Alkaline natural waters, phosphate

Conclusions Relative to Phosphate Distribution in Alkaline Natural Waters... 

Alkalinity. The alkalinity of a water sample is its acid-neutrali2ing capacity. Bicarbonate and carbonate ions are the predominant contributors to alkalinity in most waters, and their chemical equiUbria generally maintain the pH of 5—9. The presence of enough hydroxide ion to affect the alkalinity determination in natural waters is rare. SiUca, borate, or phosphate do contribute to the overall alkalinity if present in large enough quantities. 

Where At is total alkalinity and Aj are other alkalinity contributing components. In many natural waters, numerous components such as boric acid, hydrogen sulfide, phosphate, and organic compounds contribute to alkalinity, making precise determination of carbonate alkalinity difficult. It is convenient to envision components such as H+ as contributing to alkalinity in a negative manner. 

Phosphate and arsenate ions, when found in high concentrations, may also contribute to the ANC. In average natural waters, non-carbonate contributors are not present in sufficient amounts so as to affect the alkalinity or ANC determinations. 

Of course, once the ore is obtained from its deposit, the actual work of extracting the desired metal has yet to be accomplished. In addition to metals, a variety of other substances comprise natural minerals. Since aluminum and silicon are the most prevalent elements in the Earth s crust, most of the metals exist naturally as aluminates, silicates, or aluminosilicates. The most common minerals are feldspars and clays. These materials have been used since ancient times for the production of materials such as pottery, brick, and china. An example of a feldspar is K2Al2Si60i6, which corresponds to a mixture of potassium superoxide, alumina, and silica (K20-Al203 6Si02). Upon contact with water and carbon dioxide, a weathering reaction results in kaolinite, an aluminosilicate clay (Eq. 1). However, in addition to these oxidized sources of metals, there are substances such as alkaline carbonates, sulfates, phosphates, as well as organic matter that need to be removed to yield the desired metal. As you would expect, the yield for this process is quite low ores typically possess less than 1 % of the desired metal ... 

As mentioned before, the previous reaction for phosphate removal takes place in conjunction with the Al(OH)3 equilibrium. Bicarbonate alkalinity is always present in natural waters, so this equilibrium is facilitated by alum reacting with calcium bicarbonate so that, in addition to the amount of alum required for the precipitation of phosphorus, more is needed to neutralize the bicarbonate. The reaction is represented by... 

Ammonia is usually determined after being separated from a strongly alkaline medium and then absorbed in dilute H2SO4 or HCl. It is sometimes possible to carry out the indophenol reaction without separating the ammonia (e.g., in natural waters). In the presence of EDTA, moderate quantities (0.1-0.5 mg) of Ca, Mg, and A1 do not interfere. The addition of tartrate prevents the precipitation of hydrolysable metals. Phosphate interferes in the colour reaction [23]. 

Other compounds present in natural water may also make a minor contribntion to alkalinity. For example, ammonia and salts of weak inorganic acids snch as borates, silicates and phosphates may contribute to alkalinity, as may salts of organic acids (e.g., humic, acetic, propionic). 

An alternative interference removal step was developed by Ledo de Medina et al. who developed an IC method for phosphate in natural waters in the presence of high concentration of sulphates. This interference was avoided by first precipitating sulphate as lead sulphate prior to 1C analysis. Samples with high iron content were investigated by Simon. Interferences caused by the precipitation of iron hydroxides from air oxidation of ferrous iron in anoxic water samples and from the alkaline eluent used in IC, were found to affect the determination of phosphate and other inorganic anions in riverine sediment interstitial water samples with high concentrations of dissolved iron (0.5 to 2.0 mmol/1). To eliminate this interference the complexation of iron with cyanide was used prior to IC analysis. ... 

Many other organic acids important in water chemistry have p/Cg values similar to that of acetic acid, for example, propionic acid and butyric acid. Many natural waters contain silicates and organic bases that contribute to the total alkalinity. Wastewaters contain substantial quantities of organic bases, ammonia, and phosphates. Anaerobic digester supernatant often contains high concentrations of bases similar to acetate, as well as carbonates, ammonia, and phosphates. For very complex systems for which a detailed chemical analysis is not available, no attempt is made to work with the mathematical definition of alkalinity. However, if the system is chemically defined we can show that these substances will contribute to alkalinity if, during the titration, some of the base is converted to the conjugate acid. We can modify the total alkalinity definition to include these species so that... 

It was observed that organically bound phosphorus is completely decomposed to phosphate when oxidized with persulphate in an alkaline medium. Furthermore, more than 60 % of condensed phosphates are hydrolyzed. As concentrations of polyphosphates are negligible in most natural waters, a simultaneous oxidation procedure for organic phosphorus and nitrogen compounds has been developed by Korolejf (1977). Valderrama (1981) compared the procedure with former methods using separate determinations. In the simultaneous oxidation, the pH of the reaction starts at ca. 9.7 and ends at 4-5. These conditions are obtained by a boric acid-NaOH system. In seawater samples no precipitation is formed when the oxidation mixture is added. At elevated temperatures a precipitation is formed, which, however, dissolves as oxidation proceeds. 

Waters may be alkaline due to the presence of a wide variety of salts of weak acids such as carbonates, bicarbonates, borates, silicates, phosphates, etc., and also due to the presence of weak and strong bases (due to contamination with industrial wastes). Ibe major portion of alkalinity in natural water is, however, caused by presence of bicarbonates that are formed in appreciable amounts when water containing free CO2 percolates through soils containing CaC03 and/or MgC03 ... 

With borates, silicates and phosphates making only an insignificant contribution, the alkalinity of natural waters may be taken as an indication of the concentration of (i) Hydroxides, (ii) Carbonates, and (iii) Bicarbonates. 

Precipitation can occur if a water is supersaturated with respect to a solid phase however, if the growth of a thermodynamically stable phase is slow, a metastable phase may form. Disordered, amorphous phases such as ferric hydroxide, aluminum hydroxide, and allophane are thermodynamically unstable with respect to crystalline phases nonetheless, these disordered phases are frequently found in nature. The rates of crystallization of these phases are strongly controlled by the presence of adsorbed ions on the surfaces of precipitates (99). Zawacki et al. (Chapter 32) present evidence that adsorption of alkaline earth ions greatly influences the formation and growth of calcium phosphates. While hydroxyapatite was the thermodynamically stable phase under the conditions studied by these authors, it is shown that several different metastable phases may form, depending upon the degree of supersaturation and the initiating surface phase. 

The procedure used to define an equilibrium model is to (1) define all the variables and (2) define independent equilibria as a function of phase equilibria. The variables are defined as the chemical parameters typically measured in water chemistry. For the major constituents and some of the more important minor constituents, these are calcium, magnesium, sodium, potassium, silica, sulfate, chloride, and phosphate concentrations as well as alkalinity (usually carbonate alkalinity) and pH. To this list we would also add temperature and pressure. The phase equilibria are defined by compiling well-known equilibria between gas-liquid phases and solid-liquid equilibria for the solids commonly found forming in nature in sedimentary rocks. Within this framework, one can construct different equilibrium models depending upon the mineral chosen actual data concerning the formation of specific minerals therefore must be ascertained to specify a particular model as valid. 

In the 1960s, Schwartz described the phosphorylation of adenosine with trimetaphosphate to yield 2 - and 3 -AMP. The strong alkaline conditions used for this transformation were not likely to occur on the primitive Earth [137]. Similarly, all natural ribonucleosides were phosphory-lated to corresponding 2 - and 3 -nucleotide monophosphates with sodium trimetaphosphate at high pH and temperature [138,139]. When the reaction was performed under similar experimental conditions at lower pH, 2/,3/-cyclic phosphate nucleotides were recovered as the major products [140]. Magnesium ion catalyzes this transformation in neutral water solution [141]. 

Artificial enzymes with metal ions can also hydrolyze phosphate esters (alkaline phosphatase is such a natural zinc enzyme). We examined the hydrolysis of p-nitro-phenyfdiphenylphosphate (29) by zinc complex 30, and also saw that in a micelle the related complex 31 was an even more effective catalyst [118]. Again the most likely mechanism is the bifunctional Zn-OH acting as both a Lewis acid and a hydroxide nucleophile, as in many zinc enzymes. By attaching the zinc complex 30 to one or two cyclodextrins, we saw even better catalysis with these full enzyme mimics [119]. A catalyst based on 25 - in which a bound La3+ cooperates with H202, not water - accelerates the cleavage of bis-p-nitrophenyl phosphate by over 108-fold relative to uncatalyzed hydrolysis [120]. This is an enormous acceleration. 

Al ] needs to be eliminated for the equation to be expressed solely in terms of When alum is added to water, it will unavoidably react with the existing natural alkalinity. For this reason, the aluminum ion will not only react with the phosphate ion to precipitate AlP04(s), but it will also react with the OfT to precipitate Al(OH)3(j). Also, Ar will form complexes Al(OH) ", Al7(OH)i7, Ali3(OH)34, Al(OH)A and Al2(OH)2" in addition to the Al(OH)3(j). AU these interactions comphcate our objective of eliminating [Af ]. 

The amount of alum needed to precipitate the phosphate is composed of the alum required to satisfy the natural alkalinity of the water and the amount needed to precipitate the phosphate. Satisfaction of the natural alkalinity will bring the equihbrium of aluminum hydroxide. Remember, however, that even if these quantities of alum were provided, the concentration of phosphorus that will be discharged from the effluent of the unit stiU has to conform to the equilibrium reaction that depends upon the pH level at which the process was conducted. The optimum pH, we have found, is equal to or less 5. 

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