Abstract

Recently adopted federal and state advisory levels for perchlorate in drinking water pushed water treatment professionals and researchers towards the development of alternative perchlorate removal techniques. The current federal equivalent level for perchlorate in drinking water is 25 µg/L (state advisory levels vary between 1 to 20 μg/L), which renders most traditional anion removal techniques such as ion exchange and reverse osmosis inefficient for perchlorate treatment. These methods are strongly affected by source water quality and are not capable of providing adequate long term efficient treatment of perchlorate. Neither can these methods solve the perchlorate containing wastewater problem.

In this study, activated carbon based perchlorate removal technique are proposed and are thoroughly investigated. Activated carbon presents an attractive alternative for the removal of perchlorate. Although the removal of anions by activated carbon is not very efficient, results of independent studies have shown that it can be efficient for the removal of specific anions from water such as nitrate, bromate, and perchlorate. A large number of commercially available activated carbons were tested for the removal of perchlorate. The surface functional groups of activated carbons were studied by X-ray photoelectron emission spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FT-IR). The effects of activated carbon surface characteristics such as surface area and surface functionality and the effects of operational factors such as pH and the presence of other inorganic oxyanions and organic acids were investigated. The results showed that perchlorate removal by activated carbon is strongly pH dependent process with characteristic for anions decrease in removal with increase in the pH. Surface area of activated carbons was not the substantial factor in the perchlorate removal. The presence of inorganic anions in the same concentration range as perchlorate, except nitrate, did not affect the perchlorate removal by activated carbon. Nitrate was the only inorganic anion that competed with perchlorate removal. As for organic acids, the results showed that the higher protonation number organic acids affected more strongly perchlorate sorption on activated carbon. Perchlorate sorption on activated carbon was modeled by employing the surface complexation model that showed a good agreement with experimental results. The results of the experimental studies and studies by XPS and FT-IR suggested that perchlorate adsorption on activated carbon is specific, e.g. involves specific interaction with surface functional groups, aided by electrostatic forces as opposed to the widely accepted belief that the sorption of inorganic anions on activated carbons is mostly an electrostatic phenomenon.

Despite the fact that perchlorate out competed other oxyanions for sorption on activated carbon, the removal efficiency was still much lower than that of ion exchange. Therefore, to enhance perchlorate removal, activated carbons were functionalized with single chain cationic surfactants, specifically quaternary ammonium salts (quats). A series of quats of various chain lengths were used: benzyltrimethyl ammonium bromide (BTMA), hecyltrimethyl ammonium bromide (HTMA), octyltrimethyl ammonium bromide (OTMA), dodecyltrimethyl ammonium bromide (DDTMA), and hexadecyltrimethyl ammonium bromide (HDTMA). The deposition of quats on activated carbon surface resulted in significant increase in perchlorate removal. Perchlorate removal was better for activated carbons functionalized with longer chain length quats. Functionalized activated carbons also exhibited a much greater positive surface charge, and the increase in the surface charge was linearly proportional to the amount of quats deposited. The increase in the surface charge was greater for longer chain length quats. Approximately five-fold increase in the perchlorate removal was achieved on activated carbon functionalized with ODTMA and HDTMA. Moreover, perchlorate adsorption on functionalized activated carbons was less affected by pH than the on virgin activated carbon.

Perchlorate reduction by hydrogen gas was studied on activated carbon impregnated with catalysts (IAC). Since the reduction of perchlorate in aqueous solutions was slow due to high activation energies, a new gas phase hydrogenation technique was proposed and investigated. A series of Pt based monometallic and bimetallic catalysts (Pt with Co, Pt with Ni, and Pt with W) were used for the reduction of perchlorate by hydrogen gas. The effects of catalyst type, perchlorate surface concentration, and temperature on perchlorate reduction kinetics and reduction efficiency were studied. The proposed gas phase reduction techniques showed to be more feasible than in the aqueous phase and proved to be an effective way to completely destroy perchlorate.

The results of current study add to the robustness of the activated carbon treatment process. It was shown that by employing various approaches, the commonly accepted boundaries of the activated carbon removal process can be expanded beyond its traditional use in the removal of toxic contaminants from water and wastewater.