Study On Surface Properties And Adsorption And Desorption Of Copper Ion By The Micro-aggegates Of A Paddy Soil(Hangnitu)

The micro-aggregated particles of soils are the fundamental materials and functional cell of the soils, and its structure and combination plays a key role in control the physical and chemical process of soil environment. The different combination mode exist between the soil micro-aggregates of different sizes and organic matter and minerals, which resulted in the diversity on the adsorption and distribution of plant nutrient, the constraint capability of the heavy metals and toxic organic pollutants, and biological effectiveness. The micro-aggregated particles are cemented by clay mineral through radicel and mycelium, some of the sites strong affinity are shielded or weakened. On the contrary, air-dried and milled soil(whole soil)in laboratory traditionarily, which made organic matter and ferric oxide exposing on the edaphic surface, are more adsorption sites, and the ability of combining with heavy metal enhances. So there is different heavy metal adsorption result between whole soil method and field soil. We study the adsorption and desorption of the copper ion in the micro-aggregated particles of soils, and find the relations between the adsorption and the properties of soils. From micro scales, we reveal the physical and chemical processes of micro-aggregate particles in soil environment. Our studies provide science evidences to recognize the microscopic process of field soil and environmental functions.In this paper, size fractions of micro-aggregates were separated from undisturbed paddy soil samples in Tai Lake region by using low energy ultrasonic dispersion and freeze-dry method. The adsorption and desorption of Cu²⁺ in the micro-aggregates and the original soils, the dissolution aluminum, and pH decrease in the soil solution were studied by using equilibrium adsorption method and continue desorption method of CaCl₂ and HC1 solution. The effects of soil solution pH, phosphorus treatment, and phosphorus coexistence on the adsorption and desorption of copper were investigated. The effects of the bulk soil and the micro-aggregate particles on the adsorption and desorption of Cu²⁺ and soil solution pH changes in Cu²⁺ adsorption process were also investigated by using magnetic stirring method. At the same time, the mechanism of effects of soil on the adsorption and desorption were investigated by the equilibrium adsorption method and adsorption kinetics.Our main results are :1. The organic matter, ferric oxide and alumina of micro-aggegates in yellow soil are all highest content in clay and low in sand. Organic matter is correlative evidently with ferric oxide and alumina in micro-aggegates, which means that ferric oxide and alumina are propitious to fix organic matter. The quantity of cation exchange capacity and superficial anion (including alterable anion and permanent anion) in micro-aggregates are correlative with organic matter, ferric oxide and alumina, while the anion exchange capacity is negative correlated with organic matter.2. The adsorption of Cu²⁺ by the bulk soil samples fits well to the Langmuir Isothermmodel and the adsorption of Cu²⁺ by micro-aggegates fits well to the Freundlich Isothermmodel. Its indicated that the Cu²⁺ in the surface of bulk soil belonged to the monolayeradsorption, while on the surface of micro-aggregate particles could be multilayer adsorption.The Cu²⁺ adsorption capacity was in the order: clay fraction > sand fraction > bulk soil >siltfraction >coarse silt fraction, which correlated well to the contents of free Fe/Al oxide andorganic matter within them. The total adsorption capacity of different size particles was lessthan the bulk soil, while the sequence of desorption rate was reversed. The adsorption ofCu²⁺ was divided into fast adsorption and slow adsorption. The Cu²⁺ adsorbed at highlyselective adsorption sites by high affinity at fast adsorption stage, which process belongedto rapid adsorption stage and releases H⁺ result in the decrease of pH in soil suspensions.Then, a slow adsorption stage followed, Cu²⁺ was adsorbed on the non-selective sites by theweak affinity adsorption, which process belonged to non-specific adsorption, and Cu²⁺exchange Al process also occurred in the course of this stage. The pH of soil suspensionsfirst slowly declined, then slightly increased to balance. The fast adsorption rate increasedas the temperature, while the slow adsorption rate decreased as the temperature increase.3. The Al exchanged and pH dropped significantly when the bulk soil and micro-aggregates particles adsorbed Cu²⁺ under the conditions of plus copper concentrations more than 40 mg-L^-1 . The amounts of adsorbed Cu²⁺ and the dissolution of aluminum were significantly correlated with the decrease of the pH. Al exchange capacities were significantly correlated and non-specific adsorption capacity. Cu²⁺ adsorption amounts increased and dissolution of aluminum decreased as the increase of pH when pH is at the range of 2.0～6.5. Low pH helped to non-specific adsorption and high pH helped to specific adsorption.4. Adsorption amount of Cu²⁺ increased significantly when bulk soil and micro-aggregates were previously treated with phosphate, and it correlated well to the adsorption contents phosphate, while the desorption rate of Cu²⁺ decreased as the increase of phosphate adsorption amount. At the P coexistence soil solution, Cu²⁺ adsorption was restrained by P at the low P concentration, while adsorption was accelerated at high P concentration. Hence fore, the adsorption amount of Cu²⁺ was generally characterized as valley-like curves with the change of P concentration. The desorption of Cu²⁺ was promoted by P at a low P concentration, while the desorption of Cu²⁺ was inhibited by P at a high P concentration. The desorption rate of Cu²⁺ was generally characterized as mountain-like curves with the change of P concentration.According to the above results, we can conclude that the characteristic of micro-aggregates is different to bulk soil, which can be obtained from both equilibrium adsorption and kinetic results. In experimental condition, adsorption amount of Cu²⁺ in micro-aggregates is less than bulk soil, which is more observable as pH increases and is treated with phosphate. Inorganic minerals wrapped with organic matter, and the air-dried and. milled soil made organic matter and ferric oxide exposing on the edaphic surface, are more adsorption sites, bulk soil can adsorb more Cu²⁺ and the velocity of adsorption and desorption of Cu²⁺ are more rapid.