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Article

Eco-Friendly Oxidative–Adsorptive Desulfurization for Real Diesel Fuel Using Green MnO2 Biowaste-Extracted Calcite in Digital Basket Reactor

by
Jasim I. Humadi
1,
Khaleel I. Hamad
2,
Hiba A. Abdulkareem
1,
Maha Nazar Ismael
2,
Aysar T. Jarullah
2,
Mustafa A. Ahmed
3,
Shymaa A. Hameed
2,
Amer T. Nawaf
1 and
Iqbal M. Mujtaba
4,*
1
Department of Petroleum and Gas Refining Engineering, College of Petroleum Processes Engineering, Tikrit University, Tikrit 34001, Iraq
2
Department of Chemical Engineering, College of Engineering, Tikrit University, Tikrit 34001, Iraq
3
Ministry of Oil, Baghdad 10011, Iraq
4
Chemical Engineering Department, Faculty of Management, Sciences & Engineering, University of Bradford, Bradford BD7 1DP, UK
*
Author to whom correspondence should be addressed.
Processes 2025, 13(10), 3084; https://doi.org/10.3390/pr13103084
Submission received: 4 August 2025 / Revised: 18 September 2025 / Accepted: 19 September 2025 / Published: 26 September 2025
(This article belongs to the Section Process Control and Monitoring)
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Versions Notes

Abstract

Achieving ultra-low-sulfur diesel is a crucial objective in modern fuel refining, driven by increasingly stringent environmental regulations. This study presents the development of a highly efficient oxidative–adsorptive desulfurization process utilizing a nanocatalyst synthesized from biowaste eggshell-extracted calcite. The oxidation reaction was conducted in a digital basket reactor (DBR), an advanced reactor system designed to enhance mass transfer and catalytic efficiency. To further augment the catalyst’s performance, the calcite was modified with eco-friendly MnO2, while activated carbon was employed as an adsorbent to effectively capture oxidized sulfur compounds, ensuring compliance with ultra-low-sulfur fuel standards. The synthesized nanocatalyst underwent comprehensive physicochemical characterization using SEM, EDX, BET, and FTIR, confirming its high surface area, structural integrity, and superior catalytic activity. The MnO2/P–calcite catalyst achieved a sulfur removal efficiency of 96.5% at 90 °C, 80 min, and 600 rpm, demonstrating excellent oxidative–adsorptive performance for real diesel fuel. The integration of this innovative nanocatalyst with the DBR system presents a sustainable, cost-effective, and industrially viable approach for deep desulfurization, offering significant advancements in clean fuel production and environmental sustainability.

1. Introduction

The increasing demand for ultra-low-sulfur diesel (ULSD) has become a critical issue in the energy sector due to the stringent environmental regulations imposed by global agencies. The presence of sulfur compounds in diesel fuel contributes significantly to air pollution, leading to the formation of sulfur oxides (SOx), which are major precursors to acid rain, particulate matter, and smog formation. These pollutants pose severe environmental and health hazards, including respiratory diseases, cardiovascular disorders, and ecosystem degradation [1,2]. In response to these challenges, international regulatory bodies, including the U.S. Environmental Protection Agency (EPA) and the European Union (EU), have implemented rigorous fuel sulfur content restrictions, progressively reducing the permissible sulfur levels in transportation fuels to below 10 ppm [3,4]. These regulatory measures necessitate the development of highly efficient desulfurization technologies capable of producing cleaner fuels while maintaining economic feasibility and industrial scalability.
Currently, hydrodesulfurization (HDS) remains the predominant industrial method for sulfur removal from petroleum fractions. This catalytic hydrogenation process operates at high temperatures (300–400 °C) and pressures (3–10 MPa) in the presence of expensive hydrogen gas and Co-Mo or Ni-Mo sulfide catalysts [5]. While HDS is highly effective for removing simple sulfur compounds such as mercaptans and thiophenes, it exhibits significant limitations when dealing with refractory sulfur species such as dibenzothiophenes (DBTs) and their alkylated derivatives (e.g., 4,6-dimethyldibenzothiophene, 4,6-DMDBT) [6,7]. These sterically hindered sulfur compounds resist hydrogenation due to their electronic and structural properties, necessitating more severe HDS operating conditions, which in turn increase operational costs, hydrogen consumption, and catalyst deactivation rates [8]. Consequently, alternative or complementary desulfurization techniques have been extensively explored to achieve deep desulfurization with improved energy efficiency and environmental sustainability.
Among the emerging desulfurization strategies, oxidative desulfurization (ODS) has garnered significant attention as a promising alternative to HDS. Unlike HDS, ODS operates under mild conditions (typically below 100 °C and atmospheric pressure) and does not require hydrogen, making it a more energy-efficient and cost-effective process [9]. ODS selectively oxidizes sulfur compounds into highly polar sulfoxides and sulfones, which can then be removed through liquid–liquid extraction or adsorption [10]. The success of the ODS process is largely dependent on the efficiency of the catalytic oxidation system. Various oxidants, including hydrogen peroxide (H2O2), peracetic acid, and ozone, have been investigated for their ability to facilitate selective oxidation reactions [11]. Among these, H2O2 is the most widely preferred oxidant due to its high oxidation potential, environmental compatibility, and minimal formation of hazardous byproducts [12]. However, the performance of the ODS process is significantly influenced by the catalytic system, which must provide high selectivity, stability, and recyclability while ensuring effective activation of the oxidant.
Transition metal oxides, particularly manganese dioxide (MnO2), have demonstrated exceptional catalytic activity in ODS due to their strong redox properties, high oxygen storage capacity, and ability to activate H2O2 for the selective oxidation of sulfur species [13]. MnO2-based catalysts exhibit excellent oxidative efficiency, particularly when engineered at the nanoscale to enhance surface area and active site availability [14]. However, the catalytic performance of MnO2 can be further improved by selecting appropriate support materials that enhance its dispersion, stability, and overall catalytic efficiency.
In recent years, calcium carbonate (CaCO3), especially in the form of naturally extracted calcite, has emerged as a promising catalyst support due to its high surface area, biocompatibility, and ease of modification [15]. Biowaste-extracted calcite, such as that obtained from eggshells, offers a sustainable and cost-effective alternative to synthetic catalyst supports, aligning with global efforts to minimize industrial waste and promote circular economy principles [16]. The incorporation of MnO2 onto calcite surfaces can create highly efficient oxidative catalysts that leverage the synergistic effects of the metal oxide and the support material, leading to enhanced catalytic performance in ODS applications.
Despite the effectiveness of ODS in oxidizing sulfur compounds, the process alone is insufficient to meet the stringent ULSD specifications. The oxidized sulfur species, primarily sulfones and sulfoxides, must be efficiently separated from the fuel to prevent recontamination and ensure compliance with fuel quality standards [17]. Adsorptive desulfurization (ADS) has been widely explored as a complementary approach to ODS, utilizing high-surface-area materials such as activated carbon, zeolites, and metal–organic frameworks (MOFs) for the selective removal of oxidized sulfur compounds [18]. Among these adsorbents, activated carbon stands out due to its high adsorption capacity, chemical stability, and cost-effectiveness, making it an ideal candidate for post-oxidation sulfur removal [19]. The integration of ODS with ADS presents a synergistic approach to deep desulfurization, combining the selectivity of oxidative catalysis with the efficiency of physical adsorption to achieve ultra-low sulfur levels in diesel fuel [20]. However, the effectiveness of this combined process depends on optimizing key operating parameters, including oxidation temperature, reaction time, catalyst loading, and adsorbent performance.
In addition to catalyst and adsorbent optimization, reactor design plays a crucial role in enhancing the efficiency of oxidative desulfurization processes. Traditional batch and fixed-bed reactors often suffer from mass transfer limitations, catalyst deactivation, and inefficient mixing, all of which can negatively impact the overall desulfurization performance [21]. To address these challenges, this study introduces a novel digital basket reactor (DBR) designed to enhance mass transfer, improve catalyst–substrate interactions, and provide precise control over reaction conditions. The DBR system allows for better dispersion of the nanocatalyst, minimizing diffusion limitations and ensuring uniform contact between the reactants and catalytic active sites. By optimizing key operating conditions such as agitation speed, reaction time, and temperature, the DBR is expected to significantly enhance sulfur removal efficiency while maintaining process scalability and industrial feasibility [22].
Despite numerous reports regarding oxidative and adsorptive desulfurization, few investigations since 2020 have examined sustainable catalysts extracted from biowaste supports. This study introduces, for the first time, a MnO2/P–calcite catalyst synthesized from eggshell waste and utilized in a digital basket reactor (DBR) for the desulfurization of real diesel fuel. This approach combines a green catalyst with an advanced reactor design, bridging a key knowledge gap in eco-friendly ODS. This study develops an innovative oxidative–adsorptive desulfurization process integrating MnO2-modified calcite nanocatalysts from biowaste eggshells with advanced reactor design and adsorption techniques. It focuses on synthesizing and characterizing the catalysts, optimizing ODS reaction parameters to maximize sulfur conversion, and employing activated carbon adsorption for deep desulfurization. A novel digital basket reactor (DBR) is introduced to enhance mass transfer and catalyst efficiency. This study evaluates the economic and environmental feasibility of this hybrid ODS-ADS approach, aiming to establish a cost-effective, energy-efficient, and sustainable sulfur removal technology for cleaner fuel production.

2. Material Used and Experimental Design

2.1. Chemicals

2.1.1. Feedstock

Diesel fuel is used in this work (total sulfur content 9 ppm), which was provided from Pendik Company, Istanbul, Turkey. The characteristics of the diesel fuel are shown in Table 1. Dibenzothiophene (DBT) was utilized as a model sulfur compound in the diesel. DBT was purchased from Alfa Aesar, Lancashire, UK (C12H8S, purity of 98%, density at 20 °C of 1.44 g/cm3, molecular weight of 184 g/mol). It was mixed into the diesel fuel, giving a total S content of 543 ppm as the initial sulfur concentration of the feedstock. The concentration of S compounds was confirmed by analyzing the diesel oil using an X-ray sulfur content analyzer (ISO 9454, ASTM D4294-03, measuring range of 0–6.00 wt.%)

2.1.2. Oxidizing Agent

Hydrogen peroxide (H2O2, purity: 29.0–31.0%, Merck Millipore Company, Darmstadt, Germany) was employed as an oxidant in the ODS reactions.

2.1.3. Catalyst Component

Phosphoric acid (purity of 85%, Sigma Aldrich, Darmstadt, Germany) was utilized as a chemical activator in the preparation process of calcite. Eggshells were collected as biowaste materials from Iraqi eggs to prepare the calcite. Manganese (II) chloride (MnCl2·4H2O, purity of 98%, Thomas Baker, Mumbai, India) was used as a salt to provide MnO2 in the synthesis of catalyst (MnO2/calcite).

2.1.4. Adsorbent

Activated carbon was provided by Darmstadt, Germany, which was utilized as an adsorbent. Activated carbon was employed with a particle size of 1 to 3 mm.

2.2. Preparation of MnO2/Calcite Nanocatalyst

The synthesis of the nanocatalyst commenced with the pretreatment of eggshells, which were thoroughly washed, sun-dried for 24 h, and subsequently ground into an ultrafine powder. The powdered eggshell was then subjected to an activation process using phosphoric acid (H3PO4) at a 4:1 weight ratio (H3PO4 to eggshell). The mixture was stirred for 1 h to ensure homogeneous interaction, followed by a 24 h activation period. The resulting solution was then filtered, and the solid residue was thermally treated at 550 °C for 3 h to obtain activated calcite, which was subsequently characterized.
To introduce manganese dioxide (MnO2) onto the calcite support, an impregnation method was employed. Initially, manganese (II) chloride tetrahydrate (MnCl2·4H2O) was dissolved in deionized water under continuous stirring for 1 h. The prepared solution was then gradually introduced to the activated calcite to ensure uniform dispersion of the Mn precursor. The impregnated material was subjected to controlled drying at 120 °C for 12 h, followed by calcination in a nitrogen (N2) atmosphere at 550 °C for 4 h using a tubular furnace. This thermal treatment facilitated the conversion of the manganese precursor into MnO2, thereby yielding the MnO2/calcite nanocatalyst with a 5 wt.% MnO2 loading. The catalyst preparation steps are schematically illustrated in Figure 1. Each experiment was conducted using 100 mL of diesel fuel, with hydrogen peroxide added to sustain a molar ratio of 4:1 for H2O2 to sulfur. Hydrogen peroxide was incrementally introduced dropwise to prevent rapid decomposition. Following the reaction, the mixture was permitted to settle, and the aqueous phase was separated via decantation. Subsequently, the organic phase was dried with anhydrous sodium sulfate to eliminate residual moisture.

2.3. Characterization of MnO2/Calcite Nanocatalyst and AC

The synthesized MnO2/calcite nanocatalyst was extensively characterized using multiple analytical techniques to evaluate its structural, textural, and thermal properties. Brunauer–Emmett–Teller (BET) analysis was performed at the Advanced Materials Research Center and Nanotechnology in Baghdad, Iraq, to determine the surface area of the nanocatalyst. BET measurements were conducted after an 8 h reaction period to assess any changes in textural properties post-synthesis.
The surface morphology of the MnO2/calcite nanocatalyst and activated carbon (AC) was examined using scanning electron microscopy (SEM) with an accelerating voltage of 100 kV (Zeiss-EM10C, Carl Zeiss, Oberkochen, Germany), providing high-resolution imaging of the catalyst’s microstructural features. Functional group identification and molecular vibrational analysis were carried out through Fourier transform infrared (FTIR) spectroscopy using a Nicolet 6700 spectrometer (FTIR 8400 S, Shimadzu, Kyoto, Japan) at the Advanced Materials Research Center and Nanotechnology, Iraq.
The thermal stability and decomposition behavior of the catalyst were evaluated using thermogravimetric analysis (TGA) at the Petroleum Research and Development Centre (PRDC). A precisely weighed 1 µg sample was subjected to a controlled heating rate of 3 °C/min, with the temperature progressively increased up to 1000 °C. This analysis provided insights into the catalyst’s thermal resilience and mass loss characteristics under elevated temperatures.

2.4. Experimental Procedure

2.4.1. Design of Basket Reactor

This research was conducted at the College of Petroleum Processes Engineering, Tikrit University, focusing on the design and development of an advanced digital basket reactor (DBR) optimized for oxidative desulfurization (ODS) processes. The DBR is meticulously engineered to achieve exceptional catalyst dispersion within the oil matrix, ensuring superior mass transfer efficiency and enhanced reaction kinetics.
The reactor is equipped with a precision-controlled digital mixing system, capable of operating at variable speeds ranging from 0 to 5000 rpm. The mixing assembly comprises a 35 cm-long rotating shaft, terminating in four high-performance basket impellers. Each impeller features a 1:1:1 aspect ratio (length = 1 cm, depth = 1 cm, width = 1 cm) and is specifically designed to house the nano-catalyst, facilitating optimal catalytic activity. The basket impellers incorporate a network of hexagonally arranged circular perforations, strategically distributed across the metal framework to maximize fluid–catalyst interaction, thereby enhancing molecular diffusion and process efficiency.
To mitigate dead zones and ensure homogenous mixing, the reactor interior is equipped with four stainless-steel baffles, each measuring 10 cm in height and 1.5–1.8 cm in width. These baffles are precisely positioned with 38 cm interspacing and 2 cm protrusion into the reactor’s flow domain to intensify turbulence and improve mass transfer dynamics.
Constructed from high-grade stainless steel, the DBR has a working volume of 250 mL and is externally insulated with high-performance thermal wool to minimize heat loss. The system is electrically powered, capable of operating at a maximum rotational speed of 5000 rpm and sustaining temperatures exceeding 1000 °C.
A detailed schematic of the DBR experimental setup is presented in Figure 2, while Table 2 provides a comprehensive technical description of its components [23].

2.4.2. Oxidative–Adsorptive Desulfurization (ODS) Process in DBR

To evaluate the catalytic efficiency of the newly designed Mn/calcite catalyst within the digital basket reactor (DBR), real diesel fuel was utilized as the feedstock. Dibenzothiophene (DBT) was selected as the model sulfur compound and was blended into the diesel fuel to achieve an initial sulfur concentration of 543 ppm. The oxidative desulfurization (ODS) process was conducted using hydrogen peroxide (H2O2) as the oxidizing agent, with a fixed diesel volume of 100 mL per experimental run to ensure uniform reaction conditions.
A total of 25 experimental trials were performed under atmospheric pressure (1 atm), employing 4 g of Mn/calcite catalyst loaded into the basket impellers of the DBR. Each basket impeller contained 1 g of catalyst, ensuring a well-distributed catalytic reaction environment. The experiments were systematically conducted at reaction temperatures of 30 °C, 50 °C, 70 °C, and 90 °C, with reaction times of 40 min, 60 min, 80 min, and 100 min and magnetic stirring speeds of 200 rpm, 400 rpm, 600 rpm, and 800 rpm to examine the impact of various operational parameters on sulfur removal efficiency.
Post-reaction, the treated diesel samples were analyzed using X-ray fluorescence spectroscopy (ASTM D7039 method) to precisely quantify the residual sulfur content. Each experiment was repeated twice to ensure reproducibility, with the reported values representing the mean observations, maintaining a maximum deviation of 2% across all experimental runs.
In addition to oxidative desulfurization, the adsorptive desulfurization (ADS) process was integrated into the DBR, leveraging porous activated carbon (AC) to selectively adsorb and remove sulfur-containing compounds, further enhancing the overall desulfurization efficiency.

3. Results and Discussion

3.1. Characterization of the Designed Nano-Catalyst (MnO2/Calcite)

3.1.1. Surface Area Analysis

The physicochemical properties of both the nano-catalyst (MnO2/calcite) and the adsorbent (activated carbon, AC) are summarized in Table 3. A comparative evaluation of their BET surface area, total pore volume, and mean pore diameter reveals significant differences in their structural characteristics, which directly influence their functionality in the oxidative–adsorptive desulfurization (ODS) process.
Activated carbon (AC) exhibits an exceptionally high BET surface area of 912.974 m2/g and a total pore volume of 0.499 cm3/g, making it highly suitable for adsorption-based applications, particularly for the removal of small sulfur-containing molecules. In contrast, the MnO2/calcite nano-catalyst demonstrates a significantly lower BET surface area of 5.140 m2/g and a total pore volume of 0.010 cm3/g, indicative of a structure optimized for catalytic reactions rather than adsorption-dominated processes.
Furthermore, the mean pore diameter of the adsorbent (2.18 nm) is considerably smaller than that of the catalyst (7.89 nm). This distinction suggests that AC is highly efficient in capturing and retaining small sulfur species due to its microporous structure and extensive surface area. Conversely, the larger pore size of the MnO2/calcite catalyst enhances molecular diffusion within the reaction medium, making it more suitable for heterogeneous catalytic processes where larger reactant molecules require rapid diffusion to active sites.
These structural differences underscore the complementary roles of the two materials in the DBR system—with AC functioning as a high-capacity adsorbent for sulfur removal, while the MnO2/calcite catalyst facilitates efficient oxidative desulfurization through rapid diffusion and catalytic transformation of sulfur compounds.

3.1.2. Morphological and Elemental Analysis of the Synthesized Catalyst

A comprehensive structural and compositional analysis of the MnO2/calcite nano-catalyst was conducted using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The SEM micrographs, presented in Figure 3, provide critical insights into the morphology, dispersion, and surface characteristics of the synthesized catalyst.
The SEM image (Figure 3a) reveals a highly disordered and complex surface morphology, indicative of the random distribution of MnO2 nanoparticles or clusters across the calcite substrate. This non-uniform topology suggests a heterogeneous catalytic surface, which is crucial for enhancing active site availability and catalytic reactivity [24]. The observed clusters exhibit a relatively high surface area, a key parameter influencing mass transfer dynamics and catalytic performance in oxidative desulfurization (ODS) and adsorption processes [25].
Furthermore, the MnO2 particles appear well-integrated with the calcite substrate, demonstrating a uniform dispersion pattern that suggests an effective loading and deposition technique. This strong interfacial interaction between MnO2 and calcite plays a pivotal role in improving the stability and durability of the catalyst under operating conditions [26]. The structured distribution of MnO2 also prevents particle agglomeration, thereby enhancing catalytic efficiency by ensuring maximum exposure of active sites to the reactants [27].
The EDX spectrum (Figure 3b) provides a detailed compositional breakdown of the synthesized catalyst, confirming the presence of manganese (Mn), oxygen (O), and calcium (Ca) as the principal elemental constituents. Notably, the atomic ratio data suggest a relatively high concentration of various manganese oxides (MnOx), rather than pure MnO2, indicating the potential formation of Mn(III) and Mn(IV) species [28]. The presence of these higher oxidation states is particularly significant, as they contribute to enhanced redox activity, thereby improving the oxidative capabilities of the catalyst [23].
The integration of MnO2 with the calcite substrate not only provides structural support but also enhances electron transfer efficiency, a critical factor in catalytic oxidation mechanisms [29]. This synergistic interaction between the metal oxide and the support material is fundamental for achieving high catalytic activity and selectivity, making the synthesized MnO2/calcite system a highly effective catalyst for ODS applications [30].
The structural and elemental characteristics revealed by SEM and EDX analyses underscore the high potential of the synthesized catalyst in oxidative desulfurization processes. The uniform MnO2 dispersion, coupled with the heterogeneous surface morphology and the presence of multiple manganese oxidation states, provides a multi-functional catalytic environment conducive to efficient sulfur removal [31]. The high-surface-area clusters enhance reactant accessibility, while the well-defined MnO2–calcite interface ensures robust catalytic stability over extended operational cycles [32].
These findings highlight the strategic design and synthesis approach employed in developing this high-performance catalyst, positioning it as a promising candidate for industrial-scale ODS applications.
The N2 adsorption–desorption isotherm of the MnO2/P–calcite/AC composite (Figure 4) exhibits a typical Type IV profile with an H3 hysteresis loop, indicating the presence of mesopores with slit-like configurations. The marked increase in adsorbed volume at relative pressures exceeding 0.4 suggests capillary condensation within the mesopores. The hysteresis observed between the adsorption and desorption branches reflects pore connectivity and irregularity, characteristics commonly associated with porous carbon–mineral composites. The Brunauer–Emmett–Teller (BET) surface area was approximately 150 m2/g, with a total pore volume of 0.45 cm3/g and an average pore diameter of 7.5 nm. These features are indicative of a mesoporous material capable of both adsorbing and oxidizing sulfur compounds, thereby supporting the application of MnO2/P–calcite/AC in oxidative–adsorptive desulfurization. The layered pore architecture facilitates the diffusion of dibenzothiophene (DBT) molecules and enhances accessibility to active MnO2 sites, consequently improving overall desulfurization efficacy.

3.1.3. FTIR Spectral Analysis and Functional Group Identification

The Fourier transform infrared (FTIR) spectrum, illustrated in Figure 5, provides crucial insights into the functional groups present in the synthesized MnO2/calcite catalyst and activated carbon. The observed vibrational bands confirm the presence of various functional moieties that contribute to the structural and catalytic properties of the material.
A significant O-H stretching vibration was detected at 3644 cm−1, indicating the presence of hydroxyl groups, which may be attributed to surface-bound hydroxyl functionalities or physisorbed water molecules [33]. Interestingly, a weak absorption band at 60 cm−1 in the benzene sulfonic acid region further supports the possibility of hydrogen-bonded hydroxyl groups or adsorbed water species [34]. Additionally, the absorption bands at 2921.87 cm−1 and 2851.59 cm−1 correspond to the asymmetric and symmetric stretching vibrations of C-H bonds, characteristic of aliphatic hydrocarbon chains, suggesting the potential presence of organic residues or surface modifications [35]. The presence of a well-defined peak at 1797.25 cm−1 is indicative of the carbonyl (C=O) functional group, which could be linked to carbonate species or organic contaminants originating from precursor materials [36]. Moreover, the peaks observed at 1449.15 cm−1 and 885.155 cm−1 confirm the presence of carbonate ions, while an additional peak at 1164 cm−1, characteristic of P-calcite (CaCO3), further reinforces the attribution of carbonate phases within the composite material [37].
The spectrum also reveals distinct vibrational bands at 797.46 cm−1 and 693.38 cm−1, which are commonly associated with silicate impurities, possibly introduced during synthesis or from precursor sources [38]. Additionally, the characteristic metal–oxygen stretching vibration at 462 cm−1 is strongly indicative of the presence of MnO2, confirming its successful integration within the calcite matrix [39].
The FTIR spectral features suggest that the MnO2/P–calcite catalyst exhibits a heterogeneous composition, incorporating hydroxyl, carbonate, and metal–oxide functionalities. The presence of MnO2-associated metal–oxygen bonds highlights its potential catalytic activity, as these structures can facilitate redox reactions crucial for oxidative desulfurization (ODS) [40]. Furthermore, the incorporation of hydroxyl and carbonate species may enhance the material’s surface reactivity, providing additional adsorption sites for sulfur-containing compounds and improving overall desulfurization efficiency [41].
In conclusion, the FTIR results substantiate that the activated carbon encompasses hydroxyl, carbonyl, and aliphatic groups, which influence its adsorption capacity and surface reactivity, rendering it appropriate for various industrial and environmental applications [42,43]. The accompanying image displays two FTIR spectra, the black spectrum representing the MnO2/P–calcite composite and the red spectrum depicting activated carbon (AC). The black spectrum reveals prominent absorption bands indicating its dual composition: peaks at 1400 cm−1, 875 cm−1, and 712 cm−1 confirm the presence of calcite (CO32−) via characteristic asymmetric stretching and bending vibrations, while the peak between 500 and 650 cm−1 corresponds to Mn-O stretching, confirming manganese dioxide. Both materials also exhibit a broad O-H stretch around 3400 cm−1 and an H-O-H bending vibration at 1630 cm−1, reflecting adsorbed water and surface hydroxyl groups. Conversely, the AC spectrum (red line) displays typical features of carbon-based materials: a broad -OH stretching band near 3400 cm−1, minor -CH2 and -CH3 stretches around 2920 cm−1 and 2850 cm−1, respectively, and prominent C=C/C=O stretches near 1620 cm−1, along with C-O stretching and O-H bending between 1000–1400 cm−1, indicative of oxygen functional groups and aromatic structures.

3.1.4. Thermal Stability and Decomposition Behavior: TGA and DTA Analysis

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were conducted to evaluate the thermal stability, decomposition behavior, and phase transitions of the synthesized MnO2/P–calcite catalyst. Figure 6 presents the TGA (green curve), which tracks the weight loss of the sample as it is heated to 900 °C at a controlled heating rate of 20 °C per minute. The initial sample mass was approximately 6 g, and a substantial weight reduction of 13.31% was observed, primarily attributed to the thermal decomposition and volatilization of various sample constituents.
A notable weight loss event occurred at 47.07 °C, which can be linked to the evaporation of physisorbed moisture or volatile organic compounds retained within the catalyst matrix [44]. This initial mass loss is commonly observed in porous catalytic materials and is a critical factor in determining their suitability for high-temperature catalytic applications. The presence of residual water molecules or loosely bound volatiles can influence the structural integrity and adsorption capacity of the material, affecting its catalytic performance in oxidative desulfurization (ODS) reactions [45].
As the temperature increased beyond 200 °C, a progressive weight decline was observed, indicating the thermal decomposition of carbonate species and potential phase transformations within the catalyst [46]. The degradation of calcite (CaCO3) typically occurs in this temperature range, releasing CO2 and forming calcium oxide (CaO), which may modify the catalytic properties of the material [47]. This transformation is particularly relevant in heterogeneous catalysis, where the formation of reactive oxide phases can enhance oxygen mobility and catalytic efficiency [48].
The DTA profile (blue curve) in Figure 4 further elucidates the thermal events by representing the rate of weight change as a function of temperature. The presence of endothermic and exothermic peaks provides insight into phase transitions and decomposition pathways. The significant weight loss observed at higher temperatures is likely associated with the reduction of MnO2 and the subsequent formation of lower-valence manganese oxides (Mn3O4 or MnO), which are known to exhibit enhanced catalytic activity due to their superior electron transfer properties [49].
The thermal stability of the MnO2/P–calcite catalyst, as demonstrated by its gradual decomposition profile, suggests its suitability for high-temperature catalytic processes. The controlled weight loss and phase transformation behavior indicate that the material retains its structural integrity up to ~800 °C, making it a viable candidate for oxidative desulfurization and other high-temperature catalytic applications [50]. Furthermore, the observed phase evolution of manganese oxides under thermal stress aligns with literature findings, reinforcing the catalyst’s potential for enhanced oxidative performance [51].
These thermal analyses provide critical insights into the structural robustness and decomposition kinetics of the synthesized catalyst, supporting its application in high-temperature fuel purification technologies.

3.2. Characterization of the Adsorbent (Activated Carbon)

3.2.1. X-Ray Diffraction (XRD) Analysis of Activated Carbon

The X-ray diffraction (XRD) profile of the activated carbon, illustrated in Figure 7, exhibits two prominent broad peaks at 2θ = 25° and 44°, which are characteristic of an amorphous carbon structure. The broad nature of these peaks suggests a highly disordered arrangement of carbon atoms, which is advantageous for adsorption applications due to the presence of a large number of active sites and an extensive porous network [52,53].
The diffraction peak at 2θ = 25° is associated with the (002) plane, indicative of a turbostratic carbon structure where graphene layers are randomly stacked rather than forming a well-ordered graphitic arrangement. This structural disorder enhances the accessibility of adsorption sites and contributes to the superior adsorption performance of activated carbon in desulfurization processes [54]. Additionally, the broad peak at 2θ = 44° corresponds to the (100) plane, which suggests partial graphitization of the material. The presence of graphitic domains within an overall amorphous framework enhances the thermal stability and mechanical robustness of the carbon structure, which is essential for maintaining its adsorption efficiency over extended operational cycles [55,56].
The observed XRD pattern confirms that the activated carbon exhibits a predominantly amorphous structure with minor graphitic characteristics. This combination provides a balance between structural stability and high surface area, both of which are critical for applications requiring efficient adsorption and catalytic support [57,58]. The disordered nature of the carbon framework increases the availability of micro- and mesopores, thereby improving the diffusion of sulfur-containing molecules into the adsorption sites, which is crucial for oxidative desulfurization (ODS) processes [59].
These findings reinforce the suitability of the synthesized activated carbon for desulfurization applications, as its structural characteristics directly contribute to enhanced sulfur capture efficiency. The presence of a well-developed porous network, combined with a partially graphitized framework, ensures a synergistic effect between adsorption capacity and stability, making it a promising material for industrial-scale fuel purification [60,61].

3.2.2. Surface Morphology of Activated Carbon: SEM Analysis

The SEM micrograph of activated carbon, as shown in Figure 8, reveals a highly porous and irregular surface, characteristic of well-activated carbon materials used for adsorption applications. The material exhibits a heterogeneous pore structure, with pore sizes ranging from approximately 74 nm to 328.1 nm, indicating a well-developed network of micro- and mesopores. Such structural features are essential for enhancing adsorption efficiency by providing a high surface area and facilitating molecular diffusion [62].
The presence of a diverse pore size distribution suggests that the activated carbon can effectively capture a wide range of contaminants, making it highly suitable for industrial applications such as gas purification, water treatment, and pollutant removal [63]. The combination of micropores for high-capacity adsorption and larger pores for improved mass transfer dynamics enhances its overall performance in separation processes [64]. These findings confirm that the synthesized activated carbon meets the structural requirements for effective adsorption-based applications.

3.3. Effect of Experimental Parameters on ODS

3.3.1. Effect of Magnetic Stirrer Speed

The efficiency of the oxidative desulfurization (ODS) process is significantly influenced by various operational parameters, among which magnetic stirrer speed (RPM) plays a crucial role (Figure 9). Stirring intensity directly affects mass transfer, reactant dispersion, and catalyst interaction, all of which are essential for optimizing sulfur oxidation. Experimental results demonstrate a direct correlation between increasing stirrer speed and sulfur oxidation efficiency across all tested temperatures (30 °C, 50 °C, 70 °C, and 90 °C). This enhancement can be attributed to improved mass transfer, where higher RPM facilitates better reactant dispersion, ensuring uniform contact between sulfur compounds and the oxidizing agent, ultimately leading to higher oxidation rates [65]. Additionally, increasing agitation reduces diffusion limitations, allowing for greater interaction between reactants and the catalyst’s active sites, which accelerates oxidation reactions and enhances overall desulfurization performance [66]. Furthermore, efficient stirring prevents phase separation and concentration gradients, creating a more homogeneous reaction environment, which is essential for achieving maximum sulfur removal [67].
At higher stirrer speeds, increased turbulence improves oxidation kinetics, resulting in greater sulfur removal efficiency. The data indicate that the highest sulfur oxidation rates are achieved at the maximum tested RPM, suggesting that stronger agitation significantly enhances the ODS process. However, excessive turbulence at very high RPM values could lead to catalyst deactivation or emulsion formation, which may negatively impact the reaction efficiency. Therefore, while magnetic stirring is a key parameter in oxidative desulfurization, optimizing its speed is essential to balance enhanced mass transfer with potential adverse effects. These findings are consistent with previous studies on heterogeneous catalytic oxidation, which emphasize the role of agitation in improving reaction kinetics and overall process efficiency [68].

3.3.2. Effect of Oxidation Time

Figure 10 presents a set of graphs illustrating the dependence of sulfur oxidation efficiency on oxidation time, stirrer speed, and temperature in the oxidative desulfurization (ODS) process. A thorough analysis of the results reveals that increasing the oxidation time up to 100 min positively influences sulfur oxidation efficiency, particularly when combined with high stirrer speeds of 800 RPM.
At lower temperatures (30 °C and 50 °C), the extent of sulfur oxidation remains relatively low even under optimized conditions, achieving only 25% and 30%, respectively. However, as the temperature increases to 70 °C and 90 °C, sulfur oxidation becomes significantly more effective, reaching a maximum efficiency of approximately 70% at 70 °C and nearly 90% at 90 °C under 800 RPM and 100 min of reaction time. This suggests that extended oxidation time, in conjunction with higher temperature and stirrer speed, significantly enhances the ODS process.
The enhancement in sulfur oxidation over time can be attributed to the prolonged interaction between dibenzothiophene (DBT) and hydrogen peroxide (H2O2), facilitating the formation of sulfoxides and sulfones through oxidation reactions [69]. The prolonged reaction time ensures improved mass transfer and diffusion of reactants, allowing a greater number of DBT molecules to react with the oxidizing agent [70]. Additionally, as oxidation time increases, the extended exposure of reactants to catalytic sites enhances the conversion efficiency, particularly at higher temperatures where reaction kinetics are more favorable [66].
Furthermore, the results indicate that increasing the oxidation time leads to greater sulfur removal due to the enhanced contact between DBT, H2O2, and oxygen molecules within the catalyst pores [71]. This prolonged interaction not only promotes the oxidation process but also maximizes the efficiency of the catalytic reaction by ensuring a more complete transformation of sulfur-containing compounds into their oxidized derivatives [72].
Given these findings, it can be strongly argued that oxidation time plays a crucial role in optimizing the ODS process, particularly when combined with elevated temperatures and high stirrer speeds. The best performance in sulfur oxidation was achieved at 90 °C, 800 RPM, and 100 min, highlighting the significance of reaction duration in achieving high desulfurization efficiency [73].

3.3.3. Effect of Oxidation Temperature

Temperature is a critical parameter in optimizing the oxidative desulfurization (ODS) process, as it directly influences reaction kinetics, oxidation rates, and overall desulfurization efficiency. The data presented in Figure 11 indicate a strong positive correlation between increasing temperature and sulfur oxidation efficiency, particularly when combined with high stirrer speeds and extended reaction times.
As the reaction progresses from 40 to 100 min, sulfur oxidation consistently improves with rising temperatures. At 30 °C, oxidation remains relatively low, achieving only 10–20% conversion after 40 min. However, at 90 °C, oxidation efficiency significantly increases, reaching 60% within the same time frame. As the reaction time extends to 60 and 80 min, oxidation efficiency further rises, reaching 70% and 75%, respectively, at 90 °C. The highest sulfur oxidation rate, approximately 80%, is achieved at 90 °C, 800 RPM, and 100 min of reaction time. These findings underscore the crucial role of temperature in enhancing oxidative desulfurization, as higher temperatures promote faster and more complete oxidation of sulfur compounds [74].
The observed increase in sulfur oxidation with temperature can be attributed to several key factors. Firstly, higher temperatures enhance reaction kinetics, reducing activation energy barriers and accelerating the oxidation of dibenzothiophene (DBT) into sulfoxides and sulfones [75]. Secondly, elevated temperatures improve mass transfer rates, ensuring better dispersion of reactants and increased interaction between DBT and the oxidizing agent [76]. Additionally, higher thermal energy facilitates the breakdown of sulfur-containing molecules, making them more susceptible to oxidation [77].
Moreover, the data suggest that while reaction time and stirrer speed significantly impact sulfur removal, temperature exerts a more dominant influence on the overall efficiency of the ODS process. This can be explained by the fact that oxidation reactions are inherently temperature-dependent, with increased thermal energy leading to more efficient conversion of sulfur compounds [78].

3.4. Effect of Studied Variables on ADS by AC Adsorbent

Figure 12, Figure 13 and Figure 14 illustrate the influence of rotational speed (RPM), oxidation time, and temperature, respectively, on sulfur removal efficiency via adsorption desulfurization (ADS) using activated carbon (AC) as an adsorbent. A comparative analysis with Figure 11, Figure 12 and Figure 13, which depicts the oxidative desulfurization (ODS) process, reveals that the maximum sulfur removal efficiency in ODS reaches 79.6% under conditions of 90 min, 800 rpm, and 1000 °C. In contrast, ADS achieves a notably higher removal efficiency of 96.5% at 80 min, 600 rpm, and 900 °C.
The superior performance of ADS can be attributed to the inherent properties of AC as an adsorbent, which provides a high surface area, well-developed porosity, and strong affinity for sulfur-containing compounds such as thiophenes, sulfides, and mercaptans. Unlike ODS, which relies on oxidation reactions to convert sulfur species into more polar derivatives for subsequent extraction, ADS directly captures sulfur compounds through physical and chemical interactions with the AC surface. This selectivity enhances the efficiency of sulfur removal, particularly from complex hydrocarbon mixtures [79].
It is crucial to emphasize that ADS serves as a complementary rather than a standalone alternative to ODS. While ODS is effective in reducing the concentration of sulfur compounds by oxidizing them into sulfones or sulfoxides, it may not achieve complete removal, especially for sterically hindered or less reactive sulfur species. Integrating ADS post-ODS can significantly improve the overall desulfurization efficiency by selectively adsorbing residual sulfur compounds that persist after oxidation [80,81].
Furthermore, the observed variations in optimal process conditions between ADS and ODS highlight the distinct mechanisms governing each method. The lower rpm and temperature required for maximum sulfur removal in ADS suggest that excessive agitation and thermal energy may disrupt the adsorption equilibrium or cause desorption of captured sulfur species. In contrast, ODS benefits from higher temperatures and agitation, which enhance oxidation kinetics. These findings underscore the importance of process optimization to leverage the strengths of both methods in a hybrid desulfurization approach [82].
In conclusion, the results affirm the critical role of ADS as a highly efficient and selective process for sulfur removal, particularly when employed in conjunction with ODS. The integration of both methods offers a promising strategy for achieving ultra-deep desulfurization, which is essential for meeting stringent environmental regulations and improving fuel quality [83]. Dibenzothiophene (DBT) is oxidized stepwise to DBT sulfoxide and then to DBT sulfone through reactive oxygen species produced from H2O2 decomposition. Activated carbon (AC) serves a dual purpose by adsorbing DBT and its oxidation intermediates and by promoting the breakdown of H2O2 into hydroxyl and peroxy radicals, thereby boosting the oxidative desulfurization process. This synergistic effect between adsorption and oxidation is emphasized in this paper, supported by relevant literature [23].

4. Conclusions

This study presents the successful development of a highly efficient oxidative–adsorptive desulfurization (OADS) process, utilizing an eggshell-extracted MnO2/P-calcite supported green MnO2 nanocatalyst combined with activated carbon. The integration of this advanced catalyst with an innovative digital basket reactor (DBR) enabled the identification of optimal reaction conditions, resulting in an impressive 96.5% sulfur removal from diesel fuel. This high efficiency was attained under optimized settings, illustrating the synergy between the catalyst’s unique structure and the DBR’s improved mass transfer capabilities. Catalyst characterization confirmed its structural and compositional suitability, underscoring the effectiveness of the combined oxidative and adsorptive mechanisms. Despite these promising results, some limitations need to be addressed. This study did not include recyclability testing, and further research is required to assess potential catalyst leaching into the product stream. Although BET analysis provided insights into the catalyst’s surface area, a more detailed examination, including pore size distribution, is necessary to gain a better understanding of its structural properties. Looking forward, future research will focus on several key areas: conducting catalyst regeneration and recyclability studies to evaluate long-term viability and cost-effectiveness; developing comprehensive kinetic models to accurately describe reaction rates and mechanisms; and exploring scale-up possibilities for larger, continuous systems to demonstrate industrial potential. Overall, this work lays a potential basis for advancing sustainable, efficient desulfurization technologies.

Author Contributions

Conceptualization, J.I.H. and A.T.J.; methodology, M.N.I., S.A.H., A.T.N. and I.M.M.; validation, K.I.H.; formal analysis, H.A.A., M.A.A., S.A.H. and I.M.M.; investigation, M.N.I. and K.I.H.; resources, A.T.J.; writing-original draft, M.A.A.; writing-review & editing, K.I.H., J.I.H., H.A.A., A.T.J., S.A.H. and I.M.M.; visualization, A.T.N.; supervision, A.T.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Steps of MnO2/calcite nano-catalyst preparation.
Figure 1. Steps of MnO2/calcite nano-catalyst preparation.
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Figure 2. Schematic of the digital basket reactor DBBBR system.
Figure 2. Schematic of the digital basket reactor DBBBR system.
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Figure 3. (a) SEM images of MnO2/P–calcite. (b) EDX for MnO2/P–calcite.
Figure 3. (a) SEM images of MnO2/P–calcite. (b) EDX for MnO2/P–calcite.
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Figure 4. The N2 adsorption–desorption isotherm of the MnO2/P–calcite/AC composite.
Figure 4. The N2 adsorption–desorption isotherm of the MnO2/P–calcite/AC composite.
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Figure 5. FTIR for activated carbon (AC) and MnO2/P–calcite.
Figure 5. FTIR for activated carbon (AC) and MnO2/P–calcite.
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Figure 6. TGA and DTA for MnO2/P–calcite.
Figure 6. TGA and DTA for MnO2/P–calcite.
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Figure 7. XRD for activated carbon (AC).
Figure 7. XRD for activated carbon (AC).
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Figure 8. SEM for activated carbon (AC).
Figure 8. SEM for activated carbon (AC).
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Figure 9. Effect of increasing speed of mixing (RPM) on the ODS process at (a) 30 °C, (b) 50 °C, (c) 70 °C, and (d) 90 °C at different reaction times.
Figure 9. Effect of increasing speed of mixing (RPM) on the ODS process at (a) 30 °C, (b) 50 °C, (c) 70 °C, and (d) 90 °C at different reaction times.
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Figure 10. Effect of oxidation time for ODS performance at several mixing speeds and reaction temperatures (a) 30 °C, (b) 50 °C, (c) 70 °C, and (d) 90 °C using MnO2/P-calcite.
Figure 10. Effect of oxidation time for ODS performance at several mixing speeds and reaction temperatures (a) 30 °C, (b) 50 °C, (c) 70 °C, and (d) 90 °C using MnO2/P-calcite.
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Figure 11. Influence of temperature and various digital mixing speeds on ODS process for sulfur removal with reaction times of (a) 40 min, (b) 60 min, and (c) 80 min.
Figure 11. Influence of temperature and various digital mixing speeds on ODS process for sulfur removal with reaction times of (a) 40 min, (b) 60 min, and (c) 80 min.
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Figure 12. Effect of increasing speed of mixing (RPM) on the ADS process at (a) 30 °C, (b) 60 °C, (c) 90 °C for different reaction times.
Figure 12. Effect of increasing speed of mixing (RPM) on the ADS process at (a) 30 °C, (b) 60 °C, (c) 90 °C for different reaction times.
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Figure 13. Effect of oxidation time on ADS performance at several mixing speeds and reaction temperatures of (a) 30 °C, (b) 60 °C, and (c) 90 °C using MnO2/P-calcite.
Figure 13. Effect of oxidation time on ADS performance at several mixing speeds and reaction temperatures of (a) 30 °C, (b) 60 °C, and (c) 90 °C using MnO2/P-calcite.
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Figure 14. Influence of temperature influence and various digital mixing speeds on ADS process for sulfur removal for reaction times of (a) 30 min, (b) 60 min, and (c) 90 min.
Figure 14. Influence of temperature influence and various digital mixing speeds on ADS process for sulfur removal for reaction times of (a) 30 min, (b) 60 min, and (c) 90 min.
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Table 1. Characteristics of diesel oil.
Table 1. Characteristics of diesel oil.
Physical PropertyValues
Specific gravity at 15.5 °C0.8333
API at 60 F38.31
Total sulfur, (ppm)9
Kinematic viscosity, (mm2/s)3.15
Cetane index53.90
Flash point, (°C)61
Pour point, (°C)<−20
Distillation, (°C)
Initial boiling point, IBP (°C)165
10%202
50%276
90%338
Final boiling point, FBP (°C)357
Table 2. Specification of DBBBR system (adapted from [23]).
Table 2. Specification of DBBBR system (adapted from [23]).
DescriptionSpecification
Material of the reactorStainless steel
Dimension of reactorD = 8 cm, H = 10 cm
Length of the rod35 cm
Dimension of basketH = 1 cm, L = 1 cm, W = 1 cm
Impeller typeFour-basket impeller
Impeller diameter90.0 mm
Reactor working volume250 mL
Baffle4, distributed along the wall of the reactor (height 8 cm)
PreheaterElectrical heater
Insulator materialGlass wool
Table 3. Properties of adsorbent and the nano-catalyst.
Table 3. Properties of adsorbent and the nano-catalyst.
PropertyAdsorbent (AC)Catalyst (MnO2/Calcite)
BET912.974 m2/g5.140 m2/g
Total pore volume (p/p0 = 0.9900)0.499 cm3/g0.010 cm3/g
Mean pore diameter2.18 nm7.89 nm
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Humadi, J.I.; Hamad, K.I.; Abdulkareem, H.A.; Ismael, M.N.; Jarullah, A.T.; Ahmed, M.A.; Hameed, S.A.; Nawaf, A.T.; Mujtaba, I.M. Eco-Friendly Oxidative–Adsorptive Desulfurization for Real Diesel Fuel Using Green MnO2 Biowaste-Extracted Calcite in Digital Basket Reactor. Processes 2025, 13, 3084. https://doi.org/10.3390/pr13103084

AMA Style

Humadi JI, Hamad KI, Abdulkareem HA, Ismael MN, Jarullah AT, Ahmed MA, Hameed SA, Nawaf AT, Mujtaba IM. Eco-Friendly Oxidative–Adsorptive Desulfurization for Real Diesel Fuel Using Green MnO2 Biowaste-Extracted Calcite in Digital Basket Reactor. Processes. 2025; 13(10):3084. https://doi.org/10.3390/pr13103084

Chicago/Turabian Style

Humadi, Jasim I., Khaleel I. Hamad, Hiba A. Abdulkareem, Maha Nazar Ismael, Aysar T. Jarullah, Mustafa A. Ahmed, Shymaa A. Hameed, Amer T. Nawaf, and Iqbal M. Mujtaba. 2025. "Eco-Friendly Oxidative–Adsorptive Desulfurization for Real Diesel Fuel Using Green MnO2 Biowaste-Extracted Calcite in Digital Basket Reactor" Processes 13, no. 10: 3084. https://doi.org/10.3390/pr13103084

APA Style

Humadi, J. I., Hamad, K. I., Abdulkareem, H. A., Ismael, M. N., Jarullah, A. T., Ahmed, M. A., Hameed, S. A., Nawaf, A. T., & Mujtaba, I. M. (2025). Eco-Friendly Oxidative–Adsorptive Desulfurization for Real Diesel Fuel Using Green MnO2 Biowaste-Extracted Calcite in Digital Basket Reactor. Processes, 13(10), 3084. https://doi.org/10.3390/pr13103084

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