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Aquatic Geochemistry (v.16, #4)

AquaEnv : An Aqua tic Acid–Base Modelling Env ironment in R by Andreas F. Hofmann; Karline Soetaert; Jack J. Middelburg; Filip J. R. Meysman (pp. 507-546).

AquaEnv is an integrated software package for aquatic chemical model generation focused on ocean acidification and antropogenic CO₂ uptake. However, the package is not restricted to the carbon cycle or the oceans: it calculates, converts, and visualizes information necessary to describe pH, related CO₂ air–water exchange, as well as aquatic acid–base chemistry in general for marine, estuarine or freshwater systems. Due to the fact that it includes the relevant acid–base systems, it can also be applied to pore water systems and anoxic waters. AquaEnv is implemented in the open source programming language R , which allows for a flexible and versatile application: AquaEnv ’s functionality can be used stand-alone as well as seamlessly integrated into reactive-transport models in the R modelling environment. Additionally, AquaEnv provides a routine to simulate and investigate titrations of water samples with a strong acid or base, as well as a routine that allows for a determination of total alkalinity and total carbonate values from recorded titration curves using non-linear curve-fitting.

Keywords: Acid–base chemistry; pH modelling; Ocean acidification; Reactive-transport models; Marine estuarine and freshwater systems; CO₂ air–sea exchange; In silico titration; TA determination

Reductive Transformation of 2,4-Dinitrotoluene: Roles of Iron and Natural Organic Matter by Minori Uchimiya (pp. 547-562).

This study investigated the effects of redox-active and iron-coordinating functional groups within natural organic matter (NOM) on the electron transfer interactions between Fe(II) and 2,4-dinitrotoluene (2,4-DNT), an energetic residue often encountered in aqueous environments as a propellant component and impurities in 2,4,6-trinitrotoluene (TNT). Experiments were first conducted in homogeneous phases as a function of pH in the presence of ligands that (1) complex iron (e.g., citric acid, oxalic acid), (2) complex and reduce iron (e.g., caffeic acid, ascorbic acid), and (3) humic substances with known carboxyl content and electron transfer capacity. Then, effects of these NOM components on Fe(II) reactivity in heterogeneous media were investigated by introducing goethite. Our results indicate complex catalytic and inhibitory effects of NOM components on the reaction between Fe(II) and 2,4-DNT, depending upon the ability of NOM component to (1) reduce dissolved and particulate Fe(III) (e.g., ascorbic acid), (2) form kinetically labile dissolved Fe(II) reductants (e.g., tiron and caffeic acid), and (3) produce surface-associated Fe(II) species that are accessible to 2,4-DNT.

Keywords: 2,4-Dinitrotoluene; Explosives; Electron transfer; Nitroaromatic compounds; Iron; Natural organic matter

Marine Dissolved Organic Phosphorus Composition: Insights from Samples Recovered Using Combined Electrodialysis/Reverse Osmosis by Cindy L. Young; Ellery D. Ingall (pp. 563-574).

The dominant phosphorus compound classes were characterized in marine samples using a new, high recovery method for isolating and concentrating bulk dissolved organic matter (DOM) called combined electrodialysis + reverse osmosis (ED/RO). In contrast to earlier studies that use ultrafiltration (UF) to recover only the high molecular weight DOM, ED/RO is capable of isolating both low molecular weight (LMW) and high molecular weight (HMW) DOM. Samples were collected from a broad range of marine environments: along a transect incorporating coastal and offshore waters off the Southeastern United States, in Effingham Inlet, a Pacific fjord located on Vancouver Island, British Columbia and in the Amundsen Sea, Antarctica. Results from phosphorus nuclear magnetic resonance (³¹P NMR) analysis reveal a similar abundance of P compound classes among samples, phosphate esters (80–85%), phosphonates (5–10%) and polyphosphates (8–13%). These samples contain significantly higher proportions of polyphosphate P and P esters and lower proportions of phosphonates than measured in previous studies using the UF method. The much higher levels of polyphosphate detected in our samples suggests that polyphosphate is present mainly in the LMW dissolved matter fraction. Polyphosphates in dissolved matter may be present as (or derived from) dissolved nucleotides or organismal polyphosphate bodies, or both. Low molecular weight P esters are possibly composed of phosphoamino acids and small carbohydrates, like simple sugar phosphates and/or dissolved nucleotides. Phosphonates in DOM are more prevalent as HMW phosphonate compounds, which suggests that LMW phosphonates are more readily utilized in marine ecosystems. Overall, the investigation of DOM across a size spectrum that includes both the HMW and the LMW fractions reveals a new picture of phosphorus distribution, cycling and bioavailability.

Keywords: Phosphorus; Polyphosphate; Phosphonate; Marine; Amundsen Sea; Effingham inlet; Electrodialysis

The Association of Cobalt with Iron and Manganese (Oxyhydr)oxides in Marine Sediment by Anthony Stockdale; William Davison; Hao Zhang; John Hamilton-Taylor (pp. 575-585).

Formation and dissolution of authigenic Fe and Mn (oxyhydr)oxides influence cycling of trace metals in oxic/suboxic surface sediments. We used the diffusive gradients in thin films technique (DGT) to estimate the association of cobalt with iron and manganese oxides. We compared Co, Fe and Mn maxima measured by DGT in the pore waters of fresh and aged marine sediment cores and estimated the Co/Fe and Co/Mn ratios in the metal oxides. A Mn maximum was not visible in DGT concentration profiles of freshly collected sediment cores, but after ageing the sediment, we observed a distinct Mn peak, presumably due to broadening of the depth range over which the various electron acceptors occur. Estimated Co/Mn ratios from both experiments are within the range of literature values for marine sediments, but the value from the aged experiment is at the lower end of the range. This is attributed to stimulation of sulphate reduction and precipitation of cobalt sulphides. The good correlation between Co and Fe maxima in the fresh sediments is attributed to the similarity of their reactions with sulphide rather than Co being released during authigenic Fe oxide reduction.

Keywords: DGT; Porewater; Trace metal; Biogeochemical cycling

Biogeochemical Modelling of a Seasonally Anoxic Lake: Calibration of Successive and Competitive Pathways and Processes in Lake Aydat, France by F. A. Lopes; G. Michard; M. Poulin; A. Roué; F. Prévot; D. Jézéquel; E. Viollier (pp. 587-610).

A reactive transport model was developed to describe seasonal variations of biogeochemical and physical processes in Lake Aydat. The model includes physical processes such as vertical mixing, sedimentation and advection related to inflows into the lake and biogeochemical conversion processes in the water column and in the sediment surface layer. The reactions described in the model include primary redox reactions such as primary production, aerobic and anaerobic respiration, methanogenesis and secondary reactions established between oxidants and reducers produced by the primary reactions. After adjusting various kinetic constants, the model reasonably reproduced the main features of seasonal variations of dissolved oxygen and nitrate depth profiles and pH. The reactive transport model was also used to quantify the relative importance of different biogeochemical pathways. For instance, ferrous denitrification seems to play an important role when stratification is increasing.

Keywords: Eutrophic lake; Reactive-transport model; Biogeochemistry; Redox

Analytical Artifacts Associated with the Chelating Resin Extraction of Dissolved Rare Earth Elements in Natural Water Samples by Intae Kim; Seolwon Kim; Guebuem Kim (pp. 611-620).

In order to determine rare earth elements (REE) in coastal seawater and groundwater samples using an inductively coupled plasma mass spectrometer, a time-efficient chelating resin column method has been widely used for small water volumes (<100 mL). We obtained improved, quantitative (>95%) results for extracting REE from most of the seawater (including the certified reference materials) and groundwater samples at extraction conditions of pH 5.8–6.0, flow rates <1.2 mL min⁻¹, and 1.2 g of wet resin weight (Chelex 100, Bio-Rad, 7 mm diameter). However, we often encountered large fractionations of HREE (heavy REE) relative to LREE (light REE) associated with chelating resin extraction for some organic-rich coastal seawater samples. This fractionation could be due to unidentified complexation of HREE with natural organic substances in water samples. Our results imply that many previous observations reporting the natural fractionation of HREE using this method could have been misleading. Thus, we suggest that the quantitative (>95%) extraction of REE should be confirmed by using either one of the HREE (i.e., Tm) as a spike or isotope dilution for natural aqueous samples which have large matrix effects.

Keywords: Rare earth elements; Lanthanides; Chelating resin; Seawater; ICP-MS

Organic Sulfides in Hydrothermal Solution: Standard Partial Molal Properties and Role in Organic Geochemistry of Hydrothermal Environments by Mitch Schulte (pp. 621-637).

Thermodynamic properties for aqueous alkyl sulfides have been compiled and/or estimated through established methods. These properties are used to investigate reactions among various sulfur compounds in a variety of geological environments, ranging from sea floor hydrothermal systems to organic-rich sludge. Using thermodynamic data and the revised Helgeson-Kirkham-Flowers (HKF) equations of state, along with geochemical constraints imposed by the environment, it is possible to estimate the abiotic production of this class of organic sulfur compounds. For example, in hydrothermal systems in which H₂ and H₂S concentrations are buffered by the pyrite–pyrrhotite–magnetite (PPM) mineral assemblage, calculated equilibrium activities of dimethyl sulfide (DMS) are as high as 10⁻³ through formation reactions in which the environment contains millimolal concentrations of CO₂. Higher activities are obtained when DMS formation from CO is considered and when more reducing mineral assemblages are present.

Keywords: Organic sulfur compounds; Hydrothermal systems; Abiotic organic synthesis; Geochemical modeling; Thermodynamics

Phosphorus Speciation in Stream Bed Sediments from an Agricultural Watershed: Solid-Phase Associations and Sorption Behavior by Michael L. Machesky; Thomas R. Holm; James A. Slowikowski (pp. 639-662).

The sorption behavior and solid-phase associations of phosphorus (P) in fine-grained sediments (<63 μm) from two upstream tributaries and one downstream main stem site of the Spoon River in west-central Illinois were characterized to better understand phosphorus bioavailability in this agriculturally dominated watershed. The P sorption affinities, as indicated by linear distribution coefficients (K _d), of all sediments were 330–5,150 L/kg, and negatively correlated with equilibrium phosphorus concentration (EPC_o) values, which ranged between 0.2 and 2.2 μM. pH values measured at the conclusion of the sorption experiments varied only slightly (7.45–8.10) but were nonetheless strongly positively correlated to EPC_o values, and negatively correlated to K _d values, suggesting the importance of pH to the observed sorption behavior. K _d values were generally lower and EPC_o values higher at the main stem site than at the upstream tributary sites, suggesting dissolved reactive P (DRP) bioavailability (specifically orthophosphate) increased downstream. The solid phase associations of P were operationally assessed with the streamlined SEDEX (sedimentary extraction) procedure, and most sediment P (≥50%) was released during the step designed to determine iron oxide–associated P. On average, 70–90% of the total sediment P pool was potentially bioavailable, as estimated by the sum of the iron oxide-, authigenic carbonate-, and organic-associated P fractions. Considerable calcium was also extracted from some sediments during the step designed to specifically remove iron oxide–associated P. It is hypothesized that the severe drought conditions that persisted between April and October, 2005 allowed authigenic carbonates (perhaps partly amorphous) to accumulate, and that these carbonates dissolved during the iron oxide extraction step. The extensive benthic algal populations also present may have aided carbonate precipitation, which under more normal hydrologic conditions would be periodically flushed downstream and replaced by fresh sediment. This suggests antecedent hydrologic conditions played a dominant role in the P sorption and solid phase associations identified.

Keywords: Phosphorus; Sorption; Sequential extraction; Equilibrium phosphorus concentration

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Aquatic Geochemistry (v.16, #4)

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Aquatic Geochemistry (v.16, #4)