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Highly efficient detoxification of Cr(VI) by brown coal and kerogen: Process and structure studies
Fuel Processing Technology 150 (2016) 71–77
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Research article
Highly efficient detoxification of Cr(VI) by brown coal and kerogen: Process and structure studies Ting-Ting Zhao a, Wen-Zhi Ge b,c, Yan-Xin Nie b, Ying-Xiong Wang d, Fan-Gui Zeng a,e,⁎, Yan Qiao b,d,⁎ a Key Laboratory of Coal Science & Technology, Ministry of Education & Shanxi Province, Department of Earth Science & Engineering, Taiyuan University of Technology, Taiyuan 030024, People's Republic of China b Analytical Instrumentation Center, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, People's Republic of China c University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China d State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, People's Republic of China e Shanxi Key Laboratory of Coal and Coal-measure Gas Geology, Ministry of Education & Shanxi Province, Department of Earth Science & Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China
a r t i c l e
i n f o
Article history: Received 23 December 2015 Received in revised form 1 April 2016 Accepted 1 May 2016 Available online 21 May 2016 Keywords: Brown coal Kerogen Cr(VI) Detoxification Structure
a b s t r a c t In this study, different from previous reports focused on removal efficiency of Cr(VI), the structural variations of brown coal and kerogen concerned with Cr(VI) removal were studied. The process conditions of Cr(VI) removal were optimized, and the reduction of Cr(VI) to Cr(III) was evaluated. In addition, the variations of functional groups in brown coal and kerogen before and after Cr(VI) detoxification were characterized by FTIR, XPS, SSNMR, TEM-EDS, and SEM-EDS. The results indicated that brown coal and kerogen had highly adsorption efficiency for Cr(VI) (maximum 4.67 mmol/g), and 80.1% of adsorbed Cr(VI) was reduced to Cr(III) while 90.4% of them was held on the adsorbents. It also revealed that the content of –CH3, –CH2, C\\O and phenolic hydroxyl groups in adsorbents decreased, but the content of C_O and O–C_O groups increased after Cr(VI) detoxification. Overall, structure identified of brown coal and kerogen in this work could provide important basic data for further application in wastewater treatment. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Chromium compounds, in either the chromium(III) or chromium(VI) forms, are widely used in dye manufacturing, chrome plating, petroleum refining and other industrial processes [1]. Afterwards, they are usually released as hexavalent chromium in acidic raw sewage with varied concentration. As classified by the United States Environment Protection Agency, Cr(VI) is a common contaminant, which is classified as “Group A” human carcinogen, and may cause lung cancer or other organ damage [1,2]. Therefore, Cr(VI) must be detoxified or removed directly from the industrial wastewater to meet the corresponding standards, which is below 0.5 mg/L in the emission and 0.05 mg/L of total chromium in drinking water announced by World Health Organization [3]. Generally, the Cr(VI) in aqueous media was detoxified by reduction to Cr(III) which is essential nutrient and the procedure can avoid the secondary pollution. Researchers are focused on developing low-cost multifunctional reducing adsorbents, including olive stone [4], seaweed [5], activated carbons [6], etc. There are also few reports about coal-based adsorbents, for example: Arslan et al. [7,8] reported that brown coals and humic acids had good
⁎ Corresponding authors. E-mail addresses: [email protected] (F.-G. Zeng), [email protected] (Y. Qiao).
http://dx.doi.org/10.1016/j.fuproc.2016.05.001 0378-3820/© 2016 Elsevier B.V. All rights reserved.
adsorption capacity for Cr(VI) and studied the optimal adsorption conditions for Cr(VI). J. Lakatos et al. not only studied the effect of coal rank for Cr(VI) uptake but also indicated the reduction of Cr(VI) to Cr(III) [9]. These studies open the door for more possibilities associated with the applications of coal-based adsorbents. As shown in the macromolecular structure model of one type brown coal (Fig. 1) [10], it contains –COOH, C\\O, –OH and others like –SR or –NR groups [11]. Kerogen, which is extracted from brown coal, also contains a large number of aromatic rings with the quinone (\\C_O on ring), –COOH, –OH groups [12]. All these functional groups along with their surface properties make brown coal and kerogen as ideal substances to remove pollutants [7–9,13,14] especially strong oxidizing metal ions from wastewater. However, although these groups have been reported to be advantageous to the adsorption of Cr(VI) [15– 17], we do not know their roles and change in the removal process of Cr(VI). Moreover, due to the complex nature of coal, there may be more unknown structures to play key roles in the process. The previously reported procedures were followed the determination of Cr(VI), Cr(III) and total Cr concentration in the liquid phase [8,9,13], whereas almost no studies were done to detect the oxidation states of Cr bound to the adsorbents and surface chemical-physical properties of solid phase coalbased adsorbents. Therefore, there is one challenge existing that the chemical structural variations of coal-based adsorbents before and after detoxification of Cr(VI) are unclear.
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hydrochloric acid (37 wt%). The other procedure was as well as above. To this end, the above sample was washed with distilled deionized water until the filtrate was Cl-free (checked with AgNO3, no precipitation) [18]. The final demineralized XBC sample was dried to constant weight at 50 °C for 5–6 h. Finally, dried sample was extracted by soxhlet extractor at 80 °C for 48 h with CHCl3 as solvent, and the kerogen was obtained as dark solid powder. Table 1 lists the proximate and ultimate analyses of XBC and XK. 2.3. Adsorption experiments
Fig. 1. Molecular model for brown coal.
In this study, the use of brown coal and kerogen for detoxification of Cr(VI) was examined under various conditions, including pH, contact time and temperature. Most importantly, the structure of brown coal and kerogen before and after Cr(VI) detoxification was studied by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), high resolution solid state NMR (SS-NMR), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). This information is essential for understanding the detoxification mechanism of Cr(VI) by brown coal and kerogen, and could provide important basic data for their further application in wastewater treatment.
The adsorption of Cr(VI) by XBC and XK was studied by batch technique. The general method was described as follows: 0.2 g XBC or XK was added into 0.01 L certain concentration Cr(VI) solution and stirred for some hours. Then the mixture was centrifuged at 3500 rpm for 10 min, and the supernate was filtrated through a 0.22 μm pinhole filter. The solid residues (XBC or XK) were washed with distilled water until pH was nearly 7.0 and were dried naturally. The concentration of Cr(VI) in the filtrate was determined according to the standard colorimetric method [19] with a visible spectrophotometer at a wavelength of 543 nm. Total concentration of Cr in the filtrate was measured by ICP-OES (Thermo iCAP 6300). The concentration of Cr(III) was obtained by the difference between the total Cr and Cr(VI) concentration. Adsorption capacity (Q, mmol/g) of Cr(VI) was calculated by Eq. (1): Q¼
ðC0 −Cx Þ V : m
The removal ratio (R1) of Cr(VI) was calculated by Eq. (2): R1 ¼
2. Experimental
Potassium dichromate (K2Cr2O7, analytical grade) was dried at 110 °C for 2 h before use. Ethanol absolute (analytical grade), acetone (analytical grade), hydrochloric acid (HCl, 37% aqueous solution), silver nitrate (AgNO3, analytical grade), hydrofluoric acid (HF, guaranteed reagent), phosphoric acid (H3PO4, guaranteed reagent), sulfuric acid (H2SO4, guaranteed reagent), diphenylcarbazide (C13H14N4O, analytical grade) and trichloromethane (analytical grade) were used in the experiments and distilled deionized water was used in all the experiments. 2.2. Preparations of brown coal and kerogen samples The brown coal, abbreviated as XBC, used in the experiments was collected from Xilinguole, located in the middle of the Inner Mongolia Autonomous Region, China. The sample was used without additional pre-treatment except of grinding and a size classification by sieving. The fraction with grain sizes b 74 μm was used for all experiments. The corresponding kerogen (XK) was prepared according to the following procedures: 6.0 g ground XBC (b74 μm) was rinsed by 1–2 mL of ethanol absolute. The above mentioned sample was treated with 40 mL of diluted hydrochloric acid solution (5 M), soaking for 2 h at 50–60 °C and stirring once every 10 min. After cooling the solution was filtered and washed with distilled deionized water. Secondly, the HCl-washed sample was blended with 40 mL of HF for demineralization at 50–60 °C for 2 h. The other procedure was as well as above. Thirdly, the HF-washed sample was treated with 50 mL of concentrated
ðC0 −Cx Þ 100%: C0
ð2Þ
The reduction ratio (R2) of Cr(VI) to Cr(III) was calculated by Eq. (3): R2 ¼
2.1. Materials
ð1Þ
Cy −Cx þ C0 −Cy r 100%: ðC0 −Cx Þ
ð3Þ
The immobilization ratio (R3) of reduced Cr(III) was calculated by Eq. (4): R3 ¼
C0 −Cy r 100% Cy −Cx þ C0 −Cy r
ð4Þ
where C0 (mmol/L) is the initial Cr(VI) concentration, Cx (mmol/L) is the Cr(VI) concentration and Cy (mmol/L) is the total Cr concentration in the filtrate after reaction. V (L) represents the volume of Cr(VI) solution, m (g) is the weight of absorbents, and r is the ratio of Cr(III) on the surface of adsorbents. In this work, in order to quantify the reduction ability of Cr(VI) to Cr(III) by adsorbents and immobilization ability of reduced Cr(III) by adsorbents afterward, two parameters namely R2 and R3 were created during the experiment. R2 is the percentage of Cr(III) reduced by adsorbents from adsorbed Cr(VI). R3 is the percentage of
Table 1 Proximate and ultimate analyses of XBC and XK. Sample
XBC XK
Proximate analysis (wt%)
Ultimate analysis (wtdaf%)
St,d
Mad
Aad
Vdaf
C
H
N
Oa
22.0 6.8
13.7 0.5
34.3 36.5
70.9 68.3
2.6 4.5
1.2 1.1
N23.8 N24.8
1.5 1.3
ad, air dried basis; daf, dry ash-free basis; d, dry basis; St, total sulfur; M, moisture; A, ash; V, volatile. a By difference.
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Cr(III) immobilized by adsorbents from the total Cr(III) reduced by adsorbents. 2.4. Characterizations The X-ray photoelectron spectroscopy (XPS) was measured with an AXIS ULTRA DLD (Kratos). The X-ray excitation was provided by a monochromatic Al Kα (excitation energy 1486.6 eV) at a voltage of 10 kV and a current of 20 mA. The detection of the emitted photoelectrons was performed perpendicular to the surface sample. Molecular structure of samples was analyzed by 600 MHz Bruker Avance III solid state NMR spectrometer (13C = 151.0 MHz), CP/MAS 13 C spectra were recorded with 2 ms contact time, 5 s recycle delay and 10,000 scan times. Transmission electron microscopy (TEM) analysis was carried out on JEM-2010 with an EDS system at 200 kV. The samples were dispersed by ultrasound in alcohol and then deposited on a Cu grid. The surface morphology of the coated samples was visualized by a scanning electron microscopy (JSM-7001F) with an EDS analyzer at a voltage of ~10 keV. The sample was attached to 10 mm metal mounts using carbon tape under vacuum in an argon atmosphere. FTIR spectra of samples were obtained on an IR spectrometer (VERTEX 70, Bruker, Germany) at room temperature. All samples were fully ground to guarantee high homogeneity prior to tests. The samples were uniformly mixed with dried KBr powder at mass ratio of 1:100. The spectra were recorded at the wavenumber ranges from 400 to 4000 cm−1 and the baselines of the spectra were corrected. Samples for characterizations are chosen based on the maximum loading Cr. They are XBC-Cr1 (pH = 1, 96 mmol/L, 40 °C and 96 h) and XK-Cr1 (pH = 1, 100 mmol/L, 40 °C and 96 h), respectively. But high Cr-loaded samples have low signal-noise, so low Cr-contained XK-Cr2 (pH = 1, 20 mmol/L, 25 °C and 14 h) is chosen for SS-NMR. 3. Results and discussion
Fig. 3. Effect of contact time on the detoxification process of Cr(VI) by XBC and XK. Dosage of XBC or XK, 0.2 g; volume of Cr(VI) solution, 0.01 L; temperature, 25 °C; initial Cr(VI) concentration, 70 mmol/L for XBC and 90 mmol/L for XK; pH, 1.0.
To get a deeper insight into the detoxification process of Cr(VI), another parameter which is the contact time seems also indispensable. As shown in Fig. 3, the contact time has a significant influence on the removal percentage of Cr(VI), and they are positive correlation. But compared with other sorbents [8,21,22], more Cr(VI) can be removed by XBC as well as XK but longer time was also needed. As it can be seen in Fig. 4, raising the temperature could improve the adsorption capacity of Cr(VI). The maximum adsorption capacity for XBC and XK increased from 3.38 mmol/g to 4.63 mmol/g and 4.38 mmol/g to 4.67 mmol/g (pH = 1.0 and 96 h) when temperature increased from 25 °C to 40 °C, respectively. Based on the analysis of above process conditions, it was found that removal ability of XK for Cr(VI) was better than that of XBC under the same experimental conditions. This is probably because the inorganic minerals were removed and the organic-bound metal ions were replaced by H+ during acid treatment, which could be confirmed by the SEM–EDS (Fig. S1) and wide spectra of XPS (Fig. S2). Therefore, the
3.1. Effect of process conditions on detoxification of Cr(VI) Acidity of the solution is crucial in the detoxification process of Cr(VI) because it is highly related to the oxidation ability of Cr(VI), herein the XBC and XK. As shown in Fig. 2, the adsorption capacity of XBC and XK for Cr(VI) was strongly dependent upon the initial pH, and the maximum uptake amount was obtained at pH 1.0 in this study. Moreover, it is worth noting that the adsorption ability of XBC (3.38 mmol/g) and XK (4.38 mmol/g) for Cr(VI) was considerably greater than those of other materials reported such as seaweed biomass (0.88 mmol/g) [20] and humic acids (0.64 mmol/g) [8]. Hence, the advantage of using XBC and XK as adsorbents lies in their high performance.
Fig. 2. Effect of pH on the detoxification process of Cr(VI) by XBC and XK. Dosage of XBC or XK, 0.2 g; volume of Cr(VI) solution, 0.01 L; temperature, 25 °C; contact time, 96 h.
Fig. 4. Effect of temperature on the detoxification process of Cr(VI) by (a) XBC and (b) XK. Dosage of XBC or XK, 0.2 g; volume of Cr(VI) solution, 0.01 L; initial Cr(VI) concentration, 70 and 95 mmol/L for XBC, 90 and 100 mmol/L for XK; pH, 1.0.
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Fig. 5. Cr2p XPS spectra of (a) XBC-Cr1 and (b) XK-Cr1.
structure was optimized after acid treatment, which allowing XK to adsorb more Cr(VI). 3.2. Reduction of Cr(VI) to Cr(III) by XBC and XK The XPS spectrum of Cr2p was used to determine the valence state of Cr (Fig. 5). In the Cr2p spectra of XBC-Cr1 (Fig. 5a), Cr peaks can be fitted into four peaks, the ones at 588.5 and 586.9 eV were assigned to Cr2p1/2 of Cr(VI) and Cr(III), while those at 579.3 and 577.3 eV corresponded to Cr2p3/2 of Cr(VI) and Cr(III) [23,24]. Meanwhile, based on the peak processing results, it can be known that the Cr species on the surface were consisted of 79.8% of Cr(III) and 20.2% of Cr(VI). Similar results were also
Fig. 7. The distribution of Cr ions in the filtrate. Dosage of (a) XBC and (b) XK, 0.2 g; volume of Cr(VI) solution, 0.01 L; temperature, 40 °C; initial Cr(VI) concentration, 96 mmol/L for XBC and 100 mmol/L for XK; pH, 1.0.
obtained for kerogen (Fig. 5b). These results indicated that Cr(VI) was reduced to Cr(III) and they were coexisted on the surface of adsorbents. As mentioned above, Cr(III) was coexisted with Cr(VI) and they dispersed well with the solid particles of adsorbent (Fig. 6). So the distribution of Cr ions in the aqueous phase should be investigated as well. As shown in Fig. 7, the concentration of Cr(VI) was always lower than total Cr, and their difference was the concentration of Cr(III) in the solution. It also can be observed that the concentration of Cr(III) was far less than expected if Cr(III) loading was reversible. It means that only a part of the Cr(III) was released into the solution, but most of them was still held by adsorbents. The results were intriguing, because Cr(VI) was
Fig. 6. TEM image and its corresponding TEM-EDS elemental maps for XK-Cr1.
T.-T. Zhao et al. / Fuel Processing Technology 150 (2016) 71–77 Table 2 Conversion ratio in the detoxification process of Cr(VI). Sample XBC XK
75
Table 3 Assignments of different chemical shift ranges in 13C NMR spectra.
Removal ratio of Cr(VI) (%)
Reduction ratio of Cr(VI) to Cr(III) (%)
Immobilization ratio of reduced Cr(III) (%)
96.4 93.4
81.0 80.1
92.6 90.4
Fig. 8. FTIR spectra of XBC, XBC-Cr1, XK and XK-Cr1.
reduced to less-toxic Cr(III) and the Cr(III) did not exposed to environment. As Table 2 lists, 96.4% of Cr(VI) was removed in high Cr(VI) concentration by XBC, and 81.0% of adsorbed Cr(VI) was reduced to Cr(III) while 92.6% of them was immobilized by adsorbents. Similar performance also was found in the kerogen.
3.3. Structural variations of XBC and XK As mentioned above, most of Cr(VI) was reduced to Cr(III) by XBC and XK. Thus, it is of great importance to determine the structural variations of XBC and XK as reductants concerned with Cr(VI) detoxification.
Range
Chemical shift
Functional groups
1 2 3 4 5 6 7 8 9
185–220 165–185 135–165 120–135 90–120 60–90 50–60 25–50 0–25
Ketone, quinone, aldehyde Carboxyl, ester, quinone O-substituted aromatic Aromatic Aromatic Saccharide, alcohol, ether Methoxy, methyne, quaternary Methylene Methyl
C_O, HC_O COO, COOH C–O, C–OH CH, C CH CHOH, CH2OH, CH2–O– CH3O–, CH–NH, CH, C CH2 CH3
FTIR spectroscopy was used to detect vibrational frequency change in XBC and XK before and after Cr(VI) detoxification. In Fig. 8a, the broad peak at 3410 cm−1 can be ascribed to the hydrogen bonded hydroxyl groups in the adsorbents, and the varying intensity of the peaks was mainly caused by the interference of moisture [25]. The peaks at 2923 and 2851 cm−1 were attributed to the signal of –CH3 and –CH2 groups, and the strength of two peaks decreased after Cr(VI) detoxification, which was possible due to the oxidation of Cr(VI). The band at 1610 cm− 1 represented contribution from C_C and C_O stretching. After Cr(VI) detoxification, the band of 1610 cm− 1 was broadened and shifted toward a lower wavenumber, which was indicative of C_O chelate stretching with Cr ion [26]. The peak at 1278 cm−1 was partly assigned to C\\O bonds, including alcohols, ethers, etc. The decreased intensity of the peak was possibly caused by the oxidation of Cr(VI). Similar results were obtained for XK (Fig. 8b). Furthermore, in order to investigate the molecular structural change of adsorbents after Cr(VI) treatment, SS-NMR was carried out and the result was shown in Fig. 9. According to reference, the assignments of these obtained NMR peaks were listed in Table 3 [27]. Compared XK with XK-Cr2, the peak intensity of –CH3 at 16.4 ppm weakened which was consistent with the FTIR results. Additionally, it was also observed that the intensity of phenolic carbons (144.2 and 155.0 ppm) decreased. Phenolic hydroxyl has stronger reducibility and easily was oxidized to unstable quinone [28]. However, the signals of C_O and –O\\C_O were overlapped by other spinning side bands and hence their growth turned uncertain. But this information can be detected by XPS. Fig. 10 showed the C1s XPS spectra of XBC, XBC-Cr1, XK and XK-Cr1. Before adsorption (Fig. 10a), there were mainly four types of C-bonds including C\\C or C\\H (284.8 eV), C\\O (286.2 eV), C_O (287.4 eV) and O–C_O (289.1 eV) [17,29,30]. These organic functional groups were typical in the brown coal. After adsorption (Fig. 10b), the same four peaks were still evident. However, it could be found that the
Fig. 9. 13C NMR spectra of XBC and XBC-Cr1; XK and XK-Cr2. Spinning side bands are marked with asterisks.
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Fig. 10. C1s XPS spectra of (a) XBC and (b) XBC-Cr1; (c) XK and (d) XK-Cr1.
intensity of peaks had changed. As listed in Table 4, the relative amount of C\\O group decreased after Cr removal which was agreement with the result obtained by FTIR. In contrast, the proportion of C_O and O– C_O groups increased after adsorption. C\\O group had reducing capacity, together with the strong oxidizing Cr(VI), it could be speculated that C\\O group participated in the detoxification process and reduced the Cr(VI) to Cr(III), consequently it was oxidized to C_O and O–C_O groups. Similar redox behaviors were also reported [31–33]. It was noteworthy that the chemical shift of C\\O, C_O and O–C_O groups moved toward the lower binding energy after adsorption, and this is probably due to the effect of Cr. [5]. Similar results were obtained for XK (Fig. 10c and d). Overall, the structure of XBC and XK was changed significantly by the process of Cr(VI) detoxification. Reducing groups including –CH3, –CH2, C\\O and phenolic hydroxyl participated in the detoxification process and reduced the Cr(VI) to Cr(III), consequently they were oxidized to C_O and O–C_O groups. This structural information is essential for understanding the detoxification mechanism of Cr(VI) by brown coal and kerogen, and could provide important basic data for the reuse of Cr-loaded brown coal and kerogen.
Cr(VI) detoxification by brown coal as well as kerogen was a lowspeed reaction that was highly related to the pH value of solution and can be accelerated by raising the temperature. Maximum adsorption capacity of Cr(VI) can be achieved to 4.63 mmol/g for brown coal and Table 4 Components of raw and Cr-loaded adsorbents XPS C1s spectra and their relative distribution.
C–C/C–H C–O C_O O–C_O
Acknowledgements This work was financially supported by the Natural Science Foundation for Youths of Shanxi (2013021008-7). Professor Yan Qiao thanks the Chinese Academy of Sciences (2013YC002) and the Youth Innovation Promotion Association of Chinese Academy of Sciences (2011137) for financial support. Appendix A. Supplementary data
4. Conclusions
Peak
4.67 mmol/g for kerogen (pH 1.0, 96 h, 40 °C), and 80.1% of adsorbed Cr(VI) was reduced to Cr(III), while 90.4% of them was held on kerogen. The structure of XBC and XK was changed significantly by the process of Cr(VI) detoxification. The content of –CH3, –CH2, C\\O and phenolic hydroxyl groups in adsorbents decreased, but the content of C_O and O– C_O groups increased accordingly. Therefore, it is definitely redox mechanism for Cr(VI) removal by brown coal and kerogen. Deeper insight on why reduced Cr(III) is adsorbed irreversibly and its recovery based on this study will be reported soon. What more, the data obtained in this study was further utilized for investigation the treatment of Crloaded solid wastes (submitted manuscript).
BE (eV)
Amount (%)
BE (eV)
Amount (%)
XBC
XBC-Cr1
XBC
XBC-Cr1
XK
XK-Cr1
XK
XK-Cr1
284.8 286.2 287.4 289.1
284.8 286.1 287.1 288.7
55.3 30.7 8.0 6.0
63.5 15.3 9.2 12.0
284.8 286.2 287.1 288.9
284.8 286.1 287.1 288.7
71.6 15.6 4.8 8.0
68.7 13.1 6.6 11.6
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Research article
Highly efficient detoxification of Cr(VI) by brown coal and kerogen: Process and structure studies Ting-Ting Zhao a, Wen-Zhi Ge b,c, Yan-Xin Nie b, Ying-Xiong Wang d, Fan-Gui Zeng a,e,⁎, Yan Qiao b,d,⁎ a Key Laboratory of Coal Science & Technology, Ministry of Education & Shanxi Province, Department of Earth Science & Engineering, Taiyuan University of Technology, Taiyuan 030024, People's Republic of China b Analytical Instrumentation Center, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, People's Republic of China c University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China d State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, People's Republic of China e Shanxi Key Laboratory of Coal and Coal-measure Gas Geology, Ministry of Education & Shanxi Province, Department of Earth Science & Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China
a r t i c l e
i n f o
Article history: Received 23 December 2015 Received in revised form 1 April 2016 Accepted 1 May 2016 Available online 21 May 2016 Keywords: Brown coal Kerogen Cr(VI) Detoxification Structure
a b s t r a c t In this study, different from previous reports focused on removal efficiency of Cr(VI), the structural variations of brown coal and kerogen concerned with Cr(VI) removal were studied. The process conditions of Cr(VI) removal were optimized, and the reduction of Cr(VI) to Cr(III) was evaluated. In addition, the variations of functional groups in brown coal and kerogen before and after Cr(VI) detoxification were characterized by FTIR, XPS, SSNMR, TEM-EDS, and SEM-EDS. The results indicated that brown coal and kerogen had highly adsorption efficiency for Cr(VI) (maximum 4.67 mmol/g), and 80.1% of adsorbed Cr(VI) was reduced to Cr(III) while 90.4% of them was held on the adsorbents. It also revealed that the content of –CH3, –CH2, C\\O and phenolic hydroxyl groups in adsorbents decreased, but the content of C_O and O–C_O groups increased after Cr(VI) detoxification. Overall, structure identified of brown coal and kerogen in this work could provide important basic data for further application in wastewater treatment. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Chromium compounds, in either the chromium(III) or chromium(VI) forms, are widely used in dye manufacturing, chrome plating, petroleum refining and other industrial processes [1]. Afterwards, they are usually released as hexavalent chromium in acidic raw sewage with varied concentration. As classified by the United States Environment Protection Agency, Cr(VI) is a common contaminant, which is classified as “Group A” human carcinogen, and may cause lung cancer or other organ damage [1,2]. Therefore, Cr(VI) must be detoxified or removed directly from the industrial wastewater to meet the corresponding standards, which is below 0.5 mg/L in the emission and 0.05 mg/L of total chromium in drinking water announced by World Health Organization [3]. Generally, the Cr(VI) in aqueous media was detoxified by reduction to Cr(III) which is essential nutrient and the procedure can avoid the secondary pollution. Researchers are focused on developing low-cost multifunctional reducing adsorbents, including olive stone [4], seaweed [5], activated carbons [6], etc. There are also few reports about coal-based adsorbents, for example: Arslan et al. [7,8] reported that brown coals and humic acids had good
⁎ Corresponding authors. E-mail addresses: [email protected] (F.-G. Zeng), [email protected] (Y. Qiao).
http://dx.doi.org/10.1016/j.fuproc.2016.05.001 0378-3820/© 2016 Elsevier B.V. All rights reserved.
adsorption capacity for Cr(VI) and studied the optimal adsorption conditions for Cr(VI). J. Lakatos et al. not only studied the effect of coal rank for Cr(VI) uptake but also indicated the reduction of Cr(VI) to Cr(III) [9]. These studies open the door for more possibilities associated with the applications of coal-based adsorbents. As shown in the macromolecular structure model of one type brown coal (Fig. 1) [10], it contains –COOH, C\\O, –OH and others like –SR or –NR groups [11]. Kerogen, which is extracted from brown coal, also contains a large number of aromatic rings with the quinone (\\C_O on ring), –COOH, –OH groups [12]. All these functional groups along with their surface properties make brown coal and kerogen as ideal substances to remove pollutants [7–9,13,14] especially strong oxidizing metal ions from wastewater. However, although these groups have been reported to be advantageous to the adsorption of Cr(VI) [15– 17], we do not know their roles and change in the removal process of Cr(VI). Moreover, due to the complex nature of coal, there may be more unknown structures to play key roles in the process. The previously reported procedures were followed the determination of Cr(VI), Cr(III) and total Cr concentration in the liquid phase [8,9,13], whereas almost no studies were done to detect the oxidation states of Cr bound to the adsorbents and surface chemical-physical properties of solid phase coalbased adsorbents. Therefore, there is one challenge existing that the chemical structural variations of coal-based adsorbents before and after detoxification of Cr(VI) are unclear.
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hydrochloric acid (37 wt%). The other procedure was as well as above. To this end, the above sample was washed with distilled deionized water until the filtrate was Cl-free (checked with AgNO3, no precipitation) [18]. The final demineralized XBC sample was dried to constant weight at 50 °C for 5–6 h. Finally, dried sample was extracted by soxhlet extractor at 80 °C for 48 h with CHCl3 as solvent, and the kerogen was obtained as dark solid powder. Table 1 lists the proximate and ultimate analyses of XBC and XK. 2.3. Adsorption experiments
Fig. 1. Molecular model for brown coal.
In this study, the use of brown coal and kerogen for detoxification of Cr(VI) was examined under various conditions, including pH, contact time and temperature. Most importantly, the structure of brown coal and kerogen before and after Cr(VI) detoxification was studied by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), high resolution solid state NMR (SS-NMR), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). This information is essential for understanding the detoxification mechanism of Cr(VI) by brown coal and kerogen, and could provide important basic data for their further application in wastewater treatment.
The adsorption of Cr(VI) by XBC and XK was studied by batch technique. The general method was described as follows: 0.2 g XBC or XK was added into 0.01 L certain concentration Cr(VI) solution and stirred for some hours. Then the mixture was centrifuged at 3500 rpm for 10 min, and the supernate was filtrated through a 0.22 μm pinhole filter. The solid residues (XBC or XK) were washed with distilled water until pH was nearly 7.0 and were dried naturally. The concentration of Cr(VI) in the filtrate was determined according to the standard colorimetric method [19] with a visible spectrophotometer at a wavelength of 543 nm. Total concentration of Cr in the filtrate was measured by ICP-OES (Thermo iCAP 6300). The concentration of Cr(III) was obtained by the difference between the total Cr and Cr(VI) concentration. Adsorption capacity (Q, mmol/g) of Cr(VI) was calculated by Eq. (1): Q¼
ðC0 −Cx Þ V : m
The removal ratio (R1) of Cr(VI) was calculated by Eq. (2): R1 ¼
2. Experimental
Potassium dichromate (K2Cr2O7, analytical grade) was dried at 110 °C for 2 h before use. Ethanol absolute (analytical grade), acetone (analytical grade), hydrochloric acid (HCl, 37% aqueous solution), silver nitrate (AgNO3, analytical grade), hydrofluoric acid (HF, guaranteed reagent), phosphoric acid (H3PO4, guaranteed reagent), sulfuric acid (H2SO4, guaranteed reagent), diphenylcarbazide (C13H14N4O, analytical grade) and trichloromethane (analytical grade) were used in the experiments and distilled deionized water was used in all the experiments. 2.2. Preparations of brown coal and kerogen samples The brown coal, abbreviated as XBC, used in the experiments was collected from Xilinguole, located in the middle of the Inner Mongolia Autonomous Region, China. The sample was used without additional pre-treatment except of grinding and a size classification by sieving. The fraction with grain sizes b 74 μm was used for all experiments. The corresponding kerogen (XK) was prepared according to the following procedures: 6.0 g ground XBC (b74 μm) was rinsed by 1–2 mL of ethanol absolute. The above mentioned sample was treated with 40 mL of diluted hydrochloric acid solution (5 M), soaking for 2 h at 50–60 °C and stirring once every 10 min. After cooling the solution was filtered and washed with distilled deionized water. Secondly, the HCl-washed sample was blended with 40 mL of HF for demineralization at 50–60 °C for 2 h. The other procedure was as well as above. Thirdly, the HF-washed sample was treated with 50 mL of concentrated
ðC0 −Cx Þ 100%: C0
ð2Þ
The reduction ratio (R2) of Cr(VI) to Cr(III) was calculated by Eq. (3): R2 ¼
2.1. Materials
ð1Þ
Cy −Cx þ C0 −Cy r 100%: ðC0 −Cx Þ
ð3Þ
The immobilization ratio (R3) of reduced Cr(III) was calculated by Eq. (4): R3 ¼
C0 −Cy r 100% Cy −Cx þ C0 −Cy r
ð4Þ
where C0 (mmol/L) is the initial Cr(VI) concentration, Cx (mmol/L) is the Cr(VI) concentration and Cy (mmol/L) is the total Cr concentration in the filtrate after reaction. V (L) represents the volume of Cr(VI) solution, m (g) is the weight of absorbents, and r is the ratio of Cr(III) on the surface of adsorbents. In this work, in order to quantify the reduction ability of Cr(VI) to Cr(III) by adsorbents and immobilization ability of reduced Cr(III) by adsorbents afterward, two parameters namely R2 and R3 were created during the experiment. R2 is the percentage of Cr(III) reduced by adsorbents from adsorbed Cr(VI). R3 is the percentage of
Table 1 Proximate and ultimate analyses of XBC and XK. Sample
XBC XK
Proximate analysis (wt%)
Ultimate analysis (wtdaf%)
St,d
Mad
Aad
Vdaf
C
H
N
Oa
22.0 6.8
13.7 0.5
34.3 36.5
70.9 68.3
2.6 4.5
1.2 1.1
N23.8 N24.8
1.5 1.3
ad, air dried basis; daf, dry ash-free basis; d, dry basis; St, total sulfur; M, moisture; A, ash; V, volatile. a By difference.
T.-T. Zhao et al. / Fuel Processing Technology 150 (2016) 71–77
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Cr(III) immobilized by adsorbents from the total Cr(III) reduced by adsorbents. 2.4. Characterizations The X-ray photoelectron spectroscopy (XPS) was measured with an AXIS ULTRA DLD (Kratos). The X-ray excitation was provided by a monochromatic Al Kα (excitation energy 1486.6 eV) at a voltage of 10 kV and a current of 20 mA. The detection of the emitted photoelectrons was performed perpendicular to the surface sample. Molecular structure of samples was analyzed by 600 MHz Bruker Avance III solid state NMR spectrometer (13C = 151.0 MHz), CP/MAS 13 C spectra were recorded with 2 ms contact time, 5 s recycle delay and 10,000 scan times. Transmission electron microscopy (TEM) analysis was carried out on JEM-2010 with an EDS system at 200 kV. The samples were dispersed by ultrasound in alcohol and then deposited on a Cu grid. The surface morphology of the coated samples was visualized by a scanning electron microscopy (JSM-7001F) with an EDS analyzer at a voltage of ~10 keV. The sample was attached to 10 mm metal mounts using carbon tape under vacuum in an argon atmosphere. FTIR spectra of samples were obtained on an IR spectrometer (VERTEX 70, Bruker, Germany) at room temperature. All samples were fully ground to guarantee high homogeneity prior to tests. The samples were uniformly mixed with dried KBr powder at mass ratio of 1:100. The spectra were recorded at the wavenumber ranges from 400 to 4000 cm−1 and the baselines of the spectra were corrected. Samples for characterizations are chosen based on the maximum loading Cr. They are XBC-Cr1 (pH = 1, 96 mmol/L, 40 °C and 96 h) and XK-Cr1 (pH = 1, 100 mmol/L, 40 °C and 96 h), respectively. But high Cr-loaded samples have low signal-noise, so low Cr-contained XK-Cr2 (pH = 1, 20 mmol/L, 25 °C and 14 h) is chosen for SS-NMR. 3. Results and discussion
Fig. 3. Effect of contact time on the detoxification process of Cr(VI) by XBC and XK. Dosage of XBC or XK, 0.2 g; volume of Cr(VI) solution, 0.01 L; temperature, 25 °C; initial Cr(VI) concentration, 70 mmol/L for XBC and 90 mmol/L for XK; pH, 1.0.
To get a deeper insight into the detoxification process of Cr(VI), another parameter which is the contact time seems also indispensable. As shown in Fig. 3, the contact time has a significant influence on the removal percentage of Cr(VI), and they are positive correlation. But compared with other sorbents [8,21,22], more Cr(VI) can be removed by XBC as well as XK but longer time was also needed. As it can be seen in Fig. 4, raising the temperature could improve the adsorption capacity of Cr(VI). The maximum adsorption capacity for XBC and XK increased from 3.38 mmol/g to 4.63 mmol/g and 4.38 mmol/g to 4.67 mmol/g (pH = 1.0 and 96 h) when temperature increased from 25 °C to 40 °C, respectively. Based on the analysis of above process conditions, it was found that removal ability of XK for Cr(VI) was better than that of XBC under the same experimental conditions. This is probably because the inorganic minerals were removed and the organic-bound metal ions were replaced by H+ during acid treatment, which could be confirmed by the SEM–EDS (Fig. S1) and wide spectra of XPS (Fig. S2). Therefore, the
3.1. Effect of process conditions on detoxification of Cr(VI) Acidity of the solution is crucial in the detoxification process of Cr(VI) because it is highly related to the oxidation ability of Cr(VI), herein the XBC and XK. As shown in Fig. 2, the adsorption capacity of XBC and XK for Cr(VI) was strongly dependent upon the initial pH, and the maximum uptake amount was obtained at pH 1.0 in this study. Moreover, it is worth noting that the adsorption ability of XBC (3.38 mmol/g) and XK (4.38 mmol/g) for Cr(VI) was considerably greater than those of other materials reported such as seaweed biomass (0.88 mmol/g) [20] and humic acids (0.64 mmol/g) [8]. Hence, the advantage of using XBC and XK as adsorbents lies in their high performance.
Fig. 2. Effect of pH on the detoxification process of Cr(VI) by XBC and XK. Dosage of XBC or XK, 0.2 g; volume of Cr(VI) solution, 0.01 L; temperature, 25 °C; contact time, 96 h.
Fig. 4. Effect of temperature on the detoxification process of Cr(VI) by (a) XBC and (b) XK. Dosage of XBC or XK, 0.2 g; volume of Cr(VI) solution, 0.01 L; initial Cr(VI) concentration, 70 and 95 mmol/L for XBC, 90 and 100 mmol/L for XK; pH, 1.0.
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Fig. 5. Cr2p XPS spectra of (a) XBC-Cr1 and (b) XK-Cr1.
structure was optimized after acid treatment, which allowing XK to adsorb more Cr(VI). 3.2. Reduction of Cr(VI) to Cr(III) by XBC and XK The XPS spectrum of Cr2p was used to determine the valence state of Cr (Fig. 5). In the Cr2p spectra of XBC-Cr1 (Fig. 5a), Cr peaks can be fitted into four peaks, the ones at 588.5 and 586.9 eV were assigned to Cr2p1/2 of Cr(VI) and Cr(III), while those at 579.3 and 577.3 eV corresponded to Cr2p3/2 of Cr(VI) and Cr(III) [23,24]. Meanwhile, based on the peak processing results, it can be known that the Cr species on the surface were consisted of 79.8% of Cr(III) and 20.2% of Cr(VI). Similar results were also
Fig. 7. The distribution of Cr ions in the filtrate. Dosage of (a) XBC and (b) XK, 0.2 g; volume of Cr(VI) solution, 0.01 L; temperature, 40 °C; initial Cr(VI) concentration, 96 mmol/L for XBC and 100 mmol/L for XK; pH, 1.0.
obtained for kerogen (Fig. 5b). These results indicated that Cr(VI) was reduced to Cr(III) and they were coexisted on the surface of adsorbents. As mentioned above, Cr(III) was coexisted with Cr(VI) and they dispersed well with the solid particles of adsorbent (Fig. 6). So the distribution of Cr ions in the aqueous phase should be investigated as well. As shown in Fig. 7, the concentration of Cr(VI) was always lower than total Cr, and their difference was the concentration of Cr(III) in the solution. It also can be observed that the concentration of Cr(III) was far less than expected if Cr(III) loading was reversible. It means that only a part of the Cr(III) was released into the solution, but most of them was still held by adsorbents. The results were intriguing, because Cr(VI) was
Fig. 6. TEM image and its corresponding TEM-EDS elemental maps for XK-Cr1.
T.-T. Zhao et al. / Fuel Processing Technology 150 (2016) 71–77 Table 2 Conversion ratio in the detoxification process of Cr(VI). Sample XBC XK
75
Table 3 Assignments of different chemical shift ranges in 13C NMR spectra.
Removal ratio of Cr(VI) (%)
Reduction ratio of Cr(VI) to Cr(III) (%)
Immobilization ratio of reduced Cr(III) (%)
96.4 93.4
81.0 80.1
92.6 90.4
Fig. 8. FTIR spectra of XBC, XBC-Cr1, XK and XK-Cr1.
reduced to less-toxic Cr(III) and the Cr(III) did not exposed to environment. As Table 2 lists, 96.4% of Cr(VI) was removed in high Cr(VI) concentration by XBC, and 81.0% of adsorbed Cr(VI) was reduced to Cr(III) while 92.6% of them was immobilized by adsorbents. Similar performance also was found in the kerogen.
3.3. Structural variations of XBC and XK As mentioned above, most of Cr(VI) was reduced to Cr(III) by XBC and XK. Thus, it is of great importance to determine the structural variations of XBC and XK as reductants concerned with Cr(VI) detoxification.
Range
Chemical shift
Functional groups
1 2 3 4 5 6 7 8 9
185–220 165–185 135–165 120–135 90–120 60–90 50–60 25–50 0–25
Ketone, quinone, aldehyde Carboxyl, ester, quinone O-substituted aromatic Aromatic Aromatic Saccharide, alcohol, ether Methoxy, methyne, quaternary Methylene Methyl
C_O, HC_O COO, COOH C–O, C–OH CH, C CH CHOH, CH2OH, CH2–O– CH3O–, CH–NH, CH, C CH2 CH3
FTIR spectroscopy was used to detect vibrational frequency change in XBC and XK before and after Cr(VI) detoxification. In Fig. 8a, the broad peak at 3410 cm−1 can be ascribed to the hydrogen bonded hydroxyl groups in the adsorbents, and the varying intensity of the peaks was mainly caused by the interference of moisture [25]. The peaks at 2923 and 2851 cm−1 were attributed to the signal of –CH3 and –CH2 groups, and the strength of two peaks decreased after Cr(VI) detoxification, which was possible due to the oxidation of Cr(VI). The band at 1610 cm− 1 represented contribution from C_C and C_O stretching. After Cr(VI) detoxification, the band of 1610 cm− 1 was broadened and shifted toward a lower wavenumber, which was indicative of C_O chelate stretching with Cr ion [26]. The peak at 1278 cm−1 was partly assigned to C\\O bonds, including alcohols, ethers, etc. The decreased intensity of the peak was possibly caused by the oxidation of Cr(VI). Similar results were obtained for XK (Fig. 8b). Furthermore, in order to investigate the molecular structural change of adsorbents after Cr(VI) treatment, SS-NMR was carried out and the result was shown in Fig. 9. According to reference, the assignments of these obtained NMR peaks were listed in Table 3 [27]. Compared XK with XK-Cr2, the peak intensity of –CH3 at 16.4 ppm weakened which was consistent with the FTIR results. Additionally, it was also observed that the intensity of phenolic carbons (144.2 and 155.0 ppm) decreased. Phenolic hydroxyl has stronger reducibility and easily was oxidized to unstable quinone [28]. However, the signals of C_O and –O\\C_O were overlapped by other spinning side bands and hence their growth turned uncertain. But this information can be detected by XPS. Fig. 10 showed the C1s XPS spectra of XBC, XBC-Cr1, XK and XK-Cr1. Before adsorption (Fig. 10a), there were mainly four types of C-bonds including C\\C or C\\H (284.8 eV), C\\O (286.2 eV), C_O (287.4 eV) and O–C_O (289.1 eV) [17,29,30]. These organic functional groups were typical in the brown coal. After adsorption (Fig. 10b), the same four peaks were still evident. However, it could be found that the
Fig. 9. 13C NMR spectra of XBC and XBC-Cr1; XK and XK-Cr2. Spinning side bands are marked with asterisks.
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Fig. 10. C1s XPS spectra of (a) XBC and (b) XBC-Cr1; (c) XK and (d) XK-Cr1.
intensity of peaks had changed. As listed in Table 4, the relative amount of C\\O group decreased after Cr removal which was agreement with the result obtained by FTIR. In contrast, the proportion of C_O and O– C_O groups increased after adsorption. C\\O group had reducing capacity, together with the strong oxidizing Cr(VI), it could be speculated that C\\O group participated in the detoxification process and reduced the Cr(VI) to Cr(III), consequently it was oxidized to C_O and O–C_O groups. Similar redox behaviors were also reported [31–33]. It was noteworthy that the chemical shift of C\\O, C_O and O–C_O groups moved toward the lower binding energy after adsorption, and this is probably due to the effect of Cr. [5]. Similar results were obtained for XK (Fig. 10c and d). Overall, the structure of XBC and XK was changed significantly by the process of Cr(VI) detoxification. Reducing groups including –CH3, –CH2, C\\O and phenolic hydroxyl participated in the detoxification process and reduced the Cr(VI) to Cr(III), consequently they were oxidized to C_O and O–C_O groups. This structural information is essential for understanding the detoxification mechanism of Cr(VI) by brown coal and kerogen, and could provide important basic data for the reuse of Cr-loaded brown coal and kerogen.
Cr(VI) detoxification by brown coal as well as kerogen was a lowspeed reaction that was highly related to the pH value of solution and can be accelerated by raising the temperature. Maximum adsorption capacity of Cr(VI) can be achieved to 4.63 mmol/g for brown coal and Table 4 Components of raw and Cr-loaded adsorbents XPS C1s spectra and their relative distribution.
C–C/C–H C–O C_O O–C_O
Acknowledgements This work was financially supported by the Natural Science Foundation for Youths of Shanxi (2013021008-7). Professor Yan Qiao thanks the Chinese Academy of Sciences (2013YC002) and the Youth Innovation Promotion Association of Chinese Academy of Sciences (2011137) for financial support. Appendix A. Supplementary data
4. Conclusions
Peak
4.67 mmol/g for kerogen (pH 1.0, 96 h, 40 °C), and 80.1% of adsorbed Cr(VI) was reduced to Cr(III), while 90.4% of them was held on kerogen. The structure of XBC and XK was changed significantly by the process of Cr(VI) detoxification. The content of –CH3, –CH2, C\\O and phenolic hydroxyl groups in adsorbents decreased, but the content of C_O and O– C_O groups increased accordingly. Therefore, it is definitely redox mechanism for Cr(VI) removal by brown coal and kerogen. Deeper insight on why reduced Cr(III) is adsorbed irreversibly and its recovery based on this study will be reported soon. What more, the data obtained in this study was further utilized for investigation the treatment of Crloaded solid wastes (submitted manuscript).
BE (eV)
Amount (%)
BE (eV)
Amount (%)
XBC
XBC-Cr1
XBC
XBC-Cr1
XK
XK-Cr1
XK
XK-Cr1
284.8 286.2 287.4 289.1
284.8 286.1 287.1 288.7
55.3 30.7 8.0 6.0
63.5 15.3 9.2 12.0
284.8 286.2 287.1 288.9
284.8 286.1 287.1 288.7
71.6 15.6 4.8 8.0
68.7 13.1 6.6 11.6
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