Seawater cadmium

Using the equilibrium constants below, calculate the concentrations of free (uncomplexed) cadmium ion in a freshwater with a chloride concentration of 15 mg/L, and in seawater containing 17 000 mg/L chloride. Ignore com-plexation with other ions. 

Zirino, A. and Yamamoto, S., A pH-dependent model for the chemical speciation of copper, zinc, cadmium, and lead in seawater, Limnol Oceanogr, 17 (5), 661-671, 1972. 

Losses of Silver, Arsenic, Cadmium, Selenium, and Zinc from Seawater by Sorption... 

Table 1.9 shows the percentage loss as a function of time for silver, cadmium, and zinc from artificial seawater stored in polyethylene, borosilicate glass, PTFE at various pH and R values. 

Table 1.9. Sorption behaviour as percentage of silver, cadmium and zinc in artificial seawater [54]... 

Scarponi et al. [93] used anodic stripping voltammetry to investigate the contamination of seawater by cadmium, lead, and copper during filtration and storage of samples collected near an industrial area. Filtration was carried... 

The first aim of this work was to study the influence of an unwashed membrane filter on the cadmium, lead, and copper concentrations of filtered seawater samples. It was also desirable to ascertain whether, after passage of a reasonable quantity of water, the filter itself could be assumed to be clean so that subsequent portions of filtrate would be uncontaminated. If this were the case, it should be possible to eliminate the cleaning procedure and its contamination risks. The second purpose of the work was to test the possibility of long-term storage of samples at their natural pH (about 8) at 4 °C, kept in low-density polyethylene containers which have been cleaned with acid and conditioned with seawater. 

Figure 1.4 shows a typical curve demonstrating the dependence of concentrations of copper, lead, and cadmium in the filtrate on the volume of seawater sampled. Metal levels become constant after 1-1.51 of sample have been filtered, and it can be concluded that at this point, contamination of the sample by the filtration equipment is negligible. 

Apart from the observation that the concentrations are generally higher than those measured previously, which indicates contamination during conditioning and manipulation, and the necessity of frequently renewing seawater for equilibration purposes, it can be seen that there are no changes in the metal concentrations for three months for lead and copper, or for five months for cadmium. Also, after five months storage, some loss of lead and... 

Scarponi et al. [93] concluded that filtration of seawater through uncleaned membrane filters shows positive contamination by cadmium, lead, and copper. In the first filtrate fractions, the trace metal concentration maybe increased by a factor of two or three. During filtration, the soluble impurities are leached from the filter, which is progressively cleaned, and the metal concentration in the filtrate, after passage of 0.8 -11 of seawater, reaches a stable minimum value. Thus it is recommended that at least one litre of seawater at natural pH be passed through uncleaned filters before aliquots for analysis are taken... 

Low-density polyethylene containers are suitable for storing seawater samples at 4 °C and natural pH, provided that they are thoroughly cleaned (in 2 M hydrochloric acid for at least a week) and adequately conditioned (with prefiltered seawater for at least one to two weeks). Storage can be prolonged for at least three months (or five months for cadmium) without significant concentration changes. For lead and copper, adsorption losses are observed after five months. 

Schnepfe [83] has described yet another procedure for the determination of iodate and total iodine in seawater. To determine total iodine 1 ml of 1% aqueous sulfamic acid is added to 10 ml seawater which, if necessary, is filtered and then adjusted to a pH of less than 2.0. After 15 min, 1 ml sodium hydroxide (0.1 M) and 0.5 ml potassium permanganate (0.1M) are added and the mixture heated on a steam bath for one hour. The cooled solution is filtered and the residue washed. The filtrate and washings are diluted to 16 ml and 1ml of a phosphate solution (0.25 M) added (containing 0.3 xg iodine as iodate per ml) at 0 °C. Then 0.7 ml ferrous chloride (0.1 M) in 0.2% v/v sulfuric acid, 5 ml aqueous sulfuric acid (10%) - phosphoric acid (1 1) are added at 0 °C followed by 2 ml starch-cadmium iodide reagent. The solution is diluted to 25 ml and after 10-15 min the extinction of the starch-iodine complex is measured in a -5 cm cell. To determine iodate the same procedure is followed as is described previously except that the oxidation stage with sodium hydroxide - potassium permanganate is omitted and only 0.2 ml ferrous chloride solution is added. A potassium iodate standard was used in both methods. 

Spencer and Brewer [111] have reviewed methods for the determination of nitrate in seawater. Classical methods for determining low concentrations of nitrate in seawater use reduction to nitrite with cadmium/copper [ 112,116,117] or zinc powder [113] followed by conversion to an azo dye using N- 1-naphthyl-ethylenediamine dihydrochloride and spectrophotometric evaluation. Malho-tra and Zanoni [114] and Lambert and Du Bois [115] have discussed the interference by chloride in reduction-azo dye methods for the determination of nitrate. 

Spencer and Brewer [144] have reviewed methods for the determination of nitrite in seawater. Workers at WRc, UK [ 145] have described an automated procedure for the determination of oxidised nitrogen and nitrite in estuarine waters. The procedure determines nitrite by reaction with N-1 naphthyl-ethylene diamine hydrochloride under acidic conditions to form an azo dye which is measured spectrophotometrically. The reliability and precision of the procedure were tested and found to be satisfactory for routine analyses, provided that standards are prepared using water of an appropriate salinity. Samples taken at the mouth of an estuary require standards prepared in synthetic seawater, while samples taken at the tidal limit of the estuary require standards prepared using deionised water. At sampling points between these two extremes there will be an error of up to 10% unless the salinity of the standards is adjusted accordingly. In a modification of the method, nitrate is reduced to nitrite in a micro cadmium/copper reduction column and total nitrite estimated. The nitrate content is then obtained by difference. 

Several ions (e.g., manganese, iron (II), iron (III), cobalt, nickel, copper, zinc, cadmium, lead, and uranyl) react with pyrocatechol violet, and to some extent are extracted together with aluminium. The interferences from these ions and other metal ions generally present in seawater could be eliminated by extraction with diethyldithiocarbamate as masking agent. With this agent most of the metal ions except aluminium were extracted into chloroform, and other metal ions did not react in the amounts commonly found in seawater. Levels of aluminium between 6 and 6.3 pg/1 were found in Pacific Ocean and Japan Sea samples by this method. 

In the determination of cadmium in seawater, for both operational reasons and ease of interpretation of the results it is necessary to separate particulate material from the sample immediately after collection. The dissolved trace metal remaining will usually exist in a variety of states of complexation and possibly also of oxidation. These may respond differently in the method, except where direct analysis is possible with a technique using high-energy excitation, such that there is no discrimination between different states of the metal. The only technique of this type with sufficiently low detection limits is carbon furnace atomic absorption spectrometry, which is subject to interference effects from the large and varying content of dissolved salts. 

Various workers have discussed the application of graphite furnace atomic absorption spectrometry to the determination of cadmium in seawater [ 115— 124],... 

Batley and Farrah [ 120] and Gardner and Yates [118] used ozone to decompose organic matter in samples and thus break down metal complexes prior to atomic absorption spectrometry. By this treatment, metal complexes of humic acid and EDTA were broken down in less than 2 min. These observations led Gardner and Yates [ 118 ] to propose the following method for the determination of cadmium in seawater. 

Danielson et al. [119] have described a method for the determination of cadmium in seawater. The samples were analysed by graphite furnace atomic... 

As cadmium is one of the most sensitive graphite furnace atomic absorption determinations, it is not surprising that this is the method of choice for the determination of cadmium in seawater. Earlier workers separated cadmium from the seawater salt matrix prior to analysis. Chelation and extraction [ 121— 128], ion exchange [113,124,125,129], and electrodeposition [130,131] have all been studied. 

The direct determination of cadmium in seawater is particularly difficult because the alkali and alkaline earth salts cannot be fully charred away at temperatures that will not also volatilise cadmium. Most workers in the past [125,132-135] who have attempted a direct method have volatilised the cadmium at temperatures which would leave sea salts in the furnace. This required careful setting of temperatures, and was disturbed by situations that caused temperature settings to change with the life of the furnace tubes. 

Campbell and Ottaway [136] also used selective volatilisation of the cadmium analyte to determine cadmium in seawater. They could detect 0.04 pg/1 cadmium (2pg in 50 pi) in seawater. They dried at 100 °C and atomised at 1500 °C with no char step. Cadmium was lost above 350 °C. They could not use ammonium nitrate because the char temperature required to remove the ammonium nitrate also volatilised the cadmium. Sodium nitrate and sodium and magnesium chloride salts provided reduced signals for cadmium at much lower concentrations than their concentration in seawater if the atomisation temperature was in excess of 1800 °C. The determination required lower atomisation temperatures to avoid atomising the salts. Even this left the magnesium interference, which required the method of additions. 

Guevremont et al. [ 117] used a direct, selective volatilisation determination of cadmium in seawater. They used 20 pi seawater samples, 1 g/1 of EDTA, an... 

Guevremont et al. [117] studied the use of various matrix modifiers in the graphite furnace gas method of determination of cadmium in seawater. These included citric acid, lactic acid, aspartic acid, histidine, and EDTA. The addition of less than 1 mg of any of the compounds to 1 ml seawater significantly decreased matrix interference. Citric acid achieved the highest sensitivity and reduction of interference, with a detection limit of 0.01 pg cadmium per litre. 

In similar work, Sturgeon et al. [125] compared direct furnace methods with extraction methods for cadmium in coastal seawater samples. They could measure cadmium down to 0.1 pg/1. They used 10 pg/1 ascorbic acid as a matrix modifier. Various organic matrix modifiers were studied by Guevremont [116] for this analysis. He found citric acid to be somewhat preferable to EDTA, aspartic acid, lactic acid, and histidine. The method of standard additions was required. The standard deviation was better than 0.01 pg/1 in a seawater sample containing 0.07 pg/1. Generally, he charred at 300 °C and atomised at 1500 °C. This method required compromise between char and atomisation temperatures, sensitivity, heating rates, and so on, but the analytical results seemed precise and accurate. Nitrate added as sodium nitrate delayed the cadmium peak and suppressed the cadmium signal. 

Sperling [133] has reported extensively on the determination of cadmium in seawater, as well as in other biological samples and materials. He added ammonium persulfate, which permitted charring seawater at 430 °C without loss of cadmium. For workbelow 2 pg/1 cadmium in seawater he recommended extraction of the cadmium to separate it from the matrix [126,134,135]. He found no change in the measured levels over many months when the seawater was stored in high-density polyethylene or polypropylene. 

Pruszkowska et al. [135] described a simple and direct method for the determination of cadmium in coastal water utilizing a platform graphite furnace and Zeeman background correction. The furnace conditions are summarised in Table 5.1. These workers obtained a detection limit of 0.013 pg/1 in 12 pi samples, or about 0.16 pg cadmium in the coastal seawater sample. The characteristic integrated amount was 0.35 pg cadmium per 0.0044 A s. A matrix modifier containing di-ammonium hydrogen phosphate and nitric acid was used. Concentrations of cadmium in coastal seawater were calculated directly from a calibration curve. Standards contained sodium chloride and the same matrix modifier as the samples. No interference from the matrix was observed. 

Knowles [139] used extraction with ammonium pyrollidine dithiocarba-mate dissolved in methyl isobutyl ketone to extract cadmium from seawater... 

Three Zeeman-based methods for the determination of cadmium in seawater were investigated. Direct determinations can be made with or without the use of a pyrolytic platform atomisation technique. The wall atomisation methods presented were considerably faster than the platform atomisation technique. For extremely low levels of cadmium, indirect methods of analysis employing a preliminary analyte extraction can be employed. Background levels are minimal in extracted samples, and the total furnace programme time was the lowest of the methods examined. 

Lum and Callaghan [ 140 ] did not use matrix modification in the electother-mal atomic absorption spectrophotometric determination of cadmium in seawater. The undiluted seawater was analysed directly with the aid of Zeeman effect background correction. The limit of detection was 2 ng/1. 

Electrothermal atomic absorption spectrophotometry with Zeeman background correction was used by Zhang et al. [141] for the determination of cadmium in seawater. Citric acid was used as an organic matrix modifier and was found to be more effective than EDTA or ascorbic acid. The organic matrix modifier reduced the interferences from salts and other trace metals and gave a linear calibration curve for cadmium at concentrations < 1.6 pg/1. The method has a limit of detection of 0.019 pg/1 of cadmium and recoveries of 95-105% at the 0.2 pg of cadmium level. 

Lum and Callaghan [140] determined down to 2 ng/1 of cadmium directly in seawater by atomic absorption spectrometry with Zeeman correction. 

Stolzberg [143] has reviewed the potential inaccuracies of anodic stripping voltammetry and differential pulse polarography in determining trace metal speciation, and thereby bio-availability and transport properties of trace metals in natural waters. In particular it is stressed that nonuniform distribution of metal-ligand species within the polarographic cell represents another limitation inherent in electrochemical measurement of speciation. Examples relate to the differential pulse polarographic behaviour of cadmium complexes of NTA and EDTA in seawater. 

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