Effect of Specified pH Conditions on Leaching of Environmentally Important Major and Trace Elements from Fly Ash-Marl Mixtures: A Case Study of the Lignite Center of W. Macedonia

Significant environmental issues concerning the risk of soil, surface, and groundwater pollution are indicated by the presence of dissolved elements in the leachates. In the Lignite Center of Western Macedonia, the current study investigates the leaching behavior of main and trace elements from fly ash-marl mixtures, focusing on the effect of pH conditions on environmental impacts. Five mixtures of fly ash-marl were prepared in different percentage ratios by weight, simulating the possible deposition conditions to investigate the leaching of their contents in specified pH conditions. Initial fly ash is enriched in CaO, SO₃, P₂O₅, and MgO, exceeding continental crust average composition in U, Br, Sb, Ni, Cd, W, and Mo. In contrast, the original marl is enriched in CaO and Ni but depleted in other major elements. The pH significantly affects leaching, with high concentrations of Ca²⁺ both in fly ash and in mixtures with high concentrations of fly ash, particularly at pH 8. Significant leaching of Cr and Ni is observed, with Cr mainly from fly ash at pH 10–12 and Ni increasing in marl at pH 6. The findings highlight the significant role of fly ash in the environmental element release and the importance of effective management.

Introduction

One of the main sources of electricity generation in the world is coal-fired power stations. However, while lignite has historically been an important source of electricity generation in Greece, with lignite plants accounting for a large part of the country’s electricity production, Greece has taken steps to gradually transition to Renewable Energy Sources (RES).

Ash, which is a byproduct of burning coal, is the primary source of waste produced by coal-fired power plants. The process that the coal-fired plants follow to dispose of the ash is by conveyor belt dry transportation into landfills and, in some cases, in tailing pods. Fly ash, as a solid waste from combustions, is enriched in a variety of main and trace elements that can be potentially hazardous. These elements tend to be excreted in sufficient quantities to the atmosphere when they come into contact with water, ending up in the aquifers [1], [2]. The excretion and transport of those elements, in dissolved or particulate form, to surface and groundwater water often leads to environmental and public health issues [3].

Many studies on the mineralogy and geochemistry of lignite and lignite fly ash have been carried out in the last few decades [4]–[7]. The degree and the ability of constituents to be dissolved depend on several variables, such as their chemical affinity, component solubility, combustion conditions, redox conditions, and especially the prevailing pH conditions [8]–[10]. While some metals have received significant attention [11], the environmentally crucial factors of leaching have been overlooked. The pH of the leaching solution affects the quantity of leaching and solubility of elements in fly ash. Although there have been studies on fly ash leaching under specific pH settings, such as controlled laboratory trials, there has been limited comprehensive investigation into fly ash leaching across a range of pH values [12], [13].

Despite extensive previous research on the mobility and dissolution capacity of toxic metals from fly ash, as well as assessments of potential negative environmental effects on soil and water, it is essential to assess the risk associated with marl layers co-excavated with lignite during deposit exploitation. In the Greek region, the presence of marl often occurs as alternating layers with coal, primarily lignite, due to its formation conditions. The marl-lignite layer alternation is so tightly intertwined that it becomes impractical to separate them in many cases during lignite mining. The combustion of these lignite-marl layers with the main combustible material leads the non-volatile components through physicochemical changes to concentrate in the fly ash [14], [15]. Marl is often used in mines to mix and dispose of ash. As a result, the contained potentially hazardous elements are transported via aqueous media to the soil and water table or to be concentrated in solid matter. Consequently, marl can have a significant effect on the leaching of various elements [15].

The objective of this research is to investigate the release of major and trace elements at varying pH conditions, both toxic and non-toxic, from fly ash, marl, and their mixture samples. The release pattern of these components will be determined by conducting different leaching tests, which simulate related environmental conditions. Special attention will be given to pH conditions that enhance the release of elements, as well as the influence of each material on element concentration. Limited information is available regarding how pH affects the behavior of leached metals from mixtures. Therefore, the study aims to evaluate the potentially hazardous elements associated with the disposal of fly ash and marl, with particular emphasis on the leaching behavior of their constituent elements in deposition areas where water is present.

Materials and Methods

The fly ash samples were from the South Field Mine, Greece’s largest mine, which supplies the Agios Dimitrios Power Plant. Marl samples were collected at the South Field Mine in Kozani, Greece. In order to carry out the leaching studies, representative quantities of marl and fly were separated and prepared for the mixtures.

Five mixtures (MFA1-MFA5) were prepared with varying weight ratios (Table I). MFA1 and MFA2 represent the original materials, with MFA1 consisting of 100% fly ash (FA) and MFA2 consisting of 100% marl (M). The remaining mixtures, MFA3 (75FA-25M), MFA4(50FA-50M), and MFA5 (25FA-75M), are combinations of fly ash and marl, with a gradual change in the percentage of marl, allowing the evaluation of marl content in the concentrations of the elements leached from fly ash under different pH conditions.

Table I. Change of the Target pH Range for the Leaching Experiments

Sample	Mixture	Initial pH	Target pH	HCLSV (ml)	NaOHSV (ml)
MFA1	100FA	12,5	12	22.0
MFA2	100M	8,7			5.3
MFA3	75FA-25M	13,3		18.3
MFA4	50FA-50M	13,4		28.5
MFA5	25FA-75M	13,3		35.7
MFA1	100FA	12,5	10	37.4
MFA2	100M	8,7			3.3
MFA3	75FA-25M	13,3		32.0
MFA4	50FA-50M	13,4		24.6
MFA5	25FA-75M	13,3		21.2
MFA1	100FA	12,5	8	36.0
MFA2	100M	8,7		28.8
MFA3	75FA-25M	13,3		33.0
MFA4	50FA-50M	13,4		27.4
MFA5	25FA-75M	13,3		24.5
MFA1	100FA	12,5	6	37.2
MFA2	100M	8,7		21.0
MFA3	75FA-25M	13,3		33.5
MFA4	50FA-50M	13,4		34.3
MFA5	25FA-75M	13,3		18.8

Chemical analyses were conducted to determine the chemical composition of the original fly ash and marl samples. The inductively coupled plasma-mass spectrometry (ICP-MS) and-optical emission spectrometry (ICP-OES) methods were used to determine the percentages of major oxides (SiO₂, TiO₂, Al₂O₃, Fe₂O_3t, MnO, MgO, CaO, Na₂O, K₂O, P₂O₅, SO₃). The studied samples underwent fusion with a lithium metaborate/tetraborate mixture, and the resulting fused disks dissolved in a weak nitric acid solution. For the detection of trace elements (As, Au, Br, Co, Cr, Cs, Eu, Hf, Hg, Ir, La, Lu, Nd, Rb, Sb, Sc, Se, Sm, Ta, Tb, Th, U, W and Yb) of the studied samples, the Instrumental Neutron Activation Analysis (INAA) method was used. Furthermore, Ag and Ba were measured using the combined INAA/TD-ICP and INAA/FUS-ICP methods, respectively. The Total Digestion-ICP (TD-ICP) method was used for trace elements (Bi, Cd, Cu, Mo, Ni, Pb, and Zn) after complete dissolution in a solution of HClO₄, HNO₃, HCl, and HF at 200°C, followed by evaporation and dilution in aqua regia. Analyses were performed on the Activation Laboratories Ltd., (Actlabs), Ancaster, Ontario, CANADA (code package 4E-expl).

The geochemical study of the concentration of these elements in the examined samples was conducted by calculating the pollution coefficient of each major oxide element and the trace elements [16] relative to the average composition of the continental crust [17]. According to Håkanson [18], pollution coefficient values are categorized as follows: <1 for depleted samples, 1–3 for moderately enriched/polluted samples, 3–6 for significantly enriched/polluted samples, and >6 for highly enriched/polluted samples.

The leaching experiments of the studied samples and their mixtures were conducted following the European standard EN-12457/1-4 (2003). The mixtures were prepared with a liquid-to-solid ratio of 10:1 (10 L/kg) using distilled water. The samples were then rotated (15 rpm) for 24 hours to establish equilibrium between the liquid and solid phases. Afterward, the solid residue was separated from the liquid using centrifugation and filtration. The pH conditions of the original samples were recorded, and the leachates were analyzed.

The pH of the initial fly ash and marl mixtures ranged between 8.7 and 13.4. For the leaching experiments, the pH was adjusted to the target values of 6, 8, 10, and 12. Specific volumes (SV, ml) of standard solutions of hydrochloric acid (HCl_SV) and sodium hydroxide (NaOH_SV) were added to achieve the target pH values (Table I). These adjustments were based on the pH-time reference experiments conducted on the starting materials MFA1 and MFA2 [19].

The leachate pH is determined following the Standard Analytical Method 4500 − H + B. The concentrations of major and trace elements were obtained using the standard analytical methods 3111 B, 3500 Ca B, 3500 mg B, and 3113 B by Atomic Absorption Spectroscopy (AAS) on a Perkin Elmer 5000 spectrometer equipped with a graphite furnace was used for the analysis. Analyses were performed on the laboratories of the Department of Mineralogy-Petrology-Economic Geology, School of Geology, Aristotle University, Thessaloniki, Greece.

Results and Discussion

Chemical Composition of Initial Materials

Major Elements

The chemical composition of fly ash can vary considerably based on factors such as the quality of the coal burned, the size of the fly ash particles, and the efficiency of the electrostatic filters [20]. The initial fly ash sample (Table II) consists mainly of CaO (37.16% w/w), SiO₂ (23.01% w/w), and Al₂O₃ (9.53% w/w), with minor amounts of Fe₂O_3t (3.88% w/w), MgO (3.11% w/w), and SO₃ (2.46% w/w), were also measured. The remaining major elements were measured at <1% w/w (TiO₂, MnO, Na₂O, K₂O, and P₂O₅), while the loss on ignition was calculated at 14.96% w/w. The analyzed fly ash exhibits a high content of CaO (PC_FA = 6.76), significant content of SO₃ (PC_FA = 4.10), and moderate content of P₂O₅ (PC_FA = 1.33) and MgO (PC_FA = 1.00). According to the European standard EN197-1 (2000), the fly ash from Agios Dimitrios PP is classified as calcareous fly ash (W). According to ASTM C618 (2019), the studied fly ash is classified as class C based on the amount of CaO (>18.0%).

Table II. Major Element Content (% w/w) and Enrichment/Pollution Coefficients (PC) of the Major Element Oxides of the Starting Fly Ash (PC_FA) and Marl (PC_M) Samples

Oxide	Detection limit (% w/w)	Content (% w/w)			Pollution coefficients
		FA	M	Continental crust1	PCFA	PCM
SiO2	0.01	23.00	14.86	60.20	0.38	0.25
TiO2	0.005	0.45	0.18	0.70	0.64	0.26
Al2O3	0.01	9.53	5.09	15.20	0.63	0.33
Fe2O3t	0.01	3.88	1.89	6.70	0.58	0.28
MnO	0.01	0.04	0.03	0.10	0.40	0.30
MgO	0.01	3.11	1.82	3.10	1.00	0.59
CaO	0.01	37.16	39.51	5.50	6.76	7.18
Na2O	0.01	0.28	0.20	3.00	0.09	0.06
K2O	0.01	0.65	0.53	2.90	0.22	0.18
P2O5	0.01	0.32	0.06	0.24	1.33	0.25
SO3	0.001	2.46	0.13	0.60	4.10	0.22
L.O.I.	0.01	14.96	35.52
Total	–	95.90	99.82

The weathering of carbonate rocks surrounding the Ptolemais basin serves as the primary natural source of CaO enrichment in the ash (PC_FA = 6.76). This enrichment is attributed to both the detrital material contained in the mined lignite and the co-mined and burned sterile layers of marl. In addition to CaO, the examined sample demonstrates a significant presence of SO₃ (PC_FA = 4.10) and a moderate presence of P₂O₅ (SIT = 1.33), which are elements associated with the organic material contained in the mined lignite. Furthermore, the moderate enrichment of MgO (PC_FA = 1.33) can be attributed to the weathering of ophiolitic formations which surround the Ptolemais Basin, leading to the deposition of detrital material with ultrabasic composition within the lignite layers and the beds alternating with the lignite layers.

The initial marl sample (Tab. II) primarily consists of CaO (39.51 wt%), SiO₂ (14.86 wt%), and Al₂O₃ (5.09 wt%). It also contains smaller amounts of Fe₂O₃ (1.89 wt%) and MgO (1.82 wt%). The remaining major elements (TiO₂, MnO, Na₂O, K₂O, and P₂O₅) were measured at less than 1 wt%. The loss on ignition was calculated at 35.52% wt (Table II). The examined marl exhibits a high enrichment in CaO (PC_M = 7.18) and is low in all other major elements. Many thin interbedded sterile horizons that are co-excavated with lignite layers are characterized as marly limestones [20].

Trace Elements

Many trace elements are found in concentrations greater than one ppm in the initial samples of marl and fly ash (Table III). It is observed that the fly ash sample is highly enriched/polluted in contrast to the average composition of the continental crust in U (PC_FA = 15.70), Br (PC_FA = 9.00), Sb (PC_FA = 8.75), Ni (PC_FA = 8.45), and Cd (PC_FA = 7.00). Significant enrichment/pollution is observed for Mo (PC_FA = 3.33) and W (PC_FA = 3.33), while it is depleted (As, Ba, Cu, Hf, Rb, Sc, Sr, Y, Zn, Zr, Nd, Sm, and Eu) to moderately (Co, Cr, Cs, Pb, Th, V, La, and Ce) enriched/polluted in all other trace elements.

Table III. Trace Element Concentrations (ppm) and Pollution Coefficients (PC) for the Starting Fly Ash (PC_FA) and Marl (PC_M) Samples

Element	Detection limit (ppm)	FA	M	Continental crust1 (ppm)	PCFA	PCM
Au (ppb)	5	<5	<5	4	–	–
Ag	0.5	<0.5	<0.5	0.06	–	–
As	2	22	<2	108	0.20	–
Ba	3	253	151	400	0.63	0.38
Be	1	<2	<1	3	–	–
Bi	2	<2	<2	0.2	–	–
Br	1	18	<1	2	9.00	–
Cd	0.5	0.7	<0.5	0,1	7.00	–
Co	1	17	14	10	1.70	1.40
Cr	1	258	222	100	2.58	2.22
Cs	0.5	6.9	3,1	3	2.30	1.03
Cu	1	29	16	55	0.53	0.29
Hf	0.5	2.9	2	3	0.97	0.67
Hg	1	<1	<1	0.07	–	–
Ir (ppb)	5	<5	<5	0.1	–	–
Mo	2	5	<2	1.5	3.33	–
Ni	1	169	121	20	8.45	6.05
Pb	5	17	4	15	1.13	0.27
Rb	20	35	49	90	0.39	0.54
Sb	0.2	1.75	<0.2	0.2	8.75	–
Sc	0.1	9.8	5.8	11	0.89	0.53
Se	3	<3	<3	0.05	–	–
Sr	2	332	780	375	0.89	2.08
Ta	1	<1	<1	2	–	–
Th	0.5	9.3	5.6	7.2	1.29	0.78
U	0.5	31.4	2.1	2	15.70	1.05
V	5	142	53	135	1.05	0.39
W	3	5	<3	1.5	3.33	–
Y	1	18	7	33	0.55	0.21
Zn	1	54	38	70	0.77	0.54
Zr	2	114	49	165	0.69	0.30
La	0.2	30.4	13	30	1.01	0.43
Ce	3	61	18	60	1.02	0.30
Nd	5	26	<5	28	0.93	–
Sm	0.1	4.2	2.1	4.7	0.89	0.45
Eu	0.1	0.9	0.1	1.2	0.75	0.08
Tb	0.5	<0.5	<0.5	0.6	–	–
Lu	0.05	<0.05	0.06	0.3	–	0.20

The high concentration of various trace elements in the fly ash from the Agios Dimitrios Power Plant has been previously reported in several studies [21]–[26]. It is important to note that the enrichment of fly ash with various trace elements does not only arise from the elements present in the lignite but also from the intervening sterile layers that are co-mined and burned with the lignite [20], [27], [28]. Trace elements like U, Cd, Ni, and Cr are abundant among the various trace elements in both the lignite deposits and the produced fly ash [29]. The presence of dispersed fly ash, the weathering, and co-deposition of ultrabasic clastic material with organic and other inorganic elements can be responsible for the high concentrations of Cr and Ni in the soils and groundwater around the Agios Dimitrios Power Plant area [25], [26]. This ultrabasic material, along with separate layers of marl, is co-excavated from the South Field mine and burned with the lignite in the Agios Dimitrios Power Plant, resulting in the enrichment of fly ash with hazardous components.

It is observed that marl sample is highly enriched compared to the average composition of the continental crust in Ni (P_CM = 6.05). Moderate enrichment is observed for Cr (P_CM = 2.22), Sr (P_CM = 2.08), Co (P_CM = 1.40), U (P_CM = 1.05), and Cs (P_CM = 1.03), while it is depleted in Ba, Cu, Hf, Pb, Rb, Sc, Th, Y, Zn, Zr, La, Ce, Sm, Eu and Lu. Marl samples from the Ptolemais-Amyntaio mines demonstrate a low enrichment factor for most trace elements, with depleted to moderate enrichment observed in Pb [15]. The moderate to significant enrichment of elements like Cs, U, Co, Sr, Cr, Cs, and Ni may indicate the influence of the geological formations or mineral deposits in the study area. Among the main components of lignite, Ni exhibits an inorganic affinity with carbon. Trace elements like U, Ni, and Cr are found in significant abundance within both the lignite deposits and the produced fly ash [29].

Leaching Experiments

Chemical Analyses of Leachates

One crucial factor influencing the leaching behavior of various elements is the pH of the suspension [10], [30]. It is well established that the mobility of various elements in an aquatic environment is significantly influenced by the prevailing pH conditions [31]. The suspension of the fly ash sample MFA1 (100% FA) with deionized water resulted in a pH value of 12.5, while the marl sample MFA2 (100% M) showed a pH value of 8.7. Similar pH values in fly ash samples from Agios Dimitrios Power Plant have been reported elsewhere [32]–[34]. The leachate from Agios Dimitrios fly ash presents higher pH values mainly due to elevated CaO + Ca(OH)₂ contents [35]. Alkaline leachates from fly ash are a result of the hydrolysis of metal oxides, such as calcium, sodium, magnesium, and potassium, formed during coal combustion [36].

The pH is significantly influenced by the mineral content of the solid residues (e.g., lime, portlandite, periclase), as well as the dissolution rates of these minerals in aqueous solutions [19], [37]. The release of cations such Ca²⁺ and Mg²⁺ into the solution can have a notable impact on the pH.

During the leaching experiments, constant ratios of the studied mixtures were maintained, while the pH conditions were adjusted to the target values of 6, 8, 10, and 12, as illustrated in Figs. 1–4. Notably, leachable levels varied significantly among elements and samples. A broad range of elements displayed substantial mobility in fly ash-marl mixtures, whereas certain elements exhibited mixed behavior.

Fig. 1. Variation at pH 6–12 of concentrations of cations (mg/L) and trace elements (μg/L) in the studied MFA1 mixture.

Fig. 2. Variation at pH 6–12 of concentrations of cations (mg/L) and trace elements (μg/L) in the studied MFA2 mixture.

Fig. 3. Variation at pH 6–12 of concentrations of cations (mg/L) and trace elements (μg/L) in the studied MFA3 mixture.

Fig. 4. Variation at pH 6–12 of concentrations of cations (mg/L) and trace elements (μg/L) in the studied MFA4 mixture.

Effect of pH on Leaching Procedure

In the initial fly ash (MFA1) (Table IV), leaching of higher concentrations of Ca²⁺ is observed, with the highest concentration being observed at pH 8 (38711 mg/L). κ⁺, Sr²⁺, Na⁺, and Mg²⁺ also show some degree of leaching, although to a lesser extent. Previous studies have reported high concentrations of Ca²⁺ in fly ash samples [35], [38], [39]. Both Ca²⁺ and Sr²⁺ exhibit a consistent trend, with their concentrations remaining high regardless of pH changes. On the other hand, magnesium and sodium display mixed behavior in terms of leaching. Cr shows a great concentration at pH 8–10, with the greatest concentration being observed at pH 10 (1409 µg/L), followed by a decrease in leaching at pH 12, although its concentration remains high.

Table IV. Chemical Analysis of Fly Ash (MFA1) Leaching Fluids at pH 6–12

Identified parameter	Detection limit	pH 6	pH 8	pH 10	pH 12
Cations (mg/L)
Sodium (Na+)	0.005	5.1	5.7	4.8	4.6
Potassium (K+)	0.012	10.9	13.8	34	13.3
Calcium (Ca2+)	0.06	38,434	38,711	38,212	34,225
Strontium (Sr2+)	0.06	9.93	11.3	10.8	11.74
Magnesium (Mg2+)	0.0035	11.6	24.9	2.6	1.4
Trace elements (μg/L)
Cobalt (Co)	5	59	63	42	4
Manganese (Mn)	20	<20	<20	<20	<20
Lead (Pb)	100	<100	<100	<100	<100
Nickel (Ni)	1	218	210	139	37
Iron (Fe)	10	16	41	36	10
Copper (Cu)	1	12	3	<1	<1
Chromium (Cr)	50	<50	1328	1409	998
Zinc (Zn)	1	7	2	5	<1
Rubidium (Rb)	60	182	162	130	104
Tin (Sn)	2000	<2000	<2000	<2000	<2000

Lower concentrations of Ni, Rb, Co, and Fe are observed, primarily at pH 6–8. Co and Fe exhibit similar behavior in response to pH changes. The previous study [40] has reported the leaching of low amounts of Fe from fly ash during leaching tests. Cu and Zn concentrations were found to be significantly low, while Mn, Pb, and Sn were detected below the detection limit in all pH conditions. Elements such as Ni and Zn show strong aluminosilicate or iron oxide affinities in fly ash, leading to their low solubility under mild leaching conditions [31]. The alkalinity of fly ash contributes to the reduction in the release of elements of concern, such as Mn and Cu, compared to other elements [41]. In a similar study [42], elements such as Co, Cu, Mn, Ni, Rb, and Zn show a lower degree of mobility because they are included in the aluminosilicate amorphous phase and iron oxides of solid wastes (Fig. 1).

In the initial marl sample (MFA2) (Table V), higher concentrations of Ca²⁺ are leached, with the highest concentration indicated at pH 8 (1706 mg/L). and lower concentrations of Na⁺, Mg²⁺, K⁺, and Sr²⁺ were also leached. Mg²⁺, Sr²⁺, and K⁺ show a common trend, with their concentrations gradually decreasing as conditions become more alkaline. In contrast, Ca²⁺ and Na⁺ show a mixed behavior of leaching with an increased concentration of Ca²⁺ from pH 6 to pH 8 and decreased concentration from pH 10 to pH 12. At the same time, there is a significant increase in Na⁺ concentration within the pH range of 10 to 12, with the highest value of 2132.4 mg/L at pH 12. Gradual leaching of Ni is observed at high concentrations, with the highest concentration observed at pH 6–8 (315 μg/L and 227 μg/L, respectively), while at pH 10–12, a significant reduction in leaching is observed, with the concentration remaining particularly high. Co and Rb show similar behavior with the change in pH, and their concentration gradually decreases to a pH value of 12. Cu and Zn concentrations were significantly low, while under all pH conditions, the detected trace elements Mn, Pb, and Sn were identified at quantities below the detection limit (Fig. 2).

Table V. Chemical Analysis of Marl (MFA2) Leaching Fluids at pH 6–12

Identified parameter	Detection limit	pH 6	pH 8	pH 10	pH 12
Cations (mg/L)
Sodium (Na+)	0.005	4.5	4.2	160.9	213.4
Potassium (K+)	0.012	10.8	5.5	6.5	0.7
Calcium (Ca2+)	0.06	333	1706	1689	809
Strontium (Sr2+)	0.06	7.9	1.61	0.86	0.23
Magnesium (Mg2+)	0.0035	66.6	39.4	4.9	2.3
Trace elements (μg/L)
Cobalt (Co)	5	80	63	38	17
Manganese (Mn)	20	<20	<20	<20	<20
Lead (Pb)	100	<100	<100	<100	<100
Nickel (Ni)	1	315	227	142	73
Iron (Fe)	10	<10	21	36	<10
Copper (Cu)	1	14	3	3	8
Chromium (Cr)	50	<50	<50	<50	<50
Zinc (Zn)	1	5	40	12	7
Rubidium (Rb)	60	145	105	74	<60
Tin (Sn)	2000	<2000	<2000	<2000	<2000

High Ca²⁺ concentrations are detected in the leachate of the MFA3 mixture (Table VI), with the highest concentration detected at pH 6 (37215 mg/L), followed by a gradual decrease in leaching up to pH 12 (28133 mg/L). Na⁺, Sr²⁺, and K⁺ show a mixed behavior as the pH level increases. Strong Na⁺ leaching at concentrations of 160.9 and 213.4 mg/L was observed at pH values 10 and 12, respectively, while at the lower pH values (6 and 8), less leaching of Na⁺ was observed. Less leaching of Sr²⁺ and K⁺ was observed at pH 10 and 12, while at pH 6 and 8, stronger leaching was observed. It’s observed that Mg²⁺ release with a high concentration at acidic pH 6 (66.9 mg/L) and a significant decrease at pH 8–12. In addition, a significant leaching of Ni is observed at pH 6 (218 μg/L), which gradually decreases with decreasing pH values. Strong Cr leaching at concentrations of 1009 and 988 μg/L was observed at pH values 10 and 12, respectively, while at the pH 8, Cr measured at lower concentrations of 672 μg/L, and at the pH 6, Cr measured below detection limit (<50 μg/L). Co, Rb, and Cu exhibit similar behavior with changing pH, with their leaching concentrations being high under the lower pH conditions and gradually decreasing up to pH 12. Zn concentrations were significantly low, but although the trace elements Mn, Pb, and Sn were measured, they were found at concentrations below the detection limit at all pH conditions (Fig. 3).

Table VI. Chemical Analysis of the Leaching Fluids of the MFA3 Mixture at pH 6–12

Identified parameter	Detection limit	pH 6	pH 8	pH 10	pH 12
Cations (mg/L)
Sodium (Na+)	0.005	7.4	7.5	6.5	6.6
Potassium (K+)	0.012	38.5	10.9	10.9	13.9
Calcium (Ca2+)	0.06	37,215	35,111	32,564	28,133
Strontium (Sr2+)	0.06	13.5	12.6	12.3	13.9
Magnesium (Mg2+)	0.0035	66.9	16.2	8.3	3.7
Trace elements (μg/L)
Cobalt (Co)	5	90	75	50	17
Manganese (Mn)	20	<20	<20	<20	<20
Lead (Pb)	100	<100	<100	<100	<100
Nickel (Ni)	1	303	218	142	78
Iron (Fe)	10	13	<10	54	31
Copper (Cu)	1	18	9	<1	<1
Chromium (Cr)	50	<50	672	1009	988
Zinc (Zn)	1	7	10	16	2
Rubidium (Rb)	60	181	129	118	94
Tin (Sn)	2000	<2000	<2000	<2000	<2000

The leachates of MFA4 mixture (Table VII) present high concentrations of Ca²⁺, up to 32,730 mg/L at pH 6. As the pH increases, the concentration of Ca²⁺ in the leachate decreases. Lower concentrations of Mg²⁺, K⁺, Sr²⁺, and Na⁺ are detected. Mg²⁺, with a high concentration at acidic pH 6 (52.3 mg/L), shows a significant decrease at pH 12 (2.8 mg/L). In addition, a significant leaching of Ni is observed at pH 6 (303 μg/L), which gradually decreases with decreasing pH values. Similarly, significant leaching of Cu is observed at pH 6 (200 μg/L), which significantly decreases at pH 8 (15 μg/lL), while at the higher pH values (8 and 10), Cu measured below the detection limit (<50 μg/L). Strong Cr leaching at concentrations of 644 and 942 μg/L was observed at pH values 10 and 12, respectively, while at the lower pH values (6 and 8), Cr measured below the detection limit (<50 μg/L). Co and Rb demonstrate similar behavior with changing pH, with their leaching concentrations being high under the lower pH conditions and gradually decreasing up to pH 12. Fe and Zn concentrations were significantly low, but although the trace elements Mn, Pb, and Sn were measured, they were found at concentrations below the detection limit at all pH conditions (Fig. 4).

Table VII. Chemical Analysis of the Leaching Fluids of the MFA4 Mixture at pH 6–12

Identified parameter	Detection limit	pH 6	pH 8	pH 10	pH 12
Cations (mg/L)
Sodium (Na+)	0.005	10.0	7.9	7.3	6.9
Potassium (K+)	0.012	20.8	15.8	12.4	21
Calcium (Ca2+)	0.06	32,730	27,912	23,758	15,839
Strontium (Sr2+)	0.06	15.7	14.2	12.2	13.1
Magnesium (Mg2+)	0.0035	52.3	22	15.7	2.8
Trace elements (μg/L)
Cobalt (Co)	5	126	65	46	27
Manganese (Mn)	20	<20	<20	<20	<20
Lead (Pb)	100	<100	<100	<100	<100
Nickel (Ni)	1	303	220	174	90
Iron (Fe)	10	<10	39	26	39
Copper (Cu)	1	20	15	<1	<1
Chromium (Cr)	50	<50	<50	644	942
Zinc (Zn)	1	9	3	28	2
Rubidium (Rb)	60	162	132	100	89
Tin (Sn)	2000	<2000	<2000	<2000	<2000

The MFA5 mixture’s leachate (Table VIII) has high Ca²⁺ concentrations throughout the pH 6–12 range, with pH 10 indicating the highest value (18940 mg/L) and pH 12 indicating the least leaching (13734 mg/L). Na⁺ shows a high concentration at alkaline pH 12 (109.5 mg/L) and significantly lower concentrations at pH 6–10, while Mg²⁺ demonstrates a high concentration at pH 6 (54.9 mg/L) and then significantly lower concentration at pH 8–12. Lower concentrations of Sr²⁺,and K⁺, with mixed behavior while pH increases.

Table VIII. Chemical Analysis of the Leaching Fluids of the MFA5 Mixture at pH 6–12

Identified parameter	Detection limit	pH 6	pH 8	pH 10	pH 12
Cations (mg/L)
Sodium (Na+)	0.005	7.6	7.2	7.6	109.5
Potassium (K+)	0.012	11.6	15.1	20.7	13.1
Calcium (Ca2+)	0.06	17,611	17,279	18,940	13,734
Strontium (Sr2+)	0.06	13.4	12.4	13.4	12.0
Magnesium (Mg2+)	0.0035	54.9	4.3	12.6	0.9
Trace elements (μg/L)
Cobalt (Co)	5	96	80	59	21
Manganese (Mn)	20	<20	<20	<20	<20
Lead (Pb)	100	<100	<100	<100	<100
Nickel (Ni)	1	670	249	200	125
Iron (Fe)	10	<10	<10	16	31
Copper (Cu)	1	15	15	<1	<1
Chromium (Cr)	50	<50	<50	179	608
Zinc (Zn)	1	5	7	33	2
Rubidium (Rb)	60	173	136	99	79
Tin (Sn)	2000	<2000	<2000	<2000	<2000

In the leaching experiments of the MFA5 mixture, leachates show high Ni concentration, with the highest concentration noted at pH 6 (670 μg/L), which gradually decreases to 125 μg/L at pH 12. Strong chromium leaching at concentrations of 608 µg/L was observed at pH 12 and lower at pH 10 (179 µg/L), while at the lower pH values (6 and 8), chromium measured below the detection limit (<50 μg/L). Co and Rb exhibit similar behavior with changing pH, with their leaching concentrations being high under the lower pH conditions and gradually decreasing up to pH 12. Zn, Fe, and Cu concentrations were significantly low, but although the trace elements Mn, Fe, and Sn were measured, they were found at concentrations below the detection limit at all pH conditions (Fig. 5).

Fig. 5. Variation at pH 6–12 of concentrations of cations (mg/L) and trace elements (μg/L) in the studied MFA3 mixture.

Effect of FA Ratio on Leaching Products Under Constant pH Values

There are notable Ca²⁺ concentrations in leachates at pH 12 for both fly ash and mixtures with a high percentage of fly ash (Table IX). The maximum concentration of Ca²⁺ in leachates is indicated in the MFA1 (100% w/w fly ash sample) with a value of 34225 mg/L, gradually decreases as the fly ash quantity in the mixtures decreases, reaching 13734 mg/L in the mixture MFA4 with 25% w/w fly ash. The marl sample (MFA2) shows low Ca²⁺ leaching at pH value 12 (809 mg/L). From the above, it is obvious that the fly ash is the source of the Ca²⁺ leaching at pH value 12. The other cations (Na, K, Sr, Mg) leach at lower concentrations at pH 12, with Na⁺ leaching clearly affected by the percentage of marl in the samples with the highest leaching observed in the MFA2 (100% w/w M) mixture, while K, Sr, and Mg cations show mixed behavior with variations in their leaching.

Table IX. Chemical Analysis of the Leaching Fluids of the Mixtures at pH 12

Identified parameter	Detection limit	MFA1	MFA3	MFA4	MFA5	MFA2
Cations (mg/L)
Sodium (Na+)	0.005	4.6	6.6	6.9	109.5	213.4
Potassium (K+)	0.012	13.3	13.9	21.0	13.1	0.7
Calcium (Ca2+)	0.06	34,225	28,133	15,839	13,734	809
Strontium (Sr2+)	0.06	11.7	13.9	13.1	11.1	0.2
Magnesium (Mg2+)	0.0035	1.4	3.7	2.8	0.9	2.3
Trace elements (μg/L)
Cobalt (Co)	5	4	17	27	21	17
Manganese (Mn)	20	<20	<20	<20	<20	<20
Lead (Pb)	100	<100	<100	<100	<100	<100
Nickel (Ni)	1	37	78	90	128	73
Iron (Fe)	10	10	31	39	31	<10
Copper (Cu)	1	<1	<1	<1	<1	8
Chromium (Cr)	50	998	988	942	608	<50
Zinc (Zn)	1	<1	2	<1	2	7
Rubidium (Rb)	60	104	94	89	80	44
Tin (Sn)	2000	<2000	<2000	<2000	<2000	<2000

Cr is the main trace element extracted in the liquid phase from fly ash (MFA1) at pH 12, at an extremely high concentration (998 µg/L), and it is still present in significant concentrations in the leachate of fly ash mixtures MFA3 (988 μg/L), MFA4 (942 μg/L), and MFA5 (608 μg/L), clearly indicating fly ash as soluble Cr source. It is worth noting that marl (MFA2) mixture (100% w/w M) at pH 12 leaches Cr at concentrations below the detection limit (<50 μg/L).

Rb shows the same leaching trend as Cr, with a progressive decrease in its concentration as the content of fly ash in the mixture decreases–revealing a positive correlation and also demonstrating fly ash as its source. In contrast, the trace elements that were measured above the detection limit in the leachates (Co, Ni, and Fe) showed mixed behavior influenced by both fly ash and marl content in the mixtures. It is worth noting that Mn, Pb, Cu, Zn, and Sn were measured below the detection limit in all leaching experiments at pH 12, and only a small amount of Cu and Zn was leached for marl sample MFA2 (100% w/w), indicating their lithogenic origin (Fig. 6).

Fig. 6. Variation at pH 12 of cations (mg/L) and trace elements (μg/L) concentrations with studied mixtures.

There are notable Ca²⁺ concentrations in leachates at pH 10 for both fly ash and mixtures with a high percentage of fly ash (Table X). The highest concentration of Ca²⁺ in leachates is observed in the MFA1 (100% w/w fly ash sample) with value 38212 mg/L gradually decreases as the fly ash content in the mixtures decreases, reaching 23758 mg/L in the mixture MFA4 with 25% w/w fly ash. The marl sample (MFA2) shows low Ca²⁺ leaching at pH value 10 (1686 mg/L). It is obvious from the above that at pH value 10 the fly ash is responsible for the leaching of Ca²⁺. The other cations (Na, K, Sr, Mg) leach at lower concentrations at pH 10, with Na⁺ leaching clearly affected by the percentage of marl in the samples with the highest leaching observed in the MFA2 (100% w/w M) mixture, while K, Sr, and Mg cations show mixed behavior with variations in their leaching.

Table X. Chemical Analysis of the Leaching Fluids of the Mixtures at pH 10

Identified parameter	Detection limit	MFA1	MFA3	MFA4	MFA5	MFA2
Cations (mg/L)
Sodium (Na+)	0.005	4.8	6.5	7.3	7.6	160.9
Potassium (K+)	0.012	34	10.9	12.4	20.7	6.5
Calcium (Ca2+)	0.06	38,212	32,564	23,758	18,940	1686
Strontium (Sr2+)	0.06	10.8	12.3	12.2	13.4	0.86
Magnesium (Mg2+)	0.0035	2.6	8.3	15.7	12.6	4.9
Trace elements (μg/L)
Cobalt (Co)	5	42	50	46	59	38
5+ Manganese (Mn)	20	<20	<20	<20	<20	<20
Lead (Pb)	100	<100	<100	<100	<100	<100
Nickel (Ni)	1	139	142	174	200	142
Iron (Fe)	10	36	54	26	16	36
Copper (Cu)	1	<1	<1	<1	<1	3
Chromium (Cr)	50	1409	1009	644	179	<50
Zinc (Zn)	1	5	16	28	33	12
Rubidium (Rb)	60	130	118	100	99	74
Tin (Sn)	2000	<2000	<2000	<2000	<2000	<2000

Cr is the main trace element dissolved in the liquid phase from fly ash (MFA1) at pH 10, at a remarkably high concentration (1409 µg/L), and it is still present in significant quantities in the leachate of fly ash mixtures MFA3 (1009 μg/L), MFA4 (644 μg/L), and MFA5 (179 μg/L), clearly indicating fly ash as soluble Cr source. It should be highlighted that marl (MFA2) mixture (100% w/w M) at pH 10 leaches Cr at concentrations below the detection limit (<50 μg/L). Rb shows the same leaching trend as Cr, with a progressive decrease in its concentration as the percentage of fly ash in the mixture decreases, also indicating fly ash as its source.

Ni gradually increases in concentration as the proportion of marl in the mixture increases, indicating that marl is the source of the element. In contrast, the trace elements that measured above the detection limit in the leachates (Co, Zn, and Fe) have a varied behavior affected by the mixtures’ fly ash and marl concentrations. It is worth noting that the trace elements Mn, Pb, Cu, and Sn were measured below the detection limit in all leaching experiments at pH 10, and only a minor amount of Cu was leached for marl sample MFA2 (100% w/w), indicating their lithogenic origin (Fig. 7).

Fig. 7. Variation at pH 10 of cations (mg/L) and trace elements (μg/L) concentrations with studied mixtures.

At pH 8, fly ash and mixtures with a high percentage of fly ash (Table XI) indicate significant contents of Ca²⁺ in leachates. As the fly ash content in the mixtures decreases, the highest concentration of Ca²⁺ in leachates is observed in the MFA1 (100% w/w fly ash sample) with a value of 38711 mg/L. This concentration gradually decreases eventually to 27912 mg/L in the mixture MFA4 (25% w/w fly ash). At pH 10, the marl sample (MFA2) indicates moderate Ca²⁺ leaching (1706 mg/L). It is clear from the above that at pH value 8, the fly ash is responsible for the leaching of Ca²⁺. The other cations (Na, K, Sr, Mg) leach at lower concentrations at pH 8 with Mg²⁺ leaching clearly affected by the percentage of marl in the samples with the highest leaching observed in the MFA2 (100% w/w M) mixture, while K, Sr, and Na cations show mixed behavior with variations in their leaching.

Table XI. Chemical Analysis of the Leaching Fluids of the Mixtures at pH 8

Identified parameter	Detection limit	MFA1	MFA3	MFA4	MFA5	MFA2
Cations (mg/L)
Sodium (Na+)	0.005	5.7	7.5	7.9	7.2	4.2
Potassium (K+)	0.012	13.8	10.9	15.8	15.1	5.5
Calcium (Ca2+)	0.06	38,711	35,111	27,912	17,279	1706
Strontium (Sr2+)	0.06	11.3	12.6	14.2	12.4	1.6
Magnesium (Mg2+)	0.0035	24.9	16.2	22	4.3	39.4
Trace elements (μg/L)
Cobalt (Co)	5	63	75	65	80	63
Manganese (Mn)	20	<20	<20	<20	<20	<20
Lead (Pb)	100	<100	<100	<100	<100	<100
Nickel (Ni)	1	210	218	220	249	227
Iron (Fe)	10	41	<10	39	50	21
Copper (Cu)	1	3	9	15	15	3
Chromium (Cr)	50	1328	672	<50	<50	<50
Zinc (Zn)	1	2	10	3	7	40
Rubidium (Rb)	60	162	129	132	136	105
Tin (Sn)	2000	<2000	<2000	<2000	<2000	<2000

Cr is the main trace element extracted in the liquid phase from fly ash (MFA1) at pH 8, at an extremely high concentration (1328 µg/L), and it is still present in significant concentrations in the leachate of fly ash mixtures MFA3 (672 μg/L), clearly indicating fly ash as soluble Cr source. It is significant to point out that marl (MFA2) (100% w/w M) and the other mixtures (MFA4, MFA5) at pH 8 leach Cr at concentrations below the detection limit (<50 μg/L).

Rb shows the exact same leaching trend as Cr, gradually decreasing in concentration as the mixture’s fly ash content decreases, indicating fly ash as its source. In contrast, the trace elements that were measured above the detection limit in the leachates (Co, Cu, Zn, and Fe) showed mixed behavior that was affected by the mixtures’ fly ash and marl concentrations. Ni gradually increases in concentration as the amount of marl in the mixture increases, indicating that marl is the source of the element. It is worth noting that the trace elements Mn, Pb, and Sn were measured below the detection limit in all leaching experiments at pH 8 (MFA1, MFA3, MFA4, and MFA5) (Fig. 8).

Fig. 8. Variation at pH 8 of cations (mg/L) and trace elements (μg/L) concentrations with studied mixtures.

At pH 6, there are notable Ca²⁺ concentrations in the leachates of both the fly ash and the mixtures that contain a greater amount of fly ash (Table XII). The mixture MFA1 (which contains 100% w/w fly ash) has the highest concentration of Ca²⁺ in leachates (38434 mg/L). As the fly ash amount in the mixtures decreases, the concentration gradually decreases, reaching 32730 mg/L in the mixture MFA4 (25% w/w fly ash). The marl sample (MFA2) reveals the lowest Ca²⁺ leaching at pH value 6 (333 mg/L). It is obvious from the above data that at the pH 6 the fly ash has the main responsibility for the leaching of Ca²⁺. The other cations (Na, K, Sr, Mg) are leached at lower concentrations at pH 6, showing mixed behavior with variations in their leaching, with Mg²⁺ leaching obviously affected by the percentage of marl in the samples with high leaching seen in the MFA2 (100% w/w M) mixture 66.6 mg/L. The K, Sr, and Na cations show mixed behavior with variations in their leaching.

Table XII. Chemical Analysis of the Leaching Fluids of the Mixtures at pH 6

Identified parameter	Detection limit	MFA1	MFA3	MFA4	MFA5	MFA2
Cations (mg/L)
Sodium (Na+)	0.005	5.1	7.4	10	7,6	4.5
Potassium (K+)	0.012	10.9	38.5	20,8	11,6	10.8
Calcium (Ca2+)	0.06	38434	37215	32730	17611	333
Strontium (Sr2+)	0.06	9.9	13.5	15.7	13.4	7.9
Magnesium (Mg2+)	0.0035	11.6	66.9	52.3	54.9	66.6
Trace elements (μg/L)
Cobalt (Co)	5	59	90	126	96	80
Manganese (Mn)	20	<20	<20	<20	<20	<20
Lead (Pb)	100	<100	<100	<100	<100	<100
Nickel (Ni)	1	218	303	303	670	315
Iron (Fe)	10	16	13	<10	<10	<10
Copper (Cu)	1	12	18	20	15	14
Chromium (Cr)	50	<50	<50	<50	<50	<50
Zinc (Zn)	1	7	7	9	5	5
Rubidium (Rb)	60	182	181	162	173	145
Tin (Sn)	2000	<2000	<2000	<100	<2000	<2000

The leachate of fly ash mixtures MFA3 (181 μg/L), MFA4 (162 μg/L), and MFA5 (173 μg/L) each contain significant concentrations of Rb, which is the main trace element extracted in the liquid phase from fly ash (MFA1) at pH 6 at an extremely high concentration (182 µg/L). This clearly indicates fly ash as a soluble source of Rb. Ni shows a gradual increase in its concentration as the percentage of marl in the mixture increases, with the highest concentration in the mixture MFA5 (75% w/w marl 25% w/w fly ash), indicating marl as its source.

The trace elements that were measured above the detection limit in the leachates (Co, Cu, and Zn) showed mixed behavior influenced by both fly ash and marl content in the mixtures. It is worth noting that the trace elements Mn, Pb, Cr, Rb, and Sn were measured below the detection limit in all leaching experiments at pH 6 for all samples. Only a small amount of Fe was leached for fly ash sample MFA1 and mixture MFA3, while for all other samples, it was measured below the detection limit (Fig. 9).

Fig. 9. Variation at pH 6 of cations (mg/L) and trace elements (μg/L) concentrations with studied mixtures.

The concentrations of these trace elements in the soil, as well as in underground and surface waters, are significant factors affecting environmental quality [14], [21], [25], [26]. Co is found in high concentrations, up to 200 mg/kg, in basic rocks on Earth’s surface, while its concentration is low in acidic igneous rocks, ranging from 1 to 15 mg/kg. The upper continental crust contains cobalt at concentrations of 10–12 mg/kg [41]. At pH values of 9 to 10, only a small amount of Co leaches into water, whereas at pH 12, its concentration increases significantly [34], [38].

At the Earth’s surface, Ni is estimated to be around 20 mg/kg, but its content in ultramafic rocks can reach as high as 2000 mg/kg [16]. In fly ash, Ni is associated with organic carbon [42]. During coal combustion, Ni is mainly distributed in the magnetic and silicate fractions of fly ash [42], [43], with most of the Ni leached coming from the aluminosilicate fraction.

Cu is found in concentrations ranging from 25 to 75 mg/kg in the Earth’s crust [16]. Its presence in fly ash is affected by the type and content of coal-containing clay minerals. During coal combustion, clay minerals are destroyed to give an amorphous phase enriched with Cu and other ions, which are difficult to release in environment. Cu is leached at high concentrations from fly ash at acidic conditions [34].

In the earth’s crust, the average concentration of Zn is 70 mg/kg [16]. Compared to other heavy metals, Zn is characterized by higher solubility. At acidic pH conditions, the concentration of Zn leached is 3% to 9% of the total Zn content in a sample [44], [45]. Fly ash at alkaline conditions releases 0.02 to 0.2 mg/kg of Zn in the liquid phase [3], [44].

The earth’s upper crust has an average Cr value of around 100 mg/kg, with basic and ultrabasic rocks containing the highest quantities of Cr [46]. Ultramafic rocks have Cr contents exceeding 3000 mg/kg [16]. Cr enrichment in coals is influenced by the geology of the specific area. Weathering of ultramafic rocks can lead to the presence of rock fragments as well as soluble amounts of Cr and Ni in the lignite layers, where they remain organically bound [6], [47]. In bituminous coals, Cr is present as trivalent chromium [48], [49]. The solubility of Cr³⁺ is limited in moderately acidic environments, while Cr⁶⁺ shows higher solubility in both acidic and alkaline conditions [16].

The most common type of Cr that is leached from fly ash is Cr⁶⁺ [25], [26], [50], [51]. It constitutes about 2.7% of the total Cr content of fly ash [51]. The leached Cr from the fly ash of the Agios Dimitrios Power Plant is almost exclusively hexavalent, representing 4% to 6% of the total Cr [25], [26]. Cr⁶⁺ is mainly attributed to anthropogenic activities and partly to natural causes associated with the interaction between water and ophiolitic rocks [52].

Conclusions

In conclusion, this study investigates the main and trace element leaching behavior from fly ash-marl mixtures in the W. Macedonia Lignite Center, focusing on the environmental implications of specific pH conditions. Key findings could be summarized as follows:

1. The initial fly ash is highly enriched in CaO, significantly enriched in SO₃, and moderately enriched in P₂O₅ and MgO. Moreover, the initial fly ash is highly enriched relative to the average composition of the continental crust in U, Br, Sb, Ni, and Cd and significantly enriched in W and Mo. On the other hand, the initial marl is highly enriched in CaO and in Ni but depleted in other major elements.
2. Examining the effect of pH on the leaching procedure, the study points out the considerable concentration of Ca²⁺ as well as from fly ash and mixtures with a high percentage of fly ash, with the maximum concentration observed in the MFA1 with value 38711 mg/L at pH 8. Additionally, in the MFA1 sample, K⁺, Sr²⁺, Na⁺, and Mg²⁺ show some degree of leaching, although to a lesser extent, while there is strong Cr leaching at concentrations of 1328 µg/L and 1409 μg/L at pH values 8 and 10. The leachates of the MFA2 sample present a significant leaching of Ca²⁺, especially at pH 8 and pH 10, and a significant leaching of Ni (670 μg/L) at pH 6. The leachate of the MFA3, MFA4, and MF5 mixtures shows high Ca²⁺ concentrations throughout the pH 6–12 range.
3. Fly ash is the main cause of Ca²⁺ leaching in the pH range of 6–12. This is clear from an analysis of the impact of the fly ash to marl ratio at constant pH values. Other cations (Na, K, Sr, Mg) show mixed behavior within this range. Notably, Cr is the main trace element leached from fly ash at pH 10–12, indicating fly ash is a soluble Cr source. Conversely, Ni shows increased concentrations with higher marl content, designating marl as its source. The release of Cr in neutral pH conditions has been observed to be due to the dissolution of mineral phases by which Cr is bound by Cr₂O₃(s) [53]. Under acidic and basic conditions, Cr concentrations increase due to the dissolution of these oxide/hydroxide minerals. It is important to highlight that only a minor amount of Cu and Zn was leached from marl sample MFA2, indicating their lithogenic origin at pH 12. According to an amphoteric leaching pattern, Zn and Cu concentrations are at lower concentrations at neutral pH and maximum concentrations at acidic and basic pH levels [54]. The solubility of these amphoteric elements is significantly influenced by the dissolution of oxide/hydroxide minerals [54], [55].
4. The significant effect of the presence of fly ash on the release of the major and trace elements into the aquatic environment highlights the importance of its management, both in water and in soil. Furthermore, pH conditions play a critical role in the leaching behavior of elements, highlighting the need to consider specific pH levels in environmental management practices.

Environmental Implication

Fly ash, which is enriched in various elements, is characterized as a crucial leaching agent of Ca^2+, Cr, and Ni, with notable pH dependencies. The pH dependence is significant, with Ca²⁺ peaking at pH 8, Cr at pH 10, and Ni at pH 6. Marl and fly ash-marl mixtures show a varied leaching behavior, depending significantly on the percentage of the initial material. The research highlights how fly ash plays a crucial role in elements leaching from both land and water, underlining the urgency of effective management strategies. The variation of pH conditions is characterized as a major environmental management factor, urging its early consideration in environmental practices for sustainable risk mitigation. Recognizing the degree of impact of fly ash and the pH-leaching dependence becomes important for fostering a “healthier environment.”