Rate-limited sorption of simazine in saturated soil columns

Column flow experiments under saturated conditions were conducted to investigate the sorption behavior of simazine on a Tujunga sandy loam. The flow interruption technique was used to test whether rate-limited processes were present. The outflow results showed that the simazine sorption was not at equilibrium during the time of the experiment. This non-equilibrium was due to the sorption process, as the bromide tracer concentration did not change during flow interruption. Simazine breakthrough data could be best described by a two-stage, two-rate process, where the first rate is considerably faster than the second. The model predicts slow reaction kinetics, so that equilibrium was not reached even after 185 h of flow interruption. Outflow data of the first simazine breakthrough only could be described well with a one-stage, rate-limited sorption model; flow interruption was necessary to reveal the second sorption stage.