Key words: biosurfactant remediation of heavy metal polycyclic aromatic hydrocarbons
Biosurfactants are surface-active metabolites secreted by microorganisms in the process of culture and metabolism under certain conditions. Compared with chemically synthesized surfactants, biosurfactants have many unique properties, such as structural diversity, biodegradability, extensive biological activity and environmental gentleness.
Table 1 Types of biosurfactants and their producing bacteria
Biosurfactant
Producing bacteria
Trehalose ester
Arthrobacter paraffin
Corynebacterium.
Rhodococcus erythropolis
Rhamnolipid
Pseudomonas aeruginosa (Pseudomonas aeruginosa)
Sophora japonica glycolipid
Candida lipolytica and pseudosaccharomyces bombycis.
Glucose, fructose, sucrose ester
Corynebacterium.
Rhodococcus erythropolis (Rhodococcus erythropolis)
Cellulose disaccharide ester
Corn smut
lipopolysaccharide
Acinetobacter calcoaceticus rag 1
Pseudomonas.
lipopeptid
Bacillus subtilis (Bacillus subtilis)
Bacillus licheniformis and Pseudomonas fluorescens.
Ornithine, lysine, peptide
thiobacillus thiooxidans
Streptomyces schia
Gluconobacter cereus
phosphatide
T. sulfur oxide
fatty acid
Corynebacterium rabbit
Arthrobacter paraffin
2 the production of biosurfactant
At present, there are two production methods of biosurfactants: microbial fermentation and enzymatic method.
In fermentation production, the types and output of biosurfactants mainly depend on the types of producing bacteria, growth stages, properties of carbon matrix, concentrations of N, P and metal ions Mg2+ and Fe2+ in the culture medium, and culture conditions (pH, temperature, stirring speed, etc. ). For example, Davis et al. [5] found that the maximum concentration of Shafantine (439.0 mg/L) can be obtained under the condition of exhausted dissolved oxygen and limited nitrogen. Kitamoto et al [6] used resting cells of Candida antarctica to produce mannose erythritol. After optimizing the culture conditions, the highest yield can reach140 g/L. The production of biosurfactants by fermentation has the advantages of low production cost, variety and simple process, which is convenient for large-scale industrial production, but the cost of product separation and purification is high.
Compared with microbial fermentation, most of the surfactant molecules synthesized by enzymatic method are relatively simple in structure, but they also have excellent surface activity. Its advantages are low extraction cost, convenient transformation of secondary structure, easy purification, reusable immobilized enzyme, and the surfactant synthesized by enzymatic method can be used to produce high value-added products, such as pharmaceutical ingredients. Although the cost of enzyme preparation is high at present, it is expected to reduce its production cost by enhancing the stability and activity of enzyme through genetic engineering technology.
3 extraction of biosurfactant
The extraction of fermentation products (also known as downstream treatment) accounts for about 60% of the total production cost, which is a major obstacle to the commercialization of biosurfactant products. The best extraction method of biosurfactant varies with fermentation operation and its physical and chemical properties. Among them, solvent extraction is the most commonly used extraction method. For example, Kuyukina et al. [7] used methyl tert-butyl ether to extract biosurfactant produced by Rhodococcus, which can obtain higher yield10 mg/L. Ultrafiltration is a new method to extract biosurfactant. Lin et al. [8] used ultrafiltration membrane with molecular weight cut-off of 30000 Da to extract the lipopeptide biosurfactant savantidin from the fermentation broth of Bacillus subtilis, and the yield was 95%. Mattei et al. designed a set of device for continuous extraction of biosurfactants. The product can be continuously extracted by tangential flow filtration, and the yield is as high as 3 g/L[ 1]. The product extraction methods that can be matched with continuous fermentation production include foam separation method and ion exchange resin method. Davis et al. [9] used foam separation method to continuously extract the shavantin produced by Bacillus subtilis, and the yield reached 7 1.4%. The extraction process of rhamnolipid is to remove cells by centrifugal filtration, then concentrate rhamnolipid on Amberlite XAD-2 resin by adsorption chromatography, and then purify it by ion exchange chromatography. Finally, the finished product with purity of 90% can be obtained by evaporation and freeze drying, and the yield is 60%[2].
Application of biosurfactant in environmental engineering
Many chemically synthesized surfactants destroy the ecological environment because of their difficult degradation, toxicity and accumulation in the ecosystem. In contrast, biosurfactants are more suitable for pollution control in environmental engineering because of their easy biodegradation and non-toxicity to the ecological environment. For example, it can be used as a flotation collector to attract charged colloidal particles in the process of wastewater treatment to remove toxic metal ions and repair sites polluted by organic matter and heavy metals.
4. Application of1in wastewater treatment process
When wastewater is treated by biological method, heavy metal ions often inhibit or poison the microbial flora in activated sludge. Therefore, pretreatment is necessary when wastewater containing heavy metal ions is treated by biological method. At present, hydroxide precipitation method is commonly used to remove heavy metal ions from wastewater, but its precipitation efficiency is limited by the solubility of hydroxide, and its application effect is not ideal. When wastewater is pretreated by flotation, the flotation collectors (such as sodium dodecyl sulfonate, a chemically synthesized surfactant) are difficult to degrade in the subsequent treatment process, and are prone to secondary pollution, which is often limited. Therefore, it is necessary to develop biodegradable and non-toxic substitutes, and biosurfactants have this advantage. However, there are few applied researches in this field at home and abroad, and they have only been reported recently. Zouboulis et al. [10] studied the removal of two toxic metal ions: Cr4 ++ and Zn2 ++ widely existing in industrial wastewater with biosurfactants as collectors. The results showed that Savantin and Bacillus licheniformis could separate αFeO(OH) or Cr4++ from FeCl3? The chelate formed by 6H2O greatly improved the removal rate of Cr4+ (50 mg/L), almost reaching 100%. At pH 6, the removal rate of Zn2+(50 mg/L) in chelate was as high as 96%, but under the same conditions, the treatment effect of Bacillus licheniformis was not obvious, and the removal rate was about 50%.