Fate and Transport of Lead and Copper in Calcareous Soil

Abstract

Heavy metals transport to groundwater relies on the characteristics of soil, such as carbonate and clay minerals, organic matter content, soil pH, and some other factors. Most of the heavy metals in calcareous soils are precipitated as metal carbonate minerals; consequently, their transport to the groundwater is not anticipated. Therefore, the current study focused on the impacts of calcium carbonate presence on the adsorption and transport of lead (Pb) and copper (Cu) in calcareous soil using batch and column experiments. To elucidate the contaminants removal mechanisms in calcareous soils, extensive laboratory batch investigations were conducted to study the equilibrium kinetic and adsorption isotherm characteristics of the two studied heavy metals. The quick adsorption of Pb²⁺ and Cu²⁺ by soil was seen in kinetics trials. In addition, Pb²⁺ and Cu²⁺ sorption onto the soil was best described by the pseudo-second order kinetic model (R² = 0.9979 and 0.9995 for Cu²⁺ and Pb²⁺, respectively). To explain the equilibrium sorption data, the Freundlich isotherm showed the best fitness to Pb²⁺ (R² = 0.96) and Cu²⁺ (R² = 0.98), collectively. The Freundlich parameters revealed that the Pb²⁺ has favorable adsorption; however, Cu²⁺ has unfavorable adsorption onto the soil. The results of column experiments showed the higher binding of Pb²⁺ than Cu²⁺ to the top surface of the soil column, making the movement of these two metals very slow. In columns, most of the Pb²⁺ and Cu²⁺ ions were sorbed at an initial 5 and 10 cm, respectively. The findings of this study will help in understanding the fate of heavy metals in calcareous soils.

1. Introduction

Economic development and rapid growth in most fields, such as agriculture and other industries, have resulted in an invasive increase in environmental pollution worldwide [1]. The pollutants could be heavy metals and pesticides which cause the disturbance of the entire ecosystem by posing serious damage to the environment [2]. The metals with high atomic weight and a density greater than 5 g/cm³ are termed heavy metals [3]. Compared to the physical properties of heavy metals, chemical properties are more important from practical aspects. The heavy metals standards exceeding their permissible limit have gained the attention of think tanks worldwide. Cadmium (Cd), lead (Pb), copper (Cu), and zinc (Zn) are among the metals posing serious threats to the environment [4,5]. Heavy metals could be generated by a vast number of sources, such as smelting, mining, refining, sewage disposal, industrialization, pesticides application [5,6], and chemical fertilization, which consequently result in soil pollution to a greater extent [6]. Currently, the heavy metal pollution is increasing invasive due to the accumulation of heavy metals in soil and plants [2]. This heavy metal pollution is in dire need of time to be solved [6].

These metals are required by plants and humans in small amounts, but, when their concentration exceeds the permissible limit, they cause several problems [7]. These heavy metals not only cause the reduction in crop yield but also cause serious health risks to humans through ingestion, inhalation, dermal contact, and oral contact when they become involved in the food chain [8,9]. Plants can uptake the heavy metals from the soil and transport them to humans. Chronic exposure to heavy metals causes destructive consequences to humans, such as bone fractures, lung cancer, kidney dysfunctions, infertility, liver dysfunctions, and disorders in the nervous immune and endocrine systems, ultimately leading to impairments in cholesterol levels [10,11]. By considering the harms of heavy metals, researchers have focused their attention on mitigating the effects of heavy metals. It was found that several human diseases are linked with heavy metal accumulation, such as copper (Cu) exposure leading to kidney damage, brain disorders, liver cirrhosis, and intestinal irritation, whereas lead (Pb) exposure can damage the neurologic system, causing the loss of neurological functions [10]. The accumulation of heavy metals in soil due to continuous anthropogenic activities could result in the lateral transport and leaching of heavy metals to the environmental matrices, consequently contaminating surface and groundwater resources [11]. Therefore, the investigation of heavy metals’ mobility and transport is of significant importance in evaluating the fate and hazardous impacts of such heavy metals on the environment. Heavy metals’ fate and transport in the soil have been investigated intensively in the last few years. Heavy metal transport in the soil is influenced by various factors, such as moisture, soil reaction, presence of organic matter, clay particles, specific surface area, soil pH, presence of competing ions, cation exchange capacity, carbonate content, and iron oxide contents [12,13]. Moreover, water circulation, as well as soil hydrological, mineralogical, and chemical properties, can also influence the transport of heavy metals in soil [12]. Depending upon the aforementioned properties, heavy metals can be immobilized within the soil either by adsorption onto soil via selective/non-selective adsorption or can be precipitated in the presence of higher carbonate contents at higher pH levels. Calcium carbonate (CaCO₃) is an inorganic material usually found in soil and is an essential constituent of sedimentary rocks [14]. Calcium carbonate affects not only the soil’s chemical and physical processes, such as soil buffering capacity, water-holding capacity, hydraulic conductivity, and soil texture but also the biological processes [15]. The CaCO₃ in soil has a role in the sorption of Pb, Cd, Ni, and P [16]. Furthermore, it impacts the adsorption and desorption of insecticides and herbicides [17]. Therefore, in calcareous soils, heavy metals occur in the form of carbonate salts. However, with decreasing soil pH, the carbonate salt starts dissolving and results in ionic-exchange, subsequently enhancing heavy metal retention [14,18,19]. In addition to the aforementioned reasons, the presence of plants may also affect the mobility of metals in soil [20,21]. The formation of preferential pathways along the roots of plants may act as channels for the transportation of heavy metals in soil, consequently resulting in higher mobility [22]. On the other hand, the presence of plants may decrease the mobility of heavy metals by sucking the water upwards, adsorption onto roots, and microbial immobilization, which results in the reduced leaching of heavy metals into groundwater. However, the impacts of vegetation on heavy metal transport are not fully understood, specifically, in alkaline and calcareous soils [22].

Heavy metals exist in various phases in the soil, such as soluble phase, exchangeable, carbonate-associated, occluded with oxides, bonded with clay minerals, bounded with organic matter, and residual phase. The water-soluble portion of heavy metals is freely labile and prone to rapid transportation, while the residual fraction is strongly bounded and immobile in soil. Therefore, metal speciation is very critical to study when investigating heavy metals’ mobility and adsorption in soil [23]. Indeed, there are many factors affecting the adsorption properties, such as the concentration of humic acid, temperature, ion species, and ionic strength [12,13]. The transport and bioavailability of heavy metals in soil are influenced by the type of bonding between the metal and soil constituent. Therefore, the fractionation of soil heavy metals must be known in order to investigate the mobility of such metals. Hence, the current study was conducted to explore the fate and transport of heavy metals (Pb and Cu) in calcareous soil in lab-scale trials. The aims of the research were to understand the vertical transportation of Pb and Cu along with the adsorption mechanism during saturated conditions of soil with carbonate contents.

2. Materials and Methods

The soil was collected from Almohous farm, Thadiq, 120 km northwest of Riyadh city, Saudi Arabia. The farm is situated at 25°17′40″ N, 45°52′55″ E and +722 m above sea level (Figure 1). The climate of the Thadiq area is characterized by desert climate with hot and dry summers. The mean daytime temperature ranges between 43 and 45 °C; while the daytime temperature in winter ranges between 20 and 25 °C. The average summer night temperature is 28 °C, whereas winter nights are cold, and the temperature ranges between −2 °C and 5 °C. The area receives minimal to no precipitation in summer; however, only 51 mm is the average precipitation in winter. Due to the desert climate, the soils of Thadiq area are classified as Aridisols [24]. The area depends on groundwater for irrigation. Various fruits and vegetables, including cucumbers, pepper, potatoes, tomatoes, and date palm, are grown in this area. The soil samples were collected from the top (0–30 cm) from different randomly selected sites of the Almohous farm. The samples were collected with the help of a wooden auger to avoid metal contamination. After collection, a composite soil sample was prepared and brought to the laboratory for further analyses after air-drying, grounding, and sieving (<2 mm).

2.1. Analytical Methods

2.1.1. Soil Physical Characteristics

Soil physical analyses including texture, hydraulic conductivity, and infiltration rate were determined. Particle size distribution of the soil was estimated through the hydrometer method, and soil texture was determined [25]. The soil texture was loamy sand with 84%, 6%, and 10% of sand, silt, and clay fractions, respectively (

Table 1. Soil chemical properties and texture used in column and batch experiments.

Particle Size Distribution			Textural Class	CaCO3 %	O.M %	CEC (meq/100 g Soil)	Surface Area	Ks
Sand %	Silt %	Clay %					(m2 g−1)	(cm min−1)
84	6	10	Loamy sand	11	0.08	8	202	0.1

). Darcy’s law with a constant head was used to determine the hydraulic conductivity (Equation (1)).

$$\frac{Q}{At}=K_s\frac{h+L}{L}$$

(1)

where Q represents water volume collected from the soil (cm³), A stands for the area of the soil cross-section (cm²), t shows the time (min), K_s is to represent the saturated hydraulic conductivity (cm/min), h stands for water head overhead the saturated soil (cm), and L represents soil column length (cm).

The soil-specific surface area was determined by the ethylene glycol method, as described by the Soil Survey Laboratory Methods Manual (2004) (Table 1).

2.1.2. Soil Chemical Analyses

The pH and electrical conductivity (EC) of the soil were measured in saturated paste, which was then used for the analyses of soluble K⁺, Na⁺, CO₃²⁻, Mg²⁺, Cl⁻, Ca²⁺, and HCO₃⁻ and SO₄⁻² [26]. The HCO₃⁻ was measured by acid titration; on the other hand, the Cl⁻ was determined by silver nitrate titration using the proper indicator [26]. The SO₄⁻² was measured using the method of turbidity [27]. The NO₃⁻ was measured using phenoldisulfonic acid methods [26] (STM 1998). The K⁺ and Na⁺ were measured by a flame photometer (Corning 400); however, the Mg²⁺ and Ca²⁺ were determined by titration with EDTA using the proper indicator [26]. The concentrations of Cu and Pb were extracted by DTPA and water-soluble methods and determined using ICP-OES (Perkin Elmer Model 4300DV) as described by Sparks [26] (Table 1 and

Table 2. Acidity, soluble salts, nitrate, and available heavy metals of soil used in batch and column experiments.

pH	ECe (dSm−1)	Soluble Cations (meq/L)				Soluble Anions (meq/L)				NO3− (mg/L)	Heavy Metals * (mg/Kg)
		Ca++	Mg++	Na+	K+	CO3−	HCO3−	Cl−	SO4−		Cu	Pb
8.2	1.2	7	3	1.8	0.5	0	3.5	3	6	0.8	ND	ND

* Water soluble and extracted by DTPA.

).

The cation exchange capacity (CEC) of the soil was analyzed by following the procedure reported by Sparks [26]. The soil organic matter was determined by Walkley and Black [28]. The contents of calcium carbonate in the soil sample were analyzed through the calcimeter method as described by Richards [29] (Table 1).

2.1.3. Soil Calcium Carbonate Removal

Acetic acid, CH₃COONa buffer at pH 5 (the concentration of acetate 0.4 M), was used for the removal of calcium carbonate from the studied soil. The suspensions were occasionally stirred, and the acid was substituted by fresh ones until no CO₂ formation was detected when little drops of HCl were added to the soil [30]. The free calcium carbonate soil was then air-dried, ground, sieved using a 2 mm sieve, and stored for sorption isotherm study.

2.2. Heavy Metals Kinetics and Sorption Isotherms

2.2.1. Sorption Kinetics

A laboratory batch study was conducted by using a lab-grade solution of Pb²⁺ and Cu²⁺ made from lab-grade PbCl₂ and CuCl₂ compounds (Sigma-Aldrich Co., St. Louis, MO, USA). Stock solutions of Pb²⁺ and Cu²⁺ were prepared and were used for kinetics batch type studies [31]. A total of 5 g triplicate samples of calcareous soil (intact soil) or calcium carbonate free soil (sorbents) were mixed into 50 mL of Pb²⁺ and Cu²⁺ solutions of 100 mg L⁻¹ (Initial pH = 8.2) and shaken for 5, 10, 20, 30, 60, 120, 240, 480, 960, and 1920 min in polypropylene centrifuge tubes. The remaining concentrations of Pb²⁺ and Cu²⁺ in the solutions were determined using ICP-OES [23]. The following kinetic models were adopted to investigate the Pb²⁺ and Cu²⁺ adsorption kinetics of the intact soil [32,33].

• Fractional power model:

ln q_t = ln a + b ln t

(2)

where q_t is the quantity of Pb²⁺ and Cu²⁺ sorbed at a time t, and a and b are constants with b < 1. The factor ab is also a constant, being the specific sorption rate at unit time, i.e., when t = 1.

• Pseudo-second-order model:

t/q_t = 1/k₂q_e ² + t/qe

(3)

where q_e is the adsorption of Pb²⁺ and Cu²⁺ ions on the soil (mg g⁻¹), while q_t is the Pb²⁺ and Cu²⁺ concentration at equilibrium, and k₂ represents the rate constant [g/(mg min)].

• Elovich model:

q_t = β ln(αβ) + β ln t

(4)

where q_t is the amount of Pb²⁺ and Cu²⁺ sorbed at time t, β is the desorption constant (g/mg), and α is the initial Pb²⁺ and Cu²⁺ sorption rate [mg/(g min)] through any one experiment.

• Intra-particle diffusion:

q_t = a + ki t^1/2

(5)

where the rate of intraparticile diffusion is indicated by ki (mg g⁻¹ min^−0.5), while a is the plot interception and indicates surface adsorption or the effect of the boundary layer. The higher the intercept, the more influence of the surface adsorption in the rate-limiting step.

2.2.2. Sorption Isotherms

The Pb²⁺ and Cu²⁺ ions adsorption onto the intact soil or calcium-carbonate-free soil in aqueous media (pH = 8.2) were investigated by batch sorption trials [34]. The adsorption experiment was carried out in 3 replicates at room temperature using 50 mL of a solution in polypropylene centrifuge tubes containing 5, 10, 20, 50, 100, 150, 200, and 250 mg L⁻¹ of Pb²⁺ and Cu²⁺ ions. A total of 5 g of calcareous soil was used in the isotherm studies. The suspension of soil and heavy metal solution was shaken for 240 min. The quantity of Pb²⁺ and Cu²⁺ ions sorbed at equilibrium (q_e) was obtained using Equation (6):

$${\mathrm{q}}_{\mathrm{e}}=\left[\frac{\left({\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}\right)}{\mathrm{W}}\right]\times \mathrm{V}$$

(6)

where C_o is the initial concentrations, while C_e is the final concentrations of the ions in mg L⁻¹, V is the ion solution volume (mL), and W is the sorbent weight (g) [35].

The data of Pb²⁺ and Cu²⁺ sorption to the intact soil were subjected to isotherm models, such as Langmuir and Freundlich, as indicated in Equations (7) and (8):

Langmuir equation:

$$\frac{{\mathrm{C}}_{\mathrm{e}}}{\mathrm{Cs}}=\frac{1}{{\mathrm{q}}_{\mathrm{o}}{\mathrm{K}}_{\mathrm{L}}}+\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{o}}}$$

(7)

Freundlich equation:

$$\mathrm{Log}\mathrm{Cs}=\mathrm{Log}{\mathrm{K}}_{\mathrm{f}}+\frac{1}{\mathrm{n}}\mathrm{Log}{\mathrm{C}}_{\mathrm{e}}$$

(8)

where concentration related to equilibrium is indicated by C_e (mg L⁻¹); Cs represents the concetration at solid phase equilibrium (mg g⁻¹); however, the constant for sorption capacity is indicated by k_f (L mg⁻¹), the capacity of adsorption at its maximum is indicated by q_o (mg g⁻¹); the constant of intensity is indicated by n; the enthalpy constant is indicated by K_L (L mg⁻¹);

2.3. Column Experiment Setup

The vertical transport of Pb²⁺ and Cu²⁺ in soil was studied in columns. Acrylic columns of 40 cm height and 5 cm diameter were used. The bottom of the columns was closed with filter papers, and calcareous soil was filled in the columns up to 30 cm height (bulk density = 1.72 g/cm³, porosity = 35%). All the treatments were performed with 3 replications. A set of control samples, without any adsorbent, was also involved. The metals’ transport has been studied under saturated soil conditions. This is done by saturating the soil columns from the bottom with the capillary property using tap water (EC = 0.8 dS/m) and ensuring that a stable flow state occurs by adding water over the columns with the appropriate flow, further confirming that the flow rate at the bottom of the column is stable over time. Water and a solution of heavy metals were added to the columns using a syringe pump, allowing a constant rate of water and heavy metal solution to be injected. After making sure that stable flow has occurred (steady state water flux = 0.12 cm/min), the heavy metal injection started at the required concentration (200 mg/L) for a specified period of time, receiving the filtrate from the columns at specified intervals: 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0 pore volumes (PV) (1 PV = 206.06 mL). Then we estimated the heavy metal concentration in the filtrate until there were no more metals detected in the filtrate (Figure 2). Then the soils in the columns were taken out and divided into different sections [36]. Then the adsorbed heavy metals concentrations in the soil were extracted by the DTPA method and estimated by ICP-OES (Perkin Elmer Model 4300DV) as described by Sparks [26].

3. Results and Discussion

3.1. Lead and Copper Kinetics and Sorption Isotherms

3.1.1. Lead and Copper Adsorption Kinetics

Figure 3 concluded that there was the rapid sorption of Pb²⁺ than Cu²⁺ onto the calcareous soil. The soil reached equilibrium within 240 min, and then the curves became stable (32 h). In contrast to Cu²⁺, rates of adsorption exhibited by Pb²⁺ were faster, as revealed in the batch tests (Figure 3). In the first 240 min, 90% of Pb²⁺ and 70% of Cu²⁺ were adsorbed. Consequently, calcareous soil can effectively remove Pb²⁺ and Cu²⁺ from aqueous media [34,37]. Figure 3 reveals that the capacity of the soil to adsorb Pb²⁺ and Cu²⁺ deceased dramatically when the calcium carbonate was removed from it; furthermore, the equilibrium was reached for both heavy metals after only 60 min with calcium-carbonate-free soil. The results of this investigation showed that natural calcareous soil can be successfully used in the ion-sorption process to achieve high Pb²⁺ and Cu²⁺ removal from wastewater. This finding is in agreement with some of the previous reports [38,39,40]. Several investigations have revealed that many kinetic models could be valuable to define ion sorption rates from sorbents [34,41,42]. The data for Pb²⁺ and Cu²⁺ sorption onto calcareous soil were simulated by the Elovich, intra-particle diffusion, power function kinetic models, and pseudo-second-order (

Table 3. Parameters obtained from the kinetics modeling for Pb²⁺ and Cu²⁺ adsorption onto calcarous soil.

Sorbents	Initial pH	Pseudo-Second-Order			Elovich			Power Function			Intra-Particle Diffusion
		qe	K2	R2	α	β	R2	b	a	R2	ki	a	R2
Pb2+	8.2	1.03	0.03	0.9995	2.4	0.19	0.95	0.52	0.04	0.89	0.101	0.112	0.86
Cu2+	8.2	0.76	0.02	0.9979	24.0	0.05	0.97	0.29	0.03	0.85	0.042	0.143	0.78

). The results presented show that the kinetic sorption of Cu²⁺ and Pb²⁺ onto soil was described well with the pseudo-second-order kinetic models. An estimated R² of 0.99 was obtained for both Cu²⁺ and Pb²⁺, followed by other the three models (Table 3). Fifi et al. [38] reported similar findings previously. The pseudo-second-order suggested that chemisorption, demonstrating sharing or the exchange of electrons between Pb²⁺ and Cu²⁺ and calcareous soil, was involved in Pb²⁺ and Cu²⁺ removal. The intraparticle diffusion model showed R² values of 0.76 and 0.85 for Cu²⁺ and Pb²⁺, respectively (Table 3). It was seen that the adsorption of studied metals followed a rapid removal at the beginning, which slowed with time, consequently achieving equilibrium. The linear part of the data was used for the intraparticle diffusion model (Table 3). The ki, apparent and Cu²⁺ and Pb²⁺ diffusion rate coefficients, from intra-particle diffusion, were used to estimate the ion sorption relative rate. In contrast to Cu²⁺, the value of ki was higher for Pb²⁺. However, the higher value of ‘a’ was exhibited by Cu²⁺. These results suggested that the calcareous soils can efficiently adsorb Pb²⁺ and Cu²⁺. Additionally, the adsorption of Pb²⁺ was more than that of Cu²⁺. Several mechanisms are responsible for the process of slow sorption, such as: adsorption, precipitation, and diffusion reactions on sites of sorptions gaining greater activation energy than quick sorption [43]. Moreover, mass-transport (transport from the bulk liquid phase to solid phase), film diffusion, and diffusion into the pores could also be involved in the adsorption of Pb²⁺ and Cu²⁺ [34]. The R² values for the Elovich model were also in the range of 0.95 and 0.97, indicating good fitness to the adsorption data. Thus, the best fitness of pseudo-second order and Elovich kinetic models to Pb²⁺ and Cu²⁺ sorption data showed that multiple mechanisms were responsible for Pb²⁺ and Cu²⁺ adsorption onto calcareous soils [34].

3.1.2. Lead and Copper Sorption Isotherms

The isotherm batch trials were performed at an initial pH of 8.2 with an initial metal concentration of 0–250 mg L⁻¹. Figure 4 displays the sorption isotherms of Cu²⁺ and Pb² before and after calcium carbonate removal from the soil. The severe decrease (75–80%) of the heavy metal sorption after carbonate removal is evidence that the carbonate is controlling the heavy metal sorption of the studied calcareous soil [44,45].

The Freundlich and Langmuir models were applied to evaluate the data from the sorption experiments of the calcareous soil. These results suggested that Cu²⁺ and Pb²⁺ adsorption was best fitted to the Freundlich model (R² = 0.96–0.98), followed by Langmuir model (R² = 0.91–0.95) (

Table 4. Parameters obtained from isotherm models for Pb²⁺ and Cu²⁺ sorption onto calcareous soil (intact soil) pH = 8.2.

Sorpent	Freundlich				Langmuir
	Kf	R2	1/n	n	qo (mg g−1)	KL (Lm g−1)	R2
Pb2+	0.08	0.96	1.3	0.8	6.8	0.8	0.95
Cu2+	0.04	0.98	0.8	1.3	4.0	2.1	0.91

). The fitness of the Freundlich and Langmuir models suggested both multi-layer and mono-layer adsorption. The calculated maximum adsorption capacity (q_o) and bonding energy (K_L) of the models were different for both of the studied metals. The Pb²⁺ had higher values of q_o than Cu²⁺, and the q_o were 6.8 and 4.0 for Pb²⁺ and Cu²⁺, respectively (Table 4); yet, the K_L of Pb²⁺ is less than that of Cu²⁺ (0.8 and 2.1, respectively), suggesting strong complexes of Pb²⁺ in the studied soil. Very high correlation coefficients between adsorbates (Pb²⁺ and Cu²⁺) and soil were observed for the Freundlich model (R² = 0.96–0.98) (Table 4), suggesting that this model can best describe the adsorption of the studied metals onto calcareous soils. The Freundlich parameters, 1/n and K_f, were used to demonstrate the sorption intensity and capacity, respectively. The values of 1/n for Pb²⁺ are found >1, representing favorable adsorption and a better adsorption mechanism. Accordingly, there were more chances of the development of stronger bonds between soil and metals [34]. Conversely, the value of n was <1, suggesting that, at high concentrations, the adsorption of Pb²⁺ was favorable and vice versa at lower concentrations [32]. On the other hand, the value of n is greater than 1 for Cu²⁺; indicating that, at higher concentrations, the adsorption intensity is unfavorable but is far less so at lower concentrations (Table 4 and Figure 4). Similar results have previously been reported by Arias et al. [46], Tellan and Owalude [47], and Fifi et al. [39]. Thus, the best fitness of the Freundlich isotherm suggests a heterogeneous surface of the soil, followed by multilayer adsorption [31,34]. Further, current study outcomes suggested that the adsorption was increased as a consequence of the increased initial concentrations of Cu²⁺ and Pb²⁺ (Figure 4) [35,38]. This study found that the selectivity order of the calcareous soil for the investigated two metals is Pb²⁺ > Cu²⁺. The order of soil ion selectivity is affected by the ionic size and the valence of the ions [48]. The smaller ions with a similar valence, such as Cu²⁺ (0.72 Aº), compared with Pb²⁺ (1.20 Aº), usually have higher charge densities; consequently, they adsorbed more molecules of water, leading to a greater hydrated radius (HR). The greater HR metals have a weaker attraction of Columbic forces [49]. Thus, Cu²⁺ is anticipated to be more mobile and to lead to less sorption than Pb²⁺, owing to its greater hydrated radius [48].

Sposito [50] showed that the ionization potential and ionic radius of the cations were significant in the formation of covalent bonds between the cations. These concepts clarify the potential of forming stronger complexes by the larger metal cations. The order of metals according to their ability to form covalent bonds is as follows: Pb > Cd > Cu > Co > Ni > Zn.

3.2. Lead and Copper Transport in Soil Column

The results showed very low concentrations of Cu²⁺ and Pb²⁺ in the filtrate, even with increasing the PV. This indicates that Pb²⁺ and Cu²⁺ ions remained completely adsorbed in the calcareous soil. This means that the Pb²⁺ and Cu²⁺ movement is very slow in the calcareous soil [14]. The results in Figure 5 showed the distribution of Pb²⁺ and Cu²⁺ in the soil columns at the end of the experiment. It appeared from the adsorption experiments that a strong susceptibility of Pb²⁺ and Cu²⁺ to bind to the surface of the soil particles existed. The high binding of Pb²⁺ and Cu²⁺ to the surface of the soil particles indicates the slower mobility of these two metals in the soil. The two studied metals were found concentrated on the surface of the column. In the columns, Cu²⁺ and Pb²⁺ were concentrated at an initial 10 cm and 5 cm, respectively (Figure 5). These outcomes confirmed that the Pb²⁺ and Cu²⁺ have very slow mobility in the calcareous soil [18,19].

4. Conclusions

The calcareous soil was found efficient for Pb²⁺ and Cu²⁺ removal. Nonetheless, the soil was found to be more efficient for Pb²⁺ removal than for Cu²⁺ due to its chemisorption affinity. This study confirmed that Cu²⁺ and Pb²⁺, which are potential contaminants of wastewater, can effectively be removed by using calcareous soil. In this study, the adsorption of the Cu²⁺ and Pb²⁺ (in an aqueous solution) onto calcareous soil was investigated. The relatively quicker adsorption of Pb²⁺ and Cu²⁺ by the soil was observed during kinetic adsorption batch trials. The pseudo-second-order kinetic model described the adsorption of Pb²⁺ and Cu²⁺ onto calcareous soil at its best. Moreover, the Freundlich isotherm model fitted well with the adsorption data, followed by Langmuir model. The sorption capacity and intensity is demonstrated by 1/n and Kf, which are Freundlich parameters. The values of 1/n for Pb²⁺ were >1, representing favorable adsorption, subsequently generating strong bonds. Conversely, the value of n is <1, indicating the favorable adsorption of Pb²⁺ at higher initial concentrations but less favorable adsorption at lower concentrations. On the other hand, for Cu²⁺, the n is more than 1, indicating that the intensity of adsorption is unfavorable at higher concentrations but far weaker at lower concentrations. This study also explored the transport of Cu²⁺ and Pb²⁺ in columns filled with calcareous soil. The results showed the higher binding of Pb²⁺ and Cu²⁺ to the surface of the soil columns, making the movements of these two metals very slow. The two studied metals were found to be concentrated on the surface of the column. In the columns, Cu²⁺ and Pb²⁺ were concentrated at an initial 10 cm and 5 cm, respectively. Therefore, the application of calcareous soils to adsorb Pb²⁺ and Cu²⁺ in contaminated water could serve as a cheaper and environmentally friendly approach to remediate contaminated water.