Comparative efficacy of thallium adsorption by activated charcoal, prussian blue, and sodium polystyrene sulfonate

Background: Although Prussian blue is considered the antidote of choice for thallium poisoning, the lack of a Food and Drug Administration-approved ph armaceutical formulation has led to the search for other adsorbents. Activa ted charcoal has been demonstrated to adsorb thallium in vitro, and the sim ilarity between thallium and potassium has led some authors to consider the use of sodium polystyrene sulfonate as a potential adsorbent. This experim ent was designed to compare the relative thallium binding efficacy of these agents in a standard isotherm model. Methods: A standard aqueous solution of thallium acetate buffered to pH 7.0 was agitated at 25 degrees C with ac tivated charcoal, Prussian blue, or sodium polystyrene sulfonate at adsorbe nt:thallium ratios ranging from 1.5:1 to 100:1. In order to further simulat e physiologic conditions, all trials were repeated in a solution containing 4 mmol/L potassium phosphate. After thorough agitation, the mixtures were allowed to settle and were centrifuged and filtered through a 0.22-micron f ilter. Supernatant thallium concentrations were measured by atomic absorpti on spectrophotometry. Langmuir isotherms were used to calculate the maximal adsorptive capacity of each adsorbent, using linear regression with Pearso n's correlation coefficients (r). Maximal adsorptive capacities were compar ed statistically with a p < 0.05 considered significant. Results: The maxim al adsorptive capacities defined as milligrams of thallium per gram of adso rbent (shown with linear regression p and r values) were as follows: activa ted charcoal, 59.7 mg/g (p = 0.005, r = 0.995); Prussian blue, 72.7 mg/g (p = 0.004, r = 0.996); and sodium polystyrene sulfonate, 713 mg/g (p = 0.049 , r = 0.951). All three values were statistically different from each other . At a physiologic potassium concentration, the maximal adsorptive capaciti es for activated charcoal and Prussian blue were essentially unchanged (58. 3 mg/g and 69.8 mg/g, respectively, p > 0.05 for each vs trials without pot assium), while the maximal adsorptive capacity for sodium polystyrene sulfo nate fell to 39.1 mg/g (p = 0.003, r = 0.997, p = 0.005 vs sodium polystyre ne sulfonate without potassium). Conclusions: This in vitro study confirms the utility of Prussian blue and activated charcoal as thallium adsorbents. Although sodium polystyrene sulfonate demonstrates exceptional in vitro ad sorption of thallium, its greater affinity for potassium probably renders i t clinically ineffective.