The speciation of Fe(II) and Fe(III) in natural waters

The interactions of Fe(II) and Fe(III) with the inorganic anions of natural waters have been examined using the specific interaction and ion pairing models. The specific interaction model as formulated by Pitzer is used to examine the interactions of the major components (Na⁺, Mg²⁺, Ca²⁺, K⁺, Sr²⁺, Cl⁻, SO₄⁻, HCO₃⁻, Br⁻, CO₃²⁻, B(OH)₄⁻, B(OH)₃ and CO₂) of seawater and the ion pairing model is used to account for the strong interaction of Fe(II) and Fe(III) with major and minor ligands (Cl⁻, SO₄²⁻, OH⁻, HCO₃⁻, CO₃²⁻ and HS⁻) in the waters. The model can be used to estimate the activity and speciation of iron in natural waters as a function of composition (major sea salts) and ionic strength (0 to 3 M). The measured stability constants (K_FeX^*) of Fe(II) and Fe(III) have been used to estimate the thermodynamic constants (K_FeX) and the activity coefficient of iron complexes (γ_FeX) with a number of inorganic ligands in NaClO₄ medium at various ionic strengths: In(K_FeX/γ_Feγ_X) = InK_FeX − In(γ_FeX) The activity coefficients for free ions (γ_Fe, γ_x) needed for this extrapolation have been estimated from the Pitzer equations. The activity coefficients of the ion pairs have been used to determine Pitzer parameters (B_FeX, B_FeX⁰, C_FeX^φ) for the iron complexes. These results make it possible to estimate the stability constants for the formation of Fe(II) and Fe(III) complexes over a wide range of ionic strengths and in different media. The model has been used to determine the solubility of Fe(III) in seawater as a function of pH. The results are in good agreement with the measurements of Byrne and Kester and Kuma et al. When the formation of Fe organic complexes is considered, the solubility of Fe(III) in seawater is increased by about 25%.     

Investigations of iron coordination and redox reactions in seawater using Fe radiometry and ion-pair solvent extraction of amphiphilic iron complexes

Iron coordination and redox reactions in synthetic and coastal seawater were investigated at nanomolar concentrations using ⁵⁹Fe radiometry and ion-pair solvent extraction of iron chelated by sulfoxine (8-hydroxyquinoline-5-sulfonate) and BPDS (bathophenanthroline disulfonate). Using sulfoxine, we determined the rate at which the monomeric Fe(III) hydroxide species present in seawater of pH 8 are complexed by the microbial siderophore deferriferrioxamine B and the synthetic chelator EDTA (ethylenediaminetetraacetic acid). Forward rate constants of 2 × 10⁶M⁻¹s⁻¹ and 20 M⁻¹s⁻¹, respectively, were obtained. The kinetics of these reactions have not been measured previously at pH values near that of seawater. Conditional equilibrium constants measured for the Fe(III)-EDTA system are consistent with published stability constants for EDTA complexes and for Fe(III) hydrolytic equilibria minus the neutral Fe(OH)₃^o species, suggesting it is not quantitatively significant near pH 8. Commercial humic acid was found to have sufficient affinity for iron to compete with Fe(III) hydrolysis in seawater, and limited evidence was obtained for an interaction with dissolved organic matter in coastal seawater.In our investigations of redox reactions using BPDS to trap Fe(II) produced in the medium, we observed enhanced photoreduction of Fe(III) by humic acid as well as reduction induced by solutes released from phytoplankton in seawater of pH 8. Although the method is sensitive enough to work at near-oceanic levels of iron, the difficulty in distinguishing Fe(II) generated by Fe(III)-BPDS interactions from Fe(II) produced by other means limits its utility. This analytical ambiguity may be generalizable to other methods which measure ferrous iron in seawater using Fe(II)-specific ligands.     

Calibration of a chalcogenide glass membrane ion-selective electrode for the determination of free Fe in seawater: I. Measurements in UV photooxidised seawater

Calibration of a chalcogenide glass membrane, Fe(III)ISE [Fe_2.5(Ge₂₈Sb₁₂Se₆₀)_97.5], in buffered saline media has been undertaken in order to assess the suitability of this ISE for seawater analyses. The electrode slopes in saline citrate and salicylate buffers were 26.3 and 28.2 mV/decade, respectively, for Fe³⁺ concentrations ranging from 10⁻¹⁰ M to less than 10⁻²⁵ M Fe³⁺. The calibration lines in the citrate and salicylate buffers were essentially collinear with the response in unbuffered chloride-free standards containing >10⁻⁵ M Fe³⁺, demonstrating that the response of the FeISE is unaffected by chloride ions. A mechanism involving a combination of charge transfer and ion-exchange of Fe(III), at the electrode diffusion layer, can be used to explain the ≈30 mV/decade slope of the FeISE. The response of the FeISE in UV photooxidised seawater containing 8 nM total Fe was measured as the pH was changed from 8.27 to 3.51. The slope of the response was 24.2 mV/decade [Fe³⁺] calculated as a function of pH using Fe(III) hydrolysis constants for seawater. Moreover, the response was essentially collinear with that in citrate buffers and in unbuffered solutions containing >10⁻⁵ M Fe³⁺ and the slope for the combined data was 26.2 mV/decade. This study was restricted to organic-free seawater because the certainty in Fe(III)–ligand stability constants is insufficient to warrant the selection of an ideal calibration buffer system, and there is evidence that powerful chelating ligands (e.g., EDTA along with humic and fulvic acids) may alter the response of the Fe(III)ISE. The Fe dissolution rate of the FeISE in UV photooxidised seawater was found to be 1.6×10⁻² nmol Fe/min, as measured by cathodic stripping voltammetry (CSV). This would contaminate a 100-ml sample by 0.8–1.6 nM Fe over a typical measurement period of 5–10 min obtained using a stability criterion of 0.5 mV/min. Various methods are proposed for reducing the level of contamination in open ocean samples that contain sub-nanomolar concentrations of iron. The FeISE has the potential to detect free Fe³⁺ at concentrations typically found in natural seawater.     

东山湾海水中Fe(II)和Fe(III)相互转化围隔实验研究

根据2008 年8 月与11 月在东山湾海域获得的调查资料对表层水中溶解态Fe(II)和Fe(III)含量、浮游植物叶绿素a、营养元素及其浓度等环境参数进行分析。结果表明, 夏、秋季海水中Fe(II)浓度及其在总溶解铁中所占比例均与浮游植物叶绿素a 呈正相关, 其相关系数分别为0.7959、0.9219。现场围隔实验表明, 海水中总溶解态Fe 含量在24 h 内有较大的变化, 最大减少量达到17.4%。DS2 站点海水中Fe(II)浓度及其在总溶解铁中所占比例随光照强度增加而增加。最高值与初始值相比较, 叶绿素a 较高的DS2 站点海水中Fe(II)浓度增加较叶绿素a 较低的DS5 号站点高0.053μg/L。Fe(II)和Fe(III)加富实验研究了溶解态的Fe(II)和Fe(III)在海水中相互转化。高浓度的Fe(II)在海水中被氧化成Fe(III),海水中浮游植物也会引发光还原作用使Fe(III)还原成Fe(II)。     

Chemical environment for red tides due toChattonella antiqua

In order to assess the roles of Fe and Cu in outbreaks ofChattonella antiqua red tide, concentrations of these metals in the surface seawater were monitored around the Ie-shima Islands in the Seto Inland Sea during the summers of 1986–1988. Bioassay of the surface seawater with respect to Fe and Cu was also conducted using a cultured strain ofC. antiqua.Concentrations of Fe and Cu in the filtered seawater (Fe^F and Cu^F) were in the range of 3.9–10.0 and 9.3–11.2 nM, respectively. The bioassay with respect to Fe revealed that Fe at the surface layer was usually insufficient to support the maximum growth rate ofC. antiqua, except whenC. antiqua was dominant in the field. However, correlations between Fe^F and the growth rate of the control cultures (Fe, EDTA=not enriched; N, P, B₁₂=enriched at optimum levels) were not apparent, probably because Fe^F did not reflect the concentration of available Fe.The bioassay with respect to Cu was coupled with the Cu^F values obtained. The results indicated that Cu at the surface layer was detoxified by complexation with natural organic ligand(s), and that pCu (=minus log of cupric ion activity) was 11.5–11.7, optimum for the growth ofC. antiqua, throughout the survey period. It is suggested that Fe, but not Cu, is a potentially important factor in regulating the natural populations ofC. antiqua in the Seto Inland Sea.     

Measurement of the redox speciation of iron in seawater by catalytic cathodic stripping voltammetry

Catalytic cathodic stripping voltammetry (CSV) preceded by adsorptive collection of complexes of 1-nitroso-2-napthol (NN) can be used to determine iron in seawater. It is shown here that iron(II) is effectively masked in the presence of 2,2-dipyridyl (Dp) so that iron(III) is measured selectively. The concentration of iron(II) is then calculated as the difference between the concentrations of reactive iron (Fe_R) in the absence and presence of 2 μM Dp, Fe_R being defined as that which was complexed by 20 μM NN at pH 6.9 in the presence of 1.8 mM H₂O₂ and 5 ppm sodium dodecyl sulphate. A 30 min reaction time was allowed for Dp to react with iron(II) in seawater prior to the determination of reactive iron(III) using the same conditions as used for Fe_R. Detection limits of 0.08 nM, 0.077 nM and 0.12 nM were obtained for Fe_R, iron(III) and iron(II), respectively, using a 60 s deposition time.The method was utilised to determine the redox speciation of iron in the northern North Sea. Concentrations of Fe_R ranged between 0.8 and 3.5 nM with nutrient-like depth profiles. Iron(II) was found to be present at concentrations up to 1.2 nM, the highest concentrations occurring in the upper 20 m of the water column.     

Simplified synthesis of an 8-hydroxyquinoline chelating resin and a study of trace metal profiles from Jellyfish Lake, Palau

We report a simplified synthesis, and some performance characteristics, for 8-hydroxyquinoline (8-HOQ) covalently bonded to a chemically resistant TosoHaas TSK vinyl polymer resin. The resin was used to concentrate trace metals from stored, acidified seawater samples collected from Jellyfish Lake, an anoxic marine lake in the Palau Islands. The Mn, Fe, and Zn profiles determined from the 8-HOQ resin extraction were similar to those determined using Chelex-100 resin. The Zn and Cd profiles did not exhibit removal by sulfide “stripping” in contrast to other anoxic marine basins. The profiles of Co and Ni also exhibited elevated concentrations in the anoxic hypolimnion. The solution speciation and saturation states for the metals were calculated using revised metal-bisulfide stability constants. The calculations suggest that the MS(HS)⁻ species dominates the solution speciation for Mn, Co, Ni, Zn, Cd, and Pb. Cu(I) is modeled as the CuS⁻ or Cu(HS)₂⁻ species, while Fe(II) behaves as the free Fe²⁺ cation. The Mn, Co, Ni, Cu and Cd concentrations appeared to be at least 10-fold undersaturated, while the Fe(II), Zn, and Pb concentrations were close to saturation with respect to their metal sulfides.     

Reactivity of various types of iron(III) (hydr)oxides towards light-induced dissolution

Laboratory experiments were conducted on the light-induced dissolution of three well defined Fe(III) (hydr)oxide phases (γ-FeOOH, α-FeOOH, and α-Fe₂O₃) with oxalate as reductant/ligand. Upon irradiation of an aerated γ-FeOOH suspension of pH 3, photooxidation of oxalate and photochemical formation of dissolved Fe(II) occurred according to a 1:1 stoichiometry. This was not observed with aerated α-FeOOH and α-Fe₂O₃ suspensions of pH 3, where photooxidation of oxalate was not accompanied by formation of appreciable concentrations of dissolved Fe(II). We hypothesize that in aerated α-FeOOH and α-Fe₂O₃ suspensions, oxidation of surface Fe(II) outcompetes its detachment from the crystal lattice. Also in deaerated suspensions, α-FeOOH and α-Fe₂O₃ behaved differently from γ-FeOOH with regard to light-induced dissolution. We interpret our results by assuming that light-induced dissolution of α-FeOOH and α-Fe₂O₃ in deaerated suspensions of pH 3 occurred mainly through Fe(II)-catalyzed thermal dissolution of the solid phases, where Fe(II) was initially formed by photoreductive dissolution and then predominantly via photolysis of dissolved Fe(III) oxalate complexes. With γ-FeOOH, on the other hand, dissolved Fe(II) formation occurred probably mainly through photochemical reductive dissolution under photooxidation of adsorbed oxalate. From our results we conclude that the efficiency of detachment of reduced surface iron is a key parameter of the overall kinetics of photoreductive dissolution of Fe(III) (hydr)oxides in aquatic systems, and that thermodynamically stable phases such as α-FeOOH and α-Fe₂O₃ are not readily dissolved in the presence of O₂, even at low pH-values and in the presence of light and reductants like oxalate. We propose that redox cycling of iron at the surface of crystalline Fe(III) (hydr)oxide phases, i.e. reduction and oxidation of surface iron without transfer into solution, may be an important pathway of transformation of thermodynamically stable atmospheric Fe(III) (hydr)oxides into less stable and thus more soluble phases.     

Fe(OH)_3胶体与铜离子作用的研究

使用切向流超滤系统得到粒度较均匀的Fe(OH)3胶体,进而对Fe(OH)3胶体粒子与Cu2 的作用进行了研究,并将实验结果与南沙群岛海区表层水中铜与胶体物质结合状况的调查数据进行了比较分析,得到如下结果:(1)在海水介质中,铜在Fe(OH)3胶体上结合的百分率(pH>5)随pH的升高而下降,随着Fe(OH)3胶体浓度的减小而下降?(2)在天然海水介质中,使铜在Fe(OH)3胶体上结合的百分率在50%-70%的有机物浓度为1.0mg·L-1,有机物浓度的增大使铜与无机胶体结合的百分率有下降的趋势?     

Interconversion of iron(III) hydroxy complexes in seawater

Interconversion rates of the mononuclear ferric iron species Fe(OH)₃⁰ and Fe(OH)₂⁺ are derived and their implications for the behavior of these species in seawater are examined. The previously reported formation constant for Fe(OH)₃⁰ and its claimed extreme adsorptive reactivity in seawater are shown to be mutually inconsistent. Although Fe(OH)₃⁰ is probably a stoichiometrically minor dissolved iron species, its rapid formation from Fe(OH)₂⁺ could substantially enhance the rates of heterogeneous reaction rates of the [Fe(OH)₂⁺ + Fe(OH)₃⁰] pool if the latter species is very reactive.     

Polycyclic aromatic hydrocarbons in surface seawater and in indigenous mussels (Mytilus galloprovincialis) from coastal areas of the Saronikos Gulf (Greece)

Polycyclic aromatic hydrocarbons (PAHs) were identified and measured in surface seawater and in the tissues (gills and mantle) of indigenous black mussels, Mytilus galloprovincialis, collected from three coastal sites of Saronikos Gulf (Greece), a gulf that exhibits high levels of pollution. The total PAHs measured by spectrofluorometry in the surface seawater were found in the range of 425–459 ng L⁻¹ at the most polluted sites 1 and 2 (Elefsis Bay–Salamis Island) and in the range of 103–124 ng L⁻¹ at site 3 (Aegina Island). PAHs' sources in seawater were identified by application of specific PAH ratios, such as phenanthrene/anthracene and fluoranthene/pyrene. Levels of PAHs in soft tissues (gills and mantle) of indigenous mussels were much higher than those reported for seawater. Total PAH concentrations in mantle tissues were in the range of 1300–1800 ng g⁻¹ dry weight (dw) tissue at sites 1 and 2 and approximately 380 ng g⁻¹ dw at site 3. In gill tissues total PAH concentrations were in the range of 1480–2400 ng g⁻¹ dw at sites 1 and 2 and approximately 430 ng g⁻¹ dw at site 3. PAHs composition was dominated by two-, three- and four-ring compounds in seawater, where 17 different PAH compounds were identified and measured in mussel tissues. Mussels can be used as sentinel organisms to monitoring PAHs' contamination, since they concentrate PAHs from the surrounding water media and therefore making the chemical analysis simpler and less prone to error than that for water. In surface seawater possible weathering and photodegradation due to hot climates contribute to reduced PAHs concentrations.     

Oxidation of copper(I) in seawater at nanomolar levels

The oxidation and reduction of nanomolar levels of copper in air-saturated seawater and NaCl solutions has been measured as a function of pH (7.17–8.49), temperature (5–35 °C) and ionic strength (0.1–0.7 M). The oxidation rates were fitted to an equation valid at different pH and ionic strength conditions in sodium chloride and seawater solutions:

The reduction of Cu(II) was studied in both media for different initial concentrations of copper(II). When the initial Cu(II) concentration was 200 nM, the copper(I) productions were 20% and 9% for NaCl and seawater, respectively. The effect of speciation of copper(I) reduced from Cu(II) on the rates was studied. The Cu(I) speciation is dominated by the CuCl₂⁻ species. On the other hand, the neutral chloride CuCl species dominates the Cu(I) oxidation in the range of 0.1 M to 0.7 M chloride concentrations.     

Equilibrium and structural studies of silicon(IV) and aluminium(III) in aqueous solution. II. Formation constants for the monosilicate ions SiO(OH)3 and SiO2(OH)2. A precision study at 25°C in a simplified seawater medium

The hydrolysis of silicic acid, Si(OH)₄, was studied in a simplified seawater medium (0.6 M Na(Cl)) at 25°C. The measurements were performed as potentiometric titrations (hydrogen electrode) in which OH⁻ was generated coulometrically. The total concentration of Si(OH)₄, B, and log[H⁺] were varied within the limits 0.00075 B 0.008 M and 2.5 -log[H⁺] 11.7, respectively. Within these ranges the formation of SiO(OH)₃⁻ and SiO₂(OH)₂²⁻ with formation constants log β₋₁₁(Si(OH)₄ SiO(OH)₃⁻ + H⁺) = −9.472 ±0.002 and log β₋₂₁(Si(OH)₄ SiO₂(OH)₂²⁻ + 2H⁺) = −22.07 ± 0.01 was established. With B > 0.003 M polysilicate complexes are formed, however, with -log[H⁺] 10.7 their formation does not significantly affect the evaluated formation constants. Data were analyzed with the least squares computer program LETAGROPVRID.     

Determination of picomolar iron in seawater by double Mg(OH)2 precipitation isotope dilution high-resolution ICPMS

A low-blank pre-concentration procedure is described for the analysis of picomolar iron (Fe) in seawater by isotope dilution high-resolution inductively coupled plasma mass-spectrometry (HR-ICPMS). The procedure uses a two-step Mg(OH)₂ co-precipitation procedure to extract Fe from a 50 ml seawater sample into a 100 μl 4% nitric acid (HNO₃) solution followed by HR-ICPMS measurement. The high pre-concentration ratio ( 500:1) achieved by the procedure minimizes the Fe blank due to ICPMS instrumental Fe background and results in a detection limit of  2 pM and a precision of  4% at the 50 pM Fe level. The measurement of a low-Fe seawater sample spiked with gravimetric Fe standard shows that the method can clearly distinguish 0.01 nM Fe from 0.02 nM Fe in seawater with high accuracy. The method is demonstrated by the analysis of dissolved Fe in the equatorial Pacific Ocean.     

Factors controlling dissolved silica in coral reef surface water of Urasoko Bay, Ishigaki Island, Japan

From August 2006 to August 2007, the concentrations of dissolved silica (Si(OH)₄) were monitored in the surface water of Urasoko Bay and the mouth of the stream that runs into the bay. Urasoko Bay is located on the northern coast of Ishigaki Island, Okinawa, Japan, which is in a subtropical area of the North Pacific Ocean and is surrounded by a relatively poorly developed fringing reef. Added to these samples were freshwater from the upstream area and brackish water that exudes at the beach site, which were collected from April to June 2007. Rainwater samples were also collected during the study period. The concentration of Si(OH)₄ generally decreased from upstream to the bay site, and, on clear days, Si(OH)₄ data from all study sites (the bay, beach, stream mouth, and upstream) plotted against salinity fell on a single straight line. When the influence of rainwater was, the results were scattered below the straight line, which suggests dilution by rainwater with a much lower Si(OH)₄ concentration. These findings show that offshore seawater, rainwater, and upstream freshwater regulate the concentration of Si(OH)₄ in the surface water of Urasoko Bay.     

Impact of environmental factors on in situ determination of iron in seawater by flow injection analysis

A sensitive method for iron determination in seawater has been adapted on a submersible chemical analyser for in situ measurements. The technique is based on flow injection analysis (FIA) coupled with spectrophotometric detection. When direct injection of seawater was used, the detection limit was 1.6 nM, and the precision 7%, for a triplicate injection of a 4 nM standard. At low iron concentrations, on line preconcentration using a column filled with 8-hydroxyquinoline (8HQ) resin was used. The detection limit was 0.15 nM (time of preconcentration = 240 s), and the precision 6%, for a triplicate determination of a 1 nM standard, allowing the determination of Fe in most of the oceanic regimes, except the most depleted surface waters. The effect of temperature, pressure, salinity, copper, manganese, and iron speciation on the response of the analyser was investigated. The slope of the calibration curves followed a linear relation as a function of pressure (C_p = 2.8 × 10^{− 5}P + 3.4 × 10^{− 2} s nmol^{− 1}, R² = 0.997, for Θ = 13 °C) and an exponential relation as a function of temperature (C_Θ = 0.009e^0.103Θ, R² = 0.832, for P = 3 bar). No statistical difference at 95% confidence level was observed for samples of different salinities (S = 0, 20, 35). Only very high concentration of copper (1000 × [Fe]) produced a detectable interference. The chemical analyser was deployed in the coastal environment of the Bay of Brest to investigate the effect of iron speciation on the response of the analyser. Direct injection was used and seawater samples were acidified on line for 80 s. Dissolved iron (DFe, filtered seawater (0.4 μm), acidified and stored at pH 1.8) corresponded to 29 ± 4% of Fe_a (unfiltered seawater, acidified in line at pH 1.8 for 80 s). Most of Fe_a (71 ± 4%) was probably a fraction of total dissolvable iron (TDFe, unfiltered seawater, acidified and stored at pH 1.8).     

Solubility of hydrous ferric oxide and iron speciation in seawater

Iron solubility equilibria were investigated in seawater at 36.22‰ salinity and 25°C using several filtration and dialysis techniques. In simple filtration experiments with 0.05 μm filters and Millipore ultra-filters, ferric chlorides fluorides, sulfates, and FeOH²⁺ species were found to be insignificant relative to Fe(OH)₂⁺ at p[H⁺] = ?log [H⁺] greater than 6.0. Hydrous ferric oxide freshly precipitated from seawater yielded a solubility product of ${}^1\text{K}{}_{\mathrm{<mtext>so</mtext>}}\text{=}\text{[Fe}{}^{\mathrm{3+}}\text{][H}{}^{\mathrm{+}}\text{]}{}^{\mathrm{?3}}\text{= 4.7 · 10}{}^5$. Solubility studies based on the rates of dialysis of various seawater solutions and on the filtration of acidified seawater solutions indicated the existence of the Fe(OH)₃⁰ species. The formation constant for this species can be calculated as ${}^1\text{β}{}_3\text{=}\text{[Fe(OH)}{}_3{}^0\text{] [H}{}^{\mathrm{+}}\text{]}{}^3\text{/[Fe}{}^{\mathrm{3+}}\text{] = 2.4 · 10}{}^{\mathrm{?14}}$. The Fe(OH)₄^? species is present at concentrations which are negligible compared to Fe(OH)₂⁺ and Fe(OH)₃⁰ in the normal pH range of seawater. However, there is at least one other significant ferric complex in seawater above p[H⁺] = 8.0 (possibly with bicarbonate, carbonate, or borate ions) in addition to the Fe(OH)₂⁺ and Fe(OH)₃⁰ species.     

Variations in the chemical lability of iron in estuarine, coastal and shelf waters and its implications for phytoplankton

The relationship between total and chemically labile Fe has been studied in estuarine, coastal and shelf waters of the Gulf of Maine, U.S.A. Measurements of the labile fraction of total Fe, defined by complexation with 8-hydroxyquinoline in 1 h, correlate with the availability of Fe to marine phytoplankton and therefore can be used to estimate Fe availability in seawater. The results show that the relative lability (=labile/total) of Fe in seawater varied both spatially and temporally from near-zero to 100%. Although particulate Fe (>0.45 μm) was generally less labile than dissolved Fe (<0.45 μm), the particulate fraction often contributed substantially to labile Fe concentrations overall. Conversely, as much as 75% of ‘dissolved’ Fe was non-labile, and therefore was probably not available to phytoplankton. In seawater/river-water mixing experiments, aggregation diminished the relative lability of Fe by 30%, even though much of it remained in the ‘dissolved’ fraction. Considering phytoplankton nutrition, these results demonstrate that equating dissolved Fe concentrations with ‘available’ metal can be misleading. Furthermore, the large variability observed in the labile proportion of total Fe in seawater indicates that Fe availability to phytoplankton cannot be estimated by applying fixed lability-ratios to total Fe concentrations.     

Characteristics of the ratio of dissolved cadmium to phosphate in subtropical coastal waters of Ishigaki Island,Okinawa, Japan

The ratio of dissolved cadmium (Cd) to phosphate (PO₄) in the subtropical coastal area of Ishigaki Island, Okinawa, Japan, was investigated. Twenty vertical seawater samplings were carried out once a month from May 2008 to January 2010. In order to examine how the Cd/PO₄ ratio in seawater varies with the oceanographic conditions (i.e., the water temperature–salinity characteristics), the water masses at the study sites were classified into two types: group 1 with a water temperature of >25°C and a salinity range of 34.0–34.4, and group 2 with a water temperature of <25°C and a salinity of >34.4. A different phytoplankton assemblage was observed in each water mass defined. Different Cd/PO₄ ratios were obtained for the two water mass types, due to the differences between the types in terms of the environmental conditions such as the water temperature–salinity (T–S) characteristics and phytoplankton assemblages, as well as possible variations in the concentrations of dissolved iron, zinc, manganese, and CO₂ in seawater in each water mass.     

Solvent extraction of copper acetylacetonate in studies of copper(II) speciation in seawater

A liquid-liquid partition, ligand exchange procedure involving the formation of copper(II) complexes with acetylacetone is presented for the determination of stability constants and concentrations of copper chelators in seawater. Acetylacetone competes with natural ligands for copper, and the equilibrium concentration of the copper acetylacetonate complex is used in speciation calculations. The concentration of the complex is calculated by partitioning a fraction of it into an organic phase and determining the total Cu concentration in that phase by back extracting with acid, and analyzing by flameless atomic absorption spectroscopy. The concentration of Cu acetylacetonate in seawater in equilibrium with the organic phase is calculated from the partition coefficient. The simple, thermodynamically well characterized procedure offers several advantages over previous techniques. Studies using organic free seawater and model ligands show good agreement between experimental and calculated conditional stability constants. Studies from seawater in Biscayne Bay, Florida, indicate two ligand types are present; type 1, K₁ = 1.2 × 10¹², C_L1 = 5.1 × 10⁻⁹ M; type 2, K₂ = 2.8 × 10¹⁰, C_L2 = 1.1 × 10⁻⁷ M. Speciation is dominated by ligand type 1. Depth profiles of [Cu(II)]_free/[Cu(II)]_total measured with the procedure at ambient copper concentrations show an increase from < 5 × 10⁻⁵ at 50–60 m to > 1 × 10⁻³ at the surface at two stations off the Florida coast.