Stoichiometry, polarity, and organometallics in solid-phase extracted dissolved organic matter of the Elbe-Weser estuary

Abstract

Dissolved organic matter (DOM) is ubiquitous in natural waters and plays a central role in the biogeochemistry in riverine, estuarine and marine environments. This study quantifies and characterizes solid-phase extractable DOM and trace element complexation at different salinities in the Weser and Elbe River, northern Germany, and the North Sea. Dissolved organic carbon (DOC), total dissolved nitrogen (TDN), Co and Cu concentrations were analyzed in original water samples. Solid-phase extracted (SPE) water samples were analyzed for DOC (DOCSPE), dissolved organic nitrogen (DONSPE), sulfur (DOSSPE) and trace metal (51V, 52Cr, 59Co, 60Ni, 63Cu, 75As) concentrations. Additionally, different pre-treatment conditions (acidification vs. non-acidification prior to SPE) were tested. In agreement with previous studies, acidification led to generally higher recoveries for DOM and trace metals. Overall, higher DOM and trace metal concentrations and subsequently higher complexation of trace metals with carbon and sulfur-containing organic complexes were found in riverine compared to marine samples. With increasing salinity, the concentrations of DOM decreased due to estuarine mixing. However, the slightly lower relative decrease of both, DOCSPE and DONSPE (~77%) compared to DOSSPE (~86%) suggests slightly faster removal processes for DOSSPE. A similar distribution of trace metal and carbon and sulfur containing DOM concentrations with salinity indicates complexation of trace metals with organic ligands. This is further supported by an increase in Co and Cu concentration after oxidation of organic complexes by UV treatment. Additionally, the complexation of metals with organic ligands (analyzed by comparing metal/DOCSPE and metal/DOSSPE ratios) decreased in the order Cu > As > Ni > Cr > Co and thus followed the Irving-Williams order. Differences in riverine and marine trace metal containing DOMSPE are summarized by their average molar ratios of (C107N4P0.013S1)1000V0.05Cr0.33Co0.19Ni0.39Cu3.41As0.47 in the riverine endmember and (C163N7P0.055S1)1000V0.05Cr0.47Co0.16Ni0.07Cu4.05As0.58 in the marine endmember.

Particle aggregation, pH changes and metal behaviour during estuarine mixing: review and integration

“…3). Ksionzek et al (2018) measured a reduction in Cu-DOM binding with increasing salinity, which is consistent with our modelling results. Hamilton-Taylor et al (2002) also predicted that Cu-humic binding decreased substantially with increasing ionic strength under typical estuarine conditions, but they did not postulate Ca and Mg binding as a significant factor.…”

“…Cu) to colloidal and dissolved organic matter (DOM) is also very important in estuaries because a quite different geochemistry can be observed for these metals compared with the predominantly particleassociated metals (Santschi et al 1997;Tang et al 2002;Illuminati et al 2019). The binding behaviour of individual metals to DOM, and the functional composition of the DOM itself, can also vary with salinity in estuaries (Zhou et al 2016;Ksionzek et al 2018). In addition, pH and inorganic carbon speciation may vary markedly in an estuary as river water, with typically lower pH and alkalinity (except in some calcareous drainage basins), mixes with seawater.…”

Riverine mixing at the molecular scale – An ultrahigh-resolution mass spectrometry study on dissolved organic matter and selected metals in the Amazon confluence zone (Manaus, Brazil)

“…DOC content and pH exert the major controls on Cu partitioning onto riverine DOC-conditioned suspended particulate matter (SPM) (Lu and Allen, 2001). Copper is also the divalent cation with strongest affinities to DOM (Xue and Sigg, 2002;Ksionzek et al, 2018). In a follow-up study, Lu and Allen (2002) found small amounts of strong phenolic Cu binding sites in DOM based on titration data.…”

“…Cu and Fe behavior could be connected to molecular groups that were not addressed by our method. Future studies could close the gap, for instance through parallel or sequential IMAC (immobilized metal-ion affinity chromatography) protocols that are able to retain metal-specific ligands (Paunovic et al, 2005;Smith et al, 2014), or direct measurement of trace metals in DOM extracts (both in retentate and permeate, Waska et al, 2015;Ksionzek et al, 2018).…”

“…Dissolved organic compounds modify particle surfaces by conditioning (Davis, 1982;Brown and Calas, 2012) and thereby influence the partitioning of trace metals (Takahashi et al, 1999). For example, iron (Fe) and copper (Cu) are among the elements that show highest affinities towards organic matter (Xue and Sigg, 2002;Ksionzek et al, 2018). Metals are complexed by a suite of competing inorganic or organic ligands that control their bioavailability and toxicity, and there is evidence for the presence of highly specific organic ligands in aquatic systems especially regarding essential trace elements like Cu and Fe (Ross et al, 2000;Buck et al, 2012;Waska et al, 2015).…”