ABSTRACT

Perchlorate Remediation by Divalent Titanium

Perchlorate (ClO₄^-) is an emerging contaminant that is of concern because of its toxic effects on humans resulting from its interference with the functioning of the thyroid. It has been found in soils, groundwater and surface waters in over 26 states due to release from human activities and natural sources. The primary method for removing perchlorate from groundwater is ion exchange. However, perchlorate is not destroyed in this process and will ultimately contaminate the waste regenerant. Anaerobic biological processes can be used to treat perchlorate-contaminated water, but there are problems associated with treating lower concentrations and interferences by co-contaminants. Furthermore, biological methods are difficult to apply to treating waste regenerants because of their high ionic strength. Chemical reduction could avoid these problems, but perchlorate is reported to be reduced only very slowly by most chemical reductants. However, work in our labs has shown that perchlorate can be destroyed by an electrochemical process in which titanium metal is oxidized. The mechanism of this process is believed to be the production of partially oxidized titanium species that act as reductants for perchlorate. Divalent titanium (Ti(II)) is a potential specie that could reduce perchlorate. A preliminary test was conducted to test this hypothesis by preparing a solution of Ti(II) and contacting it with perchlorate. Perchlorate was destroyed with a half-life of about 1 hour, which would be very suitable in a treatment application. The cost of a treatment process based on Ti(II) cannot be reasonably estimated without additional information. However, an estimate of $0.05/1000-gal can be made for the cost of Ti(II) needed to reduce 1 mg/L of perchlorate using the theoretical stoichiometry of the process. The low cost and rapid reaction of Ti(II) with perchlorate demonstrates the potential for developing a new treatment technology based on it. Furthermore, Ti(II) should be able to destroy a wide range of other oxidized contaminants such as nitrate, chromate, bromate, chlorinated organics, arsenate, selenate and oxidized metals. The goal of this project is to determine the feasibility of developing treatment processes to destroy perchlorate and other oxidized contaminants by reaction with Ti(II). Five tasks would be carried out to achieve this goal: 1) Demonstrate suitability of analytical and experimental procedures; 2) Optimize methods for producing Ti(II); 3) Characterize destruction of perchlorate by Ti(II); 4) Determine the ability of Ti(II) to reduce other contaminants. Task 1 will evaluate ion chromatography as the technique for measuring perchlorate and chloride and a batch reactor system based on 24-mL glass vials with triple-closures as the reactor system. Task 2 will use batch reactor experiments to evaluate the effects of metallic Ti concentration, acid concentration, acid type and F/Ti ratio on production of Ti(II). Task 3 will use batch kinetic experiments to evaluate the effects of pH, initial perchlorate concentration, and initial molar ratio of Ti(II)/ClO₄^- on rates of perchlorate reduction and will use the data to develop a kinetic model of the process. This task will also conduct a chloride balance to document the extent of conversion of perchlorate to chloride. Task 4 will use batch kinetic experiments to develop preliminary data on the ability of Ti(II) to destroy other oxidized contaminants such as nitrate, chromate, arsenate, selenate, mercury and PCE. This research is expected to demonstrate that Ti(II) can be made economically and that it can be applied effectively to convert perchlorate to chloride. A kinetic model would be produced that would allow application of the process to a wide range of conditions. It is also expected to show that Ti(II) is an effective reductant for a variety of other oxidized contaminants.