Removal of Octinoxate, a UV-filter Compound, from Aquatic Environment Using Polydimethylsiloxane Sponge

Abstract

This work demonstrates the potential of polydimethylsiloxane sponges for removing organic UV filter compounds such as octinoxate from aqueous solutions. The sponges were fabricated using simple templates made of hydrophilic fused or pressed particles (sugar or NaCl salt) with an approximate particle size of 0.4 mm. Among the prepared sponges, those templated with sugar cubes or coarse salt exhibited the highest adsorption capacity, effectively adsorbing up to 0.6% of their own mass in octinoxate. The PDMS sponges were fully regenerable, allowing for the complete removal of octinoxate without any detectable changes in their adsorption properties or dry weight. Due to their simple fabrication, ease of handling, ability to float, and reusability, PDMS sponges present an environmentally friendly and low-maintenance alternative to conventional filtration systems for the removal of octinoxate and potentially other UV filter compounds from environmental surface waters and recreational water bodies.

1. Introduction

Increasing concerns about UV radiation and skin cancer have led to the widespread use of UV filter compounds. UV filter materials can be either inorganic (e.g., titanium dioxide, zinc oxide) or organic compounds (e.g., oxybenzone, avobenzone, and octinoxate). The latter group have recently gained popularity as they provide invisible coverage even if their more frequent applications are needed. The presence of organic UV filter compounds used in sunscreen cosmetics in aquatic environments represents a growing environmental concern problem. For instance, octinoxate (ethylhexyl methoxycinnamate) is one of the most commonly detected UV filters in aquatic environments, with measured concentrations ranging from ng/L to low µg/L levels, particularly in recreational areas or downstream of wastewater treatment plants [1]. Its presence raises ecological concerns due to potential endocrine-disrupting effects and developmental toxicity observed in aquatic organisms, including fish and invertebrates [2].

Polydimethylsiloxane (PDMS) is a soft, flexible silicone polymer that has an inert hydrophobic and biocompatible nature, as well as chemical and UV light resistance. The applications of PDMS range from contact lenses and shampoos to food additives (antifoaming agent) and lubricants due to its low manufacturing costs. Choi et al. [3] proposed a simple procedure to make a sponge-like porous PDMS, which provides a structure with a large specific surface area. In their work, sugar granules (sugar cube) served as a mold and were dissolved after the PDMS had cured. They proved that the PDMS sponge efficiently absorbs oils and organic solvents, and that the sponge could be reused through simple squeezing. PDMS sponges are typically fabricated using template methods, as PDMS is poured onto templates (molds) made using fused sugar/salt particles [3,4] or 3D-printing [5], but foaming techniques [6], when gas bubbles are generated by chemical reactions to form a sponge-like structure, are also known. Among these methods, the simplest, most economical, and potentially best suited to the large-scale production of PDMS sponges is the sugar/salt fusion templating method, because it does not need intricate procedures or equipment [7,8].

PDMS has a non-polar, flexible backbone of –Si–O–Si– with methyl groups on the side. These methyl groups induce van der Waals forces with other non-polar molecules. These interactions are weak individually, but surprisingly strong cumulatively. These forces depend on the size (larger molecules = stronger dispersion) and the polarizability of the analyte, and the contact surface area between the analyte and polymer [9]. PDMS can also interact with polar compounds, either through hydrogen bonds between the siloxane group and alcoholic or acid hydrogen of the analyte or through polar–polar interactions [10]. In the case of the PDMS sponge and the weakly polar octinoxate, the adsorption is based on van der Waals (mainly London dispersion) forces. The PDMS sponge can effectively and rapidly absorb and remove oils from water when it comes into contact with the oil film/droplet. The PDMS sponge showed oil absorbency (the removal of petroleum products and organic solvents) in the range from 790% to 4000% for various oils and solvents, with the maximum absorption capacity reaching up to 23 times its weight [11,12]. It was shown that the oil-filled PDMS sponge floats on water without water penetrating its structure or the oil being released over extended periods. The PDMS sponge can not only absorb oils or hydrophobic liquids/solvents in its microstructure (pores) [13], but also enables components to adsorb due to its excellence adsorptive properties (PDMS is the main constituent in stationary phases of gas chromatography (e.g., DB-1 columns) [14], solid phase extraction (SPE) [15], and solid phase microextraction (SPME) [16]) and high surface-to-volume ratio. Surface-functionalized PDMS sponges have also been developed for the selective recovery of dyes or heavy metal ions from aqueous solutions (e.g., Mo(VI) [17] or Cu(II) [18] ions were retained in amine-functionalized PDMS from water). Liu et al. coated the PDMS sponge with graphene oxide for the efficient adsorption of Pb(II) [19].

The goal of the present work is to demonstrate the applicability of PDMS sponges for the removal of organic UV filter compounds from water as an environmentally friendly and low-maintenance alternative to conventional filtration systems. For this study, octinoxate, one of the most often applied UV filters, was used.

2. Materials and Methods

2.1. Materials

Analytical grade reagents were used. Octinoxate and methanol were purchased from Sigma Aldrich (St. Louis, MO, USA). The PDMS sponge was fabricated by casting a 10:1 mixture of PDMS oligomer and cross-linking agent (Sylgard 184, Dow Corning, Midland, MI, USA) onto the templates. Sugar cubes, sugar powder, fine and course NaCl salt, and green food dye were purchased from a local shop. Dilutions were prepared prior to use in double-deionized water with a 18 MΩ·cm resistivity (Elix-3, Millipore, Darmstadt, Germany).

2.2. Instrumentation

Experiments were carried out with a UV-Vis spectrometer (HP 8453, Agilent, Santa Clara, CA, USA) at 320 nm. Although the solubility of octinoxate in water is very low (0.2 mg/L, according to the Safety data sheet of Sigma, Revision Date 08.03.2024), UV measurements after strong vortexing resulted in an absorbance signal linearly proportional to the amount of the compound up to 1.25 Abs. Both glass and plastic cuvettes with optical pathlengths of 1 cm were used. The structure of the porous PDMS was examined using a zoom microscope (180×) equipped with a CCD camera (USB-500, 5 MP) (Eakins Microscope Store, Shenzhen, China). Images were processed via the Hayear ×64 Ver. 4.10.17214.20200601 software. For these pictures, the PDMS sponge samples were embedded in paraffin blocks, and then sectioned in 0.1–0.5 mm-thick slices with a manual microtom. Paraffin was washed out by soaking the slices in hexane and then in acetone, and finally drying them in the air.

2.3. Fabrication of PDMS Sponge

PDMS sponges were fabricated using simple templates: (1) commercial sugar cube, (2) fused sugar particles, and (3) pressed NaCl salt particles. During the manufacturing process of the sugar cube, the sugar granules are slightly moistened with water to stick together and are then pressed into a cube-shaped mold. The molded cubes are dried at approximately 60 °C to eliminate the moisture content (higher temperature to be avoided to prevent melting or caramelization). Sugar particles of different sizes (granulated or powdered) can be fused in molds of various shapes, similarly to as described above—for instance, in a micropipette tip (sugar particles are layered above a compressed cotton wool layer). The PDMS sponges are most often made via the sugar fusion method [20]. Although salt (e.g., NaCl) particles cannot be easily fused/melted together, the compressed salt particles with a hydrophilic surface ensure an interconnected network when the hydrophobic PDMS seeps into the pores.

The 10:1 mixture of PDMS oligomer and cross-linking agent was degassed before casting onto the templates. First, a thin layer (about 3 mm) of PDMS was poured into a clean Petri dish and a sugar cube was placed into the liquid PDMS base. Alternatively, the uncured PDMS was cast onto an approximately 1 cm-thick layer of sugar or salt particles, compressed between cotton wool pieces inside the micropipette. The uncured PDMS was sucked up into the microchannels/pores of the cubes/compressed particles by the capillary forces, and a sugar cube with a ~1 cm height was homogenously filled with the PDMS after one hour. Then, the PDMS was heated in an oven at 60 °C for 60 min for reticulation. After complete cross-linking, the solid PDMS pieces were placed into water in order to dissolve and remove the sugar/salt content from the PDMS structure, aided by mild vortex mixing (~1 h). Then, the pure PDMS sponge was dried at 60 °C for 60 min. A schematic illustration of the PDMS sponge’s fabrication using a sugar cube is shown in Figure S1.

In Figure 1a,b, photos of a PDMS sponge made using sugar cube as a template are shown. The density of the sponges was in the range of 0.4–0.5 mg/mL. The density of the PDMS sponges was determined using the basic formula of density = mass/volume. The mass of the dry sponge was measured using an analytical balance, while the volume was calculated based on the dimensions of the sugar cube template used during fabrication. The pore sizes were approximately 0.4–0.5 mm (that is the size of the sugar particles) (Figure 1c). The obtained PDMS sponges demonstrated elastic (even up to 80%) deformation under squeezing (if it was not extremely high) and could recover their original shape after compression. When uncured PDMS was poured onto the powdered sugar (~0.1 mm particle size), the viscous liquid surrounded the fine particles, which could hardly be dissolved and removed later. This is why these PDMS sponges seemed rather homogeneously white and not highly porous (Figure 1d). The porosity looks similar for sponges made from granulated sugar and regular table salt (NaCl) (Figure 1e,f), since the particle sizes were around 0.4 mm. All sugar or salt particles could be removed via the thorough rinsing of the sponge.

2.4. Adsorption Studies

A 0.1 +/− 0.01 g clean/regenerated PDMS sponge was immersed in the cuvette of the spectrometer filled with 3.5 mL of aqueous sample (volume of the cuvette), which included octinoxate. The cuvette was gently vortexed and at certain intervals, we placed the cuvette back into the photometer for absorbance measurements. The absorbance of the solution was measured at 300 nm. Since the PDMS sponge floated on the surface of the liquid, it had not been removed from the cuvette during spectrophotometric measurements. When a large volume of aqueous solution was used for the adsorption studies, the vortexing of the PDMS sponge in the solution was performed in a glass vessel, and a small portion was transported (just during vortexing) to the cuvette for absorbance measurement.

3. Results and Discussion

3.1. Adsorption of Octinoxate on Different PDMS Sponges

Although the PDMS sponge is applied mainly for the absorption of apolar solvents and oils, the hydrophobic surface of PDMS is utilized especially in analytical chemistry (chromatography and solid phase extraction) for the adsorption of various components. PDMS is highly hydrophobic and it is among the most efficient adsorbent materials [7]; thus, the adsorption of the apolar components from the aqueous medium is obvious. Generally, the adsorption efficiency is enhanced with the increase in the porosity and the specific surface area of the solid material. The penetration of the liquid (mobile phase) through the small pores and channels is often facilitated by external pressure (hydraulic or hydrostatic). The PDMS sponges studied in the present work are intended for use in environmental surface waters, where external pressure is not applicable. Therefore, in our work, we focused on the adsorption of components present in aqueous solutions, when the solutes can diffuse into the pores or microchannels of a PDMS sponge. Obviously, the moving/stirring (e.g., due to lake waves) of the liquid/sponge can effectively assist the solutes in reaching the inner parts of a sponge.

For the study of octinoxate adsorption on PDMS, a small piece of PDMS sponge was placed in a spectrometer’s cuvette filled with octinoxate solution. The cuvette was gently vortexed and the absorbance of the solution was monitored. The adsorption experiments were carried out using PDMS sponges made with templates of sugar cubes, fine salt, coarse salt, or powdered sugar. The concentration of the octinoxate decreased exponentially over time when the PDMS sponge was vortexed in the aqueous solution (Figure 2a,c). The sugar cube and the coarse (regular) salt had similar particle sizes (around 0.4 mm); therefore, they show very similar concentration curves. The PDMS sponge made using fine salt consisted of smaller pores (larger specific surface) and showed slightly faster reduction in the octinoxate concentration compared to the other sponges, but after 30 min, only a negligible difference was found in the rates of octinoxate removal. This could be explained by the fact that, although the smaller pores provide a larger specific surface for a higher adsorption capacity, the aqueous solution (of octinoxate) could penetrate the deeper region of the sponge with more difficulty. In these experiments, 30 min was enough to almost completely (more than 95%) remove the octinoxate from the water (0.1 g PDMS sponge in 3.5 mL of liquid). The PDMS sponge made with the powdered sugar template contained such small pores (or the fine particles were surrounded by the PDMS), that they could not be removed from the reticulated PDMS and no pores were formed. This PDMS sponge could adsorb octinoxate only on the outer surface, not in the inner body of the sponge. Based on these results, the PDMS sponges made with the fine salt template were mostly applied for the subsequent investigations. Because the surface area of the cuvette (together with the cuvette cap) is relatively large compared to its volume, the adsorption of the cuvette’s wall had to be taken into consideration for the PDMS sponges’ adsorption studies. In our experiments, both glass and plastic cuvettes were used. The octinoxate adsorbed more strongly onto the more hydrophobic plastic (cyclic olefin polymer). After 30 min of vortexing the solution without the PDMS sponge, 12% or 22% of the octinoxate was adsorbed onto the glass or plastic, respectively (Figure 2b).

Obviously, the removal of a larger amount of octinoxate required a longer time. Even in the presence of a high amount of octinoxate (an oily film appeared on the surface of the unstirred liquid), the PDMS sponge was able to remove 90% of the component within 1 h (Figure 2d). In a similar experiment, PDMS sponges with the same size (0.1 g) were vortexed in different volumes of aqueous solutions (17.5, 8.5, and 250 mL). Moreover 85% of the octinoxate content was adsorbed by a sponge from a liquid volume that was 2500 larger than the volume of the sponge over a period of 20 h (Figure 2e).

Clean lakes generally contain almost exclusively water-soluble, mainly inorganic components (salts, nitrate, phosphate, and alkali and alkaline earth metal ions), which do not tend to adsorb onto the hydrophobic surface of the PDMS sponge (they do not compete for the adsorption sites). However, a recreational surface water body like a lake can potentially also include (partly) hydrophobic contaminants caused by natural or human activities. These (partly) hydrophobic components can compete for the adsorption sites on the PDMS sponges and therefore influence the adsorption efficiency of octinoxate. In our experiment, octinoxate was spiked into a recreational surface water (Tisza Lake, Tiszafüred, Hungary) in the same concentration that was applied in our adsorption experiments with model solutions, and the adsorption under these realistic conditions was measured (Figure S2). We found that the decreases in the absorbances (efficiency of the octinoxate removal) were similar, and that only a slightly (around 5%) smaller removal efficiency was obtained in the real sample. However, the amount of (partly) hydrophobic components in recreational waters can vary significantly (even by orders of magnitude) depending on human population, proximity to industrial environments, the number of swimmers, and seasonal conditions, etc.; therefore, a more detailed study on this matter is warranted.

3.2. The Adsorption Capacity and Recyclability of the PDMS Sponge

Numerous parameters can be found in the literature regarding the absorption capacity of PDMS sponges (e.g., the PDMS sponge can absorb oils even up to 23 times its weight) [11,12]. In this case, the highly hydrophobic liquid is retained within the inner pores and channels of the sponge. When small amounts of apolar components are either dissolved or dispersed in a microemulsion within an aqueous liquid, the adsorption onto the PDMS surface is likely the dominant mechanism over the absorption of the aqueous phase containing the apolar components.

A higher concentration of octinoxate solution was vortexed for 24 h to determine the total amount of octinoxate that could be adsorbed onto the PDMS sponges prepared with different templates (Figure 3a). As expected based on the previous experiments, the sponges prepared with sugar cubes or coarse salt showed a very similar adsorption capacity, while the sponge prepared using powdered sugar exhibited a 5 times smaller capacity. The slightly lower adsorption capacity of the sponge prepared with the fine salt template could be explained by its reduced ability to allow aqueous solutions to penetrate. The PDMS sponges prepared using sugar cube or coarse salt templates, which exhibited the highest adsorption capacity, were able to adsorb octinoxate up to 0.6% of their own mass. To visually demonstrate the outstanding liquid permeability and adsorption properties of the PDMS sponge, it was immersed in a solution of food dyes (mixture of E102 and E131), which have good water solubility. After 20 min of vortexing, the 7 mm-sided cube was completely saturated with the dye (Figure 3b,c and Figure S3). Similar behavior was seen for PDMS sponges prepared using coarse salt or fine salt as templates (Figure S4). Since these hydrophilic compounds have minimal adsorption onto the PDMS surface, they can be easily rinsed away with water. Hydrophobic compounds, such as Sudan III, adsorb so strongly onto the PDMS that they cannot be removed even by intensive rinsing with water or MeOH, but require hexane for removal (Figure S5).

The reusability of the PDMS sponge was assessed in the following experiment: a sponge was first used to remove octinoxate from water (10 min vortex), then it was squeezed and thoroughly rinsed with MeOH. These steps were repeated nine times (Figure 4), and after achieving approximately 75% octinoxate removal efficiency, the PDMS sponges could be fully regenerated and completely cleared from octinoxate. The amount of octinoxate in the original sample solution was consistent with the sum of the octinoxate remaining in the sample solution and rinsed out from the sponge with MeOH. Throughout the nine repetitions of the procedure, no changes in the adsorption properties of the sponge or in the weight of dry sponge were observed. Several works can be found in the literature that demonstrate the excellent recyclability of PDMS sponges used as absorbents [3].

4. Conclusions

In this work, we demonstrated that a PDMS sponge, which has recently been widely applied for the removal (absorption) of oils and apolar solvents from aqueous media, can also be efficiently utilized for the removal (mainly through adsorption) of compounds with low water solubility, like octinoxate, a UV filter compound. The fabrication procedure of the PDMS sponge is simple, and it does not need intricate processes or instrumentation. Various templates were tested, and those with 0.4–0.5 mm particle sizes (i.e., regular sugar and salt) were found to produce the optimal, highly interconnected pore structure in the PDMS sponge. Larger pores resulted in a lower surface-to-volume ratio, while pores smaller than ~0.1 mm hindered the penetration of the aqueous octinoxate solution into the deeper regions of the sponge. The adsorption capacity of the PDMS sponge did not change after repeated cycles, demonstrating the excellent reusability of the adsorbent.

Octinoxate is one of the most widely used organic UV filters and belongs to the cinnamate family. While other cinnamate derivatives—such as cinoxate, isoamyl p-methoxycinnamate, and methyl cinnamate—exist, they are less commonly used. These compounds share similar chemical structures and exhibit very low water solubility, suggesting that, like octinoxate, they likely show strong adsorption affinity toward PDMS-based materials. Other organic UV filters, including members of the salicylates, triazines, and dibenzoylmethane classes, are also mainly non-polar and can be expected to behave similarly in their interactions with PDMS. In contrast, some UV filters such as camphor derivatives and p-aminobenzoic acid are water-soluble due to their polar functional groups. These compounds contain only a limited non-polar region (typically an aromatic ring), so high adsorption efficiency on PDMS is not expected.

It should be noted that the composition and concentration of non-polar or slightly polar compounds, as well as organic matters in natural waters, can vary significantly. These variations may influence the adsorption efficiency of UV filters, as other organic components could potentially occupy the available adsorption sites on the sponge. Future research should therefore include field applications in natural waters over extended periods (days or even weeks) to better understand these effects. The PDMS sponge remains afloat on the surface waters, allowing for the efficient adsorption of the non-water-soluble UV filter compounds that accumulate there. This makes the PDMS sponge a promising passive tool for in situ remediation, especially in environmental surface waters or recreational water bodies where sunscreen-derived lipophilic pollutants such as octinoxate tend to accumulate. Moreover, the simple handling and floating property of the material may offer an environmentally friendly and low-maintenance alternative to conventional filtration systems.