Chu-Fan Wanga,
Xing Fan*a,
Fan Zhanga,
Shou-Ze Wanga,
Yun-Peng Zhaoa,
Xiao-Yan Zhaoa,
Wei Zhaoa,
Teng-Gao Zhub,
Jin-Li Luc and
Xian-Yong Wei*a
aKey Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China. E-mail: fanxing@cumt.edu.cn; wei_xianyong@163.com; Tel: +86-13337938221 Tel: +86-516-83885951
bJiangxi Key Laboratory for Mass Spectrometry and Instrumentation, East China Institute of Technology, Nanchang, Jiangxi 330013, China
cNews Center of CUMT, China University of Mining & Technology, Jiangsu 221116, China
First published on 10th April 2017
Humic acids obtained from a Chinese lignite via alkali treatment were analyzed using Fourier transform infrared spectroscopy and Orbitrap mass spectrometry coupled with an electrospray ion source (ESI-Orbitrap-MS). Raw coal and the corresponding residue were characterized via scanning electron microscopy and energy dispersive spectrometry. Over 4700 heteroatom-containing compounds with wide distributions of molecular mass and unsaturation degree were detected via the ESI-Orbitrap-MS, and around 60 percent of the detected species were found to be oxygen-containing compounds. In addition, van Krevelen diagram and double-bond equivalent (DBE) plot were introduced to provide more structural details of the compounds. For the species only containing C, H, and O (HACHO), condensed aromatic compounds with a DBE value over 20 only contained 1 or 2 oxygen atoms. Carboxyl- and hydroxyl-containing aliphatic compounds (CHCACs) were predominant in HACHO with 5 or 6 oxygen atoms. Both the CHCACs and aromatic carboxylic acids or phenols were grouped into clusters in the van Krevelen diagram to be recognized. The introduction of a nitrogen atom to the HACHO species was based on the structures of the HACHO species, which is also indicated by the van Krevelen diagram.
Among all the non-fuel utilization methods for lignite, the method acquiring humic acids plays an important role because of the high demand for humic acids. Humic acids, consisting of different humic substances derived from the degradation of organic matter in flora and fauna, exist not only in soils, natural waters, sea sediment plants, and peat, but also in lignite and oxidized bituminous coal.9,10 It is a complex mixture of various acids containing substituted aromatic rings connected by various bridges, and the main oxygen-containing groups include carboxyl, alcoholic hydroxyl, and phenolic hydroxyl groups.11,12 Generally, humic acids were produced from lignite through sequential treatments with strong alkali and acid, which account for 10–80% of the organic matter in lignite.13 Peuravuori et al. obtained humic acids from Lakhra lignite via alkali separation with 2000 mL of alkaline mixture (0.5 M NaOH + 0.1 M Na4P2O7, 1:1 in volume), followed by treatment with concentrated HCl solution.14
Understanding the characteristics of humic acids acquired from lignite at the molecular level is important for effective, clean, and value-added utilization of coal.15,16 Modern instrumental analyses such as Fourier transform infrared spectroscopy (FTIR),17 scanning electron microscopy (SEM),18 and gas chromatography/mass spectrometry (GC/MS)19 have been applied in the characterization of the compositions of complex biomass/coal. Olivella and co-workers used analytical pyrolysis coupled with GC/MS to obtain the detailed molecular structure information of humic acids from Spanish leonardite coal.20 Sulfonate and sulfate were proven to be the most occurring states of sulfur in the coal. Shi et al. studied Geting bituminous coal using SEM and FTIR to learn the surface morphology of raw coal/extraction residue and structural details such as chemical functional groups, respectively.18 The extracts obtained from Geting bituminous coal through sequential solvent extraction contained more aliphatic components and less aromatic compounds compared with raw coal and its extraction residue. The combination of different analytical methods can promote in-depth investigation on the composition of humic acids that originate from lignite at the molecular level.
In this study, the process of acquiring humic acids from Shengli lignite was optimized by orthogonal experiments to achieve the highest yield. Both the raw coal and reaction residue were characterized using SEM and energy dispersive spectrometry (EDS). Humic acids were analyzed by FTIR and high-resolution MS. Statistical analysis of the mass spectra revealed the composition of humic acids at the molecular level.
Proximate analysis | Ultimate analysis (daf) | ||||||
---|---|---|---|---|---|---|---|
Mad | Ad | Vdaf | C | H | N | S | Odiff |
a Mad = moisture (air-dried base); Ad = ash (dry base, i.e., moisture-free base); Vdaf = volatile matter (dry and ash-free base); daf = dry and ash-free base; diff: by difference. | |||||||
12.52 | 16.69 | 43.44 | 74.28 | 4.27 | 1.08 | 1.13 | 19.24 |
Aqueous sodium hydroxide (0.21 mol L−1) and concentrated hydrochloric acid (37% in weight) were used in the reaction process. The analytical grade acetone was sequentially purified by distillation using a rotary evaporator (R-134, Büchi Labortechnik AG, Flawil, Switzerland) prior to use.
Humic acids dissolved in acetone were analyzed using a high-resolution MS, ESI-Orbitrap-MS (LTQ Orbitrap XL, Thermo-Fisher Scientific, Waltham, USA). The MS was operated in a negative ion mode with 35 psi nebulizer pressure and 4 kV source voltage. Thermo Xcalibur Roadmap software Data Analysis 2.2 was used for peak selection and molecular formula assignment. The signal-to-noise ratio for peak detection was set to 3, and molecular formulas were assigned with a mass tolerance less than 3 ppm. The molecular formulas were limited to a maximum of 50 C, 100 H, 5 N, 10 O, and 2 S atoms.
Humic acids were also analyzed using FTIR spectroscopy (IR-560, Nicolet Co. Madison, USA). The spectra were obtained from 4000 to 400 cm−1 with 4 cm−1 resolution.
Compared with humins obtained from biomass sources such as chitin, there are more aromatic rings but few N-containing compounds in the humic acids obtained from lignite according to the corresponding FTIR spectra. Chitin is rich in C–N and N–H bonds,23 whereas there are many aromatic rings in lignite.24 The distribution of chemical groups in humins was determined by the origin of the feedstock. However, humins obtained from both chitin and lignite have absorption peaks attributed to the vibrations of –OH, –CH2, –CH3, C–O, and CO groups, suggesting widespread existence of –OH and –COOH, as well as aliphatic structures in humins.
Fig. 3 SEM images of (a) raw coal, (b) alkali extraction residue, (c) acid washed residue, and (d) water washed residue. |
Fig. 4 EDS spectra of (a) raw coal, (b) alkali extraction residue, (c) acid washed residue, and (d) water washed residue; ND = not detected. |
Around 60% of the detected species in the humic acids were oxygen-containing compounds (OCCs) with hydroxyl, carboxyl, and other functional groups. The OCCs are divided into six groups according to the number of oxygen atoms in a molecule: OCCs1, OCCs2, OCCs3, OCCs4, OCCs5, and OCCs6. The number of OCCs in each group is 668, 511, 535, 413, 323, and 331, respectively. The distribution of the relative content of OCCs is shown in Fig. 5. The content of OCCs1 is the highest, which accounts for 68.2% of the total content of OCCs. Hydroxyl and carboxyl groups are the main oxygen-containing groups in humic acids, which induce hydrogen bonding inside the raw coal.33,34 With the increase of oxygen atoms in OCCs, the number of oxygen-containing groups increases that strengthen the non-covalent interactions, inducing the enhancement of difficulty in separating OCCs from the raw coal.
Fig. 5 Relative content distribution of OCCs1, OCCs2, OCCs3, OCCs4, OCCs5, and OCCs6 detected in humic acids via ESI-Orbitrap-MS. |
Heteroatom-containing compounds can be classified at the molecular level according to the chemical formula. There are various classes of heteroatom-containing compounds such as Oo, OoNn, and OoSs in humic acids. The van Krevelen diagram was proposed to plot molar H/C versus O/C.35 The plot can provide the atomic ratio of the compounds and display the compounds with different composition by distinct points. In recent years, the van Krevelen plot has been introduced to analyze the complex data obtained from mass spectrometry with ultrahigh resolving power.36,37 Double bond equivalent (DBE) is widely used to estimate the unsaturation degree of organic compounds according to the chemical formula, which can be further used to provide the proposed structural information.38
The van Krevelen diagram and DBE plot for O1 to O6 classes are shown in Fig. 6 and 7, respectively. In the van Krevelen diagram, homologous lines such as lines A, B, and C, as shown in Fig. 6(c), consist of compounds (points in the diagram) from the same alkylation series. Compound CcHhOo (c, h, and o represent the number of atoms C, H, and O, respectively) and its homologous species Cc+nHh+2nOo (n = ±1, ±2, ±3…) are on the homologous line of Y = ((h − 2c)/o)X + 2, which intersects with the Y axis at (0, 2). When the oxygen number is constant, a higher slope of the homologous line means a higher saturation degree for the compounds on the line. Slopes of line A and line B are greater than 0; thus, the points on the line A or B represent a series of saturated chain organic species containing hydroxyl groups and/or ether bonds. Classes O1 and O2 have lower O/C ratios compared to O3 to O6 classes due to the lower number of oxygen. Acid groups such as –COOH and –OH are detected in the negative ion mode; thus, classes O1 and O2 are more likely attributed to the compounds with –OH and –COOH/double –OH, respectively. In Fig. 7, there are 15% of compounds with a DBE value over 20 in O1 and O2 classes, whereas almost no such compounds were identified in other Oo classes. These high DBE compounds are mainly attributed to polycyclic aromatics with acid groups or O-containing heterocycles such as furan. The van Krevelen diagram can also graphically distinguish the compounds into different clusters. The distance between the two points in the van Krevelen diagram represents the difference in the chemical structure between the two corresponding compounds,39 such that the compounds in one cluster should have a similar structure. DiDonato et al. found that compounds of humic substances derived from soil can be classified into different types of molecules with similar aromaticity or unsaturation degree by a van Krevelen diagram according to the MS data.40 In classes O5 and O6 (Fig. 6(c)), 40% of the unsaturated species are concentrated in the region that spans between H/C from 1.5 to 2.0 and O/C from 0.2 to 0.5. The point corresponding to the compound with the highest unsaturation degree locates in the lower left corner of this region. In Fig. 6(c), point D (C19H32O5) has the highest DBE value of 4 among all the compounds in the blue region. Considering the existence of –COOH, there are few aromatic ring structures for compounds because the minimum DBE number is five for the compounds containing an aromatic ring plus a –COOH group. Thus, O5 and O6 species in the blue region are mainly attributed to carboxyl- and hydroxyl-containing aliphatic compounds (CHCACs). CHCACs account for 60% of O5 and O6 species, and the similar result was also found, as shown in Fig. 7(c). For O3 and O4 classes, as shown in Fig. 6(b), there is a new region that spans between H/C from 1.0 to 1.5 and O/C from 0.1 to 0.4. Compounds in this region have DBE values from 4 to 10 that are attributed to aromatic carboxylic acids or phenols (ACAPs) with 1–2 rings.
Fig. 6 van Krevelen diagram for O1 to O6 species. Blue and red regions are defined as CHCACs and ACAPs regions, respectively. |
Organic compounds containing S and N in humic acids can develop high affinity for metal cations in soil and water, which has great significance to environmental chemistry in heavy metals.41 Thus, O1N to O4N were also studied through van Krevelen diagrams (Fig. 8) and DBE plots (Fig. 9). In Fig. 8, the points of O1N to O4N species are concentrated in clusters with a position similar to that of O1 to O4 species, as shown in Fig. 6, indicating that the introduction of a nitrogen atom into O1N to O4N species is based on the structures of the O1 to O4 species. Compared with O3 and O4 species, as shown in Fig. 7(b), the DBE distributions for O3N and O4N species are more scattered. Moreover, an increment of 1.4 in the arithmetic average DBE for O3N and O4N species compared with that for O1 to O4 species is probably attributed to the introduction of a N-containing ring. Compared with pyridine groups, pyrrole groups are easier to be ionized via deprotonation in the negative ion mode.42 The introduction of pyrrole groups also leads to a decrement of 0.03 in the average O/C ratio for O3N and O4N species compared to the species shown in Fig. 6(b).
S-containing molecules are typically less polar than N-containing compounds and not easily ionized by ESI.43,44 There is no Ss species detected under the negative ion mode because Ss species such as thioethers and thiophenes are not efficiently ionized. However, for OoS species, the existence of acid groups enhances the molecular polarity and further increases the ionization efficiency. As shown in Fig. S3,† the OoS species have a wide distribution of DBE from 0 to 32. The arithmetic average DBE value of OoS species is lower than that of the corresponding Oo species, which is probably due to few OoS species with DBE over 20. Therefore, there are few condensed aromatic compounds with over 6 rings in the OoS species.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra01497j |
This journal is © The Royal Society of Chemistry 2017 |