Measurement of Dielectric Properties in Soil Contaminated by Biodiesel-Diesel Blends Based on Radio Frequency Heating

Abstract

Several recent studies have found that measuring the dielectric permittivity of soil can be used to determine the level of environmental pollution. However, there is limited research on the measurement of dielectric properties in soil contaminated with biodiesel-diesel blends from Thailand. This paper presents to monitor the dielectric properties of soil contaminated with biodiesel-diesel blends. Specifically, we use the commercial grade diesel B7 to contaminate a sample of sand soil. We also study the measurement of dielectric properties in contaminated soil with the diesel B7 using a dual electrode plate-based radio frequency (RF) heating system. This allows us to observe the behavior of the contaminated soil before and after RF heating treatment. The experimental result showed that the dielectric properties of uncontaminated and contaminated soil were different. In addition, the RF heating system utilizing the electric field intensities of 450 kV/m resulted in the dielectric properties of the contaminated soil becoming similar to those of uncontaminated soil. These findings suggest that using RF heating on contaminated soil samples improve the air in the pore space compared to unheated contaminated soil. This approach may be effective for the treatment of soil in Thailand using an RF heating system with dual electrode plates.

1. Introduction

Soil quality is a fundamental factor in plant growth and impacts the forests, wet-lands, jungles, prairies, and grasslands that support the planet’s diverse vegetation. However, soil can become contaminated with biodiesel-diesel fuel, which can interfere with soil-forming processes [1]. When soil is contaminated with biodiesel-diesel, it is considered hazardous waste and can cause diffuse pollution in the environment. In order to properly treat contaminated soil through remediation methods, it is important to accurately measure the extent of the contamination. This helps to ensure that the soil can be restored to a healthy state and support the growth of plants and the overall health of the ecosystem.

Conventional methods for observing the difference between contaminated soil with diesel fuel and uncontaminated soil can generally be divided into two categories. The first category is chemical analysis, which includes methods such as Gas Chromatography (GC) [2], Infrared spectroscopy (IR) [3], and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) [4]. These methods involve collecting a soil sample, extracting the contaminants of interest, and analyzing the extract using specialized equipment to determine the concentration of the contaminants present. The second category is physical analysis, which involves examining the physical properties of soil, rather than its chemical composition. Physical analysis is often preferred because it can be performed quickly and non-destructively, making it a cost-effective and efficient method for detecting contaminated sites.

There have been a number of recent studies that propose various physical methods for measuring the dielectric properties of contaminated soil. For example, the authors of [5,6] introduced Time Domain Reflectometry (TDR). This method involves the use of a pulse generator and a reflectometer to measure the electrical properties of soil. TDR is a non-destructive method that can be used to measure the dielectric permittivity of soil. Dielectric Spectroscopy (DES) was proposed in [7,8]. This method involves the measurement of the dielectric properties of soil over a range of frequencies. DES is typically performed using a network analyzer or a vector network analyzer. In [9,10], Capacitance and Resistance Method (CRM) was introduced. This method involves the measurement of the capacitance and resistance of soil using a capacitor and a resistor. CRM is typically used to measure the dielectric permittivity of soil. The authors of [11] presented Frequency Domain Reflectometry (FDR). This method is similar to TDR, but it uses a continuous wave signal instead of a pulse signal. FDR is typically used to measure the dielectric permittivity of soil. Finally, Ground Penetrating Radar (GPR) methods were proposed in [12,13]. This method involves the use of radar waves to measure the electrical properties of soil. GPR is a non-destructive method that can be used to measure the dielectric permittivity of soil. In this study, we will focus on using DES to measure contaminated soil because it has a wide frequency range [14], is non-destructive [15], can provide fast measurements [16], is versatile [17], and is easy to use.

In this paper, we introduce to monitor of dielectric properties in contaminated soil with biodiesel-diesel blends. Specifically, we use the commercial grade diesel B7 (a mixture of 7% biodiesel and 93% diesel fuel, which is the main commercial diesel fuel in Thailand) to contaminate a sample of sand soil. We also study the measurement of dielectric properties in contaminated soil with the commercial grade diesel B7 heated by a dual electrode plate-based radio frequency heating system. This allows us to observe the behavior of the contaminated soil with biodiesel-diesel blends before and after Radio Frequency (RF) heating treatment. We expect the dual electrode plate-based radio frequency heating system provide greater penetration than microwave heating systems due to lower frequencies. The main contribution of this work can be summarized as follows: (1) Based on main commercial diesel fuel, measurement of dielectric properties in contaminated soil with the commercial grade diesel B7 is presented to show the difference between contaminated soil with diesel fuel and uncontaminated soil. The experimental results showed that dielectric properties could significantly provide the difference between contaminated soil with diesel fuel and uncontaminated soil. (2) To our best knowledge, measurement of dielectric properties in contaminated soil with the commercial grade diesel B7 heated by dual electrode plate-based radio frequency heating system is first investigated in this paper. The experimental results presented that the use of dual electrode plate-based RF heating system make contaminated soil with the commercial grade diesel B7 become similar with uncontaminated soil. This suggests that applying dual electrode plate-based radio frequency heating system to contaminated soil may be useful for the treatment of soil in Thailand.

The remaining sections of this paper are organized as follows: Section 2 describe the materials and methods including soil sample preparation, experimental setup for RF heating system, measurement of dielectric properties, and statistical analysis. Results are reported and discussed in Section 3. Finally, Section 4 gives the conclusions.

2. Materials and Methods

2.1. Soil Sample Preparation

Soil samples were collected from Prajuabkirikhan Province in Thailand using a clean spatula. Approximately 1 kg of soil was placed in a clean, labeled plastic bag, and dried by heating at 105 °C for 24 h [18]. The dried soil was then sieved to remove large particles and the resulting soil, with particle sizes ranging from 0.10 mm to 0.25 mm, was referred to as fine sand for the experiments. The fine sand was then contaminated with diesel fuel obtained from a commercial source (PTT gas station, Thailand). The fuel used was a mixture of 7% biodiesel and 93% diesel fuel (B7), which is commonly used in Thailand [19,20].

In this study, the commercial grade diesel B7 was used to artificially contaminate soil samples. The processed soil samples were placed in 8 glass dishes (10 cm diameter × 1.5 cm height) containing 100 g of soil each, with 4 dishes containing unsaturated soil and 4 containing saturated soil. For the unsaturated contaminated soil, the processed soil samples were contaminated with 20 g of the commercial grade diesel B7, resulting in intermediate dielectric properties between uncontaminated and saturated contaminated soil. For the saturated contaminated soil, the processed soil samples were contaminated with 80 g of the commercial grade diesel B7, exceeding the porosity value of 0.67. The configuration of the soil samples is summarized in

Table 1. The properties of soil samples.

Parameter	Value
Soil (g)	100 g
Particle size (mm)	0.10–0.25 mm
Porosity	0.67
Volume of soil jars	117.75 cm3
Bulk density	0.85 g/cm3
Diesel fuel	20, 80 g

.

To create the final test samples, the soil samples in the glass dishes were contaminated with two levels of the commercial grade diesel B7, referred to as unsaturated and saturated soil. The diesel fuel was uniformly dispersed over the top of the samples using a syringe to create the polluted soil. The dishes containing the polluted soil were then sealed to prevent evaporation and were prepared for measurement and heating in the next phase of the experiment. Figure 1 illustrates the soil samples that were used in the study and were not contaminated with diesel B7.

2.2. Experimental Setup for RF Heating System

In this research, we utilized a RF heating system with a power output of 9 kW and a frequency of 40.68 MHz. The system consists of two electrode plates, one on the top and one on the bottom, as depicted in Figure 2a. Both the top and bottom electrode plates have dimensions of 520 mm × 520 mm. The bottom electrode plate is fixed in position to secure the samples in place, while the top electrode can be adjusted to generate a range of electric field intensities, including 450 kV/m, 225 kV/m, 150 kV/m, and 112.5 kV/m. In order to achieve the most efficient heating rate with a uniform temperature distribution, as described in [21,22], the central position of the bottom electrode was chosen as the heating position for our experiment. As shown in Figure 2b, the contaminated soil samples were placed at this central position on the bottom electrode.

The power conversion in a material using RF heating depends on the operating frequency, dielectric loss factor, and electric field density inside the material. The power absorbed per unit volume in a material [23] can be calculated using the following formula:

$$P=\omega {\epsilon }_0{\epsilon }^{″}{\left|E\right|}^2$$

(1)

where P presents the power dissipated per unit volume (W/m³), $\omega$ denotes the angular frequency ($\omega =2\pi f,f$ Radio frequency), ${\epsilon }_0$ denotes the permittivity of free space ($8.85\times {10}^{-12}$ F/m), ε″ presents dielectric loss factor of the material and it represents the ability of a material to convert the RF energy into heat and $E$ is the electric field intensity (V/m).

In this paper, the radio frequency heating system was operated at a frequency of 40.68 MHz, which is within the optimized Industrial, Scientific, and Medical (ISM) band as suggested in [24,25]. The electric field intensity was calculated using the following formula:

$$E=\frac{V}{\Delta d}$$

(2)

where E denotes the electric field intensity (V/m), V is the voltage during the heating process (V), and $\Delta d$ presents the distance between electrodes (m).

2.3. Measurement of Dielectric Properties

In this study, we used a vector network analyzer (VNA) equipped with an open-ended coaxial probe to measure the dielectric properties of the soil samples used in our experiment. The VNA (Keysight E5071C ENA network analyzer, Keysight Technologies Inc., San Jose, CA, USA) had a wide frequency range of 300 kHz to 20 GHz, and we used a dielectric slim form probe (Keysight N1501A, Keysight Technologies Inc., San Jose, CA, USA) as the open-ended coaxial probe. This probe is non-destructive to materials and can be used to measure the dielectric properties of solid, liquid, and powder materials, as advised in [26]. The VNA was connected to a computer desktop with the Keysight Materials Measurement Suit software (Keysight Technologies Inc., Santa Rosa, CA, USA) for data acquisition and dielectric property calculations as suggested in [27]. The measurement of dielectric properties involves the determination of both the dielectric constant and dielectric loss factor, as described in the following equation:

ε = ε′ − jε″

(3)

where ε′ is the dielectric constant and ε″ is the dielectric loss factor.

The process of calibrating the network analyzer involved using air, a shot probe, and deionized water, as recommended in [28]. The calibrations and measurements were conducted in a closed laboratory setting with a temperature of 25 °C. The soil samples were measured on site within a frequency range of 20 MHz to 10 GHz, utilizing a VNA configured with a total of 1001 sampling points. This frequency range is relevant for industrial, scientific, and medical applications [24,25]. The soil samples were placed in a container that was wide enough to allow the probe to be inserted and the soil sample was thick enough to prevent any reflection coefficient interference from the glass dish and ground floor on the measurements and errors in the measurement of the dielectric properties. A dielectric slim form probe was used to measure the dielectric properties of the soil samples in situ. The dielectric properties values were measured with three replications, as suggested in [29], and the mean values were used for data analysis. The setup for measuring dielectric properties is depicted in Figure 3.

The complex relative permittivity, or the measure of a material’s ability to allow an electric field to pass through it, often varies with frequency when subjected to an applied electric field. The frequency-dependent behavior of the complex permittivity can be described using the Debye equation, which is expressed as follows:

$$\epsilon ={\epsilon }_{\infty }+\frac{{\epsilon }_s-{\epsilon }_{\infty }}{1+j\omega \tau }$$

(4)

where ${\epsilon }_s$ is the static dielectric constant, ${\epsilon }_{\infty }$ is the dielectric constant at high frequency, $\omega$ is the angular frequency and $\tau$ is the relaxation time. Equation (4) can be separated into the dielectric constant ε′, and dielectric loss factor ε″, as follows:

$${\epsilon }^{\prime }={\epsilon }_{\infty }+\frac{{\epsilon }_s-{\epsilon }_{\infty }}{1+{(\omega \tau )}^2}$$

(5)

$${\epsilon }^{″}=\frac{({\epsilon }_s-{\epsilon }_{\infty })\omega \tau }{1+{(\omega \tau )}^2}$$

(6)

Here, the permittivity values in this paper are obtained directly from our experimental equipment, which simplifies the process of recording results.

2.4. Statistical Analysis

Three replicate experiments were conducted, and the data was analyzed using SPSS Statistics version 20.0. A one-way analysis of variance (ANOVA) was performed to determine the statistical differences between the sample means, with a significance level of 5%. In this study, the dielectric properties of the test soil sample were considered significantly different based on Fisher’s least significant difference test.

3. Results and Discussion

In this section, we present and discuss the results of the temperature measurements, dielectric properties, and statistical analysis.

3.1. Investigation of Relative Electric Field Intensity and Temperature on Heating System

The temperature profiles with time during RF heating treatment are investigated using various electric field intensities including 112.5 kV/m, 150 kV/m, 225 kV/m, and 450 kV/m to demonstrate the difference between unsaturated contaminated soil and saturated contaminated soil. For the overall experiment, the contaminated soil sample was treated at a power of 900 W for irradiation times of up to 30 min. Here, the heating time was recorded at each min while the temperature was monitored using the infrared thermography camera (U5857A, Keysight Technologies Inc., Santa Rosa, CA, USA), and was recorded. Figure 4 shows the difference between unsaturated contaminated soil and saturated contaminated soil based on the temperature profiles with time derived from the electric field intensities of 112.5 kV/m, 150 kV/m, 225 kV/m, and 450 kV/m.

Based on the electric field intensity value of 112.5 kV/m, we can see from Figure 4a that the temperature of the contaminated soil increased quickly at the start of the RF heating treatment. For saturated contaminated soil, the temperature reached a maximum of 128.5 °C, while for unsaturated contaminated soil, the maximum temperature was 110 °C. This is because the ability of the soil to convert RF energy into heat was still high before the evaporation of the commercial grade diesel B7, a trend that has also been observed in other studies (referenced in [30,31,32]). Eventually, the temperature decreased from the maximal temperature due to the evaporation of the diesel, which caused the loss factor (ε”) of the contaminated soil to decrease and reduced its ability to convert RF energy into heat.

A similar trend to Figure 4a can be seen in Figure 4b–d, where higher electric field intensities cause the temperature to rise more quickly. The reason for this is that a higher electric field intensity value leads to an increased power absorbed per unit volume in the material, a relationship that can be described by Equation (2). From the data, it appears that the highest electric field intensity value of 450 kV/m leads to the shortest time required to reach the maximum temperature for both unsaturated and saturated contaminated soil. These findings suggest that increasing the electric field intensity value has a significant impact on the time required for RF heating treatment of contaminated soil.

3.2. Result of Dielectric Properties

This section presents the dielectric constant and loss factor of both unsaturated and saturated contaminated soil before and after RF heating treatment, using various electric field intensities of 112.5 kV/m, 150 kV/m, 225 kV/m, and 450 kV/m. The experimental results were analyzed using the polynomial curve fitting method and the MATLAB software to find the most appropriate values for the fitting polynomial model parameters, in order to obtain values that closely match the measured data. Figure 5 displays the dielectric properties of all of the tested soil samples before and after RF heating treatment, as well as a reference line for soil without RF heating treatment. Additionally, the summary of the dielectric properties at significantly highlighted frequency is shown in

Table 2. Dielectric properties of heated soil samples based on the highlighted frequencies.

Contaminated Soil Condition	Frequency (MHz)	None		112.5 kV/m		150 kV/m		225 kV/m		450 kV/m
		ε′	ε″	ε′	ε″	ε′	ε″	ε′	ε″	ε′	ε″
Unsaturated	200	3.06	0.73	2.94	0.60	2.74	0.47	2.68	0.43	2.59	0.40
	400	3.21	0.65	3.05	0.50	2.90	0.43	2.88	0.39	2.75	0.38
	600	3.27	0.53	3.13	0.43	3.00	0.37	2.98	0.36	2.85	0.34
	800	3.27	0.47	3.14	0.42	3.02	0.36	3.02	0.34	2.88	0.33
	1000	3.26	0.43	3.10	0.36	2.99	0.31	3.03	0.30	2.88	0.29
Saturated	200	3.29	0.77	3.22	0.67	3.11	0.54	3.01	0.57	2.99	0.59
	400	3.69	0.89	3.45	0.60	3.35	0.51	3.25	0.51	3.23	0.55
	600	3.80	0.82	3.54	0.50	3.45	0.43	3.35	0.44	3.32	0.46
	800	3.78	0.62	3.57	0.47	3.48	0.39	3.39	0.41	3.35	0.43
	1000	3.68	0.51	3.58	0.42	3.50	0.35	3.41	0.36	3.37	0.39

.

As shown in Figure 5, the dielectric constant of the saturated contaminated soil samples was higher than that of the unsaturated contaminated soil samples and the uncontaminated soil samples, respectively. This is because the addition of commercial grade diesel B7 into the soil penetrates deep into the soil, replacing the air in the pore space and blocking air spaces with the diesel. As a result, the volumetric content of the diesel in the soil affects the dielectric constant of the uncontaminated soil samples, making it different from the dielectric constant of the unsaturated contaminated soil and more different than the saturated contaminated soil samples. It can also be seen that the unsaturated and saturated contaminated soil samples after RF heating treatment have a lower dielectric constant than the unsaturated and saturated contaminated soil samples before RF heating treatment, at all frequencies. This is because the evaporation of the commercial grade diesel B7 in the soil occurs during RF heating treatment, reducing the volumetric content of the diesel in the soil. Here, the RF heating system using an electric field intensity of 450 kV/m provided the highest reduction in the volumetric content of the diesel in the soil. This is because a higher electric field intensity leads to improved power absorbed per unit volume in the dielectric material, as previously discussed. Statistical analysis shows that the reduction in the volumetric content of the diesel in the soil was statistically significant (p ≤ 0.05) when using the optimal electric field intensity for RF heating treatment. This result suggests that RF heating treatment is effective in reducing the volumetric content of diesel in soil. Additionally, it can be observed from Figure 5 that the dielectric loss factors are less pronounced beyond around 1 GHz, which may not be useful for the reader as this frequency range is not suitable for TDR, as summarized in [33].

To clearly see the signature of the unsaturated and saturated contaminated soil samples before and after RF heating process, Table 2 summarizes the comparison of the signature effect of the tested soil types and the volumetric content of the commercial grade diesel B7. As shown in Table 2, the difference in dielectric properties between the unheated and heated soils is noticeable and distinct. This trend suggests that the difference in dielectric properties between the unheated and heated soils should be observed at all frequencies less than 1 GHz, as previously suggested in [28,33].

4. Conclusions

In this paper, we presented a study on the monitoring of the dielectric properties of soil contaminated with biodiesel-diesel blends. The focus of the study is on using commercial grade diesel B7 to contaminate a sample of sand soil and measuring the dielectric properties of the contaminated soil using a dual electrode plate-based radio frequency heating system. This allowed us to observe the behavior of the contaminated soil with biodiesel-diesel blends before and after RF heating treatment. For our experiment, soil samples were contaminated with commercial grade diesel B7 and were heated using a dual electrode plate-based RF heating system with various electric field intensities of 112.5 kV/m, 150 kV/m, 225 kV/m, and 450 kV/m to observe the dielectric properties of contaminated soil with biodiesel-diesel blends. The results showed that the RF heating system using an electric field intensity of 450 kV/m provided the highest reduction in the volumetric content of commercial grade diesel B7 in the soil. This is because a higher electric field intensity leads to improved power absorbed per unit volume in the dielectric material. These results suggest that using RF heating on contaminated soil samples leads to the evaporation of the commercial grade diesel B7 in the soil and improves the air in the pore space and blocking air spaces compared to unheated contaminated soil samples. This strategy may be useful for soil treatment in Thailand using a RF heating system with dual electrode plates.

In future work, we plan to use a high-performance heating system that can be adjusted to electric field intensity values higher than 450 kV/m. Additionally, we will use GC [2], Fourier Transform Infrared Spectrometer [34], and Thermogravimetric Analysis [35] to further investigate the contaminated soil with biodiesel-diesel blends heated by RF heating. We will also study different types of soil and various types of other biodiesel-diesel blends used in Thailand.