Vet 1 ENVIRONMENTAL AND HUMAN HEALTH ASPECTS OF COMMERCIALLY IMPORTANT SURFACTANTS Andrew Sivak, Muriel Goyer, Joanne Perwak, and Philip Thayer Arthur D. Little, Inc. Acorn Park, Cambridge, MA. 02140 Seven types of surfactants comprise the majority of those presently used in commercial detergent formu­ lations . These are linear alkylbenzene sulfonates (LAS), alkyl sulfates (AS), alcohol ethoxylates (AE), alkyl phenol ethoxylates (APE), alcohol ethoxy sulfates (AES), alpha olefin sulfonates (AOS) and secondary alkane sulfonates (SAS). LAS surfactants, the main­ stay of detergent components, have been in use the longest and their paths of biodegradation are relatively well understood . Environmental levels of methylene blue active substances, the most commonly employed but non- specific measure of .anionic surfactant con­ centration, indicate that LAS are readily biodegradable. Nonionic (AE and APE) and the anionic AES and AS sur­ factants are also biodegradable with APE degrading somewhat more slowly than the others. Acute toxic effects to aquatic life forms generally occur in adult vertebrates and invertebrates at surfactant concen­ trations from 1 to 20 mg/L; juvenile and developmental stages show effects at somewhat lower concentrations. Studies with mammalian species have shown acute oral toxicity to occur at doses of 650 to > 25,000 mg/kg. No evidence for carcinogenic, mutagenic or teratogenic effects has been noted. The present domestic and commercial use of these surfactants appears to pre- sent no problems with respect to human health and the aquatic environment. 161 162 A. SIVAK ET AL. INTRODUCTION Five billion pounds of surface-active agents are produced in the United States annually. Sixty-five percent of this vol­ ume is of the anionic type; an additional twenty-eight percent is in the nonionic category.l Three anionic surfactants are prime components of almost all types of household detergent products, and as such, these surfactants find their way into the environment with the resulting possibility of environmental and human exposure. These three anionic surfactants are the linear alkylbenzene sulfonates, alkyl sulfates and alpha olefin sulfonates (Figure 1). The linear alkylbenzene sulfonates (LAS) represent a substantial portion of today's surfactant market; approximatell 640 million pounds were produced in the United States in 1978. First introduced in 1965 to replace the more slowly biodegradable tetrapropylene-derived alkylbenzene sulfonates (ABS), LAS are a complex mixture of isomers and homologues whose proportions are dependent on starting materials and reaction conditions. The LAS in commercial use contain linear alkyl chains ranging from 10 to 14 carbons in length with phenyl groups placed at various internal carbon positions in the alkyl chain.2 The alkyl sulfates (AS) are widely used in specialty products such as shampoos, cosmetics, toothpastes, etc., and are also extensively utilized as wool-washing agents. The bulk of AS in use are linear, prima~ AS, but some linear and branched secondary AS are also utilized. Primary AS are typically prepared by conventional sulfation of the parent alcohol with either sulfur trioxide or chlorosulfonic acid; secondary AS are more readily prepared by reacting the parent alkene with sulfuric acid. Alpha olefin sulfonates (AOS) are relative newcomers to the domestic synthetic detergent industry. However, recent development of continuous, short contact sulfur trioxide sul­ fonation processes and increased availability of high purity alpha olefin feedstock have made AOS surfactants competitive with other surfactants presently on the market. The highly exothermic, direct sulfonation of linear ~-olefins with a dilute stream of vaporized sulfur trioxide can follow several paths, leading to a variety of reaction products.z Commercial AOS formulations contain a mixture of alkene sulfonates and hydroxyalkane sulfonates. In addition, trace amounts of alkene disulfonates, hydroxyalkane disulfonates, saturated sultones and unreacted a-olefins may also be present. 2 AOS possess good detergency and foaming characteristics in hard water and are utilized in heavy duty powder detergents of low phosphate content. ENVIRONMENTAL AND HUMAN HEALTH ASPECTS 163 A fourth type of anionic surfactant, the secondary alkane sulfonates (SAS), is largely limited to the European continent at this time. SAS are predominantly linear with the sulfonate group attached to a secondary carbon and randomly positioned along the carbon chain. They are produced commercially via a 2 sulfoxidation reaction with n-paraffins in the c14-c18 range. SAS have a good detergency and foaming properties as well as high water solubility characteristics and are principally used as components of liquid detergent formulations. In addition to these four types of anionic surfactants, the balance of the household surfactant market is composed of two nonionic surfactants, the alcohol ethoxylates and alkylphenol ethoxylates and an anionic-nonionic hybrid, the alkylethoxy sul~ fates which are anionic in character. The nonionic surfactants, particularly the alcohol ethoxylates, have found wide use in newer detergent formulations due to their superior cleaning of man-made fibers, their tolerance of water hardness and their low foaming properties. Prepared commercially by reaction of an alcohol and ethylene oxide, some 476 million pounds of mixed linear alcohol ethoxylates (AE) are produced annually in the United States.l _ The second category of nonionics, the alkylphenol ethoxy­ lates (APE) have been largely replaced in domestic household products by the more rapidly biodegradable AE although they still find considerable use in industrial and agricultural applications. 2 Commercially, alkylphenols are manufactured by the addition of phenol to the double bond of an olefin in the presence of a catalyst such as boron trifluoride; the alkyl­ phenol is purified by distillation, then reacted with several moles of ethylene oxide to produce APE. Perhaps the fastest growing volume usage of the surfactant market are the alkyl ethoxy sulfates (AES).l They are known for their reduced sensitivity to water hardness, and their high foaming capabilities and are used primarily as components of liquid dishwashing ~roducts, shampoos and other household specialty products. They are prepared commercially by ethoxy­ lation of a fatty alcohol followed by sulfation with sulfur trioxide or chlorosulfonic acid. A total of 285 million pounds of AES were produced in 1978.1 ANALYTICAL PROCEDURES The data presently available on the very low residual concentrations of surfactants and their biodegradation products in the environment suggest that their use poses no threat to 164 A . SIVAK ET AL. LINEAR ALKYLBENZENE SULFONATES (LAS) CH - (CH ln- CH - OS0 Na 3 2 2 3 ALKYL SULFATES (AS) n = 10-16 CH - (CH )n -CH = CH- CH S0 Na 3 2 2 3 65-65% Alkene Sulfonate ALPHA OLEFIN SULFONATES CH - (CH )n -yH- CH CH S0 Na 3 2 2 2 3 (AOS) OH 35-40% Hydroxyalkane Sulfonate n = 12-16 CnH2 n+ 1 S03 Na SECONDARY ALKANE SULFONATES n = 14-18 (SAS) CH - (CH )n- CH - (O-CH )x OS0 Na 3 2 2 2 3 ALCOHOL ETHOXY SULFATES n = 8-16 (AES) X= 2-4 ALKYLPHENOLETHOXYLATES n =5-7 (APE) X = 4-30 CH - (CH )n- CH - (O-CH -CH )x OH 3 2 2 2 2 ALCOHOLETHOXYLATES n = 6-16 (AE) X= 3-20 Figure 1. Name and structure of representative surfactants. 2 ENVIRONMENTAL AND HUMAN HEALTH ASPECTS 165 2 environmental quality . Several analytical procedures are available for the determination of surfactant concentrations in the environment. They can be categorized into three major areas: physical methods, chemical techniques and physico~ chemical analyses . Two physical methods of analysis utilized to assess presumptive levels of both anionic and nonionic surfactants in the environment are foaming potential and surface tension. Although the residual foaming potential of a partially degraded surfactant can be used as a measure of biodegradation, the usefulness of this method is limited. A transient phenomenon, foaming can be affected by a wide variety of factors such as temperature, humidity, size of test container, etc.; further­ more, foaming often is not a linear function of surfactant concentration.2,3 Changes in surface tension can also be used as a measure of biodegradation. This procedure is based upon the fact that the presence of a few parts per million of a surfactant sig­ nifi cantly lowers the surface tension of water. The magnitude of this change increases (although not in a linear fashion) with an increase in the concentration of surfactant until a critical micelle concentration is reached; above this concen~ tration, further increases in surfactant concentration produce little or no change in surface tension.3 Measurement of sur­ face tension is quick and qualitative in that each surfactant has a characteristic ability to lower surface tension. The major drawbacks of the method are the lack of specificity, insufficient sensitivity to distinguish minute changes in surfactant concentration as biodegradation proceeds and the ease with which foreign substances can distort results.4 By far, the most widely used method for the determina­ tion of anionic surfactants in environmental samples, is the methylene blue method . z Methylene blue is a cationic dye which, in the presence of anionic materials, forms a salt which is readily extractable into organic solvents. Measurement of the intensity of the blue color in the solvent provides a measure of the amount of anionic material present. Because of its simplicity