In situ electrokinetic isolation of cadmium from paddy soil through pore water drainage: Effects of voltage gradient and soil moisture

Abstract

Novel equipment based on electrokinetic geosynthetic (EKG) technology was developed for the in situ isolation of heavy metals from paddy soil through pore water drainage. Four mutually independent field plot experiments (A, B, C, and D) were conducted to investigate the effects of soil moisture (complementation or not) and the voltage gradient (0.5, 1.0, and 2.0 V cm−1) on pore water drainage capacity, cadmium (Cd) removal performance, energy consumption, and soil chemical properties. 0.03 M FeCl3 and 0.03 M CaCl2 were used to saturate the soil for 24 h, the reduction in the total soil Cd content was 29.63%, 25.93%, 38.27%, and 41.98% after 7 days of treatment, and the corresponding energy consumption was 3.48, 16.79, 12.23, and 9.47 kWh for plots A, B, C, and D, respectively. Soil moisture complementation and an increase in the electric field intensity favored soil Cd removal, but the most economical operation condition was a voltage gradient of 1 V cm−1 and no soil moisture complementation. Soil pore water drainage through gravity and Cd electromigration were the main drivers of soil Cd isolation; higher pore water drainage resulted in higher soil Cd isolation. Compared with conventional EK technology, ferric hydroxide was precipitated in the cathode area at a pH value higher than 3.0; the precipitation blocked the guiding gutter of the EKG sheet, resulting in an increased level of pore water and a higher concentration of effluent Cd concentration in the anode area. The saturation of low concentrations of FeCl3 and CaCl2 caused 18.60%–30.41% and 366%–516% increases in the soil Fe and Cl contents compared with the untreated control; however, the residual impact was acceptable for rice growth.