Molecular Characterization and Transformation of Dissolved Organic

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Molecular Characterization and Transformation of Dissolved Organic Matters in Refinery Wastewater from Water Treatment Processes: Characterization by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Yongyong Li, Zhi Fang, Chen He, Yahe Zhang, Chunming Xu, Keng H. Chung, and Quan Shi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01446 • Publication Date (Web): 12 Oct 2015 Downloaded from http://pubs.acs.org on October 12, 2015

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Molecular Characterization and Transformation of Dissolved Organic Matters in Refinery Wastewater from Water Treatment Processes: Characterization by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Yongyong Li1, Zhi Fang1, Chen He1, Yahe Zhang1, Chunming Xu1, Keng H Chung2, and Quan Shi1*

1

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing

102249, China 2

North Huajin Chemical Industries Group Corporation, Panjin, Liaoning, 124000,

China

Corresponding author:

Dr. Quan Shi

State Key Laboratory of Heavy Oil Processing China University of Petroleum-Beijing 18 Fuxue Road, Changping, Beijing, 102249 China Tel: +86-10-89733738 Fax: +86-10-69724721 E-mail: [email protected]

Refinery wastewater

Organic phase

GC-MS

Anaerobic biodegradation

Aqueous phase

ESI FT-ICR MS

Aerobic biodegradation

O4S2 O3

Oxidation by bio-aeration

O4 O5S3

Activated carbon filtration

O4S3 O3S2

O3S4 284.95

O5 O5S1

285.00

285.05

[C16H30O4-H]-

O3S1

O4S1 O6

O5

O7 285.10

1

ACS Paragon Plus Environment

285.15

285.20

m/z

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ABSTRACT A set of wastewaters sampled in a stream-by-stream flow of the process in a refinery wastewater treatment plant were characterized to investigate the molecular composition and transformation of dissolved organic matter (DOM). The samples were separated into organic and aqueous phase DOMs by solvent extraction and solid phase extraction (SPE). Volatile and semi-volatile compounds in the organic phase were characterized by gas chromatography – mass spectrometry (GC-MS); DOMs in the organic and aqueous phase were characterized by negative ion electrospray (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The aqueous phase DOMs exhibited more complex molecular composition than other complex mixtures investigated by far, in which there were totally 76 compound class species identified in a single mass spectrum. Refinery wastewater DOMs have lower values of double bond equivalent (DBE) and O/C ratio than those of natural organic matters (NOMs) in fresh and marine waters. The organic phase DOM occupied the major TOC value, but was liable to be degraded in the biological process. Some humic-like substances presented in the aqueous DOM were found resistant for the treatment processes by the ESI FT-ICR MS based semi-quantitative results.

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1. INTRODUCTION Treatment of industrial wastewaters has attracted tremendous attention for environmental and health concerns.[1] Refinery wastewater is an important chemical industrial wastewater which containing lots of contaminants like oil, ammonia, sulfides, chlorides, mercaptans, phenols, and other hydrocarbons.[2] Along with the increase of oil consumption, refineries become high water consumers and, as a result, large wastewater producers.[2, 3] A significant volume of water is used in refinery processes, especially for distillation, hydro-treating, desalination and cooling system.[4, 5] Due to the increasing stringent environmental regulations and limited water resources, efficient treatment technologies are needed to deal with the hazardous pollutants within many refinery waste streams.[2, 6, 7] For designing and improving water treatment processes, it is necessary to better understand the molecular composition of dissolved organic matter (DOM) from the wastewaters. However, DOM probably contains thousands or even millions of different chemical compositions, making it impossible to characterize it on the basis of complete description of the individual compounds.[8, 9] In previous studies, various analytical techniques were applied to characterize the DOM composition, including spectroscopic methods such as ultraviolet-visible spectroscopy (UV/Vis),[10-13] Fourier transform infrared spectroscopy (FT-IR),[14-16] nuclear magnetic resonance spectroscopy (NMR),[17-19] fluorescence excitation/emission matrix spectroscopy (EEM),[20, 21] and mass spectrometric methods like liquid chromatography-mass spectrometry (LC-MS),[22, 3


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23] gas chromatography-mass spectrometry (GC-MS),[24, 25] and ultrahigh-pressure liquid chromatography (UPLC) coupled to quadrupole time-of-flight mass spectrometer (QTOF-MS). [26] Although great efforts have been made for the investigation on DOM by various approaches, characterization on molecular composition of DOM in wastewater still is a challenge for analytical chemists. Recently, electrospray ionization (ESI) coupled to ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), has become an important tool and has been successfully applied to the characterization of DOM from different aquatic environments such as ocean,[27-30] lake,[31] bay system,[32] pore water[33-35] and rainwater,[36] oil sands process water,[37-39] as well as the water-soluble

DOM

from

soil,[40,

41]

and

atmosphere.[42-45]

In

addition,

transformation of DOM through biodegradation,[46] photochemical degradation,[47, 48] and chlorination[49, 50] has been studied by using ESI FT-ICR MS. The ultrahigh resolving power and mass accuracy of FT-ICR MS allows complete peak resolution and unambiguous molecular composition determination of the components in complex DOM, and can therefore reveal the molecular-level changes that occur during DOM transformation.[32, 38] One the other hand, ESI can directly and selectively ionize most DOM components in water,[51, 52] which largely facilitate the characterization of DOMs. In contrast to natural water DOM, the wastewater DOMs have rarely been

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characterized,[53] especially for refinery wastewater. In our previous work, we used resin fractionation technique combined with ESI FT-ICR MS obtained a detailed molecular composition of DOM for one refinery wastewater and its fractions.[54] Here we present a comprehensive analytical method to better understand the DOM composition from the process wastewaters. The water samples from a refinery wastewater treatment plant were collected stream-by-stream. The DOMs were separated into organic phase and aqueous phase and characterized by GC-MS and negative ion ESI FT-ICR MS.

2. MATERIALS AND METHODS 2.1 Refinery wastewater samples Five refinery wastewater samples were obtained from a refinery wastewater treatment plant, which is an integrated activated sludge treatment unit consists of physicochemical, mechanical, and biological processes.[55-57] Fig. 1 shows the process flow diagram of the refinery wastewater treatment plant and the sampling points: ⋅

Sample #1 was the mixed raw water from de-oiling and flotation,

⋅

Sample #2 was the effluent from anaerobic biodegradation,

⋅

Sample #3 was the effluent after aerobic biodegradation,

⋅

Sample #4 was the effluent after bio-aeration oxidation, and

⋅

Sample #5 was the effluent from activated carbon adsorption process. De-oiling and flotation are physical processes, which can remove the suspending oil

drops and other matters from the raw wastewater. The aerobic biodegradation and 5


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anaerobic biodegradation are the biological processes that use activated sludge treatment to convert complex organic compounds into simpler ones and inorganic compounds under the condition with or without oxygen, respectively. After removing the sludge by sedimentation, the wastewater was sent to bio-aeration process, which uses biological membrane to give more effective aerobic biodegradation. The activated carbon adsorption is the last process for further purification of DOMs. The refinery wastewater samples were filtered through a 0.45 µm Pall Supor membrane filter and kept in the dark at 4°C before analysis. 2.2 Sample preparation The refinery wastewater sample preparation scheme is shown in Fig. 2. The filtered wastewater sample (200 mL) was extracted with 30 mL dichloromethane (DCM) for three times. The organic phase was combined together and subjected to rotary evaporation to remove DCM and stored in the dark at -18°C. The aqueous phase was acidified using hydrochloric acid (HCl) to pH=2 and pumped at 5 mL/min through a solid-phase extraction cartridge (Oasis HLB, 500 mg, 6 mL, Waters, USA). Prior to aqueous phase injection, the solid-phase extraction cartridge was conditioned by rinsing the cartridge with methanol followed by acidified ultrapure water (pH=2). After sample injection, the cartridge was rinsed with 20 mL acidified ultrapure water (pH=2) to remove the salt. The DOM was eluted with 10 mL methanol. The eluted sample was stored at -18°C in the dark.

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2.3 Organic carbon analysis The dissolved organic carbon (DOC) of the refinery wastewater samples were analyzed using a 5000A total dissolved organic carbon (TOC) analyzer (Shimadzu, Japan).

All the dried samples were diluted with ultrapure water to a constant volume of

200 mL prior to TOC analysis. 2.4 GC-MS Analysis The GC-MS analyses of DOM in the organic phase of the refinery wastewater samples were performed using a Bruker SCION TQ gas chromatography – mass spectrometry (GC/MS/MS) equipped with a HP-5 MS column (30 m × 0.25 mm × 0.25 µm). The GC oven was held at 80 °C for 1 min, increased to 300°C at a rate of 10°C/min, and then held constant at 300°C for 10 min. Helium was used as the carrier gas at a flow rate of 1 mL/min. The ion source temperature was maintained at 250°C, ionizing voltage was 70 eV, and the mass range was 35-500 with a 0.5 s scan period. 2.5 Negative Ion ESI FT-ICR MS Analysis The MS analyses of DOC from wastewaters were performed using a Bruker Apex Ultra FT-ICR mass spectrometer equipped with a 9.4 T superconducting magnet and Apollo II electrospray ion source. DOM samples were dissolved in methanol solution and injected into the electrospray source at 180 µL/h using a syringe pump. The typical operating conditions for negative-ion formation consisted of 4.0 kV emitter voltage, 4.5 kV capillary column introduced voltage, and -320 V capillary column end voltage. The

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mass range was set at m/z 150−1000. The data size was set to 4 M words. A data set of 128 FT-ICR scans were accumulated to enhance the signal-to-noise ratio and dynamic range. The procedures for FT-ICR MS mass calibration, data acquisition, and processing have been described elsewhere.[58, 59] Prior to FT-ICR MS analysis, a deuterated stearic acid (C18D35H1O2) was added equally to each DOM sample as an internal standard to obtain semi-quantitative results.

3. RESULTS AND DISSCUTION 3.1 Dissolved organic carbon (DOC) Since ESI MS analysis cannot provide accurate quantitative results of each compounds, it is essential to determine a bulk amount of DOM in each samples. DOC is an available and intuitive parameter to assess the performance of water treatment process on DOM removal.[60] Figure 3 shows the DOC of the wastewater samples and their organic and aqueous phases fractions, the value are tabulated in the Supporting Online Material (Table S1). The results showed that the wastewater treatment process reduced 90% DOC, while the ratios of DOM in organic and aqueous phases are varying. The DOC gradually decreased during the processes, implying the each step of the treatment is effective. The hydrophilic components in the wastewater seems more resistant than the hydrophobic components, or it can be explained as some hydrophobic compounds converted into hydrophilic compounds in the process. As shown in Table S1, the organic

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phase extract DOC decreased from 48.7% to 18.3%, while the aqueous phase extract DOC increased from 26.4% to 53.3%. Total DOC recoveries of organic phase and aqueous phase extracts account to a range of 66.7 % to 82.8% (see Table S1 in Supporting Information). Other DOC should be in the extraction raffinate and/or was lost by evaporation, as well as permanent adsorption on the SPE cartridges. 3.2 Characterization of organic phase DOM by GC-MS The GC-MS analyses of organic phase DOM (See Figure S1 in Supporting Online Material) showed that the abundant GC peaks of Sample #1 (raw water) and Sample #2 (anaerobic effluent) were phenol and its C1-C2 homologs. Phenolic compounds are major contaminants of the refinery wastewater, which can be detected by GC analysis. Hence, the effectiveness of a wastewater treatment process can be determined by the reduction in phenolic compounds before and after the process.[61] The GC-MS data showed that the phenolic compounds were almost completely removed after aerobic biodegradation, indicating that the bio-treatment process was effective for removing volatile compounds. Simultaneously, the results indicate some DOM cannot be detected by the GC-MS technique used in this study, the TOC of these DOMs account for at least of one third of the total TOC. The composition and the treatment effect is not clear solely base on the GC-MS results presented in the Supporting Information. 3.3 Molecular Characterization of organic phase DOM by FT-ICR MS The broad-band negative-ion ESI FT-ICR mass spectra and its segments at m/z 243

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and 244 normal mass of the organic phase DOMs were shown in Figure S2 (see Supporting Information) and Figure 4, respectively. The molecular weight distribution of the organic phase DOMs varied from 150 and 450 Da. Compounds with 200-300 Da were almost completely removed by the wastewater treatment plant, especially after the anaerobic treatment. We tend to provide quantitative results by normalizing the peak intensity based on the relative peak abundance of the deuterated stearic acid standard and the analytes, however, the ESI MS signal intensity was found elusive due to its sensitivity for the solvent and analyte composition, as well as the concentration of the analyte and the internal standard (C18D35H1O2). So the deuterated stearic acid standard was meant to be used as a semi-quantitative reference, which could be used to compare the change trends in molecular composition. Peaks at m/z 243 (see Figure 4) were corresponding to the O2 class species, which were dominant and almost removed by the treatment, while peaks at m/z 244 are corresponding to nitrogen-containing compounds and/or O2 class species with a 13C atom. The nitrogen-containing compounds were relatively constant over the first four treatment processes, but were removed by the activated carbon adsorption. The composition and transformation trends shown at m/z 243 and 244 is typical for the whole spectra. The results indicates that the nitrogen-containing compounds in organic phase extracts were refractory to biochemical reactions. Figure 5 shows the semi-quantitative of relative abundance of each class species in the organic phase DOMs, showing that O1-O5, N1O1-N1O5, O1S1-O5S1, O2, and O3S1 class 10


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species were predominant. The O2 class species of the samples dropped significantly during the treatment processes. Iso-abundance plots of DBE versus carbon numbers for O2 class species of DOM in the organic phase are shown in Figure S2(see Supporting Information) in which the series of DBE=3-7 with 13-19 carbon numbers were the most abundant in the raw wastewater. These compounds are likely 2-6 rings naphthenic acids which were removed by biodegradation in the first two stages of wastewater treatment plant. The results indicate that low molecular weight or fewer rings naphthenic acids were more susceptible to biodegradation.[62] The highly abundant O3S1 class species with 4 DBEs are likely alkylbenzene sulfonic acids, which are surfactant derivatives of linear alkyl benzene sulfonates.[63] The relative peak intensity of these compounds increased gradually after the biochemical and aeration oxidation in the first three treatment processes, but dropped sharply after activated carbon adsorption. The increased intensity of these compounds in the first three treatment processes could be attributed to high ESI ionization selectivity of these compounds, and/or production of these metabolic compounds in the biodegradation process. Nevertheless, these compounds were effectively removed by activated carbon adsorption.[64] 3.4 Molecular Characterization of aqueous phase DOM by FT-ICR MS The broad-band negative-ion ESI FT-ICR mass spectra of aqueous phase DOMs are shown in Figure S4 (see Supporting Information). The close-up views of expanded negative-ion ESI FT-ICR mass spectra of aqueous phase DOM at m/z 341, 340, and 285

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are shown in Figure S5, S6, and S7 (see Supporting Information), respectively. Similar to the distribution of the organic phase DOMs, the molecular weight of the aqueous phase DOMs was ranged from 150 to 450 Da, however, the molecular composition of DOM in the aqueous phase are more complex. A total of 76 class species were identified in Sample #1 (raw refinery wastewater) and the relative abundance of each class species are shown in Figure 6. Detected class species include O1-O10, N1O1-9, O1-8S1-, O1-7S2, O1-7S3, O1-7S4, N1O1-7S1, N1O1-7S2, N1O1-O7S3, and N1O1-7S4. Except most oxygen and nitrogen containing compounds that had been reported in water samples,[27-35, 63] many sulfur containing compounds are newfound in this study. This should be the most complex mixture in class species identified in a single mass spectrum. Multi-oxygen atom Ox class species DOMs in the aqueous phase DOMs exhibit higher relative abundance than that in organic phase DOMs, however the oxygen atom numbers of abundant class species are much lower than that of DOMs from fresh and marine waters. In addition, the DBE values are small, which implies naphthenic instead of condensed aromatic molecular structures are dominant. The composition feature is similar to that found in oil sand processing wastewaters.[37-39, 64] The predominance of O2 and O4 to O3 class species implies that carboxyl is one major type of oxygen atoms, while the predominance of N1O3 to N1O2 and N1O4 indicates the nitrogen compounds have NO function group, like nitryls instead of aza naphthenic acids. It should be note that the sulfur compounds composition are interesting; the most abundant sulfur

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compound class species is O1S2, but not O1S1. Considering the unusual relative abundance of multi-sulfur compounds, such as N1O4S3 and N1O4S4, we speculate these compounds could be decomposed from chemicals introduced from the oil production and/or refinery processing. Figure 7 shows the relative abundance of the identified 76 class species in the raw wastewater and its treatment products. The relative abundance were normalized by the intensity of the deuterated stearic acid. The results showed that the Ox and Nx class species with less oxygen atoms can be easily removed, while those with more oxygen atom species, which are so called humic-like substances seems resistant from the processing. The S3 and S4 class species show reverse trend for the processing, compounds with more oxygen atoms seems more liable to be removed. As shown in Figure S7 (see Supporting Information), the abundant peaks corresponding to O3S4, O5S3, O4S2, and O4S3 class species in aqueous phase DOMs at m/z 285 were removed by anaerobic biodegradation, while the Sx and Ox-containing compounds were removed by the biodegradation processes. Although the structure and the origin of so many nitrogen and/or sulfur containing compounds are unknown, fortunately, these compounds seems liable to be degraded in the treatments (see Figure S5, S6, and S7 in Supporting Information). Figure 8 shows the van Krevelen diagram of CHO class species in the raw wastewater derived organic and aqueous phase DOM extracts and Suwannee River

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natural organic matter (SRNOM) standard from International Humic Substances Society (IHSS). The red dots were corresponding to the compounds in the organic phase DOM, which distributing in relatively low O/C region (O/C

Molecular Characterization and Transformation of Dissolved Organic

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