Sorption of the cyanobacterial toxins cylindrospermopsin and anatoxin-a to sediments
Introduction
Anatoxin-a (ATX) and cylindrospermopsin (CYN) are toxins produced by certain cyanobacteria (ATX: Edwards et al., 1992, Sivonen et al., 1989; CYN: Ohtani et al., 1992, Falconer, 2005, Preußel et al., 2006). They are known to have a range of effects on human health such as tissue damage (liver, lung, gut) and cell necrosis (CYN: Hawkins et al., 1997), as well as damage to the nervous and respiratory systems (ATX: Thomas et al., 1993).
Anatoxin-a is an alkaloid toxin with a molecular weight of 165 g mol−1 (Fig. 1a). Despite its worldwide distribution it seems not to occur as frequently as microcystins and cylindrospermopsin (Osswald et al., 2007, Hedman et al., 2008). The screening of 78 German water bodies yielded maximum total ATX concentration (i.e. intracellular and extracellular) of up to 13.1 μg L−1, but higher concentrations can be expected during bloom situations (Bumke-Vogt et al., 1999). The highest anatoxin-a concentration found was 1750 μg L−1 (Hedman et al., 2008). Anatoxin-a was shown to decompose under the influence of light and alkaline pH (Stevens and Krieger, 1991) and to adsorb on lake sediments (Rapala et al., 1994).
Cylindrospermopsin is an alkaloid toxin with a cyclic guanidine moiety bridged to a hydroxymethyluracil group and a molecular weight of 415 g mol−1 mol−1 (Chiswell et al., 1999; Fig. 1b). Cylindrospermopsin has been detected in many water bodies throughout the world with concentrations in subtropical regions of Australia in the range of up to120 μg L−1 (Shaw et al., 1999, McGregor and Fabbro, 2000) and in European freshwaters with maximum concentrations between 9 and 18 μg L−1 (Bogialli et al., 2006a, Quesada et al., 2006, Rücker et al., 2007). Due to its chemical stability and slow degradation (Chiswell et al., 1999) CYN shows a high persistence in many water bodies (Wormer et al., 2008). Sorption to sandy sediments with a content of fines of 1% and 4% was found to be negligible (Klitzke et al., 2010).
If toxin contaminated surface waters are used as drinking water reservoirs, efficient elimination has to be ensured. Beside degradation, dilution and physical straining, sorption is an important process for contaminant removal during drinking water (pre-) treatment methods (Grützmacher et al., 2010) such as river bank filtration, artificial groundwater recharge, and slow sand filtration. The properties and structure of the toxins together with sediment texture determine their sorption capacity. While CYN is very hydrophilic and carries both a positive and a negative charge at neutral pH (Meriluoto and Spoof, 2008) ATX occurs as cation below pH 9.6 (pka: 9.6; Devlin et al., 1977). The adsorption of both organic zwitterions to soils and soil minerals (Carrasquillo et al., 2008) as well as the adsorption of organic cations onto clays (Narine and Guy, 1981) and sediments (Brown and Combs, 1985) are well documented phenomena. Burns et al. (2009) found strong sorption of the cyanobacterial toxin saxitoxin, which occurs as protonated ion below pH 8.2, to clays and sediments. As a cation bridging mechanism has been reported for other organic zwitterions to enhance sorption to organic matter (MacKay and Canterburry, 2005), we hypothesize higher CYN sorption in the presence of divalent cations such as Ca.
In order to gain a better understanding of the role of toxin removal by adsorption during sediment passage the objective of this study was to (i) determine the adsorption coefficients of CYN and ATX according to the Freundlich and Langmuir model for sediments of various textures and to (ii) derive relevant sediment and hydrochemical characteristics which control CYN and ATX sorption.
Section snippets
Material and methods
To determine adsorption isotherms batch experiments were conducted with sediments of varying texture as outlined in the procedure of the OECD guideline 106 (OECD, 1997).
Sorption of ATX
Anatoxin-a shows weakest sorption to sandy sediments such as UBA (99% sand, Fig. 2), Mergel (89% sand), and GW (96% sand) and highest sorption to clay-rich (SRW, 27% clay) and organic-rich (organic mud 44.5% OC; Fig. 2) sediments (Table 3). Even if we take 8% microbial degradation into account (see chapter 2.2.1.3), we cannot explain the high sorption onto the Müggel sediment (9% clay, 1% OC), which exceeds the qmax value of SRW. Sorption of ATX was mostly best described by the non-linear model
Sorption comparison and environmental relevance
Our results showed much stronger sorption of ATX than CYN. Anatoxin-a sorption also exceeds the reported Kf-values of a batch study for nodularin and microcystin-LR (Miller et al., 2005). This difference in sorption may be explained by the structure of the three toxins: While nodularin and microcystin both carry negative charges (Miller et al., 2001), leading to increased repulsion from negatively charged mineral surfaces, ATX occurs as cation (Devlin et al., 1977) enhancing sorption to
Conclusions
Our results showed that sediment texture together with the chemical structure of the respective toxin are crucial parameters to predict the fate of ATX and CYN during sediment passage. Although clay and organic carbon content proved the most important parameter in ATX sorption to sediments, ATX is still retained by sandy porous media containing low fractions of fines. This suggests at least some ATX removal through sorption during water percolation through sandy sediments. Anatoxin-a is mainly
Acknowledgements
The authors would like to thank Silke Meier, Claudia Kuntz, and Gabriele Gericke for their support with batch experiments and LC-MS-MS measurements, respectively, Michael Facklam for analysis of sediment grain size distribution, Sabine Rautenberg for C/N-analysis, Gabriele Wessel for help with data analysis, Reimo Kindler and Jaane Krüger for fruitful discussion and for proof-reading part of the manuscript.
The funding provided by the Federal Ministry of Education and Research (BMBF), Veolia
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