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Surfactants are a group of amphiphilic molecules (i.e., having both hydrophobic and hydrophilic domains) that are a vital part of nearly every contemporary industrial process such as in agriculture, medicine, personal care, food, and petroleum. In general surfactants can be derived from (i) petroleum-based sources or (ii) microbial/plant origins. Petroleum-based surfactants are obvious results from petroleum products, which lead to petroleum pollution and thus pose severe problems to the environment leading to various ecological damages. Thus, newer techniques have been suggested for deriving surfactant molecules and maintaining environmental sustainability. Biosurfactants are surfactants of microbial or plant origins and offer much added advantages such as high biodegradability, lesser toxicity, ease of raw material availability, and easy applicability. Thus, they are also termed “green surfactants”. In this regard, this review focused on the advantages of biosurfactants over the synthetic surfactants produced from petroleum-based products along with their potential applications in different industries. We also provided their market aspects and future directions that can be considered with selections of biosurfactants. This would open up new avenues for surfactant research by overcoming the existing bottlenecks in this field.
The term “surfactant” comes from “surface active agents”, which are molecules that adsorb on the water–surface interface and reduce water’s surface tension to enhance the cleaning of surfaces.1,2 They are also known as amphiphiles because they have polar heads, also known as hydrophilic heads, that have an attraction for polar solvents, and nonpolar tails, also known as hydrophobic tails.2,3 The molecular structures of these molecules help reduce the cohesive forces between water molecules, resulting in the lowering of surface tension.1−5 They possess other qualities that allow them to be used in applications other than lowering surface tension6,7 such as emulsifiers,8−11 foaming agents,12−16 corrosion inhibitors,17−21 and antistatic agents.22−25 Surfactants have been used in practically every industry because of their physicochemical characteristics.25 These include paints,26 inks,27,28 coatings,29−31 adhesives, paper and pulp, petroleum and oil, plastics, resins, textiles and fibers, detergents, agricultural, food, cosmetics, pharmaceutical, and various industrial applications.32 Originally, surfactants were only created from renewable resources such as plant oils or animal fat. The majority of surfactants in use today are either only partly or slowly biodegradable, which results in environmental damage and toxicological problems.33,34 For example, eutrophication is one of the direct effects on the environment due to the use of synthetic surfactants that include phosphates.35−38 Cleaning products and detergents with phosphate surfactants are the principal sources of phosphate in aquatic systems.39 As a result, there is a high demand for biodegradable products developed through green chemistry to prevent environmental pollution (the elimination of the use or generation of hazardous substances in the design, manufacture, and application of chemical products worldwide). The growing legal and societal pressures for these substances to be biodegradable and produced in a sustainable manner have fueled research into new degradable surfactants of synthetic or biological origin.40 Also, diminishing petrochemical stocks and environmental degradation have created a drive toward the identification of novel renewable bioresources for efficient surfactant production. Next-generation renewable surfactant inventions must be produced effectively, come from reliable and sustainable feedstocks, and have physicochemical characteristics that are on par with or better than those of petrochemical surfactants.41,42 All of these requirements must be met while achieving a low manufacturing cost. The green surfactants currently in use come from two different sources: biosurfactants, which are produced by bacteria as they increase the availability of hydrocarbons, and oleo surfactants, which are sourced from feedstocks such oils and fats of plant and animal origins.43,44 Oleochemical-based surfactants are more biocompatible and easily biodegradable than petroleum-based ones.45 Fatty acids, fatty alcohols, fatty amines, and glycerol are the main oleochemical feedstocks. The development of modern biotechnology has made it feasible to generate many forms of biosurfactants from microbes and vegetable oils that are biodegradable, have low toxicity, and behave similarly to synthetic surfactants.46
There are many negative environmental consequences of using synthetic surfactants, including their high levels of toxicity and poor biodegradability. These materials have a negative impact on wastewater treatment as well as aquatic microbial populations, fish and other aquatic life, and plant photochemical energy conversion efficiency.46 With over 15 million tons of surfactants used worldwide each year and an estimated 60% of them ending up in the aquatic environment, it is urgently necessary to find substitutes that have fewer environmental impacts.47−49 The origins and natural uses of biosurfactants are discussed, along with their benefits over synthetic alternatives, such as their low toxicity and biodegradability. This review describes the current methods of surfactant production, the future trends, cleaner and sustainable production methods, and an extensive comparison of performance parameters between green and petroleum-based surfactants.
Better management of surfactant usage and disposal has become a necessity of the hour, at both the industrial and domestic levels. Strict guidelines should be followed for properly remediating surfactants before disposal. Oxidation-based approaches, photocatalytic degradation, foam fractionation, electrochemical degradation, and microbial biodegradation are among the techniques used to treat surfactants.209 In recent years, biosurfactants have attracted prospective interest for use in the environmental remediation of organic and inorganic contaminants, particularly in the removal of heavy metals from soil and water, cosmetics, and pharmaceutical products, as well as in enhanced oil recovery.209−212 Biosurfactants have also applications as microbial-enhanced oil recovery (MEOR).210
Green surfactants, i.e., biosurfactants, are known to have properties like self-assembly, reduction of surface and interfacial tension, emulsification, and adsorption which make them applicable in various applications. Also, their low toxicity makes biosurfactants potentially more useful and attractive than traditionally used surfactants.213
Biosurfactants generally have CMC values ranging from 1 to 200 mg/L, which are on the lower side as compared to petroleum- and oleo-based surfactants. One previous study showed the comparison of CMCs of biosurfactants derived from B. subtilis EG1 with traditional surfactants. Table 5 indicates the CMCs of given biosurfactants along with HLB values.
The findings showed that B. subtilis EG1 producing biosurfactant is significantly more effective than synthetic surfactants at reducing surface tension.213
Additionally, biosurfactants have the ability to significantly lower interfacial and surface tension. In comparison to synthetic surfactants, they are even quite effective under adverse situations like high temperatures, acidic pH levels, and salinity.214,215
In applications that require low surfactant concentrations, biosurfactants may be appealing due to their low CMC values and high exhibited emulsifying abilities.213 The stabilities of the resulting water-in-oil emulsions varied between the two surfactants, according to a comparison between rhamnolipid biosurfactant and an amphiphilic quaternary ammonium salt (containing 75 wt % diacetyl dimethylammonium chloride in water–isopropanol solvent). Rhamnolipid was 83% less efficient than the quaternary ammonium salt at 0.01 wt %, and at 1.5 wt %, its emulsion stability was half that of the quaternary ammonium salt (3 min compared to 130 min).216
At greater concentrations, the emulsion stabilities of the two surfactants vary dramatically. Rhamnolipids stop the binding and aggregation of hydrate crystallites at concentrations of 0.05% or higher. In comparison to SDS, rhamnolipid has also demonstrated greater kerosene emulsification effectiveness in the pH range 6–9.
In general, biosurfactants have low toxicity. There is numerous research that has investigated the toxicities of biosurfactants in aquatic life, plants, and human cell lines. According to a study, using biosurfactants made from Candida lipolytica at concentrations 0.5–2 times the CMC had no impact on plant root length or seed germination.81 In a recent study, the toxicities of natural and synthetic biosurfactants were compared. In aquatic habitats, both a naturally occurring monorhamnolipid and a synthetic monorhamnolipid had EC50 levels that were “somewhat hazardous” according to the EPA. Additionally, a human cell line’s cytotoxicity and biodegradability (measured by the xCELLigence assay) were dependent on the stereochemistry of the synthetic rhamnolipid.217
Synthetic surfactants are substantially less expensive than biosurfactants when comparing pricing (see Table 6).217
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The prices of synthetic surfactants are significantly lower than those of biosurfactants. The high cost of production is mainly due to the fermentation and product purification steps. Rhamnolipids still cannot be recovered and purified on an industrial scale using any downstream technique that is both cost-effective and compelling. For the manufacturing of biosurfactants, downstream processing is responsible for 70–80% of total production costs. A significant barrier to the commercialization of biosurfactants is the economics of manufacturing.218
During the forecast period of 2021–2028, the global surfactants market is anticipated to expand at a CAGR of 4.9%, rising from $41.22 billion in 2021 to $57.81 billion in 2028. Surfactants are produced by the industry at a rate of approximately 17 million metric tons yearly, some of which come into direct contact with customers and most of which are eventually released as effluent. In light of this volume, solving green issues is a crucial subject for a sector that is dealing with expanding regulation and customer awareness.219 In 2020, the market for green surfactants was estimated to be worth close to USD 2.54 billion. The global green surfactants market is projected to expand at a CAGR of 5.7% from 2022 to 2027, reaching a value of $3.56 billion by 2026. The sector is expanding because of the growing demand for green surfactants made from waste biomass and agricultural raw materials.220 The booming personal care sector is contributing to the continuous rise of the market for green surfactants. This industry is expanding as a result of the increased attention being paid to health, beauty, and personal hygiene, which in turn is assisting the market for green surfactants.221Table 7 displays a list of companies that produce novel green surfactants or use them in their products advancing sustainability in the process. demonstrates a typical comparison between synthetic surfactants and biosurfactants.177
Open in a separate windowThis has led to a rise in the cultivation of natural oils, the origin of which, particularly tropical oils, is a major source of concern. Even though manufacturers have joined groups like the Roundtable on Sustainable Palm Oil (RSPO), there is still much disagreement over the true cradle-to-gate effects of using land for renewable chemical feedstocks. It is interesting to note that Clariant introduced its GlucoPure Sense surfactant in 2017, which utilizes European sunflower oil as opposed to oils from tropical regions. As an alternative to cocamidopropyl betaines generated from coconut oil, BASF has also developed amphoteric betaine surfactants for use in formulations for hair care.222
Green surfactants provide a significant and expanding contribution to the business, albeit the extent of that contribution will depend on how people define what is natural, biobased, and sustainable. Additionally, even though different customer product and personal care companies have shown interest in 100% biobased surfactants, the market has not yet evaluated whether consumers are prepared to pay premium pricing for them. A further factor driving the global market for green surfactants is the strict government laws on the use of chemicals that may have a harmful impact on the environment. These rules are encouraging manufacturers to employ more eco-friendly and sustainable goods. As businesses try to get away from surfactants made from petroleum sources, green surfactants are becoming more and more desirable.
Many of the synthetic procedures required or involved in biosurfactants’ production utilizes harsh conditions.235 It is interesting to note that several processes are still using hazardous solvents, toxic acid catalysts, or toxic base catalysts. This definitely has environmental issues regarding toxicities. Thus, one important challenge in synthesizing or purifying biosurfactants is to have proper reaction optimizations for solvents or catalysts. Recent literature also demonstrates the usage of various enzymes in reaction optimizations, which definitely aids in sustainability.236 Enzymes’ primary disadvantages are their considerably greater costs as compared to chemical catalysts and their slower reaction rates. The requirement for sustainability (reduced operating energy, less waste, and safer operating conditions) is essential, nevertheless, as energy prices are predicted to increase. One other challenge includes the higher pricing of biobased surfactants and biosurfactants; with this hurdle it is indeed difficult to meet the expectations of price-sensitive Asian customers. Further, the higher complexities and lower efficiencies of microbial surfactants also disappoint in their industrial production. For example, the average price of sophorolipids is USD 34/kg as compared to sodium dodecyl sulfate and amino acid surfactants that are priced at USD 1–4/kg.237 But with technical advancements like integrated separation, sophorolipid surfactants may be produced at a lower operating cost of USD 2530/ton, making them comparable in cost to other specialty surfactants.237 Increased biosurfactant sustainability without noticeably greater performance is not well received since typical customers will not be prepared to pay a premium for items made from biomaterials.
Therefore, improving biosurfactant manufacturing at a reduced cost is crucial for achieving an economically viable method and guaranteeing future market stability.237 Recently, our group has also optimized various procedures pertaining to a few biosurfactants with the ease of freely available raw materials.238−243 The dependence of the demand for biosurfactants on the volatility and economic meltdown of downstream end-user industries is another issue. The performance of the overall macroeconomic environment is known to have an impact on industries that are relevant for biosurfactant applications, including oil and gas, improved oil recovery, the food sector, construction, textiles, paints, pharmaceuticals, and detergents. The coronavirus disease (COVID-19) pandemic also affects end-user industrial demand and raises concerns about the sustainability of the raw material supply. The sustainability of raw materials is a significant issue since they account for up to 50% of the cost of producing glycolipids and 10–30% of the price of other biosurfactant products. Of the cost of production, 60% goes in purification; however, this may be reduced when biosurfactants in their raw forms are used.
Additionally, a number of operational elements offer crucial controls to reduce the expenses associated with the manufacturing of biosurfactants. The fermentation and purification can be improved with batch cycles’ optimization, which also aids in shortening the time between two batches during production. The most crucial element in the manufacturing economics of the manufacture of biosurfactants at industrial sizes is productivity. The most effective batch-sequencing campaign reduces the frequency of starting and shutdown to reduce production downtime and boost productivity. Last, but not least, the development of biosurfactant products will be hampered by costly and time-consuming legal restrictions. Manufacturers of biosurfactants pay a significant cost of compliance in addition to the price of product development.
The traditional usage of petroleum-based surfactants is a result of their inexpensive production costs, long shelf lives, widespread availability, and improved performance at lower temperatures. They also offer formula flexibility owing to their branching, odd, and even hydrocarbon chains. The necessity to discover a renewable alternative that is safe for the environment arises from their nonrenewability and environmental damage. The increasing interest of industries in developing environmentally safe products has led to biotechnological advances involving the synthesis of green surfactants. Biosurfactants and oleo surfactants are examples of green surfactants gaining popularity and have been looked at as a breakthrough for the replacement of synthetic surfactants. There has been a rise in the production of these surfactants over a decade, and they are likely to touch the mark of 2.6 billion in 2023.52 However, the high cost of production of these surfactants poses a problem for its scale-up.
The main obstacles to the manufacture of biosurfactants include low productivity, extensive downstream processing, and a lack of adequate understanding of the bioreactor systems.
Although these problems are a cause of concern, there has been a substantial increase in the variety of manufacturing processes of these green surfactants. Also, chemical giants like Dow, BASF, Croda, Evonik, and Clariant are investing heavily in the research and development of these products and have proved so by launching them in the market. We believe that in the coming decades there will be a complete phasing out of petroleum-based surfactants, giving rise to newer, greener, and sustainable surfactants.
The authors of this article are thankful to the Institute of Chemical Technology, Mumbai, India. H.K.A.Y. would like to thank the Deanship of Graduate Studies and Research, Ajman University, UAE, for their support in providing assistance in article processing charges for this review.
○ V.S.N., C.C., H.K.A.Y., and S.N.M.: These authors made equal contributions.
The authors declare no competing financial interest.
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