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 ^[1,2]. Although metal products have brought countless conveniences to the human, unfortunately, heavy metal ions released into water bodies which have been recognized as serious environmental hazards^[2,3,4,5,6]. For instance, cadmium is frequently found in effluents industries such as mining, smelting operations, or leather manufacturing. Due to its extreme toxicity, high mobility, high biological accumulation and carcinogenicity^[7,8,9,10], cadmium ions (Cd(II)) even at a low concentration could pose severe impairment on the human and ecosystem. As a consequence, removal of Cd(II) from wastewater is of enormous significance to maintain ecological stability and public safety.

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^{[11,12,13,14,15,16,17,18,19,20,21]}.Among these technologies, adsorption has been proved to be feasible, economical and high removal effective which is widely used in wastewater treatment contaminated by heavy metals^{[2, 7, 18-21]}. Thus it is of great importance and difficulty to seek effectual adsorbent with extraordinary efficiency, wide adaptability, environment-friendly and low cost. In this respect, popular materials such as nanosized carbon materials^{[18, 22-24]}, nanoscale zero valent iron (NZVI)^[24,25], carbon nitride^[19], layered double hydroxides (LDHs)^[26,27] and boron nitride (BN), etc.^[28-29] have been employed as adsorbents, and more materials are constantly developed for the removal of Cd(II) from aqueous solutions.

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^[28,29]. Notably, adsorption of a series of heavy metals (such as Pb, Hg, Cr, Ni, Cu) have been investigated via BN-based adsorbent^{[30,31,32,33,34,35]}. Li et al.^[32] studied Cr(III) adsorption by fluorinated activated boron nitride. Chen et al. ^[35] prepared O-doped BN nanosheets as capacitive deionization electrode for efficient removal of heavy metal ions. However, to the best of our knowledge, a comprehensive systematic study for the adsorption of Cd(II) using p-BN materials is still lacking.

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.

The p-BN microrods were synthesized by a modified two-step-synthesis method^[36]. In brief, analytical grade melamine and boric acid (mole ratio=1 : 2) purchased from Aladdin were used directly without further purification to prepare precursor and the final product was obtained by subsequent high temperature calcination (1373 K for 1 h). Experimental solutions were prepared by using 18 MΩ∙cm de-ionized water (Millipore Milli-Q water purification system) under ambient conditions. The other chemicals used in this study were of analytical grade.

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).

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 q_e and q_t(mg·g^-1). The relevant parameters are calculated by the formula (1-3):

where C₀(mg·L^-1) is the initial Cd(II) concentration,C_t and C_e 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.

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.

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^[34]. It is noteworthy that the calculated interplanar distances of (002) plane is 0.35 nm which is a little larger than that of raw h-BN. The enlarged interspace can be attributed to a similar turbostratic BN structure observed in previous report^[30]. Furthermore, after adsorption, it is found that the high-intensity peak (002) shifted to the right of +0.8°, which could relate to the emergence of new substances and/or lattice deformation. The appearance of low-intensity peaks at ~23.6°, ~30.3°, ~36.4°, ~49.9° come from a new substance, which is considered to be a highly similar compound of otavite, syn (CdCO₃, JCPDS 42-1342). The observed peaks are consistent with the corresponding lattice planes of (012)(23.49°), (104)(30.28°), (110)(36.42°) and (116)(49.9°). These contribute to analyze the possible adsorption mechanisms.

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^[37]. The characteristic adsorption peaks at ~3420 and ~1160 cm^-1 were consistent with B-OH and B-N-O stretching and peaks close-by ~3200 cm^-1 attributed to B-NH₂ stretching vibrations^[38]. The presences of B-OH/B-NH₂ groups provide abundant basic sites which facilitates electrostatic adsorption of positively-charged ions under alkaline conditions. The adsorption peaks at ~1630 and ~1090 cm^-1 can be attributed to C=O and C-O stretching vibrations, which may be related to the addition of triblock copolymers as structure directing agents^[36]. It is noteworthy that after adsorption, the broad band intensities ranges from 3000 to 3600 cm^-1 and 1200 to 1600 cm^-1 increase and are consistent with amine and imine hydrohalide N-H⁺/N-H₂⁺ stretching vibrations^[39]. We also conducted BET characterization of the material, and the results are shown in Fig. S2.

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.^[19] studied pH effect on Cd(II) sorption onto g-C₃N₄ nanosheets. Paola et al.^[22] studied pH effect on Cd(II) sorption onto modified N-doped carbon nanotubes.Huang et al.^[23] studied the effect of pH on Cd(II) sorption onto graphene oxides. And similar results were found. Fig. 3(B) presents the influence of adsorbent dosage on the adsorption. These results give a chance to expect less sorbent consumption or higher efficiencies of adsorption. It can be clearly seen that the adsorption percentage of Cd(II) rises with the increase of p-BN dosage and tends to be stable near ~80% after the adsorbent concentration reaches 0.4 g·L^-1. In contrast, the adsorption gradually decreases with the dose of p-BN increasing. The reasons for this phenomenon can be ascribed to the fact that: firstly, the surface active sites of p-BN increase with the concentration of p-BN; secondly, when the concentration of adsorbed ions in solution is low to a certain extent, the dynamic equilibrium of adsorption/desorption inhibits the increasing adsorption. It can be deduced to a chemisorption process for Cd(II) adsorption.

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 C_cd=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 q_e-t curve for C_cd = 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). k_d(g·mg^-1·h^-1/2) is the rate constant of intra-particle diffusion and k_f 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^[40,41].The detailed results of the fitting results by 4 kinetic models are shown in Fig. S4 and Table S1.

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 C_e and q_e. 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 K_L(L·mg^-1) is the Langmuir adsorption constant about the affinity of binding sites; K_F is the Freundlich constant which reflects the adsorption ability and n is non-linear factor relate to heterogeneous energy of adsorption surface. K_T(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^[19,42]. Based on analysis of Freundilich model, the smaller the numerical value of 1/n is, the better adsorption will be. It is clear that 1/n (0.22-0.23) is small, which illustrates a favorable adsorption process. Applying to the adsorption system with heterogeneous surface, the linear fit of Tempkin model (Fig. 5(D)) has a high correlation coefficient (~0.99), demonstrating that the decrease in adsorption heat with adsorption capacity is linear energy dependence rather than exponential one. This is possibly ascribed to a certain strong chemical bonding interactions between Cd(II) and p-BN adsorbent. 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 shown in Fig. S5 and Table S2.

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 Cd3d_5/2 and Cd3d_3/2 in the inset of Fig. 6(A), respectively^[43]. These results confirm the successful adsorption of Cd(II) on p-BN in consistent with the EDS results in Fig. 1(E). High resolution Cd3d XPS spectrum is depicted in Fig. 6(B), a double peak is observed at 405.7 and 412.4 eV, which is assigned to Cd3d_5/2 and Cd3d_3/2 as mentioned before. Compared with the primary peaks of Cd3d_5/2 (405.0 eV) and Cd3d_3/2 (411.7 eV) for free Cd(II) ions, the existence of a shift of +0.7 eV toward higher bonding energy can be attributed to the interaction between Cd(II) and the adsorbent^[43]. Similarly, the peaks of N1s and O1s obtained a little left shift after the adsorption, which suggest a possible interaction between B (or N) atom and Cd(II) (Fig. S6). Besides, recognized as two common cadmium precipitates in aqueous solution, the photoelectron binding energies of Cd(II) in CdCO₃ (405.4 eV) and Cd(OH)₂ (406.7 eV) are also illustrated in Fig. 6(B) for comparison. The experimental peaks located at 405.7 and 412.4 eV can be fitted into several peaks, and the split peaks can be attributed to the complexation of Cd(II) and numerous strucurual defects among p-BN adsorbent, exampling as polar bonds of B-O, B-N and C-O, etc^[44]. These results imply that p-BN adsorbent has substantial functional groups and bonding sites, which is propitious to uptake Cd(II) from wastewater.

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^[1]. Meanwhile, the corresponding Barrett-Joyner-Halenda (BJH) pore-size distribution curve of p-BN micro rods are illustrated in Fig. S2(b), the calculated pore volume is 0.070 cm³·g^-1 and the major characteristic pore size is ~2.62 nm. Besides, the BET specific surface area is 66.0 m²·g^-1, which can provide more activated site and be beneficial 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 (q_e,cal=193.1mg·g^-1) is very close to the experimental result (q_e=184.0mg·g^-1). These indicate that the adsorption process involves the sharing or transfer of electrons between the adsorbent and the adsorbate^[2], and the adsorption process is mainly controlled by chemisorption. Additionally, two distinct regions (Fig. S4(C)) are observed in the intra-particle diffusion diagram, the piecewise linear regressions suggest that the adsorption process can be divided into two stages, external mass transfer and intra-particle diffusion^[3]. The rapid adsorption of the first stage is attributed to the larger surface area and more activated adsorption sites. The second stage of adsorption relies on the molecular diffusion of Cd(II) in p-BN micropores, which is slow and has a limited rate of adsorption. The liquid membrane diffusion model has a relatively low linear fitting coefficient value (R²=0.948) for fitting the experimental data (Fig. S4(D)), demonstrating that the rate limiting step is primarily molecular diffusion of Cd(II) in the micropores of p-BN^[1].

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 K_d a is the distribution coefficient and R is the molar gas constant (8.314 J·mol^-1K^-1). Briefly, K_d 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:

[1]SONG QIAN-QIAN, FANG YI, LIU ZHEN-YA, et al. The performance of porous hexagonal BN in high adsorption capacity towards antibiotics pollutants from aqueous solution. Chemical Engineering Journal, 2017, 325: 71-79.

[2]HO YUH-SHAN, MCKAY GORDON, Pseudo-second order model for sorption processes. Process Biochemistry, 1999, 34(5): 451-465.

[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.