Environ. Sci. Technol. 1991, 25, 489-493
Occurrence of Organotins in Municipal Wastewater and Sewage Sludge and Behavior in a Treatment Plant Karl Fent" Swiss Federal Institute for Water Resources and Water Pollution Control (EAWAG), CH-6047 Kastanienbaum, Switzerland
Markus D. Muller Swiss Federal Research Station, CH-8820 Wadenswil, Switzerland
w The behavior of selected organotin species in a wastewater treatment plant of Zurich, Switzerland, was studied. In untreated wastewater, monobutyltin (MBT), dibutyltin (DBT), and tributyltin (TBT) were detected in the range of 136-564, 127-1026, and 64-217 ng/L, respectively, of which 81-9270 were associated with suspended solids. During treatment, the fraction of organotins in the particulate phase decreased with decreasing suspended solids concentration. All organotin species monitored were found to be efficiently removed from wastewater, mainly by sedimentation in the primary clarifier. In the secondary effluent, levels of different organotins were in the range of 7-47 ng/L. These compounds were transferred into sewage sludge, indicating that the most important process for the elimination of organotins was adsorption into sludge. Residues of MBT, DBT, and TBT in digested sludges were in the range of 0.10-0.97, 0.41-1.24, and 0.28-1.51 mg/kg (dry weight), respectively. Introduction Organotins have been produced since 1936, but the variety of products and consumption have increased significantly in the past decade ( I ) . The world production in 1985 was estimated to be -35000 tonnes (2). Thermal and UV stabilizers for polyvinyl chloride (PVC) make up to -70% of total consumption (3). About 23% of the total organotin production is used as agrochemicals (acaricides and fungicides), and as general biocides in a broad spectrum of applications. Tributyltin (TBT) is used as an antifouling paint biocide on ships and boats, as a fungicide in the preservation of wood, and is added to a variety of materials as a protectant against microbial attack (e.g., PVC, dispersion paints, and textiles). Monobutyltins (MBT) and dibutyltins (DBT) are used as stabilizers and as catalysts for polyurethane foams and silicones, and in industrial processes (1-4). The impact of T B T on the aquatic environment has received considerable attention during the last years (4-9). Direct entry of TBT into the aquatic environment is primarily due t o its use in antifouling paints, leading to water concentrations that are considered to be toxic for a variety of aquatic organisms (9-15). Prior research has principally been focused on TBT originating from antifouling paints. To date, there is a lack of data on additional environmental input sources, especially on the occurrence of organotins in municipal wastewater and on the behavior in the wastewater and sludge treatment process. Preliminary data on organotin residues in sewage indicate a considerable organotin load in several sewage treatment plants (16). Here, we report on the determination of a series of organotin compounds in wastewater and sewage sludge. We present results from a field investigation in *Current address (sabbatical leave, November 1, 1990, to November 1, 1991): Woods Hole Oceanographic Institution, Biology Department, Woods Hole, MA 02543. 0013-936X/91/0925-0489$02.50/0
a mechanical-biological treatment plant, describe the elimination process, and give mass flows. The results show that municipal wastewater and sewage sludge contain considerable organotin concentrations. In the treatment plant, the organotins are conserved and transferred from wastewater to the sewage sludge. Experimental Section Sampling, Water and sludge samples were collected at different sites in a treatment plant of the city of Zurich, Switzerland (350 000 inhabitants). During Monday 22 and Tuesday 23 February 1988, Wednesday 11 and Tuesday 17 January 1989, respectively, flow-proportional composite samples (24 h) were taken automatically from untreated wastewater in the influent of the plant, from primary and secondary effluent, and from the effluent, and daily grab samples were taken from raw sewage sludge and anaerobically stabilized sludge (digested sludge). On 22 and 23 February 1988, a grab sample was taken from the digestion supernatant. On Sunday 28 February 1988, flow-proportional composite 24-h samples were taken automatically from the influent and from primary effluent, and on Tuesday 7 March 1989, from the influent. Glassware was rinsed with bidistilled water, acetone, methanol, and diethyl ether prior to use. Samples (500 mL) were preserved in glass bottles by addition of 10 mL of 37% formaldehyde and stored at 4 "C in the dark. Activated sludge had a mean residence time of approximately 2 days in 1988, and 10 days in 1989. Wastewater temperature was €2 f 1 "C. Materials. Solvents of purissimum grade were purchased from Fluka AG (Buchs, Switzerland), HC1, silica gel, and ascorbic acid were from Merck (Darmstadt, FRG), and Sep-Pak C18cartridges were from Waters (Milford, MA). Dibutyltin dichloride, tributyltin chloride, tetrabutyltin, SnCl,, and tropolone were obtained from Fluka, n-butyltin trichloride was obtained from Ventron (Karlsruhe, FRG), and tripropyltin chloride from Merck. Stock solutions of 1 mg/mL in toluene were prepared and kept in the dark at -20 "C. Fresh dilutions of ethylated organotin compounds (100 pg/pL) in hexane were prepared biweekly. Magnesium and ethyl bromide were from Fluka. Grignard reagents were prepared by reacting 6.1 g of Mg (0.25 mol) in tetrahydrofuran (THF) with equimolar quantities of 28 g of ethyl bromide. The Grignard reagent solution was diluted with THF to give an approximately 2 M solution and was stored at 4 "C in the dark. Pentyltin standards were prepared by reacting dilute solutions of SnC1, with substoichiometric amounts of pentylmagnesium bromide (reaction product, i). Tetrapentyltin was then prepared from the reaction between an excess of pentylmagnesium bromide and SnC1,. Tetrapentyltin and SnC1, were then allowed to react at 180 "C (reaction product, ii). The resulting mixtures of tetra-, tri-, di-, and monopentyltin were extracted with deionized water to remove SnC1,. The pentyltin standard mixture was then prepared by combining the reaction products i and ii. Solutions containing approximately 10 ng/pL of
0 1991 American Chemical Society
Environ. Sci. Technol., Vol. 25, No. 3, 1991
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each compound in acetone were used for water, particulates, and sludge samples as internal standards (IS). Sample Analysis. The analytical method for trace determination of organotins is based on a procedure described elsewhere (161, and it has been extended to analysis of particulate matter. It speciates between butyltins (MBT, DBT, TBT), phenyltins (mono-, di-, triphenyltin), dioctyltin, and tricyclohexyltin. Prior to analysis, the water samples were filtered through Watman GF/C micropore glass fiber filters (pore size 1.2 pm) in order to separate suspended particles and dissolved water phase. Water and particulate phases were analyzed separately. Analytical sample sets consisted of four samples, a spiked blank, and a procedural blank in every two sets, consisting of all the glassware, reagents, and IS. Briefly, the analytical procedure contained four steps: (a) acid digestion of the sample, (b) extraction, (c) derivatization, (d) analysis by capillary HRGC/FPD. Extraction. Prior to extraction, all samples were spiked with the IS pentyltins and tripropyltin. Water samples (500 mL) were acidified with HC1 to pH 2; tropolone (0.5 mL of a 0.25% solution in methanol) and ascorbic acid were added and mixed. After slow passage through a Sep-Pak CIScartridge, organotin compounds were eluted with 3-4 mL of diethyl ether, dried with anhydrous CaCl,, and concentrated. Filters loaded with particles were transferred to 25-mL glass centrifugation tubes, and after addition of 10 mL of HC1 (pH 2), they were homogenized mechanically. Tropolone solution (5-10 mL, 0.25 % in diethyl ether) was added; the resultant mixture was shaken vigorously and centrifuged (2500 rpm, 10 m i d . After additional extractions with tropolone and pentane, the combined organic phases were dried with CaC12 and reduced in volume to -1 mL prior to derivatization. Experiments with spiked filters demonstrated that adsorption of organotins onto glass fiber filters was neglectible. Aliquots of sludge were dried a t 105 “C to a constant weight to determine the dry weight content. Aliquots of 10 g were weighed in flasks for organotin analysis. After addition of IS, the sample was acidified with HC1 to pH 2 in a hood. After 30 min, the sludge was homogenized mechanically. The slurry was extracted with three portions of diethyl ether containing 0.25% tropolone. After being shaken vigorously and centrifuged, the organic phases were combined, dried with CaCl,, and reduced in volume to 1-2 mL. Up to three aliquots of a sludge sample were analyzed, and mean values are given. Derivatization, Cleanup, and HRGC-FPD Analysis. Small portions of the Grignard reagent, ethylmagnesium bromide, were carefully added to the extracts, and after 10 min the excess of reagent was destroyed by dropwise addition of 2 M HC1. The organic layer was dried over anhydrous sodium sulfate, reduced in volume by a rotatory evaporator at room temperature, purified by adsorption chromatography on silica gel in a Pasteur pipet, reduced in volume, and dissolved in hexane. Samples were then analyzed with a Carlo-Erba HRGC 5160 gas chromatograph equipped with an on-column injector, a fused-silica capillary column (30 m, 0.32 mm i.d., film thickness 0.25 pm) (DB-5, J&W, Folsom, CA), and a Carlo-Erba flame photometric detector SSD 250. The detector was operated without a filter and with a hydrogen-rich flame, and the detector temperature was set at 225 “C. Hydrogen (0.55 bar) served as carrier gas. Quantitation. Tripropyltin chloride served as a quantitative IS and pentyltin trichloride, dipentyltin dichloride, and tripentyltin chloride as semiquantitative IS. Recoveries, typically 50-60’70 for monoalkylated, 60-8070
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Environ. Sci. Technol., Vol. 25, No. 3, 1991
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Figure 1. HRGC-FPD chromatograms of an extract of the particulate phase of the treatment plant influent, previously spiked with internal standard solution (top), and after secondary clarification (below). Internal standards: tripropyltin (l),monopentyltin (2), dipentyltin (3),and tripentyltin (4). No monophenyltin (a), diphenyltin (b), dioctyltin (c), triphenyltin (d), and tricyclohexyltin (e) were detected.
for dialkylated, and 50-60% for trialkylated organotins, were determined in each analytical sample. Peaks in the gas chromatograms were assigned to individual organotin compounds on the basis of retention time and were confirmed by standard addition. The concentrations of organotins in the samples were calculated from specific sensitivities as determined by external standards, taking into account the different recoveries from the addition of the IS. Peak heights instead of peak areas were found to be more reliable and thus used. The detection limits for the butyl- and phenyltins were 1-10 ng/L of water and 0.014.1 mg/kg (dry weight) of sludge. No tributylethyltin or triphenylethyltin peaks were detected in any of the blanks analyzed. All results refer to the butyltin and phenyltin species as the ion (BuSn3+,MBT; Bu2Sn2+,DBT; Bu3Sn+,TBT), and they were corrected for recovery by IS.
Results and Discussion MBT, DBT, and TBT were detected in both the dissolved and particulate phases of raw wastewater, but no phenyltins, dioctyltin, and tricyclohexyltin were found (Figure 1). On the six sampling days, the sum of MBT, DBT, and TBT in both phases ranged from 136 to 564, 127 to 1026, and 64 to 217 ng/L, respectively (sum of butyltins 489-1482 ng/L). In raw wastewater, daily variation was greatest for DBT, and butyltin species were primarily associated with suspended solids (Figure 2). The proportion in the particulate fraction ranged between 61 and 93% for MBT, 87 and 97% for DBT, and 83 and 92% for TBT. The high proportion of TBT associated with suspended solids was expected due to the reported n-octanol-water partition coefficients between 1300 and 7000 (17, 18).
Table I. Concentrations of Organotins raw wastewaterb raw wastewater primary effluent secondary effluent tertiary effluent 'In nanograms per sampling days.
MBT
DBT
TBT
total
245 f 162 181 48 69 7 30 14 9f6
523 403 456 412 92 f 43 28 f 10 6f5
157 f 53 175 f 31 59 f 12 21 f 12 2*2
924 f 452 812 f 401 219 49 80 11 17 10
elimination, % 100
*
73 90 98
liter. Average total concentrations (sum of dissolved and assoeiated with particles) fSD on 5 sampling days. 'Six February
22
1988
MBT
c 0 ._
-5 e
DBT
E
100
TBT
6
0
22
23
28
11
17
7
Februav 1988 J a n u a ~1989 March 1989 Flaure 2. Concentrations of monobwnin (MET), dibutynin (DBT). and tributynin (TBT) in the particulate (P) and dissolved (D) phase of raw wastewater on six sampling days in 1988 and 1989. Figure 3 illustrates the behavior of organotin compounds throughout treatment on two sampling days. On the other sampling days the behavior was similar. Because organotins are primarily associated with suspended solids, these compounds were removed mainly by sedimentation in the primary clarifier. On the sampling days, concentrations of MBT, DBT, and TBT in the primary effluent ranged from 60 to 77,27 to 141, and 42 to 76 ng/L, respectively (sum of dissolved and particulate phases). As expected, the relative proportions of the different butyltins did not change in the influent and primary effluent. Table I gives butyltin levels and the calculated rate of elimination a t different sites in the treatment plant. Elimination of the total hutyltin concentration during primary clarification was 73% (MBT 62%, DBT EO%, TBT 66%). After activated sludge treatment and secondary clarification, MBT, DBT, and TBT were always present as shown in Figure 1. Concentrations were lower than in the primary effluent (Figure 3, Table I), which is likely a result of both sorption onto activated sludge and aerobic degradation. MBT in the secondary effluent ranged from 15 to 47 ng/L, DBT ranged from 19 to 43 ng/L, and TBT ranged from 7 to 33 ng/L (sum of butyltins 73-96 ng/L). Thus, of the influent concentrations 83% of MBT, 94% of DBT, and 88% of TBT were eliminated in the secondary effluent. In the effluent of the treatment plant MBT, DBT, and TBT concentrations were in the range of 4-17,3-13, and
