A review study on new aspects of biodemulsifiers: Production, features and their application in wastewater treatment
Graphical abstract
Introduction
The rapid development of industries has led to environmental contamination. An oily emulsion is a blend of oil (dispersed phase) in water (continuous phase), which is generated in the wastewater of refineries (Katepalli, 2014; Parvin et al., 2019; Nematollahzadeh et al., 2021). Generally, we have 4 kinds of emulsions, including water in oil (W/O), oil in water (O/W), water in oil in water (W/O/W), and oil in water in oil (O/W/O). Emulsions generated in refinery effluents are often stable and difficult to destabilize because they contain complex compounds such as resins and asphalt (Vallejo-Cardona et al., 2017; Hashemi et al., 2019; Fouladi et al., 2021). Fig. 1 demonstrates various kinds of emulsions.
As a result, surfactants are required to produce a stable emulsion (Mohamed et al., 2017). Chemical surfactants have high emulsification efficiency, however, they have many disadvantages compared to biosurfactants. For example, chemical surfactants are harmful to the environment and have adverse impacts on marine life. Also, they are not easily degraded, which could lead to a ban on these products in the future (e Silva et al., 2017). The capability of a surfactant to decrease the interfacial tension between oil and water is an essential feature for emulsion generation. Several factors like temperature, mixing time, water content, mixing rate, biosurfactant type, and dosage of biosurfactant are effective to generate a stable emulsion. Also, the more the age of the emulsion, the more stable it is (Zhang et al., 2019; Saad et al., 2019; Chen et al., 2017). One of the most important methods to specify the stability of emulsions is to measure their droplet size distribution. The smaller the size of the droplets, the more stable the emulsion (Goodarzi and Zendehboudi, 2019). The separation of hydrocarbon compounds from a stable emulsion is difficult and requires an efficient method. There are several procedures to destabilize and separate oil from the emulsion, including physical (e.g., gravity settling, flotation, membrane, microwave, ultrasonic irradiation), thermal, chemical (e.g., adsorption, demulsifier), and biological approaches (e.g., activated sludge, lagoons). In the industry, a combination of these methods is used (Liang and Esmaeili, 2021; Yang et al., 2009). However, the use of biodemulsifiers has recently received much attention in research work (Vallejo-Cardona et al., 2017). Biodemulsifiers are environmentally friendly materials that consist of non-toxic compounds (Saad et al., 2019). Also, biodemulsifiers can be produced using low-cost substrates. Other benefits of biodemulsifiers are biodegradability, high performance in severe conditions of pH, salinity, and temperature (more tolerance in harsh conditions) (Mujumdar, 2011). Biosurfactants are classified into two groups, including low molecular weight compounds (e.g., glycolipids, lipopeptides, and phospholipids) and high molecular weight compounds (e.g., polymeric surfactants). Sophorolipids, trehalolipids, and rhamnolipids are some of the best glycolipids, which can be produced from Torulopsis bombicola, Rhodococcus erythropolis, Pseudomonas aeruginosa, respectively. Also, surfactin is the most potent biosurfactant of the lipopeptide type, which can be produced from Bacillus subtilis (e Silva et al., 2017; Shekhar et al., 2015). Moreover, emulsan, liposan, carbohydrate protein-lipid, mannan lipid-protein, and biodispersan are the most important polymeric biosurfactants, which can be produced from Arethrobacter calcoaceticus, Candida tropicalis, Pseudomonas fluorescens, Candida tropicalis, and Arethrobacter calcoaceticus, respectively (Shekhar et al., 2015). Different kinds of microorganisms (bacteria, yeasts, and fungi) can be used to generate biodemulsifiers, including Serratia marcescens (Matsuyama et al., 2010), Rhodotorula glutinis R. graminis (Amaral et al., 2006), Corynebacterium, Nocardia, Mycobacterium, Rhodococcus, Bacillus (e Silva et al., 2017), Pseudomonas aeruginosa (Jadhav et al., 2011), Aureobasidium pullulans (Garay et al., 2018), and Aspergillus ustus (Alejandro et al., 2011). A small quantity of biodemulsifier should be added to the O/W emulsion to destabilize it. After adding the biodemulsifier to the emulsion, the interfacial tension between the two phases is reduced, resulting in the removal of oil from the emulsion (Saad et al., 2019). Microorganisms can change surface charges in the interface of water/oil and thus improve emulsion properties (Coutinho et al., 2013; Karlapudi et al., 2018).
This review paper provides good information about biosurfactants, biodemulsifiers, their features, types of biosurfactants, and their application in biodemulsification. Also, different procedures for producing biosurfactant using various microorganisms (e.g., bacteria, fungi) are completely studied. Besides, the impact of various parameters like pH, temperature, different types of biodemulsifiers, concentration of biodemulsifiers, and contact time are studied on the biodemulsification process. To the best knowledge of the authors, so far, no review work has been done on the synthesis method of biodemulsifiers, properties of biodemulsifiers, and their application on the destabilization and demulsification of oily wastewater.
Section snippets
Biosurfactants
Biosurfactants are produced from biological substances and compared to chemical surfactants, they are high stability, low toxicity, simple synthesis, structural diversity, and eco-friendly (Lima and Alegre, 2009). Other advantages of biosurfactants are high biodegradability, biocompatibility, and digestibility. Biosurfactants are produced using microorganisms (e.g., fungi and bacteria) and have hydrophilic and hydrophobic groups on their structures, which are critical to stabilize or
Emulsion formation
Biosurfactants can be applied as emulsifiers or demulsifiers. Emulsions are a heterogeneous solution of two immiscible liquids, in which one liquid is dispersed in another. They may be formed by adding a small quantity of the biosurfactant. Some emulsions may be stable for a month or even a year. Liposan is a hydrophilic emulsifier that is used to produce a stable emulsion (Vijayakumar and Saravanan, 2015; Wong, 2017; Lu et al., 2019). This biosurfactant forms micelles around oil droplets and
The role of biodemulsifiers in the demulsification process
Biodemulsification with the help of biodemulsifiers is a critical process for eliminating oil from oily wastewater (Saad et al., 2019; Ly et al., 2008; Jiang et al., 2017). Common demulsifiers are dangerous to the environment and have adverse impacts on human life. Also, surfactants, unlike biosurfactants, do not degrade easily (e Silva et al., 2017; Aunsary and Chen, 2019). Therefore, the use of biodemulsifiers has received much attention to destabilize and demulsify oil/water emulsions. Also,
Mechanism of the demulsification process
The demuslification process depends on various factors like the oil content in the emulsion, temperature, and gravity field (Lima and Alegre, 2009; Zhang et al., 2020b). As the oil droplets coalesce, the droplets become larger and the number of oil droplets in the emulsion decreases. Increasing the size of the oil droplets causes the emulsion to become unstable and thus separate. Also, as the oil content in the emulsion enhances, the emulsion destabilizes faster. The interfacial feature of the
Conclusion and future perspectives
In this review study, the potential of biodemulsifiers for the removal of hydrocarbon compounds from oily wastewaters was completely studied. To this end, chemical, biochemical, physical, and structural properties of biosurfactants such as biodegradability, toxicity, cost, etc. were thoroughly investigated and compared with chemical surfactants. Also, various types of microorganisms (e.g., bacteria and fungi) and their mechanism to generate biosurfactants were studied. Moreover, efficient
Credit author statement
Lei Yao: Conceptualization, Data curation, Reviewing and Editing, Abdellatif Selmi: Methodology, Investigation Formal analysis, Editing, Hossein Esmaeili: Supervision, Writing – original draft preparation, Writing- Reviewing and Editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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