Science of The Total Environment

IF 9.8SCIEJCI 1.68JCR Q1环境科学与生态学1区TopEI
Volume 911, 10 February 2024, 168681
Science of The Total Environment

Wastewater from natural gas Cansolv desulfurization process: Comprehensive characterization and effective removal of organic compounds

https://doi.org/10.1016/j.scitotenv.2023.168681Get rights and content 获取权限和内容

Highlights 突出

  • The O3/H2O2-Fenton process exhibits superior performance.
    O /H 2 O 2 3 -Fenton工艺具有卓越的性能。

  • In O3/H2O2, Cdouble bondC, Cdouble bondO and single bondOH bonds react more preferentially than Csingle bondH bonds.
    在 O /H O 中,C C、C O 3single bond OH 键的反应比 C single bond double bond double bond H 2 键更优先。 2

  • The Kendrick mass defect theory was utilized to probe DOM degradation.

  • O3/H2O2-Fenton oxidation can effectively remove CHOS and CHONS.
    O /H 2 O 2 3 -Fenton氧化可有效去除CHOS和CHONS。

Abstract 抽象

The wastewater generated by the solvent amine desulfurization process in natural gas purification plants is characterized by its recalcitrant organic compounds and high salinity. Without effective treatment, it has the potential to inflict severe environmental harm. The composition of organic matter, however, exerts a profound influence on the outcomes of oxidation processes. To rectify the limitations associated with indiscriminate oxidation that yields suboptimal results, this investigation meticulously performed a molecular-level analysis of organic matter. Based on the organic matter composition in the influent, this study compared the treatment efficacy of three oxidation processes and determined O3/H2O2-Fenton as the optimal joint approach. After O3/H2O2 oxidation, long-chain unsaturated organic compounds (C > 40,DBE > 20) underwent degradation into short-chain aldehydes and low-molecular-weight fatty acids, with priority given to reactions involving Cdouble bondC, Cdouble bondO, and single bondOH over Csingle bondH reactions. Subsequent Fenton oxidation effectively removed the refractory organics (CHOS, CHONS) and significantly reduced the diversity of organic matter (from 7730 to 4237). The carboxylation, demethylation, and dehydrogenation reactions further facilitated the removal of recalcitrant organic compounds. In light of these findings, this study substantiates that the conversion of extended-chain unsaturated compounds into abbreviated-chain saturated compounds within the system through O3/H2O2 oxidation significantly enhances the subsequent efficacy of Fenton oxidation in organic matter removal. These insights offer valuable perspectives for the efficient remediation of analogous high-salinity organic wastewater scenarios.
天然气净化装置中溶剂胺脱硫过程产生的废水具有顽固性有机化合物和高盐度的特点。如果没有有效的治疗,它有可能造成严重的环境危害。然而,有机物的组成对氧化过程的结果产生了深远的影响。为了纠正与产生次优结果的不分青红皂白的氧化相关的局限性,本研究对有机物进行了细致的分子水平分析。本研究基于进水中有机物组成,比较了3种氧化工艺的处理效果,确定O /H 2 O 2 3 -Fenton为最佳接合方法。O 3 /H 2 O氧化后,长链不饱和有机化合物(C > 40,DBE> 20)降解为短链醛和低分子量脂肪酸,优先于C double bond C single bond 、C double bond O 2single bond OH反应。随后的芬顿氧化有效地去除了难降解有机物(CHOS、CHONS),并显著降低了有机物的多样性(从7730到4237)。羧化、去甲基化和脱氢反应进一步促进了顽固有机化合物的去除。基于这些发现,本研究证实了通过O /H 2 2 O 3 氧化将扩展链不饱和化合物转化为系统内的短链饱和化合物,显著增强了Fenton氧化在有机物去除中的后续功效。 这些见解为有效修复类似的高盐度有机废水方案提供了宝贵的视角。

Keywords 关键字

High-salinity organic wastewater
Refractory organics removal
O3/H2O2-Fenton combined process
Organic matter transformation

高盐有机废水难熔有机物去除O 3 /H 2 O 2 -Fenton联合工艺有机物转化

1. Introduction 1. 引言

Natural gas (NG), an efficient energy source, has gained attention owing to the increasing demand for NG in China (Esfahani et al., 2015; Tikadar et al., 2021). China is the world's fourth-largest producer of NG, and its production is expected to reach >230 billion cubic meters by 2025. More than 90 % of NG contains H2S and CO2, with contents ranging from 1 % to 10 %, which might be converted into SO2 during NG processing (Dong et al., 2020; Ren et al., 2012). To alleviate its current severe air pollution problem, China released the latest industry standard (GB 39728-2020) to restrict the emission concentration of SO2 in NG purification plants, which raised the limit of the SO2 content index from no >1200 mg/m3 to no >400 mg/m3.
天然气(NG)是一种高效能源,由于中国对天然气的需求不断增加,天然气(NG)受到关注(Esfahani等人,2015;Tikadar 等人,2021 年)。中国是世界第四大天然气生产国,预计到2025年产量将达到2300亿立方米>。超过 90% 的 NG 含有 H 2 S 和 CO 2 ,含量范围为 1% 至 10%,可能在 NG 加工过程中转化为 SO 2 (Dong 等人,2020 年;任等人,2012)。为缓解目前严重的大气污染问题,我国发布了最新的行业标准(GB 39728-2020),限制了天然气净化装置中SO 2 的排放浓度,将SO 2 含量指标的限值从>1200mg/m提高到>400mg/m 3 3

The Cansolv tail gas treatment process uses organic amine solvent (OAS) to absorb SO2, lowering the mass concentration of SO2 in the exhaust gas to approximately 400 mg/m3. Thermally stable salts (e.g., sulfate, nitrate, thiosulfate, chloride) are produced during the absorption of SO2 by OAS, leading to a decrease in its absorption efficiency (Farzaneh et al., 2021). To address this issue, OAS requires regeneration through a resin unit. The regeneration of OAS is accomplished via the utilization of resin, rendering the rejuvenated OAS available for subsequent use (Golubeva et al., 2020). Nevertheless, a fraction of the OAS remains confined within the resin unit, necessitating the utilization of water to cleanse the equipment, during which a voluminous flow of high-salinity organic wastewater amounting to 1100–1400 m3/d is designated as the amine purification unit (APU). According to the literature, APU wastewater may contain various organic compounds, including amines, alcohols, halogenated hydrocarbons, and long-chain unsaturated organic compounds (Golubeva et al., 2020). Furthermore, because of the high concentration of thermally stable salts in APU wastewater, the total dissolved solid (TDS) levels typically exceed 20,000 mg/L. Untreated wastewater discharged into the environment can threaten the ecosystem and public health. Therefore, developing an effective treatment strategy to remove recalcitrant organic compounds from high-salinity APU wastewater is imperative.
康世富尾气处理工艺使用有机胺溶剂(OAS)吸收SO 2 ,将废气中SO 2 的质量浓度降低到约400 mg/m 3 。在OAS吸收SO 2 的过程中会产生热稳定的盐(例如硫酸盐,硝酸盐,硫代硫酸盐,氯化物),导致其吸收效率降低(Farzaneh等人,2021)。为了解决这个问题,OAS需要通过树脂单元进行再生。OAS的再生是通过利用树脂完成的,使恢复活力的OAS可供后续使用(Golubeva等人,2020)。然而,OAS的一部分仍然被限制在树脂单元内,需要利用水来清洁设备,在此期间,大量的高盐度有机废水流为1100-1400米 3 /天,被指定为胺净化装置(APU)。根据文献,APU废水可能含有各种有机化合物,包括胺、醇、卤代烃和长链不饱和有机化合物(Golubeva等人,2020)。此外,由于 APU 废水中热稳定盐的浓度很高,总溶解固体 (TDS) 水平通常超过 20,000 mg/L。因此,制定有效的处理策略以去除高盐度APU废水中的顽固有机化合物势在必行。

Many techniques have been investigated for the removal of organic compounds from high-salt organic wastewater, including biological treatments (Tang et al., 2021), adsorption methods (Liu et al., 2021), and advanced oxidation processes (AOPs) (Lester et al., 2015). AOPs are frequently used chemical methods to treat high-salt organic wastewater due to the substantial capability of powerful oxidants to chemically break down organic pollutants (Wang and Bai, 2017; Xu et al., 2018). Examples of AOPs include persulfate oxidation (PS) (Wei et al., 2018), ozone oxidation (Bourgin et al., 2017; Fu et al., 2019; Jung et al., 2017), and Fenton oxidation (Grčić et al., 2009). Among these techniques, Ozone is a highly effective technique for treating recalcitrant organic pollutants in high-salt organic wastewater owing to its potent oxidation potential and high reactivity toward electron-rich moieties (Chen et al., 2019). Ozone rapidly reacts with benzene rings, Cdouble bondC, and Cdouble bondO, yielding intermediate products such as phenols, aldehydes, ketones, and small-molecule organic acids (Wang et al., 2020). The ozone treatment alone for high-salt organic wastewater generally results in a low mineralization rate, primarily due to the limited reactivity of intermediate products with ozone (Fu et al., 2019). Consequently, ozone is commonly combined with other treatment techniques to enhance the overall mineralization rate of organic compounds in high-salt organic wastewater (Itzel et al., 2020). Among these treatment techniques, Fenton oxidation is widely applied due to its ability to generate a substantial amount of hydroxyl radicals (Chen et al., 2023b). Compared to ozone oxidation, the hydroxyl radicals generated in the Fenton process exhibit non-selective oxidation of organic compounds (Chen et al., 2023c; He et al., 2021). Fenton oxidation is highly effective in breaking down lipids, lignin/carboxylic acid-rich alicyclic molecule organics, unsaturated hydrocarbons, carbohydrates, and aromatic compounds, but less efficient in breaking down amine organics (He et al., 2021). Consequently, the choice of treatment technology is generally closely related to the type of organic compound because different oxidation methods produce different effects on certain organic substances (Zhang et al., 2023).
已经研究了许多从高盐有机废水中去除有机化合物的技术,包括生物处理(Tang 等人,2021 年)、吸附方法(Liu 等人,2021 年)和高级氧化过程 (AOP)(Lester 等人,2015 年)。AOP是处理高盐有机废水的常用化学方法,因为强氧化剂具有化学分解有机污染物的强大能力(Wang和Bai,2017;Xu 等人,2018 年)。AOP的例子包括过硫酸盐氧化(PS)(Wei等人,2018),臭氧氧化(Bourgin等人,2017;Fu 等人,2019 年;Jung等人,2017)和Fenton氧化(Grčić等人,2009)。在这些技术中,臭氧是一种非常有效的技术,用于处理高盐有机废水中顽固的有机污染物,因为它具有强大的氧化电位和对富电子部分的高反应性(Chen等人,2019)。臭氧迅速与苯环、C C 和 C double bond double bond O 反应,产生酚、醛、酮和小分子有机酸等中间产物(Wang 等人,2020 年)。单独对高盐有机废水进行臭氧处理通常会导致低矿化率,这主要是由于中间产物与臭氧的反应性有限(Fu 等人,2019 年)。因此,臭氧通常与其他处理技术相结合,以提高高盐有机废水中有机化合物的整体矿化率(Itzel等人,2020)。在这些处理技术中,芬顿氧化因其能够产生大量羟基自由基而被广泛应用(Chen等人,2023b)。 与臭氧氧化相比,芬顿工艺中产生的羟基自由基表现出有机化合物的非选择性氧化(Chen等人,2023c;He 等人,2021 年)。芬顿氧化在分解脂质、富含木质素/羧酸的脂环分子有机物、不饱和烃、碳水化合物和芳香族化合物方面非常有效,但在分解胺有机物方面效率较低(He 等人,2021 年)。因此,处理技术的选择通常与有机化合物的类型密切相关,因为不同的氧化方法对某些有机物质产生不同的效果(Zhang等人,2023)。

In practical applications, the removal of Dissolved Organic Matter (DOM) holds significant importance (Zhang et al., 2019). However, it is imperative to precisely discern the specific nature of DOM to comprehensively comprehend and enhance the removal procedures. Prior research predominantly relied on three-dimensional excited emission (3D-EEM) spectroscopy and gas chromatography–mass spectrometry (GC–MS) for the characterization of organics in organic wastewater. Nevertheless, acquiring a comprehensive understanding of organic matter degradation and transformation necessitates a multifaceted approach, as a single characterization method may not provide a holistic perspective on the analysis of organic matter. To elucidate the alterations in functional groups during the DOM transformation process, a combination of two-dimensional correlation spectroscopy (2D-COS) and Fourier-transform infrared spectroscopy (FTIR) was employed to resolve the sequential changes in DOM structure (Zhao et al., 2023). Furthermore, Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has proven to be an effective method for the molecular-level identification of DOM (He et al., 2023). This analytical technique provides additional information, such as molecular formulas and the relative abundance of various substances. Therefore, the utilization of FT-ICR MS enables a more comprehensive analysis of DOM in wastewater. To unveil the transformation characteristics of DOM in APU wastewater from diverse perspectives, we integrated various analytical techniques, including GC–MS, EEM, 2D-COS-FTIR, and FT-ICR MS.
在实际应用中,去除溶解有机物(DOM)具有重要意义(Zhang等人,2019)。然而,必须准确辨别DOM的具体性质,才能全面理解和完善删除程序。以前的研究主要依靠三维激发发射 (3D-EEM) 光谱和气相色谱-质谱 (GC-MS) 来表征有机废水中的有机物。然而,要全面了解有机物的降解和转化,需要采取多方面的方法,因为单一的表征方法可能无法为有机物的分析提供整体视角。为了阐明DOM转化过程中官能团的变化,采用二维相关光谱(2D-COS)和傅里叶变换红外光谱(FTIR)相结合来解决DOM结构的顺序变化(Zhao et al., 2023)。此外,傅里叶变换离子回旋共振质谱法(FT-ICR MS)已被证明是分子水平鉴定DOM的有效方法(He等人,2023)。这种分析技术提供了额外的信息,例如分子式和各种物质的相对丰度。因此,利用 FT-ICR MS 可以更全面地分析废水中的 DOM。为了从不同角度揭示DOM在APU废水中的转化特性,我们整合了各种分析技术,包括GC-MS、EEM、2D-COS-FTIR和FT-ICR MS。

Currently, there is a lack of comprehensive analysis regarding the transformation of organic compounds in the APU wastewater treatment process, and an effective oxidative treatment method for APU wastewater has yet to be developed. Therefore, this study introduces, for the first time, an efficient APU wastewater treatment approach while analyzing the changes during the oxidation of organic substances. Employing various analytical techniques, we have elucidated the degradation and transformation of DOM from multiple perspectives, including its types, functional groups, molecular compositions, and fluorescence properties. This research effectively addresses the challenges associated with APU wastewater treatment, advances the application of Cansolv technology in SO2 purification from exhaust gases, and contributes to mitigating atmospheric pollution during the purification process.
目前,对APU废水处理过程中有机化合物的转化缺乏综合分析,APU废水的有效氧化处理方法尚待开发。因此,本研究首次介绍了一种高效的APU废水处理方法,同时分析了有机物氧化过程中的变化。我们运用各种分析技术,从DOM的类型、官能团、分子组成和荧光特性等多个角度阐明了DOM的降解和转化过程。该研究有效地解决了与APU废水处理相关的挑战,推动了康世富技术在废气SO 2 净化中的应用,并有助于减轻净化过程中的大气污染。

2. Materials and methods 2.材料与方法

2.1. Collection of water samples and analysis of water quality
2.1. 水样采集和水质分析

The APU wastewater is collected from the Cansolv exhaust system at a natural gas purification plant. The wastewater appeared yellow-brown and contained a small percentage of precipitates. Before each experiment, the containers were shaken for 3 min. Onsite pH and conductivity measurements were performed, and other parameters were measured in the laboratory (Text S1 presents the test method).

2.2. Experimental procedures
2.2. 实验程序

Ozone was introduced into a 100 mL sample within a glass reactor using an ozone generator (QJ-8006k, Quantitative State Ozone Technology Co., Ltd., Guangzhou, China), simultaneously injecting a specific quantity of 30 % H2O2 for the reaction. Any remaining unreacted ozone was absorbed using a KI solution. Following the ozone reaction, the solution underwent Fenton oxidation. The Fenton process was performed in batch mode using a glass reactor (VLiquid = 100 mL) and was maintained at an ambient temperature. The pH of the initial solution was adjusted by adding H2SO4 (1 mol/L) or NaOH (1 mol/L), and FeSO4-7H2O and 30 % H2O2 (Kelong, Chengdu, China) were added in the appropriate proportions. The reaction was terminated by increasing the pH to 10.0, which allowed the solution to settle for 30 min. The supernatant was filtered through a nylon membrane with a pore size of 0.45 μm and quenched by adding 1.0 M sodium thiosulfate.
使用臭氧发生器(QJ-8006k,Quantitative State Ozone Technology Co., Ltd.,Guangzhou,China)将臭氧引入玻璃反应器内的 100 mL 样品中,同时注入特定量的 30 % H 2 O 2 进行反应。使用KI溶液吸收任何剩余的未反应臭氧。在臭氧反应之后,溶液发生了芬顿氧化。Fenton工艺使用玻璃反应器(V Liquid = 100 mL)以间歇模式进行,并保持在环境温度下。通过加入H SO(1 mol/L)或NaOH(1 mol/L)调节初始溶液的pH值,并按适当比例加入FeSO 4 4 -7H 2 O和30 % H 2 2 O 2 (Kelong,Chengdu,China)。通过将pH值提高到10.0来终止反应,使溶液沉淀30分钟。上清液通过孔径为0.45μm的尼龙膜过滤,加入1.0M硫代硫酸钠淬灭。

2.3. Analytical methods 2.3. 分析方法

2.3.1. GC–MS analysis 2.3.1. GC-MS分析

The most commonly employed protocol for organic compound analysis in wastewater is GC–MS (Luek and Gonsior, 2017). Prior to GC–MS analysis, the wastewater samples were subjected to liquid-liquid extraction using dichloromethane (HPLC-grade). The initial wastewater sample (100 mL) was acidified to a pH of 2 using hydrochloric acid. Extraction was performed in three stages using 30 mL dichloromethane. Subsequently, the extract was evaporated to approximately 10 mL using a rotary evaporator, enriched with a nitrogen concentrator, and fixed to 1 mL using dichloromethane. The extract was then analyzed using a GC–MS system (7890A, Agilent Technologies, USA) equipped with an Agilent Technologies 5975c mass spectrometer and an HP-5MS capillary column (30 m in length, 0.25 mm inner diameter, 0.25 mm film thickness). A detailed description of the equipment is provided in Text S2 of the Appendix.
废水中有机化合物分析最常用的方案是GC-MS(Luek和Gonsior,2017)。在进行GC-MS分析之前,使用二氯甲烷(HPLC级)对废水样品进行液-液萃取。使用盐酸将初始废水样品(100 mL)酸化至pH值为2。使用 30 mL 二氯甲烷分三个阶段进行提取。随后,使用旋转蒸发器将提取物蒸发至约10 mL,用氮气浓缩器富集,并使用二氯甲烷固定至1 mL。然后使用配备 Agilent Technologies 5975c 质谱仪和 HP-5MS 毛细管柱(长 30 m、内径 0.25 mm、膜厚 0.25 mm)的 GC-MS 系统(7890A,Agilent Technologies,USA)分析提取物。附录的文本 S2 中提供了设备的详细说明。

2.3.2. Fluorescence measurements and EEM-PARAFAC modeling
2.3.2. 荧光测量和EEM-PARAFAC建模

Water samples were analyzed using a HORIBA Scientific spectrofluorometer equipped with a xenon arc lamp as the excitation source and a synaptic CCD detector (Horiba Jobin Yvon). Excitation wavelengths ranged from 240 to 500 nm at 10 nm intervals, and emission wavelengths ranged from 280 to 600 nm at 10 nm intervals. For the correction of internal filtering effects, the sample EEM spectra were corrected using an excitation-emission correction factor. Raman scattering was minimized by subtracting the blank. The fluorescence intensity was standardized by normalizing to the area under the water Raman peak (ex 350 nm/em 371–428 nm), and the results were expressed in Raman units (RU) (Stedmon et al., 2003). EEM data processing and parallel factor analysis (PARAFAC) modeling were conducted using the MATLAB (MATLAB R2019a, MathWorks), Dreem (version 0.2.0), and N-way (version 3.30) toolboxes. PARAFAC reduces the EEM dataset to an array of individual fluorescence components and residuals using an alternating least squares algorithm that minimizes the residual sum of squares in the trilinear model and estimates the potential EEM spectrum. The data processing is described in Text S3.
使用配备氙弧灯作为激发源的HORIBA Scientific荧光光谱仪和突触CCD探测器(Horiba Jobin Yvon)对水样进行分析。激发波长范围为240至500 nm,间隔为10 nm,发射波长范围为280至600 nm,间隔为10 nm。为了校正内部滤波效应,使用激发-发射校正因子对样品EEM光谱进行校正。通过减去空白,拉曼散射最小化。通过归一化到水下拉曼峰(ex 350 nm/em 371-428 nm)下的区域来标准化荧光强度,结果以拉曼单位(RU)表示(Stedmon等人,2003)。使用 MATLAB(MATLAB R2019a、MathWorks)、Dreem(版本 0.2.0)和 N-way(版本 3.30)工具箱进行 EEM 数据处理和并行因子分析 (PARAFAC) 建模。PARAFAC 使用交替最小二乘法将 EEM 数据集简化为单个荧光分量和残差的数组,该算法最小化三线性模型中的残差平方和并估计潜在的 EEM 光谱。文本 S3 中描述了数据处理。

2.3.3. FTIR and 2D-COS 2.3.3. FTIR 和 2D-COS

The functional groups of the samples were characterized using FTIR spectroscopy (Nicolet 382, USA) in transmission mode. After filtration, the samples were freeze-dried to produce a large amount of powder, which was mixed with 300 mg potassium bromide (KBr). The mixture was compressed using a hydraulic press (20 psi). The spectral range of all samples was measured from 4000 to 400 cm−1, with a spectral resolution of 4 cm−1. A KBr tray was scanned as a background. Each spectrum was normalized by the sum of all measured areas (from 4000 to 400 cm−1) and multiplied by a factor of 1000. Baseline correction was performed according to a protocol in the literature. Information on 2D COS is in Text S4.
在透射模式下使用FTIR光谱(Nicolet 382,USA)表征样品的官能团。过滤后,将样品冷冻干燥,生成大量粉末,与300mg溴化钾(KBr)混合。使用液压机 (20 psi) 压缩混合物。所有样品的光谱范围为4000至400厘米,光谱分辨率为4厘米 −1 −1 。将 KBr 托盘扫描为背景。每个光谱由所有测量区域(从4000到400厘米 −1 )的总和归一化,并乘以系数1000。根据文献中的方案进行基线校正。有关 2D COS 的信息在文本 S4 中。

2.3.4. FT-ICRMS analysis 2.3.4. FT-ICRMS分析

An appropriate volume of raw water sample (specific volume determined based on the TOC content in the water sample) was collected and passed through a 0.22 μm filter membrane to remove particulate matter and other impurities. Subsequently, the water sample was acidified with hydrochloric acid, gradually adding drops of hydrochloric acid until the pH of the water sample was adjusted to 2. Solid-phase extraction was performed on the DOM in the water samples using an Agilent Bond Elut PPL column (500 mg, 6 mL). The organic molecular composition was analyzed using a 9.4 T FT-ICR MS (Solarix, Bruker Daltonik, Bremen, Germany) operating in negative mode with an electrospray ionization source. The raw data were processed using Bruker Daltonics data Analysis 4.0 and MATLAB routines, with mass spectra collected in the range of 100–1000 Da. FT-ICR MS data were visualized using Van Krevelen (VK) plots based on established formulas. Typically, VK plots have O/C as the x-axis and H/C as the y-axis. The VK space was divided into seven regions based on the elemental ratios (Liu et al., 2020) (Fig. S1).
收集适量的原水样品(根据水样中的TOC含量确定的比体积),并通过0.22μm滤膜去除颗粒物和其他杂质。随后,将水样用盐酸酸化,逐渐加入盐酸滴,直到水样的pH值调节到2。使用 Agilent Bond 洗脱 PPL 色谱柱 (500 mg, 6 mL) 对水样中的 DOM 进行固相萃取。使用9.4 T FT-ICR MS(Solarix,Bruker Daltonik,Bremen,Germany)在负模式下使用电喷雾电离源分析有机分子组成。使用布鲁克道尔顿数据分析4.0和MATLAB例程处理原始数据,收集100–1000 Da范围内的质谱图。通常,VK 图的 O/C 为 x 轴,H/C 为 y 轴。根据元素比例将VK空间分为七个区域(Liu等人,2020)(图S1)。

3. Results and discussion
3. 结果与讨论

3.1. Analysis of organic matter in raw water
3.1. 原水中有机质的分析

Studies have demonstrated that GC–MS analysis enables the detection of a wide range of organic compounds in high salt organic wastewater, encompassing various classes such as olefins, alkanes, aliphatics, aromatics, halogenated compounds, amines, and esters (Luek and Gonsior, 2017). Fig. 1 illustrates the significant presence of acid esters (containing Cdouble bondC double bonds) and alkane organic compounds in the untreated water from the APU source, along with minor quantities of phenolic and halogenated hydrocarbon organic compounds, in the untreated water from the APU source. Moreover, according to Table S2, the wastewater exists within a highly alkaline environment, resulting in the deprotonation of functional groups in the majority of organic compounds (Bourgin et al., 2017). Most organic compounds in the system undergo deprotonation, leading to the formation of negatively charged functional groups and an increased degree of electron enrichment (Jung et al., 2017). Because of the oxidative characteristics of ozone (selective oxidation), electron-rich organic compounds can react more rapidly with ozone. The selective oxidation of ozone enhances the overall saturation of the system, reducing the reactivity of organic compounds with ozone, resulting in decreasing mineralization rates. By contrast, the hydroxyl radicals (radical dotOH) and sulfate radicals (SO4radical dot) generated by the Fenton and PS processes, respectively, exhibit non-selective oxidation properties, demonstrating the effective removal of the less-reactive organic compounds.
研究表明,GC-MS分析能够检测高盐有机废水中的各种有机化合物,包括烯烃,烷烃,脂肪族,芳烃,卤代化合物,胺和酯等各种类别(Luek和Gonsior,2017)。图1显示了APU源的未处理水中大量存在酸酯(含有CC double bond 双键)和烷烃有机化合物,以及APU源未处理的水中少量的酚类和卤代烃类有机化合物。此外,根据表S2,废水存在于高碱性环境中,导致大多数有机化合物中官能团的去质子化(Bourgin等人,2017)。系统中的大多数有机化合物发生去质子化,导致带负电荷的官能团的形成和电子富集程度的增加(Jung等人,2017)。由于臭氧的氧化特性(选择性氧化),富含电子的有机化合物可以更快地与臭氧发生反应。臭氧的选择性氧化增强了系统的整体饱和度,降低了有机化合物与臭氧的反应性,导致矿化速率降低。相比之下,Fenton和PS工艺产生的羟基自由基( radical dot OH)和硫酸根自由基(SO 4 radical dot )分别表现出非选择性氧化特性,证明了对反应性较低的有机化合物的有效去除。

Fig. 1
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Fig. 1. Raw Water Organic Matter. (a) Proportion of various organic substances; (b) GC–MS spectrogram Data.
图 1.原水有机质。(a) 各种有机物质的比例;(b) GC-MS光谱图数据。

3.2. Optimal oxidation process
3.2. 最佳氧化工艺

The removal efficiencies of organic compounds in the APU wastewater for various processes, including O3/H2O2-Fenton (indicating the sequence of O3/H2O2 reaction followed by Fenton reaction), Fenton, O3/H2O2, PS, O3/H2O2-PS, and Fenton-O3/H2O2 (indicating the sequence of Fenton reaction followed by O3/H2O2 reaction), were 67 %, 38 %, 20 %, 18 %, 30 %, and 44 %, respectively (Fig. S2). However, the removal efficacy of the Fenton-O3/H2O2 oxidation process was unsatisfactory. While the hydroxyl radicals generated during the Fenton process exhibit non-selective oxidation of pollutants, their reaction rate with unsaturated bonds (1010 M−1 s−1) is faster than that with saturated bonds (such as Csingle bondH bonds). This process leads to the consumption of free radicals through electrophilic addition reactions with unsaturated bonds, resulting in an insufficient quantity of free radicals for mineralizing saturated organic matter (von Gunten, 2003). In contrast, the combined O3/H2O2 and Fenton processes (i.e., O3/H2O2-Fenton) demonstrated optimal DOM treatment efficiencies (Fig. S2). Ozone, as a primary oxidant, cleaves the majority of unsaturated long-chain organic compounds (e.g., esters containing Cdouble bondC double bonds) into short-chain intermediates. Fenton, as a secondary oxidation step, effectively mineralizes short-chain intermediates. Therefore, the O3/H2O2-Fenton oxidation process was considered for APU wastewater treatment in the following study.
O /H O -Fenton(表示 O /H 2 O 反应后 Fenton 反应的顺序)、Fenton、O /H O 、PS、O /H O -PS 和 Fenton-O /H O (表示 O 2 3 /H O 反应后 O /H 2 2 2 2 2 2 O 2 3 2 3 2 3 2 3 3 的顺序)等各种工艺中有机化合物的去除效率反应),分别为67%、38%、20%、18%、30%和44%(图S2)。但Fenton-O 3 /H 2 O 2 氧化工艺的去除效果不尽如人意。虽然芬顿过程中产生的羟基自由基表现出污染物的非选择性氧化,但它们与不饱和键的反应速率(10 10 M −1 s −1 )比与饱和键(如C.H single bond 键)的反应速率快。这个过程导致自由基通过与不饱和键的亲电加成反应消耗,导致饱和有机物矿化的自由基数量不足(von Gunten,2003)。相比之下,O 3 /H O 和 Fenton 工艺(即 O /H 2 2 O 2 2 3 -Fenton)的组合表现出最佳的 DOM 处理效率(图 S2)。臭氧作为主要氧化剂,将大多数不饱和长链有机化合物(例如,含有C、C double bond 、双键的酯)裂解成短链中间体。芬顿作为二次氧化步骤,可有效地矿化短链中间体。 因此,在下文研究中,考虑了 O /H 2 O 2 3 -Fenton 氧化工艺进行 APU 废水处理。

3.3. Oxidation process parameter optimization
3.3. 氧化工艺参数优化

3.3.1. O3/H2O2 process parameter optimization
3.3.1. O /H 2 O 2 3 工艺参数优化

In the ozone reaction process, parameters such as reaction time, ozone concentration, pH, and hydrogen peroxide dosage are critical (Bourgin et al., 2017). Fig. S3(a) depicts the correlation between the reaction time of ozone and the efficacy of TOC removal. The TOC removal rate reached a plateau of 20 % after 60 min. This result can be attributed to the transformation of unsaturated long-chain organic compounds in the system into intermediate products with diminished ozone reactivity over a certain reaction duration (Brunet et al., 1984). As illustrated in Fig. S3(b), the TOC removal rate exhibited a rapid escalation within the ozone concentration range of 0–6 g/h, owing to the increased likelihood of contact between ozone and unsaturated organic compounds. However, a stabilization trend was observed after reaching an ozone concentration of 6 g/h. This result can be attributed to the escalation in the production rate of intermediates, specifically small-molecule acids, as the ozone concentration increases. Consequently, there was a rapid decrease in the pH of the solution, which hampered the autolytic decomposition of ozone into hydroxyl radicals (Wang et al., 2021a). Ozone can be classified into two reaction modes according to the acidity and alkalinity of the system. More hydroxyl radicals are generated under alkaline conditions, which can effectively increase the Organic Compounds mineralization rate (Fig. S3(c)). To increase the number of hydroxyl radicals in the ozone process, the addition of H2O2 can accelerate ozone decomposition. Optimal TOC removal was achieved when the H2O2 dosage was 1 mL/100 mL (Fig. S3(d)). When the dosage of hydrogen peroxide exceeded 1 mL/100 mL, the TOC removal rate decreased to 13 %. H2O2 acts as a catalyst to promote ozone decomposition; however, excessive doses also act as quenching agents for free radicals.
在臭氧反应过程中,反应时间,臭氧浓度,pH值和过氧化氢剂量等参数至关重要(Bourgin等人,2017)。无花果。S3(a)描述了臭氧反应时间与TOC去除功效之间的相关性。60 min后,TOC去除率达到20%的稳定期。这一结果可归因于系统中不饱和长链有机化合物转化为在一定反应持续时间内臭氧反应性降低的中间产物(Brunet等人,1984)。如图所示。S3(b)中,由于臭氧与不饱和有机化合物接触的可能性增加,TOC去除率在0-6 g/h的臭氧浓度范围内迅速上升。然而,在臭氧浓度达到6克/小时后,观察到稳定趋势。这一结果可归因于随着臭氧浓度的增加,中间体,特别是小分子酸的生产速率增加。因此,溶液的pH值迅速降低,这阻碍了臭氧的自溶分解为羟基自由基(Wang等人,2021a)。臭氧根据体系的酸度和碱度可分为两种反应模式。在碱性条件下产生更多的羟基自由基,可有效提高有机化合物的矿化速率[图S3(c)]。为了增加臭氧过程中羟基自由基的数量,添加H 2 O 2 可以加速臭氧分解。当H 2 O 2 剂量为1 mL/100 mL时,实现了最佳的TOC去除[图S3(d)]。当过氧化氢用量超过1 mL/100 mL时,TOC去除率降至13%。 H 2 O 2 作为催化剂促进臭氧分解;然而,过量的剂量也可以作为自由基的猝灭剂。

3.3.2. Fenton process parameter optimization
3.3.2. Fenton工艺参数优化

The effects of H2O2/TOC, H2O2/Fe2+, pH, and reaction time on the Fenton oxidation were investigated separately. In the Fenton process, the integration of hydrogen peroxide plays a crucial role in achieving optimal degradation of organic compounds (Ilhan et al., 2017). As illustrated in Fig. 2(a), the total organic carbon (TOC) removal efficiency demonstrated a progressive enhancement as the H2O2/TOC ratio increased from 0.5 to 4, reaching a maximum removal rate of 67 %. However, upon further increasing the ratio to 6, the TOC removal rate exhibited a decline to 65 %. Typically, an increase in the amount of H2O2 produces many hydroxyl radicals (radical dotOH), promoting the degradation of organic compounds. However, excessive H2O2 may react with radical dotOH, inhibiting further improvements in the organic removal efficiency (Bautista et al., 2014). Samet et al. found that the optimal H2O2/Fe2+ ratio plays a pivotal role in the degradation of organic compounds by using the Fenton method, with lower or higher ratios potentially impeding oxidation reactions (Samet et al., 2011). As shown in Fig. 2(b), an increase in the H2O2/Fe2+ ratio from 4 to 10 led to an increase in the TOC removal efficiency from 63 % to 68 %. However, when the ratio was further increased to 18, the TOC removal efficiency decreased to 55 %. As a pivotal catalyst in the Fenton system, an optimal quantity of Fe2+ enhances the production of radical dotOH radicals, whereas an excessive amount of Fe2+ scavenges the generated radical dotOH radicals, resulting in a reduction in mineralization efficiency. The H2O2/Fe2+ ratio in the range of 1:1–100:1, as reported by Samet et al., is consistent with the results of this study (Samet et al., 2011). In the Fenton process, the solution pH has a significant influence on the oxidation of organic pollutants. As shown in Fig. 2(c), when the pH increased to 4, the TOC removal rate reached 69 %. The optimal pH value for the Fenton process is between 2.0 and 4.5 (Bello et al., 2019). High pH values can suppress the formation of radical dotOH by inhibiting the decomposition of H2O2, promoting the self-decay of H2O2, and causing the catalytic deactivation of Fe(II) by forming solid iron hydroxide. By contrast, low pH values can slow the reaction of Fe(II) or Fe(III) with H2O2 and enhance the removal of radical dotOH by H+. Optimizing the Fenton reaction time can effectively reduce operational costs. Fig. 2(d) shows a clear correlation between the TOC removal efficiency and reaction time. Specifically, the TOC removal efficiency increases to 64 % when the reaction time is increased from 0 to 70 min. However, extending the reaction time beyond this point did not further improve the TOC removal rate.
分别研究了H O /TOC、H 2 2 O 2 2 /Fe 2+ 、pH和反应时间对Fenton氧化的影响。在Fenton工艺中,过氧化氢的整合在实现有机化合物的最佳降解方面起着至关重要的作用(Ilhan等人,2017)。如图2(a)所示,随着H 2 O 2 /TOC比从0.5增加到4,总有机碳(TOC)去除效率逐渐提高,最大去除率达到67%。然而,当该比率进一步提高到6时,TOC去除率下降到65%。通常,H 2 O 2 量的增加会产生许多羟基自由基( radical dot OH),促进有机化合物的降解。然而,过量的H 2 O 2 可能与 radical dot OH反应,抑制有机物去除效率的进一步提高(Bautista等人,2014)。Samet等人发现,通过使用Fenton方法,最佳的H 2 O 2 / Fe 2+ 比在有机化合物的降解中起着关键作用,较低或较高的比率可能会阻碍氧化反应(Samet等人,2011)。如图2(b)所示,H 2 O 2 /Fe 2+ 比从4增加到10,导致TOC去除效率从63%提高到68%。然而,当该比率进一步增加到18时,TOC去除效率下降到55%。作为芬顿体系中的关键催化剂,最佳量的铁 2+ 会增强OH自由基的产生,而过量的 radical dot Fe会 2+ 清除产生的 radical dot OH自由基,从而导致矿化效率降低。 Samet等人报告的H 2 O 2 / Fe 2+ 比在1:1-100:1范围内,与本研究的结果一致(Samet等人,2011)。在芬顿工艺中,溶液的pH值对有机污染物的氧化有显著影响。如图2(c)所示,当pH值增加到4时,TOC去除率达到69%。Fenton工艺的最佳pH值在2.0至4.5之间(Bello等人,2019)。高pH值可抑制 radical dot OH的形成,抑制H O的分解,促进H 2 2 O 2 2 的自衰,并通过形成固体氢氧化铁引起Fe(II)的催化失活。相反,低pH值可以减缓Fe(II)或Fe(III)与H O 2 的反应,并增强H 2 + 对OH的 radical dot 去除。优化Fenton反应时间可以有效降低运营成本。图2(d)显示了TOC去除效率与反应时间之间的明显相关性。具体而言,当反应时间从0增加到70 min时,TOC去除效率提高到64%。然而,将反应时间延长到该点以上并不能进一步提高TOC去除率。

Fig. 2
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Fig. 2. Fenton single-factor experiment. (a) Effect of H2O2/TOC ratio on Fenton's removal of TOC from APU. Reaction conditions: initial pH is 3, H2O2/Fe2+ is 8, and reaction time is 60 min; (b) Effect of H2O2/Fe2+ ratio on Fenton's removal of TOC from APU. Reaction conditions: initial pH is 3, H2O2/TOC is 4, and reaction time is 60 min; (c) The effect of pH on Fenton's removal of TOC from APU. Reaction conditions: H2O2/Fe2+ is 10, H2O2/TOC is 4, and the reaction time is 60 min; (d) Effect of reaction time on Fenton's removal of TOC from APU. Reaction conditions: H2O2/Fe2+ is 10, H2O2/TOC is 4, and pH is 3.
图 2.芬顿单因素实验。(a) H 2 O 2 /TOC 比率对 Fenton 从 APU 中去除 TOC 的影响。反应条件:初始pH为3,HO 2 2 /Fe 2+ 为8,反应时间60 min;(b) H 2 O 2 /Fe 2+ 比对 Fenton 从 APU 中去除 TOC 的影响。反应条件:初始pH为 2 3,HO 2 /TOC为4,反应时间60 min;(c) pH 值对 Fenton 从 APU 中去除 TOC 的影响。反应条件:H O /Fe为10,H 2 2 O 2 2 /TOC 为4,反应时间 2+ 为60 min;(d) 反应时间对Fenton从APU中去除TOC的影响。反应条件:H O /Fe为10,H 2 2 O 2 2 /TOC 2+ 为4,pH为3。

3.4. GC–MS characterization of DOM during oxidation process
3.4. 氧化过程中DOM的GC-MS表征

For investigating the changes in the types of organic compounds during the oxidation process, water samples before and after the O3/H2O2-Fenton treatment were measured using GC–MS (Fig. 3(a)). As shown in Fig. 3(b), the main categories of compounds included long-chain esters (containing Cdouble bondC), long-chain alkanes, halogenated hydrocarbons, and phenolic organic compounds. The degradation effect of ozone on alkanes is not ideal, owing to the higher bond energy of the Csingle bondH bond (Wang et al., 2021b). By contrast, esters and phenolic organic compounds in the system can be effectively broken down into more stable intermediates after the ozone reaction (electrophilic substitution and ring addition reactions) (Tizaoui et al., 2007). Additionally, Cdouble bondC bonds undergo Criegee ozonolysis with ozone (Fig. S4), producing aldehydes and ketones with lower reactivity toward ozone (Criegee, 1975). After the Fenton reaction, the intermediates in the wastewater were effectively mineralized. The contents of alkanes and phenolic organic compounds decreased significantly, and aldehyde organic compounds were completely degraded. Notably, carboxylic acid organic compounds were detected after the Fenton reaction (Chen et al., 2023a). In summary, the GC–MS analysis revealed a substantial decrease in the concentration of esters following O3/H2O2 treatment, whereas the reduction in saturated organic compounds was relatively minor. Subsequent Fenton oxidation, however, can result in a notable reduction in saturated organic compounds.
为了研究氧化过程中有机化合物类型的变化,使用GC-MS测量了O /H 2 O 2 3 -Fenton处理前后的水样[图3(a)]。如图3(b)所示,主要类别的化合物包括长链酯(含C C double bond )、长链烷烃、卤代烃和酚类有机化合物。臭氧对烷烃的降解作用并不理想,因为 C single bond H 键的键能较高(Wang 等人,2021b)。相比之下,系统中的酯类和酚类有机化合物在臭氧反应(亲电取代和环加成反应)后可以有效地分解成更稳定的中间体(Tizaoui等人,2007)。此外,C C double bond 键与臭氧发生Criegee臭氧分解(图S4),产生对臭氧反应性较低的醛和酮(Criegee,1975)。芬顿反应后,废水中的中间体被有效矿化。烷烃和酚类有机化合物含量明显下降,醛类有机化合物完全降解。值得注意的是,在芬顿反应后检测到羧酸有机化合物(Chen等人,2023a)。综上所述,GC-MS分析显示,O 3 /H 2 O 2 处理后酯类浓度显著降低,而饱和有机化合物的降低相对较小。然而,随后的芬顿氧化可导致饱和有机化合物的显着减少。

Fig. 3
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Fig. 3. (a) Organic components in APU were analyzed by GC–MS at different oxidation stages; (b) Changes in the organic composition of oxidation treatments.
图 3.(a) 采用GC-MS方法分析APU中不同氧化阶段的有机成分;(b) 氧化处理的有机成分的变化。

3.5. EEM spectra characterizations of DOM
3.5. DOM的EEM谱图表征

EEM spectra offer valuable information regarding the fingerprint characteristics of DOM. Fig. 4(a–b) illustrates the changes in the EEM spectra of DOM at different oxidation stages. After ozonation, the fluorescence intensity of the humic substances decreased significantly (Fig. 4(a)). This decrease in intensity suggests the depletion or alteration of aromatic structures and an increase in electron-absorbing groups within aromatic compounds. By contrast, the intensity of the saprotrophic species almost disappeared after Fenton oxidation (Fig. 4(b)). In the further investigation of the fluorescent substances, the complex fluorescent matrix was decomposed into individual fluorescent components using PARAFAC. Fig. 4(c) shows four distinct fluorescent components (C1–C4), all of which were cross-validated with the OpenFluor database, achieving a Tucker's correlation coefficient exceeding 0.95 (Table S2). C1 (Ex/Em = 350/433 nm) was identified as a terrestrial humic-like component. C2, with a peak at Ex/Em = 330/391 nm, was identified as a marine humic-like component. C3, with a peak at Ex/Em = 380/482 nm, was identified as xanthate. C4 (Ex/Em = 380/482 nm) was identified as a marine humic substance. The relative contents of these components are often expressed as their respective maximum fluorescence intensities (Fmax). As illustrated in Fig. 4(d), C1 and C2 exhibited the highest abundance among the four components, C3 and C4 exhibited less abundance. Ozone demonstrated a certain extent of intensity reduction for C3 and C4; however, Fenton oxidation was more effective than ozone in removing recalcitrant organic compounds. Ozone exhibited relatively high removal rates for C1 and C2, which can be attributed to the presence of a greater number of electron-rich functional groups in these compounds. In addition, C1–C4 were positively correlated with the humification index, indicating the dominant contribution of all components to the degree of humification. Notably, the negative correlation between C1 and the endogenous index indicates a contribution from external inputs, whereas C4 has an internal origin (Fig. 4(e)). Overall, the O3/H2O2-Fenton oxidation process demonstrated favorable removal rates for all four components to varying degrees.
图4(a–b)说明了DOM在不同氧化阶段的EEM谱的变化。臭氧化后,腐殖质的荧光强度显著降低[图4(a)]。这种强度的降低表明芳香族结构的消耗或改变以及芳香族化合物中吸电子基团的增加。相比之下,腐生物质的强度在芬顿氧化后几乎消失了[图4(b)]。在对荧光物质的进一步研究中,使用PARAFAC将复杂的荧光基质分解成单独的荧光成分。图4(c)显示了四种不同的荧光成分(C1-C4),所有这些成分都与OpenFluor数据库进行了交叉验证,实现了超过0.95的塔克相关系数(表S2)。C1 (Ex/Em = 350/433 nm) 被鉴定为陆地腐殖质样成分。C2 的峰值位于 Ex/Em = 330/391 nm,被鉴定为海洋腐殖质样成分。C3 的峰在 Ex/Em = 380/482 nm 处被鉴定为黄原酸盐。C4 (Ex/Em = 380/482 nm) 被鉴定为海洋腐殖质。这些组分的相对含量通常表示为它们各自的最大荧光强度(F max )。如图4(d)所示,C1和C2在4种组分中表现出最高的丰度,C3和C4表现出的丰度较低。臭氧对C3和C4表现出一定程度的强度降低;然而,芬顿氧化在去除顽固性有机化合物方面比臭氧更有效。 臭氧对C1和C2的去除率相对较高,这可归因于这些化合物中存在更多的富电子官能团。此外,C1–C4与腐殖化指数呈正相关,表明各组分对腐殖化程度的贡献占主导地位。值得注意的是,C1与内源指数之间的负相关表明来自外部输入的贡献,而C4则具有内部来源[图4(e)]。总体而言,O /H 2 O 3 2 -Fenton氧化过程在不同程度上对所有4种组分都表现出良好的去除率。

Fig. 4
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Fig. 4. (a) Changes in fluorescence in the O3/H2O2 phase; (b) Changes in fluorescence in the Fenton phase; (c) Fluorescence excitation-emission matrix contours of four components identified by PARAFAC analysis; (d) shows the composition ratios of the four separated components using PARAFAC modeling (where A represents raw water, B represents treated O3/H2O2, and C represents treated O3/H2O2-Fenton); (e) Correlation analysis based on EEM (asterisk (*) indicates significance level, “*” indicates p < 0.05, “**” indicates p < 0.01, “***” indicates p < 0.001).
图 4.(a) O/H 2 O 2 3 相荧光的变化;(b) Fenton相荧光的变化;(c) 通过PARAFAC分析确定的四个成分的荧光激发-发射矩阵轮廓;(d)使用PARAFAC模型显示了四种分离组分的组成比例(其中A代表原水,B代表处理过的O /H O,C代表处理过的O /H 2 O 2 2 3 3 -Fenton); 2 (e) 基于EEM的相关性分析(星号(*)表示显著性水平,“*”表示p < 0.05,“**”表示p < 0.01,“***”表示p < 0.001)。

3.6. Trend in conversion of DOM functional groups
3.6. DOM官能团的转换趋势

FTIR analysis was conducted on water samples collected at various oxidation stages to discern the transformations of the functional groups during the oxidation process (Fig. S5). However, the presence of diverse functional groups often leads to overlapping infrared peaks, resulting in ambiguous band assignments and complicated analyses. 2DCOS is a promising tool for overcoming this limitation (Chen et al., 2015). Fig. 5 displays the 2DCOS-FTIR of the ozone oxidation and Fenton stages. The synchronous spectra show five major autocorrelation peaks at 1630 cm−1, 1383 cm−1, 1144 cm−1, 970 cm−1, and 620 cm−1 (Fig. 5(a)). The peaks at 1630 cm−1 and 620 cm−1 exhibit the smallest sensitivity. The peaks at 1144 cm−1 and 1383 cm−1 are the most sensitive. The peak at 1630 cm−1 can be attributed to the Cdouble bondC double bond vibration of cis-alkenyl. Notably, the structures of the trans-alkenes were also detected in the GC–MS results. The peaks at 1383 cm−1 and 620 cm−1 were assigned to the single bondOH stretching of phenolic hydroxyl and single bondOH stretching of carboxyl groups, respectively. The autocorrelation peaks at 1144 cm−1 and 970 cm−1 correspond to the ester Csingle bondO stretching of polysaccharides and the out-of-plane vibration of alkenyl Csingle bondH, respectively. The detailed assignment of the frequency bands in the asynchronous mapping and the signs of the cross-peaks are shown in Table S3. According to the Noda sequence rule (Noda and Ozaki, 2004), the order of functional group changes after ozone oxidation follows 1383 (phenolic Osingle bondH) > 1630 (cis-alkenyl Cdouble bondC) > 620 (carboxylic acid-OH) > 1144 (polysaccharide ester Csingle bondO stretching) > 970 (alkene Csingle bondH). The change sequence demonstrated that the phenolic organic compounds reacted with ozone with highest priority in the system, which could be attributed to ozone's electrophilic nature facilitating its reaction with nucleophilic functional groups. The most abundant functional group in wastewater, Cdouble bondC, changed following phenolic hydroxyl groups, owing to the Cdouble bondC double bonds and phenolic hydroxyl groups being electron-rich functional groups. In addition, the change of Csingle bondH in alkenes at the end, which is consistent with the low reactivity of ozone toward saturated carbon chains reported in other studies. Compared with ozone, only four autocorrelation peaks were detected during the Fenton reaction, as shown in Fig. 5(c) and (d). The most significant changes occurred at the peaks of 1144 cm−1 and 620 cm−1, assigned to the Csingle bondO stretching of esters (polysaccharides) and the single bondOH stretching of carboxylic acids, respectively. The detailed distributions of the frequency bands and the symbols of the cross-peaks in the asynchronous mapping are shown in Table S4. Based on the Noda sequence rule, the order of change in the functional groups was deduced as follows: 1144 (polysaccharide Csingle bondO stretching) > 1383 (phenolic Osingle bondH) > 620 (carboxylic acid-OH) > 1705 (aliphatic aldehydes Cdouble bondO). Among these, the transformation of esters and phenols takes precedence in the system. In summary, the results obtained from 2D IR COS and GC–MS analyses provide evidence of the preferential reaction of ozone with unsaturated organic compounds during the oxidation process. Furthermore, the formation of carboxylic acid compounds was observed after Fenton oxidation (Chen et al., 2023a).
对在不同氧化阶段收集的水样进行FTIR分析,以辨别氧化过程中官能团的转变(图S5)。然而,不同官能团的存在通常会导致红外峰重叠,导致波段分配不明确和分析复杂。2DCOS是克服这一限制的有前途的工具(Chen等人,2015)。图5显示了臭氧氧化和Fenton阶段的2DCOS-FTIR。同步光谱显示,在1630 cm、1383 cm −1 、1144 cm、970 cm和620 cm −1 −1 −1 −1 处有五个主要的自相关峰[图5(a)]。1630 cm 和 620 cm −1 −1 处的峰值表现出最小的灵敏度。1144 cm 和 1383 cm −1 −1 的峰值是最敏感的。1630 cm −1 处的峰值可归因于顺式烯基的C C double bond 双键振动。值得注意的是,在GC-MS结果中也检测到了反式烯烃的结构。1383 cm和620 cm −1 −1 处的峰分别用于酚羟基的 single bond OH拉伸和 single bond 羧基的OH拉伸。1144 cm和970 cm −1 −1 处的自相关峰分别对应多糖的酯C O拉伸和烯基C single bond single bond H的面外振动。表S3显示了异步映射中频段的详细分配和交叉峰的符号。 根据野田序列规则(Noda和Ozaki,2004),臭氧氧化后官能团变化的顺序遵循1383(酚类O H)>1630(顺式烯基C C)>620(羧酸-OH)>1144(多糖酯C O single bond 拉伸)>970(烯烃C double bond single bond single bond H)。变化序列表明,酚类有机化合物与臭氧反应的优先级最高,这可归因于臭氧的亲电性质促进了其与亲核官能团的反应。废水中含量最丰富的官能团C double bond C随着酚羟基而变化,这是由于C C double bond 双键和酚羟基是富电子官能团。此外,烯烃末端 single bond C-H的变化,与其他研究报道的臭氧对饱和碳链的低反应性一致。与臭氧相比,Fenton反应过程中仅检测到4个自相关峰,如图5(c)和(d)所示。最显著的变化发生在1144 cm和620 cm −1 −1 的峰值,分别归因于酯类(多糖)的C single bond O拉伸和羧酸的 single bond OH拉伸。异步映射中频带的详细分布和交叉峰的符号如表S4所示。根据Noda序列规则,推导出官能团的变化顺序为:1144(多糖C O拉伸)>1383(酚类O H)>620(羧酸-OH)>1705(脂肪醛C single bond double bond O single bond )。 其中,酯类和酚类的转化在系统中占主导地位。总之,从二维红外COS和GC-MS分析中获得的结果提供了臭氧在氧化过程中与不饱和有机化合物的优先反应的证据。此外,在Fenton氧化后观察到羧酸化合物的形成(Chen等人,2023a)。

Fig. 5
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Fig. 5. (a) shows the 2D-COS synchronous and (b) asynchronous FT-IR spectra during the O3/H2O2 process; (c) Synchronous and (d) Asynchronous 2D-COS plots of FT-IR spectra during Fenton oxidation process.
图 5.(a) 显示了 O 3 /H 2 O 2 过程中的 2D-COS 同步和 (b) 异步 FT-IR 光谱;(c) Fenton氧化过程中FT-IR光谱的同步和(d)异步2D-COS图。

3.7. Molecular changes of DOM during oxidation process
3.7. 氧化过程中DOM的分子变化

3.7.1. Transformation of DOM composition
3.7.1. DOM组合的转换

A comprehensive investigation of the molecular composition and transformation attributes of organic compounds in APU wastewater was undertaken employing FT-ICR MS. The utilization of a van Krevelen diagram facilitated the depiction of compound distribution and the visual monitoring of molecular alterations within the DOM. In the VK diagram, each point represents a specific DOM with defined H/C and O/C ratios, and it delineates the compounds represented by different regions within the diagram (Fig. S1) (Kim et al., 2003). A VK diagram was used to assess the composition of DOM in the untreated APU wastewater and the APU wastewater treated by the O3/H2O2 and O3/H2O2-Fenton processes (Fig. 6). In each VK diagram, the types of DOM identified in the samples were represented by different colors, with “n” denoting the quantity of identified DOM types in the sample. Based on their heteroatom characteristics (N and S), four subclasses of compounds, CHO, CHON, CHOS, and CHONS, were distinguished (Fig. 6). As shown in Fig. 6(b), after treatment with the O3/H2O2 process, although the concentration of organic compounds decreased, the total number of organic compounds in the wastewater increased (from 7199 to 7730). Ozone reacts with carbon‑carbon double bonds, aromatic hydroxyl groups, and ester groups in the system, resulting in the formation of a significant quantity of fatty acid-like compounds (Fig. 6(e)). In this context, the inefficiency of ozone oxidation can be ascribed to the accumulation of a significant quantity of low-molecular-weight acidic compounds. Additionally, a reduction in unsaturation is observable during the oxidation process. Subsequent Fenton oxidation resulted in the substantial reduction of almost all organic matter, with the highest removal rates observed for fatty acids and lignin (Fig. 6(c) and (e)). These two types of organic compounds possess electron-rich functional groups, facilitating the attack of radical dotOH and leading to their decreased concentration. As shown in Fig. S6(a), the original water sample had a large amount of bonded long-chain organic matter containing Cdouble bondC. After oxidation by O3/H2O2, some long-chain organic compounds were cleaved, reducing the number of double bonds (Fig. S6(b)). Simultaneously, a significant quantity of oxygenated organic matter and low molecular weight compounds are generated (Fig. 7(a–b)). Additionally, the concentration of red and orange dots in the lower-left corner further indicates a positive correlation between the DBE and unsaturation (Wei et al., 2021) (Fig. S6). After the degradation of most long chain unsaturated compounds in wastewater, aldehydes or ketones, along with other residual organic compounds, are formed, leading to a decrease in the unsaturation level (Phungsai et al., 2019). As illustrated in Fig. 7(g–h), after treatment with the O3/H2O2-Fenton process, the quantity of recalcitrant organic compounds (CHOS and CHONS) decreased significantly. Additionally, the number of double bonds in the system further decreased, proving that further Fenton oxidation can significantly reduce oxygen-containing and double bond-containing organic compounds such as aldehydes (Fig. S6(c)).
采用FT-ICR MS对APU废水中有机化合物的分子组成和转化属性进行了全面研究。van Krevelen 图的使用有助于描述化合物分布和目视监测 DOM 内的分子变化。在VK图中,每个点代表一个特定的DOM,具有定义的H / C和O / C比率,并描绘了图中不同区域所代表的化合物(图S1)(Kim等人,2003)。VK图用于评估未经处理的APU废水和通过O 3 /H O和O 3 /H 2 2 O 2 - 2 Fenton工艺处理的APU废水中DOM的成分(图6)。在每个 VK 图中,样本中标识的 DOM 类型用不同的颜色表示,“n”表示样本中标识的 DOM 类型的数量。根据它们的杂原子特性(N 和 S),区分了 CHO、CHON、CHOS 和 CHONS 四个亚类化合物(图 6)。如图6(b)所示,采用O 3 /H 2 O 2 工艺处理后,虽然有机化合物的浓度降低,但废水中有机化合物的总数增加(从7199增加到7730)。臭氧与系统中的碳-碳双键、芳香族羟基和酯基发生反应,形成大量脂肪酸类化合物[图6(e)]。在这种情况下,臭氧氧化的低效率可归因于大量低分子量酸性化合物的积累。此外,在氧化过程中可以观察到不饱和度的降低。 随后的芬顿氧化导致几乎所有有机物的大幅减少,其中脂肪酸和木质素的去除率最高[图6(c)和(e)]。这两种类型的有机化合物具有富含电子的官能团,促进OH radical dot 的攻击并导致其浓度降低。如图所示。S6(a),原始水样中含有大量含有C C double bond 的键合长链有机物。经O 3 /H 2 O 2 氧化后,一些长链有机化合物被裂解,双键数量减少[图S6(b)]。同时,产生大量的含氧有机物和低分子量化合物[图7(a-b)]。此外,左下角红色和橙色点的浓度进一步表明 DBE 与不饱和度之间存在正相关(Wei 等人,2021 年)(图 S6)。废水中大多数长链不饱和化合物降解后,形成醛或酮以及其他残留有机化合物,导致不饱和度降低(Phungsai等人,2019)。如图7(g–h)所示,用O /H 2 O 3 - 2 Fenton工艺处理后,顽固有机化合物(CHOS和CHONS)的数量显着减少。此外,体系中的双键数量进一步减少,证明进一步的Fenton氧化可以显著减少含氧和含双键的有机化合物,如醛类[图S6(c)]。

Fig. 6
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Fig. 6. Van Krevilen diagram of organic matter at different stages. (a) Untreated APU wastewater (n = 7199); (b) APU wastewater treated by O3/H2O2 process (n = 7730); (c) APU wastewater treated by O3/H2O2 Fenton process (n = 4237). And (d) bar chart of organic matter quantity at different stages.
图 6.不同阶段的有机物Van Krevilen图。(a) 未经处理的APU废水(n=7199);(b) 经O/H 2 O 2 3 工艺处理的APU废水(n = 7730);(c) 经 O 3 /H 2 O 2 Fenton 工艺处理的 APU 废水 (n = 4237)。(d)不同阶段有机物数量的条形图。

Fig. 7
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Fig. 7. VK plots of CHO, CHON, CHOS and CHONS in each oxidation stage. (a–d) After oxidation of O3/H2O2; (e–h) After oxidation of O3/H2O2-Fenton; (i) CHO, CHON, CHOS and CHONS changes as a percentage (O3/H2O2); (k) CHO, CHON, CHOS and CHONS changes as a percentage (O3/H2O2-Fenton). The blue dots represent those that have not been removed after oxidation, the orange dots represent those that have been eliminated after reaction, and the pink dots represent newly generated ones.
图 7.CHO、CHON、CHOS 和 CHONS 在每个氧化阶段的 VK 图。(a-d)氧化后 O /H 2 O 2 3 ;(e-h)氧化后 O /H 2 O 2 3 -Fenton;(i) CHO、CHON、CHOS 和 CHONS 变化百分比 (O /H 2 O 2 3 );(k) CHO、CHON、CHOS 和 CHONS 变化百分比 (O /H 2 O 2 3 -Fenton)。蓝点代表氧化后未除去的,橙色圆点代表反应后已除去的,粉红圆点代表新生成的。

3.7.2. Analyzing the degradation and transformation mechanisms of DOM during the oxidation process
3.7.2. 分析DOM在氧化过程中的降解和转化机理

As per the KMD definition, DOM sharing identical KMD values belong to a series of homologous organic compounds that vary solely in molecular weight, encompassing CH2, H2, and COO groups (Gu et al., 2022). Within the same KMD value range, a higher Kendrick nominal mass signifies the presence of a greater number of recurring functional groups in the organic compounds. Conversely, within the same Kendrick nominal mass range, a lower KMD value indicates a lower count of functional groups with the same molecular weight, and vice versa (Yuan et al., 2017). As depicted in Fig. 8, it is evident that the DOM present in wastewater harbors a substantial quantity of functional groups, all of which underwent various degrees of transformation during the oxidation process. Notably, following O3/H2O2 oxidation (Fig. 8(a)), a discernible increase in the overall KMD value was observed. This phenomenon signifies the occurrence of carboxylation during the ozonation process, which can be attributed to the outcome of the ozone's addition reaction with double bonds. Simultaneously, some DOM characterized by higher molecular weights were degraded into intermediates following the oxidation process. As depicted in Fig. 8(b), it's evident that certain high-molecular-weight DOM experienced a process of dehydrogenation following ozonation. This phenomenon might be attributed to ozone's removal of H2O during the Criegee oxidation reaction. Furthermore, at the same KMD level, as illustrated in Fig. 8(c), only a limited number of long-chain organic compounds underwent demethylation, while the KMD-CH2 values of the refractory organics remained relatively stable. This observation suggests that the ozonation process encounters challenges in demethylating DOM. The results of the KMD analysis for the O3/H2O2-Fenton oxidation process are depicted in Fig. 8(d–f). As illustrated in Fig. 8(d), the overall KMD-COO value exhibited a decrease following the combined O3/H2O2-Fenton oxidation, signifying the effective degradation of carboxyl group-containing DOM during this process. Notably, this process also led to the generation of small-molecule organic acids. In comparison to O3/H2O2 oxidation, an increase in the KMD-H2 value was observed after the combined oxidation (Fig. 8(e)). This could be attributed to the substantial addition reactions (hydrogenation) between the generated hydroxyl radicals and DOM in this process. As depicted in Fig. 8(d), it's evident that a significant portion of the recalcitrant organic compounds underwent demethylation processes following O3/H2O2-Fenton oxidation. In summary, O3/H2O2 oxidation alone was capable of degrading only a fraction of the macromolecular unsaturated organics into carboxyl-containing intermediates. Conversely, the combined O3/H2O2-Fenton oxidation facilitated the demethylation, decarboxylation, and dehydrogenation processes for a substantial portion of the hard-to-degrade organics, resulting in their effective removal.
根据 KMD 定义,具有相同 KMD 值的 DOM 属于一系列仅分子量不同的同源有机化合物,包括 CH 2 、H 2 和 COO 基团(Gu 等人,2022 年)。在相同的KMD值范围内,较高的Kendrick标称质量表示有机化合物中存在更多重复的官能团。相反,在相同的Kendrick标称质量范围内,较低的KMD值表示具有相同分子量的官能团计数较低,反之亦然(Yuan等人,2017)。如图8所示,很明显,废水中存在的DOM含有大量的官能团,所有这些官能团在氧化过程中都经历了不同程度的转变。值得注意的是,在O 3 /H 2 O 2 氧化后[图8(a)],观察到整体KMD值明显增加。这种现象表示在臭氧化过程中发生了羧化,这可以归因于臭氧与双键的加成反应的结果。同时,一些具有较高分子量特征的DOM在氧化过程后被降解为中间体。如图8(b)所示,很明显,某些高分子量DOM在臭氧化后经历了脱氢过程。这种现象可能归因于臭氧在Criegee氧化反应过程中去除H 2 O。此外,在相同的KMD水平下,如图8(c)所示,只有有限数量的长链有机化合物经历了去甲基化,而难降解有机物的 2 KMD-CH值保持相对稳定。 这一观察结果表明,臭氧化过程在DOM去甲基化方面遇到了挑战。O /H 2 O 2 3 -Fenton氧化过程的KMD分析结果如图8(d–f)所示。如图8(d)所示,在O /H 2 O 2 3 -Fenton联合氧化后,总KMD-COO值表现出降低,表明在此过程中含羧基DOM的有效降解。值得注意的是,这一过程还导致了小分子有机酸的产生。与O 3 /H O 2 氧化相比,在联合氧化后观察到KMD-H 2 2 值的增加[图8(e)]。这可以归因于在此过程中生成的羟基自由基和 DOM 之间的大量加成反应(氢化)。如图8(d)所示,很明显,很大一部分顽固的有机化合物在O 3 /H 2 O-Fenton 2 氧化后经历了去甲基化过程。综上所述,仅O 3 /H 2 O 2 氧化只能将一小部分大分子不饱和有机物降解为含羧基的中间体。相反,O /H 2 O 2 3 -Fenton的联合氧化促进了大部分难降解有机物的去甲基化、脱羧和脱氢过程,从而有效地去除它们。

Fig. 8
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Fig. 8. Kendrick mass defect analysis during oxidation (a–c) O3/H2O2 oxidation; (d–f) O3/H2O2-Fenton oxidation.
图 8.氧化过程中的Kendrick质量缺陷分析(a–c) O /H 2 O 3 2 氧化;(D-F)O /H 2 O 2 3 -Fenton 氧化。

4. Conclusions 4. 结论

In this study, we conducted a comprehensive assessment of various combined processes aimed at the removal of organic matter from wastewater. Ultimately, we ascertained that the combined O3/H2O2-Fenton oxidation process yielded the most favorable outcomes. Our investigation encompassed a meticulous analysis of the transformations occurring within the DOM during the oxidation process, utilizing a suite of analytical techniques including GC–MS, EEM, 2D-COS-FTIR, and FT-ICR MS characterization. We have observed a significant transformation in the unsaturated organic constituents within the wastewater, as they were converted into aldehydes following the O3/H2O2 oxidation. Subsequently, the application of O3/H2O2-Fenton oxidation furthermore facilitated the substantial removal of both saturated and unsaturated organic matter and led to the generation of carboxylic acid compounds. Our outcomes, as derived from the EEM analysis, have unveiled the presence of four distinct constituents within the wastewater: terrestrial humus (C1), marine humus (C2), fulvic acid material (C3), and microbial humus (C4). Notably, the O3/H2O2-Fenton oxidation process demonstrated remarkable efficacy in eliminating all four constituents, with the most pronounced enhancements observed for C1 and C2. Furthermore, we conducted a meticulous molecular-level investigation into the transformation of DOM throughout the oxidation process, employing FT-ICR MS. Our findings revealed a notable decrease in CHO and CHON like organic compounds following O3/H2O2 oxidation, accompanied by an increase in recalcitrant organic compounds (CHOS and CHONS). The large-molecule unsaturated organic constituents underwent a metamorphosis, evolving into truncated aldehydes and low-molecular-weight fatty acids during the ozonation process, consequently elevating the oxygen content of DOM (carboxylation), while dehydrogenation reactions occurred, resulting in a diminishment of hydrogen content. Following the combined O3/H2O2-Fenton oxidation, a marked reduction in CHOS and CHONS DOM species was evident. The synchronized processes of decarboxylation, hydrogenation, and demethylation during this stage significantly promoted the degradation of aromatic compounds, unsaturated hydrocarbons, and lignin within the densely compacted system matrix. In contrast to alternative oxidation approaches, O3/H2O2-Fenton co-oxidation emerged as the most adept selection for the elimination of DOM within the APU wastewater milieu. In summary, our exhaustive investigation imparts profound insights into the degradation and metamorphosis of organic compounds from multifarious viewpoints, bestowing invaluable guidance for the efficacious treatment of high-salt organic wastewater housing refractory organic constituents.
在这项研究中,我们对旨在去除废水中有机物的各种组合过程进行了全面评估。最终,我们确定 O /H 2 O 2 3 -Fenton 联合氧化过程产生了最有利的结果。我们的研究包括对氧化过程中DOM内发生的转变进行细致的分析,利用一套分析技术,包括GC-MS、EEM、2D-COS-FTIR和FT-ICR MS 表征。我们观察到废水中的不饱和有机成分发生了显着的转变,因为它们在 O /H 2 O 2 3 氧化后转化为醛。随后,O /H 2 O 2 3 -Fenton氧化法的应用进一步促进了饱和和不饱和有机物的大量去除,并导致了羧酸化合物的产生。根据EEM分析得出的结果揭示了废水中存在四种不同的成分:陆地腐殖质(C1),海洋腐殖质(C2),黄腐酸物质(C3)和微生物腐殖质(C4)。值得注意的是,O /H 2 O 3 2 -Fenton 氧化过程在消除所有四种成分方面表现出显着的功效,其中 C1 和 C2 的增强最为明显。此外,我们采用FT-ICR MS,对DOM在整个氧化过程中的转变进行了细致的分子水平研究。我们的研究结果显示,在O 3 /H 2 O 2 氧化后,CHO和CHON类有机化合物显著减少,同时顽固有机化合物(CHOS和CHONS)增加。 大分子不饱和有机成分发生变质,在臭氧化过程中演变为截短的醛和低分子量脂肪酸,从而提高了DOM(羧化)的氧含量,同时发生脱氢反应,导致氢含量降低。在联合 O /H 2 O 2 3 -Fenton 氧化后,CHOS 和 CHONS DOM 物种明显减少。该阶段脱羧、氢化和脱甲基化的同步过程显著促进了致密压实体系基质中芳香族化合物、不饱和烃和木质素的降解。与替代氧化方法相比,O /H 2 O 2 3 -Fenton 共氧化成为消除 APU 废水环境中 DOM 的最熟练选择。综上所述,我们的详尽研究从多个角度对有机化合物的降解和变质提供了深刻的见解,为有效处理含有难降解有机成分的高盐有机废水提供了宝贵的指导。

CRediT authorship contribution statement
CRediT 作者贡献声明

Wanjin Hu: Investigation, Visualization, Data curation, Formal analysis; Wenshi Liu: Writing – original draft, reviewing and editing; Xin Wang: Conceptualization, Formal analysis; Yan Wu: Writing – original draft; Yang Qu: Data Processing; Writing – reviewing and editing; Xuemei Wang: Conceptualization; Jun Xiong: Funding acquisition, Project administration; Lingli Li: Writing – review and editing.
胡万金:调查、可视化、数据管理、形式分析;刘文世:写作——原稿、审稿、编辑;王昕:概念化,形式分析;吴彦:写作——原稿;杨曲:数据处理;写作 – 审阅和编辑;王雪梅:概念化;熊军:资金获取、项目管理;李玲丽:写作——审阅和编辑。

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments 确认

This work was financially supported by CNPC-SWPU Innovation Alliance Science and Technology Cooperation Project (No. 2020CX020301) and Natural Gas Purification Plant General, PetroChina Southwest Oil & Gasfield Company.

Appendix A. Supplementary data
附录 A. 补充数据

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Supplementary material

Data availability 数据可用性

The data that has been used is confidential.

References 引用