Adsorption-desorption Behaviors Of As (V) In The Red Soil

This thesis uses the red soil which is widely distributed in southern China as the materials to absorb arsenic in the water. The adsorption - desorption behavior and the impact of various factors on the adsorption is studied through the static adsorption experiment, the mechanism of the red soil's adsorption of arsenic systematically explored too. This study provides the theoretical guidance to the design of the treatment process of the drinking water with excessive arsenic contamination. In addition, the red soil as the adsorption material can be found conveniently and has low cost, which can be used and promoted for the design of treatment technology of distributed water in rural areas.The main reach contents of this thesis are as follows:(1) The single factor's impact of the adsorption of arsenic, such as the initial concentration of As (Ⅴ) solution, reaction time, pH value, reaction temperature and coexisting anions, is studied. The adsorption effect under the influence of the combination of different factors is studied by L16 (44) orthogonal.(2) The kinetics of adsorption in the system is studied. The impact of the initial concentration of As (Ⅴ) solution, pH and reaction temperature to the adsorption kinetics is studied. The adsorption mechanism and pattern are investigated through the numerical simulation of dynamic model under different factors.(3) The adsorption thermodynamics in the system is studied. The experimental results of different temperatures and different initial concentrations are simulated by the adsorption isotherm model in order to explore the adsorption mechanism. The adsorption thermodynamic analysis is conducted to study the thermodynamic effect.(4) The desorption experiments of the soil after adsorption are conducted to study the stability and the migration of the arsenic in the soil. The factors that affect desorption are analyzed, the kinetics of arsenic desorption in the soil is also conducted.The conclusions in this thesis are as follows:(1) The research of factors affecting adsorption of arsenic The arsenic adsorption in the red soil has a close relationship with the initial concentration of the solution, contacting time of the solution and soil , the temperature of the system and pH value and coexisting anion in soil solution. The effect dimension sequence is: System pH>contact time> the initial arsenic concentration>temperature. The arsenic adsorption rate of the soil rapidly increased in the beginning of the reaction, the growing rate declined and tended to become adsorption balance with the extension of time. The pH value in the system has a great impact on the arsenic adsorption of the red soil, the adsorption rate decreased with the growing of the pH value in the system. Temperature have a accelerate action to adsorption. The existence of H2PO₃^- and SiO3₃^- obviously repressed the adsorption.Addition concentration of SO42- enlarged to certain extent can just accelerate the adsorption action.The addition of HCO₃^- makes adsorption rate increase gradually, but Cl^- and NO₃^- did not show obvious impact to the adsorption rate.(2) The research of the adsorption kinetics and thermodynamics Langmuir isotherms can explain the adsorption mechanism.The maximum adsorption capacity calculated is 1250 mgkg^-1. The dimensionless constant separation factor RL is 0< RL<1, represents the adsorption about in the Red Soil is favorable; 1/n calculated by Freundlich isotherms is 0.1<1/n<0.5,showing that arsenate can be adsorbed easily in the Red Soil; The standard Gibbs free energy changes (ΔG) for the adsorption process is negative, indicating that the adsorption is spontaneous. The enthalpy changeΔH>0, expresses that the adsorption is an endothermic reaction. Entropy changeΔS>0, shows a desorption process accompany with the absorption. Because the arsenate into the adsorptive sites and exchange some more lively ions, or the displacement of the adsorbed water molecules caused the entropy increase greater than the entropy decrease.The preliminary research about adsorption thermodynamics shows that the adsorption is a complex process including the physical and chemical adsorption.The experiment results are fitted with adsorption kinetic models, showing that the process of arsenic adsorption in the Red Soil follows Lagrange Second-order reaction model.In addition to particle proliferation, there are other steps to control the adsorption process.(3) The research of desorptionThe desorption studies of adsorbed arsenic in soil have shown that the adsorption behavior of arsenic in the soil is coexistence process of specific adsorption and non-specific adsorption. With the adsorbed arsenic in soil increase the ratio of easy desorption also increased; Arsenic desorption trend along with time is: In the initial reaction stage adsorbed arsenic desorpted rapidly.With the desorption time going, desorption tended to a stabilization gradually; When the pH <7, the desorption increases slowly with pH raised in the first process, and then the rate becomes larger. When the pH> 7, the desorption amount tendes to a stabilization.