Phosphate in sea water

A commonly used procedure for the determination of phosphate in sea water and estuarine waters involves the formation of the molybdenum blue complex at 35 - 40 °C in an autoanalyser followed by spectrophotometric evaluation of the resulting colour. Unfortunately when applied to sea water samples, depending on the chloride content of the sample, peak distortion or even negative peaks occur, which make it impossible to obtain reliable phosphate values. This effect can be overcome by the replacement of the distilled water used in such methods by a solution of sodium chloride of an appropriate concentration related to the chloride concentration of the sample (see Sect. 2.22.2). 

Fig, 12.3a. Z-score presentation for the determination of phosphate in sea water (QUASIMEME project, courtesy of D. Wells, Marine Laboratory, Aberdeen, United Kingdom)... 

K.S. Johnson, R.L. Petty, Determination of phosphate in sea water by flow injection analysis with injection of reagent, Anal. Chem. 54 (1982) 1185. 

Normal superphosphate or triple superphosphate (common commercial fertilizers) are cheap sources of water-soluble phosphate. Normal superphosphate is primarily a mixture of monocalcium phosphate and calcium sulfate (gypsum), while triple superphosphate is essentially all monocalcium phosphate. Monosodium phosphate was prepared from superphosphate by first leaching superphosphate with sea water until a saturated solution of monocalcium phosphate was obtained. Then the monocalcium phosphate solution in sea water was percolated through a column of Dowex 50 (strongly acidic type resin) in the sodium form. The effluent from the column was a solution of monosodium phosphate in sea water and the resin was converted to the calcium form as shown by Equation 6. 

The high phosphate concentration in cells might indicate the abundance of phosphorus compounds in the primordial waters [85,251,252]. Since the concentration of phosphate in sea water is low (the phosphates of Ca and Mg are poorly soluble in water), it has been argued that more reduced compounds such as hypophosphite (phosphinate, P022-) and/or phosphite (phosphonate, P033-), which have better solubility in sea water, could have been abundant in the primeval, more reduced ocean [252-258]. This suggestion is supported by findings of diverse systems of hypophosphite and phosphite oxidation in prokaryotes (see [259] for a review). 

Dahllof, I., Svensson, O., and Torstensson, C., Optimising the determination of nitrate and phosphate in sea water with ion chromatography using experimental design. Journal of Chromatography A 771,163,1997. 

All methods for phosphate in sea water rely on the formation of a phospho-molybdate complex and its subsequent reduction to highly coloured blue compounds. Methods using stannous chloride as a reductant at room temperature have been fevoured as they arc most sensitive and give less interference from easily hydrolysable organic compounds than do other techniques. There are complexities in these methods due to interference from arsenic and to concealed blanks arising from the reduction of molybdate in sea water in the absence of phosphate. An excellent program of comparative tests has been described by Jones and Spencer (7. Marine Bid. Assoc. U.K., 43 251, 1963). 

The insecticide fenitrothion (0,0-dimethyl-0-4-nitro-3-methylphenyl thio-phosphate) can be measured in sea water and sediments by gas chromatography, using a flame photometric detector to determine P and S [387]. The degradation products of the organophosphorus insecticides can be concentrated from large water by collection on Amberlite XAD-4 resin for subsequent analysis [383]. 

The problem is to calculate the steady-state concentration of dissolved phosphate in the five oceanic reservoirs, assuming that 95 percent of all the phosphate carried into each surface reservoir is consumed by plankton and carried downward in particulate form into the underlying deep reservoir (Figure 3-2). The remaining 5 percent of the incoming phosphate is carried out of the surface reservoir still in solution. Nearly all of the phosphorus carried into the deep sea in particles is restored to dissolved form by consumer organisms. A small fraction—equal to 1 percent of the original flux of dissolved phosphate into the surface reservoir—escapes dissolution and is removed from the ocean into seafloor sediments. This permanent removal of phosphorus is balanced by a flux of dissolved phosphate in river water, with a concentration of 10 3 mole P/m3. 

The history of the iodides dates from the time of J. L. Gay Lussac s discovery 1 of hydriodic acid in 1813. Iodides occur in sea-water, and in the waters of many natural springs and brines. Iodides also occur in varec in the nitre beds of South America and in many natural phosphates. In whatever form iodine occurs in these substances, it is usual to extract this element as iodine, and subsequently to convert this into the iodide—generally potassium iodide. Potassium iodide is used in analytical and photographical work, and medicinally for the treatment of scrofulous, rheumatic, and syphilitic diseases. Sodium iodide is used as a precipitant for gold and silver in the treatment of weak copper ores from Tharsis, etc. 

In sea water, we encounter one phosphate group for every 106 water molecules. 

The brackets indicate the molar concentrations of the various molecular species. The empirical quantity a is defined by pH = —log a. In sea water, pH measurements do not yield a thermodynamic hydrogen ion activity due to liquid junction and asymmetry potentials a only approximates the hydrogen activity an+. For sea water of 33%>o salinity at 20 °C and at pH 8, 87% of the inorganic phosphate exist as HPO4-, 12% as PO4-, and 1% as H2POj. Of the PO - species 99.6% is complexed with cations other than Na+. The equilibrium relationship for the system is shown in Fig. 15. 

In the external environment of tunicates, most metal ions are cationic, few are anionic. Vanadium is one of the anions, as it is present predominantly as HzVOj at the pH of sea water. Only chromate, among the metal-containing anions, is significant in sea water and it is present at a fiftieth the concentration of vanadium8. Other anions to consider are sulfate and phosphate, present as SO4 and HPO4" in sea water. 

Colloidal iron consists mainly of colloidal solutions of hydroxides and phosphates of Fe (Fe, " 7 ) and colloidal solutions of the enumerated organic compounds (Fe , ). Direct determination of colloidal iron in sea waters is difficult and usually dissolved iron means the sum of Fe j + Fe. , . 

Several inorganic ion exchangers like the zirconium salts of phosphates, silicate phosphates, molybdate phosphates, and tungstate phosphates showed selective sorption properties for potassium dissolved in sea water and brines. The potassium capacity of zirconium phosphate was found to be 25 mg K+/g. The selectivity for potassium increased with higher drying temperatures of the exchangers. The potassium ion sorption rate exceeded that of other cations40). 

Other oxyanionic species are present in sea water, such as SO, CrO -, and, especially, H2PO4 and HPO2-. All these anions are taken up by tunicates1261. They are not assimilated, however, but are rapidly turned over. Laboratory attempts to measure vanadate uptake in the presence of phosphate generally show inhibition of uptake of the essential element. This observation can be explained by the formation of vanadate-phosphate complexes which are not bound at the uptake sites49). At the high levels of phosphate used, the equilibria would be shifted away from monomeric vanadate, whereas in sea water these moderately stable complexes would remain virtually fully dissociated. 

W. R. Grace Co. is working on a process for preparing a fertilizer from the calcium and magnesium in sea water. The scale-forming constituents would be removed in the form of a salable product. Ammonia, phosphoric acid, and sodium hydroxide or carbonate are added to the water to precipitate a mixture of phosphates, which when dried can be used as a fertilizer. The economics of such a process appear favorable in some circumstances, but it will take large scale tests to determine its feasibility. 

The loss of ammonia was reduced when the dehydration was carried out in a continuous manner. The slurry from the settler of the descaling step was continuously pumped into a reactor vessel fitted with a stirrer, thermometer, and heater in which the phosphates were dehydrated. The overflow was discharged by gravity into a filter or a settler. When this method of dehydration was used on a slurry containing 17% solids in sea water, the product was equivalent in composition to that obtained by the best batch procedures—i.e., with the more concentrated slurries. 

Recovery of the potassium in sea water was 90% (see Table V) when phosphate was added equivalent to the calcium and magnesium content of the sea water and the final pH was adjusted to 9.5. The hydrated product, probably a mixture of magnesium potassium phosphate diluted with other magnesium and calcium phosphates, contains approximately 3% K20 (or an estimated 4 to 5% K20 after dehydration). 

The source of the aerosol salt is the ocean. The elemental constituents occur in ratios which differ somewhat from those found in sea water. The chemical composition of the aerosol salt is influenced by several factors. When the salt aerosol is created at the sea surface by whitecaps, we And ion fractionation. That is, the sea water droplets injected into the atmosphere contain the elements in proportions different from those in sea water. Examples include different ratios for the halogens, alkali metals, sulfate, phosphate, and nitrogen (12, 44, 51, 62, 63), The causes for fractionation, both as physical and organic chemical processes are under... 

Figure 2 Depth profiles for major nufrienfs (nifrafe (Pacific only), phosphate, and silicic acid) and filterable concentrations (that passing a 0.4-nm filter) of frace nufrienf elemenfs (zinc, cadmium, nickel, copper, and manganese) in the central North Pacific (diamonds, 32.7° N, 145.0° W, Sep. 1977) and North Atlantic (squares, 34.1° N, 66.1 °W, Jul. 1979). Manganese concentrations in the Pacific were analyzed in acidified, unfiltered seawater samples. The units molkg are defined as moles per kilogram of seawater. Data from Bruland KW and Franks RP (1983) Mn, Ni, Cu, Zn and Cd in the western North Atlantic. In Wong CS, Boyle E, Bruland KW, Burton JD, and Goldberg ED (eds.) Trace Metals in Sea Water, pp. 395-414. New York Plenum.

Other additional uranium sources, associated with unconventional deposits or exploited as a by-product of other minerals (e.g. copper and gold), are those found in old mine dumps (gold mines in South Africa), phosphate rocks (Morocco, the U.S.A. and the U.S.S.R.), with a content ranging from 0.001 to 0.07%, in copper deposits, such as the porphyry coppers , in marine black shales with a content ranging from 0.001 to 0.008% (the U.S.A. and Sweden), in coal and lignite deposits with a content normally of 0.001%, exceptionally reaching 1% (the U.S.A.), in monazite deposits with 0.3% (India, Brazil, Australia and Malaysia), in igneous rocks, such as the alkaline intrusives distributed in various parts of the world, and, as has already been mentioned, in sea water. 

At equilibrium, the least soluble substance in a system that can form will precipitate. Much phosphate contained in sea water is precipitated as tricalcium orthophosphate or hydroxyl apatite, Caio(P04)6(OH)2, and fluorapatite, Caio(P04)6(F)2. Oceans floors are covered with these deposits and are referred to as marine pellets. There are many ways in which this problem may be approached, but it is obvious that if phosphates are to be leached from igneous rocks, large boulders will leach very slowly. Smaller particles of rock caused by grinding, weathering, and aging solubilize more rapidly than larger particles. As a first approximation, rates of solubilization are proportional to fresh surfaces of solubilized rocks. 

When seas became saturated with phosphates, the compound that precipitated around shorelines where rivers emptied into oceans or in evaporating inland seas was hydroxyl- and fluor-apatites. Most organisms living in these seas should have migrated to shorelines or drifted there when dead. As sedimentary deposits formed rapidly, soluble phosphates became much more abundant to all forms of life. Phosphates are useless to plant life unless solubilized. As discussed in detail below, this is the reason that mined phosphates are treated with sulfuric acid to make them soluble enough to act as fertilizers. Even after being acidified their value as a fertilizer is short-lived because they are precipitated by the minerals of soil in much the same way they were insolubilized in sea water. 

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