Comparison of heavy metal contaminants removal using EDTA and Cyanex 302 as chelating agents for supercritical CO2-based soil remediation

Abstract

Supercritical CO2-based soil remediation technology has become one new method with its advantages of high efficiency, environmental protection, and no secondary pollution. In this study, experiments on a kilogram-scale experimental system have been conducted under different pressures, temperatures, processing times, etc. A wide range of parameter conditions (∼75 °C, ∼40 MPa, ∼40 min) have been tested to explore parameter effects on the removal efficiencies (RE) of metal contaminants: lead, chromium, and arsenic. Comparisons between Cyanex 302 and Ethylene Diamine Tetraacetic Acid (EDTA) as chelators have been conducted: (1) Both Cyanex 302 and EDTA show significant role in promoting the soil remediation process under supercritical CO2 remediation process. Additionally, the removal efficiency of EDTA for the three metals is generally lower than that of Cyanex 302 under the same operating conditions. (2) Cyanex 302 exhibits selectivity for metals. For Cyanex 302, the RE of both lead (Pb) and arsenic (As) was quite high (>93 %), while the removal efficiency of chromium (Cr) showed an obvious regularity with the increase of temperature, pressure and processing time, from 37 % to 83 %. (3) For EDTA cases, the RE of lead (Pb), chromium (Cr) and arsenic (As) showed different trends: the lowest removal efficiency found to be 48 %, 15 %, 28 % at 20 MPa, 35 °C, 20 min conditions, respectively; and the highest of 83 %, 63 %, 64 %, respectively, at 30 MPa, 55 °C conditions. EDTA cases showed different results for all three metals: the influence of temperature on removal efficiency is most obvious, while the influence of processing time on efficiency is neglectable.