants and their production processes [2], such as an extracellular bio-emulsifiers produced by Fusobacterium acetogenes, and finished products are already available for sale. The development and application of biosurfactants in China started late, but in recent years, great attention has been paid to the application of biosurfactants in the enhancement of oil recovery and bioremediation, which is the most researched.
Types of biosurfactants and their production bacteria
Types of biosurfactants While chemically synthesized surfactants are usually classified according to their polar groups, biosurfactants are differentiated by their biochemical properties and producing bacteria. They are generally classified into five types: glycolipids, phospholipids and fatty acids, lipopeptides and lipoproteins, polymers and specialty surfactants .
Biosurfactant-producing bacteria Most biosurfactants are metabolites of bacteria, yeasts and fungi. Most of these producing bacteria are screened from oil polluted lakes, soils or oceans. For example, Banat et al [3] isolated two strains of biosurfactants from oil sludge contaminated soil: Bacillus sp. AB-2 and Y12-B. Table 1 lists some of the major biosurfactant species and their producing bacteria.
Production of biosurfactants Currently, biosurfactants can be produced by two ways: microbial fermentation and enzymatic methods. In the case of fermentation, the type and yield of biosurfactant mainly depend on the type and growth stage of the producing bacteria, the nature of the carbon substrate, the concentration of N, P, and metal ions Mg2+ and Fe2+ in the medium, and the culture conditions (pH, temperature, agitation speed, etc.). Such as Davis et al [5] in the batch culture of Bacillus subtilis found that in the dissolved oxygen depletion and nitrogen limitation conditions can be the maximum concentration (439.0mg / L) of Savantin. Kitamoto et al [6] use of Antarctic Pseudomallei yeast resting cells to produce mannose erythritol lipids, the optimization of culture conditions, the highest yield of up to 140g / L. Fermentation production of biosurface The advantages of fermentation for the production of biosurfactants include low production cost, variety and simple process, which is convenient for large-scale industrial production, but the cost of separation and purification of the products is high. Compared with the microbial fermentation method, the surfactant molecules synthesized by enzyme method are relatively simple molecules, but they also have excellent surface activity. The advantages are low extraction cost, convenient secondary structure improvement, easy purification and reusability of immobilized enzyme, etc. The surfactants synthesized by enzyme method can be used to produce high value-added products, such as pharmaceutical components. Although the cost of enzyme preparation is relatively high at the present stage, it is expected that the stability and activity of the enzyme can be enhanced through genetic engineering technology, which will reduce its production cost.
