Sequential washing vs. thermal treatment: heavy metal management in soil-like material derived from landfills

Landfill mining generates large quantities of potentially contaminated soil-like material (SLM) that require treatment before reuse. While most earlier studies have emphasized the characterization and valorization of SLM, the present study focuses on the critical need for contaminant reduction (predominantly heavy metals) to enable its safe off-site reuse. This study compares two cost-effective methods: sequential washing with ultrapure water and low-temperature thermal treatment (ranging from 60 °C to 500 °C), specifically targeting the mobilization and immobilization behaviour of heavy metals commonly found in landfill environments. Sequential washing with ultrapure water significantly reduced the leachable fractions of As, Cr, Co and Ni, with most metals showing a substantial reductions after the first one or two wash cycles, except for Pb. In contrast, thermal treatment at 300 °C resulted in increased mobilization of Co, Cr, Mn, and Ni, while treatment at 500 °C was more effective for immobilizing Cu and Pb. A comparative analysis revealed that seven successive washings were more effective than thermal treatment at 300 °C in mobilizing Cu, Zn, As, Pb, and Cd, whereas thermal treatment was better at immobilizing Cu and Pb. The findings of the study suggest that the choice of treatment should be guided by the target metal and the desired outcome - whether mobilization for removal or immobilization for stabilization.