Research PaperResourceful treatment of harsh high-nitrogen rare earth element tailings (REEs) wastewater by carbonate activated Chlorococcum sp. microalgae
Graphical Abstract
Introduction
Rare earth elements (REEs) are widely used in chemical manufacturing, nuclear industry, metallurgy, medicine, electronics, and computer technology because of their unique properties (Anton and Frances, 2012). China is rich in rare earth resources, accounting for more than 95% of the world’s output of REEs, including products and exports (Yang et al., 2013, Dutta et al., 2016). It has been reported that more than 2000 million tons of wastewater is produced every year by the REEs industry (Zhang et al., 2019). REEs tailings wastewater is acidic containing high ammonia nitrogen and low chemical oxygen demand (COD) (Xu et al., 2020). Each year, a large amount of REEs wastewater is discharged to the aquatic environment, which has caused serious water pollution and ecosystem damage, such as eutrophication, leading to nuisance algal growth, dissolved oxygen concentration decrease and fish death, undesirable pH shift, and cyanotoxin production. Increasing attention has been paid on the proper treatment of REEs wastewater (Moldoveanu and Papangelakis, 2013); the main concern is nitrogen removal. By comparison, biological methods are more competitive in the physical and chemical based methods in terms of cost and environmental friendliness. However, the low C/N ratio (0–0.1) makes REEs wastewater recalcitrant to biodegradation (Layer et al., 2019). Recent studies have shown that microalgae can even tolerate various wastewaters with a wide range of C/N/P ratios (Table S1) (Chiu et al., 2015, He et al., 2013). The concentrations of phosphorus and nitrogen were 0.03%−3% and 3%−12% of the dry biomass of microalgae (Reynolds, C. S., 2006; Prandini et al., 2016; Kim et al., 2016). For instance, Scenedesmus sp. is widely applied in the dairy wastewater with high phosphorus content (31 mg/L) (Hülsen et al., 2018), while Chlorella sp. can be cultivated in high nitrogen wastewater (767 mg/L) and high COD-loading (341 mg/L) landfill leachate wastewater (Chang et al., 2018). Zhang et al. (2019) isolated microalgal strains from REEs wastewater treatment sites and found that Co-flocculating microalgae (Parachlorella sp. and Scenedesmus sp.) has strong growth ability in water with low COD (6 mg/L) and high ammonia nitrogen (209 mg/L). In addition, microalgae can treat C, N, P in wastewater and integrate them into biomass without occupying land resources (Beuckels et al., 2015). In addition to providing economical and sustainable advanced wastewater treatment, microalgae can also produce products with commercial value, such as biofuels (e.g., TAG) (Arbib et al., 2014, Chang et al., 2020), pharmaceuticals and cosmetics (Skjanes et al., 2013). Therefore, microalgae based treatment can potentially achieve nutrient removal in a less expensive and ecologically safe way with the additional benefits of resource recovery and recycle (Sutherland and Ralph, 2019). These merits of microalgae inspired us to apply it in the treatment of refractory REEs wastewater; however, the slow removal contaminants rate of microalgae, as well as low yield and harvesting difficulties seriously limit the application of this method.
To accelerate the removal inorganic nitrogen rate of microalgae in REEs wastewater treatment, bacteria-algae symbiosis method, immobilization microalgae method and genetic engineering methods are mainly used. The bacteria-algae symbiosis is a low-cost method, but the strains are difficult to adapt to this low C/N wastewater and the harvested biomass is still limited (Muñoz and Guieysse, 2006). Notably, the growth rate of microalgae can be achieved by immobilization, which increases the contact surface between microalgae and pollutants, thereby improving REEs wastewater treatment (Mujtaba et al., 2017, Luo et al., 2018). Unfortunately, this method artificial and carrier consumes is costly. Another feasible method is to modify microalgae by genetic engineering, which exhibited strong pertinence and specific objective. However, gene silencing as the main obstacle, the requirements on technology and operation are demanding, which is difficult to achieve under common conditions (Kim et al., 2015, Fajardo et al., 2019). In order to overcome the drawbacks of the above methods to improve the efficiency of microalgae treatment, it is necessary to explore a method for the rapid removal of inorganic nitrogen as well as low-cost and operating easily to increase the potential of microalgae use for REEs wastewater treatment (Park et al., 2010, Perez-Garcia et al., 2011).
According to the characteristics of the REEs wastewater (Table S2) and the empirical formula of microalgae (C106H263O110N16P, carbon constitutes about 50% of the microalgal biomass), the carbon source is considered to be a key to microalgae efficiency (Markou et al., 2014). Organic carbon is a common carbon source, but considering most microalgae species are obligate autotrophs (Hamilton et al., 2016), which the intrinsic metabolic traits are photosynthetic and almost no use of organic carbon as a carbon source. In addition, adding organic carbon sources in low COD REEs wastewater will cause secondary pollution (Zhao et al., 2019). Accordingly, inorganic carbon can be the primary growth-limiting factor in microalgae cultivation (Cole et al., 2014). Thus, carbon (DIC) concentrations mechanism (CCM) is formed as microalgae cells adapt to changes in inorganic carbon (DIC) in wastewater (Meyer and Griffiths, 2013). This mechanism could converse DIC actively in their cells (Xu et al., 2019). CCM of single-cell green microalgae mechanism indicates that CO2 can enter the cell through diffusion, and HCO3− enters the cell through active transport (Kong et al., 2021). When the CO2 concentration is insufficient, part of HCO3− is dissociated into CO2 by the action of extracellular carbonic anhydrase (CA) and diffuses into the cell. Intracellular CA maintains the appropriate pH of the chloroplast stroma by adjusting the balance between HCO3−. Therefore, CA has an important influence on catalyzing the conversion of CO2 (Huang et al., 2017).
In this work, a robust microalgal species was cultured and fed with carbonate to simultaneously promote nitrogen removal from REEs wastewater and improve biomass recovery. The purposes of this work were to: (i) assess whether carbonate addition can improve nitrogen removal efficiency of microalgae towards the harsh REEs wastewater treatment; (ii) elucidate the mechanisms of carbonate utilization by microalgae for enhancing REEs wastewater treatment; (iii) evaluate the efficiency, stability, and resource recovery in a continuous-flow photobioreactor.
Section snippets
Characteristics of REEs wastewater
The wastewater samples were collected from the Longnan Foot Cave rare earth element mine area in Longnan County, Ganzhou City, Jiangxi Province, China (N24°85'03.19", E114°81'60.80"). The samples were sealed in 30 L polyethylene drums and stored in the laboratory at 4 ± 1 ℃. The characteristics of the REEs wastewater are shown in Table S2. The main characteristics were as follows: mean pH, 3.5; COD, < 18 mg/L; mean NH4+-N, 102.1 mg/L; and NO3--N, 43.5–44.3 mg/L.
Microalgae cultivation and characterization
The microalgal species used in
Nitrogen removal from REEs wastewater
REEs wastewater has a strong acidity, high NH4+-N and low COD concentrations, which lead to it difficult for common microorganisms and microalgae to survive. However, after 3 years of acclimation, a new type of microalgae (Chlorococcum robustum AY122332.1) adapted to the acidic REEs wastewater (Fig. S2a). Under light microscope, the microalgae were spherical (Fig. S2b), and a large amount of (EPS) was observed by SEM (Fig. S2c), which led to the fast sedimentation rate of 99.2% within 20 h (
Conclusions
In summary, resourceful treatment of harsh high-nitrogen rare earth element tailings (REEs) wastewater was achieved by the carbonate activated Chlorococcum sp. microalgae. The results can be summed up as following:
- (i)
The rates of TIN and NH4+-N removal from REEs wastewater using microalgae Chlorococcum sp. facilitated by carbonate addition (0.5 g/L) are as high as 4.45 mg/L h−1 and 3.5 mg/L h−1, respectively, with effluent quality which meets emission standards (GB 26451–2011).
- (ii)
The underlying
CRediT authorship contribution statement
Yanni Geng: Conceptualization, Methodology, Investigation, Writing – original draft. Dan Cui: Conceptualization, Methodology, Investigation, Writing – review & editing. Liming Yang: Conceptualization, Methodology, Writing – review & editing, Project administration. Zhensheng Xiong: Conceptualization, Methodology. Spyros G. Pavlostathis: Writing – review & editing. Penghui Shao: Investigation. Yakun Zhang: Methodology, Writing – review & editing. Xubiao Luo: Conceptualization, Methodology,
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.
Acknowledgements
This study was financially supported by the National Science Foundation of China (Nos. 51808279 and 51878009), the National Key Research and Development Program of China (No. 2019YFC1907900), General Science and Technology Project Fund of Scientific Research Program of Beijing Education Commission (No. KM201910005018), the Key Science and Technology Projects of Inner Mongolia Autonomous Region (No. 2019ZD001), the Key Project of Research and Development Plan of Jiangxi Province (No.
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These authors contributed to this work equally.