Remote Fiber-Optic Flow Cell for the Detection of Uranium(VI) in Groundwater

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Abstract
Optimization and initial characterization of a remote probe for extremely low levels (<20 μg/L) of uranium(VI) in groundwater are described. The monitor takes advantage of a small length of Nafion cation exchange membrane tubing, which permits diffusion of the uranyl ion into an internally renewable reagent solution of Arsenazo III. The sensitive, colorimetric metal complexation of uranium(VI) by Arsenazo III is measured by recording the absorbance change in a fiber-optic SMA Z-cell that is coupled to a red-light-emitting diode. The entire system is packaged on a 2 × 11 in. breadboard and attached to a 100-ft power and data transmission cord. The final optimized pH of 1.5 was utilized for maximizing sensitivity to uranium while minimizing interferences from other metal ion impurities. The addition of 1 mM diethylenetriaminepentaacetic acid to the reagent further assisted in the elimination of metal ion interferences. Two Nafion membrane tubings with different internal diameters and wall thicknesses are evaluated. Using a 20 min diffusion time and the Nafion 20 membrane, the detection limit obtained for uranium(VI) in groundwater was 1.4 ppb. Of the twenty metal ion interferents examined, all gave essentially no response, with the exception of calcium and thorium, for which a 30 mg/L and a 100 μg/L solution, respectively, gave signals representative of 4.7 and 6.0 ppb uranium(VI).