In situ electrokinetic (EK) remediation of the total and plant available cadmium (Cd) in paddy agricultural soil using low voltage gradients at pilot and full scales

Abstract

Electrokinetic (EK) remediation has been widely studied at laboratory scales. However, field-scale research is far less. In this study, a 14-day EK remediation was carried out, in a field pilot (4 m2) test and a full-scale (200 m2) application for the first time, in a cadmium (Cd) contaminated paddy agricultural field near a mining area. A low voltage of 20 V was applied at both scales; voltage gradient was 20 V m−1 and 4 V m−1 at the pilot and full scales, respectively. Samples were taken from near the anode and cathode, and in the middle of the electric field, in the soil layers 0–10 cm, 10–20 cm, and 40–50 cm. After the EK remediation, a significant portion of the total Cd was removed in all the layers at the pilot scale, by 87%, 72%, and 54% from the top down, but only in the 0–10 cm layer at the full scale by 74%. As for the plant available (exchangeable and soluble) Cd, significant removal (64%) was only observed in the 0–10 cm layer at the pilot scale. The percentage reduction of the electrical conductivity and removal efficiency of the total Cd was higher near the anode than the cathode. The soil pH was elevated near the cathode but stayed below pH 6 due to the sufficient supply of lactic acid. After the EK remediation, the concentration of the total Cd dropped below the hazard threshold, i.e. 0.4 mg (kg dry wt soil)−1 for agricultural paddy fields in China. A total energy of 2 kW·h and 0.6 kW·h was consumed at the pilot and full scales, respectively. This study showed a successful in situ EK remediation of Cd contaminated paddy agricultural soil, especially in the surface layer, with low voltage and energy demand.