Controls of fluorescent tracer retention by soils and sediments

Abstract. Fluorescent dyes like uranine (UR) and sulforhodamine B (SRB) have been used as artificial tracers in hydrological studies for decades. Recently, efforts have been intensified to enable their application to trace soil processes. Some researchers have used UR and SRB to characterise environmental compartments and derive insights into the fate of pesticides. However, existing knowledge on the controls of sorption of UR and SRB in soils is still incomplete and poorly standardized. For this reason, we aimed to identify the controls of UR and SRB adsorption in soils and to quantify their impact and possible interactions systematically. To reach this goal, we selectively controlled clay, organic matter (OM) and pH within batch experiments and examined the influence of these controls on the adsorption of UR and SRB in soils and sediments.

Tracer adsorption was studied using a sandy sediment with low content of organic matter (OM), a silty loamy top- and subsoil with 2.8 %-organic carbon (OC) and 0.6 %-OC, respectively. Sorption isotherms were determined by preparing watery solutions containing six different concentrations steps for each tracer and shaking these solutions with the suspended sorbent for 42 h (sorbent-solution ratio of 1 : 5). Afterwards, the suspension was centrifuged and the tracer fluorescence was measured in the supernatant. The effect of the pH on tracer adsorption was investigated by adjusting the pH of solution/adsorbent suspensions repeatedly during the equilibration to 5.5, 6.5 and 7.5 by adding hydrochloric acid and sodium hydroxide. Additionally, we examined the impact of OM and clay on tracer adsorption by conducting the batch experiments at pH 7.5 and manipulating the sorbents: OM was removed from top- and subsoil samples by H₂O₂-treatment and the clay mineral montmorillonite was added to the sandy sediment to achieve final clay contents of 0.1, 0.5, 1, 2, 2.5, 5 and 10 %.

The linear sorption coefficient Kd and pH showed a negative relationship which was stronger for UR than for SRB. This observation might be explained by an increasing number of negative sorption sites of sorbents and functional groups of both tracers with increasing pH. The pH-dependent carboxyl- and hydroxyl groups of the UR-molecule might be the reason for the higher extend of this effect for UR. As expected, UR and SRB adsorption increased with increasing clay content, due to more sorption sites related to an increase of the specific surface. The addition of 4 % of the clay mineral montmorillonite sufficed to adsorb nearly 100 % of both tracers. The complex sorption behaviour of UR and SRB in soils and sediments became obvious since OM influenced their sorption in opposite direction: The adsorption of UR increased with increasing OC concentration while that of SRB decreased.

Our study indicates the high relevance of physico-chemical properties of soils and sediments for the fate of applied tracers. The investigated controls determine if the respective tracer shows more conservative or non-conservative behaviour. Overall, the reported results will help to optimise the use of fluorescent tracers in terrestrial ecosystems to maximise their potential as a cheap and fast tool to gain insights into the fate of pollutants in soils and sediments.