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 -/CrO4 2-/Cr2O7 2- 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.
Measurements of simultaneously extracted metals (SEM), acid volatile sulfide (AVS), and invertebrate toxicity were combined with X-ray absorption spectroscopy (XAS) to evaluate metal speciation and ecological hazard of contaminated sediments from the Seaplane Lagoon, Naval Air Station Alameda (CA). This site is characterized by moderate to low toxicity in surface sediments and by metal concentrations in sediments and porewaters that increase with depth. Standard 1-h ΣSEM/AVS measurements for surface sediments were compared with time-series (0.25−24 h) measurements of metal and sulfide release from sediments at 30 cm. Results show that AVS is rapidly and completely evolved after 1 h, but metal extraction continues with time and is not complete after 24 h. Sediment−water interface tests of invertebrate toxicity using sand dollar embryos (D. excentricus) and adult amphipods (E. estuarius) exposed to intact cores showed no to low toxicity in surface sediments. In sediments from 30- and 60-cm depth, high toxicity in several replicates was attributed to factors other than metal concentrations, such as high dissolved ammonia or low dissolved oxygen concentrations. Metal speciation and bonding determined from XAS show that cadmium (100%), zinc (≈80%), and manganese (≈50−70%) are associated with monosulfide phases in the sediments. The remaining fraction of zinc and manganese and all of the chromium and lead are ligated by oxygen atoms, indicating association with oxide, carbonate, or silicate minerals. Iron is present in the sediments in two fractions, as Fe(II) in the sulfide phase pyrite and as oxygen-ligated octahedral iron, probably associated with clay minerals. Bulk chemical measurements of porewaters and sediments, and speciation information from XAS, suggest that AVS could be accounted for by volatilization of porewater sulfide. Our results indicate that metals are removed from porewaters by formation of monosulfide phases only for cadmium and partially for zinc and manganese but not for lead or chromium, even though these are reduced, anoxic sediments typical of a restricted marine estuary environment. Comparison of geochemical, spectroscopic, and biological data provides new insight for the interpretation of ΣSEM/AVS measurements and points out the need for synergistic biological/geochemical tests for determining potential ecological hazard.
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