1. Introduction
Essential oils are significant bioactive components found in aromatic traditional Chinese medicine (TCM) [
1]. They exhibit a wide range of biological activities, including antibacterial, antiviral, anti-inflammatory, and antioxidant properties [
2,
3]. Essential oil is the primary bioactive compound in nutmeg (
Myristica fragrans Houtt.), which belongs to the Myristica genus within the nutmeg family [
4]. The essential oil from nutmeg (EON) possesses bactericidal and analgesic properties, making it a valuable resource for treating conditions such as rheumatism, diarrhea, cholera, intestinal diseases, and stomach spasms [
5,
6]. Moreover, EON is an essential commodity employed as a flavoring agent in the food, pharmaceutical, and cosmetic industries [
7,
8]. Consequently, there is a compelling need to develop efficient methods for obtaining the essential oil.
The essential oils used in TCM are known for their volatilities and instabilities, so they are susceptible to decomposition when exposed to light, heat, and air [
9]. These specific attributes contribute to the challenges of extracting essential oils effectively. Currently, the prevalent methods used for extracting essential oils for TCM include distillation, solvent extraction, gas extraction, and supercritical fluid extraction [
10]. At present, steam distillation (SD) is often used because it does not require organic solvents and preserves the integrity of the components. During the extraction, in addition to obtaining pure essential oil, many aromatic aqueous solutions with dispersed oil, emulsified oil, and dissolved oil are produced [
11]. Consequently, there is an urgent need to develop an efficient, environmentally friendly, and safe technology for separating essential oils from water to address these challenges effectively [
12,
13].
Membrane separation is an efficient technology for oil/water separation, owing to its high efficiency, low energy consumption, simple operation processes, and minimal secondary pollution [
14,
15,
16,
17]. S.K. Gopika et al. [
18] utilized nanofiltration to separate essential oils from ginger butter extracted with n-hexane, which effectively obtained high-quality turmeric oil with improved stability. C. Du et al. [
19] prepared organic composite membranes and used them for pervaporation to extract valuable essential organic compounds from dilute aqueous solutions of Perilla frutescens. H. Xiao et al. [
20] combined ceramic membrane microfiltration and poly (dimethyl siloxane)/poly (vinylidene fluoride) composite membrane pervaporation for separation of the thioether compounds in garlic oil. These results indicate that membrane separation technology has great potential for the separation and application of essential oils in TCM [
21].
Polyacrylonitrile (PAN) membranes are widely used in industrial oil/water separations and have good chemical properties, thermal stabilities, and solvent resistance [
22,
23,
24,
25,
26]. D. Teng et al. [
27] fabricated SiO
2/zein/PAN fiber membranes, which exhibited high oil/water separation efficiencies and separation fluxes. N. Xue et al. [
28] used a collagen fiber membrane (CFM) as a multifunctional carrier and electrospun a PAN layer in situ to prepare a PAN/CFM composite membrane with an ultrahigh separation flux and high fouling resistance for preparing oil-in-water lotions.
Unlike most of the oils reported, the essential oil components of TCM are more complex [
29,
30,
31]. According to their chemical structures, essential oils can be divided into terpenoids, including monoterpenes and sesquiterpenes, and their oxygen-containing derivatives, such as α-pinene β-pinene and limonene; aromatic compounds, such as eugenol and nutmeg ether; and aliphatic compounds, such as houttuynin and n-nonyl alcohol [
32]. There are components in these complex organic compounds with similar polarity to PAN, resulting in damage to the PAN membrane. However, there is still limited research on the application of organic membranes, especially PAN membranes, in the enrichment of essential oils, and the membrane separation process still needs further investigation.
In this work, a heat-treated polyacrylonitrile (H-PAN) membrane was fabricated by annealing under inert gas to enhance the enrichment of EON. The investigation involved a comprehensive analysis of changes in the pore size distribution, microstructure, surface chemical structure, and wetting behavior of the PAN and H-PAN membranes. The enrichment of EON with the two respective membranes was compared by considering the oil rejection rates, fluxes, pollution models, and oil quality. It was extended to other TCMs used in research, such as Bupleuri Radix (BR), Magnolia Officinalis Cortex (MOC), Caryophylli Flos (CF), and Cinnamomi Cortex (CC). This research provides insight into PAN-based membranes with potential application in the enrichment of essential oils for TCM.
2. Materials and Methods
2.1. Materials
A PAN ultrafiltration membrane (molecular weight cut off 50 kDa) was purchased from RisingSun Membrane Technology, Beijing, China. The nutmeg herb was purchased from Jiangsu Chengkai Chinese Medicine Co., Ltd., Huaian, Jiangsu, China. Bupleuri Radix, Magnolia Officinalis Cortex, Caryophylli Flos, and Cinnamomi Cortex were purchased from Shaanxi Sciendan Pharmaceutical Co., Ltd., Tongchuan, Shaanxi, China. Sulfuric acid was purchased from Sinopharm Group Chemical Reagent, Shanghai, China. Ethyl acetate and diiodomethane were purchased from Aladdin, Shanghai, China. Chemical Oxygen Demand (COD) oxidant and COD catalyzer were purchased by Lvyu, Qingdao, Shandong, China. The COD was determined via a multifunctional water quality test (LY-4DB, Lvyu, Qingdao, Shandong, China). Deionized water was prepared with a water purification system (EPED-E2-20TS, Yipu Yida, Nanjing, Jiangsu, China).
2.2. Fabrication of H-PAN Membrane
First, the PAN membrane was soaked in deionized water for 24 h, and the water was changed three times to wash off the protective surface coating of the membrane. Then, the sample was dried and placed in a tube furnace. The temperature was raised to 200 °C at 2 °C/min and maintained at 200 °C for 1.5 h under argon protection.
2.3. Characterization
The thermogravimetric properties of the PAN polymer membrane were analyzed with a thermogravimetric analyzer (TG, STA 8000, PerkinElmer, Waltham, MA, USA). The chemical structure changes of the PAN polymer membrane during heat treatment were analyzed with a Fourier transform infrared spectrometer (FTIR, Nicolet iS20, Thermo Scientific, Waltham, MA, USA) and X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific, Waltham, MA, America). The surface morphology of membranes was observed by field-emission scanning electron microscopy (FE-SEM, Regulus-8100, Tokyo, Japan). Pore size was calculated with Brunauer–Emmett–Teller (BET, ASAP2460, Micromeritics, Norcross, GA, USA). The surface microstructure and roughness were observed with an atomic force microscope (AFM, Dimension ICON, Bruker, Billerica, MA, USA). Contact angle measurement (CA, DSA100, Kruss, Hamburg, Germany) was employed to characterize the wettability of the membranes. Optical photographs were taken with a biomicroscope (BX35, Olympus, Tokyo, Japan). The size distribution was measured with a nanoparticle size analyzer (Zetasizer Nano ZS90, Malvern Panalytical, Malvern, UK). The compositions of EON were analyzed by gas chromatography-mass spectrometry (GC-MS, TQ8050NX, Shimadzu, Kyoto, Japan).
2.4. Preparation of Essential Oil-in-Water Emulsion
An appropriate amount of nutmeg was accurately weighed. Then, it was crushed, passed through a No. 1 sieve, and placed in a round-bottom flask. Fourteen volumes of water were added to the herbs and soaked for 1 h. The extraction device is shown in
Figure 1. The distillate was collected by SD to obtain an essential oil-in-water emulsion. Pure essential oil was extracted according to method A, Part IV “Determination Method of Essential Oil” in the Chinese Pharmacopoeia 2020. After cooling and stratification, the essential oil was collected and stored for use in brown bottles at a low temperature. Different volumes of pure EON extracted in the previous step were taken, and deionized water was added while stirring for 2 h at 500 r/min to obtain nutmeg essential oil-in-water emulsions with different concentrations. The BR, MOC, CF, and CC essential oil-in-water emulsions were collected by SD with 10 times the volume of water.
2.5. Oil/Water Separation
The permeation performance evaluation was conducted using ultrafiltration equipment (Millipore, XFUF07601, USA), which is schematically presented in
Figure 2. The effective area of the membranes was 34.2 cm
2. The applied transmembrane pressure was constant at 0.2 MPa, and the stirring speed was 200 r·min
−1 at room temperature. The permeate flux was calculated as the following equation:
where
J is the flux (L/(m
2·h
−1·MPa
−1),
V is the filtrate volume (L),
t is per unit time (h),
A is the effective area (m
2), and Δ
P is the applied trans-membrane pressure (MPa).
2.6. Membrane Fouling Mechanism
Hermia [
33] developed four classical models of dead-end filtration based on Darcy’s law to explain the membrane fouling mechanisms: complete blocking, intermediate blocking, standard blocking, and cake filtration models [
34,
35,
36,
37]. The equation can be written as
where
t is the filtration time (h),
V is the cumulative filtration volume (L),
K is the fouling constant, and
n is the parameter that determines the type of membrane contamination [
38]. The corresponding blocking models are presented in
Table 1.
2.7. Determination of Rejection Rate
The permeating solution obtained through the membrane was determined by the classical potassium dichromate method. A 3 mL sample was added with 1 mL COD oxidant and 5 mL reducing agent, then digested for 10 min. Then, 3 mL of distilled water was added to the above solution. After cooling to room temperature, it was used to determine the COD value of organic matter in the sample. The calculation formula for the oil rejection rate is as follows:
where COD
0 is the initial oil-in-water emulsion COD value, and COD
1 is the permeate COD value.
2.8. GC-MS Analysis of the Enriched EON
The quality of EON separated by PAN and H-PAN membranes and the SD method was evaluated by GC-MS [
39]. First, the enriched oil was diluted with ethyl acetate. The conditions for GC-MS chromatographic analysis were as follows: SH-Rxi-5Sil MS capillary column (30 m × 0.25 μm, 0.25 mm); helium as carrier gas (volume fraction was 99.999%); volumetric flow rate (1.5 mL/min); split ratio (30:1). The heating program is shown in
Table 2. The spectrum library was searched through the Nist 20 MS Search data system of the chemistry workstation to confirm the chemical composition of the essential oil of the sample under test. The area normalization method was used to measure the relative percentage content of each component.
4. Conclusions
In this work, the H-PAN membrane with a smaller pore size and narrower pore size distribution was fabricated through heat treatment. Compared with the PAN membrane, the H-PAN membrane exhibited higher oil rejection rates at different oil contents. Moreover, the morphology of the separated H-PAN membrane was intact, while the pore size of the PAN membrane clearly increased, indicating that H-PAN has better stability for essential oil filtration. Based on the GC-MS data, the similarities of the essential oils enriched by the PAN and H-PAN membranes with those obtained through SD were 0.988 and 0.990, respectively. This indicated that the PAN-based membranes enriched the nutmeg essential oil almost without destructiveness. Moreover, the H-PAN membrane also exhibited a better ability to enrich the essential oils in the BR, MOC, CF, and CC oil-in-water emulsions. In summary, this study provides a design approach for obtaining organic membranes with nanoscale pore size and high stability and preliminarily demonstrates the feasibility of organic membrane enrichment of essential oils.