Modeling arsenic transformation in clay soils

This project is designed to estimate the possible transformation forms of arsenic in clay soil. A computer model was established to simulate arsenic transformations. The amount of arsenic species adsorbed, dissolved, and the predominant soluble species are calculated by the model, which is mainly based on the Freundlich adsorption isotherm, solubility data collected by Keaton and Kardos, and pH-Eh diagram established by Ferguson and Garvis. The computer language used is Turbo PASCAL version 5.5. The clay soil environment is considered to be in standard state: 25°C 1 atm. Environmental factors of the soil system which are accounted for by the model are :moisture, pH and Eh. Soil moisture affect moderate soluble species, such as more than the very soluble species ( ) and the relatively insoluble species ( As2S3, As2S5 ). The predominant soluble species is controlled by the pH-Eh condition. Factors, which are not accounted for this model and also influence arsenic transformation are discussed and explained. Processes, such as biological conversion, evaporation and volatilization, are also possible processes in clay soil system and are not considered in current version of model, which are discussed in Chapter 3. Solubility and adsorption approaches are applied in the model. However, these two approaches gives two opposite results. In the solubility approach, As2O3 ,As2O5, are very soluble. However, they are more adsorbed than dissolved in the adsorption approach. It is possible that As ( III ) and As ( V ) are highly soluble in soil water but also adsorbed largely when the presence of metal ions or other clay mineral functional groups. It is also possible that default n, k value are not good for this condition