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

The capability of Scenedesmus obliquus to remove cations (K + , Na + , Ca 2+ , Mg 2+ ) 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 + , Ca 2+ , and Mg 2+ 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 Ca 2+ , 66.14 mg for Mg 2+ ) occurred at salinity 25. The maximum removal of K + (2.28 mg), Na + (6.62 mg), Ca 2+ (1.01 mg), and Mg 2+ (0.62 mg) occurred at carbonate alkalinities of 25 mmol/L for K + , 35 mmol/L for Na + , 20 mmol/L for Ca 2+ , and 25 mmol/L for Mg 2+ , respectively. Under a salinity stress, the concentration of Na + in S. obliquus increased significantly, while that of K + decreased significantly. The concentrations of Ca 2+ and Mg 2+ decreased as well. The ratios of K + /Na + , Ca 2+ /Na + , and Mg 2+ /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 + , Ca 2+ /Na + , and Mg 2+ /Na + were significantly higher in all treatments than in the control. Moreover, the concentrations of Ca 2+ and Mg 2+ 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 Mg 2+ and Ca 2+ were removed through both biosorption and bioaccumulation.