Comparative study of organic Cd and Zn complexation in lake waters - seasonality, depth and pH dependence

Abstract

Environmental context. The bioavailability of trace metals such as zinc and cadmium strongly depends on what chemical form they are in, and not simply on the total metal concentration. Zinc is an essential micronutrient, whereas cadmium is extremely toxic, but when they occur in the same environment there is potential for the two metals to compete for the same biological binding sites. In this study we have studied the trends in Cd and Zn complexation in three alpine lakes in New Zealand. We conclude that, although the total concentration of cadmium is much lower than that of zinc and copper, it bares the highest risk of toxicity for organisms. Abstract. The variation with depth, pH dependence, seasonal variability and selectivity of strong, natural organic cadmium and zinc-binding ligands from three New Zealand alpine lakes (Hayes, Manapouri and Hauroko) have been investigated. Competitive ligand equilibration–cathodic stripping voltammetry (CLE-CSV) with APDC (ammonium pyrrolidine dithiocarbamate) was used to measure the ligand complexation by Zn2+ and anodic stripping voltammetry (ASV) for complexation of Cd2+. In all lakes, the total dissolved cadmium concentration [CdT] averaged 0.040 nM (standard deviation σ = 0.114), while the average concentration of Cd-binding ligands [LCd] was 5.17 ± 1.79 nM (1σ), with conditional stability constants relative to free Cd2+, log K′Cd2+L′, that ranged from 7.92 at depth to 10.58 at the surface. Calculated concentrations of the free aquo ion [Cd2+] averaged 0.0147 ± 0.0616 nM (1σ), and showed a higher percentage of Cd complexed by strong ligands in the summer and in surface waters. The selectivity of cadmium-binding organic ligands was low, and ligand-bound Cd2+ was easily displaced by Zn2+ and Cu2+. Total dissolved zinc concentrations [ZnT] were highly variable, and ranged from 1.04 to 10.94 nM. The corresponding ligand concentrations of strong zinc-binding ligands [LZn] were between 2.14 and 15.52 nM, with conditional stability constants log K′Zn2+L′ as low as 8.78 in deep water collected in summer from Lake Hauroko, up to a maximum of 12.41 at a depth of 5 m in Lake Hayes. The calculated concentrations of the free aquo complex [Zn2+] ranged widely between 0.001 and 1.620 nM.