Research Article pubs.acs.org/journal/ascecg
Removal of I− from Aqueous Solutions Using a Biomass Carbonaceous Aerogel Modified with KH-560 Li Sun,†,‡,§ Yujie Zhang,*,‡ Xiushen Ye,† Haining Liu,*,† Huifang Zhang,† Aiguo Wu,*,‡ and Zhijian Wu† †
CAS Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources and Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 18 Xinning Road, Xining, Qinghai 810008, P. R. China ‡ CAS Key Laboratory of Magnetic Materials and Devices, Key Laboratory of Additive Manufacturing Materials of Zhejiang Province, and Division of Functional Materials and Nanodevices, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, No. 1219 Zhongguan West Road, Zhenhai District, Ningbo, Zhejiang 315201, P. R. China § University of Chinese Academy of Sciences, 19 A Yuquan Road, Shijingshan District, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: Materials capable of separating and removing iodine ions (I−) from aqueous solutions are significant to our society, especially human health. Adsorption is one of the effective I− separation methods. In this work, a novel biomass carbonaceous aerogel (CA) modified with 3-glycidyl-oxypropyl-trimethoxy-silane (KH-560) was prepared and used for I− adsorption for the first time. This preparation process of CA@KH-560 is easily accessible. The results of characterization demonstrated that CA and CA@KH-560 have three-dimensional porous structure with the sheetlike skeletons connected to each other, and KH-560 effectively entered into the matrix of CA. Adsorption experiments were carried out systematically by changing the pH, adsorption time, ionic strength, temperature, and ion concentration. The maximal adsorption capacity for I− is 2.5 mmol/g at an initial pH of 1.5. The adsorption can reach equilibrium within 24 h, and the data of adsorption kinetics fitted the pseudo-second-order model very well. The adsorption isotherm model Langmuir, Freundlich, and Tempkin isotherms were studied, and the results showed that the Langmuir model is the best-fitting one for the experimental results. Moreover, CA@KH560 shows excellent regenerating ability, and the regeneration process is relatively simple. KEYWORDS: Iodide ion, Biomass carbonaceous aerogel, KH-560, Adsorption, Regeneration
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iodine, 129I, 127I, 131I, etc., in the radioactive liquid waste may contaminate groundwater without proper treatment.4 Overall, extracting and separating I− from water solutions is crucial to industry development and human health.1 Adsorption is an extensively used and efficient method for removing or recovering low concentrations of I−. In the past several decades, many kinds of materials were applied to adsorb I−, including ion exchange resins, activated carbons, porous
INTRODUCTION
Iodine has become an increasingly important component and is widely used in the fields of bioscience, environmental chemistry, functional materials, etc.1,2 Most commercial iodine is extracted from subsurface brines.3 Thus, effective extraction of iodine is crucial to sustainable and stable development of iodine-related industries. On the other hand, superfluous intake of iodide ions (I−) has a negative impact on human health, because I− can participate in the metabolism process of thyroid gland and excess I− in the body may cause dysfunction of the thyroid gland, which produces hormones essential for the functioning of every cell. Moreover, radioactive isotopes of © 2017 American Chemical Society
Received: April 13, 2017 Revised: July 5, 2017 Published: July 17, 2017 7700
DOI: 10.1021/acssuschemeng.7b01145 ACS Sustainable Chem. Eng. 2017, 5, 7700−7708
Research Article
ACS Sustainable Chemistry & Engineering Scheme 1. Preparation of CA and CA@KH-560
solution was used to investigate the effect of ionic strength on the adsorption of I−. Preparation of the Carbonaceous Aerogel. Hydrothermal synthesis was employed to prepare the carbonaceous aerogel from watermelon rind according to the method described previously.15,24 First, the watermelon rind was chopped into small cubes, washed with DI water, and placed in a Teflon-sealed autoclave. Then the autoclave was placed in a 180 °C oven for 12 h. After being cooled to room temperature, the hydrogel was rinsed with a mixed solution of absolute ethanol and DI water (VEtOH/VH2O = 1/1) to eliminate soluble impurities. Then the carbonaceous hydrogel (named CH) was obtained. The corresponding carbonaceous aerogel (named CA) was obtained by freezing CH in a refrigerator at −80 °C for 12 h and lyophilized at −65 °C for 50 h using a vacuum freeze-drying device (LGJ-10C, Beijing Sihuan Experimental Instrument Co., Ltd.). Preparation of the KH-560-Modified Carbonaceous Aerogel. A certain mass of CA was totally immersed in KH-560 (assay, ≥98%; form, liquid) for 24 h and then removed, followed by drying for activation at 70 °C for 12 h, yielding the KH-560-modified carbonaceous aerogel (named CA@KH-560). Structural Characterization. The functional groups present in CA before and after modification were investigated using Fourier transform infrared spectroscopy (FT-IR) (Cary660 + 620, Agilent). The morphology of CA and CA@KH-560 before and after adsorption was investigated through field emission scanning electron microscopy (FE-SEM) (S-4800, Hitachi), with a 10 kV acceleration voltage. The structural properties of CA were investigated by X-ray diffraction using a Thermo ESCALAB 250XI spectrometer (XRD, D8 Discover, Bruker AXS). The thermal stability of CA was determined through thermogravimetric and derivative thermogravimetric analysis (TGDTG) (Pyris Diamond, PerkinElmer). TG analysis was performed with an increase in temperature from room temperature to 800 °C, and the heating rate was 10 °C/min. Elements on the surface of the materials were characterized by X-ray photoelectron spectroscopy (XPS) (AXIS ULTRA DLD, Shimadzu), and a monochromatic Cu (Kα) X-ray source was used. The concentration of I− in the solution was measured by using the ultraviolet and visible spectrophotometer (T10CS, Beijing Purkinje General Instrument Co., Ltd.), and the absorption peak of I− is located at 228 nm. N2 adsorption analysis was performed on the surface area and porosimetry instrument (ASAP 2020, Micromeritics) at 77 K. Adsorption Experiments. Batch adsorption experiments were used to identify the adsorption properties of prepared CA@KH-560 for I− from an aqueous KI solution. Adsorptive experiment parameters such as the pH (1.0−5.4), contact time (0−38 h), initial concentration of I− (1−20 mmol/L), and temperature (25−45 °C) were studied. Generally, a definite amount of CA@KH-560 was placed in 100 mL of
silicas, organic polymers, and some composite adsorbents with Ag, AgCl, or Cu2O as the adsorption active component.1,3−8 Ion exchange resins have high adsorption capacity; unfortunately, their selectivity is poor. The composite adsorbents show good selectivity, but they are costly and hard to recycle.4 Therefore, it is essential to develop environmentally friendly, cost-effective, and easily prepared and regenerated adsorbents. Generally, carbon materials derived from biomass possess high specific area and lightweight properties, so that they can be used as favorable adsorbents. Using biomass-derived char as an adsorbent has been confirmed to be the most effectively lowcost technique that has good application prospects for wastewater treatment.9−12 Recently, as a promising adsorbent material, biomass carbonaceous aerogels have attracted an increased level of concern because of their ultralight mass and high surface areas, such as organic solvents, metal ions, etc.13−15 However, the adsorption of I− by using biomass carbonaceous aerogels has not yet been reported. In general, carbon materials, including carbonaceous aerogels, can be obtained from biomass directly through a hydrothermal carbonization (HTC) process with mild heating.13,15−21 The precursor biomasses are carbohydrates or carbohydrate-rich materials. Thus, the hydrothermal products contain abundant oxygen groups (e.g., −OH and −COOH), making them ideal adsorbent materials; however, these groups do not have specific interaction with concrete ions, so the modification is very essential for improving selectivity in adsorption studies.22,23 The intention of this work was to investigate the adsorption of I− on the biomass carbonaceous aerogel that was prepared through the HTC process and modified with 3-glycidyloxypropyl-trimethoxy-silane (KH-560). The adsorption properties were explored with respect to kinetics, isotherms, thermodynamics, mechanism, and recycling for the first time.
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EXPERIMENTAL SECTION
Materials. All reagents (analytical grade) used in this study were obtained from Sinopharm Chemical Reagent Co., Ltd. All glassware was washed with deionized (DI) water several times before being used. KH-560 was used as the epoxy-functional silane modifier. The I− solution was obtained by dissolution of KI in DI water. The pH of the solution was adjusted using HCl (8 mol/L), and a NaCl (1 mol/L) 7701
DOI: 10.1021/acssuschemeng.7b01145 ACS Sustainable Chem. Eng. 2017, 5, 7700−7708
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. SEM images and elemental analysis (by EDS) of (a) CA and CA@KH-560 (b) before and (c) after adsorption. a KI solution with a known concentration at a known pH in each adsorption experiment. After adsorption equilibration, CA@KH-560 was washed with 100 mL of DI water. Then the adsorbents were placed in 100 mL of an ammonia solution (8%) and kept in a thermostatic oscillator at a speed of 120 rpm for 24 h at room temperature. After desorption, the adsorbents were placed in 100 mL of HCl (8 mol/L) and kept in the oscillator under the same condition for 24 h. Then CA@KH-560 was washed several times with DI water, followed by drying at 80 °C, and used to investigate the reusability. The adsorption capacity of CA@KH-560 is calculated using eq 1:4,9,25 qt =
(C0 − Ct )V m
enough for the adsorption capacity of the adsorbent to reach its maximum. After impregnation in pure KH-560 for 24 h, CA was taken out and dried for activation at 70 °C for 12 h. During this process, KH-560 reacted with −OH on the surface of CA, and then KH-560 was modified on the surface of CA (Figure S2a).26 During the reaction process, one side effect of hydrolytic condensation between KH-560 on the surface of CA and excessive KH-560 was caused by the presence of trace moisture in the air (Figure S2b). If abundant water existed, another side effect would occur between −OCH3 of KH-560 and H2O, generating −OH groups on the surface of CA@KH560 (Figure S2c). XRD and TG-DTG Analysis of CA. The XRD pattern of CA (Figure S3) shows a broad peak that is attributed to the amorphous aerogel powder. The typical broad peaks of the sample can be found around 25° and 45°. Moreover, the absence of a sharp peak in the XRD pattern of CA indicates that it is in an amorphous state.27,28 According to the TG-DTG curves of CA (Figure S4), most of the weight loss occurred between 350 and 600 °C, and decomposition occurred in two steps with the increase in temperature. The weight loss in the first step (350 °C) may be associated with desorption of water and decomposition of the residual organic precursors. The weight loss in the second step (600 °C) was most likely due to the decomposition of CA.28−30 FT-IR Analysis. For the purpose of analyzing the change in the functional groups during the modification and adsorption, FT-IR was used to characterize CA and CA@KH-560 before
(1) −
where C0 and Ct (millimoles per liter) are I concentrations before adsorption and at contact time t, respectively, qt (millimoles per gram) is the capacity of I− adsorbed onto CA@KH-560 at contact time t, m (grams) is the adsorbent quality, and V (liters) is the solution volume.
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RESULTS AND DISCUSSION Preparation and Characterizations. Reaction between CA and KH-560. The experimental sketch of preparation is shown in Scheme 1, from which we can see that the amount of KH-560 on the surface of CA is the key to the adsorption property of CA@KH-560. The effects of the concentration of the impregnation liquid and impregnation time were investigated (Figure S1), and the results showed that when CA was impregnated in pure KH-560 and modified, the corresponding adsorbent has the highest adsorption capacity for I−. An impregnating time of 24 h for CA in KH-560 is 7702
DOI: 10.1021/acssuschemeng.7b01145 ACS Sustainable Chem. Eng. 2017, 5, 7700−7708
Research Article
ACS Sustainable Chemistry & Engineering and after adsorption. Table S1 summarizes the band positions of the functional groups on the basis of infrared spectral data (Figure S5). In the infrared spectrum of CA, the peak at 3342 cm−1 is identified as a stretching vibration of −OH, while the peaks at 2915 and 2848 cm−1 correspond to the symmetric and antisymmetric stretching vibrations of C−H (−CH2−), respectively. The peaks at 1733 and 1461 cm−1 are assigned to the stretching vibration of carbonyl functional groups (C O) and the deformation vibration of carboxylic O−H, respectively, revealing the presence of carboxyl groups (−COOH) on the CA surface. The peak at 1159 cm−1 is from the asymmetric stretching vibration of −COC− (also represents the −CH vibration).31,32 Compared with that of CA, the infrared spectrum of CA@KH-560 exhibited some new adsorption peaks at 1079, 1091, and 784 cm−1, which are ascribed to asymmetric stretching and symmetric stretching vibration of the Si−O−Si group.33 After reaction between CA and KH-560, the adsorption band intensity of −OH becomes significantly weaker, indicating that CA successfully reacted with KH-560. After adsorption, the −OH stretching vibration peak becomes significantly stronger than that before adsorption, indicating that the −OCH3 group of KH-560 (Figure S2c) was hydrolyzed during the adsorption process. Surface Morphology. Figure 1 shows the SEM images and elemental analysis (by EDS) of CA and CA@KH-560, from which we can see that the pore texture of CA was maintained after modification and adsorption. The EDS data of CA@KH560 samples displayed a strong Si signal that confirmed that the modification had occurred. The component elements (I) can be seen in Figure 1c, which confirms that I− was adsorbed onto CA@KH-560. The Cl peak in the spectrum was due to HCl that was used to adjust the pH of the KI solution before adsorption. Moreover, Cl− ions were involved in competitive adsorption.34 BET and Porosity Investigation. CA and CA@KH-560 were also studied by N2 adsorption−desorption isotherms (Figure S6 and Table S2). The results indicated that the surface areas of CA and CA@KH-560 were 8.6939 and 0.5454 m2/g, respectively. The BET surface area becomes smaller after modification, which is caused by the blockage of some pores induced by the entry of KH-560 into the channel. This result fit with the fact that the pore size distribution curve cannot be obtained. XPS Spectra. The XPS test can provide precise information about the composition of the surface element and adsorbed species of a solid material. The wide scan results (Figure 2a) showed the presence of carbon (284.6 eV, 1s) and oxygen (531.6 eV, 1s) on the surface of all the samples. In comparison with those of CA (Figure 2a1), two new characteristic peaks of CA@KH-560 (Figure 2a2) emerged, Si 2p (101.6 eV) and Si 2s (152.6 eV), which confirmed that KH-560 was successfully modified on the surface of CA. Furthermore, the peaks corresponding to the binding energies of the “2d” electrons of iodine at 619.6 (I 3d) and 631.6 (I 3d3/2) in Figure 2a3 clearly indicated that I− was successfully adsorbed on the surface of CA@KH-560. To investigate the interaction between CA and KH-560, the XPS spectra of carbon and oxygen elements in CA and CA@ KH-560 were analyzed. Panels b1 and b2 and panels c1 and c2 of Figure 2 show the high-resolution XPS spectra in the C 1s and O 1s regions, respectively. The high-resolution C 1s spectra (Figure 2b1,b2) show several specific peaks, which are ascribed
Figure 2. XPS spectra of CA and CA@KH-560: (a) full spectra of CA (a1) and CA@KH-560 before (a2) and after (a3) adsorption, (b) C 1s spectra of CA (b1) and CA@KH-560 (b2), and (c) O 1s spectra of CA (c1) and CA@KH-560 (c2).
to the carbon atoms on the aerogel backbone (C−C/C−H, 284.62 and 284.6 eV), the carbon atoms adjacent to the ester bonds (C−CO, 285.47 and 285.85 eV), and the C−O moiety (286.27 and 286.2 eV), whereas the high-resolution O 1s signal (Figure 2c1,c2) suggests several specific peaks that correspond to the oxygen atoms from the O−CO (530.82 and 531.44 eV), C−O−C (531.62 and 531.99 eV), and O− CO (531.87 and 532.79 eV) groups. The small shoulders at a lower binding energy originated from the formation of new bonds (Si−C, 288.8 eV; Si−O, 533.04 eV) of the carbon (Figure 2b2) and oxygen (Figure 2c2) atoms. All the results mentioned above indicated that KH-560 has been successfully modified on the surface of CA.35−38 Adsorption Results. Effect of pH. Solution pH, as one of the important parameters, can significantly affect the adsorption properties.12 In this study, the effect of pH adjusted by a HCl solution in the range of 1−5.4 on the adsorption of CA before and after modification was investigated under the same conditions, and the results are displayed in Figure 3. The adsorption properties of both CA@KH-560 and CA are very sensitive to solution pH; however, the adsorption capacity of CA@KH-560 is much higher than that of CA, which indicates that modification is significant for improving the adsorption property of CA. Meanwhile, at an initial pH of 1.5, the adsorption capacity for I− on CA@KH-560 is much higher than that under more acidic (pH 0.7 after five cycles, indicating that CA@KH-560 showed excellent regenerating ability for removing I− from a water solution. Figure S10 shows the morphology of the adsorbent after five cycles. As one can see from Figure S10, the adsorbent retains the original structure after multiple cycles, indicating that the adsorbent has good stability. At the same time, the regeneration process is very simple. Adsorption Mechanism. On the basis of the results described above, a mechanism for adsorption of I− by CA@ KH-560 was proposed. Adsorption of I− by CA@KH-560 was mainly caused by the electrostatic interaction between the protonated oxygen atoms on the surface of the adsorbent and I−, a finding similar to that of a previously reported work.49 The adsorption was affected differently by pH, ionic strength, and coexisting anions. The effect of pH was suggested to be related to the protonation extent of oxygen atoms. As shown in Figure 3, when the original pH is >1.5, the extent of protonation of oxygen atoms decreased and the adsorption capacity of I− decreased correspondingly. The same phenomenon took place in the adsorption of anionic dyes by utilizing electrostatic interaction generated by protonation.50,51 When the initial pH is
