Chemical conversion of dissolved organic matter in petrochemical spent caustic along a wet air oxidation pretreatment process

doi:10.11949/0438-1157.20230702

Abstract

Abstract:

Wet air oxidation (WAO) is an effective pretreatment process for high-concentration petrochemical spent caustics. In this study, an integrated analysis based on three-dimensional excitation-emission matrix fluorescence spectroscopy (3D-EEM), gas chromatography-mass spectrometry (GC-MS) and orbitrap mass spectrometry (Orbitrap MS) was established to investigate chemical conversion behaviors of petrochemical spent caustic along a WAO process at molecular level. Oxygen-dehydrogenation (+1O-2H and +2O-2H), oxygenation (+3O) and dealkylation (-C₂H₆) reactions happened in WAO process, resulting in a very significant change of DOM constituents. The molecular structures of fluorescent DOM converted from high ring number (≥5 rings) to low ring number (3 or 4 rings). Weak polar DOM, mainly thiobenzaldehyde and 3,4-bis(ethoxymethyl)-thiophene, is converted to sulfates, thiophenic acid and thioethers. The polar DOM, mainly O_2—4 and O₃S₁ species with low unsaturation (DBE_wa 3.727) and O/C_wa (0.386), is converted into O_5—9 and O_4—9S₁ species with high unsaturation (DBE_wa 5.911) and O/C_wa (0.537). Compound types of polar DOM increased from 1792 to 4909 by WAO pretreatment and the biodegradability of effluent improved by 35%.

Key words: waste water, pollution, degradation, petrochemical spent caustic, wet air oxidation, dissolved organic matter, chemical conversion

摘要：

湿式氧化工艺(WAO)是炼化企业预处理高浓度碱渣的有效手段。基于3D-EEM、GC-MS以及Orbitrap MS组合分析，在分子水平上研究了碱渣中溶解性有机物(dissolved organic matter，DOM)在WAO过程的化学转化特征。研究发现：WAO过程主要发生加氧去氢(+1O-2H和+2O-2H)、加氧(+3O)以及脱烷基化(-C₂H₆)反应，芳香性DOM分子结构由高环(≥5环)向低环(3环/4环)转化；弱极性DOM由硫基苯甲醛、3，4-双(乙氧基甲基)噻吩等转化为硫酸盐、噻吩甲酸和硫醚等小分子化合物；强极性DOM则由具有较低不饱和度(DBE_wa 3.727)和O/C_wa(0.386)的O_2~4类和O₃S₁类化合物转化为具有较高不饱和度(DBE_wa 5.911)和O/C_wa(0.537)的O_5~9类和O_4~9S₁类化合物。WAO预处理后极性DOM种类由1792种增至4909种，水质的可生化性提升了35%。

关键词: 废水, 污染, 降解, 炼化碱渣, 湿式氧化, 溶解性有机物, 化学转化

CLC Number:

TE 991

Baiyu YANG, Yue KOU, Juntao JIANG, Yali ZHAN, Qinghong WANG, Chunmao CHEN. Chemical conversion of dissolved organic matter in petrochemical spent caustic along a wet air oxidation pretreatment process[J]. CIESC Journal, 2023, 74(9): 3912-3920.

杨百玉, 寇悦, 姜峻韬, 詹亚力, 王庆宏, 陈春茂. 炼化碱渣湿式氧化预处理过程DOM的化学转化特征[J]. 化工学报, 2023, 74(9): 3912-3920.

Figures/Tables 8

Fig.1 Flow diagram of WAO unit

Table 1 Measurement methods and instruments for water quality indicators

项目	指标	测定方法与标准	仪器
DOM总量分析方法	SCOD	重铬酸钾法 HJ 828—2017	承德华通CTL-12化学需氧量速测仪
	SBOD₅	稀释与接种法 HJ 505—2009	美国HACH BODTrak Ⅱ生化需氧量测试仪
	DOC	燃烧氧化-非分散红外吸收法 HJ/T 104—2003	日本Shimadzu TOC-L CPH CN 200总有机碳测试仪
DOM组成表征方法	SUVA₂₅₄	测定样品在波长254 nm下的吸光度（UV₂₅₄），计算得到SUVA₂₅₄（UV₂₅₄×100/DOC）	日本Shimadzu UV-mini-1280UV-Vis紫外光谱仪
	3D-EEM荧光光谱	激发波长（λ_ex）和发射波长（λ_em）分别设置为200~550 nm和200~600 nm，步长为5 nm	法国Horiba Scientific Aqualog 3D-EEM荧光光谱仪
	GC-MS	样品利用二氯甲烷（色谱级，≥99.9%）液液萃取法进行富集，以超纯氦气为载气（1 ml/min），进样量为1.0 µl。离子源温度为250℃，四级杆温度为150℃	美国Agilent 7890B/GC-5977B/MSD GC-MS仪
	Orbitrap MS	样品DOM用固相萃取柱（美国Agilent，Bond Elut PPL）提取，并用甲醇（HPLC级，≥99.9%）洗脱浓缩^[12]。采用负离子电喷雾离子源模式，仪器的m/z检测范围设置为100~800 Da，相关操作参数同文献^[13]	美国ThermoFisher Scientific Orbitrap Fusion型Orbitrap MS仪

Table 2 Changes of comprehensive pollution indicators of spent caustics before and after WAO pretreatment

综合污染指标	预处理前	预处理后
DOC/(mg/L)	1147±7	947±4
SCOD/(mg/L)	9240±224	2060±12
SBOD₅/(mg/L)	2088±27	633±10
SBOD₅/SCOD	0.23±0.03	0.31±0.05
硫化物/(mg/L)	41701±601	35±2
$S O_{4}^{2 -}$ /(mg/L)	640±8	8561±93

Table 2 Changes of comprehensive pollution indicators of spent caustics before and after WAO pretreatment

综合污染指标	预处理前	预处理后
DOC/(mg/L)	1147±7	947±4
SCOD/(mg/L)	9240±224	2060±12
SBOD₅/(mg/L)	2088±27	633±10
SBOD₅/SCOD	0.23±0.03	0.31±0.05
硫化物/(mg/L)	41701±601	35±2
$S O_{4}^{2 -}$ /(mg/L)	640±8	8561±93

Fig.2 3D-EEM spectra of spent caustics before and after WAO pretreatment

Fig.3 The GC-MS results of spent caustics before and after WAO pretreatment

Table 3 Molecular types and attributes of DOM of spent caustics before and after WAO pretreatment

项目		预处理前 DOM数量1792种	预处理后 DOM数量4909种
DOM类型	CHO/%	27.0	39.4
	CHOS/%	68.8	58.5
	CHON/%	4.1	1.9
	CHONS/%	0.1	0.1
分子属性	DBE_wa	3.727	5.911
	O/C_wa	0.386	0.537
	H/C_wa	1.536	1.099
	MW_wa/Da	282	278
	NOSC_wa	-0.626	0.091

Fig.4 The Orbitrap MS results of spent caustics before and after WAO pretreatment

Fig.5 The screening results of precusor-product pairs among spent caustics before and after WAO pretreatment

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