Stability of heavy metals in soil washing residue with and without biochar addition under accelerated ageing

Highlights: • Cd^{2 +} and Pb^{2 +} in soil washing residue (SWR) exceeds TCLP regulation limit. • Heavy metal mobility in SWR was reduced by biochar addition. • Accelerated ageing mobilised heavy metals in SWR regardless of biochar addition. • Accelerated ageing show SWR poses long-term risks to the environment. Soil washing residue (SWR), which typically concentrates the washed toxic metals and is comprised of high contents of clay particles, may pose risks to the surrounding environment. This study aims to simulate accelerated ageing to assess the stability of selected metals (Cd^{2 +} (132 mg/kg), Cu^{2 +} (248 mg/kg) and Pb^{2 +} (3470 mg/kg)) in a SWR (89.68% of clay) with and without biochar treatment. The soil was incubated under constant moisture and wet-dry cycles (accelerated ageing), respectively, and the mobility and fractions of heavy metals in the soils with and without biochar treatment were examined. Under the constant moisture condition, biochar addition at 5% w/w reduced the leached Cd^{2 +} (by 1.81%) and Cu^{2 +} (by 8.70%) from SWR at day 1 and the leached Cu^{2 +} (by 51.08%) and Pb^{2 +} (by 25.36%) from SWR at day 14; however, the leached metals in the TCLP solution from the biochar-amended soils still exceed the regulatory limits (1 mg/L for Cd^{2 +}, 5 mg/L for Pb^{2 +}, no regulatory limits for Cu^{2 +}). Conversely, accelerated ageing (14 days) significantly increased the fractions of exchangeable Cd^{2 +} (from 3.63–3.94% to 6.21–6.29%) and Pb^{2 +} (from 0.025–0.027% to 0.034–0.041%) as well as the TCLP leachabilities of Cd^{2 +} (from 2.91–3.28% to 3.46–3.73%), Cu^{2 +} (from 0.08–0.10% to 0.03–0.06%) and Pb^{2 +} (from 0.25–0.35% to 0.52–0.57%) in the soils, as compared with those incubated under constant moisture, regardless of biochar addition. This study reveals challenges associated with stabilising SWR due to the presence of residual fine-grained particles.