The formation of iron hydroxide coatings in an Emscher Marl: inverse reactive transport modeling of reactive surface area

The possible effects of iron oxide coatings on the reactive surface area of calcite in column experiments have been studied and then modeled numerically. A column with six compartments separated by teflon filters was packed with Emscher Marl (essentially calcite). The marl was initially mixed with corundum as an internal standard. Hydrochloric acid with pH 3 was percolated through the column for a given period, after which the mineralogical changes were quantified by X-ray diffraction ex-situ. Then, the column compartments were re-filled and again percolated with HCl. This procedure was repeated five times. The losses and gains of calcite in the column compartments provided the data basis for modeling the entire experiment with the reactive transport code TOUGHREACT using a kinetic rate law. The experimental results showed that during the first period, loss of calcite in the first compartment is about 50 % less than that determined from the theoretical analysis. This showed the entrance of acid into the higher compartments through preferential flow paths (wormholes) which was observed at the boundary between sample and cell wall. This pattern could also be verified by the relatively high Peclet and low Damköhler numbers, showing a strongly advection-dominant system. Apart from calcite dissolution, structural Fe²⁺ released from calcite oxidized and formed iron hydroxide (FeOOH) coatings along preferential fluid pathways. Despite specific assumptions such as using pure calcite in the model, a comparison between modeling results and lab observations is instructive. The simulated calcite change patterns in the most compartments are consistent with the experiments. Some discrepancies are noted in the last compartment, which may bring the attention to a need for model improvements.