Removal of K⁺, Na⁺, Ca²⁺, and Mg²⁺ from saline-alkaline water using the microalga Scenedesmus obliquus

Abstract

The capability of Scenedesmus obliquus to remove cations (K⁺, Na⁺, Ca²⁺, Mg²⁺) from saline-alkaline water was investigated at different salinities (0, 5, 10, 15, 20, 25) and carbonate alkalinities (0, 5, 10, 15, 20, 25, 30, 35 mmol/L). K⁺, Na⁺, Ca²⁺, and Mg²⁺ in saline-alkaline water were efficiently removed by S. obliquus. The maximum removal of the cations (29.37 mg for K⁺, 185.85 mg for Na⁺, 23.07 mg for Ca²⁺, 66.14 mg for Mg²⁺) occurred at salinity 25. The maximum removal of K⁺ (2.28 mg), Na⁺ (6.62 mg), Ca²⁺ (1.01 mg), and Mg²⁺ (0.62 mg) occurred at carbonate alkalinities of 25 mmol/L for K⁺, 35 mmol/L for Na⁺, 20 mmol/L for Ca²⁺, and 25 mmol/L for Mg²⁺, respectively. Under a salinity stress, the concentration of Na⁺ in S. obliquus increased significantly, while that of K⁺ decreased significantly. The concentrations of Ca²⁺ and Mg²⁺ decreased as well. The ratios of K⁺/Na⁺, Ca²⁺/Na⁺, and Mg²⁺/Na⁺ were significantly lower in all salinity treatments than those of the control. Under alkaline stress, the concentrations of Na⁺ and K⁺ in S. obliquus decreased significantly and the ratios of K⁺/Na⁺, Ca²⁺/Na⁺, and Mg²⁺/Na⁺ were significantly higher in all treatments than in the control. Moreover, the concentrations of Ca²⁺ and Mg²⁺ in S. obliquus at alkalinities of 5–10 mmol/L were significantly higher than those of the other treatments. The removal of Na⁺ by S. obliquus mainly occurs through biosorption, and Mg²⁺ and Ca²⁺ were removed through both biosorption and bioaccumulation.