Surfactants and the Environment
Larry N. Britton
CONDEA Vista Company, Austin, Texas
ABSTRACT:A large body of information on the environmental attributes of commercial surfactants has been generated in respon se to concerns about the environmental fate and effects of these high-volume chemicals. This review examines the scientific disciplines involved i n evaluating the environmental acceptability of surfactants and highlights the current issues.
JSD 1,109-117 (1998).
KEY WORDS:
Anaerobic biodegradability, biodegradation, endocrine disrupting chemicals, environmental impact, surfactants, toxicity.
______________________________________________
Surfactants are not generally viewed as a menace to the environment. Nonetheless, their environmental attributes often receiv e as much attention as their technical properties and economic aspects. One reason may be that the mental image of foaming streams and rivers, formed over three decades ago, has not faded entirely.
Then, too, public environmental awareness has increased markedly in recent years. In fact, environmentalism has transcended from a social attitude to an almost metaphysical level, or at least to a p osition that compares with spirituality A large portion of the public wants to "do the right thing," environmentally speaking. To this e nd, regulations are enacted and products are being designed and marketed. We have entered the age of environmental marketing and are forming attitudes about environmental attributes, sometimes without adequate comprehension of the scientific disciplines that is required.
Finally, the sheer mass of surfactants that can ultimately be released into the environment gives rise to interest in their e nvironmental impact. In 1995, the estimated global use of the major classes of surfactants was (1): linear alkylbenzenesulfonates (LAS), 1.77 mill ion metric tons (MMT); alcohol ethoxylates (AE), 0.35 MMT; alkylphenol ethoxylates (APE), 0.32 MMT; alcohol sulfates (AS), 0.3 MMT.
Much of the discussion of the environmental acceptability or preferability of surfactants centers on the standard against wh ich measurement is made and on the designation of what is environmentally important. MT; and alcohol ether sulfates (AES), 0.3 MMT.
This paper undertakes to explain the scientific processes for evaluating environmental acceptability and gives a broad overview of current environmental issues concerning surfactants.
DETERMINING THE ENVIRONMENTAL ACCEPTABILITY OF SURFACTANTS
- The term "environmentally acceptable" is not likely to have the same meaning to all persons. In the regulatory context, only those surfactants that pass some environmental muster can be designated envi ronmentally acceptable. Any process that evaluates the environmental properties will result in some sort of score and hence lead to debates on the meaning of such scores. Despite the pitfalls of interpretation and value judgments, a process for scientific evaluation has emerged.
The process begins with a consideration of the production of surfactants. The environmental impact of a surfactant can be examined in a Life Cycle Inventory (LCI), which is the well-defined part of LCA (interchangeably called Life Cycle Analysis or Ass essment). The European surfactant industry undertook an LCI in 1992 for the major classes of surfactants used in Europe. An LCI quantifies the energy and raw materials consumed to produce a surfactant and its feedstocks as well as the amounts of atmospheri c, waterborne, and solid waste emissions intrinsic to these processes. It is also a balance sheet that allows manufacturers to assess opportunitie s for improving the environmental profile of their surfactants and intermediates. Although an LCI is not intended to compare products it can be us ed to highlight excessive energy and materials consumption and/or emissions associated with a particular feedstock or process.
The results of the European LCI (2) are summarized in Table 1.
Table 1 Life Cycle Inventory Summary of Total Resource Requirements and Environmental Releases (a) Requirements Releases -------------------------- ----------------------------- Surfactant Raw Energy Consump Atmosph. Waterb. Solid Material (GJ/1000 kg) kg/1000 kg/1000 waste (kg/1000 kg) kg) kg/1000 kg) kg) ___________________________________________________________________ LAS (pc) 1040 61 1661 5 65 AS Pc 1091 73 2604 7 81 Oc 2077 57 1684 26 75 Ae(3)S Pc 1310 73 2335 5 68 Oc 2167 67 2054 24 96 Soap(Oc) 2167 50 4451 45 144 SAS (Pc) 1013 52 1261 2 64 AE(3) Pc 1448 83 2366 6 67 Oc,Pc 2401 73 2024 28 66 AE(7) Pc 1570 79 2281 6 64 Oc,Pc 2264 72 2062 21 64 AE(11) (Oc,Pc) 2064 82 2299 11 63 APG (Oc) 2060 63 2015 35 142 __________________________________________________________________ (a)Pc=Petrochemical; Oc=oleochemical; Oc,Pc=derived from both petro- and oleochemicals. This table is a summary of information from Tables 2-4 of Reference 2. Data cannot be considered as different when two values differ by less than 10% for energy and less than 25% for emissions. AES, alcohol ether sulfates; LAS, linear alkylbenzene sulfonates; AE, alcohol ethoxylates; AS, alcohol sulfates.The data show that no technical or scientific basis exists to support a general environmental superiority claim, either for an individual surfactant or for various options for sourcing from petrochemical, oleochemical, or agri cultural feedstocks and minerals. Therefore, environmental acceptability is not associated with raw material source, at least in the LCI context.< /P>
Most of the issues on environmental acceptability focus on the effects on the environment associated with the use and disposal of these surfactants. These effects are taken into account by a risk assessment. The first step in a risk assessment is an estimate of the concentrations of surfactants in the environmental compartm ents of interest, such as wastewater treatment plant effluents, surface waters, sediments, and soils. This estimate is generated either by actual measurement or by prediction via modeling. The measured or predicted concentrations are then compared to the concentrations of surfactant known to be toxic to organisms living in these environmental compartments. If the measured or predicted concentration is less than the no-toxic-effect level, then a margin of safety ex ists. Just to be sure, a safety factor of 10, 100, or 1000 is often applied. That is, the margin of safety between the actual, measured and no-eff ect concentrations should differ by one, two, or three orders of magnitude.
The most thorough aquatic risk assessment of currently used surfactants was undertaken by the Dutch government in cooperation with the European detergent and surfactant industries (3). This 5-year study consisted of four phases: (i) collection and evaluation of published and unpublished data from laboratory studies and field monitoring on environmental fate an d effects; (ii) estimation of surfactant concentrations in the aquatic environments by accepted modeling techniques; (iii) actual field monitoring of LAS, AE, AES, and soap in wastewater treatment plant effluents and downstream receiving waters; and (iv) calculation of safety margins. The ec otoxicological data were normalized to C11.6-LAS; C13.3EO8.2AE (EO 3D moles of ethylene oxide); and C12.5 EO3.4 AES. The Dutch government established a Maximum Permissible Concentration (MPC) and Negligible Concentration (NC) (100-fold less than the MPC; values at which adverse effects are deemed negligible) for the four surfactants in surface waters. These are shown in Table 2 together with the predicted environmental concentrations (PEC) which were d erived from monitoring data from sewage treatment plants, the average treated wastewater dilution, and the assumed in-stream removal rates.
The monitoring and modeling data showed that sewage treatment plants removed 99.1-99.8% of the surfactants As a result, the PEC of the four surfactants, after dilution and in-stream removal, were we ll below the MPC. Therefore, the Dutch government concluded that the use of these substances in laundry and cleaning products is acceptable if dis posed to normal-functioning sewage treatment plants. Similar conclusions have been reached by the British government (4).
Table 2 Max Permissible Concentration (MPC), Negligile Concentration (NC) and predicted Environmental Concentrations (PEC) for Dutch Surface Waters ________________________________________________________________ Ingredients MPC (ug/L) NC (ug/L) PECa (ug/L) *u= Greek letter "mu" LAS 250 2.5 3.7 AE 110 1.1 0.5 AES 400 4.0 1.2 Soap 27 2.7 20 a: PEC= predicted 90th percentile river water concentration in The Netherlands at 100m below wesage outfall with an in-stream removal rate of 0.70 day-1. See Table 1 for additional abbreviations.
The risk assessment approach can be summarized in two relationships using the terminology PEC and the Predicted No-Effect Concentration (PNEC) which is analogous to the MPC used by the Dutch authorities:
If PEC/PNEC x safety factor (1, 10, 100, ...)B31,then there is a risk.
If PEC/PNEC x safety factor (1, 10, 100,...)<1 then risk is acceptable.
- The process of conducting aquatic risk assessments on the major classes of surfactants is aided by the availability of doc uments that summarize toxicity data (5-16). Also, models, such as WWTREAT , have become indispensable for calculating PEC (17).
The LCI and risk assessment studies teach two things: (i) it is possible to quantify the environmental risk, and therefore environmental accept ability, of currently used and future surfactants; and (ii) the studies to date indicate that there are essentially no differences among the major surfactant classes.
ENVIRONMENTAL PROPERTIES USED IN EVALUATING ACCEPTABILITY
This topic has created many debates ranging from how to measure toxicity and biodegradability to the relevance of anaerobic biodegradability and the issue of endocrine disrupter activity. The important environme ntal properties are discussed below, together with brief explanations of the controversies.Toxicity. For the most part, surfactants are discharged into sewers and ultimately reach aquatic environments. Aquatic toxicity therefor e is an important property. The manufacture of a new surfactant could, for example, require aquatic toxicity data in the Premanufacturing Notifica tion (PMN) using an invertebrate (e.g., Daphnia or Ceriodaphnia for fresh water or mysid shrimp for marine system), a fish (e.g., fa thead minnow or silversides minnow), and an algal species. It could be argued that these few test organisms do not cover the range of sensitivitie s of the diverse organisms in the environment and that more sensitive species exist on which the risk assessment should be based.
This consideration has prompted ecotoxicologists to conduct larger-scale, more species-diverse testing known as mesocosm testing (18,19). With a larger test population it is possible to apply statistical analyses and to make decisions analogous to the U.S. Environmental Protection Agency (EPA) Water Quality Criteria (WQC). The WQC recognize that protection of 100% of a particular population exposed to wastewater components is not a chievable. Therefore, a policy was established to protect at least 95% (or 2 Cs) of the exposed population. Single-species testing of eight or more species is the approach by WQC to define the distribution curve. In typical a quatic mesocosm studies, an artificial stream is continuously dosed with the test chemical and the response of the population and other community endpoints are measured. When the number of monitored species is above 30, good statistical evaluation of the chemical's effects can be achieved. F igure 1 is an idealized distribution response of the population to illustrate this principle.
The MPC is that concentration at which at least 95% of the population shows no toxic effects. More precisely, it is the 95% threshold, measured with 50% confidence. This criterion would concede a small portion of the population. In exceptional situations where members of the sensitive or affected population represent endangered species, or are economically important, or if they contribute significantly to the function and/or struct ure of the ecosystem, the MPC may need to be lowered. In this approach, the ecosystem is treated as an entity, with the same redundancy, adaptive response, and ability to manage change as a single organism. The focus is on the community. Mesocosm testing has proven useful in verifying predic tions of laboratory tests and in advancing an understanding of the effect of stressors at higher levels of biological organization.
Endocrine disrupting chemicals (EDC). One of the newest issues in ecotoxicology is the ability of certain chemicals to mimic hormone activity and thus disrupt chemical signaling by hormones and hence the devel opment of the host organism. EDC can act at low levels and sometimes do not produce dose/response curves typical for toxicants that cause lethal e ffects. The timing of the dose is also important, especially for reproduction and developmental effects.
Surfactants and EDC were first mentioned in the same sentence by Professor John Sumpter, Brunel University in the United Kingdom (UK) (20) in s tudies on the feminizing effect (i.e., estrogenic activity) of wastewaters on roach (freshwater fish) in an industrialized river. Prior to this st udy, there had been considerable interest in the endocrine disrupter effects from PCB (polychloro biphenyls), dioxins, and other chlorinated xenob iotics. The EDC spotlight became especially bright when scientific work in this area (21) was publicized in the popular book Our Stolen Future (22). A recent article reported that most, if not all, effluents from sewage treatment works in the UK are estrogenic to fish (23). Sumpter92 s work targeted alkylphenol ethoxylates and their biodegradation intermediates, the alkylphenols, as the xenobiotics in wastewater that exhibited estrogenic activity (24,25). In his view, the logical connection to the estrogenic activity in the sewage treatment plant effluents was the presen ce of alkylphenol ethoxylates. However, when the effluents were fractionated and estrogenic activity assays and chemical identifications were perf ormed, it was discovered that the natural female hormones estrone and 17b-estra diol and the synthetic hormone ethinyl estradiol from birth control pills were the active agents (26). These anthropogenic yet natural substances were active at 10-100 ng/L. It was hypothesized that the glycosylated inactive forms of the hormones are excreted subsequently reactivated when bi ological activity in the wastewater treatment plants and riverine environments removes the inactivating carbohydrate moiety. A review of Sumpter's original data (20) comparing estrogenic activities of alkylphenols and their ethoxylates with 17b-estradiol reveals that these surfactants and their biodegradation intermediates are only weakly estrogenic (see Table 3).
Table 3 Relative Estrogenic Potencies of Alkylphenol Ethoxylates and Their Biodegradation Products Compound Potency (relative to estradiol) 17B-Estradiol 1 B= Greek letter "Beta" Nonylphenol ethoxylate (EO=9)a 0.0000002 Nonylphenol ethoxylate (EO=2)b 0.0000060 Nonylphenol carboxylate b 0.0000063 p-Nonylphenol b 0.0000090 p-Octylphenol b 0.0000370 p-tert-Butylphenol c 0.0001600 a: Intact surfactant. b: Biodegradation intermediates of nonylphenol ethoxylate and octylphenol ethoxylate. c: Listed in Reference 20, but neither surfactant nor intermediate.
In 1995 the EPA launched an initiative to investigate EDC. An interim assessment report issued in early 1997 listed potential hormone disrupters, most of them pesticides. According to the report, a causal relationship has not been established for human health effects, and new epidemiologic and laboratory studies are needed for a better defini tion of the scope of the problem. It is clear that the EPA will proceed cautiously when relating EDC with human health effects; however, it is unc ertain what regulatory actions will be taken at the federal and state levels for EDC that are designated as ecotoxicological risks except for thos e chemicals [dioxins, PCB, dichlorodiphenyltrichloroethane (DDT), and other chlorinated pesticides] that are already regulated.
Bioconcentration factor (BCF) and biomagnification. Since surfactants have a hydrophobic component, they may partition into lipids of organisms and bioaccumulate. If the surfactants are not catabolized, the possibility exists for magnification of potential toxicological effects up the food chain. The fear of bioaccumulation and biomag nification comes mainly from experience with chlorinated compounds, espec ially pesticides and PCBs. Only minimal experimental and monitoring infor mation has been gathered on the bioaccumulation properties of currently u sed surfactants. To date, the biomagnification of commercial surfactants has not been observed or predicted in aquatic systems.
To understand the BCF of a surfactant, the quantitat ive structure activity relationship (QSAR) can be used (27):
log BCF 0.79 x log P -0.40 [3]
where log P is the octanol/water partitioning coefficient. A BC F value of >1000 poses a concern for bioaccumulation, particularly if the chemical is considered persistent in aquatic environments. This BCF v alue corresponds to a log P of >4.3, which is close to that for surfactants. Therefore, surfactants will always be scrutinized for potential bioaccumulation effects.
In addition to bioaccumulation predictions, the log P for octanol/water partition has been used in QSAR studies for ev aluating the nonspecific modes of surfactant toxicity (i.e., toxicity due to nonreactivity). General and polar narcosis modes of toxicity of surfactants can be predicted in QSAR studies using calculated log P val ues (28).
Biodegradation. No other environmental property of surfactants has created as much controversy as biodegradability. Mu ch of the debate and confusion arises from the extrapolation of laboratory data to real-world situations and the association of detectable surfact ant levels with environ-mental decline. The confusion has been augmented further by "green" advertising for certain products. To prevent deceptive green advertising claims from misleading the public, the U.S. Federal Trade Commission issued guidelines in 1992 (Guidelines on Environmental Mar keting Claims, 16 CFR Part 260). These guidelines do not rigidly define biodegradability but rather combine the terms degradable, biodegradable, and photodegradable into a single definition: "The entire product or package will completely break down and return to nature, i.e., decompose into elements found in nature within a reasonably short period of time after customary disposal." The guidelines are elegantly simple and useful. Prob lems arise when surfactants are ranked
with respect to environmental attributes. Nonetheless, such ecolabeling schemes have arisen in Europe and in the United State s. To put such rankings into perspective, one must understand the definitions related to biodegradation and the test methodology. A good set of de finitions was given by Gilbert and Watson (29). These are listed below.
- Biodegradation: molecular degradation of an organic compound by living organisms.
- Biodegradability: the tendency of an organic compound to biodegrade.
- Mineralization: complete biodegradation of an organic compound to methane, and/or carbon dioxide, minerals and wa ter.
- Primary biodegradation: biodegradation of an organic compound to the extent that some characteristic property (e.g 2E, surfactancy) is destroyed.
- Recalcitrance: resistance of an organic compound to biodegradation.
- Ultimate biodegradation: biodegradation of an organic compound to produce methane, and/or carbon dioxide, water, biomass, and minerals.
- Inherent biodegradation: biodegradation of an organic compound in environmentally relevant conditions. The "inher ent biodegradation" term comes from the three-tiered testing protocols established by the Organization for Economic Cooperation and Development (O ECD). Inherently biodegradable compounds exhibit mineralization in more favorable conditions for biodegradation, but not necessarily in stringent conditions of laboratory screening tests.
- Ready biodegradation: biodegradation of an organic compound in simple, stringent tests specified by the OECD. This is a performance definition and does not necessarily imply recalcitrance if the compound shows negative results in the ready biodegradability screening tests.
Test methods. Numerous biodegradability test methods have been standardized by organizations such as the OECD, the American Society for Testing and Materials (ASTM), EPA, the International Organization for Standardization (ISO), Environmental Centre for Ecotoxicology and Toxicology of Chemicals (ECE TOC), and the Japanese Ministry of International Trade and Industry (MITI). The most commonly used methods are those published by the OECD which involve a three-tiered approach of (i) ready biodegradability (screening) tests; (ii) inherent biodegradability tests; and (iii) simulation (confirmation) tests. Table 4 lists the protocols in these tiers.
Table 4
Title Test parameter Definition or pass criteria
___________________________________________________________________
READY BIODEGRADABILITY
301A:DOC die away %DOC removal >= 70%DOC removal within 28d
and within 10-d window after
10% DOC removal is reached
301B:C02 evolution
(modified Sturm test) %CO2 production >=60% of thoretical CO2 production
within 28 d and with 10-d winddow
after 10% CO2 is reached
301C: MITI (i) %BOD removal >=60% thoretical BOD removal with
28 d
301D: Closed bottle %BOD removal >=60% thoretical BOD removal with
28 d and within 10- or 14-d window
after 10% BOD removal is reached
301E: Modified OECD
screening %DOC removal >=70% DOC removal within 10-d
window after 10% DOC removal is reached
301F: Manometric
respirometry %BOD removal >=60% theoretical BOD removal with
28d and within 10-d window after 10%
BOD removal is reached
INHERENT BIODEGRADIBILITY
302A:modified SCAS %DOC removal
Test in daily cycles 20-70% daily DOC removal during
12-wk testing
302B: Zahn=Wellens/EMPA %DOC removal 20-70% DOC removal within 23 d
302C: Modified MITI
Test (II) %BOD removal 20-70% BOD or parent compound
and/or % loss removal within 28 d
of parent
compound
SIMULATION (CONFIRMATION)
303A: Aerobic sewage %DOC Removal Degradation rate is calculated
treatment coupled
units test
______________________________________________________________________
a: OECD, Organization for Economic Cooperation and Development; DOC,
dissolved organic carbon; MITI, Japanese Ministry of International Trade
and Industry; BOD, biochemical osygen demad; SCAS, semicontinouus
activated sludge, EMPA, Swiss Federal Institute of Materials Testing and
Research.
Biodegradation rates. Except for method 303A, none of the OECD methods calculates biodegradation rates (k values or half-lives), yet an understanding of how fast a surfactant is removed from an environmental compartment is important. To meet an increasing need for predicting concentrations of organic compounds, including surfactants, in the various environmental compartments, investigators are proposing fate models that use biodegradation rate data and physicochemical pro perties of the organic compounds (17,30-32). However, the use of screening biodegradation data (e.g., OECD Ready Biodegradability tests) is not ad vised because the rates from these screening tests do not correspond to r ates in the actual environmental compartments (33-36). Even scaling facto rs cannot be used to "correct" the rates in the field to those from the l aboratory screening tests. As quoted in one recent reference (36): "While the Ready Test accurately predicted whether biodegradation would occur in these important environmental compartments, there was surprisingly no s tatistical relationship between the mineralization rates in the Ready Test and those in the actual environmental compartments." It was succinctly stated that correlation analyses revealed almost perfect noncorrelation between the rates observed in the Ready Test and those observed in the mor e realistic tests. Moreover, there was no correlation among realistic tests, making it doubtful that data from one environmental compartment could be extrapolated to another.
In light of the studies like those above, the assignment of environmental significance to surfactants that perform exceptionally well or poorly in screening tests is difficult to support since any rankings are relevant only within the confines of the test method.
Monitoring data. If simple laboratory tests cannot quantitatively predict the fate of organic compounds in environmental compartments, the logical approach is to monitor their actual concentrations in these compartments. Monitoring data for the currently used surfactants LAS, AS, AE, AES, AOS (a-olefinsulfonate), SAS (secondary alkanesulfonate), and soap are available to varying extent (more so for LAS). Kinetic analyses, however, are meager. Table 5 compares half-lives of LAS in riverine environments and activated sludge units compared to laboratory-generated rates.
The outstanding finding from Table 5 is the gap in rates between actual monitoring (first two columns) and laboratory tests : half-lives of 1-3 h vs. 1-4 days. Most scientists have concluded that laboratory screening tests are overly conservative in estimating biodegradation rates. Although the explanation for the slower rates in laboratory tests is not precisely known, lower biomass, batch conditions (vs. continuous operation), substrate types and concentrations, an d culture acclimation have been postulated as the possible reasons.
Table 5 Comparative Biodegration Rate Data for Linear Alkylbenzenesulfonates ___________________________________________________________________ T 1/2 for rivers T 1/2 for T 1/2 in Lab Test streams (ref #) ASU a (ref #) tests b (ref#) Descrip __________________________________________________________________ <0.5-3 h (37) 3.0 h (40) c 1.2-2.2 d (41,42) Batch activated sludge 1.2 h (38) 3-4 d (43)d Sturm ready CO2 1.0 h (39) 1.0 d (41) River die-away __________________________________________________________________ a. ASU, activated sludge unit b. Technical Gudance Document in support of the European Commission Directives 93/67/EEC and 1488/94 on Ris Assessment of new and existing substances specifies a T1/2 of 360 h for substances that are readily biodegradable in OECD screening tests. c. Average percentage ultimate biodegration in seven sewage treatments plants was 83% in 5 h hydraulic retention time. d. This value is similar to numerouls values reported for ready biodegradabilitiy tests.
Risk estimates that calculate the predicted environmental concentrations based on laboratory-generated rate constants will th erefore incorrectly overestimate the theoretical concentrations of the surfactants. Opportunities exist to collect monitoring data for widely used surfactants such as LAS, AS, AE, AES, AOS, AES and soap, but not for newly developed surfactants. One solution is to devise more meaningful rate- determining methods for the different environmental compartments such as the one Federle and Itrich (44) developed for activated sludge application.
Another solution is a semiquantitative assessment th at is referred to as a "practical biodegradation" model (45). This model relates biodegradation rates (assumed to follow pseudo-first-order kinetics), which are based on estimates, laboratory or monitoring data, of the time a surfactant spends in a particular environmental compartment (i.e., residence time). The basic equation is:
C/C03D e -0.693 CRT/BHL
where C/C0 is the removal ratio for a chemical, CRT is chemical residence time in the environmental compartment, and BHL is biodegra dation half-life.
The BHL is the time for the concentration of a given chemical to decrease to 50% of its original value. It is equal to In(2)/k1 , where k1 is the pseudo-first-order biodegradation rate, in units of reciprocal time. Inclusion of the CRT in the model focus es on the environmental compartment of interest and permits the investigator to determine quickly if biodegradation is a significant removal process. The practical importance of the model is the requirement that the BHL of a chemical must be less than its CRT in a particular environmental compartment or else there will be accumulation and obvious potential for ecotoxicological problems. Results of monitoring studies on chemical compounds, including surfactants, have of ten been interpreted as evidence of recalcitrance to biodegradation. A seminal publication on the fate of organic carbon in sediments appeared in Nature in 1994, entitled "Sorptive Preservation of Organic Matter in Marine Sediments" (46). The following key points emerged from this paper:
- Organic matter preserved in marine sediments accounts for about 20% of all carbon burial and plays a key role in balancing the long-term flux of oxygen to the atmosphere.
- Preserved organic material in sediments can be either intrinsically stable or stabilized by interaction with mineral matrices. Sorption to mineral surfaces stabilizes the molecules, slowing remineralization ra tes by up to five orders of magnitude. Sorptive protection accounts for t he enigmatic preservation of intrinsically labile molecules, such as amin o acids and simple sugars.
- Organic matter preserved in sediments for as long as 500 year c an be desorbed by repeated extractions with distilled water and subsequen tly mineralized by indigenous microorganisms within six days. Thus, recal citrance to biodegradation of otherwise biodegradable compounds is the re sult of sorption onto the surface of mineral grains.
It should not be surprising to detect widely used surfactants in sediments sorbed on mineral surfaces and associated with particulate organic matter (e.g., humics). Both sorption and association in effect remove surfactants from biological activity.
This phenomenon has also been observed with hydrophobic organic compounds residing in soils (47,48). Over time, organic compounds are sequestered by noncovalent mechanisms so that both solvent extra ction and biodegradation are impaired. Sequestration makes these compounds, including surfactants, less bioavailable both for biodegradation and a lso for toxic interaction with resident organisms.
Anaerobic biodegradability. Surfactants that do not degrade anaerobically have the potential for accumulating in anoxic environmental compartments like sediments and the subsurface. Results from laboratory anaerobic biodegradati on tests and monitoring observations of municipal anaerobic sewage sludge digesters show that sulfonated surfactants (LAS, AOS, SAS) are not amenable to biodegradative attack if molecular oxygen (02) is excluded from the vessels. Molecular oxygen is required for the initial attack on these molecules by oxygenase-catalyzed hydr oxylation of their alkyl groups. (An initial attack via anaerobic desulfonation to form anaerobically degradable intermediates is a potential, but still undemonstrated, mechanism for anaerobic biodegradation of these surfactants.) Some natural compounds such as lignin are also restricted in their availability to molecul ar oxygen for biodegradation, yet the carbon turnover of plant materials in surficial sediments and soil appears to proceed without undue accumulation. Monitoring data have shown the presence of LAS in subsurface (septic system) environments (49) and in pond (50) and river sediments (51,52), all of which have been described as "anaerobic." However, the concentrations found do not reveal accumulation above levels expected from loading on these environments. Continuous organic loading onto sediments or subsurface soils from sources, such as wastewater, will result in the deposit ion of sorbed LAS. As organic matter in these environments is mineralized, a continual enrichment in anaerobically nondegradable organics would be expected to occur. This has not been observed in practice. A reasonable hypothesis is that molecular oxygen is available in these niches, at least in sufficient quantity to participa te significantly in carbon turnover via aerobic catabolism. It is well-accepted that both aerobic and anaerobic processes occur in anoxic e nvironments.
Oxygen can enter sediments by means other than the obvious occurrence of sediment scouring and resuspension in the water column. Anoxic conditions in sediment occur when oxygen consumption by microb ial aerobic respiration exceeds oxygen transport. This is a dynamic process. Oxygen is constantly entering the sediments via the processes depicted in Figure 2 (not included):
- Diffusion from overlying oxygenated water. The depth of penetration is dependent on the degree of oxygen consumption, which is controlled directly or indirectly by resident microbes and the availability of oxidizable substrates.
- Introduction of oxygenated water via bioturbation.
- Gas ebullition with water pumping. Gas from m ethanogenic decomposition of organic substrates in sediments is released; water with dissolved oxygen is subsequently transported into the space vacated by the methane bubble.
- Introduction from roots of aquatic plants.
Even though the steady-state oxygen concentration is nil, aerobic activity can proceed in these sediments along with anaerobic metabolism that leads to production of methane and sulfide.
Although the amount of oxygen entering these anoxic environments is small in comparison with the potential rate of oxygen consumption, this oxygen can be expected to effect aerobic breakdown of orga nics such as LAS. This hypothesis was tested with 14C-labeled LAS in simulated anaerobic conditions where oxygen was introduced via permeation through a polymer membrane in an amount less than the oxygen demand by microorganisms (53). LAS was observed to mineralize, as evid enced by the production of 14C02 and 14CH4, even though at rates well below those under aerobic condit ions. Therefore, LAS can potentially be degraded in anoxic sediments as a result of the introduction of oxygen by the processes described above. T his finding points to the need for timed monitoring data in these compartments, just as monitoring data have been used to estimate the biodegradat ion rates in the water column.
Summary. Interest in the environmental fate of surfactants has generated a huge body of experimental work over the past three decades. In turn, much scientific knowledge has been accumulated and an effective framework has been e stablished for risk assessment and measurement of the various interactions of surfactants with the environment. The overall conclusion is that, despite the huge volumes of surfactant s entering the environment, they do not represent a serious threat to it.
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[Received October 25, 1997; accepted November 24,1997]
Larry Britton is a research associate in the R&a mp;D group of CONDEA Vista Co. where he directs research on environmental topics. He has a Ph.D. in microbiology with 20 years experience i n biodegradation. enzymology, bioremediation, and other environmental applications.