Research PaperMicrobially induced calcium precipitation based simultaneous removal of fluoride, nitrate, and calcium by Pseudomonas sp. WZ39: Mechanisms and nucleation pathways
Graphical Abstract
Introduction
The coexistence of multiple pollutants in the aquatic environment has aroused extensive attention on the simultaneous treatment of these pollutants to adopt a cost-effective technology and save energy consumption (Rahman, 2020, Zhai et al., 2019). Among the aquatic pollutants, consumption of fluoride (F−) is an extremely important issue (Huang et al., 2020, Wimalasiri et al., 2021). F− is introduced in the groundwater by the dissolution of fluorinated minerals, disposal of industrial wastes, and manufacturing of fertilizers (Ali et al., 2019). Although the intake of F− below 1.0 mg L−1 can prevent dental caries, however, long-term ingestion of a higher level of F− will have deleterious effects on the human health such as renal, liver, thyroid, urinary tract, neurological dysfunction, and even skeletal cancer (Barberio et al., 2017, Wang et al., 2017, Zhang et al., 2020b).
Nitrate (NO3−) contamination has also become a crucial environmental issue, especially in groundwater, mainly derived from the excessive use of fertilizers and the discharge of industrial nitrogenous compounds (Cheng et al., 2020). Chronic intake of excessive NO3− can lead to methemoglobinemia (blue baby syndrome) and even gastric cancer (Aliaskari and Schäfer, 2021, Fan et al., 2021). Because of the severe harms posed by these pollutants to human health, the world health organization (WHO) has recommended the upper limit concentrations of NO3− and F− to be 50 and 1.5 mg L−1, respectively (WHO, 2017).
Considerable research efforts have been devoted to treat the co-contaminated water, such as adsorption, precipitation, electrodialysis, membrane filtration, photocatalysis, and ion exchange (Alka et al., 2021, Belkada et al., 2018, Rahman, 2020). Whereas, these physico-chemical practices are restricted due to the complicated technology, high cost, and the generation of noxious by-products (Katiyar et al., 2020, Lu et al., 2019). Biological treatment is considered a secure and economical method, which is greatly propitious for water remediation (Hou et al., 2021, Wen et al., 2019).
Microbially induced calcium precipitation (MICP) has been intensively investigated in geotechnical engineering, such as the enhancement of the soil properties, preparation of bio-bricks, and the repairing of concrete cracks (Chuo et al., 2020, Naveed et al., 2020, Tang et al., 2021). Recently, this technology has also been reported to offer a viable source for the water treatment (Su et al., 2020a, Yan et al., 2020). The main microorganisms involved in the MICP process are photosynthetic organisms, sulfate-reducing bacteria, nitrate-reducing bacteria, and urea-degrading bacteria (Ansari et al., 2020). The use of heterotrophic denitrifying bacteria as templates for mineralization can produce alkalinity during the metabolism process (Burt et al., 2018, Wang et al., 2021), thus, enabling MICP technology more effective and sustainable in the simultaneous removal of F−, Ca2+, and NO3−-N from groundwater. Although, much work so far has focused on the biomineralization mechanism of bacteria (Qin et al., 2020, Tang et al., 2021, Zhang et al., 2020a), the phenomenon still falls short of exact mechanism explanation.
In this study, the defluoridation capacities of simultaneous denitrifying and mineralizing bacterium, Pseudomonas sp. WZ39 was investigated under low C/N conditions. The effects of initial pH, F−, and Ca2+ on defluoridation were evaluated and the optimum conditions were determined employing response surface methodology (RSM). To further understand the nucleation pathways, Pseudomonas sp. HXF1 was introduced for comparative study. The extracellular polymeric substances (EPS) were isolated and the biomass was inactivated to analyze the nucleation sites. Besides, the role of calcium channels in biomineralization was also explored.
Section snippets
Chemicals and materials
All reagents were analytical grade and were purchased from the Sinopharm Chemical Reagent Co., Ltd (China) and Aladdin Chemical Reagent Co., Ltd (China). F− stock solution (1.0 g L−1) was prepared by dissolving the solid NaF in sterilized distilled water to control the initial F− concentration.
Microorganism and culture preparation
Pseudomonas sp. WZ39 was isolated from the bottom sludge samples, deriving from a high hardness well water in Yingkou (Liaoning Province, China). The composition of heterotrophic medium (HM) resembled
Isolation and identification of Pseudomonas sp. WZ39
After preliminary screening of 50 aerobic denitrifying strains, one strain (WZ39) with the best calcium precipitation induction effect was re-screened. Nitrogen balance (Table S2) and DO measurement (Fig. S2) experiments showed that strain WZ39 exhibited a remarkable nitrogen removal ability in an aerobic environment. The colonies on the agar medium exhibited smooth white dots. Strain WZ39 was identified as Pseudomonas species according to the results of the phylogenetic tree (Fig. S2).
Growth of strain WZ39 and dynamic changes of environmental factors
The 96 h
Modern challenges and future recommendations
Extensive attention has been paid to the research on MICP technology and its potential application in soil restoration and water pollution remediation. However, the mechanism of bacteria induced mineralization still remains an open question, and more exploration is needed to extend the environmentally friendly and economical technology from the laboratory to the actual field trials. Hence, few recommendations are proposed for future research direction.
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Although abundant evidence has indicated
Conclusions
In this work, Pseudomonas sp. WZ39 was identified by 16S rDNA homology comparison and used to lower soluble F−, Ca2+, and NO3−-N content through the MICP process. Under low C/N conditions (2.5), the removal efficiency of F−, Ca2+, and NO3−-N reached 81.44%, 100%, and 59.27%, respectively. According to the RSM results, a maximum defluoridation efficiency of 87.49% was predicted. Chemical adsorption and co-precipitation synergistically contributed to the removal of F− in the form of CaF2 and Ca5
CRediT authorship contribution statement
Junfeng Su: Conceptualization, Methodology, Supervision. Zhao Wang: Investigation, Formal analysis, Writing - original draft. Amjad Ali: Methodology, Writing - review and editing. Ruijie Zhang: Investigation, Validation. Wenshuo Yang: Investigation, Validation. Liang Xu: Investigation, Data curation. Tingbao Zhao: Validation.
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.
Acknowledgments
This research work was partly supported by the National Natural Science Foundation of China, China (NSFC) (Nos. 51978556 and 51678471), Shaanxi Science Fund for Distinguished Young Scholars, China (No. 2019JC-31) and the Key Research and Development Program in Shaanxi Province, China (2018ZDXM-SF-029).
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