Effect of Eisenia fetida Earthworm in the Presence of Organic Matter for Bioremediation and Bioavailability of Cadmium in Contaminated Soils

In terms of environmental hazard assessment, earthworms are ecologically important components of soil and considered as a biological indicator to maintain soil health and quality. The aim of this study was to assess the efficacy of Eisenia fetida earthworms in the bioremediation of cadmium in contaminated soils. For this purpose, this study was carried out on a contaminated soil sampled from the Ahangaran mine, 26 Km far from the Malayer town and surrounding lands. In this study, six soil samples were collected from 0-15 cm depth at different intervals around the mine lands and one soil sample (control sample) from the non-polluted area. This experiment was investigated as a factorial in the form of a complete randomized block design with the first factor of organic fertilizers (cow manure, carrot pulp, vermicompost, and control) and the second factor was existence or absence of earthworms. Twelve earthworms with 0.3 to 0.6 g were selected for each soil sample (300 g). The worms were exposed to cadmium-contaminated soils at concentrations of 1.159 to 6.658 mg kg-1 for 42 days. At the end of the experiment, the concentration of cadmium was measured in soil and whole body of earthworms. The results showed that earthworms have different capacities for absorbing and accumulating cadmium, which is the behavior of earthworms resulting from the diet of earthworms. The highest concentration of cadmium in the whole body of earthworms was found in all treatments at point S3 (soil sampled from the third point), which was more polluted than other sampling points. The addition of cow manure to other treatments also increased accumulation of cadmium in the soil worm tissue. The low concentration of cadmium in the soil led to the high bioaccumultion factor of this metal for earthworms. In this study, the amount of bioaccumultion factor for cadmium was more than one. Application of cow manure, vermicompost and carrot pulp treated with earthworms, significantly increased 1.54, 1.03, 1.38 mg kg-1 Cd in the organic fraction and increased 3.02, 2.79, 2.69 mg kg-1 Cd in the carbonate fraction. Extractable fraction, which contains exchangeable forms and carbonates and organic fraction, can be a good indicator of strong adsorption of cadmium in soil for earthworms. In general, the use of organic fertilizers with earthworms increased cadmium in the organic sector.