Pb. Kozelka et Kw. Bruland, CHEMICAL SPECIATION OF DISSOLVED CU, ZN, CD, PB IN NARRAGANSETT BAY, RHODE-ISLAND, Marine chemistry, 60(3-4), 1998, pp. 267-282
The chemical speciation of 'dissolved' Cu, Zn, Cd, and Pb was determin
ed by differential pulse anodic stripping voltammetry (DPASV) at three
sampling sites in the high salinity region (24-30 psu) of the Narraga
nsett Bay estuary, Speciation results show that organic chelates of th
ese four trace metals are the dominant forms in conventionally filtere
d (< 0.2 mu m) samples with ranges from 51 to 97% for Zn, 73 to 83% fo
r Cd, 67 to 94% for Pb and > 99% for Cu in these estuarine waters. Dis
solved Zn ([Zn-T]) varied from 72 nM at the Upper Bay site to 16 nM at
the Lower Bay site, the concentration of the Zn-binding ligands ([L-T
]) varied from 48 to 11 nM with a relatively uniform conditional stabi
lity constant (K-ZnL,Zn'(cond) = 10(9.0) M-1). At the Upper and Lower
Bay sites, where [Zn-T] > [L-T], there was less organic complexation (
50 to 60%) of [Zn-T] as compared to the Mid-Bay (bloom) station, where
[Zn-T] < [L-T] and the degree of organic complexation was considerabl
y higher (97%). The concentration of Cd-binding ligands was nearly con
stant throughout the bay (ca. 4 nM with mean K-CdL,Cd'(cond) = 10(8.9)
M-1) and present at much higher levels than the dissolved Cd ([Cd-T]
= 0.80-0.29 nM, decreasing down bay). Titration results for Pb were mo
deled by two classes of Pb-binding ligands, Dissolved Pb ranged from 0
.32 to 0.13 nM and the strong class of Pb-binding ligands (L-1) was in
slight excess of these values (ca. 0.8 nM) with a mean K-PbL1,Pb'(con
d) = 10(10.0) M-1. A weaker class of Pb-binding ligands (L-2) existed
at 4.0-8.2 nM with a mean K-PbL2,Pb'(cond) 10(8.8) M-(1). Titration da
ta for Cu were interpreted as being influenced by three classes of Cu-
binding ligands (L-1, L-2, L-3), whose sum was always in excess of the
dissolved [Cu-T] (12.7-27.9 nM). Our DPASV estimates of [L-1] were 16
-38 nM with a K-CuL1,Cd'(cond) greater than or equal to 10(12) M-1. Th
ere were higher concentrations of intermediate and weaker classes of C
u-binding ligands: [L-2] = 40 to 15 nM with a mean K-CuL2,Cu'(cond) =
10(8.8) M-1 and [L-3] = 100 to 54 nM with a mean K-CuL3,Cu'(cond) = 10
(7.7) M-1. The two stronger classes of Cu-binding ligands (L-1 and L-2
) resulted in CuL1 and CuL2 as the dominant dissolved Cu species. Free
ion concentrations were calculated for all four metals at the three s
ampling sites: [Cu2+] less than or equal to 10(-12) M, [Cd2+] less tha
n or equal to 2 X 10(-12) M, [Pb2+] similar to 10(-12) M, while [Zn2+]
ranged from 0.3-13 X 10(-9) M. For Cu, Pb, and Cd, these ambient valu
es are below the concentrations of toxic responses reported from labor
atory studies of marine microorganisms. At the Upper Bay site, any tox
ic effect of the elevated free zinc ion concentration is likely to be
alleviated by the antagonistic effect of high dissolved Mn. (C) 1998 E
lsevier Science B.V.