Article pubs.acs.org/est
Facile Fabrication of Magnetic Carbon Composites from Hydrochar via Simultaneous Activation and Magnetization for Triclosan Adsorption Xiangdong Zhu, Yuchen Liu, Gang Luo, Feng Qian, Shicheng Zhang,* and Jianmin Chen Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China S Supporting Information *
ABSTRACT: Advanced magnetic carbon composites with high specific surface area and high microporosity are required for both environmentally and agriculturally related applications. However, more research is needed for the development of a facile and highly efficient synthesis process. In the present work, a novel approach of simultaneous activation and magnetization is proposed for the fabrication of magnetic carbon composites via the thermal pyrolysis of hydrochar (i.e., a solid residue from a hydrothermal carbonization process) that has been pretreated with mixtures of ferric chloride (FeCl3) and zinc chloride (ZnCl2). The main objective of this study is the investigation of the variation of characteristics of magnetic carbon composites produced at various conditions, as well as triclosan (TCS) adsorption behavior on such composites. This presented simple one-step synthesis method has the following advantages: (a) the hydrochar is activated with high surface area and pore volume (up to 1351 m2/g and 0.549 cm3/g, respectively), (b) activation and magnetization are simultaneously achieved without further modification, (c) the magnetic particles (γ-Fe2O3) are stable under an acidic medium (pH of 3.0 and 4.0), and (d) the products have the potential to remove TCS from aqueous solutions with a maximum adsorption capacity of 892.9 mg/g. The results indicate the effectiveness of this facile synthesis strategy in converting low-value biowaste into a functional material with high performance for pollutant removal from aqueous solutions.
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INTRODUCTION In recent decades, magnetic carbon composites derived from carbonaceous materials (mainly including waste biomass, porous carbon and graphene) have been widely investigated in environmental applications, due to their capacity for separation from aqueous solutions, and have shown promising performance in pollutant removal.1−5 Several procedures have been used for introducing magnetic particles into carbonaceous materials, including hydrothermal coprecipitation reaction,6,7 microwave irradiation,8 and simultaneous carbonization methods.2,9,10 The magnetic medium derived from the hydrothermal coprecipitation reaction is unstable under an acidic medium; and, both this synthesis method and microwave irradiation are usually time-consuming and complex.5 In the simultaneous carbonization method, magnetic carbon composites can be fabricated via the thermal pyrolysis of ferric (Fe) loaded carbonaceous material.10 However, the surface area and pore volume of the resultant products cannot be enhanced effectively, due to the negative effect of the magnetic medium on the porosity of product. This issue may ultimately hinder the widespread practical application of biowaste. Zinc chloride (ZnCl2) is a porogen that is widely used to produce activated carbon with high performance.11,12 There© 2014 American Chemical Society
fore, it is expected that high-performance magnetic carbon composites can be produced through the thermal conversion of mixtures of ZnCl2 and ferric chloride (FeCl3) pretreated biowaste. To the best of our knowledge, there is no information about the preparation of magnetic carbon composites using such a method. Hydrothermal carbonization (HTC) of biomass, which is a thermochemical treatment of an aqueous solution with a dispersion of biomass, is receiving increasing attention, due to the energy shortage and environmental concerns.13−15 During the HTC process, biosolids (such as lignocellulosic biomass, agricultural and food wastes, and algae) are converted into valuable carbon materials (i.e., hydrochar) under mild conditions.16−18 The hydrochar possesses high concentrations of oxygen functional groups and low degrees of aromatic group, making this material suitable as a template for the fabrication of metal/carbon composites.15 However, the effective utilization of hydrochar for environmental applications is hindered by Received: Revised: Accepted: Published: 5840
January 30, 2014 March 26, 2014 April 16, 2014 April 16, 2014 dx.doi.org/10.1021/es500531c | Environ. Sci. Technol. 2014, 48, 5840−5848
Environmental Science & Technology
Article
Synthesis of γ-Fe2O3 Carbon Composites. The γ-Fe2O3 carbon composites were produced by using the method shown in SI Figure S2. Two g of FeCl3·6H2O and the desired amount of ZnCl2 were mixed in 50 mL of deionized water; and, 10 g of hydrochar were then added to the solution. The weight ratios of ZnCl2 to hydrochar were 0.25, 0.5, 1, and 2. The mixtures were vigorously shaken (150 rpm) for 24 h and then dried at 353 K for 4 h in air. The pretreated hydrochar was activated at the desired temperature for 90 min under N2 flow of 1 L min−1 at a heating rate of 4 K min−1, and the activation temperature was in the range of 773−1073 K. The carbonized sample was successively washed with diluted hydrochloride (HCl, 0.1 M), ethanol and water (300 mL) and finally dried in an oven at 353 K for 4 h. The synthesized γFe2O3 carbon composites are denoted as MC-X-Y, where X is the activation temperature in K (i.e., 773, 873, 973, or 1073), Y is the ZnCl2/hydrochar weight ratio (i.e., 0.25, 0.5, 1, or 2). Characterizations of γ-Fe2O3 Carbon Composite. The elemental (carbon, C; hydrogen, H; nitrogen, N; oxygen, O) composition was analyzed with an elemental analyzer (Vario EL III). The amount of ash (including γ-Fe2O3) in the composites was measured by heating the samples at 473 K for 1 h and then at 773 K for additional 4 h in air. The concentration of the total Fe ions in the aqueous solutions was analyzed using the inductively coupled plasma (ICP, P-4010) technique. The pore structure characteristics of the resulting γ-Fe2O3/ carbon composites were determined by N2 adsorption at 77 K using a Quantasorb SI (Quantachrome Instruments). The surface area was calculated from the isotherm using the Brunauer−Emmett−Teller (BET) equation. The volume and surface area of the micropores were obtained with the t-plot method. The mesopore size distributions were determined using the Barret-Joyner-Halenda (BJH) model. The functional groups of the samples were examined using Fourier transform-infrared (FTIR, Nexus 470) techniques. The phase structure was characterized with X-ray diffraction (XRD) using Cu K α radiation (λ = 1.5406 Å) at a scan rate of 4°/min and a step size of 0.02° in 2θ. Raman spectra were recorded by the LabRam-1B spectrometer, and the curve fittings were performed with the combination of Gaussian line shapes that gave the minimum fitting error. Magnetic properties were measured with a vibrating sample magnetometer (VSM, MPMS, SQUID) at room temperature. The morphology of samples was examined through scanning electron microscopy (SEM, XL30FEG) and transmission electron microscopy (TEM, H-600). Adsorption of Triclosan. Adsorption isotherms were achieved in the concentration range of 10−50 mg/L TCS (20% methanol) at 303 ± 1 K, and the background solution was 0.02 M of sodium chloride (NaCl). To initiate the experiments, a 60 mL amber glass vial with 45 mL of sorption solution received 5 mL adsorbent stock solution (500 mg/L, pure water). The vial was then shaken at 150 rpm at 303 K. The adsorption kinetics of TCS on γ-Fe2O3/carbon composite was examined under 50 mg L−1 of TCS. After the adsorption equilibrium, the solid adsorbent was separated from the liquid phase by centrifugation at 3000 rpm for 10 min; and, 1 mL supernatant was then mixed with 1 mL methanol, followed by filtration using a 0.45 μm polytetrafluoroethylene (PTFE) membrane filter for the analysis of the TCS concentration. Triclosan Analysis. The concentration of TCS was determined using an Agilent 1100 HPLC with a 4.6 × 150
their low surface area and poor porosity.19 Postactivation is, therefore, required for the high performance of porous carbon materials. Environmental contamination by pharmaceuticals and personal care products (PPCPs) is beginning to receive widespread public attention, due to their potential negative ecological effects.20 Pharmaceutical antibiotics, such as triclosan (TCS, 5-chloro-2-(2,4-dichlorophenoxy)phenol), have been produced in large quantities and extensively used in the consumer products as antimicrobial agents.21 It has been reported that the combined annual consumption of TCS and triclocarban exceeds 1220 tons in China; and, TCS has been found in five large rivers in China, with detection frequencies of 100% in the surface water and sediments.22 TCS has been reported to be highly toxic to certain aquatic organisms, particularly to green algae, and is considered to be a parent compound of even more toxic byproducts, such as dioxin and endocrine chemicals.21,23 Hence, it is of great importance to explore efficient treatment technologies for the removal of TCS from contaminated waters. This study presents a novel and facile methodology to fabricate magnetic (γ-Fe2O3) carbon composites from hydrochar via simultaneous activation and magnetization for use in TCS removal from contaminated water. This method includes (i) immersion of hydrochar pretreated with mixtures of FeCl3 (precursor of magnetic particles) and ZnCl2 (porogen) in aqueous solutions and (ii) drying and carbonization of the mixture under a nitrogen (N2) atmosphere. The effects of experimental parameters, including different activation temperatures and ZnCl2 impregnation ratios, on the performance of products have been investigated. The asprepared γ-Fe2O3 carbon composites have been further used as adsorbents to remove TCS pollutants and have exhibited excellent adsorption properties. The adsorption capacities were higher than what has been reported in previous works.24,25 The correlations between the adsorption capacity and the properties of products were also studied, in order to comprehend the TCS adsorption mechanism.
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EXPERIMENTAL SECTION Materials. Ferric chloride hexahydrate (FeCl3·6H2O), and zinc chloride (ZnCl2) were purchased from Sinopharm (Shanghai) Chemical Reagent Co., Ltd., China. Reagent grade TCS was purchased from Sigma-Aldrich Corporation. Supporting Information (SI) Table S1 shows the selected physiochemical properties of the TCS.20 For adsorption experiments, a stock solution of TCS was prepared by dissolving TCS powders into HPLC (high-performance liquid chromatography) grade methanol: the concentration of TCS stock solution was 250 mg/L. Other chemical reagents were of analytical grade, obtained from commercial suppliers, and used without further purification. The hydrochar was a solid residual of the HTC of Salix psammophila (SP) collected from our pilot-scale HTC unit. SP is one of the dominant desert shrubs in northern China. In a typical experimental run, 3.5 kg (dry weight) of SP and 44 L of water were loaded in the autoclave, which was heated up to 573 K and maintained for 60 min, and then cooled with tap water to room temperature. The variations of temperature and pressure during the HTC process are shown in SI Figure S1. The resulting solid product was recovered by filtration and washed with distilled water and ethyl acetate and dried at 373 K for 2 h. 5841
dx.doi.org/10.1021/es500531c | Environ. Sci. Technol. 2014, 48, 5840−5848
Environmental Science & Technology
Article
Table 1. Yields, Elemental Compositions and Atomic Ratios of Raw Hydrochar and γ-Fe2O3/Carbon Composites sample
yield/%a
ashb
C (%)c
O (%)c
H (%)c
O/Cd
H/Ce
hydrochar MC-773−1 MC-873−1 MC-973−1 MC-1073−1 MC-873−0.25 MC-873−0.5 MC-873−2
62.1 58.0 52.8 45.1 62.0 62.1 60.0
15.2 16.7 17.6 16.2 13.9 20.7 17.9 15.5
66.3 84.2 88.7 91.0 90.1 88.9 86.8 87.1
27.2 9.4 4.9 3.0 4.9 5.5 7.4 7.0
5.21 4.6 4.8 4.4 3.9 4.3 4.5 4.2
0.307 0.084 0.042 0.025 0.041 0.046 0.064 0.060
0.94 0.66 0.66 0.58 0.52 0.59 0.62 0.58
Yields are on a water-free basis. bAsh (including γ-Fe2O3) are on a water-free basis. cElemental compositions and atomic ratios are on a water- and ash-free basis. dO/C: atomic ratio of oxygen to carbon. eH/C: atomic ratio of hydrogen to carbon.
a
Table 2. Surface Area, Pore Size and Pore Volume Parameters for γ-Fe2O3/Carbon Composites sample
SBETa (m2/g)
Smicb (m2/g)
Smic/SBET (%)
Vmicc (cm3/g)
Vtd (cm3/g)
Vmic/Vt (%)
pore sizee (d, nm)
hydrochar MC-773−1 MC-873−1 MC-973−1 MC-1073−1 MC-873−0.25 MC-873−0.5 MC-873−2
7.16 1311 1020 1065 1110 515 719 1351
6.46 1201 953 992 1007 476 675 1101
90.2 91.6 93.4 93.1 90.7 92.4 93.9 81.5
0.004 0.588 0.466 0.487 0.494 0.238 0.335 0.549
0.055 0.715 0.551 0.573 0.634 0.315 0.412 0.754
7.27 82.2 84.6 85.0 77.9 75.6 81.3 72.8
30.5 2.18 2.16 2.15 2.29 2.45 2.29 2.23
a c
Measured using N2 adsorption with the Brunauer−Emmett−Teller (BET) method. bMicropore surface area calculated using the t-plot method. Micropore volume calculated using the t-plot method. dTotal pore volume determined at P/P0 = 0.99. ePore size in diameter means average values.
The hydrochar material exhibited low surface area (
