Biogeochemistry of redox-sensitive elements in natural waters: Chemical speciation of molybdenum and vanadium.

摘要

The transition elements, molybdenum (Mo) and vanadium (V), are essential micronutrients for plants, animals and microorganisms. Mo and V both form part of the active sites of metalloenzymes that execute key transformations in the metabolism of nitrogen, sulfur, carbon and halide compounds. These two elements have a rich redox chemistry (+II to +V for V, and +II to +VI for Mo), which partly explains why they are so biologically active. Mo and V are also relatively abundant in the ocean, with Mo (VI) and V (V) dominating under oxic conditions, while Mo (V) and V (IV) are expected to exist as the soluble forms under anoxic reducing conditions. Biologically both Mo and V belong to the group of trace elements that organisms need in minute amounts and both metals are also toxic at high concentrations. On the other hand, lack of Mo and V is also lethal for organisms. Toxicity and bioavailability are dependent on the speciation of the two metals rather than simply the total concentration: Mo (V) is expected to be the more bioavailable form than Mo (VI) in aqueous environments, while V (V) is much more toxic than V (IV).;New methods for the determination of reduced Mo and V in seawater were developed first. The new protocol for determining V (V) and V (IV) involves Chelex 100 solid-phase extraction of both species, and then stripping off V (V) with ammonium solution and finally V (IV) was removed with acid. The new protocol for reduced Mo involves complexation of Mo (V) with tartrate at pH=7.0, subsequent XAD-7 solid-phase extraction of Mo (V) complexes, and elution by acidic acetone. All V and Mo species were quantified via Graphite Furnace Atomic Absorption Spectrometry (GF-AAS). The detection limits of the protocols are approximately on the order of 0.5 nM and 0.2 nM for V and Mo respectively. Analytical precision are ∼10% in the concentration range of 10 nM for both elements. The methods were successfully applied to the determination of V (IV) and Mo (V) in coastal waters around Long Island, New York, including filtered seawater, sediment pore water, and river water samples. Both methods are sensitive, simple and reproducible, but careful handling and operation under nitrogen are critical because both reduced V and Mo may be oxidized quickly under atmospheric conditions.;By applying the new methods, field investigations of reduced forms of these two elements, V (IV) and Mo (V), along with hydrological parameters, were carried out at the head of Peconic River Estuary and in Long Island Sound to examine the existence of reduced forms of both metals. Consistent with thermodynamic calculations, reduced forms of Mo and V exist in these natural waters. Field investigations showed that V (IV) and Mo (V) were favored under low pH and low dissolved oxygen conditions. Mo and V concentrations and speciation changed dynamically both seasonally and spatially in estuarine waters in response to redox conditions. V (IV) in the Peconics and LIS apparently was formed under suboxic conditions, which may be related to sewage inputs as well as reducing environments. Mo (VI) was rapidly mobilized from carrier phases (Fe, Mn-oxides and organic particles), reduced to Mo (V), under nonsulfidic or low sulfide conditions, while Mo (V) was further reduced to Mo (IV) and precipitated as MoS2 in highly sulfidic sediments during early diagenetic processes. Mo (V), and presumably V (IV), are released as transient dissolved intermediates during the reduction and oxidation of particulate carrier phases (Fe, Mn-oxides, organic matter and Fe-sulfides). The implications of the reduced forms of Mo and V on biological processes are still unknown.;Despite the importance of reduced species of Mo and V in marine environments, few studies have been conducted on the speciation of these two elements. This is mainly due to the lack of direct methods for the determination of different redox states of Mo and V in seawater. Therefore, this dissertation research combined laboratory experiments and field investigations to develop direct methods of measuring reduced and oxidized species of Mo and V in seawater, and then successfully applied the new methods to examine the existence of the species in coastal waters of Long Island, New York. This research also investigated the early diagenetic behaviors of Mo in porewater at Flax Pond, and hence furthered our understanding of the biogeochemical cycles and paleoceanographic implications of these elements in the marine systems.