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Molecular Composition and Transformation of Dissolved Organic Matter (DOM) in Coal Gasification Wastewater Zhi Fang, Lijie Li, Bin Jiang, Chen He, Yongyong Li, Chunming Xu, and Quan Shi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04450 • Publication Date (Web): 30 Mar 2019 Downloaded from http://pubs.acs.org on April 1, 2019
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Molecular Composition and Transformation of Dissolved Organic Matter (DOM) in Coal Gasification Wastewater Zhi Fang †, Lijie Li †, Bin Jiang †,‡, Chen He †, Yongyong Li †,§, Chunming Xu †, Quan Shi †* †State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249,
China ‡State
Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry,
Chinese Academy of Sciences, Guangzhou 510640, PR China §College
of Food and Pharmaceutical Sciences, Ningbo University, Ningbo, Zhejiang 315211,
China
Abstract Coal gasification process generates huge amount high concentrated industrial wastewater. A better understanding of molecular compositions of dissolved organic matter (DOM) is necessary for the design and optimization of CGW treatment processes. In this study, we analyzed the chemical compositions of DOM in a Lurgi coal gasification wastewater treatment plant by three-dimensional excitation emission matrix (3D-EEM) fluorescence spectroscopy and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Six samples were collected from several specific treatment processes including raw CGW, effluent of hydrolytic acidification (HA), effluent of sequencing batch reactor (SBR), effluent of contact oxidation (CO), influent and effluent of NaClO bleaching process. The CGW DOM were separated into organic and aqueous phases using liquid-liquid extraction (hydrophobic part) and following solid phase extraction (hydrophilic part). The concentration of DOM decreased along the whole process, while its hydrophilicity increased. The 3D-EEM results reveal that three distinct fluorophores-aromatic protein, fulvic acid-like organics and humic acid-like organics exist in raw CGW. The intensity of three fluorophores significantly reduced along these treatment processes and only fulvic acid-like organics can be identified in the end effluent. The
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molecular compositions of DOM in organic and aqueous phase show a different pattern, CHO formulas were the dominant species in organic phase, while CHOS formulas are predominant in the aqueous phase. The chemical diversity of DOM increased with different treatment processes, especially potential disinfection byproducts (DBPs)-CHOCl compounds are formed during NaClO bleaching process. Semi-quantitative analysis using stearic-d35 acid (C18D35O2H) revealed the decreasing trend of different species, providing a detailed information of molecular transformation along the water treatment processes.
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1. Introduction Coal gasification technology has been widely used in China due to the country’s abundance of coal, but scarcity of oil and natural gas.[1] Several types of gasification process has been developed and coal based-products are varied.[2-5] Among them, middle-temperature coal gasification such as Lurgi coal gasification process was used extensively due to its strong adaptability to low-rank feed coal and high content of methane yield.[6,
7]
However, huge
amount of water is needed during gasification process and then produce large volumes of wastewater, known as coal gasification wastewater (CGW). CGW is generated during the processes such as gas washing, condensation and purification and its property varies with types of coal, operating temperature and pressures, etc.[8, 9] CGW is a type of industrial wastewater features high concentration of pollutants and high toxicity, which has challengs to be handled for dischargeing or reusing under the current environmental legislation and water quality standard requirement.[8, 9] Extensive effort has been devoted to the treatment of CGW due to the rapid growth of coal chemical industry to meet the target zero liquid discharge (ZLD).[8,
10]
For the better design and improvement of CGW
treatment processes, it is necessary to understand the molecular compositions of dissolved organic matter (DOM) in CGW.[11, 12] Various analytical methods have been used and adopted to characterize the DOM in industrial wastewater. For example, total organic carbon (TOC) and chemical oxidation demand (COD) can give the quantitative information of organics exist in wastewater. Spectroscopic analysis such as ultraviolet-visible spectroscopy (UV/vis), Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR), fluorescence excitation/emission matrix spectroscopy (EEM) can provide the information of chemical groups of DOM.[13-15] For the detailed analysis of molecular composition, mass spectrometric methods such as gas chromatography-mass spectrometry (GC-MS) were always used but limited organic compounds such as phenolics, polycyclic aromatic hydrocarbons (PAHs), heterocyclic compounds can be identified due to the inadequate mass resolution[12]. Although extensive efforts have been made to elucidate the organics in CGW, the challenge also exists
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due to the very complex molecular composition of DOM.[15-18] Recent advances in high-resolution mass spectrometry such as FT-ICR MS and Orbitrap MS and appropriate ionization methods such as electrospray (ESI), atmospheric pressure photoionization (APPI) and matrix-assisted laser desorption/ionization (MALDI) enable detailed assess the molecular composition of aquatic DOM in the environmental samples.[19, 20] The ultrahigh resolving power and mass accuracy of these techniques provide complete peak resolution and unambiguous molecular composition of the compounds in complex DOM and then the molecular-level changes during DOM transformation can be studied. Molecular compositions of aquatic DOM during various processes such as biodegradation, photochemical degradation, and chlorination have been investigated and elucidated through ESI FT-ICR MS.[21-24] However, most of these studies were about natural water samples or domestic wastewater, and collected samples are also limited to either single sample or specific water treatment process. In contrast to natural occurring DOM, the compositions of DOM in chemical industrial wastewater shows different pattern of molecular compositions. In previous studies, we found the DOM in refinery wastewater has lower DBE and O/C values compared to natural organic matter.[15] However, little is known about the nature of DOM in CGW. Herein, we present comprehensive analytical methods to characterize the DOM in a series of wastewater from a coal gasification wastewater plant. The objectives of this study were as follows: (1) to study the changes of the fluorophores along the whole CGW treatment processes; (2) to distinguish the hydrophobic and hydrophilic DOM using solvent extraction and solid phase extraction; (3) to characterize the molecular compositions of hydrophobic and hydrophilic DOM; (4) to investigate the molecular transformation of DOM during specific treatment processes. 2. Experimental section 2.1 Samples and chemicals Six CGW water samples along wastewater treatment processes were collected from a typical Lurgi coal gasification wastewater treatment plant (Henan, China) and the process flow
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is shown in Figure 1. The raw CGW was treated with the integrated processes and the end effluent was discharged into local domestic wastewater treatment plants (WTPs) for further water treatment and purification. This CGW wastewater treatment plant features several biological processes. The flotation processes (Flotation-I and Flotation-II) used along treatment processes are physical processes that can remove oil drops and other matter in wastewater. The hydrolytic acidification (HA) process is the pretreatment process for the following biological processes under anaerobic or anoxic condition. Sequencing batch reactor (SBR) is an activated sludge system for wastewater treatment. Contact oxidation (CO) process is also a biological process under aerobic conditions. Sedimentation and Flotation-II processes are used to remove sludge from the water. NaClO bleaching treatment process is an advanced oxidation process (AOP) for decoloration and organics removal. All samples were collected and stored in the high-density polyethylene (HDPE) barrels at 4℃ and filtered through the 0.45μm membrane (Pall supor, USA) prior to analysis.
1# CGW
Floatation-I
Hydrolytic acidification
Secondary sedimentation
Floatation-II
5#
NaClO
4#
6#
2#
SBR
3#
Contact oxidation
Domestic WTP
Figure 1. Schematic diagram of the CGW wastewater treatment processes.
Sample information: Sample#1 was raw CGW; Sample#2 was the effluent from hydrolysis acidification; Sample#3 was the effluent from sequencing batch reactor (SBR); Sample#4 was the effluent from aerobic contact oxidation; Sample#5 and#6 were the influent the effluent of NaClO bleaching treatment process; Pure water (H2O) with LC-MS grade was obtained from Fisher Scientific, USA. Methanol
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(CH3OH) and dichloromethane (CH2Cl2, DCM) of analytical grade and further distilled were used during all the experiments. 2.2 DOM extraction procedure The dissolved organic matter in raw and treated CGW was extracted using a combined liquid-liquid extraction (LLE) and solid phase extraction (SPE) method, which is similar to Li et al. with minor modification.[15] Firstly, the pH of 100 mL sample was adjusted to 2 by 2M HCl and extracted three times with 30mL DCM. The DCM extracts were then concentrated using rotary evaporator and regarded as organic phase. Secondly, the pH of above LLE raffinate was adjusted to 2 again and the DOM was extracted through a Bond Elut PPL (500 mg, Agilent Technologies, USA) cartridge.[25] The cartridge was cleaned and conditioned using 20 mL methanol and 20 mL acidic water (pH=2, diluted HCl solution). Samples were loaded on to the cartridge and passed through the cartridge under gravity. The cartridge was then rinsed with 20 mL acidic water to remove adsorbed inorganic salts and dried under N2 gas flow. 10mL methanol was used to elute the cartridge to get the DOM. The SPE extracts were regarded as aquatic phase and for further analysis. 2.3 Dissolved organic carbon analysis The dissolved organic carbon (DOC) of CGW samples and its DCM and SPE extracts were analyzed using total organic carbon (TOC) analyzer (TOC-V, Shimadzu, Japan). Before analysis, LLE and SPE extracts were dried and redissolved in 20 mL pure water for determining the DOC in LLE and SPE fractions of raw and treated CGW. 2.4 Excitation-emission matrix fluorescence spectroscopy analysis The EEM fluorescence spectra of the wastewater samples were carried out a F-7000 fluorescence spectrophotometer (Hitachi, Japan) with a 450-W Xenon lamp was used as the excitation source. Fluorescent data were collected every 5 nm over an excitation (Ex) range of 200-400 nm and with an emission (Em) range of 200-550 nm by 5 nm. The EEM spectra of all
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samples were obtained by subtracting the pure water blank and analyzed under the same conditions. The Origin software (OriginLab, USA) was used to process the fluorescent data. 2.5 GC-MS analysis Organic phase (LLE extracts) of raw and treated CGW were analyzed with a gas chromatography-mass spectrometer (TSQ 8000 Evo, ThermoFisher Scientific, USA) equipped with a HP-5 column (60 m × 0.25 mm × 0.25 μm) with helium as the carrier gas at the flow rate of 1 mL/min. The GC oven was programmed from 80 °C (1 min), increased to 300 °C at a rate of 10 °C/min, and then held at the final temperature for 10 min. Electron impact (EI) source with 70 eV ionization voltage was used in the mass spectrometry. The ion source temperature were set at 250 °C and scanned from 35 to 500 with a 0.5 s scan period. 2.6 Negative ESI FT-ICR MS analysis Molecular characterization of DOM extracts was performed using a negative ESI mode Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) (Apex-Ultra, Bruker, Germany) equipped with a 9.4 Tesla superconducting magnet. Diluted Suwannee River Natural Organic Matter (SRNOM, IHSS, USA) solution with known homologous oxygencontaining compounds was used for mass spectra calibration before analysis. Samples (250 μL h-1) were injected directly into electrospray ion source under negative-ion mode with voltages of the capillary and spray shield were, 4 kV and 3.5 kV, respectively. The mass range was m/z 150-800 and the mass spectra was acquired over 128 scans with the ion accumulation time of 0.5 s and the time of flight was set at 0.0011 s. Mass resolution was higher than 400,000 at m/z400 and mass accuracy across the mass range was
