Synergic effect of adsorption and biodegradation enhance cyanide removal by immobilized Alcaligenes sp. strain DN25
Graphical abstract
Introduction
As a potent metabolic inhibitor to humans and aquatic organisms, cyanide is listed as a priority pollutant by the U.S. Environmental Protection Agency (EPA) [1] as well as in China. The permitted standard level of total cyanide is set to be 0.5 mg/L for integrated wastewater discharge by the Ministry of Ecology and Environment of China. However, many industries, such as mining, dye production, synthetic fibers and plastics, use cyanide as raw material. The total worldwide discharge amount of cyanide is estimated to be nearly 14 × 106 kg per year [2], and cyanide leakage occurs unpredictably all over the world. Biotreatment mediated by microorganisms or enzymes has become a common choice for the removal of such hazardous chemicals [3,4]. Many microorganisms, including bacterial strains of Pseudomonas, Klebsiella and Alcaligenes as well as several fungi spices of Fusarium solani and Trametes versicolor were thus isolated for the treatment of cyanide and its derivatives [[5], [6], [7]], and certain microorganisms have even been employed in practical applications [8,9].
The technique of immobilization has many applications as a measure to improve the stability and efficiency in the bioremediation process [10,11]. More attractively, a proper and elaborate support would endow the immobilized cells with certain unique properties, such as the recently reported simultaneous adsorption and biodegradation (SAB) system, in which porous materials, active carbon [12], macroporous resin [13], and bamboo charcoal [14] were chosen to immobilize the cells. The observed synergistic effects are generally considered to be related to the dual-function of the support. The support would provide an interface for facilitating the biomass accumulation, and the adsorption of the target pollutant on solid surface would promote sufficient contact with the cells, which results in the acceleration of the degradation process [15,16]. Dash et al. reported the removal of ferrocyanide by employing Pseudomonas fluorescens immobilized on granular activated carbon [17]. The ferrocynide removal efficiency of 300 mg/L increased from 69.3% to 81.8% through the synergistic action of adsorption and biodegradation. Therefore, such immobilized system appears an appealing way for the establishment of this rapid treatment process.
Polyurethane foam (PUF) is a type of porous material that is recommended to be an emerging water treatment medium by the U.S. EPA because of its good mechanical properties, larger contaminant capture capacity, environmentally friendly and competitive price [18,19]. Similarly, PUF is a good carrier for enzyme or cell immobilization. Many studies have reported the applications of PUF-immobilized cells to remove toxic chemicals, such as naphthalene [20], 2-nitrotoluene [21], and chlorpyrifos [22]. A higher removal efficiency and better stability have been observed in these cases. However, the biotreatment of cyanide with PUFs as the immobilized support has not been reported as far as is known. Moreover, a deeper understanding of the SAB process must be obtained for future engineering applications.
Based on the goal of constructing the purposeful immobilized cell system adapted to cyanide removal, PUF was thus selected because it showed good adsorption abilities for cyanide in our previous work. An Alcaligenes sp. strain DN25 was immobilized to investigate the cyanide removal capacity. As expected, the immobilized cell system exhibited a strong synergistic effect and achieved higher removal efficiency, especially at high concentrations. Data relevant to the immobilized and freely suspended cells were included and compared to elucidate the synergistic effect. A continuous reactor was also constructed to evaluate the system stability. This PUF-immobilized system would provide a new strategy for the rapid elimination of this type of dangerous and toxic pollutant.
Section snippets
Bacterial strain and materials
The cyanide-degrading strain DN25, identified as Alcaligenes sp. by a 16S rDNA sequence analysis, was previously isolated in our laboratory and preserved in the China General Microbiological Culture Collection Center with the accession number CGMCC 5734. Polyurethane foam was purchased from Hu zhou H&T New Material Techonlogy Co., Ltd (Huzhou, China). All other chemicals used in this study were of analytical reagent grade or high purity. Cyanide stock solution was prepared by dissolving a
Cyanide removal
Cyanide is volatile and photolysis. In order to determine the role of adsorption, biodegradation and other abiotic factors in cyanide removal, corresponding experiments were performed, respectively. Fig. 2 presented the time curve of cyanide adsorption and desorption by the foam cubes. The maximum cyanide adsorption of 50 mg CN−/L by the foam cubes was approximate 16.1%, and the adsorption equilibrium was reached after 2 h. A similar trend as adsorption was observed for cyanide desorption,
Discussion
The immobilized cell system with synergic effect of adsorption and biodegradation exhibits broad application prospects in the field of contaminants elimination and thus prompts researchers to design and construct such a system, especially for the treatment of toxic and refractory substances [26,27]. The PUF-immobilized cell system reported here might be considered as a successful example for cyanide treatment. The higher efficiency in the SAB system than adsorption and biodegradation alone is
Conclusions
In this study, PUF was purposely selected to immobilize Alcaligenes sp. for cyanide removal considering the synergic interaction between adsorption and biodegradation. Effects of important parameters on cyanide adsorption and biodegradation process were investigated. The PUF-immobilized cell system showed strong tolerance to high temperature, pH and cyanide concentration. The cyanide removal efficiency of the SAB system was greatly enhanced compared to the freely suspended cell system, which
Conflict of interest
The authors of this study declare no conflict of interest.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 51108098) and the Scientific Research Foundation of GuangXi University (No. XJZ130360).
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