Dissolved Silver Concentration Research Articles

Solid phase extraction materials as a key for improving the accuracy of silver nanoparticle characterization with single-particle inductively coupled plasma mass spectrometry in natural waters through dissolved silver removal

The accurate characterization of silver nanoparticles (Ag NPs) in environmental samples is crucial for understanding the potential impact of these materials on the environment and human health. Single-particle inductively coupled plasma mass spectrometry (spICP-MS) is one of the few techniques available for quantifying NPs at environmentally relevant concentrations. However, the spICP-MS detection of Ag NPs can be interfered with coexisting dissolved analyte causing high background signals, which mask NP signals leading to a significant bias in NP characterization. In this paper, a simple sample pre-treatment procedure for efficient removal of dissolved silver in natural waters is proposed using solid phase extraction (SPE) materials, allowing more accurate characterization of NPs in environmental water matrices. The applicability of eight commercially available SPE materials was evaluated based on their effects on sample Ag NP properties and dissolved silver extraction efficiency in environmental waters, which were shown to depend on sample matrix. The SPE materials found most efficient in dissolved silver extraction were further studied for improving the characterization of 30 nm Ag NPs in colorless and dark-colored waters. Whereas dissolved silver concentrations of ≤1 μg kg−1 were shown to lead to a significant bias in NP characterization (up to +58% increase in NP size and − 90% decrease in particle concentration), pre-treatment of the samples with SPE materials ‘SiliaMetS Thiol’ and ‘Purolite C115HMR’ minimized the effect of dissolved silver interference. As a result, highly more accurate NP sizing (28 ± 2 nm) and relatively stable particle concentration was obtained for 30 nm Ag NPs in environmental waters.

Ionic-strength-dependent effect of suspended sediment on the aggregation, dissolution and settling of silver nanoparticles.

Suspended sediment (SS) is ubiquitous in natural waters and plays a key role in the fate of engineered nanomaterials. In this study, the effect of SS on the aggregation, settling, and dissolution of polyvinylpyrrolidone-coated silver nanoparticles (PVP-AgNPs) was investigated under environmentally relevant conditions. The heteroaggregation of AgNPs with SS was not observed at low ionic strength (≤0.01M) due to high electrostatic repulsion and steric forces. At higher NaCl concentrations (0.1 and 0.3M), PVP-AgNPs were found to attach onto the SS surface, and the formation of AgNP-SS heteroaggregates strongly promoted settling of PVP-AgNPs due to the overwhelming gravity force. PVP-AgNP dissolution was reduced after the addition of sediment to ultrapure water because the presence of sediment-associated dissolved organic matter (SS-DOM). The formation of an AgCl layer on PVP-AgNP surface in 0.01M NaCl solution resulted in the minor effect of SS on AgNP dissolution. After addition of SS, the dissolved silver concentrations of PVP-AgNP increased in 0.1 and 0.3M NaCl solution. The interactions of SS-DOM with AgNPs under different NaCl concentrations interfered the dissolution of AgNPs in sediment-laden water. This study provides new insight into the fate of AgNPs in sediment-laden water under various environmental conditions.

DETECÇÃO E QUANTIFICAÇÃO DE NANOPARTÍCULAS DE PRATA POR SPICP-MS

DETECTION AND QUANTIFICATION OF SILVER NANOPARTICLES BY spICP-MS. The growing interest in nanotechnology has led to an increase in the production and application of nanoparticles worldwide. Due to the unique functional properties of nanoparticles, these materials are being used by many industries, including the agricultural and food sectors. Among the commercially available nanomaterials, it is possible to highlight those produced with silver nanoparticles. One of the most promising techniques for the analysis of metallic nanoparticles is the Inductively coupled plasma mass spectrometry (ICP-MS) performed in single particle mode (spICP-MS). However, the use of the technique in the detection and measurement of nanoparticles requires that the equipment be operated differently than when used to analyze dissolved metal solutions. Thus, this article presents the use of the spICP-MS technique for the measurement and quantification of nanoparticles, as well as their validation. The method proved to be adequate for the purpose, presenting a satisfactory result for the selectivity test and recovery of 83.7 (40 nm) and 77.6% (80 nm). The detection limits, determined for the most frequent size, dissolved silver concentration and particle concentration, were 17.5 nm, 0.736 ng mL-1,146 particles mL-1, respectively. Thus, the results obtained indicated the possibility of using the technique to determine the size and concentration of silver nanoparticles.

Attenuation of Microbial Stress Due to Nano-Ag and Nano-TiO2 Interactions under Dark Conditions

Engineered nanomaterials (ENMs) are incorporated into thousands of commercial products, and their release into environmental systems creates complex mixtures with unknown toxicological outcomes. To explore this scenario, we probe the chemical and toxicological interactions of nanosilver (n-Ag) and nanotitania (n-TiO2) in Lake Michigan water, a natural aqueous medium, under dark conditions. We find that the presence of n-Ag induces a stress response in Escherichia coli, as indicated by a decrease in ATP production observed at low concentrations (in the μg L-1 range), with levels that are environmentally relevant. However, when n-Ag and n-TiO2 are present together in a mixture, n-TiO2 attenuates the toxicity of n-Ag at and below 20 μg L-1 by adsorbing Ag+(aq). We observe, however, that toxic stress cannot be explained by dissolved silver concentrations alone and, therefore, must also depend on silver associated with the nanoscale fraction. Although the attenuating effect of n-TiO2 on n-Ag's toxicity is limited, this study emphasizes the importance of probing the toxicity of ENM mixtures under environmental conditions to assess how chemical interactions between nanoparticles change the toxicological effects of single ENMs in unexpected ways.

New, rapid method to measure dissolved silver concentration in silver nanoparticle suspensions by aggregation combined with centrifugation.

It is unclear whether the antimicrobial activities of silver nanoparticles (AgNPs) are exclusively mediated by the release of silver ions (Ag+) or, instead, are due to combined nanoparticle and silver ion effects. Therefore, it is essential to quantify dissolved Ag in nanosilver suspensions for investigations of nanoparticle toxicity. We developed a method to measure dissolved Ag in Ag+/AgNPs mixtures by combining aggregation of AgNPs with centrifugation. We also describe the reproducible synthesis of stable, uncoated AgNPs. Uncoated AgNPs were quickly aggregated by 2 mM Ca2+, forming large clusters that could be sedimented in a low-speed centrifuge. At 20,100g, the sedimentation time of AgNPs was markedly reduced to 30 min due to Ca2+-mediated aggregation, confirmed by the measurements of Ag content in supernatants with graphite furnace atomic absorption spectrometry. No AgNPs were detected in the supernatant by UV–Vis absorption spectra after centrifuging the aggregates. Our approach provides a convenient and inexpensive way to separate dissolved Ag from AgNPs, avoiding long ultracentrifugation times or Ag+ adsorption to ultrafiltration membranes.Electronic supplementary materialThe online version of this article (doi:10.1007/s11051-016-3565-0) contains supplementary material, which is available to authorized users.

Response of three biofilm-forming benthic microorganisms to Ag nanoparticles and Ag+: the diatom Nitzschia palea, the green alga Uronema confervicolum and the cyanobacteria Leptolyngbya sp.

Although the industrial use of nanoparticles has increased over the past decade, the knowledge about their interaction with benthic phototrophic microorganisms in the environment is still limited. This study aims to characterize the toxic effect of ionic Ag+ and Ag nanoparticles (citrate-coated silver nanoparticles, AgNPs) in a wide concentration range (from 1 to 1000μgL-1) and duration of exposure (2, 5 and 14days) on three biofilm-forming benthic microorganisms: diatom Nitzschia palea, green algae Uronema confervicolum and cyanobacteria Leptolyngbya sp. Ag+ has a significant effect on the growth of all three species at low concentrations (1-10μgL-1), whereas the inhibitory effect of AgNPs was only observed at 1000μgL-1 and solely after 2days of exposure. The inhibitory effect of both Ag+ and AgNPs decreased in the course of the experiments from 2 to 14days, which can be explained by the progressive excretion of the exopolysaccharides and dissolved organic carbon by the microorganisms, thus allowing them to alleviate the toxic effects of aqueous silver. The lower impact of AgNPs on cells compared to Ag+ can be explained in terms of availability, internalization, reactive oxygen species production, dissolved silver concentration and agglomeration of AgNPs. The duration of exposure to Ag+ and AgNPs stress is a fundamental parameter controlling the bioaccumulation and detoxification in benthic phototrophic microorganisms.

Critical influence of chloride ions on silver ion-mediated acute toxicity of silver nanoparticles to zebrafish embryos

The toxicity of silver nanoparticles (AgNP) to aquatic organisms, including zebrafish (Danio rerio), has been demonstrated, but differing opinions exist on the contribution of the physical properties of the particles themselves and the free dissolved silver ions (Ag+) to the observed effects. High concentrations of chloride ions (Cl−) in the routinely used exposure media can cause precipitation of Ag+ as AgCl, as well as complexation of silver in diverse soluble chlorocomplexes, thus masking the contribution of dissolved silver to AgNP toxicity. In the present study, we formulated a zebrafish exposure medium with a low chloride content and exposed zebrafish embryos to AgNO3 or carbonate-coated AgNP. The severity of toxicity caused by both silver forms depended on the time of exposure start, with younger embryos being most sensitive. Toxicity caused by both AgNO3 and AgNP was of the same order of magnitude when compared based on the total dissolved silver concentration and could be prevented by addition of the Ag+ chelator cysteine. Further, we have analyzed the data from several previous studies to evaluate the influence of interactions between Ag+ and Cl− on silver toxicity to zebrafish embryos. Our analysis demonstrates that the acute toxicity of AgNP to zebrafish embryos is largely mediated by Ag+. The influence of particle size and coating can at least partially be explained by the differences in Ag+ dissolution. High Cl− levels in the exposure medium indeed have a pivotal influence on the resulting toxicity of AgNP, appearing to significantly attenuate toxicity in several studies. This consideration should influence the choice of exposure medium to be used when evaluating and comparing AgNP toxicity.

Silver nanoparticle toxicity to Daphnia magna is a function of dissolved silver concentration

The most persistent question regarding the toxicity of silver nanoparticles (AgNPs) is whether this toxicity is due to the nanoparticles themselves or the silver ions (Ag(+)) they release. The present study investigates the role of surface coating and the presence of dissolved organic carbon on the toxicity of AgNPs to Daphnia magna and tests the hypothesis that the acute toxicity of AgNPs is a function of dissolved Ag produced by nanoparticle dissolution. Toxicity of silver nitrate (AgNO3) and AgNPs with surface coatings-gum arabic (AgGA), polyethylene glycol (AgPEG), and polyvinylpyrrolidone (AgPVP)-at 48 h was assessed in US Environmental Protection Agency moderately hard reconstituted water alone and augmented with Suwannee River dissolved organic carbon (DOC). As expected, AgNO3 was the most toxic to D. magna and AgPVPs were the least toxic. In general, Suwannee River DOC presence reduced the toxicity of AgNO3, AgGAs, and AgPEG, while the toxicity of AgPVPs was unaffected. The measured dissolved Ag concentrations for all AgNPs and AgNO3 at the 48-h median lethal concentration in moderately hard reconstituted water were similar. The presence of Suwannee River DOC decreased the ratio of measured dissolved Ag to measured total Ag concentration. These results support the hypothesis that toxicity of AgNPs to D. magna is a function of dissolved Ag concentration from these particles.

Aggregation and Dissolution of Silver Nanoparticles in Natural Surface Water

This study investigated aggregation and silver release of silver nanoparticles suspended in natural water in the absence and presence of artificial sun light. The influence of the capping layer was investigated using uncoated particles and particles coated with citrate or Tween 80. The experiments were conducted over 15 days in batch mode using a river water matrix. Silver release was monitored over this time while the aggregation state and morphological changes of the silver nanoparticles were tracked using dynamic light scattering and transmission electron microscopy. Results indicate sterically dispersed particles coated with Tween released silver quicker than did bare- and citrate-coated particles, which rapidly aggregated. A dissolved silver concentration of 40 μg/L was reached after just 6 h in the Tween-coated particle systems, accounting for ca. 3% of the total silver. Similar levels of dissolved silver were reached in the uncoated and citrate-coated systems at the end of the 15 days. Silver release was not significantly impacted by the artificial sun light; however, the light (and citrate) imparted significant morphological changes to the particles. Their impact was masked by aggregation, which seemed to be the controlling process in this study.

Cysteine-Induced Modifications of Zero-valent Silver Nanomaterials: Implications for Particle Surface Chemistry, Aggregation, Dissolution, and Silver Speciation

The persistence of silver nanoparticles in aquatic environments and their subsequent impact on organisms depends on key transformation processes, which include aggregation, dissolution, and surface modifications by metal-complexing ligands. Here, we studied how cysteine, an amino acid representative of thiol ligands that bind monovalent silver, can alter the surface chemistry, aggregation, and dissolution of zero-valent silver nanoparticles. We compared nanoparticles synthesized with two coatings, citrate and polyvinylpirrolidone (PVP), and prepared nanoparticle suspensions (approximately 8 μM total Ag) containing an excess of cysteine (400 μM). Within 48 h, up to 47% of the silver had dissolved, as indicated by filtration of the samples with a 0.025-μm filter. Initial dissolution rates were calculated from the increase of dissolved silver concentration when particles were exposed to cysteine and normalized to the available surface area of nanoparticles in solution. In general, the rates of dissolution were almost 3 times faster for citrate-coated nanoparticles relative to PVP-coated nanoparticles. Rates tended to be slower in solutions with higher ionic strength in which the nanoparticles were aggregating. X-ray absorption spectroscopy analysis of the particles suggested that cysteine adsorbed to silver nanoparticles surfaces through the formation of Ag(+I)--sulfhydryl bonds. Overall, the results of this study highlight the importance of modifications by sulfhydryl-containing ligands that can drastically influence the long-term reactivity of silver nanoparticles in the aquatic environment and their bioavailability to exposed organisms. Our findings demonstrate the need to consider multiple interlinked transformation processes when assessing the bioavailability, environmental risks, and safety of nanoparticles, particularly in the presence of metal-binding ligands.

Occurrence and Concentration of Dissolved Silver in Rivers in England and Wales

There is a paucity of monitoring data for silver in freshwater environments in Europe. There are several reasons for this, including the relatively low levels of silver in the aquatic environment and the requirement for commensurately low levels of detection (<100 ng l⁻¹), which are generally not routinely achieved in analytical laboratories. In this study 425 separate analytical determinations for dissolved (<0.45 μm) silver from 84 Environment Agency monitoring stations were carried out. Sampling was carried out on a monthly basis over a period of 6 months. Of the 425 samples, 346 were reported as having dissolved silver concentrations below the limit of quantification (6.6 ng l⁻¹) and, of these, 280 samples were reported as below the reporting limit of detection (3 ng l⁻¹). The mean of the maximum dissolved silver concentrations reported at each station was calculated as 6.1 ng l⁻¹ using a statistical extrapolation technique to allow for the high level of censorship in the dataset. The maximum mean dissolved silver concentration recorded at a station was 19.8 ng l⁻¹. A freshwater Predicted No Effect Concentration (PNEC) of 40 ng l⁻¹ was used in this study.

A self-referencing microelectrode for real time measurements of silver flux

Silver has many diverse applications, including use as a biocide for biomedical devices and potable water systems. Increased use of silver has led to a demand for technologies which can measure dissolved species in situ. In addition to the development of tools such as electrodes for measuring dissolved silver concentration, techniques are needed which can quantify spatially and temporally dynamic transport of silver. For the first time, we demonstrate the use of a self-referencing Ag +-selective microelectrode for non-invasively quantifying Ag + flux in biomedical and environmental applications. Characterization of sensor performance included detailed analysis of microelectrode sensitivity, selectivity, response time, limit of detection, and linear range. Liquid membrane microelectrode performance was similar to modern solid state electrodes, and in self-referencing modality differential signal acquisition was sufficient for use in a wide range of applications. Self-referencing microelectrodes were used to profile concentration boundary layer transport, providing a “snapshot” of microgradients in the unstirred boundary layer. In addition, time resolved, non-invasive measurements were used to quantify Ag + release rate (efflux) from two common antimicrobial biomedical devices, and Ag + uptake rate (influx) to a mature Pseudomonas aeruginosa biofilm during disinfection. Use of this technique will increase our understanding of silver transport in engineered antimicrobial devices and systems. In addition, the technique may be used to better our understanding of Ag + release from nanoparticles.

Dissolved silver in European estuarine and coastal waters

Silver is one of the most toxic elements for the marine microbial and invertebrate community. However, little is known about the distribution and behaviour of dissolved silver in marine systems. This paper reports data on dissolved and sediment-associated silver in European estuaries and coastal waters which have been impacted to different extents by past and present anthropogenic inputs. This is the first extended dataset for dissolved silver in European marine waters. Lowest dissolved silver concentrations were observed in the Gullmar Fjord, Sweden (8.9 ± 2.9 pM; x ± 1σ), the Tamar Estuary, UK (9.7 ± 6.2 pM), the Fal Estuary, UK (20.6 ± 8.3 pM), and the Adriatic Sea (21.2 ± 6.8 pM). Enhanced silver concentrations were observed in Atlantic coastal waters receiving untreated sewage effluent from the city of A Corũna, Spain (243 ± 195 pM), and in the mine-impacted Restronguet Creek, UK (91 ± 71 pM). Anthropogenic wastewater inputs were a source of dissolved silver in the regions studied, with the exception of the Gullmar Fjord. Remobilisation of dissolved silver from historically contaminated sediments, resulting from acid mine drainage or sewage inputs, provided an additional source of dissolved silver to the estuaries. The ranges in the log particle-water partition coefficient ( K d) values of 5–6 were similar for the Tamar and Mero estuaries and agreed with reported values for other estuaries. These high K d values indicate the particle reactive nature of silver with oxic sediments. In contrast, low K d values (1.4–2.7) were observed in the Fal system, which may have been due to enhanced benthic inputs of dissolved silver coupled to limited scavenging of silver on to sediments rich in Fe oxide.

Silver in Tokyo Bay estuarine waters and Japanese rivers

Dissolved and particulate concentrations of silver in Tokyo Bay estuarine waters and Japanese rivers were determined in this study. The dissolved silver concentrations in the surface water of Tokyo Bay range from 5.9 to 15.1 pmol kg−1, which is comparable to those in the surface water of the Japan Sea, but two or three times higher than those in the surface water of the open ocean. However, elevated concentrations of dissolved silver are not found in Tokyo Bay compared with those in other highly urbanized estuaries, such as San Francisco Bay (20∼243 pmol kg−1). In the Tokyo Bay estuary, silver typically exhibits non-conservative mixing behavior, which is a common feature in the other estuaries reported previously. Dissolved silver concentrations decrease with salinity from the rivers to the mouth of Tokyo Bay. Silver is efficiently scavenged by suspended particulates, as evidenced by the high conditional distribution coefficients for silver throughout the estuary (log Kd > 5.0 ± 0.6). The silver fluxes into Tokyo Bay via inflowing rivers and atmospheric deposition were estimated as 83 kg y−1 and 15 kg y−1, respectively. A simple budget calculation shows that the silver supplied from rivers and atmosphere must be rapidly scavenged within the Tokyo Bay estuary.

Acute silver toxicity in the euryhaline copepod Acartia tonsa: Influence of salinity and food

The euryhaline copepod Acartia tonsa was exposed to silver (AgNO(3)) in either the absence or the presence of food (diatom Thalassiosira weissflogii; 2 x 10(4) cells/ml). Standard static-renewal toxicity tests that included a fixed photoperiod of 16: 8 h light:dark and temperature (20 degrees C) were run in three different salinities (5, 15, and 30 ppt) together with measurements of pH, ions (Na(+), Cl(-), K(+), SO(4)(2-), Mg(2+), and Ca(2+)), alkalinity, dissolved organic carbon, and total and dissolved (0.45 microm) silver concentrations in the experimental media. In the absence of food, the 48-h EC50 (concentration causing effect to 50% of the individuals tested) values based on total and dissolved silver concentrations were 11.6, 87.2, and 163.2 microg Ag/L and 7.1, 79.2, and 154.6 microg Ag/L at salinities 5, 15, and 30 ppt, respectively. In the presence of food, they were 62.1, 98.5, and 238.4 microg Ag/L and 48.4, 52.3, and 190.9 microg Ag/L, respectively. In all experimental conditions, most of the toxic silver fraction was in the dissolved phase, regardless of salinity or the presence of food in the water. In either the absence or the presence of food, acute silver toxicity was salinity dependent, decreasing as salinity increased. Data indicate that changes in water chemistry can account for the differences in acute silver toxicity in the absence of food, but not in the presence of food, suggesting that A. tonsa requires extra energy to cope with the stressful conditions imposed by acute silver exposure and ionoregulatory requirements in low salinities. These findings indicate the need for incorporation of both salinity and food (organic carbon) in a future biotic ligand model (BLM) version for estuarine and marine conditions, which could be validated and calibrated using the euryhaline copepod A. tonsa.

Chronic toxicity of silver to the sea urchin (Arbacia punctulata)

The chronic toxicity of silver to the sea urchin (Arbacia punctulata) was determined in 30 per thousand salinity seawater during a three-part study: A fertilization test (1-h sperm exposure), a 48-h embryo test, and a 30-d adult test. Combined data from the three tests resulted in a lowest-observed-effect concentration of 19 microg/L, a no-observed-effect concentration of 8.6 microg/L, and a maximum acceptable toxicant concentration of 13 microg/L, based on measured concentrations of dissolved silver. The 96-h median effective concentration was 40 microg/L, and the acute to chronic toxicity ratio was 3.1. During the tests, measured concentrations of free ionic silver (Ag+) were only 0.0027 to 0.0046% of dissolved silver concentrations, as predicted by ion-speciation theory. Some measured Ag+ concentrations were lower than predicted, indicating the presence of other ligands in the seawater test media. These strong sulfide ligands were exuded by the exposed sea urchins into the seawater (where Ag-sulfide complexes formed) in amounts that increased in direct proportion to the silver concentration during the toxicity test. This suggests a toxicity-defense mechanism that functioned by modifying the chemistry of the surrounding external medium.

The time course of silver accumulation in rainbow trout during static exposure to silver nitrate: physiological regulation or an artifact of the exposure conditions?

The pattern of gill silver accumulation in rainbow trout during waterborne silver exposure has been reported to be unusual, reaching a peak in the first few hours of silver exposure followed by a marked decline with continued exposure. The potential causes of the pattern were investigated. Rainbow trout (1–5 g) were exposed in a static system to 110 m Ag labeled AgNO 3 at a total concentration of 1.92 μg Ag l −1 for 24 h in synthetic soft water. Periodically throughout the exposure, gill and body 110 m Ag accumulation, gill and body 24 Na uptake (from which whole body Na + uptake was calculated), gill Na +K +-ATPase activity, plus water silver (total and dissolved), Cl − and total organic carbon (TOC) concentrations were measured. Gill silver levels rapidly increased, peaked at 3 h of exposure and then decreased until a plateau was reached at 12 h of exposure. Body (minus gills) silver levels increased steadily over the exposure period until 18 h of exposure. Whole body Na + uptake decreased, was maximally inhibited by 3 h of exposure but recovered by 12 h despite continued silver exposure. Gill Na +K +-ATPase activity was not inhibited until 5 h of exposure. The water dissolved silver concentration declined by ∼70% over the 24 h exposure period and the TOC content of the water increased over three-fold during the first 2 h of exposure. There was a decrease in the calculated contribution of Ag + (from 20.9 to 2.5%) and an increase in the calculated contribution of Ag–TOC complexes (from 77 to 97.3%) to the total water silver concentration over the first 2 h of exposure. Apical silver uptake into the gills decreased over the initial 2.5 h of exposure while basolateral silver export out of the gills to the body remained constant throughout the exposure. The results of this study suggest that: (1) physiological regulation of silver movement may explain the pattern of gill silver accumulation observed in rainbow trout, although not by a mechanism coupled to Na +K +-ATPase inhibition as originally proposed; (2) alternatively or additionally, a decreased bioavailability of silver, due to the static exposure conditions, may explain the pattern of gill accumulation; (3) the early inhibition of whole body Na + uptake observed during silver exposure occurs via a mechanism other than Na +K +-ATPase inhibition; and (4) gill silver accumulation may be an appropriate endpoint for biotic ligand modeling.

Silver speciation during chronic toxicity tests with the mysid, Americamysis bahia

A 28-day chronic toxicity test and two 7-day chronic estimation toxicity tests were conducted with silver nitrate (AgNO 3) and the marine invertebrate, Americamysis bahia, in 20‰ (parts-per thousand) salinity seawater. One 7-day test was initiated with 7-day-old mysids and the second was initiated with <24-h-old mysids. There was very good agreement between the three toxicity tests. The no-observed-effect concentration (NOEC) values from the 28-day test, the 7-day test initiated with 7-day-old mysids, and the 7-day test initiated with <24-h-old mysids were 34, 65 and 38 μg/l silver, respectively. The 96-h LC50 values from the 28-day toxicity test and the 7-day toxicity test initiated with 7-day old mysids were 260 μg/l, and the 96-h LC50 value from the 7-day toxicity test initiated with <24-h-old mysids was 280 μg/l. Free ionic silver, Ag +, concentrations measured with a silver electrode were in good agreement with concentrations calculated using total dissolved silver and chloride concentrations. Mean measured concentrations of Ag + in the test solutions ranged from 0.99 to 25 ng/l for the dissolved silver concentrations that ranged from 34 to 410 μg/l silver, indicating that free ionic silver varied from 0.003 to 0.006% of the silver dissolved in the 20‰ salinity seawater. Understanding the relationship of salinity and silver speciation, and the effect of this relationship on chronic invertebrate toxicity, will be useful for development of a marine biotic ligand model (BLM) and a water quality criterion for silver. This model could provide an important tool for improving the relationship of laboratory toxicity test results and predicted effects in natural environments, where variations in salinity may act to modify the toxicity of silver and other metals.

Silver Removal from an X-Ray Fixer Solution by means of a Potentiostatically-Controlled Rotating Cylinder Electrode

AbstractThe removal of silver from a 1 litre batch of an X-ray fixer solution (pH 4.7, 30 °C) is studied using a laboratory rotating cylinder electrode, RCE (diameter 5.0 cm, length 2.5 cm, area 40 cm2) rotating at 175 revolutions per minute. Performance is monitored by measuring the dissolved silver concentration, cell current and cell voltage as a function of time under potentiostatic, i.e., constant potential, control of the RCE (cathode potential -0.500 to -0.650 V vs SCE). The rate of concentration decay is seen to be enhanced by: (i)an increased cathode area, (ii) a roughened cathode surface, (iii) a faster rotation speed, (iv) a higher temperature, and (v) the use of a more negative cathode potential. Potentials more negative than -0.725 V vs SCE resulted in unwanted silver sulphide deposition. The experimental data are used to calculate figures of merit, including the cumulative current efficiency and the apparent mass transport coefficient, which serve to characterize the performance of the RCE.

Electrochemical behavior of pyrite in sulfuric acid solutions containing silver ions

A systematic electrochemical study of pyrite in H2SO4 solutions containing dissolved silver was undertaken to gain more information about the transfer of silver ions to pyrite and their role in enhancing the direct oxidation of pyrite. The results of cyclic voltammetry experiments provide additional evidence of the formation of metallic silver on the FeS2 surface under open-circuit conditions. A pyrite electrode held at the open-circuit potential for 2 h in the presence of 10−3m Ag+ exhibits a large and sharp anodic peak at about 0.7V. The current associated with this peak is the result of the dissolution of metallic silver deposited during the initial conditioning period. There is no evidence of silver deposition without preconditioning until the potential drops below about 0.6V for Ag+ concentrations ranging from 10−4 to 10−2m. However, subsequent silver deposition appears to be very sensitive to the dissolved silver concentration in this range. There is also evidence that the state of the pyrite surface has a pronounced influence on its interaction with silver ions. Agitation has also been found to have a significant effect on the electrochemistry of the Ag−FeS2 system.