Computer modelling of the chemical speciation of caesium, uranium(VI) and neptunium(V) in human duodenal fluids under fasting conditions

Abstract

A model simulating the human duodenal contents under physiologically realistic, fasting conditions was developed using the joint expert speciation system (JESS) computer program and database and used to investigate the chemical speciation of caesium, uranium(VI) and neptunium(V). Over the pH range 5.0–9.0, and the concentration range 5×10 −15–5×10 −5 mol dm −3, caesium was predicted to occur predominantly as the absorbable free monovalent cation Cs + (∼95%) with species such as CsHPO 4 − and CsCl representing the remainder. The presence or absence of sulphate at 2.1×10 −3 mol dm −3 did not influence the predicted speciation. Uranium was predicted to be present entirely as a soluble, highly charged species, both in the absence and in the presence of sulphate. Between pH 5.0 and ∼6.5 the UO 2H 2(PO 4) 2 2− predominated, above this pH carbonate species, either UO 2(CO 3) 4 6− or, possibly, UO 2(CO 3) 5 8−. At pH 8.0, and in the presence of sulphate, neptunium(V) was predicted to exist solely as the tetrasulphate species, whilst in the absence of sulphate, an array of negatively charged soluble carbonate species predominated. Studies over the pH range 5.0–9.0 predicted the formation of a spectrum of negatively charged carbonate and phosphate species, ∼40% of the total neptunium was predicted to be present as the electrically net-neutral species NpO 2HCO 3 at pH6.0, ∼20% at pH 7.0, ∼10% at pH 7.5 and ∼1% at pH 8.0. The observed speciation patterns of uranium and neptunium did not change over the concentration range 5×10 −15–5×10 −5 mol dm −3 and no solid species were predicted to occur under the conditions simulated. Whether the predicted electrically net-neutral neptunium species or the uranium pentacarbonate species do actually occur under true physiological conditions remains to be established. The observed speciation patterns for caesium and uranium are consistent with the observed absorption of these elements by humans; however, the observations for neptunium appear to be equivocal.