Synthesis of magnetic carbon composite by diesel engine and its application to dye removal

Abstract We report a one-step and efficient method to synthesize magnetic diesel particles (MDPs) and their application toward dye removal. The MDPs were obtained by the combustion of ferrocene-doped diesel fuel using an internal combustion engine. The diesel particles combined with ferrocene as a metal source were collected by an electrostatic precipitator. The thermogravimetric analysis confirmed the ferrocene-based catalyst and thermal stability of MDPs. FE-SEM showed irregular and aggregated morphologies, and the presence of iron oxide phases (Fe4O3 and γ-Fe2O3) was confirmed by X-ray diffraction analysis. The characteristics of MDPs as an adsorbent for organic pollutants were evaluated by methylene blue (MB) adsorption experiments. Langmuir isotherm and pseudo-second-order models best interpret the adsorption process and kinetics, respectively, with a maximum adsorption capacity of 40.81 mg/g of MB adsorbed on the surface of magnetic diesel particles.


Introduction
Conventional internal combustion engines (ICE) are used for converting the chemical energy in the fuel to generate electricity and power cars and trucks.Recently, however, there has been considerable interest in repurposing internal combustion engines as piston reactors for chemical synthesis and conversion (Ashok et al. 2023).Thanks to its unique operating conditions, namely high pressure, high temperature, extremely short residence time, and fast piston motion, ICE as a piston reactor provides unparalleled features for chemical reactions and process intensification.While repurposing ICE as a piston reactor is still in its early stages, several groups reported its application for partial oxidation and reforming of methane, dissociation and reforming of methanol, and ammonia production (Yang et al. 2009;Ezdin et al. 2020;Yun et al. 2015).ICE engine can also be used for nanoparticles synthesis; in previous works, we reported the possibility of using a diesel engine as a piston reactor to synthesize carbon nanotube (CNT), the diesel engine fueled with various biofuels containing sulfur/molybdenum as a promoter and ferrocene as a catalyst for the CNT growth.Under control of ethanol concentration in biofuel, catalyst and promoter ratio, and engine operating conditions, CNT with diameters ranging from a few nanometers to several tens of nanometers could be synthesized (Suzuki and Mori 2018a;Suzuki and Mori 2018b).
In the last decade, diesel exhaust particles have attracted much practical interest and applications in various domains such as energy storage (Sahu et al. 2017), imaging and sensing (Tripathi et al. 2014), anode electrodes for Li-ion batteries (Gregory et al. 2022), photodegradation of organic pollutants (Singh et al. 2017), and water treatment (Gunture et al. 2020;Vishvendra Pratap Singh and Vaish 2020).Several studies investigated the use of diesel particles collected from the exhaust pipe of a diesel engine for the removal of different cationic organic dyes (Gunture et al. 2020) (V.P. Singh and Vaish 2020) (Sharma et al. 2019) (Vishvendra Pratap Singh and Vaish 2020).Adsorption of organic contaminants from wastewater is one of the most widely used techniques for water treatment (Arami et al. 2005;Yagub et al. 2014).Before its utilization, diesel soot was purified and annealed to remove the unburned organic impurities.Further, to overcome the complexity of the desorption process of an absorbent in its powder form, the authors coated the diesel particles on polyurethane foam (V.P. Singh and Vaish 2020), non-woven fabric (Sharma et al. 2019), and as a composite with cement on wood (Vishvendra Pratap Singh and Vaish 2020).Although diesel particles and soot based-adsorbent coated on different substrates shows a remarkable removal capacity of different dyes, robust chemical and thermal stability (Xu et al. 2021), they require several additional processing steps, including the deposition process and materials as a substrate which may limit their reusability and strongly increase their cost.Recently, new emerging nanomaterials have been used in adsorption for remediation and water treatment, particularly magnetic nanoparticles, owing to their high surface-to-volume ratio, large removal capacity, fast kinetics, and, more importantly, their magnetism.The magnetic nanoparticles can be easily separated from the solution using a low-gradient magnetic field or a hand-held magnet and recovered for further reuse (Hosseini et al. 2016;Ateia et al. 2017;Ateia et al. 2018;Le et al. 2019;Awfa et al. 2019;Barbosa et al. 2021;Kahya and Erim 2022).However, multiple steps are required to synthesize the magnetic nanoparticles according to the previous investigations, which made the synthesis process complex and expensive.Herein, we report for the first time a one-step process to synthesize low-cost magnetic diesel particles (MDPs) using ICE as a piston reactor, the novelty of this work lies in the direct combination of ferrocene (Fe(C 5 H 5 ) 2 ) as a metal source with diesel particles to obtain magnetic carbon composite and its utilization for wastewater purification.The MDPs generated by the engine fueled with ferrocene-doped diesel were collected by an electrostatic precipitator and used for methylene blue (MB) removal.A good adsorption capacity of MB onto MDPs was obtained.In addition, MDPs can be easily separated from the solution for further reuse.This study enables a unique approach for repurposing a diesel engine as a piston reactor to synthesize high-value materials and their use in water purification.

Synthesis of magnetic diesel particles (MDPs)
Magnetic diesel particles were obtained from the exhaust gas generated by the combustion of ferrocene-doped diesel using an ICE engine.Figure 1 shows a schematic diagram of a diesel engine combined with an electrostatic precipitator to charge and collect magnetic diesel particles.The diesel engine is a direct injection, four-stroke cycle Kipor KDE2.0E, fueled with 10000 ppm by weight of ferrocene (98.0 wt%, Wako Pure Chemical Corporation).The ferrocene was previously dispersed in standard diesel JIS K2204-No.1 available in Japan, using ultrasonication for 30 min.The obtained diesel-ferrocene blend presented good stability in time without any sedimentation.Engine output power was set to 1.65 kW using an external heater.After reaching a steady state, the valve installed downstream of the diesel engine was switched to provide a flow to the electrostatic precipitator; Details of the engine specifications and experimental conditions can be found in Tables 1 and  2, respectively.After the collecting time, the engine was stopped, and the black or brown exhaust soot adhering to the aluminum plates in the electrostatic precipitator was washed off using ethanol.The mixture of ethanol-magnetic diesel soot was heated at 150 C for 1 h to collect dried magnetic diesel particles (As-collected MDPs).The latter were subjugated to thermal annealing at 300•C for 12 h to remove the soluble organic fraction (SOF).Heat-treated MDPs were purified with distilled water using ultrasonic for 60 s and collected by a permanent magnet.This purification process was repeated three times.The amount of purified MDPs is about 24%wt of the as-collected MDPs.

Characteristics of MDPs
The collected magnetic diesel particles were characterized systematically by Thermogravimetric analysis conducted under air to analyze the thermal stability of MDPs using the NETZSCH STA 2500 thermal analyzer.The CHN analysis before and after purification was obtained on an elemental analyzer (Elementar vario EL cube).X-ray diffraction spectra of MDPs were recorded after thermal treatment and magnetic purification using the X'Pert-MPD-OES-PANalytical XRD instrument.The radiation voltage and current were set at 40 kV and 15 mA, respectively.The particle size distribution of the MDPs was measured using HORIBA SZ-100 dynamic light scattering (DLS).The surface morphology was investigated using field emission scanning microscopy (JSM-7500F) operated at a voltage of 5.0 kV and a current beam of 10 mA.Elemental compositions were determined by EDAX energydispersive X-ray analyzer (Genesis XM2).

Batch adsorption experiments
Methylene blue removal efficiency of MDPs was investigated at room temperature and desired concentrations.5 mg of MDPs was added to 100 mL of water solution with a concentration of dye between 1-10 mg/L and agitated at 180 rpm for a specific time interval of up to 200 min.After agitation, a neodymium magnet was placed close to the beaker containing the solution for 30 s to attract the MDPs to the magnet side, as shown in (Figure 2).Approximately 1 mL sample was taken from the aqueous solution, and its absorbance was measured using a UV/visible Hitachi U-2910 double beam spectrophotometer.Equations ( 1) and ( 2) were used to estimate the percentage of dye removal and MB adsorption per adsorbent unit, respectively.
Where C 0 and C e are the initial and equilibrium concentration of MB in the solution (mg/L); q e (mg/g) is the equilibrium adsorption capacity of MDPs; V (mL) is the volume of the solution; and W (mg) is the dry weight of MDPs.

Adsorption kinetic models
The adsorption kinetics of MB into MDPs is essential for understanding the adsorption mechanisms in time and estimating the design parameters of MDPs.Two main kinetics models were used to analyze and fit the experimental data, the pseudo-first-order (PFO) model and the pseudosecond-order (PSO) model, expressed in Equations ( 3) and (4), respectively.
q t ðmg=gÞ ¼ q e ð1 À e Àk 1 t Þ (3) where q e (mg/g) is the equilibrium adsorbed amount of dye per unit mass of absorbent, q t (mg/g) is the absorbed amount of dye at the time (t), and k 1 and k 2 are the equilibrium rate constants of the PFO and PSO models, respectively.

Adsorption isotherm models
In order to describe the equilibrium adsorption and estimate the adsorption characteristics of MB onto MDPs, Langmuir and Freundlich isotherm adsorption models were used to interpret the experimental data.The linear form of Langmuir and Freundlich isotherm models is given in equations ( 5) and ( 6), respectively: where q e (mg/g) is the equilibrium adsorption capacity, Ce(mg/L) is the equilibrium concentration, q m (mg/g) is the maximum adsorption capacity of the adsorbent, K L as Langmuir constant is related to the energy of Adsorption, K F as Freundlich constant is related to the adsorption capacity of adsorbent, and 1/n F constant is related to the surface heterogeneity.

Results and discussion
3.1.Diesel particles (DPs) and MDPs mass concentration in a raw exhaust First, we investigated the engine load effect on diesel particles and magnetic diesel particle concentration in a raw exhaust for output power ranging from 0 to 1.65 kW.The mass concentration of particles generated by the engine fueled with diesel and ferrocene-diesel blend was measured using a mixed cellulose ester membrane filter with a high-pressure holder (Advantec Toyo Kaisha, Ltd.Japan).The filter has a plain surface, a pore size of 1.0 mm, and a diameter of 47 mm.The particles collected on the membrane filter were used for gravimetric analysis.The relatively high temperature of the diluted exhaust gas can deposit a mist or condensed droplets on the filter, which can cause an error in the measurement.To reduce this error, the filters are heat treated at 100 C for 3 h before and after exhaust gas collection.The results are depicted in Figure 3, where the weight of particles generated by the engine is given per the total amount of exhaust gas in the liter.As shown in Figure 3, the particle weight tends to be higher with the increase of engine output power regardless of fuel type (Zhang and Balasubramanian 2015).On the other hand, a slight decrease in the particle weight is observed for ferrocene-based blend fuel compared to the neat diesel, and this reduction in the presence of ferrocene can be attributed mainly to the relative reduction of elemental carbon emission due to an oxidation catalyst of iron (Zhang and Balasubramanian 2017).Considering that the highest amount of diesel particulate is generated by the engine with an applied output power of 1.65 kW, the magnetic diesel particles are synthesized and collected at this output power in the following experiments.

Thermal treatment of DPs and MDPs
In general, the DPs contains soluble organic fraction (SOF) which inhibits adsorption of organic matter to DPs.In order to maximize adsorption capacity of MDPs by removal of SOF with thermal pretreatment, we investigated the thermal stability of DPs and MDPs.The obtained thermograms for DPs and MDPs are shown in Figure 4. Two significant mass losses can be observed.The first mass loss starts at around 210 C for both DPs and MDPs, with maximum weight loss of 32% and 44%, respectively.This first degradation can be attributed to the evaporation and oxidation of SOFs.The second mass loss starts at around 380 C for MDPs and 430 C for DPs and could be assigned to soot oxidation (Amann and Siegla 1982).The difference in the maximum weight loss and the soot oxidation temperature is probably due to the ferrocene-based burning rate catalyst, which decreases the soot oxidation temperature (Bladt et al. 2012).In addition, the residue percentage of MDPs is slightly higher than DPs after soot oxidation, which can be explained by the density of iron oxide compared to the ash.Based on these results, before their application to water treatment, the MDPs were thermally treated under an air atmosphere at 300 C for 12 h to remove the soluble organic fraction (SOF) and minimize the soot oxidation at the same time.

Magnetic purification of DPs and MDPs
Subsequently, to annealing at 300 C, the obtained MDPs were dispersed in distilled water by ultrasonic for 60 s and collected with a permanent magnet.Figure 5 shows the CHN analysis results of crude DPs, crude MDPs, and MDPs after thermal treatment and magnetic purification.The main components of the residue may include, in addition to ash, iron oxides which may explain the percentage increase in the residue from 15.5% to 17.8% between DPs and MDPs.Moreover, a significant decrease in carbon and hydrogen percentages in favor of residue of MDPs after magnetic purification is observed, which is consistent with ATG results.Despite the necessary thermal posttreatment to eliminate the SOF, the diesel particles have various advantages as it can be seen as a waste to treat another waste (wastewater), economically attractive and presents a good removal efficiency, moreover its combination with ferrocene to form a magnetic composite makes it a reusable adsorbent thanks to their magnetic proprieties that allow their easy separation from the solution for further reuse.

Characterization of MDPs
The crystal structure of MDPs was investigated by XRD. Figure 6 shows XRD patterns of MDPs; different crystalline phases of iron oxides can be seen due to the complex combustion process of a diesel engine.While it is difficult to distinguish  between maghemite (c-Fe 2 O 3 ) and magnetite (Fe 3 O 4 ) phases because of their overlapping of peak position and relative intensities, the peaks at 30.29 , 35.68 , and 63.00 assigned to the diffraction planes ( 220), ( 311), and ( 440), respectively, match well with the characteristic Bragg reflexes typical of a cubic magnetite (Fe 3 O 4 ) structure, according to reference card ICDD 00-008-8146.
The same peaks at 30.24 , 35.63 , 43.28 and 62.92 may also indicate the presence of cubic maghemite (c-Fe 2 O 3 ) according to the reference card ICDD 00-39-1346.Given the diesel engine combustion process between fuel and air involving high pressure and temperature, the main phase in MDPs is likely to be maghemite, but the presence of magnetite is also possible (Kim et al. 2001;Maity and Agrawal 2007).The existence of maghemite or magnetite is also consistent with the fact that the synthesized MDPs dispersed in water can be easily recovered using a magnet, as can be seen in Figure 2. Further, we note the presence of other peaks for non-magnetic iron oxides, such as e-Fe 2 O 3 and a-Fe 2 O 3 structures, according to the reference cards ICDD 04-013-5433 and ICDD 00-071-0073, respectively.The particle size distribution of MDPs, obtained after thermal treatment and magnetic purification, is shown in In this mode, we can clearly distinguish iron oxides particles with brighter contrast from carbonaceous particles and they are homogenously distributed on the surface of diesel particles.From these results, it is concluded that we successfully synthesized magnetic DPs in which iron oxide nanoparticles were uniformly dispersed on the carbon-based particulate matter.EDX spectrum (Figure 8e) gives the chemical composition of MDPs; the spectrum depicts the presence of three significant elements, C, O, and Fe, and other elements contained generally in diesel particulate matter, such as Ca, K, and Si, with lower atomic percentages can be seen.

Adsorption kinetic models of MB onto MDPs
The UV-adsorption spectra over time are depicted in Figure 9(a), showing that the adsorption intensity decreases with time to reach the equilibrium after 120 min.Figure 9(b) shows the experimental adsorption data of MB (2.92 mg/l) onto MDPs were fitted with the PFO and PSO.The adsorption process is revealed to be relatively fast and reaches equilibrium after 120 min.Table 3 summarizes the PFO and PSO model parameters and regression coefficients (R 2 ).From the results the PSO model better represented the adsorption kinetics with a higher regression coefficient R 2 ¼0.97 compared with R 2 < 0.95 for the PFO.This suggests that the adsorption of MB on MDPs follows the second-order kinetics, a similar  result was reported for MB adsorption onto diesel soot-based adsorbent (Vishvendra Pratap Singh and Vaish 2020).Furthermore, the adsorption kinetics of synthesized MDPs follows the common characteristics of carbon based adsorbents (Santoso et al. 2020) .The fitting of experimental data at different initial MB concentrations using PFO and PSO are given in Figure S1(a) and (b), respectively, the obtained parameters are depicted in Table S2 in the supplementary material.

Adsorption isotherm models of MB onto MDPs
The equilibrium data obtained for the adsorption of MB onto MDPs were fitted to linear Langmuir and Freundlich isotherm models and presented in Figure 10(a) and (b).The fitting parameters are summarized in Table 4.A comparison of regression coefficients of both models indicates that the adsorption process of MB onto MDPs is fairly explained by Langmuir model and controlled by its surface adsorption mechanism.The maximum adsorption capacity calculated from Langmuir model is 40.81 mg/g of MB into MDPs.

Thermodynamic parameters
The spontaneity of the process can be predicted by calculating the Gibbs free energy (DG ads ).A negative value of DG ads signify that the adsorption process is spontaneous and vice versa.Gibbs free energy can be estimated using Langmuir and Freundlich constants in the following Equation (7) (Mahmoodi et al. 2011 By replacing the Langmuir and Freundlich constants value (K L and K F ) in the Equation (7),

Comparison of MDPs with other carbonbased magnetic adsorbents for MB adsorption
MB removal by magnetic diesel particles was compared with other carbon-based magnetic adsorbents reported in the literature.Table 5 shows the adsorption capacities of carbon-based magnetic particles such as CNT and graphene containing different magnetic iron oxide compounds.The adsorption capacity of MDPs in the present study is found to be greater than or comparable to that of the most of the adsorbents listed in Table 5.Although Fe 3 O 4 based magnetic adsorbents is commonly used, metal oxide containing adsorbents might results in ion leaching during the adsorption process (Mahmoodi et al. 2019), hence an in-depth analysis of ion leaching is necessary.Considering their satisfactory adsorption capacity and simple and inexpensive synthesis process, the MDPs can be expected to be an attractive and effective adsorbent material for removing MB from aqueous solutions.

Conclusion
We demonstrated that the combustion of ferrocene-doped diesel fuel using a diesel engine as a piston reactor generated magnetic diesel particles in which iron oxide nanoparticles were uniformly dispersed on the carbon-based particulate matter that can be isolated and applied for water treatment.The magnetic diesel particles presented a good adsorption capacity of MB of about 40.81 mg/g and can be easily separated from the solution using a magnet for further reuse.We opened a new frontier for a direct interfacial combination of diesel exhaust particles with other nanoparticles to synthesize high-value nanomaterials.

Figure 1 .
Figure 1.Schematic diagram of the diesel engine coupled with the electrostatic precipitator (ESP) system.

Figure 2 .
Figure 2. Schematic of Methylene blue removal approach using MDPs.

Figure 3 .
Figure 3. Weight of particles collected by filter with a pore size of 1 mm.

Figure 4 .
Figure 4. TGA curves of DPs and MDPs in air atmosphere.

Figure 7 .
With an average diameter size of 250 ± 20 nm, MDPs are characterized by a broader and polydisperse (PDI ¼ 0.112) diameter distribution, indicating a possible agglomeration of particles during the thermal annealing at 300 C and/or purification step.The morphology and chemical composition of MDPs were investigated using FE-SEM and EDX.FE-SEM images in SEI mode are shown in Figure 8(a) and (b), where irregular and aggregated structure of MDPs is clearly observed.The FE-SEM images with backscattering electron image mode (BEI mode) are shown in Figure 8 (c) and (d).

Figure 8 .
Figure 8. (a, b) FE-SEM images of MDPs (obtained in SEI mode), (c, d) FE-SEM images (obtained in COMPO or BEI mode), and e) EDS spectrum of MDPs.

Table 3 .
The kinetics of adsorption reactions for removal of MB (2.92 mg/l) dye by MDPs.

Table 4 .
Adsorption isotherm data for the removal of MB dye by magnetic diesel particles.

Table 5 .
Comparisons of the adsorption capacity of methylene blue onto MDPs with various carbon -based magnetic particles from the literature.