Bituminous Soil Remediation in the Thermal Plasma Environment

Abstract

Petroleum hydrocarbons like diesel, crude oil, and bitumen are persistent soil contaminants, necessitating urgent remediation due to their harmful effects on the soil and living organisms. Plasma remediation is a thermochemical method that is gaining attention as an alternative to soil de-pollution. Accordingly, the purpose of this experimental research is to investigate the feasibility of thermal air and water vapour plasmas to de-pollute bituminous soil. Pre-/post-remediation of soil and generated products, an analysis was carried out using SEM, EDX, TGA, and elemental and gas analysers. Despite the plasma type, the research showed that the bituminous soil’s surface morphology changed and resembled clean soil after the remediation. The EDX analysis revealed that the carbon content in the soil reduced from 70.14 wt.% to 7.70 wt.% and 5.74 wt.% and the sulphur concentration decreased from 2.64 wt.% to 0.70 wt.% and 0.74 wt.% after treatment in the environment of water vapour and air plasmas, respectively. The gas analysis revealed that bitumen was mainly decomposed into a synthesis gas (H₂ + CO) and CO₂, with concentrations ranging from 8.67% to 13.66% and 10.89% to 12.82% when air and water vapour plasma were used. Thus, both thermal plasmas effectively remediated soil, with the bitumen concentration not being detected or below the laboratory’s finding limit (<0.089 g/kg).

1. Introduction

The soil and water pollution caused by petroleum products (e.g., diesel, gasoline, crude oil, and unconventional oil, such as bitumen) is growing with the expanding energy demand in the world. Furthermore, soil that is contaminated with petroleum products is considered hazardous waste due to the presence of polycyclic aromatic hydrocarbons, resins, paraffin, benzene, toluene, xylenes, etc., which induces a negative effect on living beings (e.g., plants, animals, humans, and, in general, various environment ecosystems) [1,2]. Accidental petroleum spills during pipeline breaks, the explosion of an oil extraction well or spill during oil transportation, usage in power plants, or even inappropriate waste disposal from petrochemical industries are the most commonly known primary sources of petroleum products contaminating the soil environment [3,4]. Among environmental contamination with various petroleum products, spills of diluted bitumen in terrestrial ecosystems often require removing and cleaning polluted soil or vegetation. In addition, the diluted bitumen is strongly adhesive and tends to cover surfaces with which it has come into contact and sink into the substance (e.g., soil). Therefore, it is challenging to wash bitumen off. Moreover, bitumen is a highly persistent material that is resistant to dissolution and biodegradation. Also, the management of bitumen spills into the environment is sensitive in terms of time when traditional treatment methods, such as dispersants or in situ burning, are applied. The effectiveness of dispersant usage 6 h after bitumen spillage is equal to 50%. However, the effectiveness decreases to 0% when 12 h pass from the bitumen spillage. Applying in situ burning can ensure 50–75% effectiveness if used up to 24 h after bitumen spills. Nevertheless, this method becomes ineffective 96 h after bitumen spillage. Thus, soil that is polluted with bitumen often needs to be removed from its primary location and cleaned by applying ex situ treatment methods [5]. Various soil treatment options based on the separation (e.g., physical treatment, oil/water separation) or destruction (e.g., chemical treatment, thermal treatment) methods exist [6,7,8]. Despite the variety of existing soil treatment methods, this article intends to focus only on thermal or thermo-chemical treatment methods, particularly plasma used for the remediation of soil that is contaminated with petroleum products. Thus, thermal treatment methods are very efficient in soils polluted with organic contaminant remediation. Incineration, pyrolysis, and plasma gasification are classified as thermal or thermo-chemical treatment methods. The first two methods are well known and frequently applied for polluted soil treatment [9,10,11,12]. However, despite the popularity and benefits, incineration and pyrolysis have drawbacks. For example, bottom ash and toxic pollutants, such as dioxins and furans, are produced during the incineration process. Further, pyrolysis requires oxygen-free conditions, and during the pyrolysis process, the production of pyrolytic water from organic materials occurs. Also, the generation of pollutants, including H₂S, HCl, NH₃, and tar, takes place [13,14,15,16,17]. Moreover, the treatment of petroleum hydrocarbon-polluted soil with plasma is a new and under-researched method. Lately, thermal plasma technologies have gained attention as prospective technologies for polluted soil treatment purposes due to several advantages: a plasma medium provides the energy needed for chemical reactions and accelerates the contaminant conversion process, thus ensuring a high material transformation efficiency with short contact times. Also, applying such a de-pollution method avoids using additional reagents or an expensive catalyst. Moreover, multi-purpose plasma technologies can decompose diverse concentrations of various pollutants, including petroleum hydrocarbons. Further, plasma is also characterized by lower emissions than conventional incineration. Thus, plasma technologies can become an integral component of a greener energy landscape, because they provide solutions for converting contaminants into clean energy [18,19,20,21].

Practically, non-thermal plasma is currently used to de-pollute soil that is contaminated with widely used petroleum hydrocarbons, such as kerosene, gasoline, and diesel. Meanwhile, thermal plasma tends to be used for gasifying waste or biomass materials, including forest residue or wood pellets/chips, glycerol, refuse-derived fuel, or even solid radioactive waste [18,22,23,24,25,26]. Despite the recent growing interest in thermal plasma technologies, applying this type of plasma for soil remediation is still rare but developing. Further, as far as we know, the use of non-thermal plasma and thermal plasma technologies to remediate soil polluted with unconventional oil (e.g., bitumen) has not yet been implemented, implying a research gap in this research field. Consequently, this article’s novelty unfolds through the innovative plasma’s application for the remediation of soil that is contaminated with more complex petroleum hydrocarbon fractions (consisting of longer hydrocarbon molecule chains and with higher boiling points, making it more persistent in the environment and more challenging to decompose). Thus, this experimental research aimed to investigate the feasibility of thermal air and water vapour plasmas to de-pollute bituminous soil. Bituminous soil de-pollution was carried out by applying thermal air or water vapour plasma. The soil surface morphology analysis showed that the soil lost its metallic brightness and rounded particles (a bitumen feature) after the plasma processing. Also, the de-polluted soil surface became relatively rougher and had a granular structure (a feature of clean soil). Moreover, the soil’s carbon and sulphur concentrations were reduced after the soil treatment with both types of plasma. Also, it was noticed that the bitumen was converted into gaseous products, mainly syngas (H₂, CO) and CO₂. The performed experimental study indicated that the thermal arc plasma environment favours the bituminous soil de-pollution process, since a bitumen concentration was not detected or fell below the laboratory’s finding limit (<0.089 g/kg).

2. Materials and Methods

2.1. Ultimate and Proximate Material Assessment

The experiments were carried out using bitumen-polluted soil collected from one of the Lithuanian regions.

Table 1. The bitumen-polluted soil’s main characteristics.

Parameter	Bitumen-Polluted Soil
Carbon, wt.%	16.08
Hydrogen, wt.%	2.16
Nitrogen, wt.%	0.12
Sulphur, wt.%	0.49
Oxygen, wt.% 1	0.87
Moisture, wt.%	1.28
Volatiles, wt.%	11.70
Fixed carbon, wt.%	6.74
Ashes, wt.%	80.28
LHV, MJ/kg	11.96

¹ by difference.

summarizes the ultimate as well as proximate analysis of the bitumen-polluted soil before it was de-polluted in the thermal air or water vapour plasma environment.

The ultimate analysis showed that the majority of the bitumen-polluted soil elements were carbon (16.08 wt.%) and hydrogen (2.16 wt.%). Also, 0.49 wt.% of sulphur and 0.12 wt.% of nitrogen were detected. The oxygen content was calculated according to Equitation (1):

$$O_2=100\mathrm{\%}-\mathrm{C}-\mathrm{H}-\mathrm{N}-\mathrm{S}-\mathrm{A}\mathrm{s}\mathrm{h}\mathrm{e}\mathrm{s}$$

(1)

The proximate analysis of the bitumen-polluted soil revealed a high ash content (80.20 wt.%). The volatiles of the bituminous soil were equal to 11.70 wt.%, while the fixed carbon and moisture corresponded to 6.74 wt.% and 1.28 wt.%, respectively. The bitumen-polluted soil’s lower heating value (LHV) accounted for 11.96 MJ/kg.

2.2. The Experimental Set-Up of Bituminous Soil De-Pollution with Plasma

The experimental study parameters of the bituminous soil de-pollution with the plasma are presented in

Table 2. The parameters of the bituminous soil de-pollution in the thermal arc plasma environment.

	Bituminous Soil De-Pollution Operating with
Parameter	Air Plasma	Water Vapour Plasma
Current of the arc, A	160	160
Voltage of the arc, V	330	350
Power, kW	52.8	56
Bituminous soil content, kg	1.5	1.5
Bitumen amount in the polluted soil, g/kg	42	42
Flow of the gasifying agent, g/s	4.4	4.1
Airflow for cathode protection, g/s	0.5	0.5
Total gas flow, g/s	4.9	4.6
Mean plasma temperature, K	4100	2800
Thermal efficiency of the plasma torch, (η), %	54	74

. The de-pollution was performed at 52.8–56 kW of the plasma torch power, an airflow of 4.4 g/s, and a water vapour flow of 4.1 g/s. In both cases, an additional 0.5 g/s of airflow rate was used for cathode protection. The bituminous soil de-pollution process was handled at atmospheric pressure for 17 min. Moreover, 1.5 kg of bituminous soil was remediated for each experiment. Thus, the proposed plasma-based technology can treat approximately 5.3 kg/h of bituminous soil. The mean temperature of the operating experimental system was 4100 K and 2800 K when air or water vapour plasma was used as a soil de-pollution environment, respectively. The plasma torch’s thermal efficiency varied between 54% and 74%, indicating that the water vapour plasma exhibits higher efficiency.

Figure 1 shows the plasma-based de-pollution system for bitumen-contaminated soil. The system is composed of the plasma torch (DC arc) (1), a 1 m long plasma-chemical reactor with a 0.4 m inner diameter (2), a DC rectifier, which serves as a system for the power supply (3), a system for the air supply (4), a generator for steam (5), a superheater (6), a particulate matter water filter (7), a condenser (8), and a producer gas sampling and analysis system (9).

The bitumen-contaminated soil clump was put on the grate in the plasma-chemical reactor. The thermal arc plasma torch is located at the reactor’s top and is directed straight to the grate holding the contaminated soil clump. This type of construction creates conditions that are necessary to ensure sufficient interaction between the active species in the thermal plasma environment and the bituminous contaminant in the soil.

2.3. Facilities Used for the Soil and Produced Gas Analysis

The pre-/post-de-pollution analysis of the soil was carried out with scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), a thermogravimetric analyser (TGA), an elemental analyser, and a gas analyser. The scanning electron microscopy (Hitachi S-3400N, Tokyo, Japan) helped specify the surface morphology of the soil. The EDS (Bruker Quad 0540, Hamburg, Germany) characterized the elemental composition of the contaminated and de-polluted soil. The elemental analyser (FLASH 2000, Thermo Fisher Scientific, Breda, The Netherlands) was used to assess the soil’s ultimate analysis (CHNS). The bituminous soil’s C, H, and N measurements were based on the standard LST EN ISO 16948:2015 [27]. The S content was assessed under the methodical BM-8B/3-BO0:2012 [27]. The thermogravimetric analysis (TGA/DTG) using a NETZSCH STA 449 F3 Jupiter (Selb, Germany) analyser with a SiC furnace was used to evaluate the bitumen-contaminated soil and de-polluted soil’s thermal properties. The measurements of the gas amount obtained during the bituminous soil processing with the thermal arc plasma were carried out by using the MRU SWG 300⁻¹ (Neckarsulm, Germany) gas analyser.

To ensure the reliability of the data, a threefold soil sample analysis using the facilities mentioned above was implemented. The data given in this article are the mean values of these replicates. The relative errors of the measurements with the thermogravimetric analyser, energy-dispersive X-ray spectroscopy, as well as gas analyser were ±4%, ±1–2%, and ±2%.

2.4. Plasma Formation and Bitumen De-Pollution Process

The ability to perform the bituminous soil de-pollution in the plasma reactor begins with the plasma creation. Plasma is characterized as a partly or thoroughly ionized gas. Positive and negative ions, electrons, as well as neutral species, such as miscellaneous molecules, atoms, radicals, as well as excited species, are integral parts of the gas appearing in the plasma state. At least part of the species that create plasma are in electronically excited states. Such conditions ensure that chemical reactions will take place in highly reactive surroundings.

The contact alongside the energetic electrons emitted out of the cathode and the plasma-forming gases induces the generation of radicals (Figure 2).

The type of gas used to form plasma determines the kind of reactive radicals. For instance, whenever air is used as a material to create plasma, the reactive plasma species, such as O∙, N∙ are produced during the ongoing reactions (Equations (2) and (3)) given below [28,29,30]:

$$O_2+e^{-} \to 2O·+e^{-}$$

(2)

$$N_2+e^{-} \to 2N·+e^{-}$$

(3)

Moreover, the formed reactive plasma species (O∙, N∙) can become the foundation for nitrogen oxide formation during the reactions (Equations (4) and (5)) [29,30,31]:

$$N·+O· \to NO$$

(4)

$$NO+O· \to {NO}_2$$

(5)

When water vapour is employed as a matter for the plasma formation, the reactive species, such as H∙, O∙, and OH∙, are generated in the course of the reactions ((6)–(9)) provided below [28,32,33,34]:

$$H_{2}O+e^{-} \to H·+OH·+e^{-}$$

(6)

$$H_{2}O+O· \to 2OH·$$

(7)

$$H_2+O· \to OH·+H·$$

(8)

$$H_2+OH· \to H_{2}O+H·$$

(9)

Bitumen lacks a specific molecular formula because of its complicated internal structure and chemical composition. Bitumen’s main chemical constituents are asphaltenes and maltenes (the latter can be subdivided into aromatics, saturates, and resins). However, the general bitumen molecular formula can be represented as alkanes: $C_n{H}_{2n+2}$. Thus, the general chemical reactions of a bitumen conversion can be expressed as presented in Equations (10)–(13) [35,36,37,38]:

Partial oxidation:

$$C_n{H}_{2n+2}+\frac{n}{2}{O}_2\to nCO+(n+1)H_2$$

(10)

Complete oxidation:

$$C_n{H}_{2n+2}+\left(\frac{3n}{2}+\frac{1}{2}\right)O_2\to n{CO}_2+(n+1)H_{2}O$$

(11)

Steam reforming:

$$C_n{H}_{2n+2}+n{H}_{2}O\to nCO+(2n+1)H_2$$

(12)

Water gas shift reaction:

$$CO+H_{2}O \leftrightarrow {CO}_2{+H}_2$$

(13)

During the bitumen conversion process, the side reactions can also take place (Equations (14) and (15)):

Boudouard reaction:

$$2 CO \leftrightarrow {CO}_2+C$$

(14)

Reverse gasification:

$$CO+H_2 \leftrightarrow C+H_{2}O$$

(15)

The provided reactions will assist in giving the explanation of the obtained experimental results through the bituminous soil de-pollution process, which will be given in the upcoming section.

Furthermore, conjuring up a picture of the bituminous soil de-pollution process mechanism is also beneficial. Thus, bitumen is a strongly adhesive material that tends to stick well to the material that it comes into contact with. Firstly, the bitumen covers the material’s (e.g., soil) surface, and further, it sinks into its substance. The adhesive bitumen’s properties allow it to firmly bind to soil, forming stable joints, thus reducing the soil’s porosity. Consequently, bituminous soil looks like congealed clumps.

Moreover, the bituminous soil de-pollution process begins with creating a thermal arc plasma. The plasma torch cathode releases electrons that interact with plasma-creating gas as well as a gasifying agent (air or water vapour) to form the reactive plasma species (Equations (2)–(9)). Further, the enriched reactive species’ plasma environment assures efficacious bituminous soil de-pollution reactions, which depend on the plasma-creating gas and gasifying agent used (Equations (10)–(15)).

The topmost layer of the bituminous soil is the first place where the de-pollution process with thermal plasma starts. The active plasma species’ make contact with the top bitumen layer atop the soil grains, thus initiating its decomposition process. When the bitumen in the topsoil layer decomposes, the plasma species infiltrate through soil voids and interact with the bitumen in deeper soil layers. Accordingly, bituminous soil processing by applying the thermal arc plasma happens progressively and vertically in the soil’s layers. The interaction between bitumen and plasma causes the bitumen to decompose into producer gas, which is primarily composed of H₂, CO, and CO₂.

3. Results and Discussion

3.1. Analysis of the Surface Morphology of Bituminous and De-Polluted Soil

The images of the surface morphology of the bituminous soil and the soil that was de-polluted with air plasma or water vapour plasma are presented in Figure 3.

The polluted soil has a relatively smooth surface (Figure 3a,b), which was formed when viscous bitumen interacted with the soil and merged the single soil grains into a bigger unit. Also, rounded particles were observed in the polluted soil surface. These particles are a feature of bitumen.

Similar images of spherical particles were observed by Golubev et al. during their research on natural bitumen [39]. The researchers noted that the analysis of the raw bitumen samples with atomic force microscopy showed a globular super-molecular structure that is typical for high-metamorphic-grade bitumen. Further, the bituminous soil de-pollution process modified the soil surface. Hence, the plasma-treated soil lost its metallic brightness and spherical bubbles and distinguished itself with a relatively rougher surface and a granular structure (Figure 3c–f). Such structural changes are a feature of clean soil.

Additionally, the elemental composition of the bituminous soil and the soil treated in the air or water vapour plasma was determined using EDX to verify the effectiveness of the bituminous soil de-pollution process. The obtained results are presented in the subsection below.

3.2. Assessment of Soil’s Elemental Content

Carbon and hydrogen are two chemical elements that constitute over 90 wt.% of the bitumen. Sulphur, oxygen, and nitrogen also account for considerable amounts of the bitumen, while transition metals, such as nickel, iron, and vanadium, only account for this material’s traces (ppm) [40,41]. Accordingly, the elemental contents of bituminous soil and soil that was de-polluted in the environment using air plasma or water vapour plasma are presented in

Table 3. The elemental content of bituminous and processed soil.

Element	Bituminous Soil, wt.% *	De-Polluted Soil with Air Plasma	De-Polluted Soil with Water Vapour Plasma
Carbon	70.14	5.74	7.70
Oxygen	18.78	47.29	46.45
Silicon	4.17	27.16	27.71
Sulphur	2.64	0.74	0.70
Potassium	0.41	2.68	2.83
Calcium	1.09	6.12	4.49
Magnesium	0.25	1.46	1.32
Aluminium	0.92	4.59	4.88
Iron	0.94	2.95	2.63
Sodium	0.20	0.53	0.66
Titanium	0.38	0.63	0.47
Phosphorus	0.08	0.12	0.15

* Weight concentration of element, wt.%.

. The change in carbon amount in the soil was noticed when it was processed in the air or water vapour plasma. More specifically, the bituminous soil’s carbon concentration was 70.14 wt.%. After the soil de-pollution with air plasma or water vapour plasma, the carbon amount in the soil decreased to 5.74 wt.% or 7.70 wt.%, respectively.

Thus, the bituminous soil treatment in the plasma caused the soil’s carbon content to reduce despite the gas type (air or water vapour) employed being a plasma-creating gas, as well as a gasifying agent. The same tendency was detected for the sulphur amount in both cases. It constituted 18.78 wt.% in the bituminous soil. Still, following the soil cleansing in the air or water vapour plasma, the sulphur concentration decreased and varied in the 0.74–0.70 wt.% range. Meanwhile, the oxygen and iron concentration in the soil increased after the polluted soil remediation process. This is mainly related to the soil properties, as the soil usually consists of silicon oxides, metal oxides, or minerals.

Correspondingly, after reducing the contaminant content in the soil (reflected in the C and S concentrations), the relative concentration of other elements increased, because all elements’ final concentration must be 100% due to the peculiarities of the measuring equipment—EDX. Generalizing the elemental composition analysis, the trend of reducing the carbon and sulphur concentrations demonstrates that bitumen decomposition successfully occurred in the soil through the cleansing process in the plasma environment.

3.3. Thermal Analysis of Bituminous and De-Polluted Soil

Bitumen has a longer hydrocarbon molecule chain and a higher boiling point (above 350 °C) than lighter hydrocarbons such as naphtha-gasoline (25–205 °C) or middle distillates, including aviation fuel, kerosene, and diesel (150–380 °C). Also, it is more viscous and less volatile than these lighter crude oil fractions [42,43]. Thus, to comprehend the bitumen’s demeanour in the soil during cleaning, TG and DTG assessments were performed through combustion (Figure 4a–c).

The bituminous soil combustion process was composed of three stages of mass loss (the curves of TG and DTG in Figure 4a). In the first stage, the TG curve indicates a moisture content decrease in the bituminous soil (1.28 wt.% captured from 0 to 15.5 min). Based on the TG and DTG data, the subsequent stage of mass loss happened in the experiment’s 15.5–21.5 min range, representing a lowering in the organic matter and volatile compounds (11.70 wt.%) in the contaminated soil. The DTG peak (at 455.6 °C) could be related to the bitumen’s boiling point, indicating its vaporization, followed by combustion. The same trend was observed by other authors who performed bitumen analyses [44,45]. The third bituminous soil mass loss stage appeared in the 21.5–32 min measurement range and represented a decrease in the soil’s remaining organic part (6.74 wt.%). Finally, 80.28 wt.% of the ashes were produced during the combustion process.

Furthermore, research was carried out using processed soil in an air plasma (Figure 4b) or water vapour plasma environment (Figure 4c). Two phases of mass loss were logged for both plasma types that were used for the soil de-pollution processes. The first mass loss stage (from 0 to 24 and from 0 to 26 min) showed reduced soil moisture (0.68 wt.% and 1.05 wt.%) when air and water vapour plasma were used, respectively. Meanwhile, the initial DTG peak (the second phase of the mass loss) was measured within 26 to 30 min at 780.6 °C for the air plasma, identifying the decrease in the small amount of the soil’s organic matter (0.86 wt.%). A similar DTG peak was noted when using water vapour plasma from 24 to 27 min at 677.5 °C, when 0.41 wt.% of the soil’s organic part reduction was recorded. Also, 98.46–98.54 wt.% of the ashes resulted from the combustion of the soil samples taken after remediation with air or water vapour plasma, respectively. Additionally, the lack of a DTG peak between the 15.5^th and 21.5^th min of sample measurement (a feature of bituminous soil, Figure 4a) reveals that the contaminated soil processing in the air or water vapour plasma was successful, since there was no indication of a presence of bitumen in the soil.

3.4. Assessment of the Produced Gaseous Compounds

The producer gas concentration variation during the bituminous soil cleaning in the thermal air and water vapour plasma is presented in Figure 5. While the oxygen concentration reduced from 21 vol% to 4.30–4.52 vol%, carbon-based materials (bitumen) underwent conversion into producer gas, primarily comprising H₂, CO, and CO₂, irrespective of the type of plasma-creating gas and gasifying agent (whether air or water vapour). Therefore, the highest recorded H₂ and CO amounts were 3.74 vol% and 4.93 vol% in the case of air plasma, respectively. The ongoing partial oxidation reaction impacted the syngas (H₂ + CO) formation (Equation (10)). Also, the production of CO₂ (up to 10.89 vol%) was observed. The latter gaseous compound generation was initiated by complete oxidation (Equation (11)) and Boudouard (Equation (14)) reactions. Meanwhile, up to 12.82 vol% of CO₂, 5.77 vol% of H₂, and 7.89 vol% of CO were measured by cleaning the bituminous soil in the water vapour plasma. The steam reforming reaction (Equation (12)) initiated the formation of the synthesis gas. The water gas shift reaction (Equation (13)) and Boudouard (Equation (14)) reaction caused CO₂ formation.

Furthermore, bitumen conversion in the air plasma generated up to 8.67 vol% of synthesis gas, while its conversion in water vapour plasma resulted in 13.66 vol% of synthesis gas production. The higher syngas content was produced through the bituminous soil processing in the water vapour plasma environment. Such a trend was noted due to the nature of the water vapour plasma, which acted not only as a gasifying agent during the bitumen conversion process but also as a source of the H₂. Thus, water vapour plasma was a more favourable environment for bituminous soil cleaning.

Also, small concentrations of NO (up to 0.14–0.15 vol%) and traces of NO₂ (up to 0.02%), SO₂ (up to 0.02%), and C₃H₈ (up to 0.06%) were recorded in both experimental cases (Figure 5b,d).

Since carbon, hydrogen, nitrogen, sulphur, and oxygen are the main bitumen constituents (Table 1), NO, NO₂, SO₂, and C₃H₈ formed primarily due to the bitumen’s interaction with the active plasma species. Additionally, a modest air content (approximately 13%) was employed to preserve the thermal plasma torches’ cathode from the erosion process. Consequently, air also influenced the generation of these compounds.

Further, bitumen was mainly converted into CO₂ and synthesis gas through soil cleaning with plasma. Synthesis gas can further be employed as a feedstock for generating chemicals and liquid fuels or for producing electricity or heat [21]. Meanwhile, CO₂ can further be used as a source for CO₂ hydrogenation into valuable products such as methanol and methane.

3.5. The Thermal Arc Plasma’s Capacity to De-Pollute Bituminous Soil

The experiment’s goal was to de-pollute soil from bitumen. Thus, the air and water vapour plasma potential to treat soil polluted with bitumen is given in

Table 4. The thermal arc plasma’s capacity to de-pollute bituminous soil.

Parameter	Air Plasma Environment	Water Vapour Plasma Environment
Bitumen amount in the soil after de-pollution process	<0.089 g/kg	<0.089 g/kg
Efficiency of bitumen elimination from the soil	99.7%	99.7%

. After the remediation, the soil specimens were investigated according to the guidelines outlined in the normative document (LAND-89-2010). The bitumen content in the specimens of the de-polluted soil was either undetectable or fell below the laboratory’s detection limit (<0.089 g/kg). Thus, the results of the experimental research revealed that both thermal plasmas effectively de-polluted bituminous soil (99.7%).

Further, a brief assessment of the petroleum products’ polluted soil processing using different plasma technologies is provided in

Table 5. Comparison of de-pollution processes applying plasma technologies for soil polluted with petroleum products.

Reference	Discharge Type, Power kW	Gasifying Agent	Target Contaminant, g/kg	De-Pollution Time, h	Elimination Efficiency, %
Present work	Direct current, 52.8	Air	Bitumen, 42	0.28	99.7
Present work	Direct current, 56	Water vapour	Bitumen, 42	0.28	99.7
[46]	Dielectric barrier discharge, 0.18	-	Diesel, 5	0.67	62.0
[46]	Dielectric barrier discharge, 0.18	-	Diesel, 10	0.67	74.0
[47]	Double dielectric barrier discharge, 40 kV	Air	Naphthalene, 0.1	0.5	96.3
[47]	Double dielectric barrier discharge, 40 kV	Air	Phenanthrene, 0.1	0.5	89.1
[47]	Double dielectric barrier discharge, 40 kV	Air	Pyrene, 0.1	0.5	88.6
[48]	Dielectric barrier discharge, 0.002	Air	Kerosene, 0.074	0.07	25.0
[48]	Dielectric barrier discharge, 0.002	Air	Kerosene, 0.074	0.2	88.0
[49]	Dielectric barrier discharge, 0.105	-	Diesel, 2	0.17	25–67
[50]	Pulsed corona discharge, 30 kV	-	Gasoline, 2.5	1	81.0
[50]	Pulsed corona discharge, 30 kV	-	Gasoline, 10	1	57.0
[51]	Pulsed corona discharge, 18 kV	Air	Phenanthrene, 0.01	0.67	70.5
[52]	Multi-cylindrical dielectric barrier discharge, 0.03	Air	Diesel, 10	1	94.2
[53]	Dielectric barrier discharge, 0.081	Nitrogen	Gasoline, 5	1	59.0
[53]	Dielectric barrier discharge, 0.081	Oxygen	Gasoline, 5	1	87.0
[53]	Dielectric barrier discharge, 0.081	Air	Gasoline, 5	1	92.0
[53]	Dielectric barrier discharge, 0.081	Argon	Gasoline, 5	1	100

. Hence, bituminous soil processing with thermal plasma ensured a 99.7% pollutant removal efficiency from the soil over a 0.28 h (or 17 min) period. The pollutant elimination efficiency from the soil employing non-thermal plasma differed from 25% to 100%. Accordingly, researchers have paid attention to the de-pollution process of petroleum products’ contaminated soil in recent years by applying non-thermal plasma. Thus, a summary of several representative research studies is given below to help better comprehend non-thermal plasma usage for soil cleaning.

Zhao et al. [46] used a plasma type called a dielectric barrier discharge (DBD) to degrade diesel in the soil. During the assessment of the impact of the primary diesel content in the soil, the authors stated that a 62% diesel elimination efficiency was observed after 0.67 h of the soil de-pollution process when the soil was polluted by 5 g/kg of contaminant. In comparison, 74% of the diesel was degraded from the soil via the same de-pollution process time when the primary diesel content was 10 g/kg. Furthermore, the researchers also cleaned soil from heavily contaminated oil fields. The primary contaminant content was ~26.4 g/kg. Zhao et al. noted that 70.39% and 63.53% of saturated and aromatic hydrocarbons were degraded after 0.67 h of soil de-pollution.

Abbas et al. [47] employed a double dielectric barrier discharge (DDBD) plasma technology to de-pollute soil polluted with polycyclic aromatic hydrocarbons (PAHs) (naphthalene, phenanthrene, and pyrene). The primary PAH mixture concentration corresponded to 0.1 g/kg. The researchers noted that when the air was employed as a carrier gas in the DDBD, de-pollution efficiencies of 96.32% (naphthalene), 89.08% (phenanthrene), and 88.59% (pyrene) were reached.

Redolfi et al. [48] remediated soil that had been polluted with 0.074 g/kg kerosene by applying dielectric barrier discharge (DBD) plasma. The authors stated that 25–88% of the kerosene was de-polluted from the soil after 0.07–0.2 h of the de-pollution process.

Zhao et al. [49] carried out pre-treatment with DBD plasma and further phytoremediation with ryegrass of diesel-polluted soil (2 g/kg). The authors confirmed that DBD plasma can effectively degrade diesel in the soil. More precisely, the diesel (C₁₂–C₂₁ alkanes) removal efficiencies varied from 25% to 67% after 0.17 h of pre-treatment with DBD. During this process, low-carbon alkenes were removed from the soil. The DBD plasma and ryegrass’s synergetic effect also positively affected the removal of diesel from the soil. Accordingly, C₁₂–C₂₁ alkanes’ removal efficiency in plasma-discharged diesel soil was higher, ranging from 16% to 30% in the 50-day ryegrass growth period, compared to diesel soil not treated with plasma.

Li et al. [50] used non-thermal plasma (pulsed corona discharge) to clean soil polluted with 2.5–10 g/kg of gasoline. After being contaminated with 2.5 g/kg of gasoline, the soil underwent a 1 h treatment process, resulting in the degradation of 81% of the pollutant. However, the degradation efficiency reduced to 57% when the primary pollutant amount in the soil rose to 10 g/kg.

Lu et al. [51] performed remediation with pulsed corona discharge plasma of polycyclic aromatic hydrocarbon (phenanthrene)-contaminated soil. The contaminant content in the soil was 0.01 g/kg. The researchers noted that phenanthrene’s degradation efficiency corresponded to 70.5% after a 0.67 h remediation process with air employed as the carrier gas. The authors equivalented the contaminant degradation efficiency in the air plasma as “1 time”. Consequently, when oxygen, nitrogen, and argon were applied as carrier gases, the contaminant degradation efficiency was equivalent to 1.1, 0.7, and 0.6 times. However, the researchers did not specify the exact contaminant degradation efficiency in percentages.

Acharya et al. [52] cleaned diesel-polluted soil with multi-cylindrical dielectric barrier discharge (MC–DBD) plasma. The amount of pollutants in the soil was 10 g/kg. The researchers noted that 94.19% of the diesel removal from the soil was achieved after an hour of remediation.

Zhao et al. [53] treated gasoline-polluted soil with DBD plasma. The authors estimated the carrier gas’s influence on the 5 g/kg gasoline degradation process. Accordingly, after an hour of cleaning, the gasoline degradation efficiency corresponded to 59%, 87%, 92%, and approximately 100% when nitrogen, oxygen, air, and argon were used as carrier gases.

In conclusion, non-thermal plasma can de-pollute relatively low concentrations and small amounts of a pollutant. This is also revealed in the above-discussed representative research studies, where researchers de-polluted soil contaminated with 0.074–10 g/kg of petroleum products. Despite the lower amount of contaminating substance in the soil, the cleaning lasted longer (0.5–1 h) in most cases of non-thermal plasma usage compared to the thermal plasma (0.28 h) introduced in this article. The latter plasma type demonstrates an ability to cleanse a larger quantity of soil (kg instead of g) that is contaminated with higher pollutant concentrations (e.g., 42 g/kg) in a shorter time. Therefore, thermal plasma has a more significant scale performance capacity and is more appropriate than non-thermal plasma for promptly remediating highly contaminated soils detected and collected during pollution occurrences in places such as petroleum product storage or industrial areas.

It is important to note that the thermal arc plasma technology discussed in this study is still in its early stages of development and has only been tested on a laboratory scale. However, at this technology readiness level, it is still difficult to define the exact costs at the industrial level. Therefore, it is crucial to conduct additional future research using various thermal plasma operation parameters, including diverse pollutant content ranges and differing types and quantities of plasma-creating gas and gasifying agents. This will not only help to better understand soil cleaning with the thermal arc plasma but also will guide towards further technology development and broader practical applicability.

4. Conclusions

Bituminous soil processing with a thermal arc plasma was performed in this experimental research study.

The pre-/post-remediated soil surface morphology assessment showed that the bituminous soil’s surface morphology changed after de-pollution, whatever plasma type was applied. Its rounded particles and metallic brightness were lost, and it resembled clean soil features, including a rougher surface and granular structure.

The soil’s elemental analysis revealed that the soil’s carbon concentration reduced to 5.74 wt.% and 7.70 wt.% from the primary concentration of 70.14 wt.%, while the sulphur content decreased to 0.74 wt.% and 0.70 wt.% from the primary 2.64 wt.% after the soil cleaning with air or water vapour plasma, respectively.

The thermogravimetric analysis indicated the bituminous soil’s combustion temperature and time, which assisted in determining the bitumen’s behaviour during the soil’s combustion process. The analysis of the de-polluted soil showed that bitumen was successfully removed from the soil.

The gas analysis showed that up to 8.67% and 13.66% of the synthesis gas (H₂ + CO) and up to 10.89% and 12.82% of the CO₂ were generated during the bitumen decomposition in the air and water vapour plasma, respectively.

Finally, the results indicated that bitumen was not detected in the soil, or its amount fell below the laboratory’s finding limit (<0.089 g/kg) after the soil cleaning with air or water vapour plasma, respectively.