Environmental stability of untreated neutral metalliferous mine drainage sludge after 100 years of weathering

Untreated neutral metalliferous mine drainage sludge deposited without environmental protection for over a century was studied to assess its long-term stability. The lack of long-term stability data on such untreated sludge represents a critical knowledge gap in mine waste management. We characterized the sludge's mineralogy, chemistry, and metal(loid) mobility using XRD, SEM-EDS, and a suite of leaching tests (deionized water, 0.01 M CaCl₂, and pH-static extractions), supported by geochemical modeling. Key findings show that even after ~ 100 years, the sludge's phase composition and metal(loid) content remain largely unchanged, with high concentrations of As, Cd, Pb, and Zn comparable to those in the overlying Technosol. These metal(loid)s form their own phases (e.g., cerussite, smithsonite) or are adsorbed onto goethite and clays, limiting their release under the sludge's natural pH (6.3-7.1). Leaching is primarily confined to the sludge-Technosol interface, where the Technosol's low pH (~ 4) promotes mobilization of Cd, Pb, and Zn (e.g., up to 342 mg kg^-1 Zn and 278 mg kg^-1 Pb in CaCl₂ extracts from the Technosol). These findings are consistent with PHREEQC modeling. A remediation scenario involving raising the Technosol pH to ≥ 7 (e.g., by liming), combined with metal(loid)-binding amendments (amorphous Mn oxide, biochar, zeolite), could reduce metal(loid) migration from the sludge to the Technosol by ~ 99.9% (especially for Cd, Pb, and Zn) and significantly decrease their bioavailability to plants. This study provides the first field-based evidence that untreated neutral mine drainage sludge can remain geochemically stable for over a century, and suggests remediation strategies to ensure environmental safety of such historical mine residues.