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Aquatic Geochemistry (v.16, #2)
In Memoriam: John W. Morse (1946–2009) Texas A&M University
by Fred T. Mackenzie; Alfonso Mucci; George W. Luther III (pp. 219-221).
Natural Arsenic Attenuation via Metal Arsenate Precipitation in Soils Contaminated with Metallurgical Wastes: I. Wet Chemical and Thermodynamic Evidences by M. Villalobos; D. G. García-Payne; J. L. López-Zepeda; A. E. Ceniceros-Gómez; M. E. Gutiérrez-Ruiz (pp. 225-250).
Arsenic from natural and anthropogenic sources is a worldwide contaminant of aqueous environments, such as groundwater and soils. The present investigation was performed on Mexican soils contaminated with residues from metallurgical processes that have shown a natural As attenuation. Experimental aqueous arsenic extractions in these were successfully simulated for almost half of the soil samples using a database updated for all known metal arsenate formation constants, revealing the predominance of solubility-controlled As mobility via Pb, mixed Pb–Cu, and Ca arsenate solid formation. The relatively low total Fe/As ratios (2–13 w/w) present in the soils studied, together with the high and equivalent contents of As, Pb, and Cu in these, favor the precipitation process over As(V) adsorption to Fe oxides, despite a 2% average Fe content in the soils studied. Under these conditions bicarbonate was found to be a highly unsuitable extractant due to its indirect As release from the solid arsenates, via heavy metal carbonate precipitation processes.
Keywords: Arsenic; Metal arsenate; Solubility; Soils; Contamination; Metallurgical wastes
Geochemical Control of Heavy Metal Concentrations and Distribution Within Bahia Blanca Estuary (Argentina) by Jorge Marcovecchio; Sandra Botté; Melisa Fernández Severini; Federico Delucchi (pp. 251-266).
Chromium and lead concentrations and distribution have been fully studied within Bahía Blanca estuary inner area, which is strongly influenced by urban and industrial stress. Not only metals dissolved in estuarine water but also those included in sediments and suspended particulate matters (SPM) were measured. In all cases, internationally standardized protocols were applied to metal measurements, including analytical quality check test through analysis of certified reference materials. Total metal contents from surface sediments and SPM were compared with those from a historical database of the area, as well as with values representing the natural geochemical baseline within the system. Results showed that heavy metal pollution is mainly localized in the areas close to both industrial effluents discharge system and urban sewage outfall discharge. Data from sequential extractions indicate that metals from anthropogenic sources (i.e., Pb) are potentially more mobile than those inherited from geological parent material (i.e., Cr). The influences of other potential sources of metals (i.e., streams, runoff) were also considered. SPM was clearly identified as the main carrier of the studied heavy metals within the system, and its significance to metals input into sediments and/or biota was verified. Finally, the normalization of measured metal concentrations against background reference elements (i.e., Al or Fe) has allowed to identify that most of the measured Cr was lithogenic, while a significant percentage of Pb was from anthropogenic origin within Bahía Blanca estuary.
Keywords: Heavy metal pollution; Estuaries; Geochemical regulation; Dissolved and particulate metals; Geochemical fractionation
Mercury Redox Chemistry in the Negro River Basin, Amazon: The Role of Organic Matter and Solar Light by Wilson F. Jardim; Márcia Cristina Bisinoti; Pedro Sérgio Fadini; Gilmar Silvério da Silva (pp. 267-278).
Pristine water bodies in the Negro River basin, Brazilian Amazon, show relatively high concentrations of mercury. These waters are characterized by acidic pH, low concentrations of suspended solids, and high amounts of dissolved organic matter and are exposed to intense solar radiation throughout the year. This unique environment creates a very dynamic redox chemistry affecting the mobility of mercury due to the formation of the dissolved elemental species (Hg0). It has been shown that in this so-called black water, labile organic matter from flooded forest is the major scavenger of photogenerated H2O2. In the absence of hydrogen peroxide, these black waters lose their ability to oxidize Hg0 to Hg2+, thus increasing Hg0 evasion across the water/atmosphere interface, with average night time values of 3.80 pmol m−2 h−1. When the dry period starts, labile organic matter inputs gradually diminish, allowing the increasing concentration of H2O2 to re-establish oxidative water conditions, inhibiting the metal flux across the water/atmosphere interface and contributing to mercury accumulation in the water column.
Keywords: Amazon; Mercury; Redox chemistry; Negro river; Solar light
Modeling the Acid–Base Surface Properties of Aquatic Sediments by L. Borgnino; M. G. Garcia; M. V. del Hidalgo; M. Avena; C. P. De Pauli; M. A. Blesa; P. J. Depetris (pp. 279-291).
We propose a model that explains the acid–base surface properties of clastic sediments from two Argentinean reservoir lakes. The model uses potentiometric titration data sets and fixed parameters such as the apparent stability constants and reaction stoichiometries of acid–base equilibriums involving known mineral phases. The model considers that sediments act as a set of independent sorption surfaces, such as organic matter, clay silicate, and iron (hydr)oxides, thus the acid–base equilibrium and the correspondent protolytic constants are represented by a humic acid, a Na-illite, and a poor crystalline Fe-hydr(oxide). In agreement with experimental data, the model predicts that all sediment samples show a similar charging behavior, increasing the negative charge as the pH increases. The net charge of sediments is controlled by the presence of negatively charged minerals and/or organic matter coatings. This reveals the great influence of clays and organic matter functional groups on the acid–base surface properties of sediments, and consequently on the surface reactivity toward contaminant transport.
Keywords: pH-dependent charge; Clastic sediments; Potentiometric titrations; FITEQL; Multisurface approach
Spatial and Temporal Ion Dynamics on a Complex Hydrological System: The Lower Luján River (Buenos Aires, Argentina) by Ruben J. Lombardo; Inés O’Farrell; María dos Santos Afonso (pp. 293-309).
The interplay of the geochemistry of the lower stretch of the Luján River is analyzed with its natural geomorphology (basin features), local hydrometeorology (tidal regime and “sudestada” events) and the impact of polluted tributaries. Major ions, dissolved heavy metals and limnological variables were analyzed using multivariate techniques. The water quality of the mainstream of the Luján River, flowing through urbanized and industrialized areas is strongly and positively influenced by the input of the Paraná River through deltaic watercourses and is negatively impacted after receiving the discharge from polluted watercourses. The longitudinal spatial variations evidenced major discontinuities in the Lower Luján Basin, showing clearly the riverine and the deltaic water influenced zones. Seasonal variations were also marked and were either temperature driven or associated with the estuarine cycle and the “sudestada”.
Keywords: Major ion dynamics; Luján River; Hydrological system