Abstract
The speciation and reactivity of Zn was studied along with eight other trace metals in contaminated estuarine sediments in order to investigate the biogeochemical cycling of Zn in estuarine environment. Metal concentrations were determined for sediments, pore waters and surface waters. The enrichment factors (EF) and trace metal partitioning (Kd) of sediments were also assessed.
Sediment samples were collected from two estuaries: the Whau Estuary in Auckland (North Island) and the Kaikorai Estuary in Dunedin (South Island). Concentrations of Zn, Cr, Mn, Fe, Co, Ni, Cu, Cd and Pb were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) after sequential extraction of the sediments. The mineralogical composition of the sediments was determined by X-ray diffractometry (XRD). The adsorption-desorption phenomena of Zn, Cd, Cu and Pb were studied for both oxic and anoxic sediments by using a Freundlich isotherm model. The resulting data were used to predict the potential metal mobility in the estuarine sediments.
It has been observed that the total concentration of Zn and other trace metals (Cr, Fe, Cu, Cd, and Pb) but not Mn in the sediments decreased with increasing salinity (i.e. seaward direction). For example, concentrations of Mn, Zn and Pb at the sea end of the Kaikorai Estuary were 383 ± 13, 60 ± 3 and 12 ± 1 ppm, and towards the river end the concentrations were 133 ± 5, 120 ± 6 and 54 ± 3 ppm respectively.
In the both Whau Estuary sites the total Zn concentrations in sediments ranged from 74 ± 3 to 353 ± 10 ppm. The Zn concentration in the water of the Whau Stream tributary water was 20.5 ppb at pH 7.03. In surface water of the Whau Estuary the concentration of Zn was 5.61 ppb at pH 7.77 at the river end member, but at the sea end member the concentration at pH 8.01was below the ICP-MS detection limit. In sedimentary pore water of the Whau Estuary the concentration of Zn in three depth layers (0-1, 4-6 and 10-12 cm) ranged from12.3 ppb to 41.4 ppb collected at the river end.
The order of release of total metals from surface water and sediments in the Whau Estuary system was varied with increased salinity. The typical enrichment factors of Zn, Cd, Cu and Pb at the river end member of the Whau Estuary were 4.0, 1.9, 3.6 and 3.4, which indicated that moderate enrichment occurred in the sediments. However, at the same site Fe showed the largest distribution coefficient (log Kd) and Cd the smallest, where Zn, Cd, Cu and Pb were in the range of 3.6 to 5.6.
The adsorption and desorption phenomena of metals (Cu, Zn,Cd and Pb) on sediments were studied by addition of these metals to determine the effect of salinity gradients and redox potential on the release of Zn and other metals into the soluble phase. A significant amount of Zn and other metals was released under oxidizing conditions while the metals were effectively immobilized in slightly basic and strongly anaerobic conditions. The Freundlich model showed that the adsorption of metal ions to the Mn-Fe oxide and organic fractions of the sediments were controlled by the presence of Mn-Fe oxide and organic matter contents in sediments. The results demonstrated that the affinity of Zn (KF) was significantly greater (51 ± 13 ppm) in oxic fine (100 µm) sediment than in oxic coarse sediments where KF was 15 ± 8.4 ppm. The adsorption affinity in coarse dry oxic sediments decreased in the order of Pb>Cu>Zn>Cd whereas in wet anoxic sediment collected from the same site in the Whau Estuary the KF decreased in the order of Pb>Cu>Cd>Zn.
It is concluded that in these estuarine environments the water-particle interactions were important in metal cycling. Studies showed that relative changes occurred in partitioning of Zn between salinity gradients. However, the increased seawater salinity was not the major driver responsible for the release of Zn from sediments.
The cycling of Zn from solid phase (adsorbed onto sediment) to the liquid phase (dissolved phase of the water column) was mainly driven by the redox change of Fe and Mn oxy/hydroxides in estuarine systems. The ability of Fe and Mn oxy/hydroxides to retain Zn ions and act as scavenger in estuarine sediments has been confirmed.