INTERACTIONS BETWEEN DYES AND EARTH MATERIALS AND ITS INFLUENCE ON GROUNDWATER TRACER STUDY

Many Earth materials, such as clay minerals, have large specific surface area and high cation exchange capacity (CEC), and thus have strong interactions with polar or ionized organic compounds. Meanwhile, in groundwater study, tracers have been used to track down the groundwater flow velocity and movement. Common tracers used in groundwater study include fluorescence dyes. These dyes could be either cationic or anionic in nature. Rhodamine is a family of related chemical compounds and is extensively used in biotechnology applications, due to its high photostability. It is highly chemical stable against degradation. In this study, the interactions between a cationic dye rhodamine B (RB) or an anionic dye rhodamine WT (RWT) with different types of Earth materials were studies in batch experiments supplemented with column study. The studied Earth materials include quartz sand, common clays and oxides. The influence of solution pH to the emission wavelengths and intensity was also investigated. The strong affinity of RB for clay minerals was confirmed by the X-Ray diffraction analyses, suggesting intercalation of RB into the interlayer of montmorillonite, and by FTIR investigation. Thermal gravity analyses also conformed the uptake of RB by the clay mineral. The strong interactions between RB and the clay minerals suggested that using cationic dyes as tracers would have significant retardation effect on their transport through Earth media that have extensive negative charges. In contrast to clay minerals, negatively charged quartz has much weak interactions with the dyes, primarily due to its low charge density and minute uptake of RB. On the contrary, using RWT as tracer for Earth formations that have significant amounts of iron oxide coating could also induce retardation effects, as confirmed by slight uptake of RWT on iron oxides and red sands. These results suggest that care must be taken when using fluorescence tracers to monitor the groundwater flow.