Sorption and Transport of Iron-Cyanide Complexes in Goethite-coated Sand

Iron-cyanide complexes are present in soil and ground water because of anthropogenic inputs. We studied the sorption and the transport of the complexes ferrocyanide, [Fe(CN)(6)](4-), and ferricyanide, [Fe(CN)(6)](3-), in goethite-coated sand in column experiments under saturated conditions as influenced by flow velocity and flow interruption. Isotherm parameters obtained from batch experiments of Fe-cyanide complex sorption on goethite were used to simulate breakthrough curves in goethite-coated sand. The breakthrough curves of ferrocyanide were inversely modeled. The transport of both complexes was retarded and rate-limited. Only ferricyanide breakthrough curves revealed concentration drops after flow interruption. Simulations with batch parameters roughly reflected breakthrough curves of ferricyanide, but not of ferrocyanide. Ferricyanide sorption and desorption could not be described with the same isotherm indicating hysteresis. Since the sorption fronts of ferrocyanide breakthrough curves revealed the formation of a shoulder, it was concluded that ferrocyanide sorption in column experiments could not be described by a single isotherm, which is based on a singular sorption process. Therefore, sorption of ferrocyanide on goethite was assumed to be influenced by more than one sorption mechanism. Inverse modeling of ferrocyanide breakthrough data using the Langmuir isotherm resulted in erroneous sorption maxima.