| Since1930s, with rapid development of industry and agriculture, a large amount ofpollutants have been taken into the environment. As a result, there has been severedeterioration of environmental quality, especially the heavy metal contamination inagricultural soils. Due to the multi-source, accumulation, concealment of heavy metalcontamination is, it has become a serious and urgent environmental problem needed to solveat present.Soil heavy metal contamination has long duration. Aand it is not easy decomposition inthe biological material cycle and energy exchange, but easy to accumulate, stable property,and difficult to degrade in the environment, it can be adsorbed by crops, and inhibit thegrowth, development and production of crops, heavy metal contamination could go into theplant body through roots, and then could be transfered and accumulated through the foodchain, and finally harm to human health. Therefore, heavy metal contamination is not only asimple problem of environmental pollution, but also a crisis for food safety production in ourcountry. In the study, eighteen typical soils which represented the different climatic and zonalcharacteristics in China were collected. The cadmium was elected as a research object that is amore serious pollution in soils at present. There were primary study on the characteristics ofadsorption-desorption mechanism and kinetics of cadmium, and analysis of undertook theadsorption thermodynamic characteristics and hysteresis effect. At the end, the effects of lowmolecular weight organic acids on cadmium adsorption-desorption were studied. The mainconclusions were as follows:1. The adsorption-desorption characteristics of cadmium in the soils(1) When the equilibrium concentrations of cadmium in soil solution were lower, theadsorption capacity increased rapidly with increase of cadmium concentration. But theadsorption capacity decreased gradually as the concentrations of cadmium further increasing.Alkaline soil on cadmium adsorption capacity was greater than the acidic soil. The correlationindex (R2) and standard deviation (SE) of different models for quality judgment indicated thedifferent types of soil had the different cadmium adsorption equilibrium model. This reflectedthat the effects of soil propertied on cadmium adsorption mechanism.(2) pH and CaCO3were the main factors which influence the soil cadmium absorption. CEC, free alumina oxide and organic matter had positive contribution on cadmium adsorption;CEC, free iron oxide and clay had also positive contribution on cadmium adsorption affinity.In acidic soil the cadmium desorption rate was relatively large than alkaline soil. So when thecadmium went into the acidic soil there was a certain migration and diffusion risk. The secondorder power function was the most suitable to the relationship between cadmium adsorptionand desorption amount. Then cadmium desorption increased with the cadmium adsorptionincreasing.2. The energy feature and the hysteresis effect of cadmium adsorption in soils(1)The cadmium adsorption process was a spontaneous reaction, and the hightemperature was favorable to the cadmium adsorption. The Gibbs free energy changeΔG°values can be used to predict the cadmium desorption capacity. The main chemical bondingforce was not the same in different soils because of different soil types.(2) There was a significant hysteresis in the process of cadmium adsorption-desorption.The hysteresis effect increased with the cadmium concentrations in equilibrium increased.The calcium carbonate contents and the pH values were higher, the hysteresis index valueswere higher. Maybe it related with forming inner-complexes that was difficult to be desorbedwhen the cadmium was adsorbed by soils with high pH and high calcium carbonate content.3. The kinetic characteristics of cadmium adsorption in soils(1) At25℃and40℃,the cadmium adsorption in different soils can be divided into twoprocesses, they were fast adsorption reaction and slow adsorption reaction. In the fastadsorption process, the orders of adsorption velocity Vfastwere as follows: alkaline soil>acidic soil, which accord with the whole average adsorption velocity Vaverage. The largervelocity of the fast adsorption reactions caused the less time of the transition and slowervelocity of the slow adsorption. The fast adsorption reaction of cadmium was considered to beof chemical adsorption. And the slow adsorption reaction of cadmium was considered to be ofphysical adsorption. Therefore, soil cadmium adsorption included two kinds of adsorptionmechanisms such as chemical and physical adsorption. And then the chemical adsorptiondominated the whole reaction process.(2) Because of the different soil types, the tested soils, the best kinetics model ofcadmium adsorption reactions were not the same; this reflected that the soil propertiesinfluenced the adsorption mechanism. For the different soils, the optimum model of cadmiumadsorption were the two-constants model and Elovich model. High temperatures alsoincreased the adsorption rate and the amount of Cd adsorption by the soils. In many soilproperties, the Cadmium adsorption rate was mainly affected by pH.4. Effect of organic acids on Cadmium adsorption-desorption by soils(1) In the experimental setting concentrations range, when the cadmium concentrations were low, the effect of citric acid and malic acid on adsorption cadmium was not obvious.Along with the cadmium concentrations gradually increased, two kinds of organic acids hadinhibition on cadmium adsorption. And effect of citric acid inhibition was larger than malicacid. Comparing the fitting of cadmium adsorption isotherms that was found that theparameters1/n (presence acid)<1/n (without acid), kF(presence acid)<kF(without acid). Itindicated that the soil cadmium adsorption strength at presence organic acid was greater thanwithout organic acid. This reflected that the complexation enhanced with the cadmiumconcentrations increasing whenorganic acid ligand and soils competed to adsorpt cadmium atpresence of organic acid. The amount of cadmium adsorption was decreased and mainlyadsorption was in high-energy sites, then the adsorption strength relatively increased.(2) The desorption capacity order of three kinds of desorption agent was: citric acid>malic acid> NaNO3. Among them, citric acid desorption capacity in acidic soil was moreobvious.The total acidity of citric acid and the complex stability of citric acid formed withcadmium were greater than malic acid. So the complex ability of citric acid with cadmiumwas greater than malic acid, which caused the desorption ability of citric acid was greater thanthe malic acid and NaNO3.(3) Under a range of organic acid concentrations (0~5.0mmol/L),the adsorptioncapacity decreased gradually with the organic acid concentrations increasing in alkaline soils.And the organic acid inhibitted the cadmium adsorption. The organic acid can promote thecadmium adsorption at lower organic acid concentrations in acid soils and can inhibit thecadmium adsorption at high organic acid concentrations. The inhibitory abilities of3kinds oforganic acids were: citric acid> malic acid> tartaric acid. The influence of different organicacid on cadmium adsorption by different types of soil was not only relevant to soil properties,but also the nature of organic acid and cadmium. The distribution of organic acid in soilsolid/liquid phase and complexing ability between organic acdis and cadmium were theimportant factors that influenced the direction and degree of reaction. Compared with control(NaNO3system), the tartaric acid can promote the cadmium adsorption and the citric acid andmalic acid can inhibit the cadmium adsorption under a range of pH(2~7). The influence of3kinds of organic acids on cadmium adsorption in different types of soil was: tartaric acid>control (NaNO3system)> malic acid> citric acid. In a word, regardless of at presence oforganic acid or absence of organic acid the cadmium adsorption increased with the pHincreasing.
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