Chemical speciation of trace metals in seawater: Implication of particulate trace metals

Chemical speciation of particulate metals in seawater was examined theoretically. Mass balance considerations showed that the apparent conditional stability constant, defined for organically binding metals in suspended particles, coincides with the conditional stability constant determined for the corresponding metal-organic complexes dissolved in seawater. This hypothesis suggests that some metals, which are present as organic complexes (e.g. copper), are directly associated with particulate organic matter. Metals, whose free ion is buffered by organic and/or inorganic ligands, may be used as indicators of the presence of particulate organic matter in the marine environment.     

Determination of the complexing capacity and conditional stability constants of complexes of copper(II) with natural organic ligands in seawater by cathodic stripping voltammetry of copper-catechol complex ions

A new method is proposed for the determination of complexing capacities and conditional stability constants for complexes of copper(II) with dissolved organic ligands in seawater. This method is based on ligand competition by the added ligand catechol for free metal ions. The concentration of copper-catechol complex ions is measured with great sensitivity by cathodic stripping voltammetry. The concentration of the free copper ion is calculated from the concentration of copper-catechol complex ions. Ligand concentrations and conditional stability constants are obtained from a titration of the ligands with copper. Two techniques for treatment of the data are compared. A seawater sample, originating from open oceanic conditions, is analysed and two complexing ligands were detected, having concentrations of 1.1 × 10^?8 and 3.3 × 10^?8 M, and conditional stability constants (log K′_CuL) of 12.2 and 10.2, respectively.     

Determination of copper complexation with natural organic ligands in seawater by equilibration with MnO2 I. Theory

The theory is discussed which describes the distribution of copper ions between a weak ion exchanger, as exemplified by MnO₂, and natural organic complexing material in seawater. Application of this theory and experimental procedures are outlined in part II of this series. It is apparent from the theory that titration with Cu²⁺ of one or more organic complexing ligands can be graphically represented by straight lines; slope and y-axis intercept provide information on the conditional stability constants and the ligand concentrations. Model calculations show that measurement of metal complexation at ligand concentrations higher than normally present in seawater may produce erroneous results because of possible changes in the metal to ligand ratio in the complexes. It is therefore advisable to measure metal complexation in the original, unaltered, water sample.     

Determination of conditional stability constants of organic copper and zinc complexes dissolved in seawater using ligand exchange method with EDTA

To clarify the nature of organic metal complexes dissolved in seawater, a ligand exchange reaction between ligands of natural origin and an aminopolycarboxylic acid (EDTA) was used to determine the conditional stability constants of organic metal complexes. The results indicate that more than two organic molecules complexed with copper and zinc exist in surface seawater. It is found that the conditional stability constants of these naturally-occurring organic metal complexes are 1–3 orders of magnitude higher than those of EDTA-Cu and EDTA-Zn complexes. These estimates of the conditional stability constants for the dominant species of organic copper and zinc complexes are 10^11.8 and 10^9.3, respectively, at pH 8.1. The results indicate that these naturally-occurring organic metal complexes are stable species and not easily dissociated or displaced with others in the marine environment.     

Strong ligands for thorium complexation in marine bacteria

The interaction between thorium and marine organisms (cultured heterotrophic bacteria) was experimentally examined by using chemical equilibrium techniques. Thorium (Th) quantitatively reacts with a binding site on bacteria (Alteromonas, Vibrio, Pseudomonas and Flavobacterium) in 0.1 M HCl solution. According to mass balance analysis of adsorption experiment data, Th forms a 1:1 complex with a binding site similar in reactivity among bacteria used in this study, whose conditional stability constants are in the range from 10(6.63) to 10(7.07) M-1 under the experimental conditions of a 0.1 M HCl solution. The mole ratio of the strong ligand to organic carbon in bacteria ranged from 2.3 to 4.3 mmol/mol C. The strong ligand/carbon ratios in bacteria were more than one order of magnitude greater than in phytoplankton, zooplankton or other organic ligands in surface waters. The results suggest that the strong organic ligand reacting with Th is one of the functional groups commonly existing in oceanic microorganisms. The conditional stability constants of the Th complexes with the binding site in marine microorganisms are in the same order of magnitude as that with the strong ligand found in particulate and dissolved organic matter. These findings strongly suggest that the strong ligand in particulate and dissolved organic matter, reacting with trace metals under the conditions of seawater, originates from marine organisms.     

Voltammetric studies of the organic association of copper and lead in two canadian inlets

Anodic stripping voltammetry and gel filtration chromatography were used to examine the speciation and organic complexation of copper and lead in seawater. The extent of metal complexation in biologically different water types, the molecular weight ranges of the dissolved organic matter involved in metal-organic associations, and the metal uptake kinetics of these naturally occurring organic species were examined. Analyses of samples from Saanich and Narrows inlets, British Columbia, suggest organic complexation throughout the water column, including anoxic waters of Saanich Inlet. Several fractions of organic material from sedimentary interstitial water in the molecular weight range 500–10,000 were found to complex copper and lead, the concentration of complexing ligands decreasing with depth in the core. Rates of uptake of metals by organic material are slow, of the order of tens of minutes or longer.     

Determination of copper complexation with natural organic ligands in seawater by equilibration with MnO2 II. Experimental procedures and application to surface seawater

The MnO₂ adsorption method combined with voltammetry is proposed for the direct determination of metal complexation in seawater of various salinities as a more satisfactory alternative to direct voltammetric measurements and bioassay methods. A small quantity of MnO₂ is equilibrated with copper ions in filtered seawater. Natural organic ligands in the seawater compete for copper with the MnO₂. Total dissolved copper is measured by differential pulse anodic stripping voltammetry after filtration and acidification of the sample. Preconcentration of natural water samples is unnecessary and measurement is performed at the natural equilibrium pH of the aerated sample. The analytical limit of detection of the method depends on contamination from the filtration step, and for copper complexation a ligand concentration of 5 × 10^?8 M was obtained. The sensitivity can be increased by use of radioisotopes as tracers. The method is very versatile in that complexation of various metals may be determined by any analytical method that measures total dissolved metal concentrations. Neither organic ligands nor their complexes with copper adsorb on the MnO₂ at pH8, but at pH 1.8 MnO₂ is an efficient scavenger for electroactive organic material.Samples of surface water from the Irish Sea and the Atlantic Ocean were found to contain ligand concentrations of 1.7 × 10^?7 and 1.1 × 10^?7 M, with conditional stability constants (log values) of 9.84 ± 0.13 and 9.86 ± 0.23, respectively, at pH 8.0.     

Molecular masses and chromophoric properties of dissolved organic ligands for copper(II) in oceanic water

The distribution of molecular masses of organic ligands for copper(II) in oceanic water was investigated. The bulk dissolved organic matter (DOM) was fractionated by ultrafiltration and organic ligands were extracted from the resultant fractions by using immobilized metal ion affinity chromatography (IMAC). Contributions of total organic ligands were 2.0–4.4% of the bulk DOM in surface waters, as determined by the UV absorbance. In the distribution of molecular masses of organic ligands, relative contribution of the fraction with low molecular masses (<1000 Da) was dominant (49–62%), while 26–33% of the total organic ligands was in the 1000–10,000 Da fraction, leaving 10–19% in the >10,000 Da fraction. The distribution of molecular masses of organic ligands shifted to higher molecular masses, as compared with that of the bulk DOM. The fluorescence intensities of organic ligands were shown to be associated with carboxyl contents, based on peak excitation/emission wavelengths and the pH-dependence of fluorescence. Two ligand classes with different conditional stability constants (log K_CuL′≈7 and 9) were determined from fluorescence quenching of ligand fractions during copper(II) titration. Organic ligands in low molecular mass fractions were relatively weak and strong ligands occurred in higher molecular mass fractions. It is suggested that the weaker ligand sites would consist of two or more carboxyl groups (log K_HL′=4), whereas carboxyl groups (log =2), which are protonated at lower pH, and primary amine may additionally contribute to the formation of more stable copper(II) complexes of the stronger ligand.     

Organic complexation and its control of the dissolved concentrations of copper and zinc in the Scheldt estuary

Cathodic stripping voltammetry (CSV) is used to determine total (after UV-irradiation) and labile dissolved metal concentrations as well as complexing ligand concentrations in samples from the river Scheldt estuary. It was found that even at high added concentrations of catechol (1 m for copper and 0·4 m for iron) and of APDC (1 m for zinc) only part of the dissolved metal was labile (5–58% for copper, 34–69% for zinc, 10–38% for iron); this discrepancy could be explained by the low solubility of iron which is largely present as colloidal material, and by competition for dissolved copper and zinc by organic complexing ligands. Ligand concentrations varied between 28 and 206 n for copper and between 22 and 220 n for zinc; part of the copper complexing ligands could be sub-divided into strong complexing sites with concentrations between 23 and 121 n and weaker sites with concentrations between 44 and 131 n . Values for conditional stability constants varied between (logK′ values) 13·0 and 14·8 for strong and between 11·5 and 12·1 for weaker copper complexing ligands, whereas for zinc the values were between 8·6 and 10·6. The average products of ligand concentrations and conditional stability constants (a-coefficients) were 6 × 10² for zinc and 6 × 10⁶ for copper.The dissolved zinc concentration was found to co-vary with the zinc complexing ligand concentration throughout the estuary. It is argued that the zinc concentration is regulated, in this estuary at least, by interactions with dissolved organic complexing ligands. A similar relationship was apparent between the dissolved copper and the strong copper complexing ligand concentration. The total copper complexing ligand concentrations were much greater than the dissolved copper concentrations, suggesting that only strongly complexed copper is kept in solution.These results provide evidence for the first time that interactions of copper and zinc with dissolved organic complexing ligands determine the geochemical pathway of these metals.     

Complexation of copper and nickel in the dissolved phase of marine sediment slurries

Speciation of copper and nickel in the water phase of incubated marine slurries under aerobic conditions was performed with MnO₂ and Sep—Pak C₁₈ cartridges. Changes in time during the incubations of concentrations of dissolved organic carbon (DOC), dissolved copper and nickel and inorganic nitrogen were followed. The influence of organic complexation on the dissolved concentrations of copper and nickel was investigated as well as competition between copper and nickel for dissolved organic ligands.Two pools of dissolved organic ligands could be distinguished. With the MnO₂ method a relatively strong ligand group was determined that was subjected to degradation. The conditional stability constant for copper with the relatively strong ligand was 10^11.1. The conditional stability constant for the relatively strong nickel ligand was difficult to determine due to saturation of the ligand sites; it was found to be around 10¹⁰. However, it could not be ascertained whether nickel was reversibly com-plexed with the organic ligands.With Sep—Pak a relatively weak Hgand group was detected that was probably more resistant to degradation. The conditional stability constant of the weaker ligand could not be estimated, an approximation revealed that it was weaker than the ligand group determined with the MnO₂ method. For copper the difference between binding strength of the ligand groups was at least 100, for nickel the difference was less.Competition between copper and nickel for the ligands could not be detected. Only during the first day of the experiment, when the system was not in equilibrium was competition suspected. However, the replacement of nickel by copper from the ligand sites was not straightforward and could not be accounted for by our model.The concentration of total dissolved copper during the first week of the experiment was found to be controlled on the one hand by release from the sediment of copper already associated with dissolved organic matter (DOM) and on the other hand by concentration of the strongest ligand. The calculated free copper concentration increased from 10⁻¹² to 10⁻⁹mol l⁻¹ due to the oxidation of the strongest ligand. After saturation of the strongest ligand the relatively weak ligand controlled the free copper concentration. A continuing release of copper from the sediment by degradation of particulate organic matter (POM) will not increase the free copper concentration until the ligand sites of the weaker ligands get saturated.The total dissolved nickel concentration seemed only to be determined by the sum of the concentrations of the organic ligands. A degradation of ligands resulted in a decrease of the total dissolved nickel concentration. The calculated free nickel concentration did not change with time.     

浙江西沪港重金属铜的配位容量和形态分析

根据2000年6月10～14日在浙江省西沪港采集的海水样品,利用AA-800石墨炉原子吸收分光光度技术和阳极溶出伏安法测定样品中重金属铜的含量,获得铜在海水中受不同的有机配体控制.不同粒级的铜表观配位容量表明西沪港海水过孔径1.00μm微孔膜的(ACuCC)较高,为144.4nmol/dm3;过0.40和0.20μm滤膜的(ACuCC)分别为103.0和102nmol/dm3;铜的有机配体条件稳定常数的对数值在7.25～9.14之间.铜的总量为21.72nmol/dm3.铜全部为稳定溶解态,其中pH2酸溶态占95.0%,强有机结合态占5.0%.溶解态铜中有机结合态占过滤海水中总铜的61.6%.     

Effects of ligand speciation on determinations of the complexing abilities of strong ligands in natural waters

The presence of organic ligands (L) whose conditional stability constants (K_ML) are strong enough to allow them to form complexes with copper has been reported in oceanic waters. However, there is no general agreement on the distributional characteristics of such strong ligands in the water column. We present that these inconsistencies are derived from not only different analytical methods employed for their detection but also different oceanographic conditions. In particular, the nature and quantities of detectable natural ligands are affected by what kind of form the ligands are presentin situ in different marine environments, that is, chemical speciation of natural ligands (ligand speciation), which depends strongly on the variations in concentrations of coexisting trace metals. Using published data from observations in the laboratory and the field, we provide limits to the ranges of conditional stability constants of organic ligands for copper, zinc and cadmium that are detectable by the extensively used direct metal-titration method. For example, our model indicates that organic ligands for copper with log K_CuL(Cu)>12.4 in surface water and with log K_CuL(Cu)>9.9 in deep water might not have been detected because stronger ligands had formed complexes with ambient metalsin situ at a station in the North Pacific. The estimation suggests that there is a basinscale difference in speciation of natural organic ligands and, moreover, that this difference influences metal speciation. It is postulated that comparisons of the occurrence and levels of organic ligands might not be possible among spatially and temporally different observations without reconciliation of the effects of speciation of the ligands, even if an identical method is used in every case.     

杭州湾南岸潮滩重金属污染调查和评价

文章于杭州湾南岸潮滩布设3个采样断面并采集沉积物样本,采用地质累积指数法和潜在生态危害指数法评价重金属污染状况及其生态风险。研究结果表明:沉积物基本为粗粉砂和细砂,由海向陆粒径逐渐变细且分选性逐渐转差;所有站位重金属含量均值从高到低依次为Pb、Zn、Cu、Cr、Cd和Hg,断面重金属含量从高到低依次为高潮滩、中潮滩和低潮滩,Cu、Hg和Pb的空间分布差异较大;本次采样调查结果与邻近区域数据和同区域历史数据相比有所不同;根据地质累积指数评价结果,Cu、Pb和Zn有不同程度的污染;根据污染指数和潜在生态危害指数评价结果,Zn、Cd和Cr与地质累积指数评价结果区别较大,研究区重金属总体污染程度为低度和中度,除Cd外其他重金属处于低生态危害水平,研究区重金属总体生态危害为低度。调查和研究成果可为分析区域重金属环境影响和制定污染控制对策提供科学依据。     

Assessment of pollution and ecological risk of heavy metals in the surface sediments of Ulsan Bay,Korea

Heavy metal concentrations in the surface sediments of specially managed Ulsan Bay were investigated to determine metal distribution, pollution status and its ecological risk using pollution indices (enrichment factor and geo-accumulation index), potential ecological risk index and sediment quality guidelines (SQGs). The order of mean concentration (mg/kg) of metals was Zn (361.9) > Cu (95.6) > Pb (90.7) > Cr (64.7) > Ni (32.2) > Co (16.6) > As (15.8) > Cd (0.40) > Hg (0.16) in sediments of Ulsan Bay. Spatial distribution of metals in sediments showed a significantly higher concentration near industrial complexes, indicating that metal pollution is caused by anthropogenic sources. The results of enrichment factor (EF) and geo-accumulation index (I_geo) showed that sediments were significantly accumulated with Cu, Zn, As, Cd, Pb and Hg, indicating moderate to very severe enrichment (pollution) by these metals. Based on the potential ecological risk index, Hg and Cd posed a very high and a considerable potential ecological risk. Cu and As posed a moderate potential ecological risk, while, other metals (Cr, Co, Ni, Zn and Pb) rarely posed any potential ecological risk to the coastal environments. The sediments in Ulsan Bay showed a very high level of ecological risk, dominated by Hg and Cd. Metal concentrations in sediments were 80% for Cu, 96.7% for Zn, 50% for As, 70% for Pb and 50% for Hg above the threshold effects level (TEL), respectively.     

Metal partitioning between colloidal and dissolved phases and its relation with bioavailability to American oysters

Kinetics and the extent of metal partitioning between colloidal and dissolved phases and coagulation of metals associated with colloids were examined to determine their effects on the bioavailability of selected metals (Cd, Co, Hg, Ag, Fe, and Zn) to American oysters (Crassostrea virginica) using radiotracer and short term exposure experiments. After dispersion of radiolabeled colloids into low molecular weight (LMW, < 1 kDa) seawater, metal partitioning between dissolved (<1 kDa) and colloidal (1 kDa-0.2 microm) phases resulted in a consistent pattern, with a relatively constant percentage in the colloidal phase for each metal. On average, about 90% of Hg and Fe, approximately 60% of Ag and approximately 40% of Zn, Co, and Cd were measured in the colloidal fraction during a short term exposure experiment, consistent with their partitioning in natural waters. Controlled laboratory experiments carried out in parallel using radioactively tagged colloids showed that coagulation of colloidal species, quantified as the fraction retained by a 0.2 microm filter, was insignificant for most metals under the conditions and time periods of the uptake experiments. The bioavailability of colloidally complexed metals, measured in terms of dry weight concentration factor (DCF, ml g(-1)) and uptake rate constant (ml g(-1) h(-1)), was somewhat depressed compared with their counterpart in the LMW treatment, but could be well predicted from the results of the LMW treatment and metal partitioning. Both DCF values and uptake rate constants were higher in the LMW treatment than in the colloidal treatment. In addition, B-type metals, such as Ag, Hg, and Zn, all had higher values of DCF and uptake rate constants, regardless of treatments, except for Cd which had a lower DCF and uptake rate constant. In contrast, Co and Fe had significantly lower DCF values and uptake rate constants. Most of Hg and Ag (60-80%) were measured in the soft tissue of oysters in both LMW and colloidal treatments. In contrast, 80% of Fe, 75% of Co, and approximately 60% of Cd were observed on the shell, while Zn was found evenly distributed between shell and soft tissue of oysters. These results agree well with the variation pattern of both DCF value and uptake rate constant for these two groups of metals.     

The influence of surfactants on the measurements of copper and cadmium speciation in model seawater by differential pulse anodic stripping voltammetry

Model systems consisting of a heavy metal ion (Cu²⁺ or Cd²⁺), complexing ligand (EDTA or NTA), a surfactant (9,12-octadecadienoic acid, C₁₈H₃₂O₂, i.e., linoleic acid), and a surfactant which is at the same time a complexing ligand (tert-octylphenol etoxylate, C₈H₁₇(C₆H₄)(OC₂H₄)_9–10OH, i.e., Triton-X-100) in seawater and NaCl solution were used in order to investigate the influence of the surfactant adsorption (on the electrode surface) on the heavy metal speciation measurements. The samples were titrated with either the metal, complexing ligand or surfactant and were measured by differential pulse anodic stripping voltammetry. It was shown that the surfactant adsorption exerts a strong influence upon the overall metal redox process and, thus, changes considerably both the apparent complexing capacity and the conditional stability constant of the system. Considering the presence of high concentrations of surfactants in polluted seawaters, the danger of measuring the apparent complexing capacity with the anodic stripping voltammetry method, without a detailed knowledge of surfactant properties and influence upon the system, is discussed.     

Complexation of mercury by dissolved organic matter in surface waters of Galveston Bay, Texas

The chemical speciation of dissolved mercury in surface waters of Galveston Bay was determined using the concentrations of mercury-complexing ligands and conditional stability constants of mercury-ligand complexes. Two classes of natural ligands associated with dissolved organic matter were determined by a competitive ligand exchange-solvent solvent extraction (CLE-SSE) method: a strong class (L_s), ranging from 19 to 93 pM with an average conditional stability constant (K_HgLs) of 10²⁸, and a weak class (L_w) ranging from 1.4 to 9.8 nM with an average K_HgLs of 10²³. The range of conditional stability constants between mercury and natural ligands suggested that sulfides and thiolates are important binding sites for dissolved mercury in estuarine waters. A positive correlation between the estuarine distribution of dissolved glutathione and that of mercury-complexing ligands supported this suggestion. Thermodynamic equilibrium modeling using stability constants for HgL, HgCl_x, Hg(OH)_x, and HgCl(OH) and concentrations of each ligand demonstrated that almost all of the dissolved mercury (> 99%) in Galveston Bay was complexed by natural ligands associated with dissolved organic matter. The importance of low concentrations of high-affinity ligands that may originate in the biological system (i.e., glutathione and phytochelatin) suggests that the greater portion of bulk dissolved organic matter may not be important for mercury complexation in estuarine surface waters.     

An evaluation of a C-18 solid phase extraction technique for isolating metal-organic complexes from central North Pacific Ocean waters

This paper presents results from an investigation using a C-18 solid phase extraction (SPE) method for isolating dissolved metal-organic complexes from seawater. The vertical distribution of dissolved organically complexed copper isolated by this technique in the intermediate and deep waters of the central North Pacific is similar to that observed in the North Atlantic. Copper isolated by C-18 SPE from 125 to 5000 m in the North Pacific ranged from 11 to 30% of the total dissolved copper, whereas published concentrations for similar depths in the North Atlantic ranged from 22 to 36% of the total dissolved copper. Evidence from voltammetric studies suggests that the C-18 SPE technique may isolate the major portion of the organically complexed copper from deep waters.In oceanic surface waters, however, a major discrepancy exists — recent voltammetric and equilibration studies indicate that essentially all of the dissolved copper and zinc is associated with organic complexes, while the C-18 SPE method isolates <10% of the total dissolved copper and zinc in the North Pacific and <40% of the total dissolved copper in the North Atlantic. Thus, in surface waters, it appears that the major fraction of organically complexed copper and zinc is not isolated by the C-18 SPE approach.The fractions of total dissolved cadmium, iron, manganese, and nickel isolated by the C-18 SPE technique were also studied. The degree to which each of these metals is complexed with the fraction of the dissolved organic matter isolated by this method is smaller and not as significant as that observed for copper. These results do not imply, however, that organic complexation of these metals is not significant, only that the organic—metal complexes are not isolated by the C-18 SPE method.     

Application of the fluorescence quenching titration method to the complexation of copper (II) in the Gironde Estuary (France)

Quenching of fluorescence emission by the Cu²⁺ ion is measured in five samples from the Gironde Estuary (France). Non-linear regression analysis allows us to calculate from experimental data the values of C_L (complexing capacities or total ligand concentrations), K (conditional stability constants) and I_ML (residual fluorescence at the end of the titration). The results obtained on the Gironde estuarine samples indicate that C_L values are very low (<0.1 micromolar) and K values decrease from upstream to downstream. The relatively high values of I_ML are an indication that much of the organic fluorescing matter does not bind to Cu²⁺.