Photodissolution of copper from resuspended coastal marine sediments

Abstract

Total dissolved Cu (TDCu) was photoproduced from suspensions of estuarine and coastal marine sediments from varied biogeochemical settings after exposure to the equivalent of 1 day of sunlight. Net photorelease (defined as the difference between mean light and dark concentrations) of TDCu occurred in 10 of 13 experiments and ranged from 1.1 nM to 11.8 nM (0.88–46.1 nmol Cu g−1 dry sediment). Photorelease was observed over environmentally relevant total suspended sediment concentrations ranging from 38 mg L−1 to 460 mg L−1. The magnitude of photorelease was directly correlated to the 1 M HCl‐extractable Cu and Fe contents of sediments. Photoreleased Cu was likely associated with organic matter and/or Fe oxyhydroxides, both of which are photoreactive. Photoreleased TDCu persisted for at least 24 h after irradiation indicating that the Cu was not immediately scavenged back onto particles, most likely because it was stabilized by complexation with dissolved organic matter. Results of this study are important because a conservative estimate of the areal flux of TDCu from irradiation of a modest suspended sediment loading of 100 mg L−1 in the top 100 cm of a sunlit water column would be 460 nmol m−2 d−1, which is comparable to or significantly larger than benthic and riverine fluxes of the metal in estuarine and coastal environments. These results suggest that photolytic fluxes are a significant yet previously unrecognized source of bioactive Cu to natural waters.

Constraining the Global Ocean Cu Cycle With a Data‐Assimilated Diagnostic Model

“…Reconciling these previous estimates with our diagnosed rate of dissolved Cu input requires 0.77–1.81 Gmol Cu yr −1 of “new” dissolved Cu from shelf/slope sediments in oxygen‐replete margins or additional river input not included in prior estimates, as suggested by Richon and Tagliabue (2019). Photo‐dissolution of Cu from resuspended coastal sediments may contribute to this input as well (Skrabal et al, 2018).…”

“…A candidate source mechanism is photochemical reactions in the surface ocean, which previous studies have shown can destroy or cleave Cu-ligand complexes and increase the concentration of free Cu ions (Laglera & ven den Berg, 2006;Moffett & Zika, 1987;Shank et al, 2006;Tonietto et al, 2011). If Cu is photochemically solubilized from colloids or particles larger than 0.2-0.45 μm, this photodegradation could represent a source of dissolved Cu (smaller than 0.2-0.45 μm), as demonstrated by the photo-dissolution of organic-associated Cu from resuspended coastal sediments (Skrabal et al, 2018). (Paytan et al, 2009).…”

“…This is well below the minimal concentrations found for particulate Cu in the surface ocean (typically >10 pmol kg −1 ; e.g., Little et al, 2018), indicating that such a photochemical source of dissolved Cu is reasonable. To test this hypothesis, one could perform experiments like that in Skrabal et al (2018) in the open ocean, by exposing seawater samples to light with different durations before filtering or ultra-filtering them. This could establish if including a photochemical source of dissolved Cu is important for helping prognostic models better capture the surface distribution of dissolved Cu, particularly in the subtropical gyres (Richon & Tagliabue, 2019).…”

Determination of iron(II) by chemiluminescence using masking ligands to distinguish interferences

“…, it implies that there must be a significant pool of photoreducible iron(III) in the particulate phase. Previous studies have found a correlation between labile particulate minerals and dissolved metal concentrations following irradiation (Skrabal et al ), which suggests that in Big Fisherman's Cove, a significant amount of iron minerals may be relatively fresh oxyhydroxides. The increase in total dissolved iron concentrations throughout the day indicates that photoreduction and abiological processes are the dominant controls on iron cycling, as opposed to biological uptake and remineralization.…”