离子强度对铁质砖红壤铜离子连续解吸的影响

EFFECT OF ION-STRENGTH ON SUCCESSIVE DESORPTION OF COPPER IONS IN HYPER-RHODIC FERRALSOL

研究了昆明铁质砖红壤中水吸附性铜离子依次在包括去离子水在内的浓度从低到高的NaNO3溶液中连续解吸时，电解质浓度和有机质去除对不同pH段铜离子解吸率的影响。结果表明，当铁质砖红壤中水吸附性铜离子依次在去离子水、0.01 mol L-1 NaNO3、0.1 mol L-1 NaNO3及1 mol L-1 NaNO3中连续解吸时，去离子水和NaNO3溶液对铁质砖红壤pH-铜离子解吸率曲线形状影响截然不同。在去离子水中解吸时，解吸率曲线表现为单调下降，当解吸平衡液pH达到5.3左右时，铜离子解吸率降至基本为零，且解吸次数不影响这一规律；在NaNO3溶液中解吸时，除1 mol L-1 者外，对于0.01 mol L-1和0.1 mol L-1者，铜离子解吸率曲线均在pH4.4~4.6之间出现解吸峰。部分去除有机质对解吸率的整体变化趋势并无根本影响。研究结果表明，土壤中吸附性铜离子可在去离子水中解吸的原因与双电层重叠以及离子强度降低导致土壤表面对铜离子解吸势增加有关，而解吸峰产生的可能原因与在不同pH段，体系pH对铁质砖红壤中吸持铜离子的羟基化比例和表面电荷性质的综合影响有关。

To investigate effects of change in ion-strength and depletion of organic matter on desorption of copper ions, a test was conducted on using electrolyte solutions, including de-ionized water, varied in NaNO3 concentration from low to high, to desorb successively copper ions adsorbed by Hyper-Rhodic Ferralsol, which is a typical variable charge soil collected from Kunming, China. Results show that de-ionized water and NaNO3 electrolyte solutions （0.01 mol L-1 NaNO3, 0.1 mol L-1 NaNO3 and 1 mol L-1 NaNO3） differed in effect on Cu（II） desorption with pH of the solutions. In de-ionized water, the pH-dependent Cu（II） desorption rate displayed a linear declining curve, and almost down to zero around pH5.3, on which frequency of the desorption did not have any effect. In NaNO3 solutions, except for the solution 1 mol L-1 in concentration, Cu（II） desorption rate peaked when pH lingered in the range of 4.4~4.6. Compared with Cu（II） desorption in OM depleted Hyper-Rhodic Ferralsol, that in the original Hyper-Rhodic Ferralsol was much higher in rate during its first time desorption test, and pH-Cu（II） desorption changed in shape of its rate curve and the desorption peak appeared at a lower pH. However, the pH-desorption rate curve as a whole was not much affected in tendency. The effects of overlapping of the electronic double layers and the relationship between ion strength and surface potential on repulsive potential of the surface sorption layer to copper ions could be used to explain how de-ionized water desorbed copper ions from the soil. According to the relationship between the surface potential and ion-strength, the decrease in ion-strength always means increase in absolute value of surface potential of the sorption layer, that is to say, when the surface potential of the soil is positive, the repulsive potential will increase with increasing surface charge and on the same time, the decreasing ion-strength will also lead to expansion of the diffusion layer, which will eventually result in neutralization of more charges through overlapping of electronic double layers. The two above-mentioned causes would both trigger desorption of adsorbed copper ions in de-ionized water. The appearance of peaks of Cu（II） desorption rate could also be explained as effect of hydrolysis. As the ratio of hydroxide copper ions increases with increasing pH and hydroxide copper ion is lower than Cu2＋ in valence its electrostatic attraction to the surface of the adsorption layer is weaker than that of Cu2 , which makes it easier for adsorbed hydroxide copper ions to be replaced by Na , so the increasing ratio of hydroxide copper ions along with pH increases the desorption rate of copper ions. On the other hand, the density and amount of negative charges on soil surface increase with increasing pH. Such a change favors adsorption of Cu2 , rather than desorption of the ions. When pH of the system reaches a certain threshold value and the positive effect of hydrolysis on desorption is offset by the negative effect of increased amount and density of negative charges, the apparent Cu（II） desorption rate will be at its maximum, and begin to decline. Base on the above-mentioned observation, it is held that there is ample reason to believe that the induced hydrolysis of copper ions on soil surface is also effective on exchangeable copper ions, in other words, the copper ions that are adsorbed in the diffusion layer are also very likely to be hydrolyzed. Besides, this paper also has some lines devoted to the discussion of the relationship between pH0 of kaolinite and the threshold pH value, at which the rising slope of the adsorption rate curve with rising pH undergoes a sudden change.