| Cadmium, as an environmental pollutant, is a cumulative highly toxic element for human and animal. It can not only influence the air and water quality, but also into the soil and indirectly affect human health through the food chain. Therefore, studying the environmental behavior of cadmium in the soil is particularly important. In order to investigate the effect of single organic acid (citric acid, fulvic acid and EDTA) and mixed organic acid on the adsorption-desorption characteristics of Cd2+ on inorganic nanoparticles, we collected the yellow soil from Mingshan watershed as the studied object in the present study. By ultrasonic action, centrifuging and freezing-thawing methods, we collected the inorganic nanoparticles (≤100nm) in soils successfully. Then through equilibrium liquid adsorption method and the NaNO3、HCl as desorbent’s desorption method, we can draw the conclusion and provide the scientific reference for effective management of soil. The main research showed that:(1)The isothermal adsorption of Cd2+ for bulk soil and soil inorganic nanoparticles were increased with the Cd2+ concentration, and the increased amount of low concentration phase adsorption rate is higher than the high stage. The desorption rate of non-special sorption Cd2+ by soil inorganic nanoparticles were also increased with the Cd2+ concentration, and the desorption rate of special sorption Cd2+ were decreased with the Cd2+ concentration. Comparing the desorption rate of Cd2+ by bulk soil and inorganic nanoparticles, found that the specific adsorption was the main form of adsorption. The isothermal and dynamic adsorption of Cd2+ for soil inorganic nanoparticles were greater than the bulk soil, showed that soil inorganic nanoparticles have stronger ability of retaining of Cd2+ and other heavy metals. The dynamic processes of adsorption of Cd2+ by soil inorganic nanoparticles had both fast and slow process. Equations were used to fit adsorption isotherm, it was turned out Freundlich equation was the best one and reached extremely significant level (p<0.01), while dynamic equation with two-constant equations was more ideal.(2)Under the effect of a single organic acid, the adsorption of Cd2+ by soil inorganic nanoparticles from largest to smallest were ranking in the following order:fulvic acid> citric acid> EDTA. Low concentration organic acids promote the adsorption of Cd2+ by soil inorganic nanoparticles, and high concentration of organic acid inhibit the adsorption of Cd2+, but the concentration range of each organic acid is not consistent. When the desorption rate of non-special sorption Cd2+ by soil inorganic nanoparticles at lower concentration of Cd2+(≤10 mg/L), the fulvic acid was greater than the citric acid and EDTA, and when the concentration of Cd2+ was high (>10 mg/L), the citric acid desorption rate was higher. Under the action of the three kinds of single organic acid. equations were used to fit adsorption isotherm and the Freundlich equation fitting best. Which means the adsorption of Cd2+ by soil inorganic nanoparticles was multilayer adsorption. Under the effect of different single organic acid, the kinetic adsorption process of Cd2+ by soil inorganic nanoparticles also divided into two reactions of fast (the adsorption of 60 min before) and after the slow reaction stage. All the selected organic acid can reduce the adsorption of Cd2+, the order between the different kinds of organic acid was:fulvic acid> citric acid> EDTA, studies showed that EDTA can reduce most soil nanoparticles on the adsorption of Cd2+. The best fit of the three organic acid for the kinetics adsorption also was two-constant equations, reached extremely significant level (p<0.01).(3) Under the effect of mixed organic acid, the isothermal adsorption of Cd2+ were increased with the Cd2+ concentration, and consistented with the size of the relationship between adsorption amount and adsorption rate. Relationship between the isothermal adsorption rate was shown as: citric acid+EDTA> citric acid+fulvic acid> fulvic acid+EDTA. Under the influence of three kinds of mixed organic acid, the easy desorption rate of Cd2+ by soil inorganic nanoparticles from largest to smallest were ranking in the following order:citric acid+ fulvic acid> citric acid, EDTA> fulvic acid+EDTA. The isothermal adsorption of Cd2+ process is the best on the Freundlich equation, reached the significant level (p<0.01). Under the influence of three kinds of mixed organic acid, the dynamic adsorption of Cd2+ by soil inorganic nanoparticles increased rapidly in 60 minutes at the beginning of reaction, then increased slowly as the extension of time, finally reached adsorption equilibrium. Adsorption studies have shown that citric acid+fulvic acid and fulvic acid+EDTA combination can significantly reduce the adsorption of Cd2+ by soil nanoparticles, and combination of citric acid and EDTA had a promoting effect on the adsorption of Cd2+. Under the action of mixed organic acid, the dynamic adsorption of Cd2+ by inorganic nanoparticles fitting process was still in two-constant equations for the optimal, reached extremely significant level (p<0.01), the worst fitting effect was the first-order kinetics equation and the second -order equation, only reached significant level (p<0.05).(4) Under the action of different organic acids and their combinations, the isothermal adsorption of Cd2+ by soil inorganic nanoparticles were increased with the Cd2+ concentration. Low concentration organic acids promote the adsorption of Cd2+, high concentration of organic acid inhibit the adsorption of Cd2+.However, a tendency for 1 mmol/L combination of citric acid and EDTA had a promoting effect on the adsorption of Cd2+. The dynamic adsorption of Cd2+ by soil nanoparticles was shown as:citric acid+EDTA> fulvic acid> citric acid> citric acid+ fulvic acid>fulvic acid+EDTA> EDTA. Overall, citric acid+EDTA had the strongest ability to promote the adsorption amount and EDTA had the strongest inhibition ability and can minimize the adsorption of Cd2+ by soil nanoparticles. |
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