Biogenic manganese oxides generated by green algae Desmodesmus sp. WR1 to improve bisphenol A removal
Introduction
In nature, catalytic and oxidative processes involving manganese (III/IV) oxides are believed to play important roles in the biogeochemical cycles of organic and inorganic matter [1], [2]. Because the oxidation rate of abiotic Mn(II) is limited under nearly neutral conditions, biotic (microbial) oxidation processes are responsible for the formation of most environmental manganese oxides [3]. Microorganisms—particularly Mn-oxidizing bacteria—isolated from freshwater, marine water, soils, and sediments have been found to oxidize manganese, causing Mn-oxide precipitation in terrestrial and aquatic environments [4], [5], [6], [7]. The structural characteristics and properties of bacteria-generated Mn oxides are well-studied. These compounds tend to be nano-sized amorphous or poorly crystallized particles that are highly-reactive sorbents of metal ions (e.g., silver, lead, uranium, zinc) from water [8], [9] and possess high affinity for oxidizing various organic and inorganic compounds (e.g., humic substances, 17α-ethinylestradiol, steroid hormones, biocides, and pharmaceuticals) [6], [10], [11].
Bacterial production of biogenic manganese oxide (BioMnOx) requires aeration and organic nutrient supplementation. However, such limitations are surmountable through using algae, the main native primary producers in aquatic ecosystems. Algal growth and photosynthesis can increase dissolved oxygen (DO) levels and phycosphere pH, thus providing optimal conditions for Mn oxidation. Recently, microalgae have been used as an eco-friendly and cost-effective reclamation agent in research on topics such as the phytoremediation of contaminated water, as well as the generation of bio-energy and secondary medical byproducts [12], [13], [14], [15]. However, only a few studies have examined the role of microalgae in Mn oxidation or their capacity to perform this function (e.g., Mn uptake and oxidation by algae Scenedesmus subspicatus [16] and Chlorella sp. [17]). Furthermore, data are limited regarding the characteristics of algae-formed Mn deposits, although some work has investigated the iron and manganese deposits on Scenedesmus and Siderocelis cell walls [18], as well as biogenic manganese-calcium oxide formation on Chare coralline cell walls [19].
Therefore, the present study aims to: (i) investigate Mn oxidation capacity and its mechanisms in the algae Desmodesmus sp. WR1, (ii) evaluate the degradation reactivity of algae-formed Mn oxides towards bisphenol A (BPA), and (iii) estimate the feasibility of algal cultures to continuously regenerate Mn oxides for further environmental applications.
Section snippets
Algal culture
Samples of raw municipal wastewater (total nitrogen ≈ 23.4 mg L−1, total phosphorus ≈ 3.9 mg L−1, COD ≈ 221 mg L−1) obtained from a sewage treatment plant were treated with 5 mg L−1 BPA and 4-nonylphenol (NP) for two weeks to isolate tolerant Desmodesmus sp. WR1 algal strains that could use BPA and nonyphenol as carbon and energy sources. And their morphological and molecular characteristics were identified in our previous study (see section Morphological and molecular identification in the supplementary
Characterization of Mn oxides
Cultures turned brown after one day of incubation with Mn (supplementary material, Fig. A.2). Brown deposits were only found in the culture solution and aggregated with algal cells when the algal culture was enriched with Mn2+ (Fig. 1b). The LBB test revealed a dark blue color, reflecting a certain amount of biogenic Mn oxides in the brown particles. Previously, Furgal et al. (2015) reported that Mn2+ oxidation coupled with bacterial cultures resulted in a dark brown color [11]. SEM
Conclusion
Microalgae Desmodesmus sp. WR1 was found to oxidize Mn, forming amorphous nano-sized BioMnOx (valency of Mn3+), which significantly promoted BPA oxidative degradation. The continuous increase of BioMnOx content during BPA oxidation indicated BioMnOx regeneration. Algal growth could create conditions favorable to Mn oxidation through increasing DO and pH. Mn oxidation by microalgal Desmodesmus sp. WR1 has the potential to be used in environmental remediation. The expression of oxidase genes may
Acknowledgments
This work was supported by the National Natural Science Foundation of China (grant numbers 41476099, 41676099), the Special-funds Project for Applied Science and Technology of Guangdong Province (Project No. 2015B020235008), and the Joint Funds of the National Natural Science Foundation of China (No.U1501235). We would like to thank Professor Nora Fung-yee Tam from City University of Hong Kong for checking the revised manuscript and Editage [http://www.editage.cn] for English language editing.
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