A New Hydrotalcite-Like Absorbent OSA-LDH and Its Adsorption Capacity for Pb²⁺ Ions in Water

Abstract

Hydrotalcite-like materials (OSA-LDH) were prepared used oil shale ash (OSA), which came from a thermal power plant area, as the main raw material. The characterization results of X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and thermogravimetric-differential scanning calorimetry (TG-DSC) showed that the prepared samples were mesoporous materials in a crystal state and were layered and contained lattice oxygen and a large number of surface hydroxyl groups. The adsorption property of the prepared samples was confirmed and evaluated by adsorption experiments with Pb²⁺ as the target pollutant. The adsorption process was in accord with the Langmuir isothermal adsorption equation, and the adsorption data fitted perfectly with the pseudo-second kinetic equation. The saturated adsorption capacity for Pb²⁺ was 120.92 mg·g⁻¹ at a temperature of 298 K and initial concentration of 300 mg·L⁻¹. The main adsorption mechanisms of OSA-LDH for Pb²⁺ were chemical bond cooperation and electrostatic bond cooperation. This paper aimed to not only prepare an economical and effective adsorbent to remove heavy metal ions from the solution but also provide a new path for the treatment and utilization of OSA so as to realize efficient waste resource utilization.

1. Introduction

The rapid development of industrial activities and the large number of people in the world produce a large amount of waste [1]. Waste from commercial buildings, domestic applications, institutions and industrial activities, etc., mixes with rainwater, surface water, and liquids that penetrate groundwater [2], ultimately forming wastewater with a complex pollutant composition. The components of pollutants include refractory organic matter, dissolved inorganic solids, nutrients, biodegradable organic matter, detergent, phosphorus, and heavy metals [3]. Among many toxic pollutants, heavy metals are regarded as one of the most serious pollutants. Copper, zinc, nickel, cadmium, lead, and other heavy metals are widely used in electronic products, batteries, electroplating, and other related industries, which are of great significance in scientific development, but at the same time, environmental harm cannot be ignored [4]. Heavy metals are non-biodegradable, and their concentration in living organisms accumulates over time. Their effects are persistent; they enter the food chain and can directly or indirectly affect a variety of organisms. Therefore, how to effectively control heavy metals pollution has become one of the hot topics for environmental researchers, and relevant research is of great significance for the sustainable development of the environment.

At present, there are many methods to remove heavy metals, including chemical precipitation [5], ion exchange [6], membrane separation [7], electrolysis [8], adsorption [9], etc. Among them, adsorption has the advantages of high treatment efficiency, simple and fast operation, wide source of adsorbents, low cost, no secondary pollution, renewable recycling of adsorbents, etc. Adsorbents can be divided into natural adsorbents, synthetic adsorbents, and biological adsorbents according to their source and chemical structure. Common adsorbents include activated carbon [10], clay minerals [11], polymer adsorbents [12], biological adsorbents [13], hydrotalcite materials [14], etc. The research and development of high-efficiency and low-cost adsorbents is a main research focus.

Hydrotalcite material, also known as layered double hydroxide (LDH), is a large class of two-dimensional materials with positively charged brucite-like metal hydroxide layers and charge-balancing intercalated anions. LDH has an anion-exchange capability and feature with [M_1−x²⁺M_x³⁺(OH)₂](Aⁿ⁻)x/n·yH₂O (M²⁺ contains Mg²⁺, Ca²⁺, Zn²⁺, etc.; M³⁺ contains Al³⁺, Fe³⁺, Cr³⁺ etc.; Aⁿ⁻ represents interlayer anions such as CO₃²⁻, NO₃⁻, PO₄³⁻, etc.; x typically ranges from 0.20 to 0.33) [15,16]. It has a special layered structure, large specific surface area, simple synthesis, low cost, and easy separation and is reusable, can be used as an adsorbent in environmental pollution control, and shows good application prospects. It has been reported that LDH is used to adsorb heavy metals in water, especially heavy metal pollutants in cationic form. Liang et al. [17] prepared Mg₂Al layered double hydroxide (Mg₂Al LDH) samples intercalated with diethylenetriaminepentaacetic acid (DTPA) (Mg₂Al–DTPA LDH) by co-precipitation. The sorption behavior and mechanism of Pb²⁺ on the samples were studied in detail, and the maximum sorption amounts were about 170 and 40 mg/g for Mg₂Al–DTPA LDH and Mg₂Al–Cl LDH, respectively. The Langmuir isotherm proved to describe the sorption data better, and the pseudo-second order kinetic model fit the sorption kinetic processes better for both LDH samples. The mechanisms of Pb²⁺ sorption on Mg₂Al–DTPA LDH can be explained by Pb–DTPA chelating, while that for Mg₂Al–Cl LDH was primary surface-induced precipitation. Yan et al. [18] prepared the magnetic alginate microsphere of Fe₃O₄/MgAl-LDH (Fe₃O₄/LDH-AM) by immobilizing the Fe₃O₄/LDH with calcium alginate (CA), which was used to remove Cd²⁺, Pb²⁺, and Cu²⁺ from aqueous solutions. The adsorption kinetic data conformed to the pseudo-second-order kinetic equation, and the isotherm data fitted well with the Freundlich and Langmuir isotherm models. The adsorption mechanisms of Cd²⁺, Pb²⁺, and Cu²⁺ by the Fe₃O₄/LDH-AM were complexation and precipitation.

Oil shale ash (OSA) is the by-product of the oil shale industry. According to relevant reports, the total amount of ash discharged from oil shale in China is estimated at more than 800,000 tons per year. Because of the high alkalinity of its leachate, dumped OSA is considered to be a serious environmental pollutant and even called hazardous waste. Therefore, it is necessary to develop new methods to reduce the accumulation of OSA and further explore its new applications. The main components of OSA are quartz, clay, and other minerals [19], which have high surface adsorption activity and can prepare adsorbents and flocculants and also can be used to adsorb materials to treat heavy metal ions in wastewater. The conversion of OSA to zeolite has been reported for the removal of Cu²⁺, Cd²⁺, and Pb²⁺ from wastewater [20,21,22]. The surface roughness of OSA particles is higher. The pores are developed, and there are many cracks and lattice defects on the surface, showing a flocculent structure. These properties make it suitable for the synthesis of LDH.

In this work, oil shale ash-based hydrotalcite-like compounds (OSA-LDH) were prepared by impregnation and co-precipitation methods. The adsorption behaviors of Pb²⁺ solutions on OSA-LDH were examined. Different adsorption kinetics equations and isotherm models were used to study the adsorption kinetics and to calculate the isotherm parameters, respectively. In addition, the mechanism for Pb²⁺ removal by OSA-LDH was determined. This paper aims to not only prepare an economical and effective adsorbent to remove heavy metal ions from the solution but also provide a new path for the treatment and utilization of OSA so as to realize efficient waste resource utilization.

2. Materials and Methods

2.1. Materials

The OSA used in the experiment was collected from Huadian, Jilin Province. The OSA presented as a block solid with appearances of taupe. The samples were tested by X-ray fluorescence spectroscopy (XRF) to analyze the chemical composition. The results are shown in

Table 1. Chemical composition of OSA by XRF.

Component	O	Na	Mg	Al	Si	Ca	Fe	K	Ti
Ratio (%)	47.245	0.906	1.075	9.539	30.120	4.444	4.048	1.501	0.547

. The main elementary composition of OSA is O, Si, and Al.

The chemicals used in the experiments, including Mg(NO₃)₂·6H₂O, Pb(NO₃)₂, NaOH, and H₂SO₄, which were of analytical reagent grade, were purchased from Beijing Chemical Works. Deionized water was used in all experiments.

2.2. Preparation of OSA-LDH

First, OSA was crushed and sifted to obtain OSA powders with particle sizes smaller than 2 mm. Then, these samples were calcined in a muffle furnace at 950 °C for 8 h to remove the hydrocarbons. The products were ground and stored after calcination. Secondly, the pretreated 20.0 g OSA was completely dissolved in 200 mL 30% sulfuric acid solution and then stirred adequately for 2 h at 100 °C so that the Si components could fully gel. After the silicate gel was filtered, the Mg(NO₃)₂ solution (50 mL, 0.7 mol·L⁻¹) was added to the filtrate, and then, the pH was adjusted to 11.5 by dropping 2 mol·L⁻¹ sodium hydroxide solution at 80 °C. The resulting solution was stirred by a constant temperature magnetic mixer at the same temperature to statically crystallize for 18 h. After filtration, the crystals were washed with deionized water and dried overnight at constant temperature to produce brown OSA-LDH. In order to study the influence of synthesis temperature on the formation process of hydrotalcite, the samples were synthesized at different reaction temperatures (30 °C, 65 °C, 80 °C, and 100 °C).

2.3. Characterization Method of OSA-LDH

For all the samples, X-ray diffraction (XRD) used the D8 ADVANCE X instrument (Bruker) in the Cu Kα recording mode (k = 1.5418 Å) equipped with Ni filter radiation source (30 kV, 50 mA). The scanning electron microscopy (SEM) measurement was operated at 30 kV by using a FEI Nova Nano. The elemental compositions of the samples were determined using EDS combined with SEM. The transmission electron microscope (TEM) measurements used a high-angle annular dark field (HAADF) detector operating at 200 kV and equipped with a FEI Tecnai F20ST mirror. The textural properties of the samples were tested on a Micromeritics ASAP 2020 using N₂ adsorption–desorption with a degassing temperature of 300 °C for 12 h. Thermogravimetry (TG) was measured by a Perkin-Elmer TGA7 analyzer. X-ray photoelectron spectroscopy (XPS) analyses were recorded by a KRATOS AXIC 165 equipped with magnesium Kα rays and the hemisphere analyzer Phoibos 150 and a 3D-DLD detector.

2.4. Adsorption Experiments

Adsorption experiments were conducted in batch mode. The experiments were carried out in the same flasks by mixing OSA-LDH with Pb²⁺ aqueous solutions under different adsorption conditions. The mixture was shaken at the speed of 150 rpm, maintaining the temperature at 25 °C. The aliquots of the suspensions were extracted as a function of time and immediately filtered through a 0.45 μm membrane, and the concentrations of the residual Pb²⁺ were determined by atomic absorption spectrometer (Solaar M6, Thermo Elemental, Waltam, MA, USA). The adsorption kinetics experiments and the adsorption isotherm experiments were carried out by adding 0.1 g of OSA-LDH adsorbents into a 100 mL flask containing 50 mL of Pb²⁺ solution. The initial concentrations of Pb²⁺ were 50 mg·L⁻¹, 100 mg·L⁻¹, 200 mg·L⁻¹, and 300 mg·L⁻¹.

3. Results

3.1. Characterization of OSA-LDH

The XRD patterns of the samples prepared at different temperatures are shown in Figure 1. The OSA-LDH had a good degree of crystallization, and it showed that three characteristic peaks located at about 2θ = 11.34°, 22.92°, and 34.24° were classified into (003), (006), and (012) crystal planes, respectively, which are characteristic peaks of LDH [23]. The appearance of (003) and (006) crystal planes indicated that the synthesized products had good interlayer structure [24]. The diffraction peak (d003) was representing the interlayer spacing, which is related to the anion radius between layers and the interaction between cations on the laminates. Using the (003) peak and Bragg equation, it was possible to calculate the interlayer space (d-spacing) of the sample, and then, gallery height was calculated through the interlayer space minus layer thickness. Comparing the position of the peaks with those of Mg₄Al₂(OH)₁₄·3H₂O (PDF#35-0964), the d-spacing of OSA-LDH was expanded, which was probably due to the fact that sulfate anions had a larger effective radius than carbonate anions as interlayer anions [25]. With the increase of temperature, the gallery height also increased from 0.18 to 0.28 nm, the temperature reached 100 °C, and the gallery height narrowed. Larger gallery height can improve the absorption efficiency of LDH. Therefore, the best temperature of the synthesized OSA-LDH was 80 °C, which was used in subsequent experiments.

SEM and TEM contributed to a better understanding of the morphology of OSA-LDH. Figure 2a shows that the shape of OSA-LDH particle was irregular, the surface was rough, and there were many surface cracks and lattice defects with flocculent structure. Irregular sheet-like structures can be clearly seen growing at the edge of the lamellar structure. As we can see from Figure 2b, the OSA-LDH was mainly composed of the elements C, O, Ca, Si, Mg, and Al, and this further illustrates that the adsorbent was synthesized successfully. Figure 2c shows that the samples are in good crystalline condition and show a typical layered structure. As shown in the HRTEM diagram (Figure 2d), the lattice spacing is 0.066 nm, corresponding to the (003) plane of OSA-LDH in XRD.

In this experiment, BET analysis was used to analyze and test the specific surface area and pore characteristics of the sample. Figure 3 shows the adsorption and desorption isotherms of the OSA-LDH prepared for the BET test.

The specific surface area BET of the prepared samples was 116.5 m²·g⁻¹, the pore volume was 0.1936 cm³·g⁻¹ and the pore diameter was 6.65 nm when the relative pressure P/P₀ was 0.97. The results showed that the OSA-LDH adsorbent samples were mesoporous materials.

TG/DSC is one of the important means by which to characterize the thermal stability of materials. As shown in Figure 4 and

Table 2. The results of TG-DTA analyses.

	Transition Temperature (°C)	Weight Loss (wt%)
1st	57.7	0.7
2nd	270.9	6.0

, the TG curve was divided into three stages: The first stage was from room temperature to 200 °C. The mass loss of the samples in this stage was very small. In the second stage, the mass loss was about 6%, from 200 °C to 600 °C. In this stage, nitrate ions and -OH were decomposed successively between laminates, leading to the collapse of laminates and the destruction of hydrotalcite laminates, and finally, metal composite oxides were formed. The third stage was above 600 °C, in which there was basically no obvious weight loss, indicating that the sample was stable. Accordingly, two heat-absorbing peaks appeared on the DSC curve. One was the peak at about 57.7 °C, which represented the heat absorption and evaporation of adsorbed water and some bound water in the sample. The other peak was around 270.9 °C, which was caused by the decomposition of nitrate ions and -OH successively between laminates.

3.2. Optimization of the Batch Adsorption Conditions

The removal efficiencies of Pb²⁺ by OSA-LDH at different adsorbent dosages (0.4–3.0 g·L⁻¹) are shown in Figure 5. When the OSA-LDH adsorbent dosage was <2.0 g·L⁻¹, with the increase of OSA-LDH dosage, the removal effect of Pb²⁺ was significantly enhanced, and the removal efficiency reached 97.4%. This was due to an increase in adsorption sites and specific surface area. When the dosage of OSA-LDH was >2.0 g·L⁻¹, the removal efficiency remained almost constant. Due to consideration of removal efficiency and cost, 2.0 g·L⁻¹ OSA-LDH dosage was selected as the optimal adsorbent dosage.

The initial concentration of pollutants has obvious influence on the adsorption effect of the adsorbent. The Pb²⁺ initial concentration of the reaction was set as 50 mg·L⁻¹, 100 mg·L⁻¹, 200 mg·L⁻¹, and 300 mg·L⁻¹, respectively; the dosage of the OSA-LDH adsorbent was 2.0 g·L⁻¹; the reaction temperature was 25 °C; the pH was 5.5; and during the whole reaction process, it was continuously shaken to ensure a full reaction. The results of the adsorption removal rate and time are shown in Figure 6.

From Figure 6, when the initial concentration gradually increased, the equilibrium adsorption capacity of the adsorbent increased as well. Because the initial concentration of Pb²⁺ was very low, the adsorption sites on the surface of the adsorbent were surplus, and the saturated adsorption capacity was not reached. Correspondingly, when the concentration of metal cations in the solution was high, the probability of contact between the metal ions and the surface of the adsorbent was increased, and the adsorption probability was also increased. According to Figure 6, when the initial concentration was 50 mg·L⁻¹–300 mg·L⁻¹, the adsorption capacity of adsorbent for Pb²⁺ increased from 23.44 mg·g⁻¹ to 117.81 mg·g⁻¹. The results show that the adsorption capacity of the sample increased with the increase of the initial concentration.

The removal efficiencies of Pb²⁺ by OSA-LDH at different pH (1, 3, 5, and 7) are shown in Figure 7. With the pH value of the solution increasing from 1.0 to 5.0, the equilibrium adsorption capacity also increased. However, when the pH of the solution was adjusted to 7.0, the adsorption capacity decreased. At a lower pH value, H⁺ in the solution, which was very high, was in competition with a positive charge of Pb²⁺. As a result, the adsorption capacity was reduced. In addition, in lower-pH solutions, more hydroxyl groups on the surface of OSA-LDH were protonated, the surface was positively charged, and there was a repulsive force between OSA-LDH and Pb²⁺, which was not conducive to the adsorption of positively charged Pb²⁺, resulting in a smaller adsorption capacity. At higher pH values, due to the increased Pb(OH)₂ in the solution, and there was little Pb²⁺; thereby, the adsorption capacity was reduced.

3.3. Adsorption Kinetics

To clarify the mechanism of the adsorption process of Pb²⁺ by OSA-LDH, elucidation of kinetic parameters and sorption characteristics of the adsorbent materials are necessary. In this study, the adsorption experimental data were fit to conventional kinetic models, including the pseudo-first-order and pseudo-second-order kinetic equations.

The pseudo-first-order kinetic adsorption rate model proposed by Lagergren in the late 19th century was a very common adsorption rate model for solid–liquid systems, and it was based on the assumption that adsorption is controlled by a diffusion step [26]. The equation is as follows:

$$\ln \left(q_e-q_t\right)=ln{q}_e-k_{1}t$$

The quasi-second order kinetic adsorption rate model was established on the assumption that the adsorption rate was controlled by chemical adsorption, and the model was proposed by Ho, Y.S., and McKay, G. [27]. The equation is as follows:

$${\displaystyle \frac{\mathrm{t}}{q_t}}={\displaystyle \frac{1}{k_2{q_e}^2}}+{\displaystyle \frac{t}{q_e}}$$

where t is adsorption time (min), q_t is the adsorption amount of adsorbent per unit (mg·g⁻¹) to the adsorbent at time t, q_e refers to the adsorption amount (mg·g⁻¹) of the adsorbent to the adsorbent per unit when the adsorption equilibrium is reached, k₁ is the kinetic adsorption rate constant (min⁻¹) of the quasi-first-order kinetic adsorption rate model, and k₂ is the kinetic adsorption rate constant of the first-order kinetic adsorption rate model (g·mg⁻¹·min⁻¹).

The relevant data of the dynamic model are shown in

Table 3. Adsorption kinetic constants and correlation coefficients.

	qe,exp mg·g−1	Pseudo-First-Order			Pseudo-Second-Order
		qe,cal mg·g−1	k1 min−1	R12	qe,cal mg·g−1	k2 g·(mg min)−1	R22
50	22.92	0.7090	0.0078	0.4351	22.93	0.0465	0.9999
100	44.34	3.301	0.0177	0.5248	44.50	0.0126	0.9999
200	89.13	46.67	0.0144	0.8462	93.11	0.0005	0.9987
300	114.13	87.62	0.0138	0.9636	120.92	0.0003	0.9974

. In the table, R₁² and R₂² indicate the degree of consistency between the actual data and the fitted curve. The larger the value, the greater the consistency with this dynamic model. As can be seen from Figure 8 and Table 3, at room temperature and an initial concentration of 300 mg·L⁻¹, the equilibrium adsorption capacity of OSA-LDH on Pb²⁺ was 120.92 mg·g⁻¹. For the pseudo-first-order kinetic model, the difference between q_e,_cal and q_e,exp was relatively large, and R₁² was relatively small, which is not suitable for describing the adsorption process of metal ions by OSA-LDH. Instead, the pseudo-second-order kinetic model can well describe the actual adsorption process. Instead, the values of q_e,cal and q_e,exp were similar, and the values of R₂² were all between 0.99 and 1.000.

3.4. Adsorption Isotherms

The adsorption process is a dynamic equilibrium process. On the one hand, it is the adsorption process of the adsorbent to adsorbent (solute); on the other hand, it is the desorption process of the adsorbent (solute) from the surface of the adsorbent. When the two rates are equal, the adsorption reaches a dynamic equilibrium [28]. When adsorption reaches equilibrium at a certain temperature, the relationship between the adsorption quantity q_e (mg·g⁻¹) of the sorbent to solute and the equilibrium concentration C_e (mg·L⁻¹) of the solute in the liquid phase can be described by adsorption isotherms. In this study, the equilibrium adsorption data were analyzed by Langmuir and Freundlich models to elucidate the characteristics of Pb²⁺ removal by adsorption in OSA-LDH. The equations are as follows:

$$C_e/q_e=C_e/q_m+1/b{q}_m$$

$$ln{q}_e=ln{K}_f+{\displaystyle \frac{1}{n}}ln{C}_e$$

where C_e (mg·L⁻¹) represents the concentration of residual solution; q_e (mg·g⁻¹) denotes equilibrium adsorption quantity; q_m represents the maximum adsorption capacity of adsorbents; the constant b is related to adsorption affinity, meaning adsorption energy. K_f and 1/n represent the Freundlich constant, which is connected to adsorption capacity and intensity, respectively.

The adsorption parameters and correlation coefficients obtained after fitting are shown in

Table 4. Isotherm parameters of the Freundlich and Langmuir models.

Model	Freundlich			Langmuir
Parameter	Kf (mg·g−1)	1/n	R2	qmax (mg·g−1)	A (mg·L−1)	R2
Pb2+	0.027702	1.582	0.8845	147.93	20.47	0.9916

. It can be seen from the fitting correlation coefficient R₂ of the two isotherm models that the adsorption process of OSA-LDH can be well described by the Langmuir adsorption isotherm model with a correlation coefficient above 0.99, which indicates that the adsorption of Pb²⁺ by OSA-LDH was mainly single-molecular layer adsorption, with a maximum adsorption capacity of 147.93 mg·g⁻¹.

3.5. Adsorption Mechanisms

The adsorption mechanisms of Pb²⁺ onto OSA-LDH were investigated using the XPS method. Figure 9 displays the XPS survey spectra of OSA-LDH before and after the adsorption of Pb²⁺. In addition to the characteristic peaks of O1s, Al2p., and Mg2p in OSA-LDH, the characteristic peaks of Pb4f are shown in Figure 9(b), indicating that Pb²⁺ was successfully adsorbed on the surface of OSA-LDH.

To better explain the chemical environment of the adsorbed Pb²⁺, the XPS spectra of Pb 4f were also investigated. As shown in Figure 10, the characteristic peaks of Pb 4f appeared at 138.8 eV for Pb 4f_7/2 and at 143.6 eV for Pb 4f_5/2, indicating the formation of complexations such as Sur-OH-Pb²⁺, Sur-O-Pb²⁺, or Sur-COO-Pb²⁺. Therefore, it can be inferred that the surface of OSA-LDH has some hydroxyl groups as functional groups that can react with Pb²⁺ by chemical binding to form inner-sphere complexes. Meanwhile, there were some deprotonated hydroxyl groups (Sur-O⁻) that may have formed outer-sphere complexes with Pb²⁺ through electrostatic binding reaction. The adsorption of Pb²⁺ by OSA-LDH can be expressed as follows [29,30]:

$$Sur-OH+{Pb}^{2+}\to Sur-O-{Pb}^{2+}+H^{+}$$

$$Sur-O^{-}+{Pb}^{2+}\to Sur-O^{-}•••{Pb}^{2+}$$

4. Conclusions

Using oil shale ash as a raw material, an oil shale ash-based hydrotalcite-like compound (OSA-LDH) was prepared by impregnation and co-precipitation methods. The sample showed a good performance in the sorption of Pb²⁺ from aqueous solutions. The adsorption process accorded with the Langmuir isothermal adsorption equation and quasi-second order adsorption kinetic equation. The equilibrium adsorption capacity of Pb²⁺ was 120.92 mg·g⁻¹. The main adsorption mechanisms of OSA-LDH for Pb²⁺ included chemical bond cooperation and electrostatic bond cooperation. Therefore, OSA-LDH can be a promising sorbent for the treatment of Pb²⁺-contaminated wastewater.