Iron hydroxide sludges from the post-mining landscape of Lusatia, Germany: Prospects for their application as soil ameliorant based on geochemical and mineralogical characteristics

Opencast coal mining lastingly alters the landscape and hydrology of impacted regions, such as the Lusatian lignite mining district in Eastern Germany. As mining operations are being phased out, groundwater recharge leads to an increased exfiltration of iron- and sulphate-containing compounds into surface waters where they precipitate as iron hydroxide sludge (IHS). These processes occur either under natural conditions or induced through specific technical water treatment interventions. Thus, approximately 60,000 tons of IHS accumulate in Lusatia annually, which currently must be disposed of at great expense to the local authorities, due the absence of viable recycling solutions. Depending on the specific location and precipitation context, IHS can vary greatly in terms of their chemical and mineralogical composition. Initial applied research suggests that there is great potential for using iron-rich, fine-textured and, in some cases, organic-rich IHS in soil amelioration, aiming to stabilise soil carbon, improve water retention capacity and nutrient storage. Respective positive effects are particularly relevant for the restoration/improvement of sandy post-mining soils, prevalent in Lusatia. However, since some IHS are associated with potentially toxic elements, a detailed characterisation of these mineral residues is necessary to determine their suitability for potential uses. As part of the European Centre of Just Transition Research and Impact-Driven Transfer (Project: CO₂-Sequestration and Soil Recultivation Through Recycling of Mineral Residues), we are presenting a comprehensive geochemical and mineralogical systematisation of Lusatian IHS, relying on data derived from RFA, XRD, XPS, ⁵⁷Fe Mössbauer spectroscopy, wet chemical extractions, and BET surface area measurements. The high geochemical and mineralogical diversity of IHS, even across short geographical distances, is highlighted by the range of exemplary key parameters such as pH (2.1–7.7), total S content (0.1–3.0 %), Dithionite-Citrate-Bicarbonate-extractable iron Fe_d (124.1–388.4 mg/g), Oxalate-extractable iron Fe_o (63.0–322.6 mg/g), and Fe_o/Fe_d ratio describing the degree of crystallinity of the iron phase (0.16–1.0). This data from Lusatia is contextualized with published (inter)national case studies, and perspectives for the use of specific IHS types in soil amelioration are critically discussed.