Effects of increased sodium concentration on the re-distribution of calcium and magnesium in different media in Taihu Lake, China

Description

Taihu Lake, the third largest freshwater lake in China, has experienced rapid salinization in the past decades, however, little is known about its impact on the distribution of base cations (especially Ca and Mg) in the lake environment. In this work, the adsorption-exchange experiment was conducted using the sediment from Taihu Lake mixed with the Na solution with different concentrations. the experimental and the MINTEQ model data was used to quantitatively explain this exchange process and compared with the actual Taihu environment. The datea showed that Taihu exist obvious cation exchange process caused by salinization, and this process could alter on the distribution of Ca and Mg between the water and sediment in the lake, especially in water; i.e., elevated Na concentration can significantly increase Ca and Mg concentration in the solution, with Ca being preferably exchanged over Mg from the sediment, and the reason for this ionic preference could be attributed to the exchange affinity between base cations by MINTEQ.

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Experimental method: 1.The samples (1.5g) were placed into 50mL centrifuge tubes, and then added 30ml 100mg/L NaCl solution, which was slightly higher than the maximum concentration of Na in pore water of Taihu Lake. The tubes were shaken at 25±5℃ in a constant temperature shaker at 200r/min for different time intervals between 0.5 and 24h (0.5, 1, 1.5, 2, 3, 6, 9, 12 and 24h). The samples were centrifuged at 6500r/min for 5min, and leached by a 0.45μm membrane filter, then the cations were measured by flame atomic absorption spectrophotometer. 2.the isothermal experiments were carried: 30mL NaCl solution with the concentration of 0-1000mg/L, and about 1.5g dried sediment samples were added into the centrifuge tubes, which was shaken for 24h at 25±5°C°C and 200rpm. The suspension was then centrifuged at 6500r/min for 5min and leached by a 0.45μm membrane filter, and tested cation concentration with flame atomic absorption spectrophotometer.

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