Journal of Hazardous Materials 106B (2004) 127–132 Electrodialytic removal of cadmium from wastewater sludge Mathilde R. Jakobsen, Janne Fritt-Rasmussen, Signe Nielsen, Lisbeth M. Ottosen ∗ Department of Civil Engineering, Building 204, Technical University of Denmark, 2800 Lyngby, Denmark Received 10 June 2003; received in revised form 27 October 2003; accepted 27 October 2003 Abstract This paper presents for the first time laboratory results demonstrating electrodialytic removal of Cd from wastewater sludge, which is a method originally developed for soil remediation. During the remediation a stirred suspension of wastewater sludge was exposed to an electric dc field. The liquid/solid (ml/g fresh sludge) ratio was between 1.4 and 2. Three experiments were performed where the sludge was suspended in distilled water, citric acid or HNO 3 . The experimental conditions were otherwise identical. The Cd removal in the three experiments was 69, 70 and 67%, respectively, thus the removal was approximately the same. Chemical extraction experiments with acidic solutions showed that 5–10 times more Cd could be extracted from decomposed sludge than from fresh sludge. It is likely that the mobilization of Cd during decomposition of the sludge contributes to the efficient removal of Cd by the electrodialytic method. Extraction experiments and electrodialytic remediation using distilled water as enhancement agent showed that 0.3% Cd could be extracted from decomposed sludge during 1 week in closed flasks, whereas 69% was removed during 2 weeks of electrodialytic remediation in a stirred solution in contact with atmospheric air. A combination of aerobic decomposition and electrodialytic treatment could be a promising method for Cd removal from wastewater sludge, and thus Cd could be removed without the addition of chemicals to the sludge. © 2003 Elsevier B.V. All rights reserved. Keywords: Heavy metals; Wastewater sludge; Cadmium; Electrodialytic remediation; Decomposition 1. Introduction Cadmium and Cd compounds are, compared to other heavy metals, relatively water-soluble and are therefore more mobile in soil and generally more bio-available. This results in bio-accumulation of an element which is toxic to plant and animal life. The major route of exposure to cadmium for the non-smoking general population is via food and the human exposure from agricultural crops is susceptible to in- creases in soil Cd. Increases in soil Cd contents, e.g. due to Cd in soil amendment products, result in an increased up- take by plants. In view of the danger of the chronic accu- mulation of Cd in the human body it is important to limit the daily intake through the diet, since even slightly elevated Cd concentrations in food can have significant effects in the long term [1]. Accumulation of Cd in agricultural land is a European problem, which has been a major focus area for the CSTEE, Scientific Committee for Toxicity, Ecotoxicity and the Environment. ∗ Corresponding author. Tel.: +45-45-25-22-60; fax: +45-45-88-59-35. E-mail address: lo@byg.dtu.dk (L.M. Ottosen). Wastewater sludge has traditionally been applied as a fer- tilizer to agricultural land in Denmark. The relatively high Cd concentration in the wastewater sludge has caused con- cern and resulted in a lowered limit for Cd in wastewater sludge used for this purpose [2]. Legislation that forbids any import, use or sale of products containing Cd in Denmark [3] resulted in a decrease in Cd concentration in the wastew- ater sludge but average concentrations have now stabilized at a level that generally exceeds the limit value. The obvi- ous Cd sources, such as e.g. Ni–Cd batteries have now been eliminated, and further reduction in the Cd concentrations of the sludge are difficult to obtain because the remaining sources are diffuse and therefore difficult to identify. The upper limit of Cd in wastewater sludge for applica- tion to agricultural land varies between different countries being e.g. 0.8 mg Cd/kg DM in Denmark and 2 mg Cd/kg DM in Sweden [4]. Only a small proportion of the Danish and Swedish wastewater treatment plants fulfil the actual limit values, and it is necessary to develop methods to re- move Cd from wastewater sludge to avoid large deposit sites [4]. Formerly tested methods use ferric sulfate and natural zeolite as ion exchangers [5,6] and EDTA has been used as a complex binder [7]. 0304-3894/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2003.10.005