COMPETITIVE SORPTION OF FLORESCENT DYE AND GROUNDWATER CONTAMINANTS ON GRANULAR ACTIVATED CARBON

Dye tracer tests using granular activated carbon (GAC) offer a semi-quantitative method to passively detect dyes in water flowing through wells, springs, and streams. GAC is used to sorb fluorescent dyes used for tracing groundwater flow paths in karst aquifers. One concern regarding dye tracer tests is that natural and/or anthropogenic contaminants in aquifers may compete for sorption sites on GAC, potentially leading to non-detectable results for dye on GAC during passive tracer tests; this may lead to incorrect interpretations about groundwater flow paths. This study aims to investigate whether natural and anthropogenic contaminants affect the sorptive capacity of dye onto GAC.

Laboratory experiments using 3D-printed mixed flow reactors were used to simulate the flow of dye and sorption competitors (tannic acid, bicarbonate, and TCE) in the natural environment. Approximately 2 g of sieved GAC (2.00-1.70 mm) were placed inside a mixed flow reactor, and standard solutions of fluorescent dye were injected into the mixed flow reactor using a syringe pump with a constant injection rate of 0.5 mL/min for two and a half hours. After two and a half hours, sorption competitors (tannic acid, bicarbonate, and TCE) were injected into the mixed flow reactor using the same injection procedure. Control experiments were also conducted with deionized water following the dye injection. Dye was extracted from the GAC and analyzed using an ultraviolet visible spectrophotometer. The same procedure described above was followed in subsequent experiments in which the contaminant was injected into the mixed flow reactor prior or simultaneously to the dye.

We expect the results of these experiments will provide insight into the competitive sorption that fluorescent dyes may experience in tracer tests conducted in the field. Analyses of the contaminant-first injection will demonstrate whether or not contaminants can occupy sorption spots, inhibiting sorption of dye. Analyses of the dye-first injection will demonstrate whether or not dye is desorbed from GAC due to competitive sorption with contaminants.