Organic carbon stabilization by acid mine drainage–derived iron hydroxide sludge: Evidence from adsorption experiments and implications for soil amelioration

Soil organic carbon (SOC) accumulation is a slow but crucial process in sandy post-mining soils (sPMS), frequently limiting recultivation success. At the same time, acid mine drainage in post-mining landscapes generates high quantities of iron- and organic-rich residues (iron hydroxide sludge, IHS), which poses a severe environmental threat. At present, IHS are, due to a lack of viable usage options, typically landfilled. Recycling IHS as soil ameliorants could enhance SOC accumulation and stabilization by introducing highly reactive minerals to sandy substrates. However, concerns remain regarding both the fate of organic carbon (OC) initially bound to IHS and the material’s potential impact on nutrient availability as well as on the mobility of potentially toxic elements.

To investigate the sorption behavior of IHS, batch adsorption experiments were conducted using natural dissolved organic matter (DOM) of differing degrees of aromaticity with a set of mineral samples, including two IHS with contrasting properties, an sPMS and a synthetic goethite. Sorption was tested under different pH conditions, and the stability of bound organic matter was assessed by desorption experiments. Combined chemical and spectroscopic analyses provided mechanistic insight into interfacial processes.

Adsorption experiments revealed pronounced differences in OC sorption among the tested materials. While DOM sorbed most strongly to goethite surfaces not pre-occupied with organic matter, IHS accumulated substantially more OC than sPMS. At low DOM concentrations, IHS partly released initially bound OC. Increasing DOM aromaticity enhanced OC uptake and initial affinity across all sorbents and accentuated sorption differences between the two IHS. The IHS characterized by higher surface area, lower initial OC, and lower pH behaved as a stronger sorbent but, surprisingly, exhibits a lower dithionite-citrate-bicarbonate-extractable iron content and a lower share of oxalate-extractable iron than the less strongly sorbing IHS. Phosphorus was almost completely removed from solution when exposed to IHS, whereas sulfur was released into solution. Contrary to general concerns, arsenic was not mobilized from IHS, while zinc was released from one IHS across a wide pH range, with increasing mobilization under acidic conditions. To a large extent, the OC freshly sorbed from the DOM onto goethite and IHS was not re-solubilized after repeatedly exposing the materials to solutions containing no OC.

The findings indicate that IHS may serve as a locally available substrate to support OC sequestration in sPMS. However, the effectiveness of this process is governed by both sorbent and DOM composition, as well as by DOM concentration, and is also accompanied by other effects on the immobilization or release of substances, potentially affecting the reclamation success. Our results highlight the importance of careful selection of IHS with favorable chemical and mineralogical properties, as well as detailed investigation of their behavior under near-natural conditions, to enable targeted reuse as sustainable soil ameliorants within circular economy frameworks.