Time Dependence of Concentrations in the Two Step Oxidation of Ammonia

BOX 14-

The bacteria-catalyzed oxidation of ammonia (or of other reduced organic nitrogen compounds) to nitrate is a reaction with two main steps, with nitrite ion, N02^, an intermediate:

Step 2 N02~ + 102-» N03~

If sufficient oxygen is available, the rate of each reaction is first-order only in the concentration of the nitrogen reactant, so the sequence can be represented as

where A stands for ammonia, B for nitrite ion, and C for nitrate ion, and kt and k2 are the pseudo-first-order rate constants. Since the rate of step 1 depends on the first power of the ammonia concentration, then the rate of disappearance of this species is

Since B (nitrite) is produced at this rate by step 1, but is consumed in step 2 by a process whose rate is proportional to the first power of its concentration, we can write

These differential equations can be coupled and integrated to yield the following expressions for the evolution of [A] and [B] with time, relative to [A]0, the original concentration of A:

[B]/[A]0 = kje-x» - e-«*)l(k2 - Iq)

As can be seen from the solution to Problem 1, the concentration of B (nitrite) rises exponentially at first, reaches a peak value, then declines slowly. Thus significant concentrations of nitrite ion occur in water undergoing the two-step conversion of ammonia to nitrate.

PROBLEM I

(a) Derive a general expression relating the time at which [B] reaches a peak to kl and fe2.

(b) Draw a graph showing the evolution of [A] and of [B] with time for the values k1 = 1 and k2 = 2.

Of course, water contaminated by nitrate ion can also be treated by this latter step. A mathematical analysis of the kinetics of the transformations is given in Box 14-4-

PROBLEM 14-11

Given that the K|, value for ammonia is 1.8 X 10~5, deduce a formula giving the ratio of ammonia to ammonium ion as a function of the pH of water. What is the value of this ratio at pH values of 5, 7, 9, and 11?

The Origin and Removal of Excess Phosphate

One of the world's most famous cases of water pollution involves Lake Erie, which in the 1960s was said to be dying. Indeed, one of the authors of this book can recall visiting a once-popular beach on Lake Erie's north shore in the early 1970s and being repulsed by the sight and smell of dead, rotting fish on the shoreline. Lake Erie's problems stemmed primarily from an excess input of phosphate ion, P043-, in the waters of its tributaries. The phosphate sources were the polyphosphates in detergents (as explained in detail later), raw sewage, and the runoff from farms that used phosphate products. Since there is commonly an excess of other dissolved nutrients in lakes, phosphate ion usually functions as the limiting (or controlling) nutrient for algal growth: The larger the supply of the ion, the more abundant the growth of algae—and its growth can be quite abundant indeed. When the vast mass of excess algae eventually dies and starts to decompose by oxidation, the water becomes depleted of dissolved oxygen, with the result that fish life is adversely affected. The lake water also becomes foul-tasting, green, and slimy, and masses of dead fish and aquatic weeds rot on the beaches. The series of changes, including rapid degradation and aging, that occur when lakes receive excess plant nutrients from their surroundings is called eutrophication. When the enrichment arises from human activities, it is called cultural eutrophication.

To correct the regional problem, the United States and Canada in 1972 signed the Great Lakes Water Quality Agreement. Since that time, over $8 billion has been spent in building sewage treatment plants to remove phosphates from wastewater before it reaches the tributaries and the lake itself. In addition, the levels of polyphosphates in laundry detergents were restricted in Ontario and in many of the states that border the Great Lakes. The total amount of phosphorus entering Lake Erie has now decreased by more than two-thirds. As a result, Lake Erie has sprung back to life: Its once-fouled beaches are regaining popularity with tourists, and its commercial fisheries have been revived.

As we have pointed out, the presence of excess phosphate ion in natural waters can have a devastating effect on an aquatic ecology because it overfer-tilizes plant life. Formerly, one of the largest sources of phosphate as a pollutant was detergents, and in the material that follows, the role of such phosphates is discussed.

The reaction of synthetic detergents with calcium and magnesium ions, forming complex ions, diminishes the cleansing potential of the detergent. Polyphosphate ions, which are anions containing several phosphate units linked by shared oxygens, are added to detergents as builders that preferentially form soluble complexes with these metal ions and thereby allow the molecules of the detergent to operate as cleansing agents rather than being complexed with the Ca2+ and Mg2+ naturally present in the water. Another role of the builder is to make the wash water somewhat alkaline, which helps remove the dirt from certain fabrics. With soap itself, the ions form insoluble complexes that foul the cleaning water.

F1GURE 14-9 Structure of the polyphosphate ion:

(a) uncomplexed and

(b) complexed with calcium ion.

Great quantities of sodium tripolyphosphate (STP), Na5P3O10, were formerly added as the builder in most synthetic detergent formulations. As shown in Figure 14-9a, STP contains a chain of alternating phosphorus and oxygen atoms, with one or two additional oxygens attached to each phosphorus. In solution, one »¡polyphosphate ion can form a complex with one calcium ion by forming interactions between three of its oxygen atoms and the metal ion (Figure 14-9b),

Substances like STP, which have more than one site of attachment to the metal ion and thereby produce ring structures that each incorporate the metal, are called chelating agents (from the Greek word for "claw"). Because several bonds are formed, the resulting chelates are very stable and do not normally release their metal ions back into the free form. The use of chelating agents to remove metals from the human body is discussed in Chapter 15.

Tripolyphosphate ion, PjOjq5^, like phosphate ion itself, is a weak base in aqueous solution and thus provides the alkaline environment that is required for effective cleaning:

l\O10s + H20-> P3O10H4" + OH"

Unfortunately, when wash water containing STP is discarded, the excess tripolyphosphate enters waterways, where it slowly reacts with water and is transformed into phosphate ion (sometimes called orthophosphate):

P3O105" + 2 H20-> 3 P043" + 4H "

Note that when tripolyphosphate decomposes, STP behaves as an acid rather than a base (since H+ is formed in the reaction).

Because of environmental concerns, polyphosphates are now used only sparingly as builders in detergents in many areas of the world. In Canada and parts of Europe, STP was replaced largely by sodium nitrilotriacetate (NTA) (see Figure 14-10a). The anion of NTA acts in a similar fashion to that of STP, chelating calcium and magnesium ions using three of its oxygen atoms and the nitrogen atom (Figure 14-10b). NTA is not used as a builder in the United States because of concerns that its slow rate of degradation might lead to health hazards in drinking water. However, the early experiments with test animals that led to this concern are open to question, as are fears about its persistence and its tendency to solubilize heavy metals into water supplies.

Other builders now used include sodium citrate, sodium carbonate (washing soda), and sodium silicate. Currently, substances called zeolites are also employed as detergent builders. Zeolites are abundant aluminosilicate minerals

FIGURE 14-10 Structure of the nitrilotriacetate ion:

(a) uncomplexed and

(b) complexed with calcium ion.

O"

ilc;

(see Chapter 16) consisting of sodium, aluminum, silicon, and oxygen. The latter three elements are bonded together to form cages, which the sodium ions can enter. In the presence of calcium ion, zeolites exchange their sodium ions for Ca2+ (though not for Mg2+), thereby sequestering it in a manner similar to polyphosphates. Like polyphosphates, they also control pH, One disadvantage to the use of zeolites is that they are insoluble, so their use increases the amount of sludge that must be removed at wastewater treatment plants.

Phosphate ion can be removed from municipal and industrial wastewater by the addition of sufficient calcium as the hydroxide Ca(OH)2, so that insoluble calcium phosphates such as Ca3(P04)2 and CagiPO^OH are formed as precipitates that can then be readily removed. Phosphate removal could be a standard practice in the treatment of wastewater, but it is not yet practiced in all cities. Some policymakers believe that the optimum environmental solution is to use polyphosphates, rather than some other builder, in detergents and then to efficiently remove phosphates at wastewater treatment plants.

Geographically, phosphate ion enters waterways from both point and nonpoint sources. Point sources are specific locations such as factories, landfills, and sewage treatment plants that discharge pollutants. Nonpoint sources are numerous large land areas such as farms, logged forests, septic tanks, golf courses and individual domestic lawns, stormwater runoff, and atmospheric deposition. Although each nonpoint source may provide a small amount of pollution, on account of the large number of them involved they can generate larger total quantities than do point sources. For example, now that sewage treatment plants and detergent controls have been instituted, much of the remaining phosphate arises from nonpoint agricultural sources in many areas.

Green Chemistry: Sodium Iminodisuccinate— A Biodegradable Chelating Agent

Because most chelating agents are not biodegradable or are only slowly biodegradable, not only do they place a load on the environment (e.g., phosphates acts as nutrients), but it may be necessary to remove them during o o

NaO' NaO

maleic anhydride treatment of wastewater in a wastewater treatment plant. Unlike many chelating agents, sodium iminodi.succinate (IDS, see Figure 14-11) [also known as D,L'aspartiC'N-(1,2'dicarboxylethyl) tetrasodium salt] readily degrades in the environment. Not only is IDS biodegradable, it is also nontoxic.

IDS can be used as an effective chelating agent for absorption of agricultural nutrients, metal ion scavenging in photographic processing, groundwater remediation, and as a builder in detergents and household and industrial cleaners. The Bayer Corporation won a Presidential Green Chemistry Challenge Award in 2001 for the development of IDS as a chelating agent and for its synthesis from maleic anhydride (Figure 14-11). This synthesis is accomplished under mild conditions, in water as the only solvent. The excess ammonia is recycled back into the production of more IDS. This synthesis stands in stark contrast to typical syntheses of aminocarboxylate chelating agents, which employ hydrogen cyanide as a reagent. Bayer markets IDS as a chelating agent under the name Baypure.

O O

sodium iminodisuccinate

ONa .ONa

FIGURE 14-U Synthesis and structure of sodium iminodisuccinate, a biodegradable chelating agent.

Reducing the Salt Concentration in Water

The decomposition of organic and biological substances during the secondary phase of wastewater treatment usually results in the production of inorganic salts, many of which remain in the water even after the techniques listed above have been applied. Water can also become salty due to its use in irrigation or because water softener units have been recharged and their discharge disposed of as sewage. Inorganic ions can be removed from water by desalination by using one of the techniques listed below or using the precipitation methods mentioned above.

• Reverse osmosis As previously mentioned, this technique is also used to produce drinking water from salty water, such as seawater.

• Electrodialysis Here a series of membranes permeable either only to small inorganic cations or only to small inorganic anions are set up vertically in an alternating fashion (see Figure 14-12) within an electrochemical cell. Direct current is applied across the water, so cations migrate toward the cathode and anions toward the anode. The liquid in alternating zones becomes more concentrated (enriched) or less concentrated (purified) in ions; eventually the ion-concentrated water can be disposed of as brine and the purified water released into the environment. This technology is also used to desalinate seawater for drinking purposes.

• Ion exchange Some polymeric solids contain sites that hold ions relatively weakly, so one type of ion can be exchanged for another of the same charge that

Outgoing brine

Outgoing purified water

Cathode

Outgoing brine

Outgoing brine

Outgoing brine

o

©

+

©o

+ + + + +

^ ©

Anode

(+) Cations Anions incoming saJty water

Cation-

permeable membrane

Anion-

permeable membrane

FIGURE 14-12 Electro-dialysis unit (schematic) for the desalination of water, [Adapted from S. E. Manahan, 1994. Environmental Chemistry, 6th ed. Boca Raton, FL: Lewis Publishers.!

happens to pass by it. Ion exchange resins can he formulated to possess either cationie or anionic sites that function in this manner. The exchange sites of a cationic resin of this type are initially occupied by hT ions, and the exchange sites of an anionic exchange resin arc occupied by OH" ions. When water polluted by M and X~ ions is passed sequentially through the two resins, the H ions on the first are replaced by Mand then the OH " ions on the second resin are replaced hy X . Thus the water that has passed through contains Hr and OH" ions, rather than those of the salt, which remain behind in the resins. Of course, these two ions immediately combine to form more water molecules. Thus ion exchange can be used to remove salts, including those of heavy metals, from wastewater.

PROBLEM 14-12

Water polluted by inorganic ions could be purified by distilling it or by freezing it. Why do you think such techniques are not generally used on a mass scale to purify water?

Transition metal cations can be removed from water using either precipitation or reduction techniques, in either case to form insoluble solids. Precipitation of sulfides or hydroxides has already been mentioned; a disadvantage of the latter is the production of a voluminous sludge that must be disposed of in an acceptable manner. Electrolytic reduction of metals leads to their deposition on the cathode. If, instead of the elemental metal, a concentrated aqueous solution of it is desired, the deposited metal can be reoxidized chemically by adding hydrogen peroxide or electrolytically hy reversing the polarity of the cell.

The Biological Treatment of Wastewater and Sewage

An alternative to the processing of wastewater through a conventional treatment plant in small communities is biological treatment in an artificial marsh (also called a constructed wetland) that contains plants such as bullrushes and reeds. The decontamination of the water is accomplished by the bacteria and other microbes that live among the plants' roots and rhizomes. The plants themselves take up metals through their root systems arid concentrate contaminants within their cells. In facilities that have heen constructed to deal with sewage, primary treatment to filter out solids, etc. in a lagoon is usually implemented before the wastewater is pumped to the marsh, where the equivalent of secondary and tertiary treatment occurs. The plant growth uses up the pollutants and increases the pH—which serves to destroy some harmful microorganisms.

One advantage of biological treatment is that great amounts of sludge are not generated, in contrast to conventional treatments. Furthermore, it requires neither the addition of synthetic chemicals nor the input of commercial energy. Among the problems in such facilities are decaying vegetation, which must be limited so that the BOD of the processed water does not rise too much, and the fact that the marshes usually require a great deal of land unless they are constructed so that part of the routing is vertical.

In many rural and small communities, septic tanks are used to decontaminate sewage since central sewage facilities are not available. These underground concrete tanks receive the wastewater, often from only one home. Although solids settle in the tank, grease and oil rise to the top, from which they are periodically removed. The bacteria in the wastewater feed on the bottom sludge, thereby liquefying the waste. Partially purified water flows out of the tank into an underground drain, where further decontamination takes place. The system is relatively passive, compared to central facilities, and as in the case of artificial marshes, time is required for the processes to occur. In addition, nitrogen compounds are converted to nitrate, but the latter is not reduced to molecular nitrogen, so groundwater under the septic system can become contaminated by nitrate, as discussed earlier in this chapter.

Drugs in Wastewater from Sewage Treatment Plants

In recent years, trace concentrations of various drugs—prescription, over-the-counter, illegal, and veterinary—have been detected in the waters leading from sewage treatment plants, and in rivers and streams into which this water then flows, at concentrations up to the ppb level. About 100 substances have been detected in various rivers, lakes, and coastal waters. The substances—commonly including estradiol, ibuproferi, the antidepressant drug Prozac (fluoxetine), the anti-epileptic drug carbamazepine, and degradation products associated with cholesterol-reducing pharmaceuticals—are present in raw sewage after their excretion in urine or feces from humans and animals since most drugs are poorly absorbed and metabolized by the body. They also result from the disposal of unused or expired medication in toilets.

Most commonly, concentrations of drugs in drinking water are at the parts-per-trillion level, so their risk to human health is probably small. Research is under way to determine whether there could be effects on human health from sustained exposure to a combination of these substances. The synthetic hormones are thought to pose the greatest risk to aquatic species. Certain fish have been found to undergo some skewed sexual development due to exposure to sewage effluent containing the synthetic estrogen in birth control pills (see Chapter 12).

Continue reading here: The Treatment of Cyanides in Wastewater

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