Rcoch2

O II

O II

RC-O-CH2

3RCO Na Soap

HO-CH2

HO-CH2 Glycerol

The sodium salt of stearic acid, CH3(CH2)16C02H, is the major product released upon hydrolysis of animal fat, while the sodium salt of oleic acid, CH3(CH2)7CH = CH(CH2)7C02H, is the main product from the hydrolysis of olive oil. Hydrolysis of palm oil yields approximately equal amounts of the salts of oleic acid and palmitic acid, CH3(CH2)14C02H.

These naturally occurring fatty acids contain linear hydrocarbon chains, which are synthesized biochemically from the acetate ion (CH3C0^). They are also readily degraded back to acetate by microorganisms in the environment. Since long-chain ("fatty") acids occur naturally, it is not surprising to find some foam and suds at waterfalls, the surf at the beach, and other places of turbulence in streams, rivers, and oceans owing to the presence of natural soaps in the water.

The main disadvantage of the use of salts of carboxylic acids as cleansing agents is the insoluble precipitates that form with Ca2+ and Mg2+, which when present in water make it hard. This curdy, gray precipitate leaves a deposit on clothes or "ring" around collars or bathtubs. Not only are the deposits undesirable, but greater amounts of soap must be used to make up for that lost by precipitation. Another disadvantage of soaps is that they are salts of weak acids that protonate in mildly acidic solutions. The polarity of the "head" of the detergent decreases when protonated and, as a consequence, the product's effectiveness at solubilizing oils and greases is diminished.

Detergents are a mixture of synthetic surfactants, builders, bleaches, enzymes, and other materials designed to enhance the specific cleaning power of this mixture. Millions of pounds of these materials are produced in the United States each year (Table 7-1). The main advantage of synthetic surfactants over soaps is that the former do not precipitate in hard water. Since, however the cleansing power of synthetic surfactants is also decreased markedly by Ca2+ and Mg2+, it is necessary to add builders that enhance the cleansing action of synthetics by inactivating the Ca2+ and Mg2+ in hard water. The builders are polycarboxylic acids, silicates, zeolites (inorganic aluminosilicates), or polyphosphates (e.g., Na5P3O10; Section 10.5), which bind Ca2+ and Mg2+ thus preventing them from binding to the surfactant. Polyphosphates, which were used extensively as builders in the 1970s, were replaced in large part by zeolites because it was believed that phosphate from polyphosphates initiated algal blooms on lakes (Section 9.1.2), with the resulting eutrophication. Since the phosphate in detergents is easily removed by treatment with Fe3+ at sewage disposal plants, a process that has been in use in Sweden for many years, it may have been premature to ban the use of polyphosphates. It should also be noted

TABLE 7-1

U.S. Detergent Chemical Use in 1996

7.2.2 Detergents

Amount

Builders Surfactants

Bleaches, brighteners, and enzymes Fragrances and softeners Other

3805 2000" 190 128 260

^Surfactant use in 2001 projected to be 2400 x106 lb. Source: E. M. Kirschner, Chem. Eng. News, p. 39, Jan. 26, 1998.

that detergents account for only 20-25% of the phosphate in lakes and rivers, while the bulk of the remainder comes from fertilizer and animal waste runoff from farms. Consequently, the phosphates in detergents are not the main cause of lake eutrophication. In accordance with the view that phosphates have a minor role in lake eutrophication, these compounds have been classified as ecologically acceptable for use in home laundry detergents in Europe. Currently 25% of the home laundry detergents sold in Europe contain phosphates.

It is reported that zeolites are not as effective as polyphosphates in enhancing the cleansing power of surfactants and that they have environmental problems as well. Studies suggest that zeolites promote surface algal blooms in the Adriatic Sea. In natural waters they bind naturally occurring humic acids (Section 9.5.7) to form particles that float. Microorganisms that bind to these particles grow and produce a foul-smelling algal bloom.

7.2.3 Surfactants

7.2.3.1 The General Nature of Surfactants

It has been possible to synthesize a wide variety of surfactants by varying the structure of both the polar and nonpolar ends of the molecule. The polar end of the linear alkylsulfonates is the anionic sulfonate group RSOj. The linear alkylsulfonates [see later: reaction (7-7)] and alcohol sulfate

and the related alcohol ether sulfates

RO(CH2CH2CH2)BCH2CH2OSOiTNa+

are the major laundry surfactants used in the United States (Table 7-2). The nonionic surfactants, alcohol ethoxylates [RO(CH2CH2)BCH2CH2OH], and alkylphenol ethoxylates [see later: reaction (7-11)], which have alcohol groups as the polar entity, are also produced in large amounts (Table 7-2). Smaller amounts of cationic surfactants [CH3(CH2)BCH2N(CH3)3Cl_] are also manufactured. These are not as efficient as the anionic and nonionic cleaning agents but are used as germicides and fabric softeners.

7.2.3.2 Synthesis of Linear Alkylsulfonates (LAS)

Alkyl benzene sulfonates (ABS), the original slowly degrading surfactants, and linear alkylsulfonates are prepared by similar procedures. The alkyl chain in both is prepared by the oligomerization (formation of short polymers) of

TABLE 7-2

Laundry Surfactant Use in the United States in 1991

TABLE 7-2

Laundry Surfactant Use in the United States in 1991

Surfactant

Amounts (lb x 106)

Percent

Linear alkylsulfonates

740

40

Alcohol sulfates

220

12

Alcohol ether sulfates

260

14

Alcohol ethoxylates and alkylphenol ethoxylates

390

21

Other anionics

200

11

Source: Modified from A. M. Thayer, Chem. Eng. News, p. 33, Jan. 25, 1993.

Source: Modified from A. M. Thayer, Chem. Eng. News, p. 33, Jan. 25, 1993.

propene (propylene), but with the ABS derivatives a branched oligomer is formed in the following acid-catalyzed process:

CH3 ch3

CH3CH=CH2 + H+ -- CH3CH+ CH3CH=CH2 . (CH3)2CHCH2CH+

CH3 CH3

(CH3)2CH(CH2CH)2CHCH=CH2 + H

It is not possible to synthesize linear hydrocarbon molecules by the straightforward acid-catalyzed oligomerization of olefins because the more substituted secondary carbocation is formed, with the formation of a branched chain hydrocarbon as shown in reaction (7-2). The branched-chain oligomers were used because they were readily prepared in acid-catalyzed reactions.

Linear hydrocarbons can be synthesized or separated from petroleum. They can be synthesized from olefins by use of catalysts such as triethylaluminum to give either alkanes or alkenes. In the Alfol process, linear alcohols are formed by the air oxidation and subsequent hydrolysis of the oligomers formed initially [reactions (7-3) and (7-4)]. Olefins can be formed by pyrolysis of the trialkylaluminum adduct in the presence of ethylene in reaction (7-5).

CH2CH3

CH3CH2-A1-CH2CH3 + «ch2=ch2—►-A1-(CH2CH2)„CH2CH3

CH3CH2(CH2CH2)b-1CH=CH2 + Al(CH2CH3)3

Urea crystallizes in a unique fashion from hydrocarbon solutions, and this is the basis of a method of separating linear and branched hydrocarbons in petroleum fractions. Linear hydrocarbons may separate from the branched-chain kind in petroleum fractions by encapsulation in the urea crystals. The urea crystallizes as helices, with the linear hydrocarbons encased in the central channel of the tubes formed by these helices. Branched-chain hydrocarbons do not fit into the 5-A-diameter tube, so the linear hydrocarbons can be separated by merely filtering out the urea crystals. "Lightly branched" hydrocarbons with more than 10 carbon atoms will also form urea inclusion compounds and these are also present in the mixture of "linear" hydrocarbons separated by this procedure.

The use of molecular sieves has provided a cheaper means for the separation of linear and branched hydrocarbons than the use of urea. Molecular sieves are semipermeable zeolites that have a channel structure similar to that of urea when it is crystallized from hydrocarbons. The molecular sieves with a diameter of 5 A (0.5 nm) are used to selectively screen linear hydrocarbons from petroleum.

Virtually the same procedures are used to complete the synthesis of both the ABS and linear alkylsulfonate detergents. The propene or other olefin oligomer is attached to benzene in an acid-catalyzed Friedel-Crafts reaction (7-6), the alkylated benzene is sulfonated with sulfur trioxide, and then, in reaction (7-7), the sulfonic acid product is neutralized with NaOH to generate the anionic surfactant.

CH3 I

7.2.3.3 Synthesis of Nonionic Alcohol Ethoxylates

The alcohol and alkylphenol ethoxylates, the largest group of nonionic detergents currently in use, are prepared from the anion of the alcohol or alkylphenol and ethylene oxide.

Reactions (7-8)-(7-11) (n = 5-10) outline the synthesis of the nonylphenol ethoxylate, one of the major nonionic detergents manufactured in the United States.

7.2.4 Microbial Metabolism of Hydrocarbons, Soaps, and Synthetic Surfactants

Microorganisms can utilize the energy of oxidation released by some hydrocarbons, soaps, and surfactants as well as some of the organic fragments released during their growth. The metabolism of these compounds is affected by enzymes—high molecular weight proteins that catalyze a broad range of chemical transformations in living systems. Enzymes are characterized by their extreme catalytic efficiency (the catalyzed reactions typically proceed about 109 times as fast as the uncatalyzed reaction) and specificity. (There is usually a specific enzyme for each chemical transformation that takes place in a cell.) Many enzymes require a small, nonprotein prosthetic group, called a coenzyme, for catalytic activity. Coenzymes are bound to the enzyme during the course of the reaction, and sometimes they combine with one or more of the reacting chemical species.

Fatty acids (including protonated soaps) occur widely in biological systems and are therefore readily degraded by microorganisms in enzyme-catalyzed reactions. Linear hydrocarbons and linear detergents are similar in structure to soaps, and these are degraded in the same or similar pathways.

Ethylene oxide C9H19—f \-OCH2CH2O- +

C9H19—/ O(CH2CH2)nCH2CH2O"

c9hI9—i O(CH2CH2)nCH2CH2O" H

C9HI9—( O(CH2CH2)nCH2CH2OH

O II

RCH2CH2CO2H + CoASH -► RCH2CH2CSCoA

II II

RCH2CH2CSCoA + FAD -- RCH=CHCSCoA + FADH2

II II II II

RCCH2CSCoA + H2O -- RCOH + CH3CSCoA

RCH=CHCSCoA + H2O -- RCHCH2CSCoA

The steps in the oxidative metabolism of fatty acids are outlined in reactions (7-12)-(7-16). Each of these reactions is catalyzed by an enzyme even though the enzyme is not specified in the reaction. Coenzyme A (CoASH) is a complex structure that contains a thiol group (—SH) as the reactive center. The acetyl coenzyme A [CH3C(O)SCoA] produced in reaction (7-16) may be used by the microorganism for the synthesis of the specific fatty acids needed for growth by a pathway that is essentially the reverse of the one shown here. Alternatively, the acetyl CoA may be oxidatively degraded to carbon dioxide and water to provide the energy needed to drive microbial metabolic processes. Flavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide (NAD+) are the oxidized forms of hydrogen transfer coenzymes, while FADH2 and NADH are the respective reduced forms.

Originally it was believed that branching in the alkyl chain of alkylbenzene sulfonate (ABS) detergents impeded specific steps in reactions (7-12)-(7-16) and that this was the reason for their slow environmental degradation. It was proposed that the branched hydroxyester reaction formed in (7-17) would not be oxidized further to the keto derivative [see reaction (7-15)] and biodegradation would be halted at this roadblock. However, it has been found that microorganisms are able to get around these simple barriers either by oxidatively removing the methyl groups or by cleaving off propionyl CoA [CH3CH2C(O)SCoA] in place of acetyl CoA. Chain branching merely slows the degradative process.

II I II

CH3 ch3

The biodegradation of surfactants has been studied in considerable detail, but their structural diversity, even within specific types, makes it difficult to formulate guidelines for biodegradation based on structure. These studies are further confused by the need to "acclimate" the bacteria population with the surfactant to ensure that consistent results are obtained. The need for acclimation suggests that all the members of the population are not able to metabolize the surfactant and acclimation promotes the growth of strains of microorganisms that are able to degrade it. The following guidelines have been developed to facilitate understanding of the microbial degradation of ABS and LAS detergents.

1. Highly branched ABS detergents degrade slowly in part because the alternative pathways required to cleave the side chain methyl groups proceed more slowly than the route shown in reactions (7-12)-(7-16).

2. The ease of cleavage of the surfactant increases, the further the polar sulfate or sulfonate grouping is from the alkyl terminus of the chain. Consequently, surfactants with branched chains that are shorter than those with linear chains containing the same number of carbon atoms degrade more slowly. This rule may be related to the ease of binding of the surfactant to a degradative enzyme if the site for binding its polar group is in a hydrophilic region of the enzyme and the site for binding its hydrophobic end is in a hydrophobic region of the enzyme.

3. The rate of degradation of surfactants is slower in concentrated solutions than in dilute solutions. This finding suggests that minor constituents of the surfactant mixture inhibit the microbial degradative enzymes. Their concentrations are not high enough in dilute solutions to inhibit all the microbial enzyme present.

These guidelines also apply to the alkyl substituent of the alkyl- and alkyl-phenol ethoxylates. The alkyl chain will degrade slowly if it is highly branched, and then the ethoxylate grouping will degrade first. The rate of degradation of the alkyl chain decreases as the number of ethoxylate groups increases. For example, the extent of degradation of the ethoxylate chain of an alkylphenol ethoxylate decreases as the number of ethoxylate groupings increase from 5 to 20. This decrease may reflect the increased difficulty of transport of the more hydrophilic detergent (with more ethoxylate groupings) through the hydro-phobic cell membrane of microorganisms.

The alkylphenol ethoxylates degrade more slowly than the alcohol ethox-ylates. In some cases it has been possible to isolate what appear to be intermediates in the degradation of the alkylphenol ethoxylates in which one or two ethoxylate groupings remain attached to the alkylphenol. These compounds have little surfactant action or solubility and precipitate with the other sludge at the waste disposal plant (Section 11.5.2). Their anaerobic (nonoxygenic) degradation product, nonylphenol, has been found in sewage sludge [see reaction (7-18)].

C9H19—i \—OCH2CH2OCH2CH2OH ^ • C9H19—i \—OH

Nonylphenol

[H] = microbial degradation under reducing conditions

Nonylphenol is toxic to fish and other marine life, and it may be an environmental hazard when it is leached from sewage sludge that is dumped on land or in the ocean. It is also has estrogenic hormonal activity in mammals (Section 8.5.1). It is not clear whether the nonylphenol released by the degradation of nonionic surfactants is present in sufficient quantities in the environment to cause estrogenic effects.

While the biochemical pathway for the degradation of ethoxylate surfactants has not been determined, some insight into the mechanism can be obtained from studies on the degradation of glycols and simple ethoxylate oligomers shown in reaction (7-19).

ROCH2CH2OH ^ ROCHCH2OH ^ ROCHCH ^ ROCHCOH —

R = HOCH2CH2OCH2CH2O-, where [O] i ndicates oxidation.

The microbial breakdown of detergents and petroleum requires the terminal oxidation of the hydrocarbon chain to a carboxylic acid group to initiate the oxidation procedure outlined in reactions (7-12)-(7-16). This terminal oxidation is a well-documented process, and in some instances it has been established that the corresponding alcohol and aldehyde are reaction intermediates. Molecular oxygen is the oxidizing agent, and the oxygen is activated by binding to the iron-containing enzyme cytochrome P-450. The reaction follows the following sequence:

rch2ch2ch3 RCH2CH2CH2OH —► RCH2CH2CO2H —- (7-20)

R2CO2H + CH3CO2H

As discussed in Section 6.4, these reactions are carried out in microorganisms, but are slow, especially with high molecular weight materials that do not readily pass through the microbial cell wall.

The microbial oxidation of the cyclic hydrocarbons and aromatic compounds present in detergents and petroleum is also catalyzed by the enzyme cytochrome P-450 in microorganisms. It has been established in some cases that arene oxide intermediates are formed, as in the following reaction sequence for the oxidation of benzene.

cytochrome P-450

cytochrome P-450

Arene oxide

CH3CO2H +

II II

HOC—CCH2CO2H

Sco:H

Catechol

Catechol

Cco2h

CO:H

The microbial oxidation of the more highly condensed aromatics such as naphthalene or of the alkyl-substituted benzenes proceeds via salicylic acid, which is then oxidized to catechol in accordance with reaction (7-22) and finally to oxaloacetate.

CO:H

O^oH

Salicylic acid

Catechol

Catechol

The sulfate esters in alcohol sulfates are readily hydrolyzed by microbial sulfatase enzymes. Sulfonates present in the surfactants are subject to rapid oxidative elimination by a variety of aquatic bacteria:

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