Design of Hydrophobic Hybrid Ceramic Coatings Based on Silica Modified with Polydimethylsiloxane (SiO₂/DMS) for Sustainable Oil Removal

Abstract

Oily substances (oils, greases, lubricants, etc.) are among the most persistent pollutants for water. They mix with water to form emulsions that contaminate large volumes. Therefore, this project evaluated the use of porous systems (polyurethane foam) modified with polydimethylsiloxane-modified silica (SiO₂/DMS) hybrid ceramics as filtration membranes at the laboratory scale for vegetable oil. The polyurethane foam was modified using sol solutions with various SiO₂/PDMS ratios obtained via the sol–gel method. Tetraethyl-orthosilicate (TEOS) was used as the silica precursor. Three different polydimethylsiloxane chains were employed as the organic fragment: polydimethylsiloxane hydroxyl terminated (DMS-CH₃), aminopropyl-terminated polydimethylsiloxane (DMS-N), and copolymer polydiphenylsiloxane-polydimethylsiloxane hydroxyl terminated (PDS). The siloxane chain was added at a concentration of 20–40% w/w. The modification of the porous system was determined using different characterization techniques, including infrared spectroscopy, which was used to observe the main functional groups. Optical microscopy and SEM were used to identify the hybrid ceramic deposited into the pore structure of the polyurethane sponge. Contact angle measurements revealed the hydrophobic character of the modified material. The removal capacity was evaluated by using vegetable oil as a representative oily contaminant, with values ranging from 43.42 to 96.78 g of oil per gram of adsorbent. In the case of gasoline, removal capacities between 27 and 54 g were observed. This study demonstrated the influence of hydrophobicity on vegetable oil removal, confirming that higher hydrophobicity leads to greater adsorption capacity. Nevertheless, the use of a viscous contaminant introduced challenges in the extraction process from the PS/SiO₂-DMS system. Despite this limitation, the material maintained adequate removal performance for up to five reuse cycles. On the other hand, the removal capacity depends on the amount of polysiloxane chain in the ceramic, as well as the functional group, exhibiting the following behavior: DMS-N < DMS-CH₃ < PDS. This study demonstrates that hydrophobicity is a key property for enhancing the removal capacity of oily substances. Moreover, the control of intermolecular interactions further strengthens this effect, as evidenced in the PS/SiO₂–PDS system.

1. Introduction

Water is a polar molecule that can create hydrogen bonds with other molecules; due to this property, water is considered the “universal solvent,” as it can dilute a wide variety of substances. This property enables the entry of contaminants, particularly chemical-based ones, into water resources through spills or runoff, thereby significantly altering their quality [1,2,3,4,5]. Water quality is defined as the degree of modification in its physical and chemical properties. According to the National Water Quality Measurement Network in Mexico (RENAMECA-Mexico), the classification of water quality is determined using a traffic light system: red for poor quality water, yellow for moderate quality, and green for acceptable quality. In Mexico, between 2012 and 2024, 48.6% of water bodies were reported as red, and 19.5% were reported as yellow, while only 31.9% were reported as green [6].

Water quality statistics indicate that a significant proportion of water bodies are classified as poor quality, limiting their suitability for human consumption or agro-industrial applications. Moreover, in recent years, a long-standing drought has led to a substantial decrease in the availability of this resource, emphasizing the need to implement and optimize wastewater treatment processes stemming from industrial, agricultural, and domestic activities [7,8,9].

Water can contain a variety of contaminants, including oily substances, which have the potential to emulsify even in substantial volumes of water, making them challenging for wastewater treatment. For example, 1 milliliter of oil has the capacity to contaminate up to 1 L of water [10,11,12,13]. The fundamental stage of treatment entails the separation of suspended solids. Subsequently, the removal of oily substances involves the addition of coagulants or flocculants, resulting in the use of high doses of chemical compounds, hence increasing the cost of the process [14,15,16,17].

In this context, various authors have proposed adsorption systems for the separation of oily substances, among which the use of hydrophobic silica stands out [18,19,20,21,22]. As shown in

Table 1. Environmental applications of hydrophobic silica.

Substance Pollution	Silica-Deposited Matrix	Adsorbate	Reference
Organic solvents, oil	–CF3 Functionalized	oils	[23]
	Reduced graphene oxide	Fats and oils	[24]
	Cellulose–Silica Hybrid	Fats and oils	[25]

, there are illustrative examples of functional groups attached to the silica surface, including fluorinated groups (–CF₃), graphene, cellulose, among others [23,24,25].

The use of hydrophobic silicas as adsorbents for oily contaminants has been previously investigated; however, most of these studies employ flocculation methods, in which the contaminant is trapped in a volume of hydrophobic silica that must subsequently be removed from the aqueous system by decantation or filtration. Moreover, membrane-based filtration systems modified to achieve high surface hydrophobicity have been reported. Examples include the modification of polyurethane foams, filter paper, and cotton with hydrophobic silicas, which demonstrated good removal capacities for organic solvents such as chloroform, kerosene, n-hexane, toluene, and acetone, as well as for peanut oil [20,25]. These substrates are often functionalized with complex systems, such as hydrophobic silicas containing fluorinated groups [21]. Although fluorine is recognized as an element that imparts high hydrophobicity [26,27], its use raises concerns regarding toxicity and environmental impact [28].

Kyoung et al. [29] reported the use of hydrophobic silica modified with polydimethylsiloxane (PDMS), obtaining superhydrophobic silicas with a contact angle of approximately 163.55°. Through flocculation, these silicas achieved a pump oil removal capacity of 14.28 g/g. Building on this background, the aim of the present work is to investigate the effect of integrating various hybrid ceramics into polyurethane sponges with variations in organic fragments and functional groups of PDMS (SiO₂/DMS–CH₃, SiO₂/DMS–N, and SiO₂/PDMS), determining the effect of hydrophobicity on the ability to remove vegetable oil, as well as the number of reuse cycles that can be employed.

2. Materials and Methods

2.1. Synthesis of SiO₂/DMS Sols

The sol–gel solutions were prepared following the methodology reported by Salazar-Hernández et al. [30]. The polymerization of TEOS (99%, Sigma Aldrich North America, México) was carried out by co-condensation, using different amounts of PDMS as specified in

Table 2. TEOS/functionalized PDMS ratios for porous system modification.

	TEOS (g)	DMS-12 (g)	DMS-A11 (g)	PDS (g)
SiO2/DMS–CH3–1	10	1
SiO2/DMS–CH3–2	10	2
SiO2/DMS–CH3–4	10	4
SiO2/DMS–N–1	10		1
SiO2/DMS–N–2	10		2
SiO2/DMS–N–4	10		4
SiO2/DMS–PDS–1	10			1
SiO2/DMS–PDS–2	10			2
SiO2/DMS–PDS–4	10			4

. The functionalized PDMS used in the study (Figure 1a) was hydroxyl-terminated polydimethylsiloxane (DMS-12; 16–32 cSt, Gelest, Morrisville, PA, USA), aminopropyl-terminated polydimethylsiloxane (DMS-A11; 10–15 cSt, Gelest, Morrisville, PA, USA), and hydroxyl-terminated diphenylsiloxane-dimethylsiloxane copolymer (PDS-1615; 50–60 cSt; 14–18% diphenylsiloxane, Gelest, Morrisville, PA, USA). To obtain the solution, 10 g of TEOS was mixed with PDMS, and 0.2 g of dibutyltin dilaurate (DBTL; 99%, Sigma Aldrich North America, Mexico) was added. DBTL acted as a polycondensation catalyst [31,32] as shown in Scheme 1. The mixture was kept under constant magnetic stirring for 30 min at 40 °C. Then, it was applied to the porous system (polyurethane sponge) by simple immersion.

2.2. Impregnation of SiO₂/PDMS into a Porous System

Sol–gel solutions were impregnated into commercial polyurethane sponges that previously had been cut to 50 mm × 30 mm × 20 mm dimensions. Prior to impregnation, the sponges were washed twice with distilled water for five minutes in an ultrasonic bath, then dried at 70 °C for 24 h. The hybrid ceramic was incorporated through simple immersion, maintaining contact between the sponge and the sol solution for three minutes. Finally, the sponges were removed and left to dry upright in a temperature-controlled oven at 70 °C for 24 h.

2.3. Characterization of Chemical Structure and Hydrophobicity

The chemical structure of the polyurethane sponge and the SiO₂–PDMS hybrid material was analyzed by ATR-FTIR using a Nicolet iS10 Thermo Scientific spectrometer (Thermo Scientific, from México). Spectra were recorded as the average of 16 scans, with a resolution of 4 cm⁻¹, over the spectral range of 4000–600 cm⁻¹. The hydrophobic properties of the modified sponge were evaluated by measuring the change in water-accessible porosity (%P_H2O), following the ASTM C20-00 standard (Standard Test Methods for Apparent Porosity, Water Absorption, Apparent Specific Gravity, and Bulk Density of Burned Refractory Brick and Shapes by Boiling Water; 2015) [33]. The dry weight (D) was obtained by drying the material, with and without ceramic modification, at 100 °C for three hours. The saturation weight (W) was determined by immersing the material in distilled water for one minute.

$${\%\mathrm{P}}_{\mathrm{H}2\mathrm{O}}={\displaystyle \frac{\mathrm{W}-\mathrm{D}}{\mathrm{D}}}\times 100$$

(1)

The hydrophobicity of the sponge and the modified sponge was evaluated by measuring the water contact angle using a 1 μL drop of distilled water. The images were captured using a conventional smartphone camera, and the contact angle was determined using the open-source software IC-Measure 2.00.245, with the horizontal baseline serving as the reference. The structural modifications in the sponge resulting from the deposition of SiO₂/PDMS were examined using scanning electron microscopy (SEM), employing a JEOL JSM-6510 Plus instrument (JEOSL; from México). Samples were mounted on graphite tape prior to observation.

2.4. Evaluation of Oil Removal Capacity

The removal experiments were carried out using commercial vegetable oil or gasoline. Suspensions containing different amounts of the oily contaminant (0.5–6 mL) were prepared in a beaker with 100 mL of distilled water. The suspensions were subsequently passed through the modified sponge using a vacuum-assisted filtration system. Following the removal process, the quantity of non-removed oil was determined by decantation. To enhance the visual contrast between vegetable oil and water, a commercial dye was added.

The quantity of oil removed by the sponge was measured gravimetrically, as specified in Equation (2). In this equation, W₁ represents the weight of the oil removed, which was measured for the sponges after the removal process and subsequent drying at 100 °C for 24 h to eliminate physisorbed water. W_PS-modified corresponds to the weight of the sponge modified with ceramics prior to the removal test.

$$Q={\displaystyle \frac{{(W}_1-W_{PS-modified})}{W_{PS-modified}}}$$

(2)

2.5. Reusability Assessment of the Modified Sponge

The reuse cycles of the sponge modified with ceramics were evaluated using solutions containing 5 mL/L of vegetable oil. After the oil removal process, the sponge was immersed in hexane for one hour under orbital agitation (CVP-2000P) to perform a liquid–liquid extraction of the remaining oil. Thereafter, the solvent was removed, and the sponge was dried for two hours to eliminate any residual solvent before being reused in another removal cycle.

3. Results

3.1. Functionalization of Polyurethane Sponge

Figure 1a compares the infrared spectrum of the polyurethane sponge (P.S) unmodified and modified with ceramic SiO₂/DMS-CH₃. Where the main functional groups for P.S were identified: N–H at 3290 cm⁻¹ (ν); =C–H at 3080 cm⁻¹ (ν), observed as a very weak band; –CH₂ from the polymeric chain at 2869–2914 cm⁻¹ (ν); C=O at 1730 cm⁻¹ (ν); C=C (aromatic ring) at 1530 cm⁻¹ (ν); –C–O– at 1130 cm⁻¹ (ν); and –C–N at 1060 cm⁻¹ [34]. The sponge modified with the SiO₂/DMS–CH₃ hybrid ceramic demonstrates signals ranging from 10 to 40% w. The red circle highlights the signals corresponding to the polyurethane foam (1760–1282 cm⁻¹). The amino group in P.S decreases with increasing DMS–CH₃ content in the ceramic and disappears at a concentration of 40% w/w DMS-15 (SiO₂/DMS–CH₃-4) due to overlapping with the ceramic signal. Methyl groups (–CH₂) appeared with three moderate-intensity signals at 2962 cm⁻¹, 2954 cm⁻¹, and 2887 cm⁻¹ for all modifier concentrations. For this ceramic, DMS–CH₃ was identified by the Si–C group signal at 1261 cm⁻¹, a sharp band whose intensity increased with higher DMS–CH₃ content, and by the intense, sharp band at 794 cm⁻¹ corresponding to Si–O–Si in the polymeric chain. The inorganic fragment of SiO₂ was identified by the broad, intense bands at 1008 cm⁻¹ and 842 cm⁻¹. Furthermore, the signals corresponding to P.S at 1760–1282 cm⁻¹ decrease significantly in intensity at a 10% concentration of DMS-15 and nearly disappear at 40%.

Figure 1b shows the polyurethane foam modified with the SiO₂/PDS ceramic, where the added polysiloxane chain contains phenyl and methyl functional groups. For this ceramic, no significant spectral changes were observed with increasing concentrations of the organic modifier. In the PS/SiO₂-PDS system (dimethylpolysiloxane and diphenylpolysiloxane co-polymer), physisorbed water was detected at 3300 cm⁻¹ across all concentrations, with its intensity diminishing as the polysiloxane chain length increased. The –CH₃ groups were identified as a triplet at 2960–2870 cm⁻¹, while the –C–H groups of the aromatic ring appeared as a quadruplet at 3070–3050 cm⁻¹ (highlighted with a red line). The linear PDS chain indicated the presence of Si (D) at 1260 cm⁻¹, 843 cm⁻¹, and 791 cm⁻¹. The Si–O–Si bond of the silica network was observed at 1010 cm⁻¹. Furthermore, the presence of Si attached to the phenyl group resulted in intense signals at 762 cm⁻¹, 717 cm⁻¹, and 706 cm⁻¹ [35,36]. The P.S signal at 1760–1282 cm⁻¹ and the band at 3227 cm⁻¹, corresponding to the –NH group of the sponge, both decrease significantly in intensity for the ceramic containing 10% w/w PDS (PS/SiO₂–PDS-1) and nearly disappear at 40% (PS/SiO₂–PDS-4).

Figure 1c shows the spectrum of the polyurethane foam modified with the SiO₂/DMS-N ceramic, where the characteristic signals of the polyurethane sponge (PS) were observed between 1730 and 1390 cm⁻¹ (highlighted in the red circle). The N–H band at 3227 cm⁻¹ shows a decrease in intensity with the SiO₂/DMS–N-1 coating but increases at 40% due to the amino groups present in the structure of the hybrid ceramic (see Figure S1, chemical structure of SiO₂/DMS-functionalized hybrid ceramics). The –CH₂ groups appeared as a triplet at 2960, 2930, and 2850 cm⁻¹ (ν). The Silica D (Si–C) was identified at 1260 cm⁻¹, while the siloxane group (Si–O–Si) of the polymeric chain was observed at 788 cm⁻¹. The inorganic silica fragment was detected at 1010 and 843 cm⁻¹ (Si–O–Si). Additionally, the C–N bond linked to the polypropylene chain was observed at 1210 cm⁻¹. It is important to note that, for this coating, the PS bands between 1760 and 1282 cm⁻¹ did not decrease in the same manner as observed for the other two coatings, indicating a lower degree of coverage.

The results show that the P.S modified with SiO₂/DMS–CH₃ and SiO₂/PDS ceramics primarily exhibit the characteristic signals of the ceramic. The vibrational modes of the polyurethane sponge (PS), highlighted by the red circle, appear with very low intensity or are nearly absent. This behavior indicates a high degree of modification, which directly enhances the hydrophobic character of P.S and its capacity to remove oily substances.

The contact angle was measured to determine the degree of hydrophobicity generated in the foam by the deposited hybrid ceramic. Figure 2a shows the behavior for the three types of modifiers, where superhydrophobic characteristics were observed with contact angles ranging from 136° to 157° [37,38]. A linear increase in the contact angle (θ) was observed as the proportion of siloxane chains in the ceramic structures increased. In contrast, the unmodified sponge exhibited hydrophilic behavior, with a contact angle (θ) of 49°. As shown in Figure 2b, the droplet shape reveals superhydrophobic behavior. Thus, it is expected that enhanced hydrophobicity will increase the removal capacity of contaminants such as oil, gasoline, and diesel.

Table 3. Water-accessible porosity (%) of polyurethane foam with and without modification by hybrid ceramic.

	PH2O (Percentage)
Polyurethane Sponge	1810 ± 120
SiO2/DMS-CH3–1	45 ± 4
SiO2/DMS-CH3–2	34 ± 7
SiO2/DMS-CH3–4	17 ± 3
SiO2/DMS-N–1	49 ± 4
SiO2/DMS-N–2	37 ± 3
SiO2/DMS-N–4	22 ± 4
SiO2/DMS-PDS–1	40 ±3
SiO2/DMS-PDS–2	30 ±3
SiO2/DMS-PDS–4	14 ± 4

shows the percentage of water-accessible porosity, calculated according to Equation (1). The polyurethane foam showed a high-water adsorption capacity (1810%), which abruptly decreased after the deposition of the ceramic materials, reaching values between 40 and 50 the proportion of organic modifier in the ceramic structure increased, the water adsorption capacity continued to decrease. Furthermore, the functional group present in the DMS chain (organic modifier) also influenced the water adsorption capacity, following the trend described below.

SiO₂/PDS < SiO₂/DMS-CH₃ < SiO₂/DMS-N

Figure 3 shows the SEM micrographs of the unmodified polyurethane foam and the foam modified with the SiO₂/DMS–CH₃ ceramic, highlighting the surface obtained from frontal and lateral cuts. The polyurethane sponge (P:S) exhibited closed pores of approximately 500 μm (indicated by the red arrow) and open pores (blue arrow). The coating layer deposited was similar across all formulations. These coatings were applied under comparable conditions using sol solutions with a viscosity of ~11 cSt, resulting in similar deposition across all cases. The ceramic was deposited onto the sponge fibers, with partial pore filling highlighted by the yellow arrow. The degree of modification is directly associated with the efficiency of oily substance removal; higher modification levels correspond to greater removal capacity.

Figure 4 show the polyurethane sponge modified with the SiO₂/DMS–CH₃ ceramic at 10% and 40% organic modifier content, respectively. Similar surface finishes were observed in both frontal and lateral cuts. The ceramic adhered to the sponge structure, blocking the closed pores. As the proportion of organic modifiers increased, the number of modified pores also increased, according to the SEM results. However, no differences were observed between the functional groups (Figure S2 shows the modified sponges analyzed by optical microscopy). This is attributed to the similar mass of ceramic deposited, as confirmed in Figure 4.

3.2. Performance Assessment in Contaminant Removal

The removal tests were carried out using commercial vegetable oil or gasoline. For these contaminants, the removal occurred instantaneously, as shown in Figure 5, which shows the removal of 10 mL/L of vegetable oil with the PS/SiO₂–DMS–CH₃–1 sponge. In this example, the modified sponge removed the total amount of oil from the medium within 30 s, as demonstrated in Supplementary Video S1, showing an instantaneous removal process, which represents one of their key advantages.

Figure 6a shows the removal capacity, expressed as a percentage, for vegetable oil. Quantitative removals (>90%) were obtained at low concentrations (5 and 10 mL/L) for all hybrid ceramic formulations. However, at higher concentrations (30 and 60 mL/L), the removal capacity varies depending on the organic modifier content and the functional group. For the methyl (PS/SiO₂–DMS–CH₃) and phenyl (PS/SiO₂–PDS) functional groups, quantitative removal was achieved at 30 mL/L; however, it decreased to 70–80% at 60 mL/L for PS/SiO₂–CH₃ and to 80–85% for PS/SiO₂–PDS. In contrast, the amino group (PS/SiO₂-DMS-NH₂) showed a lower removal capacity, with 80–85% removal at 30 mL/L and 70–80% at 60 mL/L, respectively.

Figure 6b shows the removal capacity for gasoline, which exhibits behavior like that observed for vegetable oil. Quantitative removal (>90%) was achieved in the PS/SiO₂–DMS–CH₃ and PS/SiO₂–PDS systems at concentrations between 5 and 30 mL/L. At 60 mL/L, the removal percentage remained between 90% and 98% for the PS/SiO₂–DMS–CH₃ system; however, for the PS/SiO₂–PDS system, it decreased to 85–90%. In contrast, the PS/SiO₂–N system showed quantitative removal only at 5–10 mL/L, while at higher concentrations the removal percentage dropped to 50–70%.

Overall, higher vegetable oil removal was observed with increasing organic modifier content (%DMS) in the ceramic structure, and the following behavior was observed for both pollutants (vegetable oil and gasoline) in relation to the functional groups:

SiO₂/DMS-N < SiO₂/DMS-CH₃ < SiO₂/PDS

As illustrated in

Table 4. Removal capacity of polyurethane sponge (PS) modified with hybrid ceramics for oil pollution.

	Vegetal Oil		Gasoline
	Q (g/g PS-Modified)	Increase	Q (g/g PS-Modified)	Increase
Polyurethane Sponge	11.43 ± 1.24	–	11.5 ± 2.3	–
P.S/SiO2–DMS–CH3–1	46.59 ± 2.54	4.07	49.46 ± 3.43	4.30
P.S/SiO2–DMS–CH3–2	51.81 ± 3.26	4.53	52.21 ± 2.62	4.54
P.S/SiO2–DMS–CH3–4	59.74 ± 3.72	5.23	53.86 ± 3.73	4.68
P.S/SiO2–DMS–N–1	43.42 ± 3.23	3.80	27.48 ± 4.8	2.39
P.S/SiO2–DMS–N–2	45.70 ± 5.40	3.99	30.23 ± 3.88	2.63
P.S/SiO2–DMS–N–4	49.62 ± 4.78	4.34	32.98 ± 2.78	2.86
P.S/SiO2–PDS–1	91.40 ± 3.67	8.00	46.72 ± 3.15	4.06
P.S/SiO2–PDS–2	96.78 ± 2.74	8.47	47.81 ± 3.46	4.16
P.S/SiO2–PDS–4	96.78 ± 3.53	8.47	49.46 ± 3.26	4.30

, the adsorption load values (Q) have been calculated in accordance with Equation (2) for vegetal oil and gasoline. Here, W₁ represents the weight of the oil removed, determined for the sponges after the removal process and subsequently dried at 100 °C for 24 h to eliminate physisorbed water. W_PS-modified corresponds to the weight of the sponge modified with hybrid ceramics prior to the removal test.

The unmodified sponge exhibited oil removal capacity due to its high-water absorption (1810%). The oil was extracted via the physisorbed water within the porous structure; however, during the filtration process, the oily contaminant was not retained in the material, as shown in Figure 7a. In contrast, the modification of the sponge with hybrid ceramics enabled adsorption, effectively retaining the contaminant, as illustrated in Figure 7b.

The removal capacity of vegetable oil is a function of the hydrophobicity generated in the hybrid ceramic, as shown in Figure 8. The PS/SiO₂–DMS–N coating, which resulted in the lowest contact angle (θ = 137–140°), showed the lowest adsorption capacity (Q = 43.42–49.62 g vegetable oil/g PS-modified). In contrast, PS/SiO₂-PDS, with the highest contact angle (θ = 150–157°), showed the highest adsorption capacity (Q = 91.4–96.78 g vegetable oil/g PS-modified). The SiO₂–DMS–CH₃, showing an intermediate contact angle (θ = 146–150°), and a removal capacity of 46.59–59.74 g vegetable oil/g PS-modified.

Vegetable oil is composed of a glycerol molecule bound to three fatty acid chains, which may be saturated or unsaturated and are of natural origin [39]. The aliphatic chain demonstrates a propensity to interact with the hydrophobic groups present on the silica surface, thereby facilitating its removal from the aqueous medium. Therefore, an increase in hydrophobic character corresponds to a higher oil removal capacity (Figure 8).

Table 5. Oil removal capacity of SiO₂/DMS-functionalized ceramics compared with prior studies.

Removal System	Oil Removal Capacity (g/g)	Organic Solvent Capacity (g/g)	Reference
SiO2 nanoparticles coated with PDMS	14.28	–	[29]
P.S modified with graphite and PDS	22–30	40–50	[20]
P.S modified Hydrophobic silica (TMHFS-Silica) Flour as hydrophobic group	16–43	–	[21]
Cellulose modified with hydrophobic silica; methyltriemtoxysilane as hydrophobic group	24.8	–	[25]
PS modified poly(dopamine) hydrophobic silica formed from TEOS and TMMS	50	40–51	[40]
SiO2/DMS-CH3-1	46.59	–	Present study
SiO2/DMS-CH3-4	59.74	–
SiO2/DMS-N-1	43.42	–
SiO2/DMS-N-4	45.7	–
SiO2/PDS-1	91.40	–
SiO2/PDS-4	96.78	–

compares the removal capacity of the ceramics studied in this work with that of similar systems previously reported [18,19,20,21,22,23,24,25,26,27,28,29]. The removal capacity for oil is approximately 20 g/g; however, it increases substantially for organic solvents, reaching up to 50 g/g [40]. According to the results, both the presence and the amount of functional groups and polysiloxane chains in the hybrid ceramic influence the removal capacity. A system like that previously reported (SiO₂/PDMS–CH₃-1) shows a removal capacity of 46 g/g, consistent with earlier findings. In contrast, incorporating up to 40% polysiloxane chain into the ceramic increases the removal capacity to 59.74 g/g, representing a 29.86% improvement.

In general, a higher amount of organic modifier (polysiloxane chain) leads to greater removal capacity. In contrast, the PDS group showed a substantial enhancement in this capacity, attributable to the hydrophobicity generated by the presence of methyl and phenyl groups. In this system, a removal capacity of up to 96.78 g/g was achieved.

Although PDS forms the most hydrophobic systems (θ ~ 157°), its increased removal capacity is also influenced by intermolecular interactions, such as van der Waals forces, as reported by Panda (2022) [26]. Vegetable oil is composed of unsaturated triglycerides, and Scheme 2 illustrates these interactions: the hydrophobic methyl groups (SiO₂/DMS–CH₃) interact with the aliphatic chains. In the case of SiO₂/PDS (methyl and phenyl groups), additional interactions occur through the phenyl group, involving the electron cloud of the phenyl ring and the carbon of the vegetable oil acid group. A similar effect would be expected for SiO₂/DMS–N, where the amino group could protonate and interact with the oxygen of the acid group.

Despite the comparable weight gain of the coating deposited on the sponge relative to the other two ceramics (Figure 4), infrared spectroscopy revealed a lower coating capacity. The P.S bands (1730–1390 cm⁻¹) decreased by a smaller percentage compared to the other coatings. Therefore, this lower coating capacity results in removal efficiencies below the expected values.

Regarding gasoline removal, the hydrophobic nature of the modified sponge provides a significant advantage. Gasoline consists of a mixture of linear and branched aliphatic chains containing 5–12 carbons, which facilitate interactions with the SiO₂/DMS–CH₃ group. The presence of this functional group alone reveals hydrophobicity to the surface, accounting for the higher removal capacity observed in this system.

3.3. Assessment of Contaminant Removal Cycles

Finally, the potential for reusing the modified sponge was assessed by extracting the adsorbed oil using three different solvents: kerosene, tetrahydrofuran (THF), and hexane. The extraction process was carried out using a 20 mL suspension of vegetable oil in distilled water (20 mL/L), to which 4 mL of solvent was added. The mixture was stirred for one hour, after which the organic phase was separated. The amount of oil extracted was quantified (see

Table 6. Comparison of vegetable oil removal capacity by various solvents (2 mL vegetable oil in 100 mL distilled water).

	mL Recovered Oil	%Recovery
Kerosene	0.42 ± 0.01	21
THF	0.85 ± 0.04	42.5
Hexane	0.92 ± 0.05	46

). Although hexane showed the highest extraction capacity, complete recovery was not achieved. This indicates that not all the oil removed from the sponge could be recovered through liquid–liquid extraction. The results indicated a higher extraction capacity for hexane, which was therefore selected as the solvent for oil removal from the sponges modified with hybrid ceramics.

As illustrated in Figure 9a, the removal capacity demonstrated stability over three consecutive cycles. The SiO₂/DMS-CH₃ and SiO₂/PDS ceramics demonstrated higher removal capacity compared to the unmodified sponge during the cycles. In contrast, SiO₂/DMS-N demonstrated higher removal capacity than the unmodified sponge up to the second cycle, but decreased in the third cycle, reaching 40–50% of the Q value of the unmodified sponge. It is important to note that the unmodified sponge can only be used for a single removal cycle.

The decrease in removal capacity with each cycle is attributed to a residual layer of vegetable oil that cannot be fully extracted with hexane. Nevertheless, the SiO₂/DMS–CH₃ and SiO₂/PDS systems maintain an acceptable removal capacity of 18 g/g up to cycle 3, an intermediate value compared to similar systems reported in the literature (14–50 g/g) [18,19,20,21,22,23,24,25,26,27,28,29]. Another factor influencing the extraction capacity of vegetable oil is its high viscosity (e.g., sunflower oil at 32.3 mPa·s), which promotes the formation of a surface layer (oiliness) and hinders extraction. In contrast, as shown in Figure 9b, gasoline extraction cycles exhibited no change in removal capacity up to cycle 3. This behavior is explained by the complete extraction of gasoline from P.S-modified sponges, which preserves removal capacity over successive cycles with this contaminant.

4. Conclusions

The addition of hybrid ceramics (SiO₂/DMS-CH₃, SiO₂/PDS, and SiO₂/DMS-NH₂) to the polyurethane sponge has enabled the development of hydrophobic surfaces with the capacity to retain oily contaminants. This advancement overcomes the limitations of the unmodified sponge, which showed only water absorption capacity and a single use cycle.

The results show that polyurethane sponges modified with hybrid ceramics represent a sustainable and efficient alternative for removing oily contaminants from water. These modified sponges have potential applications in wastewater treatment, oil spills, and industrial processes.

The vegetable oil removal capacity was directly linked to the contact angle and, therefore, to the hydrophobic nature of each coating. SiO₂/PDS yielded the highest Q values, while SiO₂/DMS-N yielded lower performance, confirming that greater hydrophobicity correlates with greater removal efficiency.

Hybrid coatings have demonstrated the ability to maintain removal capacities superior to those of the sponge without modification for up to three consecutive cycles. However, a progressive decrease in efficiency was observed, especially in the case of SiO2/DMS-N, suggesting the need to optimize coating stability for prolonged reuse applications.

Based on the results observed for oil removal capacity, the unmodified polyurethane sponge shows a removal capacity of 11.43 g oil/g sponge. This value increased by 3–8.4 times upon modification, reaching up to 96.78 g oil/g sponge for the PS/SiO₂–PDS-4 system. The adsorption capacity (Q) increased proportionally with the content of organic modifier in the ceramic structure. In addition, the functional group in the siloxane chain influenced the Q values, showing the following trend: PS/SiO₂–DMS–N < PS/SiO₂–DMS–CH₃ < PS/SiO₂–PDS.

The selection of the surface modification of the adsorbent must be based on the interactions between the functional groups of the modified surface and the adsorbate. For vegetable oil, the most effective modifier was SiO₂/PDS, where both methyl and phenyl hydrophobic groups coexist, resulting in enhanced adsorption performance.