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Article

Effects of Different Heat Treatment Methods on Organic Pollutants and Heavy Metal Content in Oil Sludge Waste and Ecotoxicological Evaluation

by
Xuan Sun
1,2,
Tao Yu
1,2,*,
Yi Huang
2,3,
Ming Xue
1,
Chengtun Qu
1,2,*,
Penghui Yang
1,2,
Xiaofei Zhang
1,
Bo Yang
1,2 and
Jinling Li
1,2
1
State Key Laboratory of Petroleum Pollution Control, College of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an 710065, China
2
Shanxi Oil and Gas Pollution Control and Reservoir Protection Key Laboratory, College of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an 710065, China
3
College of Civil & Architecture Engineering, Xi’an Technology University, Xi’an 710065, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2022, 12(7), 3609; https://doi.org/10.3390/app12073609
Submission received: 1 March 2022 / Revised: 30 March 2022 / Accepted: 30 March 2022 / Published: 1 April 2022
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Abstract

:
The discharge of large amounts of oily sludge heat treatment residues constitutes a severe threat to the environment. However, little is known about the toxicity of these heat-treated residues. Current research has mainly focused on the toxic effects of single heavy metals or single hydrocarbons on plants, whereas the phytotoxic effects of hydrocarbon–metal mixtures have remained largely unexplored. In this study, pot experiments were conducted to evaluate the effects of different proportions of heat treatment residues (pyrolysis, heat-washing, and high-temperature oxidation residues) from three kinds of oily sludge on the physiological and biochemical parameters of mung bean plants. Higher proportions of residues decreased the germination rates and enzyme activity of mung beans compared to uncontaminated soil. When pyrolysis residue, hot-washing residue, and high-temperature thermal oxidation residue are used in green planting soil, their content must be lower than 30%, 90%, and 70%, respectively. Additionally, our findings indicated that the accumulation level of pollutants in oily sludge heat treatment residues was not high. However, the three kinds of residues exhibited different degrees of plant toxicity. The pyrolysis residue still exhibited strong ecotoxicity, even at low concentrations. In contrast, the toxicity of the hot-washing residue was much lower than that of the pyrolysis residue and the high-temperature thermal oxidation residue. Our findings indicated that mung bean is highly tolerant of contaminated soil and is therefore well suited for phytoremediation applications.

1. Introduction

Oily sludge is a kind of ecotoxic waste with a complex composition and high stability that is produced in the process of crude oil extraction, gathering, transportation, and refinement [1]. Therefore, many countries have listed it as a hazardous waste material [2,3]. Furthermore, our heavy reliance on oil-based products as sources of energy has led to increasing oily sludge production rates. It is estimated that approximately 60 million tons of oily sludge and more than 1 billion tons of untreated oily sludge are produced worldwide each year [4,5]. Because oily sludge contains high levels of hydrocarbons, aromatic compounds, heavy metals, and other pollutants [6,7], these pollutants accumulate in organisms and are biomagnified along the food chain. If not properly treated, prolonged exposure to heat treatment residues may also lead to hepatotoxic and renal toxicity in humans [8] and may also result in serious environmental impacts [9]. Therefore, several methods have been developed to manage oily sludge. The most commonly used approaches are pyrolytic technology [10], thermal-washing technology [11,12], and high-temperature oxidation residue technology [13]. Hot-washing technology, also known as the thermal desorption method, consists of diluting oil sludge with high-temperature water while adding certain amounts of different chemical agents to remove the oil from solids through either surface desorption or aggregation separation, thereby achieving solid–liquid separation. Pyrolysis technology consists of heating the oily sludge to a certain temperature in the absence of oxygen. When using this approach, the oil components are transformed into three-phase substances through complex physical and chemical reactions such as distillation and thermal decomposition for recycling. The main difference between pyrolysis and high-temperature thermal oxidation is that pyrolysis is carried out under anaerobic conditions. High-temperature thermal oxidation technology consists of treating oily sludge in a thermal oxidation furnace under aerobic conditions. The secondary combustion chamber and dust removal chamber in high-temperature thermal oxidation systems can effectively control dioxin and other harmful substances during the treatment process, and this technology can make full use of the calorific value of the crude oil in oily sludge, thereby reducing the operating costs incurred during combustion. The above-described technologies have largely enabled the reduction and resource utilization of oily sludge.
However, the composition of oily sludge is complex, and the above-described treatment technologies have their own unique limitations. Particularly, some hydrocarbons, heavy metals, and other pollutants remain in the treated residues [14]. Additionally, the excessive stacking of heat treatment residues may lead to the migration of hydrocarbons and heavy metals into the surrounding environment. Therefore, environmentally friendly approaches to treat these residues have recently garnered increasing attention [15,16]. Most related studies have focused on the treatment and utilization of the pyrolysis residue, whereas far fewer studies have assessed the treatment of hot-washing residue and high-temperature thermal oxidation residue. The residue resulting from pyrolysis has a loose porous structure and contains a certain amount of heavy metals and therefore possesses a catalytic effect [17]. Liu [18] used the pyrolysis residue to catalyze sludge pyrolysis. The liquid yield, oil yield, and gas yield increased significantly, whereas the solid yield decreased, indicating that the presence of residues promoted sludge pyrolysis. Additionally, the pyrolysis residue has a complex pore structure, a high specific surface area, and chemical inertness, and it can therefore also be a good catalyst carrier. Moreover, these residues exhibit high levels of hydrocarbons, which makes them uniquely suitable for the preparation of adsorption materials [19,20]. When the pyrolysis residue is used to treat simulated seawater containing metal ions, the Ca2+ and Mg2+ concentrations in the water can be significantly reduced by adding 5% or 10% kaolin into the sludge. The above-described studies achieved good results using the pyrolysis residue [21,22]; however, these studies did not consider the toxicity of the residue prior to its use.
Previous studies have focused on the toxic effects of single heavy metals or single hydrocarbons on plants, whereas very few studies have evaluated the effects of hydrocarbon and metal mixtures. Oil-contaminated sites are typically contaminated by both kinds of contaminants; therefore, examining their combined effects provides a more realistic assessment of the environmental impacts of oily residues [23]. According to the current National Environmental Quality Standards for Soils class II, 0.3 mg/kg of Cd is considered safe for plants and the environment, whereas its levels in agricultural sludge are restricted to 0.8 mg/kg of Cd in the same control standard. Nevertheless, if only the chemical index of a specific pollutant is used to evaluate soil quality, the degree of environmental damage caused by the pollutant in question could be severely underestimated. Plant toxicity assays can reflect the degree of soil contamination more accurately. Evaluating heat treatment residues using plants can not only remove pollutants from the residues to make them harmless, but it can also result in them being used for desert greening, a technology that uses plants to remove pollutants and is an economically viable and sustainable biotechnology [24]. However, this technique requires a high level of plant species selection, as plants must not only be resistant to pollutants but must also be able to accumulate them in large amounts. Mung beans are grown on abandoned cultivated lands at the northern edge of Shaanxi Province. These plants exhibit stable yields and can remove toxicants in soil, wastewater, and sludge. Therefore, the mung bean plant was selected as the test plant in the present study.
In this study, residues were prepared via commonly used oily sludge treatment methods, and the growth of the mung bean plants was used as an indicator of toxicity, to evaluate the effect of heavy metals and hydrocarbons in the actual oily sludge heat treatment residue on the growth of the mung bean plants as well as to determine the feasibility of using this plant to repair contaminated soil. Additionally, this approach not only removes the pollutants in the heat treatment residues and makes them harmless, but it also provides data to support residue greening.

2. Materials and Methods

2.1. Raw Oily Sludge and Residue Preparation

Oily sludge samples were obtained from Tie Bian City Oil Sludge Station, China. Approximately 500 g of sludge was added to the sampler. The temperature of the three levels of the instrument was set to 500 °C, and the rotating speed was adjusted to 50 rpm for pyrolysis residue preparation. Next, 200 g of sludge was placed in a beaker, and 800 mL of water was added to achieve a solid–liquid ratio of 1:4. The samples were then placed in a 70 °C water bath and were stirred for 30 min until uniform. Afterward, 10% sodium hydroxide was added to adjust the pH to a value of 10, followed by the addition of 0.8% Tween 80 (a surfactant) to heat wash the oil sludge. The samples were then stirred for 10 min, after which 0.8% demulsifier was added for demulsification. The samples were then stirred again for 60 min and were allowed to stand for 60 min. Next, the solution was centrifuged at 2400 r/min for 10 min to obtain the residue after thermal washing. The feed temperature was set to 470 °C, the temperature in the furnace was set to 750 °C, and the drum speed was set to 30 r/s. The temperature was then allowed to increase gradually until it reached 470 °C, after which 500 g of sludge was added to the sampler. Feeding was then initiated at a feeding speed of 5 kg/h. The materials were allowed to fully burn up in the furnace and were then discharged. The feed temperature was set to 470 °C, the temperature in the furnace was set to 750 °C, and the drum speed was set to 30 r/s. The furnace was allowed to slowly heat up to 470 °C, after which 500 g of sludge was added to the sampler, at which point feeding began. The feeding speed was controlled at 5 kg/h to ensure that the materials burned fully and that a high-temperature thermal oxidation residue was obtained. Table 1 lists the preparation conditions for the heat treatment residues used in this experiment, which were based on conventional treatment processes used in oil sludge treatment plants.
Table 2 lists the basic physicochemical properties of the heat treatment residues, including the oil content, water content, and solid content.

2.2. Potted Plant Experiment

The prepared residues were mixed with Mei Le Ke soil nutrient solution (P2O5: 0.27 g/L; organic N: 0.6 g/L; K2O: 0.36 g/L) in the proportions shown in Table 3, after which four parallel batches were prepared for each proportion for further use. Mung bean seeds were purchased from the Shaanxi Province seed station and were selected via the water selection method. In brief, the mung bean seeds were transferred to a container with water, after which the unsaturated seeds and debris were removed, and the seeds were soaked in hydrogen peroxide for 8 min to disinfect their surface prior to the initiation of the experiments. Afterward, the seeds were thoroughly rinsed three times with sterile water, and seed germination tests were performed. The seeds were placed on moist filter paper and incubated at 25 ± 2 °C in the dark. The preparation process and phytoremediation of the residues using different heat treatment methods are shown in Figure 1.

2.3. Analytical Methods

2.3.1. Detection of Heavy Metals and Organic Compounds in Heat Treatment Residues

Dichloromethane was used to extract the organic compounds from the different residues, and the components were analyzed via GC–MS on an Agilent 7890A GC system and an Agilent Technologies 5975C Inert XL mass selective detector equipped with an HP-5MS UI column (30 m × 0.25 mm × 0.25 μm). The determinations were conducted with a 1 µL sample injection volume and a 10:1 split ratio injection. The column was first kept at 50 °C for one minute and then heated to 300 °C at a 10 °C/min rate, and it was kept at this temperature for 5 min [25]. Pb, Zn, Cu, Cd, Cr, Ni, As, and Hg were detected before and after residue treatment via atomic fluorescence spectrometry and inductively coupled plasma emission spectrometry according to Chinese standard GB/T 36690-2018.

2.3.2. Plant Activity Testing

When planting, the plant seeds were evenly scattered on the soil layer and covered with the prepared soils at an optimal burial depth of 2 cm. The soil surface was moistened after sowing, and it was ensured that the bottom of the pot was wet and that there was no water on the surface. Seed germination is an important stage in the plant life cycle and is highly responsive to environmental cues. Therefore, germination rates can be used as an indicator of soil health [26]. Germination was defined as the emergence of the radicle 2 mm outside of the seed coat. Germination rate assessments and root elongation tests were conducted 48 h after seed germination; an amount of 50 mL of distilled water was added every 24 h.
After seed germination, the plants entered the seedling growth stage. Germination rates were determined for 14 d during the seedling growth stage. After 14 d, the mung beans were removed from the pots, and each plant was separated and rinsed with tap water. Next, 1 g of plant tissues were weighed, and 9 mL of physiological saline homogenization medium was added at a mass (g) to volume (mL) ratio of 1:9. Afterward, 10% tissue homogenates were prepared in an ice water bath. After centrifugation at 2000 r/min for 10 min, the supernatant was taken, and the total superoxide dismutase (SOD), peroxidase (POD), and malondialdehyde (MDA) contents were determined.

3. Results and Discussion

3.1. Toxicants in Oily Sludge Heat Treatment Residue

3.1.1. Organic Compounds in Heat Treatment Residues

The main toxicants in the residue were heavy metals and some organics. The hazardous effects of heat treatment residues on the environment and plants mainly depend on the organic composition and the heavy metal content in the residue. Therefore, we sought to investigate the organic composition of the heat treatment residues using GC–MS analysis.
Our findings indicated that the main components of the three heat treatment residues were straight-chain alkanes, of which hexadecane and octadecane had growth-promoting effects [27,28], whereas hexadecane had an inhibitory effect on Cd toxicity [29]. Additionally, these alkanes were also able to promote the immobilization of free metal ions in live tissues [20,30]. The main compounds in the pyrolytic residue were straight-chain alkanes with carbon numbers between 16 and 25 as well as a few compounds with more than 30 carbon atoms. Furthermore, the main compounds in the thermal-washing residue were straight-chain alkanes with carbon numbers between 15 and 30 as well as 2,6,10,14-tetramethylpentadecane, which were likely produced via the isomerization of lefins [31]. No compounds with more than 30 carbon atoms were detected in the high-temperature oxidation residue. However, this residue did contain 2,4-di-tert-butylphenol and dioctyl adipate. The compound 2,4-di-tert-butylphenol is toxic to aquatic organisms and may have long-term adverse effects on aquatic ecosystems.

3.1.2. Heavy Metal Contents in Heat Treatment Residues

In addition to organic compounds, the number of heavy metals in heat treatment residues also determines their toxicity, as they interact with biomolecules to produce toxic effects [32]. The alkanes and heavy metals in heat treatment residues can form a complex mixture. More importantly, the interaction of the alkanes with the heavy metals in the residue inhibits the survival of the microorganisms that degrade hydrocarbons [30], and hydrocarbons are known to affect heavy metal transport and migration [33]. Hydrocarbon decomposition leads to the release of free radicals and reactive oxygen species, which can damage the cellular structure of microorganisms, thus affecting the ecological structure [34]. The coexistence of heavy metals with alkanes in soils also leads to the conversion of productive agricultural land into wasteland [35]. This ultimately leads to oxygen depletion in water, disrupts aquatic biota [36], and increases the accumulation of heavy metals in sediments [37]. The phytoremediation of oil pollution in the presence of high concentrations of heavy metals has also been rarely studied [38]. Therefore, analyzing heavy metal levels in heat treatment residues is crucial. The analysis results are shown in Figure 2.
The order of the heavy metals in the three residues based on the highest content was Cr > Ni > Cu > Zn > Pb > As > Cd > Hg. Additionally, compared to the pyrolytic residue and high-temperature oxidation residue, the contents of Zn, Cr, Ni, As, and Cd in the thermal-washing residue were higher. According to the risk screening values of the GB/T 15618-2018 soil environment quality risk control standard for the soil contamination of agricultural land, when the pH value of agricultural soils exceeds 7.5, the Cd, Hg, As, Pb, Cr, Cu, Ni, and Zn contents should not exceed 0.8, 3.4, 25, 170, 250, 100, 190, and 30 mg/kg, respectively. Therefore, the contents of Zn, Cr, and Ni in the three treatments far exceeded this limit, and the Cu levels in the pyrolysis and thermal flushing residues also exceeded this limit.

3.2. Plant Growth

In natural environments, plants are more vulnerable to combined pollution. Plants are known to survive for a long time in the presence of heavy metals and organic pollutants; however, this can only be achieved when the activity of the corresponding antioxidant enzymes is reduced. In this study, the toxic effects of heat treatment residues on mung beans were investigated based on their germination rates, MDA content, SOD, and peroxidase activity as growth indicators.

3.2.1. Effects of Heat Treatment Residues on the Germination Rates of Mung Beans

Three different types of residues were mixed with Mei Le Ke Nutrient Soil at mass ratios of 0%, 10%, 30%, 50%, 70%, 90%, and 100%, respectively. Mung beans were then cultivated to determine their germination rates. The 14-day germination rates of mung beans cultivated in different heat treatment residues are illustrated in Figure 3.
Our findings indicated that more than 90% of the mung beans did not germinate when the pyrolytic residue content exceeded 30%, indicating that the maximum residue content should not exceed 30% when pyrolytic residue is used for greening soil. When the thermal-washing residue content was greater than 90%, more than 90% of the mung beans did not germinate, indicating that the maximum thermal-washing residue content should not exceed 90% when used for green planting. Further, the maximum high-temperature oxidation residue content should not exceed 70% when used for green planting. These results indicate that the pyrolysis residues had strong acute toxicity, even at low concentrations. In contrast, the thermal-washing residue was much less toxic than the pyrolysis and high-temperature oxidation residues.
Our results indicated that the main organic compounds in the three residues were straight-chain alkanes, whereas the thermal-washing residue had more straight-chain alkanes and longer carbon chains compared to the pyrolytic residue and high-temperature oxidation residue, suggesting that the residue toxicity might be related to the length of the carbon chains. Specifically, long-chain alkanes promote the growth of mung beans, whereas long-chain alkanes might increase the mutual inhibition of metal ions. Therefore, the thermal-washing residue was less toxic.

3.2.2. Analysis of MDA and Enzymatic Activity of Mung Beans after Heat Treatment Residue Cultivation

Plant organs tend to undergo membrane lipid peroxidation in response to adverse conditions. MDA is one of the most important products of membrane lipid peroxidation in plants, and its production also aggravates membrane damage. SOD is the only enzyme in nature that uses oxygen radicals as substrates and can quench the toxicity of superoxide anions and alleviate the toxic effects caused by superoxide anions on organisms. Therefore, this antioxidant enzyme plays a vital role in the oxidative and antioxidant balance of the body. Peroxidase can be used as a physiological indicator of tissue aging because of the high antioxidant dismutase (POD) activity in aging tissues in general. MDA, SOD, and POD were selected as indicators to determine the response of the plants to pollutant stress. The MDA, SOD, and POD activity in plants exposed to three toxic residues is illustrated in Figure 4.
When the three residues examined herein were at higher concentrations, the MDA and peroxidase content in the mung beans increased gradually, whereas SOD content decreased gradually. These results indicate that the residues caused a certain degree of plant membrane damage, resulting in an imbalance between oxidation and antioxidation in the plants. Higher residue concentrations resulted in aging, lodging, and other adverse effects after 14 days. This might be due to the inhibition or degradation of plant enzymes caused by the heavy metals in the residues.
The effect of high-temperature oxidation residues on the MDA and enzyme activity of the mung bean plants was the most significant. The MDA content of the mung beans after planting was 1.5–2 times higher than that of the control when the high-temperature oxidation residue was mixed in a proportion of 70%, meaning that the plant membrane damage was more severe. The high-temperature oxidation residue had an inhibition rate of approximately 45% for plant SOD, which severely disrupted the antioxidant balance of the mung bean plants. High-temperature oxidation residue contains dioctyl adipate and 2,4-di-tert-butylphenol, in addition to straight-chain alkanes. Dioctyl adipate is classified as a slightly toxic substance and a category three carcinogen by the World Health Organization (WHO). The compound 2,4-di-tert-butylphenol is toxic to organisms and may have long-term adverse effects on aquatic environments [39]. Therefore, the high-temperature oxidation residue was found to have the most serious impact on plant malondialdehyde and enzyme activity.

3.3. Analysis of Heavy Metal Contents in Residues before and after Planting

The heavy meatal content and migration behavior in residues are very important for later residue disposal [37]. Therefore, the soil ratio conditions with the strongest inhibitory effects on mung bean growth were selected for further analysis in the present study. The changes in the heavy metal content in the soil before and after planting were analyzed. Figure 5 shows the proportions of heavy metals in the soil after planting compared to the heavy metal content in the soil before planting.
Compared to the pyrolytic residue and high-temperature oxidation residue, the large variations in Cu, Pb, Zn, Cd, Cr, and Ni in the thermal-washing residue might be attributed to the strong adsorption of these metals by mung beans. Furthermore, the mung beans exhibited a stronger ability to accumulate As and Hg in the high-temperature oxidation residue than in the thermal-washing residue. However, Cr and Ni uptake was very weak in the mung beans that were cultivated in the high-temperature oxidation residue. Cd exerts strong genotoxic effects at low concentrations [29] and can therefore partially or fully inhibit vital enzyme activities, such as those that are responsible for ATP production [40]. Therefore, the high-temperature oxidation residue induced the highest levels of plant membrane damage and aging. Pb is highly persistent in soil, and although Pb is less toxic than Cd, it can alter biodiversity and disrupt ecological functions [41]. Moreover, Pb ions can be biomagnified through the food chain and can cause severe neurotoxicity in humans when ingested [42]. Interestingly, the mung beans exhibited the highest Pb uptake rates among all of the heavy metals.
The hazards of heat treatment residues on the environment and plants not only depend on their heavy metal content but also on the toxicity of organic compounds and their interactions with the heavy metals in the residue. The thermal-washing residue was found to contain n-hexadecane, which had an inhibitory effect on Cd toxicity [20]. Therefore, the Cd content was the highest when the plants were exposed to the thermal-washing residue, which also explained the low toxicity of this residue treatment.

4. Conclusions

The residues that are produced by different oily sludge treatment methods showed different levels of ecotoxicity due to their different organic matter and heavy metal contents. The accumulation level of pollutants in the oily sludge of heat treatment residues was not high, but the three residues showed different degrees of phytotoxicity. Our findings also indicated that heavy metals and petroleum hydrocarbons might inhibit plant growth. The adverse effects of residues, after having undergone heat treatment, on the environment and plants are not only reflected in the heavy metal content but also depend on the toxic effect of the organic compounds and heavy metals in the residues. The Cd content in the hot-washing residue was the highest after planting. However, the toxicity of this residue was lower than that of the pyrolysis residue and the high-temperature thermal oxidation residue. This may be due to the presence of n-hexadecane in the heat-washing residue, which has an inhibitory effect on Cd toxicity. Our findings demonstrated that the heat treatment residue from oily sludge affects plant growth. Therefore, the mixing ratio of heat-treated residues in soil must be strictly controlled when carrying out the large-scale greening of heat-treated residues to ensure normal plant development. An effective way to control heat-treated residue pollution in soil is to use suitable plants to degrade and remove the petroleum hydrocarbons in the soil to successfully restore soil quality. Additionally, the residue produced after pyrolysis treatment followed by that produced by high-temperature thermal oxidation treatment can be applied in large quantities after establishing a suitable blending ratio.

Author Contributions

Formal analysis, Y.H. and P.Y.; investigation, M.X. and X.Z.; resources, C.Q.; data curation, X.S. and T.Y.; writing—original draft preparation, X.S. and T.Y.; writing—review and editing, B.Y. and J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This study was partially supported by the open project of the State Key Laboratory of Petroleum and Petrochemical Pollutant Control and Treatment “Study on the Mechanism of Supercritical Hydrothermal Catalytic Oxidation of Oily Sludge” (subject number PPC2019001) and the Open Project of Shaanxi Key Laboratory of Oil and Gas Field Environmental Pollution Control Technology and Reservoir Protection “Study on Ecological Safety Evaluation of Oily Sludge Residue after Treatment”.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data were presented in the main manuscript.

Conflicts of Interest

All authors have no conflict of interest to declare. The views expressed in the present article are solely those of the authors.

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Applsci 12 03609 g001 550
Figure 1. Preparation process and phytoremediation of residues using different heat treatment methods.
Figure 1. Preparation process and phytoremediation of residues using different heat treatment methods.
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Figure 2. Heavy metal contents in the three types of heat treatment residues (HTTOR: high-temperature oxidation residue; PR: pyrolytic residue; HWR: thermal-washing residue).
Figure 2. Heavy metal contents in the three types of heat treatment residues (HTTOR: high-temperature oxidation residue; PR: pyrolytic residue; HWR: thermal-washing residue).
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Figure 3. The 14-day germination percentage of mung beans in (a) pyrolytic residue; (b) thermal-washing residue; (c) high-temperature oxidation residue.
Figure 3. The 14-day germination percentage of mung beans in (a) pyrolytic residue; (b) thermal-washing residue; (c) high-temperature oxidation residue.
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Figure 4. Effect of heat treatment residues on the enzymatic activity of mung beans. (a) the content of malondialdehyde; (b) the content of superoxide dismutase; (c) the content of peroxide dismutase.
Figure 4. Effect of heat treatment residues on the enzymatic activity of mung beans. (a) the content of malondialdehyde; (b) the content of superoxide dismutase; (c) the content of peroxide dismutase.
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Figure 5. Proportions of heavy metals in the soil before and after planting: (a) high-temperature oxidation residue, (b) pyrolytic residue, and (c) thermal-washing residue.
Figure 5. Proportions of heavy metals in the soil before and after planting: (a) high-temperature oxidation residue, (b) pyrolytic residue, and (c) thermal-washing residue.
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Table
Table 1. Preparation conditions for residues prepared by different heat treatment methods.
Table 1. Preparation conditions for residues prepared by different heat treatment methods.
Sampling VolumeTemperature (°C)Rotating SpeedMedicament
Pyrolysis500 g50050 r/min__
Thermal wash200 g70__Tween80(a surfactant); emulsion breaker
High-temperature oxidation500 g75030 r/s__
Table
Table 2. Physicochemical properties of raw sludge and prepared residues.
Table 2. Physicochemical properties of raw sludge and prepared residues.
Oil Content (%)Water Content (%)Solid Content (%)
Raw sludge17.8618.1364.01
Pyrolytic residue4.0116.3879.61
Thermal-washing residue13.2330.8955.88
High-temperature oxidation residue0.500.5198.99
Table
Table 3. Mass ratios of residues and soil nutrients.
Table 3. Mass ratios of residues and soil nutrients.
Content0%10%30%50%70%90%100%
Residue (g)050150250350450500
Soil nutrients (g)500450350250150500
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Sun, X.; Yu, T.; Huang, Y.; Xue, M.; Qu, C.; Yang, P.; Zhang, X.; Yang, B.; Li, J. Effects of Different Heat Treatment Methods on Organic Pollutants and Heavy Metal Content in Oil Sludge Waste and Ecotoxicological Evaluation. Appl. Sci. 2022, 12, 3609. https://doi.org/10.3390/app12073609

AMA Style

Sun X, Yu T, Huang Y, Xue M, Qu C, Yang P, Zhang X, Yang B, Li J. Effects of Different Heat Treatment Methods on Organic Pollutants and Heavy Metal Content in Oil Sludge Waste and Ecotoxicological Evaluation. Applied Sciences. 2022; 12(7):3609. https://doi.org/10.3390/app12073609

Chicago/Turabian Style

Sun, Xuan, Tao Yu, Yi Huang, Ming Xue, Chengtun Qu, Penghui Yang, Xiaofei Zhang, Bo Yang, and Jinling Li. 2022. "Effects of Different Heat Treatment Methods on Organic Pollutants and Heavy Metal Content in Oil Sludge Waste and Ecotoxicological Evaluation" Applied Sciences 12, no. 7: 3609. https://doi.org/10.3390/app12073609

APA Style

Sun, X., Yu, T., Huang, Y., Xue, M., Qu, C., Yang, P., Zhang, X., Yang, B., & Li, J. (2022). Effects of Different Heat Treatment Methods on Organic Pollutants and Heavy Metal Content in Oil Sludge Waste and Ecotoxicological Evaluation. Applied Sciences, 12(7), 3609. https://doi.org/10.3390/app12073609

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