Cadmium (Cd), one of the most common toxic heavy metals in wastewater, which poses the severe risk to human health even at low concentrations of 0.001-0.1 mg/L, including kidney, bone and pulmonary damages (Godt et al., 2006). Large quantities of the wastewater containing Cd2+ are introduced into the environment from industrial activities, such as mining, refining, plastic manufacturing, and electroplating. Accordingly, chemical techniques including precipitation, coagulation, ion-exchange, membrane processing, and solvent extraction have been applied to remove Cd pollutants from wastewater (Huang et al., 2013). While biological method using bacteria, fungus, yeast, and microalgae, has been recently recognized as a more economic and effective technology for the metal removal (Tsekova et al., 2010; Huang et al., 2014; Khan et al., 2016; Shen et al., 2018). In this context, immobilized microbial cell rather than free cell has multitude advantages to treat wastewater, such as stronger resistance to environmental perturbations and toxic chemicals, higher microbial number and activity, and greater efficiency of cell recovery and reuse (Zamani et al., 2018).
Many scientists have investigated systematically the environmental application of immobilized microorganism to the Cd2+ removal from aqueous solutions (Tsekova et al., 2010; Piccirillo et al., 2013; Ma et al., 2015). These studies and other investigations (Li et al., 2017; Shen et al., 2018; Liu et al., 2019) have demonstrated that immobilized microorganism has greater biosorption capacity of Cd2+ than free microorganism to some extent, in which the life-form of immobilized microorganism was always dead or resting state. To date, few studies have examined the biosorption by immobilized growing microorganism (metabolic cells) to remove Cd2+ from aqueous solutions. This is probably because the biosoprtion process of immobilized metabolic cell is more complicated than that of immobilized dead cell, where the dynamic change of metabolic cells in the biosorption, posing the challenges for understanding the biosorption mechanisms. In fact, the direct use of metabolic cells for metal removal could simplify control system and reduce operation costs, by avoiding the need for a separate biomass production process including cultivation, harvest, dry, process and storage prior to use (Chojnacka, 2010). Therefore, the immobilization of metabolic cells might provide a potential alternative for heavy metals bioremediation.
Immobilization carriers mainly include sodium alginate, polysulfone, and polyurethane, which have been applied for the entrapment of microorganism to enhance bioremediation efficiency (Tsekova et al., 2010; Barquilha et al., 2017). Compared with these materials, magnetic biochar used as microbial carrier could provide habitats for growing microbes because of large specific surface areas, thereby improving cell viability to enhance the overall biosorption of heavy metals (Zhu et al., 2017). Owing to the fact that magnetic biochar is easily separated from the solutions, it is ideally suited for the immobilization of metabolic cells to remove toxic metals from liquid medium.
The mechanisms of metal biosorption by immobilized bacterial pellet could be generally classified into two groups, namely adsorption and bioaccumulation (Robalds et al., 2016). The immobilized pellet is composed of growing microorganism and supporting matrix, during which actively growing cells could remove metal ions either by active adsorption onto cell surface or sequestration in cell cytoplasm (intracellular accumulation), whereas magnetic biochar could only remove heavy metals by passive adsorption. Specifically speaking, the overall biosorption mechanisms responsible for the Cd2+ removal include : (i) physical adsorption of Cd2+ was resulted from the electrostatic and van der Waals interactions (Sohbatzadeh et al., 2017); (ii) ion-exchange of Cd2+ with positively charged ions such as K+, Ca2+, Na+ and Mg2+ (Markou et al., 2015); (iii) complexation of Cd2+ with functional groups (e.g., -OH, -COOH) and Cπ electrons (e.g., C = C, C = N) (Gao et al., 2019); (iv) precipitation of Cd2+ with OH−, PO43−, CO32−, and SiO32− (Huang et al., 2020a). Owing to the coincidence of these mechanisms in bacteria and biochar (active or passive adsorption), it is impossible to determine whether involved mechanisms are derived from microbial cell or biochar matrix. While the remaining mechanism is intracellular accumulation (bioaccumulation) was only resulted from bacteria, in which the bioaccumulation capacity of Cd2+ would determine whether the bacterial cells are live or die. Despite the previous works, there is a lack of quantitative information regarding on the relative distribution of mechanisms involved in total metal biosorption process by immobilized pellet, which we assessed for immobilized growing bacterial cells using different magnetic biochars derived from rice straw, sewage sludge and chicken manure, respectively.
To summarize, three objectives of this study are: (i) to search for an effective and environmental friendly method to immobilize growing bacterial cells on the magnetic biochar by the optimization of preparation and biosorption conditions; (ii) to assess the biosorption characterization of Cd2+ by growing cells immobilized on different magnetic biochars, and the comparison between immobilized bacteria and free bacteria; (iii) to investigate the relative contributions of involved mechanisms to total biosorption qualitatively and quantitatively. This could provide insights into the biosorption process of Cd2+ by immobilized microorganism pellet, which is an important step towards possible metal bioremediation.