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Aquatic Geochemistry (v.4, #1)
Humic Ion-Binding Model VI: An Improved Description of the Interactions of Protons and Metal Ions with Humic Substances by Edward Tipping (pp. 3-47).
Humic Ion-Binding Model VI, a discrete site/electrostatic model of the interactions of protons and metals with fulvic and humic acids, is applied to 19 sets of published data for proton binding, and 110 sets for metal binding. Proton binding is described with a site density, two median intrinsic equilibrium constants, two parameters defining the spread of equilibrium constants around the medians, and an electrostatic constant. Intrinsic equilibrium constants for metal binding are defined by two median constants, log KMA and log KMB, which refer to carboxyl and weaker-acid sites respectively, together with a parameter, ΔLK1, defining the spreads of values around the medians. A further parameter, ΔLK2, takes account of small numbers of strong binding sites. By considering results from many data sets, a universal average value of ΔLK1 is obtained, and a correlation established between log KMB and log KMA. In addition, a relation between ΔLK2 and the equilibrium constant for metal-NH3 complexation is tentatively suggested. As a result, metal-binding data can be fitted by the adjustment of a single parameter, log KMA. Values of log KMA are derived for 22 metal species. Model VI accounts for competition and ionic strength effects, and for proton-metal exchange.
Keywords: humic substances; humic acid; fulvic acid; protons; metals; binding; model
Evidence for Strong Copper(I) Complexation by Organic Ligands in Seawater by M. Fernanda C. Leal; Constant M. G. Van Den Berg (pp. 49-75).
Thiols like glutathione and cysteine form such stable complexes with copper(I) that they preclude the presence of copper(II). Conventionally seawater is titrated with copper(II) whilst monitoring the labile or reactive copper concentration by voltammetry or with other techniques, to determine the concentration of copper(II) binding complexing ligands in seawater. Titrations of seawater to which copper(I) binding ligands have been added reveal that the copper(I) binding ligands are detected when seawater is titrated with copper(II). The copper(II) in seawater is reduced to copper(I) within 2 to 40 minutes depending on the nature of the copper(I) binding ligand. The titrations of seawater with copper(II) thus give a response to the presence of copper(I) binding ligands indiscernible from that for copper(II) binding ligands. The stoichiometry of the detected apparent ligand concentrations for given concentrations of glutathione and cysteine suggest that 2 : 1 (thiol : Cu) complexes are formed. This was confirmed using voltammetry of free glutathione. Values of 21.2 and 22.2 were found for log β″CuL for glutathione and cysteine respectively (for the reaction of Cu′ + 2L′ ↔ CuL2). The complex stability is similar to that of natural organic species in the oceanic water column. The high stability of the copper(I) complexes was apparent from values of 32.1 and 32.6 for log β″Cu(I)L2 (for the reaction Cu+ + 2L″ ↔ CuL2) for the copper(I) complexes with glutathione and cysteine in seawater. Glutathione and other thiols are common in the marine system including the water column. It is therefore possible that at least some of the ligands detected in seawater, and previously assumed to be copper(II) binding ligands, are in fact strongly complexed as copper(I). The copper(I) oxidation state may thus be stabilised in seawater.
Keywords: Copper; seawater; complexation; cathodic stripping voltammetry
The Chemistry of Actinide Behavior in Marine Systems by Gregory R. Choppin; Pamela J. Wong (pp. 77-101).
Nuclear test explosions and reactor waste releases have deposited an estimated 16 × 1015 Bq of plutonium into the world's oceans. Chemical speciation, oxidation state, redox reactions, and sorption characteristics are integral in predicting solubility of the different actinides, their migration behaviors and their potential effects on marine biota. Transport mechanisms, biosequestration, interactions with carbonate and humic substances and chemical redox and speciation influence migration patterns. Analytical methods such as ultraviolet-visible absorption spectrophotometry, laser-induced photoacoustic spectroscopy and mass spectrometry are utilized to obtain information on the chemical speciation and oxidation states at sub-tracer levels. Predictive computer models has attempted to evaluate the extensive data needed to adequately parameterize the complex, interdependent and continuous processes involved.
A Coupled Riverine-Marine Fractionation Model for Dissolved Rare Earths and Yttrium by Robert H. Byrne; Xuewu Liu (pp. 103-121).
Fractionation of yttrium (Y) and the rare earth elements (REEs) begins in riverine systems and continues in estuaries and the ocean. Models of yttrium and rare earth (YREE) distributions in seawater must therefore consider the fractionation of these elements in both marine and riverine systems. In this work we develop a coupled riverine/marine fractionation model for dissolved rare earths and yttrium, and apply this model to calculations of marine YREE fractionation for a simple two-box (riverine/marine) geochemical system. Shale-normalized YREE concentrations in seawater can be expressed in terms of fractionation factors (λ ij ) appropriate to riverine environments ( $$lambda _{ij}^{river}$$ ) and seawater ( $$lambda _{ij}^{ocean}$$ ): $$log frac{{left( {M_i }
ight)_T^{ocean} }}{{left( Y
ight)_T^{ocean} }} = log;lambda _{ij}^{ocean} + ((lambda _{ij}^{river} )^{ - 1} - 1);logfrac{{[Y]_T^{river} }}{{[Y^0 ]_T^{river} }}$$ where $$left( {M_i }
ight)_T^{ocean}$$ and $$left( Y
ight)_T^{ocean}$$ are input-normalized total metal concentrations in seawater and $$[Y]_T^{river} /[Y^0 ]_T^{river}$$ is the ratio of total dissolved Y in riverwater before $$([Y^0 ]_T^{river} )$$ and after $$([Y]_T^{river} )$$ commencement of riverine metal scavenging processes. The fractionation factors (λ ij ) are calculated relative to the reference element, yttrium, and reflect a balance between solution and surface complexation of the rare earths and yttrium.
Keywords: Rare earth; Rare earth; fractionation; model; riverine; oceanic; estuarine
ISORROPIA: A New Thermodynamic Equilibrium Model for Multiphase Multicomponent Inorganic Aerosols by Athanasios Nenes; Spyros N. Pandis; Christodoulos Pilinis (pp. 123-152).
A computationally efficient and rigorous thermodynamic model that predicts the physical state and composition of inorganic atmospheric aerosol is presented. One of the main features of the model is the implementation of mutual deliquescence of multicomponent salt particles, which lowers the deliquescence point of the aerosol phase.The model is used to examine the behavior of four types of tropospheric aerosol (marine, urban, remote continental and non-urban continental), and the results are compared with the predictions of two other models currently in use. The results of all three models were generally in good agreement. Differences were found primarily in the mutual deliquescence humidity regions, where the new model predicted the existence of water, and the other two did not. Differences in the behavior (speciation and water absorbing properties) between the aerosol types are pointed out. The new model also needed considerably less CPU time, and always shows stability and robust convergence.
Keywords: Inorganic aerosols; thermodynamic equilibrium; mutual deliquescence; ammonium salts; sodium salts; aerosol model
A Chemical Equilibrium Model for Natural Waters by Frank J. Millero; Denis Pierrot (pp. 153-199).
This paper reviews the present status of the Pitzer chemical equilibrium model, which can be used to characterize the one-atmosphere activity coefficients of ionic and non-ionic solutes in natural waters as a function of temperature and ionic strength. The model considers the ionic interactions of the major seasalt ions (H, Na, K, Mg, Ca, Sr, Cl, Br, OH, HCO3, B(OH)4, HSO4, SO4, CO3, CO2, B(OH)3, H2O) and is based on the 25 °C model of Weare and co-workers. The model has been extended by a number of workers so that reasonable estimates can be made of the activity coefficients of most of the major seasalt ions from 0 to 250 °C. Recently coefficients for a number of solutes that are needed to determine the dissociation constants of the acids from 0 to 50 °C (H3CO3, B(OH)3, H2O, HF, HSO 4 - , H3PO4, H2S, NH 4 + etc.) have been added to the model. These results have been used to examine the carbonate system in natural waters and determine the activity of inorganic anions that can complex trace metals. The activity and osmotic coefficients determined from the model are shown to be in good agreement with measured values in seawater. This model can serve as the foundation for future expansions that can examine the activity coefficient and speciation of trace metals in natural waters. At present this is only possible from 0 to 50 °C over a limited range of ionic strengths (<1.0) due to the limited stability constants for the formation of the metal complexes. The future work needed to extend the Pitzer model to trace metals is discussed.
Keywords: Activity coefficients; activity; ions