Assessment of the immobilization effectiveness of several amendments on a cadmium-contaminated soil using Eisenia fetida.

Proper protocols for assessing the remediation effectiveness of contaminated soils are an important part of remediation projects. In the present study, the residual immobilization effectiveness of hydrated lime (L), hydroxyapatite (H), biochar (B) and organic fertilizer (F) alone and in combination was assessed by Eisenia fetida. The results showed that the application of amendments had no significant effect on the death rate and average fresh weight loss of earthworms. The earthworm Cd concentration increased with prolonged exposure time, however, the significant immobilization efficacy of amendments observed on the 7th day nearly disappeared after 28 days of exposure. The immobilization efficiencies, estimated by the earthworms internal Cd concentration, of L, H and B on the 7th day were 38.6%, 37.8% and 20.7%, respectively. These values decreased to 4.9%, 19.8% and 15.1%, respectively, on the 28th day. The detoxification effect of amendments was confirmed by the Cd subcellular fractionation in earthworms with lower proportions of Cd distributed in the metal-sensitive fractions in L, H and B treatments. The level of oxidative stress response of earthworms increased with exposure duration and amendments alleviated the oxidative damage induced by Cd to the earthworms. In addition, the pH and CaCl-Cd in soils were both increased due to earthworm life activities and gut-related ingestion. In summary, the assessment of immobilization effectiveness of heavy metal-contaminated soils using Eisenia fetida was time-dependent. The immobilization efficacy of L and H performed better than B and F on the 7th day, while H and B performed better than L and F on the 28th day. Accordingly, the short-term earthworm exposure experiment (7 days) was recommended to be an alternative approach to time-consuming plant bioassays in assessment of reduced phytoavailability in chemical immobilization remediation. But the impact of earthworms on the immobilization effect of amendments needs to be considered in practical remediation.