A New Approach to Quantify Na-Fluorescein (Uranine) in Acid Mine Waters

Abstract

Sodium fluorescein (uranine) is one of the most popular fluorescent dyes for tracer experiments due to its chemical properties, low detection limits and low costs. As a limiting factor, it is generally assumed that Na-fluorescein cannot be properly detected under acid conditions because of weak fluorescence intensities at the standard excitation wavelength (490 nm). This laboratory study introduces a method to quantify Na-fluorescein in low pH waters without having to raise pH to alkaline conditions, which spares the time-and cost-intensive filtration of hydroxide precipitates prior to analysis. It was applied for recovery tests in water samples from five mining sites in Germany. These were buffered to pH 1.5 where Na-fluorescein shows a second fluorescence intensity maximum at an excitation wavelength of 438 nm. The method had satisfying recovery rates although, as expected, a higher detection limit compared to standard wavelength and pH. High Fe contents in the waters are a limiting factor-the impact of increasing Fe concentrations at a constant dye spike was evaluated and yielded a distinct negative trend between Fe and detected Na-fluorescein, probably due to Fe-fluorescein complexation. However, good recovery can be expected in mine waters with up to &100 mg L -1 Fe. The method of standard addition offers a potential calculative solution for higher Fe concentrations, resulting in significantly better Na-fluorescein recovery rates, compared to direct measurements. The method introduced here represents a promising approach for improving Na-fluorescein applicability in acid environments.

Use of Natural and Applied Tracers to Guide Targeted Remediation Efforts in an Acid Mine Drainage System, Colorado Rockies, USA

“…However, results from the laboratory investigation showed that uranine fluorescence recovered when acidic conditions were re-neutralized to the pH values observed in samples collected at the St. Louis Tunnel portal (pH 7.34), which were likely maintained by reversible ion exchange reactions [18,23]. The study results therefore support previous recommendations from Naurath et al [23] that uranine can be successfully used in mine tracer studies when samples analyzed for tracer recovery are alkaline.…”

“…The RWT dye was run with 550/580 nm excitation/emission and the uranine was run with 492/512 nm excitation/emission. The excitation and emission values for both dyes were within the expected variability [19] of values previously reported for fluorescence analysis of these dyes [8,18,23]. For RWT, a 5-point calibration curve was developed using standards between 1 and 400 ppb concentration.…”

“…However, as reported by Käss [18], the pH effects were reversible; when the standards were re-neutralized to the slightly alkaline conditions of the original standards (pH 8.2 and 7.4 for RWT and uranine, respectively) there was full recovery of the dye fluorescence emissions for both uranine and RWT. Although the uranine dye reaches maximum fluorescence under alkaline conditions (pH ≥ 9) [16,18,23], the dye standards (for calibration and pH response measurements) were made with the actual mine water at the sampling location, so the above mentioned results should be representative of the range of pH the dye will be exposed to in the study.…”

“…The fluorescence intensity of dyes is known to have a variable response to changes in pH [16,18,20,21,23,29,34,35]. To test for the effects of acidification on fluorescence, high concentration (400 ppb) standards of both uranine and RWT were acidified to a pH of 2.5 for 24 h and then filtered.…”

“…Secondly, applied tracers (salts and fluorescent dyes) were introduced, either simultaneously or at separate discrete locations, to provide a comparative analysis of tracer approaches in an AMD setting. To expand on previous research by Naurath et al [23], this study aims to further investigate the effectiveness of uranine as a tracer in acid mine waters by performing a dual tracer application with uranine and lithium salt.…”