Environmental Engineering Reference
In-Depth Information
water partitioning of butyltins in the environment is controlled by the
suspended solids concentration [30], pH, salinity and presence of natural
DOM [31, 32], and therefore on sediment characteristics. Organic material
in sorbents plays a pivotal role for sorption at ambient pH [21]. In muni
cipal wastewater, organotins are predominantly associated with particulates,
because of the much higher suspended solids concentrations [30] compared
to surface water. TBT and TPT are very persistent in sediments and stored
for years up to a decade, or longer [14, 33, 34]. Sediments act as important
reservoirs, but ready desorption of organotins occurs [21, 31]. Resuspension
by tide, storms or dredging will lead to increased organotin levels in the water
column.
3.1
Degradation in Water and Persistence in Sediments
As abiotic transformation (photolysis and hydrolysis) is of minor importance,
environmental degradation of TBT and TPT is principally biologically medi-
ated [4, 35]. Decomposition occurs via microbial and algal degradation, and
by a range of aquatic organisms including
Chironmids
[25]. TBT and TPT
are degraded in water with half-lives in the order of 7-30 days found at high,
but two months or longer at cold water temperatures [17]. Biodegradation
is variable and dependent on environmental conditions and composition of
micro-organisms [34, 36, 37]. Only a limited number of microbes are known
to be able to degrade TBT [38]. Assuming zero-order kinetics, rapid half-
lives from 3-19 days for the disappearance of TBT were recorded in warm
water [37], where microalgae are important [39]. Complete mineralization
proceeded only slowly with half-lives of 50-75 days. In deep sea and cold
waters half-lives are in the order of several years [17]. Generally, low degra-
dation rates of TBT and TPT were reported [33, 34, 40-42] with estimated
half-lives in the order of several years, in particular under anaerobic condi-
tions (0.9-5.2 years) [4]. The half-life in northern sediments was estimated
to be more than 80 years [33]. However, fast degradation was reported in
the marine sediments of San Diego Bay [43], and after a major organotin
spill into a freshwater system [44], which must be regarded as exceptions.
TBT degradation proceeds via successive dealkylation from TBT to DBT and
MBT via hydroxylated metabolites, eventually leading to Sn
4+
.Furthermore,
methylation of organotins occurs in sediments, leading to remobilization to
the atmosphere [45, 46]. Some bacteria develop resistance to high levels of
organotins in sediments by an adaptation process. They express genes for
a multidrug efflux pump and excrete toxic organotins [47].
Degradationismuchmorerapidinthewaterthaninsediments,where
TBT and TPT were persistent over long periods of time [14, 19, 48, 49]. High-
est TBT and TPT levels occur at the top of sediment cores as shown in Fig. 3.
High TBT concentrations reflect recent inputs, and the decrease below the
