Sources, Fluxes, and Biogeochemical Cycling of Silver in the Oceans - Reviews of Environmental Contamination and Toxicology - page 32

But as previously noted, environmental inputs of silver nanoparticles have been
increasing exponentially, and the bioavailability and toxicity of those materials are
poorly understood.
Silver in estuarine waters generally exhibits a non-conservative behavior, with
dissolved silver concentrations decreasing with salinity (Smith and Flegal 1993 ;
Wen et al. 1997 ; Zhang et al. 2008 ). This pattern has been attributed to silver's high
affi nity for suspended particulates, as demonstrated by its relatively high partition
coeffi cient (Kd ~10 5 ) (Sañudo-Wilhelmy et al. 1996 ; Wen et al. 1997 ; Zhang et al.
2008 ; Tappin et al. 2010 ). At low salinities, silver is strongly associated with iron
and manganese oxyhydroxide/sulfi de phases, organic macromolecules, and colloids
(Wen et al. 1997 ; Reinfelder and Chang 1999 ). In more saline waters, the macromo-
lecular fraction decreases and dissolved silver chloro-complexes become more
important (Turner et al. 1981 ; Miller and Bruland 1995 ). However, some “dissolved”
(<0.45

m) silver remains tightly bound to refractory organics in estuarine and
marine waters (Ndung'u et al. 2006 ).
Silver may also be present as a soluble sulfi de in fresh and estuarine waters, as
reported by Rozan and Luther ( 2009 ). They proposed this was due to the high asso-
ciation constant of silver sulfi des (pK 12-30) for multi-nuclear metal clusters with
stoichiometries of 2:1 and 3:3. The presence of those silver sulfi des was associated
with anoxic (sulfi dic) sediments and wastewater discharges, which would not be a
factor in most oceanic waters. However, those complexes could be important in
marine hydrothermal plumes.

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3 Measurements of Silver in the Oceans

3.1 Waters

Silver concentrations observed in oceanic waters display marked spatial differences
(Fig. 2 ). Recorded silver concentrations show a systematic increase along the
oceanic conveyor belt circulation, whereas the nutrient-type vertical profi le of silver
is retained (Bruland and Lohan 2004 ). As previously noted, silver's nutrient-type
distribution is evidenced by its covariance with the distribution of silicate, which is
illustrated in Fig. 3 , suggesting that both geochemical cycles are linked (Martin et al.
1983 ; Flegal et al. 1995 ; Ndung'u et al. 2001 ).
Silver is hypothesized to be sequestered within a refractory organic phase that is
associated with biogenic silica, and to follow a parallel biological scavenging and
subsequent remineralization (Ndung'u et al. 2006 ). However, departures from the
linear correlation between the two elements emphasize that the exact mechanisms of
the silver marine cycle are still poorly understood (Zhang et al. 2004 ; Ranville and
Flegal 2005 ; Kramer et al. 2011 ). Silver levels exceed those of silica when silver
concentrations are 20 pmol/kg, suggesting that silver may be relatively enriched

from aeolian deposition and/or is more slowly remineralized than silica. There may also
be a relatively greater diagenetic remobilization of silver from bottom sediments,

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