Elsevier

Geochimica et Cosmochimica Acta

Volume 74, Issue 23, 1 December 2010, Pages 6779-6787
Geochimica et Cosmochimica Acta

Sediment–water interactions of thallium under simulated estuarine conditions

https://doi.org/10.1016/j.gca.2010.09.004Get rights and content

Abstract

Thallium(I) has been added to estuarine sediment suspended in various natural and artificial aqueous samples in order to examine its reactivity under simulated estuarine conditions. In river water and sea water, adsorption of Tl to sediment was so rapid that a period of desorption–relaxation succeeded instantaneous adsorption. Entire time-courses could not be fitted with a conventional kinetic model, but pseudo-first-order forward and reverse rate constants of 0.0044 and 0.30 h−1, respectively, were derived for river water by omitting measurements defining the adsorption “overshoot” observed at the onset of the experiment. The extent of adsorption after a 16 h equilibration period was considerably greater in river water than in sea water, and displayed a quasi-linear increase with increasing pH over the range 2–9 in the former but no clear dependence on pH in the latter. A logarithmic reduction in the sediment–water distribution coefficient, KD, was observed on estuarine mixing from river water to sea water. Experiments conducted in electrolyte solutions coupled with inorganic equilibrium speciation modeling revealed that the effect was the combined result of a reduction in the activity of Tl+, an increase in the proportion of TlCl0 and increasing competition for adsorption sites from K+ with an increase in salinity. Overall, there was little experimental evidence for either the oxidation of Tl+ or its complexation by dissolved organic matter. The findings of the investigation are discussed in terms of the likely behavior of Tl in estuaries.

Introduction

Although highly toxic, thallium has received little attention in respect of its environmental distributions and behavior compared with other trace metals like Cd, Cr, Cu, Hg, Ni, Pb and Zn. This has been attributed to detection problems at environmental levels and its relatively low global economic value (Peter and Viraraghavan, 2005, Meeravali and Jiang, 2008). Thallium and its salts have had a variety of uses over the past one hundred years, but the principal, contemporary applications are in the specialized electronics industry.

Average crustal concentrations of Tl are 0.3–3 μg g−1 and the metal is mainly encountered as a trace element in sulfide ores of Cu, Zn and Pb and in coal (Kaplan and Mattigod, 1998). Consequently, the principal anthropogenic sources of Tl are metal mining, coal combustion, ferrous- and non-ferrous metal smelting and cement production. Concentrations of dissolved Tl in unpolluted surface waters are typically on the order of a few ng L−1 (Lin and Nriagu, 1999a, Cheam, 2001, Nielsen et al., 2005, Altundag and Dundar, 2009), but concentrations up to about 30 μg L−1 have been documented in heavily mined areas (Xiao et al., 2004).

Thallium exists in two oxidation states: Tl(I) and Tl(III). Dissolution of anthropogenic particulates, such as Tl2O, TlOH and Tl2SO4, ensures that the univalent form enters the aquatic environment. Except under highly acidic conditions in the presence of extremely strong oxidizing agents, Tl(I) is thermodynamically stable up to concentrations of at least 200 μg L−1 because the s-electrons display a low propensity to be displaced or bound covalently (Lin and Nriagu, 1997; Kaplan and Mattigod, 1998; Gao et al., 2007). In both oxygenated and anoxic systems, Tl(I) is predicted to exist predominantly as the free ion in fresh waters and, additionally, as TlCl0 in saline environments. Thallium(I) forms very few strong complexes (Manners et al., 1971) and, although empirical evidence for its interaction with natural organic ligands is lacking, it is believed to associate with polyelectrolytes, like humics and fulvics, only loosely at exchange sites (Jacobson et al., 2005).

Despite the thermodynamic stability of Tl(I), it has been claimed that the trivalent form of Tl has been detected in the aquatic environment following its retention on Chelex-100 at pH <2 (Batley and Florence, 1975, Lin and Nriagu, 1999a, Lin and Nriagu, 1999b). In some cases, reported concentrations of Tl(III) exceed those of Tl(I), although Cheam (2000) suggests that the contribution of the former may have been significantly overestimated because of sample acidification and the nature of the standards employed to validate the method. Inorganic equilibrium calculations predict that the higher oxidation state exists mainly as the sparingly soluble and relatively unreactive Tl(OH)30, and as chloro and hydroxy complexes (Lin and Nriagu, 1997). Covalent Tl complexes are more stable in the trivalent form and experimental evidence suggests that dimethylthallium, Tl(CH3)2+, may also be produced under anaerobic conditions (Thayer, 2002). Although the precise mechanisms by which Tl appears to be oxidized against thermodynamic gradients are unclear, experiments performed in pond and lake waters suggest that it is mediated by suspended bacteria and that net oxidation proceeds as a pseudo-first-order reaction over a period of hours to days rather than minutes (Twining et al., 2003). Since Tl(III) is a strong oxidant, subsequent reduction is predicted via coupling with sulfides and organic matter.

The trivalent cation, Tl3+, is considerably more reactive and toxic than its univalent equivalent (Lan and Lin, 2005), but the relative abundance of the former is so vanishingly small that, regardless of the valency distribution, Tl+ is considered the most geochemically- and bio-active (inorganic) form of the element in the aquatic environment (Ralph and Twiss, 2002). Because the ionic radius of Tl+ is similar to that of K+ (and, to a lesser extent, Rb+ and Cs+), in both living and non-living systems Tl+ behaves as a potassium ion analog. Thus, Tl+ participates in or interferes with potassium-dependent processes at the cellular level (e.g. the K channel, Na+K+- and H+K+-adenosine triphosphatase and Na+/K+/Cl cotransport systems; Hassler et al., 2007) and, because of its greater electronegativity, displaces K+ in clays and other potassium-bearing minerals (Rehkämper and Nielsen, 2004). Clearly, therefore, one of the main environmental controls on the behavior and fate of Tl is likely to be the concentration and availability of K+ (Borgmann et al., 1998, Twiss et al., 2004).

Although, in several studies, interactions of Tl with natural particulate matter are inferred (or sometimes required) as a means of removal from or desorption to the aqueous phase (e.g. Lin and Nriagu, 1999b, Nielsen et al., 2005), there is a lack of quantitative and mechanistic information on these interactions. Twiss et al. (2003) added Tl(I) as a radiotracer to various lake waters sampled during summer and, after a few days’ incubation, established partition coefficients of about 103 ml g−1, or removal of less than 5% of added metal at the suspended particle (mainly sestonic) concentrations encountered. The quantities of Tl reported in ferromanganese deposits and pelagic clays in the marine environment suggest that Tl is much more reactive than its alkali element analogs in sea water (Hein et al., 2000). Here, it has been proposed that Tl+ preferentially adsorbs to hydrous Mn oxides (rather than Fe oxides) and that, subsequently, rapid oxidation and precipitation of Tl2O3 takes place (Bidoglio et al., 1993).

In order to gain a better insight into the interactions of Tl with natural particles, we examine its adsorption to sediment under controlled but variable conditions. Specifically, we add Tl(I) as a tracer to suspensions of estuarine particles in river water and sea water and mixtures thereof, and to pure water amended with different salts representative of the ionic components of sea water. The kinetics and pH-dependencies of these reactions are also explored, and experimental results are interpreted with the aid of inorganic equilibrium speciation calculations.

Section snippets

Materials and methods

All plasticware used for sampling, sample processing and sample storage was soaked in 1 M HCl for at least 24 h before being rinsed in ultra pure Milli-Q water (MQW) for about 5 min. Unless otherwise specified, reagents were purchased from VWR and Fisher Scientific and were of analytical grade or better.

Sample characteristics

The pH of filtered Plym river water was 6.5 and its chlorinity, potassium ion concentration and organic carbon concentration were 1.4, 23 and 3.0 mg L−1, respectively. The salinity, pH and organic carbon concentration of filtered English Channel sea water were 32.1, 8.0 and 0.84 mg L−1, respectively. The physico-chemical characteristics of the fine fraction of Plym estuary intertidal sediment are given in Table 2.

ICP-MS analysis of filtered river water revealed an ambient Tl concentration of 19 ng L−1

Acknowledgments

We thank Andy Arnold, Andrew Tonkin and Richard Hartley, UoP, for technical and analytical assistance throughout. AC received a bursary from the University of Plymouth. We are grateful to three anonymous reviewers for their insightful comments about the study.

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