Immobilization of iron- and manganese-oxidizing bacteria with a biofilm-forming bacterium for the effective removal of iron and manganese from groundwater
Graphical abstract
Introduction
The presence of high concentrations of iron and manganese ions in groundwater, which provides the drinking water supply of a population, will cause a negative impact on people unless the groundwater is appropriately treated (Roth, 2006). The metal ions cause an unpleasant metallic taste, and metal oxides can clog pipes and discolor clothes (Guzinis et al., 1998). Manganese ions can even cause neurotoxicity in humans (Tekerlekopoulou and Vayenas, 2007). The concentrations of iron and manganese in drinking water should be limited; the standard values for these ions in China are 0.3 and 0.1 mg L−1, respectively (SEPA, 2002). Therefore, the purification of groundwater with high iron and manganese contents plays an important role in environmental safety (Qin et al., 2009). Some treatment methods concerning the removal of iron and manganese ions in the groundwater, including physicochemical and biological methods, have been published. The most effective physicochemical methods include chlorination, potassium permanganate (KMnO4) addition, ion exchange, and membrane filtration (Oehmen et al., 2006, Vaaramaa and Lehto, 2003). However, these methods generally removed iron more effectively than manganese, which was unsatisfactory. Moreover, the use of chemical reagents should be minimized because of the increase in the operational costs and because of the secondary impacts of residual and by-product formation (Cai et al., 2015). The application of ion exchange and membrane filtration is also restricted due to higher capital and operational costs (Mouchet, 1992).
Biological methods have drawn much attention in recent years because they do not require additional chemical oxidants. More importantly, the simultaneous removal of iron and manganese ions could be achieved using a biofilter, which simplifies the treatment process, increases the treatment capacity and reduces investment costs (Pacini et al., 2004). Considerable research has been conducted on Fe-oxidizing bacteria, including Siderocapsa sp., Gallionella sp., Leptothrix sp., Janthinobacterium sp., Sphaerotilus sp., Flavobacterium sp., (Qin et al., 2009, Cai et al., 2015, Miller et al., 2012), and Mn-oxidizing bacteria, including Clonothrix sp., Crenothrix sp., and Sphingomonas sp. (Cahyani et al., 2007, Burger et al., 2008, Carmichael et al., 2013). However, most of these strains have the capacity to only oxidize iron or manganese, not both. To date, several iron- and manganese-oxidizing bacteria with high iron removal ratio have also been isolated, but the manganese removal efficiency was not ideal (Li et al., 2005). Thus, it is important to obtain iron- and manganese-oxidizing bacteria with high oxidizing capabilities for the simultaneous removal of iron and manganese ions.
It also should be noted that an unavoidably longer start-up period of the biofilter and unsatisfactory removal efficiency due to the loss of the oxidizing microorganisms, which are washed out of the system, are the main drawbacks for the application of the biological method. Furthermore, it is difficult for the water quality to meet the requirements for iron and manganese in groundwater, especially the manganese content (Burger et al., 2008). Therefore, the immobilization of microorganisms with the capability of oxidizing iron and manganese in a groundwater treatment system has been suggested as a strategy for decreasing bacterial washout. Different methods for immobilizing the microorganisms to improve the effectiveness of water treatment have been developed (Cruz et al., 2013). However, some of these methods may be cost prohibitive and/or require complex execution. The biofilm method provides an inexpensive and simple way to immobilize microorganisms in water treatment systems (Li et al., 2013). Biofilms are communities composed of bacterial cells that adhere to inert or active entities and are encased by a substrate that can be hydrated (Douterelo et al., 2014). Some bacteria with strong biofilm-forming capabilities aggregate with other microorganisms to form biofilms, and these other microorganisms can be retained in the biofilm (Rickard et al., 2004), effectively shielding all of the microorganisms from harmful toxic substances and environmental stresses to improve the adaptability of microorganisms in the environment. Thus, simultaneously inoculating iron- and manganese-oxidizing strains and biofilm-forming bacteria into a biofilter is an efficient method for removing the iron and manganese ions in groundwater.
In this study, we isolated strains that had both Fe- and Mn-oxidizing activities from groundwater well sludge. To improve the iron- and manganese-oxidizing efficiency of the strains, we optimized the cultivation conditions via response surface methodology (RSM) (Pal et al., 2009, Xu et al., 2013) The iron- and manganese-oxidizing strains and the biofilm-forming bacterium Bacillus mojavensis M1 were used as inocula for the biofilter start-up. Four lab-scale biofilters with the mixture media were studied in parallel for the removal of the iron and manganese ions from groundwater. The distribution of the iron- and manganese-oxidizing strains and the removal performance were investigated in the layers of the biofilters. Our data demonstrated that the biofilm-forming bacterium enhanced the immobilization of the iron- and manganese-oxidizing bacteria and improved the iron and manganese removal ratios in groundwater. This approach provides a novel strategy for removing iron and manganese ions from groundwater.
Section snippets
Bacterial strains and media
Bacillus mojavensis M1 is a biofilm-forming bacterium with a high biofilm-forming capacity. A iron- and manganese-oxidizing bacterium, Bacillus cereus P1 (KP241859), with high iron- and manganese-oxidizing capacities (Fe and Mn removal ratios of 99.97% and 93.21%, respectively), was selected from a sludge sample of a groundwater well with high iron and manganese contents from a farm in Harbin and was stored in our laboratory (Fan et al., 2016). A selective culture medium (SCM) for isolating
Screening and identification of effective iron- and manganese-oxidizing strains
A total of eight morphologically distinct strains were isolated from the samples. Three strains that had high Fe- and Mn-oxidizing capabilities (P2, P3, and P5) were selected for characterization and further analysis. In a test of their oxidizing capabilities, the Fe and Mn removal ratios of the three strains were P2 (72.87%, 67.93%), P3 (78.45%, 67.93%) and P5 (75.68%, 69.42%), respectively, after 60 h of cultivation. A phylogenetic analysis of strains using 16S rRNA gene nucleotide sequences
Conclusions
In this study, an optimal combination of iron- and manganese-oxidizing bacteria and a biofilm-forming bacterium (Bacillus mojavensis M1) was used to inoculate biofilters and was applied in an iron and manganese groundwater treatment process. The Fe and Mn removal ratios were both approximately 95% at equilibrium. This study demonstrated that the biofilm-forming bacterium could promote the immobilization of iron- and manganese-oxidizing bacteria in the biofilters, which would enhance the removal
Acknowledgment
This research was supported by the National Natural Science Foundation of China (41271504).
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