New perspectives in post-mining soil restoration: Use of iron hydroxide sludge from acid mine drainage as soil ameliorant

Lignite open-pit mining causes long-term disturbances of natural soils with losses of their environmental functions. Oxidation of sulfide minerals in the dumped overburden of coal mines and subsequent leaching increase iron and sulfate concentrations in groundwater. If these waters reach the surface and become exposed to oxygen or elevated pH conditions, iron precipitates as iron hydroxide sludge (IHS). The large volumes of IHS generated annually pose a significant environmental challenge due to their complex and costly disposal. Soils developing on post-mining dump substrates in Lusatia are often characterized by high sand contents, poor structure, low water-holding capacity, and a limited ability to retain nutrients and, in particular, organic matter. In contrast, iron hydroxides provide highly reactive surfaces that can effectively bind soil organic matter (SOM), thereby improving soil structure as well as water and nutrient retention. The potential use of IHS in post-mining soil reclamation therefore warrants systematic scientific investigation. The proposed approach offers opportunities and challenges. While IHS may enhance carbon storage and other soil functions, there is a risk of releasing associated potentially toxic elements (PTEs) or immobilizing nutrients on iron oxide surfaces. Here we report results from the first year of a three-year lysimeter experiment evaluating IHS application under field conditions. We quantified potential PTE release as well as water balance, nutrient availability, and effects on SOM contents. Overall, our study provides first evidence on whether the use of acid mine drainage-derived IHS can contribute to the improvement of previously unproductive and low SOM post-mining soils.