Environ. Sci. Technol. 1997, 31, 3629-3636
Solid Phase Microextraction To Study the Sorption of Organotin Compounds onto Particulate and Dissolved Humic Organic Matter† J U E R G E N P O E R S C H M A N N , * ,‡ FRANK-DIETER KOPINKE,‡ AND JANUSZ PAWLISZYN§ Department of Remediation Research, Center for Environmental Research, Leipzig-Halle, Permoserstrasse 15, 04318 Leipzig, Germany, and University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Solid phase microextraction (SPME) was used to investigate the sorption behavior of ionogenic and non-ionogenic alkylated organotin compounds onto humic organic matter (HOM). The basic idea of these investigations is to measure selectively the freely dissolved proportion of the target analytes rather than the proportion bound to the polymer. This allows partition coefficients of non-ionic organic chemicals to be determined without any disturbance of the sorption equilibrium in both particulate and dissolved HOM matrices. For SPME, the ionogenic organotin compounds have to be alkylated. This can easily be achieved after centrifugation of particulate HOM, whereas for SPME in the presence of dissolved HOM an in-fiber derivatization is proposed. The sorption coefficients of a given ionogenic or non-ionogenic organotin compound are similar for particulate and dissolved HOM from the same source, indicating that the same type of interaction accounts for the sorption process. The higher the degree of alkylation in the target analyte, the more pronounced the nonspecific interactions and the faster the sorption equilibrium are established. Kinetic studies performed by SPME indicate that the sorption equilibrium on dissolved HOM, is reached within a few minutes; on particulate HOM the process takes longer due to diffusion pathways.
Introduction Environmental Importance of the Target Analytes. Growing public concern about organotin compounds and their breakdown products, among them mono-, di-, tri-, and tetrabutyl tin (MBT, DBT, TrBT, and TeBT, respectively) and triphenyl tin (TrPhT), has been based on their potential environmental accumulation and harmful environmental effects (1-3). Generally, in the water column the concentrations of organotin compounds occur in the low to medium ppt range for open waters (3, 4); the concentrations in sediments are considerably higher (5, 6). The chemistry and ecotoxicology of ionogenic (mono-, di-, and tri-) organotin compounds are related to their bivalent properties: they have an organic moiety and a hydrophilic † Dedicated to Prof. Gerhard Werner on the occassion his 65th birthday. * Corresponding author e-mail:
[email protected]; fax: 0341-2352492. † Center for Environmental Research. § University of Waterloo.
S0013-936X(97)00377-5 CCC: $14.00
1997 American Chemical Society
component (7, 8). Once released in the environment, TrBT and TrPhT can undergo biotic (3, 9) and abiotic (primarily UV) degradation, consisting in a progressive dealkylation down to inorganic tin (10). On the other hand, abiotic or biotic methylation reactions can occur (11). Toxicity of the organotins varies significantly according to the nature and number of substituents (12, 13). The elemental Sn and its inorganic compounds are of minor toxicity due to their low solubility in lipids. High bioconcentration factors in marine mammals were observed for TrBT and TrPhT (14). Sorption of Organotin Compounds on Sediments/Soils and Dissolved Organic Matter. The environmental distribution and fate of organic chemicals including organotin compounds depend on their sorption onto inorganic particulates such as clay minerals, oxyhydroxides, and hydrous iron oxides (see refs 15 and 16 and refs cited therein) and onto humic organic matter (HOM) in sediments, soils, and aquifers (see refs 17 and 18 and refs cited therein). Important parameters that affect the sorption include the pH value and the salinity (18, 19). Due to the pH-dependent hydrolysis of the ionogenic organotin hydroxo complexes (19), tin cations dominate at pH < pKhydrolysis [for example: pKhydrolysis ) 6.71 for TrBTOH (7, 19)]. Obviously, only uncharged species will take part into hydrophobic sorption on HOM. The sorption tendency of organotin compounds generally decreases with increasing salinity. This can be explained by complex competition phenomena and a decrease of the volume of the particulate matter double layer, whereas the salting-out effect acts contrarily (18, 20). Sorption can be described by the general sorption coefficient Kd (mL g-1) and the carbon-normalized sorption coefficients KOC and KDOC for particulate and dissolved organic carbon (OC and DOC), respectively, which are defined by the following equation (see ref 17 and 21):
KOC )
Kd x ) fOC fOC msorbentCe
(1)
where fOC is the mass fraction of organic carbon, x is the quantity of sorbate (µmol), Ce is the equilibrium concentration of the sorbate (µmol mL-1), and msorbent is the mass of sorbent (g). Partitioning of ionogenic organotin compounds between particulate matter and water as well as their sorption onto sediment/soil matrices have been extensively described by Fent et al. (15). As an example, 95-99% of the total TrBT in Lake Lucerne water was found to be present in the dissolved phase (
