Morphology and transformation of heavy metal during ammonia nitrogen recovery from landfill leachate by struvite: Correlation analysis and risk assessment

The recovery of ammonia nitrogen from landfill leachate by struvite (MAP) precipitation is hindered by the co-precipitation of heavy metals. This study developed an optimized ethylenediaminetetraacetic acid (EDTA) washing process to mitigate this risk. A systematic study regarding “ammonia nitrogen recovery-morphological analysis-risk control” was conducted. Optimal precipitation achieved 98.15 % ammonia nitrogen recovery. However, a substantial enrichment of heavy metals with 107.62, 8.13, 4.86, and 15.33 µg/g of Cr (Ⅲ), As (Ⅲ), Cd (Ⅱ), and Pb (Ⅱ) was observed in the precipitate, respectively. The optimized EDTA washing system (pH=5.5, 35 ℃, 120 min) effectively reduced their concentrations to 4.83, 1.00, 0.01, and 0.54 µg/g. The correlation analysis indicated that washing agent type exerted a significant regulatory effect on the removal of As (Ⅲ), Cd (Ⅱ), and Pb (Ⅱ). Morphology analysis confirmed the transition of these metals from active to stable states. The Nemerow comprehensive pollution index was reduced from 4.38 to 0.017. The Hakanson potential ecological risk was lowered from moderate to low. This research established an efficient strategy for detoxifying recovered MAP, which presented a pragmatic strategy for facilitating the safe recovery of ammonia from landfill leachate.