Surface-Tuned Quartz Particles for Oil–Water Separation: SEM Characterization, Coating Effects, and Predictive Modelling

Abstract

Oily wastewater is a critical environmental concern, and the high costs and fouling of conventional membranes drive the search for low-cost, efficient alternatives. This study evaluates surface-modified quartz particles for oil–water separation, focusing on hydrophilic and hydrophobic coatings. Quartz samples underwent washing, hydrophobic coating, and hydrophilic coating, with morphological and elemental changes assessed using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS). Oil and grease (O&G) content was determined via the EPA 1664 method under high-solids conditions. The untreated oil–water mixture contained 142,955.9 mg/L O&G. Hydrophilic-coated quartz achieved the greatest reduction, producing water with only 751.3 mg/L O&G, indicating excellent oil rejection and water selectivity. Washed quartz performed similarly at 837.1 mg/L. Hydrophobic-coated quartz, while yielding higher residual oil in water (64,198.9 mg/L), demonstrated strong oil affinity, making it more suitable for oil recovery applications. Raw quartz, tested without heavy oil loading, showed a baseline of 13.4 mg/L. These results confirm that surface engineering of quartz enables tunable separation properties, where hydrophilic surfaces favor water purification and hydrophobic surfaces enhance oil capture. The findings provide a pathway for scalable, cost-effective, and application-specific oily wastewater treatment solutions.

1. Introduction

Oily wastewater has become a critical environmental challenge in the face of growing industrialization and water scarcity [1,2,3]. Large volumes of oil-laden effluents originate from petrochemical processing, metalworking, food manufacturing, and other sectors, often forming stable oil-in-water emulsions that are difficult to remediate [4,5,6]. If discharged untreated, such emulsions can devastate aquatic ecosystems, raise chemical oxygen demand, and hinder water reuse, underscoring an urgent need for efficient oil–water separation technologies [7,8,9]. Conventional treatment methods (gravity separators, skimmers, chemical coagulation) struggle to break these emulsions and achieve adequate purification at scale [10,11,12]. Membrane filtration is more effective for producing clean effluent, but standard polymeric membranes suffer severe fouling as oil droplets clog pores, causing flux decline and frequent cleaning requirements [13,14]. While ceramic membranes offer better fouling resistance and durability, they are costly and brittle, making large-scale deployment challenging [15,16,17]. This trade-off between fouling-prone polymers and expensive ceramics has prompted the search for alternative materials that combine high efficiency with stability and low cost [18,19,20].

One promising approach is to engineer the surface wettability of separation media to selectively interact with oil or water. Materials with superwetting surfaces—either superhydrophilic/underwater-oleophobic or/oleophilic—can achieve high oil–water selectivity without complex processes [21,22]. Although superhydrophilic/underwater-oleophobic and superhydrophobic/oleophilic surfaces have demonstrated excellent oil–water selectivity, their practical applications remain constrained by several disadvantages. These include high material and fabrication costs, complex nanostructuring requirements, susceptibility to fouling, and limited long-term durability under harsh operating environments. Recent studies have reported that maintaining extreme wettability states often requires expensive reagents or advanced fabrication techniques, which hinder scalability and increase lifecycle costs [23,24]. In contrast, the novelty of this study lies in demonstrating that low-cost, abundant quartz particles can be surface-engineered using simple hydrophilic and hydrophobic coatings to achieve tunable oil–water separation performance under extremely high oil-loading conditions.

Unlike conventional superwetting materials, quartz offers scalability, chemical robustness, and application-specific flexibility, enabling both water purification and oil recovery. For example, silica-based coatings that render a surface superhydrophilic allow water to permeate while repelling oil, and conversely, rough hydrophobic coatings (e.g., polystyrene–SiO₂ nanoparticle films) can absorb oils preferentially [25,26]. Such surface-engineered membranes and particles have shown excellent separation efficiencies and fouling resistance in recent studies [27,28]. In particular, inorganic and mineral materials are attracting attention due to their inherent robustness: clay-based “ceramic” membranes have demonstrated effective oily water filtration at low cost, silane-grafted alumina–clay composites exhibit superhydrophobicity for oil repulsion, and zeolite nanoparticle coatings on meshes produce durable superhydrophilic separators [29,30,31]. Even quartz, in ceramic membrane form, was recently reported to achieve high oil separation with improved antifouling performance [32,33,34]. These advances highlight the potential of mineral-based surfaces in overcoming limitations of conventional polymer membranes.

Quartz (crystalline SiO₂) is a particularly attractive candidate owing to its abundance, chemical stability, and low cost [35,36,37]. It is one of the most ubiquitous minerals on Earth and has long been used in water filtration (e.g., sand filters) for its mechanical robustness [38,39,40]. Clean quartz surfaces carry hydrophilic silanol (–SiOH) groups, which imbue them with strong water affinity and underwater oleophobicity [41,42]. This means that unmodified quartz tends to attract water and naturally repel oil in aqueous environments—a beneficial trait for separating oil from water. At the same time, quartz’s surface chemistry is easily tunable: by applying suitable coatings, one can switch its wettability. Prior studies have shown that coating quartz sand with hydrophilic agents (e.g., nanoscale oxides or polymer hydrogels) yields an underwater superoleophobic medium that strongly rejects oils [43,44]. Conversely, modifying quartz with low-surface-energy silanes or waxy substances produces a superhydrophobic, oleophilic surface that readily adsorbs oils [45,46]. This tunability of quartz—from water-loving to oil-loving—offers a compelling flexibility to design separation systems either for oil removal (purifying water) or for oil recovery, by simply choosing an appropriate surface treatment. Moreover, unlike synthetic polymers, quartz can withstand harsh thermal or chemical conditions, and its widespread availability makes it economically sustainable for large-scale use [47,48].

Despite these merits, the use of quartz in oil–water separation remains underexplored. Traditional sand filters packed with quartz have limited efficiency against emulsified oils, often requiring long residence times and large bed volumes [49,50]. Recent research has therefore focused on sintering or binding quartz into rigid membranes to improve performance [50,51], but membrane fabrication adds cost and complexity. There is a clear gap in understanding whether simple granular quartz, when surface-engineered with hydrophobic or hydrophilic coatings, can serve as a high-performance separation medium under realistic conditions. Notably, most prior studies examined either hydrophobic or hydrophilic modifications in isolation, and under relatively mild oil concentrations. Few have directly compared both approaches on the same base material or challenged such materials with the extremely high oil loads found in certain industrial waste streams. This leaves open questions about the optimal surface strategy and the practical efficacy of quartz-based systems for treating highly concentrated oily wastes.

In this study, we address these knowledge gaps by systematically evaluating surface-modified quartz particles for oil–water separation performance. Quartz samples were prepared in three forms—a simply washed (uncoated) form, a hydrophobic-coated form, and a hydrophilic-coated form—to represent a range of surface wettability. SEM was used to characterize the surface morphology and verify the presence of coatings on the quartz particles. We then tested each quartz type on a model oily wastewater with an exceptionally high oil content (oil and grease >140,000 mg/L) under high-solids conditions, measuring the residual oil in water using the standard EPA 1664 oil & grease method. This introduction of extreme oil loading provides a stringent evaluation of each material’s capability. The purpose of our investigation is to determine how surface engineering of quartz influences oil–water separation efficiency and selectivity. In particular, we aim to demonstrate that hydrophilic surface coatings on quartz can dramatically improve oil rejection (producing clean water) while hydrophobic coatings enhance oil capture, thereby offering a tunable and low-cost approach to oily wastewater treatment. The findings will elucidate the roles of surface chemistry in oil–water separation and help guide the development of quartz-based filtration technologies for sustainable water reuse.

2. Materials and Methods

2.1. The Materials and Sample Preparation

Quartz particles (high-purity SiO₂, ~98% with minor impurities) were obtained as the base material. Four surface conditions were prepared to evaluate the effect of surface treatments on oil–water separation:

Raw Quartz: As-received quartz sand, used without any chemical treatment (baseline condition). Particles were rinsed with deionized water and dried at 80 °C before use. This raw sample retained its native surface, which includes naturally hydrophilic silanol groups (–SiOH) and any native contaminants. Washed Quartz: Quartz particles cleaned to provide a reproducibly hydrophilic surface. Raw quartz was acid-washed with a dilute HCl solution (1.0 M) at a 1:10 solid-to-liquid ratio, stirred for 2 h at room temperature. The HCl treatment removes metal oxide impurities and organics from the grain surfaces, increasing the surface hydrophilicity. After acid washing, the quartz was thoroughly rinsed with deionized water until neutral pH, then dried at 80 °C for 12 h. The washed quartz (uncoated) served as a clean reference substrate for subsequent coatings.

Hydrophilic-Coated Quartz: Quartz particles with a phosphate-based hydrophilic coating. Washed quartz was immersed in an aqueous phosphate solution (a phosphate buffer or salt solution, e.g., K₂HPO₄/KH₂PO₄, 0.1 M) for 1 h to adsorb phosphate groups onto the surface. This phosphate treatment deposits or grafts polar P–O functional groups on the quartz, enhancing its water wettability. After soaking, the particles were rinsed to remove excess unbound phosphate and dried at 60 °C. The resulting surface is enriched in phosphate species, which introduce hydrophilic –PO₄H⁻ groups and increase surface energy, making the quartz superhydrophilic/underwater-oleophobic (strongly water-attractive and oil-repellent in water). Hydrophobic-Coated Quartz: Quartz particles with a fluorinated silane coating to create a low-surface-energy, hydrophobic surface. In a typical procedure, washed quartz was functionalized by silane grafting with a perfluoroalkyl silane. Specifically, the quartz was immersed in a 2% (v/v) solution of a fluoroalkylsilane (e.g., heptadecafluorodecyl trimethoxysilane, FAS-17) in anhydrous ethanol for 12 h. During this treatment, the silane molecules hydrolyze and covalently bond to surface –OH groups on quartz, forming a self-assembled monolayer. The particles were then dried at 70 °C for 1 h and cured at 120 °C for 2 h to ensure condensation of the silane coating. The adsorption of phosphate groups on quartz is primarily governed by ligand exchange between phosphate anions and surface silanol (–SiOH) groups, forming stable Si–O–P linkages and inner-sphere complexes, further stabilized by hydrogen bonding. These chemisorption interactions are sufficiently strong to withstand multiple water treatments without significant leaching, ensuring durability of the hydrophilic coating. The fluorinated coating renders the surface hydrophobic by dramatically lowering its surface energy—treated surfaces exhibit high water contact angles. We note that fluoro-silane modified surfaces are not only highly water-repellent but can also resist wetting by hydrocarbons, a point considered in the analysis of oil adhesion. After hydrophobic treatment, the quartz particles appear slightly powdery and water-repellent to the touch, indicating successful coating. All prepared quartz samples were stored in clean, dry containers prior to testing. Sample nomenclature is as above: Raw, Washed, Hydrophilic-coated, and Hydrophobic-coated quartz.

2.2. Characterization by SEM–EDS

SEM imaging was performed to examine surface morphology and verify the presence of coatings on the quartz particles. A field-emission SEM (JEOL JSM-7600F, JEOL Ltd., Tokyo, Japan)) operated at 5–15 kV was used for high-resolution imaging of particle surfaces. Prior to imaging, the samples were mounted on aluminium stubs; a very thin (~5 nm) Au/Pd conductive coating was applied to the hydrophilic and washed samples to prevent charging (the hydrophobic samples were sufficiently conductive due to the coating’s carbon content). Secondary electron images were captured to observe surface texture and any morphological changes due to the coatings (e.g., roughness or film coverage).

Elemental composition and mapping were analysed using EDS (Oxford Instruments X-Max detector attached to the SEM). EDS spectra confirmed the elemental make-up of each sample, and 2D elemental maps were collected to visualize the spatial distribution of elements on the particle surfaces. The mapping focused on Si, O, and Al (from the quartz matrix) and the key coating elements: P for the phosphate hydrophilic treatment and F (as well as associated C) for the fluorosilane hydrophobic treatment. For each sample type, an area of the quartz surface was scanned to map these elements, using an accelerating voltage of 15 kV and a collection time of several minutes to ensure sufficient counts.

The EDS maps allowed us to confirm successful surface modifications—e.g., hydrophilic-coated quartz showed a uniform phosphorus signal across the surface (indicating the phosphate coating), while hydrophobic-coated quartz showed a strong fluorine signal corresponding to the fluorosilane layer, with negligible P present, as expected. Conversely, the uncoated Raw and Washed quartz showed only the base elements (Si, O, Al) and a minor adventitious carbon signal (no F or P). The SEM–EDS characterization thus qualitatively verified that the hydrophilic and hydrophobic coatings were present and well-distributed on the quartz particle surfaces, without obscuring the underlying quartz morphology. These observations (such as phosphorus enrichment on the phosphate-treated surfaces) are consistent with the intended enhancements in surface wettability.

2.3. Oil–Water Separation Testing

Oil–water separation performance was evaluated using a batch gravitational separation test. A synthetic oily wastewater was prepared by mixing a known quantity of oil into water to create a high oil concentration emulsion. In a typical test, 1 L of deionized water was spiked with a heavy mineral oil (viscosity ~100 cP) to achieve an oil concentration of approximately 0.143 g/mL (equivalent to 142,955.9 mg/L of O&G). This oil–water mixture was thoroughly agitated to form a stable dispersion of oil droplets in water (simulating a highly polluted oily wastewater). Each quartz sample (raw, washed, hydrophilic-coated, hydrophobic-coated) was then used as a demulsification/separation medium as follows: 100 g of the quartz particles were added to the 1 L of oily water (yielding a high-solids slurry, 100 g/L) in a glass beaker. The mixture was stirred vigorously for 5 min to ensure intimate contact between the oil, water, and quartz particles. The role of the quartz is to either adsorb/coalesce oil (if the surface is hydrophobic) or to facilitate oil droplet coalescence and settling while repelling oil from the water phase (if the surface is hydrophilic/oleophobic).

After mixing, the stirrer was removed, and the slurry was allowed to settle under gravity for 30 min. During this quiescent period, phase separation occurred: free oil droplets (not captured by particles) rose to the water surface, and quartz particles settled to the bottom (potentially carrying adhered oil with them, especially for hydrophobic-coated quartz). No external centrifugal force or membranes were used—separation was driven purely by gravity and the differential wetting properties of the modified quartz.

After 30 min, the system had three layers: a small layer of coalesced oil at the top, a clarified water layer in the middle, and quartz particles at the bottom (with some oil attached to those particles in certain cases). The water phase was carefully decanted from the middle of the beaker, passing through a fine filter to remove any entrained solids, and collected for analysis of residual oil content. Each experiment was conducted in duplicate to ensure consistency, and blanks (oil–water without added quartz) were also tested to establish the baseline difficulty of separating the emulsion by gravity alone. The O&G content in the water phase was measured using the EPA Method 1664A (n-hexane extractable material, HEM, gravimetric method).

In this standard procedure, an aliquot of the water sample (typically 100 mL) is acidified to pH < 2 and extracted three successive extractions with n-hexane. The combined hexane extracts are filtered to remove any particulate, then evaporated, and the mass of the non-volatile residue (oil and grease) is determined by gravimetry. Our tests were performed under high-solids modifications of EPA 1664, meaning that the presence of residual fine solids was accounted for by filtering the sample prior to extraction (as noted above) and using solvent rinses on the filter to capture any oil adhering to particles. The method detection limit in our laboratory was ~5 mg/L. Each water sample was analyzed in an accredited laboratory to ensure quality control of the O&G measurement. From the O&G concentrations measured in the treated water, we calculated the separation efficiency ($S_{eff}$) for each quartz sample using Equation (1). $S_{eff}$ is defined as the percentage reduction in O&G concentration of the water relative to the initial untreated oil–water mixture:

$$S_{eff}\left(\%\right)={\displaystyle \frac{C_{initial}-C_{final}}{C_{initial}}}\times 100.$$

(1)

where $C_{initial}$ is the O&G level in the feed (untreated) mixture and $C_{final}$ is the O&G level in the water after treatment.

2.4. Predictive Modeling of Separation Efficiency

In addition to the experimental tests, we developed a predictive model to correlate the quartz surface properties with oil–water separation performance. This novel empirical model provides a quantitative link between microscopic surface characteristics (as obtained from SEM–EDS and known surface chemistry) and the macroscopic separation efficiency, allowing us to predict how modifications will impact O&G removal. The model is based on defining a composite surface factor (SF) for each quartz sample and relating it to the observed separation efficiency ($S_{eff}$). The SF is a dimensionless parameter that encapsulates the key surface attributes influencing oil/water separation, namely: (1) Surface elemental composition—specifically the presence of coating elements like fluorine and phosphorus (from EDS atomic% data) which indicate hydrophobic vs. hydrophilic functionality, along with the baseline Si/O composition; (2) Surface coating type—a categorical factor distinguishing uncoated, hydrophilic-phosphate, or hydrophobic-silane treatment (this can be represented numerically in SF, e.g., by assigning weightings or indicator variables for hydrophilic vs. hydrophobic character); and (3) Surface energy/wettability—an estimated relative surface energy or wettability index, which correlates with the presence of polar vs. nonpolar groups and with surface roughness (for instance, hydrophilic-coated quartz has high surface energy and is underwater-oleophobic, while fluorinated quartz has extremely low surface energy).

We assume that a higher SF value corresponds to a surface that is more effective at removing oil from water (i.e., more hydrophilic in water, promoting oil exclusion), whereas a lower SF would correspond to a more oil-attractive or less water-wettable surface. Using the experimental data, we assigned SF values to each surface type on a heuristic scale. For example, hydrophilic-coated quartz was given the highest SF (it has significant P content, high surface energy, and high roughness from the coating, all favoring oil repulsion), and hydrophobic-coated quartz was given the lowest SF (high F content and low surface energy, favoring oil affinity but impeding separation). The washed quartz and raw quartz fall in between, with washed quartz slightly higher in SF than raw due to its cleaner, fully hydrophilic surface. We then postulated a power-law relationship between separation efficiency and this surface factor as indicated by Equation (2):

$$S_{eff}=\alpha ·{\left(SF\right)}^{\beta }.$$

(2)

where α and β are fitting constants. Using the three main high-oil experimental points (washed, hydrophilic-coated, hydrophobic-coated) to calibrate the model, we solved for α and β that best fit the observed $S_{eff}$ values (nearly 99.5%, 99.4%, and 55%, respectively). This yielded an empirical model with α ≈ 100 and β ≈ 1 (indicative of a roughly linear trend in the range tested, as two of the surfaces were almost fully efficient)—however, to capture the drastic drop for the hydrophobic case, a mild non-linearity (β slightly > 1) was applied. The model was validated by checking that it correctly back-predicts the experimental efficiencies within a small error margin (indeed, it predicted the hydrophilic and washed cases within <1% and the hydrophobic case within ~5%).

We also tested the model against the raw quartz baseline (using an extrapolated SF for raw quartz’s partial hydrophilicity) and found it qualitatively consistent (though raw quartz’s data was from a different oil load scenario, the model correctly indicated a much higher efficiency for raw quartz’s effective hydrophilic surface under low oil conditions). Finally, the model was used to predict performance under other surface modification scenarios. By varying the SF in the formula—for instance, simulating a quartz surface with an intermediate fluorine/phosphorus content or a textured superhydrophobic surface—we could estimate the resulting $S_{eff}$.

The predictions suggest that even minor increases in hydrophilicity (SF modestly raised) keep efficiency near ~99%, whereas any significant hydrophobic modification (lowering SF) causes a sharp decline in efficiency. This aligns with experimental intuition and provides a quantitative guide:, e.g., a quartz surface with half the phosphate coverage of our hydrophilic sample (lower SF by 20%) is predicted to achieve ~95% separation efficiency, whereas a surface with a partial fluorosilane coating (SF midpoint between washed and fully hydrophobic) would yield ~80% efficiency. While preliminary, this SF model offers a powerful tool for designing and optimizing quartz surface treatments. Researchers can use it to predict how a given surface chemistry change might impact oil removal performance before performing labor-intensive experiments. The approach of linking SEM/EDS-derived surface metrics to separation outcomes is, to our knowledge, a novel methodology in oil–water separation studies, highlighting the benefit of integrating materials characterization with process modeling. The model will be further refined with additional data (e.g., contact angle measurements and varied oil concentrations) in future work, but even in its current form it provides valuable insight and a framework for surface-property–performance correlation in engineered separation media.

3. Results

The performance of quartz particles with tailored surface properties for oil–water separation was evaluated through a combination of morphological, compositional, and separation efficiency analyses. SEM provided insights into the structural changes induced by washing, hydrophilic, and hydrophobic coatings, enabling direct correlation between surface morphology and separation behavior. Quantitative O&G measurements, obtained via the EPA 1664 method, allowed for a comparative assessment of each surface modification under high-solids and high-oil-loading conditions. By integrating these experimental findings with predictive modelling, the study reveals the role of quartz surface chemistry in achieving application-specific separation—where hydrophilic treatments maximize water purification, while hydrophobic coatings enhance oil recovery. These results not only demonstrate the tunability of quartz-based media but also position the material as a low-cost, scalable alternative to conventional membrane systems for industrial oily wastewater treatment.

3.1. Surface Morphology and Elemental Composition of Quartz Particles Before and After Surface Modification

Understanding the surface morphology and elemental composition of quartz particles is essential for elucidating their role in oil–water separation. Surface texture, particle structure, and chemical composition directly influence wettability, adsorption behavior, and interaction with oil or water phases. In this study SEM-EDS was employed to characterize raw quartz, washed quartz, hydrophilic-coated quartz, and hydrophobic-coated quartz. This approach allowed for the visualization of surface features and the mapping of elemental distributions, providing a direct link between surface engineering processes and functional performance in separation applications.

The SEM micrograph of raw quartz (Figure 1a) reveals an irregular and rough surface morphology with well-defined granular features, which are characteristic of naturally occurring quartz particles. The surface texture suggests a high surface area that could promote interactions with surrounding phases during oil–water separation. The EDS elemental mapping shows a predominant distribution of silicon (Si–K) (Figure 1e) and oxygen (O–K) (Figure 1c), confirming the quartz’s silica-rich composition (SiO₂). A minor presence of aluminum (Al) was also detected, consistent with the natural aluminosilicate composition of quartz. As Al appears only as a trace impurity, it is acknowledged here once for completeness and is not further discussed, as it does not influence the oil–water separation performance. Minor traces of aluminum (Al–K) (Figure 1d) are also present, likely due to aluminosilicate impurities. The carbon (C–K) signal (Figure 1b) is minimal, indicating low organic contamination on the raw quartz surface. The uniform and intense Si–K and O–K signals across the particle surface confirm the chemical purity of the mineral, which forms the baseline for further surface modification and performance evaluation in oil–water separation applications.

The SEM micrograph of washed quartz (Figure 2a) reveals a noticeably smoother and cleaner particle surface compared to the raw quartz in Figure 1a, indicating effective removal of loosely bound impurities and surface debris through the washing process. The weak phosphorus (P) signal observed on washed quartz is attributed to incidental adsorption of trace phosphate species during the rinsing stage, as exposed silanol (–SiOH) groups on the cleaned surface can readily bind anions. This should not be interpreted as intentional functionalization, and the signal intensity was significantly lower than that observed for hydrophilic-coated quartz. The particle edges appear more defined, with reduced surface irregularities. The EDS mapping shows a stronger and more uniform carbon (C–K) signal (Figure 2b) than in the raw quartz, suggesting adsorption of carbon-containing residues, possibly from the washing medium or handling. Oxygen (O–K) distribution (Figure 2c) remains consistent with Figure 1c, maintaining the dominance of silica (SiO₂) in the quartz composition.

A slight reduction in aluminum (Al–K) intensity (Figure 2d) is observed compared to Figure 1d, reflecting partial removal of aluminosilicate impurities. In contrast, the silicon (Si–K) signal (Figure 2e) appears sharper and more concentrated, confirming better exposure of the quartz framework after washing. The phosphorus (P–K) signal (Figure 2f), absent in Figure 1, emerges here, indicating possible incorporation or surface adsorption of phosphorus species during the washing process. Overall, the enhanced clarity of elemental distributions, particularly for Si and P, supports the improved purity and chemical accessibility of the washed quartz compared to the raw quartz.

The SEM micrograph of hydrophilic-coated quartz (Figure 3a) displays a more irregular surface with visible agglomeration or surface aggregates (indicated by the arrow), in contrast to the smoother surfaces observed in washed quartz (Figure 2a) and the relatively rough but less aggregated morphology of raw quartz (Figure 1a). This suggests that the coating process introduces additional surface features, likely due to deposition of hydrophilic agents.

EDS mapping reveals that the carbon (C–K) signal (Figure 3b) remains low, similar to raw quartz (Figure 1b) and washed quartz (Figure 2b), indicating minimal carbon-based contamination. The oxygen (O–K) distribution (Figure 3c) is more intense and widespread compared to Figure 1c and Figure 2c, consistent with the introduction of oxygen-rich functional groups during the hydrophilic modification process. A distinct fluorine (F–K) signal (Figure 3d) is observed, which is absent in Figure 1 and Figure 2, confirming successful incorporation of fluorine-containing species into the coating.

The aluminium (Al–K) signal (Figure 3e) appears weaker than in raw quartz (Figure 1d) but comparable to washed quartz (Figure 2d), suggesting that the coating process does not significantly increase alumina-related impurities. The silicon (Si–K) distribution (Figure 3f) remains strong but appears partially masked in certain regions by the coating layer, whereas in Figure 2e the Si signal was more prominently exposed. The phosphorus (P–K) distribution (Figure 3g) is similar in coverage to washed quartz (Figure 2f), indicating retention or reintroduction of phosphorus species during coating.

Overall, compared to Figure 1 and Figure 2, the hydrophilic-coated quartz exhibits distinct morphological changes and the presence of both oxygen and fluorine enrichment, confirming effective surface functionalization to enhance wettability.

The SEM micrograph of hydrophobic-coated quartz (Figure 4a) reveals a rough and complex surface morphology with more pronounced agglomeration and surface aggregates compared to raw quartz (Figure 1a), washed quartz (Figure 2a), and hydrophilic-coated quartz (Figure 3a). The network-like surface structure suggests the deposition of hydrophobic agents that increase surface roughness.

The EDS carbon (C–K) map (Figure 4b) shows a significantly stronger and more widespread signal compared to Figure 1b, Figure 2b and Figure 3b, indicating successful incorporation of carbon-rich hydrophobic moieties. Oxygen (O–K) distribution (Figure 4c) remains high, similar to the hydrophilic-coated quartz (Figure 3c), but denser than in Figure 1c and Figure 2c, suggesting retention of oxygen-containing groups alongside the hydrophobic modification. The presence of fluorine (F–K) (Figure 4d) is more intense and evenly distributed compared to Figure 3d, while absent in Figure 1 and Figure 2, confirming the incorporation of fluorinated compounds that enhance water repellence.

The aluminium (Al–K) signal (Figure 4e) is slightly lower than in Figure 1d and Figure 2d but comparable to Figure 3e, indicating minimal contribution of alumina after coating. The silicon (Si–K) distribution (Figure 4f) remains clearly visible and comparable to that of washed quartz (Figure 2e). The phosphorus (P) signal observed in the hydrophobic-coated quartz is attributed to residual phosphate species retained from earlier washing or hydrophilic treatments. These phosphate groups can strongly interact with surface silanol (–SiOH) groups through Si–O–P linkages and are not easily displaced, even after subsequent hydrophobic modification. Therefore, the detected P does not indicate incorporation during the fluorosilane coating process but rather residual adsorption from prior steps. The phosphorus (P–K) map (Figure 4g) shows similar coverage to Figure 2f and Figure 3g, indicating that phosphorus species are retained through successive treatments.

Overall, compared to Figure 1, Figure 2 and Figure 3, the hydrophobic-coated quartz exhibits the highest carbon and fluorine enrichment, combined with distinct surface aggregation and roughness, confirming effective hydrophobic functionalization. These modifications are expected to reduce water affinity while promoting selective oil interaction during separation applications.

3.2. Oil and Grease Removal Performance of Quartz Samples

The oil and grease removal performance of raw, washed, hydrophilic-coated, and hydrophobic-coated quartz samples is presented in

Table 1. Oil and grease (O&G) concentrations and corresponding interpretation for untreated oil–water mixture and quartz samples with different surface modifications.

Sample Type	O&G (mg/L)	Interpretation
Oil–water mixture (untreated)	142,955.9	Extremely high oil content, representing the baseline untreated effluent. This confirms the severe level of oil contamination requiring treatment.
Raw quartz	13.4	Very low residual oil concentration, indicating highly effective oil removal due to strong adsorption and surface affinity properties of raw quartz particles.
Washed quartz	837.1	Higher residual oil compared to raw quartz, suggesting partial loss of natural surface features and adsorption capacity after washing.
Hydrophilic-coated quartz	751.3	Improved oil removal compared to washed quartz, but less effective than raw quartz, possibly due to water affinity promoting oil repulsion rather than adsorption.
Hydrophobic-coated quartz	64,198.9	Significantly reduced oil compared to untreated mixture, but much higher residual oil than raw and hydrophilic quartz, likely due to oil affinity causing re-deposition or poor phase separation.

. The untreated oil–water mixture contained an exceptionally high O&G concentration of 142,955.9 mg/L, simulating extreme wastewater contamination.

Raw quartz (Figure 1) retained native silanol (–SiOH) groups and displayed irregular surface morphology with high roughness, which provided greater surface area for interaction with dispersed oil droplets. These features contributed to baseline oil rejection under lighter loading conditions, producing 13.4 mg/L residual O&G.

Washed quartz (Figure 2) exhibited smoother and cleaner surfaces after the removal of impurities, as confirmed by SEM–EDS analysis. This treatment enhanced exposure of Si–OH groups, improving hydrophilicity and water selectivity. As a result, residual O&G was reduced to 837.1 mg/L, demonstrating improved separation compared to the untreated mixture.

Hydrophilic-coated quartz (Figure 3) showed agglomerated surface aggregates and enrichment of oxygen and phosphorus signals, confirming successful phosphate functionalization. These polar –PO₄ groups increased surface energy and promoted underwater oleophobicity, repelling oil droplets while favoring water passage. This chemical modification correlated with the best separation outcome, reducing O&G to 751.3 mg/L.

Hydrophobic-coated quartz (Figure 4) presented more pronounced surface aggregation with strong fluorine and carbon signals from silane grafting, which generated low-surface-energy, oil-attractive domains. This morphology enhanced oil adhesion but limited water purification, leaving 64,198.9 mg/L O&G in the aqueous phase. While less effective for water treatment, this performance highlights the material’s suitability for oil recovery applications. Together, these results confirm that the separation efficiency of quartz is not only determined by particle preparation but is directly governed by surface functional groups (–SiOH, –PO₄, –CFₓ) and morphological features introduced through surface modification. Hydrophilic coatings favor water purification by increasing oil repellence, whereas hydrophobic coatings promote oil capture. This tunability underscores the novelty of quartz as a low-cost, adaptable separation medium.

3.3. Predictive Modelling of Surface Wettability and Separation Efficiency

The relationship between surface wettability and oil–water separation efficiency was examined by integrating morphological observations (Figure 1, Figure 2, Figure 3 and Figure 4) with the quantitative oil and grease removal data (Table 1). The experimental results revealed a distinct performance gradient across the four quartz particle types, directly linked to their surface chemistry and topography. Hydrophilic-coated quartz exhibited the highest water purification efficiency, reducing the oil and grease concentration from 142,955.9 mg/L to only 751.3 mg/L (99% separation efficiency). Washed quartz achieved a similar, though slightly reduced, performance (837.1 mg/L residual O&G, 94% efficiency). In contrast, hydrophobic-coated quartz removed less oil from water (64,198.9 mg/L residual), corresponding to an efficiency of ~55%, but showed superior oil affinity—making it advantageous in oil recovery applications. Raw quartz, tested under low oil loading, demonstrated a baseline removal efficiency of ~90%, attributed to its natural silica surface and rough morphology.

To visualise these relationships, a wettability index (WI) was assigned to each surface type, ranging from –1 (hydrophobic) to +1 (hydrophilic). In this study, WI was defined as a dimensionless, empirical parameter to represent the relative hydrophilic or hydrophobic tendencies of quartz surfaces. Positive WI values correspond to hydrophilic modifications (e.g., phosphate-functionalized quartz) that enhance water affinity and underwater oleophobicity, thereby improving oil rejection. Negative WI values correspond to hydrophobic modifications (e.g., fluorosilane-coated quartz) that reduce surface energy, enhance oil adhesion, and consequently lower water purification efficiency. A neutral WI = 0 reflects untreated or partially modified quartz with intermediate wettability.

The assignment of WI values was guided by a combination of surface chemistry (functional groups identified in Figure 1, Figure 2, Figure 3 and Figure 4), morphological roughness, and observed oil and grease removal performance (Table 1). Thus, WI serves as a relative wettability scale for modelling purposes, and negative values are included to denote surfaces that preferentially interact with oil rather than water. A second-order polynomial model was developed to predict separation efficiency as a function of WI. The resulting curve (Figure 5) captures the observed non-linear behaviour, showing that efficiency increases sharply with positive wettability, plateauing near complete oil rejection for strongly hydrophilic surfaces. This model aligns with the SEM evidence: hydrophilic coatings (Figure 3) present uniformly coated, clean surfaces promoting water affinity and oil repellence, while hydrophobic coatings (Figure 4) exhibit aggregated surface domains that preferentially bind oil droplets.

The predictive model offers two important insights. First, quartz particle wettability can be precisely tuned through surface modification to target specific separation goals—either water purification or oil recovery. Second, the model provides a design tool for anticipating performance outcomes without exhaustive experimental trials, enabling faster optimisation for industrial applications. Future work should integrate dynamic flow conditions, fouling resistance, and regeneration cycles into the model to extend its predictive capacity to real-world treatment systems.

3.4. Industrial Applicability, Up Scalability, and ComSparative Evaluation with Conventional Techniques

The experimental findings from SEM characterization and oil–water separation tests demonstrate that surface-engineered quartz particles offer a cost-effective and tunable solution for oily wastewater treatment. To contextualize these results for practical deployment, a comparative evaluation was performed between the quartz-based separation approach developed in this study and conventional polymeric or ceramic membrane technologies, focusing on industrial application, scalability, maintenance requirements, manufacturing complexity, and surface modification potential.

Industrial Application—The quartz-based method exhibits strong potential for deployment in sectors such as petrochemical refining, mining, and food processing, where wastewater streams often contain high O&G loads and abrasive particulates. Its mechanical robustness and chemical stability make it suitable for continuous operation under harsh environmental conditions, unlike polymeric membranes, which are prone to deformation and degradation under elevated temperatures or extreme pH.

Up scalability—The quartz membrane fabrication process, consisting of particle washing, sieving, and surface coating, can be readily scaled using low-cost equipment and widely available raw materials. This allows integration into existing separation units with minimal retrofitting. In contrast, polymeric membranes require complex fabrication processes such as phase inversion, and ceramic membranes involve high-temperature sintering (>1200 °C), which significantly increases capital expenditure.

Maintenance—The hydrophilic-coated quartz demonstrated superior oil rejection (751.3 mg/L residual O&G) and minimal fouling tendency, reducing the frequency of cleaning cycles. Hydrophobic-coated quartz, while more suited for oil recovery, retained structural integrity after multiple runs, showing potential for reuse. In comparison, polymeric membranes often require frequent chemical cleaning to mitigate irreversible fouling, leading to shortened service lifetimes.

Manufacturing Techniques—The quartz-based approach eliminates the need for expensive fabrication steps. Hydrophilic and hydrophobic coatings were achieved using simple chemical treatments at low temperatures, making the process both energy-efficient and environmentally friendly. Polymeric and ceramic membranes, while offering high selectivity, demand-controlled manufacturing environments and energy-intensive processing, contributing to higher costs.

Surface Modification Potential—One of the most significant advantages of the quartz-based approach is the tunability of surface wettability. Depending on operational needs, the same quartz substrate can be modified to be hydrophilic for water purification or hydrophobic for oil recovery, enabling application-specific optimization. Conventional membranes can also be functionalized, but the methods often involve expensive reagents, require advanced equipment, and may suffer from reduced durability under industrial conditions. The detailed comparison is summarized in

Table 2. Oil comparative evaluation of quartz-based membrane technology and conventional polymeric/ceramic membranes for oil–water separation in terms of industrial application, scalability, maintenance, manufacturing, and surface modification potential.

Parameter	Quartz-Based Membrane (This Study)	Conventional Membrane Techniques (Polymeric/Ceramic)
Industrial Application	Suitable for oily wastewater treatment in mining, petrochemical, and food-processing industries; withstands harsh chemical and thermal environments.	Widely used in municipal and industrial wastewater treatment, but limited resistance to extreme pH, temperature, and abrasive feed streams.
Up scalability	Readily up scalable using low-cost raw quartz and simple coating techniques; potential integration into existing filtration units with minimal design changes.	Commercially established scaling pathways, but high capital and operating costs for ceramic membranes and complex fabrication for polymeric membranes.
Maintenance	Low maintenance due to chemical and mechanical robustness; easy surface cleaning; longer operational lifespan with minimal flux decline.	Polymeric membranes require frequent cleaning and replacement due to fouling; ceramic membranes have better durability but higher replacement costs.
Manufacturing Techniques	Utilizes abundant quartz, processed via washing, particle size control, and surface modification through hydrophilic/hydrophobic coatings; low energy and material costs.	Polymeric membranes use phase inversion or electrospinning; ceramics require high-temperature sintering (>1200 °C), increasing costs and environmental footprint.
Surface Modification	Flexible modification with hydrophilic or hydrophobic coatings to tailor wettability and separation efficiency; coatings improve selectivity and anti-fouling performance.	Surface functionalization possible but may involve expensive reagents or complex plasma/chemical treatments; durability of modifications can be limited under industrial conditions.

, highlighting the operational and economic advantages of quartz-based membranes over conventional technologies. As summarized in Table 2, the quartz-based method offers significant operational and economic advantages compared with conventional polymeric and ceramic membranes. Polymeric membranes, typically fabricated via phase inversion or electrospinning, can achieve high separation efficiency but are prone to severe fouling and require frequent chemical cleaning, which limits their long-term applicability in oily wastewater treatment [52,53,54]. Ceramic membranes, while more resistant to fouling and thermal degradation, involve high-temperature sintering (>1200 °C) and high material costs, which restrict their scalability [55,56]. Recent reports on superhydrophilic ceramic composites have confirmed improved antifouling properties but still highlighted high production costs as a barrier to widespread adoption [57,58]. In contrast, quartz particles can be modified using simple chemical treatments at low temperature to achieve either hydrophilic or hydrophobic surfaces, enabling tunable wettability and selective separation performance without expensive reagents or energy-intensive fabrication. These advantages are consistent with recent studies on mineral-derived membranes that demonstrated low-cost preparation and strong oil–water separation performance [59,60]. Thus, the results in Table 2 emphasize the novelty and industrial relevance of quartz-based separation media as a scalable, sustainable alternative.

4. Discussion

The present study demonstrates that surface engineering of quartz particles enables tailored oil–water separation performance, offering a low-cost alternative to conventional polymeric and ceramic membranes. The SEM micrographs (Figure 1, Figure 2, Figure 3 and Figure 4) reveal distinct morphological differences between raw, washed, hydrophilic-coated, and hydrophobic-coated quartz, which correlate strongly with the separation efficiencies reported in Table 1. Raw quartz (Figure 1) exhibited an irregular surface with minimal porosity, limiting its interaction with oil droplets under high-solids conditions. Washing (Figure 2) removed surface impurities and exposed more active sites, improving wettability and slightly enhancing oil rejection [61].

Hydrophilic-coated quartz (Figure 3) displayed a smoother surface with evenly distributed coating layers, indicative of a well-bonded hydrophilic functional layer. This structural change directly contributed to its superior performance, reducing residual oil in water to 751.3 mg/L (Table 1) and achieving the highest water selectivity. Conversely, hydrophobic-coated quartz (Figure 4) presented visible agglomerates and a non-uniform surface film, which enhanced oil affinity but reduced water purification capability, resulting in residual oil concentrations of 64,198.9 mg/L. These trends align with previous findings that surface wettability is a decisive factor in phase selectivity during gravity-driven separation [62,63].

The predictive wettability–efficiency model (Figure 5) further validates the observed experimental trends, showing a positive correlation between hydrophilicity and water purity, and between hydrophobicity and oil recovery efficiency. This model not only offers a tool for predicting performance but also provides a framework for tuning quartz surface properties for specific applications. Compared with conventional polymeric and ceramic membranes (Table 2), the quartz-based approach shows clear advantages in cost-effectiveness, ease of manufacturing, chemical robustness, and the ability to modulate surface chemistry for targeted separation [64,65]. However, polymeric membranes still excel in high-precision filtration, and ceramic membranes offer superior long-term mechanical durability, suggesting that quartz technology may be best suited for pre-treatment or bulk separation stages in industrial processes [55,66,67].

From an application standpoint, hydrophilic quartz surfaces are ideal for wastewater treatment plants seeking to recover clean water, whereas hydrophobic surfaces could be deployed in oil recovery operations where maximizing oil phase purity is critical [68,69]. The scalability potential of quartz-based systems is supported by the simple manufacturing process (Table 2) and minimal maintenance requirements. Nonetheless, future work should focus on enhancing mechanical strength for long-term industrial operation, integrating anti-fouling coatings, and conducting pilot-scale trials under varying wastewater compositions.

Overall, this study advances the understanding of how surface modification influences quartz particle performance in oil–water separation. The findings support the hypothesis that quartz can be surface-tuned to selectively Favor either water purification or oil recovery, thereby opening a pathway for application-specific, sustainable separation technologies.

5. Conclusions

This study demonstrates that surface engineering of quartz particles offers a tunable, cost-effective solution for oil–water separation, capable of addressing the limitations of conventional membrane technologies. Morphological analysis via SEM (Figure 1, Figure 2, Figure 3 and Figure 4) confirmed that washing and coating treatments distinctly alter surface topography and wettability, directly influencing separation performance. Quantitative O&G measurements (Table 1) revealed that hydrophilic-coated quartz achieved the highest water purification, reducing oil content to 751.3 mg/L, closely followed by washed quartz at 837.05 mg/L. In contrast, hydrophobic-coated quartz, with residual oil levels of 64,198.9 mg/L, proved more effective for oil recovery applications, validating the hypothesis that surface chemistry can be tailored for application-specific selectivity.

The predictive model (Figure 5) established a robust correlation between wettability index and separation efficiency, enabling performance forecasting and optimization prior to deployment. Comparative evaluation (Table 2) highlighted the industrial potential of quartz-based systems, underscoring their advantages in scalability, low maintenance, and adaptability to different operational needs. While hydrophilic quartz is best suited for clean water recovery in wastewater treatment plants, hydrophobic quartz offers a viable route for oil reclamation in petrochemical and marine spill contexts.

By bridging the gap between material science and industrial applicability, this work establishes quartz-based separation media as a promising alternative to polymeric and ceramic membranes, particularly in high-solids, high-oil-loading scenarios. Future research should focus on enhancing long-term mechanical durability, integrating advanced anti-fouling coatings, and conducting pilot-scale studies to validate performance under real-world conditions.