Designing a High-Performance Oil–Water Filtration System: Surface-Enhanced Quartz with Hydrophilic Nanoparticles for Sustainable Water Reuse and Global Water Scarcity Solutions

Abstract

The increasing demand for freshwater resources, coupled with industrial pollution, necessitates improved water treatment technologies. This study investigates the potential of quartz-based filtration systems enhanced with hydrophilic nanoparticles for efficient oil-water separation. The quartz material, abundant and cost-effective, was processed and modified through sequential coatings to enhance its hydrophilicity and separation efficiency. Comprehensive characterization techniques, including SEM, XRD, and Raman spectroscopy, were employed to evaluate surface morphology, chemical composition, and structural integrity at different stages of coating. The findings demonstrated that the first coating achieved the most uniform nanoparticle distribution, significantly improving hydrophilicity and separation efficiency, reducing oil content in filtrates to 17.3 mg/L. Subsequent coatings resulted in agglomeration and pore clogging, leading to diminished performance. Validation through mathematical models corroborated experimental observations, confirming the first coating’s superior balance of nanoparticle integration, permeability, and separation efficiency. This research highlights the potential of surface-engineered quartz as a scalable, cost-effective solution for sustainable water reuse. Future work will focus on optimizing coating techniques, scaling up, and integrating the system with complementary technologies to enhance water treatment processes.

1. Introduction

The availability of clean water is increasingly becoming a global concern, with growing demand driven by population expansion, industrialization, and climate change [1]. A significant challenge in maintaining water quality is the contamination of freshwater resources with oily wastewater, which originates from petrochemical industries, manufacturing processes, and transportation sectors [2,3]. The presence of oil in water disrupts natural ecosystems, reduces water reuse potential, and complicates wastewater treatment efforts, necessitating the development of effective oil–water separation technologies [4].

Traditional oil–water separation methods, including gravity separation, skimming, and chemical coagulation, often face limitations in handling stable emulsions, achieving high separation efficiency, and ensuring cost-effectiveness in large-scale applications [5,6]. Membrane-based filtration systems, particularly those utilizing polymeric and ceramic membranes, offer enhanced separation efficiency but are often constrained by high fabrication costs, susceptibility to fouling, and energy-intensive operation [7,8]. As a result, research efforts are increasingly focused on developing alternative filtration materials that are both efficient and economically viable.

Quartz, a naturally abundant and mechanically robust material, has emerged as a potential low-cost alternative for oil–water separation due to its chemical stability and ease of processing [9,10]. However, its inherent hydrophobic nature limits its effectiveness in water purification applications. To enhance its separation capability, surface modification techniques have been explored to improve hydrophilicity and oil repellency, thereby increasing the efficiency of quartz-based filtration systems.

A critical component of designing advanced filtration systems is the integration of mathematical modeling. Mathematical models provide a systematic framework for understanding and optimizing key parameters, including the distribution of nanoparticles, the impact of surface modifications on filtration performance, and the interaction dynamics between oil droplets and the filtration medium. These models enable researchers to simulate operational conditions, predict performance outcomes, and refine system designs before implementation. Incorporating modeling into the development process ensures that the system is not only theoretically sound but also practically viable for real-world applications.

This study centers on the development of a high-performance oil–water filtration system utilizing quartz particles enhanced with hydrophilic nanoparticles. The research aims to balance efficiency, scalability, and environmental sustainability, positioning the system as a practical solution to combat global water scarcity. By incorporating advanced surface engineering techniques alongside mathematical modeling, the study offers a comprehensive approach to enhancing oil–water separation technologies. The outcomes align with the broader objective of fostering water reuse and advancing sustainable resource management in the face of growing global water challenges.

2. Materials and Methods

This study utilized dry-graded quartz particles (SiO₂: 98%, Fe₂O₃: 0.18%, particle size: 0.8–1.8 mm) as the primary filtration medium for oil–water separation. Quartz was selected due to its abundance, chemical stability, and cost-effectiveness, particularly in South Africa, where it is readily available and suitable for water treatment applications. The dry-graded quartz particles used on this study was obtained from Sallies Silica Mine, Brits, South Africa. To ensure the quartz particles met the required specifications, they underwent a structured preparation process involving several stages. Initially, the quartz was mined and transported to the processing facility. It was then subjected to size reduction through a two-stage crushing process, where a jaw crusher was used for primary crushing, followed by further refinement using a cone crusher to achieve the desired particle size. The crushed quartz was then washed with purified water to remove surface contaminants, dust, and residual impurities. After washing, the quartz was air-dried for three days under controlled conditions before undergoing a heating process to eliminate residual moisture, enhancing its stability for further modification.

To improve the filtration performance, the quartz surface was modified using hydrophilic nanoparticles. The incorporation of nanoparticles was aimed at increasing the hydrophilicity and oil-repelling properties of the filtration medium. A high-pressure spray coating technique was used for nanoparticle deposition, ensuring a controlled and uniform surface modification process. The quartz samples were divided into six groups: raw quartz, washed quartz, quartz with one nanoparticle coating, quartz with two coatings, quartz with three coatings, and quartz with four coatings. The coating process was standardized by maintaining a fixed nozzle-to-quartz distance of 5 cm, a 90-degree spray angle, and precise timing and pressure settings to ensure uniform nanoparticle dispersion. Each coated sample was air-dried at room temperature (approximately 25 °C) for 48 h to allow for nanoparticle stabilization and adhesion.

To assess the effectiveness of the surface modification, the coated quartz samples underwent detailed characterization using multiple analytical techniques. Scanning Electron Microscopy (SEM) was employed to examine the nanoparticle size, morphology, and distribution on the quartz surface. The analysis confirmed that the nanoparticles ranged between 23 and 78 nm, ensuring stable and well-dispersed surface modification. Raman Spectroscopy and X-Ray Diffraction (XRD) were used to verify the chemical composition and structural stability of the coated quartz, ensuring that the modification process did not alter the material’s fundamental integrity. Additionally, mathematical modeling was applied to evaluate the coating uniformity and nanoparticle distribution patterns, optimizing the surface modification process for enhanced filtration performance.

2.1. Performance Evaluation for Oil–Water Separation

Figure 1 illustrates the custom-designed test rig developed to evaluate the oil–water separation performance of quartz particles. This system was engineered as a cost-effective and scalable solution, utilizing filtration beds as the primary separation mechanism. To enhance efficiency, integrated cartridges were incorporated into the design, facilitating multi-stage filtration to ensure the production of high-quality permeate. The resulting treated water meets the necessary standards for reuse, making this system a promising approach to addressing global water scarcity and sustainability challenges.

Filtration experiments were conducted using a three-stage integrated cartridge system, developed to simulate real-world filtration conditions. The experimental setup was assessed at varying flow rates (50–200 mL/min), cross-flow velocities (0.1–0.5 m/s), and pressure differentials (0.05–0.35 bar) to examine the influence of nanoparticle coatings on permeability and separation efficiency.

For the first nanoparticle coating, the highest flux was recorded at 75.3 L/m²·h, demonstrating optimal permeability and efficient oil rejection. However, as additional coatings were applied, hydraulic resistance increased, leading to a decline in flux to 12.8 L/m²·h by the fourth coating. This suggests that excessive nanoparticle deposition can impede water transport due to pore blockage and agglomeration.

The first coating also significantly enhanced hydrophilicity, reducing the contact angle of quartz surfaces and promoting stronger water affinity while simultaneously reducing oil adhesion. This modification resulted in an 83.7% oil removal efficiency within the first 30 min of filtration. However, with increased nanoparticle layering, oil detachment was hindered, reducing efficiency to 47.2% by the fourth coating.

Overall, the highest separation efficiency was achieved with a single nanoparticle coating, whereas additional layers led to clogging and reduced performance due to excessive deposition. These findings emphasize the need for precise nanoparticle integration to maintain an optimal balance between permeability, separation efficiency, and long-term operational stability in oil–water filtration systems.

2.1.1. Comparative Analysis of Quartz-Based Filtration with Conventional Technologies

To contextualize the performance of the quartz-based filtration system, it is essential to compare its efficiency, cost-effectiveness, and operational feasibility against established oil–water separation technologies. Various filtration methods, including polymeric membranes, ceramic membranes, activated carbon adsorption, and electrocoagulation, have been widely used for oil–water separation. Each technology presents distinct advantages and limitations based on separation efficiency, fouling resistance, operational complexity, and cost sustainability.

Comparison of Filtration Technologies and Advantages of Quartz-Based Filtration

As presented in

Table 1. Comparison of Filtration Technologies.

Filtration Technology	Separation Efficiency (%)	Flux (L/m2·h)	Fouling Resistance	Energy Consumption	Cost Considerations	References
Polymeric Membranes	85–99% (varies with material)	10–120	Moderate (prone to fouling)	High (pressure-driven)	High (frequent replacement)	[11,12]
Ceramic Membranes	90–99%	50–300	High (resistant to fouling)	Moderate	Very High (fabrication costs)	[13,14]
Activated Carbon Adsorption	60–90%	N/A	Moderate (clogging issues)	Low	Medium (replacement needed)	[15,16]
Electrocoagulation	75–95%	N/A	High (self-cleaning ability)	High	Medium to High	[17,18]
Quartz-Based System (This Study)	83.7% (first coating)	75.3 (first coating)	High (hydrophilic nanoparticle surface)	Low (operates at 0.05–0.35 bar)	Low (cost-effective quartz material)	Current study

, quartz-based filtration system developed in this study offers several advantages over conventional methods:

High Permeability and Moderate Oil Removal Efficiency: The system achieves a flux of 75.3 L/m²·h, surpassing most polymeric membranes while maintaining a comparable oil removal efficiency.

Lower Energy Requirements: Unlike pressure-driven polymeric and ceramic membranes, this system operates at low pressure (0.05–0.35 bar), significantly reducing energy consumption and making it more suitable for off-grid applications.

Superior Fouling Resistance: The hydrophilic nanoparticle-modified quartz surface resists oil adhesion, reducing membrane clogging and prolonging operational lifespan—an issue commonly encountered in polymeric membranes.

Cost-Effective and Scalable: Unlike ceramic membranes, which involve high fabrication costs, quartz is an abundant, low-cost material that can be easily processed for large-scale applications. Overall, the quartz-based filtration system presents a cost-effective, energy-efficient, and scalable alternative for oil–water separation, particularly in resource-limited settings. Compared to conventional polymeric membranes, it offers higher permeability and lower operational costs, while its fouling resistance and low-energy operation make it a viable choice for sustainable water reuse applications.

2.1.2. Scalability and Practical Implementation of the Quartz-Based Filtration System

Table 2. Scalability Assessment of Quartz-Based Filtration System.

Scalability Factor	Key Considerations
Material Availability	Quartz is naturally abundant and cost-effective, making large-scale implementation feasible.
Fabrication Complexity	Simple mechanical processing and nanoparticle coating allow for scalable manufacturing.
Operational Efficiency	Modular design enables easy expansion for high-throughput applications.
Energy Consumption	Operates at low pressure (0.05–0.35 bar), significantly reducing energy demands compared to polymeric membranes.
Maintenance Requirements	Exhibits low fouling rates, requiring minimal maintenance and longer operational lifespan.
Cost Considerations	Lower initial and operational costs than polymeric and ceramic membranes; potential for oil recovery.
Environmental Impact	Supports sustainable water reuse, reduces secondary pollution, and minimizes waste generation.
Industrial Application Potential	Suitable for wastewater treatment plants, petroleum industries, and decentralized water purification systems.

highlights the key scalability factors that determine the real-world applicability of filtration technologies, particularly in industrial wastewater treatment, oil spill remediation, and sustainable water reuse initiatives. The successful deployment of any filtration system at a large scale depends on its material availability, fabrication complexity, operational efficiency, energy consumption, maintenance requirements, cost implications, environmental impact, and industrial adaptability.

The quartz-based filtration system developed in this study offers high separation efficiency and low operational costs, making it a promising candidate for large-scale implementation. However, its scalability must be critically assessed in terms of resource availability, manufacturing feasibility, energy efficiency, and long-term economic sustainability to ensure its viability in commercial and industrial applications. The insights provided in Table 2 establish a structured evaluation of these factors, positioning the quartz-based system within the broader landscape of filtration technologies.

2.2. Development of Mathematical Models for Oil–Water Filtration Systems

To design advanced filtration systems that integrate surface-enhanced quartz with hydrophilic nanoparticles, mathematical modeling is a critical tool. Below are developed mathematical models that address key parameters in the filtration process.

Equation (1) represents the Nanoparticle Distribution Model. This model captures the uniformity and density of hydrophilic nanoparticles on the quartz surface, optimizing their coverage for maximum water affinity, where $D_{np}\left(x,y\right)$ is nanoparticle density at coordinates $\left(x,y\right)$, $A$ is the total quartz surface area, $N$ is the number of nanoparticles distributed, $(x_i,y_i)$ is the coordinates of individual nanoparticles, and σ is the standard deviation controlling nanoparticle spread:

$$D_{np}\left(x,y\right)={\displaystyle \frac{1}{A}}{\sum }_{i=1}^N\left[exp\left({\displaystyle \frac{{\left(x-x_i\right)}^2+{\left(y-y_i\right)}^2}{2{\sigma }^2}}\right)\right].$$

(1)

Equation (2) represents the Surface Modification Efficiency Model. This model quantifies the improvement in surface wettability due to hydrophilic nanoparticle coatings, where ($E_s$) is the surface modification efficiency, $\Delta \theta$ is the reduction in contact angle $\left({\theta }_r-{\theta }_c\right)$, ${\theta }_r$ is initial contact angle of raw quartz, ${\theta }_c$ is the contact angle after coating, $D_{vac}$ being the void area coefficient (uncoated regions), and $D_{max}$ is the maximum area available for coating:

$$E_s={\displaystyle \frac{\Delta \theta }{{\theta }_r}}\times \left(1-{\displaystyle \frac{D_{vac}}{D_{max}}}\right).$$

(2)

Equation (3) indicates the Oil–water Interaction Dynamics Model. This model examines the interaction between oil droplets and the filtration surface, focusing on the adhesion force and removal efficiency, where $F_{adh}$ is the adhesion force between oil droplet and surface, ${\gamma }_{ow}$ is the interfacial tension between oil and water, ${\theta }_c$ being the contact angle of the modified quartz surface, $k_e$ is the empirical constant for surface-oil interaction, $d$ is the distance between oil droplet and surface, and $\beta$ being the decay rate of interaction force with distance:

$$F_{adh}={\gamma }_{ow}cos\left({\theta }_c\right)-{\displaystyle \frac{k_e}{d}}·exp\left(-\beta d\right).$$

(3)

Equation (4) represents the Permeability and Flux Model. This model calculates the water flux through the filtration system and evaluates the permeability of the quartz-based medium, where $J_w$, is the water flux, $\Delta P$ being the pressure drop across the medium, $k$ is the permeability constant of the quartz-nanoparticle composite, $\mu$ is the viscosity of water, and $L$ being the thickness of the filtration medium:

$$J_w={\displaystyle \frac{\Delta P·k}{\mu ·L}}.$$

(4)

Equation (5) represents the Separation Efficiency Model. This model evaluates the overall efficiency of the system in separating oil from water, where ${ɳ}_s$ is the separation efficiency, $C_{in}$ is the oil concentration in the feed water, and $C_{out}$ being the oil concentration in the permeate water:

$${ɳ}_s={\displaystyle \frac{\left(C_{in}-C_{out}\right)}{C_{in}}}\times 100.$$

(5)

The proposed mathematical models are interlinked, forming a comprehensive framework for the design and optimization of the filtration system. The models allow for precise customization, ensuring that the system can be tailored to diverse water contamination scenarios. This adaptability makes the filtration system a valuable tool for addressing global water scarcity challenges, particularly in regions where water reuse is critical for sustaining communities and industries.

3. Results

The experimental outcomes represent the culmination of detailed characterization and rigorous performance testing to evaluate the effectiveness of the developed filtration system. A suite of advanced analytical methods and practical assessments was applied to gain a thorough understanding of the material’s properties and separation performance. Scanning Electron Microscopy (SEM) was utilized to analyze the surface morphology of the quartz samples, both in their untreated form and after modification with hydrophilic nanoparticles. Raman spectroscopy was employed to identify chemical bonds, and functional groups present on the quartz surface, while X-Ray Diffraction (XRD) provided critical insights into the crystalline structure of the material before and after nanoparticle integration. Additionally, oil and grease analysis quantified the residual oil content in the permeate, offering a precise measure of the system’s separation efficiency.

3.1. Scanning Electron Microscopy (SEM) Quartz Surface Analysis

The following SEM images provide a detailed representation of the quartz surface at various stages of preparation and coating. The sequence begins with the untreated raw quartz surface, showcasing its natural morphology. Next, the surface is displayed after being thoroughly washed with a descaling chemical to remove impurities and enhance its cleanliness. The progression continues with the application of the first nanoparticle coating, revealing the initial layer of modification. Subsequent images capture the quartz surface after the addition of the second, third, and fourth coatings, illustrating the incremental buildup of material layers and changes in surface structure as each coating is applied.

The Scanning Electron Microscopy (SEM) image of raw quartz in Figure 2 reveals a heterogeneous surface morphology, characterized by irregularities, sharp edges, micro-cracks, and layered formations. The presence of these features suggests that the quartz has undergone mechanical stress, likely during the mining and crushing process [19]. The rough surface lacks significant porosity, which may influence its interaction with water and oil droplets during filtration. Additionally, the layered structure observed in the image is characteristic of silica-rich minerals, which can play a role in nanoparticle adhesion when the quartz is subjected to surface modification [20]. The high surface roughness could also impact oil–water separation efficiency, affecting how water permeates through the system.

The elemental mapping analysis provides insights into the chemical composition and distribution of key elements present in the raw quartz. The carbon (C-K) mapping indicates a low presence of carbonaceous material, suggesting that the quartz is relatively free from organic contaminants or hydrocarbon deposits. This is beneficial for filtration applications, as it minimizes unwanted fouling. The oxygen (O-K) mapping shows a high and uniform distribution, which aligns with the expected silicon dioxide (SiO₂) composition of quartz. The strong oxygen signal confirms the oxidized state of the material, indicating no significant elemental impurities affecting its chemical stability. Additionally, aluminum (Al-K) mapping reveals minor traces of alumina (Al₂O₃), which are common in natural quartz deposits. The presence of aluminum could slightly influence the hydrophilicity of the quartz, as alumina-based compounds tend to alter surface wettability. Finally, the silicon (Si-K) mapping demonstrates a dominant and homogenous presence of SiO₂, verifying that the quartz is structurally stable and chemically pure, with no noticeable non-silica-based inclusions.

From a filtration perspective, the observed morphological and chemical properties have several implications. The rough and uneven surface of the raw quartz provides an advantageous platform for nanoparticle adhesion, which is crucial for hydrophilic modification in oil–water separation systems [21]. However, excessive surface roughness could lead to non-uniform coating distribution, necessitating precise nanoparticle deposition techniques. The low carbon presence ensures that the quartz is free from organic fouling, reducing the likelihood of oil absorption during filtration. Additionally, the layered microstructure suggests that the material is mechanically durable, although prolonged high-pressure flow conditions could contribute to surface erosion over extended operational cycles [22]. The minor presence of alumina (Al₂O₃) may introduce surface charge variations, which could influence the hydrophilicity and water permeability of the quartz material.

The scanning electron microscopy (SEM) image of washed quartz in Figure 3 presents a smoother and more refined surface morphology compared to raw quartz, indicating the removal of surface impurities, residual debris, and fine dust through the washing process. The surface appears to have undergone structural rearrangement, exhibiting reduced microfractures and fewer surface irregularities. However, while the roughness has diminished, some surface undulations and minor imperfections remain, which may influence nanoparticle adhesion and coating uniformity in later modification steps [23]. The reduction in surface irregularities suggests that the washing procedure effectively eliminated weakly bound particles, thereby improving the structural stability of the quartz material. This enhanced smoothness is expected to facilitate more uniform hydrophilic nanoparticle coating, which is critical for optimizing oil–water separation performance in filtration applications.

Elemental mapping analysis through energy-dispersive X-ray spectroscopy (EDS) reveals key chemical composition characteristics of the washed quartz. The carbon (C-K) signal appears more prominent than in raw quartz, possibly due to adsorption of trace organic residues from the washing solution. However, the carbon presence remains minimal, indicating that the quartz surface is still largely free from hydrocarbon contamination, which is beneficial for maintaining high water affinity. The oxygen (O-K) mapping displays an intensified and homogeneously distributed signal, consistent with the silicon dioxide (SiO₂) composition of quartz. This suggests that surface silica groups are more exposed after washing, enhancing reactivity and potential bonding sites for further nanoparticle deposition.

The presence of aluminium (Al-K) remains detectable, though its distribution is relatively sparse, indicating that alumina (Al₂O₃) impurities persist despite washing. These residual aluminium traces could subtly influence surface charge properties, potentially affecting water permeability and oil repellence. Meanwhile, the silicon (Si-K) signal exhibits a well-defined, structured distribution, confirming a high-purity quartz framework with no significant phase separations or structural degradation. The phosphorus (P-K) mapping introduces a new element that was absent in raw quartz, likely resulting from descaling agents or washing additives. The phosphorus appears to be concentrated in localized regions, suggesting possible surface interactions with quartz that may influence its hydrophilicity and long-term chemical stability.

The changes observed in surface morphology and elemental composition have significant implications for the filtration performance of the quartz material [24]. The smoother and cleaner surface provides a more uniform substrate for nanoparticle adhesion, ensuring consistent hydrophilic modifications that are essential for oil–water separation applications. However, the persistence of aluminium impurities may introduce variations in wettability, while the presence of phosphorus suggests potential secondary surface interactions that require further investigation. The washing process appears to have successfully enhanced the surface purity of quartz, but minor structural imperfections and elemental variations may still influence filtration efficiency and long-term stability.

The scanning electron microscopy (SEM) image of the first-coated quartz in Figure 4 demonstrates a well-distributed nanoparticle layer, making it the optimal coating stage for filtration applications. The surface appears significantly smoother and more structured than uncoated quartz, indicating that the nanoparticles have adhered effectively without excessive agglomeration. This uniform distribution enhances the hydrophilicity of the surface, ensuring efficient water permeability while repelling oil droplets, which is a critical factor in oil–water separation systems. The moderate surface roughness observed in the micrograph promotes enhanced interaction with water molecules, preventing oil accumulation and fouling, a common limitation in traditional filtration systems.

The energy-dispersive X-ray spectroscopy (EDS) elemental mapping further confirms the efficacy of the first coating by highlighting the balanced chemical composition of the modified quartz surface. The oxygen (O-K) signal is strong and well-distributed, reinforcing the presence of hydrophilic functional groups, which play a crucial role in improving water attraction and oil repellence. The silicon (Si-K) mapping remains dominant, ensuring that the structural integrity of the quartz is maintained, preventing excessive alteration of the material’s mechanical stability. Notably, the fluorine (F-K) and phosphorus (P-K) signals are homogeneously dispersed, suggesting enhanced surface modification without localized clustering, which is essential for ensuring consistent performance across the entire filtration medium. Unlike higher coatings where nanoparticle agglomeration leads to pore clogging, the first coating achieves an optimal balance, allowing high flux rates while maintaining excellent separation efficiency.

The minimal presence of aluminum (Al-K) and carbon (C-K) suggests that surface contamination is negligible, ensuring that the filtration system remains stable over extended operational cycles. The combination of structural integrity, optimal nanoparticle dispersion, and well-regulated surface roughness makes the first coating the most efficient stage in enhancing the performance of quartz-based filtration membranes. Its high permeability, improved water affinity, and controlled nanoparticle integration ensure long-term durability, minimal fouling, and sustained oil rejection efficiency, making it the ideal candidate for scalable oil–water separation technologies [25].

The scanning electron microscopy (SEM) image of the second-coated quartz surface in Figure 5 reveals a more complex morphology, with visible changes in surface texture and nanoparticle distribution compared to the first coating. The surface exhibits increased roughness and irregularities, indicating that the additional nanoparticle layer has altered the structural uniformity. Unlike the first coating, where nanoparticle dispersion was relatively uniform, the second coating shows localized agglomeration, suggesting non-homogeneous deposition in certain regions. This could impact filtration performance, as excessive nanoparticle accumulation can lead to higher surface resistance and reduced permeability in oil–water separation applications.

The energy-dispersive X-ray spectroscopy (EDS) elemental mapping provides further insights into the chemical composition of the coated quartz. The oxygen (O-K) signal remains strong and widely distributed, confirming the continued presence of hydrophilic surface modifications, which enhance water attraction and improve oil rejection efficiency. The silicon (Si-K) mapping still dominates, ensuring that the structural integrity of the quartz remains intact despite the additional coating. However, the carbon (C-K) intensity has slightly increased, indicating potential organic residue accumulation, which could result from nanoparticle bonding agents or surface contaminants introduced during the second coating process. The presence of fluorine (F-K) and phosphorus (P-K) suggests that surface interactions between the nanoparticles and the quartz are occurring, potentially modifying the surface energy and wettability properties. While fluorine remains evenly dispersed, phosphorus appears more clustered, reinforcing the observation of localized agglomeration.

The increased surface roughness and non-uniform particle distribution observed in the second coating may have implications for filtration performance. While the enhanced hydrophilic properties promote water permeability, the excessive aggregation of nanoparticles in certain areas could contribute to higher resistance to fluid flow, thereby reducing the overall filtration efficiency compared to the first-coated quartz. Additionally, agglomerated nanoparticles may be more prone to detachment under high-flow conditions, potentially affecting the long-term stability of the coated membrane [26]. Despite these challenges, the strong elemental signals for oxygen, silicon, and fluorine suggest that the coating remains chemically stable, making it a viable candidate for further optimization in oil–water separation applications.

The Scanning Electron Microscopy (SEM) image of the third-coated quartz in Figure 6 exhibits a notable increase in surface roughness and complexity, indicating a higher degree of nanoparticle accumulation compared to previous coatings. The surface appears more irregular, with visible clusters of nanoparticle agglomeration, suggesting that the deposition process has reached a saturation point where additional coatings contribute to excessive particle aggregation rather than uniform dispersion. This increased agglomeration could lead to partial pore blockage, potentially reducing filtration efficiency by restricting water permeability and increasing hydraulic resistance during oil–water separation.

The energy-dispersive X-ray spectroscopy (EDS) elemental mapping provides further insight into the chemical composition and distribution of the coated surface. The oxygen (O-K) signal remains consistently strong, reinforcing the presence of hydrophilic surface properties, which are essential for water affinity and oil repellence. The silicon (Si-K) mapping continues to dominate, confirming that the quartz structure remains intact despite multiple coatings. However, carbon (C-K) intensity is noticeably higher, indicating that the third coating may have introduced more organic residues, likely from excessive nanoparticle deposition or bonding agents used during the modification process.

A key observation is the increased presence of fluorine (F-K) and phosphorus (P-K), both of which appear more dispersed yet concentrated in specific regions, further confirming nanoparticle clustering. The fluorine signal is well-distributed, suggesting that it continues to contribute to surface modifications, whereas phosphorus shows dense accumulation, potentially leading to localized regions of varying surface energy. The presence of aluminum (Al-K) remains sparse, indicating that the coating process has not significantly altered the underlying quartz composition, but rather affected surface interactions with nanoparticles.

The structural and chemical characteristics of the third coating highlight both advantages and challenges for filtration applications. While the hydrophilic nature is preserved, the excessive aggregation of nanoparticles may reduce the membrane’s overall efficiency by limiting water permeability and increasing resistance to flow, similar results were observed by Jiaonan et al. [27]. This suggests that beyond a certain number of coatings, performance gains begin to diminish, emphasizing the need for controlled deposition techniques to maintain an optimal balance between surface coverage and functional efficiency. Despite these challenges, the coated quartz remains chemically stable, making it suitable for oil–water separation, provided that agglomeration control strategies are implemented to sustain consistent and efficient filtration performance.

The scanning electron microscopy (SEM) image of the fourth-coated quartz in Figure 7 reveals a highly irregular and rough surface morphology, indicating a substantial increase in nanoparticle accumulation and agglomeration. Compared to previous coatings, the surface texture appears more fragmented, with significant nanoparticle clustering, suggesting that the deposition process has exceeded optimal dispersion levels. The excessive accumulation of nanoparticles may lead to pore blockage, thereby reducing the water permeability and efficiency of the filtration system. Additionally, the formation of uneven clusters introduces potential structural instability, which could impact the long-term durability of the coated quartz when subjected to high-flow filtration conditions.

The energy-dispersive X-ray spectroscopy (EDS) elemental mapping highlights further chemical changes associated with the fourth coating. The oxygen (O-K) signal remains prominent, confirming the continued presence of hydrophilic characteristics. However, silicon (Si-K) intensity appears less defined, likely due to the thicker nanoparticle coverage obscuring the underlying quartz structure. The carbon (C-K) mapping shows an increase in scattered intensity, suggesting the possible adsorption of organic compounds or remnants of binding agents from the coating process. The presence of fluorine (F-K) remains consistent with previous coatings, reinforcing its role in modifying surface energy to enhance oil–water separation properties.

A notable concern is the increased presence of phosphorus (P-K), which exhibits a dense and non-uniform distribution, further supporting the observation of nanoparticle agglomeration. The phosphorus clusters could influence surface energy variations, potentially creating localized regions of higher surface resistance, thereby negatively affecting fluid dynamics during filtration. Additionally, aluminum (Al-K) intensity remains low, indicating that the coating primarily affects the surface layer without significant interaction with the underlying quartz structure.

The findings from the fourth coating suggest a decline in filtration efficiency due to excessive nanoparticle deposition and surface congestion. While the hydrophilic nature of the coated quartz is maintained, the agglomeration and reduced surface uniformity introduce higher resistance to fluid flow, which may hinder the effectiveness of the membrane in oil–water separation applications [28]. Moreover, the structural irregularities indicate that further coatings beyond this stage may not yield additional performance benefits, highlighting the need for optimized nanoparticle deposition techniques to prevent excessive accumulation. Despite these challenges, the fourth-coated quartz still retains hydrophilic properties, making it suitable for selective filtration applications, provided that coating stability and uniformity are controlled to enhance long-term operational efficiency.

Element Composition of Quartz Samples

The data presented in

Table 3. Elemental Composition Analysis.

Row Labels	O (%)	F (%)	Al (%)	Si (%)	P (%)
A. RAW	61.65	0.00	0.94	37.41	0.00
B. WASHED	69.82	0.00	0.35	18.58	11.25
C. COAT 1	82.54	1.14	0.46	9.97	5.89
D. COAT 2	62.93	3.72	0.47	22.27	10.62
E. COAT 3	64.07	4.06	1.02	20.60	10.25
F. COAT 4	65.81	3.85	0.83	22.45	7.05
Grand Total	67.01	2.11	0.69	22.46	9.21

and Figure 8, in conjunction with the Scanning Electron Microscopy (SEM) results, highlight that the first-coated quartz (C. COAT 1) demonstrates the best filtration efficiency. The elemental composition analysis in Table 3 shows that the oxygen percentage significantly increases to 82.54% after the first coating, indicating a high degree of hydrophilicity compared to the raw (61.65%) and washed quartz (69.82%). This suggests that the first coating successfully enhances water affinity, a critical property for oil–water separation. Additionally, the fluorine content remains relatively low at 1.14%, ensuring that the surface modification does not introduce excessive fluorine-based alterations, which could otherwise impact the chemical stability of the filtration membrane.

Comparing this to the SEM analysis, the first coating exhibits a uniform nanoparticle distribution with minimal agglomeration, ensuring optimal surface coverage without clogging the filtration pores [29]. Unlike the second, third, and fourth coatings, which show increasing nanoparticle clustering and structural irregularities, the first coating maintains an even dispersion of nanoparticles, allowing for maximum permeability while ensuring efficient oil rejection. This balance between surface modification and structural integrity prevents excessive resistance to fluid flow, a common issue observed in higher coatings, where excessive agglomeration creates localized areas of reduced permeability.

Furthermore, phosphorus content (5.89%) in the first coating remains moderate, reinforcing the hydrophilic nature of the quartz while preventing over-modification. As observed in subsequent coatings, an increase in phosphorus content leads to higher surface energy variations and greater particle aggregation, which negatively affects filtration efficiency by increasing resistance to fluid passage. The silicon percentage decreases significantly in the first coating (9.97%) compared to raw quartz (37.41%), indicating that the nanoparticle layer has effectively adhered to the quartz surface, modifying its structure for enhanced oil–water separation without compromising its stability.

The second, third, and fourth coatings show a decline in performance, as observed in Figure 8, where the oxygen percentage decreases after the first coating, stabilizing around 62–65% in higher coatings. This suggests that additional coatings do not further enhance hydrophilicity but rather introduce excessive nanoparticle accumulation, leading to structural irregularities that reduce filtration efficiency. The fluorine and phosphorus contents continue to increase in later coatings, confirming the progressive clustering of nanoparticles, which negatively impacts the membrane’s permeability.

The combined analysis from Table 3, Figure 8, and SEM results clearly indicates that the first coating provides the optimal balance between nanoparticle adhesion, hydrophilicity, and surface stability. The controlled nanoparticle distribution ensures high water permeability and effective oil rejection, making it the most efficient coating for quartz-based filtration systems. Beyond this stage, the increased nanoparticle agglomeration and chemical surface variations reduce membrane efficiency, confirming that one coating is optimal for sustainable and scalable oil–water separation applications.

3.2. X-Ray Diffraction (XRD) for Quartz Surface Analysis

The data presented in

Table 4. Phase Identification and Composition by XRD Analysis.

Sample ID	${\mathbf{S}\mathbf{i}\mathbf{O}}_2$ (%)	${\mathbf{A}\mathbf{l}}_2{\mathbf{O}}_3$(%)	${\mathbf{O}}_5$	P (%)	Total (%)
A. RAW	94.8	5.2	-	-	100
B. WASHED	87.2	5.0	5.6	2.2	100
C. COAT 1	74.7	17.6	7.7	-	100
D. COAT 2	82.6	6.9	10.5	-	100
E. COAT 3	81.5	5.2	11.8	1.5	100
F. COAT 4	86.5	6.6	5.8	1.1	100

, in conjunction with Table 3, Figure 8, and SEM results, further reinforce that the first-coated quartz (C. COAT 1) exhibits the best filtration performance due to its optimal elemental composition, nanoparticle distribution, and surface modification properties. The X-ray Diffraction (XRD) analysis in Table 4 highlights significant changes in the phase composition of the quartz samples following different coating treatments. Notably, the silicon dioxide (SiO₂) content decreases from 94.8% in raw quartz to 74.7% after the first coating, indicating a substantial surface modification through nanoparticle deposition. This transformation is crucial, as it suggests a well-integrated hydrophilic layer that enhances water permeability while maintaining structural integrity, a key feature required for effective oil–water separation membranes.

The aluminium oxide (Al₂O₃) percentage in the first coating increases significantly to 17.6%, compared to 5.2% in raw quartz, indicating the successful incorporation of alumina-based nanoparticles. This plays a pivotal role in surface energy modulation, improving the hydrophilicity and fouling resistance of the filtration medium. The increase in phosphorus pentoxide (P₂O₅) to 7.7% in the first coating further suggests an enhancement in surface reactivity, promoting strong interactions with water molecules while repelling oil droplets. However, subsequent coatings show an inconsistent trend in SiO₂, Al₂O₃, and P₂O₅ content, which correlates with the surface agglomeration observed in SEM images, where excessive nanoparticle accumulation led to localized clustering, reduced permeability, and increased hydraulic resistance.

When comparing this with Table 3 and Figure 8, it becomes evident that the first coating maintains the highest oxygen content (82.54%), which aligns with the hydrophilic nature of the surface observed in SEM results. Additionally, the moderate fluorine content (1.14%) in the first coating ensures that the surface modification remains controlled, avoiding excessive alterations that could compromise membrane stability. The subsequent coatings (Coat 2, 3, and 4) exhibit increasing fluorine and phosphorus concentrations, which correlate with the progressive nanoparticle agglomeration and uneven distribution seen in SEM images. These coatings also show higher surface irregularities, leading to reduced water flux and compromised filtration efficiency.

The SEM analysis of the first coating further supports these findings, revealing a well-dispersed nanoparticle layer with minimal clustering, which is essential for maintaining consistent permeability and effective separation performance. Unlike the second, third, and fourth coatings, where nanoparticle accumulation leads to blocked pores and increased surface roughness, the first coating provides an optimal balance between surface modification and functional efficiency [30]. The reduced silicon percentage in XRD analysis confirms that the coating has effectively adhered to the quartz, ensuring long-term durability without excessive deposition that could hinder performance.

In conclusion, the combined findings from Table 4, Table 3, Figure 8, and SEM results strongly support the conclusion that the first-coated quartz offers the best filtration performance. The optimal balance of SiO₂ reduction, Al₂O₃ incorporation, and controlled P₂O₅ presence ensures that the surface remains hydrophilic, structurally stable, and highly efficient in oil–water separation. Additional coatings introduce unwanted surface agglomeration, increasing hydraulic resistance and reducing permeability, confirming that a single nanoparticle coating is the most effective approach for developing high-performance quartz-based filtration membranes.

3.3. Raman Spectroscopy for Quartz Surface Analysis

Figure 9, Figure 10, Figure 11, Figure 12, Figure 13 and Figure 14 present the Raman spectroscopy analysis of the quartz samples at different stages of treatment and modification. The analysis begins with the raw quartz sample, providing a baseline spectrum to identify its characteristic vibrational modes. The subsequent spectrum for the washed quartz sample highlights changes in the surface composition and potential removal of impurities following the cleaning process.

The coated samples, represented by the first, second, third, and fourth coatings, display progressive modifications to the quartz surface as each layer of coating is applied. These spectra reveal the introduction of new functional groups or bonds, as well as variations in the intensity and position of key Raman peaks, which correspond to changes in crystalline structure. This progression demonstrates how the coating process alters the material’s chemical and physical properties, making Raman spectroscopy a valuable tool for monitoring these transformations.

The Raman spectroscopy spectrum of the raw quartz sample in Figure 9 reveals distinct intensity peaks in the lower wave number region (0–500 cm⁻¹), which correspond to the characteristic vibrational modes of SiO₂. These peaks are indicative of the crystalline structure of quartz, confirming its high purity and well-ordered atomic arrangement. The sharp intensity at approximately 465 cm⁻¹ is attributed to the symmetric stretching of the Si-O-Si bond, a fundamental vibrational mode of quartz. Additionally, smaller peaks within the 100–400 cm⁻¹ range represent rotational and bending modes, further validating the structural integrity of the raw quartz material. Beyond 500 cm⁻¹, the spectrum exhibits minimal intensity variations, suggesting a low presence of additional crystalline phases or impurities, which is consistent with the XRD results from Table 4, where raw quartz was found to contain 94.8% SiO₂ with minor Al₂O₃ content.

When comparing this spectrum to the first-coated quartz sample, significant modifications in surface chemistry and vibrational characteristics are evident. The SEM results confirmed a uniform nanoparticle distribution in the first coating, which is further supported by the Raman spectrum of the coated quartz, where peak broadening and intensity changes indicate enhanced surface interactions due to nanoparticle integration. The increase in Al₂O₃ (17.6%) and P₂O₅ (7.7%) in the first coating contributes to surface modifications that enhance hydrophilicity, as also observed in Table 3 and Figure 8, where oxygen content peaked at 82.54%. These compositional changes optimize water permeability and oil rejection, reinforcing the efficacy of the first coating as the best filtration stage.

In contrast, additional coatings (Coat 2, Coat 3, and Coat 4) introduce increasing nanoparticle agglomeration, leading to less uniform spectral features, as observed in their respective Raman spectra and SEM images. This excessive deposition reduces surface homogeneity, increases hydraulic resistance, and negatively impacts filtration efficiency [31]. Furthermore, the Raman spectra for these higher coatings exhibit increased spectral noise beyond 500 cm⁻¹, indicating structural inconsistencies caused by non-uniform nanoparticle layering. This aligns with the SEM findings, where excessive nanoparticle accumulation compromised surface smoothness, creating irregularities that hinder water permeability.

The Raman spectroscopy results of raw quartz, in combination with SEM, XRD (Table 4), and elemental composition analysis (Table 3, Figure 8), strongly support that the first-coated quartz is the optimal filtration membrane. The controlled surface modification achieved in the first coating enhances hydrophilicity while maintaining structural stability, ensuring efficient oil–water separation. Unlike subsequent coatings that suffer from surface congestion and reduced permeability, the first coating preserves the natural crystalline integrity of quartz while introducing sufficient surface reactivity for enhanced separation efficiency. This comprehensive analysis confirms that one coating is the most effective approach for optimizing quartz-based filtration membranes, balancing nanoparticle distribution, chemical stability, and functional performance in sustainable water treatment applications.

The Raman spectroscopy spectrum of the washed quartz sample in Figure 10 reveals several key vibrational modes characteristic of silicon dioxide (SiO₂), indicating a high-purity material with minimal contamination. The strong peak at approximately 465 cm⁻¹ corresponds to the symmetric stretching of the Si-O-Si bonds, which is a signature of crystalline quartz. Additionally, smaller peaks observed in the 100–400 cm⁻¹ range represent the rotational and bending modes of silica, confirming the structural stability of the washed quartz after the cleaning process. Compared to the raw quartz spectrum, the washed quartz shows a slight sharpening of peaks and a reduction in background noise, indicating the successful removal of surface impurities and improved sample homogeneity.

When compared to the Raman spectrum of the first-coated quartz sample, significant differences in surface chemistry and vibrational properties become evident. The first coating introduces broadening and slight shifts in the characteristic Si-O-Si peak, suggesting enhanced surface interactions due to nanoparticle deposition. The XRD analysis (Table 4) complements this observation, showing a reduction in SiO₂ content from 94.8% in raw quartz to 74.7% in the first coating, indicating the successful integration of hydrophilic nanoparticles. These modifications enhance the surface energy and hydrophilicity of the quartz, making it ideal for oil–water separation applications.

The elemental composition (Table 3 and Figure 8) further supports the effectiveness of the first coating. The oxygen content peaks at 82.54%, and the fluorine and phosphorus percentages remain low and controlled, minimizing surface agglomeration. The SEM analysis confirms a uniform nanoparticle distribution in the first coating, whereas washed quartz lacks any additional surface modification, resulting in a lower hydrophilicity and reduced filtration efficiency. Subsequent coatings (second, third, and fourth) introduce surface agglomeration, as evident in their SEM results and Raman spectra, which compromises permeability and efficiency.

The washed quartz demonstrates improved surface cleanliness and structural integrity compared to raw quartz, but it lacks the functional enhancements provided by nanoparticle coating [32]. The first coating optimizes surface chemistry, balancing nanoparticle coverage and hydrophilicity while avoiding the agglomeration seen in higher coatings. These properties make the first-coated quartz the most effective material for oil–water separation filtration, offering superior performance through controlled nanoparticle distribution and enhanced water affinity.

The Raman spectroscopy spectrum of the first-coated quartz sample in Figure 11 reveals significant modifications in surface chemistry and structural properties compared to raw and washed quartz. The dominant peak at approximately 465 cm⁻¹, attributed to the symmetric stretching of the Si-O-Si bonds, remains prominent, confirming that the quartz crystalline framework is preserved despite surface modifications. However, compared to the spectra of raw and washed quartz, the first coating introduces peak broadening and intensity variations, particularly in the low-wavenumber region (100–500 cm⁻¹), indicating the successful integration of nanoparticles onto the quartz surface. This spectral modification is associated with enhanced surface interactions between the hydrophilic nanoparticles and the quartz substrate, which is essential for improving water affinity in oil–water separation applications.

The higher intensity of the Raman signal compared to the washed quartz spectrum in Figure 12 suggests an increase in surface-active sites, confirming that the first coating has effectively altered the quartz surface energy [33]. This aligns with the elemental composition analysis (Table 3, Figure 8), where oxygen content peaked at 82.54%, reinforcing the hydrophilic nature of the coated quartz. Additionally, the moderate fluorine and phosphorus presence ensures that the coating remains stable without excessive chemical alterations, preventing issues such as membrane fouling or loss of permeability. The XRD results (Table 4) further support this observation, as the SiO₂ content decreases to 74.7% in the first coating, indicating successful nanoparticle adhesion without compromising the structural integrity of the quartz.

A crucial advantage of the first coating is its controlled nanoparticle distribution, as confirmed by SEM analysis, which revealed a well-dispersed hydrophilic layer with minimal agglomeration. Unlike higher coatings, where excessive nanoparticle deposition led to clustering and pore blockage, the first coating maintains a balanced modification, ensuring high water flux while preserving selective oil rejection properties. The Raman spectrum of higher coatings (Coat 2, Coat 3, and Coat 4) exhibits increased spectral noise and peak distortions, confirming non-uniform surface modifications that reduce membrane efficiency due to excessive aggregation.

The Raman spectroscopy results, when analyzed alongside SEM, XRD, and elemental composition data, confirm that the first-coated quartz is the most effective filtration material. It achieves an optimal balance between structural integrity, surface hydrophilicity, and permeability, making it the best choice for oil–water separation applications. The controlled surface modification in the first coating maximizes filtration performance, preventing the issues seen in subsequent coatings, where agglomeration and structural inconsistencies hinder efficiency. This confirms that a single nanoparticle coating provides the most effective solution for scalable and sustainable water treatment applications.

The Raman spectroscopy spectrum of the second-coated quartz sample in Figure 13 exhibits distinct changes compared to the first-coated quartz, highlighting the impact of additional nanoparticle deposition on the surface structure and chemical properties. The sharp peak at approximately 465 cm⁻¹, characteristic of Si-O-Si symmetric stretching, remains dominant, indicating that the quartz crystalline framework is still intact despite the second coating. However, compared to the first coating, the second coating introduces broader peaks and slight intensity variations, particularly in the low-wavenumber region (100–500 cm⁻¹), suggesting increased surface interactions due to nanoparticle accumulation. The presence of higher intensity fluctuations beyond 500 cm⁻¹ further indicates the onset of surface agglomeration, which is a critical factor in filtration performance.

The SEM results of the second-coated quartz confirm the non-uniform nanoparticle distribution, with visible localized clustering, which correlates with the peak broadening observed in this Raman spectrum [34]. Unlike the first-coated quartz, which maintained a homogeneous dispersion of nanoparticles, the second coating introduces areas of excessive deposition, which can reduce permeability and increase resistance to water flux. This aligns with the elemental composition analysis (Table 3, Figure 8), where a decline in oxygen content (62.93%) compared to the first coating (82.54%) suggests reduced hydrophilicity due to excessive surface coverage. Additionally, the increased fluorine content (3.72%) in the second coating may have influenced the surface energy, further impacting the material’s wettability and filtration efficiency.

Comparing this with XRD data (Table 4), the SiO₂ content in the second coating (82.6%) remains lower than in raw quartz (94.8%) but is higher than in the first-coated quartz (74.7%), indicating that nanoparticle integration is less controlled, leading to inconsistent surface modifications. This inconsistent deposition is reflected in the Raman spectrum, where the peak shifts and intensity variations suggest an increase in heterogeneity. Unlike the first coating, which provided a balance between hydrophilicity and structural integrity, the second coating shows a decline in spectral uniformity, confirming non-optimal surface modification for filtration applications.

In contrast, the first-coated quartz demonstrated the best filtration efficiency due to uniform nanoparticle distribution and controlled surface modifications, as seen in the SEM images and Raman spectra. The broader peaks and intensity fluctuations in the second coating indicate excessive nanoparticle accumulation, which could block filtration pores, increasing hydraulic resistance and ultimately reducing permeability. This confirms that the first coating remains the most effective in maintaining an optimal balance between hydrophilicity, permeability, and structural stability, making it the best choice for oil–water separation applications.

The Raman spectroscopy spectrum of the third-coated quartz sample in Figure 13 exhibits further spectral broadening and intensity variations compared to both the first and second coatings, indicating progressive nanoparticle accumulation and surface modifications. The strong peak at approximately 465 cm⁻¹, corresponding to the Si-O-Si symmetric stretching mode, remains present but appears slightly broadened, suggesting increased surface interactions due to the denser nanoparticle layer. The additional coatings have introduced higher intensity fluctuations beyond 500 cm⁻¹, which correlates with the formation of agglomerates, as also observed in the SEM images of the third coating.

The increased surface agglomeration in the third coating reduces the uniformity of nanoparticle distribution, leading to localized clustering and potential pore blockage. This aligns with the elemental composition analysis (Table 3, Figure 8), where the oxygen percentage (64.07%) has slightly decreased compared to the first coating (82.54%), indicating that the hydrophilic functionality of the surface is becoming less effective. The fluorine content (4.06%) is at its highest concentration among all coatings, which suggests that the coating is beginning to exhibit excessive modifications that could alter surface energy in an unfavorable manner.

The XRD analysis (Table 4) supports this observation, where the SiO₂ content in the third coating (81.5%) remains higher than the first coating (74.7%) but lower than the second coating (82.6%), implying inconsistent nanoparticle adhesion. The phosphorus pentoxide (P₂O₅) content (11.8%) is also at its highest, which may indicate over-modification of the quartz surface, leading to a reduction in efficiency. These factors collectively contribute to a less effective oil–water separation performance, as excess nanoparticles may hinder fluid permeability while providing diminishing improvements in hydrophilicity.

Compared to the first-coated quartz, which demonstrated controlled nanoparticle distribution, uniformity, and minimal surface resistance, the third coating introduces performance limitations due to excessive deposition. The broader peaks and higher intensity variations in the Raman spectrum confirm increased heterogeneity, making the third-coated quartz less efficient in filtration applications [35]. In contrast, the first coating ensures the optimal balance of nanoparticle integration, hydrophilicity, and structural stability, making it the most effective choice for oil–water separation membranes.

The Raman spectroscopy spectrum of the fourth-coated quartz sample in Figure 14 exhibits notable spectral broadening and reduced peak intensity compared to the previous coatings, confirming the adverse effects of excessive nanoparticle deposition. The characteristic Si-O-Si symmetric stretching peak at approximately 465 cm⁻¹ remains present but appears less sharp and more diffused, indicating progressive surface modifications that have altered the quartz structure. The increased background noise beyond 500 cm⁻¹ suggests that the coating process has reached a saturation point, where excessive nanoparticle accumulation is leading to surface heterogeneity and agglomeration. These spectral distortions correlate with SEM observations, where the fourth-coated quartz exhibited significant nanoparticle clustering, resulting in reduced porosity and increased hydraulic resistance.

The elemental composition analysis (Table 3, Figure 8) further supports this observation, as the oxygen content (65.81%) remains lower than that of the first coating (82.54%), confirming a decline in surface hydrophilicity due to excessive nanoparticle buildup. Additionally, the fluorine and phosphorus concentrations (3.85% and 5.8%, respectively) remain relatively high, reinforcing the idea that further coatings do not contribute to improved filtration efficiency but instead introduce unnecessary surface modifications that reduce permeability. The XRD results (Table 4) reveal that SiO₂ content in the fourth coating (86.5%) has increased compared to the second and third coatings but remains lower than in raw quartz, suggesting inconsistent nanoparticle adherence and partial detachment from the surface.

The progressive decline in spectral clarity and structural uniformity in the fourth coating confirms that additional layers beyond the first coating do not enhance filtration performance. Instead, they introduce non-uniform nanoparticle deposition, which blocks pores, increases flow resistance, and reduces water flux efficiency [36]. This contrasts sharply with the first-coated quartz, where controlled nanoparticle distribution ensured optimal permeability and oil–water separation efficiency. Unlike the first coating, which maintained a balance between structural stability and hydrophilicity, the fourth coating suffers from excessive modifications, making it less effective for sustainable water treatment applications.

The Raman spectrum, along with SEM, XRD, and elemental composition analysis, confirms that the first-coated quartz remains the most effective filtration material. The fourth coating introduces unnecessary complexities, reduces structural uniformity, and diminishes permeability, reinforcing that a single nanoparticle layer provides the best balance between surface hydrophilicity, structural integrity, and long-term filtration efficiency.

3.4. Oil and Grease Analysis

The oil and grease analysis results presented in

Table 5. Oil and Grease Analysis Results.

Quartz Material	Oil and Grease (mg/L)	Effects of Hydrophilic Nanoparticles
Raw	376.9	The untreated quartz showed limited separation efficiency, with significant oil content remaining in the filtrate.
Washed	139.9	Washing removed surface impurities, moderately improving oil–water separation compared to raw quartz.
First coating	17.3	The first nanoparticle coating significantly enhanced hydrophilicity, leading to high oil removal efficiency.
Second coating	2067.4	Excessive nanoparticle layers began to obstruct the filtration system, reducing separation performance.
Third coating	4986.6	Additional coatings further exacerbated system inefficiency, with increased clogging and reduced filtration capacity.
Fourth coating	39,455.6	Over-coating resulted in severe degradation of separation performance, likely due to nanoparticle agglomeration and pore blockage.

, obtained using the hexane extraction method, provide a comprehensive evaluation of the filtration efficiency of quartz materials with varying nanoparticle coatings. The results indicate a significant reduction in oil and grease content in the filtrate upon the introduction of a hydrophilic nanoparticle coating, with the first-coated quartz demonstrating the highest separation efficiency.

The raw quartz sample exhibited an oil and grease concentration of 376.9 mg/L, highlighting its limited natural separation capabilities. The washed quartz sample showed moderate improvement, reducing the oil concentration to 139.9 mg/L, likely due to the removal of surface impurities and enhanced surface wettability, which allowed for slightly better oil rejection [37]. However, the most significant improvement was observed with the first-coated quartz, where the oil and grease content dropped drastically to 17.3 mg/L. This result strongly correlates with previous SEM, XRD, and Raman spectroscopy findings, where the first coating demonstrated uniform nanoparticle distribution, enhanced hydrophilicity, and minimal agglomeration, providing optimal surface conditions for efficient oil–water separation. The presence of a well-dispersed hydrophilic nanoparticle layer improved water affinity, ensuring that oil was effectively repelled while allowing water to pass through the filtration system efficiently.

In contrast, subsequent coatings (second, third, and fourth) exhibited a progressive decline in separation performance, with oil concentrations increasing drastically due to excessive nanoparticle accumulation. The second-coated quartz showed a substantial increase in oil concentration to 2067.4 mg/L, suggesting that the additional coating introduced surface irregularities and partial clogging, reducing water permeability and leading to inefficient separation. The third coating exacerbated this effect, with oil concentrations rising to 4986.6 mg/L, as excessive nanoparticle layers began obstructing the filtration system, increasing flow resistance, and reducing separation efficiency. The fourth-coated quartz exhibited the worst performance, with oil concentrations surging to 39,455.6 mg/L, indicating complete filtration failure due to severe pore blockage and nanoparticle agglomeration.

These findings reinforce that the first coating remains the optimal modification for quartz-based filtration systems, achieving the best balance between surface hydrophilicity, nanoparticle distribution, and minimal hydraulic resistance. Unlike higher coatings, where nanoparticle aggregation leads to structural irregularities and clogged pores, the first-coated quartz maintains a stable surface with uniform hydrophilic modifications, ensuring consistent oil rejection while preserving high water flux. This aligns with previous elemental composition results (Table 3, Figure 8), where the first coating exhibited the highest oxygen content (82.54%), directly correlating with its superior hydrophilic behaviour and filtration efficiency.

The oil and grease analysis, in conjunction with structural and chemical characterizations, confirms that the first-coated quartz provides the most effective filtration performance. The optimized surface chemistry and controlled nanoparticle integration allow for maximum oil rejection without compromising permeability, making the first coating the ideal candidate for scalable and sustainable oil–water separation applications.

3.5. Validation of Model Analysis

The experimental findings, validated through mathematical modeling, confirm the performance trends of the quartz-based filtration system. Figure 15 and Figure 16 provide crucial insights into nanoparticle distribution, coating uniformity, separation efficiency, and clogging behaviour, reinforcing that the first-coated quartz achieves the optimal balance for oil–water separation applications.

Figure 15, which illustrates the nanoparticle distribution uniformity across different coating layers, highlights that the first coating achieves the highest level of uniformity. The distribution uniformity increases sharply from the raw quartz to the first-coated quartz, reaching its peak at approximately 0.85 relative uniformity. This confirms SEM observations, where the first-coated quartz exhibited a well-dispersed hydrophilic nanoparticle layer without significant agglomeration. Beyond the first coating, uniformity progressively declines, indicating uneven nanoparticle deposition and increasing surface irregularities. By the fourth coating, the distribution uniformity has significantly deteriorated, supporting previous findings that excessive coatings lead to inconsistent nanoparticle layering, reduced permeability, and inefficient filtration performance.

Figure 16, which examines separation efficiency and clogging trends as a function of coating layers, further substantiates the superior performance of the first-coated quartz. The separation efficiency curve follows a similar trend to the nanoparticle distribution, peaking at the first coating and then steadily declining with additional coatings. This pattern correlates with the oil and grease analysis results (Table 5), where the first-coated quartz exhibited the lowest oil concentration (17.3 mg/L), demonstrating maximum filtration efficiency. The mathematical model validates that the first coating optimally enhances the hydrophilicity of the quartz surface, promoting water permeability while efficiently repelling oil contaminants.

In contrast, clogging behaviour follows an inverse trend, increasing with additional coatings. Initially, the clogging index remains low for the first coating, confirming that the nanoparticle layer is thin enough to maintain effective filtration pathways. However, as additional coatings are applied, surface agglomeration and pore blockage become more prominent, leading to a rapid rise in clogging resistance. This aligns with SEM and Raman spectroscopy findings, where higher coatings exhibited excessive nanoparticle clustering, irregular deposition, and reduced water permeability. By the fourth coating, clogging reaches its peak, corresponding to the highest oil retention (39,455.6 mg/L) observed in Table 5, indicating a severe decline in filtration efficiency due to blocked pathways and increased hydraulic resistance.

The combined experimental and mathematical analyses conclusively demonstrate that the first coating is the most effective modification for quartz-based filtration membranes. It ensures optimal nanoparticle distribution, minimal clogging, and maximum separation efficiency, making it the ideal balance between hydrophilicity, permeability, and structural stability. Additional coatings negatively impact performance by introducing surface heterogeneity and reducing functional efficiency, reaffirming that a single nanoparticle layer provides the best approach for sustainable and scalable oil–water separation applications.

4. Discussion

The experimental findings, supported by scanning electron microscopy (SEM), X-Ray diffraction (XRD), Raman spectroscopy, elemental composition analysis, oil and grease quantification, and mathematical modeling, confirm that the first-coated quartz exhibits the highest efficiency in oil–water separation. The systematic evaluation of nanoparticle distribution, surface morphology, hydrophilicity, and filtration performance reveal that a single nanoparticle coating achieves the optimal balance between surface modification and structural integrity, making it the most effective filtration system among all coatings.

The SEM analysis revealed that raw quartz had an unmodified, relatively rough surface with limited interaction sites for hydrophilic nanoparticle adhesion. Upon applying the first coating, a homogeneous distribution of nanoparticles was observed, with minimal agglomeration and even surface coverage, enhancing water permeability and oil repellency. However, as additional coatings were applied, nanoparticles began clustering, resulting in localized deposition and increasing surface roughness. By the third and fourth coatings, significant agglomeration was observed, leading to blocked filtration pores, increased hydraulic resistance, and reduced permeability [38]. These structural changes are evident in Figure 15, where nanoparticle distribution uniformity peaked at the first coating and declined with successive layers, confirming that over-coating compromises filtration efficiency rather than enhancing it.

The elemental analysis (Table 3, Figure 8) provided further evidence that the first coating optimally modifies the quartz surface without excessive alteration. The oxygen content increased from 69.82% in washed quartz to 82.54% in the first-coated sample, demonstrating a substantial enhancement in surface hydrophilicity [39]. This modification enabled stronger interactions with water molecules, improving oil–water separation while maintaining permeability. Subsequent coatings introduced higher concentrations of fluorine and phosphorus, which contributed to surface over-modification and energy distortions, reducing filtration efficiency [40]. These findings align with XRD results (Table 4), where the SiO₂ content decreased in the first coating due to successful nanoparticle integration but remained within a functional threshold for effective separation.

The oil and grease analysis (Table 5) reinforced the structural findings by quantifying oil retention across different coatings. The raw quartz exhibited high oil content (376.9 mg/L) in the filtrate, while washed quartz showed moderate improvement (139.9 mg/L) due to surface cleaning [41]. However, the first-coated quartz achieved the most significant reduction in oil concentration (17.3 mg/L), highlighting its superior filtration capabilities. This is further validated by Figure 16, where separation efficiency reached its peak at the first coating and subsequently declined with additional layers. The clogging index followed an inverse trend, remaining low for the first coating but increasing dramatically in later coatings due to pore blockage and excessive nanoparticle accumulation [42]. These results confirm that beyond a single coating, additional nanoparticle layers do not enhance oil removal but instead hinder system performance by reducing permeability and increasing hydraulic resistance.

The Raman spectroscopy results further corroborate the effectiveness of the first coating. While the Si-O-Si vibrational peak (~465 cm⁻¹) remained prominent across all samples, the first-coated quartz exhibited moderate broadening, indicating controlled surface interactions that improved hydrophilicity without compromising quartz integrity [43]. In contrast, second, third, and fourth coatings exhibited excessive spectral noise and distortions, suggesting surface inconsistencies and localized clustering of nanoparticles. These findings align with mathematical modeling, which demonstrated that one layer of nanoparticles provides optimal interaction sites for oil–water separation while preventing excessive flow resistance [44].

The comprehensive analysis across multiple characterization techniques conclusively demonstrates that the first-coated quartz is the most effective filtration membrane. The controlled surface modification ensures high hydrophilicity, uniform nanoparticle dispersion, minimal surface roughness, and superior oil rejection properties, all while maintaining efficient water flux and minimal clogging. Additional coatings negatively impact performance by introducing excessive surface roughness, increased resistance, and structural inconsistencies, confirming that a single-layer nanoparticle modification is the optimal approach for scalable oil–water separation applications [45]. These findings provide a strong foundation for developing advanced quartz-based filtration technologies, emphasizing the importance of balancing material modification with functional efficiency to achieve sustainable and high-performance water treatment solutions. A comparison of these findings with developed models is presented in

Table 6. Comparative overview of the developed models and their alignment with experimental findings.

Model	Description	Alignment with Findings
Nanoparticle Distribution Model	Highlights that a uniform distribution of nanoparticles in the first coating enhances filtration performance.	Observations confirmed that the initial coating achieved the most uniform nanoparticle coverage, improving results.
Surface Modification Efficiency Model	Demonstrates that the first coating enhances wettability and hydrophilicity of the surface.	Experimental results showed improved hydrophilicity and wettability after applying the initial coating.
Permeability and Flux Model	Explains how additional coatings increase resistance, leading to reduced system performance.	Data confirmed that subsequent coatings reduced permeability due to increased resistance, consistent with the model.
Separation Efficiency Model	Validates that the first coating provides the best performance for oil–water separation.	Tests revealed that the first coating achieved maximum oil separation efficiency, aligning with model projections.

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5. Conclusions

This study demonstrates that a single-layer hydrophilic nanoparticle coating on quartz filtration membranes provides the optimal balance between permeability, oil–water separation efficiency, and structural stability. The first-coated quartz exhibited the most uniform nanoparticle distribution, minimizing surface roughness and preventing agglomeration, which maintained open filtration pathways and enhanced water permeability. Elemental composition analysis confirmed that the first coating significantly improved surface hydrophilicity (oxygen content: 82.54%), leading to efficient oil rejection. Raman spectroscopy results indicated that the structural integrity of quartz remained intact with the first coating, while additional coatings introduced surface irregularities and excessive nanoparticle clustering. Oil and grease analysis further validated this, showing that oil concentration in the filtrate dropped to 17.3 mg/L for the first coating, while additional coatings led to clogging and performance degradation.

Mathematical modeling aligned with experimental findings, confirming that the first coating maximized separation efficiency while maintaining permeability, whereas subsequent coatings increased resistance and reduced effectiveness. Overall, this study establishes that a single nanoparticle coating is the most effective modification for quartz filtration membranes, ensuring sustainable and high-performance oil–water separation while avoiding the inefficiencies caused by excessive coatings. These insights contribute to the development of scalable and efficient filtration technologies for industrial wastewater treatment and environmental applications. Future studies could focus on investigating advanced nanoparticle deposition techniques to improve uniformity and reduce agglomeration during multi-layer coating applications.