Electrosorption of Chromium Ions on Carbon Aerogel Electrodes as a Means of Remediating Ground Water

Abstract

An electrically regenerated separation process has been developed for removing unwanted ions from aqueous waste streams as a minimally polluting, energy-efficient, and potentially cost-effective alternative to ion exchange, reverse osmosis, electrodialysis, and evaporation. Ground water containing various anions and cations is passed through a stack of carbon aerogel electrodes, each having a very high specific surface area (400−1100 m2 g-1) and exceptionally low electrical resistivity (≤40 mΩ·cm). After polarization of the stack, impurity ions are removed from the electrolyte by the imposed electric field and adsorbed on the electrode surfaces. Field tests have shown that hexavalent chromium in the form of HCrO4-/CrO42-/Cr2O72- can be selectively removed from contaminated ground water with a 530 ppm total dissolved solids (TDS) background. The concentration of Cr(VI) can be lowered from 35 to 2 ppb, well below the acceptable level for the regulatory surface water discharge limit of 11 ppb. The mechanism for Cr(VI) separation involves chemisorption on the carbon aerogel anode, a process that can be reversed by cathodic polarization. Cr(VI) removal is not based upon simple double-layer charging.