Revealing the long-term behaviour of nZVI and biochar in metal(loid)-contaminated soil: focus on Fe transformations

The long-term behaviour of stabilising amendments for soil remediation is rarely being tested. Therefore, we conducted time-dependent experiments using contaminated soil from a post-mining area. The soil was individually incubated for 1, 3, 12, and 15 months with i) biochar (BC), ii) nano zero-valent iron (nZVI), and iii) a composite of nZVI and BC (nZVI-BC). Two experimental designs were realised: i) mixing of the soil with the amendments and ii) applying the amendments as a layer between the soil and silica sand. With this dual approach, both the immobilisation efficiency and the solid phase transformations of the amendments were investigated under the effect of time. Solid-state (SEM/EDS, XAS, XRD) and liquid phase (pore water sampling, soil extractions) analyses were employed for a holistic assessment of the amendments. The three tested amendments demonstrated different efficiencies for metal(loid) immobilisation in this soil. Biochar and nZVI-BC were mostly efficient for long-term immobilisation, especially for Zn and Cd, while the efficiency of nZVI was instant but rather short-term and preferably towards Pb and As. The oxidation of nZVI was not directly proportional to time, and the nZVI products, such as lepidocrocite, ferrihydrite, and magnetite, were identified in the same proportions regardless of Fe⁰ oxidation. Implications of natural attenuation were also noticed in the control soil. However, enhancement of the contaminated soil with amendments is still recommended since the important metal(loid) scavengers (Fe and/or Mn oxides) in the soil occurred at least 2× more often after the amendment application.