Prediction of soil bases and aluminum ion leaching by the acid rain infiltration.

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The chemical equilibrium equations which describe the acid buffering capacity of soil were combined with mixing cell model, describes the water movement in soil approximately, to predict the soil bases and aluminum ion leaching owing to pH2 and pH3 sulfuric solution infiltration. The equilibrium equations were composed of aluminum dissolution, ion exchange between dissolved aluminum and bases, bicarbonate formation, sulfuric ion adsorption, charge balance and mass balance of sulfuric ion and bases, respectively. Ion exchange between dissolved aluminum ion and soil bases were lumped together with mineral weathering as acid buffering terms. Mixing cell size depends on dispersivity and that value was decided by the effluent curves of relative sulfuric ion concentration measurements.Reactions between soil and sulfuric solution depend on both contact surface area and surface area of the soil is different by the size of the soil particle. Clarify the buffering effects on soil particle size, soil column was packed with less than 2mm or 0.4mm aggregate volcanic ash soil, respectively. Infiltration measurements with pH2 and pH3 sulfuric acid solution, amount of leaching bases and that of adsorbed sulfuric ion was large in 0.4mm aggregate soil column compared with 2mm ones. These results implied that acid buffering capacities were depend on the soil-solution contact surface area and these effects must be considered in the simulations.The simulation results considering the contact surface area between acid solution and soil, measured and simulated effluent pH were well correspondent in the case of pH2 sulfuric acid solution leaching. However, simulation results of pH3 leaching were not correspondent well. This discrepancy may reflect an underestimation of sulfuric ion adsorption in low concentration sulfuric solution range. It is necessary to construct the anion adsorption curves as the function pH and concentration in detail.