Chelating agents desorb soil acid-soluble and iron‑manganese oxide-bound Cd into dissolved Cd-chelate complexes yet with low phytoavailability

The use of chelating agents in remediation of cadmium (Cd)-contaminated soils by leaching has gained popularity. However, the environmental risks associated with this practice, particularly its impact on soil Cd phytoavailability, remain poorly understood. This study investigated the effects of three common chelating agents, including citric acid, nitrilotriacetic acid (NTA), and glutamate diacetate acid (GLDA), on soil Cd phytoavailability through soil incubation (14 days) and Sedum plumbizincicola pot experiments (160 days). Water extraction, Ca(NO₃)₂ extraction, and the Community Bureau of Reference (BCR) sequential extraction were employed for soil, and plant Cd contents were analyzed. NTA and GLDA increased water-soluble Cd concentrations by over 50-fold, whereas citric acid had no significant effect. Amendment with NTA and GLDA resulted in reductions in acetic acid-soluble Cd (from 36–52% to 16–17%) and iron‑manganese oxide-bound Cd (from 31–35% to 11–22%) during water extraction followed by BCR extraction. This suggests that these chelating agents can mobilize carbonate-bound Cd and iron-manganese oxide-bound Cd. However, the increased water-soluble Cd concentrations did not enhance Cd uptake in Sedum plumbizincicola, indicating the low phytoavailability of these strong organic Cd complexes. Importantly, this study also advances our understanding of soil water-soluble Cd species as an indicator for phytoavailability: i.e., only free and weakly complexed Cd, but not strongly chelated Cd, are phytoavailable.