Geology Reference
In-Depth Information
processes to regenerate intermediates, maximized profits and, in theory,
prevented environmental harm from unwelcome discharges.
Unfortunately, however, sales of CaCl
2
from the Onondaga Lake
plant failed to match those of Na
2
CO
3
, with the result that excess CaCl
2
was allowed to be released into a tributary of the lake. In addition,
substantial quantities of unmarketable salts (mainly CaCl
2
) from the
Solvay Process were dumped daily into the lake. Furthermore, waste
slurries pumped into diked beds along the lake shoreline resulted in
substantial leaching of Ca
21
,Na
1
, and Cl
ions into the lake by
rainwater runoff. The Ca
21
ions reacted with CO
3
2
ions, resulting from
2H
(aq)
þ
CO
2
3 (aq)
H
2
O
(1)
þ
CO
2(g)
"
H
2
CO
3(aq)
"
(3.107)
to precipitate CaCO
3
via
Ca
2
þ
(aq)
þ
CO
2
"
CaCO
3(s)
(3.108)
3(aq)
The effect of the deposition of CaCO
3
, at up to 1.7
10
4
t per year, was
to increase the sedimentation rate of the lake several-fold, with a large
CaCO
3
delta where the major tributary, Ninemile Creek, flows into the
lake, and create a layer of CaCO
3
about 1 m thick over the bottom
sediments. As a result, Onondaga Lake became effectively a saturated
solution of CaCO
3
, with a pH of 7.6-8.2, and, most unusually for a lake
in the north-eastern USA, immune to both acid precipitation and
phosphate-induced eutrophication, the latter largely being avoided as
a consequence of
3Ca
2
þ
(aq)
þ
2PO
2
4 (aq)
"
Ca
3
(PO
4
)
2(s)
(3.109)
Since the closure of the Solvay Plant in 1980, the water quality has
improved as salt concentrations have decreased from 3500 to 1000 mg L
1
.
The other major industrial process which has polluted Onondaga
Lake has been the production of sodium hydroxide (NaOH) and
chlorine, two of the world's most important chemicals, from sodium
chloride via the electrolysis of NaCl solution and subsequent amalgama-
tion of the sodium metal produced with the mercury cathode [i.e.
Na(Hg)], prior to spraying into water to yield NaOH. With a single
electrolysis cell containing up to 4 t Hg and a single plant containing
dozens of cells, the escape of mercury (e.g. through leakage) at a rate of
5-10 kg per day from 1946-1970 into the lake resulted in mercury-
contaminated sediments (440 mg kg
1
) and fish, much of the mercury
in the latter being in the form of the highly toxic CH
3
Hg
1
. Despite the
closure of the chlor-alkali plant in the late 1980s, mercury is still flowing
into the lake from tributaries, although concentrations of soluble
