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Aquatic Geochemistry (v.9, #4)
Geochemical Processes Controlling the Formation of As-Rich Waters Within a Tailings Impoundment (Carnoulès, France) by Corinne Casiot; Marc Leblanc; Odile Bruneel; Jean-Christian Personné; Kouadio Koffi; Françoise Elbaz-Poulichet (pp. 273-290).
The distribution of arsenic (As(III), As(V)) and iron (Fe(II), Fe(III)) species was monitored during 1 year in a borehole drilled in the Carnoulès tailings impoundment which contains As-rich pyrite. The concentrations of total As and Fe in subsurface waters exhibited strong variations over one year, which were controlled by dissolved oxygen concentrations. At high oxygen levels, extremely high As (up to 162 mM) and Fe (up to 364 mM) concentrations were reached in the borehole, with the oxidised species predominant. As and Fe concentrations decreased 10-fold under oxygen-deficient conditions, as a result of pH increase and subsequent precipitation of As(V) and Fe(III). From drill core sections, it appeared that at low dissolved oxygen levels, As(III) was primarily released into water by the oxidation of As-rich pyrite in the unsaturated zone. Subsequent As and Fe precipitation was promoted during transport to the saturated zone; this reaction resulted in As enrichments in the sediment below the water table compared to the original content in pyrite, together with the formation of As-rich (up to 35 wt% As) ferruginous material in the unsaturated zone. High amounts of As(V) were released from these secondary phases during leaching experiments with oxygenated acid sulfate-rich waters; this process is believed to contribute to As(V) enrichment in the subsurface waters of the Carnoulès tailings during periods of high dissolved oxygen level.
Keywords: arsenic; acid mine drainage (AMD); speciation; mine tailings; pyrite oxidation
Kinetics of Disproportionation of Inorganic Polysulfides in Undersaturated Aqueous Solutions at Environmentally Relevant Conditions by A. Kamyshny; A. Goifman; D. Rizkov; O. Lev (pp. 291-304).
The rate of the reversible homogeneous disproportionation of polysulfides was studied by following the optical absorbance of polysulfide solutions in a continuous plug flow reactor equipped with an on-line photometric detector. In order to avoid heterogeneous slow reactions involving sulfur colloids or precipitate, the reaction was initiated by an abrupt pH change from an undersaturated solution containing predominantly tetrasulfide species to a pH where pentasulfide is the dominant species. The disproportionation was found to follow first order reversible reaction dynamics. At environmentally relevant conditions the characteristic time of the disproportionation reaction is of the order of 10 s. This characteristic time implies that necessary conditions for speciation of the different polysulfide species by chromatography or another separation and subsequent quantification scheme should be of the order of 1 s.
Keywords: inorganic polysulfides; disproportionation kinetics; activation energy; reduced sulfur compounds
Formation of Colloidal Silica and Alumina During Experimental Granodiorite Weathering by Jiří Faimon (pp. 305-341).
The role of aluminum and silica in the formation of colloids during granodiorite weathering was studied on the basis of long-term experiments in batch reactors. Rock samples were dissolved in un-buffered solutions of initial pH ∼ 3.2, 5.4, and 9.9 at ambient conditions for 500 days. During weathering, extremely high supersaturation with respect to various secondary solids was attained in the solutions. Consequently, new solids, part of which was conserved in solutions as colloids, condensed. The mean concentrations of colloidal Si reached values of 70, 50, and 48 μmol 1−1 in the alkaline, neutral, and acid solutions, respectively. The mean concentrations of colloidal Al, reached values of 34, 22, and 12 μmol 1−1 in the alkaline, neutral, and acid solutions, respectively. The concentration of colloids gradually decreased after 200-400 days of experiment. This phenomenon was interpreted as being due to the competition between homogeneous nucleation and crystal growth. At the initial stages of the experiments, the colloidal species (predominantly colloidal Al) comprised a large proportion of the total amounts of aqueous species. Their share, however, decreased with time. The molar Al/Si-ratios of colloids were as high as 2–2.5 at the early stages of the experiment. After 250–300 days of experiments, on the other hand, these ratios decreased to values of about 0.5 in both the neutral and alkaline solutions and to a value of 0.15 in the acid solution. The evolution of colloids was consistent with the evolution of secondary solids in the sequence Al-hydroxides – clay minerals (illite, chlorite), in both the neutral and alkaline solutions. In acid solutions, the evolution of Al/Si-colloids was influenced by the presence of sulfate ion and Al-sulfate precipitation. Besides Al and Si, other elements, in particular Ca or Mg as a major component and Na, K, P, S, and Cl as minor components, readily participated in the formation of colloids.
Keywords: aluminum; colloids; dynamics; granodiorite; silica; supersaturation; weathering
Buffering Mechanisms in Acidic Mining Lakes – A Model-Based Analysis by Oliver Totsche; Rosemarie Pöthig; Wilfried Uhlmann; Heike Büttcher; Christian E.W. Steinberg (pp. 343-359).
Buffering mechanisms of acidic mining lakes were studied on an acidic open cast lignite mining lake (Plessa 111) in Lusatia, eastern Germany. The investigations were based upon laboratory experiments, hydrochemical modelling and mineralogical structure determination. The experiments were carried out as titrations of the lake water. The modelling was done using the hydrogeochemical simulator PHREEQC. Three different phase models were applied:(1) an iron- and aluminium hydroxide precipitation model, (2) an iron- and aluminium hydroxysulphate precipitation model and (3) an iron hydroxide and aluminium hydroxysulphate precipitation model with sorption on iron hydroxide. As a result of this study four different buffers in four successive sections of the titration curve could be detected in acidic mining lake water:(1) hydrogen sulphate buffering, (2) iron buffering, (3) aluminium buffering and (4) an additional buffering (solid phase buffer) which is based on ion exchange and mineral transformation. Mineralogical analysis showed that iron precipitates as schwertmannite, an iron hydroxysulphate. Aluminium precipitates are also hydroxysulphates. These results allow formulation of process-oriented description of the buffering processes in acidic mine waters. This could lead to better reliability in lake water quality and pH-development predictions of acidic mining lakes.
Keywords: pH buffering; titration; schwertmannite; PHREEQC modelling; acidic mining lake