Abstract

The sheet-like adsorbent of the eggshells wastes was prepared by the thermal hydrolysis method. The structure of the adsorbent was characterized by scanning electron microscope, Brunauer-Emmett-Teller, X-ray diffraction, transmission electron microscope, and X-ray photoelectron spectrometer. The adsorption capacity was investigated in a Pb2+ solution. The effects of initial pH, salt concentration, contact time, and adsorbate concentration on the adsorption of lead ions were investigated in detail. The morphology of the adsorbent was sheet-like microspheres. Zinc selenide/zinc oxide could be uniformly loaded onto the eggshell waste surface, which could effectively enhance the specific surface area of the eggshells wastes. The adsorption kinetics and isotherm followed the pseudo-second-order and Langmuir-Freundlich isotherm model, respectively. The synthesized adsorbent showed a maximum lead adsorption capacity of 1,428.78 mg/g at room temperature. Ion-exchange was the main adsorption mechanism.

INTRODUCTION

With the development of modern industry, much wastewater containing heavy metals is discharged into the environment. It is a serious threat to human health through the food chain (Yang et al. 2019). The lead ion is one of the common and highly toxic heavy metal ions. It could seriously endanger human organs, nervous system, and blood system, and even cause death (Ren et al. 2019). Lead ion pollution mainly comes from lead mining, electronics assembly plants, urban rainwater runoff, etc. The World Health Organization stipulates that the concentration of lead ions in drinking water must be less than 0.01 mg/L (Venkateswarlu & Yoon 2015). So far, there are many techniques and methods used to deal with lead contaminants, such as adsorption, membrane separation, etc (Chen et al. 2019; Dong et al. 2019). Adsorption technology, which has simple operation, high efficiency, and low-cost advantages, is widely used in lead ions' wastewater treatment. The accumulation and bonding of heavy metal ions at the solid-liquid interface can be studied by adsorption (Karthik & Meenakshi 2015). Especially, non-toxic or low-toxic ions can remove highly toxic ions from aqueous solutions by ion exchange in the adsorption process (Ma et al. 2019).

Many inorganic materials are applied to remove heavy metal ions from aqueous solution, such as ZnO and ZnSe. Due to their excellent cation exchange capacity, they are considered one of the most promising adsorbent materials. The adsorption capacity of the lead ions on ZnO could reach 833 mg/g in the early report (Xu et al. 2016). It is difficult to separate adsorbent from the aqueous solution because of their nano-size and high surface hydrophilicity. The metal oxide is retained in the water for a long time, which is harmful to human health (Choi et al. 2019). The chemical bonds of ZnSe are weaker than ZnO, implying that the ion-exchange capacity of ZnSe is greater than ZnO. Moreover, the nanomaterials were applied to improve adsorption performance in wastewater treatment (Wang et al. 2018b; Chen et al. 2019; Yin et al. 2019). Recently, elastomeric nanocomposite foams containing ZnSe were successfully prepared, and were used to remove lead ions from water (Chavan et al. 2015). More than 90% of lead ions could be removed within 1 h. However, it was quite expensive and complicated to synthesize, which limits its industrial application.

Eggshells are a waste of agricultural products whose composition is Bio-CaCO3, which has a porous interface, is green, cheap, and easy to get, etc. It has attracted scholar's attention. Calcium ions are easily ion-exchanged with lead ions in aqueous solutions to achieve efficient wastewater treatment (López-Marzo et al. 2012). Eggshells are also a large-sized porous support. Eggshells could remove contaminants from aqueous solutions, such as methylene blue, Cr(III), Ca2+, Cd2+, Pb2+, and Li+ (Pettinato et al. 2015; De Angelis et al. 2017; Tizo et al. 2018; Wang et al. 2018a). However, the adsorption capacity was low. In this work, ZnSe/ZnO was loaded onto the eggshell to obtain a composite material with high adsorption performance for lead ions. Its preparation process was relatively simple and inexpensive. It is a new attempt to apply eggshell wastes to treat heavy metal pollution. Especially, after the ion-exchange, the color of composite materials changed from yellow to black, which could be used to identify lead ions in aqueous solutions.

EXPERIMENTAL SECTION

Materials and reagents

Zinc acetate (99.0%), triethanolamine (99%–110%), ethanol (99.7%), hydrochloric acid (36.0%–38.0%), and hydrazine monohydrate (≥80.0%) were purchased from Aladdin Chemical Co. Sodium selenite pentahydrate (98%), lead nitrate (99.0%), sodium nitrate (99.0%), zinc nitrate hexahydrate (99%), calcium nitrate tetrahydrate (99.0%) were obtained from J&K Scientific Co. All of the chemical reagents were not further purified. The college canteens provided the eggshell waste. The shell membrane was removed in warm water. Then, eggshells wastes (Bio-CaCO3) were washed with distilled water and dried in the oven at 60 °C for 30 minutes. After calcination at the different temperatures (100, 200 and 300 °C, Figure S1) for 5 h, the eggshells were ground in an agate mortar and sieved by 200 mesh, and were named Bio-CaCO3(100), Bio-CaCO3(200), and Bio-CaCO3(300), respectively.

Synthesis of ZnSe/ZnO/Bio-CaCO3

ZnSe/ZnO/Bio-CaCO3 was prepared by the thermal hydrolysis method. Firstly, eggshells (1.0 g) and (CH3COO)2Zn.2H2O (1.0 g) were added into 8% (HOCH2CH2)3N aqueous solution and stirred at 90 °C for 2 h. After washing with ethanol solution and drying, white ZnO/Bio-CaCO3 was obtained. Secondly, ZnO/Bio-CaCO3 (0.5 g) and Na2SeO3·5H2O (0.25 g) were added into 45% N2H4.H2O aqueous solution at 90 °C for 4 h. After washing, a yellow product was obtained. Finally, the yellow products (ZnSe/ZnO/Bio-CaCO3(100), ZnSe/ZnO/Bio-CaCO3(200) and ZnSe/ZnO/Bio-CaCO3(300)) were annealed to be kept at 400 °C for 2 h. ZnSe/ZnO was synthesized by the same method without eggshells.

Characterization of ZnSe/ZnO/Bio-CaCO3

The morphology and mapping of the element of ZnSe/ZnO/Bio-CaCO3 were characterized using a scanning electron microscope (SEM) (S4800, Hitachi Ltd., Japan). The specific surface area and pore size of ZnSe/ZnO/Bio-CaCO3 were characterized by Brunauer-Emmett-Teller (BET) (version 3.0, Quantachrome Instruments Ltd., USA). The powder X-ray diffraction (XRD) of samples was a Dmax/Ultima IV (Rigaku Corporation, Japan) at a scanning velocity of 5°/min in the range of 5–80°. The morphology of the materials was analyzed using a transmission electron microscope (TEM, Tecnai G2 F20, FEI). The XPS spectrum of the ZnSe/ZnO/Bio-CaCO3-Pb was recorded through an X-ray photoelectron spectrometer (Thermo Fisher k-alpha).

Adsorption experiments

The effect factor, adsorption kinetics and isotherm were investigated by the batch adsorption experiments. The typical adsorption process was as follows. ZnSe/ZnO/Bio-CaCO3 (0.1 g/L) was dispersed in Pb2+ aqueous solutions and the values of the initial pH (3.15, 3.78, 4.11, 4.91, and 6.51) were adjusted by HCl solution. The suspension was stirred at room temperature. After filtration, the concentration of Pb2+ in the solution was measured by atomic absorption spectrometer (Analytik Jena). The adsorption capacity of qt (mg/g) was calculated by Equation (1) and the average relative error (ARE) by Equation (2). 
formula
(1)
 
formula
(2)
where C0 is the initial concentration of Pb2+ (mg/L), Ct is the concentration of Pb2+ in the adsorption time of t (mg/L), m is the mass of the ZnSe/ZnO/Bio-CaCO3 (g), and V is the volume of the Pb2+ solution (L).

RESULTS AND DISCUSSION

Characterization of ZnSe/ZnO/Bio-CaCO3(300)

The microstructures of Bio-CaCO3(300) and ZnSe/ZnO/Bio-CaCO3(300) were analyzed by SEM (Figure 1(a)–1(d)). The surface morphology of the Bio-CaCO3 after calcination at 300 °C is shown in Figure 1(a) and 1(b). It has a multi-layered block structure. After the ZnSe/ZnO loaded onto the Bio-CaCO3(300), the surface morphology of the product had been changed dramatically. It was a petal-like microspheres structure, which was made up of some nanosheets with sizes of about 500 nm. It was obvious that ZnSe/ZnO was successfully loaded onto the surface of the Bio-CaCO3(300), and the change of morphology was beneficial to increase the specific surface area and the active site of the adsorbent. The specific surface area of Bio-CaCO3(300) was about 2 m2/g. It was increased to ∼28 m2/g after loading ZnSe/ZnO (Table 1 and Figure S2). It was not a simple addition of the value of multiple substances, but the structure had changed. The mappings of elements of ion-exchange, such as Ca and Zn, are shown in Figure 1(e) and 1(f) and Figure S3. All the elements ZnSe/ZnO/Bio-CaCO3(300) were uniformly dispersed on the surface, which was beneficial for the adsorption of active sites exposed to contaminated media. Moreover, the content of Ca, Zn and Se was about 39.31%, 14.58%, and 12.43% on the surface of ZnSe/ZnO/Bio-CaCO3(300) (Mapping), respectively. It indicated that the ratio of ZnSe:ZnO: Bio-CaCO3(300) of the adsorbent was about 16:6:39 and most of the ZnO was converted to ZnSe. Especially, the content of Ca on the surface of ZnSe/ZnO/Bio-CaCO3(300) was much larger than that of Zn, which was advantageous for exposure to lead solutions. It implied that the ion-exchange capacity of Ca in the adsorbent could be greater than that of Zn in a specific condition.

Table 1

The BET result of Bio-CaCO3(300), ZnSe/ZnO and ZnSe/ZnO/Bio-CaCO3(300)

SamplesSurface area (m2/g)Pore volume (cm3/g)Pore diameter (nm)
Bio-CaCO3(300) 2.055 0.0130 3.825 
ZnSe/ZnO 22.46 0.1922 3.407 
ZnSe/ZnO/Bio-CaCO3(300) 28.328 0.3015 30.448 
SamplesSurface area (m2/g)Pore volume (cm3/g)Pore diameter (nm)
Bio-CaCO3(300) 2.055 0.0130 3.825 
ZnSe/ZnO 22.46 0.1922 3.407 
ZnSe/ZnO/Bio-CaCO3(300) 28.328 0.3015 30.448 
Figure 1

SEM images of Bio-CaCO3(300) (a and b) and ZnSe/ZnO/Bio-CaCO3(300) (c and d), Mapping of ZnSe/ZnO/Bio-CaCO3(300). (Ca (e), and Zn (f)) and TEM and high-resolution TEM (HRTEM) images of ZnSe/ZnO/Bio-CaCO3(300) (g and h).

Figure 1

SEM images of Bio-CaCO3(300) (a and b) and ZnSe/ZnO/Bio-CaCO3(300) (c and d), Mapping of ZnSe/ZnO/Bio-CaCO3(300). (Ca (e), and Zn (f)) and TEM and high-resolution TEM (HRTEM) images of ZnSe/ZnO/Bio-CaCO3(300) (g and h).

The HRTEM of ZnSe/ZnO/Bio-CaCO3(300) was depicted in Figure 1(g) and 1(h). ZnSe/ZnO/Bio-CaCO3(300) was made up of some sheets and their diameters were above 500 nm (Figure 1(g)). This result was very consistent with the morphological analysis of the SEM image. Figure 1(h) showed that ZnSe/ZnO could be well dispersed on the Bio-CaCO3. Three individual nanoparticles show their respective clear lattice fringes. The d-spacing of 0.25, 0.28 and 0.33 nm were attributed to the (110) plane of CaCO3, (100) plane of ZnO and (111) plane of ZnSe, respectively (Chen et al. 2016).

The XRD analyses of Bio-CaCO3(100), Bio-CaCO3(200), Bio-CaCO3(300), ZnO/Bio-CaCO3(300), and ZnSe/ZnO/Bio-CaCO3(300) are shown in Figure S4 and Figure 2. It is well known that the main component of Bio-CaCO3 is CaCO3. The characteristic diffraction peak of Bio-CaCO3 at about 23.0°, 29.3°, 31.4°, 35.9°, 39.4°, 43.2°, 47.6°, 48.6°, 56.7°, and 57.5° corresponded to the crystal face of (012), (104), (006), (110), (113), (202), (018), (116), (211) and (122) of Bio-CaCO3, respectively (PDF #05-0586) (Zhang et al. 2017). With increasing calcination temperature, the characteristic diffraction peaks at 39.4° and 43.2° gradually enhanced, indicating that the structure of Bio-CaCO3(300) was close to that of pure CaCO3. ZnO was loaded onto the Bio-CaCO3(300). Many new characteristic diffraction peaks were shown in the ZnO/Bio-CaCO3(300), which were due to the crystal face of ZnO (JCPDS No. 80-0074). In the curve of ZnSe/ZnO/Bio-CaCO3(300), the characteristic diffraction peaks at 27.14°, 45.05°, 53.39°, 65.61°, and 72.37° were assigned to the (111), (220), (311), (400) and (331) planes of cubic sphalerite ZnSe, respectively (PDF #00-037-1463 of the ICDD database). This indicated that ZnSe/ZnO/Bio-CaCO3(300) was prepared by the thermal hydrolysis method. It was easy to observe that the characteristic diffraction peaks of ZnO still existed in the ZnSe/ZnO/Bio-CaCO3(300) curve. This implied that ZnO had not been completely converted to ZnSe. While many characteristic diffraction peaks of ZnO were weakened, or even disappeared, indicating that ZnSe was the main form of Zn in the composite. This result agreed with the analysis of mapping.

Figure 2

XRD pattern of Bio-CaCO3(300), ZnO/Bio-CaCO3(300) and ZnSe/ZnO/Bio-CaCO3(300).

Figure 2

XRD pattern of Bio-CaCO3(300), ZnO/Bio-CaCO3(300) and ZnSe/ZnO/Bio-CaCO3(300).

Effect of pH

To a certain extent, the initial pH of the solution affects the presence of the adsorbate in the aqueous solution. Under alkaline conditions, Pb2+ ions are present in the form of PbOH+, Pb(OH)20 (Liu et al. 2019). In the range of pH < 7.0, lead is present in an aqueous solution in the form of Pb2+ ions. It is essential to investigate the adsorption behavior of Pb2+ ions in this range. Figure 3(a) and 3(b) show the effect of the initial pH on the adsorption capacity.

Figure 3

(a) Effect of pH on the adsorption capacity of Pb2+ by ZnSe/ZnO/Bio-CaCO3, (b) the solution pH value before and after adsorption using ZnSe/ZnO/Bio-CaCO3(300). (Dosage = 0.1 g/L, C0 = 20 mg/L, and t = 6 h).

Figure 3

(a) Effect of pH on the adsorption capacity of Pb2+ by ZnSe/ZnO/Bio-CaCO3, (b) the solution pH value before and after adsorption using ZnSe/ZnO/Bio-CaCO3(300). (Dosage = 0.1 g/L, C0 = 20 mg/L, and t = 6 h).

Figure 3(a) shows the relationship between the initial pH and the adsorption capacity. It was easy to observe that the adsorption capacity increased with increasing initial pH, especially in the range of pH 3.0 ∼ 5.0. In a strong acid solution, most of the adsorption sites on the surface were occupied by the H+ions in the solution. The positive charge of lead ions produced a huge electrostatic repulsion, which seriously hindered the adsorption of lead ions on the material (Gao et al. 2019). Above pH 5.0, the adsorption capacity increased slightly. The reason was due to the result of decreased hydrogen ion concentration. This resulted in the ability of the H+ ions to occupy the adsorption site being reduced and the degree of damage to the structure being weakened. The negative charge on the surface of the adsorbent increased and the electrostatic attraction between Pb2+ and ZnSe/ZnO/Bio-CaCO3 was enhanced.

The concentration of H+ ions in the solution was reduced after adsorption (Figure 3(b)). This implied that the main constituents of the adsorbent were consumed by H+ ions. It destroyed the surface structure of ZnSe/ZnO/Bio-CaCO3 and reduced the dosage of the adsorbent. Also, the adsorption performance of ZnSe/ZnO/Bio-CaCO3(300) to Pb2+ was much better than that of ZnSe/ZnO/Bio-CaCO3(100) and ZnSe/ZnO/Bio-CaCO3(200) at the same pH value, indicating that high-temperature calcination could improve the adsorption performance of ZnSe/ZnO/Bio-CaCO3. The reason was that Bio-CaCO3 could be activated at high temperatures (Seyahmazegi et al. 2016). ZnSe/ZnO/Bio-CaCO3(300) was selected as an adsorbent for the following experiments.

Effect of salt

There are many kinds of ions in wastewater. The presence of coexisting ions in water can affect the adsorption capacity of ZnSe/ZnO/Bio-CaCO3. Investigating the effect of salt on the adsorption performance is important. Figure 4 shows the effect of NaNO3, ZnNO3, and Ca(NO3)2 on the adsorption process of Pb2+ by ZnSe/ZnO/Bio-CaCO3.

Figure 4

Effect of salt (NaNO3, Ca(NO3)2, Zn(NO3)2) on the adsorption capacity of Pb2+ by ZnSe/ZnO/Bio-CaCO3(300). (Dosage = 0.1 g/L, C0 = 20 mg/L, and t = 6 h).

Figure 4

Effect of salt (NaNO3, Ca(NO3)2, Zn(NO3)2) on the adsorption capacity of Pb2+ by ZnSe/ZnO/Bio-CaCO3(300). (Dosage = 0.1 g/L, C0 = 20 mg/L, and t = 6 h).

It was obvious that NaNO3 and Ca(NO3)2 had a little effect on the adsorption of Pb2+ ions in 20 mg/L Pb2+ solution, especially NaNO3. Pb2+ was more like a typical soft acid ion (boundary soft) and Se2− (soft alkaline) interacted more strongly with soft acid ions Pb2+ than the hard ion Na+ (Manos & Kanatzidis 2016). The ion-exchange of ZnSe/ZnO/Bio-CaCO3 in the adsorption process of Pb2+ ions could be summarized as the following two equations (Chavan et al. 2015): 
formula
(3)
 
formula
(4)

In the equation, the increased concentration of Ca2+ ions would hinder the ion-exchange of Pb2+ ions with Bio-CaCO3. Therefore, the effect of Ca2+ ions on the adsorption process was greater than that of Na+ ions. While the hindering effect of Ca2+ ions was weaker than Zn2+ ions, which may be due to the low value of the relative solubility constant Ksp of ZnSe. This implied that Zn2+ and Pb2+ ions were the main ion-exchange reaction during the adsorption process. The result also showed that the Ca2+ ions' reactivity was weaker in 20 mg/L Pb2+ solution.

Adsorption kinetics

The adsorption kinetic model is used to investigate the influencing factors in the adsorption process and to describe the adsorption mechanism. Figure 5 shows the effect of contact time on the adsorption at 20 mg/L and 200 mg/L of Pb2+, respectively.

Figure 5

Effect of contact time on the adsorption of Pb2+ at different initial concentration ((a) CPb2+ = 20 mg/L, (b) CPb2+ = 200 mg/L; dosage = 0.1 g/L, pH = 6.51).

Figure 5

Effect of contact time on the adsorption of Pb2+ at different initial concentration ((a) CPb2+ = 20 mg/L, (b) CPb2+ = 200 mg/L; dosage = 0.1 g/L, pH = 6.51).

The adsorption capacity of Pb2+ on ZnSe/ZnO/Bio-CaCO3 increased rapidly in the first 20 min and then the increase gradually reduced until equilibrium in the 20 mg/L Pb2+ solution (Figure 5(a)). The equilibrium time was about 1 h. The initial stage of the adsorption rate was quick. This was due to the adsorption process on the surface, which had a large number of adsorption sites (Gao et al. 2019). With active adsorption sites gradually occupied, the adsorption rate increased slowly until equilibrium. The C0 of Pb2+ was expanded to 10 times. Then, the equilibrium time was extended to 7 h (Figure 5(b)). This indicated that the adsorption equilibrium time was prolonged with increased initial concentration.

The pseudo-first-order and pseudo-second-order kinetic models were used to analyze the adsorption mechanism (Figure S6a and S6b).

Pseudo-first-order: 
formula
(5)
Pseudo-second-order: 
formula
(6)
where qe and qt are the adsorption capacity (mg/g) at equilibrium and at any time t and K1 (min−1) and K2 (g/(mg·min)) are the pseudo-first-order and pseudo-second-order rate constants. The kinetic parameters and coefficient of determination (R2) are shown in Table 2. It was obvious that the R2 values of the pseudo-second-order model were all above 0.999, which was better than that of the pseudo-first-order model (R2 = 0.691 and 0.922), and the calculated qt of the pseudo-second-order model was consistent with the experimental data. The pseudo-second-order model could better describe the kinetic data than the pseudo-first-order model, implying that chemisorption was the rate-determining step of adsorption (Li et al. 2016) and the adsorption rate was affected by the number of solutes at equilibrium. The adsorption process was primarily controlled by the affinity between Pb2+ ions and the binding sites of the ZnSe/ZnO/Bio-CaCO3, which could be ion-exchange (Zhang et al. 2015).
Table 2

Pseudo-first-order kinetics and the pseudo-second-order kinetics model constants of ZnSe/ZnO/Bio-CaCO3(300)

ModelsC (mg/L)k1qe (mg/g)R2qexp (mg/g)ARE (%)
Pseudo-first-order 20 0.075 ± 0.008 113.06 ± 2.78 0.9613 120.30 ± 5.71 4.8 
200 0.024 ± 0.004 1,659.67 ± 54.77 0.9897 1,827.25 ± 28.05 10.0 
Pseudo-second-order  k2 × 104 qe (mg/g) R2   
20 9.00 ± 0.75 119.76 ± 1.69 0.9894  2.4 
200 0.17 ± 0.015 1,810.47 ± 29.43 0.9982  3.7 
ModelsC (mg/L)k1qe (mg/g)R2qexp (mg/g)ARE (%)
Pseudo-first-order 20 0.075 ± 0.008 113.06 ± 2.78 0.9613 120.30 ± 5.71 4.8 
200 0.024 ± 0.004 1,659.67 ± 54.77 0.9897 1,827.25 ± 28.05 10.0 
Pseudo-second-order  k2 × 104 qe (mg/g) R2   
20 9.00 ± 0.75 119.76 ± 1.69 0.9894  2.4 
200 0.17 ± 0.015 1,810.47 ± 29.43 0.9982  3.7 

Adsorption isotherms

So as to measure the adsorption capacity of ZnSe/ZnO/Bio-CaCO3 and demonstrate the enhancement effect of ZnSe/ZnO on the adsorption of Pb2+ ions by Bio-CaCO3, the equilibrium data were investigated using the Langmuir and Langmuir–Freundlich adsorption isotherms.

The Langmuir isotherm is suitable for adsorption on a uniform surface. The model is based on the following assumptions: (1) The activity of each adsorption site is the same, and each active site captures only one contaminant. (2) The adsorption energy is constant, and is independent of the surface coverage. (3) After the adsorbate is captured, it will be fixed at the adsorption site (Sarma et al. 2016).

The Langmuir–Freundlich isotherm is an extension of the Langmuir model, which is the Freundlich isotherm at low surface coverage or Langmuir isotherms at high surface coverage. Adsorption of heavy metal ions generally follows the Langmuir model, but ion-exchange materials follow the Langmuir–Freundlich isotherm model. 
formula
(7)
 
formula
(8)
where q (mg/g) is the adsorption capacity of Pb2+ at the equilibrium concentration, qm is the maximum sorption capacity, b (L/mg) is the Langmuir constant, Ce (mg/L) is the equilibrium concentration and n are a constant.

Figure 6(a)–6(c) showed the adsorption isotherm of Bio-CaCO3, ZnSe/ZnO and ZnSe/ZnO/Bio-CaCO3 and the equilibrium constants and parameters are shown in Table 3. It was easy to observe that all R2 values of Langmuir–Freundlich isotherm were superior to the Langmuir isotherm and the maximum adsorption capacity (qmax) was close to the experimental value. The Langmuir–Freundlich isotherm could better describe the adsorption isotherm of Pb2+ ions. The qm values of Pb2+ could reach 709.78, 542.96 and 1,378.28 mg/g by Bio-CaCO3, ZnSe/ZnO and ZnSe/ZnO/Bio-CaCO3, respectively. The adsorption capacity of ZnSe/ZnO/Bio-CaCO3 was much larger than that of the previously reported adsorbent (Table 4). The reason was as follows. The adsorption capacity was not a simple mathematical sum of qt of both Bio-CaCO3 and ZnSe/ZnO. It should be attributed to the huge changes in the surface structure of the Bio-CaCO3, which greatly improved the adsorption performance of Bio-CaCO3. Besides, the concentration of Ca2+ and Zn2+ ions in the adsorption isotherm varied with the initial concentration, as shown in Figure 6(d). At low concentrations, ZnSe/ZnO on the surface of the Bio--CaCO3 was preferentially associated with Pb2+ ion exchange. With the increase of Pb2+ concentration, Pb2+ ions diffused into the ZnSe/ZnO/Bio-CaCO3, and the ion exchange of Ca2+ and Pb2+ ions gradually played a leading role.

Table 3

Langmuir and Langmuir-Freundlich isotherm constants of Pb2+ adsorption on ZnSe/ZnO/Bio-CaCO3(300), ZnSe/ZnO and Bio-CaCO3(300)

ModelsSamplesqm (mg/g)b (L/mg)R2 ARE (%)qexp
Langmuir ZnSe/ZnO/Bio-CaCO3(300) 1,854.70 ± 669.09 0.06 ± 0.04 0.5191  84.3 1,424.78 ± 35.10 
 ZnSe/ZnO 627.85 ± 59.66 0.09 ± 0.03 0.7040  27.6 549.02 ± 10.11 
 Bio-CaCO3(300) 854.89 ± 102.02 0.05 ± 0.020 0.7220  31.1 708.01 ± 15.12 
qm (mg/g)b (L/mg)nR2
Langmuir-Freundlich ZnSe/ZnO/Bio-CaCO3(300) 1,324.92 ± 110.49 0.31 ± 0.01 0.13 ± 0.02 0.9286 27.1  
 ZnSe/ZnO 534.93 ± 11.24 0.11 ± 0.002 0.12 ± 0.02 0.9699 42.4  
 Bio-CaCO3(300) 706.98 ± 17.67 0.08 ± 0.002 0.18 ± 0.03 0.9702 11.5  
ModelsSamplesqm (mg/g)b (L/mg)R2 ARE (%)qexp
Langmuir ZnSe/ZnO/Bio-CaCO3(300) 1,854.70 ± 669.09 0.06 ± 0.04 0.5191  84.3 1,424.78 ± 35.10 
 ZnSe/ZnO 627.85 ± 59.66 0.09 ± 0.03 0.7040  27.6 549.02 ± 10.11 
 Bio-CaCO3(300) 854.89 ± 102.02 0.05 ± 0.020 0.7220  31.1 708.01 ± 15.12 
qm (mg/g)b (L/mg)nR2
Langmuir-Freundlich ZnSe/ZnO/Bio-CaCO3(300) 1,324.92 ± 110.49 0.31 ± 0.01 0.13 ± 0.02 0.9286 27.1  
 ZnSe/ZnO 534.93 ± 11.24 0.11 ± 0.002 0.12 ± 0.02 0.9699 42.4  
 Bio-CaCO3(300) 706.98 ± 17.67 0.08 ± 0.002 0.18 ± 0.03 0.9702 11.5  
Table 4

Adsorption capacity of different adsorbents for Pb2+ ions in aqueous solutions

Adsorbentsqmax (mg/g)References
Chlorella sp. QB − 102 635.8 Li et al. (2019b)  
CMMB 263.6 Li et al. (2019a)  
BC/FM 154.94 Zhang et al. (2019)  
Fe3O4/poly(C3N3S3232.6 Fu & Huang (2018)  
MIL − 96(Al) 301.5 Mehdinia et al. (2018)  
NZVI/AC 59.35 Liu et al. (2019)  
Magnetic biochar/ZnS 367.65 Yan et al. (2015)  
ZnSe/ZnO/Bio-CaCO3(300) 1,424.78 This work 
Adsorbentsqmax (mg/g)References
Chlorella sp. QB − 102 635.8 Li et al. (2019b)  
CMMB 263.6 Li et al. (2019a)  
BC/FM 154.94 Zhang et al. (2019)  
Fe3O4/poly(C3N3S3232.6 Fu & Huang (2018)  
MIL − 96(Al) 301.5 Mehdinia et al. (2018)  
NZVI/AC 59.35 Liu et al. (2019)  
Magnetic biochar/ZnS 367.65 Yan et al. (2015)  
ZnSe/ZnO/Bio-CaCO3(300) 1,424.78 This work 
Figure 6

Adsorption isotherm of Pb2+ on Bio-CaCO3 (a), ZnSe/ZnO (b), ZnSe/ZnO/Bio-CaCO3 (c). Concentration of Ca2+, Zn2+ and Pb2+ ions in the aqueous solution of adsorption isotherm (d). (Dosage = 0.1 g/L, pH = 6.51 and t = 12 h).

Figure 6

Adsorption isotherm of Pb2+ on Bio-CaCO3 (a), ZnSe/ZnO (b), ZnSe/ZnO/Bio-CaCO3 (c). Concentration of Ca2+, Zn2+ and Pb2+ ions in the aqueous solution of adsorption isotherm (d). (Dosage = 0.1 g/L, pH = 6.51 and t = 12 h).

Adsorption mechanism

XRD and XPS investigated the adsorption mechanism of Pb2+ on the ZnSe/ZnO/Bio-CaCO3.

Compared with ZnSe/ZnO/Bio-CaCOCaCO3, some new peaks were shown in the diffraction peak of ZnSe/ZnO/Bio-CaCO3/Pb-20 (after adsorption in 20 mg/L Pb2+ ions), which were due to PbSe (Chavan et al. 2015), PbO (Yuan et al. 2016) (Figure 7(a)). This indicated that the surface-loaded ZnSe and ZnO were involved in the ion exchange (Equations (3), (4) and (9)). 
formula
(9)
Figure 7

XRD patterns of ZnSe/ZnO/Bio-CaCO3 and ZnSe/ZnO/Bio-CaCO3/Pb (v Bio-CaCO3, c PbCO3, e ZnSe, s PbSe, o ZnO, p PbO) and high-resolution spectrum of Pb (b).

Figure 7

XRD patterns of ZnSe/ZnO/Bio-CaCO3 and ZnSe/ZnO/Bio-CaCO3/Pb (v Bio-CaCO3, c PbCO3, e ZnSe, s PbSe, o ZnO, p PbO) and high-resolution spectrum of Pb (b).

It was almost impossible to find the diffraction peaks of PbCO3, indicating that Bio-CaCO3 did not participate in the ion exchange. At low concentrations, Pb2+ ions did not diffuse into the interior of the adsorbent, which reacted only with the surface layer of ZnSe/ZnO. After increasing the C0, some significant new diffraction peaks appeared in ZnSe/ZnO/Bio-CaCO3/Pb(200) (after adsorption in 200 mg/L Pb2+ ions), which were attributed to the PbCO3 (Yuan et al. 2016). At high concentrations, Pb2+ ions not only interacted with the surface of the ZnSe/ZnO, but also diffused into the adsorbent, reacting with Bio-CaCO3. The ion exchange between Ca2+ and Pb2+ ions played an important role in the high concentration solution. The Ksp of CaCO3 and PbCO3 was about 2.8 × 10−9 and 7.4 × 10−14, respectively (Zhang et al. 2016). Bio-CaCO3 was easily converted to PbCO3 in the high concentration of Pb2+ ions solution.

The survey spectrum of ZnSe/ZnO/Bio-CaCO3/Pb(200) exhibited elemental composition (Figure S7). The peaks at 291.28, 352.88, 537.93, 144.07, 61.00, 1,029.27 eV were attributed to C 1s (CaCO3), Ca 2p (CaCO3), O 1s (CaCO3, ZnO), Pb 4f (PbCO3, Pb(OH)2, PbSe), Se 3d (ZnSe, PbSe), and Zn 2p (ZnO, ZnSe), respectively. A high-resolution spectrum of Pb is shown in Figure 7(b). The two peaks at 148.90 and 144.07 eV were due to the 4f of Pb (Zhu et al. 2019). The band at 144.07 eV was divided into three peaks (143.62, 143.81 and 145.01 eV), which could be assigned to the different chemical bonds, such as Pb-O and Pb-Se etc (Zhou et al. 2017; Beygi et al. 2018; Zhu et al. 2019). All the above results confirm that Pb2+ was adsorbed on the surface of ZnSe/ZnO/Bio-CaCO3 by ion-exchange.

CONCLUSIONS

An efficient adsorbent of ZnSe/ZnO/Bio-CaCO3 was successfully prepared by a simple thermal hydrolysis method. It appeared as sheet-like microspheres and the specific surface area was about ∼28 m2/g. It could effectively remove Pb2+ from the aqueous solutions. It was beneficial to adsorb Pb2+ at an initial pH value = 6.5 and the qmax could reach 1,424.78 mg/g at room temperature. It was superior to many of the reported adsorbent materials. The pseudo-second-order model could well fit the adsorption kinetics and ion exchange played an important role in the adsorption process. It provides an effective way for the recycling of biomass.

ACKNOWLEDGEMENTS

The authors are grateful to the Applied Basic Research Programs of Science and Technology Department of Sichuan Province (2018JY0115), the Application Technology Research and Development Special Project of Nanchong, China (18YFZJ0035), the Meritocracy Research Funds of China West Normal University (17YC013, 17YC139).

CONFLICTS OF INTEREST

The authors declare no conflicts of interest.

SUPPLEMENTARY MATERIAL

The Supplementary Material for this paper is available online at https://dx.doi.org/10.2166/wst.2020.081.

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Author notes

Y.Y. and S.Y. contributed equally to this work.

Supplementary data