Co-metabolic enhancement of 1H-1,2,4-triazole biodegradation through nitrification
Graphical abstract
Introduction
Triazole fungicides (TFs) are commonly-used commercial pesticides, having higher consumption as compared to other fungicides available worldwide due to their excellent antifungal activity by impairing the biosynthesis of fungal ergosterol (Zhou et al., 2016). 1H-1,2,4-triazole (TZ) has been extensively used as the parent compound for the synthesis of TFs (Fan et al., 2010, Wang et al., 2011). With the increasing consumption of TFs, the occurrence and fate of TZ in the environment has become a great concern due to its highly toxic, cancerigenic and teratogenic nature (Konwick et al., 2006). Besides, TZ is not prone to biodegradation, hydrolysis or adsorption onto soil particles, leading to a rather long half-life in the environment (Wu et al., 2016). Thus, the exposure of the recalcitrant and persistent TZ and its derivates to surface water and groundwater would cause serious ecological problems (Wu et al., 2018a). Hence, there is an urgent need to develop effective and economically feasible methods to remove TZ from wastewater.
Biological treatment has been proved to be a favorable alternative because of its high efficiency, cost-effectiveness and environmental friendliness (Jiang et al., 2018, Liang et al., 2018). Conventional wastewater treatment plants are primarily designed to remove easily and moderately biodegradable carbon, nitrogen and phosphorus compounds. However, for the treatment of the poorly biodegradable pollutants such as TZ, biological process is quite ineffective (Rivera-Utrilla et al., 2013). For example, a system treating TZ containing wastewater, which is located in Changzhou Fengdeng Environmental Technology Service Co. Ltd (Jiangsu Province, China), is consisted of air flotation, microelectrolysis, Fenton, coagulation, sedimentation, up-flow anaerobic sludge blanket (UASB) and powdered activated carbon treatment tank (PACT). The influent of UASB contains 85–150 mg L−1 TZ, while the effluent of PACT still contains 50–90 mg L−1 TZ. Therefore, further disposal by activated carbon adsorption is required in order to achieve environmentally acceptable discharge, leading to the increase of treatment cost (Wu et al., 2018b). Therefore, optimization of conventional biological process through bioaugmentation is rather crucial for formulating an effective strategy to achieve higher TZ removal efficiency (Tuo et al., 2012). In our previous study, a novel strain capable of utilizing TZ as the sole carbon and nitrogen source was isolated and named after Shinella sp. NJUST26 (Wu et al., 2016). However, TZ biodegradation rate by NJUST26 was below 6.25 mg L−1 d−1 at initial TZ concentration of 100 mg L−1, which deserved to be improved in order to develop an effective bioaugmentation strategy.
Recently, enhanced removal of various refractory pharmaceuticals, such as triclosan, bisphenol A and ibuprofen, has been observed in nitrifying activated sludge system (Roh et al., 2009). This phenomenon could be attributed to the activity of ammonia oxidizing bacteria (AOB), which could co-metabolize various organic pollutants via a non-specific enzyme namely ammonia monooxygenase (AMO). AMO has a remarkably broad substrate range, and the oxidation of various organic pollutants can be catalyzed by the oxygenated form of AMO when the nitrification process occurred simultaneously (Arp et al., 2001, Yi and Harper, 2007). The key role of AOB demonstrated in previous studies suggested a potential possibility on enhancing TZ biodegradation through AOB. However, up to our knowledge, studies regarding the enhancement of TZ removal by enriched AOB have never been reported. The underlying mechanisms involved in enhanced TZ removal through the co-metabolism of AOB has not been revealed.
Therefore, the main objective of the present study is to investigate the feasibility of bioaugmentation strategy through co-metabolic nitrification for TZ biodegradation in a lab-scale activated sludge tank. TZ removal performance at the presence of nitrification and at the absence of nitrification was compared in order to confirm the key role of AOB in TZ biodegradation. The possible biotransformation pathway and mechanism involved in enhanced TZ removal was also investigated.
Section snippets
Substrate and the seed sludge
The synthetic wastewater used in this study was prepared as follows: 50 mg L−1 TZ, 380 mg L−1 KH2PO4, 1530 mg L−1 Na2HPO4·12H2O, 100 mg L−1 MgSO4·7H2O, 100 mg L−1 Na2CO3 and 10 mL L−1 trace element solution. NH4Cl and allylthiourea (ATU) was added at desired concentration. ATU was added as the selective and effective inhibitor of AOB activity to suppress nitrification in this study (Ali et al., 2013). The trace element stock solution was prepared according to Wang et al. (2018). KH2PO4 and Na2
Reactor performance
As demonstrated in Fig. 1a, within 12 days’ operation, TZ concentration in the effluent was always remained in the range of 43.2–44.1 mg L−1, resulting in TZ removal efficiencies always lower than 20%. Correspondingly, TOC slightly decreased from 17.1 to 17.7 mg L−1 in the influent to 15.5–16.5 mg L−1 in the effluent. These results confirmed the thorough recalcitrance of TZ, even in the biosystem initially inoculated with the acclimated sludge. However, after the addition of NH4Cl into the
Conclusion
In this study, co-metabolic enhancement of TZ biodegradation was achieved in a continuous flow bioreactor through the assistant of nitrification. Co-metabolic degradation of TZ could result in the enhanced removal of TZ, TOC and DOM. Based on the identified intermediates, a new co-metabolic pathway of TZ was proposed. Functional species related to nitrification and functional degrading species could be enriched with the supplement of NH4+, which may be a major reason for the improved
Acknowledgements
This research is financed by Natural Science Foundation of Jiangsu Province (BK20170038) and National Natural Science Foundation of China (No. 51478225).
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These authors contributed to the paper equally.