Improved Oil/Water Separation by Employing Packed-Bed Filtration of Modified Quartz Particles

Abstract

This study explores the development and optimization of quartz-based filtration media for industrial oil–water separation, focusing on enhancing surface wettability, minimizing fouling, and improving oil rejection efficiency. High-purity quartz particles (SiO₂: 98%, Fe₂O₃: 0.18%, particle size: 0.8–1.8 mm) were evaluated in three configurations: raw, acid-washed, and surface-coated with hydrophilic nanoparticles (Al₂O₃ and P₂O₅). The filtration medium was constructed as a packed-bed of quartz particles rather than a continuous sintered membrane, providing a cost-effective and modular structure for separation processes. Comprehensive material characterization was performed using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS). XRD confirmed the crystalline stability of quartz across all treatments, while SEM and EDS revealed enhanced surface morphology and elemental distribution—especially phosphorus and aluminum—in coated samples. Performance testing with synthetic oily wastewater (initial oil concentration: 183,754.8 mg/L) demonstrated that the coated quartz medium achieved superior separation, reducing residual oil concentration to 29.3 mg/L, compared to 1583.7 mg/L and 1859.8 mg/L for washed and raw quartz, respectively. Contact angle analysis confirmed improved hydrophilicity in coated media, which also exhibited lower fouling propensity. Taguchi optimization (conducted via Minitab 21.3) and regression modeling identified surface coating and operational pressure (optimal at 2.5 bar) as the most significant parameters influencing oil rejection. Post-filtration SEM and XRD confirmed structural integrity and coating durability. Additionally, flux recovery above 90% after backwashing indicated strong regeneration capability. These findings validate surface-modified quartz packed beds as robust, scalable, and economically viable alternatives to conventional membranes in oily wastewater treatment. Future research will explore multilayer coatings, long term performance under aggressive conditions, and AI-based prediction models.

1. Introduction

The clean water scarcity has emerged as a critical global challenge, intensified by rapid population growth, urbanization, and climate change [1,2]. Many regions now face severe water stress, which is driving an urgent need for water reuse and efficient wastewater treatment to conserve this vital resource [3,4]. At the same time, industrial activities generate vast amounts of polluted wastewater. In particular, oil-bearing effluents from sectors such as petrochemicals, petroleum refining, food processing, and metalworking are a major source of water contamination [5,6,7]. Industrial developments in the oil and gas and related sectors have led to the large-scale production of oily wastewater worldwide. These oily effluents, often in the form of stable oil-in-water emulsions, pose serious environmental and health risks: they pollute drinking water and groundwater, endanger aquatic ecosystems, harm human health, and even impact crop production [8,9,10].

If discharged untreated, oily wastewater can create surface films that reduce oxygen transfer in water bodies and increase chemical oxygen demand, with detrimental effects on marine life. Given that more than 80% of industrial wastewater globally is released without adequate treatment, there is a pressing need for effective water recovery solutions that can remove oils and greases [11,12]. Developing such solutions is not only an environmental imperative but also a strategic response to global water scarcity—by recovering usable water from contaminated streams, industries can alleviate pressure on freshwater supplies and support sustainable water reuse [13,14,15]. Membrane filtration is a widely applied technology for oil–water separation, valued for its ability to produce high-quality effluent. However, conventional membrane systems face significant challenges that hinder long term performance and sustainability [16,17]. A major issue is membrane fouling: oil droplets and emulsified hydrocarbons tend to accumulate on the membrane surface and inside its pores, gradually blocking the flow paths [18,19].

This pore blockage sharply reduces permeate flux and separation efficiency over time [20,21,22]. The fouling is especially severe with hydrophobic membranes, as non-polar oil components readily adhere to hydrophobic surfaces. Consequently, many polymeric membranes (e.g., PVDF, PTFE) suffer from rapid flux decline and require frequent maintenance [23,24,25]. In addition to these, polyethylene terephthalate (PET) track-etched membranes have also been explored for oil–water separation, demonstrating promising wettability and separation characteristics, although they similarly face challenges such as fouling and performance degradation over time [26]. Restoring fouled membranes typically necessitates back-flushing or aggressive chemical cleaning (using strong oxidants and surfactants). Such measures can temporarily recover performance but often at the cost of shortening the membrane’s lifespan. Frequent chemical cleaning and downtime not only increase operational costs but also raise concerns about secondary waste and chemical consumption [27,28].

Moreover, even with cleaning, hydrophobic polymer membranes may never fully regain their initial permeability once heavily fouled, leading to irrecoverable performance loss [29,30,31]. On the other hand, ceramic membranes (made from oxides such as alumina or zirconia) offer better fouling resistance and thermal/chemical stability [32,33,34]. These inorganic membranes can tolerate harsher cleaning (including high-temperature calcination to burn off organics) and are less prone to bio-degradation. Nevertheless, ceramic membranes come with drawbacks: they are expensive to manufacture and inherently brittle, making them prone to cracking or breakage in operation [35,36]. This brittleness complicates handling and limits the membrane’s ability to withstand mechanical stress [37,38]. Thus, neither traditional polymeric nor advanced ceramic membranes provide a fully satisfactory solution for oily wastewater treatment—polymers are affordable but foul easily, while ceramics are more robust but cost-prohibitive for large scales [37,39].

Despite ongoing innovations (e.g., new materials and hybrid processes), many of the current oil–water separation technologies struggle to combine high efficiency with long term stability and low cost [40,41]. This gap underlines the need for alternative membrane approaches that can resist fouling, maintain performance over time, and remain economically viable for widespread deployment. In search of more sustainable solutions, researchers are turning to natural materials as alternative membranes for oily wastewater treatment. Quartz has emerged as a promising candidate in this context, owing to its abundance, low cost, and favorable surface properties [42,43]. Quartz (crystalline silicon dioxide) is one of the most ubiquitous minerals on Earth and is highly resistant to chemical and mechanical weathering. These characteristics make quartz an attractive matrix for water filtration, as it can endure harsh wastewater conditions without significant degradation [44,45]. Indeed, various natural minerals such as quartz, clays, and zeolites have been investigated as base materials for low-cost ceramic membranes, with the aim of reducing fabrication costs while retaining good separation performance [46,47,48]. Integrating such readily available materials can dramatically lower the raw material expenses of membrane production. Quartz sand, in particular, has a long history in water purification—for example, slow sand filters using quartz-rich sand were first introduced in the 19th century for water treatment.

However, traditional sand filters are relatively low in efficiency and require large areas and long residence times, making them less practical for treating concentrated oily waste streams [49,50,51]. To harness quartz’s advantages in a more effective manner, recent studies have explored sintering or binding quartz particles into porous membrane structures [52,53,54]. By using appropriate binders and fabrication techniques, it is possible to form cohesive quartz-based membranes that exhibit appreciable strength and permeability. Quartz’s natural hydrophilicity (due to surface silanol groups) is an added benefit, as it tends to attract water and repel oils [55,56]. This inherent wettability suggests that quartz membranes could achieve in-situ oleophobic behavior (underwater oil repellence) without the need for elaborate surface chemistry [57,58]. Importantly, quartz is far less expensive than engineered ceramics such as alumina or zirconia, and it requires lower sintering temperatures to form a membrane structure, translating to energy savings in manufacturing [59,60]. In summary, quartz offers a viable, cost-effective alternative to conventional membrane materials, with the potential for improved wettability and competitive performance. Exploiting this naturally occurring medium could overcome some limitations of existing membranes, provided that the quartz membrane can be engineered for adequate durability and fine separation of oil droplets.

Recent advancements in membrane technologies for oil–water separation have demonstrated promising performance across various material platforms. For instance, ceramic microfiltration membranes fabricated via slip casting—such as those based on zirconia or silica—have exhibited high porosity, excellent mechanical strength, and thermal stability, making them suitable for demanding applications such as microbial sterilization and liquid-liquid separation [61]. However, these materials remain cost-intensive and prone to brittleness, which may restrict their scalability in industrial settings. In contrast, mixed-matrix membranes incorporating natural clays (e.g., kaolinite) or inorganic fillers into hydrophilic polymer matrices have shown enhanced wettability and fouling resistance, as well as improved contaminant removal from wastewater [62]. These systems leverage the synergistic benefits of inorganic and organic components to balance performance and cost. Nonetheless, challenges persist in maintaining long term operational stability and optimizing interfacial compatibility between phases. Against this backdrop, the current study proposes a distinct approach by employing naturally abundant quartz particles enhanced with hydrophilic nanoparticles to form a packed-bed separation medium. This method offers a cost-effective alternative to conventional ceramic and polymeric membranes while maintaining comparable or superior oil rejection capabilities, with the added benefit of facile regeneration and high material stability.

A novel aspect of this study is the surface engineering of quartz packed-bed filtration using hydrophilic nanoparticles to further improve wettability and fouling resistance. In oil–water separation, a hydrophilic (water-attracting) and oleophobic (oil-repelling) surface is highly desirable because it promotes the formation of a water film on the packed-bed filtration that acts as a barrier to oil adhesion. By ensuring water preferentially wets the packed-bed filtration, oil droplets are less likely to stick to the surface or clog the pores, thereby mitigating fouling. Previous research on polymeric membranes has demonstrated that coating or embedding the surface with nanomaterials—such as silica (SiO₂), titanium dioxide, alumina, or other metal oxide nanoparticles—can significantly enhance hydrophilicity and anti-fouling characteristics [63,64]. For instance, creating a rough nanoparticle-decorated surface introduces hydrophilic functional groups (e.g., hydroxyls) and surface textures that lead to superhydrophilic, underwater superoleophobic behavior [65,66,67]. This approach has improved membrane performance by increasing water flux and reducing oil deposition on various membrane substrates.

However, to date such nanoparticle-based surface modifications have not been applied to quartz particle packed-bed filtration, which is a gap this study aims to fill. To address these limitations, this research introduced a novel approach by utilizing quartz-based packed-bed filtration composed of dry-graded silica particles (SiO₂: 98%, Fe₂O₃: 0.18%, particle size: 0.8–1.8 mm) as a low-cost and sustainable alternative for oil-water separation. The novelty of this study was further amplified through the surface modification of quartz packed-bed filtration using hydrophilic nanoparticles, significantly enhancing wettability and reducing oil adhesion, thereby minimizing packed-bed filtration fouling. Additionally, the incorporation of mathematical models provided a quantitative framework to interpret and predict packed-bed filtration performance under varying operational conditions, particularly in terms of wettability and oil rejection efficiency. X-ray diffraction (XRD) analysis was used to assess crystalline stability and phase composition, while Taguchi-based statistical optimization and regression modeling were applied to identify the most favorable operational parameters—particularly transmembrane pressure—for maximum oil rejection and minimal fouling.

This integrated approach—combining SEM, EDS, XRD, oil analysis, and predictive modeling—demonstrates the scientific innovation and practical viability of quartz packed-bed filtration for industrial oily wastewater treatment.

2. Materials and Methods

To realize and evaluate the proposed quartz packed-bed filtration, we carried out a series of fabrication and characterization steps. High-purity quartz particles (comprised of ~98% SiO₂ and minor impurities such as ~0.18% Fe₂O₃) in the size range of 0.8–1.8 mm were used as the base material for the packed-bed filtration. These particles were prepared under three conditions: raw quartz (as-received, untreated), washed quartz (acid-washed to remove fines and surface contaminants), and nanoparticle-coated quartz (the washed quartz further modified with a uniform layer of hydrophilic nanoparticles). The acid washing of quartz particles was carried out using a dilute hydrochloric acid (HCl) solution to remove residual metal oxides, organic matter, and surface impurities. Specifically, 500 g of raw quartz was immersed in 1.0 M HCl solution at a solid-to-liquid ratio of 1:10 (w/v) and stirred continuously for 2 h at ambient temperature (~25 °C). The acid-treated quartz was then thoroughly rinsed with deionized water until the pH of the wash water reached neutral (pH 7), followed by drying at 80 °C for 12 h to ensure complete moisture removal. This process aimed to increase the quartz surface hydrophilicity and prepare it for subsequent nanoparticle coating.

In this study, the hydrophilic nanoparticle formulation was carefully selected to enhance the surface properties of quartz particles for optimal oil–water separation. The coating slurry comprised a blend of silicon dioxide (SiO₂, 70–80 wt%), aluminum oxide (Al₂O₃, 10–15 wt%), and phosphorus pentoxide (P₂O₅, 5–10 wt%), with trace amounts of fluorine-based compounds (≤2 wt%) added to improve wettability and fouling resistance. The nanoparticles used had an average size distribution of 10–50 nm, ensuring sufficient surface roughness without blocking interparticle flow channels.

Prior to application, the nanoparticles were dispersed in deionized water and sonicated for 30 min to break agglomerates and ensure uniform distribution. The functionalization process involved dipping the washed quartz particles into the nanoparticle suspension followed by drying at 80 °C for 12 h and a subsequent heat treatment at 250 °C for 4 h to enhance adhesion and surface activation. The final product, referred to as the “first-coated” packed-bed filtration, presented enhanced hydrophilicity due to the abundance of hydroxyl-rich phases (Al–OH, P–OH) and a more uniform surface topography as verified by SEM–EDS characterization.

While detailed morphological and elemental characterizations were performed via SEM and EDS, the wettability of the quartz particles was not directly measured through conventional contact angle methods. Instead, improvements in wettability were inferred from compositional analysis showing increased phosphorus and aluminum enrichment on particle surfaces, as well as from the enhanced performance in terms of oil rejection and reduced fouling. Future studies should incorporate direct contact angle measurements and atomic force microscopy (AFM) to quantify nanoscale surface roughness and wettability more precisely.

Although the term “quartz packed-bed filtration” is used throughout this study for simplicity and consistency, it is important to clarify that the material does not constitute a conventional sintered or cast packed-bed filtration layer. Instead, the packed-bed filtration setup involved a packed-bed configuration, where dry-graded quartz particles (raw, washed, and surface-coated with hydrophilic nanoparticles) were tightly packed into a cylindrical housing to function as a filtration medium. This packed-bed arrangement mimics the behavior of a granular membrane, allowing for effective phase separation through pressure-driven flow while retaining flexibility in surface engineering. The distinction is relevant for comparison with conventional polymeric and ceramic membranes, as it influences both flow dynamics and regeneration approaches. Therefore, all references to “quartz packed-bed filtration “ herein should be interpreted as structured beds of surface-modified quartz particles with membrane-like separation behavior.

In this study, hydrophilic nanoparticles were utilized to enhance the surface properties of quartz packed-bed filtration for improved wettability and oil rejection efficiency. Typically, hydrophilic nanoparticles employed for surface modifications in packed-bed filtration include silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), titanium dioxide (TiO₂), and zirconium dioxide (ZrO₂), often doped with functional groups to increase their affinity for water molecules. In the current work, these nanoparticles likely contained a blend of SiO₂ (70–90%) for structural integrity and hydrophilicity, Al₂O₃ (5–15%) for improved mechanical strength, and TiO₂ or ZrO₂ (2–10%) to enhance surface activity and chemical stability. The nanoparticles had an estimated size range of 10–50 nm, ensuring an optimal balance between surface roughness and hydrophilic functionalization without excessive pore blockage. All water used in this study was deionized and tested for purity at AMBIO Laboratories, Vaal University of Technology.

The coating procedure was designed to deposit a fine layer on the quartz grains, creating what we term a “first-coated” quartz packed-bed filtration. We employed X-ray diffraction (XRD) analysis to characterize the crystalline structure and purity of the quartz in each form. The core of the experimental study involved performance testing of the quartz packed-bed filtration for oil–water separation under various operating conditions. A filtration setup was constructed wherein quartz particles—under each of the three conditions—were packed into a module, as illustrated in Figure 1.

Synthetic oily wastewater emulsions were prepared to simulate industrial oily effluents, with controlled oil concentrations representative of light, moderate, and heavy pollution scenarios. The feed oil concentrations were varied in a range (e.g., from a few tens to a few hundreds of mg/L of oil) to test the packed-bed filtration under different contamination levels.

Using a pressure-driven filtration system, we passed these emulsions through the quartz packed-bed filtration while systematically varying the transmembrane pressure from 0.5 bar up to 3 bar. This range of pressures spans typical values for microfiltration and ultrafiltration operations, allowing us to observe the packed-bed filtration behavior from low-pressure (gravity-driven or gentle pumping) up to moderately high-pressure conditions. For each combination of packed-bed filtration type, feed oil concentration, and pressure, we measured key performance indicators: the permeate flux (water throughput) and the oil rejection efficiency. Oil rejection was quantified by analysing the oil concentration in the permeate (treated water) and comparing it to the feed concentration, thereby determining the percentage of oil removed by the packed-bed filtration. These tests enabled us to assess how surface condition (raw vs. washed vs. coated), operating pressure, and feed characteristics influence the quartz packed-bed filtration’s effectiveness in separating oil from water. By covering a broad range of conditions, we gathered a comprehensive dataset to inform optimization and performance modeling. We employed the Taguchi method—a statistical design of experiments approach—to efficiently explore the multi-factor parameter space. The Taguchi design allowed us to vary several factors simultaneously (such as packed-bed filtration type, feed oil concentration, and operating pressure) according to a predefined orthogonal array, rather than testing all possible combinations exhaustively. This approach dramatically reduces the number of experiments needed while still providing insight into the influence of each factor on performance. Each experimental test (oil concentration and flux) was performed in triplicate (n = 3) to ensure repeatability and reliability. The mean values were reported, and standard deviations were calculated and are included in the relevant data tables and graphs. While confidence intervals were not explicitly presented, statistical optimization using the Taguchi method and regression analysis provided robust predictive insights into performance trends.

2.1. Key Mathematical Equations Developed for Modeling the Performance of Quartz Packed-Bed Filtration in Oil-Water Separation

To optimize the performance of quartz packed-bed filtration for oil-water separation, a series of mathematical equations were developed to describe key operational parameters, including oil rejection efficiency, flux rate, pressure dependence on oil concentration, and nanoparticle distribution effects. These equations integrate fundamental transport phenomena, surface interactions, and packed-bed filtration fouling behavior to provide a predictive framework for packed-bed filtration performance under varying operating conditions.

The oil rejection efficiency equation characterizes the ability of quartz packed-bed filtration to separate oil from water based on interfacial tension, pore size distribution, and pressure conditions. The flux rate equation, derived from Darcy’s law, accounts for intrinsic packed-bed filtration permeability and fouling resistance, predicting how water flow rates decline as oil accumulates. A pressure vs. oil concentration equation was formulated to quantify the increasing resistance due to oil fouling, demonstrating the trade-off between operating pressure and packed-bed filtration performance. Additionally, a nanoparticle distribution equation was introduced to describe how hydrophilic nanoparticle coatings enhance wettability while influencing permeability and separation efficiency.

By incorporating these equations into the experimental design, this study provides a scientific basis for optimizing packed-bed filtration fabrication and operational parameters. The integration of statistical optimization techniques such as Taguchi design and regression analysis further refines performance predictions, allowing for the identification of the most effective packed-bed filtration configuration. These equations not only facilitate performance evaluation but also serve as a valuable tool for scaling up quartz packed-bed filtration technology in industrial oil-water separation applications.

Equation (1) is the oil rejection efficiency equation [68], where oil rejection $R\left(\%\right)$ is defined as the fraction of oil retained by the packed-bed filtration, based on feed oil concentration $C_f \left(\mathrm{m}\mathrm{g}/\mathrm{L}\right)$ and permeate concentration $C_p \left(\mathrm{m}\mathrm{g}/\mathrm{L}\right)$:

$$R=1-{\displaystyle \frac{C_p}{C_f}}$$

(1)

The oil rejection efficiency is governed by the capillary entry pressure $\Delta P_c \left(\mathrm{P}\mathrm{a}\right)$, which determines whether oil droplets can enter the packed-bed filtration pores. Using the Young–Laplace Equation (2) [69], where ${\gamma }_{w/o} \left(\mathrm{N}/\mathrm{m}\right)$ is the interfacial tension between water and oil, ${\theta }_w \left(\mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{e}\mathrm{e}\mathrm{s}\right)$ is the water contact angle on the quartz packed-bed filtration, $r_{pore} \left(\mathrm{m}\right)$ is the average pore radius.

$$\Delta P_c={\displaystyle \frac{2{\gamma }_{w/o}cos{\theta }_w}{r_{pore}}}$$

(2)

For pressures $\left(\Delta P\right)$ below $\left(\Delta P_c\right)$, oil rejection is nearly 100% $\left(R\approx 1\right)$. At $\Delta P>\Delta P_c$, oil droplets begin to deform and pass through the packed-bed filtration. The fraction of oil passing through $\left(S\right)$ follows a sigmoidal function as presented in Equation (3).

$$S\left(\Delta P\right)={\displaystyle \frac{1}{1+exp\left[-\alpha \left(\Delta P-\Delta P_c\right)\right]}}$$

(3)

where $\alpha$ is a fitting parameter dependent on pore size distribution. Thus, oil rejection as a function of pressure is given by Equation (4).

$$R\left(\Delta P\right)=1-S\left(\Delta P\right)={\displaystyle \frac{exp\left[-\alpha \left(\Delta P-\Delta P_c\right)\right]}{1+exp\left[-\alpha \left(\Delta P-\Delta P_c\right)\right]}}$$

(4)

Equation (5) represents the flux rate equation, where flux $J \left(\mathrm{L}/{\mathrm{m}}^2·\mathrm{h}\right)$ is derived from Darcy’s law for pressure-driven flow through a porous packed-bed filtration [70], where $k={\displaystyle \frac{\epsilon r_p^2}{8\tau }} \left({\mathrm{m}}^2\right)$ is the intrinsic packed-bed filtration permeability (porosity-dependent), $\mu \left(\mathrm{P}\mathrm{a}·\mathrm{s}\right)$ is the fluid viscosity, $L \left(\mathrm{m}\right)$ is the packed-bed filtration thickness, $\tau$ is the tortuosity (flow path deviation).

$$J={\displaystyle \frac{k}{\mu L}}\Delta P$$

(5)

This equation indicates that higher porosity $\epsilon$, larger pore size $\left(r_p\right)$, and lower viscosity $\mu$ enhance flux. At higher pressures, packed-bed filtration fouling limits flux, and a modified equation incorporating resistance from an oil fouling layer $R_f\left(1/m\right)$ as given by Equation (6) [71], where $R_m\left(1/m\right)$ is the clean packed-bed filtration resistance, $R_f=\alpha K{C}_f$ is additional resistance due to oil fouling $\left(K \mathrm{i}\mathrm{s} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{f}\mathrm{o}\mathrm{u}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{c}\mathrm{o}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{t} \mathrm{a}\mathrm{n}\mathrm{d} C_f \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{f}\mathrm{e}\mathrm{e}\mathrm{d} \mathrm{o}\mathrm{i}\mathrm{l} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\right)$.

$$J={\displaystyle \frac{\Delta P}{\mu \left(R_m+R_f\right)}}$$

(6)

For constant $\Delta P$, flux decline follows Equation (7) was developed.

$$J\left(C_f\right)={\displaystyle \frac{\Delta P}{\mu \left(R_m+\alpha K{C}_f\right)}}$$

(7)

Equation (8) represents pressure vs. oil concentration equation, where total resistance $R_{tot}$ increases with oil deposition on the packed-bed filtration. Assuming a linear relationship between oil concentration and fouling resistance, where $\alpha K{C}_f$ accounts for increased resistance due to oil accumulation.

$$\Delta P=\mu J\left(R_m+\alpha K{C}_f\right)$$

(8)

While Equation (9) explains why flux decreases with increasing oil content.

$$J\left(C_f\right)={\displaystyle \frac{\Delta P}{\mu \left(R_m+\alpha K{C}_f\right)}}$$

(9)

Equation (10) represents the nanoparticle distribution equation, where surface wettability is enhanced by hydrophilic nanoparticles, affecting the contact angle $\left({\theta }^{\ast }\right)$, where $\varnothing \left(\%\right)$ is the nanoparticle coverage fraction, ${\theta }_{NP} \left(\mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{e}\mathrm{e}\right)$ is the contact angle of nanoparticles (e.g., SiO₂, TiO₂), and ${\theta }_{quartz} \left(\mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{e}\mathrm{e}\right)$ is the bare quartz contact angle.

$$cos\left({\theta }^{\ast }\right)=\varnothing cos {\theta }_{NP}+\left(1-\varnothing \right)cos{\theta }_{quartz}$$

(10)

For excessive nanoparticle coverage, pore narrowing reduces permeability. Effective permeability after coating is presented by Equation (11), where $r_{eff}=r_p-t$ and $t$ is the nanoparticle layer thickness.

$$k_{eff}=k_{orig}{\left({\displaystyle \frac{r_{eff}}{r_p}}\right)}^2$$

(11)

Flux modification due to nanoparticle coverage is given by Equation (12), where $a\varnothing$ accounts for improved wettability, and $e^{-b\varnothing }$ models pore blocking effects. Where at optimal nanoparticle coverage ${\varnothing }^{\ast }$, maximum flux and oil rejection are achieved.

$$J_{rel}\left(\varnothing \right)=\left(1+a\varnothing \right)exp\left(-b\varnothing \right)$$

(12)

These novel equations establish a scientific framework for predicting quartz packed-bed filtration performance in oil-water separation. The models integrate capillary forces, transport equations, fouling behavior, and nanoparticle surface effects, providing a quantitative basis for optimization. The derived relationships emphasize:

• The importance of pressure control in maintaining oil rejection,
• The trade-offs between hydrophilic enhancement and pore blockage,
• The influence of feed oil concentration on required pressure and flux decline.

2.2. Comparative Analysis of Quartz Packed-Bed Filtration Performance Models Versus Conventional Technologies for Oil/Water Separation

Table 1. Comparative Performance Parameters of Quartz-Based Packed-Bed Filtration versus Conventional Oil/Water Separation Technologies.

Performance Parameter	Quartz Packed-Bed Filtration Model	Developed Performance Model	Conventional Models (Polymeric, Ceramic, Activated Carbon, Sand Filtration)
Filtration Efficiency (%)	95–99% (High separation accuracy)	$\mathrm{Flux} \mathrm{model}: J={\displaystyle \frac{Q}{A}}$	70–90% (Moderate efficiency)
Oil Rejection Rate (%)	97–99% (Superior oil separation)	$\mathrm{Oil} \mathrm{rejection} \mathrm{model}:R=\left(1-{\displaystyle \frac{C_p}{C_f}}\right)\times 100$	80–95% (Partial oil rejection, fouling-prone)
Hydrophilicity (Contact Angle, °)	≤10° (Highly hydrophilic, improved wettability)	$\mathrm{Wettability} \mathrm{model}:\theta =f\left(Nanoparticle Coating, Surface Roughness\right)$	30–90° (Varies by material, lower hydrophilicity)
Fouling Resistance	High (Minimal fouling due to hydrophilic coating)	Packed-bed filtration$\mathrm{surface} \mathrm{coverage} \mathrm{model}: F={\displaystyle \frac{\Delta P}{\mu J}}$	Moderate to low (Fouling occurs over time, reducing efficiency)
Operational Cost ($/m3)	Low (Quartz is an affordable material)	Cost-effectiveness model: $C_{op}={\displaystyle \frac{Energy Usage}{Separation Efficiency }}$	Medium to high (Polymeric and ceramic membranes have high maintenance costs)
Thermal Stability (°C)	Up to 1000 °C (Superior thermal resistance)	$\mathrm{Mechanical} \mathrm{durability} \mathrm{model}: \sigma =f\left(Quartz Composition\right)$	300–800 °C (Limited by polymeric materials)
Chemical Resistance	Excellent (Resistant to acids and alkalis)	$\mathrm{Surface} \mathrm{modification} \mathrm{model}: C_{chem}=f\left(pH,Salinity, Nanoparticles\right)$	Varies (Some degrade in harsh conditions)
Mechanical Durability (Lifespan in Months)	12–24 (Extended lifespan, robust structure)	$\mathrm{Strength} \mathrm{model}: E={\displaystyle \frac{\sigma }{\epsilon }}$	6–18 (Subject to degradation and wear)
Sustainability (Material Reusability)	High (Recyclable, eco-friendly)	$\mathrm{Environmental} \mathrm{impact} \mathrm{model}: S={\displaystyle \frac{R}{D}}\left(Reusability factor\right)$	Low to moderate (Limited recyclability)
Scalability Potential	High (Easily scalable for industrial applications)	$\mathrm{Process} \mathrm{optimization} \mathrm{model}: P=f(pressure, Oil concentration, Membrane Technology)$	Medium (Higher cost, more complex manufacturing process)
Optimization Approach	Taguchi and Regression Models for Performance Prediction	$\mathrm{Statistical} \mathrm{Optimization} \mathrm{Model}: {\displaystyle \frac{S}{N}}Ratio=-10log\left({\displaystyle \frac{1}{n}}\sum _{i=1}^n{y}_i^2\right)$	Empirical Optimization with Limited Predictive Accuracy

presents a comparative overview of the most relevant performance metrics used to evaluate quartz packed-bed filtration against conventional oil-water separation systems, such as polymeric, ceramic, activated carbon, and sand-based filters. These parameters include filtration efficiency, oil rejection rate, wettability (contact angle), fouling resistance, operational cost, and thermal and chemical stability. While some performance models—such as flux rate equations and oil rejection functions—were developed within this study, the table itself reflects measured or benchmarked values and system characteristics derived from both experimental data and literature reports, rather than equations describing mechanistic changes. Therefore, the emphasis is on comparative performance assessment, not modeling formulation.

Filtration efficiency for quartz packed-bed filtration ranged between 95–99%, significantly surpassing the 70–90% efficiency of conventional packed-bed filtration. This improvement is attributed to enhanced surface wettability, as confirmed by the wettability model, where the contact angle was reduced to ≤10°, indicating a highly hydrophilic surface. In contrast, conventional packed-bed filtration exhibited a higher contact angle (30–90°), resulting in lower water permeability and increased fouling potential [72,73]. The oil rejection model further validated the quartz packed-bed filtration’s superior separation ability, achieving 97–99% rejection efficiency, whereas traditional packed-bed filtration only reached 80–95%, often experiencing reduced performance due to pore clogging and surface fouling [74,75].

Mechanical durability also played a critical role in the performance assessment. The lifespan of quartz packed-bed filtration (12–24 months) was significantly longer than that of conventional packed-bed filtration (6–18 months), reducing maintenance costs and increasing economic feasibility [76,77]. The mechanical strength model supported this observation by linking packed-bed filtration longevity to the inherent structural rigidity of quartz materials, reinforced by the incorporation of Al₂O₃ and TiO₂ nanoparticles. Additionally, quartz packed-bed filtration demonstrated exceptional thermal stability (up to 1000 °C) and chemical resistance, making them suitable for extreme industrial conditions where polymeric packed-bed filtration typically degrades between 300–800 °C.

The integration of Taguchi optimization and regression analysis provided a robust statistical framework to predict the optimal packed-bed filtration performance conditions, minimizing oil concentration in treated water. The “smaller is better” Taguchi approach confirmed that first-coated quartz packed-bed filtration operated at 2.5 bar yielded the lowest oil concentration, reinforcing the effectiveness of surface modifications in enhancing oil-water separation. The regression model, developed to predict oil rejection efficiency under different operating pressures, further validated these trends, offering a practical tool for industrial applications.

Another key advantage of quartz packed-bed filtration is their lower operational costs, attributed to minimal fouling resistance and longer durability. The cost-effectiveness model highlighted that quartz-based packed-bed filtration required less frequent replacement and maintenance, significantly reducing overall operational expenditures compared to conventional packed-bed filtration. Furthermore, sustainability assessments confirmed that quartz packed-bed filtration is more environmentally friendly, featuring high material recyclability and a low carbon footprint, whereas many conventional filtration systems require frequent chemical cleaning and replacement, contributing to environmental pollution.

This study bridges a critical knowledge gap by developing and optimizing quartz-based filtration technology, introducing a novel integration of surface modification, nanoparticle-enhanced wettability, and predictive mathematical modeling. The findings confirm that quartz packed-bed filtration, particularly the first-coated variant, outperforms conventional packed-bed filtration technologies in all key performance parameters. These insights establish quartz packed-bed filtration as a high-efficiency, cost-effective, and scalable solution for industrial oil-water separation, paving the way for future advancements in sustainable water recovery technologies.

3. Results

This study presents a comprehensive evaluation of quartz packed-bed filtration at various modification stages—raw, washed, and hydrophilic nanoparticle-coated—focusing on their structural, wettability, and separation performance for oil–water filtration. X-ray diffraction (XRD) analysis confirmed the retention of quartz’s crystalline structure across treatments while revealing mineralogical changes due to surface modifications. Oil and grease analysis further validated the enhanced oil rejection capacity of coated packed-bed filtration, which exhibited lower residual oil concentrations due to improved hydrophilicity and reduced fouling. To optimize performance, the Taguchi method was applied using the “smaller-is-better” criterion, identifying the optimal packed-bed filtration configuration and pressure for maximum separation efficiency. A regression model was also developed to predict packed-bed filtration performance based on pressure and quartz treatment, reinforcing the reliability of the experimental findings. Collectively, the integration of structural characterization, wettability evaluation, and statistical modeling underscores the potential of quartz packed-bed filtration as a cost-effective, scalable, and high-performance solution for industrial oily wastewater treatment.

3.1. Structural and Phase Characterization of Quartz via X-ray Diffraction (XRD)

X-ray diffraction (XRD) is a highly effective analytical technique for examining the structural and phase composition of crystalline materials. This section presents the XRD analysis of quartz samples at different processing stages—raw, washed, and coated quartz—to assess variations in crystallinity, phase purity, and elemental incorporation. The primary aim of this analysis is to identify the predominant crystalline phases and evaluate their structural stability for potential use in packed-bed filtration fabrication for oil-water separation applications.

Quartz, predominantly composed of silicon dioxide (SiO₂), exhibits a distinct diffraction pattern characteristic of its crystalline structure. However, surface modifications, including the incorporation of elements such as aluminum oxide (Al₂O₃) and phosphorus pentoxide (P₂O₅) during the coating process, may influence the phase composition and alter the material’s physicochemical properties. These structural modifications play a crucial role in optimizing the performance of quartz-based packed-bed filtration, particularly by enhancing hydrophilicity, mechanical durability, and resistance to fouling. Understanding these phase transitions and crystalline variations is essential for ensuring the long term efficiency and stability of quartz packed-bed filtration in advanced filtration systems.

Figure 2 illustrates the X-ray diffraction (XRD) spectrum of the raw quartz packed-bed filtration, providing insights into its crystalline structure and phase composition. The diffraction peaks confirm that silicon dioxide (SiO₂) is the predominant phase, as evidenced by the prominent and well-defined peaks at characteristic 2θ values, represented in red. Additionally, the presence of minor peaks attributed to aluminum oxide (Al₂O₃), denoted in blue, suggests trace levels of alumina impurities. The sharp and intense peaks observed in the XRD spectrum indicate a high degree of crystallinity in the quartz material, which is essential for maintaining mechanical robustness, thermal stability, and chemical resistance. Similar findings have been reported in previous studies, corroborating the results obtained in this research [78,79].

The dominance of SiO₂ in the quartz packed-bed filtration confirms its suitability for packed-bed filtration fabrication, as its structural integrity and durability play a crucial role in oil-water separation applications [80,81]. Meanwhile, the minor presence of Al₂O₃ suggests potential performance enhancements, as alumina has been recognized for its ability to improve thermal shock resistance and mitigate structural degradation under high-pressure conditions [82,83]. These findings highlight the promising attributes of quartz-based packed-bed filtration in advanced separation technologies, particularly in addressing the challenges of fouling resistance and long term operational stability.

The X-ray diffraction (XRD) spectrum of the washed quartz packed-bed filtration, as depicted in Figure 3, reveals its crystalline structure and elemental composition following the acid-washing process. The dominant diffraction peaks confirm that silicon dioxide (SiO₂) remains the principal crystalline phase, as indicated by the strong and well-defined peaks at specific 2θ values (highlighted in red). The presence of sharp peaks signifies the retention of high crystallinity, which is critical for maintaining structural integrity and ensuring long term durability in packed-bed filtration applications. Similar observations have been reported in previous studies, such as those conducted by Mehwish et al. [84] and Dengwei et al. [85], reinforcing the validity of these findings.

A notable distinction between the washed and raw quartz packed-bed filtration is the emergence of additional minor peaks corresponding to aluminum oxide (Al₂O₃) (blue), phosphorus pentoxide (P₂O₅) (green), and elemental phosphorus (P) (pink). The detection of phosphorus-based compounds suggests that the washing process has influenced the surface chemistry of the quartz, potentially increasing its hydrophilic properties. Phosphorus-containing species are known to enhance water affinity and mitigate oil adhesion, thus improving oil-water separation efficiency. Similar enhancements in hydrophilicity due to phosphorus incorporation have been previously documented by researchers such as Najib et al. [36] and Umair et al. [25], further supporting the results of this study.

Moreover, the presence of trace amounts of Al₂O₃ aligns with previous research findings, which indicate that minor alumina content in ceramic packed-bed filtration contributes to increased thermal stability and mechanical strength [86,87]. These enhancements reinforce the potential of the washed quartz packed-bed filtration as a promising material for oil-water separation applications. The comparative analysis of the washed and raw quartz packed-bed filtration underscores the beneficial effects of the washing process in modifying the packed-bed filtration’s surface characteristics, thereby optimizing its separation efficiency and long term stability.

Figure 4 illustrates the X-ray diffraction (XRD) spectrum of the first-coated quartz packed-bed filtration, providing insights into its crystalline structure and phase composition following surface modification. The predominant silicon dioxide (SiO₂) phase remains clearly defined, as evidenced by the sharp and intense diffraction peaks at characteristic 2θ values (highlighted in red). The presence of well-resolved peaks confirms the high crystallinity of the material, which is essential for ensuring structural stability, mechanical strength, and resistance to chemical degradation in packed-bed filtration applications [88,89].

A key distinction between the first-coated quartz and the untreated (raw) and washed quartz samples is the enhanced presence of phosphorus pentoxide (P₂O₅) (marked in green) and aluminum oxide (Al₂O₃) (marked in blue). The incorporation of P₂O₅ indicates a significant improvement in hydrophilicity, which plays a crucial role in increasing water permeability and reducing oil adhesion—key factors in optimizing the performance of oil-water separation packed-bed filtration. Phosphorus-based surface modifications have been extensively reported to improve packed-bed filtration wettability and mitigate fouling, thereby enhancing long term filtration efficiency [90,91].

Additionally, the presence of Al₂O₃, though in relatively minor quantities, suggests that the coating introduces alumina reinforcements. This inclusion has been shown to improve the packed-bed filtration’s thermal stability, mechanical robustness, and overall durability under operational stresses. Similar findings have been documented by Haleemah et al. [78], further validating the beneficial effects of alumina incorporation. Overall, the XRD analysis confirms that the first coating process effectively enhances the quartz packed-bed filtration’s structural and functional properties, making it a highly suitable candidate for advanced oil-water separation applications.

3.2. Surface Morphology and Elemental Composition Analysis via SEM and EDS Findings

To complement the X-ray diffraction (XRD) findings presented in Section 3.1, Scanning Electron Microscopy (SEM) coupled with Energy-Dispersive X-ray Spectroscopy (EDS) was employed to analyze the surface morphology and elemental distribution of raw quartz packed-bed filtration. This investigation provides insight into the microstructural and compositional attributes critical to understanding packed-bed filtration performance in oil–water separation.

Figure 5a–e illustrates the SEM micrograph and corresponding EDS elemental mapping of raw quartz. The unprocessed quartz surface, shown in Figure 5a, displays a heterogeneous morphology with pronounced surface roughness and irregular grain boundaries. These features suggest a limited capacity for uniform oil repellence, potentially contributing to increased fouling and reduced separation efficiency. The elemental mapping confirms the presence of several key elements integral to quartz’s structure and performance.

In Figure 5b, carbon (C-K) is detected in minor concentrations, likely associated with trace organic contaminants or carbonates adsorbed on the surface. The oxygen distribution (Figure 5c) is widespread and homogenous, as expected for silicon dioxide (SiO₂)-based materials. Figure 5d reveals localized occurrences of aluminum (Al-K), supporting the XRD findings of minor alumina content, which could contribute to enhanced mechanical strength. Most notably, Figure 5e highlights a dense and uniform distribution of silicon (Si-K), reinforcing the quartz’s silicon-rich composition and structural integrity.

The compositional data from SEM mapping is corroborated by the EDS spectrum shown in Figure 6, which quantifies the elemental intensities present in the raw quartz. The spectrum prominently features silicon (Si-K) and oxygen (O-K) peaks, consistent with the dominance of SiO₂ in the material. Smaller peaks corresponding to aluminum (Al-K) and carbon (C-K) validate the presence of trace alumina and possible surface-bound carbonaceous materials. These findings affirm that the raw quartz packed-bed filtration retains its fundamental crystalline structure while exhibiting minor impurities, which may influence its baseline hydrophilicity and separation performance.

Collectively, the SEM and EDS analyses provide crucial evidence that while raw quartz maintains structural coherence and elemental purity, its surface lacks the functionalization necessary for effective oil-water separation. This aligns with the XRD observations and sets a foundational benchmark for comparing the improvements achieved through washing and nanoparticle coating, discussed in the subsequent sections. The morphological and compositional limitations identified here reinforce the importance of surface engineering in enhancing quartz packed-bed filtration performance for sustainable water treatment applications.

Figure 7a–f displays SEM and elemental mapping images of the acid-washed quartz packed-bed filtration. The SEM micrograph in Figure 7a reveals a rugged and textured surface topography, distinct from the more compact structure observed in raw quartz. The increased surface roughness and porosity are indicative of successful surface cleaning, which may facilitate enhanced fluid interaction during filtration.

Elemental mapping results show the spatial distribution of key elements across the quartz surface. Figure 7b,c confirm the widespread presence of carbon (C-K) and oxygen (O-K), respectively, which are common in naturally occurring quartz and residual organic films. Figure 7d shows a sparse distribution of aluminum (Al-K), suggesting trace alumina contamination likely retained during pre-treatment. More significantly, Figure 7e illustrates a homogenous spread of silicon (Si-K), verifying the material’s silica-rich composition. Notably, Figure 7f reveals the incorporation of phosphorus (P-K), not observed in the raw quartz, confirming that the acid-washing process introduced phosphorus-containing species, such as phosphates, onto the surface.

These elemental observations are corroborated by the EDS spectrum shown in Figure 8, which quantifies the detected elements in the washed quartz sample. Peaks corresponding to O-K and Si-K dominate the spectrum, consistent with the quartz mineral’s chemical makeup. Additional, albeit smaller, peaks for Al-K and P-K support the successful deposition or exposure of alumina and phosphorus compounds. These elemental enhancements are crucial, as both Al₂O₃ and P₂O₅ have been associated with improved packed-bed filtration hydrophilicity, oil rejection efficiency, and resistance to fouling.

The phosphorus enrichment in washed quartz indicates a functional surface modification that contributes to increased packed-bed filtration wettability—essential for repelling oil and facilitating water transport. Compared to the raw quartz (Figure 5 and Figure 6), the washed sample demonstrates not only a cleaner and rougher surface but also an altered elemental composition more favorable for separation applications.

The SEM and EDS analyses of the washed quartz packed-bed filtration confirm that acid washing does more than remove physical impurities; it also modifies the packed-bed filtration’s surface chemistry. These structural and compositional enhancements are pivotal in advancing the packed-bed filtration’s performance, setting a foundational improvement stage before the nanoparticle coating explored in the subsequent section.

Figure 9a–g presents the SEM image and EDS elemental maps of the first-coated quartz packed-bed filtration. The SEM image in Figure 9a illustrates a compact yet textured morphology, suggesting successful nanoparticle adherence. Compared to raw and washed quartz, the coated surface appears denser with finer textures, likely enhancing packed-bed filtration wettability and reducing fouling potential.

The EDS elemental distribution maps (Figure 9b–g) reveal the presence of key elements including carbon (C-K), oxygen (O-K), fluorine (F-K), aluminum (Al-K), silicon (Si-K), and phosphorus (P-K). The uniform presence of oxygen and silicon (Figure 9c,f, respectively) confirms the structural continuity of the quartz matrix. Notably, the appearance of phosphorus and fluorine in Figure 9d,g signifies successful integration of hydrophilic additives such as P₂O₅ and possibly F-doped components, which are known to improve surface energy and wettability. The aluminum distribution (Figure 9e) suggests the incorporation of Al₂O₃, which has been reported to enhance mechanical durability and stability under thermal stress.

Figure 10 further corroborates these observations, showing a pronounced increase in the EDS peaks corresponding to Si-K and O-K, which confirms the preservation of the quartz backbone. Importantly, the spectrum also displays additional peaks for P-K, F-K, and Al-K, reinforcing the successful deposition of the intended nanoparticle coatings. The presence of fluorine is particularly significant as it indicates possible surface passivation, which contributes to anti-fouling characteristics—a crucial factor in packed-bed filtration longevity and performance consistency.

The SEM and EDS analyses of the first-coated packed-bed filtration demonstrate the effective integration of hydrophilic nanoparticles, resulting in a structurally intact and compositionally enriched packed-bed filtration surface. These enhancements not only increase the packed-bed filtration’s water affinity but also reduce fouling susceptibility, aligning well with the performance trends observed in oil rejection and permeate flux. The morphological and elemental improvements observed in the coated packed-bed filtration underscore the pivotal role of surface modification in advancing quartz packed-bed filtration technology for efficient oil-water separation.

3.3. Oil and Grease Analysis Findings

Table 2. Oil and Grease Analysis Results.

Quartz Material	Oil Concentration (mg/L ± SD)	Impact of Hydrophilic Nanoparticle Coating
Oil and water mixture	183,754.8 ± 1035.6	Baseline oil concentration before filtration, representing the initial contamination level.
Raw	1859.8 ± 45.6	Limited oil removal due to inherent surface roughness and lack of hydrophilic functionalization, leading to weak water affinity and increased oil adhesion.
Washed	1583.7 ± 38.9	Moderate oil rejection due to surface cleaning and removal of organic contaminants, improving wettability and reducing oil deposition on the packed-bed filtration surface.
First coating	29.3 ± 2.5	Achieved the highest oil removal efficiency, significantly enhanced hydrophilicity, reduced packed-bed filtration fouling, and increased water flux. Hydrophilic nanoparticles optimized surface interactions.

presents the results of the oil and grease analysis, highlighting the effectiveness of quartz packed-bed filtration at different processing stages in reducing oil concentration. The initial oil concentration in the oil-water mixture before filtration was measured at 183,754.8 mg/L, serving as the baseline for evaluating the filtration efficiency of the raw, washed, and first-coated quartz packed-bed filtration. The findings demonstrate a progressive improvement in oil removal as the quartz packed-bed filtration undergoes surface modifications, with the first-coated packed-bed filtration exhibiting the highest efficiency.

The raw quartz packed-bed filtration, with an oil concentration of 1859.8 mg/L post-filtration, showed limited oil removal performance. This inefficiency can be attributed to the inherent surface roughness and lack of hydrophilic functionalization, which reduced its affinity for water and limited its capacity to repel oil. These observations align with the XRD findings, where the raw quartz primarily exhibited silicon dioxide (SiO₂) peaks, with no significant presence of hydrophilic phases such as phosphorus pentoxide (P₂O₅) or aluminum oxide (Al₂O₃). SEM analysis of the raw quartz revealed a rough and heterogeneous surface morphology, while EDS mapping (Figure 5) confirmed the dominance of silicon and oxygen with negligible traces of carbon and aluminum, indicating minimal surface modification. These structural and compositional features corroborate the packed-bed filtration’s limited ability to repel oil, a limitation also reported in the literature by Yang et al. [91] and Zhuang et al. [92].

The washed quartz packed-bed filtration demonstrated an improved oil rejection capability, with a reduced oil concentration of 1583.7 mg/L. The enhanced separation efficiency can be linked to the removal of organic contaminants and surface impurities during the washing process. XRD analysis of the washed quartz revealed the presence of minor peaks corresponding to Al₂O₃, P₂O₅, and elemental phosphorus (P), indicating slight modifications to the quartz surface that contributed to its improved hydrophilicity. SEM imaging (Figure 7) showed a smoother surface morphology compared to raw quartz, while EDS spectra confirmed a more prominent presence of aluminum, phosphorus, and oxygen. These enhancements suggest improved surface wettability, facilitating better interaction with water molecules. Literature reports by Najib et al. [93] and Umair et al. [26] also support the role of surface cleaning in enhancing packed-bed filtration performance by increasing water affinity and minimizing oil adhesion.

The first-coated quartz packed-bed filtration exhibited the most significant reduction in oil concentration, with only 29.3 mg/L remaining in the permeate. This remarkable performance improvement is attributed to the incorporation of hydrophilic nanoparticles, which substantially enhanced packed-bed filtration wettability, reduced fouling, and increased water permeability. XRD analysis confirmed the presence of high levels of P₂O₅ and Al₂O₃, which have been reported in the literature as key contributors to hydrophilic surface modification [94,95]. SEM micrographs (Figure 9) revealed a highly textured and compact surface morphology, indicating successful nanoparticle adherence. EDS analysis supported these findings, with marked peaks for phosphorus, fluorine, and aluminum, suggesting an enriched surface chemistry conducive to oil-water separation. Additionally, post-experimental SEM and EDS analyses were performed to examine any morphological and compositional changes in the packed-bed filtration after repeated use.

The results revealed that the first-coated quartz packed-bed filtration maintained its compact and textured surface morphology, with no significant signs of structural degradation or pore deformation. EDS spectra further confirmed the persistence of key surface elements, including Si, O, P, and Al, indicating that the hydrophilic nanoparticle coatings remained intact after multiple filtration cycles. No noticeable leaching or elemental loss was observed, affirming the chemical stability of the surface modification. These findings suggest that the packed-bed filtration’s surface chemistry and morphology were resilient to the mechanical and chemical stresses induced by filtration and backflushing.

The stability of the coated packed-bed filtration underlines its suitability for prolonged industrial use, where both consistent performance and durability are critical. The integration of Al₂O₃ in trace amounts further reinforced the packed-bed filtration’s structural durability and resistance to thermal and chemical degradation, as observed in studies by Najah et al. [96].

Overall, the findings of the oil and grease analysis strongly correlate with the results from XRD, SEM, and EDS analyses, reinforcing the conclusion that surface modification significantly enhances packed-bed filtration efficiency. The first-coated quartz packed-bed filtration emerges as the optimal candidate for oil-water separation, outperforming conventional packed-bed filtration by achieving a balance between high separation efficiency, improved hydrophilicity, and long term operational stability. These results align with previous research that emphasizes the importance of surface engineering in optimizing packed-bed-filtration-based separation technologies [97,98].

The stability of quartz-based packed-bed filtration following oil–water separation was critically assessed to ensure their feasibility for long term industrial applications. Post-experimental characterization revealed that the structural integrity of the first-coated quartz packed-bed filtration was preserved, as confirmed by consistent XRD peak patterns and SEM surface morphology across repeated filtration cycles. Additionally, the coated packed-bed filtration demonstrated excellent fouling resistance, which minimized irreversible oil accumulation and reduced the need for aggressive chemical cleaning.

The packed-bed filtration was subjected to multiple cycles of filtration and backflushing with deionized water, after which performance recovery remained above 90% of the original flux, indicating strong regeneration capacity. The hydrophilic nanoparticle coating (comprising primarily P₂O₅ and Al₂O₃) retained its functional properties without significant degradation, contributing to sustained oil rejection efficiency.

These findings suggest that quartz packed-bed filtration—particularly the coated variant—exhibits stable physicochemical properties under repeated use and moderate cleaning procedures, affirming its suitability for prolonged deployment in oily wastewater treatment. Future studies should focus on evaluating packed-bed filtration stability under more aggressive cleaning protocols and harsher wastewater compositions to further confirm long term durability and regeneration efficiency.

3.4. Statistical Optimization and Performance Assessment of Quartz Packed-Bed Filtration (Raw, Washed, and First Coated) Using Minitab: Taguchi Method and Regression Analysis

The Taguchi L18 orthogonal array presented in

Table 3. Taguchi L18 Orthogonal Array for Assessing Quartz Packed-Bed Filtration Performance.

Run	Quartz Status	Pressure (bar)	Volume Flow Rate (L/m)	Oil Concentration
1	1 (Raw)	0.5	0.76	149.25
2	1 (Raw)	1	0.68	130.8
3	1 (Raw)	1.5	0.64	115.5
4	1 (Raw)	2	0.56	98.75
5	1 (Raw)	2.5	0.52	82.4
6	1 (Raw)	3	0.44	65.9
7	2 (Washed)	0.5	0.76	120.1
8	2 (Washed)	1	0.68	105.3
9	2 (Washed)	1.5	0.64	91.25
10	2 (Washed)	2	0.56	78.6
11	2 (Washed)	2.5	0.52	65.8
12	2 (Washed)	3	0.44	50.2
13	3 (Coated)	0.5	0.85	10.5
14	3 (Coated)	1	0.75	8.2
15	3 (Coated)	1.5	0.72	6.7
16	3 (Coated)	2	0.64	5.1
17	3 (Coated)	2.5	0.6	2.3
18	3 (Coated)	3	0.5	1.8

provides a structured experimental approach to evaluate the performance of quartz packed-bed filtration under varying operational conditions. Minitab Statistical Software (version 21.3) was used to perform the Taguchi design of experiments, applying an L18 orthogonal array to evaluate the influence of pressure and quartz surface condition on packed-bed filtration performance. In this study, the Taguchi method was employed to statistically optimize the operational conditions of quartz packed-bed filtration for oil–water separation. The core objective was to minimize the residual oil concentration in the permeate, as lower oil levels directly correlate with higher separation efficiency and improved water quality. Therefore, the “smaller-is-better” signal-to-noise (S/N) ratio was selected to represent this performance goal. This design incorporates two critical variables: quartz packed-bed filtration type (raw, washed, and first coated) at three levels and operating pressure at six levels (ranging from 0.5 to 3 bar). By systematically varying these factors, the study aims to assess their impact on key performance metrics, including volume flow rate (L/m) and oil concentration (mg/L). The Taguchi method was employed for its effectiveness in handling multi-factorial experiments with a reduced number of trials, allowing for optimal parameter identification while minimizing resource consumption. The use of an L18 mixed-level array ensures a balanced representation of different parameter interactions, offering valuable insights into the optimal operating conditions for quartz packed-bed filtration in oil-water separation applications. Similar pressure conditions have been examined in previous studies by Antusia et al. [43] and Umair et al. [25], further validating the significance of pressure variation in influencing packed-bed filtration performance.

From the analysis of the data presented in Table 3, several trends can be observed. The first-coated quartz packed-bed filtration consistently exhibited the lowest oil concentration values, confirming its enhanced hydrophilicity, superior oil rejection efficiency, and reduced fouling tendencies. In contrast, raw quartz demonstrated the highest oil retention, indicating limited surface functionalization and lower separation efficiency. The washed quartz packed-bed filtration exhibited intermediate performance, with moderate improvements in oil rejection due to the removal of surface impurities and increased surface energy. Furthermore, the results revealed a distinct inverse relationship between operating pressure and oil concentration, particularly for the first-coated quartz packed-bed filtration, which showed significant oil rejection improvements at higher pressures. At pressure levels of 2.5–3 bar, oil retention was considerably reduced, indicating that higher driving forces facilitate more efficient phase separation.

In addition to oil rejection efficiency, variations in volume flow rate were also observed among the different packed-bed filtration types. First-coated quartz demonstrated the highest permeability, maintaining a stable and high flux under optimal pressure conditions. Raw quartz exhibited the lowest permeability, likely due to its higher surface roughness and lower wettability, which hinder effective water passage. Washed quartz, on the other hand, displayed moderate permeability, further reinforcing the influence of surface modification on filtration efficiency. These findings underscore the importance of packed-bed filtration surface enhancement in improving overall performance in oil-water separation applications.

The results of this study confirm that first-coated quartz packed-bed filtration outperforms raw and washed quartz packed-bed filtration, making them the most promising candidates for high-efficiency oil-water separation applications. The integration of the Taguchi experimental design and statistical optimization enabled the identification of the best-performing packed-bed filtration configurations and operating conditions. This study makes a novel contribution to packed-bed filtration technology by demonstrating the effectiveness of surface modification with hydrophilic nanoparticles and optimizing operational parameters using Taguchi and regression analysis. These insights serve as a foundation for further refinement and large-scale application of quartz packed-bed filtration in industrial water recovery and wastewater treatment processes.

The analysis of Figure 11 illustrates the impact of operational pressure on oil concentration across different quartz packed-bed filtration types, namely raw, washed, and first-coated quartz. The observed trend indicates a consistent inverse relationship between pressure and oil concentration, with a noticeable decrease in oil content as pressure increases. This trend underscores the enhancement of oil-water separation efficiency with increasing pressure. However, the degree of improvement varies depending on the packed-bed filtration modification state, with the first-coated quartz exhibiting the most efficient separation performance.

For the raw quartz packed-bed filtration, oil concentration remains relatively high across all pressure levels, starting at its peak at 0.5 bar and gradually decreasing with increasing pressure up to 3 bar. Despite this decline, the oil concentration in the treated water remains significantly high, indicating that raw quartz lacks the necessary surface properties for efficient oil rejection. The minimal reduction suggests that the intrinsic surface roughness and absence of hydrophilic functionalization limit its oil-repelling capabilities, resulting in ineffective phase separation.

Washed quartz, in comparison, demonstrates a moderate improvement in oil rejection. The oil concentration reduction is more pronounced at lower pressures, highlighting the effectiveness of surface cleaning and the removal of organic contaminants [92,93]. The washing process enhances the surface hydrophilicity of the packed-bed filtration, improving its affinity for water molecules and slightly enhancing its oil rejection efficiency compared to raw quartz. However, despite these improvements, washed quartz still exhibits limitations in achieving significant oil removal at lower pressures, necessitating higher pressures to attain improved separation efficiency.

The first-coated quartz packed-bed filtration exhibits the most remarkable reduction in oil concentration across all pressure levels. Even at the lowest pressure (0.5 bar), the oil content is substantially lower compared to both raw and washed quartz. As pressure increases, the oil concentration further decreases, nearing complete oil rejection at 2.5–3 bar. This superior performance can be attributed to the enhanced surface modification achieved through nanoparticle coating, which significantly improves packed-bed filtration hydrophilicity, permeability, and resistance to fouling [94,95]. The presence of aluminum oxide (Al₂O₃) and phosphorus pentoxide (P₂O₅), as confirmed by X-ray diffraction (XRD) analysis, contributes to structural reinforcement and optimized surface chemistry, facilitating superior phase separation [96,97].

A key observation is that the most significant reduction in oil concentration occurs within the pressure range of 0.5–2 bar, beyond which the rate of decline slows. This diminishing return suggests that beyond a certain threshold, increasing pressure further does not yield substantial improvements in separation efficiency, likely due to packed-bed filtration resistance and surface saturation effects. The distinct contrast in performance across the different quartz packed-bed filtration highlights the predominant role of surface modification over pressure alone, demonstrating that optimized coating significantly enhances separation efficiency without necessitating excessive pressure application.

Furthermore, the findings align with the mathematical models developed in this study, where regression analysis effectively predicts the relationship between pressure and oil rejection efficiency. The Taguchi analysis further supports these results by identifying the coated packed-bed filtration as the optimal configuration for enhanced separation performance. The statistical optimization results emphasize that surface treatment plays a more critical role than pressure alone in achieving effective oil removal.

Overall, the first-coated quartz packed-bed filtration emerges as the most efficient oil-water separation candidate, achieving the lowest oil concentration across all operating pressures. The study reinforces that surface modification, rather than pressure alone, is the dominant factor influencing packed-bed filtration performance, making the first-coated quartz packed-bed filtration the optimal choice for industrial-scale oil-water separation applications.

The Analysis of Means (ANOM) plot presented in Figure 12 provides a detailed evaluation of the influence of operating pressure and quartz packed-bed filtration type on oil separation efficiency. The application of the Taguchi method, using the “smaller-is-better” criterion, highlights a clear distinction in performance among the three quartz packed-bed filtration conditions: raw, washed, and first-coated quartz. The results demonstrate a strong inverse relationship between operating pressure and oil concentration, with increasing pressure leading to a notable decline in residual oil content. However, the extent of this improvement is highly dependent on the surface modifications of the packed-bed filtration, reinforcing the critical role of material engineering in packed-bed filtration efficiency [98,99].

Among the three quartz packed-bed filtration types, raw quartz consistently exhibits the highest mean oil concentration across all pressure levels, confirming its limited oil rejection capabilities. This outcome is likely due to its unmodified surface, which lacks the hydrophilic functionalization necessary for effective oil-water separation. The rough and untreated quartz surface allows oil droplets to adhere more easily, reducing separation efficiency and requiring higher pressure to achieve moderate performance improvements [100,101]. In contrast, washed quartz shows a measurable improvement, with a more pronounced reduction in oil concentration, particularly at lower pressure levels. This enhancement can be attributed to the removal of surface impurities and organic contaminants during the washing process, which improves the surface hydrophilicity of the packed-bed filtration. However, while washed quartz performs better than raw quartz, it still does not achieve optimal oil rejection at all pressure conditions.

The most significant reduction in oil concentration is observed in the first-coated quartz packed-bed filtration, which demonstrates superior separation efficiency at every pressure level. The coating process enhances the surface properties of the packed-bed filtration, promoting increased hydrophilicity and minimizing oil adhesion. This improvement results in a substantial drop in oil concentration at lower pressure levels, with the most drastic reduction occurring between 0.5 and 2.5 bar. Beyond this threshold, the rate of oil concentration decline stabilizes, indicating a diminishing effect of pressure on separation performance. This suggests that while an increase in pressure initially facilitates oil removal, excessive pressure application may not yield significant additional benefits. Instead, the coated packed-bed filtration’s surface properties play a more dominant role in achieving high oil rejection efficiency, reducing the reliance on pressure-driven separation.

The statistical significance illustrated in the ANOM plot underscores that surface modification is a key determinant in oil separation efficiency. The distinct gap in mean oil concentration values between coated quartz and the other two packed-bed filtration conditions highlights the effectiveness of functionalized surface coatings. The findings suggest that engineered surface properties significantly enhance the interaction between oil droplets and the packed-bed filtration surface, preventing fouling and improving phase separation. These results further validate that optimizing packed-bed filtration surface characteristics can achieve superior separation efficiency while minimizing energy consumption. Thus, the first-coated quartz packed-bed filtration emerges as the most efficient candidate for industrial oil-water separation applications, providing a practical, cost-effective, and high-performance alternative to conventional filtration technologies.

The regression equation presented in Equation (13) serves as a predictive model for estimating oil concentration (OC) in the permeate as a function of applied pressure (P) and quartz packed-bed filtration status (Q). Developed using Minitab statistical software, this equation provides a robust mathematical framework to quantify the influence of pressure variations and surface modifications on oil-water separation efficiency. Where OC represents the oil concentration in mg/L in the permeate, P denotes the applied pressure (bar), ranging from 0.5 to 3 bar. Q represents the quartz packed-bed filtration condition, where Q = 1 corresponds to raw quartz, Q = 2 to washed quartz, and Q = 3 to first-coated quartz.

$$OC=187.3-41.0 P-24.3 Q-6.99 \mathrm{P}\ast P+0.3 \mathrm{Q}\ast \mathrm{Q}+6.85P\ast \mathrm{Q}$$

(13)

The negative coefficients of P (−41.0) and Q (−24.3) indicate that increasing pressure and improving quartz surface treatment both contribute significantly to reducing oil concentration. This aligns with the experimental results, where a consistent decrease in oil concentration was observed as pressure increased and further declined when quartz was washed or coated.

The quadratic term −6.99 P² suggests a non-linear relationship between pressure and oil concentration, implying that while an increase in pressure enhances oil removal, the effect diminishes beyond a critical threshold. This observation is consistent with the trends depicted in Figure 4 and Figure 5, where the most substantial oil concentration reductions occurred at pressures below 2.5 bar, with only marginal improvements beyond this level. This phenomenon is also reflected in the ANOM analysis, which revealed that oil rejection efficiency gains plateau at higher pressures, reinforcing the notion that excessive pressure application does not yield proportionate improvements in performance.

Additionally, the interaction term 6.85 P · Q suggests that the combined effect of pressure and quartz status is not merely additive. Instead, at higher pressures, the impact of surface modification becomes increasingly pronounced. This finding underscores the dominant role of surface engineering in enhancing oil rejection efficiency, as opposed to relying solely on pressure-driven separation.

From the regression model, first-coated quartz (Q = 3) consistently exhibits the lowest predicted oil concentration across all pressure levels, further validating the results obtained from XRD. These characterization techniques confirmed that the first-coated packed-bed filtration had superior surface wettability, minimal agglomeration, and enhanced structural integrity, leading to optimal oil-water separation performance. In contrast, raw quartz (Q = 1) predicts significantly higher oil concentrations, reflecting its unmodified hydrophobic nature, which lacks the necessary functional groups for effective oil rejection. Washed quartz (Q = 2) demonstrates improved performance compared to raw quartz due to the removal of organic impurities and enhanced hydrophilicity. However, it still underperforms relative to the first-coated packed-bed filtration, indicating that surface cleaning alone is insufficient to achieve optimal separation efficiency.

The regression equation effectively validates the experimental findings from Figure 4 and Figure 5, confirming that pressure and surface modification are the primary determinants of oil concentration in the filtrate. Notably, the first-coated quartz packed-bed filtration consistently demonstrates the most efficient oil rejection, reinforcing the assertion that surface engineering provides a more sustainable, efficient, and energy-conserving solution for oil-water separation compared to pressure-based enhancements alone.

4. Discussion

The results presented in this study offer an in-depth understanding of how surface engineering enhances the performance of quartz packed-bed filtration in oil–water separation. The integrated use of XRD, SEM-EDS, oil and grease analysis, and statistical optimization enables a holistic assessment of packed-bed filtration behavior across different treatment stages—raw, acid-washed, and nanoparticle-coated.

The structural insights provided by Figure 2, Figure 3 and Figure 4 confirm the crystalline stability of quartz and reveal elemental enhancements post-treatment. In particular, Figure 4 shows increased P₂O₅ and Al₂O₃ peaks in the coated packed-bed filtration, correlating with its high hydrophilicity. These findings are further supported by SEM and EDS results (Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10), which highlight improved surface morphology and the successful integration of hydrophilic nanoparticles in the coated packed-bed filtration [102,103]. Specifically, Figure 9a–g demonstrates uniform nanoparticle distribution and denser surface packing, essential for oil repellence and reduced fouling.

These structural and morphological improvements directly translated into enhanced separation performance. As summarized in Table 2, the coated packed-bed filtration reduced the residual oil concentration to 29.3 ± 2.5 mg/L, compared to 1859.8 ± 45.6 mg/L (raw) and 1583.7 ± 38.9 mg/L (washed), highlighting the direct impact of surface chemistry. The EDS spectrum in Figure 10 confirms the sustained presence of phosphorus and fluorine post-filtration, reinforcing packed-bed filtration stability and fouling resistance [50,51,52].

The link between pressure and packed-bed filtration performance was quantitatively established via the Taguchi method (Table 3). Here, packed-bed filtration type (coded as 1 = raw, 2 = washed, 3 = coated) and pressure were varied, with the coated packed-bed filtration consistently outperforming others at every pressure level. Figure 11 graphically illustrates the inverse relationship between pressure and oil concentration, validating the regression model in Equation (13). As pressure increased to 2.5 bar, oil concentration dropped sharply—particularly for the coated packed-bed filtration—before plateauing. This trend was further confirmed in the ANOM plot Figure 12, reinforcing the conclusion that surface modification, more than pressure alone, drives performance.

Moreover, packed-bed filtration regeneration and durability were confirmed through post-filtration SEM and EDS analyses, which revealed no significant structural degradation or leaching [104,105,106]. A >90% flux recovery rate after backwashing underscores the long term usability and low maintenance requirements of the coated packed-bed filtration. Finally, the comparative overview in Table 1 positions the first-coated quartz packed-bed filtration as a superior alternative to conventional ceramic and polymeric membranes, offering 95–99% filtration efficiency, excellent fouling resistance, and extended operational lifespan (12–24 months). These characteristics make it both a technically and economically viable solution for industrial oily wastewater treatment.

Potential Industrial Applications and Practical Implementation of Quartz-Based Packed-Bed Filtration: The quartz particle-based packed-bed filtration system demonstrated in this study offers substantial potential for practical industrial implementation, particularly in sectors producing significant volumes of oily wastewater, including petrochemicals, metalworking, automotive manufacturing, food processing, and petroleum refining. Compared to conventional polymeric membranes such as PVDF and PTFE, which often suffer rapid fouling and require frequent chemical cleaning, the quartz-based filtration medium substantially reduces operational maintenance due to its robust antifouling properties and simple physical backwashing capability [107,108,109]. Moreover, quartz-based filtration outperforms traditional ceramic membranes (alumina or zirconia-based systems), which typically exhibit brittleness and higher production costs [110,111,112]. This novel quartz-based system offers comparable mechanical durability and chemical resistance at significantly lower costs, enhancing its economic feasibility for widespread industrial adoption.

Industrially, this filtration system can be seamlessly integrated by utilizing standard pressure-driven filtration modules that align with current plant infrastructure, including existing pumps, pressure control, and automated backwashing systems. Its chemical and thermal stability makes it particularly suitable for challenging industrial environments with elevated temperatures or chemically aggressive wastewater streams. Industries can reliably achieve stringent wastewater discharge standards due to the demonstrated high oil rejection (>98%) and excellent flux recovery (>90%) over extended operational periods (12–24 months). Recent literature supports these advantages, emphasizing growing industrial interest in sustainable and cost-effective separation technologies such as modified quartz filtration, particularly in resource-constrained settings seeking sustainable wastewater management solutions.

5. Conclusions

This study conducted a comprehensive evaluation of quartz packed-bed filtration at various stages of modification—namely raw, acid-washed, and nanoparticle-coated—with an emphasis on how surface engineering influences oil–water separation performance. The surface modification with hydrophilic nanoparticles containing primarily P₂O₅ and Al₂O₃ emerged as a critical advancement, delivering notable improvements in packed-bed filtration structure, wettability, and separation efficiency.

Firstly, SEM and EDS analyses revealed that the nanoparticle-coated packed-bed filtration developed a smoother and more compact surface morphology, with uniform elemental distribution. The elemental enrichment in phosphorus and aluminum played a central role in enhancing packed-bed filtration hydrophilicity and anti-fouling characteristics, distinguishing the coated packed-bed filtration from the raw and acid-washed variants.

Secondly, filtration performance testing using synthetic oily wastewater demonstrated that the coated packed-bed filtration achieved the highest oil rejection efficiency (98.4%) and the lowest residual oil concentration (29.3 mg/L), substantially outperforming the raw (1859.8 mg/L) and acid-washed (1583.7 mg/L) packed-bed filtration. These performance gains were attributed to increased water affinity and reduced oil adhesion conferred by the nanoparticle layer.

Thirdly, Taguchi optimization and regression modeling confirmed that surface modification exerted a greater influence on performance than pressure variation alone. The optimum performance was recorded at an operational pressure of 2.5 bar for the coated packed-bed filtration, consistent with the “smaller-is-better” design objective used in the optimization framework.

Lastly, stability and regeneration assessments indicated that the coated packed-bed filtration retained over 90% of its original flux following multiple backwashing cycles, confirming the robustness of the nanoparticle coating and its capacity for repeated use without structural degradation.

The integration of hydrophilic nanoparticles significantly improved quartz packed-bed filtration performance by enhancing structural uniformity, increasing hydrophilicity, and reducing fouling. These improvements not only contribute to superior oil–water separation but also support the long term viability and economic feasibility of quartz-based packed-bed filtration for industrial wastewater treatment. The coated quartz packed-bed filtration, in particular, represents a sustainable and scalable alternative to conventional polymeric and ceramic systems, with strong potential for real-world application in the context of global water scarcity and environmental sustainability. Building upon the promising results of this study, future research will focus on multilayer and gradient coatings using advanced nanomaterials to improve fouling resistance, permeability, and mechanical resilience. The improvements in oil rejection performance strongly suggest enhanced surface wettability due to hydrophilic nanoparticle coatings. However, it is essential to clarify that this inference was made indirectly, based on SEM-EDS compositional evidence and filtration results rather than direct contact angle measurements. Direct measurement of surface wettability is recommended for future research to explicitly confirm these findings.