Seawater arsenic

Thermodynamic considerations suggest that in oxygenated seawater, arsenic should exist almost entirely as arsenate (71). It was apparent from the early work on arsenic in seawater, however, that arsenite was also present in significant concentrations and could at times predominate over arsenate (7,8, 72, 73). Marine bacteria (74) and marine phytoplankton (75) were shown to reduce arsenate to arsenite, thereby providing an explanation for the observed As(III)/As(V) ratio in seawater. The compounds MMA and DMA also occur in seawater, generally as minor constituents (9, 34, 71, 76). The concentrations of As(III), MMA, and DMA are positively correlated with primary productivity,... 

Arsenic concentration in seawater is typically 1-3 pg/L (5-7,34,35), although in surface seawater, arsenic species may be subject to some seasonal changes... 

Investigations into the effects of arsenic and phosphorus in single-phase brasses on their susceptibility to intergranular attack and stress-corrosion cracking in seawater have shown that the normal addition of arsenic to... 

The range of processes that must be considered in the cycle of metals is described in Fig. 15-10 (Nelson et al., 1977). Both the complexity of metal cycle analysis in a real system and the importance of speciation are well-stated by Andreae (1979) in his overview of the arsenic cycle in seawater ... 

The biological cycle of arsenic in the surface ocean involves the uptake of arsenate by plankton, the conversion of arsenate to a number of as yet unidentified organic compounds, and the release of arsenite and methylated species into the seawater. Biological demethylation of the methyl-arsenicals and the oxidation of arsenite by as yet... 

Andreae, M. O. (1979). Arsenic speciation in seawater and interstitial waters the role of biological-chemical interactions on the chemistry of a trace element. Limnol. Oceanog. 24,440-452. 

The As (arsenic) concentration of seawater is controlled by input of rivers, sedimentation on the seafloor, weathering of the seafloor, exchange between atmosphere and seawater, volcanic gas input, and hydrothermal input. Previous studies on the geochemical cycle of As have not taken into account the hydrothermal flux of As. Therefore, hydrothermal flux of As from back-arc, island arc and midoceanic ridges to ocean is considered below. 

Arsenic removal from seawater to sediments is mainly governed by pyrite formation in the seafloor sediments. Production rate of sedimentary pyrite is 2.5 x 10 g S/year (Holland, 1978). Therefore, As removal by pyrite from seawater is (1.3-2.9) x lO g/year. This is the same order of magnitude as As input to ocean by river which is equal to 0.7 x 10 g/year. 

Shikazono, N. (1993) Influence of hydrothermal flux on arsenic geochemical balance of seawater. Chikyuka-gakii (Geochemistry), 27. 135-139 (in Japanese). 

Pigna M, Colombo C, Violante A (2003) Competitive sorption of arsenate and phosphate on synthetic hematites (in Italian). Proceedings XXI Congress of Societa Italiana Chimica Agraria SICA (Ancona), pp 70-76 Quirk JP (1955) Significance of surface area calculated from water vapour sorption isotherms by use of the B. E. T. equation. Soil Sci 80 423-430 Rancourt DG, Fortin D, Pichler T, Lamarche G (2001) Mineralogical characterization of a natural As-rich hydrous ferric oxide coprecipitate formed by mining hydrothermal fluids and seawater. Am Mineral 86 834-851 Raven K, Jain A, Loeppert, RH (1998) Arsenite and arsenate adsorption on ferrihydrite kinetics, equilibrium, and adsorption envelopes. Environ Sci Technol 32 344-349... 

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

Yamamoto et al. [6] studied preservation of arsenic- and antimony-bearing samples of seawater. One-half of the sample (201) was acidified to pH 1 with hydrochloric acid immediately after sampling, and the remaining half was kept without acidification. In order to clarify the effect of acidification on storage, measurements were made over a period of a month after sampling. Results are given in Table 1.1. In this study, a standard addition method and calibration curve method were used for comparison and it was proven that the two gave the same results for the analyses of seawater. 

Table 1.1. Effects of storage on the concentration of arsenic and antimony in seawater (mg/1) [6]...

Yamamoto et al. [6] conclude that their method was quite successful for the species-specific determination of arsenic and antimony in seawater. These methods, especially those for the determination of arsenic (III) and antimony (III), are quite satisfactory, as the method is almost free from interference of foreign ions. 

Haywood and Riley [14] have described a spectrophotometric method for the determination of arsenic in seawater. Adsorption colloid flotation has been employed to separate phosphate and arsenate from seawater [15]. These two anions, in 500 ml filtered seawater, are brought to the surface in less than 5 min, by use of ferric hydroxide (added as 0.1 M FeC 2 ml) as collector, at pH 4, in the presence of sodium dodecyl sulfate [added as 0.05% ethanolic solution (4 ml)] and a stream of nitrogen (15 ml/minutes). The foam is then removed and phosphate and arsenate are determined spectrophotometrically [16]. Recoveries of arsenate and arsenite exceeding 90% were obtained by this procedure. 

Isaeva [181] described a phosphomolybdate method for the determination of phosphate in turbid seawater. Molybdenum titration methods are subject to extensive interferences and are not considered to be reliable when compared with more recently developed methods based on solvent extraction [182-187], such as solvent-extraction spectrophotometric determination of phosphate using molybdate and malachite green [188]. In this method the ion pair formed between malachite green and phosphomolybdate is extracted from the seawater sample with an organic solvent. This extraction achieves a useful 20-fold increase in the concentration of the phosphate in the extract. The detection limit is about 0.1 ig/l, standard deviation 0.05 ng-1 (4.3 xg/l in tap water), and relative standard deviation 1.1%. Most cations and anions found in non-saline waters do not interfere, but arsenic (V) causes large positive errors. 

Afansev et al. [61] have described an extraction photometric method for the determination of arsenic at the xg/l range in seawater. This method uses diantipyrilmethane as the chromogene reagent. The coefficient of variation is... 

Bermejo-Barrera et al. [64] studied the use of lanthanum chloride and magnesium nitrate as modifiers for the electrothermal atomic spectrometric determination of p,g/l levels of arsenic in seawater. 

Howard and Comber [63] converted arsenic in seawater to its hydride prior to determination by atomic absorption spectrometry. 

The neutron activation method for the determination of arsenic and antimony in seawater has been described by Ryabin et al. [66]. After coprecipitation of arsenic acid and antimony in a 100 ml sample of water by adding a solution of ferric iron (10 mg iron per litre) followed by aqueous ammonia to give a pH of 8.4, the precipitate is filtered off and, together with the filter paper, is wrapped in a polyethylene and aluminium foil. It is then irradiated in a silica ampoule in a neutron flux of 1.8 x 1013 neutrons cm-2 s 1 for 1 - 2 h. Two days after irradiation, the y-ray activity at 0.56 MeV is measured with use of a Nal (Tl) spectrometer coupled with a multichannel pulse-height analyser, and compared with that of standards. 

Yusov et al. [67] separated arsenic (III) and arsenic (V) in seawater using a chloroform solution of ammonium pyrrolidine diethyldthiocarbamate. The separated fractions were then analysed by neutron activation analysis. 

Creed et al. [68] described a hydride generation inductively coupled plasma mass spectrometric method featuring a tubular membrane gas-liquid separator for the determination of down to 100 pg of arsenic in seawater. 

Klane and Blum [69] showed that inductively coupled plasma spectrometry was able to determine below 1000 ng/1 of arsenic in seawater. Ion exclusion chromatography coupled with inductively coupled plasma mass spectrometry has been used to determine several arsenic species in seawater [ 947 ]. Down to 3 ng/1 arsenic can be determined using hydride generation prior to this technique. 

Hua et al. [71] carried out automated determination of total arsenic in seawater by flow constant-current stripping with gold fibre electrodes in which the sample was acidified and pentavalent arsenic was reduced to the trivalent form with iodide. The arsenic was then deposited potentiostatically for 4 min on a 25 xm gold fibre electrode, and subsequently stripped with constant current in 5 M hydrochloric acid. Cleaning and regeneration of the gold electrode were fully automated. 

Tao et al. [658] have described a procedure in which antimony and arsenic were generated as hydrides and irradiated with ultraviolet light. The broad continuous emission bands were observed in the ranges about 240-750 nm and 220 - 720 nm, and the detection limits were 0.6 ng and 9.0 ng for antimony and arsenic, respectively. Some characteristics of the photoluminescence phenomenon were made clear from spectroscopic observations. The method was successfully applied to the determination of antimony in river water and seawater. The apparatus used in this technique is illustrated in Fig. 5.16. 

Nakashima et al. [719] detail a procedure for preliminary concentration of 16 elements from coastal waters and deep seawater, based on their reductive precipitation by sodium tetrahydroborate, prior to determination by graphite-furnace AAS. Results obtained on two reference materials are tabulated. This was a simple, rapid, and accurate technique for determination of a wide range of trace elements, including hydride-forming elements such as arsenic, selenium, tin, bismuth, antimony, and tellurium. The advantages of this procedure over other methods are indicated. 

Berman et al. [735] have shown that if a seawater sample is subjected to 20-fold preconcentration by one of the above techniques, then reliable analysis can be performed by ICP-AES (i.e., concentration of the element in seawater is more than five times the detection limit of the method) for iron, manganese, zinc, copper, and nickel. Lead, cobalt, cadmium, chromium, and arsenic are below the detection limit and cannot be determined reliably by ICP-AES. These latter elements would need at least a hundredfold preconcentration before they could be reliably determined. 

Alves et al. [744] determined vanadium, nickel, and arsenic in seawater in the 10-20 000 ppt range using flow injection cryogenic desolvation ICP-MS. 

Stroh and Voellkopf [746] utilised flow injection analysis coupled to ICP-MS to determine down to 0.6 ppt of antimony, arsenic, and mercury in seawater. 

Advantages High analysis rate 3-4 elements per hour Applicable to many more metals than voltammetric methods Superior to voltammetry for mercury and arsenic particularly in ultratrace range Disadvantages Nonspecific absorption Spectral interferences Element losses by molecular distillation before atomisation Limited dynamic range Contamination sensitivity Element specific (or one element per run) Not suitable for speciation studies in seawater Prior separation of sea salts from metals required Suspended particulates need prior digestion About three times as expensive as voltammetric equipment Inferior to voltammetry for cobalt and nickel... 

Murthy and Ryan [823] used colloid flotation as a means of preconcentration prior to neutron activation analysis for arsenic, molybdenum, uranium, and vanadium. Hydrous iron (III) oxide is floated in the presence of sodium decyl sulfate with small nitrogen bubbles from 1 litre of seawater at pH 5.7. Recoveries of arsenic, molybdenum, and vanadium were better than 95%, whilst that of uranium was about 75%. 

Arsenic in Potable and Seawater by Spectrophotometry, 1978, Tentative Method, (1980) Department of the Environment/National Water Council Standing Committee of Analysts, HMSO, London... 

In the method for [17] inorganic arsenic the sample is treated with sodium borohydride added at a controlled rate (Fig. 10.1). The arsine evolved is absorbed in a solution of iodine and the resultant arsenate ion is determined photometrically by a molybdenum blue method. For seawater the range, standard deviation, and detection limit are 1—4 xg/l, 1.4%, and 0.14 pg/1, respectively for potable waters they are 0-800 pg/1, about 1% (at 2 pg/1 level), and 0.5 pg/1, respectively. Silver and copper cause serious interference at concentrations of a few tens of mg/1 however, these elements can be removed either by preliminary extraction with a solution of dithizone in chloroform or by ion exchange. 

The precision of the method was tested by carrying out replicate analyses (10) on 150 ml aliquots of two seawater samples from the Irish Sea. Mean ( sd) arsenic concentrations of 2.63 0.05 and 2.49 0.05 pg/1 amounts of were found. The recovery of arsenic was checked by analysing 150 ml aliquots of arsenic-free seawater which had been spiked with known amounts of arsenic (V). The results of these experiments shows that there is a linear relationship between absorbance and arsenic concentration and that arsenic could be recovered from seawater with an average efficiency of 98.0% at levels of 1.3-6.6 pg/1. Analagous experiments in which arsenic (III) was used gave similar recoveries. 

Although purely thermodynamic considerations suggest that arsenic should exist in oxic seawaters practically entirely in the pentavalent state, equilibrium... 

Haywood and Riley [17] found that arsenic (III) can be separated from arsenic (V) even at levels of 2 xg/l by extracting it as the pyrrolidine dithiocar-bamate complex with chloroform. They applied this technique to samples of seawater spiked with arsenic (V) and arsenic (III) and found that arsenic (V) could be satisfactorily determined in the presence of As111. 

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