Molecular insights into the catalytic oxidation of methanol-to-olefins wastewater with phosphoric acid modified sludge biochar
Graphical abstract
Introduction
Methanol-to-olefin (MTO) is a potential alternative of nonoil resources (i.e. natural gas and coal) for producing basic chemicals (Xiang et al., 2015; Yang et al., 2019). In past decades, olefin-based petrochemicals and relevant downstream processes have been well developed. Therefore, MTO is believed to be a crucial technology linking the coal chemical industry and modern petrochemical industry, but also a heavy pollution technology. After the world's first MTO plant was constructed and successfully operated, more attention has been paid on the reaction mechanism and catalysis principle (Tian et al., 2015; Yang et al., 2019). In recent years, the pollution originated from the MTO has also been emphasized by researchers in environmental field. The wastewater from purification and other production process in the MTO process is characteristic of complicated composition, high toxicity and chemical oxygen demand (COD) (Lv et al., 2015).
The main disposal process for the refractory organic wastewater is pretreatment followed by biochemical and advanced oxidation treatment (Ji et al., 2016; Monteil et al., 2019; Shi et al., 2018). Among various advanced oxidation processes, photo-Fenton process is one promising advanced oxidation process due to its ability in degrading refractory organic pollutants with almost no pollution (Matafonova and Batoev, 2018; Kim and Kan, 2015; Li et al., 2018; Liu et al., 2012). In decades, the development of heterogenous photo-Fenton process has overcome the drawbacks of homogenous photo-Fenton process such as the production of iron sludge, the requirement of acid pH and the loss of catalyst (Han et al., 2020). Various catalysts have been synthesized for heterogeneous photo-Fenton reaction, including solid iron hydroxides, biogenic Fe–Mn oxides and sludge-based mesoporous material (Du et al., 2020; Hinojosa Guerra et al., 2019; Xu et al., 2017; Yuan and Dai, 2014). Sewage sludge which is considered as a waste discharged form wastewater treatment plant, usually consists of organics, dead bacterial cells and inorganic components (Yuan and Dai, 2014). From an environmental aspect, the reuse of sewage sludge is widely acceptable. The carbonation of sludge favors the immobilization of heavy metals and the carbon. Sludge derived carbon has been researched for decades as catalyst carrier or directly catalyst in advanced oxidation processes such as catalytic wet oxidation, catalytic ozonation and catalytic wet peroxide oxidation (Yu et al., 2016, 2020a, 2020b). The success application of sludge carbon proves that it has high catalytic efficiency in removing organics in wastewater.
During the photo-Fenton reaction, the molecular compositions and characteristics of dissolved organic matters (DOMs) in MTO wastewater are crucial for shedding light on its degradation pathways and its key treatment technology. However, there is little information on the deep analysis of DOMs in MTO wastewater. The complexity of the multicomponent wastewater and the degradation intermediates hinders the understanding of the degradation mechanism (Zark et al., 2017). Gas chromatography-mass spectrometry is used for the analysis of individual compounds but only 20% of the DOMs could be explained (Bianco et al., 2016). In recent years, much attention has been devoted to Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) due to its ultrahigh resolution and mass accuracy (Lv et al., 2016; Ning et al., 2019; Yuan et al., 2017). The excellent performance of FT-ICR MS in resolving up thousands of different elemental compositions makes it possible to decipher transferring characteristic of diverse structural type compounds/chemically distinct components simultaneously in a single mass spectrum, which was especially suited for complex mixture such as petroleum crude oil (Yuan et al., 2017), wastewater (Andersson et al., 2020; Lv et al., 2016), and sewage (Geng et al., 2018).
In this study, the MTO wastewater from the biological aerated filter was further disposal with photo-Fenton reaction. A biochar prepared from anaerobic granular sludge was used as catalyst, and its efficiency was tested. Moreover, evolution of DOMs in the treatment process of MTO wastewater was explored. The molecular composition and characteristics of DOMs in wastewater were analyzed detailly by the state of art of FT-ICR MS. The revelation of the chemical composition during the treatment process would help design better catalysts for efficient wastewater disposal.
Section snippets
Catalyst preparation
Anaerobic granular sludge was collected from an anaerobic reactor for treating the wastewater discharged from the citric acid plant. The anaerobic granular sludge was first washed with deionized water and ethanol. Subsequently, the sludge was dried in an oven for 24 h at 120 °C. Thereafter, the received solid was pyrolyzed in a revolving furnace at 800 °C for 120 min with a heating rate of 5 °C/min under N2 at a flow rate of 300 mL/min. The speed of revolving furnace was 5 r/min. The obtained
Characteristics of sludge-based biochar
As can be seen from Table 1, after acid modification, the contents of metal elements in sludge-based biochar decrease, especially Ca, K and Al. In addition to Ca, Fe was an important metal with a high content, which had an important influence on the catalytic effect of sludge-based biochar. After the treatment of H3PO4, the iron content in GSC-P only decreased by 11.56%, and the mass percentage was still as high as 5.308 wt%.
In order to investigate the surface compositions of the two catalysts,
Conclusions
The catalyst GSC-P was prepared from the pyrolysis of anaerobic granular sludge along with the modification by H3PO4. In the photo-Fenton treatment of MTO wastewater, the TOC and COD removal rates at pH of 6 was 75.4% and 62.5%, respectively. The degradation mechanism was catalytic oxidation during the photo-Fenton process because of a low adsorption by the catalyst GSC-P. The efficiently catalytic degradation of the MTO wastewater with GSC-P was owing to a high content of iron on the surface
CRediT author statement
Li Yu: Conceptualization, Methodology, Data curation, Writing - Original Draft, Writing - Review & Editing, Project administration, Funding acquisition. Li Wang: Conceptualization, Methodology, Writing - Original Draft. Huangzhao Wei: Conceptualization, Writing - Review & Editing, Funding acquisition. Hongze Chang: Formal analysis, Data curation, Investigation, Visualization. Ying Zhao: Investigation, Methodology. Xinxin Duan: Investigation, Methodology. Jiaxun Zhu: Formal analysis,
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.
Acknowledgements
This study was supported by the National Natural Science Foundation of China (Grant No. 51878643 and 21974138); the Natural Science Foundation of Shanxi Province, China (Grant No. 201901D211029); the Youth Innovation Promotion Association CAS (Grant No. 2020190).
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These authors contributed equally to this work and should be considered co-first authors.