The Effect of pH on Stability of an Isolation Barrier Made of Dolomite Post-Floatation Waste

Abstract

Dolomite post-floatation waste has been proposed as an alternative material for the construction of separation barriers. The aim of this study was to determine the effect of the pH of leaching solutions on the stability of such barriers. The present research included the determination of selected physical and chemical properties of waste, i.e., density, grain composition, and filtration coefficient. Column tests of leaching by solutions of different pH values modeling varying environmental conditions were performed. Selected ions were determined in the eluates. Grain analyses were carried out for the column material after leaching to determine the changes in grain composition of dolomite due to washing with leaching solutions. The determined value of the filtration coefficient is 6.52 × 10⁻⁹ m∙s⁻¹, which confirms the impermeability of the waste. The material is fine-grained, with a grain diameter of d ≤ 200 µm. During leaching, a decrease in the content of the analyzed ions and the diameter of grains and their movement down the barrier, resulting in its sealing, was observed. The central part of all columns showed more grains with a diameter of 7 μm, which is probably due to secondary precipitation of CaSO₄. Irrespective of the initial pH of the leaching solution, the reaction of all eluates obtained was slightly alkaline (pH 7.52–8.20). Dolomite post-floatation waste has properties that ensure the tightness and durability of the separation barrier, which, combined with its ability to alkalize solutions and the sealing process, ensures its effectiveness.

1. Introduction

One of the main sources of hazards to the soil and water environment is pollutants leached from the waste collected at landfills and transferred to leachate. Isolation barriers are commonly used to minimize this risk. In the absence of a natural barrier in the landfill in the form of clay rocks of sufficient thickness, the barrier can be replaced by an artificial geological barrier, such as the use of compacted clay liners (CCLs) [1,2]. The preferred raw material for its construction is natural clay minerals with a low filtration coefficient and high sorption capacity, enabling the capture of pollutants migrating with infiltrating waters. However, the use of natural clay and loam minerals generates waste from their extraction and processing, changes in water relations and soil quality in the vicinity of excavations, landscape degradation, and additional costs of energy and technological research. An alternative to them, both technologically and economically, is a fine-grained mineral industrial waste [3]. This way of using waste makes it possible to reduce the amount of waste deposited both on an ongoing basis as well as the amount deposited previously. The use of materials deposited in the environment as waste is in line with the principles of the currently preferred circular economy.

The expected characteristics of the material used for isolation barriers include good sorption, isolating properties, and stability in the environment. Earlier studies by the author [3,4] have shown that fine-grained post-floatation sludge from Zn-Pb ore processing deposited in settling tanks, of which dolomite is a major component, exhibits such characteristics.

One of the major pollutants leached from waste into the environment is heavy metals, which enter the food chain and can cause numerous diseases and disorders in humans and animals [5,6,7,8]. Adsorption methods using natural and synthetic sorbents are commonly used in the removal of heavy metal ions from solutions [9,10,11]. An alternative in relation to costly active coals and synthetic sorbents is inexpensive, easy to obtain, and effective mineral sorbents, especially waste, e.g., dolomite post-floatation waste. Due to its crystalline structure, dolomite has low porosity [12], but despite its relatively small specific surface area (~1.50 m²∙g⁻¹), it shows good sorption properties [13]. Due to the negative charge of the crystal lattice, dolomite adsorbs heavy metals (e.g., Pb and Zn) in the form of cations [14,15]. Sorption involves physical adsorption, surface precipitation, and ion exchange with the Mg²⁺ and Ca²⁺ ions contained in dolomite and metal cations present in an aqueous solution [12,16,17]. Good sorption properties of dolomite towards heavy metal cations, i.e., Pb, Cd, Zn, Cr, Cu, Sr, As, and Ba, have been confirmed by numerous studies [15,17,18,19,20,21,22,23,24,25], including the author’s study [3]. Examinations of the effect of dolomite grain size on the efficiency of the adsorption of heavy metal ions conducted by Farmaki et al. [26] showed no significant relationship between their size and the level of sorption of pollutants. Therefore, when using fine-grained dolomite waste for the construction of physical and chemical barriers, it will not be necessary to grind or sieve it beforehand, which will greatly simplify the preparation of material for the construction of an isolation barrier and reduce the cost of technology.

One of the basic parameters affecting the leaching of heavy metals and grain degradation of alkaline waste material is the pH of the leaching solution. The durability of dolomite as an isolation barrier element depends on the nature of the leachate coming into contact with this material. This is due to the increased susceptibility of carbonate minerals to decomposition under acidic environmental conditions. Furthermore, due to its genesis, dolomite post-floatation waste may show elevated contents of heavy metals, which can be released to the environment due to leaching if the pH and/or oxidation potential of the migrating solutions are changed [27,28,29]. This process can be a source of surface water and groundwater pollution [30]. When some metals (e.g., Fe, Mn) precipitate as hydroxides, hydrogen ions are produced, leading to an increase in the acidity of the environment [31].

The pH value of leaching solutions, which determines the nature of the environment, has a very significant effect on the degradation of dolomite and thus on the stability of barriers made of this material. Dolomite consists mainly of CaCO₃, which is highly susceptible to acidifying agents. Singh et al. [32] found that the strength of rocks decreases linearly with increasing acidity. This is confirmed by the study of Singh et al. [33], who demonstrated that the strength of marble is highest at pH 7, and the reduction in strength is greater under acidic conditions compared to alkaline environments. At the same time, the susceptibility of the rock to degradation increases with decreasing grain size [34]. Low pH accelerates dolomite leaching by dissolving calcium compounds. Soluble salts that are leached in water are formed, and the degradation mechanism depends on the type of acid. In the case of oxalic or phosphoric acid, insoluble salts can form and then precipitate on the surface of the leached material. Sulfuric acid is particularly aggressive on calcium-rich rocks. The sulfate anion causes the formation of ettringite or gypsum [35].

One of the most commonly used tests to determine the durability of rocks under different pH conditions is the test proposed by Franklin and Chandra [36]. This test consists in placing 10 weighed pieces of rock (40–60 g each, with a diameter of about 30–45 mm) in a rotary drum and pouring a solution of a specified pH and temperature of about 20 °C. The drum rotates for 10 min at 20 rpm. After the cycle, the rock pieces are dried for 24 h at 110 °C and then cooled and weighed. The durability index is calculated as the percentage ratio of the final and initial dry masses of the rock. Ghobadi et Kapelehe [37] used this test to determine the stability of limestones and marls over a wide range of pH of leaching solutions from 2 to 12. The authors found that rocks with high CaCO₃ content (more than 65%) are more degradable in acidic solutions. This is due to the chemical reaction of calcium carbonate with the acidic solution, causing a reduction in the bond strength between the molecules. The study also confirmed the relationship between degradation index and grain size. Fine-grained limestone has a higher degradation rate compared to coarse-grained limestone. The Franklin and Chandra test was also used by Yagiz [38], who analyzed the degradation of selected types of rock (including dolomite). Dolomite was found to be characterized by high strength that resists degradation. This was confirmed by the stability indices, which reached values above 99% in all pH ranges. The lowest value of the index was obtained for pH 2, which confirms that the susceptibility of the rock to degradation is highest in a strongly acidic environment.

Another important feature of dolomite regarding its use for the construction of isolation barriers is its high mechanical strength, which has been confirmed by studies of dolomite as an additive to concrete mixtures [39,40]. This parameter is important when forming the isolation layer and giving it proper geomechanical properties.

The aim of the presented research was to evaluate the effect of the chemical character of the environment modeled by changing pH on the stability and effectiveness of the barrier made of dolomite post-floatation waste. In order to verify the reaction of the barrier material with the soil and water environment, the possibility of leaching possible contaminants contained in the waste was analyzed. The novelty of the present study is the approach utilized to assess the susceptibility of dolomite to degradation when in contact with solutions of varying pH; this approach is different from that proposed by Franklin and Chandra [36]. The method proposed in this paper is based on the analysis of the phenomenon of chemical grain degradation occurring as a result of the flow of solutions through the material rather than mechanical interactions between the grains. The method uses column tests to simulate the flow of solutions of different pH through the material. It was assumed that the change in grain size of the material is a measure of degradation. It was assumed that the contact of the alkaline material with acidic solutions would cause grain degradation, which would increase the tightness of the insulating layer.

2. Materials and Methods

2.1. Research Materials

Dolomite waste from the floatation enrichment of zinc-lead ores, accumulated in over-ground settling ponds located in southern Poland in the area of the city of Bytom, was used in this study (Figure 1). The choice of the material collection site was due to the fact that:

• The area of post-floatation tanks in Bytom is the largest among all Zn-Pb ore mining regions in Poland—the amount of waste deposited at the time of the discontinuation of the use of tanks, i.e., in the early 1990s, was about 36 million Mg [41];
• The waste was produced in a short period of time, in the same treatment processes, and the ore subjected to floatation came exclusively from the deposits in Bytom, which guarantees its mineralogical homogeneity.

Waste samples were taken at three points from the central part of the settling tank’s top from a depth of about 1 m. After being transported to the laboratory, the material was brought to an air-dried state. The collected samples were mixed, and one test sample was prepared using the quartering method.

2.2. Research Procedure and Analytical Methods Used

2.2.1. Physical and Chemical Analyses

The prepared waste sample was subjected to chemical and phase composition analysis, and basic physical properties were determined (color, specific and volumetric density, grain composition, and filtration coefficient). The chemical composition was determined using a Bruker WD XRF S8 Tigerfluorescence spectrometer (Bruker Optics Inc, Billerica, Massachusetts, USA). The waste sample was ground in a MM400 Retsch ball mill (Retsch, Hahn, German)in zirconium oxide vessels and then pressed without a binder in an aluminum cap. The analyses were performed in three replications. To determine the phase composition, an X-ray analysis was performed with the use of an X-ray diffractometer, the PANalytical X’PERT PRO-PW 3040/60 (Malvern Panalytical B.V., Almelo, The Netherlands), using the following measurement conditions: Cu lamp kα₁, range: 3–75° 2Θ, time limit 30 s, step size 0.02°2Θ. The data obtained were processed using HighScore+ software (version 4.9) and the ICSD (version 2015) and PDF4+ ICDD (version 2019) databases. The specific and volumetric densities were determined according to the following standards: Geotechnical investigation and testing—Laboratory testing of soil—Part 3: Determination of particle density [42] and Geotechnical investigation and testing—Laboratory testing of soil—Part 2: Determination of bulk density [43]. The filtration properties of the tested waste were analyzed by the flow-pump method, and the filtration coefficient was determined. The test stand was equipped with a constant pressure hydraulic chamber, a Sage Instruments infusion pump, Hamilton injectors, and a digital recorder.

2.2.2. Column Tests

The main part of the present research concerned the analysis of the durability of dolomite waste as an element of an isolation barrier. To model the reaction of the surface of dolomite grains with water infiltrating through this material at different pH values simulating varied environmental conditions, column (lysimetric) tests on leaching and grain degradation were carried out. The use of column tests to assess metal release enables the monitoring of metal leaching over longer times and the identification of temporal changes in contaminant concentrations as they are transported through the waste [44].

A separate grain class ranging from 50 to 200 μm was used for the research. It was obtained by wet sieving using deionized water. This preparation of the material was due to the high proportion of grains with diameters below 50 μm (more than 70% by weight). The fine grains may have impeded the free flow of the liquid through the column and, as a result of siltation, made the experiment impossible. Equal weighted amounts of material were placed in 6 glass columns of 20 mm inner diameter, forming a layer 20 cm high in each column. The base of the layer was a glass wool filter compressed between two metal mesh discs with a mesh diameter of φ = 40 μm. At the bottom of each column, a glass tube was installed with a valve to regulate the outflow of the filtering liquid. During the experiment, the waste layer in the columns was flooded with pH-controlled aqueous solutions to model different environmental conditions. The following solutions were used for the tests: column 1—pH = 3.05; column 2—pH = 4.00; column 3—pH = 5.09; column 4—pH = 6.14; column 5—pH = 7.11; column 6—pH = 8.20. A solution of the amount equivalent to the infiltration through a hypothetical isolation barrier over 10 years was passed through the columns. The average annual precipitation in the city of Bytom, determined based on archival data, was assumed to be about 700 mm. According to this assumption, 2 L of the solution was used per column. The waste layer was flooded with the solution in batches of 100 mL, keeping the flow rate constant and the bed completely submerged in the column. The column effluent (eluate) was collected in portions for which the pH and concentration values were determined for Zn²⁺, Fe²⁺, Ca²⁺, Mg²⁺, Mn²⁺, SO₄²⁻. The pH was measured using a multifunction meter (Hanna Instruments, model HI9828, Woonsocket, USA). The eluates collected from the columns were filtered through membrane filters (0.45 μm), and the content of the studied metal ions was determined using an emission spectrometer with ICP-OES plasma induction IRIS Interpid II XSP ThermoICP (Plasma Induction, Gujarat, India). Fe²⁺ ions were determined as a chelate complex with 1.10-phenanthroline using the spectrophotometric method according to the following standard: Water quality—Determination of iron—Spectrometric method using 1.10-phenanthroline [45]. SO₄²⁻ ions were determined by the weighing method according to the following standard: Water quality—Determination of sulphates (VI)—Gravimetric method with barium chloride [46].

After the completion of the leaching test cycle, grain analyses of the material from selected columns were carried out to determine if there was a change in grain composition caused by the reaction of the post-floatation waste with leaching solutions of different pH. Columns 1, 3, and 6 were used for this study, assuming that the greatest changes would occur at extreme pH values of the leaching solutions. Three samples were obtained from each column:

• Sample 1 was obtained from the top of the column;
• Sample 2 was obtained from the center of the column;
• Sample 3 was obtained from the bottom of the column.

Grain analyses were performed using a laser grain size analyzer type LAU-10 using isopropyl alcohol as a dispersion liquid. All analyses were performed in triplicate.

2.2.3. Statistical Analyses

The aim of the analyses was to verify the effect of the solution’s pH on the leaching of selected ions and grain degradation as confirmed by changes in grain composition. The independent variable was the initial pH of the leaching solution. Statistical analyses were performed using the IBM SPSS Statistics 26 software package based on the Pearson’s r correlation analysis. The level of significance was set at α = 0.05. Test statistical significance results in the range of 0.05 < p < 0.1 were considered significant at the level of the statistical trend.

3. Results and Discussion

3.1. Physical and Chemical Analyses

Based on the chemical analysis (

Table 1. Chemical composition of waste dolomite.

Chemical Component	Contents, %
CO2	31.90 ± 0.854
CaO	25.8 ± 0.252
MgO	12.813 ± 0.290
Fe2O3	10.267 ± 0.368
SO3	10.073 ± 0.284
ZnO	5.136 ± 0.103
SiO2	2.327 ± 0.097
PbO	0.814 ± 0.006
Al2O3	0.535 ± 0.007
MnO	0.424 ± 0.005
As2O3	0.311 ± 0.002
Na2O	0.088 ± 0.006
K2O	0.046 ± 0.001
Cl	0.035 ± 0.004
CdO	0.020 ± 0.001
P2O5	0.017 ± 0.0009
Cr2O3	0.008 ± 0.0004

), it can be concluded that the basic chemical constituents of floatation waste are CO₂, CaO, and MgO. These values are in agreement with the studies of other authors [47,48,49,50,51,52], according to which the CaO content in dolomite varies within a wide range from 11.69% to 55.19%, while the MgO content varies within a range from 8.39% to 37.71%. The high iron content (10.267%) results from the frequent conversion of Fe²⁺ for Mg²⁺ in the dolomite structure, which places it in the mineralogical series as an iron dolomite. A high value for iron (9.12%) was also obtained in the study by Aral and Cihan [47]. The average content of basic heavy metals found in the ore, i.e., zinc and lead, is 5.136% and 0.814%, respectively.

The main mineral phases contained in the dolomite floatation tailings were determined by means of X-ray analysis and are presented in Figure 2 and

Table 2. The main mineral phases in dolomite wastes.

Ref. Code	Mineral Name	Chemical Formula	SemiQuant, %
01-081-8229	Dolomite	CaMg(CO3)2	61.9
98-002-0179	Calcite	CaCO3	5.6
04-006-2810	Pyrite	FeS2	6.7
98-003-4652	Bassanite	CaSO4(H2O)0.5	12.4
98-001-7789	Sphalerite	Zn0.73Fe0.27S	0.8
98-004-6153	Smithsonite	ZnCO3	9.3
98-000-5729	Quartz low	SiO2	3.3

. The basic constituent of the waste is dolomite, which is present in the amount of 61.9%. There is also a large amount of bassanite (12.4%). The remaining mineral phases are present in amounts below 10%. According to Tangviroon et al. [17], the dolomite mineral content in dolomite rock can reach more than 95%, with a Ca to Mg molar ratio of 1.2.

Table 3. Selected physicochemical properties of waste dolomite.

Colour	Particle Density, g·cm−3	Bulk Density, g·cm−3	pHH20, (-)	Filtration Coefficient, m·s−1
Honey-rusty	2.83 ± 0.087	1.67 ± 0.055	7.61 ± 0.08	6.52 × 10−9 ± 0.79

shows the selected properties of the waste dolomite. The significant presence of iron compounds in the material (Table 1), especially in their hydrated form, causes a rusty-honey coloration of the dolomite post-floatation waste. The specific and bulk density values determined in the study do not differ from those obtained for this type of waste by other authors [53,54], which were 2.78–2.89 g·cm⁻³ and 1.43–1.78 g·cm⁻³, respectively. The reaction of the solution in contact with the tested waste is slightly alkaline, which is consistent with the chemical properties of this material.

The analysis of grain size composition (Figure 3) shows that this material belongs to fine-grained formations. The size of the grains does not exceed 200 µm, which is related to the technological process in which this waste was created. Ore ground to grains below 200 µm was supplied to the floatation process.

Analysis of grain size distribution in the waste revealed that the dominant grain class was grains < 56 µm (76.96%), with more than 17% of this group consisting of grains up to 1 µm. Based on the uniformity coefficient, it is possible to characterize the test sample. This coefficient was calculated according to the Terzaghi formula because the sample contains mostly grains with a diameter of ˂0.1 mm:

$$\mathrm{U}=\frac{{\mathrm{d}}_{70}}{{\mathrm{d}}_{20}}$$

(1)

where d₂₀ and d₇₀ represents the diameter of particles, which, together with smaller ones, account for 20% and 70%, respectively.

The values of particular grain diameters (d₂₀ and d₇₀), determined based on Figure 3, allowed for the calculation of the uniformity coefficient U, which reached the value of 20.0, indicating a sample with very different grains. This is a desirable characteristic because the more different grains in the material, the better it can be compacted for better isolation.

Another property of waste dolomite that is important in considering this material for the construction of isolation barriers is the filtration coefficient. The obtained value of this parameter (Table 3) indicates that it is an impermeable material. According to the provisions of Annex 1 to European Council Directive 1999/31/EC [55], the filtration coefficient for the isolation barrier at hazardous waste landfills should be k ≤ 1.0 × 10⁻⁹ m·s⁻¹. The tested dolomite post-floatation waste meets the requirements of the Directive.

3.2. Column Tests

Factors affecting the leaching of heavy metals include the chemical form of the metal, the shape and particle size of the material, the temperature of the process environment, and the contact time between the material and the solution [56,57]. However, pH is considered one of the main parameters affecting the amount of leaching of heavy metals [58,59]. Metal compounds are naturally dissolved by slow weathering over time. However, this process is accelerated if the pH of the leaching solution deviates from neutral, i.e., at low or high pH [31]. Cation release increases towards low pH values, whereas anion release increases towards high pH values [60,61]. In general, the mobility of heavy metals decreases with increasing solution pH according to the sequence of Cd > Zn > Ni > Cu > Pb [62]. During column tests, ion concentrations were continuously determined in the eluates for Zn²⁺, Fe²⁺, Ca²⁺, Mg²⁺, Mn²⁺, and SO₄²⁻ leached as the solution of different initial pH values migrated through the column material. Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9 show the amount of ions leached as a function of the volume of leaching solution introduced into the columns and its initial pH.

Two leaching phases can be observed for each analyzed ion (Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9). In the first phase, ion concentrations are high, while in the second phase, equilibrium concentrations are observed. The second phase is characterized by a fixed concentration of the leached ion. An exception is the iron leaching process, whose concentrations fluctuate for reasons that cannot be clearly identified. It is likely that iron sulfides, mainly the pyrite contained in the waste, are more susceptible to oxidation than sulfides of other metals, which translates into variable amounts of soluble iron salts. Analysis of the graphs in the equilibrium phase (in the eluate volume above 300 mL) leads to the conclusion that the highest leaching of Ca²⁺, Mg²⁺, Zn²⁺, and Mn²⁺ ions occurs when using a leaching solution with pH = 3.05. Only the first portions of the eluate do not show this pattern due to the fact that they leach the soluble salts that have accumulated before the leaching process, e.g., during sample preparation, under relatively intense oxidation conditions. Studies by other authors confirmed that acidic environmental conditions induce dissolution and intensive leaching of heavy metals such as As, Cu, Fe, Mn, and Zn [44,63]. Król et al. [61] found that the highest concentrations of Zn, Ni, Pb, and Cu in the leaching process occurred in a highly acidic environment (pH = 3) and the lowest concentrations occurred at pH levels of 7 to 10. In acidic environments, the release of heavy metals occurs mainly due to the so-called acid attack [44]. According to Ospina-Alvarez et al. [29], even small changes in the pH of the solution can significantly affect the leaching intensity of heavy metals associated with carbonate and sulfide minerals.

Since the course of the dolomite leaching process is mainly affected by the pH of the leaching solution, the value of this parameter was determined in the eluates collected from the lysimetric columns. The analysis of the results (

Table 4. pH of successive portions of eluate from column lysimeter tests.

Column Number		1	2	3	4	5	6
Initial pH		3.05	4.00	5.09	6.14	7.11	8.20
		pH of the eluate after successive portions of the leaching solution
Successive portions of the leaching solution, mL	0–100	7.52	7.80	7.72	7.80	7.67	7.75
	100–200	7.65	7.83	7.78	7.80	7.56	7.82
	200–300	7.81	7.85	7.89	7.87	7.85	7.86
	300–400	7.90	7.89	7.89	7.96	7.96	7.97
	400–500	8.05	8.02	8.07	8.05	8.04	8.06
	500–600	7.97	7.99	7.98	7.98	7.99	7.97
	600–700	7.88	7.86	7.83	7.90	7.87	7.86
	700–800	7.80	7.82	7.84	7.82	7.83	7.84
	800–900	8.02	8.00	8.01	8.03	8.00	8.04
	900–1000	8.09	8.07	8.11	8.12	8.15	8.20
	1000–1100	8.17	8.18	8.14	8.18	8.14	8.16
	1100–1200	8.16	8.17	8.16	8.19	8.18	8.16
	1200–1300	8.11	8.09	8.07	8.04	8.09	8.12
	1300–1400	8.10	8.10	8.07	8.04	8.08	8.11
	1400–1500	8.14	8.14	8.15	8.17	8.15	8.17
	1500–1600	8.14	8.14	8.13	8.12	8.13	8.16
	1600–1700	8.13	8.10	8.11	8.14	8.12	8.14
	1700–1800	8.15	8.13	8.16	8.14	8.16	8.16
	1800–1900	8.14	8.10	8.09	8.12	8.10	8.11
	1900–2000	8.13	8.05	8.03	8.04	8.06	8.10

) reveals that irrespective of the initial pH of the leaching solution, the reaction of all eluates obtained was slightly alkaline and the pH values were in the range of 7.52–8.20. For lysimetric columns 1–5, a significant increase in pH from the initial pH (alkalinization) was already observed in the first eluate collected. Only in the case of column 6 was slight acidification of the eluates recorded during the first leaching stage. After a total of 1000 mL of leaching solution was passed through the column, the pH reached a value equal to the initial pH, which remained very similar until the end of the experiment.

The alkalinization potential of dolomite waste found during the research is connected with the hydrolysis of dolomite, occurring particularly intensively in contact with acid solutions. Due to the hydrolysis of dolomite, more CO₃²⁻ ions appear in the leaching solution. When heavy metal ions are simultaneously present in the solution, alkaline carbonate precipitation can occur. This results in a decrease in the metal ion content of the solution migrating between the dolomite grains. For example, the presence of Zn²⁺ ions in the solution may cause precipitation of hydrozincite according to the following chemical reaction:

5Ca²⁺ + 5CO₃²⁻ + 6H₂O + 5Zn²⁺ + 5 SO₄²⁻ ↔ Zn₅(CO₃)₂(OH)₆ + 5Ca²⁺ + 5SO₄²⁻ + 3CO₃ + 6H⁺
🠛
hydrozincite

(2)

The presence of this mineral in shallow parts of Zn-Pb ore deposits was found by Górecka [64].

The decrease in zinc leaching observed during the column tests may confirm this phenomenon (Figure 4).

As demonstrated, dolomite waste has the ability to alkalize wash solutions, regardless of the original pH and the associated ability to immobilize heavy metals. These properties indicate that this waste can be used to build an isolation barrier around potential sources of environmental pollution.

Taking into account the process of dolomite dissolution in contact with solutions of different pH, the next stage of the research involved the analysis of the grain composition of the column filling material. Analyses were performed according to the procedure described in Section 2.2.2. For column tests, a material composed of a separate grain class of 50–200 μm was used. The grain analysis of this material is shown in Figure 10.

The analysis of the results confirmed the correct method of preparation of the material for column tests, in accordance with the assumptions of the adopted methodology. The grain distribution shown in Figure 10 provided a reference for the results obtained for the material from the selected columns. The results of grain analyses of material taken from the top, center, and bottom of the columns of each selected column (columns 1, 3 and 6) are presented in Figure 11, Figure 12 and Figure 13.

The results confirm the process of grain degradation of dolomite, observed especially in column 1 (pH of leaching solution: 3.05). Grains with a diameter of ˂ 50 μm appeared in the upper part of this column and accounted for 49.7%. It should be noted that these grains were not present in the initial material for the tests. The amount of grains ˂ 50 μm in the central part of column 1 was 22.48%, and this increased again to a value of 51.7% in the bottom part. This is connected with the movement of fine grains down the column with successive portions of the leaching solution. In the case of column 3 (pH of leaching solution: 5.09), no such significant increase in the number of grains ˂ 50 μm is observed, and their proportion is 23.4% at the top of the column, 37.8% in the center, and 37.6% at the bottom. For column 6 (pH of leaching solution: 8.20), the grains with a diameter of ˂50 μm were also observed at all three levels. The proportion of these grains was 26.3% for the top of the column, 29.57% for the center, and 17.9% for the bottom. Analysis of the proportion of grains with a diameter of 50–200 μm demonstrated that regardless of the pH of the leaching solution and the level of sampling in the column, grains with a diameter of 71–90 μm were the dominant grain size. The central part of all columns showed more grains with a diameter of 7 μm. This is likely due to secondary precipitation of CaSO₄ and/or accumulation of other reaction products occurring at the boundary between the solution and the waste grain surface. This process is confirmed by Ca²⁺ and SO₄²⁻ concentrations observed in the first leachates from the lysimeter columns which exceed the solubility product of CaSO₄.

The formation of grains with diameters of ˂ 50 μm in the process of leaching of the tested material and the possibility of their gravitational transfer down the deposit can lead to the sealing effect. This is an advantageous phenomenon due to the need to meet the condition of impermeability of the material used for the construction of the isolation layer.

To evaluate changes in grain composition at each column level (top, center, and bottom) in relation to the grain composition of the initial sample, grains were grouped into three fractions: clay (grains ˂ 2 µm), silt (grains 2–20 µm), and fine sand (grains 20–200 µm), and the percentage proportions of grains in these classes were calculated. The obtained results are presented in

Table 5. Proportion of grains in the fraction at the indicated level: columns 1, 3, and 6 and in the starting material.

Type of Sample	Place of Sampling	% of Grains in the Range
		<2 μm (Clay)	2–20 μm (Silt)	20–200 μm (Fine Sand)
Initial sample for testing	-	0	0	100
Column 1 (pH = 3.05)	Top column	15.82 ± 0.81	30.1 ± 1.03	54.08 ± 1.24
	Center column	0.68 ± 0.07	13.83 ± 0.37	85.49 ± 0.77
	Bottom column	0.37 ± 0.04	18.18 ± 0.58	81.45 ± 0.65
Column 3 (pH = 5.09)	Top column	0	1.82 ± 0.04	98.18 ± 0.29
	Center column	1.21 ± 0.07	28.22 ± 0.49	70.57 ± 0.62
	Bottom column	0	2.22 ± 0.17	97.78 ± 0.73
Column 6 (pH = 8.2—)	Top column	0	7.98 ± 0.21	92.02 ± 0.99
	Center column	0.74 ± 0.03	17.53 ± 0.31	81.73 ± 0.93
	Bottom column	0	2.97 ± 0.18	97.03 ± 0.83

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The greatest increase in the proportion of the finest grains (<2 μm) and grains in the range of 2–20 μm was in the upper part of column 1, which increased compared to the initial sample by 15.82% and 30.1%, respectively. Therefore, under acidic environmental conditions, the greatest degradation of dolomite grains should be expected, with a simultaneous decrease in the proportion of grains in the fine sand fraction. With the increasing pH of the leaching solution, the intensity of grain degradation in the top and bottom regions of columns 3 and 6 decreased. However, a definite increase in the proportion of grains from the silt fraction in the central part of these columns was observed.

3.3. Statistical Analysis

Pearson’s r correlation analysis was performed to investigate the relationship of the pH of the initial solution leaching the column-filling material with the content of selected ions. The results of the statistical analysis are presented in

Table 6. Correlation of initial pH with the content of selected ions determined in the eluate.

Dependent Variable		Independent Variable—pH Initial Solution
Zn2+ contents	Pearson’s r	−0.38
	Significance	0.115
Fe2+ contents	Pearson’s r	−0.23
	Significance	0.362
Ca2+ contents	Pearson’s r	−0.54
	Significance	0.021
Mg2+ contents	Pearson’s r	−0.56
	Significance	0.016
Mn2+ contents	Pearson’s r	−0.73
	Significance	<0.001
SO42− contents	Pearson’s r	−0.33
	Significance	0.186

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The calculations performed showed statistically significant, negative, and strong correlations of pH with contents of Ca²⁺, Mg²⁺, and Mn²⁺ ions. This confirms the observed relationship that the amount of these ions determined in the eluate decreased with an increasing pH value. For the other ions, the increase in pH was not related to their content in the eluate.

Pearson’s r correlation analysis was performed to investigate the relationship between the initial pH of the leaching solution and the percentage content of grains grouped into three fractions: clay, silt, and fine sand. The calculations were done according to the location from which the sample was taken: either the top, center, or bottom part of the column. The results of the statistical analysis are presented in

Table 7. Correlation of initial pH with percentage content of grains from selected fractions by the location from which the sample was obtained.

Dependent Variable		Independent Variable—pH Initial Solution
		Top Column	Center Column	Bottom Column
Percentage of the fraction: clay (˂2 µm)	Pearson’s r	−0.80	−0.01	−0.80
	Significance	0.010	0.980	0.010
Percentage of the fraction: silt (2–20 µm)	Pearson’s r	−0.66	0.13	−0.77
	Significance	0.054	0.736	0.014
Percentage of the fraction: fine sand (20–200 µm)	Pearson’s r	0.72	−0.12	0.77
	Significance	0.030	0.751	0.015

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The results of the analysis conducted for the top column area showed that pH:

• Was statistically significantly, negatively, and very strongly related to the percentage content of grains from the clay fraction;
• Was statistically significantly, positively, and very strongly related to the percentage content of grains from the fine sand fraction;
• Was negatively and strongly correlated at the level of statistical tendency with the percentage content of grains from the silt fraction.

Calculations for the bottom column area revealed that pH:

• Was statistically significantly, negatively, and very strongly related to the percentage content of grains from the clay and slit fractions;
• Was statistically significantly, positively, and very strongly correlated with the percentage content of grains from the fine sand fraction.

In the center column area, there were no statistically significant relationships between the variables studied. This result indicates that as the pH increases, the percentage content of grains from clay and silt fractions decreases, and the percentage of fine sand fractions grains increases, although this relationship applies only to the top and bottom column areas.

4. Conclusions

The primary protection against contaminants leached from the waste collected at landfills is isolation barriers. In this paper, post-floatation dolomite waste was examined in the aspect of the possibility to use it as a material for the construction of such barriers. The material was found to combine the characteristics of a physical and chemical barrier. Dolomite wastes can alkalize solutions migrating through them, which is extremely important, especially when depositing contaminants in acidic environments. This feature enables the immobilization of heavy metals by precipitating them as stable and hardly soluble carbonates. The dolomite waste is essentially impermeable, as confirmed by the measured filtration coefficient k = 6.52 × 10⁻⁹ m·s⁻¹. Furthermore, the observed decrease in grain diameter during leaching by infiltrating solutions and their movement down the barrier caused additional sealing of the barrier. The added value is the possibility to reduce the amount of this waste deposited in the environment. Furthermore, this material is easy to obtain (it is stored in post-floatation tanks) and does not require additional costs in the technological process of its use. The examinations presented in this paper confirm the stability and effectiveness of an isolation barrier made of dolomite post-floatation waste in various environmental conditions. Such utilization of waste fits the circular economy cycle that is implemented, among others, by the EU member states [65], and the objectives of sustainable development by reducing the environmental impact.

In conclusion, it should be stated that:

• The dolomite post-floatation waste proposed for the isolation barrier have supra-additive characteristics and combine the advantages of a chemical and physical barrier;
• Buffering properties of dolomites stabilize the pH of solutions infiltrating through them, thus limiting leaching of heavy metals;
• With its low filtration coefficient, this waste can be considered an impermeable material;
• The grain degradation occurring with migrating solutions leads to sealing of the barrier consisting of this waste.