Environ. Sci. Technol. 1990, 2 4 , 1241-1246
(23) Sakata, K.; Katayama, A. J . Colloid Interface Sci. 1987, 116, 177-181. (24) Sakata, K.; Katayama, A. J . Colloid Interface Sci. 1988, 123, 129-135. (25) Hough, D. B.; Rendall, H. M. In Adsorption from Solution
at the SolidlLiquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: London, 1983; pp 247-319. (26) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; Wiley: New York, 1974; pp 136-238. (27) Ghosal, D. N.; Mukherjee, S. K. J. Indian Chem. SOC.1972, 49, 569-572. (28) Hayes, M. H. B.; Pick, M. E.; Toms, B. A. J . Colloid Interface Sci. 1978, 65, 254-265. (29) Narine, D. R.; Guy, R. D. Clays Clay Miner. 1981, 29, 205-2 12. (30) Karickhoff, S. W.; Brown, D. S. J. Environ. Qual. 1978, 7, 246-252. (31) Brown, D. S.; Combs, G. J. Environ. Qual. 1985,14,195-199. (32) Bayer, 0.;Hoffman, H.; Ulbricht, W. In Surfactants and
Solutions; Mittal, K. L., Bothorel, P., Eds.; Plenum: New York, 1986; Vol. 4, pp 343-348. (33) Ford, W. P. J.; Ottewill, R. H.; Parreira, H. C. J . Colloid Interface Sci. 1966, 21, 522-533. (34) Fuller, C. C.; Davis, J. A. Geochim. Cosmochim. Acta 1987,
(39) Adamson, A. W. Physical Chemistry of Surfaces, 3rd ed.; Wiley: New York, 1976; p 389. (40) Westall, J. In Geochemical Processes at Mineral Surfaces; Davis, J. A., Hayes, K. F., Eds.; A Symposium Series 323; American Chemical Society: Washington DC, 1986; pp 54-78. (41) Westall, J. In Aquatic Surface Chemistry; Stu", W., Ed., Wiley: New York, 1987; pp 3-32. (42) Westall, J. C. FITEQL: A Computer Program for Determination of Chemical Equilibrium Constants from Equilibrium Data; Version 1.2. Report 82-01; Oregon State University: Corvallis, OR, 1982. (43) Westall, J. C. FITEQL: A Computer Program for Determination of Chemical Equilibrium Constants from Equilibrium Data; Version 2.0. Report 82-02; Oregon State University: Corvallis, OR, 1982. (44) Fuerstenau, D. W.; Healy, T. W.; Somasundaran,P. Trans. Metall. SOC.AIME 1964, 229, 321-325. (45) Chen, H.; Zhang, W.; Collier, J. M.; Brownaweil, B. J.;
Westall, J. C., manuscripts in preparation. (46) Tanford, C. The Hydrophobic Effect: Formation of Mi-
celles and Biological Membranes; Wiley-Intencience: New York, 1980; pp 96-105. (47) Hand, V. C.; Williams, G. K. Environ. Sci. Technol. 1987,
51, 1491-1502. (35) Hassett, J. J.; Means, J. C.; Banwart, W. L.; Wood, S. G.
Sorption Properties of Sediments and Energy-Related Pollutants; EPA-600/3-80-041; U S . Environmental Protection Agency; National Technical Information Service:
Springfield, VA, 1980. (36) van Olphen, H., Fripiat, J. J., Eds. Data Handbook for Clay Materials and Other Non-Metallic Minerals; Permagon:
New York, 1979. (37) Gschwend, P. M.; Wu, S.-C.Environ. Sci. Technol. 1985, 19,90-69. (38) Travis, C. C.; Etnier, E. L. J . Environ. Qual. 1981,10,8-17.
21. 370-373.
Received for review June 5, 1989. Accepted March 27, 1990. Although the research described in this article has been funded in part by the United States Environmental Protection Agency through assistance agreement CR-814501 to Oregon State University, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred, The grant was administered through the R. S. Kerr Environmental Research Laboratory in Ada, OK.
Structure-Activity Relationships for Biodegradation of Linear Alkyl benzenesulfonates Robert J. Larson Environmental Safety Department, The Procter and Gamble Company, Ivorydale Technical Center, Cincinnati, Ohio 45217
The kinetics of mineralization of uniformly 14C-ringlabeled linear alkylbenzenesulfonate (LAS) were studied in river water and sediments for a series of pure chainlength homologues and phenyl isomers containing 10-14 carbon atoms in the alkyl chain. Degradation of LAS in river water and sediments showed little variation among different homologues and isomers. Half-lives for mineralization of the benzene ring varied a maximum of 2-fold (15-33 h) and were not significantly different in either water or sediments. Competition experiments indicated that degradation of long- and short-chain homologues occurred independently and was not significantly affected by high concentrations of a competing homologue.
Introduction Linear alkylbenzenesulfonate (LAS), e.g., 1, is an anionic surfactant that has been widely used by the detergent industry in laundry and cleaning products. It was initially developed in the mid-1960s as a readily biodegradable replacement for branched-chain alkylbenzenesulfonates (ABS). LAS is currently the major surfactant used in the United States, Western Europe, and Japan, with annual production volumes of approximately 1.4 million tons (1). Since its introduction, the biodegradability of LAS has been studied in a variety of screening test systems (2-6). 0013-936X/90/0924-1241$02.50/0
n = 7 - 1 1
SO3- No+ 1
Laboratory studies have also been conducted to characterize the metabolic pathway for LAS biodegradation and the influence of LAS structure on biodegradation rates (1, 7). Many of these studies, however, have utilized relatively artificial experimental conditions (e.g., high LAS concentrations, synthetic nutrient media, pure cultures of microorganisms), which do not simulate the conditions found in natural environmental systems. Much of the work has also focused on primary degradation of mixtures of LAS homologues. Relatively little information is available on the kinetics of ultimate biodegradation (mineralization to COz) of the LAS benzene ring. This latter step, however, is quite important, since it represents the final step in the LAS biodegradation pathway prior to complete mineralization (7). This paper reports the results of studies to characterize the kinetics of biodegradation of a range of pure chain
0 1990 American Chemical Society
Environ. Sci. Technol., Vol. 24, No. 8, 1990
1241
Table I. Characterization Data for River Water and Sediment Samples Used in Biodegradation Studies parameter
valuea
River Water PH total organic carbon, mg/L total suspended solids, mg/L hardness (as CaCo3), mg/L total viable organisms, CFU/mL
(7.8-8) (3-5) (8-160) (279-468) (0.8-2.4 X lo4)
Sediment total organic carbon, 9'0 sand, 70 silt, % clay, % cation-exchange capacity, mequiv/ 100 g
1.0 2.2 76.8 21.0 15.4
a Numbers in parentheses represent range of values for all samd e s tested.
length LAS homologues in river water and river water/ sediment systems. The LAS homologues, which contained 10-14 carbon atoms in the alkyl chain, were uniformly radiolabeled with 14C in the benzene ring. The specific objectives of the study were 3-fold: (1)to characterize the kinetics of mineralization of the LAS benzene ring in river water/sediment systems a t microgram per liter concentrations, which approximate realistic environmental levels; (2) to determine the effect of alkyl chain length and phenyl position (point of attachment of the benzene ring to the alkyl chain) on the rate and extent of LAS mineralization; and (3) to determine the effect of high concentrations of suspended solids and competing homologues on the rate and extent of LAS mineralization.
Experimental Section Materials. A total of 10 different homologues of linear sodium alkyl[UJ4C]benzenesulfonate were synthesized by New England Nuclear. Radiochemical purities, as determined by thin-layer chromatography and gas-liquid chromatography with radiochemical detection, were >95%. Specific activities ranged from 12 to 28 pCi/mg (4.2-13.4 mCi/mmol). For five of the LAS materials tested, the alkyl chains were attached to the benzene ring a t a single site. These included a Clo-2-phenyl LAS, Clo-5-phenyl LAS, ClZ-2-phenylLAS, Clz-6-phenyl LAS, and a C14-2phenyl LAS. The remaining five materials (ClotCll, C12, C13,and C14 LAS) contained a mixture of phenyl distributions typical of commercial LAS. More detailed information on the exact phenyl composition and physiochemical properties of the LAS materials used in testing has been reported previously (8). River water and bottom sediments used in biodegradation studies were collected over a 2-year period from Rapid Creek, a first-order stream located in southwestern South Dakota. A tributary of the Cheyenne River, Rapid Creek has been the subject of comprehensive modeling and monitoring studies to characterize the fate of LAS in aquatic ecosystems (9). Water and sediment samples were collected approximately 7 km downstream from the Rapid City municipal wastewater plant, a small (-8 million gal/day) trickling filter facility treating predominantly domestic wastewater. Samples were shipped at 4 "C in 1-gal plastic containers and used for testing within 24 h of collection. Pertinent characterization data for water and sediment samples used in biodegradation studies are given in Table I. Biodegradation Assays. Biodegradation assays were conducted in a closed flow-through shake flask system as previously described (10). Briefly, various concentrations 1242
Environ. Sci. Technol., Vol. 24, No. 8, 1990
of LAS were added to duplicate or triplicate 2-L Erlenmeyer flasks containing, 1 L of river water or river water with added bottom sediments (loo0 mg/L). The ['%]LAS concentrations added to water (5-500 pg/L) and sediment samples (100 pg/g) are in the range of concentrations measured in natural aquatic and benthic environments (11). Test flasks were incubated at 24 "C in a constanttemperature room (*2 "C) and constantly agitated on a rotary platform shaker. The head space of test flasks was continuously aerated with COz-free air and 14C02was trapped in external base traps containing 100 mL of 1.5 N KOH. At various intervals, 10-mL aliquots were taken by syringe from each flask and filtered (0.2 pm) to remove particulate matter. The filters were washed with 5 mL of deionized water or 50% ethanol and the 15-mL filtrate was added to 125-mL biometer flasks and acidified with approximately 1mL of concentrated HC1. After acidification, the 14C02present in solution as radiolabeled carbonate or bicarbonate was released and trapped in 2 mL of 1.5 N KOH contained in the biometer sidearm. Aliquots from the acidified filtrate (10 mL) and the washed filters were then counted by standard liquid scintillation techniques to quantitate the amount of radiolabel in solution and in microbial biomass, respectively. Aliquots from the biometer sidearm and external base traps (1 mL) were also assayed to determine the amount of I4CO2produced and complete the mass balance of radiolabel at a specific sampling time. Correction factors were used to normalize all disintegrations per minute data to a 10-mL basis (10). Biodegradation studies were terminated by the addition of 1.0 mL of concentrated HCl to the test flask. After acidification and quantitation of nonvolatile and volatile radioactivity for mass balance purposes, aliquots (50 mL) from water and sediment/water samples were collected, extracted twice with ethyl ether (60 mL), and evaporated to dryness. The ethyl ether extracts were resuspended in scintillation cocktail and counted to determine the amount of radiolabel (expressed as the percentage of initial 14C activity added) present with the same solubility characteristics as parent LAS. Data for percent biodegradation as a function of time (expressed as either the amount of 14Cconverted to 14C02 or the amount of 14Cremaining in solution) were analyzed by nonlinear regression models to estimate the pseudofirst-order rate constants for mineralization or removal, normalized for the extent of biodegradation observed. All parameter estimates and their associated 95% confidence intervals were obtained by least-squares analysis using iterative techniques, as previously described (12). Half-life ( t l I 2 values ) (h) for mineralization or removal from solution were calculated from the corresponding pseudo-first-order rate constants (k,) by the relationship tllz = (0.693/k1) X 24.
Results Biodegradation Kinetics. Typical results of timecourse studies to characterize the rate and extent of mineralization of LAS homologues and phenyl isomers in Rapid Creek river water and sediments are shown in Figure 1. The specific data presented are for Clz-2-phenyl LAS, tested at two initial concentrations in river water, 10 and 100 pg/L. Similar results, however, were obtained in both water and sediment systems for the other LAS materials tested. In general, final mass balances of radioactivity were high and consistent in different experiments. The overall mean mass balance across 12 separate studies (*standard deviation) was 104 f 6%, with a range of 96-112%. In general, 14C02production and removal of I4C activity from solution exhibited comparable kinetic patterns in all
Table 11. Kinetic Parameters for Biodegradation of Homologues and Phenyl Isomers in River Water
A-A 14C02 Produced I-I 14C Removal
60
e-@
compound
14C Biomoss
Clo LAS
50
10 100 10 100 10 100
Cll LAS C12 LAS 0
--
PO
2
4
,
,
,
,
,
6
8
10
12
14
I
16
18
10
100 10 100 10 100 5 50 500
22
20
i-4-
C13LAS AA 14C02 Produced I--. 14C Removal
10
100 10 100 10
C14 LAS
e-@ 14C Biomass
100
Clo-2-phenyl LAS
o r : 0
2
:
4
: 6
:
:
8
10
:
12
:
14
:
:
16
18
:
20
-
Clo-5-phenyl LAS
I
22
Clz-2-phenyl LAS
INCUBATION TIME (doyr)
Flgure 1. Kinetics of biodegradation of C,,-2-phenyl LAS in river water at initial concentrations of 100 (up r) and 10 j@L (bwer). Procedures for measuring I4CO2production, removal, and Incorporationof ''C into biomass are described in the ExperimentalSection. The error bars represent 1 standard deviation from triplicate flasks.
experiments. Lag phases for biodegradation were not observed, and both mineralization and removal could be adequately described by a simple pseudo-first-order kinetic model. The first-order rate constants for mineralization and removal were in good agreement and did not vary significantly ( P
