Iron is a source of interference in the spectroscopic analysis of dissolved organic matter (DOM); however, its effects on commonly employed ultraviolet and visible (UV–vis) light adsorption and ...fluorescence measurements are poorly defined. Here, we describe the effects of iron(II) and iron(III) on the UV–vis absorption and fluorescence of solutions containing two DOM fractions and two surface water samples. In each case, regardless of DOM composition, UV–vis absorption increased linearly with increasing iron(III). Correction factors were derived using iron(III) absorption coefficients determined at wavelengths commonly used to characterize DOM. Iron(III) addition increased specific UV absorbances (SUVA) and decreased the absorption ratios (E 2:E 3) and spectral slope ratios (S R) of DOM samples. Both iron(II) and iron(III) quenched DOM fluorescence at pH 6.7. The degree and region of fluorescence quenching varied with the iron:DOC concentration ratio, DOM composition, and pH. Regions of the fluorescence spectra associated with greater DOM conjugation were more susceptible to iron quenching, and DOM fluorescence indices were sensitive to the presence of both forms of iron. Analyses of the excitation–emission matrices using a 7- and 13-component parallel factor analysis (PARAFAC) model showed low PARAFAC sensitivity to iron addition.
Dissolved organic matter (DOM) is generally thought to lower metal bioavailability in aquatic systems due to the formation of metal–DOM complexes that reduce free metal ion concentrations. However, ...this model may not be pertinent for metal nanoparticles, which are now understood to be ubiquitous, sometimes dominant, metal species in the environment. The influence of DOM on Hg bioavailability to microorganisms was examined under conditions (0.5–5.0 nM Hg and 2–10 μM sulfide) that favor the formation of β-HgS(s) (metacinnabar) nanoparticles. We used the methylation of stable-isotope enriched 201HgCl2 by Desulfovibrio desulfuricans ND132 in short-term washed cell assays as a sensitive, environmentally significant proxy for Hg uptake. Suwannee River humic acid (SRHA) and Williams Lake hydrophobic acid (WLHPoA) substantially enhanced (2- to 38-fold) the bioavailability of Hg to ND132 over a wide range of Hg/DOM ratios (9.4 pmol/mg DOM to 9.4 nmol/mg DOM), including environmentally relevant ratios. Methylmercury (MeHg) production by ND132 increased linearly with either SRHA or WLHPoA concentration, but SRHA, a terrestrially derived DOM, was far more effective at enhancing Hg-methylation than WLHPoA, an aquatic DOM dominated by autochthonous sources. No DOM-dependent enhancement in Hg methylation was observed in Hg–DOM–sulfide solutions amended with sufficient l-cysteine to prevent β-HgS(s) formation. We hypothesize that small HgS particles, stabilized against aggregation by DOM, are bioavailable to Hg-methylating bacteria. Our laboratory experiments provide a mechanism for the positive correlations between DOC and MeHg production observed in many aquatic sediments and wetland soils.
Dissolved organic carbon (DOC) concentration and chromophoric dissolved organic matter (CDOM) parameters were measured over a range of discharge in 30 U.S. rivers, covering a diverse assortment of ...fluvial ecosystems in terms of watershed size and landscape drained. Relationships between CDOM absorption at a range of wavelengths (a254, a350, a440) and DOC in the 30 watersheds were found to correlate strongly and positively for the majority of U.S. rivers. However, four rivers (Colorado, Colombia, Rio Grande and St. Lawrence) exhibited statistically weak relationships between CDOM absorption and DOC. These four rivers are atypical, as they either drain from the Great Lakes or experience significant impoundment of water within their watersheds, and they exhibited values for dissolved organic matter (DOM) parameters indicative of autochthonous or anthropogenic sources or photochemically degraded allochthonous DOM and thus a decoupling between CDOM and DOC. CDOM quality parameters in the 30 rivers were found to be strongly correlated to DOM compositional metrics derived via XAD fractionation, highlighting the potential for examining DOM biochemical quality from CDOM measurements. This study establishes the ability to derive DOC concentration from CDOM absorption for the majority of U.S. rivers, describes characteristics of riverine systems where such an approach is not valid, and emphasizes the possibility of examining DOM composition and thus biogeochemical function via CDOM parameters. Therefore, the usefulness of CDOM measurements, both laboratory‐based analyses and in situ instrumentation, for improving spatial and temporal resolution of DOC fluxes and DOM dynamics in future studies is considerable in a range of biogeochemical studies.
Key Points
Deriving DOC concentration from CDOM absorption in U.S. rivers
Linking DOM composition to CDOM parameters in U.S. rivers
Improving DOC fluxes and DOM dynamics via CDOM measurements
The fluorescent properties of dissolved organic matter (DOM) are often studied in order to infer DOM characteristics in aquatic environments, including source, quantity, composition, and behavior. ...While a potentially powerful technique, a single widely implemented standard method for correcting and presenting fluorescence measurements is lacking, leading to difficulties when comparing data collected by different research groups. This paper reports on a large-scale interlaboratory comparison in which natural samples and well-characterized fluorophores were analyzed in 20 laboratories in the U.S., Europe, and Australia. Shortcomings were evident in several areas, including data quality-assurance, the accuracy of spectral correction factors used to correct EEMs, and the treatment of optically dense samples. Data corrected by participants according to individual laboratory procedures were more variable than when corrected under a standard protocol. Wavelength dependency in measurement precision and accuracy were observed within and between instruments, even in corrected data. In an effort to reduce future occurrences of similar problems, algorithms for correcting and calibrating EEMs are described in detail, and MATLAB scripts for implementing the study’s protocol are provided. Combined with the recent expansion of spectral fluorescence standards, this approach will serve to increase the intercomparability of DOM fluorescence studies.
The dissolution of zinc oxide (ZnO) nanoparticles (NPs) is a key step of controlling their environmental fate, bioavailability, and toxicity. Rates of dissolution often depend upon factors such as ...interactions of NPs with natural organic matter (NOM). We examined the effects of 16 different NOM isolates on the dissolution kinetics of ZnO NPs in buffered potassium chloride solution using anodic stripping voltammetry to directly measure dissolved zinc concentrations. The observed dissolution rate constants (k obs) and dissolved zinc concentrations at equilibrium increased linearly with NOM concentration (from 0 to 40 mg C L–1) for Suwannee River humic and fulvic acids and Pony Lake fulvic acid. When dissolution rates were compared for the 16 NOM isolates, k obs was positively correlated with certain properties of NOM, including specific ultraviolet absorbance (SUVA), aromatic and carbonyl carbon contents, and molecular weight. Dissolution rate constants were negatively correlated to hydrogen/carbon ratio and aliphatic carbon content. The observed correlations indicate that aromatic carbon content is a key factor in determining the rate of NOM-promoted dissolution of ZnO NPs. The findings of this study facilitate a better understanding of the fate of ZnO NPs in organic-rich aquatic environments and highlight SUVA as a facile and useful indicator of NOM interactions with metal-based nanoparticles.
Specific UV absorbance (SUVA) is defined as the UV absorbance of a water sample at a given wavelength normalized for dissolved organic carbon (DOC) concentration. Our data indicate that SUVA, ...determined at 254 nm, is strongly correlated with percent aromaticity as determined by 13C NMR for 13 organic matter isolates obtained from a variety of aquatic environments. SUVA, therefore, is shown to be a useful parameter for estimating the dissolved aromatic carbon content in aquatic systems. Experiments involving the reactivity of DOC with chlorine and tetra- methylammonium hydroxide (TMAH), however, show a wide range of reactivity for samples with similar SUVA values. These results indicate that, while SUVA measurements are good predictors of general chemical characteristics of DOC, they do not provide information about reactivity of DOC derived from different types of source materials. Sample pH, nitrate, and iron were found to influence SUVA measurements.
The fate of terrestrially-derived dissolved organic carbon (DOC) is important to carbon (C) cycling in both terrestrial and aquatic environments, and recent evidence suggests that climate warming is ...influencing DOC dynamics in northern ecosystems. To understand what determines the fate of terrestrial DOC, it is essential to quantify the chemical nature and potential biodegradability of this DOC. We examined DOC chemical characteristics and biodegradability collected from soil pore waters and dominant vegetation species in four boreal black spruce forest sites in Alaska spanning a range of hydrologic regimes and permafrost extents (Well Drained, Moderately Well Drained, Poorly Drained, and Thermokarst Wetlands). DOC chemistry was characterized using fractionation, UV-Vis absorbance, and fluorescence measurements. Potential biodegradability was assessed by incubating the samples and measuring CO₂ production over 1 month. Soil pore water DOC from all sites was dominated by hydrophobic acids and was highly aromatic, whereas the chemical composition of vegetation leachate DOC varied significantly with species. There was no seasonal variability in soil pore water DOC chemical characteristics or biodegradability; however, DOC collected from the Poorly Drained site was significantly less biodegradable than DOC from the other three sites (6% loss vs. 13-15% loss). The biodegradability of vegetation-derived DOC ranged from 10 to 90% loss, and was strongly correlated with hydrophilic DOC content. Vegetation such as Sphagnum moss and feathermosses yielded DOC that was quickly metabolized and respired. In contrast, the DOC leached from vegetation such as black spruce was moderately recalcitrant. Changes in DOC chemical characteristics that occurred during microbial metabolism of DOC were quantified using fractionation and fluorescence. The chemical characteristics and biodegradability of DOC in soil pore waters were most similar to the moderately recalcitrant vegetation leachates, and to the microbially altered DOC from all vegetation leachates.
A better understanding is needed of how hydrological and biogeochemical processes control dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) composition from headwaters ...downstream to large rivers. We examined a large DOM dataset from the National Water Information System of the US Geological Survey, which represents approximately 100 000 measurements of DOC concentration and DOM composition at many sites along rivers across the United States. Application of quantile regression revealed a tendency towards downstream spatial and temporal homogenization of DOC concentrations and a shift from dominance of aromatic DOM in headwaters to more aliphatic DOM downstream. The DOC concentration–discharge (C-Q) relationships at each site revealed a downstream tendency towards a slope of zero. We propose that despite complexities in river networks that have driven many revisions to the River Continuum Concept, rivers show a tendency towards chemostasis (C-Q slope of zero) because of a downstream shift from a dominance of hydrologic drivers that connect terrestrial DOM sources to streams in the headwaters towards a dominance of instream and near-stream biogeochemical processes that result in preferential losses of aromatic DOM and preferential gains of aliphatic DOM.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The link between composition and reactivity of dissolved organic matter (DOM) is central to understanding the role aquatic systems play in the global carbon cycle; yet, unifying concepts driving ...molecular composition have yet to be established. We characterized 37 DOM isolates from diverse aquatic ecosystems, including their stable and radiocarbon isotopes (δ13C-dissolved organic carbon (DOC) and Δ14C-DOC), optical properties (absorbance and fluorescence), and molecular composition (ultrahigh resolution mass spectrometry). Isolates encompassed end-members of allochthonous and autochthonous DOM from sites across the United States, the Pacific Ocean, and Antarctic lakes. Modern Δ14C-DOC and optical properties reflecting increased aromaticity, such as carbon specific UV absorbance at 254 nm (SUVA254), were directly related to polyphenolic and polycyclic aromatic compounds, whereas enriched δ13C-DOC and optical properties reflecting autochthonous end-members were positively correlated to more aliphatic compounds. Furthermore, the two sets of autochthonous end-members (Pacific Ocean and Antarctic lakes) exhibited distinct molecular composition due to differences in extent of degradation. Across all sites and end-members studied, we find a consistent shift in composition with aging, highlighting the persistence of certain biomolecules concurrent with degradation time.
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