Multicomponent transport of sulfate in a goethite-silica sand system at variable pH and ionic strength

In this work, existing models for adsorption of protons and sulfate on goet hite and silica were used in combination with a one-dimensional mass-transp ort model to predict the transport of sulfate at variable pH acid ionic str ength in a goethite-silica system. The predicted multicomponent transport p henomena are discussed, and an evaluation is made of the sensitivity of the results to different input parameters. This information was used to select optimal conditions for independent experimental testing of the model. Colu mn experiments were carried out in which solutions of known pH, sulfate con centration, and salt concentration were infiltrated into a goethite-coated quartz sand column. The agreement between the experiments and the predictio ns is very good, especially considering the fact that no fitting of model p arameters to the actual experimental system was involved. According to the model calculations, transport behavior of sulfate in a goethite system is v ery sensitive to relatively small amounts of adsorbed sulfate initially pre sent. This could he an explanation for the remaining differences between th e predicted and the measured curves. It also implies that, for the applicat ion of such mechanistic models to soils, accurate information on the initia l composition of the soil is essential.