Review
Application of biochar immobilized microorganisms for pollutants removal from wastewater: A review

https://doi.org/10.1016/j.scitotenv.2022.155563Get rights and content

Highlights

  • Wastewater treatment methods based on biochar are summarized.

  • Electrostatic interaction is the main mechanism of adsorption immobilization.

  • Ion grid formation is the main mechanism of embedding immobilization.

  • The applications of biochar immobilized microorganisms are reviewed.

  • Future studies on carrier performance and microbial selection are proposed.

Abstract

Microbial immobilization technology (MIT) has been rapidly developed and used to remove pollutants from water/wastewater in recent years, owing to its high stability, rapid reaction rate, and high activity. Microbial immobilization carrier with low cost and high removal efficiency is the key of MIT. Biochar is considered to be an efficient carrier for microbial immobilization because of its high porosity and good adsorption effect, which can provide a habitat for microorganisms. The use of biochar immobilized microorganisms to treat different pollutants in wastewater is a promising treatment method. Compared with the other biological treatment technology, biochar immobilized microorganisms can improve microbial abundance, repeated utilization ratio, microbial metabolic capacity, etc. However, current research on this method is still in its infancy. Little attention has been paid to the interaction mechanisms between biochar and microorganisms, and many studies are only carried out in the laboratory. There are still problems such as difficult recovery after use and secondary pollution caused by residual pollutants after biochar adsorption, which need further clarification. To have comprehensive digestion and an in-depth understanding of biochar immobilized microorganisms technology in wastewater treatment, the wastewater treatment methods based on biochar are firstly summarized in this review. Then the mechanisms of immobilized microorganisms were explored, and the applications of biochar immobilized microorganisms in wastewater were systematically reviewed. Finally, suggestions and perspectives for future research and practical application are put forward.

Introduction

With the development of industrialization, water pollution has become a major global problem (Liao et al., 2020). The quality of global water resources has been declining, and the water environment has been deteriorating over the past years. Thus the governance and control of water pollution are imminent (Jia et al., 2019). The main methods of wastewater treatment include the physicochemical method (Merma et al., 2020; Zhao et al., 2020a), membrane technology (Wu, 2019), and microbial method (Cecconet et al., 2018), etc. Table S1 summarizes the advantages, disadvantages, and application fields of different wastewater treatment technologies. Among them, the microbial method has attracted more and more attention because of its high removal efficiency and wide application fields (Wu et al., 2012). The microbial method for wastewater treatment is to decompose organic matter in wastewater through the metabolic activity of microorganisms, to achieve the purpose of purifying wastewater (Pires et al., 2021). However, the microbial method still has some shortcomings, such as the reduction or loss of microbial activity, and the difficulty of separation and recovery after adsorption (Wu et al., 2012). Therefore, how to effectively improve the treatment effect of the microbial method and expand its application scope and way has become the focus of current research. Microbial immobilization technology (MIT) is considered to be a potential and effective method. It can maintain high microbial activity and protect cells from the damage of the external environment, so the treatment of wastewater by MIT has become a global research hotspot (Gan et al., 2020; Yu et al., 2020; Zhao et al., 2019).

MIT is a type of technology fixing microorganisms on specific carriers by physicochemical methods, and this technology can prolong the lifespan of microorganisms and keep cell activity (Wang et al., 2020). MIT overcomes the shortcomings of difficult to separate microorganisms and easily causes secondary pollution. Moreover, the particles prepared by the gel embedding method are also conducive to recycling (Wang et al., 2018b). Microorganisms in the immobilization process have the advantages of high and stable biomass, not easy to lose, fast reaction speed, strong resistance to toxicity, and easy separation (Ting et al., 2013). These characteristics are conducive to the industrial applications of MIT (Boura et al., 2022), such as food fermentation and winemaking (Kosseva, 2011; Perez et al., 2022). However, the matrix may produce inhibitors resulting in the loss of activity of microorganisms during fermentation due to the weak mechanical stability of the carrier material. Since the activity and application effect of microorganisms are affected by the performance of the carrier materials, it is essential to select carrier materials suitable for immobilized microorganisms in industrial applications. A variety of immobilization carriers have been studied, such as porous ceramics, polyvinyl alcohol (PVA), silica and carbon materials, etc. (Mulinari et al., 2020; Ribes et al., 2020; Zhou et al., 2015). Some of them still have defects such as high cost, violent reaction, poor fixation effect, and weak applicability (Table S2). The research progress of microbial immobilization on carbon materials has been reviewed (Wu et al., 2021). While there are still some problems that need to be further studied, especially in terms of the selection of carrier materials (Murshid and Dhakshinamoorthy, 2020). Ideal immobilized carrier materials should be non-toxic, easily available, highly stable, as well as possess large specific surface area (SSA), low cost, and good mass transfer. Therefore, the selection of ideal carrier materials is the focus and difficulty in this research field.

Compared with other carriers, biochar has been widely concerned because of its relatively large SSA, high porosity, and strong adsorption capacity (Wang et al., 2021). As an excellent immobilized carrier, biochar has been increasingly used in wastewater treatment by MIT (Ma et al., 2020; Qi et al., 2021; Sun et al., 2020). The current methods of biochar immobilized microorganisms mainly include adsorption (Lou et al., 2019), embedding (Guo et al., 2021b), and electrochemical methods (Cheng et al., 2020). Among them, the embedding method has been received high attention (Guo et al., 2021b; Zheng et al., 2021). Biochar immobilized microorganisms technology combines the advantages of biochar and microorganism, which has a good removal effect and little secondary pollution (Cheng et al., 2020), so it has good prospects in wastewater treatment. However, the recovery of the complex of powdered biochar and microorganisms after wastewater treatment is difficult. As a carrier material, the adsorption capacity of original biochar is poor and the adsorption range is narrow. Meanwhile, current research is almost carried out in the laboratory, lacks of application in practical engineering. It is necessary to make a comprehensive summary of the current research status and progress of biochar immobilized microorganisms technology. Due to the above reasons, this paper not only reviews the wastewater treatment methods based on biochar, but also summarizes the mechanisms of immobilized microorganisms. On this basis, the applications of biochar immobilized microorganisms technology are reviewed, which could provide suggestions and perspectives for further research.

Section snippets

Physicochemical method

Physicochemical method is widely used in wastewater treatment because it is simple and practical. At present, many methods have been studied, such as the Fenton-like process (Ochando-Pulido et al., 2017), and coagulation/flocculation-adsorption (Cheng et al., 2021d).

The Fenton-like process is a common advanced oxidation process for treating complex biodegradable wastewater. Fe2+ salt is usually used as a catalyst, persulfate and H2O2 as oxidants (Ochando-Pulido et al., 2017). In addition to the

Mechanisms of microbial immobilization

Microbial immobilization is a kind of biological technology that locates planktonic microorganisms in a limited space area by physical or chemical means, which can improve microbial activity and reuse them (Karel et al., 1985). Table S3 summarizes the advantages and disadvantages of the adsorption, embedding, and covalent binding methods in detail. In MIT, the application of immobilized carriers can not only improve the activity and stability of microorganisms but also maintain the high purity

Environmental applications of biochar immobilized microorganisms

The carriers of MIT not only affect the immobilization effect, but also has a great relationship with whether the MIT plays an effective role. Biochar has attracted much attention due to its diverse sources, and high removal efficiency (Shen et al., 2021; Xiong et al., 2017). Biochar immobilized microorganisms have been used to remove pollutants in various water environments (Table S4).

Conclusions and future perspectives

This review mainly summarizes the mechanisms of action between microorganisms and carriers (focus on biochar) as well as the applications of biochar immobilized microorganisms. Electrostatic interaction and ion grid formation are considered to be the main immobilization mechanisms due to their mild immobilization conditions and high stability, respectively. The application of MIT with biochar as the carrier in wastewater treatment highlights the great potential of this technology in different

Credit authorship contribution statement

Rui Li: Methodology, Formal analysis, Writing- Original draft preparation. Bing Wang: Conceptualization, Methodology, Formal analysis, Writing – Review & Editing. Ning Cheng, Xueyang Zhang, Shengsen Wang: Formal analysis, Writing – review & editing. Miao Chen, Aping Niu: Review & Editing.

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 work was supported by the National Natural Science Foundation of China (41977297), the Key Project of Science and Technology Department of Guizhou Province [ZK(2022)016], the Special Research Fund of Natural Science (Special Post) of Guizhou University [(2020)01], and the Key Cultivation Program of Guizhou University [2019(08)].

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