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Aquatic Geochemistry (v.3, #3)
Determination of the Electrochemical Properties of a Soluble Aqueous FeS Species Present in Sulfidic Solutions by Stephen M. Theberge; George W. Luther III (pp. 191-211).
Field and laboratory data are presented that show a soluble FeS species(FeSaq) exists in sulfidic seawater solutions, and is observedwhen the IAP exceeds the Ksp of amorphous FeS. TheFeSaq yields a discrete signal (double peak) using square-wavevoltammetry and two one-electron waves in sampled DC polarographyexperiments at the Hg electrode. The aqueous FeS species reacts irreversiblyat the electrode as a single FeS subunit and not as a polymeric entity. Thepeak potential of FeSaq occurs at -1.1 V whereas the peakpotential of Fe $$(H_2 O)_6^{2 + } $$ occurs at-1.45 V; the positive shift for Fe2+ reduction inFeSaq indicates a change in geometry for Fe2+from octahedral to tetrahedral. The kinetics of electron transfer at theelectrode are determined to be similar for both Fe2+ andFeSaq. Molecular orbital energy diagrams, further indicatethat Fe(II) does change from octahedral to tetrahedral geometry in solution.First, Fe(II) exists as octahedralFe $$(H_2 O)_6^{2 + } $$ in solution whichundergoes a substitution reaction of bisulfide for water. The resultingcomplex, Fe(H2O)5(HS)+, thentransforms to a tetrahedral complex on further addition of sulfide. Thisgeometry change is consistent with the formation of amorphous FeS thatconverts to mackinawite which has tetrahedral Fe(II). The process is entropydriven because of the water loss that occurs. The overall sequence can berepresented as: $$egin{gathered} 3Fe(H_2 O)_6^{2 + } + 3HS^ - o 3Fe(H_2 O)_5 (HS)^ + + 3H_2 O hfill \ 3Fe(H_2 O)_5 (HS)^ + + 3HS^ - o Fe_3 S_3 (H_2 O)_6 + 3H_2 S + 9H_2 O hfill \ end{gathered} $$ Soluble FeS species are important asreactants in the formation of iron-sulfide minerals including pyrite.
Characterizing Colloidal Material in Natural Waters by J.R. Lead; W. Davison; J. Hamilton-Taylor; J. Buffle (pp. 213-232).
Sampling has inherent uncertainties when applied tothe sub-micron fraction of natural waters.Processes of aggregation, biological activity andchemical transformation potentially effect changesimmediately after a sample is taken from the waterbody and there is no suitable method of samplestabilization. The size distribution and associatedphysical and chemical parameters of colloids in theaquatic environment can be effectively stable overshort periods of time (about two days under idealconditions, but frequently much shorter timeintervals). To achieve accurate representations of thesize distribution and associated colloidalcharacteristics in situ techniques are required,although adequate approximations may be obtained undersome circumstances if separation is done immediatelyafter sampling. This paper reviews the currentlyavailable strategies for separation and analysis ofcolloids from natural waters (primarily filtration andcentrifugation) and discusses their uses andlimitations, as well as potential uses of promisingtechniques (voltammetry, gels, field-flowfractionation, SPLITT). For small colloids, thetechniques of voltammetry, dialysis, DET and DGT maybe used to obtain in situ information. Forlarger colloids it is more difficult to performmeasurements in situ and a combination of rapidfractionation procedures, including filtration,field-flow fractionation and SPLITT, may still berequired.
Keywords: in situ analysis; colloids; natural waters
Water Geochemistry of Three Mountain Streams from Carbonate Watersheds in the Southern French Alps by Gérard Sarazin; Jean-Pierre Ciabrini (pp. 233-265).
The chemical composition of water from three streamsflowing through a carbonate watershed wasinvestigated. Although the study area is not spatiallyvery large (a few km2), local inhomogeneitieswithin the lithology appreciably affect the chemicalcomposition of the water and the geochemical gradientas a function of elevation. The main chemical processwhich leads to the observed water chemistry is thedissolution of calcium carbonate by atmospheric andmetabolic CO2. In the stream La Sigouste, thewater dissolves nearly pure calcite and this reactionproceeds until an equilibrium with respect to calciteis reached. In Le Lauzon stream, local inhomogeneitiesin the lithologic composition prevent theestablishment of an unambiguous weathering budgetwhile, for Le Rif de l'Arc a stoichiometric modelshows that the weathering process is adequatelyrepresented by the dissolution of calcite associatedwith a minor incongruent dissolution of chloritepresent in marls. For the two last streamssupersaturation with respect to calcite is observed.For both streams, supersaturation with respect toatmospheric CO2 is nearly permanent. The observedaltitudinal gradients of sulfate are interpretedthrough a set of biogeochemical redox reactions.
Keywords: weathering; carbonate equilibria; watersheds
The Distribution of Redox Sensitive Elements (U, As, Sb, V and Mo) along a River-Wetland-Lake System (Balaton Region, Hungary) by Françoise Elbaz-Poulicheet; Attila Nagy; Tibor Cserny (pp. 267-282).
The distribution of redox sensitive elements (U, As,Sb, Mo and V) has been investigated in Lake Balatonand two man-made reservoirs (Lake Kis-Balaton-1 andKis-Balaton-2) built along the River Zala, the mainriver discharging into it. These elements underwentremoval in Lake Kis Balaton-2 (KB-2) during anoxicconditions (May and September).Similarities were found between the distributions ofdissolved As and Sb. Both elements had elevateddissolved concentrations in Lake Kis-Balaton-1 (KB-1)and in Lake Balaton in September. This increasedmobility could be due to a change of speciation (viareduction and the formation of methylated species).Such changes in speciation are well documented in theliterature and are generally enhanced during periodsof warmth and high primary productivity.Dissolved V and Mo distributions also showedsimilarities with higher concentrations in LakeBalaton than in other parts of the system.Although removal is not clearly detected from thedissolved concentrations, U, As, Sb and Mo aresignificantly enriched in the organic-rich sedimentsof Lake KB-1, while V is mainly associated with thealumino-silicate fraction. In Lake KB-1, theconcentrations of all elements in SPM (suspendedparticulate matter) are lower than in the depositedsediments (except for As) suggesting that enrichmentprocesses are occuring in the deposited sediments.Additionnally the cycling of As and Mo can be alsopartly controlled by uptake and scavenging processesin the water column, as suggested by the elevated Asand Mo concentrations observed in SPM seasonnally forAs in Lake KB-1 and at Z.mouth-1 station and for Mo atZ.mouth-1 station and Lake Balaton.
Keywords: redox sensitives elements-lake-wetland-water-sediment-suspended matter