连续解吸中离子强度对可变电荷土壤和高岭石体系pH的影响

摘要

研究了两种可变电荷土壤中在去离子水中和0.1 mol L-1 NaNO3溶液中吸附铜离子和钙离子后依次被浓度从低到高的NaNO3溶液连续解吸时,离子强度变化对每次解吸前后体系pH变化(ΔpH)的影响,为了进一步了解其相关机理,作为对照,也研究了各种条件下,离子强度变化对高岭石ΔpH的影响.结果表明,无论是否有吸附性二价阳离子的存在,或者吸附性阳离子属性(专性吸附型离子或电性吸附型离子)如何,当样品依次被去离子水、0.01 mol L-1 NaNO3、0.1 mol L-1 NaNO3以及1 mol L-1 NaNO3解吸时,在去离子水中和NaNO3溶液中的解吸过程对ΔpH的影响截然不同.总体而言,如果解吸时体系离子强度趋于降低,ΔpH将为正值,反之则为负值,且ΔpH变幅开始增大时的起始点所对应的pH吸附基本相同,该起始点应该与高岭石ZPC(电荷零点)紧密相关.对上述结果的分析表明,解吸过程中,引起上述ΔpH变化规律的根本原因之一是连续解吸过程中的离子强度变化导致的可变电荷表面的表面电位变化.%To investigate effect of change of ion-strength on change of pH(ΔpH)during successively desorption,a serious desorption tests were conducted on using electrolyte solutions,including de-ionized water,varied in NaNO3 concentration from low to high,to desorbed successively copper ions or calcium ions adsorbed by two variable charge soils(Ali-Haplic Acrisol from Jinxian of Jiangxi and Hyper-Rhodic Ferralsol from Kunming of Yunnan). To explore relative mechanism further,kaolinite is also be used,and the similar experience processes have also been conducted without the addition of bivalent adsorbing cations. Results showed that de-ionized water and NaNO3 electrolyte solutions differed in effect on ΔpH,when samples were successively desorbed in de-ionized water,0.01 mol L-1 NaNO3,0.1 mol L-1 NaNO3,1 mol L-1 NaNO3,respectively,no matter if the addition of bivalent adsorbing cations,or the type of cations adsorbed (specific adsorption ions or electrolyte adsorption ions). Although the value of ΔpH is different in different condition of desorption and different samples,the tendency of change of ΔpH is just the same. In general,ΔpH was always positive,when the samples were desorbed in de-ionized water,and was generally negative when desorbed in NaNO3. ΔpH will decrease with the increasing times of desorption in de-ionized water, and will be maintain not change or slight increase when desorbed in the same concentration of NaNO3 solution for the second or the third times. There is seemly the similar point of beginning of pHad(pH of equilibrium suspension of adsorption)for the variable charge soils and kaolinite,which is relative to the ZPC(Zero Point of Charge)of kaolinite,when desorption occurred in de-ionized water and 0.01 mol L-1 NaNO3 for the first time. The tendency of change of value of ΔpH of which supporting electrolyte is 0.1 mol L-1 NaNO3,is sharper than that supporting electrolyte is de-ionized water,when pHad is above the special point,no matter the desorption was occurred in de-ionized water or in 0.01 mol L-1 NaNO3. As pHad is higher than that point that above-mentioned,the absolute value of ΔpH will become larger in most situation. For the first desorption in 0.01 mol L-1 NaNO3,ΔpH will get the largest absolute value. Because the pH of desorption liquid is just the same as that of the equilibrium suspension of the previous adsorption or desorption,the change of ΔpH should not be attributed to the difference of pH before and after desorbing,it can only attribute to the process of desorption,during which excluded the effect of other factors on ΔpH,for example,the hydrolysis of copper,the similar changes of ion-strength is always existed. According to the theory of four layer and newest report that was about the effect of change of ion-strength on the surface potential of variable charge surface, the increasing ion-strength is always lead to decrease of absolute value of surface potential,vice versa. Base on the above-mentioned observation,it suggest that one of the root causes that lead to this phenomena should be contributed to the change of surface potential of variable charge soils and kaolinite which caused by the change of ion-strength,on the other hand,considering the similar law of change of ΔpH with the increasing pHad,and kaolinite is the main component of the two variable charge soils tested,it can be assumed that kaolinite is the main factors that decide the regular pattern of change of ΔpH caused by the change of ion-strength in variable charge soils.