Regulation mechanism of cyanide production and conversion of Pseudomonas fluorescens commonly used in bio-cyanidation of precious metals from waste printed circuit boards
Graphical abstract
Introduction
Nowadays, carbon emission reduction has become a global consensus to protect the environment (Abeydeera et al., 2019). To achieve the ambitious goal, transition toward more sustainable production system and establishment of closed-loop resource recycling system are necessary. Advocating urban mining instead of virgin mining is an attractive avenue (Wang et al., 2016; Zeng et al., 2020). Waste printed circuit boards are a type of typical anthropogenic mineral, which is one of the fastest growing waste streams (Zeng et al., 2018). They are enriched in metal resources especially precious metals, which is with high content and high grade (Chen et al., 2019; Zeng et al., 2016). At the same time, there are also heavy metals and hazardous organic and inorganic substances in waste printed circuit boards, posing extreme risk to human health and environment (Fu et al., 2013; Ni et al., 2009). Consequently, how to recover the vast quantity of waste printed circuit boards in an efficient and environmentally-friendly way is a critical issue (Li et al., 2020a; Esmaeili et al., 2022; Becci et al., 2020; Chu et al., 2021).
The recovery of precious metals of waste printed circuit boards is crucial because of their high value (Wang et al., 2015). Many literatures have studied or reviewed the method of recovering precious metals (Hao et al., 2020; Mir and Dhawan, 2022; Li et al., 2018; Isildar et al., 2018). However, due to complexity of waste printed circuit boards and the high reduction potential of precious metals, the reported technologies still possess certain defects. Generally, pretreatment is necessary for metal enrichment, which includes dismantling, crushing and separation (Lu and Xu, 2016). A typical flowchart of metal enrichment is presented in Fig. S1. For the obtained metallic particles, pyrometallurgy and hydrometallurgy are the two broadly adopted technologies to recover the precious metals (Ning et al., 2017). Pyrometallurgy is a mature technology but requires high energy consumption and discharge harmful gas (Vermesan et al., 2020; Wang et al., 2017). Hydrometallurgy is preferred over pyrometallurgy for recovering precious metals (Lu and Xu, 2016). Cyanide lixiviant is considered as industrially preferable to other non-cyanide lixiviant for its cost-effectiveness (Isildar et al., 2018). However, the detoxification of toxic cyanide containing solutions is an enormous challenge (Li et al., 2018). Therefore, both the two technologies are difficult to meet the current needs. In contrast, bio-cyanidation is recommended as one of the most promising and environmentally friendly technologies to recover precious metals from waste printed circuit boards due to its self-cleaning ability (Kwok, 2019).
In bio-cyanidation process, the cyanide produced by microorganisms is combined with the precious metals to make them converted into free state and entered the solution, achieving recovery. Currently, bio-cyanidation technology is mainly applied in laboratory scale investigations, in which Chromobacterium violaceum and Pseudomonas fluorescens are the two commonly used bacteria (Kumar et al., 2021; Li et al., 2020b). Cyanide production and conversion are the key links in this process. The low cyanide production efficiency and toxicity of unused cyanide impede its environmental application. For cyanide production, cyanide was the secondary metabolite of bacteria, of which the precursor is glycine (Castric et al., 1977; Pradhan et al., 2012). The process from glycine to cyanide was completed under the action of cyanide synthase, encoded by the gene hcnABC (Laville et al., 1998). For cyanide conversion, most cyanide-producing bacteria were able to convert the cyanide to β-cyanoalanine by β-cyanoalanine synthase (β-CAS), which is called self-cleaning ability (Kwok, 2019; Baniasadi et al., 2019). Clarifying the regulation mechanism of cyanide production and conversion can help cope the main problems for industrial applications of bio-cyanidation to recover precious metals from waste printed circuit boards.
Quorum sensing (QS), a cell-to-cell communication process (Mukherjee et al., 2019), is assumed as the driver of cyanide production. QS refers to the capacity of bacteria to regulate the gene expression and collective behaviors according to their density (Grandclement et al., 2016; O'Loughlin et al., 2013). It depends on the self-produced QS signal, which can activate the transcription of some specific genes when accumulated to certain concentrations (Whiteley et al., 2018). Acyl-homoserine lactones (AHLs) usually serve as the QS signal of Gram-negative bacteria to coordinate the collective genes expression (Lv et al., 2021).
In this study, Pseudomonas fluorescens was selected as experimental strain to investigate the regulation mechanism of cyanide production and conversion. Firstly, the bacteria growth and cyanide production were monitored. Then, the QS signal concentration and the β-CAS activity were determined. Finally, RNA sequencing and RNA-seq data analysis were conducted to reveal the gene expression. This work provides a novel insight to the working mechanism of cyanide-producing bacteria. It contributes to promoting the industrial application of bio-cyanidation technology to recovery precious metals from waste printed circuit boards.
Section snippets
Monitoring bacteria growth and cyanide production
Pseudomonas fluorescens with cyanide production capacity was used in this experiment, which was screened from reed root soil (Ruan et al., 2014; Yuan et al., 2019). It belongs to Gram-negative bacteria and were rod-shaped, 1.1–2.9 µm long, 0.27–0.56 µm wide (Fig. S2). The bacteria seed was stored at –20 °C and activated before the experiment. Luria-Bertani (LB) broth (10 g/L peptone, 5 g/L sodium chloride, 5 g/L yeast extract, and 1 g/L glucose) was employed for bacterial culture. 1 mL of the
Dynamic monitoring of cyanide production
The bacteria growth and cyanide production of Pseudomonas fluorescens within 26 h are presented in Fig. 1. Normally, during the experiment, the bacteria enter lag phase, logarithmic phase, and stationary phase. For the cyanide concentration, the curve showed a trend of rising first and then falling, which is consistent with the previously reported experimental results (Yuan et al., 2018, 2019). In the first 6 h, the bacterial density and cyanide concentration were maintained at a low level.
Conclusion
In this study, Pseudomonas fluorescens was selected as a representative to study the regulation mechanism of cyanide production and conversion of the bacteria with cyanide-producing ability. The cyanide concentration increased first and then decreased, and the peak appeared at about 18 h. During this process, the QS signal concentration was always increase while the β-CAS concentration decreased first and then increased. The valley of β-CAS concentration corresponded to the peak of cyanide
CRediT authorship contribution statement
Mi Lin: Writing – original draft, Supervision. Zichun Yao: Supervision. Pengcheng Wang: Writing – review & editing, Supervision. Yonggao Fu: Writing – review & editing, Supervision. Jujun Ruan: Conceptualization, Supervision.
Declaration of Competing Interest
The authors declare no competing interest.
Acknowledgments
This work was supported by the National Key R&D Project of China (2019YFC1904400) and the National Natural Science Foundation of China (52170146). The authors are grateful to the reviewers who helped us improve the paper through many pertinent comments and suggestions.
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