Distinguished Cd(II) Capture with Rapid and Superior Ability using Porous Hexagonal Boron Nitride: Kinetic and Thermodynamic Aspects

LI Li, GUO Xiaojie, JIN Yang, CHEN Chaogui, Abdullah M Asiri, Hadi M Marwani, ZHAO Qingzhou, SHENG Guodong

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)

In present work, a systematical and comprehensive understanding for the adsorption of Cd(II) on porous hexagonal boron nitride (p-BN) was studied. The chemical compositions, morphology and surface functional groups of p-BN before and after adsorption were characterized by SEM, HRTEM, BET, XRD, and FT-IR. The effects of pH, adsorbent dosage, contact time and temperature on Cd(II) adsorption were investigated. The maximum adsorption capacity for Cd(II) achieves 184 mg·g–1at pH 7.0 and 313 K. The kinetic data fitted well with pseudo-second-order model and intra-particle diffusion model, indicating that the adsorption is mainly controlled by chemisorption, and the rate-limiting step is the molecular diffusion. The adsorption isotherms are in accordance with Freundlich and Langmuir model respectively, suggesting Cd(II) adsorbed on the heterogeneous surface through multilayer and monolayer adsorption. The thermodynamic parameters are calculated to confirm the spontaneous and endothermic process of Cd(II) sorption. Spectroscopic results from XPS imply that p-BN adsorbent had substantial functional groups and bonding sites, which is propitious to uptake Cd(II) from wastewater. These results revealed that p-BN is a promising candidate for Cd(II) scavenging.

boron nitride; adsorption; cadmium; heavy metal ion

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,[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[2-6]. 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[7-10], 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, have been developed to remove Cd(II) from wastewater[11-21].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),[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,[28-29]. Notably, adsorption of a series of heavy metals (such as Pb, Hg, Cr, Ni, Cu) have been investigatedBN-based adsorbent[30-35]. Li[32]studied Cr(III) adsorption by fluorinated activated boron nitride. Chen[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.

1 Materials and methods

1.1 Materials

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.

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 The-rmo 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–1HCl 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 solutionfiltration 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.

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.

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 2=~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.

Fig. 1 (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

Fig. 2 XRD patterns (A) and FT-IR spectra (B) of p-BN before and after adsorption

The surface functional groups of p-BN before and af-ter adsorption were estimated by FT-IR (Fig. 2(B)). Typi-cally, 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–1were consistent with B–OH and B–N–O stretching and peaks close-by ~3200 cm–1attributed to B–NH2stretching vibrations[38]. The presences of B–OH/B–NH2groups provide abundant basic sites which facilitates electrostatic adsorption of positively-charged ions under alkaline conditions. The adsorption peaks at ~1630 and ~1090 cm–1can 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–1and 1200 to 1600 cm–1increase 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.

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 sig-nificant 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 pro-duce protonate amino/hydroxyl groups. Hence, the elec-trostatic 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 dis-tribution of the morphological species of Cd(II) is af-fected by the concentration of Cd(II) and pH. An exces-sive alkaline environment may result in strong hydro-lyzed 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[19]studied pH effect on Cd(II) sorption onto g-C3N4nanosheets. Paola.[22]studied pH effect on Cd(II) sorption onto modified N-doped carbon nanotubes. Huang[23]studied the effect of pH on Cd(II) sorp-tion onto graphene oxides. And similar results were found. Fig. 3(B) presents the influence of adsorbent dos-age 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 ad-sorbent 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 ad-sorption/desorption inhibits the increasing adsorp-tion. It can be deduced to a chemisorption process for Cd(II) adsorption.

Fig. 3 (A) Effect of initial pH on Cd(II) adsorption capacity (qe) and adsorption percentage at equilibrium, and (B) effect of p-BN dosage on the adsorption capacity (qe) and adsorption per-centage of Cd(II)

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 re-moval percentage decreases (Fig. 4(B)). It is noteworthy that at a constant dosage of p-BN adsorbent, the adsorp-tion capacity increases rapidly with Cd(II) concentration increasing, indicating that the adsorption behavior does not depend on amounts of surface active groups. On theother 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–1atcd=80 mg·L–1. Furthermore, the varia--tion of adsorption capacity with contact time can be util--ized for constructing adsorption kinetic models, reflect-ing the relationship between the structure of adsorbent and adsorption performance. The adsorption and conse--quence can also be predicted or verified. Particularly, subsequent kinetic analysis is achieved from experimen--tal datae-curve forcd= 60 mg·L–1. In the present work, the adsorption kinetics was simulated by 4 ki-netic models to investigate the possible mechanism for re-moval process, which named as pseudo-first-order model, pseudo-second-order model, intra-particle diffu-sion 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:

2.5 Adsorption isotherm and thermodynamics

Fig. 4 (A) Adsorption capacities of Cd(II) with various contact time 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)

Langmuir model:

Freundlich model:

Tempkin model:

2.6 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 Cd3d5/2and Cd3d3/2in 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 Cd3d5/2and Cd3d3/2as mentioned before. Compared with the primary peaks of Cd3d5/2(405.0 eV) and Cd3d3/2(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,[44]. These results imply that p-BN adsorb-ent has substantial functional groups and bonding sites, which is propitious to uptake Cd(II) from wastewater.

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–1at 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.

Fig. 5 (A) Adsorption isotherms of Cd(II) on p-BN at T=303, 313 and 323 K, equilibrium adsorption isotherms fitted by (B) Langmuir model, (C) Freundlich model, (D) Tempkin model

Experimental conditions: Initial pH at 7.0,0=60 mg·L–1,=10.0 mg,=50 mL

Table 1 Adsorption isotherm models parameters of Cd(II) on p-BN

Fig. 6 (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

Supporting materials

Supporting materials related to this article can be found at https://doi.org/10.15541/jim20190371.

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氮化硼納米片吸附Cd(II)的動力學和熱力學研究

李麗1, 郭筱潔2, 金陽1, 陳朝貴1, Abdullah M Asiri3, Hadi M Marwani3, 趙輕舟4, 盛國棟1

(1. 紹興文理學院 化學與化工學院, 紹興 312000; 2. 杭州電子大學 材料與環境工程學院, 杭州 310018; 3. Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia; 4. 中國科學院大學 資源與環境學院, 北京 100049)

本工作對Cd(II)在多孔六方氮化硼(p-BN)上的吸附行為和機理進行了系統而全面的研究, 考察了溶液pH、吸附劑用量、接觸時間和溫度等條件對于Cd(II)吸附的影響, 并采用不同手段表征了吸附前后p-BN的化學組成、形態和表面官能團的變化, 進而研究其吸附機理。研究結果顯示, 在pH 7.0和313 K條件下, Cd(II)的最大吸附容量可達到184 mg·g–1, 其動力學數據與擬二級模型和顆粒內擴散模型吻合, 表明吸附主要受化學吸附控制, 限速步驟主要是分子擴散。Cd(II)在p-BN上的吸附是一個自發和吸熱過程, 吸附等溫線分別符合Freundlich和Langmuir模型, 說明Cd(II)通過多層和單層吸附而吸附在非均相表面上。XPS的光譜結果顯示, p-BN吸附劑具有大量的B–N, B–O等結構用作鍵合位點, 有利于從廢水中吸收Cd(II)。這些結果表明, p-BN有希望作為吸附材料用于清除水體中的Cd(II)。

氮化硼; 吸附; Cd(II); 重金屬

TQ174

A

1000-324X(2020)03-0284-09

10.15541/jim20190371

date:2019-07-22;

date: 2019-09-11

National Natural Science Foundation of China(21777102)

LI Li(1995–), male, Master candidate. E-mail: 2740033871@qq.com

李麗(1995–), 女, 碩士研究生. E-mail: 2740033871@qq.com

Corresponding author:CHEN Chaogui, PhD, lecturer. E-mail: cgchen2014@sinano.ac.cn; SHENG Guodong, PhD, associate professor. E-mail: gdsheng@usx.edu.cn

陳朝貴, 博士, 講師. E-mail:cgchen2014@sinano.ac.cn; 盛國棟, 博士, 副教授. E-mail:gdshen@usx.edu.cn