Role of polyurethane-modified layered double hydroxides on SCFAs extraction from waste activated sludge fermentation liquid for elevating denitrification: Kinetics and mechanism
Graphical abstract
Introduction
Denitrification process was a vital procedure to alleviate water eutrophication by converting nitrite and nitrate to nitrogen gas (N2) in the wastewater treatment process (Michal et al., 2002). During the process, sufficient carbon sources were in the need as electron donor, especially for small molecules such as acetic acid, glucose and methanol. However, the available carbon sources in the influent of municipal wastewater treatment plants (WWTPs) was relatively low with chemical oxygen demand to nitrogen ratio (COD/N) of 3~4, limiting improved efficiency of nitrogen removal. Thus, large amounts of external carbon sources were added in the biological nutrient removal (BNR) process, increasing the economic burden of WWTPs (Park et al., 2009; Sun et al., 2017). Recently, exploration of internal carbon source from waste activated sludge (WAS) was disposed by anaerobic fermentation has attracted increasingly attention due to its abundant organics. Short-chain fatty acids (SCFAs) produced from WAS fermentation was proved to be alternative carbon sources for denitrification, reducing the environmental risk caused by sludge disposal as well (Wang et al., 2017). It revealed that the denitrification rate raised by approximately 55% when SCFAs were used as electron donor (Dong et al., 2010; Reyes-Avila et al., 2004). Fermentation liquid from alkaline pretreated WAS were believed to be efficient as carbon source in nitrogen removal due to high proportion of available carbon sources, acetate acid, for instance (Sun et al., 2017). However, the waste activated sludge fermentation liquid (SFL) contains not only SCFAs, but also large molecules bio-metabolites and refractory organics, e.g., proteins, lipids and humus, which increase the operation overhead of biological nutrients process and are detrimental to microbes in wastewater treatment (Wang et al., 2019). Therefore, seeking for an efficient and reusable adsorbent to extract SCFAs is the key bottleneck for the in situ application of SFL as carbon source for denitrification.
Layered double hydroxides (LDHs) have a great anion adsorption capacity due to their M(II)-M(III) cation-pair layered structure (Theiss et al., 2016; Chen et al., 2017). LDHs was wildly used in wastewater treatment industries to remove pollutants such as heavy metal ions, bromate, dyes, and phosphate (Li et al., 2014b; Peng et al., 2014; Yu et al., 2015; Shen et al., 2017; Kou et al., 2018). Wei et al. found that F− could be removed by Ca-Al-CO3, and results showed that the adsorption competitions of F− with Cl− and NO3− were not observed during the adsorption process (Wei et al., 2020). Ca/Al LDHs was also showed an excellent selenium removal performance. Its desorption and regeneration results showed that Ca/Al LDHs could be reused at least three times (Li et al., 2020). Recently, LDHs, as an efficient carrier of SCFAs with high adsorption capacity and slow-release property, have attracted more attentions. It was reported that Zn/Al intercalated with SCFAs performed better than SCFAs-Na as carbon source of denitrification (Li et al., 2017). Mg/Al, Ni/Al, and Ni/Fe LDHs were also believed with good properties of slow-release, when they were used as carbon sources with acetate intercalated (Jiang et al., 2018). However, powdery LDHs were always used as adsorbents, which mean large cost in material preparation due to limited recyclability. Noting that Ca/Al LDHs was relatively untapped as carrier of carbon sources, although several studies have explored its excellent performance to remove F−, Cl−, NO3− and selenium with high efficiency (Li et al., 2020). In addition, calcium could be easily obtained from wide natural and rich source. Eggshell waste, as the common raw material of food processing industry (its production is estimated to be 31.79 million tons in 2020), owing abundant calcium carbonate (94%–99.63%), can be regarded as a low-cost calcium source (Lunge et al., 2012). Thus, in this study, the pretreated eggshell waste was used to release calcium ions for Ca/Al LDHs preparation.
Generally, powdery LDHs were used as adsorbents, which mean large cost in material preparation due to limited recyclability. Therefore, it was a reasonable solution by loading the powdery LDHs onto other materials. Polyurethane (PU) is a suitable adsorbent for chemical matters uptake due to its excellent thermal resistance and mechanical properties, porous structure and high specific surface area (Saeed et al., 2013). It was reported that the polar and non-polar groups in PU's structure in favor of the retention of free anions with high polarizability. Although, PU had been utilized a lot in separation (Mohammadi et al., 2014; Saranya et al., 2017), there are few studies considering the possibility of the combination of PU and LDHs to solve the bottleneck of non-recyclability of powdery LDHs.
In this study, PU-modified LDHs was synthesized to extract SCFAs from SFL. Scanning electron microscopy (SEM) and fourier transformed infrared spectroscopy (FTIR) were used to characterize the prepared PU-LDHs. Adsorption parameters were optimized by a series of adsorption tests using artificial fermentation liquid (AFL), including contact time, mass ratios of LDHs to PU (LDHs/PU), pH value, temperature and initial concentrations of SCFAs. The adsorption thermodynamics and isotherms were also used to reveal the mechanism of the extraction process of SCFAs by PU-LDHs. Finally, application tests were performed, including SFL adsorption tests, PU-LDHs regeneration tests and denitrification tests with PU-LDHs-loaded SFL as carbon sources.
Section snippets
Chemicals
Sodium bicarbonate (NaHCO3), sodium hydroxide (NaOH), acetic acid (HAc), propionic acid (HPr), n-butyric acid (n-HBu), isobutyric acid (iso-HBu), n-valeric acid (n-HVa), isovaleric acid (iso-HVa) were purchased from Sinopharm Chemical Regent Co.,Ltd. Aluminium chloride hexahydrate (AlCl3·6H2O), Polyether polyol (CP) was purchased from GZRXHG Co., Ltd. Toluene diisocyanate (TDI) was purchased from Chemreagent Co., Ltd. Simethicone and dibutyltin dilaurate were purchased from Cdkelong Co., Ltd.
Characterization of PU-LDHs samples
The SEM images of PU, PU-LDHs and SCFAs-adsorbing PU-LDHs are shown in Fig. 2. Fig. 2a showed the porous surface of pure PU. Ca/Al LDHs can be seen clearly on the PU-LDHs surface (Fig. 2b). The enlargement of red circle showed the Ca/Al LDHs with layered structures, demonstrating that PU-LDHs had been successfully synthesized and it had more active sites due to the heterogeneity of surface, compared to pure PU. In Fig. 2c, some substances with different sizes can be clearly seen on the surface,
Conclusion
PU-LDHs was proved to be an effective adsorbent for extraction of SCFAs from SFL with the actual maximum adsorption capacity of 140.7 ± 2.1 mg COD/g (Vsol = 300 mL; agitation speed = 150 rpm; T = 20 ± 1 °C; pHsol = 6). In addition, PU-LDHs-loaded SFL performed better as electron donor in denitrification than glucose, with a denitrification rate of 0.014 mg NO3−-N/mg MLSS∙d. Furthermore, the cost advantages were prominent because the carbon-released PU-LDHs can be repeatedly used at least three
Credit author statement
Mengxuan Deng: Conceptualization; Methodology; Formal analysis; Data Curation; Writing - Original Draft; Investigation; Writing - Review & Editing. Aijuan Zhou: Supervision; Writing - Review & Editing; Funding acquisition; Validation; Project administration. Chen Cheng: Resources. Sufang Wang: Resources. Yanqing Duan: Writing - Review & Editing. Xiuping Yue: Supervision; Funding acquisition; Validation; Project administration.
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 research was supported by the National Natural Science Foundation of China (NSFC, Nos. 51608345), by the State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRF17021), by the Key Research and Development (R&D) Project of Shanxi Province (No. 201903D321057 and 201903D321055) and the Ministry of Environmental Protection of the People's Republic of China (Major Science and Technology Program, No. 2019YFC04086).
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