Doehlert experimental design as a tool to study liquid–liquid systems for the recovery of Uranium (VI) traces

•Doehlert design allowed finding an effective model for the recovery of uranyl ion.•The validity of the model is assured by the high values of RAdj.2 and Q2.•Experimental results agree with the prediction of the model.•D2EHPA low concentration favors the removal of traces of the uranyl ion.•A mixture of citric and phosphoric acid improves the recovery of uranyl ion.

The objective of this study was the development and optimization of a method for the liquid–liquid extraction and recovery of Uranium (VI) traces (0.10 mg/L). A comparison was made of three different extracting agents at 0.10 mol/L: tri-n-butyl phosphate, methyltrioctylammonium chloride, and bis(2-ethylhexyl) phosphoric acid. As the optimized chemical conditions of the liquid–liquid systems are very important for the future applications for the extraction, recovery and pre-concentration of Uranium (VI) traces, whether in bulk liquid membranes or in supported liquid membranes systems, a response surface methodology was adopted. A Doehlert design is used to obtain a quadratic function relationship between the percentage of recovery of uranyl ion and the amount of bis(2-ethylhexyl) phosphoric acid in kerosene, as extracting agent (0.001–0.050 mol/L) and that of phosphoric (0.192–0.718 mol/L) and citric (0.206–0.698 mol/L) acids, mixed as recovery agents. From the results, the most important effects on the recovery are the concentration of bis(2-ethylhexyl) phosphoric acid in the organic phase and the concentration of phosphoric acid in the stripping solution, while the concentration of citric acid only favors the extraction of Uranium (VI) when the other compounds are at opposite levels of concentration. From the mathematical model, optimal conditions for the back-extraction of Uranium (VI) were found with predicted recoveries of 99%, which were later corroborated experimentally.