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Aquatic Geochemistry (v.16, #3)

Preface by Virender K. Sharma (pp. 313-315).

The Hydrolysis of Al(III) in NaCl solutions: A Model for M(II), M(III), and M(IV) Ions by Ryan J. Woosley; Frank J. Millero (pp. 317-324).

Understanding the identity and stability of the hydrolysis products of metals is required in order to predict their behavior in natural aquatic systems. Despite this need, the hydrolysis constants of many metals are only known over a limited range of temperature and ionic strengths. In this paper, we show that the hydrolysis constants of 31 metals [i.e. Mn(II), Cr(III), U(IV), Pu(IV)] are nearly linearly related to the values for Al(III) over a wide range of temperatures and ionic strengths. These linear correlations allow one to make reasonable estimates for the hydrolysis constants of +2, +3, and +4 metals from 0 to 300°C in dilute solutions and 0 to 100°C to 5 m in NaCl solutions. These correlations in pure water are related to the differences between the free energies of the free ion and complexes being almost equal $$ Updelta { ext{G}}^circ left( {{ ext{Al}}^{3 + } } ight) - Updelta { ext{G}}^circ left( {{ ext{Al}}left( { ext{OH}} ight)_{j}^{{left( {3 - j} ight)}} } ight) cong Updelta { ext{G}}^circ left( {{ ext{M}}^{n + } } ight) - Updelta { ext{G}}^circ left( {{ ext{M}}left( { ext{OH}} ight)_{j}^{{left( {n - j} ight)}} } ight) $$ The correlation at higher temperatures is a result of a similar relationship between the enthalpies of the free ions and complexes $$ Updelta { ext{H}}^circ left( {{ ext{Al}}^{3 + } } ight) - Updelta { ext{H}}^circ left( {{ ext{Al}}left( { ext{OH}} ight)_{j}^{3 - j} } ight) cong Updelta { ext{H}}^circ left( {{ ext{M}}^{n + } } ight) - Updelta { ext{H}}^circ left( {{ ext{M}}left( { ext{OH}} ight)_{j}^{n - j} } ight) $$ The correlations at higher ionic strengths are the result of the ratio of the activity coefficients for Al(III) being almost equal to that of the metal. $$ gamma left( {{ ext{M}}^{n + } } ight)/gamma left( {{ ext{M}}left( { ext{OH}} ight)_{j}^{n - j} } ight) cong gamma left( {{ ext{Al}}^{3 + } } ight)/gamma left( {{ ext{Al}}left( { ext{OH}} ight)_{j}^{3 - j} } ight) $$ The results of this study should be useful in examining the speciation of metals as a function of pH in natural waters (e.g. hydrothermal fresh waters and NaCl brines).

Keywords: Hydrolysis constants; Divalent, trivalent, and quadrivalent metals

Complexation of Pb(II) by Chloride Ions in Aqueous Solutions by Robert H. Byrne; Wensheng Yao; Yanxin Luo; Frank J. Millero (pp. 325-335).

Lead chloride formation constants at 25°C were derived from analysis of previous spectrophotometrically generated observations of lead speciation in a variety of aqueous solutions (HClO₄–HCl and NaCl–NaClO₄ mixtures, and solutions of MgCl₂ and CaCl₂). Specific interaction theory analysis of these formation constants produced coherent estimates of (a) PbCl⁺, $$ { ext{PbCl}}_{2}^{0} $$ , and PbCl ₃ ⁻ formation constants at zero ionic strength, and (b) well-defined depictions of the dependence of these formation constants on ionic strength. Accompanying examination of a recent IUPAC critical assessment of lead formation constants, in conjunction with the spectrophotometrically generated formation constants presented in this study, revealed significant differences among various subsets of the IUPAC critically selected data. It was found that these differences could be substantially reduced through reanalysis of the formation constant data of one of the subsets. The resulting revised lead chloride formation constants are in good agreement with the formation constants derived from the earlier spectrophotometrically generated data. Combining these data sets provides an improved characterization of lead chloride complexation over a wide range of ionic strengths: $$ egin{gathered} {log},{}_{ ext{ Cl}} eta_{ 1} = 1. 4 9 1- 2.0 4,I^{ 1/ 2} left( { 1+ 1. 5,I^{ 1/ 2} } ight)^{ - 1} +,0. 2 3 8,I hfill \ {log},{}_{ ext{ Cl}} eta_{ 2} = 2.0 6 2- 3.0 6,I^{ 1/ 2} left( { 1+ 1. 5,I^{ 1/ 2} } ight)^{ - 1} +,0. 3 6 9,I hfill \ {log},{}_{ ext{ Cl}} eta_{ 3} = 1. 8 9 9- 3.0 6,I^{ 1/ 2} left( { 1+ 1. 5,I^{ 1/ 2} } ight)^{ - 1} +,0. 4 3 9,I. hfill \ end{gathered} $$

Keywords: Lead chloride formation constants; Lead speciation in natural waters

Carbonate Chemistry Dynamics of Surface Waters in the Northern Gulf of Mexico by Nina Keul; John W. Morse; Rik Wanninkhof; Dwight K. Gledhill; Thomas S. Bianchi (pp. 337-351).

This paper presents the results of two cruises in the Northern Gulf of Mexico in 2008 that investigated local and short-term factors influencing the carbonate chemistry dynamics and saturation state with respect to aragonite (Ω_aragonite) of surface seawater in this region. One cruise covered much of the northern half of the Gulf, and the other focused on the coastal zone west of the Atchafalaya Bay outlet of the Mississippi River—the region where the hypoxic “dead zone” occurs on the Louisiana shelf. Offshore waters (>100 m depth) exhibited only small variations in CO₂ fugacity (fCO₂), total alkalinity (TA) and Ω_aragonite. Values were close to those typically observed in subtropical Atlantic Ocean and Caribbean Sea waters of similar salinity. However, inner shelf waters (<50 m depth) exhibited large variations in fCO₂, TA, and Ω_aragonite that were not directly related to salinity or distance from the Mississippi River plume. Changes in TA values were not the result of simple mixing of end-member freshwater and seawater TA concentrations but exhibited a minimum in values near salinity of 25. This minimum could be the result of microbial recycling across salinity gradients, biological removal of alkalinity by formation of calcium carbonate or mixing of a third end-member with a low alkalinity such as Terrebonne Bay. All waters were supersaturated with respect to aragonite. Offshore waters had an average Ω_aragonite of 3.86 with a standard deviation of only ±0.06 and inner shelf waters had a range in Ω_aragonite values from 3.9 to 9.7 with a median of 4.3. Shelf water Ω_aragonite values were elevated relative to the offshore as a consequence of both high TA input from Mississippi River and biological drawdown of CO₂. A dominant factor controlling Ω_aragonite distribution in offshore waters with relatively constant temperatures was fCO₂, with higher supersaturation occurring in areas with low fCO₂.

Keywords: Gulf of Mexico; Aragonite saturation state; Ocean acidification; Mississippi/Atchafalaya Rivers

Abundance and Chemical Speciation of Phosphorus in Sediments of the Mackenzie River Delta, the Chukchi Sea and the Bering Sea: Importance of Detrital Apatite by Jia-Zhong Zhang; Laodong Guo; Charles J. Fischer (pp. 353-371).

Utilizing a sequential extraction technique this study provides the first quantitative analysis on the abundance of sedimentary phosphorus and its partitioning between chemically distinguishable phases in sediments of the Bering Sea, the Chukchi Sea and the Mackenzie River Delta in the western Arctic Ocean. Total sedimentary phosphorus (TSP) was fractionated into five operationally defined phases: (1) adsorbed inorganic and exchangeable organic phosphorus, (2) Fe-bound inorganic phosphorus, (3) authigenic carbonate fluorapatite, biogenic apatite and calcium carbonate-bound inorganic and organic phosphorus, (4) detrital apatite, and (5) refractory organic phosphorus. TSP concentrations in surface sediments increased from the Chukchi Sea (18 μmol g⁻¹ of dried sediments) to the Bering Sea (22 μmol g⁻¹) and to the Mackenzie River Delta (29 μmol g⁻¹). Among the five pools, detrital apatite phosphorus of igneous or metamorphic origin represents the largest fraction (~43%) of TSP. The second largest pool is the authigenic carbonate fluorapatite, biogenic apatite as well as CaCO₃ associated phosphorus (~24% of TSP), followed by the Fe-bound inorganic phosphorus, representing ~20% of TSP. The refractory organic P accounts for ~10% of TSP and the readily exchangeable adsorbed P accounts for only 3.5% of TSP. Inorganic phosphorus dominates all of phosphorus pools, accounting for an average of 87% of the TSP. Relatively high sedimentary organic carbon and total nitrogen contents and low δ¹³C values in the Mackenzie River Delta together with the dominance of detrital apatite in the TSP demonstrate the importance of riverine inputs in governing the abundance and speciation of sedimentary phosphorus in the Arctic coastal sediments.

Keywords: Phosphorus; Sediment; Apatite; Arctic Ocean; Mackenzie River Delta

Release of Arsenic from Volcanic Rocks through Interactions with Inorganic Anions and Organic Ligands by Barbara Casentini; Maurizio Pettine; Frank J. Millero (pp. 373-393).

The effects of a number of inorganic anions (F⁻, HCO₃ ⁻, B(OH) ₄ ⁻ , Cl⁻, I⁻) and of the siderophore DFO-B on the release of As from volcanic rocks were investigated in batch experiments. While previously reported field and laboratory data support a role of inorganic anions on As mobilization into aquifers, the role of siderophores on As-induced mobilization was less investigated. Fluoride, bicarbonate and DFO-B have shown a significant influence on the release of As from the rocks. Lava was mostly affected among the investigated rocks at pH 6 and 20°C by releasing 4% of its initial As content in the presence of 0.01 M F⁻and 10% in the presence of 500 μM DFO-B. The effect of fluoride was larger at pH 6 than at pH 8.5 for all the rocks. In the case of DFO-B, there was also a larger effect at pH 6 compared to pH 8 for the various rocks except tuff. Bicarbonate played a role under alkaline conditions while its effect was negligible at pH 6. Anion exchange processes in the presence of fluoride and bicarbonate and complexation processes in the presence of the siderophore DFO-B appear to be the major processes responsible for the release of arsenic from the rocks. The siderophore DFO-B plays mainly an indirect role on the As release by complexing Al, Fe and Mn, thus favoring the dissolution of the rocks and the consequent release of As bound to surface Al, Fe and Mn oxy-hydroxides. These findings suggest that ionic interactions with fluoride, bicarbonate and siderophore may be a further triggering factor in the mobilization of As from aquifer rocks.

Keywords: Arsenic mobilization; Groundwater; Inorganic anions; Siderophores

The Role of One- and Two-Electron Transfer Reactions in Forming Thermodynamically Unstable Intermediates as Barriers in Multi-Electron Redox Reactions by George W. Luther III (pp. 395-420).

In the aquatic geochemical literature, a redox half-reaction is normally written for a multi-electron process (n > 2); e.g., sulfide oxidation to sulfate. When coupling two multi-electron half-reactions, thermodynamic calculations indicate possible reactivity, and the coupled half-reactions are considered favorable even when there is a known barrier to reactivity. Thermodynamic calculations should be done for one or two-electron transfer steps and then compared with known reactivity to determine the rate controlling step in a reaction pathway. Here, thermodynamic calculations are presented for selected reactions for compounds of C, O, N, S, Fe, Mn and Cu. Calculations predict reactivity barriers and agree with one previous analysis showing the first step in reducing O₂ to O₂ ⁻ with Fe²⁺ and Mn²⁺ is rate limiting. Similar problems occur for the first electron transfer step in these metals reducing NO₃ ⁻, but if reactive oxygen species form or if two-electron transfer steps with O atom transfer occur, reactivity becomes favorable. H₂S and NH₄ ⁺ oxidation in a one-electron transfer step by O₂ is also not favorable unless activation of oxygen can occur. H₂S oxidation by Cu²⁺, Fe(III) and Mn(III, IV) phases in two-electron transfer steps is favorable but not in one-electron steps indicating that (nano)particles with bands of orbitals are needed to accept two electrons from H₂S. NH₄ ⁺ oxidation by Fe(III) and Mn(III, IV) phases is generally not favorable for both one- and two-electron transfer steps, but their reaction with hydroxylamine and hydrazine to form N₂O and N₂, respectively, is favorable. The anammox reaction using hydroxylamine via nitrite reduction is the most favorable for NH₄ ⁺ oxidation. Other chemical processes including photosynthesis and chemosynthesis are considered for these element–element transformations.

Keywords: Thermodynamics of electron transfer; Oxygen; Nitrogen; Sulfur; Iron; Manganese; Copper

Carbon Cycling and the Coupling Between Proton and Electron Transfer Reactions in Aquatic Sediments in Lake Champlain by Wei-Jun Cai; George W. Luther III; Jeffrey C. Cornwell; Anne E. Giblin (pp. 421-446).

We used fine-scale porewater profiles and rate measurements together with a multiple component transport–reaction model to investigate carbon degradation pathways and the coupling between electron and proton transfer reactions in Lake Champlain sediments. We measured porewater profiles of O₂, Mn²⁺, Fe²⁺, HS⁻, pH and pCO₂ at mm resolution by microelectrodes, and profiles of NO₃ ⁻, SO₄ ²⁻, NH₄ ⁺, total inorganic carbon (DIC) and total alkalinity (TA) at cm resolution using standard wet chemical techniques. In addition, sediment–water fluxes of oxygen, DIC, nitrate, ammonium and N₂ were measured. Rates of gross and net sulfate reduction were also measured in the sediments. It is shown that organic matter (OM) decomposes via six pathways: oxic respiration (35.2%), denitrification (10.4%), MnO₂ reduction (3.6%), FeOOH reduction (9.6%), sulfate reduction (14.9%), and methanogenesis (26.4%). In the lake sediments, about half of the benthic O₂ flux is used for aerobic respiration, and the rest is used for the regeneration of other electron acceptors produced during the above diagenetic reactions. There is a strong coupling between O₂ usage and Mn²⁺ oxidation. MnO₂ is also an important player in Fe and S cycles and in pH and TA balance. Although nitrate concentrations in the overlying water were low, denitrification becomes a quantitatively important pathway for OM decomposition due to the oxidation of NH₄ ⁺ to NO₃ ⁻. Finally, despite its low concentration in freshwater, sulfate is an important electron acceptor due to its high efficiency of internal cycling. This paper also discusses quantitatively the relationship between redox reactions and the porewater pH values. It is demonstrated here that pH and pCO₂ are sensitive variables that reflect various oxidation and precipitation reactions in porewater, while DIC and TA profiles provide effective constraints on the rates of various diagenetic reactions.

Keywords: Microelectrodes; Carbon cycling; Acid–base reactions; Electron transfer reactions; Lake sediments

Dissociation Constants of Protonated Oxidized Glutathione in Seawater Media at Different Salinities by Pasquale Crea; Concetta De Stefano; Frank J. Millero; Silvio Sammartano; Virender K. Sharma (pp. 447-466).

Oxidized glutathione (GSSG), which has four carboxylic and two amino groups, interacts with metal ions and may affect the bioavailability and geochemistry of metals in natural waters. In the present paper, six stepwise protonation constants $$ K^{ ext{H}}_{i} $$ for GSSG were measured as a function of salinity, S = 5–35‰ at t = 25°C (and in NaCl/MgCl₂ mixtures at different ionic strengths), in order to provide thermodynamic data for their acid base properties, which are useful for studying the interaction with metals in these media. The protonation enthalpies (ΔH i/kJ mol⁻¹) were also determined at t = 25°C. The results were interpreted using the SIT model and Pitzer equations. The seawater model with the interaction parameters accounts for the differences between the values in NaCl and seawater. The results suggest that it is important to consider all of the ionic interactions in natural waters in examining the proton dissociation of GSSG.

Keywords: Oxidized glutathione; Protonation constants; Synthetic seawater; Medium effects; SIT model; Pitzer coefficients

Fe(III) Reduction in the Presence of Catechol in Seawater by J. Magdalena Santana-Casiano; M. González-Dávila; A. G. González; F. J. Millero (pp. 467-482).

Fe(II)-Fe(III) redox behavior has been studied in the presence of catechol under different pH, ionic media, and organic compound concentrations. Catechol undergoes oxidation in oxic conditions producing semiquinone and quinone and reduces Fe(III) in natural solutions including seawater (SW). It is a pH-dependent process. Under darkness, the amount of Fe(II) generated is smaller and is related to less oxidation of catechol. The Fe(II) regeneration is higher at lower pH values both in SW with log k = 1.86 (M⁻¹ s⁻¹) at pH 7.3 and 0.26 (M⁻¹ s⁻¹) at pH 8.0, and in NaCl solutions with log k of 1.54 (M⁻¹ s⁻¹) at pH 7.3 and 0.57 (M⁻¹ s⁻¹) at pH 8.0. At higher pH values, rate constants are higher in NaCl solutions than in SW. This is due to the complexation of Mg(II) present in the media with the semiquinone that inhibits the formation of a second Fe(II) through the reaction of this intermediate with other center Fe(Cat)⁺.

Keywords: Iron; Catechol; Oxidation; Reduction; Kinetics; Seawater

The Kinetics of the Interaction Between Iron(III)-Ethylenediaminetetraacetate and Peroxynitrite by Virender K. Sharma; Ria A. Yngard; Zoltan Homonnay; Abhishek Dey; Chun He (pp. 483-490).

The kinetics of the formation of the purple-colored species between Fe^III-EDTA and peroxynitrite were studied as a function of pH (10.4–12.3) at 22°C in aqueous solutions using a stopped-flow technique. A purple-colored species was immediately formed upon mixing, which had an absorbance maximum at 520 nm. The increase in absorbance with time could be fit empirically by a power function with two parameters a and b. The power equation determined was absorbance = a*t b, where a increased linearly with pH and the concentration of peroxynitrite, while b almost remained constant with a value of ~0.25. The molar extinction coefficient ε_520 nm for the colored species was determined as 13 M⁻¹cm⁻¹, which is much lower than ε_520 nm = 520 M⁻¹ cm⁻¹ for the [Fe^III(EDTA)O₂]³⁻, a purple species observed in the Fe^III–EDTA–H₂O₂ system. The results of kinetics and spectral measurements of the present study are briefly discussed and compared with those of the reaction between Fe(III)-EDTA and hydrogen peroxide.

Keywords: Iron(III); EDTA; Peroxynitrite; Kinetics; Time-resolved spectra

Photolysis of 2,4-Dinitrotoluene and 2,6-Dinitrotoluene in Seawater by Daniel W. O’Sullivan; Jeffrey R. Denzel; Dianne J. Luning Prak (pp. 491-505).

The purpose of this experiment was to determine the rate of photolysis for 2,4-dinitrotoluene (2,4-DNT) and 2,6-dinitrotoluene (2,6-DNT) in salt water and pure water, examine the influence of salt concentration on the photolysis rate, and to identify the products of the photolysis of 2,4-DNT and 2,6-DNT in seawater. For the photolysis of 2,4-DNT in pure water, the half-life was longer than 100 h, whereas the photolysis half-life in salt water was ~15 h. For 2,6-DNT, the pure water photolysis half-life was close to 20 h, but the salt water photolysis half-life was only 5 h. The photolysis rate for 2,6-DNT in seawater was affected by the ionic strength of the parent solution and was independent of the identity of the salt. The photolysis of both DNTs in seawater resulted in a complex mixture of products that exhibit a distinct yellow color. Analysis by gas and liquid chromatography mass spectroscopy (GCMS, LCMS) following liquid–liquid extraction or solid phase extractions resulted in the confirmation of the corresponding dinitrobenzaldhydes and dinitrobenzoic acids as photolysis products and a probable azoxy compound among the photolysis products in seawater.

Keywords: Explosives; Solar simulator; Nitroaromatic compounds; Photolysis; Seawater

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Aquatic Geochemistry (v.16, #3)

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Aquatic Geochemistry (v.16, #3)