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

Although Prussian blue is considered the antidote of choice for thallium poisoning, the lack of a Food and Drug Administration-approved pharmaceutical formulation has led to the search for other adsorbents. Activated charcoal has been demonstrated to adsorb thallium in vitro, and the similarity between thallium and potassium has led some authors to consider the use of sodium polystyrene sulfonate as a potential adsorbent. This experiment was designed to compare the relative thallium binding efficacy of these agents in a standard isotherm model.

A standard aqueous solution of thallium acetate buffered to pH 7.0 was agitated at 25 degrees C with activated charcoal, Prussian blue, or sodium polystyrene sulfonate at adsorbent:thallium ratios ranging from 1.5:1 to 100:1. In order to further simulate 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 filter. Supernatant thallium concentrations were measured by atomic absorption spectrophotometry. Langmuir isotherms were used to calculate the maximal adsorptive capacity of each adsorbent, using linear regression with Pearson's correlation coefficients (r). Maximal adsorptive capacities were compared statistically with a p < 0.05 considered significant.

The maximal adsorptive capacities defined as milligrams of thallium per gram of adsorbent (shown with linear regression p and r values) were as follows: activated 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 capacities 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 potassium), while the maximal adsorptive capacity for sodium polystyrene sulfonate fell to 39.1 mg/g (p = 0.003, r = 0.997, p = 0.005 vs sodium polystyrene sulfonate without potassium).

This in vitro study confirms the utility of Prussian blue and activated charcoal as thallium adsorbents. Although sodium polystyrene sulfonate demonstrates exceptional in vitro adsorption of thallium, its greater affinity for potassium probably renders it clinically ineffective.