Remediation Efficiency and Soil Properties of TCE-Contaminated Soil Treated by Thermal Conduction Heating Coupled with Persulfate Oxidation

Abstract

Less attention was paid to the remediation of volatile organic compounds (VOCs) contaminated soil treated by thermal conduction heating (TCH) coupled with chemical oxidization. In this study, the lab-scale remediation experiments of trichloroethylene (TCE)-contaminated soil by TCH and TCH coupled with persulfate (TCH + PS) were performed to explore the influences of PS usage, temperature, reaction time, and the variation of soil properties. TCE was removed from contaminated soils using TCH with a temperature lower than boiling point, and the removal ratio of TCE reached 78.21% with a reaction time of 6h at 60 °C. In the TCH + PS treatments, the removal ratio increased to 87.60~99.50% when the PS dosage was increased from 7.0 mmol/kg to 17.5 mmol/kg at 60 °C. However, the usage efficiency of PS had no positive relationship with oxidant usage and temperature. The treatment with 14 mmol/kg PS after 3h at 50 °C had the highest PS usage ratio of 3.05. In addition, soil pH and soil organic matter (SOM) did not decrease significantly in the TCH-only treatment, while the content of SOM declined by almost 50% after the TCH + PS treatment. Overall, it was concluded that TCH + PS achieved higher removal efficiency, whereas TCH had less disturbance on soil pH and SOM. As such, the applicability of TCH-only or TCH + PS treatments is site-specific.

1. Introduction

In-situ thermal desorption (ISTD) technology is widely used in the world because of its high removal efficiency. The contaminant removal mechanism of ISTD is to heat the contaminated site to induce the volatilization, evaporation, and desorption of pollutants [1]. Thereby, ISTD was suitable for the treatment of most volatile and semi-volatile contaminants with a lower boiling point. Usually, the heating methods of ISTD contained steam-enhanced extraction (SEE), electrical resistance heating (ERH), and thermal conductive heating (TCH) [1]. In particular, the application of TCH has increased in recent years due to its strong applicability to contaminated sites [2]. ISTD is more appropriate for the removal of volatile chlorinated compounds if the shortcomings in energy consumption and process cost are overcome [3]. However, high energy consumption and costs must also be taken into account for TCH because a large amount of energy has been found to be consumed for a long time to heat the site before contaminants are partitioned into the gas phase and removed [2]. Therefore, decreasing the target temperature was a direct approach to reducing energy consumption. However, the lower temperatures may lead to low efficiencies. Previous studies suggested that heating could reduce the concentration of volatile pollutants to approximately 10–100 μg/L for a sufficiently long period [4]. Only 55% trichloroethylene (TCE) was removed when the soil was heated to 80–90 °C during 28 days of heating [5]. To enhance the removal efficiency of contaminants, other measures should be taken, such as applying chemical oxidization.

In-situ chemical oxidization is one of the most feasible and cost-effective remediation methods to remove organic pollutants in soil and groundwater, which converts target pollutants to low-toxicity or non-toxic substances through oxidation. In recent years, more researchers have paid attention to persulfate (PS). There are many activation methods for PS to generate sulfate radical (E0 = 2.6 V), including thermal activation, transition metal ion activation, photo activation, ultrasonic activation, and others. Thermal treatment was an effective method for the degradation of pollutants by activated PS [6]. Zhao et al. [7] demonstrated that the hydroxyl radical was the main species in the thermal activation of PS, followed by some superoxide radicals and sulfate radicals. These radicals were capable of oxidizing many volatile organic compounds (VOCs), especially with “C=C” bonds or with benzene rings bonded to reactive functional groups [8,9], such as Equations (1)–(4). Although the energy consumption of thermal activation is higher, the activation efficiency is better [7]. Li et al. [10] found that the degradation efficiency of the total petroleum hydrocarbon (TPH) was 72.64% for the thermally activated PS system, greater than that for the ultrasonically activated PS system.

S₂O₈²⁻ + heat → 2SO₄•⁻

(1)

SO₄•⁻ + H₂O → SO₄²⁻ + •OH + H⁺

(2)

SO₄•⁻ +C₂HCl₃ + 4H₂O → 2CO₂ + 9H⁺ + 3Cl⁻ + SO₄²⁻

(3)

2•OH +C₂HCl₃ → CCl₂CHOOH + 3Cl⁻ + H⁺

(4)

As mentioned above, each method has its pros and cons. Waldemer et al. [11] found that coupling in-situ chemical oxidation (ISCO) with in-situ thermal remediation (ISTR) was advantageous because S₂O₈²⁻ is relatively stable in unheated zones and heat activation can be focused on contaminated zones. Yang et al. [12] demonstrated that the sustainability performance of TCH-ISCO was better than that of TCH by using a quantitative life cycle assessment approach coupled with the best management practices model. However, there are few studies on the effects of TCH + PS on soil properties. SOM had a complex composition, including volatile constituents, lignin, hemicellulose, humic and fulvic acid, alkyl aromatics, lipids, and sterols [13]. SOM was degraded and the content of SOM was reduced drastically above 300 °C as heating time increased, while SOM had no significant varieties at a low temperature (at or below 300 °C) [14]. Similarly, soil pH was not substantially affected when heated to a low temperature (at or below 250 °C) [14], owing to the formation of HCO₃⁻ following the mineralization of CO₂ and the combustion of SOM [15]. Low temperature and heating have negligible adverse impacts on soil properties [16], while the introduction of PS may lead to the degradation of soil properties [17]. PS preferentially oxidizes SOM because SOM can be consumed not only by free radicals but also by PS [18,19]. As mentioned above, the hydrolysis of PS releases plenty of H⁺, thus lowering the pH of soil during the oxidation processes [20].

To explore the applicability and sustainability of TCH coupled with persulfate oxidation (TCH + PS) for the treatment of soils contaminated with volatile organic contaminants (VOCs), TCH and TCH + PS experiments were carried out in the laboratory using a three-dimensional apparatus, which could be used for conducting in-situ heat treatment coupled chemical oxidization. In this study, we investigated (a) the removal efficiency of TCE by TCH alone and by TCH + PS, (b) the efficiency of PS addition to soil TCH treatment, (c) the acceleration of the removal process, and (d) the variations of soil properties under different factors, such as heating temperature, duration time, and the usage of PS.

2. Material and Methods

2.1. Soil Preparation

Uncontaminated soil was collected from a depth of 0–30 cm in Jiangsu province. The soil samples were ground and passed through a 10-mesh (2 mm) stainless steel screen. Soil properties are presented in

Table 1. Basic physical and chemical properties of the tested soil.

pH	The Content of SOM/%	Total Nitrogen (g/kg)	Total Phosphorus (g/kg)	Total Potassium (g/kg)	Size Distribution
					<0.002 mm	0.02–0.002 mm	2.0–0.02 mm
7.53	2.399	0.787	3.557	9.530	26.32%	55.13%	18.55%

. Briefly, 1100 mL of TCE solution with a concentration of 0.0080 g/mL was evenly added layer by layer to the soil to make 7.2 kg of contaminated soil. After 7 days of aging, the average concentration of TCE was 620.25 mg/kg.

2.2. Description of the Experimental Reactor

TCH-only and TCH + PS experiments were performed in a tempered glass container (300 mm × 200 mm × 150 mm), and the schematic and physical diagrams of the reactor are depicted in Figure 1. A quartz sleeve (diameter: 30 mm, length: 200 mm, thickness: 2 mm) was connected to a resistance heating rod of 0.3 kW (14 mm × 150 mm) and placed in the center of the device for heating. The heating rod was a nickel-chromium resistance wire. The temperature monitoring points were located 15 cm away from the heating rod. Additionally, there were four injection wells and one extraction well in the device. The detailed locations are shown in Figure S1. Similarly, the oxidization agent was injected into soil through injection as well as in the actual engineering. The injection well was PVC pipe (10 cm in length and 20 mm in diameter) with uniform holes. The PVC pipe was wrapped in nylon to prevent blockage and placed 6 cm from the bottom of the device. The exhaust gas was pumped from the extraction well (PVC pipe) through a peristaltic pump and an extraction line (PVC line) and was discharged into a conical flask containing methanol solution.

2.3. Description of Experiments

Several in-situ experiments were conducted in the device mentioned above in order to compare the removal efficiencies of TCE in TCH treatments to those in TCH + PS treatments under different conditions. The AC power was used to heat the soil and hold the temperature at 40 °C, 50 °C, and 60 °C for 3 h, 4 h, 5 h, and 6h, respectively. Different concentrations of Na₂S₂O₈ (PS: 0, 7, 10.5, 14, and 17.5 mmol/kg) were added to the soil. The detailed procedures are introduced below.

The container was filled using quartz sand, contaminated soil, and unpolluted soil. The container had a layer of 4 cm thick quartz sand (4–8 mm) at the bottom to act as an aquifer, followed by 10 cm of aged TCE-contaminated soil to serve as a vadose zone, the particle size of which was 2 mm. Lastly, a 1-cm layer of unpolluted soil was laid to minimize TCE volatilization, the particle size of which was less than 0.15 mm. The device was sealed with tape to ensure air tightness.

Second, the heating rod was turned on. Intermittent heating was started, and the heating duration time was measured when the soil temperature at the temperature monitoring point reached the set temperature to maintain a constant temperature.

Third, to investigate the effect of coupled chemical oxidant, PS was injected into the soil layer. A total of 200 mL of groundwater was extracted from the extraction well by a peristaltic pump and placed in a conical flask. The measured persulfate was added and mixed using a magnetic stirrer for 1 h. Then, the persulfate solution was injected into the soil through the injection well. After cooling, the soils were sampled from the top, middle, and bottom of the soil layer and mixed for analysis.

2.4. Analytical Methods

2.4.1. Analysis Method for TCE in Soil

The TCE content of the soil was measured by purge and trap (Eclipse 4552&4660, OI, a Xylem brand, Yellow Springs, OH, USA) and gas chromatography (7890A, Agilent Technologies, Santa Clara, CA, USA) 30 min after methanol extraction at a solid: liquid ratio of 1:2 at 180 r/min. The prepared reaction samples were centrifuged at 2000 r/min for 5 min and filtered with a 0.45 μm polyethersulfone (PES) membrane filter. Briefly, 1 mL of supernatant combined with 39 mL of ultra-pure water was prepared and the mixture was analyzed by GC. The chromatographic column was a DB-VRX quartz capillary column under BFB tuning mode. The flow rate was 1.2 mL/min, and the temperature of the injector was 230 °C. The initial oven temperature was set to 40 °C and held for 2 min and then raised to 250 °C at a rate of 20 °C/min and held for 3 min. The sample was injected and analyzed in split mode at a ratio of 15:1. Mass spectrometry conditions: Solvent delay time was 1.5 min, and interface and ion source temperatures were 250 °C and 230 °C, respectively.

2.4.2. Soil Properties

To investigate the influence of the TCH/TCH + PS treatment on soil properties, soil samples were collected from experiments treated with only TCH at 50 °C and 100 °C and from experiments treated with TCH coupled with chemical oxidants at the optimum concentration of PS (14 mmol/kg) and duration time of 5h at 40 °C, 50 °C, and 60 °C, respectively. pH and soil organic matter (SOM) were determined by the standard methods [21,22].

2.5. Evaluation of Effectiveness by TCH-Only and TCH + PS Treatments

The removal ratio of TCE was calculated as follows:

$$\mathrm{Removal} \mathrm{ratio} (\%)=\frac{{\mathrm{C}}_0-\mathrm{C}}{{\mathrm{C}}_0}\mathrm{\%}$$

(5)

where C₀ is the initial TCE concentration (mg/kg) and C is the residual TCE concentration after treatment (mg/kg).

To evaluate the efficiency of PS, the usage ratio was applied as Equation (6),

U = (R_TCH+PS − R_TCH)/C

(6)

where U is the usage ratio of the PS, R_TCH+PS and R_TCH are the TCE removal efficiency by TCH + PS and TCH, respectively, and C is the concentration of PS in mmol/kg.

The TCE removal kinetics conform to the model as presented in Equation (7),

ln(C_t/C₀) = −kt

(7)

where C₀ and C_t are the initial TCE content and the TCE content at time t (in mg/kg), t is time (in h), and k is the first-order kinetics.

3. Results and Discussion

3.1. Removal of TCE in the TCH-Only Treatment

Thermal desorption was applied to remove TCE from contaminated soils under the TCH temperature range of 40~60 °C with a duration time of 3~6 h. The TCE removal ratio was 36.27~78.21% even though the TCH treatment temperatures were lower than the boiling point of TCE (87 °C) (Figure 2). Previous studies indicated that both temperature and reaction time were key parameters of thermal desorption [16]. With the increase in temperature from 40 °C to 60 °C, the TCE removal ratio increased from 36.27 to 54.90% within 3 h to 48.92~78.21% within 6 h (Figure S2). Similarly, the TCE removal ratio increased linearly with the increase in reaction times (Figure S3), from 36.27 to 48.92% at 40 °C to 54.90~78.21% at 60 °C.

In general, positive linear relationships were found between the TCE removal ratio and temperature (R², 0.989~1.000)/reaction time (R², 0.912~0.974). A study by Heron et al. [5] showed that the accumulated mass of TCE recovered from the central vent linearly increased with increasing reaction times at a relatively constant temperature. The main mechanisms for the removal of contaminants treated by thermal desorption were volatilization and desorption [23]. Moreover, the diffusion coefficients increased significantly because of the gradual decrease in soil moisture content and the increase in temperature [24,25]. In addition, the temperature provided more energy to overcome the activation energy barrier more easily due to the low activation energy of diffusion; therefore, TCE sorption and desorption diffusivity increased with temperature [26]. However, TCE removal efficiency only increased by 2.29% at 40 °C and by 0.57% at 60 °C when the reaction time was increased from 5 h to 6 h. It indicated that the increase in removal efficiency was not significant when the heating temperature and reaction time increased to a certain extent. Gilot et al. [27] found a similar trend, where the weight loss of pyrene was a linear function of time, with the exception of the trend observed at the end of the thermal treatment. As Gao et al. [28] demonstrated, soils with high moisture content had a strong heat transfer during the initial heating phase owing to heat conduction; however, due to the evaporation of water, the rate of the increase in soil temperature decreased with the increase in reaction time. This suggested that more energy was consumed in the evaporation of water rather than the volatilization of contaminants with the increase in reaction time. Therefore, the increased rate of the removal efficiency of TCE in soils with high moisture content also gradually decreased during the treatment process.

3.2. TCE Removal in the TCH + PS Treatment

3.2.1. Influence of PS Dosage on TCE Removal Efficiency

The influence of PS dosage on the TCH + PS treatment for soils contaminated with TCE has been provided in Figure 3. The results showed that PS could significantly improve the TCE removal of the TCH treatment. The TCE removal ratio increased to 44.28~60.02%, 53.94~73.60%, 68.27~87.18%, and 72.99~89.82% after adding 7.0, 10.5, 14, and 17.5 mmol/kg PS, respectively, compared to the TCE removal ratios of 36.27~48.92% with TCH alone under 40 °C for 3 to 6 h. This demonstrated the contributions of TCE removal due to the coupling effects of volatilization and oxidation introduced by the usage of PS [29], generating hydroxyl and sulfate radicals upon activation by heat [6,7,8,9], which can react with TCE as described in Equations (1)–(4). In addition to the effect of PS usage, temperature exerted significant influences on the TCE removal ratio in the TCH + PS treatment. With the addition of 14 mmol/kg PS and 5 h reaction time, the TCE removal ratio increased from 84.75% to 99.50% when the temperature was increased from 40 °C to 60 °C. Both the higher usage of PS and temperature may have promoted the production of more free radicals in favor of the degradation of TCE [30]. At a constant temperature and PS addition, the TCE removal ratio increased with the increase in reaction time. However, the increase in removal ratio was not significant at the higher temperature and PS addition.

The results of orthogonal experiments showed that the influencing factors of TCE removal efficiency treated by TCH + PS followed the order of PS usage > temperature > reaction time, consistent with the order of importance of the independent variables on removal efficiency treated by heat-activated PS reported by Ulucan-Altuntas et al. [31]. The optimum conditions of TCH + PS treatment were 17.5 mmol/kg PS addition, a temperature of 60 °C, and a reaction time of 6h, yielding a removal ratio of 99.55%.

3.2.2. PS Usage Efficiency in the TCH + PS Treatment

The PS usage efficiency was influenced by various factors, such as temperature, PS dosage, and reaction time (Figure 4). However, there were no positive relationships between the usage ratio of PS and temperature/PS usage/reaction time, unlike that of the TCE removal ratio. The usage ratio of PS was 1.50~3.05 under 50 °C, larger than that under 40 °C (1.14~2.73) and 60 °C (1.02~2.81). Wang et al. [32] observed that an increase in temperature does not always enhance the removal ratio and oxidant utilization. However, the oxidant consumption was accelerated with the increase in temperature. With increasing PS addition, the usage ratio of PS first increased and then decreased. This result is inconsistent with Han et al. [29] because the usage of PS (50~2500 mmol/kg) was larger than that (7~17.5 mmol/kg) of this study. At 40 °C, the hotspot of usage ratio (2.73) appeared with 14 mmol/kg PS after 6h, at 50 °C, the hotspot of usage ratio (3.05) appeared with 14 mmol/kg PS after 3h, and at 60 °C, the hotspot of usage ratio (2.81) appeared with 10.5 mmol/kg PS after 6 h. Among them, the highest usage ratio of PS was obtained after 3 h of TCH at 50 °C with the addition of 14 mmol/kg PS. That is, further rising temperatures and dosage of PS had a negative impact on oxidant utilization because excessive SO₄•⁻ generated by PS in response to an increase in temperature could cancel each other out [10].

On the other hand, soil components, including organic matter, metal oxides, and some inorganic ions in the environment competed with chlorinated compounds for the reaction with persulfate [33,34,35]. The decrease in oxidant utilization was large due to excessive radicals reacting with soil components and intermediates [11]. For example, the chloride ions produced by TCE oxidization (Equations (3) and (4)) could react with SO₄•⁻ according to Equations (8) and (9) [36]. The resultant Cl• and Cl₂•⁻ could react with the chlorinated ethenes or degradation products to produce more highly chlorinated products. The optimum conditions for the maximum removal ratio were not the same as the largest oxidant utilization of PS. Therefore, the pros and the cons should be taken into consideration for the usage of PS to achieve cost-effective treatment of contaminants in field applications.

SO₄•⁻ + Cl⁻ → Cl• + SO₄²⁻

(8)

Cl• +Cl⁻ ⇌ Cl₂•⁻

(9)

3.3. TCE Removal Kinetics in TCH-Only and TCH + PS Treatments

As shown in Figure 5, the removal of TCE treated by TCH coupled with different PS dosages was fitted with the pseudo-first-order kinetic model. Among them, the distinctive curvature was exhibited at higher temperatures (50 and 60 °C) and larger dosages of PS (>10.5 mmol/kg PS) (Figure 5b,c). A similar curvature was observed by Waldemer et al. [11]. They proposed that the combined reactivity of SO₄•⁻ and other reactive species occurred, which was generated by the oxidization of chlorinated ethenes. The observed reaction rate constants (k_obs) of TCE removal were 7.72~35.3 × 10⁻² h⁻¹, 17.6~83.2 × 10⁻² h⁻¹, and 26.4~100 × 10⁻² h⁻¹ at 40 °C, 50 °C, and 60 °C (

Table 2. Dynamic parameters (k) and correlation coefficient (R²) of the coupling repair process under different conditions.

Type	PS Usage (mmol/kg)	40 °C		50 °C		60 °C
		kobs	R2	kobs	R2	kobs	R2
Pseudo-first-order ln(Ct/C0) = −kt	0	7.72 × 10−2	0.98	1.76 × 10−1	0.94	2.64 × 10−1	0.91
	7	1.16 × 10−1	0.97	2.21 × 10−1	0.96	3.17 × 10−1	0.99
	10.5	1.96 × 10−1	0.97	6.32 × 10−1	0.99	4.66 × 10−1	0.75
	14	3.27 × 10−1	0.95	8.69 × 10−1	0.89	1.10	0.89
	17.5	3.53 × 10−1	0.95	8.32 × 10−1	0.88	1.00	0.85

), larger than those observed by Huang et al. [30]. This is because thermal treatment was not only the activation method of persulfate to degrade TCE but also the remediation method for removing TCE based on volatilization in this study. The k_obs value of TCH + PS was enhanced significantly (Table 2) compared to those by only TCH. Generally, the value of k_obs was increased significantly with the increasing concentrations of PS and temperature [37]. The results indicated that high temperature and usage of PS could greatly promote the oxidization of TCE in the soil by activating PS to produce large amounts of oxidizing radicals. However, when the amount of the oxidant exceeds 14 mmol/kg, the reaction rate does not increase anymore and even decreases at a high temperature (60 °C).

The calculated activation energy of TCE degradation was 38.10~53.00 kJ/mol with the addition of 7~17.5 mmol/kg PS (Figure S4). Reactions with low activation energies are relatively temperature-insensitive [6]. As mentioned above, temperatures do not always have a positive impact on the reaction rate. It is worth noting that our values of the activation energy (Ea) were lower than the Ea of TCE degradation (94~108 kJ/mol) in aqueous systems treated by thermally activated PS oxidation [6,11], and were close to the Ea (68.02 kJ/mol) in the soil systems under the same treatment [30]. Our experiments were conducted in a multi-dimension reactor with contaminated soil containing higher soil organic matter (2.4%) to simulate in situ thermal desorption coupled with chemical oxidant, which was more complex than their reaction system. The competition with other reductants in soils could explain why the apparent activation energy was lower than in a pure liquid system without reductants.

3.4. Soil pH and SOM Content Comparisons in TCH-Only and TCH + PS Treatments

For the TCH-only treatment, soil pH slightly decreased with the increase in temperature (Figure 6). A potential explanation is that the degradation of TCE may occur under TCH alone [38]. The degradation rate of TCE by aqueous oxidization was significantly elevated under the influence of heating [39]. With the addition of PS, soil pH varied significantly from 7.53 to 5.84 at 50 °C. As Equations (2) and (3) showed, a large number of H⁺ was released due to the reactions between sulfate radicals and water or TCE. In addition, for TCH coupled with chemical oxidant, soil pH decreased significantly with the increase in temperature. In particular, soil pH was 5.52 with the addition of PS at 60 °C.

The content of SOM was 2.376% and 2.177% in the TCH-only experiment at 50 °C and 100 °C, respectively (Figure 6b). The variation in SOM content was low at 50 °C, while there was 0.2% less SOM at 100 °C. SOM could not decompose and carbonize, while some volatile constituents could be released at a low temperature (<200 °C) [13]. With the addition of PS, the content of SOM was 1.283~1.616% at 40~60 °C. As the temperature increased, the content of SOM decreased. However, the difference in SOM content between TCH coupled with chemical oxidant experiments at 50 °C and 60 °C was not significant (Figure 6b). This indicates that SOM could be decomposed by free radicals generated by oxidants, especially when thermally activated, as discussed above.

4. Conclusions

This study compared and assessed the feasibility of applying only thermal conduction heating (TCH) and TCH coupled with persulfate (TCH + PS) to remediate soils contaminated with VOCs. TCE could be removed by TCH at a low temperature (40~60 °C). The removal rates of TCE were 36.27~78.21%, positively relating to temperature and reaction time. Nevertheless, the removal rate did not always increase significantly, especially at a high temperature and with a long reaction time. However, the addition of PS could enhance the removal rate of TCE from 78.21% to 87.60~99.55% when treated by TCH at a temperature of 60 °C and with a reaction time of 6h. Unlike the removal rate of TCE, further raising temperatures and usage of PS showed a negative impact on oxidant utilization due to the mutual quenching of excessive free radicals and the competition of soil components with chlorinated compounds. The optimum conditions of the highest usage ratio of PS occurred after 3h of TCH at 50 °C, with the addition of 14 mmol/kg PS. Similarly, the reaction rate was promoted by high temperature and PS usage. Yet, the reaction rate was inhibited when PS exceeded 14 mmol/kg and when the temperature was more than 60 °C. That is, from a cost-benefit point of view in practical engineering, the condition of maximum oxidization utilization efficiency, instead of the condition of the highest removal rate, should be explored. Notably, TCH + PS had a negative effect on soil properties, reducing pH and the content of SOM, while TCH at a low temperature showed no significant influence on those properties. In summary, TCH + PS demonstrates superior removal efficiency, while the TCH-only treatment leads to much lower disturbance of soil pH and SOM. Consequently, the appropriateness of TCH-only or TCH + PS treatments for VOC-contaminated soils is site-specific. This depends on how concerned land developers are about energy consumption, costs, and secondary pollution from the remediation technologies. Future research can include the development of a quantitative assessment method that takes into account the above factors to guide land developers in selecting the best technology.