The Mechanisms Of Arsenic Removal From Soil By Electrokinetic Process Coupled With Permeable Reaction Barrier

The present research was aimed at investigating the mechanisms of arsenic removal from soil by electrokinetic（EK） process coupled with iron permeable reaction barrier（PRB）. Batch tests were conducted to evaluate the adsorptivity of arsenic on the PRB media of Fe⁰. Experiments were designed to demonstrate the influence of different conditions, including the position of PRB, p H value of solution in cathode, the coexistence of ions, processing fluid types, on arsenic removal. Then the migration and transformation rule of arsenic after disposing by EK-PRB and its reaction with PRB were analyzed and discussed to bridge the gap between the concept and practical application. The highlighted results and conclusions are listed as follows:（1）Arsenic in water could be adsorbed by the PRB media of Fe⁰ effectively and quickly with removal efficiency of 80%. Adsorption quantity of arsenic increased with increasing p H from 2 to 5 and gained the best performance at p H of 5⁹ while the adsorption capacity decreased obviously at the range of 9¹1. The presence of cations(Ca²⁺、Mg²⁺) in the solutin could promote the adsorption of the arsenic to Fe⁰ with removal efficiency increasing from 64% to 75%. By contrast, the existence of HCO₃^- significantly inhibited the adsorption process with 15% decrease. Dynamics studies indicated that the adsorption process of As（III） by Fe⁰ fitted well with the Freundlich and pseudo-second order model.（2）EK-PRB could effectively remove the arsenic from the soil with removal efficiency of 24-40% which increased by 8-15% compared with single EK. Putting PRB with a distance of 15 cm to anode achieved best removal efficiency and an increase of 9%. HA and EDTA addition could improve the efficiency of arsenic removal and simultaneously reduce the energy consumption from 8.15 k Wh/g to 5.73 k Wh/g.（3）The existing states of the arsenic in soil was analyzed by BCR method. The results showed that the acid-soluble arsenic increased from 29.66% to 56.18%. Meanwhile, the reducible arsenic decreased from 78.81% to 33.89% while residual arsenic reduced as well from 11.98% to 5.96%. The coexistence of ions increased the residual arsenic content from 11.98% to 39.81% whereas EDTA could promote the transformation of residual arsenic to acid soluble with a conversion rate of 80%⁹0%.（4）Results indicated that the removal of As in EK-PRB systems was much more contributed by surface adsorption/precipitation on PRB media than by EK process. The surface characteristics of PRB media, which were qualified by X-ray photoelectron spectroscopy（XPS）, clearly confirmed the existence of arsenic（III） and arsenic（V） instead of arsenic（0）. It demonstrated that arsenic mainly removed by adsorption with the oxide on the surface of Fe⁰.