1. Introduction
There is a great industrial demand for petroderivatives, which are used by many companies and industries, such as thermoelectric power plants, to perform their activities. Thermoelectric plants commonly use heavy oils, such as low-sulfur, viscous fuel oils and diesel, to produce energy, which entails a major problem, i.e., the management of hazardous waste and dirt in the industrial environment. In addition to thermoelectric plants, other industries have the same problem, as conventional detergents cannot clean surfaces impregnated with petroderivatives; therefore, they resort to products that contain toxic, expensive and environmentally harmful petroleum-derived solvents [
1,
2,
3].
The products used to clean heavy oils currently found on the market have highly toxic solvents in their compositions, which compromises the safety of employees who will be in direct contact with them when cleaning surfaces impregnated with petroderivatives, in addition to posing a great environmental risk during disposal [
1,
3,
4,
5]. The components of these cleaners often also corrode metal surfaces, damaging the equipment. Therefore, industries that use these products can pay a very high price, not only for their cost, but also for the consequences of their use, in addition to the fact that these products do not guarantee the desired cleaning and removal efficiency when used with petroderivatives [
4,
5]. Therefore, the development of ecofriendly technology has stimulated the use of natural, biodegradable detergents that are able to reduce the environmental impact as well as protect the health of workers who handle these products. Products formulated with natural raw materials, with non-corrosive properties, low cost and which are efficient at cleaning petroderivatives have been described in the literature [
1,
2,
3,
4,
5,
6].
A detergent is usually made up of a blend of surfactants (1–50%), organic solvent(s), stabilizer(s) and additive(s) that are able to reduce water surface–interfacial tension [
1,
3]. Most chemical surfactants used in marketed cleaning and degreasing preparations are toxic and nonbiodegradable. In the last 15 years, bio-based surfactants, including the microbial ones, have been developed as an attractive and effective alternative to those already available [
7,
8]. Being prepared using renewable resources, they are atoxic, biodegradable and stable even in hard conditions [
9].
Regarding toxicity, several works have already described ecotoxicity studies of biosurfactants, especially to assess the environmental application of these biomolecules. Phototoxicity studies, for example, determine the germination index, which involves the development of seeds and roots, to assess the toxic effects of biosurfactants to various types of vegetables. The larvae of the
Artemia salina microcrustacean have also been used as standard indicators of the toxicity of these biomolecules [
5,
6,
8,
10].
Biosurfactants are usually blended with cheap synthetic surfactants because blends of surfactants generally provide better interface properties compared with single surfactants [
11]. However, even if synthetic surfactants are economically profitable, their utilization can lead to more toxic preparations. Therefore, binary systems based on non-toxic surfactants and biosurfactants are needed in a more ecological way [
12]. For instance, binary blends of synthetic surfactants and tea saponin (a biosurfactant) showed enhanced interface and foaming properties compared to the single constituents [
13]. A similar synergistic action of a conventional surfactant (sodium dodecyl benzene sulfonate) and an
Enterobacter cloacae biosurfactant was reported by Hajibagheri et al. [
11]. A blend of sophorolipid with rhamnolipid, Tween 85, sorbeth-40 tetraoleate and 2-butoxyethanol resulted in a preparation that was able to effectively disperse crude oil [
14]. An oil dispersant made up of a binary blend of lauroylcholine and a biosurfactant from
Starmerella bombicola displayed satisfactory crude oil-emulsifying activity in the pH range from 2 to 10 at salt concentrations up to 20% (
w/
v) [
12]. A completely natural, ecofriendly biodetergent, whose formulation consisted of cotton oil, saponin and two stabilizers, was shown to be stable, non-toxic and very effective, being able to completely remove heavy oil from glassy or metallic surfaces [
15]. Helmy et al. [
16] successfully applied a rhamnolipid biosurfactant in the formulation of a washing biodetergent, whose stain removal efficiency was close to that of a standard detergent, thus showing potential as a promising substitute for synthetic surfactants. Arpornpong et al. [
17] prepared cleaning agents at variable salinity values by changing the concentrations of a lipopeptidic biosurfactant, the natural surfactant Dehydol LS7TH (D) and butanol as a hydrophobic binder. The best formulation led to an oil-in-water (O/W) microemulsion with polyolefin containing no more than 20% (
v/
v) foam and 2% (
v/
v) D. Thanks to the synergism of anionic lipopeptides with nonionic D, the preparation was able to remove, in a jar test, > 90% of total petroleum hydrocarbons adsorbed on drill cuttings and fragments. Baharuddin et al. [
18] formulated a water-based dispersant, made up of ionic liquids, which showed excellent biodegradability and facilitated the dispersal of different light and heavy petroleum samples with efficiencies in the range of 70.75–94.71%, thus showing its potential to replace hazardous chemical dispersants in the near future.
Recently, the authors of this work developed a biodetergent that was produced with natural and non-toxic materials, formulated with a bacterial biosurfactant isolated from
Pseudomonas aeruginosa ATCC 10145, with a petroderivative cleaning efficiency equal or superior to other products available on the market [
15,
19]. To scale up the production of the biodetergent, initially produced on a bench scale, it is essential to study the parameters of the production process, which implies considering several factors and variables that influence the final quality of the product, such as efficiency and stability under different storage conditions. In this scenario, the purpose of this study was to enable the production of the biodetergent on a large scale, in addition to verifying its long-term stability as an emulsion, as well as its application in an industrial environment.
2. Materials and Methods
2.1. Formulation of the Biodetergent
The biodetergent previously described by Farias et al. [
19] was evaluated for the feasibility of production at scale and application in an industrial environment. The product was formulated with biodegradable and non-toxic components (20% natural organic solvent, 2.0% thickening long-chain alcohol, 0.5% emulsion stabilization gum and 0.5%
Pseudomonas aeruginosa ATCC 10145 biosurfactant). The components of the formulation were dissolved in water and added to the natural solvent until it reached 100% of the total mixture. The preparation was carried out under 3200 rpm mechanical stirring (TE-139, Tecnal, Piracicaba, Brazil) at 80 °C, and the conditions of stirring time and volume were studied.
2.2. Assessment of the Interaction between Physical Factors
Tests were prepared separately, varying the stirring time (5, 6, 7, 8, 9 and 10 min) at different volumes (4, 5, 6, 7, 8, 9 and 10 L) to assess the interaction between these two physical factors in the biodetergent production process, generating a total of 42 samples. They were prepared with the biodetergent produced at a temperature of 80 °C and 3200 rpm stirring speed, ensuring the dispersion of the formulation in immiscible phases and the stable emulsion appearance. After mixing, the batches were evaluated for stability, i.e., to analyze if there was any phase formation, for 48 h at room temperature. To ensure the maintenance of product efficiency, tests involving the removal of fuel heavy oil with low sulfur content and viscosity (OCB1) (Petrobras, Rio de Janeiro Brazil) from smooth and metallic surfaces were carried out to evaluate the visual characteristics and performance of the formulation. OCB1 is a complex hydrocarbon blend with 620-cSt momentum diffusivity at 60 °C, a 66 °C flashpoint, and 0.968-g/mL density at 20 °C.
2.2.1. Determination of Biodetergent Stability
To check biodetergent stability, each sample was tested for the extent of formulation phase separation. First, the total fluid height and the stable phase height were recorded in cm. The stability index was defined as the ratio of stable phase height to total fluid height, multiplied by 100 and expressed as a percentage. The evaluation was performed after resting for 48 h, and all the analyses were carried out in triplicate.
2.2.2. Evaluation of the Efficiency of Heavy Oil Removal from the Impregnated Surface
A portion of the surface of a glass slide with a known weight was evenly impregnated with OCB1 (0.1 mL). The impregnated slide portion was submerged in the biodetergent test solution for 3.0 min. After that, the slide was dipped in distilled water to remove any excess of the test solution or destabilized residue on the surface and oven dried at 40 °C for 30 min. After weighing, the removal index, expressed as a percentage, was determined according to the equation [
14]:
where
Wc is the contaminated slide weight,
Wl is the post-wash slide weight and
Wi is the initial slide weight.
The metal surface was washed using metallic parts (nuts) evenly contaminated by immersing them in OCB1. Contaminated portions were then submerged in the biodetergent test solution for 30 min, and pieces were dipped in distilled water as described above. The removal efficiency was evaluated via visual examination [
15].
2.3. Chemical Improvement of Biodetergent
Chemical methods were developed to help improve the characteristics of the biodetergent formulation, i.e., increase its stability (no phase formation) and reduce its viscosity to obtain a more fluid product. Thus, for chemical adaptations, different non-ionic synthetic surfactants (Span 20 and 80, Tween 80 and Triton X-100) with low hydrophilic lipophilic balance (HLB) were tested in proportions of 0.5–5.0% in a mixture of water and natural solvent to find the optimal value and concentration of surfactants to form a stable emulsion. In this sense, the emulsification index was determined as a function of HLB and surfactant content.
2.3.1. Preparation of Emulsions
To evaluate the HLB, the selected amount of surfactant mixture was placed in a 250 mL beaker, and 30 g of natural solvent and water were added so that the total mass of the system reached 150 g. The mixture was stirred at 150 rpm by means of an impeller and remained under agitation for 5 min. After this time, 20 mL aliquots were transferred to test tubes to assess the emulsification index as described in the next item.
2.3.2. Emulsification Index Determination
The emulsification index (
EI), expressed as a percentage, was calculated 1 day after the preparation of the emulsions, taking into account the system total height (
Ht) and that of the emulsified one (
Hem) according to the following equation [
20]:
The final value of the emulsification index was determined as the average of three experiments.
2.3.3. Final Hydrophilic Lipophilic Balance
The final HLB balance of the binary mixture (
HLBf) against the tested chemical surfactants (
Section 2.3) was calculated by the equation proposed by Griffin [
21]:
where
M1 and
M2 are the masses of the first and second surfactants,
HLB1 and
HLB2 are their respective tabulated indexes, and
Mt is the total mass of the mixture.
2.4. Long-Term Stability of the Biodetergent in Adverse Environments
Biodetergent stability tests were performed to evaluate and select the appropriate way to store the product in stock (shelf life). Samples containing 1 L of biodetergent produced were stored in plastic containers at room temperature (28–38 °C), with exposure to the sun, shade and dark environment, as well as at a controlled temperature (5, 30, 40 and 50 °C), using an electric stove (model TLK48, DeLeo, Porto Alegre, Brazil) and BOD incubator (model TE-371/240L, Tecnal, Piracicaba, Brazil). The samples were evaluated for 365 days, with the removal of aliquots (50 mL) at intervals of 30 days to evaluate the organoleptic and toxicological characteristics, their dispersing capacity for motor oil in water and their efficiency at removing petroderivatives, using either diluted (1:1, 1:3, 1:5 and 1:7 (
v/
v)) or pure (undiluted) biodetergent [
22].
2.4.1. Evaluation of Organoleptic Characteristics
The organoleptic characteristics of the biodetergent, i.e., visual changes, such as color, odor, homogeneity and consistency, were evaluated during the storage period. The behavior of the product in stock was investigated following the methods described by D’Leon [
23], and the characteristics were categorized based on (a) color as milky, transparent and pearlescent, (b) odor as pleasant and unpleasant, (c) consistency as creamy and fluid and (d) homogeneity as hetero- and homogeneous.
2.4.2. Toxicity Tests on Brine Shrimp as an Indicator
Toxicity tests were carried out on microcrustacean larvae (
Artemia salina) as a toxicity indicator, utilizing 1 and 2% biodetergent solutions, diluted at 1:5 or 1:10 (
v/
v) in seawater. Larvae were tested within 24 h after the hatch. Tests were carried out in triplicate in 15 mL penicillin tubes holding ten
A. salina larvae suspended in 10 mL of sea water. Larvae were examined for 1 day to determine their mortality [
24].
2.4.3. Phytotoxicity Tests on Vegetable Seeds
The biodetergent phytotoxicity was evaluated through a static assay in terms of seed germination and root growth of cabbage (
Brassica oleracea var.
capitata) and tomato (
Solanum lycopersicum) as previously reported [
25]. The same biodetergent solutions described in the previous section were used. Sterile 10 cm Petri dishes containing Whatman No. 1 filter paper disks were used to determine toxicity. Each dish was symmetrically seeded with ten seeds previously washed with sodium hypochlorite, supplemented with the test solution (5 mL) and kept at 28 °C for five days. We employed distilled water as a control. After the dark incubation period, relative seed germination, relative root elongation (≥5 mm) and germination index (GI) were determined as follows:
in which
nE and
LE are the number of germinated seeds and the mean root length in the extract, while
nC and
LC are those in the control, respectively.
The mean and standard deviations of the triplicate trials were determined at every concentration.
2.4.4. Ability to Disperse Petroderivatives in Water
The size of the clear area appearing after introducing a detergent into a thin layer of oil deposited on a water surface is used to express its ability to disperse or aggregate petroleum-derived stains. For this purpose, sea water (30 mL) was placed in a Petri dish (15 cm in diameter), to which motor oil was added. Then, the biodetergent was added to oil at up to 1:2, 1:8 and 1:25 (
v/
v) ratios. The central clear area that appeared at room temperature was measured using a 300 mm stainless steel digital caliper after 30 s. A larger clear diameter demonstrated greater detergent surface activity [
22].
2.4.5. Tests of Oily Washing with Metal Surfaces
Wash tests were conducted on metal parts (nuts and washers) impregnated with OCB1 and diesel oil, previously colored with scarlet red dye to facilitate the visualization of the petroderivative and left to rest for 20 days. After the rest period, pieces were immersed in a beaker containing 30 mL of the biodetergent diluted at either 1:1, 1:3, 1:5 or 1:7 (
v/
v), or left pure (undiluted), for 30 min. Pieces were then submerged in distilled water as previously described. Oil removal was calculated according to the equation [
15]:
where
Mc is the weight of contaminated metal surface,
Mw is that after washing and
Mi is the initial weight of the metal surface.
2.5. Biodetergent Application in Industry
Due to the biodetergent characteristics and its high power to remove high molar mass petroderivatives, some possible typical applications in different industrial environments were selected, especially in thermoelectric plants, because they use large volumes of fossil fuels in their process chain and in parts of the electric motor generators.
2.5.1. Factory Floor Cleaning
The cleaning process was carried out using two different types of flooring, a reinforced concrete floor and another composed of interlocking concrete bricks. Both surfaces received a direct application of OCB1. In both types of flooring, a specific application area (10 m2) was used after impregnation with this petroderivative. The floor composed of interlocking concrete bricks received an initial treatment with washed sand to remove excess OCB1 before the direct application of the biodetergent (250 mL/m2), simulating an oily surface with old impregnation. For application of the biodetergent and better spreading of the product, brooms and squeegee-type liquid concentrators were used. The results were visually observed, as the surfaces analyzed turned clean after the application of the biodetergent.
2.5.2. Cleaning of Parts in Thermoelectric Plants
In some motor generators, there are intercoolers, i.e., equipment used to exchange heat with the intake air of OCB1 power generator engines. Thus, cleaning was carried out on the fins and the entire external body of the intercooler, as these are usually heavily impregnated with petroderivatives, making it difficult to exchange heat with the medium. In this specific application, compressed air was used for a better contact surface on the fin housings. Surface cleaning was also carried out with the direct application of the biodetergent on the plates present in some types of heat exchangers for OCB1 fuel engines. The results were visually observed with the cleanliness of the applied parts and surfaces.
2.6. Statistical Analysis
Data were statistically analyzed according to the one-way procedure in Statistica® (version 7.0) and then subjected to linear one-way analysis of variance (ANOVA). Results collected in triplicate were expressed as means ± standard deviations. Differences were assessed using Tukey’s post hoc test at a 95% significance level.
4. Discussion
In this work, the possibility of scaling up the production of an ecological biodetergent based on non-toxic and biodegradable components and a promising biosurfactant from Pseudomonas aeruginosa ATCC 10145 was investigated, aiming to implement its direct application in an industrial environment to remove the petroderivatives embedded in parts and equipment.
According to the data collected on the destabilization of the emulsions, we highlighted the importance of carrying out tests with different proportions of surfactants and with different degrees of polarity. Stability is a critical property of emulsion-type systems because it depends on their texture and microstructure, which are essential for obtaining emulsions with minimal and controlled droplets [
29,
30,
31]. The destabilization can be characterized by phase separation, droplet size and viscosity. The developed formulation was found to be stable under the conditions of volume × agitation time that we investigated in this study. Tests showed approximately 100% stability for all volumes tested, especially under 7 min stirring at 3200 rpm. The evaluation of the interaction of the physical factors involved in the biodetergent production process helped to approximately identify the conditions able to ensure the best dispersion between the droplets of the immiscible phases, allowing a greater control over the process.
The results of this work obtained using the biodetergent formulation with the best proportion of components showed that, in addition to an adequate rotational speed, a reduction in the emulsion size or micelle formation favored physical stability. Such a finding agrees with those of Coutinho et al. [
29], who produced stable creams at 4000, 10,000 and 16,000 rpm in a rotor–stator-type homogenizer.
Chemical stability is fundamental for selecting storage conditions (temperature, light, moisture content), to choose the appropriate container material (glass, amber, clear or opaque plastic) and the type of lid, as well as to predict interactions with the product and excipients [
32,
33]. In this article, a comprehensive stability study was presented, where extensive information about the product was made available over a long shelf life (365 days). The product remained stable regarding its organoleptic and toxicological properties and maintained its efficiency at destabilizing complex petroderivatives. Similar studies have shown promising storage stability results for ecofriendly detergent formulations [
34]. Silva et al. [
35] formulated a green detergent using a biosurfactant from
S. bombicola ATCC 22214 and evaluated the formulation stability after 30 days. The results were satisfactory, since the formulation did not exhibit any phase separation and kept its original color, pleasant odor and homogeneous consistency.
The biodetergent did not present toxicity to the
Artemia salina larvae when diluted at 1:5 and 1:10 (
v/
v), nor to seeds of
Brassica oleracea and
Solanum lycopersicum when diluted at 1:1 and 1:2 (
v/
v). Similarly, Rocha and Silva et al. [
15] formulated a natural detergent that showed no toxicity to
A. salina larvae under the same conditions. Gálvez et al. [
36] performed phytotoxicity tests on onion (
Allium cepa) and lettuce (
Lactuca sativa) seeds with commercial surfactants considered to be detergents (Tween 80, sodium dodecyl sulfate, Aerosol 22, Triton X-100, Brij 35, Tween 80 and Glucopon 600) at different concentrations. The highest surfactant concentration (32.5 g/L) completely suppressed seed germination. Sodium dodecyl sulfate was the most phytotoxic compound, while Tween 80 for onion and Brij 35 for both plant seeds were the least phytotoxic ones. Both nonionic surfactants and Aerosol 22 were recommended at low concentrations (< 0.65 g/L).
The dispersing ability is one of the most utilized properties to assess the quality of a detergent. In this respect, the excellent capacity of the biodetergent formulation to disperse motor oil during its 365 days of storage, revealed by dispersion indexes above 90%, agrees with the results reported by Silva et al. [
35] for the dispersion of a heavy fuel oil by a formulated green detergent. Rocha and Silva et al. [
15] prepared a biodetergent able to disperse up to three times more hydrocarbon slick in a 1:1 ratio (
v/
v) of detergent to oil, proving to be an excellent dispersant.
Industrial processes, which are increasingly automated and necessary for human needs, use large amounts of different hydrocarbons to enable their activities that have a strong impact on the environment. Therefore, efficient, non-toxic products capable of contributing to the environmental sustainability are well accepted in the industrial sector. The biodetergent formulation developed in this study showed high efficiency when applied in an industrial environment, in addition to preserving the well-being of workers, given that it is based on water and non-toxic natural components and has a light odor. These benefits demonstrate the possibility of replacing toxic products with biodetergents to clean machines and installations. The results obtained in this work in the cleaning of reinforced concrete floors and interlocking concrete bricks agree with the findings of Rocha e Silva et al. [
15], who stressed that the application of natural detergents to clean parts and equipment contaminated by hydrocarbons or oily residues can be a more ecofriendly way of cleaning because, when making use of non-toxic, natural and anti-allergic materials, they do not cause damage to the health of employees. In addition to safety and chemical neutrality, these natural detergents proved stable and capable of completely removing the oily residues generated during industrial processes from different surfaces. Similar findings were obtained by Almeida et al. [
37], who tested the application of a vegetable biosurfactant for use in cleaning motor oil and demonstrated that the biosurfactant was able to achieve excellent removal percentages regardless of the surfaces tested and the study conditions.
5. Conclusions
The understanding of the interaction between physical (stirring time and volume) and chemical parameters (combination of components) in the production of detergents stimulated the elaboration of a study aimed at the large-scale production of a biodetergent, considering that the parameterization of production control is essential to improve the system’s performance and to reach a stable commercial formulation. The results showed that a vigorous agitation at a specific time, i.e., 7 min of agitation at 3200 rpm in the present case, was the most effective way to decrease the particle size of the solvent dispersed in the aqueous phase and to ensure formulation stability for the scale process up to 10 L. It is also important to say that the greatest benefits of a well-controlled production process are the reduction of industrial costs, effective management of the flow of inputs to better meet demand, balance in the use of labor and equipment, greater control over internal activities, shorter lead time of products and improvement in the level of customer service. Meeting these prerequisites is important for improving process performance. The study of production conditions on a laboratory scale allowed the development and scaling up of the process to the industrial reality, as well as its application according to the needs of parts impregnated with contaminating oils in the cleaning and maintenance environment. It is important to highlight that the field tests allowed us to safely define the best ways of application and demonstrated the efficiency of the product at an industrial level, enabling its future insertion in the market. The biodetergent achieved 100% stability during 365 days of storage under the selected experimental conditions and was able to completely remove heavy oil impregnated on metal surfaces and floors. Furthermore, the use of microbial metabolites with decontamination capabilities and excellent properties are essential to offer an attractive biotechnological product. This makes it possible to reduce the impacts on the environment and the health of workers and to obtain consequent economic gains compared to toxic products normally used in industrial cleaning. Therefore, the biodetergent meets the criteria of efficiency, cost-effectiveness and commercial acceptability.