Abstract
With the rapid development of industrialization and urbanization, metals are in high demand and widely used in various fields such as batteries, electroplating, steel industry, etc [
Fortunately, various methods, such as chemical co-precipitation, ion exchange, bioremediation, membrane filtration, reverse osmosis, electrochemical treatment and adsorption etc., have been developed to remove Cd(II) from wastewater[
Hexagonal boron nitride (h-BN) exhibits an isostructure of carbon and possesses unique physical and chemical properties, especially numerous strucurual defects, chemical durability and oxidation resistance as compared to carbon materials. These features render porous h-BN (p-BN) outstanding adsorption properties. Consequently, p-BN has been demonstrated to treat a wide range of pollutants, such as dyes, organic solvent, heavy metals and harmful gas, etc[
In present work, the adsorption of Cd(II) on p-BN was systematically evaluated. The p-BN compounds before and after adsorption were specifically characterized. The adsorption performance was evaluated by batch experiments, in which the effects of initial pH, adsorbent dosage, contact time and temperature were investigated in details. The adsorption kinetics and thermodynamic were also discussed to understand the mechanism of adsorption.
1 Materials and methods
1.1 Materials
The p-BN microrods were synthesized by a modified two-step-synthesis method[
1.2 Characterization
The morphology and the element contents were analyzed by scanning electron microscope (SEM, JSM-6360LV) and high resolution transmission electron microscope (HRTEM, JEM-2100F), respectively. The surface area, volume of micropore were determined by the Brunauer- Emmett-Teller method (BET, Empyrean, Micromeritics) at 77 K. Before measurement, the samples were activated in vacuum at 573 K for 8 h. The crystal structure was recorded by X-ray diffraction (XRD, Empyrean) with Cu Kα radiation. The changes of functional groups of compounds before and after adsorption were identified by a Fourier transform infrared spectrometer (FT-IR, NEXUS) with a scan range of 400-4000 cm-1. The X-ray photoelectron spectrum (XPS) was conducted by a Thermo ESCALAB250 with a focused monochromatized Al Kα X-ray source (hm=1486.6 eV). The concentration of Cd(II) was measured by flame atomic absorption spectroscope (AAS, AA-7000).
1.3 Batch experiment
Cd(NO3)2·4H2O as the sources of Cd(II) was used to prepare aqueous stock Cd(II) solution. Cd(II) solutions with different initial concentrations were prepared by diluting Cd(II) stock solutions with fixed ratio. Small volumes of 0.10 mol·L-1 HCl and/or NaOH solutions were used to adjust the initial pH of test solutions. pH was set at 7.0 after optimization. The Batch adsorption experiment was conducted by adding a certain amount of p-BN adsorbent to 50.0 mL Cd(II) solutions with different initial concentrations, and then shaken at 313.0 K for 24 h to ensure adsorption equilibrium. For the adsorption kinetic study, p-BN adsorbent was set at 10.0 mg and Cd(II) solutions with different initial concentrations (40, 60, 80 mg·L-1) were employed. To obtain sorption isotherms of Cd(II) on p-BN, the operation temperature was set at 303, 313 and 333 K. Finally, the solid was separated from aqueous solution via filtration through 0.22-μm polyethersulfone membrane filters. And then 0.5 mL supernatant and 0.5 mL HNO3 (pH=2) were diluted to 25.0 mL for determination.
The adsorption of p-BN is represented by removal (Ads%) and the adsorption capacity (q) which is the amount of Cd(II) adsorbed on the sorbent per unit weight. The adsorption capacity at equilibrium and any time t are indicated by qe and qt(mg·g-1). The relevant parameters are calculated by the formula (1-3):
where C0(mg·L-1) is the initial Cd(II) concentration,Ct and Ce are the concentration at time t and equilibrium, m (g) is the amount of adsorbent and V (L) is the volume of solution. All batch experiments were performed in duplicates or triplicates in order to ensure the data accuracy and repeatability. The average of the acquired data was used for subsequent analysis.
2 Results and discussion
2.1 Morphology and structure
The morphologies and structures of p-BN materials before and after adsorption were characterized by SEM, HRTEM, as shown in Fig.1. As depicted in SEM image (Fig. 1(A)), the bare p-BN is composed of a large number of irregular micro rods, with a length ranging from a few micrometers to tens of micrometers. Meanwhile, the corresponding HRTEM image (Fig. 1(B)) shows that p-BN has a homogenous porous structure, for which numerous visible nanopores (Fig. 1(B) dotted circle) are evenly distributed on the micro rods. In comparison, no significant changes are found in low-magnification images of p-BN after adsorption, as shown in Fig. 1(C). Because of the ultrasonic treatment, p-BN micro rods appear to be more fragmented. However, a large number of unidentified black dots appear on the surface of the adsorbed p-BN micro rods in HRTEM image (Fig. 1(D)). The high-resolution HRTEM image (Fig. 1(F)) indicates that the dots are nano-sized particles (Fig. 1(F) dotted circle) containing high-component Cd element (Fig. S1). The EDS pattern (Fig. 1(E)) shows that the adsorbed boron nitride showed a distinct peak of Cd, which means that Cd(II) has adsorbed onto p-BN material.
Figure .Energy spectrum analysis (ESA) for unidentified black dots adsorbed on p-BN
Figure .(A) SEM and (B) HRTEM images of p-BN, (C) SEM and (D) HRTEM images of p-BN after adsorption, (E) EDS analysis and (F) high-magnification HRTEM image of p-BN after adsorption
2.2 Crystal structure and surface functional groups
Fig. 2(A) presents the XRD patterns of p-BN before and after adsorption. It can be seen that there exists two broad peaks at 2θ=~25.5° and ~42.5°, which could be assigned to (002) and (100) fringes of h-BN with poor crystallization[
Figure .XRD patterns (A) and FT-IR spectra (B) of p-BN before and after adsorption
The surface functional groups of p-BN before and after adsorption were estimated by FT-IR (Fig. 2(B)). Typically, corresponding to the sp2-bonded B-N and B-N-B bending vibration, two strong characteristic absorption bands were observed at ~1400 and ~800 cm-1, respectively, indicating the main crystalline structures of hexagonal BN existed[
Figure .BET measurement of p-BN
2.3 Influence of pH and sorbent dosage
The role of acidity in Cd(II) adsorption on p-BN was studied with pH ranging from 1.0 to 7.0 and presented in Fig. 3(A). It can be revealed that the initial pH has a significant effect on the Cd(II) adsorption. Low pH is not favor of the adsorption, since in that range cadmium ions are present in free form as Cd2+ ions (Fig. S3), while more proton are available on the surface of p-BN to produce protonate amino/hydroxyl groups. Hence, the electrostatic repulsion force between p-BN surface and the cation of Cd2+ inhibits the adsorption. With the increase of pH, the deprotonation of the surface functional groups result in more adsorption activity sites for Cd(II), and the adsorption gradually increases. Additionally, the distribution of the morphological species of Cd(II) is affected by the concentration of Cd(II) and pH. An excessive alkaline environment may result in strong hydrolyzed precipitation of cadmium ions which causes large deviations from experimental results (Fig. S3). Based on all the considerations, pH of the solution is set at 7.0, which is close to the pH of the surface water, for our subsequent experiments. In previous reports, Liao et al.[
Figure .(A) Effect of initial pH on Cd(II) adsorption capacity (
Figure .Variation of distribution of Cd(II) hydrolyzate species with pH
2.4 Adsorption kinetics
Fig. 4 presents the uptake of Cd(II) on p-BN as a function of contact time at various initial concentrations of Cd(II). Obviously, as the concentration of Cd(II) increases, the adsorption capacity increases (Fig. 4(A)) while removal percentage decreases (Fig. 4(B)). It is noteworthy that at a constant dosage of p-BN adsorbent, the adsorption capacity increases rapidly with Cd(II) concentration increasing, indicating that the adsorption behavior does not depend on amounts of surface active groups. On the other hand, the adsorptive quantity of Cd(II) increases quickly during the first 3 h, and then gradually increases until equilibrium. The adsorption capacity can reach 219.7 mg·g-1 at Ccd=80 mg·L-1. Furthermore, the variation of adsorption capacity with contact time can be utilized for constructing adsorption kinetic models, reflecting the relationship between the structure of adsorbent and adsorption performance. The adsorption and consequence can also be predicted or verified. Particularly, subsequent kinetic analysis is achieved from experimental data qe-t curve for Ccd = 60 mg·L-1. In the present work, the adsorption kinetics was simulated by 4 kinetic models to investigate the possible mechanism for removal process, which named as pseudo-first-order model, pseudo-second-order model, intra-particle diffusion model and liquid-film diffusion. The linearized forms of the 4 models are expressed by the equations (4-7):
Pseudo-first-order model:
Pseudo-second-order model:
Intra-particle diffusion model:
Liquid-film diffusion model:
where k1(h-1) and k2(g·mg-1·h-1) are the rate constants of the first and the second order adsorption for Cd(II). kd(g·mg-1·h-1/2) is the rate constant of intra-particle diffusion and kf is the rate constant of liquid-film diffusion. I is a parameter relate to the thickness of the boundary layer, A is liquid-film diffusion constant[
Figure .(A) Adsorption capacities of Cd(II) with various contact times at different initial concentrations of Cd(II), and (B) adsorption percentages of Cd(II) on p-BN with various contact time at different initial concentrations of Cd(II)
Figure .Kinetics models for adsorption of Cd(II) on p-BN
Adsorbate | Δ | Δ | Δ | |||
---|---|---|---|---|---|---|
303 K | 313 K | 323 K | ||||
Cd(II) | 50 | 16.51 | 72.81 | -5.55 | -6.28 | -7.01 |
60 | 17.58 | 73.40 | -4.66 | -5.39 | -6.13 | |
70 | 14.25 | 61.39 | -4.35 | -4.97 | -5.58 | |
80 | 16.21 | 66.54 | -3.95 | -4.62 | -5.28 | |
90 | 16.08 | 64.60 | -3.49 | -4.14 | -4.79 |
Table 2.
Values of thermodynamic parameters for the adsorption of Cd(II) on p-BN
2.5 Adsorption isotherm and thermodynamics
Adsorption isotherms of Cd(II) on p-BN at T=303, 313 and 323 K are depicted in Fig. 5(A), describing the relation between Ce and qe. Not surprisingly, adsorption capacity of adsorbent increases with the increase of temperature, which confirm that high temperature is propitious to adsorption behavior. Most importantly, the variation of the adsorption isotherm contributes to analyze the interaction between adsorbent and adsorbate, and the structural characteristics of the adsorbed layer. Therefore, Langmuir, Freundlich and Tempkin models are adopted for fitting experimental data and the linearized forms of the three models are expressed by the equations (8-10):
Langmuir model:
Freundlich model:
Tempkin model:
where KL(L·mg-1) is the Langmuir adsorption constant about the affinity of binding sites; KF is the Freundlich constant which reflects the adsorption ability and n is non-linear factor relate to heterogeneous energy of adsorption surface. KT(L·mg-1) is the constant associated with the adsorption heat and f is the Tempkin equilibrium constant corresponding to maximum binding energy. Fig. 5(B-D) show the fitting results by isotherm models and the corresponding parameters and correlation coefficients are listed in Table 1. In contrast to Langmuir model (Fig. 5(B)), Freundilich model (Fig. 5(C)) fits the isotherm better with high correlation coefficient, indicating that Cd(II) are adsorbed on a heterogeneous surface through multilayer and monolayer adsorption[
Figure .(A) Adsorption isotherms of Cd(II) on p-BN at
Figure .Linear plots of ln
Cd(II)/p-BN | Model | |||||||
---|---|---|---|---|---|---|---|---|
Pseudo-first-order | Pseudo-second-order | Intra-particle diffusion | Liquid-film diffusion | |||||
Parameters | qe,cal=/(mg·g-1) | 111.7 | qe,cal=/(g·mg-1·h-1) | 193.1 | 60.2 | 0.524 | ||
K1-1 | 0.524 | K2(g·mg-1·h-1) | 1.00×10-3 | kd/(g·mg-1·h-1/2) | 49.4 | -0.499 | ||
0.948 | 0.999 | 0.872 | 0.948 |
Table 1.
Adsorption kinetics models parameters
Table Infomation Is Not Enable2.6 XPS analysis
In order to identify the bonding states and their compositions of p-BN before and after absorption, XPS spectra were also investigated and presented in Fig. 6. It can be clearly seen that the main constituent elements (B, N, O) are detected in XPS spectra of total surveys for p-BN (Fig. 6(A)). Meanwhile, the presence of double peaks at around ~410 eV is due to the orbital spin splitting of the 3d layer electrons of Cd(II), corresponding to Cd3d5/2 and Cd3d3/2 in the inset of Fig. 6(A), respectively[
Figure .(A) XPS surveys for p-BN and adsorbed p-BN(inset: high resolution Cd3d XPS spectrum and background); (B) Experimental bonding enerygy peaks of Cd(II) and the comparisons of primary peaks of Cd3d5/2 and Cd3d3/2 for free Cd(II), CdCO3, Cd(OH)2
Figure .High resolution spectra of B1s (a) and O1s (b) for p-BN before and after adsorption
3 Conclusions
In summary, the adsorption performances and mechanism of p-BN for Cd(II) were systematically studied. The results demonstrate that p-BN is an effective adsorbent for Cd(II) in aqueous solution, and its adsorption capacity highly depends on contact time, pH and temperature. In a neutral water environment (pH=7.0), the maximum adsorption capacity can reach 184 mg·g-1 at 313 K. The pseudo-second-order equation fits well with the adsorption process, indicating that the adsorption process is mainly controlled by chemisorption. The intra-particle diffusion model demonstrates that the rate limiting step is primarily molecular diffusion of Cd(II) in the micropores of p-BN. Isotherm studies present that Cd(II) sorption on p-BN complies well with Freundlich and Langmuir model, respectively, implying Cd(II) were adsorbed on the heterogeneous surface through multilayer and monolayer adsorption. The thermodynamic parameters confirm that the adsorption of Cd(II) on p-BN is a spontaneous endothermic process and high temperature is beneficial to the adsorption. Considering the above results, p-BN is a very promising candidate for Cd(II) removal from aqueous solution.
Supporting materials
Supporting materials related to this article can be found at https://doi.org/10.15541/jim20190371.
Distinguished Cd(II) Capture with Rapid and Superior Ability using Porous Hexagonal Boron Nitride: Kinetic and Thermodynamic Aspects
LI Li1, GUO Xiaojie2, JIN Yang1, CHEN Chaogui1, Abdullah M Asiri3, Hadi M Marwani3, ZHAO Qingzhou4, SHENG Guodong1
(1. College of Chemistry and Chemical Engineering, Shaoxing University, Shaoxing 312000, China; 2. College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China; 3. Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia; 4. College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China)
BET characterization
Fig. S2 illustrates N2 adsorption-desorption isotherm (77 K) and the corresponding pore size distribution of p-BN. The measured isotherm (Fig. S2(a)) can be classified as a type I isotherm and an H4 type broad hysteresis loop between the relative pressure (p/p0) of 0.47 and 0.99. This indicates p-BN material is filled with micro porosity and narrow slit-shaped mesopores, which would increase the contact probability and be favorable for adsorption[
Fitting results by four kinetic models
Fig. S4(A-D) illustrate the fitting results by 4 kinetic models, the corresponding kinetic parameters are listed in Table S1. The results (Fig. S4(A-B)) obtained from the analysis of experimental data reveals that adsorption of Cd(II) on p-BN is best described by pseudo-second-order model. The linear fitting coefficient is as high as 0.999 and the theoretically calculated adsorption capacity (qe,cal=193.1mg·g-1) is very close to the experimental result (qe=184.0mg·g-1). These indicate that the adsorption process involves the sharing or transfer of electrons between the adsorbent and the adsorbate[
Adsorption thermodynamic
The thermodynamic parameters of the adsorption, i.e. the values of standard Gibbs free energy ($\Delta G^{\theta}$), enthalpy ($\Delta H^{\theta}$) and entropy ($\Delta S^{\theta}$) are calculated by the following formulas (11-13):
Where Kd a is the distribution coefficient and R is the molar gas constant (8.314 J·mol-1K-1). Briefly, Kd is linear with 1/T and presented in Fig. S5. The slope and intercept of the fitted curve can be used to calculate $\Delta H^{\theta}$ and$\Delta S^{\theta}$, and then calculate $\Delta G^{\theta}$. The thermodynamic parameters are summarized in Table S2. The negative values of $\Delta G^{\theta}$ and the positive values of $\Delta H^{\theta}$ confirm that the adsorption of Cd(II) on p-BN is a endothermic and spontaneous process. The increase of temperature made $\Delta S^{\theta}$ more negative, indicating that high temperature is beneficial to the adsorption, which is consistent with the results of adsorption capacities at different temperatures.
References:
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[3]ZHANG LEI, SONG XIAO-YAN, LIU XUE-YAN, et al. Studies on the removal of tetracycline by multi-walled carbon nanotubes. Chemical Engineering Journal, 2011, 178: 26-33.
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