Perchlorate removal by modified activated carbon

Abstract

Perchlorate's (C104-) prevalent use as the propellant in rocket fuel has led to the C104- contamination of considerable amounts of water thr6ughout the United states. 1 Perchlorate contamination is especially a problem in the arid Southwestern U.S. where alternate perchlorate-free water sources are not always readily available. Perchlorate, while thermodynamically unstable, is kinetically non-reactive at low concentrations, 1 as typically found in ground and surface water conditions where contamination has occurred. In addition, perchlorate is extremely soluble in water and is a poor complexing agent, 1 which makes it a very mobile, persistent contaminant in the environment. Perchlorate has been shown to inhibit iodide uptake by the thyroid gland; and in large doses, it has been linked to anemia and fetal brain damage2. While there is no nationally established drinking water standard for perchlorate, several states, including California, Massachusetts, and Texas have taken the initiative to establish statewide action levels for C1O4- as low as 1-6 parts per billion (ppb). 3,4 Though ion exchange 5,6 and microbial methods 7,8 have been shown to be effective technologies to treat perchlorate contaminated water, the use of tailored granular activated carbon (GAC) to treat ppb perchlorate contaminated water has some advantages over these methods. Tailored GAC appears to be cost competitive compared to ion exchange (specific costs values can be obtained from tailored GAC and ion exchange suppliers) and the skeletal GAC can also be thermally reactivated once it is spent, allowing its reuse. If an ion exchange resin is regenerated for reuse, a brine solution is required which then subsequently must be handled. In contrast, ion exchange resins can not regenerated via a thermal process. Microbial means may not be well suited for perchlorate concentrations in the low ppb range. Water utilities in the U.S. are also unaccustomed to using microbial methods to treat drinking water. Unlike microbial means, many water utilities are comfortable with the use of GAC. Powdered or granular activated carbon is used in over half of the water treatment facilities in the US.; and its prevalent use dates back more than 50 years. Annually, 300,000 tons of activated carbon is used by the water treatment industry worldwide9 In this chapter, the remediation of groundwaters contaminated with ppb levels of perchlorate by virgin and tailored granular activated carbon is discussed. Rapid small scale column tests (RSSCTs) were designed and operated with both virgin (as a control) and tailored GAC to simulate fullscale GAC contactors and appraise their feasibility as means to treat ppb level perchlorate contamination. It was hypothesized that virgin GAC could be pre-loaded with cationic surfactants to appreciably increase the GAC's capacity for perchlorate. The cationic surfactants utilized in this research contained a quaternary ammonium functional group, which is comprised of a positively charged nitrogen atom bonded solely to carbon atoms. Quaternary ammonium-based compounds have a high pKa rendering the positive charge virtually independent of pH in natural waters. 10 Once the cationic surfactant is pre-loaded, the positively charged quaternary ammonium head-group of the surfactant can then act as an ion exchange site where perchlorate can be adsorbed. The quaternary ammonium functional group is the active site in strong-base anion exchange resins 5,11 which have been shown to be effective in the removal of perchlorate from water. 5,6,12 The use of cationic surfactants has also been shown to significantly increase the removal of perchlorate, as well as nitrate and chromate, in ultrafiltration processes. 13,14,15 In addition, quaternary ammonium based cationic surfactants have been shown to form ion pairs with perchlorate in electrospray ionization mass spectrometry.16. © 2006 Springer Science+Business Media, Inc.