| In order to investigate the effect of low molecular weight organic acids, the low molecular weight organic acids(citric acid, malic acid and oxalic acid)on adsorption and desorption of Ca2+ by soil inorganic nanoparticles of the soil of Mingshan garden was studied by Ultrasound-centrifugal-freezing and thawing tests to collect the soil inorganic nanoparticals (≤100nm) successfully, thermodynamic tests and dissolution by using NaNO3 and HCl.Finally, it could provide scientific reference for effective management of tea garden soil. The main research showed that:(1) The dynamic processes of adsorption and desorption of Ca2+ by Soil Inorganic Nanoparticles had both fast and slow process. As for the adsorption, the equilibrium reached after 120min of reaction, while the equilibrium of desorption reached at 720min of desorption reaction. Through comparing the maximum amount of adsorption and desorption, in addition to electrostatic attraction of Ca2+ by soil inorganic nanoparticles, the specific adsorption could also be found. By comparing five different kinetic equations, which used to fit the process of the dynamic adsorption of Ca2+ by soil inorganic nanoparticles, the second kinetic equation is the best.(2) In the thermodynamic adsorption of Ca2+ by Monsanto tea garden soil inorganic nanoparticles, the adsorption amount increased with the Ca2+ concentration. Langmuir, Freundlich and Temkin equation were used to fit adsorption isotherm, it was turned out Freundlich equation is the best one, which means the adsorption isotherm of Ca2+ by Monsanto tea garden soil inorganic nanoparticles was multilayer adsorption. The desorption rate (non- specific adsorption) of Ca2+ by Monsanto tea garden soil inorganic nanoparticles increased with external Ca2+ concentration; while the desorption rate (specific adsorption) decreased. Specific adsorption was the main form of adsorption by comparing the desorption rate.(3) In the single action of low molecular weight organic acids, citric acid and malic acid promoted adsorption kinetics of Ca2+ by Monsanto tea garden soil inorganic nanoparticles, and the promote capacity of citric acid was larger than malic acid; while oxalate inhibit adsorption kinetics. Second dynamic equation fitted the adsorption kinetics (under single action of low molecular weight organic acids) of Ca2+ by soil inorganic nanoparticles better. Both citric acid and malic acid promoted desorption of non-specific adsorption of Ca2+ by soil inorganic nanoparticles. The promote capacity of citric acid was larger than malic acid; while oxalate inhibitted desorption of non-specific adsorption.(4) Under the combined effect of low molecular weight organic acids, adsorption of Ca2+ by Monsanto tea garden soil inorganic nanoparticles increased with time. Under the combined effect of citric acid and malic acid, in the same time gradient range, these amounts of adsorption of Ca2+ by Monsanto tea garden soil inorganic nanoparticles were larger than these under acid-free conditions. The adsorption amounts of various periods of time show, under different combination of low molecular weight organic acids, the adsorption of Ca2+ consistent with the increasing of time. The adsorption rate had different result. The adsorption rate, from largest to smallest were the ones under the effect of citric acid and malic acid combination, oxalic acid and citric acid combination, and the malic acid and oxalic acid combination. Second dynamic equation was the best one to fit the dynamic adsorption of Ca2+ by soil inorganic nanoparticles under the different combined effect of low molecular weight organic acids.(5) Low concentrations (0.1～1mmol/L) of citric acid and malic acid promoted the desorption of Ca2+ by soil inorganic nanoparticles,while oxalic acid, high concentrations (>1mmol/L) of citric acid and malic acid inhibitted the desorption. Under the same conditions the concentration of low molecular weight organic acids, the promotion capacities were ranking in the following order:citric acid> malic acid> oxalic acid; The inhibition capacities were oxalic> malic acid> citric acid. Through thermodynamics method, Freundlich equation fitted the characteristics of adsorption of Ca2+ by soil inorganic nanoparticles under the effects of citric acid, oxalic acid and malic acid best.(6) Different combination of acid had different effect on the adsorption of Ca2+ by soil inorganic nanoparticles. Under the effect of combination between low concentrations (0.1-1 mmol/L) citric acid and malic acid, the adsorption was larger than acid-free condition, while high concentrations (1-10 mmol/L) acid effect showed the opposite result. When the concentrations of 0.1 mmol/L malic acid and oxalic acid were both added, or oxalic acid and citric acid were both added, the adsorption was promoted, while the concentration was lower than 0.1 mmol/L, the adsorption was inhibited. The adsorption of Ca2+ by soil inorganic nanoparticles under different concentrations of combined acid was multilayer adsorption. The Freundlich equation was best equationto fitted this absorption. |
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