Thallium (TI) is an uncommon, but highly toxic element to mammals. Thallium has forty-seven isotopes having atomic masses between 179 and 210 grams, and all isotopes of thallium are toxic.
Thallium is a very prominent pollutant and spreads through the environment as waste from drilling in sulfur deposits. The United States’ accepted maximum contaminant value for thallium in drinking water is 5 μg/L. In dig sites, ores are extracted through heap leaching, which places mixtures of ores on a liner and adds chemicals as separating factors to the ores. The target ore, often sulfur in cases of thallium pollution, is extracted and the excess ores are replaced back into sediment beds. As water also flows through these sediment beds, thallium is added to the runoff.
Humans take up significant amounts of TI either directly through drinking water or absorbed through the skin and mucous membranes. Once inside the body, thallium becomes deposited in bones and the renal medulla. Symptoms of thallium poisoning include hair loss, changes in blood pH, decreased kidney function and intestinal and liver problems. The maximum contaminant level (MCL) is the highest concentration that is allowed in drinking water, when considering current methods of removal. The current MCL of thallium is 2 ppb. The EPA cannot enforce any contamination levels below 2 ppb because current water treatment protocols cannot remove these trace amounts of thallium. The maximum contaminant level goal (MCLG), however, is 0.5ppb. The MCLG is the amount of contaminant in drinking water that will cause no negative health effects. The estimated average concentration of thallium in drinking water is 7.2 ppb, almost fifteen times the MCLG. Currently, the only methods approved by the EPA for thallium removal are activated alumina and ion exchange. Unfortunately, activated alumina and ion exchange are not necessarily the most effective techniques for thallium removal.
Activated alumina is a highly porous material that is created by dehydroxylating aluminum hydroxide. Depending on pH, various ions can adsorb onto the activated alumina. Activated alumina devices have been able to lower thallium levels below 2 ppb. However, various constraints hinder its ability to effectively remove thallium. Suspended solids in water can obstruct the activated alumina, and the washes required to unsaturate the activated alumina device are detrimental to the ion removal efficiency. Alternatively, activated alumina devices can be replaced upon saturation, but disposal of used activated alumina can be hazardous, depending on what is bound to the activated alumina. Furthermore, storing activated alumina devices as replacements is not feasible for long periods of time. The activated alumina particles cement together when left unused for several days. As a result, only short-term storage of the device is feasible. Ion exchange columns are an alternative to activated alumina.
Ion exchange columns contain resin beads which store positive or negative ions for anion or cation exchange, respectively. Due to the presence of positively or negatively charged species on the beads, ions of opposite charge bind to the resin beads through electrostatic interactions. Ion exchange columns, as with the activated alumina, are able to lower thallium levels below 2ppb. However, ion exchange columns, like the activated alumina, face the same problem: Suspended particles can accumulate on the resin, hindering its ability to effectively interact with ions in the water15. Suspended particles are prevalent in water. A majority of particles in wastewater are approximately 40-50um in diameter. River water contains many minerals and organic matter in the form of loose sediment. Drinking water contains small amounts of solid particles that range from “colloidal sizes to 100um”. In summary, water samples for purification will most likely contain species which can hinder the ion exchange column. After considering the disadvantages of both the activated alumina particles and ion exchange columns, one can see that an improved method of thallium removal is needed.
The radioactive isotope Tl-201(ionic form: Tl+) was first implicated for clinical use by Lebowitz, et al. 1976. Scintigraphy is a form of nuclear medicine in which a radioisotope is administered to the patient at low, nontoxic doses. In cardiac scintigraphy, Lebowitz et al discovered that Tl-201 can be used as part of diagnostic stress test, now known as the thallium stress test. The thallium stress test allows clinicians to perform non-invasive myocardial screenings. Tl-201 is used because, in the blood, Tl-201 is in its ionic form, Tl+, which acts as a potassium (K+) analog, allowing Tl-201 uptake by cardiac cells’ Na/K-ATPases.. The utility of the Tl-201 in myocardial scintigraphy as a potassium analog indicates that similar potassium transport systems in bacteria can potentially be used to facilitate uptake of thallium.