Assessment of Remediation Efficiency for Soils Contaminated with Metallic Mercury in Hydrocarbon Extraction Zones

Abstract

Reducing mercury emissions to individual environmental compartments is now a global priority. However, undefined industrial sectors still pose a risk for mercury pollution, including the extraction, processing, and transport of crude oil and natural gas. Mercury contamination in hydrocarbon extraction areas can occur around blocking and bleeding systems, gas pressure reduction and metering points, gas purification devices, and reservoir water separators. The soil mercury content depends on the quality of the extracted fuel and can vary widely. This article reviews methods for remediating mercury-contaminated soils, including washing, acid washing, thermal desorption, removal and disposal, and soil stabilization to convert mercury into less harmful forms. The main objective of the work was to present the results of a pilot process of soil remediation contaminated with metallic mercury conducted in an industrial area. This paper presented laboratory and field test results evaluating the efficiency of a pilot soil remediation method at an industrial facility. Mercury contamination at the site was localized, primarily around blocking and bleeding systems, with soil mercury levels ranging from 1.6 mg/kg to 1116 mg/kg. In 80% of the samples, the mercury levels were 2–8.5 times above the acceptable industrial soil limits. Speciation studies indicated that over 50% of the samples contained mercury capable of emissions. The remediation method involved stabilizing the mercury in the soil by adding sulfur, forming stable mercury sulfide (cinnabar). The post-remediation measurements showed significant reductions in mercury emissions to the air, demonstrating the effectiveness of the mercury immobilization procedure.

1. Introduction

Mercury, due to its toxic properties, is a major environmental pollutant. Its occurrence and emissions levels are extensively documented by the Environmental Protection Agency (EPA) and various scientific publications. Global policies aimed at reducing mercury emissions have successfully decreased its prevalence. Review studies categorize industrial sectors into two groups: those where mercury is a by-product or results in “fugitive emissions” and those that use mercury in technological processes (Figure 1). The largest sources of global mercury emissions are gold mining (37%), fossil fuel combustion (25%), and cement production. Although mercury removal processes exist in natural gas and oil extraction facilities, information on emissions from these processes remains limited [1,2,3,4,5,6,7,8,9,10,11].

The available literature data indicate that the scale of mercury contamination in soils occurs in areas of chlor-alkali production facilities [1,6,7,12,13], cinnabar mines [14,15,16], mercury mines [17], gold and silver mines [1,4,5,6,7,12], and large, urban, agricultural, and forestry areas [1,2,4,5,12]. Example mercury concentrations in soils from areas of various industrial activities are summarized in

Table 1. Summary of exemplary results from studies on mercury in soil in areas related to industrial activities worldwide [1,2,4,5,6,7,12,14,15,16,17,18,19,20,21,22,23].

Type of Activity	Contaminated Area	Determined Soil Mercury Content [mg/kg]	Source of Data
Mercury mines	China, Spain, Slovenia, USA, Turkey	0.05–9000	[17]
Gold and silver mines	China, Venezuela, Mexico	0.05–1100	[1,4,5,6,7,17,19]
Cinnabar and sulfide mines	Spain, Italy, Portugal	1–1709	[4,14,15,16,17,19]
Chemical factory	Portugal, Romania, the Netherlands	0.074–1150	[4,12,17,18]
Discharge lamp production plant	Poland	0.03–39.25	[4]
Aerometer factory	Warsaw (Poland)	62–393	[2,4]
Coal power plant Siersza	Poland	0.02–0.18	[2,4]
Natural gas mines	Polish Lowland Region (Poland)	0.05–495	[21,22,23]

.

It should be noted that the typical mercury content in uncontaminated soils worldwide ranges from 0.008 to 1.11 mg/kg [1,21]. In Poland, typical mercury concentrations in soil range from 0.002 to 1.5 mg/kg [1,21]. However, as shown in Table 1, the mercury content in contaminated soils is highly variable, ranging from 0.05 to even 9000 mg/kg. The test results described in the literature most commonly refer to the total mercury content in the soil. Specific studies have also investigated areas contaminated with metallic mercury, with metallic mercury constituting up to 38–98% of the soil contamination [12,13,18,19].

Increased mercury levels in soils can also result from dust emissions from cement plants and other sources of dust. The cement significantly contributes to both European and global mercury emissions [4,5]. As mentioned earlier, industrial areas involved in hydrocarbon exploitation and extraction are seldom described as mercury-polluted zones. However, the 2013 Global Mercury Assessment: Sources, Emissions, Releases, and Environmental Transport by UNEP highlights the oil and gas industry as a significant contributor to mercury emissions into the air [6,7,8]. Another report, the IKIMP Mercury Knowledge Exchange [20], extensively discusses the harmful effects of mercury and the sources and extent of mercury in technologies for the extraction, processing, and use of fluid and gaseous fuels. Previous studies also indicate the potential for local mercury contamination in soils of industrial areas associated with hydrocarbon extraction [21,22,23]. Given the global presence of mercury contamination in soils, it is necessary to develop methods for the disposal of mercury and the remediation of soils contaminated with mercury. There are many ways to remediate mercury-contaminated soils [1,2,3,17,24,25,26,27,28,29,30,31,32,33,34,35]. These methods can be categorized based on whether they are applied directly at the contaminated site, known as in situ methods (methods applied directly in the area where the contamination occurred), or applied outside the contaminated area, known as ex situ methods. Another criterion for categorizing the remediation methods is the nature of the method used. Based on this criterion, we can distinguish between the engineering methods (including “traditional” methods, based on the removal and storage of contaminated matrix in a landfill or the use of appropriate barriers) and process methods (including physical, biological, and chemical stabilization or solidification processes and thermal treatment). To properly select the method for removing or neutralizing mercury in soil, it is essential first to characterize the properties of Hg and its compounds and to assess their interaction with the contaminated soils. The literature indicates that the neutralization of soil contamination with mercury (the treatment of waste from mercury-contaminated soil) can take the form of:

• Removing mercury (both elemental and bound) from the soil to an acceptable level;
• Reducing the mobility of mercury in the environment to prevent its emission into the atmosphere or migration into water;
• Landfilling at an appropriate site (hazardous waste landfill or non-hazardous waste landfill, depending on waste classification).

The methods used to remove mercury from soil can be divided into three groups: chemical, biological, and physical methods. A brief characterization of each method, along with the technologies involved, is summarized in

Table 2. Overview of methods used to remove mercury from the soil [2,3,17,19,24,25,26,28,37].

	Chemical Methods	Biological Methods	Physical Methods
The main technology	Mainly chemical methods: chemical stabilization, soil washing, oxidation, reduction, reduction dichlorination, solvent extraction, and others.	Mainly divided into plant remediation and microbial remediation. Phytoremediation includes phytostabilization, phytoextracion, phytotransphormation, phytovoltalization, and rhizofiltration methods.	Soil replacement, physical separation, soil vapor extraction, fixed/stabilized soil, vitrification, thermal desorption, electrokinetic remediation.
Main characteristic	This type of remediation involves methods in which chemical reagents, reactions, and principles are used to remove contaminants. These processes typically cause pollutants to degrade, which either removes or reduces the soil’s toxicity.	These methods reduce, remove, or immobilize harmful contaminants in the soil and purify it using biological agents. They involve the use of plants and their associated rhizospheric microorganisms to remove contaminants. Contaminants in soil are controlled by introducing specific microorganisms. Additionally, the activities of some lower animals can be utilized to enhance bioremediation.	This type of remediation requires the addition of chemical reagents to improve remediation efficiency. The main physical operations used are soil cleaning/washing, vapor pressure reduction, separation by heating, and establishing an electric field gradient.
Examples of used reagents or physical techniques	HCl, HNO3, H2SO4, H3PO4, NaCl, Na2S2O3, sulfide, phosphate	Hyperacumulators (plant), bacteria, earthworms.	High temperature, establishing an electric field gradient, reduction in the vapor pressure.
Advantages and disadvantages	Rapid and effective but depends on the type of soil, chemical, and metal.	Economical, eco-friendly, but time-consuming, limited to moderately contaminated sites.	Laborious and highly costly, but it can be applied to highly contaminated sites.

. It should be noted, however, that some of the remediation methods described in the literature are not commercialized, and many have been detailed in American literature and reports prepared for the European Commission [1,2,3,17,19,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46].

The selection of appropriate and effective methods for remediating soils contaminated with various forms of mercury depends on several factors. Key considerations include the efficiency of the process, costs, and the technical feasibility of implementing the method. Taking these factors into account, it is observed that available remediation methods are chosen based on the scale of contamination and the availability and location of the contaminated area. Additionally, the temporal perspective of the remediation process and its impact on the environment are also considered.

The data presented in Table 2 show that the effective methods for remediating mercury-contaminated soils vary. The results of projects described in the literature, undertaken to remediate various areas contaminated with mercury and other metals, indicate that remediation methods should be selected individually, depending on their advantages and disadvantages, as well as the type of site and soil to be remediated. For example, during the reclamation of agricultural soils, thermal desorption should be applied with caution because excessive thermal decomposition resulting from the use of high temperatures in the process of mercury removal from soil may lead to the degradation of organic matter in the soil, thus altering its structure and rendering it unsuitable for agricultural use. On the other hand, during soil washing and the application of chemical stabilization techniques, the possibility of secondary soil contamination from chemical reagents used in the process should be considered. It should also be noted that the chemical stabilization of mercury in soil only changes the form of the mercury present in the soil (usually to a less harmful one). Another aspect to consider when using chemical methods is the necessity of monitoring the effectiveness of the remediation process, and the reagents used should be added in such a way that the remediation process remains effective over time [19,24,25]. Information about the speciation of mercury in soil is crucial for determining the appropriate remediation method. For high concentrations of bioavailable mercury in the soil, methods such as soil washing or phytoextraction are required. In cases of low bioavailable mercury concentrations, the availability of the metal should first be increased to facilitate the use of these methods. For soils contaminated with elemental mercury, methods such as stabilization (S/S), vitrification, and immobilization are necessary. On the other hand, for soils contaminated with large amounts of biologically unavailable mercury, high-temperature mercury release methods (if technically feasible) seem to be the best option [2,17,19,24,25]. Effective mercury soil cleanup processes (acid washing, thermal desorption, or electrokinetic remediation) must be conducted in high-capacity facilities to be cost-effective, where large volumes of soil can be treated. The use of ex situ methods requires transporting contaminated soil, which further increases the costs. If soil contamination affects small areas, it is preferable to remediate on-site (in situ methods). However, this requires installing the necessary equipment (low-temperature desorption tunnels, electrodes for electrokinetic remediation, scrubbers, pumps, and absorbers) and is often economically unfeasible or even impossible due to industrial infrastructure or safety regulations.

The stabilization of mercury in soil, unlike the methods for neutralizing mercury discussed above, does not involve removing mercury from the soil but rather chemically or physically binding it. These measures reduce the mobility of mercury, thus decreasing its leaching to an acceptable level. The methods used to stabilize mercury in soils or waste can be categorized into three groups: methods that involve substances containing sulfur ligands, methods with reducing agents, and methods with the use of substances with absorbent properties. As a result, mercury in the second stage of oxidation is classified as a weak Lewis acid and readily combines with sulfur to form a solid compound, HgS (very poorly soluble in water and much less volatile than other mercury compounds, making it less harmful to the environment, health, and human life) [2,17,25,26,29]. Stabilization is typically used when the mercury concentrations in soil are lower than 260 mg/kg [17,28,37,39]. Sulfur, fly ash, Portland cement, dolomites, zeolites, and nanomaterials can all play a stabilizing role [2,25]. The process is carried out in industrial mixers, either at the place of contamination or elsewhere, usually dry, under atmospheric pressure, at an ambient temperature (20 °C), or an elevated temperature (max. up to 100 °C). The method can be modified by applying stabilizing chemicals in a suspension or solution directly to the soil. The costs are estimated at USD 40 to USD 2000 per ton, and the capacity of an industrial installation may be up to 40 tons/day. The method of mercury stabilization in soil was also studied in Poland within the framework of the aforementioned project financed by the US DOP. In the treated industrial area, mercury concentrations in the upper soil layer ranged from over 100 to over 3500 mg/kg. In deeper layers, these concentrations were significantly smaller, and to the depth of 110 cm, they exceeded, at most, 1.5 times the permissible concentration, and in even deeper layers, they were much lower than that. Stabilization by adding 0.5% granulated sulfur after six weeks caused the immobilization of 78% of the leached mercury fraction. The addition of 0.5% zeolite resulted in the binding of 49% of mercury at the same time.

The process of stabilizing mercury in the soil can be supported by the cultivation of plants that take the mercury from the soil through the root system and retain it in the roots—this process is called phytostabilization. Willow (Salix virminalis L.) has the best properties in this respect, and grasses, which, although retaining less mercury, can also be used with advantage because they form a root mat that stabilizes soil well (preventing erosion and reducing the penetration of rainwater). Another stabilization process consisted of the use of sewage sludge, which is characterized by a large sorption capacity. Laboratory experiments have shown that sewage sludge (although it sometimes causes a decrease in soil pH) reduces the solubility of the metals contained in soil, thus preventing their migration to soil and water [2,17,28,29].

Selecting an appropriate, effective, and economically justified remediation method for a given contaminated area (including the oil and gas industry) is crucial. Since mercury-contaminated soil areas are relatively small but located in regions with extensive underground and above-ground infrastructure, and contamination influx is periodic/continuous, choosing the right method, along with monitoring and an effective approach for cleaning mercury-emitting soil, presents a challenge. This article reviews various methods for remediating mercury-contaminated soils and presents laboratory and field test results evaluating the efficiency of a pilot soil remediation method at an industrial facility.

2. Materials and Methods

This chapter outlines the materials and methods employed in the study to address mercury contamination, detailing the approaches used for soil remediation, sampling, and analysis to ensure effective and accurate results.

2.1. Chemicals and Analytical Methods

The AMA-254 apparatus was used to measure the mercury content in soils and absorbed on gold-platinum sensors. The device operates within a temperature range of 0 to 35 °C and a maximum relative humidity of 80%. The carrier gas is oxygen at an input pressure of 200–250 kPa. The mercury measurements are taken at a wavelength of 253.65 nm. The instrument has two measurement ranges: Range I covers concentrations from 0.05 to 40 ng of Hg/sample, while Range II measures from 50 to 600 ng of Hg/sample. To ensure measurement quality, certified reference materials (CRM BCR^®-320R produced by IRMM and Mercury Standard Solution 1000 mg Hg/L (Certipur CRM Merck)) were used in atomic absorption spectroscopy (AAS) with the AMA-254. The operating conditions of the device during the mercury measurements in soil and absorbed on gold sensors are summarized in

Table 3. The operating conditions of the apparatus during mercury measurements in soil and mercury absorbed on gold sensors.

Matrix	Time of Drying [s]	Time of Mineralization [s]	Time of Waiting [s]
Soil	0	150	45
Gold element	0	200	45

[40].

Soil samples taken from hydrocarbon exploration and exploitation sites were analyzed in a laboratory to determine both the total mercury content in the soil and the mercury content that could be emitted. Measurements of the total mercury content in soil were carried out with the use of the mercury analyzer AMA-254, a method that utilizes atomic absorption. The measurements were conducted with an accredited method, as recommended by Polish law for determining the total mercury content in soil.

Measurements of the content of metallic mercury (Hg⁰), which can be emitted from the soil, were carried out using a proprietary method based on the correlation between mercury emissions to the air and the content of metallic mercury in soil.

This method involves measuring the mercury emissions from a given amount of soil under strictly defined and controlled conditions (temperature, sorption time, volume of the measuring vessel, and size of the gold element), as determined during the validation of the method. A summary of the key parameters for measuring emissions-capable mercury using the author’s method with a gold sensor in a flask is presented in

Table 4. A summary of the key parameters for measuring emissions-capable mercury using the author’s method with a gold sensor in a flask.

Parameter
Mass of sample (soil)	2 g * ± 0.0001 g
Temperature	13 °C ± 0.2 °C
Time of sorption	3 min ± 10 s
Size of gold element (sampler)	1 cm2

* Assuming that the soil moisture content ranges from 3–10% of water.

.

The assessment of the distribution of mercury contamination was also conducted in the field based on the results of the emissions measurements from the studied area.

Emissions measurements were performed using diffusion chambers and passive sensors of our own design, where the emitted mercury was sorbed onto gold elements. The mercury content was then measured in a laboratory using the mercury analyzer AMA-254.

The results of the mercury emissions were evaluated using a proprietary algorithm, which was based on the emissions levels determined for areas free of metallic mercury contamination and the values determined for areas contaminated with metallic mercury.

The emissions rate E (in ng/h/m²) was calculated by dividing the mass of absorbed mercury m (in ng) by the area of soil covered by the chamber (A) or passive sensor and the measurement duration (t—time in hours).

$$\mathrm{E}={\displaystyle \frac{\mathrm{m}}{\mathrm{A}\ast \mathrm{t}}}$$

where

E—determined the mercury emission rate, [ng/m²/h].

m—is the mass of the mercury adsorbed on the sensor during measurement, [ng].

A—is the area of soil covered by the diffusion chamber and or passive sampler (in the case of chambers during the present work, A = 0.1963 m²), [m²].

t—is the time of the conducted measurement of Hg emissions expressed in hours, [h].

An evaluation of the emissions rate (the obtained results of the mercury content sorbed on the gold sensor) was converted to an emissions rate value [ng Hg/h/m²] and compared with the experimentally determined limit value for the areas not contaminated with mercury. The determined limit value for the emissions rate of mercury for the areas not contaminated with metallic mercury is 100 ng Hg/h/m².

If the determined emissions value for mercury is greater than the limit value, the area under investigation should be considered as potentially contaminated with metallic mercury capable of being emitted.

The emission assessment should be carried out with reference to guidelines for the estimation of metallic mercury pollution. The assessment guidelines have been prepared by the authors on the basis of a multidirectional evaluation of the results obtained experimentally in the course of the research work. On the basis of the measurements carried out for the areas not contaminated by mercury for the methods used, an emissions limit value was determined for which the site can be considered not to be contaminated by metallic mercury.

This study showed that for the results obtained in the areas not contaminated by mercury, a limit value of 100 ng/h/m² can be established for the measurements carried out using the diffusion chamber and passive sensor methods.

2.2. Description of the Sampling and Measurement Procedures

To assess the level of mercury contamination of the soil at a selected industrial site, 34 soil samples were collected for four separate areas of the technological process:

• Area near the gas drilling installation—samples were taken at 2 points (2 samples from 0 to 0.3 m below the surface and 2 samples from 0.3 to 0.6 m below the surface depth);
• Area at the in-line shut-off and relief valve systems—samples were taken at 8 points (8 samples from 0 to 0.3 m below the surface, 8 samples from 0.3 to 0.6 m below the surface, and 6 samples from 0.6 to 1.2 m below the surface);
• Area near the deposit water reservoir—samples were taken at 2 points (2 samples from 0 to 0.3 m below the surface and 2 samples from 0.3 to 0.6 m below the surface);
• Remaining area of the object (industrial facilities)—12 sampling points (12 samples in total from 0 to 0.6 m below the surface).

To identify the mercury-contaminated areas and determine whether the contamination is due to metallic mercury, laboratory tests were conducted on the collected soil samples. The analysis was performed using two methods: one for measuring the total mercury content in the soil (Hg measurement via AAS) and another for assessing the emission-capable mercury content using a gold sensor in a flask. The averaged samples were tested in at least 3 replicates according to the laboratory’s current procedure. Outliers in the test series were statistically evaluated.

2.3. Selection and Developing a Method for Remediation of Soil Contaminated with Metallic Mercury in a Natural Gas Extraction Area

The remediation procedure, developed in the laboratory and tested on the pilot facility, involved mixing soil contaminated with metal mercury soil with sulfur to a depth of up to 1 m.

Information on the speciation of mercury in the soil is essential to establish an appropriate remediation method. For high concentrations of bioavailable mercury in soil, methods such as soil washing and phytoextraction are necessary. In soils with low concentrations of bioavailable mercury, increasing the metal’s availability should be prioritized to facilitate the use of these methods; for soils contaminated with elemental mercury, techniques such as stabilization (S/S), vitrification, and immobilization are required. Conversely, for soils contaminated with large amounts of mercury that are biologically unavailable, applying high-temperature mercury release methods (if technically feasible) seems to be the best solution [1,3,17,19,24,25]. Efficient soil remediation processes (acid washing, thermal desorption, or electrokinetic remediation) must be carried out in high-capacity installations capable of treating large volumes of soil to be cost-effective. The use of the ex situ methods requires transporting the contaminated soil, which further increases the costs. For soil contamination in small areas, on-site remediation (in situ methods) should be considered. However, this approach requires installing the necessary equipment (low-temperature desorption tunnels, electrodes used in electrokinetic remediation, scrubbers, pumps, and absorbers) and is economically unfeasible and, in some cases, impractical due to industrial infrastructure or safety regulations.

During the selection of the method for the neutralization of mercury in the soil in hydrocarbon mining and exploitation areas, the nature of the soil’s contamination with this element (metallic mercury contamination, local, in small areas) and economic considerations were taken into account. Based on these factors, the optimal method chosen for remediating soil contaminated with metallic mercury in the investigated industrial area was the in situ immobilization of mercury using sulfur as a stabilizing agent. This method is both simple and cost-effective, making it suitable for industrial-scale applications.

Powdered sulfur, which is readily available, inexpensive, or can be obtained as a by-product of the Clauss process of natural gas desulphurization, is used to stabilize metallic mercury in the soil. The analysis of the sulfur safety data sheet used in the tests showed that it does not contain impurities (heavy metals, mono- and bi-cyclic aromatic hydrocarbons (BTEX), or poly and polycyclic aromatic hydrocarbons (PAHs)), which are listed, along with the permissible concentrations, in the documents concerning the soil quality standards in Poland [47].

The amount and manner of sulfur application for stabilizing mercury contamination in the soil were determined based on the laboratory test results. These tests evaluated the effect of Hg stabilization in soil with the addition of the optimal amount of sulfur (5–10%) and estimated the time required to achieve the best reduction in mercury emissions from soil to air [30]. The remediation procedure was designed so that the sulfur content introduced would constitute 5–10% of the soil involved in the process. The contaminated soil was loosened and mechanically mixed with the added sulfur. Due to the existing underground infrastructure of the facility, the entire remediation process was carried out manually. It was also crucial that the soil masses from the entire work area were displaced and mixed thoroughly during the treatment.

To evaluate the effectiveness of the applied remediation procedure, the experimental area was divided into five sections, with measurement points established within each section to assess treatment efficiency. The effectiveness of the remediation was assessed from multiple angles, including:

• Analysis of the total mercury content and mercury content capable of emissions in soil samples;
• Measurements of mercury emissions to air using a passive sensor meter and the diffusion chamber method.

The studies on mercury emissions to the air were designed to track changes over time. To achieve this, three series of measurements were conducted: before the remediation procedure, 21 days after the procedure, and 90 days after the procedure.

3. Results

This chapter presents the findings from the mercury emissions measurements conducted in the remediated area. The data provide insights into the effectiveness of the remediation process and the extent of mercury reduction achieved.

Laboratory tests were performed on soil samples to identify mercury-contaminated sites and determine whether the contamination involved metallic mercury. These tests included a method for measuring the total mercury content in the soil and a proprietary method for assessing the emissions-capable mercury content using a flask sensor. The results of the total mercury content and emissions-capable mercury content in soil samples from the industrial site are summarized in

Table 5. Summary of mercury content results in soil samples collected from the industrial site where the remediation treatment was conducted.

Sampling Location		Results of Soil Sample Analysis from the Depth of 0.0–0.6 m bts		Field Measurements Emissions Levels
		Total Mercury Content in Soil [mg/kg Dry Matter]	Emission-Capable Mercury Content [mg/kg Dry Matter]
Potential Contamination Area	Area near the blocking and bleed systems	1.6–1200	<15–>>300	Emissions levels for the contaminated area
	Area around the mercury removal facility	0.6–3.61	<15	Emissions levels in mercury-free areas
	Area near the reservoir water separator	26	<15	Emissions levels in mercury-free zones
Pollution-Free Area	The remaining area of the industrial facility	0.6 ÷ 27	<15	Emissions levels in mercury-free zones

. Based on these results, areas contaminated with mercury were delineated.

The distribution of mercury content in soil samples from the analyzed industrial facility is shown in Figure 2. The analysis revealed a diverse distribution of mercury contamination in the studied area (Table 5). The site was characterized by two distinct zones: a larger area free of mercury contamination and a smaller area of approximately 100 m² with significant mercury contamination.

As previously mentioned, the soil samples were also analyzed to determine the emissions-capable mercury content. The comparison of the obtained research results and measurements is illustrated in Figure 3.

The results of the mercury emissions measurements for the remediated area are presented graphically in Figure 4 (measurements with passive sensors) and Figure 5 (measurements with diffusion chambers).

Additionally, studies were conducted on the soil samples from the area undergoing remediation to evaluate the potential migration of mercury in contaminated soil to deeper soil layers and groundwater, given its solubility. The analysis of water extracts from the soil samples assessed the content of soluble forms of mercury. Tests were performed on the soil samples with a total mercury content ranging from 70 to 800 mg/kg dry matter. The results indicated that in the soil samples contaminated with metallic mercury, the concentrations of soluble mercury ranged from 0.028 to 1.23 mg/kg dry matter. However, in the soil samples where mercury was stabilized with sulfur, the concentration of soluble mercury decreased to between 0.017 and 0.1 mg/kg dry matter. This stabilization with sulfur led to a tenfold reduction in the leaching of mercury from the soil.

4. Discussion

4.1. Pilot Industrial Facilities—Characteristics of the Contaminated Area

As shown in Table 3 and Figure 2, the results of the total mercury content and emissions-capable mercury content in the soil samples from the industrial site are diverse. The data presented in Table 3 were categorized based on their distribution, allowing for the identification of areas with clear metallic mercury contamination capable of emissions. The distribution of the mercury content in soil samples from the analyzed industrial facility is illustrated in Figure 2. The analysis revealed that the mercury contamination in the studied area is varied, as detailed in Table 3.

The designated contaminated area is located around the in-line shut-off and relief valve systems, where mercury contamination in the soil was significantly higher compared to the rest of the industrial site under investigation. In the soil samples from this area, the mercury content ranged from 1.6 mg/kg dry matter to 1120 mg/kg dry matter. It should be noted that the mercury content determined in 60% of the examined samples from this area was 2 to 8.5 times higher than the acceptable mercury content in soils for industrial areas, which is 30 mg/kg dry matter.

Samples from the remaining area of the analyzed hydrocarbon extraction facility showed a mercury content in the soil ranging from 0.6 to 27 mg/kg dry matter. However, 30% of the results for the samples taken from the locations directly adjacent to the designated area of the in-line shut-off and relief valve systems indicated a mercury content in the range of 45 to 100 mg/kg dry matter. The analysis revealed that this area is generally free from significant mercury with potential for emissions. Nevertheless, for 30% of the tested samples, the total mercury content was in the range of 30 to 100 mg/kg dry matter. (Figure 2).

Mercury contamination of the soil environment can also occur in industrial areas, such as former gasworks and drilling rigs, as well as in oil and gas mines, particularly in locations where raw materials requiring mercury removal are extracted [21,22,40]. In natural gas mines, mercury contamination has been found in the soil around dam and mouth units, reduction and measurement nodes, devices used to purify gas from mercury, and in the vicinity of reservoirs and reservoir water separators. In one such area, the mercury content in soil has reached several hundred mg/kg dry matter, while the permissible mercury concentration in soils for Class C areas is set at 30 mg/kg dry matter. [21,22,47,48]. Potential mercury contamination areas in Poland align with the data reported for potentially contaminated sites in Great Britain, Norway, and Malaysia [21,22,42,43].

4.2. Carrying Out a Mercury-Contaminated Soil Remediation Treatment on the Pilot Site

For the remediation of mercury-contaminated soil at the pilot facility, a 100 m² site was selected within an industrial area. This site was first evaluated for the mercury content in the soil. The contamination in this area was localized and primarily concentrated around the blocking and bleeding systems. Preliminary investigations indicated that this area was contaminated with metallic mercury. The total mercury content in over 60% of the soil samples from this area was found to be 2 to 8.5 times higher than the acceptable mercury content for soils in industrial areas. Additionally, studies were conducted to determine the mercury content capable of emissions in the soil samples. The comparison of the obtained research results and measurements is presented in Figure 3.

The analysis of the obtained measurement results revealed that soil samples from the area around the blocking and bleeding systems (contaminated area) contain significant amounts of mercury capable of emissions. In 57% of the examined samples, the content of emission-capable mercury was found to exceed 30 mg Hg/kg d.m. Mercury contamination was present in soil at depths ranging from 0 to 0.6 m below the ground. Field tests confirmed the presence of metallic mercury contamination. Measured mercury emissions to the air ranged from 21 to 2700 ng/h/m².

The soil samples taken after remediation showed a significant reduction in mercury emissions across the entire remediation area. The estimated content of mercury capable of emissions in the soil decreased from over 600 mg/kg dry matter to less than 15 mg/kg dry matter. These results indicate that the metallic mercury in the soil was effectively bound and converted into stable mercury sulfide(II) after the addition of sulfur. The results of laboratory tests confirm the efficiency of the applied procedure of mercury immobilization in soil [21,41,46].

At the pilot facility where the remediation procedure was carried out, field measurements of mercury emissions into the air were also carried out using passive sensors and diffusion chambers. The results from these tests, performed in three measurement series, provided a basis for assessing the efficiency of the remediation procedure. This assessment was based on the analysis of the emission measurement results collected from the designated measurement points established at the beginning of the experiment.

When evaluating the results of the measurements of mercury emissions into the air, it should be noted that during the remediation procedure, the entire soil contaminated with mercury was thoroughly mixed and displaced. For this reason, the results of tests and emissions measurements, despite the planned sampling scheme at the same locations, were difficult to compare directly. The results of the mercury emissions measurements to the air, carried out in the remediated area using passive sensors (Figure 4), indicated the following distribution before the remediation procedure:

• At six out of seventeen measurement points (18% of the results), emissions were in the range of 50 to 100 ng/h/m²;
• For 35% of the measurements, emissions ranged from 100 to 1000 ng/h/m²;
• The remaining 47% of the results showed emissions ranging from 1000 to 5000 ng/h/m².

Based on the results obtained, it can be concluded that the area to be remediated is unevenly contaminated, as is also confirmed by the laboratory testing of the soil samples from that area

The measurements of mercury emissions into the air, conducted 21 days after the remediation procedure, showed the following distribution:

• For 30% of the measurements, the mercury emissions ranged from 0 to 50 ng/h/m²;
• In 59% of the measurements, the emissions were between 50 and 100 ng/h/m²;
• A total of 18% of the results indicated emissions in the range of 100 to 200 ng/h/m².

Comparing the changes in mercury emissions values over time, it was observed that for 70% of the results, there was a reduction in mercury emissions in the contaminated area by 66 to 97%.

One measurement point showed a 59% reduction in emissions and one a 27.9% reduction in emissions. Only at three measurement points was there an increase in emissions values of approximately 1%, 24%, and 58% observed (however, the measured emissions values at these three points were low and did not exceed emissions values of 85 ng/h/m²). The results of the control measurements carried out 90 days after the remediation procedure showed that the value of mercury emissions from the examined area does not exceed 50 ng/h/m². All measurement points showed a significant decrease in mercury emissions. Notably, the highest mercury emissions measurement result obtained in this series was less than the lowest emissions value recorded before the procedure. This result is also 40 times lower than the highest mercury emissions level recorded in the test area prior to the remediation procedure.

As part of the works carried out, the degree of reduction of mercury emissions at individual measurement points was also assessed. It was found that at fourteen out of seventeen measurement points (82%), the mercury emissions decreased by 91.6% to as much as 99.3%. At the remaining three measurement points, a reduction in mercury emissions ranging from 60% to 88% was observed. These changes were noted at the measurement points where emissions after the remediation procedure did not exceed 22.8 ng/h/m². The results obtained using diffusion chambers (shown in Figure 4) also confirm the effectiveness of the remediation procedure.

When immobilizing mercury in soil by means of ground sulfur, it is essential to periodically assess the effectiveness of this procedure. Relying solely on direct methods for determining the total mercury content in the soil is insufficient for evaluating the effectiveness of metallic mercury stabilization in the form of HgS. To accurately assess the effectiveness of mercury immobilization in soil, it is necessary to use a speciation method that can differentiate between the mercury permanently bonded as HgS and unbound mercury with the potential to emit. A laboratory method based on speciation measurements of metallic mercury content in the soil, using an indirect method of measuring mercury emissions with gold sensors in flasks, can be employed to assess the mercury emissions levels and thus evaluate the effectiveness of the laboratory tests for using sulfur as a stabilizing agent in mercury-contaminated soils [18,31]. To estimate the amount of mercury capable of being emitted from the soil under field conditions, methods using a passive sensor of its own design and diffusion chambers can be used. The latter two methods were used to assess the effectiveness of the remediation procedure.

Additionally, it should be noted that the effectiveness of the mercury immobilization method not only reduced its emissions into the air but also decreased its solubility in water. Studies of water extracts from the soil samples taken from the contaminated area before and after treatment showed that mercury bound as HgS is less soluble in water. Specifically, after immobilization with sulfur, the content of soluble mercury forms ranged from 0.017 to 0.1 mg/kg dry matter. This stabilization with sulfur resulted in a tenfold decrease in the leaching of mercury from the soil.

5. Conclusions

This study evaluates various methods for remediating mercury-contaminated soils, highlighting both established industrial techniques and experimental approaches. The choice of remediation method depends on several factors, including the extent and depth of contamination, the physicochemical form of the contaminant, and the existing infrastructure.

This publication presents the results of work on the pilot remediation treatment applied to mercury-contaminated soil in an industrial area. Soil samples from this site revealed mercury concentrations ranging from nearly 2 to over 1000 mg/kg dry matter, significantly exceeding the acceptable levels for industrial soils. Speciation studies indicated that more than 30 mg Hg/kg dry matter was in a form capable of emissions.

The contamination was detected at depths ranging from 0 to 0.6 m below ground level. The remediation involved stabilizing mercury by adding sulfur to the contaminated soil, converting mercury into stable mercury sulfide (cinnabar). This process required mixing the soil with sulfur to a depth of up to 1 m, with sulfur constituting 5–10% of the treated soil.

The post-remediation measurements demonstrated that the application of this immobilization procedure reduced mercury emissions to the air by up to 40 times. The highest mercury emissions value recorded after 90 days of treatment was lower than the lowest value measured before the remediation. Consequently, the use of sulfur for mercury stabilization proves to be an effective method for mitigating mercury contamination in soil.

The results of the remediation indicate that sulfur stabilization not only significantly reduces mercury emissions to the atmosphere but also decreases its solubility in water, which mitigates potential risks to groundwater and the broader environment. This suggests that sulfur stabilization is effective not only in the long-term reduction of mercury emissions but also in improving the overall environmental quality in contaminated areas. Additionally, the use of sulfur as a stabilizing agent could be beneficial for various types of industrial sites and may be considered as part of broader contamination management strategies.