Abstract
Abstract. Fluorescent dyes like uranine (UR) and sulforhodamine B (SRB) have been used for decades as artificial tracers in hydrological studies. Recently, attempts have been made to trace organic pollutants in soil with such dyes, but the knowledge of the controls of sorption of UR and SRB in soils is still incomplete and poorly standardised. For this reason, we selectively controlled clay, organic matter (OM) and pH within batch experiments and quantified systematically the impact and possible interactions of these controls on the adsorption of UR and SRB. Sorption isotherms were determined using a sandy sediment and a silty loamy subsoil and topsoil (0.6–2.8 % organic carbon (OC)) at pH values of 5.5, 6.5 and 7.5. Additionally, OM was removed from topsoil and subsoil samples by H2O2 treatment and the clay mineral montmorillonite was added to the sandy sediment. We found a negative relationship between the linear sorption coefficient Kd and pH that was stronger for UR than for SRB. Increasing repulsion forces between negative sorption sites and tracer functional groups at higher pH values might explain these results. Adsorption of UR and SRB increased with increasing clay content due to more specific surface area and associated sorption sites. An addition of 4 % of the clay mineral montmorillonite sufficed to adsorb nearly 100 % of both tracers. The influence of OM was more complex: while the adsorption of UR increased with increasing OC concentration, the opposite was observed for SRB. Our study indicates the high relevance of physico-chemical properties of soils and sediments for the fate of applied tracers and for their more conservative or non-conservative behaviour. Overall, the reported results will help to optimise the use of fluorescent tracers in terrestrial ecosystems and to increase their potential as a cheap and fast tool to gain insights into the fate of pollutants in soils and sediments.
Highlights
Fluorescent dyes are used in various applications, such as chemical sensing (Basabe-Desmonts et al, 2007), dye lasers (Li and Psaltis, 2008) or fluorescent labeling of biomolecules (Giepmans et al, 2006; Resch-Genger et al, 2008; Gonçalves, 2009)
We analysed the content of dissolved organic carbon (DOC) in centrifuged (2490 g, 1 h, Heracus Megafuge 40 Centrifuge, Thermo Scientific, MA, USA) and filtered soil extracts with a soil–solution ratio of 1 : 5 af
In contrast to OC, the clay contents of topsoil and subsoil are similar (24.0 % and 22.4 %) and differ clearly from that of the sediment (0.1 %), which was intended to examine the influence of the clay on the adsorption of the tracers
Summary
Fluorescent dyes are used in various applications, such as chemical sensing (Basabe-Desmonts et al, 2007), dye lasers (Li and Psaltis, 2008) or fluorescent labeling of biomolecules (Giepmans et al, 2006; Resch-Genger et al, 2008; Gonçalves, 2009) They are detectable at low concentrations, simple to handle and characterised by low toxicity (Flury and Wai, 2003; Leibundgut et al, 2009). For this reason, fluorescent dyes have been used for decades as artificial tracers in hydrological studies to investigate transport processes in surface waters and groundwater (Omoti and Wild, 1979b; Sabatini and Austin, 1991; Vanderborght et al, 2002; Hillebrand et al, 2015, e.g.).
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