Metal Fractionation and Leaching in Soils from a Gold Mining Area in the Equatorial Rainforest Zone

Sarpong, Lilian; Boadi, Nathaniel Owusu; Akoto, Osei

doi:https://doi.org/10.1155/2023/3542165

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Abstract Introduction Materials and Methods Results and Discussion Conclusion Data Availability Ethical Approval Consent Conflicts of Interest Authors’ Contributions Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 3542165 | https://doi.org/10.1155/2023/3542165

Metal Fractionation and Leaching in Soils from a Gold Mining Area in the Equatorial Rainforest Zone

Lilian Sarpong,¹Nathaniel Owusu Boadi,¹and Osei Akoto¹

Academic Editor: Rabia Rehman

Received19 Aug 2022

Revised15 Jan 2023

Accepted27 Jan 2023

Published21 Feb 2023

Abstract

In this article, a modified BCR procedure and a column leaching test were used to examine the bioavailability and mobility of heavy metals in soils collected from a gold mining area in Ghana. The results for the fractionation of Cd, Cr, Fe, and Mn indicated that high percentages of metals were found in the residual fraction. This implies that the concentrations of metals in the soil are stable under normal environmental conditions. The bioavailability of metals in the soils declined in the following order: Mn (92.4%) > Cd (64.6%) > Cr (46.4%) > Fe (39%). However, the concentrations of labile metals may pose no risk to the environment. In the column test, different rainwater conditions (i.e., natural rainwater and acidified rainwater) were used to imitate the leaching potential of the metals in the actual field. The pH of the soil primarily controlled metal migration into deeper layers. Cumulative metal concentrations in the leachates showed that Fe, Mn, and Cd were high in the tested soils but present at low concentrations, except for Cd. Cadmium showed a higher concentration than the WHO guideline for drinking water, and its seepage into deeper layers may affect the quality of groundwater.

1. Introduction

Waste material (sludge and dust) generated from mining activities contributes to the levels of metal contamination in the environment, especially soil pollution [1]. Soil serves as a storehouse for metals present naturally as well as those added directly through man-made activities, such as mining, or indirectly through the atmospheric deposition of industrial contaminants and automobile emissions [2, 3]. The ability of soil to retain metals and other contaminants is because its surface is chemically active to attract metals in soil solutions. However, factors such as soil pH, organic matter content, mineralogical composition, temperature, microbial activity, ion exchange processes, redox potential, and leaching play a key role [4, 5]. Other factors that control the release, transport, and availability of metal contaminants in the soil environment include the chemical form and whether metals are bound to the reactive fractions [6]. The different mechanisms that aid in the retention of metal ions in these fractions in soil include sorption-desorption (outer- and inner-sphere surface complexation), oxidation-reduction, precipitation-dissolution reactions, and solution complexation [5, 7].

There is a growing concern to study the bioavailability and toxicity of metals in soils to plants, grazing animals, and humans; the effectiveness of soil to act as a sink for metals; and the likely capacity of a metal to be transported from the soil to other environmental compartments [3]. Thus, bioavailability, which is the fraction of the total metal concentration that is made of the dissolved metal species that can be absorbed by plants or other organisms from the soil at a given period, gives a better estimation of the risk posed by metals in the environment [8]. This is because man-made inputs have enhanced the background concentration of metals in soils. Therefore, an evaluation to predict and understand the fate of metals and their likely impact on the environment requires information on the total metal concentration, the biogeochemical forms of the metals, and the possible physiochemical conditions in the environment that may influence transformations within the different phases of the soil [9].

The sequential extraction method has been used to estimate metals with mobility and potential bioavailability in soil and sediments by characterization into different fractions [10]. It involves using a sequence of chemical extraction reagents of varying strength or dissolution ability to release metals associated with operationally defined fractions [5, 6]. The fractionation process is achieved by varying conditions such as pH, degradation of organic matter, and redox potential to separate the metals in the soil into different fractions [11]. The scheme by the European Commission, now called the standards, measurements, and testing program (previously the modified BCRthe Community Bureau of Reference), was employed to recover metals attached to specific soil phases [12]. This scheme involves the following fractionsacid-extractable fraction, reducible fraction, oxidizable fraction, and residual fraction [13].

Metals found in the acid-soluble fraction represent those held to the soil surface by comparatively weak electrostatic forces and those precipitated with carbonates, which may be readily released into the environment due to variations in pH [5, 14]. The reducible fraction targets metal species bound to the Fe and Mn oxyhydroxide constituents and may be made available for plant uptake under reducing conditions in the soil. The oxidizable fraction targets metal species bound to organic and sulphide constituents; this fraction represents metals made available under oxidizing conditions due to the decay of organic material such as detritus or living organisms [13]. The residual fraction refers to those bound to silicates in the mineral lattice or found in the structure of the primary or secondary minerals [6, 15]. Even though this sequential extraction method was initially developed for fluvial bottom sediments, it has been used to characterize soil to assist in waste management, and it is a commonly used method [6, 16].

A laboratory model for mimicking the long-term transport of metals in soil using an infiltrating liquid is achieved through the performance of a column leaching test [17]. Leaching occurs when the infiltrating liquid moves contaminants in soil pore water into deeper soil layers [17]. The metal constituents found in the collected leachate are potentially released from pore water into the environment for uptake by plants or runoff into the surface and groundwater. As discussed by Naka et al. [18], interlaboratory comparisons have proven that column tests are reproducible and repeatable for soil materials and waste.

Amansie district represents one of the most extensive gold mining zones in Ghana. The upsurge of unregulated surface mining activities in the district has left behind heaps of scattered mine waste at the end of mining operations. These abandoned mines’ wastes are exposed to oxidation and other weather conditions, which may lead to chemical transformations that increase metal availability and mobility [19]. These tend to contaminate surface waters used for drinking or irrigational purposes and lead to the buildup of metals in nearby soils [20]. The consequences of these metals and metalloids on the health of humans and animals include carcinogenicity, teratogenicity, immunosuppression, deprived body conditions, and a negative effect on reproduction [21]. Sensitive groups of people living in these communities are likely to be exposed to metal contaminants in the soil due to the rapid expansion of the gold mining activities in this district.

Therefore, this study aimed to identify the geochemical labile fractions and distribution of selected metals and investigate their leaching characteristics in soils. It was hypothesized that the high rainfall pattern in the district could have leached elements into deeper soil layers and resulted in the low concentration of elements in the topsoil from our previous study. The findings would contribute to a better understanding of the potential mobility and transport of elements from the topsoil to other soil layers below in the gold mining district. This was the first comprehensive study to be carried out in the present study area that used the modified BCR procedure to fractionate Cd, Cr, Fe, and Mn in the soil and looked at the mobility of Cd, Cr, Cu, Fe, Mn, and Zn in these soils using a column leaching test.

It was hypothesized that the high rainfall pattern in the district could have leached metals into deeper soil layers and resulted in the low concentration of metals in the topsoil from our previous study. The findings would contribute to a better understanding of the potential mobility and transport of metals from the topsoil to other soil layers below in the gold mining district and aid in the management of mined sites. This was the first comprehensive study to be carried out in the present study area that used the modified BCR procedure to fractionate metals in the soil and looked at the solubility/leachability of the metals in these soils.

2. Materials and Methods

2.1. Description of the Study Area

The Amansie South district in Ghana is well known for intensive artisanal gold mining (AGM) activities because of the rich deposits of gold that attract investors (both local and foreign). The district has an elevation of 210 m above the sea level and is drained by the Oda River. The climate of the Amansie mining district is characterized by a double rainfall trendmaximum from March to July and minimum from September to November and a dry season from December to March [22]. It has an average yearly rainfall in the range of 855 of 1,500 mm. Over the year, temperatures are generally warm, with an average monthly temperature of around 27°C. The vegetation is largely of the rain forest kind and depicts moist, semideciduous features. Soils are classified as ochrosols in soil taxonomy and ferric fluvisols in the WRB (the World Reference Base for Soil Resources) [22]. The geology is characterized by the West African Birimian rocks with granitic gold lode deposits [23]. Gold is found in the veins and nearby host rocks, and it has a strong connection with pyrite and arsenopyrite [23].

2.1.1. Sample Collection and Preparation

Fifteen (15) soil samples were collected from abandoned mined land, close farms, and residences between the coordinates of N 06°15′27.5″–W 001°54′18.5″ and N 06°17′37.6″–W 001°54′20.9″ (Figure 1). During sample collection, a stainless-steel knife was used to loosen the soil from the surface to a depth of 10 cm, and five subsamples were collected at each site. These were mixed thoroughly to obtain a composite sample and packed into ziplock bags and labeled before being transported to the laboratory. Soil samples were air-dried on plastic trays and passed through a 2 mm nylon sieve to obtain fine grain particles. These were packed into ziplock bags and stored in a cool place. The pH of the soil samples was determined with a calibrated pH meter in a suspension of a 1 : 2 (w/v) soil/water ratio.

2.2. Apparatus and Chemical Reagents

Throughout the experiment, distilled water with a pH of 7.1 and a conductivity of 0.3 μS cm⁻¹ at 25°C was obtained using a Fistreem cyclone-distilled water plant. The chemicals used were analytical reagent grade from VWR chemicals and BDH Limited (Poole, England). To ensure minimal contamination, the glassware was precleaned by immersing it in a 10% nitric acid solution for 48 h and then rinsing with distilled water.

2.3. Total Metal Concentration

Total element concentrations in the soil samples were determined by digesting a 1.0 g soil sample with HNO₃ and HCl (3 : 1) on a hot plate for 1 h at 95 °C, following the protocol described by [24]. The digested mixture was cooled, filtered through Whatman filter paper into a 100 mL volumetric flask, and topped up with distilled water to the 100 mL mark.

2.4. Fractionation of Metals

The modified BCR procedure initially recommended by Rauret et al. [25] was employed to extract metals from the soil into four different fractions as described below.

2.4.1. Acid-Soluble Fraction: Fraction 1 (F1)

To 1.0 g of the soil sample weighed into a cleaned 50 mL centrifuge tube, 40 mL of 0.11 M acetic acid was added and shaken at room temperature for 16 h on a mechanical shaker. Then, the tubes were centrifuged for 20 min at 1200 rpm, and the extracts were decanted into clean 100 mL polyethylene containers and stored at 4 °C before analysis. The residues were washed with 20 mL distilled water, shaken for 15 min on the mechanical shaker, and centrifuged, and the supernatants were discarded to avoid lingering components from causing an error [9, 25, 26].

2.4.2. Reducible Fraction: Fraction 2 (F2)

Forty mL of 0.5 M hydroxylamine hydrochloride, freshly made, was added to the residues in the centrifuge tubes from procedure 1 [25]. The centrifuge tubes were shaken at room temperature for 16 h on a mechanical shaker and centrifuged at 1200 rpm for 20 min. The extracts were again decanted into clean 100 mL polyethylene containers and stored at 4 °C before analysis. The residues were washed with 20 mL distilled water, shaken for 15 min on the mechanical shaker, and centrifuged, and the supernatants were discarded [9].

2.4.3. Oxidizable Fraction: Fraction 3 (F3)

Ten mL of 8.8 M hydrogen peroxide was added gradually to the residues in the centrifuge tubes from procedure 2. The centrifuge tubes were digested at room temperature for 1 h with occasional manual shaking while the tubes were loosely covered with their caps. The digestion was continued for 1 h in a water bath at 85 °C until the volumes were reduced to about 2 mL. Additional 10 mL of 8.8 M hydrogen peroxide was added and digested for 1 h at 85 °C to reduce the volume to about 1 mL. The mixture was then cooled and extracted with 50 mL of 1.0 M ammonium acetate (adjusted to pH = 2), which was added to the cooled residue. The centrifuge tubes were shaken for 16 h on a mechanical shaker, and the extracts were decanted after centrifugation at 1200 rpm for 20 min and stored at 4 °C before analysis. 20 mL of distilled water was added to the residues, shaken for 15 min on the mechanical shaker, and centrifuged, and the supernatants were discarded [25].

2.4.4. Residual FractionFraction (F4)

The residues left behind after procedure 3 were digested with HNO₃ and HCl (3 : 1) to determine the residual fraction. The mixtures were heated on a hot plate at 95 °C for 1 h [24]. The digested mixtures were cooled, filtered into a 100 mL volumetric flask, and topped up with distilled water to the 100 mL mark.

2.5. Column Leaching Test

A column test was carried out to study the leaching behaviour of the selected metals in the soils of the study area, following the procedure below.

2.5.1. Experimental Set-Up

Glass columns of 1.5 cm in diameter and 60 cm in height were used in this experiment and maintained at room temperature. Two soils were usedsoil A (sandy soil) was collected from a mined site (sampling coordinates of N 06°15′57.8″ and W 001°54′51.1″) and soil B (clay loam soil) was collected from a farm (sampling coordinates of N 06°16′04.9″ and W 001°53′16.2″) within the study area. To ensure the absence of residual metals, the columns were soaked in an acid bath and thoroughly rinsed with distilled water before the starting of the experiment. A small piece of cotton was placed on top of the frit inside the bottom of the column to minimize the loss of soil particles into the leachate. To mimic field conditions, the experiment was done under unsaturated conditions because the study area was not a flood zone [27]. Aliquots of 45.0 g of the soil samples were gradually packed into the columns to ensure uniformity. The total volume of rainwater required for the leaching cycle (V) was calculated as described by Kossoff et al. [28] using equations (1)–(4):where is the average yearly rainfall in the district; 855 mm (85.5 cm) (MOFA 2019), and A is the cross-sectional area of the column and was calculated using the following equation:where r is the radius of the column; r = (1.5 cm ÷ 2) = 0.75 cm.

Therefore, the total volume of rainwater required for the leaching cycle (V) was calculated as shown in the following equation.

For the five subcycle additions, 150 mL of water was divided into five (5) portions of 30 mL each to mimic the rainfall pattern in the study area. 1.5 L of natural rainwater (NRW) (having pH values of 6.81 and 6.43, respectively) were collected into clean plastic buckets during different rainfall events at Watreso, one of the communities in the study area that engaged in intensive gold mining. This was transported into the laboratory in sealed plastic bottles. Acidified rainwater (ARW) was prepared by adding drops of diluted H₂SO₄ solution to a portion of the NRW to adjust the pH from 6.81 to 4.41 and 6.43 to 2.71 for soils A and B, respectively. Thus, soil A was subjected to NRW of a pH of 6.81 and ARW of a pH of 4.41 in separate columns, while soil B was subjected to NRW of a pH of 6.43 and ARW of a pH of 2.71, also in separate columns. 30 mL of rainwater were added slowly as droplets to the top of the columns. The rainwater took 36 h to leach through the packed columns. To simulate a dry season, the columns were allowed to stand for two days without the addition of rainwater. This cycle of wetting and drying was repeated continuously over three weeks to simulate field conditions. An empty column was subjected to the same treatment without a soil sample to serve as a reference. The leachate volume and pH were measured after collection.

2.5.2. Cumulative Metal Concentration in Leachate and Percentage Leached

The cumulative metal concentration (CMC) released into the leachates was calculated using the following equation [29, 30]:where CMC is the cumulative metal concentration released in soil upon the introduction of the infiltrating water (mg kg⁻¹), is the metal concentration (μg mL⁻¹), V is the volume of leachate collected (mL) during the test, and m is the mass of the soil placed in the column (g).

The percentage of metals that were leached was estimated using the following equation:where M is the percentage of metals that were leached (%), CMC is the cumulative metal concentration released in soil by infiltrating water (mg kg⁻¹), and T is the total concentration of metal in the soil (mg kg⁻¹).

2.6. Instrumental Analysis

Metal concentrations in the extracts were measured using an atomic absorption spectrometer (Perkin Elmer, USA). An air-acetylene flame was used alongside the appropriate lamps to focus a beam of light from the analyte at a specific wavelength on the target metal. The instrument was calibrated as instructed by the manufacturer with standard solutions of the desired concentrations by diluting an aliquot of 1000 ppm stock solution with 2% HNO₃ solution. For all analyses, triplicate measurements were carried out, and blank samples were taken through the same procedures to control the experiment. A plot of absorbance against concentration gave r² values from 0.999 to 1.000 for the metals.

2.6.1. Quality Assurance

The BCR procedure was validated by the analysis of the total concentration and calculating the recovery rate of the metals as in the following equation [7, 30]:

The sum of the four fractions obtained using the sequential extraction procedure was compared to the total concentration of the metals to validate the results [16, 31, 32]. The results were in good agreement, suggesting the accuracy of the extraction method. The recoveries were found to be satisfactory and were as follows137% for Cd, 139% for Cr, 116% for Fe, and 118% for Mn, respectively.

Standard buffer solutions of pH 4, 7, and 10 were used to calibrate the pH meter (OHAUS, Starter 3100) used to measure the pH of the collected leachate from the column. The leachates were filtered when necessary, as most were clear solutions, and a few drops of HNO₃ were added to the leachate to stabilize it at pH < 2 and stored in acid-washed sealed bottles [17]. The concentrations of selected metals (Cd, Cr, Cu, Fe, Mn, and Zn) were determined with an atomic absorption spectrometer (AAS).

2.6.2. Statistical Analysis

Statistical analysis of the data was done using Microsoft Excel, and graphs were plotted. The correlation between the concentrations of selected metals extracted from each fraction was estimated using paleontological statistics software (Past) to evaluate the relationship between metal concentrations obtained by the BCR sequential method.

3. Results and Discussion

3.1. Distribution of the Metals in the Different Soil Fractions

Results of the fractionation analysis for Cd, Cr, Fe, and Mn concentrations in soils from the gold mining district in Ghana are shown in Figure 2. It depicts the distribution pattern of metal levels as percentages in the different fractions extracted using the modified BCR sequential extraction procedure (Figure 2). The fractions represent the potential risk of metal solubility over a long period of time and estimate the metals to be potentially released under varying environmental conditions [14].

Fractionation results for levels of Fe indicate that about 61% of Fe (representing a median value of 2126 mg kg⁻¹) in soil from the study area was found bound to the residual fraction. The high content of Fe associated with the residual fraction suggests that most of the Fe in the soil of the study area is sealed in the structure of minerals (resistant phases like silicate and crystalline iron oxide) and may not be accessible for uptake by plants [9, 33]. On the other hand, the outstanding 39% of Fe was found distributed among the mobile fractions in the order: reducible fraction (20.8% ≈ 725 mg kg⁻¹) > oxidizable fraction (18% ≈ 626 mg kg⁻¹) > acid-extractable fraction (0.22% ≈ 7.61 mg kg⁻¹). Under reducing conditions, the Fe concentration associated with Fe-Mn oxides may be released, while the Fe associated with organic matter may be released into the soil solution for uptake under oxidizing conditions. The acid-soluble fraction combined the water-soluble, exchangeable, and carbonate-bound fractions and formed the reactive fraction, which usually accounts for the lowest percentage of total metals in most soils [6, 12]. Similarly, this fractionation pattern of Fe was observed in a recent study of soils from Pakistan in the order of residual fraction (73–89%) > reducible fraction (3–17%) > oxidizable fraction (4–7%) > exchangeable fraction (<1%) [11]. This suggests that Fe in the soil of the study area is considerably immobile and insoluble because a high proportion is bound to the residual fraction [33].

Chromium concentrations in the soils from the study area were found in the order: residual fraction (53.61% ≈ 2.59 mg kg⁻¹) > oxidizable fraction (28.02% ≈ 1.35 mg kg⁻¹) > reducible fraction (13.68% ≈ 0.66 mg kg⁻¹) > acid-extractable fraction (4.68% ≈ 0.23 mg kg⁻¹). The results suggest that most of the Cr (53.61%) in the soil in the studied area is insoluble under natural conditions. Hence, Cr in soil could be considered inactive, biologically and chemically stable as it is usually of lithogenic origin [11]. The residual fraction is considered unavailable for plant uptake and therefore poses little or no risk to the ecosystem because of its association with silicate minerals [10]. This is consistent with a recent study in South Africa, where the residual fraction dominated and suggested that Cr in the soils of the study area posed low risk [3]. The oxidizable fraction contained the second highest percentage of metals in the study area. Metals found here may be released into the soil solution upon decomposition of organic matter (which comprises litter, decaying roots, living soil organisms, and stable humus often associated with clay minerals) [35]. Nonetheless, because of the stability of the high molecular weight humic component, which regulates the slow release of these metals for uptake by plants, the metals in this fraction tend to remain in the soil for longer [14]. Similarly, an earlier study in a mining area in Nigeria found that Cr was mainly bound to the residual fraction in soil [36]. Another study on soil from industrial Serbian cities indicated that the highest percentage of Cr was mainly found bound to the residual fraction [34]. Nonetheless, a different fractionation pattern is reported in a recent study from Pakistan in this orderresidual fraction (59–81%) > exchangeable fraction (1–21%) > reducible fraction (3–20%) > oxidizable fraction (2–13%) [11].

Cadmium concentrations in the study area were found in the order: residual fraction (35.39% ≈ 0.26 mg kg⁻¹) > acid-extractable fraction (30.06% ≈ 0.22 mg kg⁻¹) > reducible fraction > (26.15% ≈ 0.19 mg kg⁻¹) > oxidizable fraction (8.40% ≈ 0.06 mg kg⁻¹). The acid-extractable fraction consists of metal species that are imperative in carrying out a risk assessment because they are loosely bound to carbonates and readily available for uptake [20]. However, the low concentration of Cd in the acid-extractable fraction implies that Cd in the soil of the study area may not pose a significant risk when released into the soil solution. An earlier study in South Korea also reported the highest percentage of Cd in the residual fraction (46.2% ≈ 38.1 mg kg⁻¹), followed by the acid-soluble fraction (27.2% ≈ 22.5 mg kg⁻¹) in soil surrounding an abandoned mine [26]. Similarly, an early study near a petrochemical complex observed that the residual fraction recorded the highest fraction at 35.7% and the oxidizable fraction recorded the lowest percentage fraction at 7.7% [13]. However, a different fractionation pattern was reported in soils from Pakistan in the orderresidual fraction (22–55%) > reducible fraction (16–35%) > exchangeable fraction (11–28%) > oxidizable fraction (6–15%) [11].

Manganese concentrations in the soils from the study area were found in the order: reducible fraction (69.01% ≈ 14.70 mg kg⁻¹) > acid-extractable fraction (22.47% ≈ 4.79 mg kg⁻¹) > residual fraction (1.63% ≈ 7.64 mg kg⁻¹) > oxidizable fraction (0.88% ≈ 0.19 mg kg⁻¹). The results suggest that under more reducing conditions, the solubility of Mn will increase [36]. The dominance of the reducible fraction suggests that Mn was bound to the hydrous oxides of Fe and Mn, which may occur as coatings on mineral surfaces [5]. The release of Mn (II) into the soil solution for plant uptake may occur under reducing conditions that lead to the reduction of insoluble Mn oxides [38]. In soils, a reducing condition occurs during saturation due to flooding, whereby the absence of atmospheric oxygen in submerged soil causes anaerobic microorganisms to utilize oxidized compounds as electron acceptors by converting them to reduced forms for respiration [37, 39]. A concern of reducing conditions in soil includes a competitive demand for oxygen between roots and microorganisms, and others may consist of changes in nutrient availability needed for the survival of plants [40]. The acid extractable had the second highest percentage of Mn, and this association is reported with other elements such as Ca and K [14]. On the contrary, a different fractionation pattern was reported in soils from Pakistan in the orderreducible fraction (48–63%) > residual fraction (21–35%) > exchangeable fraction (9–19%) > oxidizable fraction (1–4%) [11].

Except for Mn, all other metals in the soil were found to be mainly associated with the residual fraction. The metals in this fraction are immobile and may be considered to originate from the parent material in the study area [41]. The acid-soluble fraction was second to the residual fraction in terms of the percentage dominance of selected metals in the soil of the study area. This suggests that the acid-soluble proportions of Cd, Cr, Fe, and Mn would become soluble in soil solutions when environmental changes in either pH or ionic composition occur [11, 42]. However, their concentrations were low and could not pose a risk.

3.1.1. Estimated Mobility Sequence of the Metals in Soil

The addition of the first three fractions (labile fractions), which represent the metals that may be made available in soil for uptake was in the order: Mn (92.4%) > Cd (64.6%) > Cr (46.4%) > Fe (39%). The results indicated that Mn is the most mobile metal in the soil of the study area due to its strong association with the non-residual fraction, followed by Cd. The high percentages of Mn and Cd in the available fractions also suggest an anthropogenic source for these metals in soil [43]. Similarly, an earlier report by Sungur et al. [44] in Turkey indicated that Mn and Cd were very mobile in soil. These were attributed to human input on agricultural soils. Mn and Cd may pose a toxicity risk to plants grown in such soils over time because they can be transformed into other forms for easy uptake and transport [11, 16]. The combined mechanisms of ion exchange, adsorption, precipitation, and surface complex formation also influence Mn mobility in soil [14]. However, Mn is a known abundant metal with a low potential to cause harmful effects in soil and animal species; it is vital for general physiologic functioning [45, 46]. Moreover, the concentration of mobile Cd (0.47 mg kg⁻¹) was within the 1.4 mg kg⁻¹ limit set for agricultural use [47]. This finding is vital as it estimates the concentration of free ions in soil solutions that are likely to interact with plants or microorganisms to cause an adverse effect [45].

Cr and Fe are usually naturally occurring and associated with the silicate structure, particularly Fe, hence the high percentage in the residual fraction [48]. Despite this, the Cr concentration in the mobile phase (46.4%) of soil from the study area suggests that a significant portion of Cr may be available for plant uptake. A recent study indicated that Cr in soil was found in higher concentrations in leafy vegetables. It recommended altering the soil pH and organic matter to regulate their availability and buildup in the food chain [49].

3.1.2. Total Concentration of Selected Metals and pH of Soil

The median value of the total concentration of metals in soil samples from the study decreased in the order: Fe (3013 mg kg⁻¹) > Mn (18.1 mg kg⁻¹) > Cr (3.47 mg kg⁻¹) > Cd (0.53 mg kg⁻¹). These values were found to be generally lower than the Canadian soil quality limit values for soil used for agricultural purposes [47]. The total concentration of metal in the soil depends on several factors, such as the mineral composition and weathering of the parent material, erosion, leaching, and the history of the study area in relation to human activities [50]. The median pH value of the soil samples was 5.04, which is slightly acidic, and the pH values were within the range of 3.5–7.9. According to Ng et al. [51], the high rainfall and temperatures in the tropics may lead to acidic soil formation due to the leaching of cations such as Ca²⁺ and K⁺ and substitution by Fe²⁺ and H⁺ at the adsorption sites. The low pH of the soils in the study area may contribute to the mobility and release of soluble metals into the environment. For instance, according to Sungur et al. [44], Cd²⁺ species are produced and made available in soils with lower pH values because the bonding with the soil’s solid phase is weaker.

3.2. Column Leaching Test

To provide information on the transport of metal contaminants and the potential effect of leachate seepage in soil [50], a column leach test was used in this study. The column leaching test offers a method to extract metals present in the water-soluble fractions of the soil and, through interlaboratory comparisons, has been proven to be reproducible for soil and waste materials [18, 52]. Two selected soils were subjected to a leaching test in a column to evaluate the mobility of metals such as Cd, Cr, Cu, Fe, Mn, and Zn in rainwater. The pH values of leachate collected during the test are presented in Table 1. Table 1 shows that the soil A had a median pH value of 4.66 for natural rainwater and 4.62 for acidified rainwater, respectively, during the three-week leaching. The low pH of leachate could point to an oxidation reaction in the soil A during the test [18]. The pH values of leachate were found in the range of 2.97 to 6.10 (highly acidic to slightly acidic). The possible explanation for the acidic nature of the leachate collected could be the oxidation of sulphide minerals in the soil A that subsequently enhanced the release of metals into the leachates [20, 53]. This is likely because the soil A was collected from an abandoned mine in the study area. According to Stjernman Forsberg et al. [53], the mobilization of metals in column tests is significantly influenced by pH. An earlier study by Rieuwerts et al. [54] found that there is a decrease in metal adsorption at lower pH due to the decomposition of soil structure and competition for adsorption sites by H⁺. Thus, the low pH values of leachate collected after the introduction of NRW to the soil A could indicate that more metal cations would be extracted from the leachate.

After adding acidified rainwater, the first three leachates collected recorded a gradual decline in pH (6.65, 5.20, 4.19, 4.38, and 4.62). Overall, the volume of leachate showed good recovery compared to the initial 30 mL of rainwater introduced during the test, except when the leachate volume was reduced by evaporation after collection. A slight buffering effect of soil A on the pH of the infiltrating rain was observed.

Soil sample B, which had a pH value of 8.10, was used for the next phase of the column leaching test. The pH values of the first to fifth leachates collected after adding NRW (pH 6.43) were all neutral to slightly basic leachates (7.46, 7.39, 7.09, 8.94, and 8.40, respectively). The pH values of the first to seventh leachates collected after adding ARW (2.71) were 8.82, 8.336, 7.80, 7.65, 5.95, 7.71, and 7.70, respectively. The addition of ARW resulted in a higher pH except for the fifth leachate, which was slightly acidic. The probable explanation for the increase in the leachate pH could be attributed to the existence of reducing conditions in the column during the test and the acid-neutralizing capacity of the soil involved in the test [17].

The results indicate that soil pH plays a vital role in the pH of the leachate collected, irrespective of the pH of the NRW and ARW introduced. Hence, the leaching of metals from the soil by infiltrating liquid depended strongly on the pH value of the soil understudy. This observation is consistent with an earlier study, which indicated that the migration of metals and their biological effects in soil were strongly dependent on soil pH [55]. The variations in pH of the collected leachate during the test could also be associated with the soil’s natural buffering capacity, as this controls the mobility of metals [26]. Thus, in this study, an alkaline soil B had the highest buffering capacity because of its ability to neutralize the ARW, which may form when the burning of fossil fuels emits sulphur dioxide and nitrogen oxide, which react in the air with water droplets to make nitric and sulphuric acid, resulting in acid rain.

3.2.1. Cumulative Metal Concentration in Leachate

A summary of the estimated CMC for the targeted metals is shown in Table 2. The results displayed in Table 2 indicate that Fe had the highest CMC from soil A under NRW infiltration, while Cd had the highest CMC from soil A under ARW infiltration. In soil B, Fe had the highest CMC under NRW infiltration, while Mn had the highest CMC under ARW infiltration. The soil in the study area is rich in Fe, with a total concentration of about 3013 mg kg⁻¹.

However, it was observed that a lower concentration of metals was leached using both NRW and ARW (Table 2). This could imply that the soil of the study area has a purification system that regulates the migration of metals in the soil during rainfall. According to Makó et al. [56], soil reduces the impact of contamination through its decomposing, detoxifying, and buffering potential and hence behaves as a natural filtering medium that reduces the harmfulness of contaminants. Moreover, the strong adsorption capacity of soil controlled the release of metals into the leachate collected. The results indicated that the CMC of Cd in the leachate was above the 0.003 mg/L guideline for drinking water [57]. This suggests that the amount of Cd leached by rainwater into deeper soil may affect the quality of groundwater used for drinking. Cr, Cu, Fe, Mn, and Zn may not pose any health risk because they are needed in small quantities by humans and animals. Moreover, the general low concentration of metals in leachate suggests that the labile pool of metals in the soils of the study area is quite small [17].

3.2.2. The Effect of the pH of Infiltrating Rainwater on Metals in the Soil

Figures 3 and 4 depict the concentrations of metals released in leachates from soils A and B under natural and simulated rainfall conditions. The graph in Figure 3 shows the variations in the concentration of metals in the leachate collected from the columns under both NRW and ARW infiltration during the column leaching test. It was observed from the graphs in Figure 3 that, except for Fe, the highest concentration of metals in the leachate was during the ARW infiltration during the three-week cycle.

It was observed from the graphs in Figure 4 that, except for Cu and Fe, the ARW resulted in higher concentrations of all other metals than the NRW. The release of other metals into leachates could be due to the lower pH, which led to their dissolution. Higher metal concentrations were observed in the third leachate and the seventh leachate when the test period was extended to a four-week cycle in soil B under ARW infiltration. Overall, the metal concentrations in leachates had different patterns in terms of their release from soils A and B under both NRW and ARW. Generally, the concentration in leachate increases steeply to reach a maximum concentration, then falls steeply, and levels off towards the end of the column.

A plausible explanation for the low concentration of metals in the soils of the study area was that most of the metals could have already been flushed out from the topsoil into deeper soil layers during rainfall (leached) and lost through surface runoff or dispersed by wind. This is because the mines had been abandoned for some time [58]. Erosion by wind or water may also account for the movement of metal along the horizontal lane, while leaching accounts for the vertical movement of metal down the soil profile. These may have contributed to the low concentrations of metals in surface soils. In a recent study, erosion by wind or water was responsible for the transport and distribution of metals in soils close to a mining area in South Korea [59]. Likewise, an earlier study in South Africa indicated that the concentrations of metals were higher in the soil at a distance of 100 to 200 m from mine waste, while a lower concentration was reported in the mine waste itself [60]. The likely explanation for this observation was attributed to the displacement of contaminated soil particles to nearby soils during water and wind erosion. Finally, the high acidity of the soil aided in the mobilization of these metals into surrounding soils [60].

3.2.3. The Percentage of Metals That Were Leached

A summary of the percentage of metals that were leached from soils A and B is presented in Table 3. Generally, it was observed that the concentrations of metals that were leached were higher when ARW was used. This could be explained by its low pH or high oxidizing power. The percentage of Cd leached was the highest among the metals investigated for soil A, and this suggests that Cd is not strongly adsorbed to soil particles, and hence rainfall tends to displace it easily into deeper soil profiles. Similar to our results in Table 3, an earlier study reported that the leaching of metals in soil is influenced by adsorption, such that in soil, Cd and Zn have higher mobility when compared to Cr and Cu [59]. The result was consistent with a previous study where Cd migrated easily from the soil during a column leaching test [28]. The differences in the migration behaviour of metals in soils A and B may be caused by several variables, including the soil composition, sorption-desorption process, porosity, and pH of the soil [18]. However, the processes involved in releasing metals from surface soil to deeper layers are complex and regulated by several factors [28]. Overall, lower concentrations of metals were found in the collected leachates, and this could be attributed to the natural buffering capacity of the soil that helps to purify itself. Similarly, a recent study reported on the low content of metals in leachate from a leaching test [61]. The significant role of pH was evident as it influenced the concentration of metals leached in soils A and B.

The alkaline nature of soil B (pH = 8.10) indicates the presence of carbonate, which tends to increase the soil’s adsorptive capacity and decrease the migration of metals from the soil [32]. Hence, an ARW of pH 2.71 was used to enhance the leaching of metals from soil B. However, the type of adsorption complex in soils may be influenced by several factors, such as pH, ionic strength, surface loading, time, and the type of sorbent [62]. Thus, the adsorption sites on the soil surfaces of soil B were many, and the retained metals were released slowly upon either the introduction of NRW or ARW. Hence, the leached metals were lower in soil B than in soil A. The acidic nature of the soil A (pH = 3.52) caused it not to retain its metals upon the introduction of either the NRW or ARW.

3.3. Comparison of the Results of the Two Extraction Procedures

Comparatively, metals extracted into the acid-soluble fraction from the fractionation procedure were expected to be higher than metals extracted under the column leaching test. The likely explanation for this result could be that the acid-soluble fraction accounts for metals in the water-soluble, exchangeable, and those bound to carbonate constituents of soil, while the column accounts for metals found in the water-soluble fraction of soil. Similar results were observed in a study in the United Kingdom, where the concentration of metals in the acid-soluble fraction was higher than that extracted with water in a column leaching test [52]. The results suggest that a higher concentration of metals could be extracted from soil because the modified BCR procedure employed reagents with increased extraction ability and had a longer contact time between the extracting agent and soil [52]. Moreover, there is a problem of suspension forming in column tests involving rainwater instead of infiltrating liquid of high extracting ability because of the influence of organic matter content on the soil and water interactions [5]. Hence, only the easily soluble fraction of the metals bound to soil is found in leachate after a column test. The results show that the modified BCR procedure and the column test give detailed information about the metal bioavailability and leaching behaviour and are useful for assessing the risk posed by metals in an ecological sample such as soil and highlighting areas that require immediate remediation to mitigate pollution of the environment.

4. Conclusion

This study employed different methods to better understand the factors that regulate metals’ extractability and leaching behaviour in soils from a gold mining district in Ghana. The results from the fractionation of metals in soils showed that the highest concentrations of metals were associated with the residual fraction (stable form), which thus suggested that the metals were naturally present in the soil of the study area. Manganese was the most bioavailable and hence potentially available for uptake by plants or soil organisms, followed closely by Cd. Despite this, the levels of metals in the soil were low and suggested a minimal subsequent negative effect.

On the other hand, results from the column test indicated that the pH of the soil was a major factor that regulated the migration of metals in soil profiles. Cadmium and mercury were the most mobile metals in soils A and B, respectively, under NRW and ARW infiltration. The CMC of Cd in leachate was above the WHO guideline for drinking water, but the other metals could not pose any risk to groundwater. The low pH and low metal content of the soil in the study area indicated that most of the solubilized metals were removed from the topsoil due to the high rainfall and temperature in the district. Erosion by wind or water and leaching into deeper soil layers may be major contributing factor to the low concentration of metals in the topsoil. The results from this study have shown that selected metals in the soils of the study area are potentially less available for plant uptake, and their low concentrations suggest that the detected metals would pose a considerably low risk to the environment. Thus, the data generated in this study have given a better understanding of metals' behaviour, which has helped assess their potential impact on soils in the study area. Knowledge gained from this study will provide helpful information about the risks that mined soils pose to other ecological media and to human health.

Data Availability

All the data generated or analyzed during this study are included within this article.

Ethical Approval

Not applicable.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Authors’ Contributions

OA and NB conceptualized, designed, and supervised the study. LS carried out the experiment, analyzed, and interpreted the data regarding the statistical analysis. All authors made substantial contributions to the writing of the manuscript. All authors read and approved the final manuscript.

Acknowledgments

This research was carried out at the Department of Chemistry, Kwame Nkrumah University of Science and Technology.

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