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Aquatic Geochemistry (v.4, #2)
Effect of Complexing Agents (EDTA and ATMP) on the Remobilization of Heavy Metals from a Polluted River Sediment by François Bordas; Alain C. M. Bourg (pp. 201-214).
EDTA and ATMP are synthetic chelatingagents used in many industrial applications. Becauseof their low biodegradability, they are present inmany aquatic systems where their strong complexingcapacities can modify the transfer of toxic heavymetals between solid and liquid phases.The remobilization of Cu, Pb and Cd from pollutedsuperficial river sediments in the presence of twosynthetic organic ligands (a polycarboxylate, EDTA,and a phosphonate, ATMP) is investigated as a functionof reaction time to determine the time necessary toreach dissolutive equilibrium, and as a fonction ofthe complexing agent concentration.The dissolutive equilibrium times are similar (10 to20 hours) and the solubilizing power of ATMP isslightly lower than that of EDTA. The curves ofsolubilization of metals in the presence of thecomplexing agents present a shape of a “chelateconcentration” desorption edge, for a chelate rangebetween 10-5 to 10-3 mol L-1, thesolubilised metals increasing dramatically. Thisremobilization is the result of a competition betweenassociation with the solid phase and dissolvedcomplexation. With EDTA, the desorption order ofmetals seems to be imposed by association with thesolid. For ATMP is it the chelate complexing strengthwhich determines this order.
Keywords: sediment; heavy metals; remobilization; EDTA; ATMP
Complexation of Gold(III)-Chloride at the Surface of Hematite by Olga N. Karasyova; Lyudmila I. Ivanova; Leonid Z. Lakshtanov; Lars Lövgren; Staffan Sjöberg (pp. 215-231).
The acid/base properties of the hematite/water interface and surfacecomplexation with gold has been studied by potentiometric titrations in thefour component system H+ – ≡FeOH –AuCl 4 - – Cl-. Equilibrium measurementswere performed in NaCl media at 298.2 K. In the evaluation of equilibriummodel from experimental data the constant capacitance model was applied. Theacid/base properties were investigated in 0.1 M NaCl in the range 2.6 ≤pH ≤ 7.4. The resulting intrinsic constants for protonation anddeprotonation of hydroxyl groups at the surface were logβ 1,1,0,0(int) s = 7.10 ± 0.06 andlog β -1,1,0,0(int) s = - 7.80 ±0.06. The density of proton active surface sites was 2.85nm-2 and the specific capacitance 2.5 C V-1 m-2. In the investigation of surface complexation of gold, thepotentiometric titrations were performed in the range 2.0 ≤ pH ≤ 10.2.Titration data was supplemented with analysis of Au in the aqueous phase byatomic absorbance spectrometry. The equilibrium model proposed consists ofthe following monodentate surface complexes: ≡FeOHAuCl3(log β 0,1,1,-1 s = 1.45 ± 0.03),≡FeOHAuCl2OH (logβ -1,1,1,-2 s = -3.89 ± 0.02), ≡FeOHAu(OH)3 (logβ -3,1,1,-4 s = -21.94 ± 0.05). Aslightly better fit could be obtained by assuming formation of a bidentatecomplex with the composition(≡FeO)2Au(OH)H2O. However, based onstructural arguments this complex was rejected.
Keywords: Gold; hematite surface; surface complexation; adsorption; potentiometric titration
Arsenic and other Trace Metals in Sediments from the Kara Sea and the Ob and Yenisey Estuaries, Russia by D.H. Loring; S. Dahle; K. Naes; J. Dos Santos; J.M. Skei; G.G. Matishov (pp. 233-252).
Major (Al and Fe), minor (Mn) andtrace (As, Cd, Co, Cr, Cu, Hg, Li, Mo, Ni, Pb, Sb, Vand Zn) metals along with material of grain size<63 μm, TOC and TN have been determined insediment grab and core samples from the Kara Sea, andthe Ob and Yenisey estuaries, Russia. Surprisingly,the levels of trace metals, with the exception of As,were much lower than was anticipated from speculativereports of extensive contamination in the Arcticmarine areas adjacent to the Siberian coastline ofRussia. Lithium normalization indicates that theabundance and distribution of the metals, with theexception of As and Mo, are controlled by theaccumulation of their fine grained aluminosilicatehost minerals at sites determined by hydrodynamicconditions in the Kara Sea and in the estuaries. Metallevels in the Kara Sea and the Ob and Yeniseyestuaries, except for some anomalous As, Cu and Nivalues, are close to natural baseline levels of otherEurasian Arctic shelf sediments. High levels of As,however, occur in surface and subsurface sediments.The accumulation of As, as well as Mo, can beattributed to the post-depositional diagenetic effectsof Fe-Mn cycling both at and near the sediment waterinterface. Subsurface As and Fe maxima and minimasuggest alternating oxic and anoxic water conditionsduring post-glacial rises in sea level. In contrast tothe results from the adjacent Pechora Sea, in the KaraSea there is no correlation between the levels of Asand radionuclides in the sediments.
Keywords: Arsenic; trace metals; sediments; normalization; diagenesis; Kara Sea; Ob estuary; Yenisey estuary; Russia
Redox Processes in Groundwater Impacted by Landfill Leachate by Leif Basberg; David Banks; Ola M. Sæther (pp. 253-272).
Groundwater downgradient from Trandum municipallandfill was investigated in summer 1996, with a viewto assessing the distribution of redox-sensitivespecies. The water table at Trandum is 10–26 metersbelow the surface. Monitoring well filters have beeninstalled at depths of up to 15 m below water table,covering an area of 400 m by 200 m. Groundwaterinfluenced by the landfill seems to be confined tothis area. Pristine groundwater has a chemicalsignature indicative of calcite and pyrite weathering.Groundwater influenced by leachate from the landfillexhibits an excess of alkalinity relative to calcium,which is likely to be derived from degradation oforganic matter in the landfill. Groundwaterimmediately below and downstream of the landfillcontains elevated concentrations of Fe and Mn, largelymobilized under reducing conditions from the aquifermatrix and reflected in depleted HNO3-extractable Feand Mn in sediment samples from the same area.Groundwater samples allow the tentative identificationof redox zones based on oxidized and reduced forms ofFe, Mn, N and S. A methanogenic zone is notobserved.
Keywords: Inorganic tracers; dilution factor; leachate attenuation; redox zones; groundwater
Seasonal Variability of Adsorption and Exchange Equilibria in Soil Waters by Robert a. Berner; Ji-Long Rao; Soobum Chang; Rachel O'brien; C. Kent Keller (pp. 273-290).
Chemical analyses for major ions have been conducted on waters,collected on an approximately weekly basis over the period April, 1993 toNovember, 1996, that drain three small experimental ecosystems(“sandboxes”) at Hubbard Brook, New Hampshire. One sandbox is planted withpine trees, another with grass, and the third is left “bare” (actually itis covered sporadically by bryophytes and lichens). Results show linearcorrelations, independent of discharge, between the concentrations ofdissolved Na+ and K+ on the one hand andCa++ and Mg++ on the other for all threesandboxes. No correlations between singly charged and doubly chargedcations were found. These correlations are interpreted to represent cationexchange equilibria between soil waters and clay minerals plus soil organicmatter. The correlation slope, representing the exchange constant, for Na vsK is different for the pine-covered sandbox than for the other two whereasfor Ca vs Mg the correlation is independent of the presence or absence oftrees. We interpret this as representing a shift of cation exchangeequilibria in the pine sandbox by the activities of growing trees.Concentrations of Na, K, Ca, Mg, and H4SiO4from the barren and grass-lined sandboxes were found to vary seasonally witha marked sinusoidal pattern which was independent of the discharge from eachsandbox. (The discernment of a similar pattern in the tree lined sandbox wasdifficult due to a lack of discharge over much of the year.) Concentrationmaxima occurred in August and minima in February, and there is a closeparallelism with soil temperature. We interpret this as representingtemperature induced variations in cation exchange equilibria and silicaadsorption. Independence from highly varying water discharge, e.g.,. thataccompanying severe rainstorms, indicates rapidly re-attained equilibrium.Variations in the concentrations of cations are likely due to exchange withunmeasured cations, probably H+ or dissolved Al species, as aresult of possible seasonal changes in internal acid production and externalinput of acid rain to the sandboxes. Internal production may represent aresponse to seasonal changes in respiration rate as it responds toseasonally varying temperature. Added to this is the effect of temperatureon exchange equilibrium. Seasonal variations in dissolved silica are mostlikely due to the dependence of adsorption/desorption equilibria ontemperature. The temperature dependence of a number of silica-consumingreactions are consistent with the measured values.