Interpretation of in situ speciation measurements of inorganic and organically complexed trace metals in freshwater by DGT

The dynamic speciation technique, diffusive gradients in thin-films (DGT), has been used in freshwater to determine simultaneously, from a single set of in situ measurements, (1) the equilibrium distribution of metal ions between simple inorganic complexes and larger organic complexes and (2) information on the rates of dissociation of these complexes. DGT devices with different diffusion layer thicknesses (0.3, 0.54, 1.34, and 2.14 mm) were used to estimate the in situ dissociation kinetics. Information on the species distribution was obtained by using two types of gel, which allow relatively free (polyacrylamide, APA) and more retarded (restricted, RES) diffusion of the metal complexes. The full theoretical basis of the technique is developed and applied to in situ measurements of Mn, Fe, Co, Ni, Cu, Cd, and Pb in a pristine river (Wyre, U.K.), with high DOC(15mg L(-1)), assuming that organic complexes are dominated by fulvic acid. These first DGT measurements that do not rely on assumptions about complex lability or the distribution of species, are compared to total dissolved measurements, previously reported speciation calculations and measurements using alternative speciation techniques. Examination of calculation consistency suggests that the effective mean diffusion coefficients of metal complexes with organic matter under in situ conditions may be larger than those measured in the laboratory using extracted fulvic acid.     

Measurement and dynamic modeling of trace metal mobilization in soils using DGT and DIFS

The technique of diffusive gradients in thin-films (DGT) accumulates metals on a Chelex resin after their diffusive transport through a hydrogel. It lowers metal concentrations in soil solution adjacent to the device and induces resupply of metal associated with the solid phase. DGT devices were deployed in an alluvial gley soil for 21 different time periods between 4 h and 19.5 d. The accumulated masses of Cu, Cd, Ni, and Zn were used to calculate the distribution coefficient for labile metal, Kdl, and adsorption and desorption rate constants. Calculations were performed using a dynamic numerical model of DGT-induced fluxes in soils (DIFS). It assumes first-order exchange between solid phase and solution and diffusional transport in both the soil solution and the hydrogel. The DIFS model fitted changes in accumulated mass with time very well. Values of Kdl calculated from DIFS of 100 (Cd), 250 (Cu), 150 (Ni), and 150 (Zn) were larger than values of distribution coefficients estimated by exchange with Ca(NO3)2 but similar to those estimated by isotopic exchange (Cd and Zn only). These results suggest that the solid-phase pool of metal affected by the removal of labile metal by DGT, which operates on a time scale of minutes, is similar to the solid-phase pool of metal that can isotopically exchange with solution on a time scale of 2 d. Response times of minutes were consistent with interaction rates with surfaces, and desorption rate constants agreed with other reported values. An appraisal of the DIFS model demonstrated the importance of the labile pool size in the solid phase for controlling supply to a sink, such as DGT or a plant. As values of Kdl and kinetic parameters are obtained using DGT with minimal soil disturbance and by a similar mechanism to that involved in plant uptake, they may be pertinent to bioavailability studies.     

Chemical speciation of Zn,Cd, Cu,and Pb in pore waters of agricultural and contaminated soils using Donnan dialysis

Knowledge of trace metal speciation in soil pore waters is important in addressing metal bioavailability and risk assessment of contaminated soils. Numerous analytical methods have been utilized for determining trace metal speciation in aqueous environmental matrixes; however, most of these methods suffer from significant interferences. The Donnan dialysis membrane technique minimizes these interferences and has been used in this study to determine free Zn2+, Cd2+, Cu2+, and Pb2+ activities in pore waters from 15 agricultural and 12 long-term contaminated soils. The soils vary widely in their origin, pH, organic carbon content, and total metal concentrations. Pore water pM2+ activities also covered a wide range and were controlled by soil pH and total metal concentrations. For the agricultural soils, most of the free metal activities were below detection limit, apart from Zn2+ for which the fraction of free Zn2+ in soluble Zn ranged from 2.3 to 87% (mean 43%). Five of the agricultural soils had detectable free Cd2+ with fractions of free metal ranging from 59 to 102% (mean 75%). For the contaminated soils with detectable free metal concentrations, the fraction of free metal as a percentage of soluble metal varied from 9.9 to 97% (mean 50%) for Zn2+, from 22 to 86% (mean 49%) for Cd2+, from 0.4 to 32.1% (mean 5%) for Cu2+, and from 2.9 to 48.8% (mean 20.1%) for Pb2+. For the contaminated soils, the equilibrium speciation programs GEOCHEM and WHAM Model VI provided reasonable estimates of free Zn2+ fractions in comparison to the measured fractions (R2 approximately 0.7), while estimates of free Cd2+ fractions were less agreeable (R2 approximately 0.5). The models generally predicted stronger binding of Cu2+ to DOC and hence lower fractions of free Cu2+ as compared with the observed fractions. The binding of Cu2+ and Pb2+ to DOC predicted by WHAM Model VI was much strongerthan that predicted by GEOCHEM.     

Titanium dioxide-based DGT technique for in situ measurement of dissolved reactive phosphorus in fresh and marine waters

A new diffusive gradients in a thin film (DGT) technique for measuring dissolved reactive phosphorus (DRP) in fresh and marine waters is reported. The new method, which uses a commercially available titanium dioxide based adsorbent (Metsorb), was evaluated and compared to the well-established ferrihydrite-DGT method (ferrihydrite cast within the polyacrylamide gel). DGT time-series experiments showed that the mass of DRP accumulated by Metsorb and ferrihydrite was linear with time when deployed in simple solutions. Both DGT methods showed predictable uptake across the pH (4.0-8.3) and ionic strength (0.0001-1 mol L(-1) NaNO(3)) ranges investigated, and the total capacity of the Metsorb binding phase (～40,000 ng P) was 2.5-5 times higher than the reported total capacity of the ferrihydrite binding phase. The measurement of DRP in synthetic freshwater and synthetic seawater by Metsorb-DGT over a 4 day deployment period showed excellent agreement with the concentration of DRP measured directly in solution, whereas the ferrihydrite-DGT method significantly underestimated (23-30%) the DRP concentration in synthetic seawater for deployment times of two days or more. Field deployments of Metsorb-DGT samplers with various diffusive layer thicknesses allowed accurate measurement of both the diffusive boundary layer thickness and DRP concentration in situ. The Metsorb-DGT method performs similarly to ferrihydrite-DGT for freshwater measurements but is shown to be more accurate than the ferrihydrite method for determining DRP in seawater.     

Measuring free metal ion concentrations in situ in natural waters using the Donnan Membrane Technique

Metal toxicity is not related to the total but rather to the free or labile metal ion concentration. One of the techniques that can be used to measure several free metal ion concentrations simultaneously is the Donnan Membrane Technique (DMT) in combination with the inductively coupled plasma-mass spectrometer (ICP-MS). However, free metal ion concentrations in natural waters are commonly below the detection limit of ICP-MS. We decreased the detection limit by making use of a ligand, and we developed a field DMT cell that can be applied in situ in natural waters. A kinetic approach can be used to calculate free metal ion concentrations when the equilibrium time becomes too large. The field DMT measured in situ in natural waters a free metal ion concentration ranging from 0.015% (Cu) to 13% (Zn) of a total metal concentration ranging from 0.06 nM (Cd) to 237 nM (Zn). The free metal ion concentrations were difficult to predict using an equilibrium speciation model, probably due to the uncertainty in the nature of the dissolved organic matter or the presence of other reactive colloids. It is shown that DMT can follow changes in the free metal ion concentration on times scales less than a day under certain conditions.     

In-situ speciation of Ni and Zn in freshwaters: comparison between DGT measurements and speciation models

The technique of DGT (diffusive gradients in thin films) was used for the first time to measure in situ the distribution of Zn and Ni between inorganic species and complexes with fulvic and humic acids in natural waters. With DGT, metals are bound to a resin embedded in a layer of hydrogel after diffusive transport through an adjacent layer of hydrogel. The metal concentrations in the waters can be quantified using simple diffusion equations. By using devices with hydrogels of different pore size, large and small complex species were discriminated. Inorganic species diffuse freely through all gels, but larger organic complexes with fulvic and humic acids diffuse less freely through more restricted gels (gels with smaller pore size). Systematic differences between DGT devices containing gels of different pore size were obtained. Their calibration for the diffusion of fulvic and humic complexes allowed calculation of the concentrations of labile inorganic (Zn, 34.6 +/- 2.5 nM; Ni, 23.5 +/- 0.9 nM) and labile organic (Zn, 43.1 +/- 2.9 nM; Ni, 11.2 +/- 0.7 nM) complexes. The concentration of Zn measured by anodic stripping voltammetry in samples returned to the laboratory lay between the DGT-measured inorganic concentration and the total dissolved concentration, consistent with partial measurement of organic complexes of Zn. The speciation model WHAM successfully predicted the species distribution of Ni, Zn, and Cu, provided that competitive binding by Fe(III) was considered. Using the speciation models WHAM and ECOSAT, free ion activities of Zn and Ni were calculated from (1) the total inorganic species measured by DGT and (2) the total dissolved species and dissolved organic carbon. The calculations confirmed the good model predictions of metal-humic binding but highlighted problems with default databases used for the speciation of inorganic components.     

Comparison of analytical techniques for dynamic trace metal speciation in natural freshwaters

Several techniques for speciation analysis of Cu, Zn, Cd, Pb, and Ni are used in freshwater systems and compared with respect to their performance and to the metal species detected. The analytical techniques comprise the following: (i) diffusion gradients in thin-film gels (DGT); (ii) gel integrated microelectrodes combined to voltammetric in situ profiling system (GIME-VIP); (iii) stripping chronopotentiometry (SCP); (iv) flow-through and hollow fiber permeation liquid membranes (FTPLM and HFPLM); (v) Donnan membrane technique (DMT); (vi) competitive ligand-exchange/stripping voltammetry (CLE-SV). All methods could be used both under hardwater and under softwater conditions, although in some cases problems with detection limits were encountered at the low total concentrations. The detected Cu, Cd, and Pb concentrations decreased in the order DGT > or = GIME-VIP > or = FTPLM > or = HFPLM approximately = DMT (>CLE-SV for Cd), detected Zn decreased as DGT > or = GIME-VIP and Ni as DGT > DMT, in agreement with the known dynamic features of these techniques. Techniques involving in situ measurements (GIME-VIP) or in situ exposure (DGT, DMT, and HFPLM) appear to be appropriate in avoiding artifacts which may occur during sampling and sample handling.     

Numerical approach to speciation and estimation of parameters used in modeling trace metal bioavailability

Speciation affects trace metal bioavailability. One model used to describe the importance of speciation is the biotic ligand model (BLM), wherein the competition of inorganic and organic ligands with a biotic ligand for free-ion trace metal determines the ultimate metal availability to biota. This and similar models require natural ligand concentrations and conditional stability constants as input parameters. In concept, the BLM is itself an analogue of some analytical approaches to the determination of trace metal speciation. A notable example is competitive ligand equilibration/cathodic stripping voltammetry, which employs an artificial ligand for comparative assessment of natural ligand concentrations and discrete conditional stability constants (i.e., BLM parameters) in a natural sample. Here, we report a new numerical approach to voltammetric speciation and parameter estimation that employs multiple analytical windows and a two-step optimization process, simultaneously generating both parameters and a complete suite of corresponding species concentrations. This approach is more powerful, systematic, and flexible than those previously reported.     

In situ two-dimensional high-resolution profiling of sulfide in sediment interstitial waters

A recently developed technique of diffusive gradients in thin films (D6T)-computer-imaging densitometry (CID) was improved to study in situ two-dimensional distributions of sulfide in sediment interstitial waters adjacent to the DGT device. The in situ profiler accumulates sulfide from the sediment interstitial water through a diffusive gel onto a AgI binding gel to form black Ag2S. The amount of sulfide bound in the binding gel is then determined from the gray scale density of the dried binding gel. New gel-making procedures were employed to produce stable, evenly dispersed AgI binding gels and to minimize the darkening of the AgI gel upon exposure to light The improved DGT-CID technique was used to profile the distribution of sulfide in sediment interstitial waters at Delta Marsh, a highly productive Prairie wetland on the south shore of Lake Manitoba, at a vertical and lateral resolution of < or = 0.4 mm. The in situ high-resolution microprofiles revealed unprecedented two-dimensional heterogeneity in sulfide concentrations in the sediment interstitial waters adjacent to the DGT device. The mosaic distribution of oxic and sulfidic microenvironments suggested not only the complexity and heterogeneity of the biogeochemistry of sulfur species and sulfide-binding metals (e.g., Cd, Cu, Pb, Hg, Zn) in sediments but also the capability of aquatic organisms for coping with the sulfidic environment     

Model predictions of metal speciation in freshwaters compared to measurements by in situ techniques

Measurements of trace metal species in situ in a softwater river, a hardwater lake, and a hardwater stream were compared to the equilibrium distribution of species calculated using two models, WHAM 6, incorporating humic ion binding model VI and visual MINTEQ incorporating NICA-Donnan. Diffusive gradients in thin films (DGT) and voltammetry at a gel integrated microelectrode (GIME) were used to estimate dynamic species that are both labile and mobile. The Donnan membrane technique (DMT) and hollow fiber permeation liquid membrane (HFPLM) were used to measure free ion activities. Predictions of dominant metal species using the two models agreed reasonably well, even when colloidal oxide components were considered. Concentrations derived using GIME were generally lower than those from DGT, consistent with calculations of the lability criteria that take into account the smaller time window available forthe fluxto GIME. Model predictions of free ion activities generally did not agree with measurements, highlighting the need for further work and difficulties in obtaining appropriate input data.     

In situ measurement of polychlorinated biphenyls in the waters of San Diego Bay,California

Sediments of San Diego Bay (SDB) are known to contain elevated levels of polychlorinated biphenyls (PCBs). While numerous efforts have been made to monitor the spatial or temporal trends of contamination in SDB, no studies have been directed toward measurements of water column contamination. We measured PCB concentrations in the water column of SDB with an in situ sampling system. Except for one sample collected near the bay mouth, all other samples contained higher PCB concentrations than the 30-day average discharge limit established by the California Ocean Plan. The highest concentrations of water column PCBs occurred in the Central Bay, consistent with the previous findings that sediments in the Central Bay contained higher PCB levels than those in the North and South Bays. Based on the water volume of 2.2 x 10(8) cm3 at a lower water level, it was estimated that approximately 1,000 g of PCBs is transported out of SDB via tidal exchange annually. This indicates that water column transport driven by tidal exchange is a viable mechanism reducing PCB contamination within SDB.     

In situ measurements of metal complex exchange kinetics in freshwater

Trace metals were measured in situ in a freshwater river draining a peat catchment (DOC = 15 mg L(-1)) using diffusive gradients in thin-films (DGT) devices with a range of gel layer thicknesses (0.16-2.0 mm). The reciprocal of the accumulated mass of each metal varied linearly with the thickness of the diffusive layer. These plots allowed calculation of the thickness of an apparent diffusive boundary layer (ADBL). A constant value was obtained from the plots of Cd, Pb, and Zn. The observed increase in the ADBL for the other metals (Mn相似文献   

Ab initio and in situ comparison of caffeine, triclosan, and triclocarban as indicators of sewage-derived microbes in surface waters

Three organic wastewater compounds (OWCs) were evaluated in theory and practice for their potential to trace sewage-derived microbial contaminants in surface waters. The underlying hypothesis was that hydrophobic OWCs outperform caffeine as a chemical tracer, due to their sorptive association with suspended microorganisms representing particulate organic carbon (POC). Modeling from first principles (ab initio) of OWC sorption to POC under environmental conditions suggested an increasing predictive power: caffeine (0.2% sorbed) < triclosan (9-60%; pH 6-9) < triclocarban (76%). Empirical evidence was obtained via analysis of surface water from three watersheds in a rural-to-urban gradient in Baltimore, MD. Mass spectrometric OWC detections were correlated to microbial plate counts for 40 monitoring sites along 14 streams, including multiple chronic sewage release sites and the local wastewater treatment plant. Consistent with ab initio calculations, correlation analyses of 104 observations for fecal coliforms, enterococci, and Escherichia coli in natural surface waters showed that the particle-active antimicrobials triclosan and triclocarban (R2 range, 0.45-0.55) were indeed superior to caffeine (0.16-0.37) for tracking of microbial indicators. It is concluded that chemical monitoring of microbial risks is more effective when using hydrophobic OWCs such as triclosan and triclocarban in place of, or in conjunction with, the traditional marker caffeine.     

Disinfection byproduct relationships and speciation in chlorinated nanofiltered waters

The formation and speciation of disinfection byproducts (DBPs) resulting from chlorination of nanofilter permeates obtained from various source water locations and membrane types are examined. Specific ultraviolet absorbance and bromide utilization are shown to decrease following nanofiltration. Both dissolved organic carbon (DOC) concentration and ultraviolet absorbance at 254 nm were found to correlate strongly with trihalomethane (THM), haloacetic acid (HAA), and total organic halide (TOX) concentrations in chlorinated nanofilter permeates, suggesting that they can be employed as surrogates for DBPs in nanofiltered waters. Because smooth curves were obtained for individual THM and HAA species as well as bromine and chlorine incorporation into THMs and HAAs as a function of Br-/DOC molar ratio, it is likely that mole fractions of these DBPs are more strongly influenced by chlorination conditions, Br-, and DOC concentrations than NOM source and membrane type. Mole fractions of mono-, di-, and trihalogenated HAAs were found to be independent of Br-/DOC. Even at a very low Br-/DOC of 2.9microM/mM, the mixed bromochloro- and tribromoacetic acids constituted 20% of total HAAs on a molar basis. This increased to approximately 50% as Br-/DOC increased to approximately 25microM/mM or more, proving that a large fraction of HAAs may not be covered under existing federal regulations. Total THM and HAA9 concentrations decreased in permeate waters with increasing Br-/DOC suggesting that nanofilter permeates are limited with respect to DBP precursors.     

Interactions of trace metals with hydrogels and filter membranes used in DET and DGT techniques

Equilibrium partitioning of trace metals between bulk solution and hydrogels/filter was studied. Under some conditions, trace metal concentrations were higher in the hydrogels or filter membranes compared to bulk solution (enrichment). In synthetic soft water, enrichment of cationic trace metals in polyacrylamide hydrogels decreased with increasing trace metal concentration. Enrichment was little affected by Ca and Mg in the concentration range typically encountered in natural freshwaters, indicating high affinity but low capacity binding of trace metals to solid structure in polyacrylamide gels. The apparent binding strength decreased in the sequence: Cu > Pb > Ni approximately to Cd approximately to Co and a low concentration of cationic Cu eliminated enrichment of weakly binding trace metal cations. The polyacrylamide gels also had an affinity for fulvic acid and/or its trace metal complexes. Enrichment of cationic Cd in agarose gel and hydrophilic polyethersulfone filter was independent of concentration (10 nM to 5 microM) but decreased with increasing Ca/ Mg concentration and ionic strength, suggesting that it is mainly due to electrostatic interactions. However, Cu and Pb were enriched even after equilibration in seawater, indicating that these metals additionally bind to sites within the agarose gel and filter. Compared to the polyacrylamide gels, agarose gel had a lower affinity for metal-fulvic complexes. Potential biases in measurements made with the diffusive equilibration in thin-films (DET) technique, identified by this work, are discussed.     

Investigating arsenic speciation and mobilization in sediments with DGT and DET: a mesocosm evaluation of oxic-anoxic transitions

Mobilization of arsenic from freshwater and estuarine sediments during the transition from oxic to anoxic conditions was investigated using recently developed diffusive sampling techniques. Arsenic speciation and Fe(II) concentrations were measured at high resolution (1-3 mm) with in situ diffusive gradients in thin films (DGT) and diffusive equilibration in thin films (DET) techniques. Water column anoxia induced Fe(II) and As(III) fluxes from the sediment. A correlation between water column Fe(II) and As(III) concentrations was observed in both freshwater (r(s) = 0.896, p < 0.001) and estuarine (r(s) = 0.557, p < 0.001) mesocosms. Porewater sampling by DGT and DET techniques confirmed that arsenic mobilization was associated with the reductive dissolution of Fe(III) (hydr)oxides in the suboxic zone of the sediment; a relationship that was visible because of the ability to measure the coincident profiles of these species using combined DGT and DET samplers. The selective measurement of As(III) and total inorganic arsenic by separate DGT samplers indicated that As(III) was the primary species mobilized from the solid phase to the porewater. This measurement approach effectively ruled out substantial As(V) mobilization from the freshwater and estuarine sediments in this experiment. This study demonstrates the capabilities of the DGT and DET techniques for investigating arsenic speciation and mobilization over a range of sediment conditions.     

ICP-MS法测定食用明胶中重金属元素

利用电感耦合等离子体质谱法(ICP-MS)测定食用明胶中Cr、Mn、Ni、As、Cd、Sb、Hg、Pb等8种重金属元素的分析方法。通过优化仪器的工作参数、选择适宜待测元素的同位素以及选用干扰元素校正方程可以克服质谱干扰,以Sc、Rh、Bi为内标元素,样品的基体效应和仪器漂移得到了很好地校正。结果表明,8种重金属元素的检出限在0.002～0.091μg/L之间,RSD<2.11%,回收率在91.5%～109.0%之间,方法与其他分析对照,结果一致。     

鱼油中重金属元素的质谱分析

建立了电感耦合等离子体质谱(ICP-MS)法测定鱼油中Cr、Co、Ni、As、Mo、Cd、Sn、Sb、Hg、Pb等10种重金属元素的分析方法。采用微波消解法消解样品后直接进行分析,应用碰撞/反应池(ORS)技术有效地消除了多原子离子对待测元素的干扰,选用Sc、Rh、Bi等元素为内标混合液校正基体干扰和漂移。结果表明,该方法的检出限为0.008～0.073μg/L,相对标准偏差均小于4.4%,通过添加标准回收实验,回收率在89.6%～103.8%。该法简便、快速,能满足鱼油中多种微量重金属元素同时测定的分析需求。     

Determination of silver speciation in natural waters. 2. Binding strength of silver ligands in surface freshwaters

Three competing ligand methods were compared to determine characteristics of Ag(I) complexation by dissolved and colloidal ligands present in three rivers and one sewage treatment plant effluent. Iminodiacetate groups on Chelex-100 resin (used in batch and column experiments) and diethyldithiocarbamate (DDC) were used as competing ligands. Results of batch Chelex and DDC competition experiments show good agreement with regard to relative extent of Ag binding by natural ligands among the three river systems. Results of both methods also show a possible correlation between extent of Ag(I) complexation and organic matter concentration and/or Fe concentration. Fraction of Ag(I) associated with Chelex in both batch and column Chelex experiments was similar in each of the four systems tested, indicating that lability of Ag complexes does not change significantly on time scales ranging from seconds to 24 h. Results of Chelex and DDC competition were compared using a model based on a hypothetical single natural ligand. Under the experimental conditions used, this model quantified Ag(I) complexes with log Kcond values from approximately 12 to 14. For the three rivers studied, ligands with silver-association characteristics similar to those of reduced sulfur groups (log K = 14-16) present at subnanomolar concentrations likely dominate Ag(I) speciation in these systems. A weaker ligand (e.g., log Kcond < 12) at concentrations > 0.7 nM dominated Ag(I) speciation in the treatment plant effluent. This may result from elevated concentration of metals that compete for reduced sulfur groups rather than from a lower total concentration of these groups.