- Email: [email protected]
Adsorptive behavior of butyltin compounds under simulated estuarine conditions
The Science of the Total Environment, 57 (1986) 191-203 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
191
A D S O R P T I V E B E H A V I O R OF B U T Y L T I N C O M P O U N D S U N D E R SIMULATED ESTUARINE CONDITIONS
LOUISE RANDALL and J A M E S H. WEBER*
Chemistry Department, University of New Hampshire, Parsons Hall, Durham, NH 03824 (U.S.A.) (Received April 4th, 1986; accepted May 7th, 1986)
ABSTRACT A 23 + 1 factorial design was used to study adsorption of BuSnC13, Bu2SnC12, and Bu3SnC1 under simulated estuarine conditions. The variables included artificial seawater and its dilutions (salinity 5~35 g kg - 1), pH (6.2~.2), and hydrous iron oxide concentration (10-1000 mg 1- 1). Fulvic acid concentration was constant at 10 mg 1-1, and initial concentration of butyltin compounds was 10 ng m l - 1 (as Sn). Adsorption of butyltin compounds varied from 72 to 100°/0 for BuSnC13, 0 to 56#/0 for Bu2SnC12, and 57 to 95% for BusSnC1. At the 95#/0 confidence level all three variables were significant for BuSnC13 adsorption, pH was significant for Bu2SnC12 adsorption, and pH and salinity were significant for Bu3SnC1 adsorption. Discussion includes the importance of, and reasons for, differing adsorptive behavior of the three butyltin compounds. INTRODUCTION
The relatively high concentrations of butyltin species in the aquatic environment probably originate from the use of very toxic tributyltin compounds in antifouling marine paints [1]. Tributyltin compounds appear to be moderately persistent in aquati c systems, but Maguire and Tkacz [2] found dibutyltin, monobutyltin, and inorganic tin degradation products. In an estuary tributyltin compounds can remain in the aqueous phase of the water column, adsorb on suspended particulates, or partition to biota, sediment, or the surface microlayer [3]. The recently reported [4] octanol/water partition coefficient of 5500 for tributyltin in seawater (25 g kg -1 salinity) suggests that tributyltin concentrations in biota, suspended particulates, and sediments would be higher than the water concentration. If tributyltin adsorbs on suspended particulate matter and settles to t h e bottom, it would not be available as a toxicant in the aqueous phase of the water column. However, if the settled particulate matter is ingested by filter-feeding organisms or detritivores, the ingested tributyltin might immediately be fatal to the animal or bioaccumulate to a lethal or sublethal level. Alternatively the tributyltin might not be retained in the body tissues, but could be excreted in its toxic form and thereby recycled into the aqueous phase of the water column [3]. There are few studies of organotin compounds exposed to estuarine con* Author to whom correspondence should be addressed.
0048-9697/86/$03.50
© 1986 Elsevier Science Publishers B.V.
192
ditions [1]. Donard and Weber [5] carried out a study of methyltin compounds under simulated estuarine conditions and found that adsorption on hydrous iron oxide was in the order: monomethyltin > dimethyltin > trimethyltin. Their conclusion was that the very toxic trimethyltin would not settle out in the estuary. Maguire [6] has shown that in 75% of water column-sediment systems studied, the sediment contained more than 50% of the butyltin species or inorganic tin. Maguire and Tkacz [2] found relatively strong binding of Bu3Sn ÷ to sediments. However, degradation rates, pathways, and products in sediments are not well documented [3]. This study incorporates a 23 + 1 factorial design to assess partitioning of mono-, di-, and tributylin compounds between artificial seawater solution phases and a fulvic acid-coated hydrous iron oxide solid phase. Variables are pH, salinity, and hydrous iron oxide concentration; and fulvic acid concentration is constant. The overall results are t h a t adsorption on hydrous iron oxide is in the order: monobutyltin > tributyltin > dibutyltin. This order suggests t h a t monobutyltin compounds are most likely to settle in an estuary and that dibutyltin compounds are most likely to remain in solution. The most toxic tributyltin compounds are likely to be present in both the water column and the sediment. EXPERIMENTAL
Apparatus The analytical technique, described elsewhere [7], allows speciation of butyltin compounds by hydride generation-atomic absorption spectrometry.
Materials and methods Reagents used and the preparation of standards were described previously [5, 7]. A trimethyltin chloride standard (185 mg l-l) was prepared in deionized distilled water spiked with l m l of 5 M HNO3 and was stored in a Hypo-vial sealed with a crimp-on Teflon-lined septum. Artificial seawater was prepared as described by Kester et al. [8]. The appropriate amounts (Table 1) of artificial seawater (salinity, S), deionized, distilled water, hydrous iron oxide (particulate matter, PM), and fulvic acid (FA) were mixed in 250ml amber glass bottles. One set of 12 factorial experiments was performed for each butyltin compound. Experiments were performed in random order to avoid systematic errors such as personal bias or aging of solutions. The pH was determined with an Orion 91-04 pH electrode after adjustment with 0.1 M Na~ CO3 or 0.1 M HC1. After solutions were shaken for 4 h, the pH values were within _+0.2 pH units. Addition of 1 ttl of a butyltin compound solution resulted in a concentration of 10 ng ml- 1 (as Sn). After the experimental set was shaken for 12 h, the pH was still within _+0.2 pH units. Each sample was filtered through 0.4ttm Nucleopore polycarbonate filters
193 TABLE 1 P e r c e n t b u t y l t i n c o m p o u n d r e m o v a l by h y d r o u s i r o n o x i d e a n d i t s a d s o r p t i o n efficiency i n t h e p r e s e n c e of 10 mg 1-1 f u l v i c a c i d a Parameter Level
S ( g k g 1)
P M (mg1-1)
-
5
10
0 +
20 35
100 1000
% Removalb
pH 6.2 7.2 8.2 R e m o v a l efficiencyc (pmol g - 1)
Exp t.
S
PM
pH
Bu
Bu s
Bu 3
1 2 3 4 5 6 7 8 9 10 11 12
+ + + + 0 0 0 0
+ + + + 0 0 0 0
+ + + + 0 0 0 0
100 100 72 86 100 100 81 90 93 92 91 92
19 0.0 0.0 18 56 41 52 43 23 20 31 28
68 60 78 57 81 75 95 61 72 77 67 79
Bu
Bu 2
Bu 3
0.035 0.035 2.541 3.030 0.035 0.035 2.881 3.200 0.328 0.326 0.321 0.329
0.006 0.000 0.000 0.596 0.018 0.013 1.718 1.419 0.077 0.066 0.103 0.091
0.021 0.019 2.393 1.751 0.025 0.023 2.919 1.862 0.220 0.236 0.204 0.241
a A b b r e v i a t i o n s : S, s a l i n i t y (g k g - ~); PM, h y d r o u s i r o n o x i d e c o n c e n t r a t i o n (mg 1-1); Bu, BuSnCla; Bus, Bu2SnC12; Bu3, Bu3SnC1. b P e r c e n t r e m o v a l is t h e p e r c e n t c o m p o u n d r e m o v e d by f i l t r a t i o n . c R e m o v a l efficiency is t h e m i c r o m o l e s of c o m p o u n d r e m o v e d pe r g r a m of h y d r o u s i r o n oxide.
using an apparatus described by Truitt and Weber [9], and the filtrate was collected in a 250 ml amber glass bottle. Samples were spiked immediately with 1 ml of 5 M HNOa to bring the pH below 2 and decrease adsorption on the walls, diluted (if necessary) to fit the linear part of the calibration curve, and immediately analyzed. Three points from the calibration curve were run between samples. Calculations were made from internal trimethyltin chloride standards, and results were expressed as percent butyltin compound removed by filtration. The relative standard deviation (RSD) for the four center points was 1% for BuSnC13, 19% for Bu2SnC12, and 7% for Bu3SnC1. Adsorption on walls was measured at the high and low S, PM, and pH levels and at one center point (Table 1) for each butyltin compound. After a 12h equilibration, solutions were removed from the amber glass bottles. The bottles were rinsed with 25 ml of deionized distilled water and then filled with 100 ml of 1:20 HNOa/deionized water. After 4 h the leaching solutions were analyzed.
194 FACTORIAL EXPERIMENTAL DESIGN
Donard and Weber [5] previously discussed the rationale for factorial design experiments. This study used the three parameters salinity, particulate matter concentration, and pH at levels commonly found in estuaries. These three are very important in non-biological processes that determine partitioning between solution and particulate matter phases. Salinity is the reference scale for estuarine processes, hydrous iron oxide is an important scavenger of heavy metals in aquatic environments, and pH is important in regulating surface/ solution distribution [10]. The constant 10 mg 1-1 fulvic acid concentration is appropriate for estuaries, and the concentration of butyltin compounds was sufficient to allow measurement after high percent removal (indicated by preliminary experiments). RESULTS
Background losses and decomposition Adsorptive losses to walls, which were greatest for low S, PM, and pH levels, were less than 5% for BuSnC13, 2% for Bu2SnC12, and 1% for Bu3SnC1. Minor degradation reactions of butyltin compounds during these experiments were due to adsorption-related phenomena not to the method of determination [7]. Mono- and dibutyltin (up to 3 and 11%, respectively) occurred in tributyltin solutions, monobutyltin (up to 8%) occurred in dibutyltin solutions, and dibutyltin (up to 6%) occurred in monobutyltin solutions. The maximum loss was 14% of added tributyltin. Maguire and co-workers observed degradation of tributyltin species in biologically active harbor water and sediments [11], but not in distilled water [2].
General results Each butyltin chloride showed a different pattern of removal by adsorption under simulated estuarine conditions (Table 1). The removal of BuSnC13 was nearly complete (72-100%), but removal of Bu2SnC12 (0--56%) and BusSnC1 (57-95%) had greater ranges. Removal efficiencies in Table 1 paralleled percent removal. For each butyltin compound efficiency decreased as PM concentration increased, but was virtually unaffected by S or pH. Figures 1-3 show the separate effects of S, PM, and pH on butyltin compound removal. The lines between pairs of points indicate general trends. The slope of a line is significantly nonzero if its high and low values vary more than the ~/o RSD for the compound determined from the center points. Underlined values in Table 2 indicate parameters and their interactions significant at the 95% confidence level. All three parameters and several interactions are significant for BuSnC13 removal. The very high PM value (803.01) indicates its extreme importance, which steep slopes in Fig. 2a also demonstrate. For Bu2SnC12 pH is the only significant variable. Significant
195 (a) 100- mm-o
95.
90.
85.
80
75.
70
Ib'
llj
2b
2(s
,b
,(s
(b) 60.
50
40 > o
30
w
g
20
~
~
I0-
0,:
ib
J i6
.1----m
j J 2b
2~
.b
.-%
2(s
3b
3(i
(c) 95908580-
7570656055
1(;
~
2b S(g/kg)
Fig. 1. I n f l u e n c e of s a l i n i t y (S) o n r e m o v a l of (a) m o n o b u t y l t i n chloride, (b) d i b u t y l t i n chloride, a n d (c) t r i b u t y l t i n c h l o r i d e . + P M , + p H (m), + P M , - p H (D), - P M , + p H (e), - P M , - pH (o).
196
(a) 95
~
90
85
80-
75 7°I
,do
1obo
(b)
60-
50-
40-
30-
20-
iO-
0 l
1do
~o
(c)
95.
~
85-9°"
-
~
_
~
80-
75.
~
7o.
~
B5.
~
60.
~
55
1do PM
tdbo
(mg/L)
Fig. 9..Influence of particu|ate matter concentration (PM) on remora] o£(a) monobutyltin chloride, Co) dibutyltin chloride and (c) tributy]tin chloride. + pH, + S (I), + pH, - S (D), -pH, + S (o), -pH,
-S(O).
197 (a)
iO0 95
90-
85-
80-
75-
I '
I
I
I
I
I
I
I
I
8.12
(b)
60
50
40
30-
20-
tO
,-
,
.--I
,
,
,
,
,
,
t
I
I
I
I"--'~'~.12
(c) 959085 80 75 70 65. 60.
556.
-o I
I
I
I pH
Fig. 3. I n f l u e n c e of p H o n r e m o v a l of (a) m o n o b u t y l t i n c h l o r i d e , (b) d i b u t y l t i n c h l o r i d e a n d (c) t r i b u t y l t i n c h l o r i d e . + S, + P M ( i ) , + S, - P M (D), - S, + P M ( o ) , - S, - P M (o).
198 parameters for Bu3SnC1 are S and pH. The relatively low values for S and pH indicate t h a t these parameters were not as important for Bu3SnC1 as t hey were for BuSnC13. Sometimes parameters are not statistically significant because of opposing slopes. Salinity for Bu2SnC12 (Fig. lb) is an example. The lack of fit (LOF) term, which tests c u r v a t u r e of experimental response surfaces, is insignificant for all three butyltin compounds. This means t h a t responses to all three parameters are linear. S ed imen t- wat e r partition coefficients (K,) in Table 3 indicate t h a t tributyltin co n cen tr atio ns on the solid phase were much higher t han those in ambient water. They varied in the order: BuSnC13 >> Bu3SnC1 > Bu2SnC12 in agreement with percent removal (Table 2). The Kp values for di- and tributyltin varied considerably with PM, but relatively little with S or pH. For di- and tributyltin K, increases with decreasing PM.
Influence of parameters on butyltin removal Increasing S (Fig. 1) generally increased removal of Bu3SnC1 and Bu2SnC12, but decreased removal of BuSnC13. Increasing particulate m a t t e r concentration (Fig. 2) clearly increased removal of BuSnC13, but there are opposing trends for Bu2SnC12 and Bu3SnC1. Increasing pH (Fig. 3) decreased removal of all three butyltin chlorides. DISCUSSION Removal of butyltin compounds in our experiments could occur by many processes. Leaching experiments ruled out loss of butyltin compounds due to adsorption onto vessel walls. Precipitation of the butyltin compounds is also unlikely because of their solubility [1, 11]. Therefore, removal of butyltin compounds from solution was due predominantly to adsorption on hydrous iron oxide. TABLE 2 Statistical significance of parametersa Effectb
BuSnC13
Bu2SnC12
Bu3SnC1
S PM pH S.PM S.pH PM.pH S.PM.pH LOF
83.77 803.01 33.59 82.31 3.51 32.67 3.84 1.73
3.39 0.04 127.97 9.97 2.83 0.00 5.27 1.08
20.00 0.16 10.18 7.61 0.58 0.23 1.01 0.20
aUnderlined values indicate the significance at the 95% confidencelevel. The literature F value for 95% confidencelevel is 10.13. bAbbreviations: S, salinity; PM, hydrous iron oxide concentration; LOF, lack of fit.
199
TABLE Partition Expt.
1 2 3 4 5 6 7 8 9 10 11 12
3 c o e f f i c i e n t s (Kp), c a l c u l a t e d S
+ + + _ + 0 0 0 0
PM
+ + + + 0 0 0 0
for butyltin compounds a pH
+ + + + _ 0 0 0 0
Kp ( ~ g k g
1 ] / [ , u g l - ~ ] ) × 10 -4
BuSnC13
Bu2SnC12
Bu3SnC1
100 100 25 59 100 b 44 93 12 11 9.5 12
0.02 0.00 0.00 2.2 0.13 0.07 11 7.6 0.30 0.25 0.45 0.38
0.21 0.15 35 13 0.42 0.30 190 15 2.5 3.3 2.0 3.7
a A b b r e v i a t i o n s : S, s a l i n i t y ( g k g - 1 ) ; P M , h y d r o u s i r o n o x i d e c o n c e n t r a t i o n b 100% adsorbed.
(mgl-1).
Two major phenomena affecting the distribution of metal species between dissolved and particulate phases are adsorption onto particulate matter and flocculation in which the species are trapped by newly formed particulate matter. Because the two mechanisms are not distinguishable in this study we will use the term 'adsorption' for both processes. Removal mechanisms
The strength and nature of interactions between hydrous iron oxide and butyltin species depend on the structure, charge, and dipole moment of the butyltin compounds in water; the nature of the adsorbing surface; and the types of dissolved species-particulate matter attractive forces. The following paragraphs discuss the nature of butyltin compounds and hydrous iron oxide in artificial seawater and the nature of the attractive forces between them. The chemical nature of the aqueous butyltin compounds in these experiments is unknown. However, by analogy to methyltin analogs it is likely that the aqueous species are predominantly neutral and five- or six-coordinate due to coordination of hydroxide ions, chloride ions, and water [5]. Laughlin et al. [4] speciated tributlytin species in chloroform by extraction of bis(tributyltin) oxide from seawater and found tributyltin chloride, oxide, and carbonate species. Because of shifting of equilibria during the extraction, these results give no information about the nature of the species in seawater. Because the zero point of charge of hydrous iron oxide is at pH 8.4 [12], it will have a slightly positive charge in pH 8.2 experiments and a considerable positive charge in pH 7.2 and 6.2 experiments. However, the 10mg 1-1 fulvic
200 acid (FA) imparts a negative charge to the particles [13, 14]. At 10mg1-1 hydrous iron oxide, FA is in excess and some remains in solution, but at the 100 and 1000mg1-1 hydrous iron oxide concentrations FA was nearly 100% removed from solution [5]. Thus, neutral butyltin species reacted with negatively-charged, FA-coated hydrous iron oxide particles in our experiments. The usual adsorption model for metal ions [15] cannot explain our results because the butyltin compounds are predominantly neutral, and changes in adsorption with pH are not like those for metal ions (see below). In addition, Davis [16] showed that humic matter coating on alumina can control adsorptive behavior of metal ions. Our model developed for methyltin compounds [5] is an appropriate starting point for discussion. In this model the effectiveness of removal is proportional to the order of polarity and thus dipole moments of methyltin compounds: MeSnC13 > Me2SnC12 > Me3SnC1. However, this model is incomplete for butyltin compounds because of the mixed trend in percent removal: BuSnC13 > Bu3SnC1 > Bu2SnC12. Such a mixed trend means two or more opposing processes. The first process is due to dipole moments that predominate for methyltin compounds [5], and the second is due to nonpolar forces that are important for butyltin compounds. We will use the term 'hydrophobicity' to approximate the nonpolar nature of butyltin compounds [17]. Octanol/water partition coefficients (Kow) are used to approximate hydrophobicity. Since Kow is about 106 times higher for Bu3SnC1 than for Me3SnC1 [1], it is clear that butyltin compounds will bond much more strongly by hydrophobic interactions than analogous methyltin compounds. For this reason methyltin compounds' adsorption trends are explainable on the basis of dipoles alone. Because the hydrophobicity of alkyltin compounds increases with an increasing number of carbon atoms and number of alkyl groups bonded to tin, the observed mixed adsorption trend for butyltin compounds results from tributyltin adsorption favored by hydrophobicity and monobutyltin adsorption favored by polarity. Schellenberg et al. [18] in studies with chlorinated phenols found strong evidence that adsorption of undissociated phenols is dominated by partitioning between the aqueous phase and the organic phase in the adsorbent and is strongly correlated to Kow. Butyltin compounds might adsorb in a similar manner since they are also polar, hydrophobic compounds.
Partition coefficients Partition coefficients are important because of their relationship to physical adsorption on solids, biomagnification, and lipophilic storage [19]. In agreement with Toro et al. [20] our Kp values (Table 3) are variable. This result is not surprising because the number and nature of butyltin binding sites on the fulvic acid-coated hydrous iron oxide depend on experimental conditions. Excess FA remained in solution in experiments at low PM, but not at high PM. This means that at low PM only the most strongly-attracted FA molecules adsorbed to the hydrous iron oxide, and that the surface is different than in the case of high PM.
201 Maguire and Tkacz [2] determined the partition coefficient of Bu3Sn + to be (2.18 + 0.35)× 103 (equilibrium Bu3Sn + concentration 5ngSn1-1, ionic strength 0.77M, sediment organic carbon content 2.4%). At high PM concentrations the K, determined in this work is of the same order of magnitude (0.15-0.42 × 104 [#gkg 1]/[ggl-1]). But at low or intermediate PM concentrations our K, values were 1, 2, or 3 orders of magnitude higher. An increase in Kp values with decreased PM clearly occurs for di- and tributyltin, and is due to only selected, strongly-attracted FA molecules adsorbed at low PM values. This behavior agrees with results for titrations of fulvic acid by a metal ion such as Cu 2+ [21]. Initially the stability constant is high because Cu 2÷ binds only to the strongest sites in the fulvic acid mixture. As more and more Cu 2÷ binds, weaker and weaker sites are used and the average stability constant decreases. There is no reason for adsorbed fulvic acid not to bind butyltin compounds at sites of varying strengths. The reason that monobutyltin, for which K, values vary by only one magnitude, did not distinguish between high and low PM is t h a t its high affinity for the particulate matter overcame the differences in the strengths of binding sites.
Importance of salinity At high S less adsorption is expected due to competition of chloride ions with the PM for the butyltin species and competition of sodium ions with the butyltin compound for carboxylate sites on the PM [5]. Thus, Na ÷ partially neutralizes - COO groups of FA bound to the PM leading to a less negative PM charge. The expectation of reduced butyltin compound removal with increasing salinity was met for BuSnC13 (Fig. la) when PM was low. Bu2SnC12 and Bu3SnC1, in contrast, exhibited a general trend of increasing removal at high salinity. In these cases increasing salinity accentuates the importance of nonpolar interactions by neutralization of - COO of adsorbed FA by Na ÷.
Importance of particulate matter concentration Increasing concentrations of hydrous iron oxide, which is important for the transport of trace metals in n a t u r a l water [22], generally increases metal ion removal by providing more adsorption sites. Monobutyltin (Fig. 2a) exhibits the expected behavior, but di- and tributyltin generally do not. Significantly increased percent removal at high PM occurs for dibutyltin only when both S and pH are high and for tributyltin only when both S and pH are low. For other experimental conditions the usual trend for these two compounds is decreased removal at high PM. Since the trends are anomalous with different S and pH, the differing behavior of the butyltin compounds must be related to the difference among mono-, di-, and tributyltin compounds. The higher polarity of the monobutyltin chloride compared with the di- and tributyltin chlorides might explain the high adsorption of the monobutyltin chloride to the PM. The general trend of decreasing adsorption with increasing PM for di- and tributyl-
202 tin chloricle is probably due to their enhanced capability for nonpolar binding and lessened capability for dipolar binding relative to monobutyltin. Possibly non-adsorbed FA t h a t occurs at low PM binds hydrophobically to di- and tributyltin compounds and the resulting complexes are preferentially adsorbed.
Importance of pH The strong effect of a nar r ow pH range on metal ion adsorption is well known [15]. In this study pH was statistically significant for all three butyltin compounds, and particularly so for dibutyltin (Table 2). For mono-, di-, and tributyltin compounds the general trend, in contrast to t h a t for metal ions, was decreased removal with increasing pH (Fig. 3). Because of - COO- groups of the FA coating on the hydrous iron oxide, the particulate m at t er will be negatively-charged at all pH values in these experiments. Thus, in contrast to most studies with metal ions in the absence of fulvic acid, the PM charge did not change from positive to negative within the pH range of the experiments. However, as pH increased, higher concent r a t i ons of hydroxide and carbonate ions would more effectively compete with chloride ion for cationic butyltin species. The results suggest decreased adsorption of hydroxide and carbonate complexes relative to chloride complexes. CONCLUSIONS During estuarine processes m onobut yl t i n will be present mainly in the adsorbed phase, and dibutyltin will occur predominantly in the water column. Significant amounts of t r i but yl t i n will remain in solution and on the particulate matter. This ubiquitous presence of the very toxic t ri but yl t i n implies t h a t it might be available to both pelagic and benthic organisms. ACKNOWLEDGEMENTS We t h a n k Mar k Anderson for writing the computer program for statistical analysis and Rachel Kennedy for helpful discussions. This research was partially supported by National Science F o u n d a t i o n grant CEE 81-16960 and Environmental P r o t e c t i on Agency grant R-809416. REFERENCES 1 J.A.J. Thompson, M.G. Sheffer, R.C. Pierce, Y.K. Chau, J.J. Cooney, W.R. Cullen and R.J. Maguire, Organotin Compounds in the Aquatic Environment, NRCC/CNRC No. 22494 Otta w a , C a n a d a K1A OR6, 1985. 2 R.J. Maguire and R.J. Tkacz, Degradation of the tri-n-butylin species in water and sediment from Toronto Harbor, J. Agric. Food Chem., 33 (1985) 947-953. 3 EnvironmentalAssessment: Fleetwide Use of Organotin Antifouling Paint: December 1984, Commander, Naval Sea Systems Command, SEA 56YP, Washington, DC 20362, pp. 76~1. 4 R.B.Laughlin, Jr, H.E. Guard and W.M. Coleman,III, Tributyltin in seawater: speciation and octanol-water partition coefficient,Environ. Sci. Technol., 20 (1986)201 204.
203 5 6 7
8 9 10 11 12 13 14 15 16 17 18 19 20
21
22
O.F.X. Donard and J.H. Weber, Behavior of methyltin compounds under simulated estuarine conditions, Environ. Sci. Technol., 19 (1985) 1104-1110. R.J. Maguire, Butyltin compounds and inorganic tin in sediments in Ontario, Environ. Sci. Technol., 18 (1984) 291-294. L. Randall, O.F.X. Donard and J.H. Weber, Speciation of n-butyltin compounds by atomic absorption spectrometry using electrothermal quartz furnace after hydride generation, Anal. Chim. Acta, in press. D.R. Kester, I.W. Duedall, D.N. Connors and R.M. Pytkowicz, Preparation of artificial seawater, . Limnol. Oceanogr., 12 (1967) 176-179. R.E. Truitt and J.H. Weber, Trace metal ion filtration losses at pH 5 and 7, Anal. Chem., 51 (1979) 2057-2059. A.C.M. Bourg, in C.S. Wong, E. Boyle, K.W. Bruland, J.D. Burton and E.D. Goldberg (Eds), Trace Metals in Sea Water, Plenum Press, New York, 1983, pp. 195-208. R.J. Maguire, J.H. Carey and E.J. Hale, Degradation of the tri-n-butyltin species in water, J. Agric. Food Chem., 31 (1983) 1060-1065. F.W. Page, Synthesis and Characterization of Model Aquatic Particulates for Use in Sorption Studies, Ph.D. Thesis, University of New Hampshire, 1982. K.A. Hunter and P.S. Liss, Organic matter and the charge of suspended particulates in estuarine waters, Limnol. Oceanogr., 27 (1982) 322-335. E. Tipping and D. Cooke, The effects of adsorbed humic substances on the surface charge of Goethite (a-FeOOH) in freshwaters, Geochim. Cosmochim. Acta, 46 (1982) 75-60. W. Salomons and U. FSrstner, Metals in the Hydrosphere, Springer-Verlag, Berlin, 1984, Chapts. 2, 5 and 6. J.A. Davis, Complexation of trace metals by adsorbed natural organic matter, Geochim. Cosmochim. Acta, 48 (1984) 679~91. C. Tanford, The Hydrophobic. Effect: Formation of Micelles and Biological Membranes, J. Wiley and Sons, New York, 1973, Chapt. 1. K. Schellenberg, C. Leuenberger and R.P. Schwatzenbach, Sorption of chlorinated phenols by natural sediments and aquifer materials, Environ. Sci. Technol., 18 (1984) 652-657. C.C.T. Chiou, V.H. Freed, D.W. Schmedding and R.L. Kohnert, Partition coefficient and bioaccumulation of selected organic chemicals, Environ. Sci. Technol., 11 (1977) 475-478. D.M. Di Toro, J.D. Mahony, P.R. Kirchgraber, A.L. O'Byrne, L.R. Pasquale and D.C. Piccirilli, Effects of nonreversibility, particle concentration, and ionic strength on heavy metal sorption, Environ. Sci. Technol., 20 (1986) 55-61. W.T. Bresnahan, C.L. Grant and J.H. Weber, Stability constants for the complexation of copper(II) ions with water and soil fulvic acids measured by an ion selective electrode, Anal. Chem., 50 (1978) 1675-1679. J.C. Duinker, in E. Olausson and I. Cata (Eds), Chemistry and Biogeochemistry of Estuaries, John Wiley and Sons, Chichester, 1980, Chapt. 4.
191
A D S O R P T I V E B E H A V I O R OF B U T Y L T I N C O M P O U N D S U N D E R SIMULATED ESTUARINE CONDITIONS
LOUISE RANDALL and J A M E S H. WEBER*
Chemistry Department, University of New Hampshire, Parsons Hall, Durham, NH 03824 (U.S.A.) (Received April 4th, 1986; accepted May 7th, 1986)
ABSTRACT A 23 + 1 factorial design was used to study adsorption of BuSnC13, Bu2SnC12, and Bu3SnC1 under simulated estuarine conditions. The variables included artificial seawater and its dilutions (salinity 5~35 g kg - 1), pH (6.2~.2), and hydrous iron oxide concentration (10-1000 mg 1- 1). Fulvic acid concentration was constant at 10 mg 1-1, and initial concentration of butyltin compounds was 10 ng m l - 1 (as Sn). Adsorption of butyltin compounds varied from 72 to 100°/0 for BuSnC13, 0 to 56#/0 for Bu2SnC12, and 57 to 95% for BusSnC1. At the 95#/0 confidence level all three variables were significant for BuSnC13 adsorption, pH was significant for Bu2SnC12 adsorption, and pH and salinity were significant for Bu3SnC1 adsorption. Discussion includes the importance of, and reasons for, differing adsorptive behavior of the three butyltin compounds. INTRODUCTION
The relatively high concentrations of butyltin species in the aquatic environment probably originate from the use of very toxic tributyltin compounds in antifouling marine paints [1]. Tributyltin compounds appear to be moderately persistent in aquati c systems, but Maguire and Tkacz [2] found dibutyltin, monobutyltin, and inorganic tin degradation products. In an estuary tributyltin compounds can remain in the aqueous phase of the water column, adsorb on suspended particulates, or partition to biota, sediment, or the surface microlayer [3]. The recently reported [4] octanol/water partition coefficient of 5500 for tributyltin in seawater (25 g kg -1 salinity) suggests that tributyltin concentrations in biota, suspended particulates, and sediments would be higher than the water concentration. If tributyltin adsorbs on suspended particulate matter and settles to t h e bottom, it would not be available as a toxicant in the aqueous phase of the water column. However, if the settled particulate matter is ingested by filter-feeding organisms or detritivores, the ingested tributyltin might immediately be fatal to the animal or bioaccumulate to a lethal or sublethal level. Alternatively the tributyltin might not be retained in the body tissues, but could be excreted in its toxic form and thereby recycled into the aqueous phase of the water column [3]. There are few studies of organotin compounds exposed to estuarine con* Author to whom correspondence should be addressed.
0048-9697/86/$03.50
© 1986 Elsevier Science Publishers B.V.
192
ditions [1]. Donard and Weber [5] carried out a study of methyltin compounds under simulated estuarine conditions and found that adsorption on hydrous iron oxide was in the order: monomethyltin > dimethyltin > trimethyltin. Their conclusion was that the very toxic trimethyltin would not settle out in the estuary. Maguire [6] has shown that in 75% of water column-sediment systems studied, the sediment contained more than 50% of the butyltin species or inorganic tin. Maguire and Tkacz [2] found relatively strong binding of Bu3Sn ÷ to sediments. However, degradation rates, pathways, and products in sediments are not well documented [3]. This study incorporates a 23 + 1 factorial design to assess partitioning of mono-, di-, and tributylin compounds between artificial seawater solution phases and a fulvic acid-coated hydrous iron oxide solid phase. Variables are pH, salinity, and hydrous iron oxide concentration; and fulvic acid concentration is constant. The overall results are t h a t adsorption on hydrous iron oxide is in the order: monobutyltin > tributyltin > dibutyltin. This order suggests t h a t monobutyltin compounds are most likely to settle in an estuary and that dibutyltin compounds are most likely to remain in solution. The most toxic tributyltin compounds are likely to be present in both the water column and the sediment. EXPERIMENTAL
Apparatus The analytical technique, described elsewhere [7], allows speciation of butyltin compounds by hydride generation-atomic absorption spectrometry.
Materials and methods Reagents used and the preparation of standards were described previously [5, 7]. A trimethyltin chloride standard (185 mg l-l) was prepared in deionized distilled water spiked with l m l of 5 M HNO3 and was stored in a Hypo-vial sealed with a crimp-on Teflon-lined septum. Artificial seawater was prepared as described by Kester et al. [8]. The appropriate amounts (Table 1) of artificial seawater (salinity, S), deionized, distilled water, hydrous iron oxide (particulate matter, PM), and fulvic acid (FA) were mixed in 250ml amber glass bottles. One set of 12 factorial experiments was performed for each butyltin compound. Experiments were performed in random order to avoid systematic errors such as personal bias or aging of solutions. The pH was determined with an Orion 91-04 pH electrode after adjustment with 0.1 M Na~ CO3 or 0.1 M HC1. After solutions were shaken for 4 h, the pH values were within _+0.2 pH units. Addition of 1 ttl of a butyltin compound solution resulted in a concentration of 10 ng ml- 1 (as Sn). After the experimental set was shaken for 12 h, the pH was still within _+0.2 pH units. Each sample was filtered through 0.4ttm Nucleopore polycarbonate filters
193 TABLE 1 P e r c e n t b u t y l t i n c o m p o u n d r e m o v a l by h y d r o u s i r o n o x i d e a n d i t s a d s o r p t i o n efficiency i n t h e p r e s e n c e of 10 mg 1-1 f u l v i c a c i d a Parameter Level
S ( g k g 1)
P M (mg1-1)
-
5
10
0 +
20 35
100 1000
% Removalb
pH 6.2 7.2 8.2 R e m o v a l efficiencyc (pmol g - 1)
Exp t.
S
PM
pH
Bu
Bu s
Bu 3
1 2 3 4 5 6 7 8 9 10 11 12
+ + + + 0 0 0 0
+ + + + 0 0 0 0
+ + + + 0 0 0 0
100 100 72 86 100 100 81 90 93 92 91 92
19 0.0 0.0 18 56 41 52 43 23 20 31 28
68 60 78 57 81 75 95 61 72 77 67 79
Bu
Bu 2
Bu 3
0.035 0.035 2.541 3.030 0.035 0.035 2.881 3.200 0.328 0.326 0.321 0.329
0.006 0.000 0.000 0.596 0.018 0.013 1.718 1.419 0.077 0.066 0.103 0.091
0.021 0.019 2.393 1.751 0.025 0.023 2.919 1.862 0.220 0.236 0.204 0.241
a A b b r e v i a t i o n s : S, s a l i n i t y (g k g - ~); PM, h y d r o u s i r o n o x i d e c o n c e n t r a t i o n (mg 1-1); Bu, BuSnCla; Bus, Bu2SnC12; Bu3, Bu3SnC1. b P e r c e n t r e m o v a l is t h e p e r c e n t c o m p o u n d r e m o v e d by f i l t r a t i o n . c R e m o v a l efficiency is t h e m i c r o m o l e s of c o m p o u n d r e m o v e d pe r g r a m of h y d r o u s i r o n oxide.
using an apparatus described by Truitt and Weber [9], and the filtrate was collected in a 250 ml amber glass bottle. Samples were spiked immediately with 1 ml of 5 M HNOa to bring the pH below 2 and decrease adsorption on the walls, diluted (if necessary) to fit the linear part of the calibration curve, and immediately analyzed. Three points from the calibration curve were run between samples. Calculations were made from internal trimethyltin chloride standards, and results were expressed as percent butyltin compound removed by filtration. The relative standard deviation (RSD) for the four center points was 1% for BuSnC13, 19% for Bu2SnC12, and 7% for Bu3SnC1. Adsorption on walls was measured at the high and low S, PM, and pH levels and at one center point (Table 1) for each butyltin compound. After a 12h equilibration, solutions were removed from the amber glass bottles. The bottles were rinsed with 25 ml of deionized distilled water and then filled with 100 ml of 1:20 HNOa/deionized water. After 4 h the leaching solutions were analyzed.
194 FACTORIAL EXPERIMENTAL DESIGN
Donard and Weber [5] previously discussed the rationale for factorial design experiments. This study used the three parameters salinity, particulate matter concentration, and pH at levels commonly found in estuaries. These three are very important in non-biological processes that determine partitioning between solution and particulate matter phases. Salinity is the reference scale for estuarine processes, hydrous iron oxide is an important scavenger of heavy metals in aquatic environments, and pH is important in regulating surface/ solution distribution [10]. The constant 10 mg 1-1 fulvic acid concentration is appropriate for estuaries, and the concentration of butyltin compounds was sufficient to allow measurement after high percent removal (indicated by preliminary experiments). RESULTS
Background losses and decomposition Adsorptive losses to walls, which were greatest for low S, PM, and pH levels, were less than 5% for BuSnC13, 2% for Bu2SnC12, and 1% for Bu3SnC1. Minor degradation reactions of butyltin compounds during these experiments were due to adsorption-related phenomena not to the method of determination [7]. Mono- and dibutyltin (up to 3 and 11%, respectively) occurred in tributyltin solutions, monobutyltin (up to 8%) occurred in dibutyltin solutions, and dibutyltin (up to 6%) occurred in monobutyltin solutions. The maximum loss was 14% of added tributyltin. Maguire and co-workers observed degradation of tributyltin species in biologically active harbor water and sediments [11], but not in distilled water [2].
General results Each butyltin chloride showed a different pattern of removal by adsorption under simulated estuarine conditions (Table 1). The removal of BuSnC13 was nearly complete (72-100%), but removal of Bu2SnC12 (0--56%) and BusSnC1 (57-95%) had greater ranges. Removal efficiencies in Table 1 paralleled percent removal. For each butyltin compound efficiency decreased as PM concentration increased, but was virtually unaffected by S or pH. Figures 1-3 show the separate effects of S, PM, and pH on butyltin compound removal. The lines between pairs of points indicate general trends. The slope of a line is significantly nonzero if its high and low values vary more than the ~/o RSD for the compound determined from the center points. Underlined values in Table 2 indicate parameters and their interactions significant at the 95% confidence level. All three parameters and several interactions are significant for BuSnC13 removal. The very high PM value (803.01) indicates its extreme importance, which steep slopes in Fig. 2a also demonstrate. For Bu2SnC12 pH is the only significant variable. Significant
195 (a) 100- mm-o
95.
90.
85.
80
75.
70
Ib'
llj
2b
2(s
,b
,(s
(b) 60.
50
40 > o
30
w
g
20
~
~
I0-
0,:
ib
J i6
.1----m
j J 2b
2~
.b
.-%
2(s
3b
3(i
(c) 95908580-
7570656055
1(;
~
2b S(g/kg)
Fig. 1. I n f l u e n c e of s a l i n i t y (S) o n r e m o v a l of (a) m o n o b u t y l t i n chloride, (b) d i b u t y l t i n chloride, a n d (c) t r i b u t y l t i n c h l o r i d e . + P M , + p H (m), + P M , - p H (D), - P M , + p H (e), - P M , - pH (o).
196
(a) 95
~
90
85
80-
75 7°I
,do
1obo
(b)
60-
50-
40-
30-
20-
iO-
0 l
1do
~o
(c)
95.
~
85-9°"
-
~
_
~
80-
75.
~
7o.
~
B5.
~
60.
~
55
1do PM
tdbo
(mg/L)
Fig. 9..Influence of particu|ate matter concentration (PM) on remora] o£(a) monobutyltin chloride, Co) dibutyltin chloride and (c) tributy]tin chloride. + pH, + S (I), + pH, - S (D), -pH, + S (o), -pH,
-S(O).
197 (a)
iO0 95
90-
85-
80-
75-
I '
I
I
I
I
I
I
I
I
8.12
(b)
60
50
40
30-
20-
tO
,-
,
.--I
,
,
,
,
,
,
t
I
I
I
I"--'~'~.12
(c) 959085 80 75 70 65. 60.
556.
-o I
I
I
I pH
Fig. 3. I n f l u e n c e of p H o n r e m o v a l of (a) m o n o b u t y l t i n c h l o r i d e , (b) d i b u t y l t i n c h l o r i d e a n d (c) t r i b u t y l t i n c h l o r i d e . + S, + P M ( i ) , + S, - P M (D), - S, + P M ( o ) , - S, - P M (o).
198 parameters for Bu3SnC1 are S and pH. The relatively low values for S and pH indicate t h a t these parameters were not as important for Bu3SnC1 as t hey were for BuSnC13. Sometimes parameters are not statistically significant because of opposing slopes. Salinity for Bu2SnC12 (Fig. lb) is an example. The lack of fit (LOF) term, which tests c u r v a t u r e of experimental response surfaces, is insignificant for all three butyltin compounds. This means t h a t responses to all three parameters are linear. S ed imen t- wat e r partition coefficients (K,) in Table 3 indicate t h a t tributyltin co n cen tr atio ns on the solid phase were much higher t han those in ambient water. They varied in the order: BuSnC13 >> Bu3SnC1 > Bu2SnC12 in agreement with percent removal (Table 2). The Kp values for di- and tributyltin varied considerably with PM, but relatively little with S or pH. For di- and tributyltin K, increases with decreasing PM.
Influence of parameters on butyltin removal Increasing S (Fig. 1) generally increased removal of Bu3SnC1 and Bu2SnC12, but decreased removal of BuSnC13. Increasing particulate m a t t e r concentration (Fig. 2) clearly increased removal of BuSnC13, but there are opposing trends for Bu2SnC12 and Bu3SnC1. Increasing pH (Fig. 3) decreased removal of all three butyltin chlorides. DISCUSSION Removal of butyltin compounds in our experiments could occur by many processes. Leaching experiments ruled out loss of butyltin compounds due to adsorption onto vessel walls. Precipitation of the butyltin compounds is also unlikely because of their solubility [1, 11]. Therefore, removal of butyltin compounds from solution was due predominantly to adsorption on hydrous iron oxide. TABLE 2 Statistical significance of parametersa Effectb
BuSnC13
Bu2SnC12
Bu3SnC1
S PM pH S.PM S.pH PM.pH S.PM.pH LOF
83.77 803.01 33.59 82.31 3.51 32.67 3.84 1.73
3.39 0.04 127.97 9.97 2.83 0.00 5.27 1.08
20.00 0.16 10.18 7.61 0.58 0.23 1.01 0.20
aUnderlined values indicate the significance at the 95% confidencelevel. The literature F value for 95% confidencelevel is 10.13. bAbbreviations: S, salinity; PM, hydrous iron oxide concentration; LOF, lack of fit.
199
TABLE Partition Expt.
1 2 3 4 5 6 7 8 9 10 11 12
3 c o e f f i c i e n t s (Kp), c a l c u l a t e d S
+ + + _ + 0 0 0 0
PM
+ + + + 0 0 0 0
for butyltin compounds a pH
+ + + + _ 0 0 0 0
Kp ( ~ g k g
1 ] / [ , u g l - ~ ] ) × 10 -4
BuSnC13
Bu2SnC12
Bu3SnC1
100 100 25 59 100 b 44 93 12 11 9.5 12
0.02 0.00 0.00 2.2 0.13 0.07 11 7.6 0.30 0.25 0.45 0.38
0.21 0.15 35 13 0.42 0.30 190 15 2.5 3.3 2.0 3.7
a A b b r e v i a t i o n s : S, s a l i n i t y ( g k g - 1 ) ; P M , h y d r o u s i r o n o x i d e c o n c e n t r a t i o n b 100% adsorbed.
(mgl-1).
Two major phenomena affecting the distribution of metal species between dissolved and particulate phases are adsorption onto particulate matter and flocculation in which the species are trapped by newly formed particulate matter. Because the two mechanisms are not distinguishable in this study we will use the term 'adsorption' for both processes. Removal mechanisms
The strength and nature of interactions between hydrous iron oxide and butyltin species depend on the structure, charge, and dipole moment of the butyltin compounds in water; the nature of the adsorbing surface; and the types of dissolved species-particulate matter attractive forces. The following paragraphs discuss the nature of butyltin compounds and hydrous iron oxide in artificial seawater and the nature of the attractive forces between them. The chemical nature of the aqueous butyltin compounds in these experiments is unknown. However, by analogy to methyltin analogs it is likely that the aqueous species are predominantly neutral and five- or six-coordinate due to coordination of hydroxide ions, chloride ions, and water [5]. Laughlin et al. [4] speciated tributlytin species in chloroform by extraction of bis(tributyltin) oxide from seawater and found tributyltin chloride, oxide, and carbonate species. Because of shifting of equilibria during the extraction, these results give no information about the nature of the species in seawater. Because the zero point of charge of hydrous iron oxide is at pH 8.4 [12], it will have a slightly positive charge in pH 8.2 experiments and a considerable positive charge in pH 7.2 and 6.2 experiments. However, the 10mg 1-1 fulvic
200 acid (FA) imparts a negative charge to the particles [13, 14]. At 10mg1-1 hydrous iron oxide, FA is in excess and some remains in solution, but at the 100 and 1000mg1-1 hydrous iron oxide concentrations FA was nearly 100% removed from solution [5]. Thus, neutral butyltin species reacted with negatively-charged, FA-coated hydrous iron oxide particles in our experiments. The usual adsorption model for metal ions [15] cannot explain our results because the butyltin compounds are predominantly neutral, and changes in adsorption with pH are not like those for metal ions (see below). In addition, Davis [16] showed that humic matter coating on alumina can control adsorptive behavior of metal ions. Our model developed for methyltin compounds [5] is an appropriate starting point for discussion. In this model the effectiveness of removal is proportional to the order of polarity and thus dipole moments of methyltin compounds: MeSnC13 > Me2SnC12 > Me3SnC1. However, this model is incomplete for butyltin compounds because of the mixed trend in percent removal: BuSnC13 > Bu3SnC1 > Bu2SnC12. Such a mixed trend means two or more opposing processes. The first process is due to dipole moments that predominate for methyltin compounds [5], and the second is due to nonpolar forces that are important for butyltin compounds. We will use the term 'hydrophobicity' to approximate the nonpolar nature of butyltin compounds [17]. Octanol/water partition coefficients (Kow) are used to approximate hydrophobicity. Since Kow is about 106 times higher for Bu3SnC1 than for Me3SnC1 [1], it is clear that butyltin compounds will bond much more strongly by hydrophobic interactions than analogous methyltin compounds. For this reason methyltin compounds' adsorption trends are explainable on the basis of dipoles alone. Because the hydrophobicity of alkyltin compounds increases with an increasing number of carbon atoms and number of alkyl groups bonded to tin, the observed mixed adsorption trend for butyltin compounds results from tributyltin adsorption favored by hydrophobicity and monobutyltin adsorption favored by polarity. Schellenberg et al. [18] in studies with chlorinated phenols found strong evidence that adsorption of undissociated phenols is dominated by partitioning between the aqueous phase and the organic phase in the adsorbent and is strongly correlated to Kow. Butyltin compounds might adsorb in a similar manner since they are also polar, hydrophobic compounds.
Partition coefficients Partition coefficients are important because of their relationship to physical adsorption on solids, biomagnification, and lipophilic storage [19]. In agreement with Toro et al. [20] our Kp values (Table 3) are variable. This result is not surprising because the number and nature of butyltin binding sites on the fulvic acid-coated hydrous iron oxide depend on experimental conditions. Excess FA remained in solution in experiments at low PM, but not at high PM. This means that at low PM only the most strongly-attracted FA molecules adsorbed to the hydrous iron oxide, and that the surface is different than in the case of high PM.
201 Maguire and Tkacz [2] determined the partition coefficient of Bu3Sn + to be (2.18 + 0.35)× 103 (equilibrium Bu3Sn + concentration 5ngSn1-1, ionic strength 0.77M, sediment organic carbon content 2.4%). At high PM concentrations the K, determined in this work is of the same order of magnitude (0.15-0.42 × 104 [#gkg 1]/[ggl-1]). But at low or intermediate PM concentrations our K, values were 1, 2, or 3 orders of magnitude higher. An increase in Kp values with decreased PM clearly occurs for di- and tributyltin, and is due to only selected, strongly-attracted FA molecules adsorbed at low PM values. This behavior agrees with results for titrations of fulvic acid by a metal ion such as Cu 2+ [21]. Initially the stability constant is high because Cu 2÷ binds only to the strongest sites in the fulvic acid mixture. As more and more Cu 2÷ binds, weaker and weaker sites are used and the average stability constant decreases. There is no reason for adsorbed fulvic acid not to bind butyltin compounds at sites of varying strengths. The reason that monobutyltin, for which K, values vary by only one magnitude, did not distinguish between high and low PM is t h a t its high affinity for the particulate matter overcame the differences in the strengths of binding sites.
Importance of salinity At high S less adsorption is expected due to competition of chloride ions with the PM for the butyltin species and competition of sodium ions with the butyltin compound for carboxylate sites on the PM [5]. Thus, Na ÷ partially neutralizes - COO groups of FA bound to the PM leading to a less negative PM charge. The expectation of reduced butyltin compound removal with increasing salinity was met for BuSnC13 (Fig. la) when PM was low. Bu2SnC12 and Bu3SnC1, in contrast, exhibited a general trend of increasing removal at high salinity. In these cases increasing salinity accentuates the importance of nonpolar interactions by neutralization of - COO of adsorbed FA by Na ÷.
Importance of particulate matter concentration Increasing concentrations of hydrous iron oxide, which is important for the transport of trace metals in n a t u r a l water [22], generally increases metal ion removal by providing more adsorption sites. Monobutyltin (Fig. 2a) exhibits the expected behavior, but di- and tributyltin generally do not. Significantly increased percent removal at high PM occurs for dibutyltin only when both S and pH are high and for tributyltin only when both S and pH are low. For other experimental conditions the usual trend for these two compounds is decreased removal at high PM. Since the trends are anomalous with different S and pH, the differing behavior of the butyltin compounds must be related to the difference among mono-, di-, and tributyltin compounds. The higher polarity of the monobutyltin chloride compared with the di- and tributyltin chlorides might explain the high adsorption of the monobutyltin chloride to the PM. The general trend of decreasing adsorption with increasing PM for di- and tributyl-
202 tin chloricle is probably due to their enhanced capability for nonpolar binding and lessened capability for dipolar binding relative to monobutyltin. Possibly non-adsorbed FA t h a t occurs at low PM binds hydrophobically to di- and tributyltin compounds and the resulting complexes are preferentially adsorbed.
Importance of pH The strong effect of a nar r ow pH range on metal ion adsorption is well known [15]. In this study pH was statistically significant for all three butyltin compounds, and particularly so for dibutyltin (Table 2). For mono-, di-, and tributyltin compounds the general trend, in contrast to t h a t for metal ions, was decreased removal with increasing pH (Fig. 3). Because of - COO- groups of the FA coating on the hydrous iron oxide, the particulate m at t er will be negatively-charged at all pH values in these experiments. Thus, in contrast to most studies with metal ions in the absence of fulvic acid, the PM charge did not change from positive to negative within the pH range of the experiments. However, as pH increased, higher concent r a t i ons of hydroxide and carbonate ions would more effectively compete with chloride ion for cationic butyltin species. The results suggest decreased adsorption of hydroxide and carbonate complexes relative to chloride complexes. CONCLUSIONS During estuarine processes m onobut yl t i n will be present mainly in the adsorbed phase, and dibutyltin will occur predominantly in the water column. Significant amounts of t r i but yl t i n will remain in solution and on the particulate matter. This ubiquitous presence of the very toxic t ri but yl t i n implies t h a t it might be available to both pelagic and benthic organisms. ACKNOWLEDGEMENTS We t h a n k Mar k Anderson for writing the computer program for statistical analysis and Rachel Kennedy for helpful discussions. This research was partially supported by National Science F o u n d a t i o n grant CEE 81-16960 and Environmental P r o t e c t i on Agency grant R-809416. REFERENCES 1 J.A.J. Thompson, M.G. Sheffer, R.C. Pierce, Y.K. Chau, J.J. Cooney, W.R. Cullen and R.J. Maguire, Organotin Compounds in the Aquatic Environment, NRCC/CNRC No. 22494 Otta w a , C a n a d a K1A OR6, 1985. 2 R.J. Maguire and R.J. Tkacz, Degradation of the tri-n-butylin species in water and sediment from Toronto Harbor, J. Agric. Food Chem., 33 (1985) 947-953. 3 EnvironmentalAssessment: Fleetwide Use of Organotin Antifouling Paint: December 1984, Commander, Naval Sea Systems Command, SEA 56YP, Washington, DC 20362, pp. 76~1. 4 R.B.Laughlin, Jr, H.E. Guard and W.M. Coleman,III, Tributyltin in seawater: speciation and octanol-water partition coefficient,Environ. Sci. Technol., 20 (1986)201 204.
203 5 6 7
8 9 10 11 12 13 14 15 16 17 18 19 20
21
22
O.F.X. Donard and J.H. Weber, Behavior of methyltin compounds under simulated estuarine conditions, Environ. Sci. Technol., 19 (1985) 1104-1110. R.J. Maguire, Butyltin compounds and inorganic tin in sediments in Ontario, Environ. Sci. Technol., 18 (1984) 291-294. L. Randall, O.F.X. Donard and J.H. Weber, Speciation of n-butyltin compounds by atomic absorption spectrometry using electrothermal quartz furnace after hydride generation, Anal. Chim. Acta, in press. D.R. Kester, I.W. Duedall, D.N. Connors and R.M. Pytkowicz, Preparation of artificial seawater, . Limnol. Oceanogr., 12 (1967) 176-179. R.E. Truitt and J.H. Weber, Trace metal ion filtration losses at pH 5 and 7, Anal. Chem., 51 (1979) 2057-2059. A.C.M. Bourg, in C.S. Wong, E. Boyle, K.W. Bruland, J.D. Burton and E.D. Goldberg (Eds), Trace Metals in Sea Water, Plenum Press, New York, 1983, pp. 195-208. R.J. Maguire, J.H. Carey and E.J. Hale, Degradation of the tri-n-butyltin species in water, J. Agric. Food Chem., 31 (1983) 1060-1065. F.W. Page, Synthesis and Characterization of Model Aquatic Particulates for Use in Sorption Studies, Ph.D. Thesis, University of New Hampshire, 1982. K.A. Hunter and P.S. Liss, Organic matter and the charge of suspended particulates in estuarine waters, Limnol. Oceanogr., 27 (1982) 322-335. E. Tipping and D. Cooke, The effects of adsorbed humic substances on the surface charge of Goethite (a-FeOOH) in freshwaters, Geochim. Cosmochim. Acta, 46 (1982) 75-60. W. Salomons and U. FSrstner, Metals in the Hydrosphere, Springer-Verlag, Berlin, 1984, Chapts. 2, 5 and 6. J.A. Davis, Complexation of trace metals by adsorbed natural organic matter, Geochim. Cosmochim. Acta, 48 (1984) 679~91. C. Tanford, The Hydrophobic. Effect: Formation of Micelles and Biological Membranes, J. Wiley and Sons, New York, 1973, Chapt. 1. K. Schellenberg, C. Leuenberger and R.P. Schwatzenbach, Sorption of chlorinated phenols by natural sediments and aquifer materials, Environ. Sci. Technol., 18 (1984) 652-657. C.C.T. Chiou, V.H. Freed, D.W. Schmedding and R.L. Kohnert, Partition coefficient and bioaccumulation of selected organic chemicals, Environ. Sci. Technol., 11 (1977) 475-478. D.M. Di Toro, J.D. Mahony, P.R. Kirchgraber, A.L. O'Byrne, L.R. Pasquale and D.C. Piccirilli, Effects of nonreversibility, particle concentration, and ionic strength on heavy metal sorption, Environ. Sci. Technol., 20 (1986) 55-61. W.T. Bresnahan, C.L. Grant and J.H. Weber, Stability constants for the complexation of copper(II) ions with water and soil fulvic acids measured by an ion selective electrode, Anal. Chem., 50 (1978) 1675-1679. J.C. Duinker, in E. Olausson and I. Cata (Eds), Chemistry and Biogeochemistry of Estuaries, John Wiley and Sons, Chichester, 1980, Chapt. 4.