Alkalinity, of natural waters

The pH of a system determines the reactions that define the concentration of many dissolved chemical species in water containing salts and minerals, supplied by weathering reactions, rain, runoff, and lixiviating processes. The pH is a key parameter for biological growth and for the sustainment of life for the different aquatic flora and fauna species. As discussed in Chapter 2 the contribution of the different species will affect the final pH and vice versa (i.e., the pH on its own often determines the form of the species present). That is why the distribution diagrams of chemical species are frequently defined as functions of pH (Section 2.1.2). In summary, the main environmental processes that affect the pH and the alkalinity of natural waters include ... 

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. 

One standard method for determining the dissolved O2 content of natural waters and wastewaters is the Winkler method. A sample of water is collected in a fashion that prevents its exposure to the atmosphere (which might change the level of dissolved O2). The sample is then treated with a solution of MnS04, and then with a solution of NaOH and KI. Under these alkaline conditions Mn + is oxidized to Mn02 by the dissolved oxygen. 

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. 

In principle, the alkalinity of the water will also be affected by the balance of nutrient ions consumed and released by organisms in the water. But in practice these have a minor affect compared with CO2. The average composition of the algal biomass in natural waters is given by the Redfield formula (Redfield, 1934) as C106H263O110N16P. Therefore for the complete stoichiometry of algal photosynthesis and respiration, we have with NO3 as the source of N... 

Bottom ash from power stations is less of a problem compared with fly ash for the contamination of natural waters firstly because the proportions of fly ash to bottom ash are approximately three to one and a greater proportion of the bottom ash is used (ECOBA 2003). Secondly, the volatile elements are depleted compared with fly ash (Clarke Sloss 1992). Other combustion residues include fluidized-bed boiler ashes and the products from flue gas desulphurization (FGD). The non-regenerable FGD systems commonly use limestone, slaked lime, or a mixture of slaked lime and alkaline fly ash that are sprayed into the flue gases to remove SO2 (Clarke Sloss 1992). Although 90 wt% of the product is used to replace natural gypsum in plasters and wallboards, there is currently a small excess production in Europe of that is disposed of in landfill and equivalent sites (ECOBA 2003). Because the FGD plant treats the cooled flue gases volatile elements are concentrated and there will be similarities with fly ash. 

Alkalinity is defined as the capacity of natural water to react with H+ to reach pH 4.5, which is the second equivalence point in the titration of carbonate (CO5 ) with H. To a good approximation, alkalinity is determined by OH-, CO j, and HCOf ... 

Acid-Base. The pH of natural waters is determined primarily by the carbonate equilibria. However, organisms may produce amounts of organic matter or ammonia sufficient to influence the pH and buffer capacity of the waters. It would be of interest to determine titration curves of high organic, high color, low alkalinity waters leached from some marshes. It is possible that these waters contain sufficient amounts of organic acids to be significant. 

Under the pH and temperature conditions of natural waters, the various 2-chloro-, 2-methylthio-, and 2-methoxy-.v-triazine herbicides are generally considered to be stable in solution between pH 5 and pH 9, stable in neutral, weakly acidic and weakly alkaline media, and stable to hydrolysis at 20°C in neutral, weakly acidic, and weakly alkaline media (Pesticide Manual, 1997). Metribuzin, metamitron, and hexazinone are also considered to be stable under these conditions (Pesticide Manual, 1997). Such hydrolytic stability of the triazine herbicides is supported by hydrolysis studies (Rhodes, 1980 Widmer et al., 1993 Noblet et al, 1996 Hequet et al, 1997) that indicate either very slow rates of hydrolysis or no measurable hydrolysis (Table 23.3) under these conditions. 

One term now used interchangeably with acid neutralizing capacity (ANC) of natural waters is alkalinity (Aik)—defined as the concentration of negative charge that will react with H+. 

The most common parameters that define chem-ical/environmental characteristics of natural waters are pH, alkalinity, ionic strength, conductivity, salinity, hardness, and the composition and concentration... 

A sixth source of difference, which was unavoidable in preparation of the test cases, is the various ways each program handles the carbonate system. From a practical standpoint, the inorganic carbon system of natural waters is usually determined from the titration alkalinity. Because the titration alkalinity includes both carbonate and non-carbonate alkalinity, the titration alkalinity must -be corrected for noncarbonate alkalinity. Most models correct for the presence of H2BO0 and H SiOT but many other minor species should be considered and there is no general agreement as to the precise correction for non-carbonate alkalinity. In devising the correction used originally in WATEQ, Truesdell and Jones also... 

Let the current pH be pH r and the pH to which it is to be adjusted (the destination pH) be pH. If pH, is greater than pH an acid is needed. No matter how insignificant, a natural water will always have an alkalinity in it. Alkalinities of surface water can vary from 10 to 800 mg/L (Sincere, 1968). Until it is all consumed, this alkalinity will resist the change in pH. Let the current total alkalinity l>e [A lgeq in gram equivalents per liter. Let the total acidity to be added be [A aM eti in gram equivalents per liter. 

Now, consider the situation where pH r is less than pH, . In this case, a base is needed. As in the case of alkalinity, a natural water will always have acidity. Until it is aU consumed, this acidity will resist the change in pH when alkalinity is added to the water. Let the current total acidity be [A r]geq in gram equivalents per hter. Also, let the total alkalinity be in gram equivalents per liter. 

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. 

Hemond H. F. (1990) Acid neutralizing capacity, alkalinity, and acid-base status of natural waters containing organic acids. Environ. Set Technol. 24, 1486-1489. 

In carbonate systems and in natural waters, [ANC] is referred to as alkalinity, while [BNC] is called acidity. In the context of natural waters, these terms will be discussed in the next chapter. 

Hemond, H. F. (1990). ANC, Alkalinity, and the Acid-Base Status of Natural Waters Containing Organic Acids. Environ. Sci. Technol. 24(10), 1486—1489. 

The mechanisms for this reaction are discussed in the chapter on kinetics (Chapter 9). It is a combination of first- and second-order reactions, which is not solvable anal5dically because of the nonlinear terms following the rate constants koH and kcoj.r- The rate constants were determined in the laboratory by choosing the experimental conditions in which one of the two mechanisms predominated. pH values of natural waters, however, often fall in the range 8-10, in which the reaction with both water and OH can be important. To determine the life time of CO2 as a function of pH, one must derive the solution to the reaction rate equation. This is facilitated by employing the DIG and carbonate alkalinity, Ac, (Eqs. (4.15) and (4.26)) to eliminate the concentration of bicarbonate [HCOJ], in the CO2 reaction rate equation. This substitution results in an expression... 

We will focus first on immediate contributions to acidity and alkalinity. For the acidity of natural waters, the contributors are strong and weak acids, salts of strong acids and weak bases, hydrolysis of Fe and AP, and oxidation and hydrolysis of Fe " and Mn +. 

Acidity and alkalinity titrations determine the total capacity of natural waters to consume strong bases or acids as measured to specified pH values defined by the endpoints of titrations. Of more interest for many purposes is the ability of a water or water-rock system to resist pH change when mixed with a more acid or alkaline water or rock. This system property is called its buffer capacity. Buffer capacity is important in aqueous/environmental studies for reasons that include ... 

Recent applications have shown the potential of flow titration as a modem tool in analytical chemistry. As the required amount of titrand is associated with the analytical signal, important parameters, e.g., oxidis-ability in wastewaters [339], bromine number in foodstuffs [340], bitterness of beers and similar [341], total acidity in wines and vinegars [342] and total alkalinity in natural waters [343], are efficiently determined. In addition, the total concentration of several analytes belonging to the same family, e.g., amines [344], can be determined. The entire titration curve is generally available, allowing the determination of weak acids, complex stability constants and acid dissociation constants [345]. The determination of humidity by the Karl Fischer method [346] is another important application of flow titrations. For single analyte determinations, the analytical characteristics inherent to titrimetric procedures, such as enhanced accuracy and precision, should be emphasised. 

D.R. Turner, S. Knox, M. Whitfield, M.M.C. Santos, Flow injection titration of alkalinity in natural waters, Anal. Proc. 24 (1987) 360. 

The content of the most important ions in these wastewaters considerably differs from those of natural waters. The most frequently occurring cation is Na" rather than Ca which occurs in high amounts in natural waters, and of anions it is Cl followed by SO , instead of HCO3. Alkalinity of these waters is caused by OH or CO3 ions the HCO3 ion is absent in more concentrated acid or alkaline waters. 

A sample of natural water that has been equilibrated with CaCOsfs) is isolated from its surroundings. Indicate whether the addition of small quantities of the following will increase, decrease, or have no effect on the total alkalinity or total acidity and state very briefly why. Neglect ionic strength effects. 

The hydrolysis reactions depicted in Eqs. 5-9 to 5-12 are acid-base (proton transfer) reactions. Because of this, the pH of the solution will influence the distribution of the various species. In general, the percentage of the hydrolyzed species increases as the pH increases, just as the concentration of a conjugate base would increase if the pH of a solution containing its conjugate acid were raised. Ail trivalent and most divalent metal ions are complexed to some extent with OH at the pH of natural waters. The alkaline earth metals hydrolyze significantly only at high pH... 

CRECEP and LHRSP used natural waters because they are more convenient for taste evaluation. Alkalinity of natmal waters has been corrected to obtain 20 ppm and 200 ppm CaCOs. The differences were alw s significant and very large with soft waters, but differences also appeared with medium mineralized waters. Almninimn leaehing is lower in natural waters than in synthetic waters. 

Coagulants are often added in conjunction with lime to increase the settling rate of calcium carbonate and magnesium hydroxide. Most of these coagulants are acidic in nature and react with the alkalinity of the water. Commonly used coagulants include aluminium sulphate (alum), sodium aluminate, ferric sulphate and ferrous sulphate (Table 6.8). Alum reacts with natural alkalinity in water to form aluminium hydroxide floe (Equations 2.5—2.8) [14]. About 1 ppmofalumdecreaseswater alkahnity by 0.5 ppm and produces 0.44 ppm of CO2 ... 

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