Speciation and Reactivity of Cisplatin in River Water and Seawater
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The adsorption of the cytostatic anticancer drug, cisplatin (cis-PtCl(2)(NH(3))(2)), has been studied after its addition to suspensions of estuarine sediment in river water and seawater. After a 16 h reaction period, adsorption was significantly greater in river water (sediment-water distribution coefficient, K(D), of 400 mL g(-1)) than that in seawater (K(D) approximately 150 mL g(-1)) because of the ready aquation of cisplatin to the more reactive monoaquacisplatin (cis-PtCl(OH(2))(NH(3))(2)(+)) at low chloride ion concentrations. Adsorption in river water was enhanced (K(D) approximately 2000 mL g(-1)) by a 24 h period of preincubation in the aqueous phase in which aquation proceeded further. The effects of pH on adsorption were relatively small, presumably because protonation-deprotonation of the particle surface was accompanied by near-equivalent shifts in the charge of hydrolysis products of aquated cisplatin. Kinetic experiments revealed a period of slow protracted uptake (up to about 60 h), followed by gradual desorption in both river water and seawater. Results were interpreted in terms of the formation of monoaquacisplatin, its adsorption to the particle surface, and the subsequent desorption of undefined, unreactive species. Kinetic data were modeled with a sequence of pseudofirst-order reactions and fits were obtained with forward and reverse rate constants for aquation of 1.79 x 10(-5) and 1.84 x 10(-5) s(-1) in river water and 5.50 x 10(-6) and 5.84 x 10(-6) s(-1) in seawater, and adsorption and desorption rate constants of 1.75 x 10(-5) and 0.20 x 10(-5) s(-1) in river water and 0.98 x 10(-5) and 2.8 x 10(-5) s(-1) in seawater. Environmental conditions favoring the retention of cisplatin and its degradation products are low chloride ion concentrations, high turbidities, and long residence or transit times; dispersion of the drug is favored in saline, coastal waters.
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