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Sorption and biodegradation of nonionic surfactants by activated sludge
The Science of the Total Environment, Supplement 1993 Elsevier Science Publishers B.V., Amsterdam
417
Sorption and biodegradation of nonionic surfactants by activated sludge A.T. Kiewiet a,b, A.R. Weiland a and J.R. Parsons a
aDept, of Entironmental Chemistry and Toxicology and bChemistryshop, Universityof Amsterdam, Nieuwe A chtergracht 166, 1018 WV Amsterdam, Netherlands
ABSTRACT Nonionic surfactants are high-volume chemicals and are used in detergent applications. Biodegradation and sorption are important environmental processes which largely determine the fate of these compounds. It is often assumed that sorption inhibits biodegradation by lowering the concentrations in the water phase. In this study the relationship between sorption and biodegradation was investigated by comparing the biodegradation in water with and without added sterilised activated sludge. It appeared that the biodegradation of nonionic surfactants was not inhibited by the presence of sterilised activated sludge, although the disappearance of the nonionic surfactant was slower in the sludge than in the water phase. Key words: Sorption; Biodegradation, Nonionic suffactants; Activated sludge
INTRODUCTION Nonionic surfactants are used in large quantities in detergent applications. The alcohol ethoxylates make up the greatest part of this group of chemicals and are replacing the poorly biodegradable alkylphenol ethoxylates. Most of the research on the environmental fate of surfactants concerns their biodegradation, though sorption is also a relevant environmental process. Sorption lowers the concentration of compounds in solution and therefore it is assumed that biodegradation, which is often concentration dependent, is slowed by sorption. This hypothesis was confirmed by Steen et al. (1980) and Flenner et al. (1992). However, others have found that the presence of a solid phase enhanced or had no effect on the biodegradation (Larson and Payne, 1981; Larson and Vashon, 1983; Shimp and Young, 1988). The studies of Larson and Payne (1981) and Larson and Vashon (1983) concern the surfactants LAS and D T D M A C . 1993 Elsevier Science Publishers B.V.
418
A.T. KIEWIET ET AL.
In this study the influence of sorption on biodegradation of the nonionic surfactant C12E10by activated sludge was investigated. The activated sludge was sterilised to prevent it acting as a source of active biomass.
EXPERIMENTAL As a model compound the nonionic surfactant C12E10 was used:
CH 3-(CH 2)11-0- (CH2-CH2-O) 10H To assess the adsorbing characteristics of the activated sludge an adsorption isotherm was determined. The activated sludge was sterilised with 15% formaldehyde and 80 mg dry weight of the sterilised material were shaken for 48 h with a 100-ml solution of C12E10 (0-32 m g / l ) at 28°C. The organic matter content of the activated sludge was 80% as determined by combustion at 500°C for 16 h. The organic carbon content was 43% as determined with the Allison method. To determine the influence of sterilised activated sludge on the biodegradation of C12E10 two parallel experiments were carried out: A, biodegradation of C12E10 in water; B, biodegradation of C12E10 in water with activated sludge. The composition of the solutions used in these experiments was in general similar to the solutions used in the sorption experiment except for the addition of inoculum and minerals and trace elements. The 100 ml solution consisted of 10 mg/1 C12E10, 250 /zl effluent from a sewage treatment plant and minerals and trace elements according to the OECD Closed Bottle procedure (OECD, 1981). In the experiment with sterilised activated sludge 65 mg dry weight of this were added to the solution. The bottles were shaken for 7 days at 28°C.
Analysis Water and sludge were analysed separately after 10 min centrifugation at 6000 rpm. Water was extracted with ethyl acetate. The sludge was freezedried and soxhlet-extracted with basic methanol. The soxhlet residue was dissolved in water and extracted with ethyl acetate. The extracts were derivatised with phenyl isocyanate and analysed with RP-HPLC and U V detection. A detailed description of the analytical method will be reported elsewhere (Kiewiet and Parsons, in preparation).
RESULTS The data of the sorption experiment were fitted to the Freundlich equation (Fig. 1): C s = K ("~l/n
419
S O R P T I O N A N D B I O D E G R A D A T I O N O F N O N I O N I C SURF A CT A N T S
4.50
4.10 o
3.70 O
IZl 3.30 6 0
o 0 0
2.90
2.50
t
-1.00
-0.50
0.00
t
I
0.50
1.00
1.50
LOG cone. Cl2EI0 in water (nag/l) Fig. 1. Adsorption isotherm for C12E10 and sterilised activated sludge.
in which C s = concentration C12E10 in sludge ( m g / k g ) , C w = concentration C12E10 in water (mg/1), K , n -- constants The data of the adsorption isotherm were fitted to the Freundlich equation (r = 0.95). The values of the constants were d e t e r m i n e d by linear regression of the log-log form of the Freundlich equation (Fig. 1) in which 1 / n represents the slope and K the intercept: K = 2827 + 117, n = 1.57 + 0.1. W h e n it is assumed that n = 1 and concentrations are very small, the distribution coefficient can be calculated: K o = C s / C w = 2827 1/kg. The corresponding Koc is calculated from Koc = Kd/fo ~ = 6574 l / k g , in which Koc is the organic carbon normalised partition coefficient and fo¢ the organic carbon fraction. The biodegradation of C12E10 in water with and without sterilised activated sludge is shown in Figs. 2 and 3. In Fig. 3 the data are the sums of the m e a s u r e d concentrations in water and in sludge, which are shown in Fig. 4 separately.
420
A.T. K I E W I E T E T A L .
10
8 0 ,.4
m e~
6'
t3 0
4 e~ o
o0
2
0 0
1
2
3
4
5
6
7
time (days)
Fig. 2. Biodegradation of C12E10 in water.
5
O
4"
m ¢q O
o O u
0
I
I
I
I
I
I
I
1
2
3
4
5
6
7
8
time (days)
Fig. 3. Biodegradation of Ca2Ea0 in water in the presence of sterilised activated sludge.
SORPTION
AND BIODEGRADATION
OF NONIONIC
421
SURFACT.MVFS
DISCUSSION
From the results of the biodegradation experiment it is clear that the biodegradation is not inhibited by the presence of sterilised activated sludge. When the biodegradation rates are compared, it is seen that the biodegradation of C12E10 is in fact stimulated in the presence of activated sludge. Because the activated sludge was sterilised, the observed stimulation is not the result of an extra quantity of biomass at the start of the experiment. A more probable explanation is that the activated sludge is used as an extra carbon a n d / o r nutrient source. The graph of the biodegradation of C12E10 in the presence of activated sludge seems to have a sigmoid shape. This might result from a lag phase in the initial part of the degradation and a delay of the degradation in the latter part. From Fig. 4 it can be seen that the disappearance of C12E10 in water is much faster than in sludge. If the rate of desorption of the nonionic surfactant is much higher than that of biodegradation, equilibrium between the concentration of C12E10 in water and sludge can be assumed. Then the ratio between the concentrations of the water and sludge phases would follow the Freundlich model. With this assumption of equilibrium the measured water concentrations of C12E10 c a n be transformed via the Freundlich equation to equilibrium concentrations of C12E10 in sludge. If these equilibrium concentrations C12E~0 are compared with the measured
~o 4 O
eq L) 0 •~
0
=
3
2
1
0 o
0
1
2
3
4
5
6
time (days) Fig. 4. Disappearance of
C12E10in
the water and sludge phases.
7
422
A.T. KIEWIET ET AlL.
concentrations, the latter show a slower decrease. This indicates that there is a lowering of the rate of biodegradation due to sorption. The observed decrease of C12E10 in the sludge phase can be attributed to desorption. This decrease is relatively fast so that biodegradation is not severely inhibited by sorption onto activated sludge. Sorption will have a greater influence on biodegradiation when desorption rates are much lower than the degradation rates. REFERENCES Flenner, C.K, J.R. Parsons, S.M.Schrap and A. Opperhuizen, 1991. Influence of suspended sediment on the biodegradation of alkyl esters of p-aminobenzoic acid. Bull. Environ. Contam. Toxicol., 47: 555-560. Kiewiet, A.T. and J.R. Parsons, RP-HPLC determination of nonionic surfactants in sewage water, sewage sludge and sediment samples. (in preparation) Larson, R.J. and A.G. Payne, 1981. Fate of the benzene ring of linear alkylbenzene sulfonate in natural waters. Appl. Environ. Microbiol., 41: 621-627. Larson, R.J. and R.D. Vashon, 1983. Adsorption and biodegradation of cationic surfactants in laboratory and environmental systems. Dev. Ind. Microbiol., 24: 425-434. OECD, 1981. OECD Guidelines for Testing of Chemicals. Paris, France. Shimp, R.J. and R.L. Young, 1988. Availibility of organic chemicals for biodegradation in settled bottom sediments. Ecotox. Environ. Saf., 15: 31-45. Steen, W.C., D.F. Paris and G.L. Baughman, 1980. Effects of sediment sorption on microbial degradation of toxic substances. In: R.A Baker, (Ed.), Contaminants and Sediments. Ann Arbor Science Pub., Ann Arbor, MI, Vol. 1, pp. 477-482.
417
Sorption and biodegradation of nonionic surfactants by activated sludge A.T. Kiewiet a,b, A.R. Weiland a and J.R. Parsons a
aDept, of Entironmental Chemistry and Toxicology and bChemistryshop, Universityof Amsterdam, Nieuwe A chtergracht 166, 1018 WV Amsterdam, Netherlands
ABSTRACT Nonionic surfactants are high-volume chemicals and are used in detergent applications. Biodegradation and sorption are important environmental processes which largely determine the fate of these compounds. It is often assumed that sorption inhibits biodegradation by lowering the concentrations in the water phase. In this study the relationship between sorption and biodegradation was investigated by comparing the biodegradation in water with and without added sterilised activated sludge. It appeared that the biodegradation of nonionic surfactants was not inhibited by the presence of sterilised activated sludge, although the disappearance of the nonionic surfactant was slower in the sludge than in the water phase. Key words: Sorption; Biodegradation, Nonionic suffactants; Activated sludge
INTRODUCTION Nonionic surfactants are used in large quantities in detergent applications. The alcohol ethoxylates make up the greatest part of this group of chemicals and are replacing the poorly biodegradable alkylphenol ethoxylates. Most of the research on the environmental fate of surfactants concerns their biodegradation, though sorption is also a relevant environmental process. Sorption lowers the concentration of compounds in solution and therefore it is assumed that biodegradation, which is often concentration dependent, is slowed by sorption. This hypothesis was confirmed by Steen et al. (1980) and Flenner et al. (1992). However, others have found that the presence of a solid phase enhanced or had no effect on the biodegradation (Larson and Payne, 1981; Larson and Vashon, 1983; Shimp and Young, 1988). The studies of Larson and Payne (1981) and Larson and Vashon (1983) concern the surfactants LAS and D T D M A C . 1993 Elsevier Science Publishers B.V.
418
A.T. KIEWIET ET AL.
In this study the influence of sorption on biodegradation of the nonionic surfactant C12E10by activated sludge was investigated. The activated sludge was sterilised to prevent it acting as a source of active biomass.
EXPERIMENTAL As a model compound the nonionic surfactant C12E10 was used:
CH 3-(CH 2)11-0- (CH2-CH2-O) 10H To assess the adsorbing characteristics of the activated sludge an adsorption isotherm was determined. The activated sludge was sterilised with 15% formaldehyde and 80 mg dry weight of the sterilised material were shaken for 48 h with a 100-ml solution of C12E10 (0-32 m g / l ) at 28°C. The organic matter content of the activated sludge was 80% as determined by combustion at 500°C for 16 h. The organic carbon content was 43% as determined with the Allison method. To determine the influence of sterilised activated sludge on the biodegradation of C12E10 two parallel experiments were carried out: A, biodegradation of C12E10 in water; B, biodegradation of C12E10 in water with activated sludge. The composition of the solutions used in these experiments was in general similar to the solutions used in the sorption experiment except for the addition of inoculum and minerals and trace elements. The 100 ml solution consisted of 10 mg/1 C12E10, 250 /zl effluent from a sewage treatment plant and minerals and trace elements according to the OECD Closed Bottle procedure (OECD, 1981). In the experiment with sterilised activated sludge 65 mg dry weight of this were added to the solution. The bottles were shaken for 7 days at 28°C.
Analysis Water and sludge were analysed separately after 10 min centrifugation at 6000 rpm. Water was extracted with ethyl acetate. The sludge was freezedried and soxhlet-extracted with basic methanol. The soxhlet residue was dissolved in water and extracted with ethyl acetate. The extracts were derivatised with phenyl isocyanate and analysed with RP-HPLC and U V detection. A detailed description of the analytical method will be reported elsewhere (Kiewiet and Parsons, in preparation).
RESULTS The data of the sorption experiment were fitted to the Freundlich equation (Fig. 1): C s = K ("~l/n
419
S O R P T I O N A N D B I O D E G R A D A T I O N O F N O N I O N I C SURF A CT A N T S
4.50
4.10 o
3.70 O
IZl 3.30 6 0
o 0 0
2.90
2.50
t
-1.00
-0.50
0.00
t
I
0.50
1.00
1.50
LOG cone. Cl2EI0 in water (nag/l) Fig. 1. Adsorption isotherm for C12E10 and sterilised activated sludge.
in which C s = concentration C12E10 in sludge ( m g / k g ) , C w = concentration C12E10 in water (mg/1), K , n -- constants The data of the adsorption isotherm were fitted to the Freundlich equation (r = 0.95). The values of the constants were d e t e r m i n e d by linear regression of the log-log form of the Freundlich equation (Fig. 1) in which 1 / n represents the slope and K the intercept: K = 2827 + 117, n = 1.57 + 0.1. W h e n it is assumed that n = 1 and concentrations are very small, the distribution coefficient can be calculated: K o = C s / C w = 2827 1/kg. The corresponding Koc is calculated from Koc = Kd/fo ~ = 6574 l / k g , in which Koc is the organic carbon normalised partition coefficient and fo¢ the organic carbon fraction. The biodegradation of C12E10 in water with and without sterilised activated sludge is shown in Figs. 2 and 3. In Fig. 3 the data are the sums of the m e a s u r e d concentrations in water and in sludge, which are shown in Fig. 4 separately.
420
A.T. K I E W I E T E T A L .
10
8 0 ,.4
m e~
6'
t3 0
4 e~ o
o0
2
0 0
1
2
3
4
5
6
7
time (days)
Fig. 2. Biodegradation of C12E10 in water.
5
O
4"
m ¢q O
o O u
0
I
I
I
I
I
I
I
1
2
3
4
5
6
7
8
time (days)
Fig. 3. Biodegradation of Ca2Ea0 in water in the presence of sterilised activated sludge.
SORPTION
AND BIODEGRADATION
OF NONIONIC
421
SURFACT.MVFS
DISCUSSION
From the results of the biodegradation experiment it is clear that the biodegradation is not inhibited by the presence of sterilised activated sludge. When the biodegradation rates are compared, it is seen that the biodegradation of C12E10 is in fact stimulated in the presence of activated sludge. Because the activated sludge was sterilised, the observed stimulation is not the result of an extra quantity of biomass at the start of the experiment. A more probable explanation is that the activated sludge is used as an extra carbon a n d / o r nutrient source. The graph of the biodegradation of C12E10 in the presence of activated sludge seems to have a sigmoid shape. This might result from a lag phase in the initial part of the degradation and a delay of the degradation in the latter part. From Fig. 4 it can be seen that the disappearance of C12E10 in water is much faster than in sludge. If the rate of desorption of the nonionic surfactant is much higher than that of biodegradation, equilibrium between the concentration of C12E10 in water and sludge can be assumed. Then the ratio between the concentrations of the water and sludge phases would follow the Freundlich model. With this assumption of equilibrium the measured water concentrations of C12E10 c a n be transformed via the Freundlich equation to equilibrium concentrations of C12E10 in sludge. If these equilibrium concentrations C12E~0 are compared with the measured
~o 4 O
eq L) 0 •~
0
=
3
2
1
0 o
0
1
2
3
4
5
6
time (days) Fig. 4. Disappearance of
C12E10in
the water and sludge phases.
7
422
A.T. KIEWIET ET AlL.
concentrations, the latter show a slower decrease. This indicates that there is a lowering of the rate of biodegradation due to sorption. The observed decrease of C12E10 in the sludge phase can be attributed to desorption. This decrease is relatively fast so that biodegradation is not severely inhibited by sorption onto activated sludge. Sorption will have a greater influence on biodegradiation when desorption rates are much lower than the degradation rates. REFERENCES Flenner, C.K, J.R. Parsons, S.M.Schrap and A. Opperhuizen, 1991. Influence of suspended sediment on the biodegradation of alkyl esters of p-aminobenzoic acid. Bull. Environ. Contam. Toxicol., 47: 555-560. Kiewiet, A.T. and J.R. Parsons, RP-HPLC determination of nonionic surfactants in sewage water, sewage sludge and sediment samples. (in preparation) Larson, R.J. and A.G. Payne, 1981. Fate of the benzene ring of linear alkylbenzene sulfonate in natural waters. Appl. Environ. Microbiol., 41: 621-627. Larson, R.J. and R.D. Vashon, 1983. Adsorption and biodegradation of cationic surfactants in laboratory and environmental systems. Dev. Ind. Microbiol., 24: 425-434. OECD, 1981. OECD Guidelines for Testing of Chemicals. Paris, France. Shimp, R.J. and R.L. Young, 1988. Availibility of organic chemicals for biodegradation in settled bottom sediments. Ecotox. Environ. Saf., 15: 31-45. Steen, W.C., D.F. Paris and G.L. Baughman, 1980. Effects of sediment sorption on microbial degradation of toxic substances. In: R.A Baker, (Ed.), Contaminants and Sediments. Ann Arbor Science Pub., Ann Arbor, MI, Vol. 1, pp. 477-482.