1. Introduction
Surfactants are used as active principles in formulations for the manufacture of a wide variety of consumers’ finished products, as well as auxiliaries in many chemical and environmental technologies. Overall, about 60% of EU chemical sales include products containing surfactants [
1]. The global surfactant market size is estimated at about 48 billion USD/yr. and is expected to reach 66 billion USD/yr. in 2027 [
2] with an estimated production of 13–17 Mt/yr. [
3]. Surfactants act as cleaning, wetting, dispersing, emulsifying, foaming, and anti-foaming agents in many practical applications and products, including detergents, personal products, food, pharmaceuticals, plant protection, agriculture, and textiles [
4,
5].
A key driver of the surfactant market is the growing demand for sustainable and eco-friendly products. With increasing awareness about environmental issues and the need to reduce carbon footprints, consumers are looking for products that are made using renewable resources and do not harm the environment. Surfactants derived from natural sources, such as vegetable oils and sugar, have gained popularity as they are biodegradable, non-toxic, and do not contribute to water pollution. Manufacturers are investing in research and development to produce surfactants that are both sustainable and effective, driving the growth of the market. Relevantly, the biosurfactant market size is currently at the level of USD 8 billion and is expected to reach USD 14 billion by 2023, with a USD 3 billion share of Rhamnolipids sales [
6].
Rhamnolipids and sophorolipids are high-performance biosurfactants produced by microorganisms, e.g.,
Pseudomonas aeruginosa and
Rhodococcus erythropolis, respectively [
7]. Natural surface-active compounds, including the above microbial biosurfactants and products extracted from plants and vegetable residues, exhibit very low surface tension values (28–45 mN/m) at 0.4–2 g/L concentration in water, which makes their performance in real operational conditions very competitive with synthetic surfactants from fossil sources. However, the cost of the surfactants from renewable resources ranges from tens to hundreds EUR/kg [
7], compared to 1–6 EUR/kg for most commercial synthetic or natural surfactants from petrochemical or natural feedstock [
5,
8,
9]. Yet, in spite of the high cost, the exceptionally low 28 mN/m achievable surface tension and 0.8 g/L operational concentration make rhamnolipids sharing a significantly high USD 3 billion [
6] portion of the global surfactant market turnover [
2,
3,
4,
5].
In the above context, several papers have been published [
10,
11,
12,
13,
14,
15,
16,
17] on the properties and performance of new soluble polymeric biosurfactants obtained by hydrolysis and oxidation of collected and fermented municipal biowaste (MBW).
The new polymeric biosurfactants are a heterogeneous mixture of molecules with a molecular weight in the 5 to >750 kDa range and different chemical compositions, constituted by aliphatic C chains and aromatic lignin-like C moieties, and various functional acidic and basic groups of different strengths bonded to several mineral elements, such as Na, K, Ca, Mg, Al, Fe, and Si in 0.2–9 w/w % and Cu, Ni, and Zn in 20–180 ppm content [
17]. These features are memories of the chemical moieties and macro-molecularity of the pristine MBW polysaccharide, protein, and lignocellulosic approximates from which the polymeric new biosurfactants are derived [
16].
The product obtained by hydrolysis of the MBW anaerobic digestate was characterized by a surface tension (γ) of 49 mN/m at 1 g/L concentration in water compared to γ 61–63 mN/m at 2–3 g/L for the hydrolysates of three different MBW composts [
10,
11]. The product obtained by hydrolysis of the MBW anaerobic digestate was further processed by oxidation in plain water in the presence and absence of added hydrogen peroxide at 0.1–3 H
2O
2 moles per mole of organic C contained in the pristine digestate and in the presence or absence of irradiation with simulated solar light [
17]. The measured surface tension of these products at 2 g/L in water ranged from 69 to 46 mN/m, with the lowest values being 46–49 mN/m measured for the products obtained in the presence of 0.25–0.5 H
2O
2/C mol/mol, with or without irradiation [
17]. Better surface activity properties were exhibited by the soluble products isolated from the collected MBW slurry by simple centrifugation and by hydrolysis and ozonization of the pristine MBW slurry [
16]. In these cases, the surface tension values at 2 g/L concentration in water were 47 mN/m for the product obtained by centrifugation, 42 mN/m for the product obtained by hydrolysis, and 37–39 mN/m for the product obtained by ozonization [
16]. None of these products matched the properties of the product obtained by ozonization of the anaerobic digestate hydrolysate, for which the measured surface tension at 0.47 g/L concentration in water was 38 mN/m [
12].
The development of high-performance biosurfactants is an integral part of a wider research framework aiming to develop biorefineries for the production of new value-added chemical specialties from renewable feedstock [
18,
19]. The production of biosurfactants from MBW, as described in the above cited work [
10,
11,
12,
13,
14,
16,
17] has, in principle, several advantages arising from the nature of the material used as feedstock and from the fermentation and/or chemical processes adopted. First, MBW is the most available negative cost feedstock produced worldwide [
16]. Secondly, MBW fermentation occurs by the native microbial population [
20,
21] and does not require the production and maintenance of selected microorganisms [
22,
23]. Thirdly, the hydrolysis and oxidation reactions belong to a well-assessed technology in the conventional chemical industry [
23,
24,
25,
26]. These features are most likely to attract the interest of major chemical companies in undertaking the implementation of MBW-based products at the industrial and commercial levels [
27,
28,
29]. Yet, for this perspective to be realized, a number of criticalities need to be overcome.
Recently, soluble biopolymer (SBP) obtained by hydrolysis of the MBW anaerobic digestate has been reported to react with plain water to catalyze the production of O and OH radicals and undergo self-oxidation [
17]. The reaction caused some depolymerization of the pristine SBP, production of organic carboxylic moieties, and partial mineralization of the SBP organic matter to CO
2. The recovered oxidized polymer had a molecular weight in the range of 150–750 kDa, compared to the molecular weight > 750 kDa of the pristine SBP. Depolymerization was enhanced in the reaction performed under light irradiation [
17]. In this case, the recovered oxidized polymer had a molecular weight in the 20–50 kDa range. For the recovered polymers, the measured surface tension values at 2 g/L concentration in water were in the range of 69–56 mN/m. Depolymerization and mineralization of the pristine SBP organic matter were found to increase for the reaction performed in the presence of hydrogen peroxide at 0.25–0.5 H
2O
2/C mol/mol. However, the low yields of the recovered oxidized polymers were offset by their improved surface activity properties; the measured surface tension for these products was 46–49 mN/m at 2 g/L concentration in water solution.
The above results [
17] offered scope for investigating further the autocatalytic properties of SBP in order to develop a process that enabled better control of the rate of SBP chemical oxidation, reduced the depolymerization and mineralization degree, and improved the surface activity properties of the oxidized polymer. The present work reports the results obtained for the reaction of SBP with water containing hydrogen peroxide in catalytic amounts at H
2O
2/C µmol/mol level. Undoubtedly, obtaining valuable products by just keeping SBP in a water solution is the most desirable of low-cost processes, as it would not involve energy and reagent consumption and process waste needing secondary treatments for their disposal. To the authors knowledge, a similar oxidation process has never been reported.
5. Conclusions
The experimental work reported in the present work provides proof-of-concept of a most sustainable chemical oxidation process for the production of value-added chemical specialties from renewable feedstock. The process has been demonstrated in the case study of municipal biowaste, which is the most sustainable negative-cost feedstock available. The applied integrated biochemical–chemical process comprises conventional MBW fermentation by the native microbial population and conventional chemical hydrolysis and photochemical oxidation of the MBW anaerobic digestate in water without added oxidizing agents. The products obtained through the photochemical oxidation process are shown to be biosurfactants with chemical compositions and properties close to those of the highest-performance rhamnolipids and sophorolipids bacterial surfactants. Due to the wide use of surfactants in the chemical industry and particularly the high market value of bacterial surfactants, the obtained results lay the basis for a feasible implementation of the process at the industrial level and the realization of a biorefinery using not only MBW but also biowastes from other sources (e.g., agriculture and the agro-industry) as feedstock.
To obtain hints for further R&D work aiming to improve the composition and properties of the photo-oxidized biosurfactants, a comparative review of the composition, properties, and production cost of the chemical and bacterial surfactants has also been carried out in the present work. The results in
Section 4.4 show that the photo-oxidized biosurfactants derived from the MBW anaerobic digestate may not reach the performance level of the bacterial surfactants. Yet, the former biosurfactants obtained in the present work, according to scheme 1, have much more chance to be marketed products at large scales due to their low production costs. Contextually, without a change in paradigm in the R&D work on rhamnolipids and sophorolypids from biotechnology to integrated cheap biochemical–chemical processes, the bacterial surfactants may remain niche products.
Aside from the authors’ opinion on the above change in paradigm, the present proof-of-concept study introduces a highly sustainable chemical process for the production of biosurfactants from renewable biowastes and does not claim quality better than this. It is unquestionable that further R&D work on the integrated chemical–biochemical paradigm proposed in the present work for the implementation of SBP at the industrial level should answer important questions, as, for example, (i) what are, and how can be identified and/or isolated from the SBP heterogeneous molecular pool the molecules performing as active principles in surfactant-based consumers’ finished products? (ii) Can the presence of potentially harmful molecules in the SBP molecular pool be excluded? (iii) What is the stability (durability) of the obtained products in the application context? (iv) How can the biowaste type and site-specific compositional variability affect the SBP composition, property, and performance? For SBP derived from different biowastes, and used as biosurfactants [
10], biocatalysts [
71], and agrochemicals and animal feed supplements [
78], the results of previous work have evidenced no critical stability problems over 12 months storage in the dry solid state or in solution kept in the dark, nor toxicity or performance effects. However, the identification and/or isolation from the SBP heterogeneous molecular pool of molecules performing as active principles for specific tailored applications is likely to take very long and costly R&D work. The research risk in this case lies in the obtainment of low-value results relative to the amount of applied effort or in the production of high-performance surfactants for niche applications that are too expensive for use in place of synthetic surfactants for the manufacture of consumers’ products with high market turnover.