Migration and transformation mechanisms of iron in clayey sediments during compaction: studies using simulation experiments

The behavior of iron (Fe) in clayey aquitards has a significant effect on the groundwater environment. However, the release processes and impact of Fe within clayey sediments during compaction remain unknown. Two groups of simulation experiments were carried out to demonstrate the migration and transformation mechanisms of Fe during clayey sediment compaction. Experiment A, which simulated a natural deposition condition, revealed that pressurization changed the reaction environment from oxidative to reductive by isolating oxygen. Oxidation of ferrous ions was followed by reduction dissolution of poorly crystalline Fe (III) and crystalline Fe (III) oxides. Under the microbial utilization of organic matter, the main transformation process of sediment Fe was the dissimilatory reduction of poorly crystalline Fe (III) oxides. The total Fe concentration in pore water was 0.09–11.61 mg/L, with ferrous ions predominating among the Fe species. The lower moisture content ( \(\text{<}\) $\text{<}$ ~36%) in the later stage of compaction inhibited the dissimilatory reduction of Fe (III), and the formation of Fe (II) minerals resulted in a decrease in Fe concentration. Experiment B, which simulated an artificial compaction state, revealed that the sediment Fe was primarily released by physical dissolution because of changes in pore structure and solubility. The concentration of total Fe in pore water was 0.02–1.96 mg/L, with a significant increase in response to a rapid increase in pressure. According to the estimates in the Chen Lake wetland (eastern China), the contribution of clay pore water release accounted for 19.9–31.9% of the average Fe concentration in groundwater during natural deposition.