Copper Complexation by Thiol Compounds in Estuarine Waters
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Marine Chemistry 82 (2003) 71–89 www.elsevier.com/locate/marchem Copper complexation by thiol compounds in estuarine waters Luis M. Laglera, Constant M.G. van den Berg* Oceanography Laboratories, University of Liverpool, Liverpool L69 7ZL, UK Received 14 May 2002; received in revised form 10 February 2003; accepted 21 March 2003 Abstract The stability of copper complexes with thiol substances in estuarine waters was determined for the first time using a new procedure based on cathodic stripping voltammetry (CSV). The free thiol concentration was monitored during titrations with copper in the presence of a competing ligand salicylaldoxime (SA); concentrations of copper-complexing ligands and conditional stability constants were determined simultaneously but independently. The decrease in the free thiol concentration with increasing copper concentration was used as an independent measure of the thiol-complex stability. The conditional stability constant of the thiol complexes (log KCuThiolV ) was between 12.3 and 14.1, and decreased with increasing salinity. The copper complexing titrations were found to fit to two complexing ligands: L1 with concentrations between 10 and 33 nM, and L2 between 14 and 300 nM. The complex stability of most of the thiols was similar to that of CuL2. Titrations at different detection windows showed a shift in the thiol complex stability suggesting that a second thiol species was present. It is therefore possible that L1 is also a thiol species. The estimated thiol concentrations can account for up to half of the total ligand concentration at low to intermediate salinities and for all of the ligands at high salinities. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Copper; Thiol; Salicylaldoxime; Metal speciation; Estuarine waters 1. Introduction metals to organisms, for instance by facilitating the uptake of iron (Maldonado and Price, 1999); Complexation by organic matter dominates the there is some evidence that the Cu2+ ion induces chemical speciation of the biogenic metals copper, the release of complexing ligands by sensitive zinc, cobalt and iron in sea water (Ellwood and phytoplankton like cyanobacteria (Moffett and van den Berg, 2001; Gledhill and van den Berg, Brand, 1996) and several other species in sea water 1994; Moffett, 1995; Saito and Moffett, 2001; van (Croot et al., 2000; Leal et al., 1999) as well as den Berg et al., 1987). This complexation is freshwater (van den Berg et al., 1979) probably in important because it affects the availability of order to ameliorate toxic effects of copper. Organic complexation may also control the geochemistry of metals by reducing the rate of scavenging; a relationship between the concentrations of dissolved * Corresponding author. Tel.: +44-151-794-4096; fax: +44-151- copper and zinc with that of their respective ligand 794-4099. concentrations in the Scheldt estuary (van den Berg E-mail address: [email protected] (C.M.G. van den Berg). et al., 1987) has suggested that interactions between 0304-4203/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-4203(03)00053-7 72 L.M. Laglera, C.M.G. van den Berg / Marine Chemistry 82 (2003) 71–89 sediments, suspended particles and dissolved organ- bridge filled with 3 M KCl, and the counter ic complexation controlled the residual metal con- electrode was a glassy carbon rod. pH measure- centrations in estuarine waters with a high supply ments were with a Metrohm 605 pH meter. For pH of metals. monitoring experiments, a narrow-neck pH electrode Little is known about the source of the ligands in was inserted in the voltammetric cell for the dura- estuarine waters, and the studies into this have been tion of the experiment. Calibrations were against sparse. There is some evidence that the ligands may NBS pH buffers. be sulphide species (Rozan et al., 2000), or thiol Water used for dilutions was Milli-Q (MQ); compounds (specifically for copper) (Leal and van methanol and HCl were purified by sub-boiling den Berg, 1998), but also colloidal (Tang et al., distillation on a quartz condenser. A stock solution 2001); the ligands may originate from the sediments of 0.01 M SA (BDH) was prepared in 0.1 M HCl. (Skrabal et al., 1997) but also from the freshwater Metal contamination of SA was removed by recrys- end of the estuary (Gerringa et al., 1996). The tallisation from methanol into HCl followed by presence of specific thiols like glutathione in ocean MQ-water addition. The pH buffer contained 1 M waters (Le Gall and van den Berg, 1998) and HEPES (N-2-hydroxyethylpiperazine-NV-2-ethanesul- estuarine waters (Tang et al., 2000), and the high fonic acid, Aristar grade) and 0.55 M NaOH. The stability of thiol complexes specifically with cop- addition of 100 Al of the buffer solution to the per(I) (Leal and van den Berg, 1998) suggest that samples yielded a pH of 7.8 (NBS scale). Standard these are candidates for the identity of the copper- copper solutions were prepared by dilution of an complexing ligands. Humic and fulvic acids can atomic absorption standard solution (Spectrosol, also function as copper complexing ligands forming BDH) with MQ, and acidified to pH 2.2 using 6 quite stable complexes (Kogut and Voelker, 2001; M HCl. Xue and Sigg, 1999). Stock solutions of thioacetamide, glutathione, thio- In this work, we have determined the chemical urea, cysteine (all from BDH) and 3-mercaptopro- speciation of copper in the estuary of the river pionic acid (Fluka) were prepared freshly before use Scheldt by cathodic stripping voltammetry (CSV) by dissolution in MQ water. with ligand competition, and at the same time determined the complex stability with thiols present 2.2. Sample collection in these waters using a novel, independent, voltam- metric procedure. The copper-thiol complex stability Samples for this study were collected during a was determined by following the voltammetric peak cruise with RS Navicula (April 2001) on the estuary height of the natural thiols during titrations with of the river Scheldt in the Netherlands. Water was copper, whilst at the same time monitoring the continuously pumped on-board ship from a depth copper speciation from the CSV peak height for of f 1 m using a peristaltic pump via a 20-m copper complexed with salicylaldoxime (SA). The poly(vinyl chloride) (PVC) hose, which was at- method and the study of the Scheldt estuary are tached to an epoxy-coated, iron, ‘‘fish’’. The water presented here. was passed through two filtration cartridges, one containing two filters, of 0.8 and 0.4 Am pore-size, and a second with a 0.2 Am pore-size filter. Sam- 2. Materials and methods ples were collected in 500-ml acid-cleaned polyeth- ylene bottles and then immediately frozen until the 2.1. Instrumentation and reagents day before analysis. Samples were thawed overnight at 4j and carefully swirled to ensure dissolution of Voltammetric equipment consisted of an Autolab any particulates that might have formed by the PGStat 10 voltammeter connected to a Metrohm VA freezing process. 663 electrode stand, and controlled by a computer The study area was the estuary of the river (PC). The reference electrode was double junction, Scheldt, which was sampled at 13 stations covering Ag/AgCl, KCl (3 M), saturated AgCl, with a salt- the entire salinity range, the final sample being L.M. Laglera, C.M.G. van den Berg / Marine Chemistry 82 (2003) 71–89 73 taken in the North Sea (salinity 33), close to the mV, pulse height 25 mV, from 0 to À 0.8 V. The coast, north of the mouth of the Scheldt estuary sensitivity was calibrated by standard copper addi- (Fig. 1). tions to each sample. 2.3. Copper determinations 2.4. Copper complexing ligand titrations The concentration of dissolved copper was de- The procedure used for the copper titrations was termined using a procedure similar to that described similar to that described before (Campos and van before (Campos and van den Berg, 1994). Seawater den Berg, 1994). A 130-ml thawed sample was was UV-digested (1 h) after acidification to pH 2.2 transferred to a Teflon bottle, and HEPES buffer by addition of 10 Al 6 M HCl per 10 ml of sample (0.01 M) and SA (usually 10 AM, except when the in acid cleaned silica tubes. A 10-ml sample aliquot detection window was varied) were added. The 10- was pipetted into the voltammetric cell and ammo- ml aliquots were pipetted into 12, 30 ml, polysty- nia was used to approximately neutralise the pH; rene vials (Bibby, Sterilin), previously spiked with HEPES buffer (100 Al, final concentration 0.01 M) copper to give a concentration range between 0 and and SA (30 Al, final concentration 30 AM SA) were up to 300 nM added copper (actual range depend- added. The solution was deaerated by purging (5 ing on the initial copper concentration and the min) with nitrogen. Voltammetric parameters were: determined ligand concentration), and allowed to deposition 30 s at À 1.1 V whilst stirring; 8 s equilibrate overnight at room temperature. Prior to quiescence at À 0.1 V; potential scan using the the first titration the tubes were conditioned twice square-wave modulation: 10 Hz, step height 2.5 with water containing the same range of copper Fig. 1. Map showing the estuary of the Scheldt and the sample locations. 74 L.M. Laglera, C.M.G. van den Berg / Marine Chemistry 82 (2003) 71–89 concentrations. The equilibrium concentration of used to test the data for the presence of a single copper bound by SA was determined by CSV using complexing ligand: an adsorption potential of À 0.1 V and a deposition time of 60 s.