| Taihu watershed is one of the most developed areas in China,with the economic development and population increase,a large amount of wastewater containing NaCl has entered Taihu Lake,which has obvious salinization trend in the past decades.In order to reveal the influence of salinization on the distribution of Ca and Mg in different media,the exchange characteristics of Ca and Mg in Taihu sediment under different concentrations NaCl solution were studied through indoor simulated NaCl solution leaching experiments.Meanwhile,the parameters provided by MINTEQ model were used to quantitatively describe this exchange process,and the influence of salinization on the re-distribution of Ca and Mg in different media is determined by combining the monitoring data of Na,Ca and Mg in Taihu Lake.The research results show that:(1)The rapid growth of population and economy and the massive discharge of domestic sewage in recent 60 years had led to the increase in Na+in Taihu Lake at a rate of 0.94mg·L-1y-1,which was about five times higher than that in 1950s,there is a tendency of salinization in Taihu Lake.The high Na+concentration in water would alter the distribution of Ca and Mg in sediment and pore water,that is,with the increasing Na+concentration,the Ca and Mg concentration would increase in pore water and decrease in sediment.In addition,cation exchange capacity and Ca/Mg in sediment have significant effects on the process of Na exchanging ion-exchangeable Ca and Mg.This is mainly manifested as follows:in the same NaCl solution,ion-exchangeable Ca and Mg in the northwest lake and surface sediment were easier to be exchanged into water by Na+adsorption than that in the southeast lake and deep sediment due to the relatively higher cation exchange capacity and Ca/Mg ratio.This result is also consistent with the distribution characteristics of base ions in our pore water and sediments.(2)The MINTEQ modelling further indicated that exchangeable Ca in sediments is easier to be exchanged into pore water than Mg under the same NaCl concentration,which is more manifested by the increase of Ca2+/Mg2+in water and the decrease of exchangeable Ca/Mg in sediment.the relatively low electrostatic attraction,hydration energy and hydration radius of Ca lead to the preferential exchange between Na and Ca.(3)There was no significant difference between the MINTEQ model results and the measured values of Ca and Mg exchange quantity(p>0.05),which further proves that the exchange processes are a chemical reaction,which occur in the outermost layer of the electric double layer,and the main mechanisms that play a role in the exchange process are ion exchange and external surface complexation.In the exchange process,Ca and Mg are adsorbed on the solid surface in the structure of ≡X2M and ≡XMCl.When the exchange reaches equilibrium,Na+ is adsorbed on the sediment surface in the form of ≡XNa,while Ca and Mg are adsorbed in the form of Ca2+,CaCl2,CaCl+,CaHCO3+,Mg2+,MgCl2,MgCl+and MgHCO3+in aqueous solution.When the concentration of NaCl exceeds 1000mg/L,the Ca in the sediment will continue to enter the lake,which will make the Ca in the overlying water constantly saturated,and the Ca will form and precipitate into the sediment.(4)When NaCl concentration in lake was 100mg/L,the model calculation also showed that the Ca exchange quantity was 99mg/kg and the Mg exchange quantity was about 11mg/kg,accounting for about 4%and 5%of the ion-exchange Ca and Mg in the sediment respectively,which was relatively consistent with the Ca2+ and Mg2+ concentrations in lake.However,Na+induced ion-exchange between the water and sediment is far from being balanced in the lake due to the current Na+ concentration much lower than the saturated exchange Na+concentration computed by the model.When the initial NaCl concentration exceeded 6000mg/L,it was predicted that exchange adsorption reached saturation and cation exchange by Na+could release 1300mg/kg Ca and 250mg/kg Mg from the sediment to lake,accounting for about 60%-70%of exchangeable Ca and Mg in sediments.Salinization has a significant effect on the re-distribution characteristics of Ca and Mg in the different media in Taihu Lake.At present,Ca2+ and Mg2+ in lake had the same upward trend,and base cations are positively correlated with Na+concentration in the water.If not controlled,it will cause drinking water safety problems.Therefore,the study has scientific significance for evaluating and predicting the influence of the salinization process on the distribution of main ions and the potential impact on the aquatic ecosystem. |
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