Leachability, availability and bioaccessibility of Cu and Cd in a contaminated soil treated with apatite, lime and charcoal: A five-year field experiment

• Soil pH and SOC increased with application of amendments, but decreased with time.
• Amendments decreased leachability, availability and bioaccessibility of Cu and Cd.
• Apatite was more effective in decreasing availability of Cu than lime and charcoal.

This study evaluated the efficiency of apatite, lime and charcoal in regulating Cu and Cd leachability (toxicity characteristic leaching and synthetic precipitation leaching procedures), availability (CaCl2 and MgCl2) and bioaccessibility (simplified bioaccessibility extraction test) in a heavy metal-contaminated soil. Both soil pH and soil organic carbon content were investigated during the five-year field study. The results showed that soil pH and soil organic carbon content increased with application of amendments, but decreased with time in both the control and amended plots. Moreover, the leachability, availability and bioaccessibility of Cu and Cd in amended soils all significantly decreased compared with the control, but increased over time. Pearson's correlation analysis showed that soil pH was significantly negatively correlated with the concentrations of available, leachable and bioaccessible Cu and Cd. Bioaccessible Cu and Cd were positively correlated with the concentrations of available and leachable Cu and Cd, but they were not significantly correlated with soil total Cu and total Cd. Stepwise multiple regression analysis indicated that the variability in bioaccessible Cu and Cd was well explained by MgCl2-extractable Cu, CaCl2-extractable Cd and pH, respectively. Although the longevity of amendments decreased with time, apatite was the most effective in decreasing the availability of Cu, compared with lime and charcoal. These findings provide valuable insights for risk management during long-term in situ immobilization of heavy metals in contaminated soils.