Chapter 24
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Sorption Behavior of Butyltin Compounds in Estuarine Environments of the Haihe River, China S. G. Dai, H. W. Sun, Y. Q. Wang, W. P. Chen, and N. Li College of Environmental Science and Engineering, Nankai University, Tianjin, 300071, Peoples Republic of China
Experiments were performed to disclose sorption behavior of tributyltin (TBT) and its breakdown products, dibutyltin (DBT) and monobutyltin (MBT) using sediments of the Haihe River Estuary, Tianjin, China. Sorption constants varied with butyltin species, changing in an order of MBT (2.79x10 L/Kg) >DBT (1.52x10 L/Kg) >TBT (5.76x10 L/Kg). These sorption constants were pH-dependent, with highest sorption occurring in an acidic pH range, indicating that butyltins bind to sediment mainly through the corresponding cations. Distribution percentage of butyltins decreased in residue sediment and increased in humic matter with reduction in the number of butyl groups. Sorption constant of TBT varied with the type of sediment. Sediment -3, which contained the highest content of organic matter, clay, and permanent negative charge gave the highest sorption. Sorption of TBT decreased as salinity increased. It is concluded that at acidic condition (pHDBT>TBT. It has long been noticed that the sorption behaviors of organotin compounds are affected by their molecular structures (20,26). However, results are conflicting due to the difference in absorbent, water-phase constitution, and other experimental conditions used in different studies. Randall and Weber reported a sorption affinity sequence of MBT>TBT>DBT on
Cai and Braids; Biogeochemistry of Environmentally Important Trace Elements ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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hydrous Fe oxide (26), while a sequence of TBT>DBT>MBT was recently reported by Berg et al. for a sediment-pore water system (17). This indicates that several processes may contribute to control the sorption behavior of butyltins, and sorption constants of butyltins may vary greatly under different situations.
Sorption Constants of TBT on the Three Sediments under Different Conditions A sorption isotherm of TBT was determined using the three sediments in different water phases (Table III). The original ambient water phases in the natural environment used for sediment -1, -2 and -3 are underlined. In order to investigate the influence of water-phase salinity on the sorption behavior of TBT, sorption isotherms were also measured for sediment-2 and -3 at high salinity (30.8%o). Sorption of TBT in these systems followed the Freundlich equation well, with correlation coefficients being 0.988-1.000 (data not shown).
Table III. Sorption Constants of TBT on Different Sediments Sediment
Water phase Sorption constant Freundlich equation (L/Kg) S(%o) pH LnCs=6.356+0.895LnCw Sediment-1 30.8 7.57 576.5 LnCs=7.667+0.923LnCw Sediment-2 22.6 7.24 2137.3 LnCs=7.500+0.925LnCw Sediment-2 30.8 7.57 1807.5 LnCs=8.253+0.902LnCw Sediment-3 3Ό 6.58 3837.5 LnCs=7.250+0.904LnCw Sediment-3 30.8 7.57 1408.1 *Original ambient water phases of the sediments are underlined; S means salinity. Sorption constants of TBT on the three sediments varied greatly. Under the natural aqueous conditions, sorption constants were 576.5, 2137.3 and 3837.5 L/Kg for sediment -1,-2 and -3, respectively. Sediment-3 gave the highest sorption constant. In a recent review paper, Hoch pointed out that cation exchange processes of triorganotin at permanently negative charged surfaces of clays and at deprotonated surface hydroxyl groups sufficiently describe the overall sorption of these compounds to minerals (2). Arnold et al. (18) proposed that sorption of TBT to humic matter is governed by complexation of TBT* cation by negatively charged ligands of the humic acids. Sediment-3 had the highest humic matter content, clay content and permanent negative charge, and all favorite high sorption of TBT. Overall, TBT was moderately sorbed as indicated by its sorption constants, which means that TBT is likely to be present
Cai and Braids; Biogeochemistry of Environmentally Important Trace Elements ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
379 in the water column as well as in the sediment phase, and therefore it might be available to both pelagic and benthic organisms. When water-phase salinity was elevated, sorption of TBT on sediment-2 and -3 decreased greatly. For sediment-3, when the water phase was changed from fresh water (salinity of 3.0%c and pH of 6.58) to salt water (salinity of 30.8%o and pH of 7.57), the TBT sorption constant decreased by a factor of 2.72. Both changes in salinity and pH are considered to contribute to the sorption reduction. However, based on sorption-pH curves (see Figure 3), 93% of the reduction was attributed to the influence of salinity. This result agrees with the reports that salinity has a negative influence on sorption of TBT when salinity is less than 40%o (19, 24). A decrease in sorption constant by a factor of 2 over salinity range 0 to 34% was reported by Uger et al. (19).
Titration curve of Humic Matter The primary differential potential titration curve of the extracted humic matter from sediment-1 is shown in Figure 2. It can be seen that there are four active ligand groups in the humic matter. pKa values and contents of each ligand group are listed in Table IV. At the pH below the corresponding pKa, a majority of ligands exist as =ROH, whereas at pH above pKa, negative charge site =RO" is predominant. Elevating pH increases the amount of =RO. It can be seen that at pH >5.73 the first two ligands, which account for 61% of total ligands, exist predominately as =RO" group. These pKa values provide us a good insight into sorption mechanisms of TBT. Effect of pH on Sorption of Butyltins Influence of pH on sorption of TBT, DBT and M B T was investigated (Figure 3). It can be seen that the highest sorption occurred at an acidic pH range for all three butyltins (6.0, 5.5 and 5.0 for TBT, DBT and MBT, respectively). A hydrolysis constant of 6.3 was cited for TBTOH (18). Using this constant, at pH of 6.0, 60% of the total TBT should be TBT*" ion. A second peak of sorption Table IV. pKa and Content of Ligand Groups in Humic Matter pKa Content (mol/L)
U 4.26 2.36
U 5.73 1.47
L, 8.22 1.79
U 9.97 0.65
Cai and Braids; Biogeochemistry of Environmentally Important Trace Elements ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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4
6
8
10
12
14
16
18
20
mol/Kg
Figure 2. Primary differential potential titration curve of humic matter occurred after pH 10 for TBT. Under this pH, TBT should exist as neutral molecules. Because this experiment was conducted at a salinity of 30.8%c, TBTC1 and TBTOH are expected to be the main species. The two peaks of TBT sorption with pH indicate that sorption of TBT was most likely controlled by both hydrophobic and cationic characteristics. Not only was speciation affected by pH, but also charge properties of mineral and humic matter of the sediment were pH dependent as described above. The highest peak of sorption (6.0) of TBT occurred between pKa of TBTOH (6.3), and PZC (4.12) of the mineral and pKa of the first (4.26) and second ligands (5.73) of the humic matter of the sediment. This indicates that concentration of TBT*, concentrations of =XO" groups in the mineral, and =RO" groups in humic matter of e sediment play important roles on sorption behavior. At this pH range, TBT sorption was mainly controlled by cationic character. At pH above 10, TBT is thought to exist as neutral molecules, with sorption controlled mainly by hydrophobic partitioning into humic matter. This is perhaps why a second peak of sorption occurred after pH 10. Hence, at pH4, TBT* cation is associated with the mineral by cation exchange at the sites of =XOH and with humic matter by complextion at the sites of =ROH. As pH is elevated, neutral molecules, such as TBTC1 and TBTOH become dominant, and hydrophobic partitioning gets involved. There are few reports on fine speciation of DBT and MBT, Ionization constants for DBT and MBT are not acquired. It has been reported that at pH
