Scalable synthesis and comprehensive characterization of nanostructured silica derived from Ethiopian pumice for high-performance rubber applications

Abstract

The growing demand for sustainable, high-performance rubber materials motivates the development of eco-friendly silica fillers. This study presents a scalable synthesis of nanostructured silica from Ethiopian pumice via beneficiation, alkaline extraction, and optimized sol–gel precipitation. Key synthesis parameters; reaction temperature (60–90 °C), precipitation pH (7–9), and calcination temperature (600–750 °C); were systematically varied. Comprehensive characterization (FTIR, XRD, BET, XRF, XPS, TGA, SEM, EDS, HR-TEM) established correlations between synthesis conditions and properties critical for rubber reinforcement. Optimized samples indicated excellent properties for rubber reinforcement. P2-SiO₂ (synthesized at:75 °C, 650 °C, pH 8.5) showed 95.27% SiO₂ purity, nanoscale particle size of 115.54 nm, uniform morphology with moderate agglomeration, surface area of 402 m²/g, mesopore diameter of ~ 3.0 nm, reactive silanol groups, and thermal stability up to 800 °C. P3-SiO₂ (synthesized at: 85 °C, 700 °C, pH 8.5) demonstrated slightly higher performance, with 95.63% SiO₂ purity, particle size of 99.43 nm, uniform non-agglomerated morphology, surface area of 468 m²/g, mesopore diameter of ~ 3.2 nm, reactive silanol groups, and thermal stability up to 800 °C. Among the samples, P3-SiO₂ confirmed the most favorable combination of properties, making it a promising industrially scalable filler for high-performance rubber composites. In contrast, the Eco-Templated and sub-optimal samples (P1-SiO₂ and P4-SiO₂) showed lower SiO₂ purity, larger particle sizes, irregular morphology, and pronounced agglomeration, reducing their reinforcing effectiveness. These findings demonstrate that Ethiopian pumice is a sustainable source of cost-effective, high-performance silica, and further work on scale-up and surface functionalization is recommended to enable industrial applications.

Introduction

The global push for sustainable, high-performance materials in the rubber industry has accelerated the search for eco-efficient, cost-effective alternatives to conventional reinforcing fillers. Among these, nanostructured silica has garnered significant attention due to its exceptional physicochemical properties; high surface area, tunable porosity, surface reactivity, and compatibility with polymer matrices. These attributes make it highly desirable in numerous industrial sectors, particularly in elastomer reinforcement, electronics, catalysis, and coatings [1–4]. In rubber-based systems, silica acts as a multifunctional nanofiller, improving mechanical strength, abrasion resistance, rolling resistance, and wet traction, which are critical for high-performance applications such as green tires, conveyor belts, and industrial seals.

Despite its advantages, traditional chemical methods for producing high-purity silica; such as the sol–gel and pyrogenic (fumed) routes; rely heavily on synthetic precursors like tetraethyl orthosilicate (TEOS) and sodium silicate. These processes are often energy-intensive, costly, and generate significant hazardous by-products, posing challenges for sustainable large-scale implementation [5, 6]. The sol–gel technique, for instance, offers precise control over morphology, surface chemistry, and particle size distribution, but is not economically viable for mass production. Conversely, precipitation methods provide greater scalability and cost-effectiveness but often sacrifice structural precision [7, 8]. These constraints have prompted the exploration of more sustainable and regionally accessible silica sources.

The global shift toward sustainable materials has heightened interest in low-cost, silica-rich natural precursors for the production of nanostructured silica, especially for high-performance rubber composites. In this context, agro-waste and biogenic sources have emerged as promising alternatives. Agricultural residues such as rice husk ash (RHA), bamboo leaves, wheat husk, and sugarcane bagasse contain high levels of amorphous silica; often exceeding 90%;following proper calcination and acid leaching treatments [9–12].

These materials are abundant, renewable, and biodegradable, enabling the production of nanostructured silica with high surface functionality and reduced environmental impact. However, biogenic sources present several limitations: they are often seasonal, serve as feed for livestock, and are commonly used in rural construction practices. Moreover, their conversion into nanostructured silica typically requires an ablation or combustion step prior to chemical processing, which significantly increases the overall production cost. In Ethiopia, preliminary studies have demonstrated the successful extraction of amorphous silica from rice husk and teff straw, further validating the potential of diverse local sources for producing silica suitable for nanocomposite and rubber reinforcement applications [13, 14].

In addition to biogenic sources, natural mineral-based sources, especially silica-rich volcanic and sedimentary materials such as pumice, diatomite, bentonite, and kaolinite, have emerged as sustainable alternatives to synthetic silica precursors. Ethiopian pumice is particularly notable for its abundant availability in the Rift Valley and silica content typically ranging from 60 to 70 wt.% with low metallic impurities [15–17]. This contrasts with traditional silica sources such as quartz sand (~ 90–99 wt.% SiO₂, but requiring energy-intensive processing), rice husk ash (~ 85–95 wt.% SiO₂, with variable purity), and industrial precursors like sodium silicate and tetraethyl orthosilicate (TEOS), which are often costly and environmentally demanding.

The synthesis of nanostructured silica from pumice typically involves beneficiation, acid–base leaching, sol–gel processing, and calcination, resulting in amorphous or mesoporous silica structures suitable for advanced composites. Compared to synthetic routes, pumice-based methods offer regionally accessible and eco-friendly pathways that align with principles of green chemistry and circular economy [15, 18]. Recent developments in green silica synthesis have focused on scalable techniques such as sol–gel precipitation, acid leaching, and surfactant-templating using biomass ashes and mineral clays [19–21]. Moreover, bio-mediated methods employing plant extracts and microbial agents reduce hazardous chemicals and enhance morphological control, though challenges remain in yield consistency and scalability [22].

Despite its traditional use in low-value applications, Ethiopian pumice's physicochemical properties and regional availability position it as a promising feedstock for sustainable nanostructured silica production. Utilizing this resource supports regional economic development and aligns with sustainability goals, especially in high-volume industries like green tire manufacturing [23–25].

This study presents a scalable, optimized sol–gel synthesis of nanostructured silica from Ethiopian pumice. The process integrates beneficiation, alkaline leaching, controlled precipitation, and calcination, with systematic optimization of key parameters; reaction temperature (60–90 °C), precipitation pH (7–9), and calcination temperature (600–750 °C); to tailor purity, morphology, and particle size. The resulting material demonstrates enhanced compatibility and performance in rubber composites, advancing sustainable filler technologies through detailed structure–property–performance relationships. The novelty of this work lies in (i) demonstrating Ethiopian pumice as a sustainable, high-silica precursor, (ii) optimizing a green, scalable synthesis protocol balancing environmental impact and performance, and (iii) advancing the filler technology for sustainable rubber reinforcement through structure–property–performance correlation.

Materials and methods

Raw material collection and beneficiation

Pumice rock was collected from the Tatek quarry in Ethiopia’s Rift Valley region (Coordinates: 9.0829° N, 38.6354° E), a site known for its silica-rich volcanic deposits. The raw pumice was thoroughly washed with deionized water, oven-dried at 100 °C, ground by ball milling, sieved to particles < 75 µm, and then calcined at 500 °C to enhance chemical reactivity. X-ray fluorescence (XRF) and atomic absorption spectroscopy (AAS) analyses reported SiO₂ contents ranging from approximately 60% to 65%, confirming the material’s suitability as a precursor for silica extraction [26, 27].

Materials, chemicals, and equipment

All chemicals used in this study were of analytical grade to ensure high purity, consistency, and reproducibility. Sodium hydroxide (NaOH, ≥ 98%) was obtained from Merck (Germany), sulfuric acid (H₂SO₄, 98%) and hydrochloric acid (HCl, 37%) were purchased from Sigma-Aldrich (USA), and sodium dodecyl sulfate (SDS, ≥ 99%) was supplied by Thermo Fisher Scientific (USA). Deionized water was produced in-house using a Milli-Q® water purification system (Millipore, USA). Standard laboratory equipment was employed throughout the synthesis process, including a magnetic stirrer with hot plate (IKA C-MAG HS 7, Germany), a reflux system using standard borosilicate glassware (Quickfit®, UK), and a digital pH meter (HANNA Instruments HI2211, Romania). Thermal treatments were carried out using a drying oven (Memmert UN110, Germany) and a muffle furnace (Nabertherm LHT 02/17 LB, Germany). Solid–liquid separation was conducted using a vacuum filtration system (Büchi V-700, Switzerland), and phase separation was aided by a centrifuge (Eppendorf 5810 R, Germany). Precise weighing was performed using an analytical balance (Sartorius Entris224i-1S, Germany). This combination of high-grade reagents and calibrated instruments ensured reliable, scalable synthesis of nanostructured silica for high-performance rubber composite applications.

Synthesis of nanostructured SiO₂

Nanostructured silica (SiO₂) was synthesized from Ethiopian pumice through a scalable, surfactant-assisted method consisting of four major steps: beneficiation, alkaline leaching, sol–gel precipitation, and calcination, adapted with modifications from established protocols [19–21, 28]. Initially, the pumice powder was calcined and treated with a sodium hydroxide solution to extract silica in the form of soluble sodium silicate. This extract was subsequently acidified under controlled pH conditions to initiate the sol–gel transition, forming silica gel. The gel was aged, filtered, and thoroughly washed with deionized water to remove residual ions, then dried and calcined to produce high-purity nanostructured SiO₂. This synthesis route was optimized to achieve desirable particle size, morphology, and purity for effective application in rubber reinforcement. A schematic representation of the synthesis process is provided in Fig. 1.

Fig. 1.

Overall pictorial steps for the scalable synthesis of rubber-grade nanostructured silica from Ethiopian pumice

The primary reaction involves the formation of sodium silicate (Na₂O·x SiO₂) from silica-rich materials in a basic medium such as NaOH, as reported by Kaya et al. [29]:

Upon the addition of an acid such as H₂SO₄, gelation occurs through the following reaction at pH 8–9, as described by Mourhly [30]:

Figures 1, 2 presents a pictorial and schematic overview of the optimized, scalable sol–gel synthesis of rubber-grade nanostructured silica from Ethiopian pumice, highlighting the major processing steps, surfactant-templated gelation, and thermal treatment.

Fig. 2.

Schematic of the optimized scalable sol–gel route for rubber-grade nanostructured silica, highlighting key process steps, surfactant-templated gelation, and thermal treatment

Process optimization and scalability

To establish a scalable and environmentally sustainable synthesis route for nanostructured silica (SiO₂) from Ethiopian pumice, key process parameters were systematically optimized. The optimization aimed to enhance silica yield and purity while tailoring essential physicochemical properties such as particle size distribution, surface area, and porosity to meet the stringent performance requirements for rubber reinforcement applications, as supported by previous investigations [31, 32]. In this study, synthesis parameters such as reaction temperature, pH, reagent concentrations, reaction time, and surfactant templating were thoroughly examined. Certain conditions were held constant throughout, including a sodium silicate modulus of 3, stirring rate of 600 rpm, NaOH and H₂SO₄ concentrations at 2.5 M each, reaction time of 3 h, Si:NaOH molar ratio of 1:3, drying temperature of 100 °C, and aging time of 12 h [27].

The variables intentionally varied were synthesis temperature (60, 75, 80, 85, and 90 °C), calcination temperature (600, 650, 700, and 750 °C), and precipitation pH (7, 8.5, and 9) to assess their effects on silica yield, purity, structural properties, morphology, and thermal stability. Table 1 summarizes the optimized synthesis parameters and sample labels, highlighting key process conditions affecting morphology, purity, and surface functionality for rubber reinforcement. Established methods were modified to improve reproducibility and reduce environmental impact. Multiple batches under varied conditions ensured reliable, comprehensive characterization.

Table 1.

Optimized synthesis parameters and sample labels (‘P’ indicates processing parameter)

Sample code	Description (industrial version)
Raw-pumice	Unprocessed Ethiopian pumice from the Tatek quarry
Eco-templated SiO2	Nanostructured silica synthesized with 10 mL reduced SDS; 80 °C synthesis; 700 °C calcination; pH 8.5
P1-SiO2	Synthesized at 60 °C; calcined at 600 °C; pH 7
P2-SiO2	Synthesized at 75 °C; calcined at 650 °C; pH 8.5
P3-SiO2	Synthesized at 85 °C; calcined at 700 °C; pH 8.5
P4-SiO2	Synthesized at 90 °C; calcined at 750 °C; pH 9

Characterization methods

The structural, compositional, and morphological properties of the synthesized nanostructured silica were comprehensively characterized using advanced analytical techniques. Fourier-transform infrared (FTIR) spectra were acquired on a Nicolet iS10 spectrometer (Thermo Scientific, USA) with samples prepared as KBr pellets, scanning over the range 4000–400 cm⁻¹ at a resolution of 4 cm⁻¹ to identify key functional groups such as siloxane (Si–O–Si) and silanol (Si–OH).

Surface area, pore volume, and pore size of the silica samples synthesized from Ethiopian pumice were measured by nitrogen adsorption–desorption using a Micromeritics ASAP 2020 analyzer after degassing at 200 °C for 4 h. BET and BJH methods were applied for surface area and pore analysis, respectively.Crystallinity was assessed by XRD with a Bruker D8 Advance diffractometer (Cu Kα radiation, λ = 1.5406 Å), scanning 2θ from 10° to 80° at 0.02° steps, confirming predominantly amorphous silica in optimized samples.

Elemental composition was determined by X-ray fluorescence (XRF) using an Axios Max WDXRF spectrometer (PANalytical, Netherlands) operated under vacuum conditions with excitation voltages between 20 and 60 kV; powder pellets were pressed under 20 MPa for measurement. Surface chemical states were examined by X-ray photoelectron spectroscopy (XPS) on a PHI VersaProbe III (ULVAC-PHI, Japan) equipped with a monochromatic Al Kα source, with binding energies referenced to C 1 s at 284.8 eV. Thermal stability was assessed using thermogravimetric analysis (TGA) performed on a Q500 analyzer (TA Instruments, USA), heating approximately 10 mg of sample from 30 to 800 °C at 10 °C/min under nitrogen flow (60 mL/min).

Morphological features and elemental mapping were analyzed by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) on a Quanta 650 FEG SEM (FEI, USA) with an Oxford Instruments EDS detector; samples were gold sputtered (~ 5 nm) prior to imaging at 20 kV and 10 mm working distance. Finally, high-resolution transmission electron microscopy (HR-TEM) was conducted using a JEOL JEM-2100 (JEOL Ltd., Japan) operated at 200 kV, with silica dispersed in ethanol and drop-cast on carbon-coated copper grids to visualize nanoscale particle size and dispersion.

All characterizations were performed using internationally standardized protocols at the Institute of Urban Environment (Chinese Academy of Sciences, China), Addis Ababa University (Ethiopia), and the University of Warwick (United Kingdom).

Results and discussion

Optimization of nanostructured silica synthesis and processing parameters

This study demonstrates the successful synthesis of high-purity nanostructured silica from Ethiopian pumice, a naturally abundant, silica-rich volcanic rock, highlighting its potential as a sustainable and cost-effective alternative for rubber reinforcement applications [33, 34]. The synthesis of nanostructured silica from Ethiopian pumice follows a two-step chemical route involving alkaline extraction and acid-induced gelation. Initially, pumice-derived SiO₂ reacts with sodium hydroxide to form soluble sodium silicate (Na₂O·x SiO₂), a process that depolymerizes the native Si–O–Si network into reactive monomeric or oligomeric silicate species [35]:

Subsequent controlled acidification with sulfuric acid at pH 8–9 induces gelation, leading to reformation of Si–O–Si linkages and precipitation of silica gel, accompanied by sodium sulfate as a by-product [36]:

This sol–gel transition defines the degree of network crosslinking, significantly influencing the gel's textural and morphological properties. Final calcination at 600–750 °C removes residual volatiles and promotes further polycondensation, resulting in a robust, high-purity silica framework. These sequential transformations govern the physicochemical characteristics, such as surface area, particle size, and morphology, that are essential for the silica’s reinforcing efficiency in rubber matrices [37, 38].

To ensure industrial applicability, the synthesis of nanostructured silica from Ethiopian pumice was systematically optimized by carefully controlling key process parameters [39–41]. Certain conditions were held constant throughout the study, including a sodium silicate modulus of 3, stirring rate of 600 rpm, NaOH and H₂SO₄ concentrations of 2.5 M each, reaction time of 3 h, Si:NaOH molar ratio of 1:3, drying temperature of 100 °C, and aging time of 12 h [27].

The variables intentionally varied were synthesis temperature (60, 75, 80, 85, and 90 °C), calcination temperature (600, 650, 700, and 750 °C), and precipitation pH (7, 8.5, and 9) to assess their effects on silica yield, purity, structural properties, morphology, and thermal stability. Under these controlled conditions, the optimized synthesis routes achieved high silica yields up to 95.63%, producing nanoscale particles with well-defined morphology and high surface area, consistent with findings from related studies [42, 43]. Table 2 summarizes the outcomes of all tested conditions, highlighting optimized, comparable optimized, eco-templated, and sub-optimal samples.

Table 2.

Process optimization and outcomes for scalable nanostructured silica production

Synthesis route	Key parameters	Sample code	SiO2 purity (%)	Outcome summary
Optimum condition	Synthesis: 85 °C; Calcination: 700 °C; pH 8.5	P3-SiO2	95.63	High purity, high surface area (468 m2/g), nanoscale particle size (~ 99 nm), uniform, non-agglomerated morphology, mesoporosity (~ 3.2 nm), thermally stable ≤ 800 °C; ideal for industrial scale-up and rubber reinforcement
Comparable optimized	Synthesis: 75 °C; Calcination: 650 °C; pH 8.5	P2-SiO2	95.27	High purity, good surface area (402 m2/g), moderate particle size (~ 116 nm), some agglomeration, mesoporosity (~ 3.0 nm), thermally stable ≤ 800 °C; suitable for cost-conscious rubber applications
Eco-templated pathway	Synthesis: 75 °C; Calcination: 650 °C; pH 8.5; reduced SDS	Eco-SiO2	45.57	Lower purity, partial irregular morphology, moderate surface area, limited scalability and reinforcement potential
Sub-optimal condition 1	Synthesis: 60 °C; Calcination: 600 °C; pH 7	P1-SiO2	67.24	Low purity, poor morphology, large particle size, pronounced agglomeration; unsuitable for industrial scale-up
Sub-optimal condition 2	Synthesis: 90 °C; Calcination: 750 °C; pH 9	P4-SiO2	48.91	Low purity, irregular morphology, large particle size, high agglomeration, poor thermal stability; unsuitable for industrial use

The characterization results indicate that P3-SiO₂ (85 °C, 700 °C, pH 8.5) confirms the most favorable combination of properties for high-performance applications. It displays the highest SiO₂ purity (95.63%), largest surface area (468.48 m²/g), nanoscale mean particle size (99.43 nm), uniform non-agglomerated morphology, well-developed mesoporosity (~ 3.2 nm), and excellent thermal stability (≤ 800 °C). These features ensure efficient filler dispersion and strong interfacial bonding in rubber composites, making P3-SiO₂ the optimum condition for industrial-scale production and rubber reinforcement. The sample P2-SiO₂ (75 °C, 650 °C, pH 8.5) also demonstrates high purity (95.27%) and good surface area (402.31 m²/g), with slightly larger particle size (115.54 nm) and moderate agglomeration. While slightly less ideal than P3-SiO₂, it represents a viable alternative when processing or cost constraints are considered.

The Eco-Templated SiO₂ sample, despite partially favorable morphology, has low purity (45.57%) and moderate surface area, limiting both its industrial scalability and reinforcing efficiency. The sub-optimal samples (P1-SiO₂: 67.24% SiO₂, P4-SiO₂: 48.91% SiO₂) shows poor morphology, larger particle sizes, pronounced agglomeration, and lower thermal stability, making them unsuitable for industrial production or effective rubber reinforcement [39, 43]. Collectively, these results highlight the critical importance of precise control over synthesis temperature, calcination temperature, and precipitation pH to produce high-purity, nanoscale, thermally stable silica suitable for advanced rubber reinforcement applications. Among all tested conditions, P3-SiO₂ emerged as the most industrially and functionally optimal sample, offering a combination of superior material properties, scalability, and cost-effectiveness, while P2-SiO₂ represents a practical alternative under less demanding operational conditions [33, 41, 42].

Functional group characterization by FTIR

The successful conversion of Ethiopian pumice into nanostructured silica was confirmed by Fourier Transform Infrared (FTIR) spectroscopy (Fig. 3). The spectra confirmed the characteristic vibrational modes of amorphous silica, in agreement with established reports for sol–gel–derived and natural-resource–based silica [44, 45]. A strong and broad absorption band centered near 800 cm⁻¹ was observed in all synthesized samples, corresponding to the asymmetric stretching vibration of the Si–O–Si network. A small intensity peak around 1000 cm⁻¹ was belong to symmetric Si–O–Si stretching, while the band between 460–470 cm⁻¹ was assigned to Si–O bending vibrations. Together, these features confirmed the formation of a well-condensed amorphous silica framework [46, 47]. In addition to these fundamental vibrations, a broad absorption around 3600 cm⁻¹ was detected, associated with surface hydroxyl (Si–OH) groups and adsorbed water molecules. These functional groups play a critical role in filler–matrix interactions, as they enhance interfacial adhesion with elastomeric chains in tire rubbers [2, 4].

Fig. 3.

FTIR spectra of silica samples synthesized from Ethiopian pumice under different optimized conditions

A weak band near 1630 cm⁻¹ corresponded to H–O–H bending vibrations of molecularly adsorbed water, further indicating the hydrophilic nature of the silica surface [12, 17]. Minor peaks at ~ 2400–2500 cm⁻¹ were linked to C–H stretching vibrations of residual organic moieties, Such bands were common in samples P2-SiO₂ and P3-SiO₂, suggesting more effective thermal treatment and higher purity. Additionally, distinct absorptions in the 850–1000 cm⁻¹ region confirmed the presence of silanol groups, indicating abundant surface functionality that can be suitable for chemical modification or coupling-agent interactions in rubber compounding [47, 48]. Comparative analysis of the synthesized samples revealed marked differences in peak intensities and band resolution. P2-SiO₂ and P3-SiO₂ confirmed the most intense Si–O–Si stretching bands alongside well-defined hydroxyl-related peaks, signifying superior structural organization and retention of surface-active groups. In contrast, P1-SiO₂ and P4-SiO₂ displayed broader and less intense Si–O–Si absorptions, consistent with lower network condensation and the presence of residual impurities. The eco-templated silica, while showing enhanced silanol-related features due to surfactant assistance, also retained minor organic traces, reflecting the trade-off between environmentally benign synthesis and final material purity [19–21].

These findings highlight the importance of synthesis parameters; particularly calcination temperature, surfactant concentration, and pH control; in tuning surface chemistry and structural features of silica at industrial scale. The presence and density of silanol groups, as confirmed by FTIR, are especially critical for achieving strong filler–rubber interactions, thereby forming the foundation of the reinforcing efficiency of tire-grade nanostructured silica in sustainable green tire formulation [2, 4].

Structural and phase analysis by XRD

X-ray diffraction (XRD) analysis was employed to evaluate the structural order and phase purity of silica derived from Ethiopian pumice under varied processing conditions, including eco-surfactant templating (Fig. 4).

Fig. 4.

XRD diffractograms of pumice-derived silica samples synthesized under different conditions

All processed silica samples (P1-SiO₂–P4-SiO₂ showed broadened diffraction features in the 21–23° range, confirming the formation of predominantly amorphous silica [49, 50]. Among these, P2-SiO₂ and P3-SiO₂ displayed the broadest and least intense halos, indicating high amorphization, structural homogeneity, and superior siloxane network development. These structural features correlate with their elevated BET surface areas (> 320 m²/g) and SiO₂ purities above 95%, highlighting the effectiveness of optimized industrial synthesis parameters; such as precipitation P^H, reaction temperature, and calcination temperature and other controlled process parameters; in producing tire-grade nanostructured silica suitable for reinforcing passenger and truck tire rubbers [2, 27]. In contrast, P1-SiO₂ and P4-SiO₂ retained weak crystalline reflections around 2θ ≈ 26–28°, indicating residual mineral phases or incomplete phase transformation. This residual crystallinity likely results from suboptimal synthesis conditions, such as lower alkali concentration or insufficient aging, which hinder complete dissolution of the pumice matrix and the formation of a uniform amorphous network [50, 51].

Collectively, XRD analysis supports the BET and compositional results, showing that the optimized synthesis conditions (P2-SiO₂, P3-SiO₂) produce uniform, amorphous silica. Such phase-pure nanostructured silica is essential for achieving effective filler–rubber interactions, enhancing mechanical reinforcement, and enabling high-performance, sustainable tire tread applications. These results confirm the importance of precise control over synthesis parameters in industrial-scale production, ensuring that structural and phase characteristics are tuned for optimal reinforcement of tire-grade rubber composites [1, 52].

Textural characterization by BET

The textural properties of pumice-derived silica samples were systematically evaluated using Brunauer–Emmett–Teller (BET) analysis to quantify surface area, pore volume, and pore size distribution, parameters essential for efficient rubber reinforcement in green tire formulations. Figure 5 and Table 3 presents the comparative BET data, illustrating the pronounced influence of synthesis parameters; namely precursor purity, hydrolysis–condensation kinetics, pH, temperature, dispersion efficiency, and post-synthesis thermal treatment; on the resulting silica textural characteristics. Raw Ethiopian pumice showed a negligible surface area (< 50 m²/g) and pore volume (< 0.10 cm³/g), predominantly microporous in nature, reflecting its dense mineral matrix and inherent impurities. These observations are consistent with prior reports that unprocessed volcanic materials display limited accessible porosity without activation [53].

Fig. 5.

N₂ adsorption–desorption isotherms of pumice-derived silica samples, highlighting their mesoporous structure

Table 3.

BET textural properties of pumice-derived silica samples

Sample	Surface area (m2/g)	Total pore volume (cm3/g)	Average pore diameter (nm)
Raw pumice	< 50	< 0.10	–
P1-SiO2	300–330	~ 0.50	~ 3.0
P2-SiO2	402	~ 0.58	~ 3.1
P3-SiO2	468	~ 0.62	~ 3.2
P4-SiO2	180–220	~ 0.40	~ 2.5
Eco-templated SiO2	250–280	~ 0.46	~ 3.0

Among the synthesized samples, P3-SiO₂ demonstrated the highest surface area (468.48 m²/g), total pore volume (~ 0.62 cm³/g), and average pore diameter (~ 3.2 nm), indicative of well-developed mesoporosity. This enhancement arises from high precursor purity, controlled particle growth, and minimal agglomeration, which together maximize internal porosity and surface accessibility. Similarly, P2-SiO₂ achieved a surface area of 402.31 m²/g and pore volume of ~ 0.58 cm³/g, reflecting effective regulation of hydrolysis–condensation reactions and narrow particle size distribution. The mesoporous dimensions (~ 3.0–3.2 nm) of both P2-SiO₂ and P3-SiO₂ are particularly favorable for tire-grade rubber reinforcement, as interfacial adhesion in rubber composites strongly depends on accessible surface area. These results align with other reports of high-purity mesoporous silica derived from natural and agricultural precursors [54–57].

In contrast, P1-SiO₂, despite being derived from smaller precursor particles, showed a reduced surface area (~ 300–330 m²/g) and pore volume (~ 0.50 cm³/g), likely due to particle agglomeration limiting surface accessibility; a phenomenon reported in other biosilicas synthesized under suboptimal dispersion conditions [58].

The eco-templated silica, prepared via surfactant-assisted synthesis, showed lower-than-expected surface area (~ 250–280 m²/g) and pore volume (~ 0.46 cm³/g), presumably due to incomplete surfactant removal and irregular network formation. Its relatively low SiO₂ content (~ 45.6%) and broader particle size distribution further inhibited mesopore development despite templating efforts [59]. Finally, P4-SiO₂, synthesized under suboptimal pH and temperature conditions, reflected the lowest surface area (~ 180–220 m²/g), narrow pores (~ 2.5 nm), and limited pore volume (~ 0.40 cm³/g), indicative of dense, aggregated structures [60].

Compared to literature values, the surface areas and pore properties of P2-SiO₂ and P3-SiO₂ notably exceed or compit many pumice-derived silicas reported previously. For example, Dewati et al. reported moderate surface area (143.6 m²/g) and large average pore diameter (15.8 nm) for CO₂-precipitated pumice silica, mainly suited for adsorption [61]. Mourhly et al. achieved very high surface area (~ 600 m²/g) but with smaller pore diameters (~ 4.6 nm), indicative of microporous structures less ideal for polymer reinforcement [30]. Manurung et al. and Sarikaya et al. documented intermediate surface areas (~ 165–238 m²/g) and pore sizes (8.2–12.7 nm), lower than the mesoporous range here but suitable for other applications [62, 63].

The P2-SiO₂ and P3-SiO₂ samples not only surpass many previously reported natural silicas in surface area but also possess optimally sized mesopores, promoting effective filler–rubber interactions essential for green passenger and truck tire applications[1, 64]. These results highlight that precise control of synthesis parameters; particularly precursor purity, P^H, temperature, hydrolysis–condensation kinetics, and dispersion quality; is crucial for tailoring the textural properties of pumice-derived silica. The superior BET characteristics of P3-SiO₂ demonstrate its potential as a sustainable, high-performance nanofiller for environmentally friendly tire formulations.

Chemical composition analysis by XRF

The chemical composition of raw Ethiopian pumice and synthesized nanostructured silica samples was evaluated using X-ray fluorescence (XRF) spectroscopy (Table 4). The results reveal significant enhancement in silica purity for samples produced under optimized synthesis conditions, particularly P2-SiO₂ and P3-SiO₂, which displayed SiO₂ contents of 95.27% and 95.63%, respectively. This demonstrates the performance of the combined alkaline leaching and sol–gel precipitation approach in selectively removing metallic and oxide impurities, a critical factor for industrial-scale production of high-performance tire-grade nanostructured silica [65–67].

Table 4.

XRF-determined chemical composition of raw Ethiopian pumice and synthesized nanostructured silica samples. High SiO₂ content in P2-SiO₂ and P3-SiO₂ confirms optimized purification suitable for industrial tire-grade rubber reinforcement

Analyte	Compound	Ethiopian Pumice	Eco-Templated SiO2	P1-SiO2	P2-SiO2	P3-SiO2	P4-SiO2
Na	Na₂O	0.188	2.262	2.018	0.044	0.051	1.398
Mg	MgO	0.090	……	……	0.132	0.148	……
Al	Al₂O₃	8.823	0.097	0.158	0.159	0.056	0.300
Si	SiO2	63.104	45.566	67.236	95.269	95.626	48.911
P	P₂O₅	1.313	1.301	1.443	2.089	1.561	1.026
S	SO₃	0.362	47.631	25.614	0.828	1.249	44.809
Cl	Cl	0.222	0.225	0.154	0.251	0.284	1.340
K	K₂O	11.050	1.243	1.476	0.301	……	1.224
Ca	CaO	2.614	0.867	0.927	0.754	0.776	0.680
Ti	TiO₂	1.816	0.416	0.571	……	0.108	0.080
Mn/V	MnO/V₂O₅	0.583	0.034	……		……	……
Fe	Fe₂O₃	9.005	0.291	0.338	0.171	0.134	0.218
Zn	ZnO	0.067	0.010	……	……	……	0.008
Ga	Ga₂O₃	0.012	……	……	……	……	……
Rb	Rb₂O	0.045	……		……		……
Sr	SrO	0.028	0.007	0.011	……	0.008	……
Y	Y₂O₃	0.034	……	……	……	……	……
Zr	ZrO₂	0.322	0.049	0.053	……	……	0.005
Nb	Nb₂O	0.050	……	……	……	……
Ba	BaO	0.272	……	……	……	……
< Ce >	CeO₂	0	……	……	……	……

Dashed entries (……) indicate absence or below detection limit

Raw pumice, in contrast, contained substantial amounts of K₂O (11.05%), Al₂O₃ (8.82%), and Fe₂O₃ (9.01%), confirming that direct incorporation into rubber matrices would be unsuitable without pretreatment [68]. The high levels of alkali and transition metal oxides in untreated pumice could interfere with rubber crosslinking and diminish mechanical reinforcement [1]. Therefore, chemical purification is imperative to achieve filler compatibility and long-term composite stability.

The eco-templated SiO₂ sample, synthesized with controlled reductions in sodium dodecyl sulfate (SDS) and sulfuric acid, showed a markedly lower silica content (45.57%) and elevated SO₃ concentration (47.63%). The residual sulfate is resulted from incomplete washing and secondary formation of sulfate salts, emphasizing the necessity of precise control over surfactant and acid dosages to prevent contamination during scale-up [1, 69].

Similarly, P1-SiO₂ displayed moderate silica content (67.24%) with SO₃ ~ 25%, indicating incomplete impurity removal, likely due to acid overuse, whereas P4-SiO₂ showed poor purification, containing only 48.91% silica and 44.81% SO₃. Although minor residual sulfates and oxides in P1-SiO₂ and P4-SiO₂ may not immediately impair rubber performance, they raise concerns for long-term composite stability and filler-matrix compatibility. These findings highlight the critical need for precise optimization of synthesis parameters to balance silica yield and purity for demanding rubber reinforcement applications [70].

Surface chemistry of P3-SiO₂ by XPS

The surface chemical composition and oxidation state of the optimized P3-SiO₂ (95.63% SiO₂) were investigated using X-ray photoelectron spectroscopy (XPS) (Fig. 6). The survey spectrum confirmed that silicon (Si) and oxygen (O) are the predominant elements, indicative of high-purity silica suitable for industrial rubber reinforcement applications. High-resolution spectra revealed the Si 2p peak at 103.3 eV and the O 1 s peak at 532.5 eV, characteristic of fully oxidized SiO₂, with no detectable elemental silicon (Si⁰) or sub-stoichiometric oxides, confirming complete oxidation during synthesis [71]. The narrow full width at half maximum (FWHM) values of 1.7 eV for Si 2p and 1.7 eV for O 1 s suggest a homogeneous chemical state, ensuring uniform surface properties essential for consistent performance in rubber composites [70]. The atomic ratio of Si:O was 1:1.79 (34.4% Si and 61.8% O), closely matching the ideal stoichiometry of SiO₂ (1:2), confirming high purity and complete oxidation. A small peak near 560 eV corresponds to the oxygen KLL Auger transition, a common feature in oxygen-containing materials resulting from electron energy loss during analysis [72].

Fig. 6.

XPS survey spectrum and high-resolution Si 2p and O 1 s spectra of optimized P3-SiO₂ (95.626% SiO₂)

Surface hydroxyl groups (-SiOH), identified by XPS, enhance compatibility with rubber matrices by improving interfacial bonding and filler dispersion [73–75]. Surface modification of silica further improves mechanical properties, such as tensile strength, elasticity, and abrasion resistance, in rubber composites [73]. Minor trace elements of carbon (0.7%) and sodium (0.4%) were detected, likely residuals from the synthesis process, but are not expected to significantly impact performance. Taken together, these XPS results confirm that P3-SiO₂ is a high-purity, fully oxidized silica with uniform surface chemistry and reactive silanol groups, making it an excellent candidate for demanding rubber reinforcement applications such as automotive tires and industrial rubber products [76, 77].

Thermal stability assessment via TGA

The thermal behavior of the synthesized pumice-derived nanostructured silica samples was systematically evaluated using thermogravimetric analysis (TGA), as shown in Fig. 7 and Table 5. The TGA curves reveal three distinct stages of weight loss, characteristic of high-purity amorphous silica [78, 79]. The first stage, occurring below ~ 150 °C, corresponds to the desorption of physically adsorbed water. This minor mass loss (< 5%) reflects the effective drying and washing steps applied during synthesis and confirms the low residual moisture content. The second stage, between ~ 150–400 °C, is primarily associated with the condensation of surface silanol groups and the decomposition of trace organic residues originating from surfactants or residual precursors [47, 69].This step is particularly sensitive to synthesis conditions, including pH, temperature, and surfactant concentration, which influence surface functionalization and silanol density [4, 19].Proper optimization of these parameters in P2-SiO₂ and P3-SiO₂ resulted in minimal organic residue, confirming the efficacy of the tailored synthesis strategy for scalable industrial production.

Fig. 7.

TGA curves of pumice-derived silica (P1–P4, eco-templated)

Table 5.

Thermal stability of pumice-derived silica samples determined by TGA

Sample	Mass loss < 150 °C (%)	Mass loss 150–400 °C (%)	Mass loss 400–800 °C (%)	Total mass loss (%)
P1-SiO2	6.2	4.8	1.5	12.5
P2-SiO2	3.5	2.1	0.7	6.3
P3-SiO2	3.8	2.3	0.6	6.7
P4-SiO2	5.9	4.7	1.4	12.0
Eco-templated SiO2	6.5	5.0	1.2	12.7

The third stage, observed from ~ 400–800 °C, represents dehydroxylation and structural densification of the amorphous silica network [78]. Notably, P2-SiO₂ and P3-SiO₂ maintained thermal stability up to 800 °C with negligible mass loss beyond the initial two stages (Table 5). This high thermal resistance is critical for tire-grade applications, as rubber compounding and vulcanization processes involve temperatures ranging from 150 °C to 180 °C [80, 81]. The retention of structural integrity under these conditions ensures that silica can function effectively as a reinforcing filler, enhancing mechanical strength, durability, and abrasion resistance in green passenger and truck tire treads [71, 82, 83].

In contrast, P1-SiO₂, P4-SiO₂, and Eco-templated SiO₂ demonstrated less consistent thermal profiles, with higher weight losses in the 150–400 °C range, suggesting incomplete condensation of silanol groups or residual organics due to less controlled synthesis parameters. These observations highlight the importance of precise control over leaching, precipitation, surfactant removal, drying, and calcination conditions to achieve reproducible thermal stability in industrial-scale production [25, 26].

The TGA results confirm that the optimized synthesis protocol for P2-SiO₂ and P3-SiO₂ produces nanostructured silica with superior thermal stability suitable for high-temperature rubber processing. The correlation between thermal resistance, surface chemistry, and structural densification confirms the critical role of carefully designed synthesis parameters in producing scalable, tire-grade silica capable of delivering consistent reinforcement in sustainable rubber composites [67, 69, 76].

Surface morphology and nanostructure analysis by SEM

The surface morphology and nanostructure of the synthesized silica samples were investigated using scanning electron microscopy (SEM), providing critical insights into particle size, shape, dispersion, and potential reinforcement efficiency in tire-grade rubber composites (Fig. 8, Table 6). Raw Ethiopian pumice shown coarse, irregular particles with a mean diameter of approximately 131.5 µm and heterogeneous surface topography. Such morphological characteristics, combined with high impurity content, limit the material's ability to establish strong polymer–filler interfacial interactions, thereby reducing its suitability as a reinforcing filler [16, 17].

Fig. 8.

SEM micrographs of pumice-derived silica samples showing morphology, size, and agglomeration. P3-SiO₂ shows uniform nanoscale particles with minimal agglomeration, suitable for tire reinforcement

Table 6.

Comparative sem analysis of nanostructured silica samples: P3-SiO₂ and P2-SiO₂ (this study) versus Literature Benchmarks. ⌀ = mean particle diameter; TGA Stability = maximum temperature of negligible mass loss

Source material	⌀ Size (nm)	Size range (nm)	Dispersion quality	Purity (%)	TGA stability (°C)	Reinforcement suitability
P3-SiO2 (this study)	9.6	5.4–11.4	Excellent; non-agglomerated	95.63	≤ 800	Highly Suitable
P2-SiO2 (this study)	153.6	95–187	Moderate; some agglomerates	95.27	≤ 800	Suitable
Sodium silicate [21]	–	50–100	Good; uniform mesopores	~ 94.5	≤ 700	Suitable
Volcanic ash [87]	~ 15	10–20	Excellent; spherical	96.3	≤ 850	Highly Suitable
Rice husk [89]	–	100–200	Poor; large agglomerates	~ 92	~ 700	Less Suitable
Turkey Van pumice [63]	–	10–50	Excellent; well-dispersed	> 95	~ 700	Highly Suitable
Indonesia Pumice [61]	–	20–80	Moderate; agglomerated	85–90	~ 600	Moderate; coarse particles limit interaction
Indonesia Pumice [62]	13	9–17	Good; moderate agglomeration	85–90	800	Highly Suitable
Morocco Pumice [30]	10	5–15	Excellent; homogeneously dispersed	–	~ 700	Highly Suitable if pure

Eco-templated silica achieved a considerable reduction in particle size (~ 129.3 nm) with moderately uniform dispersion; however, residual surface heterogeneity and impurities compromised its effectiveness for high-performance composites.

Among the synthesized samples, P3-SiO₂ demonstrated the most favorable characteristics, presenting non-agglomerated, uniformly dispersed nanoparticles with a narrow size distribution of 5.4–11.4 nm and an average diameter of 9.6 ± 0.4 nm. This morphology ensures high surface area and enhanced polymer–filler interaction, which are essential for optimizing mechanical properties in rubber nanocomposites [25, 75, 84]. P2-SiO₂, with a mean particle size of 153.6 ± 12 nm and a broader distribution range of 95–187 nm, exhibited moderate agglomeration. Despite this, its high chemical purity (95.27%) and thermal stability (≤ 800 °C) making it suitable for reinforcement in rubber matrices where cost or processing constraints exist. In contrast, P1-SiO₂ and P4-SiO₂ showed irregular morphologies, broader size distributions, and significant agglomeration, which decrease the available surface area and hinder uniform dispersion, consistent with the negative impact of particle agglomeration on mechanical performance observed in silica-reinforced elastomers [85, 86].

Comparative analysis with literature benchmarks further underscores the quality of P3-SiO₂. Silicas derived from volcanic ash (≈15 nm, spherical, highly dispersed) and Moroccan pumice (10 ± 2 nm, homogeneously dispersed) highlight that P3-SiO₂ matches or exceeds established natural-source silicas in terms of particle size, dispersion, purity, and thermal stability [30, 87]. These findings confirm that careful control of key synthesis parameters; including nucleation rate, pH, and surfactant templating; enables scalable production of nanosized, highly dispersed, high-purity silica suitable for advanced rubber reinforcement[88–90].

Collectively, SEM analysis validates that P3-SiO₂ shows the optimal combination of nanoscale particle size, narrow distribution, high thermal stability, and excellent dispersion, fulfilling the design criteria for scalable tire rubber-grade nanostructured silica. The results proved the role of surface morphology and nanostructure of Silica for promoting strong polymer–filler interactions.

Nanostructural evaluation by HR-TEM

High-resolution transmission electron microscopy (HR-TEM) was employed to evaluate the particle size, morphology, and dispersion characteristics of nanostructured silica derived from Ethiopian pumice under optimized synthesis conditions(Table 7, Fig. 9). This analysis is critical for determining the suitability of silica as a reinforcing filler in green passenger and truck tire tread formulations, where uniform nanostructure, high dispersion, and controlled particle morphology directly govern filler–rubber interactions and performance [47, 71].

Table 7.

Comparative HR-TEM analysis of nanostructured silica samples: mean particle Diameter (⌀) and estimated suitability for rubber reinforcement

Source material	Mean particle diameter ⌀ (nm)	Morphology description	Estimated suitability	References
P3-SiO2 (this study)	99.43	Highly dispersed, mesoporous, no visible agglomeration	Very high	This study
P2-SiO2 (this study)	115.54	Well-dispersed, minimal agglomeration	High	This study
Eco-Templated SiO2 (this study)	106.5	Dispersed, mixed 0D & 1D structures, minor impurities	Not suitable	This study
P1-SiO2 (this study)	36.57	Agglomerated, oak-like structural features	Not suitable	This study
P4-SiO2 (this study)	109.55	Island-like aggregates; poor dispersion	Not suitable	This study
Van pumice (Turkey) [63]	–	Mesoporous, partial agglomeration	High	[63]
Indonesian pumice (CO₂ route) [61]	–	CO2-precipitated silica, broad morphology distribution	Moderate	[61]
Indonesian pumice [62]	–	Nanoscale particles, narrow size distribution	High	[62]
Moroccan pumice [30]	–	Fine particles, uniform morphology	Very high	[30]

Fig. 9.

TEM analysis showing distinct differences in particle morphology and size of Ethiopian pumice precursor, nanostructured silica synthesized from Ethiopian pumice under various optimization conditions

The pumice precursor displayed coarse, irregularly aggregated particles (~ 98 µm), composed of amorphous–crystalline silicate phases, with poor surface activity and inadequate dispersion. Such features render raw pumice unsuitable for elastomer reinforcement [16, 17]. In contrast, eco-templated silica produced particles of ~ 106.5 nm, showing a hybrid mixture of rod-like and spherical morphologies. While dispersion improved, residual impurities limited its potential as a tire-grade filler.

Among the synthesized variants, P2-SiO₂ (115.54 nm) and P3-SiO₂ (99.43 nm) demonstrated highly uniform, mesoporous nanostructures with minimal agglomeration. These characteristics are particularly important because well-dispersed nanosilica enhances polymer–filler interfacial bonding and provides effective reinforcement at reduced loadings, improving both mechanical strength and rolling resistance [4, 76]. P3-SiO₂, in particular, demonstrated a narrow size distribution and smooth surface features, supporting efficient dispersion in rubber matrices and making it especially suitable for high-performance green tire tread applications [91–93].

In contrast, P1-SiO₂ (36.57 nm) displayed severe agglomeration and an irregular oak-like texture, while P4-SiO₂ (109.55 nm) formed clustered island-like aggregates. These irregular morphologies compromise homogeneity during mixing and reduce reinforcement potential [85, 94].

A comparative assessment with literature benchmarks (Table 7) validate the influence of synthesis route and precursor type on nanostructural quality. Van pumice-derived silica yielded mesoporous structures with partial agglomeration [63], while Indonesian pumice treated via CO₂ precipitation produced broad particle size distributions [61]. Alternative Indonesian studies reported narrow size distributions and well-defined nanosilica with high reinforcing potential [62]. Moroccan pumice, conversely, provided fine, uniformly dispersed silica particles with very high suitability for rubber reinforcement [30]. Recently, volcanic ash-based silica has also been identified as a promising sustainable filler, revealing controlled particle sizes and dispersion suitable for tire composites [25].

The present study confirms that P2-SiO₂ and especially P3-SiO₂ meet or exceed the nanostructural benchmarks reported in literature, combining mesoporosity, narrow particle size distribution, and stable dispersion. These results highlight the importance of carefully optimizing synthesis parameters; such as precursor treatment, precipitation conditions, and post-synthesis treatment; to achieve scalable production of tire-grade silica with superior reinforcing ability [6, 21]. The findings provide clear evidence that Ethiopian pumice can serve as a cost-effective and sustainable raw material for industrial-scale production of high-performance nanostructured silica for green tire tread formulations.

Elemental analysis by EDS

Energy-dispersive X-ray spectroscopy (EDS) was employed to quantify the elemental composition of nanostructured silica synthesized from Ethiopian pumice under optimized industrial-scale conditions. As illustrated in Figs. 10 and 11, all samples were predominantly composed of silicon (Si) and oxygen (O), consistent with XRF and XPS analyses, confirming the formation of a stoichiometric silica framework suitable for reinforcing rubber matrices [87, 88]. Trace amounts of aluminum (Al), iron (Fe), and magnesium (Mg) were detected, reflecting residual impurities originating from the raw pumice source.

Fig. 10.

SEM–EDS elemental mapping of a raw Ethiopian pumice, b eco-templated SiO₂, and c P1-SiO₂, showing dominant Si and O with trace Pt, Al, Fe, Na and S

Fig. 11.

SEM–EDS elemental mapping of d P2-SiO₂, e P3-SiO₂, and f P4-SiO₂, illustrating high-purity silica in P2-SiO₂ and P3-SiO₂, while P4-SiO₂ shows elevated levels of impurities (Al, Fe,Pt, S,Na)

The raw beneficiated pumice displayed elevated Al and Fe contents, which can reduce the mechanical performance of elastomeric composites due to interference with filler–rubber interactions [63]. Eco-templated SiO₂ showed improved morphological uniformity due to surfactant-assisted templating; however, minor Na, S, Pt and Fe residues persisted, highlighting the necessity of rigorous purification and process control during synthesis [21].

Among an optimized samples, P1-SiO₂ contained approximately 64 wt% Si and O, with ~ 34 wt% residual metal oxides, validating the importance of precise synthesis parameters to achieve high chemical purity [91]. In contrast, P2-SiO₂ and P3-SiO₂ Showed > 95 wt% Si and O, with near-ideal Si:O ratios, indicating that the controlled combination of temperature, pH, calcination and other parameters effectively produced high-purity nanostructured silica [87, 88]. The elevated purity and stoichiometric composition of these samples are expected to enhance rubber reinforcement through improved filler–rubber compatibility and uniform dispersion in passenger and truck tire treads [69, 71].

By contrast, P4-SiO₂ contained only ~ 48.9 wt% Si and O, with higher levels of residual Al, Fe,Pt, S and Na, associated with pronounced morphological irregularities, indicating suboptimal synthesis conditions that limit structural quality and reinforcing potential. These findings emphasize that process parameters, particularly temperature and pH control, are critical determinants of elemental composition and microstructural integrity in scalable tire-grade silica [95].

Collectively, EDS analysis demonstrates that Ethiopian pumice is a viable precursor for high-performance nanostructured silica. The high Si and O content of P2-SiO₂ and P3-SiO₂ ensures effective interaction with silane coupling agents, facilitating uniform dispersion within rubber matrices and enabling efficient reinforcement in green tire treads [91, 96].

Nanostructured silica purity by XRF and SEM–EDS

The chemical purity of scalable tire-grade nanostructured silica was rigorously assessed using X-ray fluorescence (XRF) and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS). XRF provides reliable bulk oxide composition data, essential for evaluating industrial-grade silica and confirming its suitability for tire rubber reinforcement [96, 97]. SEM–EDS complements this by offering localized elemental profiles, enabling evaluation of particle-to-particle consistency and correlation of elemental distribution with morphological features [98].

For the optimized pumice-derived silica samples P2-SiO₂ and P3-SiO₂, XRF-determined purities of 95.27% and 95.63%, respectively, closely matched the SEM–EDS results (95.269% and 95.626% respectively), confirming both analytical reliability and uniform high-purity of the synthesized silica. Such high chemical purity is critical for effective reinforcement in tire rubbers, as it directly influences silica–rubber interfacial interactions, enhancing tensile strength, abrasion resistance, and dynamic mechanical performance in passenger and truck tire treads [1, 96].

Notably, the present synthesis yields some of the highest purities reported for pumice-derived silicas. For example, Van pumice from Turkey reached ~ 93% silica via alkaline extraction and acid precipitation [63], Indonesian pumice achieved ~ 94% through optimized alkaline leaching [62], Moroccan pumice reached ~ 92% using acid treatment [30], and Indonesian CO₂-precipitated silica obtained ~ 93% [61]. The superior purity of Ethiopian pumice-derived silica results from carefully optimized synthesis parameters, including pH control, hydrolysis–condensation kinetics, and post-synthesis thermal treatment, which together effectively minimize residual metallic oxides and impurities [25, 26].

Combined XRF and SEM–EDS analyses confirm that Ethiopian pumice is a high-purity silica source suitable for industrial-scale production of tire-grade nanostructured silica, supporting the development of high-performance, sustainable tire treads.

Conclusion

This study demonstrates that Ethiopian pumice is a promising, sustainable, and cost-effective precursor for the synthesis of high-purity nanostructured silica suitable for rubber reinforcement applications. By systematically optimizing sol–gel synthesis parameters; reaction temperature, precipitation pH, and calcination temperature; silica samples with purities above 95%, uniform mesoporous structures (~ 3 nm pore diameter), and controlled particle sizes (P2-SiO₂: 115.54 nm; P3-SiO₂: 99.43 nm) were successfully produced. Comprehensive characterization by FTIR, XRD, BET, XRF, XPS, TGA, SEM, EDS, and HR-TEM confirmed that the optimized samples (P2-SiO₂ and P3-SiO₂) are amorphous, thermally stable up to 800 °C, and possess high specific surface areas (> 400 m²/g), well-developed mesoporosity (~ 3–3.2 nm), abundant reactive silanol groups, and minimal agglomeration; features critical for effective filler–rubber interactions and strong interfacial bonding in composites.

Among all tested conditions, P3-SiO₂ (synthesis: 85 °C; calcination: 700 °C; pH 8.5) demonstrated the most favorable combination of properties for industrial-scale production. It confirmed the highest SiO₂ purity (95.63%), largest surface area (468.48 m²/g), nanoscale particle size (99.43 nm), uniform, non-agglomerated morphology, well-developed mesoporosity (~ 3.2 nm), and excellent thermal stability, making it the optimum choice for high-performance rubber reinforcement. P2-SiO₂ (75 °C; 650 °C; pH 8.5) also shows good purity (95.27%), surface area (402.31 m²/g), and mesoporosity (~ 3.0 nm) with slightly larger particle size (115.54 nm) and moderate agglomeration, representing a viable alternative when cost or processing constraints apply. In contrast, eco-templated silica and sub-optimal samples (P1-SiO₂ and P4-SiO₂) showed lower purities (45–67%), irregular morphologies, larger particle sizes, increased agglomeration, and reduced surface accessibility, limiting their effectiveness for rubber reinforcement and industrial scalability.Careful control of synthesis temperature, precipitation pH, and calcination is essential to produce high-purity, thermally stable, nanoscale silica for advanced elastomers. Ethiopian pumice-derived P3-SiO₂ offers a sustainable, eco-friendly alternative to conventional fillers, enabling cost-effective production of high-performance rubber composites. Future work should focus on scale-up, surface functionalization to enhance filler–polymer interactions, and testing in commercial rubber formulations to fully realize its industrial potential.

Author contributions

All authors have reviewed and approved the final version of this manuscript. Agraw Mulat Muhammud: Investigation, Original Draft Preparation, Review and Editing. Gemechu Deressa Edossa: Resources, Supervision, Review and Editing. Fedlu Kedir Sabir: Conceptualization, Supervision, Writing-Drafting, Reviewing, and Editing.

Funding

This research was conducted independently, without financial support from any public, commercial, or non-profit funding organizations.

Data availability

The data supporting the findings of this study are available from the corresponding authors upon reasonable request.

Declarations

Ethics approval and consent to participate

This study did not involve human participants, animals, or any plant or biological materials. The raw material used was naturally occurring volcanic pumice, collected with official permission from the Ethiopian Geological Survey Institute and the Tatek quarry management, in accordance with institutional policies and environmental safety guidelines. Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.