اقدامات هم افزایی میکروارگانیسم ها برای حذف موثر فلزات سنگین
Abstract
1- Introduction
2- Materials and methods
3- Results and discussion
4- Conclusion
References
Abstract
In this study, two bacteria strains (Enterobacter sp. SL and Acinetobacter sp. SL-1) and waste molasses (carbon source) were used to remove Zn(II), Cd(II), Cr(VI), and Cr(Total) in the liquid solution (87 mg·L). The results showed the removal efficiencies of Cr(Total) and Cr(VI) could reach over 98.00% after reaction, and the removal efficiencies of Zn(II) and Cd(II) were all about 90.00% by the synergistic actions of microorganisms and waste molasses. In this process, waste molasses provides nutrients for microorganisms and has the characteristics and capability of Cr, Zn, and Cd. Microorganisms mainly use biological adsorption (36.95% and 45.69%) and metabolism (24.37% and 17.05% by producing humic-acid and fulvic-acid like substances) to remove Zn(II) and Cd(II), while waste molasses could to remove Cr(Total) (81.24%) and Cr(VI) (75.90%). This study has potential application value for the treatment of wastewater containing high concentrations of heavy metals.
Introduction
Heavy metal-containing wastewaters are produced in many industries, and their direct discharge has caused serious impacts on human health and environment (Zhu et al., 2018). The popular methods for treatment of heavy metal-containing wastewater include the ion exchange method (Dai et al., 2015), electrochemical method (Kobya et al., 2017), photocatalysis (Luo et al., 2017), and biological method (Wen et al., 2018). Among them, biological method has been widely studied due to the low energy consumption, small secondary pollution, and environmental friendliness. (Zhou et al., 2013). However, applying survivable microbes for biological treatments in toxic environments has become the main challenge (Pradhan et al., 2019) because heavy metal-containing wastewater not only has complex heavy metal compositions but also may contain other toxic substances such as cyanide (a phenolic organic matter) (Wen et al., 2018). In addition, the characteristics of heavy metal-containing wastewater may vary moderately; especially the change of pH can significantly inhibit the bacterial activities (Alexandrino et al., 2014). Moreover, biological treatments of heavy metals normally have been used for low concentration ranges (10–50 mg·L−1) of heavy metals (Guo et al., 2017), and some bacteria can only work with specific single heavy metal [e.g., Zn(II) or Cr(VI)] in the wastewater (Peng et al., 2019; Singh et al., 2011; Wang et al., 2018). Due to the high concentration of heavy metals on microbial stress, it is extremely difficult for microorganisms to remove many kinds of heavy metals simultaneously (Dhal et al., 2013; Guo et al., 2017). Therefore, previous studies have used chemical pretreatment (Mejias Carpio et al., 2016) or the addition of substances such as iron ore (Guo et al., 2017) to reduce heavy metal content or toxicity. Although these methods can improve the activity of microorganisms, they also bring trouble to the subsequent treatment of the final effluent. Alexandrino et al. (2014) and Yan et al. (2018) applied a high concentration of sodium lactate (7.0 g·L−1) as a carbon source to stimulate bacteria growth and heavy metal removal. However, these expensive carbon sources burden wastewater treatment (Li et al., 2019; Wang et al., 2018). In addition, it has been reported that microorganisms need specific nutrient composition to be able to function, and the complex wastewater system (containing heavy metals) will inhibit its activity (Mejias Carpio et al., 2016; Peng et al., 2019; Pradhan et al., 2019). Therefore, biotreatments are usually limited in industrial application due to the multi heavy metals contained wastewater. So, it is important that to develop an economic biotreatment method with multi heavy metals tolerance performance. Moreover, it is critical to differentiate the contributions of biotic (e.g., via microbial metabolisms) and abiotic (the chemical properties of the carbon source) reactions for heavy metals removal in complex wastewater system.