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| Titel |
Controlling parameters of fluorescent tracer sorption on soils and sediments |
| VerfasserIn |
Marcus Bork, Markus Graf-Rosenfellner, Jens Lange, Friederike Lang |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250144805
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| Publikation (Nr.) |
 EGU/EGU2017-8674.pdf |
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| Zusammenfassung |
| Fluorescent dyes like uranine (UR) and sulforhodamine B (SRB) have been widely used,
especially for tracing hydrological processes. In the recent past, efforts have intensified to use
fluorescent tracers also in soils, for example as proxies for organic pollutants. However, the
sorption properties of both organic pollutants and fluorescent tracers have to be exactly
known to succeed. Yet existing knowledge for soils is still incomplete and poorly
standardized. For this reason, we carried out laboratory batch experiments to determine
sorption isotherms of UR and SRB with varying pH, soil texture and organic carbon content
(OC).
As sorbents we used a sandy sediment with low OC, a silty loamy topsoil with 2.8 %-OC
and a similar textured subsoil containing 0.6 %-OC. For both tracers six concentration steps
each were prepared and shaken with the suspended sorbent for 42 h using a sorbent:solution
ratio of 1:5. During the equilibration, the pH was repeatedly adjusted to 5.5, 6.5, and 7.5 by
adding hydrochloric acid (HCl) or sodium hydroxide (NaOH). Subsequently, the
tracer-sorbent-suspension was centrifuged and the fluorescence of the tracer in the
supernatant was measured. In order to examine the influence of OC and the clay
fraction on the tracer sorption, batch-experiments at pH 7.5 were also conducted with
manipulated sorbents: top- and subsoil samples were treated with H2O2 to remove
organic matter 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 %
clay.
We observed a negative relationship between the linear sorption coefficient Kd and pH,
which was stronger for UR than for SRB. Increasing numbers of negative sorption sites and
functional groups of both tracers and sorbents with increasing pH might be the reason for this
observation. Besides the pH-value, quantity and quality of clay and OC had a crucial
influence on the sorption of UR and SRB in soils and sediment. As expected, increasing clay
content, which is associated with an increasing specific surface and therefore more
sorption sites, led to an increasing sorption of UR and SRB. Here, after the addition
of 4 % of the clay mineral montmorillonite, nearly 100 % of both tracers were
sorbed. Furthermore, OC influenced the sorption of UR and SRB in different ways:
while the sorption of UR increased, the sorption of SRB decreased with increasing
OC.
In conclusion, the sorption behaviour of the fluorescent tracers UR and SRB
in soils is very complex, and for appropriate application, the physico-chemical
properties of the respective soils or sediments have to be considered. These conditions
essentially determine if the respective tracer shows a conservative or non-conservative
behaviour. With these aspects in mind, applying SRB and UR has the potential to be
a cheap and fast method to estimate the fate of pollutants in soils or sediments. |
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