Graphite particles as third electrodes to enhance metal removal and energy saving in a stationary electrodialytic soil system

This study reports the effects of the graphite particles as third electrodes on the performance of the stationary electrodialytic remediation (EDR) for As, Cr and Cu contaminated original soil(<1 mm), from the aspects of mechanisms and energy consumption. A series of eight tests regarding graphite addition (0–10%) and treatment duration (10–28 days) were conducted. The results demonstrated that introducing graphite particles were most favorable for Cu removal probably through generating abundant H⁺ at multiple sites and promoting Cu desorption simultaneously. The removal of amphoteric Cr and As was not significantly improved as initially expected due to the narrow soil pH range (2.8–6.7) caused by electrolysis on particle electrodes. The arsenic oxidation state changes, analyzed by XPS, indicated a disordered oxidizing/reducing environment aroused in the three-dimensional electrode (3D) EDR system was one of the main causes of low As removal. In addition, the presence of electrode particles could decelerate the growth of electrical resistance of the electrodialytic cell, and thus reduce the total electric energy consumption. Using 10% graphite particles during the stationary EDR process resulted in the increased extraction of Cu and Cr by 17% and 2%, respectively, while energy consumption to be reduced by 19 Wh. Studies indicated the use of graphite particles as third electrodes possesses a good potential to be applied for the stationary EDR system for optimization of both metal removal, especially Cu, and energy-saving.