Resourceful treatment of harsh high-nitrogen rare earth element tailings (REEs) wastewater by carbonate activated Chlorococcum sp. microalgae

Highlights

• Resourceful treatment of harsh high-nitrogen REEs wastewater is first achieved.

• Carbonate activated Chlorococcum sp. microalgae enhances NH₄⁺-N and TIN removal.

• The microalgae shows an excellent settle ability and biomass yield.

• The carbonate activated microalgae photobioreactor is stably operated.

Abstract

The discharge of rare earth element (REE) tailings wastewater results in serious ecological deterioration and health risk, due to high ammonia nitrogen, and strong acidity. The low C/N ratio makes it recalcitrant to biodegradation. Recently it has been shown that microalgal technology has a promising potential for the simultaneous harsh wastewater treatment and resource recovery. However, the low nitrogen removal rate and less biomass of microalgae restricted its development. In this work, Chlorococcum sp. was successfully isolated from the rare earth mine effluent. The microalgae was capable of enhancing nitrogen contaminants removal from REEs wastewater due to the carbonate addition, which simulated the activity increase of carbonic anhydrase (CA). The total inorganic nitrogen (TIN) removal rate reached 4.45 mg/L h⁻¹, which compared to other microalgal species, the nitrogen removal rate and biomass yield were 7.8- and 4.9-fold higher, respectively. Notably, high lipid contents (mainly triglycerides, 43.85% of dry weight) and a high biomass yield were obtained. Meanwhile, the microalgae had an excellent settleability attributed to higher extracellular polymeric substance (EPS) formation, leading to easier resource harvest. These results were further confirmed in a continuous-flow photobioreactor with a stable operation for more than 30 days, indicating its potential for application.

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.