离子强度对可变电荷表面吸附性铜离子解吸的影响:高岭石

Effect of Ion-strength on Desorption of Copper Ions Adsorbed on Surface of Variable Charge: Kaolinite

为了解连续性解吸对可变电荷表面吸附性铜离子解吸的影响,研究了高岭石在去离子水和0.1 mol L^(-1) NaNO_3溶液中吸附铜离子后,依次在去离子水以及浓度由低到高的NaNO_3溶液中连续解吸时,离子强度变化对不同pH段铜离子解吸的影响。结果表明:在去离子水中和不同浓度NaNO_3中解吸吸附性铜离子时,pH-解吸分值曲线的变化趋势完全不同。在去离子水中解吸时可出现重吸附现象,而在NaNO_3中解吸时出现解吸峰现象。高岭石pH-铜离子解吸分值曲线的拐点pH与pH吸附有着对应关系,且pH特征与高岭石pH0关系密切。离子强度变化导致的吸附表面电位变化、高岭石边面的诱导水解作用和土壤表面电荷性质随pH升高的变化被认为是导致这些现象的原因。

【Objective】To investigate effect of ionic strength on desorption of Cu（Ⅱ） pre-adsorbed on the surface of a variable charge, kaolinite was put into in de-ionized water or 0.1 mol L-1 NaNO3 solution to adsorbed copper ions first and then into NaNO3 solution varying in concentration from low to high for sequential desorption. 【Method】In this study, kaolinite was pretreated with electrodialysis and then underwent a series of adsorption and desorption tests with varying pH to explore characteristics of copper ions（Cu（Ⅱ）） desorption from the clay mineral.【Result】Similar to the findings of previous studies, the fraction of Cu（Ⅱ） adsorbed by kaolinite increased rapidly from 0.05 to nearly 1 in solutions with pH varying within the set range（pH 3.0～6.3）. Regardless of concentrations of the electrolyte used, all the adsorption fraction curves could be fitted with Fischer equation with the degree of fitting being over 0.999. Also noteworthy, when Cu（Ⅱ） adsorption was carried out separately in de-ionized water and in 0.1 mol L-1 NaNO3 solution, the same in pH, the fraction of Cu（Ⅱ） adsorbed was always higher in de-ionized water than in 0.1 mol L-1 NaNO3 solution, which was attributed to the effect of the electrolyte of high concentration in the solution inhibiting Cu（Ⅱ） adsorption. Results of this experiment demonstrate that firstly, the adsorbed copper ions can be desorbed in de-ionized water, and the fraction of desorbed Cu ion declines with the desorption going on round by round in de-ionized waters the same in pH; secondly, in most cases, when the fraction of desorbed Cu ions approaches near zero, the equilibrium solutions are basically the same in pH（about pH 5.0）; this phenomenon is underatandable when the fraction of readsorbed Cu ions is taken into account; and the last, after the first round of desorption in de-ionized water, the phenomenon of readsorption will begin to appear only when the pH in the solution gets higher than a certain pH value, which means that copper ions is adsorbed rather than desorbed when the equilibrium solution goes above a certain value in pH. Results of the experiment to desorb Cu（Ⅱ） that was pre-adsorbed either in de-ionized water or 0.1 mol L-1 NaNO3 solution with de-ionized water for three rounds and then with NaNO3 solutions varying in concentration from low to high demonstrate that firstly, Cu（Ⅱ） that could not obviously be desorbed by de-ionized water could be desorbed by NaNO3 solution, and with rising pH, the pH-desorption fraction exhibited a curve of ascending first and descending, which has nothing to do with concentration of the solution and rounds of the desorption; secondly, the fraction of desorption did not vary much in 0.01 mol L-1 NaNO3 or in 0.1 mol L-1 NaNO3, which is fully understandable when the fraction of re-adsorbed in de-ionized water is taken into account; and the last, regardless of the concentration of NaNO3, the rising trend of the fraction of adsorption had a relatively gental start.In most cases the desorption fraction curve had an apparent turning point where the desorption fraction abruptly turned upwards, regardless of concentration of NaNO3 and rounds of the desorption. Although the pH of the desorption equilibrium solution at the turning point of the desorption fraction curve was not consistent, while the pH（pHch） of the adsorption equilibrium solution at the turning points of the adsorption fraction curve was quite consistent, being around pH 3.6, which means that with the system rising in pH, a similar desorption trend was observed around the turning point of the pH-adsorption curve in the desorption tests, i.e. Cu ion desorption begins with rise and then fall regardless of adsorption condition.The findings also demonstrate that pH（pHch） at the turning point of the desorption curve is related to the zero point of zero salt effect（PZSE） of kaolinite.【Conclusion】 All the above described can be attributed to changes in potential of the variable-charge surface caused by ion strength as well as hydrolysis of the edges of the kaolinite induced by increasing pH.