The fate of arsenic in the aquatic environment is influenced by dissolved natural organic matter (DOM). Using an equilibrium dialysis method, conditional distribution coefficients (Dom) for As(III) and As(V) binding onto two commercial humic acids were determined at environmentally relevant As/dissolved organic carbon (DOC) ratios and as a function of pH. At all pH values, As(V) was more strongly bound than As(III). Maximum binding was observed around pH 7, which is consistent with H+ competition for binding sites at low pH values and OH- competition for the arsenic center at high pH. For both oxidation states, Dom values increased with decreasing As/DOC ratios. Dom values were fitted as a function of the As/DOC ratio for As(III) and As(V). Compared to the aquatic humic acid, the terrestrial humic acid had a higher affinity for arsenic binding with 1.5-3 times higher Dom values under the same conditions. Al3+ in excess to arsenic successfully competed for strong binding sites at low As/DOC ratios. Under environmentally relevant conditions, about 10% of total As(V) may be bound to DOM, whereas >10% of As(III) is bound to DOM at very low As/DOC ratios only. Binding of arsenic to DOM should be considered in natural systems.

Measurements of trace metal species in situ in a softwater river, a hardwater lake, and a hardwater stream were compared to the equilibrium distribution of species calculated using two models, WHAM 6, incorporating humic ion binding model VI and visual MINTEQ incorporating NICA−Donnan. Diffusive gradients in thin films (DGT) and voltammetry at a gel integrated microelectrode (GIME) were used to estimate dynamic species that are both labile and mobile. The Donnan membrane technique (DMT) and hollow fiber permeation liquid membrane (HFPLM) were used to measure free ion activities. Predictions of dominant metal species using the two models agreed reasonably well, even when colloidal oxide components were considered. Concentrations derived using GIME were generally lower than those from DGT, consistent with calculations of the lability criteria that take into account the smaller time window available for the flux to GIME. Model predictions of free ion activities generally did not agree with measurements, highlighting the need for further work and difficulties in obtaining appropriate input data.

Several techniques for speciation analysis of Cu, Zn, Cd, Pb, and Ni are used in freshwater systems and compared with respect to their performance and to the metal species detected. The analytical techniques comprise the following:  (i) diffusion gradients in thin-film gels (DGT); (ii) gel integrated microelectrodes combined to voltammetric in situ profiling system (GIME−VIP); (iii) stripping chronopotentiometry (SCP); (iv) flow-through and hollow fiber permeation liquid membranes (FTPLM and HFPLM); (v) Donnan membrane technique (DMT); (vi) competitive ligand-exchange/stripping voltammetry (CLE−SV). All methods could be used both under hardwater and under softwater conditions, although in some cases problems with detection limits were encountered at the low total concentrations. The detected Cu, Cd, and Pb concentrations decreased in the order DGT ≥ GIME−VIP ≥ FTPLM ≥ HFPLM ≈ DMT (>CLE−SV for Cd), detected Zn decreased as DGT ≥ GIME−VIP and Ni as DGT > DMT, in agreement with the known dynamic features of these techniques. Techniques involving in situ measurements (GIME−VIP) or in situ exposure (DGT, DMT, and HFPLM) appear to be appropriate in avoiding artifacts which may occur during sampling and sample handling.

Transport into sediments of the trace elements Cu, Zn, Pb, Cd, Cr, and Sr by settling particles was investigated in Lake Zurich; the concentrations of these elements in settling particles collected in sediment traps and in the water column were determined at different times of year. Correlations of trace element concentrations with the various main components of the settling particles (biological material, calcium carbonate, manganese and iron oxides, silicate minerals) and examination of seasonal variations both of these concentrations in particles and of particle fluxes to the sediments show that biological material is an important carrier phase (especially for Cu and Zn). Iron and manganese oxides also contribute to trace element transport; calcium carbonate is inefficient as a carrier material.

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Free aquo copper ion and zinc ion concentrations were determined in water samples from eutrophic Lake Greifen by means of ligand exchange with catechol and cathodic stripping voltametry (for Cu) and ligand exchange with EDTA and anodic stripping voltametry (for Zn). The ratios of total dissolved Zn (1 O-40 nM) to dissolved Cu (7-20 nM) were [Zn] : [Cu] = 0.5-3 in samples taken at different seasons of a year; the ratios of the free aquo ion concentrations [Zn'-&] : [Cu2 i ] were -1 06. pCu was in the range of 14.5-l 5.9 and pZn 8.6-9.5 at different times and depths. The release of Zn from electrochemically inert complexes upon addition of Cu suggested direct competition of Cu and Zn for ligands in all lake-water samples examined. The selectivity of the natural ligands for Cu over Zn was evaluated as the conditional constant for the reaction Cu2 k + ZnL, = Zn2 I + CuL,; K = (1.4-tO.9) x lo6 (average in the euphotic zone; pH 8). These highly selective ligands are probably of biological origin.Cu and Zn are essential elements for biological activity, but they may be toxic at elevated concentrations, as has been demonstrated in marine phytoplankton (Brand et al. 1986; Sunda and Huntsman 1992). Phytoplankton may, in turn, affect trace metal chemistry by releasing metal-complexing ligands as well as by metal binding on surfaces and uptake and sedimentation in natural waters (Bruland et al. 199 1;Sunda 1994; Xuc and Sigg 1993). The fret metal ion concentration-a key parameter for the reactivity, bioavailability, and effects of metals (Sunda 1994; Sunda and Guillard 1976)-is regulated by complex interactions between trace metal ions, ligands, and major ions and particles (Whitfield and Turner 1987; Bruland et al. 199 1). To assess the fate and biological effects of trace metals in natural waters, it is thus essential to determine metal speciation and especially to evaluate the free aquo metal ion concentrations.Cu and Zn complexation in seawater have been extensively studied (e.g. Sunda and Huntsman 199 1; Coale and Bruland 1990; Donat and Bruland 1990) but few studies on freshwater systems are available. Cu is strongly complexed by organic ligands in seawater; Zn is also present mostly in organic complcxcs in the upper layers of the central Pacific (Bruland 1989; Bruland ct al. 199 1).Our recent studies on Cu and Zn speciation in frcshwater Sigg 1993, 1994) showed that in the water column of a eutrophic lake (Lake Greifen), Cu is strongly complexed by organic ligands, which arc probably biologically produced, giving log[Cu2+ ] = -16 to -14, whereas a substantial part of Zn is present as free Zn ions and weak organic complexes. The ratios of the free aquo ion concentrations [Zn'+] : [Cu2' ] were estimated to be in the range of 105-lo6 in this lake; however, the results Acknowledgments

Cu 2 +] and Cu complexation parameters in some selected freshwater systems in Switzerland were determined by the technique of ligand-exchange and DPCSV. Results from the water columns of some eutrophic and oligotrophic lakes are presented and compared to small acid lakes. Cu is strongly complexed by organic ligands which with very high stability constants at low concentrations are probably biologically produced, as indicated by the seasonal variations in the eutrophic lakes and by the relationship between Cu complexation and algal activity in the eutrophic (pCu=15-16), oligotrophic (pCu=13-14) and acidic (pCu=9-10) lakes. The extent of Cu complexation in river waters was generally lower than in the eutrophic lakes, at similar DOC levels. No obvious correlation between Cu complexation and DOC was observed, indicating that Cu complexing ligands are specific organic compounds.

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