1. Introduction
Lignite is a kind of low-rank coal with a low degree of coalification, and it is a macromolecular network structure compound that is composed of aromatic hydrocarbon rings and aliphatic chains, connected by bridge bonds. The molecular structure of lignite contains a large number of active oxygen-containing functional groups such as phenolic hydroxyl, carboxyl, ether bonds, and carbonyl, which impart the characteristics of weak acidity, hydrophilicity, and a cation exchange property to the coal [
1]. Compared with other coal types, lignite has a more developed internal pore structure, a larger specific surface area, more rich oxygen-containing functional groups, and a higher porosity, which causes better adsorption properties of lignite and makes it useful as adsorbent to remove heavy metal ions and organic matter.
Shuying et al. [
2] used Shulan lignite as adsorbent to handle wastewater that contained Ni
2+, Zn
2+, and Pb
2+. The results showed that lignite can remove Ni
2+, Zn
2+, and Pb
2+ from an aqueous solution, and the adsorption capacities of Ni
2+, Zn
2+, and Pb
2+ were 28.18, 28.36, and 29.29 mg·g
−1, respectively. Lumin et al. [
3] used Longkou lignite to adsorb mercury-containing wastewater. The mercury removal rate in wastewater was 99%, and the saturated adsorption capacity of lignite was 24.34 mg·g
−1 under room temperature conditions and a pH = 5. On the other hand, natural porous lignite also shows good adsorption properties for phenol, p-nitrophenol, reactive dyes, and nonionic surfactants. Polat. et al. [
4] adsorbed phenol-containing wastewater by Soma lignite. Experiments showed that the saturated adsorption capacity of lignite was 10 mg·g
−1, and its adsorption capacity per unit area (1.3 mg·m
−2) was much higher than that of activated carbon (0.05~0.3 mg·m
−2). The content of pollutants in the wastewater after adsorption was lower than the emission standard.
Although lignite has certain adsorption properties, in actual application there are some problems in the adsorption process, including low mechanical strength, high impurity content, poor chemical stability, and a tendency to shrink or swell. In order to overcome the aforementioned problems, some attempts have been done to prepare modified coal, including demineralization, nitration, sulphonation, pyrolysis, oxidation, granulation, and so on. Daocheng et al. [
5] found that sulfonic acid groups can be introduced into the molecular structure of lignite coal by sulfonation with concentrated sulfuric acid. At the same time, more oxygen-containing functional groups, such as hydroxyl groups and phenolic hydroxyl groups, were formed on the surface of lignite, which made the lignite change from a weak acid-type ion exchange to a strong acid-type ion exchange, and greatly improved the adsorption capacity of lignite to metal ions. Choudhury et al. [
6] studied the adsorption capacities of heavy metals on two low-rank Indian coals, which was oxidized by dilute nitric acid. Nitric acid oxidation of coal incorporates oxygen and nitrogen atoms into the coal matrix. The experimental data revealed good adsorption capacities of some heavy metal ions, such as Ni, Cu, Cd, and Pb. Kus et al. [
7] studied the characteristics of oxidized coal, and found that coal oxidation lead to chemisorption of oxygen at the coal surface along with the formation of acid functional groups such as hydroxyl, carbonyl, carboxyl, and others, which led to subsequent thermal decomposition and a decrease in the aliphatic and alicyclic carbons. And during advanced stages of coal oxidation, vitrinite in coal will develop very extensive microcracks and microfissures accompanied by oxidation rims. During the modification process, the pore structure and surface chemistry of the adsorbent will change greatly and cause changes in adsorption performance. Thus, it is very important to study the changes of the pore structure and surface chemistry of modified coal to evaluate their effects on the adsorption properties of the adsorbent.
In this paper, different concentrations of nitric acid were used to modify lignite to get different pore structures and surface chemistry level samples. The pore structure, surface chemistry, and adsorption properties of different modified lignite samples were studied. Through the study we can explore the mechanism of lignite modification and the relationship between coal characteristics and adsorption performance. Furthermore, this study provides a basis for further research on the mechanism of lignite modification to improve the adsorption performance of lignite.
2. Experimental Parameters
2.1. Raw Materials and Experimental Coal Sample Preparation
The experimental lignite was obtained from Xilinhaote, Inner Mongolia. The raw coal was crushed and sieved, and the particles with a 45~75 μm size range were selected as the experimental samples. Moisture (M
ad), ash (A
ad), volatiles (V
ad), and fixed carbon (FC
ad) content based on air drying were tested according to GB/T 212-2008. The proximate analysis results of raw coal are listed in
Table 1.
Ten raw coal samples with a mass of 10 g were weighed and added into 100 mL of nitric acid with concentrations of 0, 0.1, 0.3, 0.5, 0.7, 1, 3, 5, 7, and 10 mol·L−1, marked as N0M (the raw coal), N0.1M, N0.3M, N0.5M, N0.7M, N1M, N3M, N5M, N7M, and N10M, respectively. Then, the mixtures were placed in a shaking machine at 25 °C for 2 h. After the modification, the coal samples were washed with deionized water to a neutral state, and they were dried in a vacuum drying oven at 70 °C for subsequent use.
2.2. Coal Sample Mineral Composition Analysis
The mineral compositions of the brown coal before and after modification were measured by an X-ray diffractometer (SmartLab, Japan Science, Tokyo, Japan). The coal sample was sufficiently dried and ground before scanning with a speed of 15°·min−1, in the range of 5° to 90°.
2.3. Coal Sample Pore Structure Characterization
The specific surface area and pore structure characteristics of coal samples under different nitric acid modification conditions were characterized by a static nitrogen adsorption instrument (JW-BK122W, Jingwei Gaobo, Beijing, China). The adsorption and desorption isotherms of the samples were determined by the static capacity method using nitrogen as the adsorption medium at −195.15 °C under the relative pressure of 0.001~1. All samples were degassed at 105 °C for 4 h before the measurement. The specific surface area, pore volume, and pore size distribution of the coal samples were calculated by the BET (Brunauer-Emmett-Teller), BJH (Barret-Joyner-Halenda) and HK (Horvaih-Kawazoe) models.
2.4. Coal Sample Surface Morphology Analysis
The surface morphology characteristics of coal samples under different nitric acid modification conditions were characterized by a scanning electron microscope (JSM-7800F, JEOL, Tokyo, Japan). In the operation, firstly, the conductive adhesive was pasted on the sample holder, and the coal powders were scattered on the conductive adhesive. Next, the coal powders that did not adhere to the conductive adhesive were blown away by a blowing device, and then they were scanned by a scanning electron microscope after the gold sputtering treatment. The acceleration voltage was set to 15 KV, the resolution was set to 0.8 nm, and the magnification was 5000×.
2.5. Coal Sample Surface Functional Group Analysis
The surface functional groups of lignite under different nitric acid modification conditions were determined using a Fourier transform infrared spectroscopy analyzer (Nicolet iS10, Thermo Fisher Scientific, Waltham, MA, USA). The coal samples to be tested were ground and mixed with KBr at a ratio of 1:160, and then they were pressed into thin slices. The coal samples were fully dried in a vacuum drying oven before compression, and the test range was 4000~400 cm−1.
2.6. Coal Surface Potential Test
The surface potential of lignite samples under different nitric acid modification conditions were determined using a potential analyzer (Zetasizer Nano ZS90, Malvern, UK). One gram each of coal sample was added into 50 mL deionized water, mixed well, settled for 24 h, and the supernatant was used to determine results.
2.7. Analysis of the Existence of the Types of Coal Surface Elements
The types of coal elements existing on the surface of lignite samples were analyzed by an X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA). The coal samples to be tested were ground and dried in a 50 °C drying oven for 1 h at a constant temperature, and they were then cooled in a desiccator to room temperature. After cooling, an X-ray photoelectron spectroscopy survey scan and narrow zone scan were performed.
2.8. Determination of the Adsorption Capacity of Pb2+ by Coal Samples
In this paper, Pb2+ was taken as the target pollutant to explore the adsorption efficiency of modified lignite. The Pb(NO3)2 stock solution was prepared in ultrapure water at a concentration of 200 mg·L−1. Coal samples weighing 0.6 g and a 100 mL Pb(NO3)2 solution were added into a 250 mL conical flask. The conical flask was then placed in a constant temperature shaking machine set at room temperature for 3 h. The mixture was filtered by a 45 μm membrane to obtain a clear liquid. The Pb2+ concentration in the filtrate was measured by a plasma emission spectrometer (ICPE-9000, Shimadzu, Kyoto, Japan) to calculate the Pb2+ removal efficiency.
4. Conclusions
To evaluate pore structure and surface chemistry effects on the adsorption of Pb2+ from aqueous solution, a matrix of lignite with different pore structure and surface chemistry levels was studied. From the relationship observed between the adsorbent characteristics and the contaminate adsorption data in ultrapure water, a number of conclusions can be drawn.
The adsorption of Pb2+ on the surface of lignite is the result of both the pore structure and the surface chemistry of the adsorbent. The adsorption capacity of lignite to Pb2+ increased from 14.45 mg·g−1 to 30.68 mg·g−1 after modification. From the X-ray diffraction, static nitrogen adsorption, and Scanning electron microscope characterizations of the nitric acid-modified lignite, it was found that nitric acid entered the pores of coal and reacted with the mineral components, and the pore structure of the coal body was destroyed. The pores in the coal collapsed in a high, modified concentration. Both of the specific surface areas and pore volumes of each sample were decreased. This can partly explain the decrease in adsorption capacity of lignite in the low, modified concentration.
From the analysis of Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and the zeta potential test, it was confirmed that nitric acid modification of coal incorporated oxygen and nitrogen atoms in coal matrix. The surface of lignite was rapidly oxidized to produce a large number of polar oxygen-containing functional groups, such as hydroxyl groups and carboxyl groups, which enhanced the surface-level, negative electrical properties of lignite. The nitrification reaction between nitric acid and the lignite organic components introduced nitro groups on the surface of lignite, which also enhanced the surface polarity of lignite and its adsorption properties to metal ions.
The physical parameters of lignite, such as specific surface areas, pore volume, and pore size, do not characterize the lignite adsorption properties well. The adsorption model of Pb2+, the types of functional groups on the surface of adsorbent, and the quantity plays key roles in the adsorption of Pb2+ by lignite, which are determined between the adsorption capacity of Pb2+ and the number of oxygen-containing functional groups of lignite in the corresponding relationship.