Nanoporous Carbon (NPC) with high surface area of 1379 M2/G and high proportion of micropore and mesopore volume of 2.90 Cm3/G was prepared by carbonization of metal?organic frameworks ZIF-8. The adsorption of NPC towards the representative sulfonamide antibiotics sulfamethoxazole (SMX) from aqueous solutions was explored, in comparison with powder active carbon (AC). The adsorption kinetics and isotherms showed that the maximum adsorption capacity (Qm) of NPC toward SMX was 757 Mg/G, around 2 times than that of AC adsorption. The high adsorption affinity of NPC was related to the high surface area and special Mic/Mesopore structure. The pore-filling mechanism as well as electrostatic interaction had important influence on the high adsorption of NPC. The results implied that nanoporous carbon derived from mofs could remove the contaminants from aqueous solutions effectively, and would be a promising adsorbent for the removal of contaminants in the future.
In recent years, many researchers found that some Pharmaceutical and Personal Care Products (PPCPs) and Endocrine Disrupting Chemicals (EDCs) exist in natural water and drinking water. This kind of contaminants can affect the growth and reproduction of aquatic organisms, having negative effects on human health through the biological accumulation [
Traditional wastewater treatment methods include Gas floatation process, adsorption, membrane technology and advanced oxidation. As a wide-applica- tion and good-effect processing method, adsorption is high efficiency, low energy consumption and without by-product. Carbon materials are widely used in the field of adsorption due to their large specific surface area and pore structure, represented by activated carbon, silica gel and zeolite. According to different morphology, the types of activated carbon are divided into powder, granular and fibrous. The pore size distribution of activated carbon, used for the adsorption of pollutants in water, is very wide. Metal organic frameworks (MOFs), polymers with three-dimension pore structure, have been developed rapidly in last ten years. MOFs have the highest specific surface areas reported to date for porous materials (up to 6200 m2/g) [
ZIF-8 was synthesized referring to the reported method, zinc salts (Zn(NO3)2 or Zn(CH3COO)2) and 2-methylimidazole (Hmim) were dissolved in methanol to generate clear solutions, respectively [
In order to get the nanoporous carbon, a quartz boat containing ZIF-8 powder was placed into the vacuum tube furnace. Next, in order to ensure the optimum temperature of carbonization, the vacuum tube furnace was heated from room temperature to 950˚C, 1000˚C and 1100˚C under the protection of argon, with a heating rate of 10˚C/min. Then the temperature was maintained for 7 h. After natural cooling, the nanoporous carbon (NPC) was obtained.
The AC was purchased from Sinopharm Chemical Regent Co., Ltd., China. Before using, the AC was pretreated by HCl, and then washed with milli-Q water for several times.
The morphology and pore structure of NPC were observed by field emission scanning electron microscope (FE-SEM, Hitachi S-4800). The specific surface area was obtained by the method of multi-point Brunauer-Emmett-Teller (BET), calculated by the adsorption-desorption isotherms of N2 at 77 K. The total volume of mesopore and macropore was the difference between total pore volume and micropore volume which was calculated by the method of V-t plot. The pore width distribution was calculated by the DFT method. The method of X-ray photoelectron spectroscopy (XPS), implemented on a surface analysis system (Thermal ESCALAB 250) using Al Kα radiation, was used to measure the element content of NPC. Fourier transform infrared spectroscopy (FT-IR) was recorded by using a Vertex70, Bruker Vector FTIR spectrometer. The Raman spectrum was obtained via a Renishaw micro-Raman system 2000 spectrometer (Renishaw, UK), which was operated with He?Ne laser excitation.
Zeta potential at different pH was measured by zetasizer nano-ZS equipment (Malvern instrument company, USA). Before the measurement, NPC, SWNT and PAC were dispersed in 1 mM KCl solution, followed by sonicating for 30 min. The solution pH was adjusted by HCl (1 M) and NaOH (0.1 M) before measurement.
The batch adsorption experiments on different-sized NPCs of SMX were carried out in 50 mL quartz vessels with 30 mL adsorption solution. Background solution containing 0.5 mM CaCl2 and 200 mg/L NaN3 was mixed with about 2 mg NPCs. The batch adsorption experiments were conducted at 200 rpm on an orbital shaker at fixed temperature of 20˚C ± 1˚C. The concentration of solution was in the range of 5 mg/L to 100 mg/L. The volume ratio of methanol in solution of each vial was controlled to be less than 0.1% to minimize cosolvent effect. The vials were sealed and placed in the shaker for 48 hours (kinetics experiments indicated that apparent equilibrium was reached before 48 hours) [
The pH value of solution was 4.82 for SMX, which didn’t change during the adsorption process. After adsorption experiments, the adsorbents and the aqueous solution were separated by centrifugation. The concentrations of SMX was determined by a high-performance liquid chromatograph (HPLC, Waters 2695, Waters, USA) equipped with a C18 column (15 cm × 4.6 mm × 5 μm) and a UV detector at a column temperature of 30˚C. SMX was detected at 271 nm, with a mobile phase of 45:55 (v:v) acetonitrile: 0.1% acetic acid aqueous solution, at flow rate of 1 mL/min.
In order to investigate the effect of pH, the adsorption experiments were carried out at the concentrations of 10 mg/L for SMX. HCl (1 M) and NaOH (0.1 M) were applied to adjust the pH value of the solution in the range of 2.0-11.0 [
NPC was prepared through carbonization by different-sized ZIF-8, which were named as S-NPC (the traditional structure), M-NPC and L-NPC.
According to the pore structure comparison diagram of S-ZIF8 and S-NPC, the specific surface area of S-ZIF8 becomes bigger after carbonization. The size of M-ZIF8 and L-ZIF8 showed same tendency. The zeta potential of these three materials is 3.6, 4.8 and 6.3, respectively. The isoelectric point, the important factor of adsorption, increases with the increase of the crystal size.
Name | S-NPC | M-NPC | L-NPC | AC |
---|---|---|---|---|
Specific surface area S/(m2/g) | 1378.7 | 998.4 | 872.9 | 685.8 |
Pore volume V/(mL/g) | 2.90 | 0.62 | 0.55 | 0.60 |
Pore width D/(nm) | 0.61 | 0.61 | 0.61 | 0.57 |
the Raman spectrogram and the FTIR spectra of three NPCs and AC. In these kinds of grams, D-band corresponds to defect, while G band relates to graphite. The relative ratios of G bands to D bands (IG/ID) in Raman spectra are important parameters for understanding the crystallization degree of graphitic carbon.
For adsorption of SMX on different-sized NPCs, we can see the secondary kinetic fitting is linear (
It was obvious that the adsorption equilibrium was reached within 10 h for all of the samples, and the rapid adsorption occurred during the first 3 hours, indicating fast initial transfer of chemicals to the adsorbents surface (
Adsorption isotherms of SMX on the four adsorbents were displayed in
Adsorbent | C (mg/L) | Pseudo-second-order parameter | |||
---|---|---|---|---|---|
qe(mmol∙g−1) | V0(mmol∙h−1∙g−1) | K2(g∙mmol∙h−1) | R2 | ||
AC | 20 | 0.33 | 1.15 | 10.58 | 0.997 |
S-NPC | 20 | 0.56 | 0.63 | 1.79 | 0.997 |
M-NPC | 20 | 0.55 | 1.00 | 1.82 | 0.999 |
L-NPC | 20 | 0.54 | 0.71 | 2.35 | 0.998 |
For AC, the maximum adsorption capacities (qm) of SMX were lower than those of NPCs, demonstrating the excellent adsorption capacity of NPCs. As shown in
Adsorbent | Langmuir constants | Freundlich constants | ||||
---|---|---|---|---|---|---|
qm(mmol/g) | b(l/mmol) | R2 | KF(mmol(1−n)Ln/g) | n | R2 | |
S-NPC | 2.70 | 27.98 | 0.988 | 4.36 | 0.38 | 0.909 |
M-NPC | 2.98 | 32.39 | 0.986 | 4.83 | 0.36 | 0.931 |
L-NPC | 2.29 | 14.69 | 0.973 | 3.43 | 0.44 | 0.986 |
AC | 1.54 | 15.85 | 0.960 | 2.14 | 0.40 | 0.868 |
carbon derived from MOFs would be a promising adsorbents used for wastewater treatment in the future.
This work was supported by National Natural Science Foundation of China (grant no. 21590813), and the Fundamental Research Funds for the Central Universities (grant no. DUT16TD02).
Li, Y.X. and Quan, X. (2017) Adsorption of Sulfamethoxazole on Nanoporous Carbon Derived from Metal-Organic Frameworks. Journal of Geoscience and Environment Protection, 5, 1-8. https://doi.org/10.4236/gep.2017.56001