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Proceeding Paper

Salt-Induced Recovery of Volatile Organic Acids Using Non-Ionic Surfactants †

by
Kristel M. Gatdula
1,2,* and
Emmanuel D. Revellame
1,3,*
1
Energy Institute of Louisiana, Department of Chemical Engineering, University of Louisiana at Lafayette, Lafayette, LA 70504, USA
2
Department of Chemical Engineering, University of the Philippines Los Baños, Laguna 4031, Philippines
3
Department of Engineering Technology, University of Louisiana at Lafayette, Lafayette, LA 70504, USA
*
Authors to whom correspondence should be addressed.
†
Presented at the 3rd International Electronic Conference on Applied Sciences, 1–15 December 2022; Available online: https://asec2022.sciforum.net/.
Eng. Proc. 2023, 31(1), 20; https://doi.org/10.3390/ASEC2022-13817
Published: 5 December 2022
(This article belongs to the Proceedings of The 3rd International Electronic Conference on Applied Sciences)
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Abstract

:

Featured Application

This research could serve as a theoretical foundation for the advancement of the extraction of volatile organic acids using surfactants.

Abstract

Non-ionic surfactants are one of the most useful surfactants as they are the second largest group, produced by volume at about 35%. Due to their low toxicity, the demand for them is escalating due to an extensive use of these amphiphilic materials for an efficient, non-energy-requiring recovery of volatile organic acids (VOAs) from aqueous mixtures. This separation process is mainly due to the cloud point property of surfactants, which is referred to as the temperature of the system at which two phases are formed. One of the phases is micellar-rich and the other is micellar-poor. In these micelles, the surfactant molecules are oriented in such a way that the hydrophilic heads shield the hydrophobic tails from the other water molecules in the system. This assembly partitions the organic compounds within the interior of the micelles, which act as the pseudo-organic phase. This work elucidates how salting-out affects the cloud point of ethoxylated non-ionic surfactants, resulting in VOA separation. Studies suggest the sensitivity of the cloud point to the presence of electrolytes and its dependence on the parameters’ hydrophile–lipophile balance (HLB) and on the number of ethylene oxide (EON) units in the surfactant molecule. Electrolyte addition, in the form of salt, causes the dehydration of micelles as salt is a water-structure maker. The salt changes the solvent structure through aggregation and formation of larger micelles. This translates into a lipophilic shift, which reduces the cloud point and the surfactant’s HLB. As the HLB decreases, the more hydrophobic the surfactant becomes, resulting in better separation. The type of salt influences the characteristics of the interphase that separates the phases formed. Typically, polyvalent cations such as Al3+ and Ca2+ are more effective in decreasing the HLB than the monovalent cations (e.g., Na+ and K+) because of their higher surface charge densities. Since the surfactant’s HLB is dictated by its ethylene oxide component (i.e., HLB decreases with EON), it follows that non-ionic surfactants with a lower EON could achieve better separation in the presence of salt. Although the actual separation of VOAs could possibly be affected by other parameters (e.g., amount of added surfactant and salts and mass transfer rates), the response of surfactant’s properties (i.e., cloud point, HLB, and EON) to salt addition could be utilized to establish an enhanced VOAs extraction from aqueous systems.

1. Introduction

Volatile organic acids (VOAs) are short-chain fatty acids with a wide range of applications in the pharmaceutical, petrochemical, food, cosmetic, tanning, and chemical industries. These compounds can be derived from wastewater through bacterial fermentation, typically in the form of acetic, propionic, butyric, and lactic acids, each at a concentration of 2.5 to 10 g/L [1]. The processing of wastewater into a spectrum of marketable products is the concept of biorefinery. Instead of a waste stream to be treated and disposed, wastewaters are used to produce precursors of high value-added products. Similar to its derivatives, the synthesis of these VOAs as platform chemicals is profitable, with their current market prices of USD 550/ton for acetic acid [2], USD 1600–2000/ton for propionic acid [3], USD 1600–5000/ton for butyric acid [4], and USD 3000 to 4000/ton for lactic acid [5]. With approximately 34 billion gallons of wastewater treated daily in the United States alone [6], the recovery of these VOAs could be economically viable.
Due to increasing demand and usage of these VOAs, there are several recovery methods that have been explored. These include absorption, adsorption, electrodialysis, reverse osmosis, nanofiltration, membrane contactor, and solvent extraction [7]. Solvent extraction is one of the most popular strategies to recover VOAs from dilute aqueous solutions because of its effectiveness. However, the downside of solvent extraction is the use of extractants (e.g., amines, C5-C12 alcohols, C5-C8 ketones, benzene derivatives, isoamyl acetate, and di-isopropyl ether) that may be toxic or inhibitory to microorganisms if this method is to be used for the in-line recovery of VOAs (i.e., extractive VOAs fermentation or in situ VOAs extraction) [8]. Hence, the use of non-ionic surfactant as an extractant is recommended. Non-ionic surfactants are also generally less toxic than the ionic surfactants. The absence of a charge in the head of the non-ionic surfactant molecules reduces their toxicity effect on the negatively charged surfaces of bacterial cells [9]. In contrast, charged or ionic surfactants could interact with the charged bacterial cells, making them more inhibitory to microbial growth.
This work was performed to elucidate the effect of electrolytes (salts) on the recovery of VOAs using ethoxylated, non-ionic surfactants. Recovery improvement was anticipated due to salting-out, which is considered a valuable mechanism to induce substances to separate out of aqueous solutions by the addition of salt [10]. This happens when the water solubility of non-electrolyte substances such as VOAs decreases as the salt concentration increases [11]. This paper expounds how salting-out influences the behavior of surfactant–water solutions in terms of the cloud point, hydrophile–lipophile balance (HLB), and the number of ethylene oxide (EON) units. Recommendations on the selection of suitable, non-ionic surfactants to effectively recover VOAs from aqueous solutions are then provided based on the identified and anticipated effect of salts to the above-mentioned surfactant properties.

2. Mechanism of Salting-Out

The recovery of VOAs using a non-ionic surfactant proceeds through three major steps: adsorption, aggregation, and solubilization. When a surfactant is introduced in an aqueous solution of VOAs, the surfactant molecules accumulate at the surface of the solution [12]. This is termed adsorption. The orientation of the hydrophilic heads and hydrophobic tails of the surfactant molecules on the surface of the solution depends on the composition of the mixture in the bulk phase [13,14,15,16]. If the solution is too polar, the hydrophilic heads will interact with the water molecules of the solution, reducing the surface tension [17,18,19,20,21]. Once the surface is saturated with the surfactant, the surfactant molecules bind together and begin to form micelles. The aggregation behavior of the surfactant transforms the molecules into a dense state with the lipophilic groups (i.e., hydrophobic tails) at the interior of the micelles and the hydrophilic groups at the outside [22]. Finally, in solubilization, the lipophilic core of the micelle interacts with the hydrophobic side groups [23] of the VOAs, incorporating these acids in the aggregates of surfactant molecules.
In salt-assisted extraction, the separation of VOAs from a mixture is governed by the combination of electrostatic repulsion and enhancement of hydrophobic effect [24]. Salts in the bulk phase create a strong electric field [25], which leads to a screening-out of the electrostatic repulsion between the surfactant’s hydrophilic head groups and the formation of a hydration shell that makes water molecules unavailable for surfactant hydration [26]. This therefore leads to a decrease in the solubility of the VOAs in the aqueous solution with the increasing salt concentration. In selecting a particular salt for this application, the salt should have a high solubility in water and a low solubility in the surfactant. Typically, salts used are small, multiple-charged anions such as those that contain sulfates, phosphates, and carbonates [24].

3. Adjustment on the Cloud Point due to Electrolytes

Among different amphiphilic materials, non-ionic surfactants are frequently used in organics extraction due to their unique property, called the cloud point. Cloud point occurs due to the decrease in affinity for water of hydrophilic heads [27]. Below the cloud point temperature is the existence of a micellar solution [28], and above it is a sudden onset of turbidity in the surfactant solution [29]. This signifies that the mixture starts to phase separate. The cloudy dispersion is attributed to the dehydration of the surfactant’s polyoxyethylene chain (EON) [30]; hence, the formation of larger aggregates, which means a higher solubilization capacity to recover the VOAs.
Non-ionic surfactant exhibits optimal effectiveness if used for extraction near or below their cloud point [31]. Cloud point depression is known to occur in the presence of salt electrolytes as this property is highly affected by salinity [32]. When introduced into the aqueous solution, the salt dissociates and interacts directly with water molecules, forming strong bonds and thereby increasing the enthalpy and entropy of the mixture. The surfactant affinity for the polar medium is then reduced, promoting the formation of the coacervate droplets that trigger phase separation [27].

4. Relationship between Cloud Point and HLB

Non-ionic surfactants are comprised of hydrophobic tail and the hydrophilic headgroups with ethylene oxide units, as represented in Figure 1 [33]. The strength and size of these groups in the molecule determines the surfactant’s HLB [34], which dictates how the surfactant will interact with the components in the solution. A higher number means that the surfactant is more hydrophilic, or water soluble. In particular, surfactants with an HLB value of less than 10 are water insoluble, while those with an HLB value of greater than 10 are otherwise (i.e., water soluble). Hence, for the recovery of VOAs, surfactants with HLB values of less than 10 should be selected to form a wrapping layer that will isolate the acid molecules from the more polar components.
The effect of added electrolytes on the cloud point of non-ionic surfactants has been the subject of many studies. Most of this research determined that the HLB value changes with the cloud point [35,36,37,38]. In a study by Arkhipov et al. [39], the equation representing the change in cloud point of a neonol surfactant, upon addition of NaCl (Equation (1)), had the same form as the equation for its effective HLB (Equation (2)). This clearly shows the dependence of the HLB on the cloud point. In both equations, the cloud points and the HLBs decrease exponentially with the addition of salt. The notations tcp and tcp0 represent the cloud point of the aqueous solutions with and without salt, respectively, C is the salt concentration, N and N0 refer to the HLB numbers in the presence and absence of salt, respectively, and k is the slope coefficient that dictates the boundary conditions for the cloud point or HLB based on C. Due to the processes of dehydration of the non-ionic surfactant molecules and the competing action of electrolyte ions, the micelle size increases with a decreasing number of aggregates and the value of the cloud point decreases along with the HLB [39].
tcp = tcp0 exp (−kC)
N = N0 exp (−kC)

5. Response of Hydrophilic Units to Kosmotropicity

Kosmotropicity refers to the efficiency of most common ions as promoters of salting-out. It follows the decreasing orders of the Hofmeister series [40,41]. For anions, the hierarchy is OH− > SO42−, CO32− > ClO4− > BrO3− > Cl− > H3CCOO− > IO3−, IO4− > Br−, I− > SCN− > NO3−; while for cations, the hierarchy is Na+ > K+ > Li+ > Ba2+ > Rb+ > Ca2+ > Ni2+ > Co2+ > Mg2+ > Fe2+ > Zn2+ > Cs+ > Mn2+ > Al3+ > Fe3+, Cr3+ > NH4+ > H+. A more kosmotropic ion promotes an enhanced degree of hydrogen bonding with water molecules in a solution. On the other hand, the ions that diminish the water molecule structure by disrupting the hydrogen bonding pattern are termed as chaotropic ions [42]. The order of interaction of these types of ions is not always in accordance with the Hofmeister series as it varies depending on the nature of the system.
The variation in the salting-out ability of the ions depends on the competition between the hydrogen bonding of the ion–surfactant’s hydrophilic group, ion–water, ion-ion, water–water, and the water–surfactant’s hydrophilic group. In a molecular dynamic simulation study on the effects of addition of monovalent salt to polyethylene oxide surfactant solutions, the interaction was initiated with the ion binding with water as a single entity [43]. At the air–ion solution interface, the phase is negatively charged [44] because of the stronger adsorption tendencies of the anions than the cations [44,45,46]. Within the solution, the anions dominate the cations in terms of salting power in majority of the cases [47,48,49]. After the dissociation of salts, short-ranged interactions among various species take place, favoring most of the hydrogen bonding interactions. The hydration of the hydrophilic groups in the form of ethylene oxide units in ethoxylated surfactants is then highly influenced by the charge density of ions [43]. With an increasing salt concentration, the cloud point decreases with the HLB [45]. Since higher surface charge density ions are much more effective at decreasing the surfactant’s cloud point and HLB, this therefore means that polyvalent cations have a higher salting power than the monovalent cations [50].
Since salts bring about the dehydration of the hydrophilic moiety of the surfactant, causing the enhancement of micelle formation, the ethylene oxide number (EON) should also be considered in the selection of a non-ionic surfactant for the recovery of VOAs. The EON dictates the hydrophilicity of the surfactant, while the propylene oxide content is typically used to measure the hydrophobicity [51]. The HLB is directly proportional to the EON. When the EON value is high, the solubility of the surfactant in the aqueous solution increases [52]. This is not ideal since the aim is to dehydrate the surfactant entities to attain the separation of VOAs.
In some cases, manufacturers of surfactants do not provide the EON value. Since the EON increases with the critical micelle concentration (CMC) [53], the CMC can also be used as a guide in selecting the suitable surfactant for the recovery of VOAs. The CMC is a parameter that defines the surfactant concentration at the onset of micelle formation [54]. It is a useful indicator of propensity of the surfactant to assemble in an aqueous system in terms of micellization ability and micellar stability. With a low CMC, the easier it is to form micelles and the more stable they will be [55].

6. Conclusions and Recommendations

In selecting a suitable, non-ionic surfactant for the recovery of VOAs, the nature of the surfactant and the type of organic acid to be recovered should be considered. Since the mechanisms involved in the separation of VOAs from other components of an aqueous mixture include the dehydration of micelles and the hydration of the hydrophilic groups, the target acid to be separated should be less polar when compared with the other components. This favors the binding of the hydrophobic tails of the micelles with the acid’s hydrophobic chain. With the aim of increasing the solubilization capacity of the surfactant, which corresponds to more acid molecules being entrapped by the micelles, the HLB value should be low. This is because the solubilization capacity is larger for a surfactant with a longer alkyl chain, or if the surfactant is lipophilic. The HLB is directly proportional to the CMC and EON. However, between the CMC and EON, it is more straightforward to use the CMC in selecting the suitable, non-ionic surfactant since it specifies the amount of surfactant required to reach the maximum surface tension reduction. Hence, the lower the value of the CMC, the less amount of surfactant is required to effectively recover the VOAs. Aside from the HLB and CMC, the cloud point temperature of the surfactant should be checked to ensure the occurrence of two-phase partitioning and the optimal effect of using the surfactant. The recovery of the VOAs can still be enhanced by salting-out, wherein the selection of appropriate salt type can be initially based on the anions and the charged density of ions.

Author Contributions

Conceptualization and investigation, K.M.G. and E.D.R.; original draft preparation, K.M.G.; writing—review and editing, K.M.G. and E.D.R.; supervision and funding acquisition, E.D.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Louisiana Board of Regents (Contract No.: LEQSF (2021-24)-RD-A-21).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Acknowledgments

The authors would like to thank the Louisiana Board of Regents for the opportunity and the financial support to this research, and the staff and students at the Energy Institute of Louisiana for their assistance in the conduct of this research.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study, in the collection, analyses or interpretation of data, in the writing of the manuscript or in the decision to publish the results.

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Figure 1. Molecular models of the alkyl and ethylene oxide chains of oligo (ethylene oxide) monoalkyl ether non-ionic surfactants [33].
Figure 1. Molecular models of the alkyl and ethylene oxide chains of oligo (ethylene oxide) monoalkyl ether non-ionic surfactants [33].
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Gatdula, K.M.; Revellame, E.D. Salt-Induced Recovery of Volatile Organic Acids Using Non-Ionic Surfactants. Eng. Proc. 2023, 31, 20. https://doi.org/10.3390/ASEC2022-13817

AMA Style

Gatdula KM, Revellame ED. Salt-Induced Recovery of Volatile Organic Acids Using Non-Ionic Surfactants. Engineering Proceedings. 2023; 31(1):20. https://doi.org/10.3390/ASEC2022-13817

Chicago/Turabian Style

Gatdula, Kristel M., and Emmanuel D. Revellame. 2023. "Salt-Induced Recovery of Volatile Organic Acids Using Non-Ionic Surfactants" Engineering Proceedings 31, no. 1: 20. https://doi.org/10.3390/ASEC2022-13817

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