Potential of Fe sludge addition to increase the water holding capacity of sandy soil substrates

Iron (Fe) hydroxide sludge is a by-product of open-pit lignite mining that accumulates in large quantities during acid mine drainage (AMD) treatment, where Fe²⁺ is precipitated to mitigate its environmental impact on aquatic ecosystems. Large quantities accrue, and the majority of Fe sludge is currently landfilled, although it may hold potential for beneficial reuse, for example, as a soil amendment. Hence, this study investigated the potential of Fe sludge to improve the water-holding capacity of sandy soils. A pure quartz sand and three sandy soil substrates were mixed with three different amounts of Fe oxide sludge (15, 30, and 60 t ha^-1) in pelletized and powdered form. Plant-available water-holding capacity (AWHC) was measured for all treatments, and results were compared to controls without Fe oxide sludge addition. The quartz sand's AWHC increased at all application rates of Fe sludge. In natural sandy soil substrates, Fe sludge increased AWHC at the highest application rate only in the soil material with an initial AWHC of <10 vol%. The application of powdered Fe sludge was found to be more effective than pelletized sludge. We conclude that Fe oxide sludge applied as powder has the potential to enhance the AWHC of soils with an initial AWHC <10 vol%, thus improving the quality of sandy substrates in post-mining areas. Yet, application of Fe sludge to improve soil physical properties should always consider their simultaneous impact on soil chemical properties, such as pH buffering, carbon accumulation, and effects on potentially harmful trace elements.