Molybdenum-contaminated soil stabilization/solidification by mixtures of magnesium oxide and limestone calcined clay cement

Large quantities of soil excavated from civil engineering projects are found to contain leachable contaminants, including metallic oxyanions like molybdate. This study presents a novel treatment approach by identifying a mixture of magnesium oxide (MgO) and hydraulic binders for the stabilization and solidification (S/S) of molybdenum-contaminated limestone tunnel sludge. XRD analysis and geochemical modeling revealed the presence of brucite in all samples, suggesting that the immobilization mechanism was likely molybdate sorption onto brucite. The addition of cement enhanced the geotechnical properties and improved sulfate retention through the formation of ettringite. Considering efficiency and a low CO₂ footprint, the S/S formulation consisting of 1 wt% MgO and 3 wt% limestone calcined clay cement (LC3) was selected for this study. Environmental assessments were conducted using leaching test standards, including US EPA 1313 to assess pH sensitivity, US EPA 1311 (TCLP) and EN 12457-2 to evaluate the effects of curing time, at intervals from 6 hours to 90 days. Results showed that molybdenum release remained below 0.05 mg/L (the European inert waste threshold) after just 24 hours of curing, with further improvements over time under alkaline conditions. However, the treatment was ineffective at neutral or acidic pH due to brucite dissolution. The S/S of molybdenum-contaminated sludge using MgO and LC3 proved highly efficient for Mo immobilization and is suitable for industrial applications.