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

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 (Cd²⁺ (132mg/kg), Cu²⁺ (248mg/kg) and Pb²⁺ (3470mg/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 Cd²⁺ (by 1.81%) and Cu²⁺ (by 8.70%) from SWR at day 1 and the leached Cu²⁺ (by 51.08%) and Pb²⁺ (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 (1mg/L for Cd²⁺, 5mg/L for Pb²⁺, no regulatory limits for Cu²⁺). Conversely, accelerated ageing (14days) significantly increased the fractions of exchangeable Cd²⁺ (from 3.63-3.94% to 6.21-6.29%) and Pb²⁺ (from 0.025-0.027% to 0.034-0.041%) as well as the TCLP leachabilities of Cd²⁺ (from 2.91-3.28% to 3.46-3.73%), Cu²⁺ (from 0.08-0.10% to 0.03-0.06%) and Pb²⁺ (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.