ABSTRACT
Water treatment receives wide attention due to its close relationship with the environment and its protection from pollutants. This study investigates the effectiveness of applying chitosan/essential oil/nanoemulsion (NE) hydrogel beads in water treatment, which were produced by combining chitosan and various NEs containing different fractions of mixed surfactants of tween 80 and span 20, oils of either clove or peppermint, and distilled water. The physicochemical properties were determined using the scanning electron microscope (SEM), differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR). SEM images exhibited that the clove beads presented sharp edges with parallel lines of networks and uniform pores (6.947 ± 1.45 μm, coefficient of variation (CV) = 20%), whereas peppermint beads displayed a spongy surface with spherical pores (11.203 ± 3.34 μm, CV = 29%). DSC analysis revealed that both clove and peppermint beads were thermally stable until 100 °C. The FTIR curve of the beads displayed new peaks confirming chemical interactions between the chitosan and the constituents of the NE. The adsorption capacity of the beads for the copper sulfate and bromophenol blue was more than 50% achieved within 1 h at 25 °C. In conclusion, chitosan–NE hydrogel beads have promising potential for use in water treatment.
HIGHLIGHTS
Synthesis of chitosan/peppermint oil/nanoemulsion (NE) beads.
Synthesis of chitosan/clove oil/NE beads.
The new materials have great potential to adsorb copper sulfate from the wastewater.
The new materials have great potential to adsorb bromophenol blue from the wastewater.
The new materials are inexpensive and easy to manufacture for water treatment applications.
INTRODUCTION
Water treatment is considered a key component of environmental sustainability because it reduces the waste that is released into the ecosystem as well as generates energy by producing biogases such as methane while empowering the water treatment complex (Crini & Lichtfouse 2019; Ahmed et al. 2024). The primary goal of water treatment ensure the continuation of life on the planet by providing clean water to various living organisms. Water treatment can also contribute to the production of fertilizers by using the resulting biomass that forms from the sludge phase of water treatment. Some other environmental sustainability aspects can be achieved through water treatment such as disease prevention, reusing and recovering water resources, ensuring long-term availability and accessibility of water resources, and preserving the quality and quantity of different freshwater resources (Saleh et al. 2022; Obiuto et al. 2024).
Water treatment involves a diverse range of techniques, with some methods requiring less energy, such as coagulation and sedimentation, while others demand more energy, like membrane processes that provide superior removal efficiencies (Cheremisinoff 2019; Silva 2023).
The formation of hydrogels for wastewater treatment has recently attracted the manufacturing industries due to their cost-effectiveness and ease of production (Etale et al. 2023; Hossain et al. 2024). Hydrogel is a novel kind of polymer material that has a three-dimensional network porous structure. It may be used to remove heavy metal ions, dyes, and auxiliaries from dye effluent by fully utilizing its high specific surface area, high porosity, and surface activity (functional groups). In the meantime, the hydrogel's strong water absorption helps to enhance its capacity to absorb dyes and heavy metal ions, making it an excellent adsorbent. Adding polymers (polyacrylate, alginate, chitosan sugar, and cellulose derivatives) to the hydrogel improved its adsorption ability and further improved the hydrogel's advanced treatment impact on heavy metal ions and dye wastewater (Chen et al. 2020). In addition, the hydrogel has significant benefits in water absorption and transportation due to its strong hydrophilicity and excellent pore structure. Hydrogels possess exceptional water absorption capabilities because they may contain a multitude of hydrophilic groups, including –OH, –COOH, –NH2, and –SO3H. Hydrogels are becoming a promising material in the collection of water resources with high water retention and excellent recyclability (Zhou et al. 2019, 2020; Akter et al. 2021).
Chitosan is a linear polysaccharide composed of randomly distributed β-(1 → 4)-linked D-glucosamine (the deacetylated unit) and N-acetyl-D-glucosamine (the acetylated unit). It 's like a molecular dance of sugar molecules. As an alkaline hydrolytic derivative of chitin, chitosan has amine, hydroxyl, and acetamido functional groups that provide it a higher solubility profile, reduced crystallinity, and chemical modification flexibility. The chemical modification of chitosan is intriguing because it preserves the organic structure of the material, imparts new or enhanced qualities, and does not alter its basic physicochemical and biological characteristics (Li et al. 2020; Aranaz et al. 2021; Wang & Zhuang 2022).
Nanoemulsions (NEs), double phasic dispersion, can be water-in-oil (W/O) or oil-in-water (O/W), both of which consist of two immiscible liquids stabilized together by the right surfactant. Typically, the mean droplet diameter obtained in NEs is less than 500 nm. Their appearance is hazy or transparent due to their small droplet size, which contrasts with the milky white color of a coarse emulsion, where the droplets are micron-sized and participate in multiple light scattering. While submicron and mini emulsions are occasionally used interchangeably with the term ‘nanoemulsion,’ the two terms should not be confused. Even though they both have a similar droplet size range, NEs and microemulsions differ greatly in terms of long-term thermodynamic stability and structural features (Alkhatib et al. 2012; Ozogul et al. 2022; Mushtaq et al. 2023).
Essential oils solubilized in NEs may add beneficial properties to the hydrogels (Barradas & de Holanda e Silva 2012; Herman et al. 2019). Clove oil is well-known for its antibacterial, analgesic, and anesthetic qualities in medicine. Its antibacterial and antifungal qualities make it effective against warts, parasites, scars, and acne. Peppermint oil has antioxidant and antimicrobial activities in addition to its unique flavor and aroma.
It may be feasible to create innovative NE hydrogel beads that exhibit characteristics including significant adsorption capacity, decreased sensitivity to pH variations, and thermal stability. These beads have the potential to offer both cost-effectiveness and efficiency, making them attractive to water treatment plant enterprises. Importantly, such beads have not been previously employed in water treatment applications. The aim of the present study was to synthesize hydrogel beads consisting of chitosan, tween 80, span 20, and natural oils (clove and peppermint). It also intended to characterize the synthesized hydrogel beads and quantify the hydrogel bead's adsorption capacity both physically and chemically.
MATERIALS AND METHODS
Preparation of the hydrogel beads
Schematic presentation for the formation of the chitosan/essential oil/NE hydrogel beads.
Schematic presentation for the formation of the chitosan/essential oil/NE hydrogel beads.
Testing the adsorption efficiency of the produced beads



For further investigation of the adsorption capacity of the successful hydrogel beads, the temperature and time effects were determined. A 1.0 g of clove or peppermint beads was added to 0.1 M copper (II) sulfate or 10 ppm of bromophenol blue. Then, the adsorption capacity was measured at a certain period of time or temperature.
RESULTS AND DISCUSSION
Formation of the chitosan/essential oils/NE beads
Different combinations of chitosan and NE were produced by mixing 1 mL of 20 mg/mL of chitosan solution (CH) and 1 mL of various essential oil NEs consisting of fixed percentages (15%) of mixed surfactants of span 20 and tween 80 at a ratio of 1:2, respectively, and several volume fractions of oil and water. Three NEs of each oil [clove (C) or peppermint (P)] were produced as follows: (1) NE with (75%) high (H) oil content relative to (10%) water (NECH or NEPH); (2) NE with equal (E) oil and water contents at (42.5%) for each (NECE or NEPE); and (3) NE with (10%) low (L) oil content relative to (75%) water (NECL or NEPL). During the process of manufacturing the beads by adding each combination of (NE–CH) to a cold solution containing 100 mL of the basic alcohol cold solution, not all NE and chitosan mixtures were suitable for forming the beads. The beads formation resulted from the mixing of the chitosan (CH) and the NEs with low (NEL), equal (NEE), or high (NEH) oil contents, relative to the water amount, are illustrated in Table 1.
Nanoemulsion–chitosan (NE–CH) mixture that resulted in the formation of beads
Type of bead . | NEL–CH . | NEH–CH . | NEE–CH . |
---|---|---|---|
Clove (C) beads | Formed | Formed | Not formed |
Peppermint (P) beads | Not formed | Formed | Formed |
Type of bead . | NEL–CH . | NEH–CH . | NEE–CH . |
---|---|---|---|
Clove (C) beads | Formed | Formed | Not formed |
Peppermint (P) beads | Not formed | Formed | Formed |
Adsorption capacity of the produced beads
Concentration effect
The adsorption capacity of the hydrogel beads at different concentrations of CuSO4.
The adsorption capacity of the hydrogel beads at different concentrations of CuSO4.
Temperature effect
Temperature effect on the adsorption capacity of peppermint and clove oil beads in (a) CuSO4 solution (0.1 M) and (b) bromophenol blue (10 ppm).
Temperature effect on the adsorption capacity of peppermint and clove oil beads in (a) CuSO4 solution (0.1 M) and (b) bromophenol blue (10 ppm).
Time effect
Time effect on the adsorption capacity percentage of peppermint and clove beads in (a) CuSO4 (0.1 M) and (b) bromophenol blue (10 ppm).
Time effect on the adsorption capacity percentage of peppermint and clove beads in (a) CuSO4 (0.1 M) and (b) bromophenol blue (10 ppm).
A rise in the adsorption capacity over time for both bead types added to the bromophenol blue is depicted in Figure 4(b). Initially, there is a notable peak in the adsorption capacity during the first hour, attributed to the high absorbance percentage of stain from the water. Subsequently, the absorbance percentage gradually decreases as time progresses. Also, the values of R2 were determined to be 0.94 for peppermint beads and 0.93 for clove beads, confirming a linear relationship between the adsorption capacity and time in both types of beads. Interestingly, the adsorption capacity of the clove beads was considerably larger than that of peppermint ones at all times, implying that the structure of the clove beads' pores is more hydrophobic since they attracted the bromophenol blue and adsorbed it. Additionally, the presence of eugenol, the major constituent of clove oil that has a similar structure to the bromophenol, may potentiate the adsorption capacity of the clove beads.
Characterization of the selected hydrogel beads
Scanning electron microscopy for morphology visualization
The scanning electron microscope (SEM) allows us to analyze samples with a diameter of up to 200 mm and a height of 80 mm. The magnification of the device ranges from x to 300,000 x. Materials that can be used in SEM are organic and solid inorganic materials including metals and polymers (Zu et al. 2012). While preparing the beads for scanning, it was important to cover the beads with materials that could conduct electricity, which was a thin layer of gold in our study. This layer prevented the surface of the beads from accumulating electric charge and also increased the emission of secondary electrons to make the surface more suitable for analysis and imaging (Fourie 1982).
SEM images of the external shape, pores, and surface of clove (a, c, and e, respectively) and peppermint (b, d, and f, respectively) beads.
SEM images of the external shape, pores, and surface of clove (a, c, and e, respectively) and peppermint (b, d, and f, respectively) beads.
Differential scanning calorimetry for thermal stability determination
DSC thermographs of (a) chitosan, (b) clove NE, (c) peppermint NE, (d) clove beads, and (e) peppermint beads. The heat ranges from (0 to 300) °C with a heat rate of 10 °C/min. The temperatures of crystallization, evaporation, formation, melting, and glass transition are signified as Tc, Te, Tf, Tm, and Tg, respectively.
DSC thermographs of (a) chitosan, (b) clove NE, (c) peppermint NE, (d) clove beads, and (e) peppermint beads. The heat ranges from (0 to 300) °C with a heat rate of 10 °C/min. The temperatures of crystallization, evaporation, formation, melting, and glass transition are signified as Tc, Te, Tf, Tm, and Tg, respectively.
Analyzing the DSC curves depends on the baseline that indicates the physical and chemical transitions of the sample. The region over the baseline shows exothermic behavior, which means the sample releases energy as it crystallizes or crosslinks. Conversely, the region below the baseline shows endothermic behavior, which means the sample absorbs energy as it melts from a solid state to a liquid state or evaporates from liquid to vapor. The chitosan solution has undergone evaporation at 110 °C followed by crystallization at 120 °C as revealed in Figure 6(a). By contrast, the average evaporation temperatures of both clove and peppermint NE were around 125 °C (Figure 6(b) and 6(c)). In fact, the nanosuspensions of peppermint NE were less stable as bond formation was constructed among them evidenced at the temperature of 65 °C. Both clove and peppermint beads were thermally stable until 100 °C as they endured glass transition, but their melting temperature differed, to 127 and 139 °C for clove and peppermint beads, respectively (Figure 6(d) and 6(e)). Although there were some differences in the determined temperatures at which the physical and chemical changes occur, the shape of the thermographs of the peppermint NE and peppermint beads were very similar and differed from the shape of the chitosan solution indicating that the NE effect was more dominant. By contrast, the shape of the thermographs of the clove beads differed from that of clove NE and chitosan solution, implying that distinct beads were produced with different chemical and physical properties.
FTIR for chemical structure characterization
Fourier transform infrared (FTIR) spectroscopy involves passing infrared (IR) light through a material. A portion of the IR light is transmitted while a part of it is absorbed by the sample. The resulting spectrum provides a molecular fingerprint of the sample by illustrating the molecule's absorption and transmission. Similar to how two fingerprints are never the same, no two different molecular structures can have the same infrared spectrum. This makes infrared spectroscopy applicable to various kinds of analysis. The spectrum is divided into two regions: the fingerprint region and the functional group region. The fingerprint region displays the distinct pattern for each compound and can be used to positively identify an unknown sample by comparing it to a known one. However, the functional group region is utilized to forecast a molecule's chemical structure and indicate whether or not it has common functional groups (Nandiyanto et al. 2019).
FTIR spectra of the chitosan, NE, and both clove (a) and peppermint (b) beads.
CONCLUSION
In conclusion, water scarcity is a global challenge that impacts countries worldwide, especially emerging countries such as the Sultanate of Oman. The present study produced new hydrogel beads made from chitosan, tween 80, span 20, and essential oils (clove and peppermint) that exhibited desirable physicochemical properties and adsorption performance. Further investigations are recommended to comprehensively assess the scalability, treatment efficiency, and any potential environmental impacts of this innovative water treatment technology. In particular, the environmental compatibility, long-term stability, and regeneration potential of the beads need to be examined. It is also highly recommended to test the produced beads on a broader range of pollutants, which will enhance the applicability of the beads at a larger scale in real-world water treatment plants. Additionally, the mechanism of pollutant adsorption onto the bead surface should be further investigated to better understand the process.
FUNDING
This research did not receive any specific funding.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.