Environmental Engineering Reference
In-Depth Information
the most effective in producing photodegradation. An aqueous photolysis
study [80] based on US EPA guidelines under simulated natural sunlight
showed that photolysis of zinc pyrithione in pH 9 buffer or artificial sea-
water at pH 8.2 occurred very rapidly (first-order half-life from the first
hour of incubation was
13-18 min). In contrast, similarly slow degra-
dation rates to those in the hydrolysis study were found in samples incu-
bated in the dark (half-lives extrapolated from 30 days of incubation were
83-91 days).
ZnPT and CuPT hydrolysis and photolysis [81] were also studied in ster-
ile synthetic seawater and fresh water. Whereas the half life for hydrolysis of
pyrithiones in dark sterile waters ranged from 7 to more than 90 days, the
photolysis half life ranged from 15 to 30 min. Photodegradation of ZnPT is
much faster than CuPT [82]. Under natural solar irradiation (
E
= 880 Wm
2
),
the half-lives of ZnPT and CuPT were estimated as approximately 2 and
20 min, respectively [82]. The degradation half-life of ZnPT and CuPT was
estimated to be between 7 and 9 min [79]. The degradation times for ZnPT
and CuPT ranged from 7 to 9 min and did not differ significantly from one
another (ANOVA,
p
= 0.897). Environmental photolysis rates for compounds
with high quantum yields (e.g., the pyrithiones) depend upon various factors
including angle and intensity of incident surface solar radiation, water depth
and clarity, presence of photosensitizers and particulate matter, and wave ac-
tion. Photolysis rates are strongly influenced by the photon flux density and
spectral distribution of the light sources [83]. This may explain the differ-
ences in half-lives in the different experiments mentioned above. The amount
of ZnPT and CuPT that will photodegrade is, therefore, dependent on the wa-
ter depth to which the 320-355 nm wavelengths can penetrate in the water
column, together with the photon flux density. It is clear, however, that radia-
tion in the 300-355 nm range, which causes photodegradation of pyrithione,
can penetrate to significant depths under favorable conditions. Therefore, it
is likely that photochemical processes play an important role in the environ-
mental degradation of pyrithiones. They can therefore be marketed as being
environmentally neutral, non-persistent antifouling compounds based on the
fact that they easily photolyze [76], and thus rapidly degrade into less toxic
compounds [79, 80, 84, 85].
Several degradation products were formed during ZnPT photodegrada-
tion (Fig. 7). Turley et al. [76] have reported that pyridine-2-sulfonic-acid
(1), is the major photolysis product of the parent compound. Other degrada-
tion products formed included pyridine sulfinic acid (2), pyrithione disulfide
(3), pyridine disulfide (4), and the pyridine
∼
pyrithione mixed disulfide (5)
(Fig. 7). Some byproducts formed with light exposure, but not in the dark.
An earlier work [80] also reported rapid degradation of related pyrithione
biocides (pyrithione, sodium pyrithione, pyrithione disulfide, and a tertiary-
butylamine pyrithione derivative, generally tested at 100 ppm) in seawater
exposed to sunlight, at a range of pHs.
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