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Zero Valent Iron Reduction or Adsorption on Fe0 and GoethiteSan Diego, CASource: Gurol, Mirat D., and Kyehee Kim, 2000. "Investigation of Perchlorate Removal in Drinking Water Sources by Chemical Methods", Chapter 10, in Perchlorate in the Environment, Edited by Edward T. Urbansky, Kluwer Academic/Plenum Publishers, New York, New York, 2000. Project Summary: The following text was excerpted from Gurol, Mirat D., and Kyehee Kim, 2000. "Investigation of Perchlorate Removal in Drinking Water Sources by Chemical Methods", Chapter 10, in Perchlorate in the Environment, Edited by Edward T. Urbansky, Kluwer Academic/Plenum Publishers, New York, New York, 2000: INTRODUCTION Soon after a new IC method that achieved a method detection limit of approximately 1 ppb was developed in 1997, perchlorate has been detected in many drinking water wells and surface and groundwater in the western states, including Colorado River water. Perchlorate is a chemical of health concern due to its interference with the activity of the thyroid gland, and therefore its removal from drinking water sources is very desirable. A few promising technologies are being developed for removal of perchlorate, however, the stability of perchlorate makes treatment technologies difficult, especially at low concentration levels. There is no doubt that perchlorate can be removed from water by separation processes, such as ion exchange and reverse osmosis. Many researchers have been also investigating biological techniques to reduce perchlorate by organic chemicals, e.g., ethanol, acetate, and lactate under anaerobic conditions. A recent literature review indicates that many metals, including Ti(IlI), V(lI), Cr(lI), Mo(lII) are capable of reducing perchlorate to chloride or chlorate 1 However, perchlorate reduction by common reducing agents, e.g , Fe0, S2032, and 5Q32, is believed to be too sluggish to be practically useful. The present study was designed to evaluate the interaction of perchlorate with metallic iron and an iron oxide mineral. The objectives were to investigate the feasibility of (1) perchlorate reduction by metallic iron (Fe0) using ultraviolet light to promote the reaction, and (2) adsorption of perchlorate on the surfaces of metallic iron and goethite crystal(alpha-Fe(O)OH) under various conditions. BACKGROUND Perchlorate ion is the conjugate base of perchloric acid with a pI(. value of-7, existing in ground and surface waters as the salts of ammonium, potassium, magnesium, or sodium. The chlorine atom in perchlorate has an oxidation valence of +7, and its reduction to chloride or chlorate is thermodynamically very favorable. However, the reduction reactions of perchlorate are kinetically controlled by their large activation energies. An input of energy as heat or light or the presence of a catalyst would be needed to accelerate the reaction rate. Metallic (Fe0) iron was used as the reducing agent of choice in our study, because zero-valent metals, mainly Fe, Sn, and Zn, have been effective in enhancing the rate of removal of a wide range of heavy metals and halogenated compounds under anoxic conditions. These reactions involve the dissolution of ions, e.g., Fe2', from the metal surface coupled with release of electrons, which provide reduction and dehalogenation of the targeted chemicals. However, the reaction rates are generally quite low, and depend upon such parameters as pH, temperature and surface area of the metal under consideration In fact, reduction of perchlorate by metallic iron has been tried earlier by Yarmoff and Amrhein,6 who reported no observable change in perchlorate concentration. In the present study, ultraviolet (UV) light was used as a catalyst since perchlorate is known to absorb UV in the wavelength range shorter than 185 nm.7 Furthermore, radiolytic decomposition of perchlorate by the action of X-rays and y-rays on the alkali metal and alkaline earth perchlorates has been reported by Prince and Johnson.8 Chemically identifiable fragments, including 02-, C103-, C102-, C102-, C10-, CL-, 0-, ClOO, ClO3- and ClO4- have been detected in radiated samples. RESULTS AND DISCUSSION Perchlorate Removal in Fe-UV System. While no appreciable reduction was observed in the presence of 10 g L-1 Fe0, about 37% of 1000 ug L-1 of perchlorate was removed within 3 h, indicating the feasibility of the following reactions: 4Fe0 + ClO4- + 8H+ = Cl- + 4Fe2+ + 4H20 (1) Fe0 + C104- + 2H+ = C103- + Fe2+ + H20 (2) The direct role of Fe0 as a reactant implies the involvement of reactive sites on the metal and, therefore, the quantity and the condition of the metal surface is expected to strongly influence the rate of perchlorate reduction. Irradiation by UV light has been tried to accelerate removal of perchlorate by Fe0. While UV light without any Fe0 could not reduce the perchlorate concentration, simultaneous application of Fe0 and UV light was very effective on perchlorate removal. Furthermore, increasing the Fe0 concentration for a constant light intensity increased the perchlorate removal rate. In the presence of UV light, perchlorate was reduced by 77% by 100 gL-1 of Fe0 in 3 h, whereas the removal was only 37% without UV. These experiments were conducted in unbuffered neutral solutions (pH about 6.6) where the pH has increased by up to 2.0 units at the end of the experiments. The results of additional experiments showed that removal rate of perchlorate was a function of the UV intensity. For example, 77% removal of perchlorate was achieved with 100 g L-1 of Fe0 and a total UV intensity of 0.9 W cm~2, while only 40% of perchlorate was removed using the same concentration of Fe0 , but a total UV intensity of 0.6 W cm2. Furthermore, ion chromatograms of the treated samples showed significant increase in peaks that belong to C1- and C103-. Conversion of perchlorate to these ions was complete, with more than 99 % of perchlorate reduced to Cl while less than 1% converted to ClO3-;. Hence, it is apparent from these results that 1) UV light acts as a catalyst, 2) both the concentration of Fe0 and dosage of UV affect the reaction rate significantly, and 3) perchlorate is reduced quantitatively to Cl-, with less than 1% reduced to CLO3-;. Mechanistically, it is conceivable that perchlorate ion first adsorbs on the surface of Fe0, and then undergoes an electron transfer process that is facilitated by UV excitation. Perchlorate Removal in Fe0-H3P04 and FeOOH-H3P04 Systems The metallic iron in the presence of phosphoric acid was capable of removing large amounts of perchlorate from water. However, within about 5 min of contact, perchlorate concentration started to increase, indicating that the removal was due to a reversible adsorption on the metallic surface. The initially low pH of about 1, which was provided by phosphoric acid, started to increase in parallel with perchlorate desorption, which is coupled with pH increase. The pH increase might be due to the dissolution of reduce the metallic iron to ferrous ion and the reduction of water. It should be noted that no removal very effective of perchlorate was observed when sulfuric or hydrochloric acids were used in amounts to also reduce the p11 to less than 2. The same process was repeated for goethite. Orthophosphate is known to have high affinity towards iron and iron oxide surfaces. Hence, it is conceivable that the removal of perchlorate is due to a complexation between perchlorate and phosphoric acid followed by adsorption of the complex on the surfaces of the particles. However, as in the case for metallic iron, proved to be very effective in removing perchlorate in the presence of phosphoric acid perchlorate concentration stared to increase upon prolonged contact. The desorption rate was however very slow for goethite for the same initial pH of 2.5, and particle concentration of 12 g L-1. This could be due to stronger binding of the complex on goethite compared to metallic iron, or more likely goethite particles having mostly internal surface provided by the porous structure, which provides resistance to back diffusion of the complex. Perchlorate removal was observed only at acidic pH values. In fact, the desorption during the contact is very likely due to pH of the suspension increasing from initial pH values of about 2 to more than 3 during prolonged contact, for both metallic iron and goethite. This pH dependence might be explained in the context of speciation of the surface and phosphate Hence, when the pH is about 2, the surface becomes positively charged due to the dominance of FeOH2+ sites, whereas the dominant phosphate species is phosphoric acid, which is neutral. In the case of complexation between H3P04 and C104-, the complex will have a net charge of -1, and therefore will exhibit an electrostatic attraction towards the positively charged surface. However, the negatively charged phosphate species that become dominant at elevated pH values may not necessarily form a complex with C104-, and there should be no removal of perchlorate at higher pH. It should be noted that C104 did not adsorb well on the positively charged surface in the absence of phosphoric acid, as checked by reducing the pH to 2 with HCI and H2S04 acids. Additional experiments were conducted using FeOOH-H3P04 system for lower initial perchlorate concentrations of 200 and 500 ug L-1, keeping all other conditions the same. The removal of perchlorate as C/C0, where C0 is the initial concentration, is discussed for three different initial concentrations of perchlorate. About 70-75% of perchlorate was removed for all three cases within the first few minutes. However, it was released back to the solution upon prolonged contact, although more slowly for the lowest perchlorate concentration. The percent removal of perchlorate was independent of its initial concentration. CONCLUSIONS Two innovative chemical processes were investigated to determine the feasibility of perchlorate removal from water. The first process involves the exposure of perchlorate simultaneously to metallic iron and UV light under anoxic conditions. Despite the concerns of many researchers regarding the high kinetic inertness of perchlorate, it was shown that perchlorate can be reduced by metallic iron, and furthermore that UV light can accelerate the reaction rate to levels that could make the process viable for practical applications.* The results can be summarized as follows: (1) UV light promotes the reaction, while metallic iron provides electrons for reduction of perchlorate, (2) both the concentration of metallic iron and dosage of UV affect the reaction rate significantly, and (3) more than 99% of perchlorate is reduced to CI, with less than 1% reduced to C103. It is believed that perchlorate ion is adsorbed on the surface of metallic iron, and then undergoes an electron transfer process that is facilitated by UV excitation. It should be noted that C104 absorbs light at wavelengths shorter than 185 nm; however, the low pressure mercury lamps used in this study generate light primarily at 254 nm (99%), with only 1% emitted at 185 rim. Thus, these lamps are not efficient for C104 excitement. Better results can be obtained by using lamps that emit primarily at lower wavelength. The second process involves the contact of perchlorate with the surfaces of metallic iron or an iron oxide mineral (goethite) in the presence of phosphoric acid. The experimental results suggest that perchlorate can be removed up to almost 100% during the initial phases of the contact in the pH ranges of 2.0-2.5. This removal is believed to be due to formation of a complex between perchlorate and phosphoric acid that subsequently adsorbs to particle surfaces. At higher pH values very little removal of perchlorate can be observed. However, even at acidic pH, continuous contact with the surface-coupled with agitation and pH rise-seems to release the perchlorate back to the solution. It is obvious that the particles must be separated from solution before desorption of perchlorate if this is to be used as a treatment method. Unfortunately, the requirement of very acidic conditions and subsequent neutralization for pH restoration might make this process relatively expensive for typical applications. * Patent application date: August 1999. Additional Info Source: Gurol, Mirat D., and Kyehee Kim, 2000. "Investigation of Perchlorate Removal in Drinking Water Sources by Chemical Methods", Chapter 10, in Perchlorate in the Environment, Edited by Edward T. Urbansky, Kluwer Academic/Plenum Publishers, New York, New York, 2000. |
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