Composite Sorbent Prepared from Layered Double Hydroxide Nanoparticles to Remediate Simulated Groundwater Polluted with Lead and Cadmium Ions

Life on earth is sustained by soil, an invaluable and non-renewable resource [1]. However, anthropogenic activities have precipitated extensive soil degradation and pollution [2]. Heavy metals, a group of toxic metals and metalloids, pose significant threats to ecosystems and human health due to their presence in soil [3, 4]. Issues surrounding heavy metal contamination in groundwater and soil, propelled by rapid industrialization and urbanization, have garnered significant attention in recent times. These metals can bioaccumulate in plants and animals, subsequently infiltrating the human food chain.

Despite the diverse chemical properties of heavy metals, their non-biodegradable nature and inorganic composition render them as persistent environmental hazards when released into ecosystems. Metals such as cadmium, mercury, lead, chromium, cobalt, copper, nickel, and zinc are particularly detrimental to environmental health [5]. They are often introduced into the environment through mismanaged industrial waste and mine tailings, posing a global environmental risk. Mine tailings, in particular, often contain high levels of heavy metals, resulting in soil and groundwater contamination [6].

Recent global studies have underscored the severity of heavy metal pollution in groundwater, emphasizing the necessity for ongoing development of treatment technologies. Consequently, the primary objective of this study is to design and synthesize novel reactive materials that meet key requirements for permeable reactive barriers (PRBs), particularly with regard to groundwater flow and contaminant sequestration [7].

Layered double hydroxides (LDHs) have drawn significant interest in their synthesis phase due to their potential applications in diverse fields such as electrochemistry, magnetism, and environmental science. LDHs are cost-effective and simple to produce on both laboratory and commercial scales, making them an attractive reactive medium for various anions [8], cations [9], radionuclides [10] and dyes [11].

The gypsum industry produces large amounts of plaster kiln dust by-product waste, and the steel and iron industries generate significant quantities of iron-slag, a byproduct that amounts to approximately 50×10⁶ tons annually. These industrial wastes can be repurposed to produce new sorbents for treating contaminated water, exemplifying the sustainable and green synthesis approach that mitigates the adverse impacts of industrial by-products and processes on the environment and human health. Alum, or aluminum sulphate, is a low-cost coagulant commonly used in water treatment processes and can also contribute to the creation of the proposed coated slag.

Therefore, this study aims to produce LDH nanoparticles through a chemical reaction involving calcium (extracted from the readily available and inexpensive by-product, plaster kiln dust) and aluminum (derived from alum dissolution), in the presence of a well-known surfactant (cetyltrimethylammonium bromide, CTAB). These particles are then precipitated onto the surface of iron-slag to create a "coated slag". The performance of this coated slag is subsequently evaluated for its potential use in PRBs to limit the propagation of contamination plumes.

Laboratory column experiments were completed to evaluate the permeability and reactivity of the covered slag in order to verify the geochemical and hydraulic performance of the produced sorbent. Figure 1 shows experimental simulation experiments using the column technique that reflect a one-dimensional model of the barrier's real operation and column setup. Two aspects—contaminant elimination effectiveness, coated slag durability, and hydraulic conductivity—were examined in order to assess the reactive medium's efficacy in PRB. The adsorbent continuous setup that was used in this work was made of two PVC columns that were arranged in a row-like fashion, along with a number of accessories like a water holding tank, a peristaltic pump, valves, and pipelines. Each column's wall is 5 mm thick, with a 50-centimeter height and a 2.5 cm interior diameter. To test the efficacy of (Ca/Al-CTAB)-LDH nanoparticles in reducing contaminants content, an iron-slag-coated column was filled.

Distilled water was poured into the slag layer at room temperature from the bottom up to prevent the incidence of trapped air. A particular hydraulic gradient was used to introduce contaminated water to the bed at a rate of 5, 10, or 15 mL/min once saturation was reached. Each column has openings 1, 2, and 3 that are situated 10, 30, and 40 centimeters, respectively, from the inlet. Outlets have been used to remove samples of purified water at predetermined intervals to check the level of metals.

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Figure 1. Components and configuration of column set-up used for continuous operation; schematic diagram of column set-up used for continuous operation

5.1 Manufacturing of sorbent

Four specific parameters have an impact on the manufacturing of "iron-slag coated with (Ca/Al-CTAB)-LDH". On the prepared sorbent, the CTAB mass, aqueous solution pH, molar ratio of Ca to Al, and iron-slag mass have all been investigated. The appropriate indicators that can be utilized to determine the best magnitudes of the aforementioned parameters were those with the highest adoption contaminant removal efficiencies. In order to produce coated slag, three CTAB masses 0.0, 0.035, and 0.05 g were used in conjunction with a pH of 10 and ration of Ca/Al 2 and a mass 1 g of iron slag per 50 milliliters. For the purpose of determining how this coated slag would react with an aqueous solution that was tainted with Pb⁺² and Cd⁺² ions, sorption tests have applied with Co 50 mg/L, pH 5, sorbent mass 0.5 g/50 mL, time period of three hours, and speed of agitation 250 rpm.

The existence of CTAB, which has significant effect in increasing the spacing for Ca/Al inter-layer and can improve the adsorptive characteristics of prepared slag, may be the cause of this increase in removal efficiency. Forces of electrostatic repulsion can generate between the cations present in the synthesized slag and the contaminant can result in a significant decrease in the amount of metal removed [20].

Preparation tests were carried out with three pH values —specifically 9, 10, and 11 - at the same values of synthesis and sorption conditions mentioned in the study to determine the effect of CTAB mass, with the exception of the mass of surfactant, which must be taken the best one (=0.035 g). According to the measurements, the solution pH of 10 has the highest removal efficiency. Due to the fact that not all nanoparticles will adhere to the iron-slag or because the diameter of the nanoparticles is increasing, the removal percentage can be slightly reduced by increasing or decreasing the pH from 10; consequently, the coating process at pH 10 is optimal [21].

For same conditions of sorption tests, three values of this ratio specifically 1, 2, and 3 were used. An increase or decrease in this molar ration can be resulted in a decrease in Pb⁺² and Cd⁺² ions removals. LDH's disordered, hydrotalcite-like composition or variations in the radius difference between Al and Ca could be the primary cause of this decrease in removal efficiency [22].

The final parameter examined in the synthesis of the current sorbent was the iron slag dosage. Three different slag masses 0.5, 1, and 1.5 g/50 mL on the production of composite sorbent under the same optimal preparation conditions. It seems that the 1 g of slag per 50 mL of solution utilized to prepare coated slag can remove the maximum quantity of Pb⁺² and Cd⁺². A decrease in the quantity of iron slag can associate with a clear decrease in the metal removal; this can explain by the fact that many (Ca/Al-CTAB) nanoparticles do not have the opportunity to adhere with iron slag; as a result, they will be washed away. Additionally, the increase in surface area for the same quantity of nanoparticles as compared to the best one may be the cause of the decrease in removals for iron slag dosages greater than 1 g.

5.2 Description of present sorbent

The "X-ray diffraction (XRD) analysis" is very essential for specifying and identifying the mineralogical composition of materials used in the preparation of present sorbent. In Figure 2(a), the PKD, virgin iron-slag, and coated iron-slag XRD patterns were displayed. In order to create a plot relating the intensity of the diffracted beam to two Theta (2θ), the material sample in this test is subjected to a collimated beam of parallel X-rays and diffracted beams of varying intensities. With Cu K radiation and an X-ray diffractometer (40 mA, 40 kV), the records with 2θ ranged from 5° to 80°. Based on International Centre for Diffraction Data, ICDD databases, the XRD pattern of PKD had peaks that correspond to many compounds like gypsum, montmorillonite, calcium sulfate, iron oxide, and quartz as shown in Figure 2(a). Also, this figure certifies the presence of many metal oxides in the composition of iron slag like manganese (MnO), aluminum (Al₂O₃), calcium (CaO), iron (Fe₂O₃), and silica (SiO₂). Depended on the "Joint Committee on Powder Diffraction Standards, JCPDSs", the XRD analysis for coated slag proved the appearance of several reflections at intensities of 22.693, 25.501, 31.741, 34.081, 38.865, 40.983, 45.469, 48.589, 52.541, 56.467, 57.317, and 78.983 which identical to the nanoparticles of Ca+ Al + CTAB precipitated on the slag surfaces. The formation of new sites on the coated sand can be supported the removal of Pb⁺² and Cd⁺² ions onto the prepared sorbent [23].

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Figure 2. X-ray diffraction (XRD) analysis; FT-IR analysis

The results of the FT-IR analysis for wavenumbers ranging from 400 to 4000 cm^-1 (Figure 2(b)) was used to identify the working functional groups for PKD, coated iron slag, coated iron slag loaded with Pb⁺² and Cd⁺² ions, and virgin iron slag. According to the spectra, the OH group is represented by a broad and strong peak at wavenumber 3442.3 cm^-1 [24]. The C–C group vibration can be linked to peak values of 554.4,675.93, 704.9, 811.8, and 894.7 cm^-1 that were found. C–O group vibration can be used to identify the peak values of 1149.4, 1260.25, 1265.07, 1290.14, and 1308.5 cm^-1that were identified [25]. At 1560.13 cm-1, an olefinic C=C stretching band can be seen, and its conjugation with a C=O or C=C bond can cause a clear shift to lower wavenumbers. As a result, the principal functional groups found in the current materials are O–H, C=C, C=O, C-C, and C–O. Appearance of peaks at 2345.16 and 2358.5 cm^-1 demonstrated to the existing of C-N vibration in tertiary amines. The removal of Pb⁺² ions onto coated iron-slag is mainly controlled by electrostatic interaction and hydrogen bonding, according to the FT-IR spectrum of a reactive material's chemistry.

As shown in Figure 3, FESEM graphs are useful for describing the morphological characteristics of PKD, virgin iron slag, coated iron slag, and coated slag loaded with lead and cadmium ions. Iron slag's porous surface exhibits irregular morphology, giving the impression of roughness and cracks. The formation of layers with irregular orientations and sizes is depicted in Figure 3(a) [26, 27]. However, the sorbent's efficiency at capturing lead and cadmium ions can be improved by increasing the surface area exposed for interaction. In contrast to the sorbent before the interaction with metal ions (Figure 6 (b)), the sorbent's morphology changed clearly (Figure 3 (c, d)) after the binding of lead ions. The findings of the BET surface area test proved that the current sorbent had a significant increase in area, going from 0.488 m²/g for iron-slag to 10.21 m²/g after the coating process.

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Figure 3. FESEM images for virgin iron-slag; coated iron-slag; coated iron-slag loaded with Pb⁺² ions; and coated iron-slag loaded with Cd⁺² ions

As depicted in Figure 4, EDS produces elemental composition graphs for PKD, virgin iron-slag, and composite sorbent before and after the sorption process. Representative iron slag grains clearly contain Ti, O, Si, Fe, k, Al, C, Ca, Mg, Na, and Mn as major elements. According to the tables in Figure 4, the coated iron slag contained 18.0 and 17.7 percent Ca and Al, respectively. This rise demonstrates the plantation's success with layered double hydroxides. As illustrated in Figure 4(c, d), interaction with iron-slag particles can result in the formation of a cloud of lead and cadmium ions.

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Figure 4. EDS images for virgin iron-slag; coated iron-slag; coated iron-slag loaded with Pb⁺² ions; and coated iron-slag loaded with Cd⁺² ions

5.3 Performance of coated slag bed

Using coated slag as the PRB, the movement through the column arrangement was evaluated. The goal of the tracking is to determine how well the barrier performs in removing the heavy metals lead and cadmium from aqueous solutions while keeping a reasonable hydraulic conductivity coefficient under changing inlet concentration, water flow rate, and mass of coated slag. Figure 5 illustrates the effect of the input percentage of adopted metals through the barrier bed at a flow rate of 5 mL/min for ports 1, 2, and 3. Variations in the concentration gradient may have an impact on the bed's rate of absorption. Due to the incidence of delayed sorption, it appears that the form of the curve is less obvious at lower influent concentrations. However, as influent concentration rises and the sorbent becomes saturated, the slope steepness of the parabola increases. Additionally, low concentration gradients cause chemical species to propagate slowly through pores because the mass transfer coefficient is reduced [28], which extends the time for saturation. The breakthrough time corresponded to 5% of the normalized concentration (C/Co) for adopted contaminant can be specified from the plotted curves and this time represents the bed longevity to ensure the outlet concentration below the acceptable limit. For example, Figure 5 port 1 (a-c) for Pb⁺² contaminant certified that the breakthrough time for bed depth of 40 cm (P3) was equal to 180 h when inlet concentration of 50 mg/L and flow rate of 5 mL/min; however, this time was decreased significantly to become 120 and 84 h for 100 and 150 mg/L respectively. The lowest longevity (≈42 h) can be occurred at P1 when inlet concentration of 150 mg/L for mentioned flow rate, the relationship between "breakthrough time" and "inlet concentration" can be seen to follow a similar pattern. Due to difference in affinity and adsorption capacity of Pb and Cd with coated iron-slag, the longevities of the barrier in the case of Cd were smaller than that of Pb.

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Figure 5. Measured breakthrough curves of Pb⁺² ions in comparison with predictions of models for different inlet concentration at ports 1, 2 and 3

Applying three rate values 5, 10, and 15 mL/min to the sorption of Pb⁺² and Cd⁺² onto covered slag barriers allowed for monitoring the effect of water flowrate on that sorption. Plots of breakthrough curves (Figure 6) were made using the aforementioned flow rates at Ports 1, 2, and 3 and an input concentration of 50 mg/L. It is obvious that a higher discharge will result in shorter breakout times and steeper slopes. Thus, the influent metal molecules depart before reaching the equilibrium state [29] because the retention period is insufficient. As water velocity rises, solute adhesion to solid particulates decreases, which may result in a noticeable decline in clearance percentage [30]. As a consequence, the solute's retention period in the packed bed was insufficient to allow for the achievement of sorption equilibrium at a particular discharge. As a result, the solute will exit the bed before it has an opportunity to bind to the coated slag's active sites. Furthermore, reversible and porous interactions between the sorbent surface and some of the connected solute molecules may make some of the molecules desorbed under high flow rates. As a consequence, the metal content in effluent quickly rises, which causes an early breakthrough time. In the case of inlet Pb content of 50 mg/L at Port3, increasing flow rate from 5 to 15 mL/min can result in a notable reduction in breakthrough time of 180 to 72 h; however, the same reduction in time (156 to 42 h) can be noted for Cd under the same circumstances. Because the chemical will exit the bed before it achieves equilibrium, an increase in discharge will hasten the emergence of the chemical front, reduce "breakthrough time," and steepen "breakthrough curves." A chemical equilibrium is defined as the absence of an observable change in the system after a period of time.

It is clear in the graph mentioned that the adsorption rate was very fast initially and it's increased with increasing of contact time until reached the equilibrium time. indicating a gradual blinding of sorption sites on the adsorbent surface. While the residual concentrations of these chemical species are being kept relatively constant with insignificant can't changes. The high rate of adsorption can be attributed to the existence of adequate numbers of the reactive binding sites on the sorbent surface. This means that the amount of these contaminants sorbed per unit mass of sorbent decreased with the increase in initial concentration. This represents saturation of the active sites available on the iron-slag samples for interaction with contaminants, indicating that less favorable sites became involved in the process with increasing concentration. For weak and reversible bonds, the high discharge may also desorb many attached molecules from the sorbent surface. As a result, the effluent's metal concentration rises rapidly, accelerating the "breakthrough time", low flow rate helped removal of metals in the packed bed due to its higher capacity of adsorption. Present results were in agreement with those reported in previous studies [31, 32].

Figures 5 and 6 for Pb⁺² demonstrate that bed depth can affect the spread of contaminant fronts, with a notable delay in the emergence of the metal fronts due to an increase in the amount of sorbent material compressed in the column. This implies that a deeper layer provides solute molecules enough time to diffuse within particle holes, increasing the amount of metal that is adsorbed. These figures display standard S-shaped breakthrough curves, which indicate a rise in collected effluent and breakthrough duration with bed depth. For a flow rate of 5 mL/min and a concentration of 50 mg/L at intake, the "breakthrough time" can significantly rise from 2.25 to 7 days (Pb) and from 1.75 to 6 days (Cd) with a change in bed depth from 10 cm (Port 1) to 40 cm (Port 3). It is essential to note that, for a given inlet concentration, increasing bed depth will result in a reduction in bed adsorption capacity because, at a greater depth, a given amount of metal can spread over a larger surface area. Sorbents become depleted, that is, they reach a point where they are no longer able to remove molecular species. Time lapse taken to attain this exhaustion stage is called “exhaust time”, at which effluent C/C_o equals to 0.9. This time is increased with higher bed height because of presence of more active sites for adsorption. Capacity of adsorption for higher bed height is decreased because of; i) overlap of active sites can be limited their effective use in the removal of chemicals [32], and ii) generation of channels especially in the region between the wall of column and packed bed; so, the solution will find its way to escape from the bed without treatment.

Metals ions breakthrough curves along coated slag bed at different discharges and inlet concentrations for depths ranging from 10 to 40 cm are compared with experimental measurements in Figures 4 and 5. Bohart-Adams, Thomas-BDST, Belter-Cussler-Hu, Yan and Clark models were used to simulate these curves at ports 1, 2, and 3. "solver option in Microsoft Excel 2016 for nonlinear regression" was applied to determine fit between the models mentioned and the measurements. Also use mathematical simulation for experimental measurements of breakthrough curves using advection-dispersion-sorption equation, which is solved numerically by finite element method with the aid of Computer Solution (COMSOL) software. Very similar results were obtained with satisfactory agreement between the two methods.

The output of fitting process (like parameters of models with measures of goodness) has been listed in Tables 1 and 2 for cadmium. With the lowest sum squared errors (SSE) and higher coefficient of determination, the Bohart-Adams, Thomas-BDST, Belter-Cussler-Hu, and Clark models accurately describe the experimental measurements.

Hydraulic conductivities of coated slag packed in column at various time intervals were calculated using hydraulic gradient and accumulative volume of treated water. Results showed that hydraulic conductivity coefficients remained relatively constant throughout experiments, with a mean value of 2.7*10^-2 cm/s. The fact that there is a low likelihood of precipitate formation within packed bed can be used to explain this behavior. Additionally, gaps accessible for the movement of aqueous solutions retain their accessibility in the transfer of solution and, subsequently, dissolved metals along bed. For mattresses in PRB, conductivity must be higher than 2.1*10^-2 cm/s.

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Figure 6. Measured breakthrough curves of Pb⁺² ions in comparison with predictions of models for different water flow rates at ports 1, 2 and 3

Table 1. Outputs for fitting of measured Cd⁺² contaminate breakthrough curves with adopted models at Ports 1, 2, and 3 for different inlet concentrations

Table 2. Outputs for fitting of measured Cd⁺² contaminate breakthrough curves with adopted models at Ports 1, 2, and 3 for different flow rates

We would like to gratefully acknowledge the technical support of Environmental Engineering Department/ University of Baghdad provided during the present work.