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Article

Oxidation Enhancement of Gaseous Elemental Mercury Using Waste Steel Slag under Various Experimental Conditions

by
Joo Chan Lee
1,2,
Se-Won Park
1,
Hyun Sub Kim
2,
Tanvir Alam
1,3,* and
Sang Yeop Lee
1,*
1
Department of Environmental Engineering, Yonsei University, Wonju 26493, Gangwon-do, Republic of Korea
2
Wonju Regional Environment Office, Ministry of Environment, Wonju 26461, Gangwon-do, Republic of Korea
3
Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia
*
Authors to whom correspondence should be addressed.
Sustainability 2023, 15(2), 1406; https://doi.org/10.3390/su15021406
Submission received: 22 September 2022 / Revised: 27 December 2022 / Accepted: 9 January 2023 / Published: 11 January 2023
(This article belongs to the Special Issue Sustainable and Clean Chemical Engineering Technologies)
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Abstract

:
In this study, the oxidation characteristics of elemental mercury were assessed based on the input gas environment, temperature, and particle size distribution of the steel slag. Experiments were performed at room temperature, 100 °C and 200 °C, under air and simulated gas environments. The oxidation reaction of elemental mercury was conducted using steel slag samples of 1 mm, 2.36 mm, and 4.75 mm at various conditions. From the basic characteristic analysis of the steel slag, it was found that the steel slag exhibits a similar composition to that of fly ash, and it can be utilized as an oxidizing agent. Results show that regardless of the temperature and the particle size distribution of steel slag, the oxidation reaction of elemental mercury rarely occurred in the air environment. However, in the case of the HCl gas environment, it was observed that the smaller the steel slag particle size, the stronger the oxidation reaction. It is believed that the oxidation efficiency of the steel slag increased as the contact area between the gas and particles increased. The oxidation reactivity was nearly two times higher in the temperature range of 100 °C to 200 °C than it was at room temperature. It is advised that further research be undertaken in order to determine the precise temperature range at which the oxidation reaction occurs.

1. Introduction

Only a small amount of mercury compounds exist in nature, however, regular exposure to it can have a negative influence on human health [1]. Particularly, it has recently been recognized as an intensive management subject due to the risks and severity of anthropogenic emissions of mercury compounds from industry [2]. Accordingly, the Minamata Convention came into effect in August 2017 and comprehensive measures for mercury management in major fields such as monitoring, emission source management, product management, waste treatment, exposure reduction, and technology development were established for the implementation of the agreement [3]. Annex D of the Minamata Convention lists anthropogenic mercury emissions sources in the atmosphere and is shown in Table 1 [3]. Anthropogenic mercury emission facility means that mercury emits from the process, although mercury was not used in the raw material. Among the facilities falling under Annex D, the main emission facilities are coal-fired power plants, primary smelting facilities for non-ferrous metals, cement clinker production facilities, and waste incineration facilities [4].
In the case of mercury compounds emitted from anthropogenic emission sources, there is elemental mercury (Hg0) and oxidized mercury (Hg2+) in the atmosphere. Mercury oxide shows a short residence time of less than several days, while elemental mercury has low solubility. It has relatively low reactivity and shows a long residence time; more than a year. Due to the various chemical characteristics of mercury compounds, it is important to understand the mercury emission characteristics for each emission source using the Ontario Hydro Method (OHM) and Continuous Emission Monitor (CEM), which can analyze the types of mercury compounds in exhaust gas. In general, the chemical varieties of mercury emitted from anthropogenic sources are greatly affected by the composition of the raw materials and the air pollution control devices used in the facilities. The various air pollution control devices used in different facilities for anthropogenic emission sources in South Korea are shown in Table 2. The bituminous coal-fired power plant consists of three types of air pollution control device facilities, and the control efficiency of mercury was increased by the flue gas desulfurization (FGD) installed to control SOx in the air pollution control device facility configuration. The anthracite coal-fired power plant has one type of air pollution control device facility and consists of selective catalytic reduction (SCR) for controlling NOx, electrostatic precipitator (ESP) for controlling dust, and flue gas desulfurization (FDG) for controlling SOx. The heavy oil power plant has two types of air pollution control device facilities. The higher the number of air pollution control device facilities, the higher the control efficiency of mercury. Non-ferrous metal smelting facilities (zinc, copper, lead) have a wide variety of air pollution control device facilities, and the mercury control efficiency is as high as 99.99%. Cement clinker production facilities and steel smelting facilities have simpler air pollution control devices than other facilities. Therefore, the control efficiency of mercury is relatively low compared with other facilities [5].
Mercury compounds are classified into particulate mercury (Hgp) and gaseous mercury compounds, which exist in the form of elemental mercury (Hg0) and mercury oxide (Hg2+) [6]. Since mercury oxide (Hg2+) is water-soluble it can be removed by existing air pollution control device facilities, but elemental mercury (Hg0) has only about a 50% removal rate when an adsorption facility is not installed. It is commonly known that elemental mercury (Hg0) does not bind with other substances; it emits into the atmosphere [7]. On the other hand, mercury oxide (Hg2+) is partially adsorbed to fly ash in air pollution control device facilities, or mostly captured by alkaline substances used as a de-sulfurizer [7,8].
Various factors affect the oxidation of mercury, such as the properties and types of the fuel, combustion conditions, exhaust gas temperature and composition, fly ash components, characteristics of particle control facilities, and desulfurization facilities. In particular, studies have been conducted to understand the oxidation and adsorption characteristics of mercury emitted from anthropogenic facilities using fly ash [9,10]. Serre and Silcox [11] tested the effect of coal fly ash by injecting mercury into nitrogen background gas as the initial stage of the study. As a result, the adsorption amount of elemental mercury increased with the unburned carbon content of the fly ash. However, it was not linearly proportional. Dunham et al. [12] evaluated the adsorption characteristics of elemental mercury (Hg0) and mercury chloride (HgCl2) in the reactor by using 16 types of coal fly ash and found that the content of iron oxide and unburned carbon in the fly ash played a major role in the oxidation and adsorption of mercury. However, the research to date has been mainly limited to fly ash. Thus, in this study, air pollution control device facilities were inadequately installed. Moreover, the research was conducted using steel slag in facilities similar to fly ash components where industrial byproducts are steadily increasing.
Steel production facilities are one of the major anthropogenic mercury emission facilities [13,14]. As shown in Figure 1, the global steel production capacity has increased steadily every year to approximately 1.87 billion tons in 2019. As a result, industrial by-products, such as steel slag, have also increased since 2013 [15]. Korean steel slag is also showing an increasing trend, out of which more than 99 wt.% is currently being recycled. However, it has been used in relatively low value-added products, except for the use of cement raw materials and fertilizers, as shown in Figure 2. Moreover, the demand for concrete admixture and siliceous fertilizer materials used as cement and fertilizer materials are currently limited. Therefore, it is necessary to develop an alternative source to utilize steel slag for various purposes. Thus, in this study, a basic characteristics analysis on steel slag was conducted to assess its feasibility as a mercury oxidizer. Furthermore, the oxidation characteristics of elemental mercury were assessed based on the input gas environment, temperature, and particle size distribution of steel slag to increase the control efficiency of mercury emitted into the atmosphere from steel facilities and cement clinker production facilities.

2. Materials and Methods

2.1. Samples Selection

A particle size separation experimental device (Sieve Shaker JI-403, Jeil Precision Co. Ltd., Seoul, Korea) consisting of six stages was constructed in order to conduct oxidation reactions of elemental mercury with each steel slag particle size. As shown in Figure 3, the size of the steel slag was separated with a sieving device with sieve sizes of 8.65 mm, 4.75 mm, 2.36 mm, 1 mm, 850 µm, and 150 µm. The Standard Specification for Woven Wire Test Sieve Cloth and Test Sieves Method (ASTM E11-13) was used for this particular study.
Steel slag larger than >8.65 mm sieve size represented 25.70 wt.% of the weight fraction, and sieve size ranging between 8.65–1 mm represented 62.4 wt.% of the weight fraction. Table 3 denotes the steel slag distribution.

2.2. Experimental Device

The experimental device shown in Figure 4 was constructed in order to find the oxidation characteristics of elemental mercury using steel slag. The experimental device can be divided into three categories, namely the simulated gas injection unit, the fixed bed reaction unit, and the mercury analysis unit. The elemental mercury gas of 100 µg/m3 was made from the elemental mercury gas generator (Hg0, Permeation device) in the injection unit. The details of experimental conditions are summarized in Table 4. The oxidation reaction occurs between 100 °C and 200 °C, and this is the main reason behind selecting the temperature, as at this temperature range mercury is oxidized by HCl. This is why the temperature was set to 100 °C, 200 °C, and 25 °C (denoting the room temperature). In the case of gas conditions, HCl gas (0.04%) and N2 basis simulation gas (CO2 30%, SO2 0.1%, NO 0.01%) were injected to simulate the composition ratio of combustion gas at room temperature, 100 °C, and 200 °C for 1 L/min gas (permeation device: 0.8 mL/min, HCl gas: 0.1 mL/min, mixed gas: 0.1 mL/min). In the case of the air condition, gas was injected at room temperature, 100 °C and 200 °C at 1 L/min (permeation device: 0.8 mL/min, air-gas: 0.2 mL/min). Moreover, a mass flow controller (MFC) was used to precisely control the flow rate of gas from each cylinder. In the fixed bed reaction unit, 5 g of each steel slag, separated by particle size, was placed in the reactor and the experiment was conducted at room temperature at 100 °C and 200 °C. The mercury analysis unit consists of eight impingers. Impinger 1, 2, 3 hold 1N KCl and impinger 4 holds 5 wt.% HNO3/10 wt.% H2O2. 10 wt.% H2SO4/4 wt.% KMnO4 was placed in impinger 5, 6, 7. Silica gel was placed on the last impinge so that the moisture in the absorption solution did not flow out to the low gas pump (LGP). In addition, a dry gas meter (DGM) was placed behind the mercury analysis unit to check the flow rate of gas injected from the injection unit. Impingers 1 to 3 captured mercury oxide and impingers 4 to 7 captured elemental mercury. Mercury concentration was measured according to the US EPA Ontario-Hydro Method (OHM) and EPA Method 7471A [18].

2.3. Analysis Method

Following the recovery method specified in the US EPA OHM and ASTM D 6784, the recovery of the absorbent liquid was performed by collecting the sample from the exhaust gas. For the mercury compound analysis, several methods, such as atomic absorptional spectrometry (AAS), atomic fluorescence spectrometry (AFS), or inductively coupled plasma atomic emission spectrometry (ICP-AES) were available [19]. However, to analyze the concentration of mercury compounds, Lumax Ra 915+ of the cold vapor atomic absorption (CVAA) method was used in this study after collecting and recovering the sample [20]. The minimum sensitivity was 0.5 ng/L. Figure 5 shows the oxidation and reduction treatment methods for mercury oxide (KCl absorbent), elemental mercury (H2O2, H2SO4 absorbent), and the pretreatment process before analyzing the samples.

3. Results and Discussion

3.1. Characteristic Analysis Result in Steel Slag

In this study, the steel slag samples were analyzed using X-ray fluorescence (XRF) spectrometry. Table 5 depicts the XRF analysis results of steel slag and fly ash. The results showed that the key components of steel slag were Fe2O3, CaO, MgO, and SiO2, and the key components of fly ash were SiO2, Al2O3, Fe2O3, and CaO. This indicates that the key components of steel slag and fly ash are the same, however, the concentrations are different.
Ghorishi et al. [21] conducted a study to investigate the effect of fly ash on elemental mercury oxidation efficiency using a fixed bed reactor. Synthetic fly ash samples were prepared by mixing components such as aluminum oxide (Al2O3), silicon dioxide (SiO2), iron oxide (Fe2O3), copper oxide (CuO), and calcium oxide (CaO). Results showed that among the components, oxides of transition elements such as Fe2O3 and CuO showed a very high oxidizing power of elemental mercury, while Al2O3, SiO2, and CaO hardly oxidized elemental mercury [21].
Bhardwaj et al. [22] investigated the reaction characteristics of fly ash components such as SiO2, Al2O3, MgO, CaO, TiO2, Fe2O3, and micro-carbonate powder on elemental mercury oxidation rate. The variety analysis of mercury was also conducted. Results depicted that Al2O3, SiO2, CaO, MgO, and TiO2 showed very low oxidation and adsorption efficiency of elemental mercury. Fe2O3 and micro-carbon powder showed high efficiency for the oxidation of elemental mercury [22]. Among the key components of steel slag, transition elements oxide such as Fe2O3 and CuO were expected to have very high elemental mercury oxidizing power. Thus, in this study, steel slag was selected as an elemental mercury oxidizer.

3.2. Elemental Mercury Oxidation Reaction of Steel Slag

In order to determine the oxidation reaction of elemental mercury in steel slag, the experiment was conducted by changing the experimental conditions for each input gas, temperature, and particle size distribution. The input gas condition was either air or simulated environment. The experimental temperatures were at room temperature, 100 °C and 200 °C, respectively. The particle size of the sample was 1 mm, 2.36 mm, and 4.75 mm, respectively. The results of the steel slag experiment for the particle size of 1 mm, 2.36 mm, and 4.75 mm under air conditions at room temperature, 100 °C, and 200 °C are shown in Figure 6. Although there was a slight difference in each temperature condition, the concentration of elemental mercury remained 80 wt% to 90 wt% in each case, and only 10 wt% to 20 wt% of the mercury was oxidized. Figure 7 shows the result of the simulated gas environment instead of air under the same sample and temperature conditions. At room temperature, elemental mercury and mercury oxide concentrations were between 50–60 wt%. At 100 °C and 200 °C, elemental mercury concentration was between 15–25 wt%, and mercury oxide concentration was 75–85 wt%, respectively. From the above experimental results, it is possible to determine the oxidation reactivity of the steel slag with respect to the input gas. The oxidation reaction hardly activated regardless of the temperature and particle size of the sample under air conditions. Unlike the air condition, in the case of the HCl gas and mixed gas conditions, it was found that the oxidation reactivity and the elemental mercury removal rate were high. In general, HCl gas is known for its good oxidizing power [23] and from this study it was found that HCl gas has a significant effect on mercury oxidation.
However, according to the literature, the observed mercury oxidation rates were different in our study. Li et al. [24] reported an oxidation efficiency of more than 90 wt% at 10 ppm HCl, but Zhuang et al. reported an oxidation efficiency of 45 wt% to 70 wt% at 50 ppm HCI. Hong et al. [25] reported an oxidation efficiency of 20% at 10 ppm HCl. This happened due to the difference in experimental conditions between the studies. In order to predict the mercury oxidation efficiency of the denitrification catalyst in the exhaust gas of the actual process, it is thought that an experiment under the same conditions as the actual exhaust gas process is necessary. The simulation gas of N2 basis and HCl gas (0.04%) was injected to simulate the composition ratio of combustion gas of the actual process in this study. Although a difference existed in each condition, the oxidation efficiency varied between 70 wt% and 90 wt%. In the case of the oxidation reaction under different temperature conditions, the oxidation reactivity in the temperature range of 100 °C to 200 °C was almost two times greater than that of room temperature. Yang et al. [26] investigated the efficiency of elemental mercury removal using a new regenerable magnetic catalyst based on cobalt oxide-loaded magnetospheres from fly ash (Co−MF) at various temperature conditions. Results showed that the highest mercury removal efficiency was observed in the temperature range of 100 °C to 150 °C. However, oxidation efficiency decreased as the temperature increased from 150 °C to 300 °C. This may occur due to the desorption of mercury adsorbed on the fly ash surface through temperature-programmed desorption (TPD) experiments [26]. According to the results of this study, although there was some variation in the oxidation reaction in the 100~200 °C, it is believed that the injection of gas in the temperature range of 100~350 °C in the exhaust gas process would be more advantageous for the steel slag oxidation reaction. In the case of oxidation reaction based on particle size distribution, it was found that the smaller the steel slag particle size, the greater the oxidation reactivity, and the larger the non-surface area, the more oxidation reactions generated. This is expected to increase efficiency as the area of contact between gases and particles increases.

4. Conclusions

This study evaluated the oxidation characteristics of elemental mercury depending on the input gas environment, temperature, and particle size distribution of steel slag. From the basic characteristic analysis of the steel slag, it was found that steel slag exhibits a similar composition to that of fly ash. Thus, it can be utilized as an oxidizing agent. Therefore, steel slag waste recycled from steel facilities and cement clinker production facilities was used in this study to enhance the oxidation efficiency of elemental mercury. The key conclusions that came from this study are as follows.
In the oxidation reaction of elemental mercury using steel slag, the oxidation reaction rarely occurred in the air condition regardless of the temperature or particle size distribution of steel slag. However, in the case of the HCl gas environment, the smaller the particle size, the larger the oxidation reaction generated. It is believed that the efficiency increased as the contact area between the gas and particles increased. In addition, the oxidation reactivity in the temperature range of 100 °C to 200 °C was almost two times greater than that of room temperature. There were some variations observed in the oxidation reaction in the temperature range of 100~200 °C, therefore, it is suggested that additional experiments are conducted to find the exact temperature section in which the oxidation reaction takes place. To commercialize the process, it is necessary to find the difference in oxidation reactivity of the mixed gas. Thus, it is suggested that further experiments should be conducted to understand the difference in the oxidation reactivity of the HCl gas and the mixed gas by making a difference in the flow rate for each component of the mixed gas.

Author Contributions

Conceptualization, J.C.L.; methodology, J.C.L. and S.-W.P.; validation, J.C.L. and H.S.K.; investigation, J.C.L., S.-W.P., H.S.K., and S.Y.L.; resources, S.Y.L.; data curation, J.C.L.; writing—original draft preparation, J.C.L. and T.A; writing—review and editing, J.C.L. and T.A.; supervision, T.A. and S.Y.L.; project administration, S.Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Steel production and recycling rates [16].
Figure 1. Steel production and recycling rates [16].
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Figure 2. Rates of utilizing steel slag in South Korea [17].
Figure 2. Rates of utilizing steel slag in South Korea [17].
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Figure 3. The process of separating of steel slag.
Figure 3. The process of separating of steel slag.
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Figure 4. Schematic diagram of the experimental device.
Figure 4. Schematic diagram of the experimental device.
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Figure 5. Process of Mercury Sampling and Preparation (drawn based on the EPA 7471A Method [18]).
Figure 5. Process of Mercury Sampling and Preparation (drawn based on the EPA 7471A Method [18]).
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Figure 6. Elemental mercury oxidation reaction at different temperatures using air environment for different sizes of steel slag.
Figure 6. Elemental mercury oxidation reaction at different temperatures using air environment for different sizes of steel slag.
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Figure 7. Elemental mercury oxidation reaction at different temperatures using the simulated gas environment for different sizes of steel slag.
Figure 7. Elemental mercury oxidation reaction at different temperatures using the simulated gas environment for different sizes of steel slag.
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Table
Table 1. Anthropogenic mercury emission source.
Table 1. Anthropogenic mercury emission source.
Large CategoryMiddle CategoryEmission Source
Anthropogenic use and emission facilitiesFuel and energy-related facilitiesCoal and combustion facilities, oil extraction, refining facilities, other fossil fuel extraction facilities, utilization facilities, biomass power generation, and heat power generation facilities, geothermal power plant
Primary metal production facilitiesMercury extraction, processing facility, gold silver extraction, processing facilities, zinc, copper, lead, aluminum, steel extraction, processing facilities
Metal recycling facilityMercury recovery, iron recovery, and other metal recovery facilities
Other pollution and material production facilitiesCement production facilities, pulp paper production facilities, lime production facilities, carbon black production facilities, coke production facilities
Crematorium and graveyardCrematorium and graveyard
Other anthropogenic emissionsMobile pollutant
Waste facilitiesWaste incineration facilitiesHousehold wastes incineration facility, designated waste, incineration facility, hazardous waste incineration facility, sewage incineration facility, and other waste incineration facilities
Waste landfill, wastewater facilitiesManagement type landfill facility, industrial waste dumping, general waste dumping, wastewater facilities
Table
Table 2. Details of air pollution control devices used in different facilities.
Table 2. Details of air pollution control devices used in different facilities.
FacilitiesAir Pollution Control DeviceControl Efficiency (%)
Coal-fired power plantBituminousSCR-ESP-FGD-Stack77.12
ESP-Stack68.57
ESP-FGD-Stack83.51
AnthraciteSCR-ESP-FGD-Stack73.17
Oil-fired power plantESP-Stack-
SCR-ESP-FGD-stack-
Non-ferrous metal smeltingZincESP-Venturi Scrubber-ESP-Boliden Norzink-Dry tower-SO2 1st adsorption tower-SO2 2nd adsorption tower-Scrubber-Stack99.99
CupperWet scrubber-ESP-dry tower-SO2 1st adsorption tower-SO2 2nd adsorption tower-FGD-Stack99.99
LeadESP-Venturi Scrubber-ESP-Boliden Norzink-Dry tower-SO2 1st adsorption tower-SO2 2nd adsorption tower-Scrubber-Stack99.99
Cement productionSpray tower-BF-Stack
Iron manufacturing plantSintering furnaceESP-Stack22.42
ESP-BF-SCR-Stack-
Electric furnaceBF-Stack-
Waste incinerationMunicipal solid wasteSDR-SCR-SNCR-BF-Stack98.14
SDA-ACI-BF-SCR-Stack92.92
ESP-WS-ACI-BF-SCR-Stack71.01
SDA-ACI-BF-Stack86.95
SDA-BF-SCR-Stack93.95
SDA-BF-Stack68.56
Industrial solid wasteCy-BF-Stack41.79
Cy-BF-WS-Stack60.52
SNCR-SDR-BF-WS-Stack43.41
Hospital wasteSDR-BF-WS-Stack-
Sewage sludgeBF-SDR-WS-Stack-
Table
Table 3. Distribution of steel slag samples.
Table 3. Distribution of steel slag samples.
Sieve RangeWeight (g)Weight Fraction (wt.%)
>8.65 mm77.125.70
4.75–8.65 mm75.9125.30
2.36–4.75 mm58.2519.42
1–2.36 mm53.0517.68
850 µm–1 mm11.283.76
150–850 µm12.94.30
<150 µm11.513.84
Total300100
Table
Table 4. Experimental conditions.
Table 4. Experimental conditions.
ParameterValue
Size of Steel Slag (mm)4.75, 2.36, 1
Temperature (°C)25, 100, 200
Gas
(mL/min)		N2 basisPermeation Device: 0.8
HCl: 0.1
Mixed gas: 0.1
AirPermeation Device: 0.8
Air: 0.2
※ Mixed Gas Composition: CO2 30%, SO2 0.1%, NO 0.01%
Table
Table 5. XRF analysis results of steel slag and fly ash.
Table 5. XRF analysis results of steel slag and fly ash.
Steel Slag Composition and DistributionFly Ash Composition and Distribution
CompoundsDistribution (wt.%)CompoundsDistribution (wt.%)
Fe2O336.4SiO244.6
CaO31.9Al2O322.4
MgO11.1Fe2O38.1
SiO29.5CaO7.3
MnO4.0MgO1.3
Al2O33.5TiO21.4
P2O51.3CuO0.0
K2O, TiO2, SO3, V2O5, Cr2O3, Cl<1--
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Lee, J.C.; Park, S.-W.; Kim, H.S.; Alam, T.; Lee, S.Y. Oxidation Enhancement of Gaseous Elemental Mercury Using Waste Steel Slag under Various Experimental Conditions. Sustainability 2023, 15, 1406. https://doi.org/10.3390/su15021406

AMA Style

Lee JC, Park S-W, Kim HS, Alam T, Lee SY. Oxidation Enhancement of Gaseous Elemental Mercury Using Waste Steel Slag under Various Experimental Conditions. Sustainability. 2023; 15(2):1406. https://doi.org/10.3390/su15021406

Chicago/Turabian Style

Lee, Joo Chan, Se-Won Park, Hyun Sub Kim, Tanvir Alam, and Sang Yeop Lee. 2023. "Oxidation Enhancement of Gaseous Elemental Mercury Using Waste Steel Slag under Various Experimental Conditions" Sustainability 15, no. 2: 1406. https://doi.org/10.3390/su15021406

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