Phosphate formation sediments

The destiny of most biological material produced in lakes is the permanent sediment. The question is how often its components can be re-used in new biomass formation before it becomes eventually buried in the deep sediments. Interestingly, much of the flux of phosphorus is held in iron(lll) hydroxide matrices and its re-use depends upon reduction of the metal to the iron(ll) form. The released phosphate is indeed biologically available to the organisms which make contact with it, so the significance attributed to solution events is understandable. It is not clear, however, just how well this phosphorus is used, for it generally remains isolated from the production sites in surface waters. Moreover, subsequent oxidation of the iron causes re-precipitation of the iron(lll) hydroxide floes, simultaneously scavenging much of the free phosphate. Curiously, deep lakes show almost no tendency to recycle phosphorus, whereas shallow... 

Emerson, S. and Widmer, G. (1978). Early diagenesis in anaerobic lake sediments II. Equilibrium and kinetic factors controlling the formation of iron phosphate. Geochim. Cosmochim. Acta 42,1307-1316. 

By far the most important ores of iron come from Precambrian banded iron formations (BIF), which are essentially chemical sediments of alternating siliceous and iron-rich bands. The most notable occurrences are those at Hamersley in Australia, Lake Superior in USA and Canada, Transvaal in South Africa, and Bihar and Karnataka in India. The important manganese deposits of the world are associated with sedimentary deposits the manganese nodules on the ocean floor are also chemically precipitated from solutions. Phosphorites, the main source of phosphates, are special types of sedimentary deposits formed under marine conditions. Bedded iron sulfide deposits are formed by sulfate reducing bacteria in sedimentary environments. Similarly uranium-vanadium in sandstone-type uranium deposits and stratiform lead and zinc concentrations associated with carbonate rocks owe their origin to syngenetic chemical precipitation. 

Nitrogen pollution has received far more attention than that of phosphorus for two reasons. First, it has been considered as the nutrient-limiting primary production in estuaries and coastal waters. Second, its loading into the coastal zone has been far greater than that of phosphorus (Figure 24.21). It is also more efficiently exported into the ocean due in part to formation of iron phosphate minerals in anoxic estuarine sediments. 

Bodies of Water and the Chemical Sediments ,— The chemistry of the deposition of salts from sea-water has already been made the subject of special research, and van t Hoff s results in this field are already familiar. The deposition of calcium carbonate awaits a similar thorough study. Allied questions are the formation of dolomite, the deposition of various salts from inclosed bodies of water, the deposition of phosphate rocks, the precipitation of colloidal suspensions of clay and other substances, and the origin of the great deposits of sedimentary iron ore. 

Nriagu, J. O. 1984. Formation and stability of base metal phosphates in soils and sediments. In Nriagu, J. O. Moore, P. B. (eds) Phosphate Minerals. Springer-Verlag, Berlin, 318-329. 

As noted above, Sumner and co-workers were unable to determine the diffusion coefficient of urease unless they added Na2S03 and NaHSO-to the phosphate buffer (40) used. Nichol and Creeth, employing identical concentrations (60), measured both the sedimentation coefficient and the electrophoretic mobility of sulfite-modified urease. They concluded that sulfite contributed to the formation of -S-S03 groups attached to the (16n) species. Some of these groups they ascribed to the scission of intermolecular disulfide bonds of aggregated forms others, they suggested, arose from the 22 reactive sulfhydryl groups that react with 02 (air) to form transitory disulfides that can, in turn, react with sulfite. 

Redox Reactions. Aquatic organisms may alter the particular oxidation state of some elements in natural waters during activity. One of the most significant reactions of this type is sulfate reduction to sulfide in anoxic waters. The sulfide formed from this reaction can initiate several chemical reactions that can radically change the types and amounts of elements in solution. The classical example of this reaction is the reduction of ferric iron by sulfide. The resultant ferrous iron and other transition metals may precipitate with additional sulfide formed from further biochemically reduced sulfate. Iron reduction is often accompanied by a release of precipitated or sorbed phosphate. Gardner and Lee (21) and Lee (36) have shown that Lake Mendota surface sediments contain up to 20,000 p.p.m. of ferrous iron and a few thousand p.p.m. of sulfide. The biochemical formation of sulfide is undoubtedly important in determining the oxidation state and amounts of several elements in natural waters. 

The strong dissolution effects on heavy metals of increasing phosphate concentrations in river water (Fig. 8-19) indicate the strong remobilization effects which result from complex formation. This tendency for complex formation with triphosphates is also described in the literature [GMELIN, 1965]. Increasing the concentration of phosphate in water from 0.38 to 0.68 mg L 1 increases the iron concentration by 648 pg L 1 in water in contact with sediment in which the iron concentration is 16220 mg kg ... 

The greater proportion of unextractable residues in the unshaded system may reflect formation of degradation products in the water column which are subsequently irreversibly bound to sediments or sedimented detritus. Phosphate diesters, in particular, are difficult to extract from sediments as well as water (13) and may account much of this unextractable residue. 

Allen M.R. (1985) The origin of dolomites in the phosphatic sediments of the Miocene Pungo River Formation North Carolina. M.S. thesis, Duke Univ., Durham, North Carolina. 

Gaudette, H.E., and Lyons, W.B. (1980) Phosphate geochemistry in nearshore carbonate sediments suggestion of apatite formation. Soc. Econ. Paleon. Min. Spec. Publ. 29, 215-225. 

Roden, E.E., and Edmonds, J.W. (1997) Phosphate mobilization in iron-rich anaerobic sediments microbial Fe(III) oxide reduction versus iron-sulfide formation. Arch. Hydrobiol. 139, 347-378. 

As was mentioned before, Arey et al. [19] conducted batch equilibration experiments to evaluate the ability of hydroxyapatite to remove uranium from contaminated sediments at the Savannah River Site of DOE and showed that removal of U was due to secondary phosphate minerals that had solubility even lower than autunite (Ca(U02)2(P04)2- IOH2O). The authors suggest formation of Al/Fe secondary phosphate. A similar conclusion was reached by Fuller et al. [20], who showed that uranyl ions can be removed by using hydroxyapatite. 

Sediment phosphorus extraction analyses show that hydrous iron oxides (extracted by (NH4)2C204) play a major role in the transport of sediment phosphorus. In northern areas of the Genesee River watershed calcium carbonate formation also appears to be Involved in phosphorus fixation. Ion activity product calculations for water column samples from the Genesee River consistently exhibit subsaturation with respect to the stable calcium phosphate phase, hydroxyapatite. Calcium carbonate, which can serve as a substrate for phosphate mineralization, shows an ion activity product below the solubility product in the Genesee River except during the summer low-rainfall season. 

CFA is omnipresent in pelagic sediments, ferromanganese nodules, and seamount crusts, and marine phosphorites on continental margins and seamounts. Supply of phosphate by oxidation of organic matter and pre-concentration of phosphate by iron oxyhy dr oxides, supply of calcium by dissolution of carbonates or carbonates acting as substrates, and supply of fluorine from seawater are all important factors in the formation of marine phosphorites. 

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